very, very simple WP with midi CC out

.gitignore

@@ -0,0 +1,4 @@
+build/**
+.cache
+viseq
+compile_commands.json

Makefile

@@ -0,0 +1,33 @@
+CXX = g++-13
+CXXFLAGS = -std=c++20 -Wall -g -fno-omit-frame-pointer -gdwarf-4 -Og -DLIBREMIDI_ALSA=1
+CXXINC = -I/usr/lib/gcc/x86_64-pc-linux-gnu/13.3.0/include -Iinclude
+
+LDLIBS = -lsfml-graphics -lsfml-window -lsfml-system 
+LDFLAGS = -L/usr/lib -L/usr/local/lib
+
+hdr_files = ${wildcard src/*.h}
+
+src_files = ${wildcard src/*.cpp}
+src_files += ${wildcard include/libremidi/*.cpp}
+
+all: viseq
+	
+viseq: $(src_files) $(hdr_files)
+	mkdir -p build
+	$(CXX) ${wildcard build/*.o} -o viseq $(LDFLAGS) $(LDLIBS)
+	
+$(src_files): %.cpp : %_src
+	$(CXX) -c $@ -o build/$(*F).o $(CXXFLAGS) $(CXXINC)
+
+$(hdr_files): %.h : %_hdr
+	@echo "G++  >---------- $(*F).h -----------<"
+
+%_src:
+	@echo "G++  >---------- $(*F).cpp -----------<"
+
+%_hdr:
+	@echo "G++  >---------- $(*F).h -----------<"
+
+clean:
+	rm -f build/*
+	rm -f viseq

include/libremidi/api.hpp

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+#pragma once
+#include <libremidi/config.hpp>
+
+#include <string_view>
+#include <vector>
+
+namespace libremidi
+{
+//! MIDI API specifier arguments.
+//! To get information on which feature is supported by each back-end, check their backend file
+//! in e.g. backends/winmm.hpp, etc.
+enum class API
+{
+  UNSPECIFIED, /*!< Search for a working compiled API. */
+
+  // MIDI 1.0 APIs
+  COREMIDI,    /*!< macOS CoreMidi API. */
+  ALSA_SEQ,    /*!< Linux ALSA Sequencer API. */
+  ALSA_RAW,    /*!< Linux Raw ALSA API. */
+  JACK_MIDI,   /*!< JACK Low-Latency MIDI Server API. */
+  WINDOWS_MM,  /*!< Microsoft Multimedia MIDI API. */
+  WINDOWS_UWP, /*!< Microsoft WinRT MIDI API. */
+  WEBMIDI,     /*!< Web MIDI API through Emscripten */
+  PIPEWIRE,    /*!< PipeWire */
+
+  // MIDI 2.0 APIs
+  ALSA_RAW_UMP,          /*!< Raw ALSA API for MIDI 2.0 */
+  ALSA_SEQ_UMP,          /*!< Linux ALSA Sequencer API for MIDI 2.0 */
+  COREMIDI_UMP,          /*!< macOS CoreMidi API for MIDI 2.0. Requires macOS 11+ */
+  WINDOWS_MIDI_SERVICES, /*!< Windows API for MIDI 2.0. Requires Windows 11 */
+
+  DUMMY /*!< A compilable but non-functional API. */
+};
+
+/**
+ * \brief A function to determine the available compiled MIDI 1.0 APIs.
+
+  The values returned in the std::vector can be compared against
+  the enumerated list values.  Note that there can be more than one
+  API compiled for certain operating systems.
+*/
+LIBREMIDI_EXPORT std::vector<libremidi::API> available_apis() noexcept;
+
+/**
+ * \brief A function to determine the available compiled MIDI 2.0 APIs.
+
+  The values returned in the std::vector can be compared against
+  the enumerated list values.  Note that there can be more than one
+  API compiled for certain operating systems.
+*/
+LIBREMIDI_EXPORT std::vector<libremidi::API> available_ump_apis() noexcept;
+
+//! A static function to determine the current version.
+LIBREMIDI_EXPORT std::string_view get_version() noexcept;
+
+//! Map from and to API names
+LIBREMIDI_EXPORT std::string_view get_api_name(libremidi::API api);
+//! Map from and to API names
+LIBREMIDI_EXPORT std::string_view get_api_display_name(libremidi::API api);
+//! Look-up an API through its name
+LIBREMIDI_EXPORT libremidi::API get_compiled_api_by_name(std::string_view api);
+
+namespace midi1
+{
+//! Returns the default MIDI 1.0 backend to use for the target OS.
+inline constexpr libremidi::API default_api() noexcept
+{
+#if defined(__APPLE__)
+  return API::COREMIDI;
+#elif defined(_WIN32)
+  return API::WINDOWS_MM;
+#elif defined(__linux__)
+  return API::ALSA_SEQ;
+#elif defined(__emscripten__)
+  return API::EMSCRIPTEN_WEBMIDI;
+#else
+  return API::DUMMY;
+#endif
+}
+}
+
+namespace midi2
+{
+//! Returns the default MIDI 2.0 backend to use for the target OS.
+inline constexpr libremidi::API default_api() noexcept
+{
+#if defined(__APPLE__)
+  return API::COREMIDI_UMP;
+#elif defined(_WIN32)
+  return API::WINDOWS_MIDI_SERVICES;
+#elif defined(__linux__)
+  return API::ALSA_SEQ_UMP;
+#elif defined(__emscripten__)
+  return API::DUMMY;
+#else
+  return API::DUMMY;
+#endif
+}
+}
+}

include/libremidi/backends.hpp

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+#pragma once
+#include <tuple>
+
+#if !__has_include(<weak_libjack.h>) && !__has_include(<jack/jack.h>)
+  #if defined(LIBREMIDI_JACK)
+    #undef LIBREMIDI_JACK
+  #endif
+#endif
+#if !defined(LIBREMIDI_ALSA) && !defined(LIBREMIDI_JACK) && !defined(LIBREMIDI_COREMIDI) \
+    && !defined(LIBREMIDI_WINMM)
+  #define LIBREMIDI_DUMMY
+#endif
+
+#if defined(LIBREMIDI_ALSA)
+  #include <libremidi/backends/alsa_seq.hpp>
+  #include <libremidi/backends/linux/alsa.hpp>
+
+  #if LIBREMIDI_ALSA_HAS_RAMWIDI
+    #include <libremidi/backends/alsa_raw.hpp>
+  #endif
+
+  #if LIBREMIDI_ALSA_HAS_UMP
+    #include <libremidi/backends/alsa_raw_ump.hpp>
+    #include <libremidi/backends/alsa_seq_ump.hpp>
+  #endif
+#endif
+
+#if defined(LIBREMIDI_JACK)
+  #include <libremidi/backends/jack.hpp>
+#endif
+
+#if defined(LIBREMIDI_PIPEWIRE)
+  #include <libremidi/backends/pipewire.hpp>
+#endif
+
+#if defined(LIBREMIDI_COREMIDI)
+  #include <libremidi/backends/coremidi.hpp>
+  #include <libremidi/backends/coremidi_ump.hpp>
+#endif
+
+#if defined(LIBREMIDI_WINMM)
+  #include <libremidi/backends/winmm.hpp>
+#endif
+
+#if defined(LIBREMIDI_WINUWP)
+  #include <libremidi/backends/winuwp.hpp>
+  #if __has_include(<winrt/Microsoft.Devices.Midi2.h>)
+    #include <libremidi/backends/winmidi.hpp>
+  #endif
+#endif
+
+#if defined(LIBREMIDI_EMSCRIPTEN)
+  #include <libremidi/backends/emscripten.hpp>
+#endif
+
+#include <libremidi/backends/dummy.hpp>
+
+namespace libremidi
+{
+// The order here will control the order of the API search in
+// the constructor.
+template <typename unused, typename... Args>
+constexpr auto make_tl(unused, Args...)
+{
+  return std::tuple<Args...>{};
+}
+
+namespace midi1
+{
+static constexpr auto available_backends = make_tl(
+    0
+#if defined(LIBREMIDI_ALSA)
+    ,
+    alsa_seq::backend{}
+  #if LIBREMIDI_ALSA_HAS_RAMWIDI
+    ,
+    alsa_raw::backend{}
+  #endif
+#endif
+#if defined(LIBREMIDI_COREMIDI)
+    ,
+    core_backend{}
+#endif
+#if defined(LIBREMIDI_WINMM)
+    ,
+    winmm_backend{}
+#endif
+#if defined(LIBREMIDI_WINUWP)
+    ,
+    winuwp_backend{}
+#endif
+#if defined(LIBREMIDI_EMSCRIPTEN)
+    ,
+    emscripten_backend{}
+#endif
+#if defined(LIBREMIDI_JACK)
+    ,
+    jack_backend{}
+#endif
+#if defined(LIBREMIDI_PIPEWIRE)
+    ,
+    pipewire::backend{}
+#endif
+    ,
+    dummy_backend{});
+
+// There should always be at least one back-end.
+static_assert(std::tuple_size_v<decltype(available_backends)> >= 1);
+
+template <typename F>
+auto for_all_backends(F&& f)
+{
+  std::apply([&](auto&&... x) { ((x.available() && (f(x), true)), ...); }, available_backends);
+}
+
+template <typename F>
+auto for_backend(libremidi::API api, F&& f)
+{
+  static constexpr auto is_api = [](auto& backend, libremidi::API api) {
+    return backend.available() && backend.API == api;
+  };
+  std::apply([&](auto&&... b) { ((is_api(b, api) && (f(b), true)) || ...); }, available_backends);
+}
+}
+
+namespace midi2
+{
+static constexpr auto available_backends = make_tl(
+    0
+#if defined(LIBREMIDI_ALSA) && LIBREMIDI_ALSA_HAS_UMP
+    ,
+    alsa_seq_ump::backend{}, alsa_raw_ump::backend{}
+#endif
+#if defined(LIBREMIDI_COREMIDI)
+    ,
+    coremidi_ump::backend{}
+#endif
+#if defined(LIBREMIDI_JACK)
+#endif
+#if defined(LIBREMIDI_WINUWP)
+  #if __has_include(<winrt/Microsoft.Devices.Midi2.h>)
+    ,
+    winmidi::backend{}
+  #endif
+#endif
+#if defined(LIBREMIDI_EMSCRIPTEN)
+#endif
+    ,
+    dummy_backend{});
+
+// There should always be at least one back-end.
+static_assert(std::tuple_size_v<decltype(available_backends)> >= 1);
+
+template <typename F>
+auto for_all_backends(F&& f)
+{
+  std::apply([&](auto&&... x) { (f(x), ...); }, available_backends);
+}
+
+template <typename F>
+auto for_backend(libremidi::API api, F&& f)
+{
+  static constexpr auto is_api
+      = [](auto& backend, libremidi::API api) { return backend.API == api; };
+  std::apply([&](auto&&... b) { ((is_api(b, api) && (f(b), true)) || ...); }, available_backends);
+}
+}
+
+namespace midi_any
+{
+
+template <typename F>
+auto for_all_backends(F&& f)
+{
+  midi1::for_all_backends(f);
+  midi2::for_all_backends(f);
+}
+
+template <typename F>
+auto for_backend(libremidi::API api, F&& f)
+{
+  midi1::for_backend(api, f);
+  midi2::for_backend(api, f);
+}
+
+}
+}

include/libremidi/backends/alsa_raw.hpp

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+#pragma once
+#include <libremidi/backends/alsa_raw/midi_in.hpp>
+#include <libremidi/backends/alsa_raw/midi_out.hpp>
+#include <libremidi/backends/alsa_raw/observer.hpp>
+
+// Credits: greatly inspired from
+// https://ccrma.stanford.edu/~craig/articles/linuxmidi/alsa-1.0/alsarawmidiout.c
+// https://ccrma.stanford.edu/~craig/articles/linuxmidi/alsa-1.0/alsarawportlist.c
+// Thanks Craig Stuart Sapp <craig@ccrma.stanford.edu>
+
+namespace libremidi::alsa_raw
+{
+struct backend
+{
+  using midi_in = midi_in_impl;
+  using midi_out = midi_out_impl;
+  using midi_observer = observer_impl;
+  using midi_in_configuration = alsa_raw_input_configuration;
+  using midi_out_configuration = alsa_raw_output_configuration;
+  using midi_observer_configuration = alsa_raw_observer_configuration;
+  static const constexpr auto API = libremidi::API::ALSA_RAW;
+  static const constexpr auto name = "alsa_raw";
+  static const constexpr auto display_name = "ALSA (raw)";
+
+  static inline bool available() noexcept
+  {
+    static const libasound& snd = libasound::instance();
+    return snd.available && snd.rawmidi.available;
+  }
+};
+}

include/libremidi/backends/alsa_raw/config.hpp

@@ -0,0 +1,79 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <chrono>
+#include <functional>
+#include <optional>
+#include <span>
+#include <thread>
+
+#if __has_include(<poll.h>)
+  #include <poll.h>
+namespace libremidi
+{
+using poll_descriptors = pollfd;
+}
+#else
+namespace libremidi
+{
+struct poll_descriptors
+{
+  int fd;
+  short int events;
+  short int revents;
+};
+}
+#endif
+
+namespace libremidi
+{
+
+/**
+ * Used to determine how large sent messages will be chunked.
+ */
+struct LIBREMIDI_EXPORT chunking_parameters
+{
+  std::chrono::milliseconds interval{};
+  int32_t size{};
+
+  /**
+   * @brief Will be called by the chunking code to allow the API user to wait.
+   *
+   * By default just calls sleep.
+   * Arguments are: the time that must be waited, the bytes currently written.
+   * Return false if you want to abort the transfer, and true otherwise.
+   */
+  std::function<bool(std::chrono::microseconds, int64_t)> wait = chunking_parameters::default_wait;
+
+  static bool default_wait(std::chrono::microseconds time_to_wait, int64_t /*written_bytes*/)
+  {
+    std::this_thread::sleep_for(time_to_wait);
+    return true;
+  }
+};
+
+struct manual_poll_parameters
+{
+  std::span<poll_descriptors> fds;
+  std::function<int64_t(std::span<poll_descriptors> fds)> callback;
+};
+
+struct alsa_raw_input_configuration
+{
+  std::function<bool(const manual_poll_parameters&)> manual_poll;
+};
+
+struct alsa_raw_output_configuration
+{
+  /**
+   * For large messages, chunk their content and wait.
+   * Setting a null optional will disable chunking.
+   */
+  std::optional<chunking_parameters> chunking;
+};
+
+struct alsa_raw_observer_configuration
+{
+  std::chrono::milliseconds poll_period{100};
+};
+}

include/libremidi/backends/alsa_raw/helpers.hpp

@@ -0,0 +1,328 @@
+#pragma once
+#include <libremidi/backends/linux/alsa.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <chrono>
+#include <functional>
+#include <iostream>
+#include <optional>
+#include <span>
+#include <string>
+#include <string_view>
+#include <thread>
+#include <vector>
+
+// Credits: greatly inspired from
+// https://ccrma.stanford.edu/~craig/articles/linuxmidi/alsa-1.0/alsarawmidiout.c
+// https://ccrma.stanford.edu/~craig/articles/linuxmidi/alsa-1.0/alsarawportlist.c
+// Thanks Craig Stuart Sapp <craig@ccrma.stanford.edu>
+
+namespace libremidi
+{
+namespace
+{
+struct alsa_raw_port_id
+{
+  int card{}, dev{}, port{};
+  std::string to_string() const noexcept
+  {
+    return "hw:" + std::to_string(card) + "," + std::to_string(dev) + "," + std::to_string(port);
+  }
+};
+inline constexpr port_handle raw_to_port_handle(alsa_raw_port_id id) noexcept
+{
+  return (uint64_t(id.card) << 32) + (uint64_t(id.dev) << 16) + id.port;
+}
+inline constexpr alsa_raw_port_id raw_from_port_handle(port_handle p) noexcept
+{
+  alsa_raw_port_id ret;
+  ret.card = (p & 0x00'00'FF'FF'00'00'00'00) >> 32;
+  ret.dev = (p & 0x00'00'00'00'FF'FF'00'00) >> 16;
+  ret.port = (p & 0x00'00'00'00'00'00'FF'FF);
+  return ret;
+}
+static_assert(raw_from_port_handle(raw_to_port_handle({102, 7, 3})).card == 102);
+static_assert(raw_from_port_handle(raw_to_port_handle({12, 7, 3})).dev == 7);
+static_assert(raw_from_port_handle(raw_to_port_handle({12, 7, 3})).port == 3);
+}
+
+namespace alsa_raw
+{
+struct alsa_raw_port_info
+{
+  std::string device;
+  std::string card_name;
+  std::string device_name;
+  std::string subdevice_name;
+  int card{}, dev{}, sub{};
+
+  std::string pretty_name() const
+  {
+    return device + ": " + card_name + " : " + device_name + " : " + subdevice_name;
+  }
+
+  bool operator==(const alsa_raw_port_info& other) const noexcept = default;
+};
+
+struct enumerator;
+struct snd_ctl_wrapper
+{
+  const libasound& snd;
+  snd_ctl_t* ctl{};
+  inline snd_ctl_wrapper(enumerator& self, const char* name);
+
+  ~snd_ctl_wrapper()
+  {
+    if (ctl)
+    {
+      snd.ctl.close(ctl);
+    }
+  }
+
+  snd_ctl_t& operator*() const noexcept { return *ctl; }
+  snd_ctl_t* operator->() const noexcept { return ctl; }
+  operator snd_ctl_t*() const noexcept { return ctl; }
+};
+struct enumerator
+{
+  const libasound& snd = libasound::instance();
+  std::vector<alsa_raw_port_info> inputs;
+  std::vector<alsa_raw_port_info> outputs;
+
+  std::function<void(std::string_view)> error_callback;
+  std::function<void(std::string_view)> warn_callback;
+
+  template <typename... Args>
+  void warning(Args&&... args)
+  {
+    std::string s;
+    ((s += args), ...);
+    if (warn_callback)
+    {
+      warn_callback(std::move(s));
+    }
+    else
+    {
+      std::cerr << s << std::endl;
+    }
+  }
+  template <typename... Args>
+  void error(Args&&... args)
+  {
+    std::string s;
+    ((s += args), ...);
+    if (error_callback)
+    {
+      error_callback(std::move(s));
+    }
+    else
+    {
+      throw std::runtime_error(s.c_str());
+    }
+  }
+
+  // 1: is an input / output
+  // 0: isn't an input / output
+  // < 0: error
+  int is(snd_rawmidi_stream_t stream, snd_ctl_t* ctl, int card, int device, int sub)
+  {
+    snd_rawmidi_info_t* info;
+
+    snd_rawmidi_info_alloca(&info);
+    snd.rawmidi.info_set_device(info, device);
+    snd.rawmidi.info_set_subdevice(info, sub);
+    snd.rawmidi.info_set_stream(info, stream);
+
+    const int status = snd.ctl.rawmidi.info(ctl, info);
+    if (status == 0)
+    {
+      return 1;
+    }
+    else if (status < 0 && status != -ENXIO)
+    {
+      error(
+          "alsa_raw_helpers::enumerator::is: cannot get rawmidi information:", card, device, sub,
+          snd.strerror(status));
+      return status;
+    }
+    else
+    {
+      return 0;
+    }
+  }
+
+  int is_input(snd_ctl_t* ctl, int card, int device, int sub)
+  {
+    return is(SND_RAWMIDI_STREAM_INPUT, ctl, card, device, sub);
+  }
+
+  int is_output(snd_ctl_t* ctl, int card, int device, int sub)
+  {
+    return is(SND_RAWMIDI_STREAM_OUTPUT, ctl, card, device, sub);
+  }
+
+  std::string get_card_name(int card)
+  {
+    char* card_name{};
+    snd.card.get_name(card, &card_name);
+
+    std::string str = card_name;
+    free(card_name);
+    return str;
+  }
+
+  static std::string device_identifier(int card, int device, int sub)
+  {
+    std::string s;
+    s.reserve(12);
+    s += "hw:";
+    s += std::to_string(card);
+    s += ",";
+    s += std::to_string(device);
+    s += ",";
+    s += std::to_string(sub);
+    return s;
+  }
+
+  void enumerate_cards()
+  {
+    int card = -1;
+
+    int status = snd.card.next(&card);
+    if (status < 0)
+    {
+      error(
+          "alsa_raw_helpers::enumerator::enumerate_cards: "
+          "cannot determine card number: ",
+          snd.strerror(status));
+      return;
+    }
+
+    if (card < 0)
+    {
+      error(
+          "alsa_raw_helpers::enumerator::enumerate_cards: "
+          "no sound cards found");
+      return;
+    }
+
+    while (card >= 0)
+    {
+      enumerate_devices(card);
+
+      if ((status = snd.card.next(&card)) < 0)
+      {
+        error(
+            "alsa_raw_helpers::enumerator::enumerate_cards: "
+            "cannot determine card number: ",
+            snd.strerror(status));
+        break;
+      }
+    }
+  }
+
+  virtual void enumerate_devices(int card) = 0;
+};
+
+inline snd_ctl_wrapper::snd_ctl_wrapper(enumerator& self, const char* name)
+    : snd{self.snd}
+{
+  int status = snd.ctl.open(&ctl, name, 0);
+  if (status < 0)
+  {
+    self.error(
+        "alsa_raw_helpers::enumerator::snd_ctl_wrapper: "
+        "cannot open control for card",
+        name, snd.strerror(status));
+  }
+}
+
+struct midi1_enumerator : enumerator
+{
+  void enumerate_devices(int card) override
+  {
+    char name[128];
+
+    sprintf(name, "hw:%d", card);
+
+    // Open card.
+    snd_ctl_wrapper ctl{*this, name};
+    if (!ctl)
+      return;
+
+    // Enumerate devices.
+    int device = -1;
+    do
+    {
+      const int status = snd.ctl.rawmidi.next_device(ctl, &device);
+      if (device == -1)
+        return;
+
+      if (status < 0)
+      {
+        error(
+            "alsa_raw::midi1_enumerator::enumerate_devices: "
+            "cannot determine device number: ",
+            snd.strerror(status));
+        break;
+      }
+
+      if (device >= 0)
+        enumerate_subdevices(ctl, card, device);
+
+    } while (device >= 0);
+  }
+
+  void enumerate_subdevices(snd_ctl_t* ctl, int card, int device)
+  {
+    snd_rawmidi_info_t* info;
+    snd_rawmidi_info_alloca(&info);
+    snd.rawmidi.info_set_device(info, device);
+
+    snd.rawmidi.info_set_stream(info, SND_RAWMIDI_STREAM_INPUT);
+    snd.ctl.rawmidi.info(ctl, info);
+    const int subs_in = snd.rawmidi.info_get_subdevices_count(info);
+
+    snd.rawmidi.info_set_stream(info, SND_RAWMIDI_STREAM_OUTPUT);
+    snd.ctl.rawmidi.info(ctl, info);
+    const int subs_out = snd.rawmidi.info_get_subdevices_count(info);
+
+    alsa_raw_port_info d;
+    d.card = card;
+    d.dev = device;
+    d.card_name = get_card_name(card);
+    d.device_name = snd.rawmidi.info_get_name(info);
+
+    auto read_subdevice_info = [&](int sub) {
+      snd.rawmidi.info_set_subdevice(info, sub);
+      snd.ctl.rawmidi.info(ctl, info);
+
+      d.device = device_identifier(card, device, sub);
+      d.subdevice_name = snd.rawmidi.info_get_subdevice_name(info);
+      d.sub = sub;
+    };
+
+    if (subs_in > 0)
+    {
+      snd.rawmidi.info_set_stream(info, SND_RAWMIDI_STREAM_INPUT);
+      for (int sub = 0; sub < subs_in; sub++)
+      {
+        read_subdevice_info(sub);
+        inputs.push_back(d);
+      }
+    }
+
+    if (subs_out > 0)
+    {
+      snd.rawmidi.info_set_stream(info, SND_RAWMIDI_STREAM_OUTPUT);
+      for (int sub = 0; sub < subs_out; sub++)
+      {
+        read_subdevice_info(sub);
+        outputs.push_back(d);
+      }
+    }
+  }
+};
+}
+}

include/libremidi/backends/alsa_raw/midi_in.hpp

@@ -0,0 +1,350 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/config.hpp>
+#include <libremidi/backends/alsa_raw/helpers.hpp>
+#include <libremidi/backends/linux/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+#include <alsa/asoundlib.h>
+
+#include <chrono>
+#include <thread>
+
+namespace libremidi::alsa_raw
+{
+class midi_in_impl
+    : public midi1::in_api
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , alsa_raw_input_configuration
+  {
+  } configuration;
+
+  const libasound& snd = libasound::instance();
+
+  explicit midi_in_impl(input_configuration&& conf, alsa_raw_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    fds_.reserve(4);
+  }
+
+  ~midi_in_impl() override { }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_in_alsa_raw: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_alsa_raw: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_alsa_raw: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW; }
+
+  // Must be a string such as: "hw:2,4,1"
+  [[nodiscard]] int do_init_port(const char* portname)
+  {
+    constexpr int mode = SND_RAWMIDI_NONBLOCK;
+    if (const int err = snd.rawmidi.open(&midiport_, nullptr, portname, mode); err < 0)
+    {
+      error<driver_error>(this->configuration, "midi_in_alsa_raw::open_port: cannot open device.");
+      return err;
+    }
+
+    snd_rawmidi_params_t* params{};
+    snd_rawmidi_params_alloca(&params);
+
+    if (const int err = snd.rawmidi.params_current(midiport_, params); err < 0)
+      return err;
+    if (const int err = snd.rawmidi.params_set_no_active_sensing(midiport_, params, 1); err < 0)
+      return err;
+#if LIBREMIDI_ALSA_HAS_RAWMIDI_TREAD
+    if (configuration.timestamps == timestamp_mode::NoTimestamp)
+    {
+      if (const int err
+          = snd.rawmidi.params_set_read_mode(midiport_, params, SND_RAWMIDI_READ_STANDARD);
+          err < 0)
+        return err;
+      if (const int err
+          = snd.rawmidi.params_set_clock_type(midiport_, params, SND_RAWMIDI_CLOCK_NONE);
+          err < 0)
+        return err;
+    }
+    else
+    {
+      if (const int err
+          = snd.rawmidi.params_set_read_mode(midiport_, params, SND_RAWMIDI_READ_TSTAMP);
+          err < 0)
+        return err;
+      if (const int err
+          = snd.rawmidi.params_set_clock_type(midiport_, params, SND_RAWMIDI_CLOCK_MONOTONIC);
+          err < 0)
+        return err;
+    }
+#endif
+
+    if (const int err = snd.rawmidi.params(midiport_, params); err < 0)
+      return err;
+
+    return init_pollfd();
+  }
+
+  [[nodiscard]] int init_port(const port_information& p)
+  {
+    return do_init_port(raw_from_port_handle(p.port).to_string().c_str());
+  }
+
+  [[nodiscard]] int init_pollfd()
+  {
+    const int num_fds = snd.rawmidi.poll_descriptors_count(this->midiport_);
+
+    this->fds_.clear();
+    this->fds_.resize(num_fds);
+
+    return snd.rawmidi.poll_descriptors(this->midiport_, fds_.data(), num_fds);
+  }
+
+  ssize_t do_read_events(auto parse_func, std::span<pollfd> fds)
+  {
+    // Read events
+    if (fds.empty())
+    {
+      return (this->*parse_func)();
+    }
+    else
+    {
+      unsigned short res{};
+      const int err = snd.rawmidi.poll_descriptors_revents(
+          this->midiport_, fds.data(), static_cast<unsigned int>(fds.size()), &res);
+      if (err < 0)
+        return err;
+
+      // Did we encounter an error during polling
+      if (res & (POLLERR | POLLHUP))
+        return -EIO;
+
+      // Is there data to read
+      if (res & POLLIN)
+        return (this->*parse_func)();
+    }
+
+    return 0;
+  }
+
+  ssize_t read_input_buffer()
+  {
+    static const constexpr int nbytes = 1024;
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = false,
+        .absolute_is_monotonic = false,
+        .has_samples = false,
+    };
+
+    unsigned char bytes[nbytes];
+
+    ssize_t err = 0;
+    // err is the amount of bytes read
+    while ((err = snd.rawmidi.read(this->midiport_, bytes, nbytes)) > 0)
+    {
+      const auto to_ns = [this] { return absolute_timestamp(); };
+      decoder_.on_bytes({bytes, bytes + err}, decoder_.timestamp<timestamp_info>(to_ns, 0));
+    }
+    return err;
+  }
+
+#if LIBREMIDI_ALSA_HAS_RAWMIDI_TREAD
+  ssize_t read_input_buffer_with_timestamps()
+  {
+    static constexpr int nbytes = 1024;
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = true,
+        .has_samples = false,
+    };
+
+    unsigned char bytes[nbytes];
+    struct timespec ts;
+
+    ssize_t err = 0;
+    // err is the amount of bytes read
+    while ((err = snd.rawmidi.tread(this->midiport_, &ts, bytes, nbytes)) > 0)
+    {
+      const auto to_ns = [ts] {
+        return static_cast<int64_t>(ts.tv_sec) * 1'000'000'000 + static_cast<int64_t>(ts.tv_nsec);
+      };
+      decoder_.on_bytes({bytes, bytes + err}, decoder_.timestamp<timestamp_info>(to_ns, 0));
+    }
+    return err;
+  }
+#else
+  ssize_t read_input_buffer_with_timestamps() { return read_input_buffer(); }
+#endif
+
+  void close_port() override
+  {
+    if (midiport_)
+      snd.rawmidi.close(midiport_);
+    midiport_ = nullptr;
+  }
+
+  timestamp absolute_timestamp() const noexcept final override { return system_ns(); }
+
+  snd_rawmidi_t* midiport_{};
+  std::vector<pollfd> fds_;
+  midi1::input_state_machine decoder_{this->configuration};
+};
+
+class midi_in_alsa_raw_threaded : public midi_in_impl
+{
+public:
+  midi_in_alsa_raw_threaded(input_configuration&& conf, alsa_raw_input_configuration&& apiconf)
+      : midi_in_impl{std::move(conf), std::move(apiconf)}
+  {
+    if (this->termination_event < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_in_alsa::initialize: error creating eventfd.");
+    }
+  }
+
+  ~midi_in_alsa_raw_threaded() override
+  {
+    // Close a connection if it exists.
+    this->midi_in_alsa_raw_threaded::close_port();
+  }
+
+private:
+  void run_thread(auto parse_func)
+  {
+    fds_.push_back(this->termination_event);
+
+    for (;;)
+    {
+      // Poll
+      ssize_t err = poll(fds_.data(), fds_.size(), -1);
+      if (err == -EAGAIN)
+        continue;
+      else if (err < 0)
+        return;
+      else if (termination_event.ready(fds_.back()))
+        break;
+
+      err = do_read_events(parse_func, {fds_.data(), fds_.size() - 1});
+      if (err == -EAGAIN)
+        continue;
+      else if (err < 0)
+        return;
+    }
+  }
+
+  [[nodiscard]] ssize_t start_thread()
+  {
+    try
+    {
+      if (configuration.timestamps == timestamp_mode::NoTimestamp)
+      {
+        this->thread_ = std::thread{[this] { run_thread(&midi_in_impl::read_input_buffer); }};
+      }
+      else
+      {
+        this->thread_ = std::thread{
+            [this] { run_thread(&midi_in_impl::read_input_buffer_with_timestamps); }};
+      }
+    }
+    catch (const std::system_error& e)
+    {
+      using namespace std::literals;
+
+      error<thread_error>(
+          this->configuration,
+          "midi_in_alsa::start_thread: error starting MIDI input thread: "s + e.what());
+      return false;
+    }
+    return true;
+  }
+
+  bool open_port(const input_port& port, std::string_view /*name*/) override
+  {
+    if (const int err = midi_in_impl::init_port(port); err < 0)
+      return false;
+    if (!start_thread())
+      return false;
+    return true;
+  }
+
+  void close_port() override
+  {
+    termination_event.notify();
+    if (thread_.joinable())
+      thread_.join();
+    termination_event.consume(); // Reset to zero
+
+    midi_in_impl::close_port();
+  }
+
+  std::thread thread_;
+  eventfd_notifier termination_event{};
+};
+
+class midi_in_alsa_raw_manual : public midi_in_impl
+{
+public:
+  using midi_in_impl::midi_in_impl;
+
+  ~midi_in_alsa_raw_manual()
+  {
+    // Close a connection if it exists.
+    this->close_port();
+  }
+
+private:
+  void send_poll_callback()
+  {
+    if (configuration.timestamps == timestamp_mode::NoTimestamp)
+    {
+      configuration.manual_poll(manual_poll_parameters{
+          .fds = {this->fds_.data(), this->fds_.size()},
+          .callback = [this](std::span<pollfd> fds) {
+            return do_read_events(&midi_in_impl::read_input_buffer, fds);
+          }});
+    }
+    else
+    {
+      configuration.manual_poll(manual_poll_parameters{
+          .fds = {this->fds_.data(), this->fds_.size()},
+          .callback = [this](std::span<pollfd> fds) {
+            return do_read_events(&midi_in_impl::read_input_buffer_with_timestamps, fds);
+          }});
+    }
+  }
+
+  bool open_port(const input_port& p, std::string_view /*name*/) override
+  {
+    if (midi_in_impl::init_port(p) < 0)
+      return false;
+    send_poll_callback();
+    return true;
+  }
+};
+}
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<midi_in_api> make<alsa_raw::midi_in_impl>(
+    libremidi::input_configuration&& conf, libremidi::alsa_raw_input_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_raw::midi_in_alsa_raw_manual>(std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_raw::midi_in_alsa_raw_threaded>(std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/alsa_raw/midi_out.hpp

@@ -0,0 +1,177 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/config.hpp>
+#include <libremidi/backends/alsa_raw/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+#include <alsa/asoundlib.h>
+
+#include <atomic>
+#include <thread>
+
+namespace libremidi::alsa_raw
+{
+class midi_out_impl final
+    : public midi1::out_api
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , alsa_raw_output_configuration
+  {
+  } configuration;
+
+  const libasound& snd = libasound::instance();
+
+  midi_out_impl(output_configuration&& conf, alsa_raw_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+  }
+
+  ~midi_out_impl() override
+  {
+    // Close a connection if it exists.
+    midi_out_impl::close_port();
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW; }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: set_port_name unsupported");
+  }
+
+  int connect_port(const char* portname)
+  {
+    constexpr int mode = SND_RAWMIDI_SYNC;
+    int status = snd.rawmidi.open(NULL, &midiport_, portname, mode);
+    if (status < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_out_alsa_raw::open_port: cannot open device.");
+      return status;
+    }
+    return status;
+  }
+
+  bool open_port(const output_port& p, std::string_view) override
+  {
+    return connect_port(raw_from_port_handle(p.port).to_string().c_str()) == 0;
+  }
+
+  void close_port() override
+  {
+    if (midiport_)
+      snd.rawmidi.close(midiport_);
+    midiport_ = nullptr;
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    if (!midiport_)
+      error<invalid_use_error>(
+          this->configuration,
+          "midi_out_alsa_raw::send_message: trying to send a message without an open "
+          "port.");
+
+    if (!this->configuration.chunking)
+    {
+      write(message, size);
+    }
+    else
+    {
+      write_chunked(message, size);
+    }
+  }
+
+  bool write(const unsigned char* message, size_t size)
+  {
+    if (snd.rawmidi.write(midiport_, message, size) < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_out_alsa_raw::send_message: cannot write message.");
+      return false;
+    }
+
+    return true;
+  }
+
+  std::size_t get_chunk_size() const noexcept
+  {
+    snd_rawmidi_params_t* param;
+    snd_rawmidi_params_alloca(&param);
+    snd.rawmidi.params_current(midiport_, param);
+
+    std::size_t buffer_size = snd.rawmidi.params_get_buffer_size(param);
+    return std::min(buffer_size, (std::size_t)configuration.chunking->size);
+  }
+
+  std::size_t get_available_bytes_to_write() const noexcept
+  {
+    snd_rawmidi_status_t* st{};
+    snd_rawmidi_status_alloca(&st);
+    snd.rawmidi.status(midiport_, st);
+
+    return snd.rawmidi.status_get_avail(st);
+  }
+
+  // inspired from ALSA amidi.c source code
+  void write_chunked(const unsigned char* const begin, size_t size)
+  {
+    const unsigned char* data = begin;
+    const unsigned char* end = begin + size;
+
+    const std::size_t chunk_size = std::min(get_chunk_size(), size);
+
+    // Send the first buffer
+    std::size_t len = chunk_size;
+
+    if (!write(data, len))
+      return;
+
+    data += len;
+
+    while (data < end)
+    {
+      // Wait for the buffer to have some space available
+      const std::size_t written_bytes = data - begin;
+      std::size_t available{};
+      while ((available = get_available_bytes_to_write()) < chunk_size)
+      {
+        if (!configuration.chunking->wait(
+                std::chrono::microseconds((chunk_size - available) * 320), written_bytes))
+          return;
+      };
+
+      if (!configuration.chunking->wait(configuration.chunking->interval, written_bytes))
+        return;
+
+      // Write more data
+      len = end - data;
+
+      // Maybe until the end of the sysex
+      if (const auto sysex_end = static_cast<const unsigned char*>(memchr(data, 0xf7, len)))
+        len = sysex_end - data + 1;
+
+      if (len > chunk_size)
+        len = chunk_size;
+
+      if (!write(data, len))
+        return;
+
+      data += len;
+    }
+  }
+
+  snd_rawmidi_t* midiport_{};
+};
+}

include/libremidi/backends/alsa_raw/observer.hpp

@@ -0,0 +1,240 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/config.hpp>
+#include <libremidi/backends/alsa_raw/helpers.hpp>
+#include <libremidi/backends/dummy.hpp>
+
+#if LIBREMIDI_HAS_UDEV
+  #include <libremidi/backends/linux/helpers.hpp>
+  #include <libremidi/backends/linux/udev.hpp>
+  #include <libremidi/detail/observer.hpp>
+
+  #include <stdexcept>
+
+namespace libremidi::alsa_raw
+{
+template <typename Enumerator>
+class observer_impl_base : public observer_api
+{
+public:
+  struct
+      : observer_configuration
+      , alsa_raw_observer_configuration
+  {
+  } configuration;
+
+  const libasound& snd = libasound::instance();
+
+  explicit observer_impl_base(
+      observer_configuration&& conf, alsa_raw_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (!configuration.has_callbacks())
+      return;
+
+    fds[0] = this->udev;
+    fds[1] = termination_event;
+    fds[2] = timer_fd;
+
+    // Set-up initial state
+    if (configuration.notify_in_constructor)
+      this->check_devices();
+
+    // Start thread
+    thread = std::thread{[this] { this->run(); }};
+  }
+
+  ~observer_impl_base()
+  {
+    termination_event.notify();
+
+    if (thread.joinable())
+      thread.join();
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    std::vector<libremidi::input_port> ret;
+    Enumerator new_devs;
+
+    new_devs.enumerate_cards();
+    for (auto& d : new_devs.inputs)
+    {
+      ret.push_back(to_port_info<true>(d));
+    }
+    return ret;
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    std::vector<libremidi::output_port> ret;
+    Enumerator new_devs;
+
+    new_devs.enumerate_cards();
+    for (auto& d : new_devs.outputs)
+    {
+      ret.push_back(to_port_info<false>(d));
+    }
+    return ret;
+  }
+
+private:
+  void run()
+  {
+    for (;;)
+    {
+      if (int err = poll(fds, 3, -1); err < 0)
+      {
+        if (err == -EAGAIN)
+          continue;
+        else
+          return;
+      }
+
+      // Check udev
+      if (fds[0].revents & POLLIN)
+      {
+        udev_device* dev = udev.udev.monitor_receive_device(udev.monitor);
+        if (!dev)
+          continue;
+
+        std::string_view act = udev.udev.device_get_action(dev);
+        std::string_view ss = udev.udev.device_get_subsystem(dev);
+        if (!act.empty() && ss == "snd_seq")
+        {
+          if (act == "add" || act == "remove")
+          {
+            // Check every 100 milliseconds for ten seconds
+            this->timer_fd.restart(configuration.poll_period.count());
+            timer_check_counts = 100;
+          }
+        }
+
+        udev.udev.device_unref(dev);
+
+        fds[0].revents = 0;
+      }
+
+      // Check eventfd
+      if (fds[1].revents & POLLIN)
+      {
+        break;
+      }
+
+      // Check timer
+      if (fds[2].revents & POLLIN)
+      {
+        if (this->timer_check_counts-- <= 0)
+          this->timer_fd.cancel();
+        fds[2].revents = 0;
+
+        check_devices();
+      }
+    }
+  }
+
+  template <bool Input>
+  auto to_port_info(alsa_raw::alsa_raw_port_info p) const noexcept
+      -> std::conditional_t<Input, input_port, output_port>
+  {
+    return {
+        {.client = 0,
+         .port = raw_to_port_handle({p.card, p.dev, p.sub}),
+         .manufacturer = p.card_name,
+         .device_name = p.device_name,
+         .port_name = p.subdevice_name,
+         .display_name = p.subdevice_name}};
+  }
+
+  void check_devices()
+  {
+    Enumerator new_devs;
+
+    new_devs.enumerate_cards();
+
+    for (auto& in_prev : current_inputs)
+    {
+      if (auto it = std::find(new_devs.inputs.begin(), new_devs.inputs.end(), in_prev);
+          it == new_devs.inputs.end())
+      {
+        if (auto& cb = this->configuration.input_removed)
+        {
+          cb(to_port_info<true>(in_prev));
+        }
+      }
+    }
+
+    for (auto& in_next : new_devs.inputs)
+    {
+      if (auto it = std::find(current_inputs.begin(), current_inputs.end(), in_next);
+          it == current_inputs.end())
+      {
+        if (auto& cb = this->configuration.input_added)
+        {
+          cb(to_port_info<true>(in_next));
+        }
+      }
+    }
+
+    for (auto& out_prev : current_outputs)
+    {
+      if (auto it = std::find(new_devs.outputs.begin(), new_devs.outputs.end(), out_prev);
+          it == new_devs.outputs.end())
+      {
+        if (auto& cb = this->configuration.output_removed)
+        {
+          cb(to_port_info<false>(out_prev));
+        }
+      }
+    }
+
+    for (auto& out_next : new_devs.outputs)
+    {
+      if (auto it = std::find(current_outputs.begin(), current_outputs.end(), out_next);
+          it == current_outputs.end())
+      {
+        if (auto& cb = this->configuration.output_added)
+        {
+          cb(to_port_info<false>(out_next));
+        }
+      }
+    }
+    current_inputs = std::move(new_devs.inputs);
+    current_outputs = std::move(new_devs.outputs);
+  }
+
+  udev_helper udev{};
+  eventfd_notifier termination_event{};
+  timerfd_timer timer_fd{};
+  int timer_check_counts = 0;
+  std::thread thread;
+  std::vector<alsa_raw_port_info> current_inputs;
+  std::vector<alsa_raw_port_info> current_outputs;
+
+  pollfd fds[3]{};
+};
+}
+#else
+  #include <libremidi/backends/dummy.hpp>
+namespace libremidi::alsa_raw
+{
+template <typename Enumerator>
+struct observer_impl_base : observer_dummy
+{
+  explicit observer_impl_base(
+      [[maybe_unused]] observer_configuration&& conf,
+      [[maybe_unused]] alsa_raw_observer_configuration&& apiconf)
+      : observer_dummy{dummy_configuration{}, dummy_configuration{}}
+  {
+  }
+};
+}
+#endif
+
+namespace libremidi::alsa_raw
+{
+struct observer_impl : observer_impl_base<alsa_raw::midi1_enumerator>
+{
+  using alsa_raw::observer_impl_base<midi1_enumerator>::observer_impl_base;
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW; }
+};
+}

include/libremidi/backends/alsa_raw_ump.hpp

@@ -0,0 +1,30 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/observer.hpp>
+#include <libremidi/backends/alsa_raw_ump/midi_in.hpp>
+#include <libremidi/backends/alsa_raw_ump/midi_out.hpp>
+#include <libremidi/backends/alsa_raw_ump/observer.hpp>
+#include <libremidi/backends/dummy.hpp>
+
+namespace libremidi::alsa_raw_ump
+{
+struct backend
+{
+  using midi_in = midi_in_impl;
+  using midi_out = midi_out_impl;
+  using midi_observer = observer_impl;
+  using midi_in_configuration = alsa_raw_ump::input_configuration;
+  using midi_out_configuration = alsa_raw_ump::output_configuration;
+  struct midi_observer_configuration : alsa_raw_observer_configuration
+  {
+  };
+  static const constexpr auto API = libremidi::API::ALSA_RAW_UMP;
+  static const constexpr auto name = "alsa_raw_ump";
+  static const constexpr auto display_name = "ALSA (raw UMP)";
+
+  static inline bool available() noexcept
+  {
+    static const libasound& snd = libasound::instance();
+    return snd.available && snd.rawmidi.available && snd.ump.available;
+  }
+};
+}

include/libremidi/backends/alsa_raw_ump/config.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/config.hpp>
+
+namespace libremidi::alsa_raw_ump
+{
+struct input_configuration
+{
+  std::function<bool(const manual_poll_parameters&)> manual_poll;
+};
+
+struct output_configuration
+{
+  /**
+   * For large messages, chunk their content and wait.
+   * Setting a null optional will disable chunking.
+   */
+  std::optional<chunking_parameters> chunking;
+};
+
+struct observer_configuration
+{
+  std::chrono::milliseconds poll_period{100};
+};
+}

include/libremidi/backends/alsa_raw_ump/helpers.hpp

@@ -0,0 +1,66 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/helpers.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/detail/observer.hpp>
+
+namespace libremidi::alsa_raw_ump
+{
+struct midi2_enumerator : alsa_raw::enumerator
+{
+  void enumerate_devices(int card) override
+  {
+    char name[128];
+
+    sprintf(name, "hw:%d", card);
+
+    // Open card.
+    alsa_raw::snd_ctl_wrapper ctl{*this, name};
+    if (!ctl)
+      return;
+
+    // Enumerate devices.
+    int device = -1;
+    do
+    {
+      const int status = snd.ctl.ump.next_device(ctl, &device);
+      if (device == -1)
+        return;
+
+      if (status < 0)
+      {
+        error(
+            "alsa_raw_ump::midi2_enumerator::enumerate_devices: "
+            "cannot determine device number: ",
+            snd.strerror(status));
+        break;
+      }
+
+      if (device >= 0)
+      {
+        enumerate_blocks(ctl, card, device);
+        enumerate_endpoints(ctl, card, device);
+      }
+
+    } while (device >= 0);
+  }
+
+  void enumerate_endpoints(snd_ctl_t* ctl, [[maybe_unused]] int card, [[maybe_unused]] int device)
+  {
+    snd_ump_endpoint_info_t* info{};
+    snd_ump_endpoint_info_alloca(&info);
+    snd.ctl.ump.endpoint_info(ctl, info);
+
+    fprintf(stderr, "UMP endpoint: %s", snd.ump.endpoint_info_get_name(info));
+  }
+
+  void enumerate_blocks(snd_ctl_t* ctl, [[maybe_unused]] int card, [[maybe_unused]] int device)
+  {
+    snd_ump_block_info_t* info{};
+    snd_ump_block_info_alloca(&info);
+    snd.ctl.ump.block_info(ctl, info);
+
+    fprintf(stderr, "UMP block: %s", snd.ump.block_info_get_name(info));
+  }
+};
+
+}

include/libremidi/backends/alsa_raw_ump/midi_in.hpp

@@ -0,0 +1,383 @@
+#pragma once
+#include <libremidi/backends/alsa_raw_ump/config.hpp>
+#include <libremidi/backends/alsa_raw_ump/helpers.hpp>
+#include <libremidi/backends/linux/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+#include <alsa/asoundlib.h>
+
+#include <atomic>
+#include <thread>
+
+namespace libremidi::alsa_raw_ump
+{
+class midi_in_impl
+    : public midi2::in_api
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::ump_input_configuration
+      , alsa_raw_ump::input_configuration
+  {
+  } configuration;
+
+  const libasound& snd = libasound::instance();
+
+  explicit midi_in_impl(
+      libremidi::ump_input_configuration&& conf, alsa_raw_ump::input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    fds_.reserve(4);
+    assert(snd.ump.available);
+  }
+
+  ~midi_in_impl() override { }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "alsa_raw_ump::ump: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "alsa_raw_ump::ump: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "alsa_raw_ump::ump: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW_UMP; }
+
+  // Must be a string such as: "hw:2,4,1"
+  [[nodiscard]] int do_init_port(const char* portname)
+  {
+    constexpr int mode = 0;
+    SND_RAWMIDI_NONBLOCK;
+    if (int err = snd.ump.open(&midiport_, 0, portname, mode); err < 0)
+    {
+      error<driver_error>(
+          this->configuration, "alsa_raw_ump::ump::open_port: cannot open device.");
+      return err;
+    }
+
+    snd_rawmidi_params_t* params{};
+    snd_rawmidi_params_alloca(&params);
+
+    auto rawmidi = snd.ump.rawmidi(midiport_);
+
+    if (int err = snd.ump.rawmidi_params_current(midiport_, params); err < 0)
+      return err;
+    if (int err = snd.rawmidi.params_set_no_active_sensing(rawmidi, params, 1); err < 0)
+      return err;
+
+    if (configuration.timestamps == timestamp_mode::NoTimestamp)
+    {
+      if (int err = snd.rawmidi.params_set_read_mode(rawmidi, params, SND_RAWMIDI_READ_STANDARD);
+          err < 0)
+        return err;
+      if (int err = snd.rawmidi.params_set_clock_type(rawmidi, params, SND_RAWMIDI_CLOCK_NONE);
+          err < 0)
+        return err;
+    }
+    else
+    {
+      if (int err = snd.rawmidi.params_set_read_mode(rawmidi, params, SND_RAWMIDI_READ_TSTAMP);
+          err < 0)
+        return err;
+      if (int err
+          = snd.rawmidi.params_set_clock_type(rawmidi, params, SND_RAWMIDI_CLOCK_MONOTONIC);
+          err < 0)
+        return err;
+    }
+
+    if (int err = snd.ump.rawmidi_params(midiport_, params); err < 0)
+      return err;
+
+    return init_pollfd();
+  }
+
+  [[nodiscard]] int init_port(const port_information& p)
+  {
+    return do_init_port(raw_from_port_handle(p.port).to_string().c_str());
+  }
+
+  [[nodiscard]] int init_pollfd()
+  {
+    int num_fds = snd.ump.poll_descriptors_count(this->midiport_);
+
+    this->fds_.clear();
+    this->fds_.resize(num_fds);
+
+    return snd.ump.poll_descriptors(this->midiport_, fds_.data(), num_fds);
+  }
+
+  ssize_t do_read_events(auto parse_func, std::span<pollfd> fds)
+  {
+    // Read events
+    if (fds.empty())
+    {
+      return (this->*parse_func)();
+    }
+    else
+    {
+      unsigned short res{};
+      ssize_t err = snd.ump.poll_descriptors_revents(
+          this->midiport_, fds.data(), static_cast<unsigned int>(fds.size()), &res);
+      if (err < 0)
+        return err;
+
+      // Did we encounter an error during polling
+      if (res & (POLLERR | POLLHUP))
+        return -EIO;
+
+      // Is there data to read
+      if (res & POLLIN)
+        return (this->*parse_func)();
+    }
+
+    return 0;
+  }
+
+  ssize_t read_input_buffer()
+  {
+    static const constexpr int nbytes = 1024;
+
+    unsigned char bytes[nbytes];
+
+    ssize_t err = 0;
+    while ((err = snd.ump.read(this->midiport_, bytes, nbytes)) > 0)
+    {
+      std::cerr << "We read: " << nbytes << "bytes !!!!";
+      return 1;
+      // err is the amount of bytes read
+      // decoder_.add_bytes(bytes, err);
+    }
+    return err;
+  }
+
+  void set_timestamp(struct timespec ts, timestamp& res)
+  {
+    static constexpr int64_t nanos = 1e9;
+    switch (configuration.timestamps)
+    {
+      // Unneeded here
+      case timestamp_mode::NoTimestamp:
+        break;
+      case timestamp_mode::Relative: {
+        const auto t = static_cast<int64_t>(ts.tv_sec) * nanos + static_cast<int64_t>(ts.tv_nsec);
+        if (firstMessage == true)
+        {
+          firstMessage = false;
+          res = 0;
+        }
+        else
+        {
+          res = t - last_time;
+        }
+        last_time = t;
+        break;
+      }
+      case timestamp_mode::Absolute:
+      case timestamp_mode::SystemMonotonic:
+        res = static_cast<int64_t>(ts.tv_sec) * nanos + static_cast<int64_t>(ts.tv_nsec);
+        break;
+      case timestamp_mode::Custom:
+        res = configuration.get_timestamp(
+            static_cast<int64_t>(ts.tv_sec) * nanos + static_cast<int64_t>(ts.tv_nsec));
+        break;
+    }
+  }
+
+  ssize_t read_input_buffer_with_timestamps()
+  {
+    static const constexpr int nbytes = 1024;
+
+    unsigned char bytes[nbytes];
+    struct timespec ts;
+
+    ssize_t err = 0;
+    while ((err = snd.ump.tread(this->midiport_, &ts, bytes, nbytes)) > 0)
+    {
+      // err is the amount of bytes read
+      int64_t ns{};
+      set_timestamp(ts, ns);
+      std::cerr << "We read: " << nbytes << "bytes !!!!";
+      return 1;
+      // decoder_.add_bytes(bytes, err, ns);
+    }
+    return err;
+  }
+
+  void close_port() override
+  {
+    if (midiport_)
+      snd.ump.close(midiport_);
+    midiport_ = nullptr;
+  }
+
+  timestamp absolute_timestamp() const noexcept override { return system_ns(); }
+
+  midi2_enumerator get_device_enumerator() const noexcept
+  {
+    midi2_enumerator device_list;
+    device_list.error_callback
+        = [this](std::string_view text) { this->error<driver_error>(this->configuration, text); };
+    return device_list;
+  }
+
+  snd_ump_t* midiport_{};
+  std::vector<pollfd> fds_;
+  // midi_stream_decoder decoder_{this->configuration.on_message};
+  int64_t last_time{};
+};
+
+class midi_in_impl_threaded : public midi_in_impl
+{
+public:
+  midi_in_impl_threaded(
+      libremidi::ump_input_configuration&& conf, alsa_raw_ump::input_configuration&& apiconf)
+      : midi_in_impl{std::move(conf), std::move(apiconf)}
+  {
+    if (this->termination_event < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_in_alsa::initialize: error creating eventfd.");
+    }
+  }
+
+  ~midi_in_impl_threaded()
+  {
+    // Close a connection if it exists.
+    this->close_port();
+  }
+
+private:
+  void run_thread(auto parse_func)
+  {
+    fds_.push_back(this->termination_event);
+
+    for (;;)
+    {
+      // Poll
+      ssize_t err = poll(fds_.data(), fds_.size(), -1);
+      if (err == -EAGAIN)
+        continue;
+      else if (err < 0)
+        return;
+      else if (termination_event.ready(fds_.back()))
+        break;
+
+      err = do_read_events(parse_func, {fds_.data(), fds_.size() - 1});
+      if (err == -EAGAIN)
+        continue;
+      else if (err < 0)
+        return;
+    }
+  }
+
+  [[nodiscard]] int start_thread()
+  {
+    try
+    {
+      if (configuration.timestamps == timestamp_mode::NoTimestamp)
+      {
+        this->thread_ = std::thread{[this] { run_thread(&midi_in_impl::read_input_buffer); }};
+      }
+      else
+      {
+        this->thread_ = std::thread{
+            [this] { run_thread(&midi_in_impl::read_input_buffer_with_timestamps); }};
+      }
+    }
+    catch (const std::system_error& e)
+    {
+      using namespace std::literals;
+
+      error<thread_error>(
+          this->configuration,
+          "midi_in_alsa::start_thread: error starting MIDI input thread: "s + e.what());
+      return false;
+    }
+    return true;
+  }
+
+  bool open_port(const input_port& port, [[maybe_unused]] std::string_view name) override
+  {
+    if (int err = midi_in_impl::init_port(port); err < 0)
+      return false;
+    if (!start_thread())
+      return false;
+    return true;
+  }
+
+  void close_port() override
+  {
+    termination_event.notify();
+    if (thread_.joinable())
+      thread_.join();
+    termination_event.consume(); // Reset to zero
+
+    midi_in_impl::close_port();
+  }
+
+  std::thread thread_;
+  eventfd_notifier termination_event{};
+};
+
+class midi_in_impl_manual : public midi_in_impl
+{
+public:
+  using midi_in_impl::midi_in_impl;
+
+  ~midi_in_impl_manual()
+  {
+    // Close a connection if it exists.
+    this->close_port();
+  }
+
+private:
+  void send_poll_callback()
+  {
+    if (configuration.timestamps == timestamp_mode::NoTimestamp)
+    {
+      configuration.manual_poll(manual_poll_parameters{
+          .fds = {this->fds_.data(), this->fds_.size()},
+          .callback = [this](std::span<pollfd> fds) {
+            return do_read_events(&midi_in_impl::read_input_buffer, fds);
+          }});
+    }
+    else
+    {
+      configuration.manual_poll(manual_poll_parameters{
+          .fds = {this->fds_.data(), this->fds_.size()},
+          .callback = [this](std::span<pollfd> fds) {
+            return do_read_events(&midi_in_impl::read_input_buffer_with_timestamps, fds);
+          }});
+    }
+  }
+
+  bool open_port(const input_port& p, [[maybe_unused]] std::string_view name) override
+  {
+    if (midi_in_impl::init_port(p) < 0)
+      return false;
+    send_poll_callback();
+    return true;
+  }
+};
+}
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<midi_in_api> make<alsa_raw_ump::midi_in_impl>(
+    libremidi::ump_input_configuration&& conf, libremidi::alsa_raw_ump::input_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_raw_ump::midi_in_impl_manual>(std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_raw_ump::midi_in_impl_threaded>(std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/alsa_raw_ump/midi_out.hpp

@@ -0,0 +1,107 @@
+#pragma once
+#include <libremidi/backends/alsa_raw_ump/config.hpp>
+#include <libremidi/backends/alsa_raw_ump/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+#include <alsa/asoundlib.h>
+
+#include <atomic>
+#include <thread>
+
+namespace libremidi::alsa_raw_ump
+{
+class midi_out_impl final
+    : public midi2::out_api
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::output_configuration
+      , libremidi::alsa_raw_ump::output_configuration
+  {
+  } configuration;
+
+  const libasound& snd = libasound::instance();
+
+  midi_out_impl(
+      libremidi::output_configuration&& conf,
+      libremidi::alsa_raw_ump::output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    assert(snd.ump.available);
+  }
+
+  ~midi_out_impl() override
+  {
+    // Close a connection if it exists.
+    midi_out_impl::close_port();
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW; }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_alsa_raw: set_port_name unsupported");
+  }
+
+  int connect_port(const char* portname)
+  {
+    constexpr int mode = SND_RAWMIDI_SYNC;
+    int status = snd.ump.open(NULL, &midiport_, portname, mode);
+    if (status < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_out_alsa_raw::open_port: cannot open device.");
+      return status;
+    }
+    return status;
+  }
+
+  bool open_port(const output_port& p, std::string_view) override
+  {
+    return connect_port(raw_from_port_handle(p.port).to_string().c_str()) == 0;
+  }
+
+  void close_port() override
+  {
+    if (midiport_)
+      snd.ump.close(midiport_);
+    midiport_ = nullptr;
+  }
+
+  void send_ump(const uint32_t* ump_stream, std::size_t count) override
+  {
+    if (!midiport_)
+      error<invalid_use_error>(
+          this->configuration,
+          "midi_out_alsa_raw::send_message: trying to send a message without an open "
+          "port.");
+
+    write(ump_stream, count * sizeof(uint32_t));
+  }
+
+  bool write(const uint32_t* ump_stream, size_t bytes)
+  {
+    if (snd.ump.write(midiport_, ump_stream, bytes) < 0)
+    {
+      error<driver_error>(
+          this->configuration, "midi_out_alsa_raw::send_message: cannot write message.");
+      return false;
+    }
+
+    return true;
+  }
+
+  snd_ump_t* midiport_{};
+};
+
+}

include/libremidi/backends/alsa_raw_ump/observer.hpp

@@ -0,0 +1,14 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/observer.hpp>
+#include <libremidi/backends/alsa_raw_ump/helpers.hpp>
+
+namespace libremidi::alsa_raw_ump
+{
+
+class observer_impl : public alsa_raw::observer_impl_base<midi2_enumerator>
+{
+  using alsa_raw::observer_impl_base<midi2_enumerator>::observer_impl_base;
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_RAW_UMP; }
+};
+
+}

include/libremidi/backends/alsa_seq.hpp

@@ -0,0 +1,42 @@
+#pragma once
+//*********************************************************************//
+//  API: LINUX ALSA SEQUENCER
+//*********************************************************************//
+
+// API information found at:
+//   - http://www.alsa-project.org/documentation.php#Library
+
+// The ALSA Sequencer API is based on the use of a callback function for
+// MIDI input.
+//
+// Thanks to Pedro Lopez-Cabanillas for help with the ALSA sequencer
+// time stamps and other assorted fixes!!!
+
+#include <libremidi/backends/alsa_seq/midi_in.hpp>
+#include <libremidi/backends/alsa_seq/midi_out.hpp>
+#include <libremidi/backends/alsa_seq/observer.hpp>
+
+namespace libremidi::alsa_seq
+{
+
+struct backend
+{
+  using midi_in
+      = alsa_seq::midi_in_impl<libremidi::input_configuration, alsa_seq::input_configuration>;
+  using midi_out = alsa_seq::midi_out_impl;
+  using midi_observer = alsa_seq::observer_impl<alsa_seq::observer_configuration>;
+  using midi_in_configuration = alsa_seq::input_configuration;
+  using midi_out_configuration = alsa_seq::output_configuration;
+  using midi_observer_configuration = alsa_seq::observer_configuration;
+  static const constexpr auto API = libremidi::API::ALSA_SEQ;
+  static const constexpr auto name = "alsa_seq";
+  static const constexpr auto display_name = "ALSA (sequencer)";
+
+  static inline bool available() noexcept
+  {
+    static const libasound& snd = libasound::instance();
+    return snd.available && snd.seq.available;
+  }
+};
+
+}

include/libremidi/backends/alsa_seq/config.hpp

@@ -0,0 +1,63 @@
+#pragma once
+#include <libremidi/backends/alsa_raw/config.hpp>
+
+#if __has_include(<alsa/asoundlib.h>)
+  #include <alsa/asoundlib.h>
+#else
+extern "C" {
+typedef struct _snd_seq snd_seq_t;
+typedef struct snd_seq_event snd_seq_event_t;
+typedef struct snd_seq_addr
+{
+  unsigned char client;
+  unsigned char port;
+} snd_seq_addr_t;
+}
+#endif
+
+namespace libremidi::alsa_seq
+{
+
+// Possible parameters:
+// - Direct / non-direct output
+// - Timer source used (high resolution, etc)
+// - Timestamping
+// - Tempo, ppq?
+
+struct poll_parameters
+{
+  snd_seq_addr_t addr{};
+  std::function<int(const snd_seq_event_t&)> callback;
+};
+
+struct input_configuration
+{
+  using poll_parameters_type = poll_parameters;
+
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+  std::function<bool(const poll_parameters&)> manual_poll;
+  std::function<bool(snd_seq_addr_t)> stop_poll;
+
+  static constexpr int midi_version = 1;
+};
+
+struct output_configuration
+{
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+
+  static constexpr int midi_version = 1;
+};
+
+struct observer_configuration
+{
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+  std::function<bool(const poll_parameters&)> manual_poll;
+  std::function<bool(snd_seq_addr_t)> stop_poll;
+
+  static constexpr int midi_version = 1;
+};
+
+}

include/libremidi/backends/alsa_seq/helpers.hpp

@@ -0,0 +1,306 @@
+#pragma once
+#include <libremidi/backends/linux/alsa.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <sys/time.h>
+
+#include <cstring>
+#include <optional>
+#include <string>
+
+namespace libremidi::alsa_seq
+{
+struct event_handle
+{
+  const libasound& snd;
+  snd_seq_event_t* ev{};
+
+  explicit event_handle(const libasound& snd) noexcept
+      : snd{snd}
+  {
+  }
+
+  explicit event_handle(const libasound& snd, snd_seq_event_t* ev) noexcept
+      : snd{snd}
+      , ev{ev}
+  {
+  }
+
+  event_handle(const event_handle&) noexcept = delete;
+  event_handle& operator=(const event_handle&) noexcept = delete;
+  event_handle(event_handle&&) noexcept = delete;
+  event_handle& operator=(event_handle&&) noexcept = delete;
+
+  void reset(snd_seq_event_t* new_ev) noexcept
+  {
+    if (ev)
+      snd.seq.free_event(ev);
+    ev = new_ev;
+  }
+
+  ~event_handle() { snd.seq.free_event(ev); }
+};
+
+namespace
+{
+inline constexpr port_handle seq_to_port_handle(uint64_t client, uint64_t port) noexcept
+{
+  return (client << 32) + port;
+}
+
+inline constexpr std::pair<int, int> seq_from_port_handle(port_handle p) noexcept
+{
+  int client = p >> 32;
+  int port = p & 0xFFFFFFFF;
+  return {client, port};
+}
+
+inline void for_all_ports(
+    const libasound& snd, snd_seq_t* seq,
+    std::function<void(snd_seq_client_info_t&, snd_seq_port_info_t&)> func)
+{
+  snd_seq_client_info_t* cinfo{};
+  snd_seq_client_info_alloca(&cinfo);
+  snd_seq_port_info_t* pinfo{};
+  snd_seq_port_info_alloca(&pinfo);
+
+  snd.seq.client_info_set_client(cinfo, -1);
+  while (snd.seq.query_next_client(seq, cinfo) >= 0)
+  {
+    int client = snd.seq.client_info_get_client(cinfo);
+    if (client == 0)
+      continue;
+
+    // Reset query info
+    snd.seq.port_info_set_client(pinfo, client);
+    snd.seq.port_info_set_port(pinfo, -1);
+    while (snd.seq.query_next_port(seq, pinfo) >= 0)
+    {
+      func(*cinfo, *pinfo);
+    }
+  }
+}
+
+// This function is used to count or get the pinfo structure for a given port
+// number.
+inline unsigned int iterate_port_info(
+    const libasound& snd, snd_seq_t* seq, snd_seq_port_info_t* pinfo, unsigned int type,
+    int portNumber)
+{
+  snd_seq_client_info_t* cinfo{};
+  int count = 0;
+  snd_seq_client_info_alloca(&cinfo);
+
+  snd.seq.client_info_set_client(cinfo, -1);
+  while (snd.seq.query_next_client(seq, cinfo) >= 0)
+  {
+    const int client = snd.seq.client_info_get_client(cinfo);
+    if (client == 0)
+      continue;
+
+    // Reset query info
+    snd.seq.port_info_set_client(pinfo, client);
+    snd.seq.port_info_set_port(pinfo, -1);
+    while (snd.seq.query_next_port(seq, pinfo) >= 0)
+    {
+      const unsigned int atyp = snd.seq.port_info_get_type(pinfo);
+      if (((atyp & SND_SEQ_PORT_TYPE_MIDI_GENERIC) == 0) && ((atyp & SND_SEQ_PORT_TYPE_SYNTH) == 0)
+          && ((atyp & SND_SEQ_PORT_TYPE_APPLICATION) == 0))
+        continue;
+
+      const unsigned int caps = snd.seq.port_info_get_capability(pinfo);
+      if ((caps & type) != type)
+        continue;
+      if ((caps & SND_SEQ_PORT_CAP_NO_EXPORT) != 0)
+        continue;
+      if (count == portNumber)
+        return 1;
+      ++count;
+    }
+  }
+
+  // If a negative portNumber was used, return the port count.
+  if (portNumber < 0)
+    return count;
+  return 0;
+}
+}
+
+// A structure to hold variables related to the ALSA API
+// implementation.
+struct alsa_data
+{
+  const libasound& snd = libasound::instance();
+  snd_seq_t* seq{};
+  int vport{-1};
+  snd_seq_addr_t vaddr{};
+
+  snd_seq_port_subscribe_t* subscription{};
+  snd_midi_event_t* coder{};
+
+  [[nodiscard]] int init_client(auto& configuration)
+  {
+    // Initialize or use the snd_seq client
+    if (configuration.context)
+    {
+      seq = configuration.context;
+      return 0;
+    }
+    else
+    {
+      // Set up the ALSA sequencer client.
+      int ret = snd.seq.open(&seq, "default", SND_SEQ_OPEN_DUPLEX, SND_SEQ_NONBLOCK);
+      if (ret < 0)
+        return ret;
+
+      // Set client name.
+      if (!configuration.client_name.empty())
+        snd.seq.set_client_name(seq, configuration.client_name.data());
+
+#if __has_include(<alsa/ump.h>)
+      if (snd.seq.ump.set_client_midi_version)
+      {
+        switch (configuration.midi_version)
+        {
+          case 1:
+            snd.seq.ump.set_client_midi_version(seq, SND_SEQ_CLIENT_LEGACY_MIDI);
+            break;
+          case 2:
+            snd.seq.ump.set_client_midi_version(seq, SND_SEQ_CLIENT_UMP_MIDI_2_0);
+            break;
+        }
+      }
+#endif
+
+      return 0;
+    }
+  }
+
+  void set_client_name(std::string_view clientName)
+  {
+    snd.seq.set_client_name(seq, clientName.data());
+  }
+
+  void set_port_name(std::string_view portName)
+  {
+    snd_seq_port_info_t* pinfo;
+    snd_seq_port_info_alloca(&pinfo);
+    snd.seq.get_port_info(seq, vport, pinfo);
+    snd.seq.port_info_set_name(pinfo, portName.data());
+    snd.seq.set_port_info(seq, vport, pinfo);
+  }
+
+  unsigned int get_port_count(int caps) const
+  {
+    snd_seq_port_info_t* pinfo;
+    snd_seq_port_info_alloca(&pinfo);
+
+    return alsa_seq::iterate_port_info(snd, seq, pinfo, caps, -1);
+  }
+
+  std::optional<snd_seq_addr_t> get_port_info(const port_information& portNumber)
+  {
+    auto [client, port] = alsa_seq::seq_from_port_handle(portNumber.port);
+    // FIXME check that the {client, port} pair actually exists
+    // snd_seq_port_info_t* src_pinfo{};
+    // snd_seq_port_info_alloca(&src_pinfo);
+    // snd.seq.port_info_set_client(src_pinfo, client);
+    // snd.seq.port_info_set_port(src_pinfo, port);
+
+    // {
+    //   self.template error<invalid_parameter_error>(
+    //       self.configuration,
+    //       "alsa::get_port_info: invalid 'portNumber' argument: " + std::to_string(portNumber));
+    //   return {};
+    // }
+    snd_seq_addr_t addr;
+    addr.client = client;
+    addr.port = port;
+    return addr;
+  }
+
+  [[nodiscard]] int create_port(
+      auto& /*self*/, std::string_view portName, unsigned int caps, unsigned int type,
+      std::optional<int> queue)
+  {
+    if (this->vport < 0)
+    {
+      snd_seq_port_info_t* pinfo{};
+      snd_seq_port_info_alloca(&pinfo);
+
+      snd.seq.port_info_set_name(pinfo, portName.data());
+      snd.seq.port_info_set_client(pinfo, 0);
+      snd.seq.port_info_set_port(pinfo, 0);
+      snd.seq.port_info_set_capability(pinfo, caps);
+      snd.seq.port_info_set_type(pinfo, type);
+
+      if (type & SND_SEQ_PORT_TYPE_MIDI_GENERIC)
+      {
+        snd.seq.port_info_set_midi_channels(pinfo, 16);
+      }
+
+      if (queue)
+      {
+        snd.seq.port_info_set_timestamping(pinfo, 1);
+        snd.seq.port_info_set_timestamp_real(pinfo, 1);
+        snd.seq.port_info_set_timestamp_queue(pinfo, *queue);
+      }
+
+      if (int err = snd.seq.create_port(this->seq, pinfo); err < 0)
+        return err;
+
+      this->vport = snd.seq.port_info_get_port(pinfo);
+      if (int err = snd.seq.get_port_info(this->seq, this->vport, pinfo); err < 0)
+        return err;
+
+      if (auto addr = snd.seq.port_info_get_addr(pinfo))
+        this->vaddr = *addr;
+      else
+        return -1;
+
+      return this->vport;
+    }
+    return 0;
+  }
+
+  int create_connection(auto& self, snd_seq_addr_t sender, snd_seq_addr_t receiver, bool realtime)
+  {
+    // Create the connection between ports
+    // Make subscription
+    if (int err = snd.seq.port_subscribe_malloc(&this->subscription); err < 0)
+    {
+      self.template error<driver_error>(
+          self.configuration, "create_connection: ALSA error allocation port subscription.");
+      return err;
+    }
+
+    snd.seq.port_subscribe_set_sender(this->subscription, &sender);
+    snd.seq.port_subscribe_set_dest(this->subscription, &receiver);
+
+    if (realtime)
+    {
+      snd.seq.port_subscribe_set_time_update(this->subscription, 1);
+      snd.seq.port_subscribe_set_time_real(this->subscription, 1);
+    }
+
+    if (int err = snd.seq.subscribe_port(this->seq, this->subscription); err != 0)
+    {
+      snd.seq.port_subscribe_free(this->subscription);
+      this->subscription = nullptr;
+      return err;
+    }
+    return 0;
+  }
+
+  void unsubscribe()
+  {
+    if (this->subscription)
+    {
+      snd.seq.unsubscribe_port(this->seq, this->subscription);
+      snd.seq.port_subscribe_free(this->subscription);
+      this->subscription = nullptr;
+    }
+  }
+};
+}

include/libremidi/backends/alsa_seq/midi_in.hpp

@@ -0,0 +1,575 @@
+#pragma once
+#include <libremidi/backends/alsa_seq/config.hpp>
+#include <libremidi/backends/alsa_seq/helpers.hpp>
+#include <libremidi/backends/linux/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+namespace libremidi::alsa_seq
+{
+struct dummy_processing
+{
+  explicit dummy_processing(auto&&...) { }
+};
+
+template <typename ConfigurationImpl>
+using midi_in_base
+    = std::conditional_t<ConfigurationImpl::midi_version == 1, midi1::in_api, midi2::in_api>;
+template <typename ConfigurationImpl>
+using midi_in_processing = std::conditional_t<
+    ConfigurationImpl::midi_version == 1, midi1::input_state_machine, dummy_processing>;
+
+template <typename ConfigurationBase, typename ConfigurationImpl>
+class midi_in_impl
+    : public midi_in_base<ConfigurationImpl>
+    , protected alsa_data
+    , public error_handler
+{
+public:
+  struct
+      : ConfigurationBase
+      , ConfigurationImpl
+  {
+  } configuration;
+  midi_in_processing<ConfigurationImpl> m_processing{this->configuration};
+
+  bool require_timestamps() const noexcept
+  {
+    switch (configuration.timestamps)
+    {
+      case timestamp_mode::NoTimestamp:
+      case timestamp_mode::SystemMonotonic:
+      case timestamp_mode::AudioFrame:
+        return false;
+      case timestamp_mode::Absolute:
+      case timestamp_mode::Relative:
+      case timestamp_mode::Custom:
+        return true;
+    }
+    return true;
+  }
+
+  explicit midi_in_impl(ConfigurationBase&& conf, ConfigurationImpl&& apiconf)
+      : midi_in_base<ConfigurationImpl>{}
+      , configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (init_client(configuration) < 0)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_alsa::initialize: error creating ALSA sequencer client "
+          "object.");
+      return;
+    }
+
+    // Create the input queue
+    if (require_timestamps())
+    {
+      this->queue_id = snd.seq.alloc_queue(seq);
+      // Set arbitrary tempo (mm=100) and resolution (240)
+      snd_seq_queue_tempo_t* qtempo{};
+      snd_seq_queue_tempo_alloca(&qtempo);
+      snd.seq.queue_tempo_set_tempo(qtempo, 600000);
+      snd.seq.queue_tempo_set_ppq(qtempo, 240);
+      snd.seq.set_queue_tempo(this->seq, this->queue_id, qtempo);
+      snd.seq.drain_output(this->seq);
+    }
+
+    // Create the event -> midi encoder
+    {
+      int result = snd.midi.event_new(0, &coder);
+      if (result < 0)
+      {
+        error<driver_error>(
+            this->configuration, "midi_in_alsa::initialize: error during snd_midi_event_new.");
+        return;
+      }
+      snd.midi.event_init(coder);
+      snd.midi.event_no_status(coder, 1);
+    }
+  }
+
+  ~midi_in_impl() override
+  {
+    // Cleanup.
+    if (this->vport >= 0)
+      snd.seq.delete_port(this->seq, this->vport);
+
+    if (require_timestamps())
+      snd.seq.free_queue(this->seq, this->queue_id);
+
+    snd.midi.event_free(coder);
+
+    // Close if we do not have an user-provided client object
+    if (!configuration.context)
+      snd.seq.close(this->seq);
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_SEQ; }
+
+  [[nodiscard]] int create_port(std::string_view portName)
+  {
+    return alsa_data::create_port(
+        *this, portName, SND_SEQ_PORT_CAP_WRITE | SND_SEQ_PORT_CAP_SUBS_WRITE,
+        SND_SEQ_PORT_TYPE_MIDI_GENERIC | SND_SEQ_PORT_TYPE_APPLICATION,
+        require_timestamps() ? std::optional<int>{this->queue_id} : std::nullopt);
+  }
+
+  void start_queue()
+  {
+    if (require_timestamps())
+    {
+      snd.seq.control_queue(this->seq, this->queue_id, SND_SEQ_EVENT_START, 0, nullptr);
+      this->queue_creation_time = std::chrono::steady_clock::now();
+      snd.seq.drain_output(this->seq);
+    }
+  }
+
+  void stop_queue()
+  {
+    if (require_timestamps())
+    {
+      snd.seq.control_queue(this->seq, this->queue_id, SND_SEQ_EVENT_STOP, 0, nullptr);
+      snd.seq.drain_output(this->seq);
+    }
+  }
+
+  int connect_port(snd_seq_addr_t sender)
+  {
+    snd_seq_addr_t receiver{};
+    receiver.client = snd.seq.client_id(this->seq);
+    receiver.port = this->vport;
+
+    return create_connection(*this, sender, receiver, false);
+  }
+
+  std::optional<snd_seq_addr_t> to_address(const port_information& p)
+  {
+    return alsa_data::get_port_info(p);
+  }
+
+  int init_port(std::optional<snd_seq_addr_t> source, std::string_view portName)
+  {
+    this->close_port();
+
+    if (!source)
+      return -1;
+
+    if (int ret = create_port(portName); ret < 0)
+    {
+      error<driver_error>(configuration, "midi_in_alsa::create_port: ALSA error creating port.");
+      return ret;
+    }
+
+    if (int ret = connect_port(*source); ret < 0)
+    {
+      error<driver_error>(
+          configuration, "midi_in_alsa::create_port: ALSA error making port connection.");
+      return ret;
+    }
+
+    start_queue();
+
+    return 0;
+  }
+
+  int init_virtual_port(std::string_view portName)
+  {
+    this->close_port();
+
+    if (int ret = create_port(portName); ret < 0)
+      return ret;
+
+    start_queue();
+    return 0;
+  }
+
+  void close_port() override
+  {
+    unsubscribe();
+    stop_queue();
+  }
+
+  void set_client_name(std::string_view clientName) override
+  {
+    alsa_data::set_client_name(clientName);
+  }
+
+  void set_port_name(std::string_view portName) override { alsa_data::set_port_name(portName); }
+
+protected:
+  void set_timestamp(const auto& ev, auto& msg) noexcept
+  {
+    static constexpr int64_t nanos = 1e9;
+    switch (configuration.timestamps)
+    {
+      case timestamp_mode::NoTimestamp:
+        msg.timestamp = 0;
+        return;
+      case timestamp_mode::Relative: {
+        const auto t1 = static_cast<int64_t>(ev.time.time.tv_sec) * nanos
+                        + static_cast<int64_t>(ev.time.time.tv_nsec);
+        const auto t0 = static_cast<int64_t>(last_time.tv_sec) * nanos
+                        + static_cast<int64_t>(last_time.tv_nsec);
+        const auto time = t1 - t0;
+
+        last_time = ev.time.time;
+
+        if (this->firstMessage == true)
+        {
+          this->firstMessage = false;
+          msg.timestamp = 0;
+        }
+        else
+        {
+          msg.timestamp = time;
+        }
+        return;
+      }
+      case timestamp_mode::Absolute: {
+        msg.timestamp = ev.time.time.tv_sec * nanos + ev.time.time.tv_nsec;
+        break;
+      }
+      case timestamp_mode::Custom: {
+        msg.timestamp
+            = configuration.get_timestamp(ev.time.time.tv_sec * nanos + ev.time.time.tv_nsec);
+        break;
+      }
+      case timestamp_mode::SystemMonotonic: {
+        namespace clk = std::chrono;
+        msg.timestamp = system_ns();
+        break;
+      }
+    }
+  }
+
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return std::chrono::duration_cast<std::chrono::nanoseconds>(
+               std::chrono::steady_clock::now() - this->queue_creation_time)
+        .count();
+  }
+
+  int process_event(const snd_seq_event_t& ev)
+  {
+    if constexpr (ConfigurationImpl::midi_version == 1)
+    {
+      switch (ev.type)
+      {
+        case SND_SEQ_EVENT_PORT_SUBSCRIBED:
+        case SND_SEQ_EVENT_PORT_UNSUBSCRIBED:
+          return 0;
+        case SND_SEQ_EVENT_SYSEX: {
+          if (configuration.ignore_sysex)
+            return 0;
+          else if (ev.data.ext.len > decoding_buffer.size())
+            decoding_buffer.resize(ev.data.ext.len);
+          break;
+        }
+      }
+
+      static constexpr timestamp_backend_info timestamp_info{
+          .has_absolute_timestamps = true,
+          .absolute_is_monotonic = false,
+          .has_samples = false,
+      };
+
+      const auto to_ns = [ts = ev.time.time] {
+        return static_cast<int64_t>(ts.tv_sec) * 1'000'000'000 + static_cast<int64_t>(ts.tv_nsec);
+      };
+      auto buf = decoding_buffer.data();
+      auto buf_space = decoding_buffer.size();
+
+      // FIXME according to the doc snd_midi_event_decode can apparently return multiple events????
+      const auto avail = snd.midi.event_decode(coder, buf, buf_space, &ev);
+      if (avail > 0)
+      {
+        m_processing.on_bytes(
+            {buf, buf + avail}, m_processing.template timestamp<timestamp_info>(to_ns, 0));
+        return 0;
+      }
+      else
+      {
+        return avail;
+      }
+    }
+    return 0;
+  }
+
+  int process_events()
+  {
+    if constexpr (ConfigurationImpl::midi_version == 1)
+    {
+      snd_seq_event_t* ev{};
+      event_handle handle{snd};
+      int result = 0;
+      while ((result = snd.seq.event_input(seq, &ev)) > 0)
+      {
+        handle.reset(ev);
+        if (int err = process_event(*ev); err < 0)
+          return err;
+      }
+      return result;
+    }
+    else
+    {
+      return 0;
+    }
+  }
+
+#if __has_include(<alsa/ump.h>)
+  int process_ump_event(const snd_seq_ump_event_t& ev)
+  {
+    // Filter the message types before any decoding
+    switch (ev.type)
+    {
+      case SND_SEQ_EVENT_PORT_SUBSCRIBED:
+      case SND_SEQ_EVENT_PORT_UNSUBSCRIBED:
+        return 0;
+
+      case SND_SEQ_EVENT_QFRAME: // MIDI time code
+      case SND_SEQ_EVENT_TICK:   // 0xF9 ... MIDI timing tick
+      case SND_SEQ_EVENT_CLOCK:  // 0xF8 ... MIDI timing (clock) tick
+        if (configuration.ignore_timing)
+          return 0;
+        break;
+
+      case SND_SEQ_EVENT_SENSING: // Active sensing
+        if (configuration.ignore_sensing)
+          return 0;
+        break;
+
+      case SND_SEQ_EVENT_SYSEX: {
+        if (configuration.ignore_sysex)
+          return 0;
+        break;
+      }
+    }
+
+    // MIDI 2 : no decoder, we can just send the UMP data directly, yay
+    libremidi::ump ump;
+    std::memcpy(ump.data, ev.ump, sizeof(ev.ump));
+    set_timestamp(ev, ump);
+    configuration.on_message(std::move(ump));
+    return 0;
+  }
+
+  int process_ump_events()
+  {
+    snd_seq_ump_event_t* ev{};
+    event_handle handle{snd};
+    int result = 0;
+    while ((result = snd.seq.ump.event_input(seq, &ev)) > 0)
+    {
+      handle.reset((snd_seq_event_t*)ev);
+      if (int err = process_ump_event(*ev); err < 0)
+        return err;
+    }
+    return result;
+  }
+#endif
+
+  int queue_id{}; // an input queue is needed to get timestamped events
+  snd_seq_real_time_t last_time{};
+
+  // Only needed for midi 1
+  std::vector<unsigned char> decoding_buffer = std::vector<unsigned char>(4096);
+  std::chrono::steady_clock::time_point queue_creation_time;
+};
+
+template <typename ConfigurationBase, typename ConfigurationImpl>
+class midi_in_alsa_threaded : public midi_in_impl<ConfigurationBase, ConfigurationImpl>
+{
+public:
+  midi_in_alsa_threaded(ConfigurationBase&& conf, ConfigurationImpl&& apiconf)
+      : midi_in_impl<ConfigurationBase, ConfigurationImpl>{std::move(conf), std::move(apiconf)}
+  {
+    if (this->termination_event < 0)
+    {
+      this->template error<driver_error>(
+          this->configuration, "midi_in_alsa::initialize: error creating eventfd.");
+    }
+  }
+
+  ~midi_in_alsa_threaded() { this->close_port(); }
+
+private:
+  bool open_port(const input_port& pt, std::string_view local_port_name) override
+  {
+    if (int err = this->init_port(this->to_address(pt), local_port_name); err < 0)
+      return false;
+
+    if (!start_thread())
+      return false;
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    if (int err = this->init_virtual_port(portName); err < 0)
+      return false;
+
+    if (!this->start_thread())
+      return false;
+    return true;
+  }
+
+  void close_port() override
+  {
+    midi_in_impl<ConfigurationBase, ConfigurationImpl>::close_port();
+
+    stop_thread();
+  }
+
+  [[nodiscard]] int start_thread()
+  {
+    try
+    {
+      this->thread = std::thread([this] { thread_handler(); });
+    }
+    catch (const std::system_error& e)
+    {
+      using namespace std::literals;
+      this->unsubscribe();
+
+      this->template error<thread_error>(
+          this->configuration,
+          "midi_in_alsa::start_thread: error starting MIDI input thread: "s + e.what());
+      return false;
+    }
+    return true;
+  }
+
+  void stop_thread()
+  {
+    termination_event.notify();
+
+    if (this->thread.joinable())
+      this->thread.join();
+
+    termination_event.consume();
+  }
+
+  void thread_handler()
+  {
+    int poll_fd_count = alsa_data::snd.seq.poll_descriptors_count(this->seq, POLLIN) + 1;
+    auto poll_fds = (struct pollfd*)alloca(poll_fd_count * sizeof(struct pollfd));
+    poll_fds[0] = this->termination_event;
+    alsa_data::snd.seq.poll_descriptors(this->seq, poll_fds + 1, poll_fd_count - 1, POLLIN);
+
+    for (;;)
+    {
+      if (alsa_data::snd.seq.event_input_pending(this->seq, 1) == 0)
+      {
+        // No data pending
+        if (poll(poll_fds, poll_fd_count, -1) >= 0)
+        {
+          // We got our stop-thread signal
+          if (termination_event.ready(poll_fds[0]))
+          {
+            break;
+          }
+        }
+        continue;
+      }
+
+      int res{};
+      if constexpr (ConfigurationImpl::midi_version == 1)
+      {
+        res = this->process_events();
+      }
+#if __has_include(<alsa/ump.h>)
+      else if constexpr (ConfigurationImpl::midi_version == 2)
+      {
+        res = this->process_ump_events();
+      }
+#endif
+
+      (void)res;
+#if defined(__LIBREMIDI_DEBUG__)
+      if (res < 0)
+        std::cerr << "midi_in_alsa::thread_handler: MIDI input error: " << snd.strerror(res)
+                  << "\n";
+#endif
+    }
+  }
+
+  std::thread thread{};
+  eventfd_notifier termination_event{};
+};
+
+template <typename ConfigurationBase, typename ConfigurationImpl>
+class midi_in_alsa_manual : public midi_in_impl<ConfigurationBase, ConfigurationImpl>
+{
+public:
+  midi_in_alsa_manual(ConfigurationBase&& conf, ConfigurationImpl&& apiconf)
+      : midi_in_impl<ConfigurationBase, ConfigurationImpl>{std::move(conf), std::move(apiconf)}
+  {
+    assert(this->configuration.manual_poll);
+    assert(this->configuration.stop_poll);
+  }
+
+  [[nodiscard]] int init_callback()
+  {
+    using poll_params = typename ConfigurationImpl::poll_parameters_type;
+    this->configuration.manual_poll(
+        poll_params{.addr = this->vaddr, .callback = [this](const auto& ev) {
+                      if constexpr (ConfigurationImpl::midi_version == 1)
+                        return this->process_event(ev);
+#if __has_include(<alsa/ump.h>)
+                      else
+                        return this->process_ump_event(ev);
+#endif
+                    }});
+    return 0;
+  }
+
+  ~midi_in_alsa_manual() { this->close_port(); }
+
+  bool open_port(const input_port& pt, std::string_view local_port_name) override
+  {
+    if (int err = this->init_port(this->to_address(pt), local_port_name); err < 0)
+      return false;
+
+    if (int err = init_callback(); err < 0)
+      return false;
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view name) override
+  {
+    if (int err = this->init_virtual_port(name); err < 0)
+      return false;
+
+    if (int err = init_callback(); err < 0)
+      return false;
+    return true;
+  }
+
+  void close_port() override
+  {
+    this->configuration.stop_poll(this->vaddr);
+
+    midi_in_impl<ConfigurationBase, ConfigurationImpl>::close_port();
+  }
+};
+}
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<midi_in_api>
+make<alsa_seq::midi_in_impl<libremidi::input_configuration, alsa_seq::input_configuration>>(
+    libremidi::input_configuration&& conf, libremidi::alsa_seq::input_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_seq::midi_in_alsa_manual<
+        libremidi::input_configuration, alsa_seq::input_configuration>>(
+        std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_seq::midi_in_alsa_threaded<
+        libremidi::input_configuration, alsa_seq::input_configuration>>(
+        std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/alsa_seq/midi_out.hpp

@@ -0,0 +1,174 @@
+#pragma once
+#include <libremidi/backends/alsa_seq/config.hpp>
+#include <libremidi/backends/alsa_seq/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi::alsa_seq
+{
+
+class midi_out_impl final
+    : public midi1::out_api
+    , private alsa_data
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::output_configuration
+      , alsa_seq::output_configuration
+  {
+  } configuration;
+
+  midi_out_impl(libremidi::output_configuration&& conf, alsa_seq::output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (init_client(configuration) < 0)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_alsa::initialize: error creating ALSA sequencer client "
+          "object.");
+      return;
+    }
+
+    if (snd.midi.event_new(this->bufferSize, &this->coder) < 0)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_out_alsa::initialize: error initializing MIDI event "
+          "parser.");
+      return;
+    }
+    snd.midi.event_init(this->coder);
+  }
+
+  ~midi_out_impl() override
+  {
+    // Close a connection if it exists.
+    midi_out_impl::close_port();
+
+    // Cleanup.
+    if (this->vport >= 0)
+      snd.seq.delete_port(this->seq, this->vport);
+    if (this->coder)
+      snd.midi.event_free(this->coder);
+
+    if (!configuration.context)
+      snd.seq.close(this->seq);
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_SEQ; }
+
+  [[nodiscard]] int create_port(std::string_view portName)
+  {
+    return alsa_data::create_port(
+        *this, portName, SND_SEQ_PORT_CAP_READ | SND_SEQ_PORT_CAP_SUBS_READ,
+        SND_SEQ_PORT_TYPE_MIDI_GENERIC | SND_SEQ_PORT_TYPE_APPLICATION, std::nullopt);
+  }
+
+  bool open_port(const output_port& p, std::string_view portName) override
+  {
+    unsigned int nSrc = this->get_port_count(SND_SEQ_PORT_CAP_WRITE | SND_SEQ_PORT_CAP_SUBS_WRITE);
+    if (nSrc < 1)
+    {
+      error<no_devices_found_error>(
+          this->configuration, "midi_out_alsa::open_port: no MIDI output sources found!");
+      return false;
+    }
+
+    auto sink = get_port_info(p);
+    if (!sink)
+      return false;
+
+    if (int err = create_port(portName); err < 0)
+    {
+      error<driver_error>(configuration, "midi_out_alsa::create_port: ALSA error creating port.");
+      return false;
+    }
+
+    snd_seq_addr_t source{
+        .client = (unsigned char)snd.seq.client_id(this->seq), .port = (unsigned char)this->vport};
+    if (int err = create_connection(*this, source, *sink, true); err < 0)
+    {
+      error<driver_error>(
+          configuration, "midi_out_alsa::create_port: ALSA error making port connection.");
+      return false;
+    }
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    if (int err = create_port(portName); err < 0)
+      return false;
+    return true;
+  }
+
+  void close_port() override { unsubscribe(); }
+
+  void set_client_name(std::string_view clientName) override
+  {
+    alsa_data::set_client_name(clientName);
+  }
+
+  void set_port_name(std::string_view portName) override { alsa_data::set_port_name(portName); }
+
+  void send_message(const unsigned char* message, std::size_t size) override
+  {
+    int64_t result{};
+    if (size > this->bufferSize)
+    {
+      this->bufferSize = size;
+      result = snd.midi.event_resize_buffer(this->coder, size);
+      if (result != 0)
+      {
+        error<driver_error>(
+            this->configuration,
+            "midi_out_alsa::send_message: ALSA error resizing MIDI event "
+            "buffer.");
+        return;
+      }
+    }
+
+    std::size_t offset = 0;
+    while (offset < size)
+    {
+      snd_seq_event_t ev;
+      snd_seq_ev_clear(&ev);
+      snd_seq_ev_set_source(&ev, this->vport);
+      snd_seq_ev_set_subs(&ev);
+      // FIXME direct is set but snd_seq_event_output_direct is not used...
+      snd_seq_ev_set_direct(&ev);
+
+      const int64_t nBytes = size; // signed to avoir potential overflow with size - offset below
+      result = snd.midi.event_encode(this->coder, message + offset, (long)(nBytes - offset), &ev);
+      if (result < 0)
+      {
+        warning(this->configuration, "midi_out_alsa::send_message: event parsing error!");
+        return;
+      }
+
+      if (ev.type == SND_SEQ_EVENT_NONE)
+      {
+        warning(this->configuration, "midi_out_alsa::send_message: incomplete message!");
+        return;
+      }
+
+      offset += result;
+
+      result = snd.seq.event_output(this->seq, &ev);
+      if (result < 0)
+      {
+        warning(
+            this->configuration,
+            "midi_out_alsa::send_message: error sending MIDI message to port.");
+        return;
+      }
+    }
+    snd.seq.drain_output(this->seq);
+  }
+
+private:
+  uint64_t bufferSize{32};
+};
+}

include/libremidi/backends/alsa_seq/observer.hpp

@@ -0,0 +1,340 @@
+#pragma once
+#include <libremidi/backends/alsa_seq/config.hpp>
+#include <libremidi/backends/alsa_seq/helpers.hpp>
+#include <libremidi/backends/linux/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <alsa/asoundlib.h>
+
+#include <bitset>
+#include <map>
+
+namespace libremidi::alsa_seq
+{
+
+struct port_info
+{
+  std::string client_name;
+  std::string port_name;
+  int client{};
+  int port{};
+  bool isInput{};
+  bool isOutput{};
+};
+
+template <typename ConfigurationImpl>
+class observer_impl
+    : public observer_api
+    , public alsa_data
+{
+public:
+  struct
+      : libremidi::observer_configuration
+      , ConfigurationImpl
+  {
+  } configuration;
+
+  explicit observer_impl(libremidi::observer_configuration&& conf, ConfigurationImpl&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    using namespace std::literals;
+    if (int err = init_client(configuration); err < 0)
+    {
+      throw driver_error("observer_alsa: snd_seq_open failed");
+    }
+
+    if (!configuration.has_callbacks())
+      return;
+
+    // Init with the existing ports
+    if (configuration.notify_in_constructor)
+      init_all_ports();
+
+    // Create the port to listen on the server events
+    {
+#if __has_include(<alsa/ump.h>)
+      constexpr int midi2_cap
+          = ConfigurationImpl::midi_version == 2 ? SND_SEQ_PORT_CAP_UMP_ENDPOINT : 0;
+#else
+      constexpr int midi2_cap = 0;
+#endif
+
+      constexpr int caps = SND_SEQ_PORT_CAP_READ | SND_SEQ_PORT_CAP_WRITE
+                           | SND_SEQ_PORT_CAP_SUBS_READ | SND_SEQ_PORT_CAP_SUBS_WRITE | midi2_cap;
+      int err = alsa_data::create_port(
+          *this, "libremidi-observe", caps, SND_SEQ_PORT_TYPE_APPLICATION, false);
+      if (err < 0)
+      {
+        throw driver_error("observer: ALSA error creating port.");
+      }
+    }
+
+    // Connect the ALSA server events to our port
+    {
+      int err
+          = snd.seq.connect_from(seq, vport, SND_SEQ_CLIENT_SYSTEM, SND_SEQ_PORT_SYSTEM_ANNOUNCE);
+      if (err < 0)
+      {
+        throw driver_error("observer_alsa: snd_seq_connect_from failed");
+      }
+    }
+  }
+
+  std::optional<port_info> get_info(int client, int port) const noexcept
+  {
+    port_info p;
+    p.client = client;
+    p.port = port;
+
+    snd_seq_client_info_t* cinfo;
+    snd_seq_client_info_alloca(&cinfo);
+    if (int err = snd.seq.get_any_client_info(seq, client, cinfo); err < 0)
+      return std::nullopt;
+
+    snd_seq_port_info_t* pinfo;
+    snd_seq_port_info_alloca(&pinfo);
+    if (int err = snd.seq.get_any_port_info(seq, client, port, pinfo); err < 0)
+      return std::nullopt;
+
+    const auto tp = snd.seq.port_info_get_type(pinfo);
+    bool ok = this->configuration.track_any;
+    if ((tp & SND_SEQ_PORT_TYPE_HARDWARE) && this->configuration.track_hardware)
+      ok = true;
+    else if ((tp & SND_SEQ_PORT_TYPE_SOFTWARE) && this->configuration.track_virtual)
+      ok = true;
+    if (!ok)
+      return {};
+
+    if (auto name = snd.seq.client_info_get_name(cinfo))
+      p.client_name = name;
+
+    if (auto name = snd.seq.port_info_get_name(pinfo))
+      p.port_name = name;
+
+    auto cap = snd.seq.port_info_get_capability(pinfo);
+    p.isInput = (cap & SND_SEQ_PORT_CAP_DUPLEX) | (cap & SND_SEQ_PORT_CAP_READ);
+    p.isOutput = (cap & SND_SEQ_PORT_CAP_DUPLEX) | (cap & SND_SEQ_PORT_CAP_WRITE);
+
+    return p;
+  }
+
+  template <bool Input>
+  auto to_port_info(port_info p) const noexcept
+      -> std::conditional_t<Input, input_port, output_port>
+  {
+    static_assert(sizeof(this->seq) <= sizeof(libremidi::client_handle));
+    static_assert(sizeof(std::uintptr_t) <= sizeof(libremidi::client_handle));
+    return {
+        {.client = std::uintptr_t(this->seq),
+         .port = alsa_seq::seq_to_port_handle(p.client, p.port),
+         .manufacturer = "",
+         .device_name = p.client_name,
+         .port_name = p.port_name,
+         .display_name = p.port_name}};
+  }
+
+  void init_all_ports()
+  {
+    alsa_seq::for_all_ports(
+        snd, this->seq, [this](snd_seq_client_info_t& client, snd_seq_port_info_t& port) {
+          int clt = snd.seq.client_info_get_client(&client);
+          int pt = snd.seq.port_info_get_port(&port);
+          register_port(clt, pt);
+        });
+  }
+
+  libremidi::API get_current_api() const noexcept override
+  {
+    if constexpr (ConfigurationImpl::midi_version == 1)
+      return libremidi::API::ALSA_SEQ;
+    else
+      return libremidi::API::ALSA_SEQ_UMP;
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    std::vector<libremidi::input_port> ret;
+    alsa_seq::for_all_ports(
+        snd, this->seq, [this, &ret](snd_seq_client_info_t& client, snd_seq_port_info_t& port) {
+          int clt = snd.seq.client_info_get_client(&client);
+          int pt = snd.seq.port_info_get_port(&port);
+          if (auto p = get_info(clt, pt))
+            if (p->isInput)
+              ret.push_back(to_port_info<true>(*p));
+        });
+    return ret;
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    std::vector<libremidi::output_port> ret;
+    alsa_seq::for_all_ports(
+        snd, this->seq, [this, &ret](snd_seq_client_info_t& client, snd_seq_port_info_t& port) {
+          int clt = snd.seq.client_info_get_client(&client);
+          int pt = snd.seq.port_info_get_port(&port);
+          if (auto p = get_info(clt, pt))
+            if (p->isOutput)
+              ret.push_back(to_port_info<false>(*p));
+        });
+    return ret;
+  }
+
+  void register_port(int client, int port)
+  {
+    auto pp = get_info(client, port);
+    if (!pp)
+      return;
+    auto& p = *pp;
+    if (p.client == snd.seq.client_id(seq))
+      return;
+
+    knownClients_[{p.client, p.port}] = p;
+    if (p.isInput && configuration.input_added)
+    {
+      configuration.input_added(to_port_info<true>(p));
+    }
+
+    if (p.isOutput && configuration.output_added)
+    {
+      configuration.output_added(to_port_info<false>(p));
+    }
+  }
+
+  void unregister_port(int client, int port)
+  {
+    auto it = knownClients_.find({client, port});
+    if (it != knownClients_.end())
+    {
+      auto p = it->second;
+      knownClients_.erase(it);
+
+      if (p.isInput && configuration.input_removed)
+      {
+        configuration.input_removed(to_port_info<true>(p));
+      }
+
+      if (p.isOutput && configuration.output_removed)
+      {
+        configuration.output_removed(to_port_info<false>(p));
+      }
+    }
+  }
+
+  void handle_event(const snd_seq_event_t& ev)
+  {
+    switch (ev.type)
+    {
+      case SND_SEQ_EVENT_PORT_START: {
+        register_port(ev.data.addr.client, ev.data.addr.port);
+        break;
+      }
+      case SND_SEQ_EVENT_PORT_EXIT: {
+        unregister_port(ev.data.addr.client, ev.data.addr.port);
+        break;
+      }
+      case SND_SEQ_EVENT_PORT_CHANGE:
+      default:
+        break;
+    }
+  }
+
+  ~observer_impl()
+  {
+    if (seq)
+    {
+      if (vport)
+        snd.seq.delete_port(seq, vport);
+
+      if (!configuration.context)
+        snd.seq.close(seq);
+    }
+  }
+
+private:
+  std::map<std::pair<int, int>, port_info> knownClients_;
+};
+
+template <typename ConfigurationImpl>
+class observer_threaded : public observer_impl<ConfigurationImpl>
+{
+public:
+  observer_threaded(libremidi::observer_configuration&& conf, ConfigurationImpl&& apiconf)
+      : observer_impl<ConfigurationImpl>{std::move(conf), std::move(apiconf)}
+  {
+    // Create relevant descriptors
+    auto& snd = alsa_data::snd;
+    const auto N = snd.seq.poll_descriptors_count(this->seq, POLLIN);
+    descriptors_.resize(N + 1);
+    snd.seq.poll_descriptors(this->seq, descriptors_.data(), N, POLLIN);
+    descriptors_.back() = this->termination_event;
+
+    // Start the listening thread
+    thread = std::thread{[this] {
+      auto& snd = alsa_data::snd;
+      for (;;)
+      {
+        int err = poll(descriptors_.data(), descriptors_.size(), -1);
+        if (err >= 0)
+        {
+          // We got our stop-thread signal
+          if (descriptors_.back().revents & POLLIN)
+            break;
+
+          snd_seq_event_t* ev{};
+          event_handle handle{snd};
+          while (snd.seq.event_input(this->seq, &ev) >= 0)
+          {
+            handle.reset(ev);
+            this->handle_event(*ev);
+          }
+        }
+      }
+    }};
+  }
+
+  ~observer_threaded()
+  {
+    termination_event.notify();
+
+    if (thread.joinable())
+      thread.join();
+  }
+
+  eventfd_notifier termination_event{};
+  std::thread thread;
+  std::vector<pollfd> descriptors_;
+};
+
+template <typename ConfigurationImpl>
+class observer_manual : public observer_impl<ConfigurationImpl>
+{
+public:
+  observer_manual(libremidi::observer_configuration&& conf, ConfigurationImpl&& apiconf)
+      : observer_impl<ConfigurationImpl>{std::move(conf), std::move(apiconf)}
+  {
+    this->configuration.manual_poll(
+        poll_parameters{.addr = this->vaddr, .callback = [this](const auto& v) {
+                          this->handle_event(v);
+                          return 0;
+                        }});
+  }
+
+  ~observer_manual() { this->configuration.stop_poll(this->vaddr); }
+};
+}
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<observer_api>
+make<alsa_seq::observer_impl<alsa_seq::observer_configuration>>(
+    libremidi::observer_configuration&& conf, libremidi::alsa_seq::observer_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_seq::observer_manual<alsa_seq::observer_configuration>>(
+        std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_seq::observer_threaded<alsa_seq::observer_configuration>>(
+        std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/alsa_seq/shared_handler.hpp

@@ -0,0 +1,197 @@
+#pragma once
+#if __has_include(<boost/lockfree/spsc_queue.hpp>)
+  #include <libremidi/backends/alsa_seq/config.hpp>
+  #include <libremidi/backends/alsa_seq/helpers.hpp>
+  #include <libremidi/backends/linux/helpers.hpp>
+  #include <libremidi/shared_context.hpp>
+
+  #include <boost/lockfree/spsc_queue.hpp>
+
+  #include <variant>
+
+namespace libremidi::alsa_seq
+{
+
+struct shared_handler : public libremidi::shared_context
+{
+  const libasound& snd = libasound::instance();
+  struct equals_addr
+  {
+    constexpr bool operator()(const snd_seq_addr_t& lhs, const snd_seq_addr_t& rhs) noexcept
+    {
+      return lhs.client == rhs.client && lhs.port == rhs.port;
+    }
+  };
+
+  explicit shared_handler(std::string_view v)
+  {
+    if (int err = snd.seq.open(&client, "default", SND_SEQ_OPEN_DUPLEX, SND_SEQ_NONBLOCK); err < 0)
+    {
+      client = nullptr;
+      // fixme throw?
+      return;
+    }
+
+    if (!v.empty())
+      snd.seq.set_client_name(client, v.data());
+
+    // Last descriptor is the eventfd one
+    int fds_size = snd.seq.poll_descriptors_count(client, POLLIN);
+    fds.reserve(fds_size + 2);
+    fds.resize(fds_size);
+    snd.seq.poll_descriptors(client, fds.data(), fds_size, POLLIN);
+    fds.push_back(termination_event);
+    fds.push_back(queue_event);
+  }
+
+  void start_processing() override
+  {
+    thread = std::thread{[this] { process(); }};
+  }
+
+  void stop_processing() override
+  {
+    termination_event.notify();
+    if (thread.joinable())
+      thread.join();
+    termination_event.consume();
+  }
+
+  static shared_configurations make(std::string_view client_name)
+  {
+    auto clt = std::make_shared<shared_handler>(client_name);
+
+    auto cb = [client = std::weak_ptr{clt}](const libremidi::alsa_seq::poll_parameters& params) {
+      if (auto clt = client.lock())
+      {
+        clt->events.push(
+            {.type = shared_handler::event_type::callback_added, .payload = std::move(params)});
+        clt->queue_event.notify();
+      }
+      return true;
+    };
+
+    auto stop_cb = [client = std::weak_ptr{clt}](snd_seq_addr_t id) {
+      if (auto clt = client.lock())
+      {
+        clt->events.push(
+            {.type = shared_handler::event_type::callback_removed, .payload = std::move(id)});
+        clt->queue_event.notify();
+      }
+      return true;
+    };
+    return {
+        .context = clt,
+        .observer = alsa_seq::
+            observer_configuration{.context = clt->client, .manual_poll = cb, .stop_poll = stop_cb},
+        .in = alsa_seq::
+            input_configuration{.context = clt->client, .manual_poll = cb, .stop_poll = stop_cb},
+        .out = alsa_seq::output_configuration{.context = clt->client},
+    };
+  }
+
+  int64_t index_of_address(snd_seq_addr_t addr)
+  {
+    auto it = std::find_if(addresses.begin(), addresses.end(), [=](snd_seq_addr_t other) {
+      return equals_addr{}(addr, other);
+    });
+    if (it != addresses.end())
+    {
+      return std::distance(addresses.begin(), it);
+    }
+    else
+    {
+      return -1;
+    }
+  }
+
+  void process()
+  {
+    for (;;)
+    {
+      int err = poll(fds.data(), fds.size(), -1);
+      if (err < 0)
+        return;
+      // Check for termination signal
+      if (termination_event.ready(fds[fds.size() - 2]))
+        return;
+
+      // Check for queue processing signal
+      if (queue_event.ready(fds[fds.size() - 1]))
+      {
+        this->queue_event.consume();
+
+        event ev;
+        while (this->events.pop(ev))
+        {
+          switch (ev.type)
+          {
+            case callback_added: {
+              auto [addr, cb]
+                  = std::move(*std::get_if<libremidi::alsa_seq::poll_parameters>(&ev.payload));
+              addresses.push_back(addr);
+              callbacks.push_back(std::move(cb));
+              break;
+            }
+            case callback_removed:
+              auto addr = std::move(*std::get_if<snd_seq_addr_t>(&ev.payload));
+              if (auto index = index_of_address(addr); index >= 0)
+              {
+                addresses.erase(addresses.begin() + index);
+                callbacks.erase(callbacks.begin() + index);
+              }
+              break;
+          }
+        }
+      }
+
+      // Look for who's ready
+      for (int64_t i = 0, N = std::ssize(fds) - 2; i < N; i++)
+      {
+        if (fds[i].revents & POLLIN)
+        {
+          // Read alsa event
+          snd_seq_event_t* ev{};
+          event_handle handle{snd};
+          int result = 0;
+          while ((result = snd.seq.event_input(client, &ev)) > 0)
+          {
+            handle.reset(ev);
+
+            if (auto index = index_of_address(ev->dest); index >= 0)
+            {
+              // Dispatch the event to the correct observer or midi_in object
+              int err = callbacks[index](*ev);
+              if (err < 0 && err != -EAGAIN)
+                return;
+            }
+          }
+        }
+      }
+    }
+  }
+
+  ~shared_handler() { snd.seq.close(client); }
+
+  enum event_type
+  {
+    callback_added,
+    callback_removed,
+  };
+  struct event
+  {
+    event_type type;
+    std::variant<libremidi::alsa_seq::poll_parameters, snd_seq_addr_t> payload;
+  };
+
+  snd_seq_t* client{};
+
+  boost::lockfree::spsc_queue<event> events{16};
+  std::vector<snd_seq_addr_t> addresses;
+  std::vector<std::function<int(const snd_seq_event_t&)>> callbacks;
+  std::vector<pollfd> fds;
+  eventfd_notifier termination_event, queue_event{false};
+  std::thread thread;
+};
+}
+#endif

include/libremidi/backends/alsa_seq_ump.hpp

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <libremidi/backends/alsa_seq/midi_in.hpp>
+#include <libremidi/backends/alsa_seq/observer.hpp>
+#include <libremidi/backends/alsa_seq_ump/config.hpp>
+#include <libremidi/backends/alsa_seq_ump/midi_out.hpp>
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<observer_api>
+make<alsa_seq::observer_impl<alsa_seq_ump::observer_configuration>>(
+    libremidi::observer_configuration&& conf,
+    libremidi::alsa_seq_ump::observer_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_seq::observer_manual<alsa_seq_ump::observer_configuration>>(
+        std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_seq::observer_threaded<alsa_seq_ump::observer_configuration>>(
+        std::move(conf), std::move(api));
+}
+
+template <>
+inline std::unique_ptr<midi_in_api> make<
+    alsa_seq::midi_in_impl<libremidi::ump_input_configuration, alsa_seq_ump::input_configuration>>(
+    libremidi::ump_input_configuration&& conf, libremidi::alsa_seq_ump::input_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<alsa_seq::midi_in_alsa_manual<
+        libremidi::ump_input_configuration, alsa_seq_ump::input_configuration>>(
+        std::move(conf), std::move(api));
+  else
+    return std::make_unique<alsa_seq::midi_in_alsa_threaded<
+        libremidi::ump_input_configuration, alsa_seq_ump::input_configuration>>(
+        std::move(conf), std::move(api));
+}
+
+}
+
+namespace libremidi::alsa_seq_ump
+{
+struct backend
+{
+  using midi_in = alsa_seq::midi_in_impl<
+      libremidi::ump_input_configuration, alsa_seq_ump::input_configuration>;
+  using midi_out = alsa_seq_ump::midi_out_impl;
+  using midi_observer = alsa_seq::observer_impl<alsa_seq_ump::observer_configuration>;
+  using midi_in_configuration = alsa_seq_ump::input_configuration;
+  using midi_out_configuration = alsa_seq_ump::output_configuration;
+  using midi_observer_configuration = alsa_seq_ump::observer_configuration;
+  static const constexpr auto API = libremidi::API::ALSA_SEQ_UMP;
+  static const constexpr auto name = "alsa_seq_ump";
+  static const constexpr auto display_name = "ALSA (sequencer, UMP)";
+
+  static inline bool available() noexcept
+  {
+    static const libasound& snd = libasound::instance();
+    return snd.available && snd.seq.available && snd.seq.ump.available && snd.ump.available;
+  }
+};
+}

include/libremidi/backends/alsa_seq_ump/config.hpp

@@ -0,0 +1,53 @@
+#pragma once
+#include <libremidi/backends/alsa_seq/config.hpp>
+
+#if __has_include(<alsa/asoundlib.h>)
+  #include <alsa/asoundlib.h>
+#endif
+
+// Necessary for the versions of ALSA that did have UMP
+// through snd_seq_event, before snd_seq_ump_event existed.
+extern "C" {
+typedef struct snd_seq_ump_event snd_seq_ump_event_t;
+}
+
+namespace libremidi::alsa_seq_ump
+{
+
+struct poll_parameters
+{
+  snd_seq_addr_t addr{};
+  std::function<int(const snd_seq_ump_event_t&)> callback;
+};
+
+struct input_configuration
+{
+  using poll_parameters_type = poll_parameters;
+
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+  std::function<bool(const poll_parameters&)> manual_poll;
+  std::function<bool(snd_seq_addr_t)> stop_poll;
+
+  static constexpr int midi_version = 2;
+};
+
+struct output_configuration
+{
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+
+  static constexpr int midi_version = 2;
+};
+
+struct observer_configuration
+{
+  std::string client_name = "libremidi client";
+  snd_seq_t* context{};
+  std::function<bool(const libremidi::alsa_seq::poll_parameters&)> manual_poll;
+  std::function<bool(snd_seq_addr_t)> stop_poll;
+
+  static constexpr int midi_version = 2;
+};
+
+}

include/libremidi/backends/alsa_seq_ump/midi_out.hpp

@@ -0,0 +1,151 @@
+#pragma once
+#include <libremidi/backends/alsa_seq/helpers.hpp>
+#include <libremidi/backends/alsa_seq_ump/config.hpp>
+#include <libremidi/backends/alsa_seq_ump/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+#include <libremidi/detail/ump_stream.hpp>
+
+namespace libremidi::alsa_seq_ump
+{
+
+class midi_out_impl final
+    : public midi2::out_api
+    , private alsa_seq::alsa_data
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::output_configuration
+      , alsa_seq_ump::output_configuration
+  {
+  } configuration;
+
+  midi_out_impl(
+      libremidi::output_configuration&& conf, alsa_seq_ump::output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    assert(snd.seq.ump.available);
+    if (init_client(configuration) < 0)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_alsa::initialize: error creating ALSA sequencer client "
+          "object.");
+      return;
+    }
+
+    if (snd.midi.event_new(this->bufferSize, &this->coder) < 0)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_out_alsa::initialize: error initializing MIDI event "
+          "parser.");
+      return;
+    }
+    snd.midi.event_init(this->coder);
+  }
+
+  ~midi_out_impl() override
+  {
+    // Close a connection if it exists.
+    midi_out_impl::close_port();
+
+    // Cleanup.
+    if (this->vport >= 0)
+      snd.seq.delete_port(this->seq, this->vport);
+    if (this->coder)
+      snd.midi.event_free(this->coder);
+
+    if (!configuration.context)
+      snd.seq.close(this->seq);
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::ALSA_SEQ; }
+
+  [[nodiscard]] int create_port(std::string_view portName)
+  {
+    return alsa_data::create_port(
+        *this, portName,
+        SND_SEQ_PORT_CAP_READ | SND_SEQ_PORT_CAP_SUBS_READ | SND_SEQ_PORT_CAP_UMP_ENDPOINT,
+        SND_SEQ_PORT_TYPE_MIDI_GENERIC | SND_SEQ_PORT_TYPE_APPLICATION, std::nullopt);
+  }
+
+  bool open_port(const output_port& p, std::string_view portName) override
+  {
+    unsigned int nSrc = this->get_port_count(SND_SEQ_PORT_CAP_WRITE | SND_SEQ_PORT_CAP_SUBS_WRITE);
+    if (nSrc < 1)
+    {
+      error<no_devices_found_error>(
+          this->configuration, "midi_out_alsa::open_port: no MIDI output sources found!");
+      return false;
+    }
+
+    auto sink = get_port_info(p);
+    if (!sink)
+      return false;
+
+    if (int err = create_port(portName); err < 0)
+    {
+      error<driver_error>(configuration, "midi_out_alsa::create_port: ALSA error creating port.");
+      return false;
+    }
+
+    snd_seq_addr_t source{
+        .client = (unsigned char)snd.seq.client_id(this->seq), .port = (unsigned char)this->vport};
+    if (int err = create_connection(*this, source, *sink, true); err < 0)
+    {
+      error<driver_error>(
+          configuration, "midi_out_alsa::create_port: ALSA error making port connection.");
+      return false;
+    }
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    if (int err = create_port(portName); err < 0)
+      return false;
+    return true;
+  }
+
+  void close_port() override { unsubscribe(); }
+
+  void set_client_name(std::string_view clientName) override
+  {
+    alsa_data::set_client_name(clientName);
+  }
+
+  void set_port_name(std::string_view portName) override { alsa_data::set_port_name(portName); }
+
+  void send_ump(const uint32_t* ump_stream, std::size_t count) override
+  {
+    snd_seq_ump_event_t ev;
+
+    memset(&ev, 0, sizeof(snd_seq_ump_event_t));
+    snd_seq_ev_set_ump(&ev);
+    snd_seq_ev_set_source(&ev, this->vport);
+    snd_seq_ev_set_subs(&ev);
+    snd_seq_ev_set_direct(&ev);
+
+    auto write_func = [this, &ev](const uint32_t* ump, int64_t bytes) {
+      std::memcpy(ev.ump, ump, bytes);
+      const int result = snd.seq.ump.event_output_direct(this->seq, &ev);
+      if (result < 0)
+      {
+        warning(
+            this->configuration,
+            "midi_out_alsa::send_message: error sending MIDI message to port.");
+        return libremidi::segmentation_error::other;
+      }
+      return libremidi::segmentation_error::no_error;
+    };
+    segment_ump_stream(ump_stream, count, write_func, []() {});
+
+    snd.seq.drain_output(this->seq);
+  }
+
+private:
+  unsigned int bufferSize{32};
+};
+}

include/libremidi/backends/coremidi.hpp

@@ -0,0 +1,22 @@
+#pragma once
+#include <libremidi/backends/coremidi/midi_in.hpp>
+#include <libremidi/backends/coremidi/midi_out.hpp>
+#include <libremidi/backends/coremidi/observer.hpp>
+
+namespace libremidi
+{
+struct core_backend
+{
+  using midi_in = midi_in_core;
+  using midi_out = midi_out_core;
+  using midi_observer = observer_core;
+  using midi_in_configuration = coremidi_input_configuration;
+  using midi_out_configuration = coremidi_output_configuration;
+  using midi_observer_configuration = coremidi_observer_configuration;
+  static const constexpr auto API = libremidi::API::COREMIDI;
+  static const constexpr auto name = "core";
+  static const constexpr auto display_name = "CoreMIDI";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+}

include/libremidi/backends/coremidi/config.hpp

@@ -0,0 +1,41 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <cstdint>
+#include <functional>
+#include <optional>
+#include <string>
+
+#if defined(__APPLE__)
+  #if __LP64__
+typedef unsigned int UInt32;
+  #else
+typedef unsigned long UInt32;
+  #endif
+typedef UInt32 MIDIObjectRef;
+typedef MIDIObjectRef MIDIClientRef;
+#else
+using MIDIClientRef = uint32_t;
+#endif
+namespace libremidi
+{
+
+struct coremidi_input_configuration
+{
+  std::string client_name = "libremidi client";
+  std::optional<MIDIClientRef> context{};
+};
+
+struct coremidi_output_configuration
+{
+  std::string client_name = "libremidi client";
+  std::optional<MIDIClientRef> context{};
+};
+
+struct coremidi_observer_configuration
+{
+  std::string client_name = "libremidi client";
+  std::function<void(MIDIClientRef)> on_create_context{};
+};
+
+}

include/libremidi/backends/coremidi/helpers.hpp

@@ -0,0 +1,324 @@
+#pragma once
+#include <libremidi/detail/memory.hpp>
+#include <libremidi/detail/midi_api.hpp>
+#include <libremidi/input_configuration.hpp>
+
+#include <CoreMIDI/CoreMIDI.h>
+#include <CoreServices/CoreServices.h>
+
+#include <cmath>
+
+#include <bit>
+
+#if TARGET_OS_IPHONE
+  #include <CoreAudio/CoreAudioTypes.h>
+  #include <mach/mach_time.h>
+  #define LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME mach_absolute_time
+#else
+  #include <CoreAudio/HostTime.h>
+  #define LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME AudioGetCurrentHostTime
+#endif
+
+namespace libremidi
+{
+using CFString_handle = unique_handle<const __CFString, CFRelease>;
+using CFStringMutable_handle = unique_handle<__CFString, CFRelease>;
+namespace
+{
+static inline std::string get_string_property(MIDIObjectRef object, CFStringRef property) noexcept
+{
+  CFStringRef res;
+  MIDIObjectGetStringProperty(object, property, &res);
+
+  char name[256];
+  CFStringGetCString(res, name, sizeof(name), kCFStringEncodingUTF8);
+  CFRelease(res);
+  return name;
+}
+
+static inline int32_t get_int_property(MIDIObjectRef object, CFStringRef property) noexcept
+{
+  SInt32 res;
+  MIDIObjectGetIntegerProperty(object, property, &res);
+  return res;
+}
+
+static inline CFString_handle toCFString(std::string_view str) noexcept
+{
+  return CFString_handle{CFStringCreateWithCString(nullptr, str.data(), kCFStringEncodingASCII)};
+}
+
+#if TARGET_OS_IPHONE
+inline uint64_t AudioConvertHostTimeToNanos(uint64_t hostTime)
+{
+  static const struct mach_timebase_info timebase = [] {
+    struct mach_timebase_info theTimeBaseInfo;
+    mach_timebase_info(&theTimeBaseInfo);
+    return theTimeBaseInfo;
+  }();
+  const auto numer = timebase.numer;
+  const auto denom = timebase.denom;
+
+  __uint128_t res = hostTime;
+  if (numer != denom)
+  {
+    res *= numer;
+    res /= denom;
+  }
+  return static_cast<uint64_t>(res);
+}
+#endif
+// This function was submitted by Douglas Casey Tucker and apparently
+// derived largely from PortMidi.
+inline CFStringRef EndpointName(MIDIEndpointRef endpoint, bool isExternal)
+{
+  CFMutableStringRef result = CFStringCreateMutable(nullptr, 0);
+
+  static constexpr auto getProp = [](MIDIObjectRef prop) {
+    CFStringRef str = nullptr;
+    MIDIObjectGetStringProperty(prop, kMIDIPropertyName, &str);
+    return CFString_handle{str};
+  };
+
+  // Begin with the endpoint's name.
+  if (auto endpoint_name = getProp(endpoint))
+  {
+    CFStringAppend(result, endpoint_name.get());
+  }
+
+  // some MIDI devices have a leading space in endpoint name. trim
+  CFStringTrim(result, CFSTR(" "));
+
+  MIDIEntityRef entity = 0;
+  MIDIDeviceRef device = 0;
+  MIDIEndpointGetEntity(endpoint, &entity);
+  if (entity == 0)
+    goto finish;
+
+  if (CFStringGetLength(result) == 0)
+  {
+    // endpoint name has zero length -- try the entity
+    if (auto entity_name = getProp(entity))
+    {
+      CFStringAppend(result, entity_name.get());
+    }
+  }
+
+  // now consider the device's name
+  MIDIEntityGetDevice(entity, &device);
+  if (device == 0)
+    goto finish;
+
+  if (auto dev_name = getProp(device))
+  {
+    const auto dev_strlen = CFStringGetLength(dev_name.get());
+
+    // if an external device has only one entity, throw away
+    // the endpoint name and just use the device name
+    if (CFStringGetLength(result) == 0
+        || (isExternal && MIDIDeviceGetNumberOfEntities(device) < 2))
+    {
+      CFStringAppend(result, dev_name.get());
+      goto finish;
+    }
+
+    // does the entity name already start with the device name?
+    // (some drivers do this though they shouldn't)
+    // if so, do not prepend
+    if (CFStringCompareWithOptions(result, dev_name.get(), CFRangeMake(0, dev_strlen), 0)
+        != kCFCompareEqualTo)
+    {
+      // prepend the device name to the entity name
+      if (CFStringGetLength(result) > 0)
+        CFStringInsert(result, 0, CFSTR(" "));
+      CFStringInsert(result, 0, dev_name.get());
+    }
+  }
+
+finish:
+  if (CFStringGetLength(result) == 0)
+    return CFSTR("No name");
+  else
+    return result;
+}
+
+// This function was submitted by Douglas Casey Tucker and apparently
+// derived largely from PortMidi.
+inline CFStringRef ConnectedEndpointName(MIDIEndpointRef endpoint)
+{
+  CFMutableStringRef result = CFStringCreateMutable(nullptr, 0);
+  CFStringRef str{};
+
+  // Does the endpoint have connections?
+  CFDataRef connections = nullptr;
+  std::size_t nConnected = 0;
+  bool anyStrings = false;
+  MIDIObjectGetDataProperty(endpoint, kMIDIPropertyConnectionUniqueID, &connections);
+  if (connections != nullptr)
+  {
+    // It has connections, follow them
+    // Concatenate the names of all connected devices
+    nConnected = CFDataGetLength(connections) / sizeof(MIDIUniqueID);
+    if (nConnected)
+    {
+      const SInt32* pid = (const SInt32*)(CFDataGetBytePtr(connections));
+      for (std::size_t i = 0; i < nConnected; ++i, ++pid)
+      {
+        MIDIUniqueID id = CFSwapInt32BigToHost(*pid);
+        MIDIObjectRef connObject;
+        MIDIObjectType connObjectType;
+        auto err = MIDIObjectFindByUniqueID(id, &connObject, &connObjectType);
+        if (err == noErr)
+        {
+          if (connObjectType == kMIDIObjectType_ExternalSource
+              || connObjectType == kMIDIObjectType_ExternalDestination)
+          {
+            // Connected to an external device's endpoint (10.3 and later).
+            str = EndpointName((MIDIEndpointRef)(connObject), true);
+          }
+          else
+          {
+            // Connected to an external device (10.2) (or something else,
+            // catch-
+            str = nullptr;
+            MIDIObjectGetStringProperty(connObject, kMIDIPropertyName, &str);
+          }
+          if (str != nullptr)
+          {
+            if (anyStrings)
+              CFStringAppend(result, CFSTR(", "));
+            else
+              anyStrings = true;
+            CFStringAppend(result, str);
+            CFRelease(str);
+          }
+        }
+      }
+    }
+    CFRelease(connections);
+  }
+  if (anyStrings)
+    return result;
+
+  CFRelease(result);
+
+  // Here, either the endpoint had no connections, or we failed to obtain names
+  return EndpointName(endpoint, false);
+}
+
+inline MIDIObjectRef
+locate_object(auto& self, const port_information& info, MIDIObjectType requested_type)
+{
+  auto uid = std::bit_cast<std::int32_t>((uint32_t)info.port);
+  MIDIObjectRef object{};
+  MIDIObjectType type{};
+  auto ret = MIDIObjectFindByUniqueID(uid, &object, &type);
+  if (ret != noErr)
+  {
+    self.template error<invalid_parameter_error>(
+        self.configuration, "coremidi::locate_object: cannot find port: " + info.port_name);
+    return 0;
+  }
+
+  if (type != requested_type || object == 0)
+  {
+    self.template error<invalid_parameter_error>(
+        self.configuration, "coremidi::locate_object: invalid object: " + info.port_name + " : "
+                                + std::to_string(object));
+    return 0;
+  }
+
+  return object;
+}
+}
+
+// A structure to hold variables related to the CoreMIDI API
+// implementation.
+struct coremidi_data
+{
+  MIDIClientRef client{};
+  MIDIPortRef port{};
+  MIDIEndpointRef endpoint = 0;
+
+  [[nodiscard]] OSStatus init_client(auto& configuration)
+  {
+    if (configuration.context)
+    {
+      client = *configuration.context;
+      return noErr;
+    }
+    else
+    {
+      // Set up our client.
+      return MIDIClientCreate(
+          toCFString(configuration.client_name).get(), nullptr, nullptr, &client);
+    }
+  }
+
+  static uint64_t time_in_nanos(MIDITimeStamp tp) noexcept
+  {
+    if (tp == 0)
+    { // this happens when receiving asynchronous sysex messages
+      return clock_gettime_nsec_np(CLOCK_UPTIME_RAW);
+    }
+    else
+    {
+      return AudioConvertHostTimeToNanos(tp);
+    }
+  }
+
+  static void set_timestamp(auto& self, MIDITimeStamp packet, timestamp& msg) noexcept
+  {
+    // packet.timeStamp is in mach_absolute_time units
+    // We want a timestamp in nanoseconds
+
+    switch (self.configuration.timestamps)
+    {
+      case timestamp_mode::NoTimestamp:
+        msg = 0;
+        return;
+      case timestamp_mode::Relative: {
+        if (self.firstMessage)
+        {
+          self.firstMessage = false;
+          msg = 0;
+          return;
+        }
+        else
+        {
+          if constexpr (requires { self.continueSysex; })
+          {
+            if (self.continueSysex)
+              return;
+          }
+
+          auto time = time_in_nanos(packet);
+          time -= self.last_time;
+          msg = time;
+        }
+        break;
+      }
+      case timestamp_mode::Absolute:
+      case timestamp_mode::SystemMonotonic:
+        if constexpr (requires { self.continueSysex; })
+        {
+          if (self.continueSysex)
+            return;
+        }
+        msg = time_in_nanos(packet);
+        break;
+
+      case timestamp_mode::Custom: {
+        if constexpr (requires { self.continueSysex; })
+        {
+          if (self.continueSysex)
+            return;
+        }
+        msg = self.configuration.get_timestamp(time_in_nanos(packet));
+        break;
+      }
+    }
+  }
+};
+
+}

include/libremidi/backends/coremidi/midi_in.hpp

@@ -0,0 +1,178 @@
+#pragma once
+#include <libremidi/backends/coremidi/config.hpp>
+#include <libremidi/backends/coremidi/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+namespace libremidi
+{
+class midi_in_core final
+    : public midi1::in_api
+    , private coremidi_data
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , coremidi_input_configuration
+  {
+  } configuration;
+
+  midi_in_core(input_configuration&& conf, coremidi_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (auto result = init_client(configuration); result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_core: error creating MIDI client object: " + std::to_string(result));
+      return;
+    }
+  }
+
+  ~midi_in_core() override
+  {
+    // Close a connection if it exists.
+    midi_in_core::close_port();
+
+    if (this->endpoint)
+      MIDIEndpointDispose(this->endpoint);
+
+    close_client();
+  }
+
+  void close_client()
+  {
+    if (!configuration.context)
+      MIDIClientDispose(this->client);
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_core: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_core: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI; }
+
+  bool open_port(const input_port& info, std::string_view portName) override
+  {
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+
+    auto source = locate_object(*this, info, kMIDIObjectType_Source);
+    if (source == 0)
+      return false;
+
+    // Create our local sink
+    MIDIPortRef port;
+    OSStatus result = MIDIInputPortCreate(
+        this->client, toCFString(portName).get(), midiInputCallback, (void*)this, &port);
+
+    if (result != noErr)
+    {
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_in_core::open_port: error creating macOS MIDI input port: "
+                                   + std::to_string(result));
+      return false;
+    }
+
+    // Make the connection.
+    if (result = MIDIPortConnectSource(port, source, nullptr); result != noErr)
+    {
+      MIDIPortDispose(port);
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_in_core::open_port: error connecting macOS MIDI input port.");
+      return false;
+    }
+
+    // Save our api-specific port information.
+    this->port = port;
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    // Create a virtual MIDI input destination.
+    MIDIEndpointRef endpoint;
+    OSStatus result = MIDIDestinationCreate(
+        this->client, toCFString(portName).get(), midiInputCallback, (void*)this, &endpoint);
+
+    if (result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_core::open_virtual_port: error creating virtual macOS MIDI "
+          "destination.");
+      return false;
+    }
+
+    // Save our api-specific connection information.
+    this->endpoint = endpoint;
+    return true;
+  }
+
+  void close_port() override
+  {
+    if (this->endpoint)
+    {
+      MIDIEndpointDispose(this->endpoint);
+      this->endpoint = 0;
+    }
+
+    if (this->port)
+    {
+      MIDIPortDispose(this->port);
+      this->port = 0;
+    }
+  }
+
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return coremidi_data::time_in_nanos(LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME());
+  }
+
+  static void midiInputCallback(const MIDIPacketList* list, void* procRef, void* /*srcRef*/)
+  {
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = false,
+        .has_samples = false,
+    };
+
+    auto& self = *(midi_in_core*)procRef;
+
+    const MIDIPacket* packet = &list->packet[0];
+    for (unsigned int i = 0; i < list->numPackets; ++i)
+    {
+      // My interpretation of the CoreMIDI documentation: all message
+      // types, except sysex, are complete within a packet and there may
+      // be several of them in a single packet.  Sysex messages can be
+      // broken across multiple packets and PacketLists but are bundled
+      // alone within each packet (these packets do not contain other
+      // message types).  If sysex messages are split across multiple
+      // MIDIPacketLists, they must be handled by multiple calls to this
+      // function.
+
+      if (packet->length == 0)
+      {
+        packet = MIDIPacketNext(packet);
+        continue;
+      }
+
+      auto to_ns = [packet] { return time_in_nanos(packet->timeStamp); };
+      self.m_processing.on_bytes_multi(
+          {packet->data, packet->data + packet->length},
+          self.m_processing.timestamp<timestamp_info>(to_ns, 0));
+
+      packet = MIDIPacketNext(packet);
+    }
+  }
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+}

include/libremidi/backends/coremidi/midi_out.hpp

@@ -0,0 +1,203 @@
+#pragma once
+#include <libremidi/backends/coremidi/config.hpp>
+#include <libremidi/backends/coremidi/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi
+{
+class midi_out_core final
+    : public midi1::out_api
+    , private coremidi_data
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , coremidi_output_configuration
+  {
+  } configuration;
+
+  midi_out_core(output_configuration&& conf, coremidi_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (auto result = init_client(configuration); result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_out_core: error creating MIDI client object: " + std::to_string(result));
+      return;
+    }
+  }
+
+  ~midi_out_core()
+  {
+    midi_out_core::close_port();
+
+    if (this->endpoint)
+      MIDIEndpointDispose(this->endpoint);
+
+    close_client();
+  }
+
+  void close_client()
+  {
+    if (!configuration.context)
+      MIDIClientDispose(this->client);
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_core: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_core: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI; }
+
+  bool open_port(const output_port& info, std::string_view portName) override
+  {
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+
+    // Find where we want to send
+    auto destination = locate_object(*this, info, kMIDIObjectType_Destination);
+    if (destination == 0)
+      return false;
+
+    // Create our local source
+    MIDIPortRef port;
+    OSStatus result = MIDIOutputPortCreate(this->client, toCFString(portName).get(), &port);
+    if (result != noErr)
+    {
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_out_core::open_port: error creating macOS MIDI output port.");
+      return false;
+    }
+
+    // Save our api-specific connection information.
+    this->port = port;
+    this->destinationId = destination;
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    if (this->endpoint)
+    {
+      warning(
+          configuration,
+          "midi_out_core::open_virtual_port: a virtual output port already "
+          "exists!");
+      return false;
+    }
+
+    // Create a virtual MIDI output source.
+    MIDIEndpointRef endpoint;
+    OSStatus result = MIDISourceCreate(this->client, toCFString(portName).get(), &endpoint);
+
+    if (result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_out_core::initialize: error creating macOS virtual MIDI source.");
+      return false;
+    }
+
+    // Save our api-specific connection information.
+    this->endpoint = endpoint;
+    return true;
+  }
+
+  void close_port() override
+  {
+    if (this->endpoint)
+    {
+      MIDIEndpointDispose(this->endpoint);
+      this->endpoint = 0;
+    }
+
+    if (this->port)
+    {
+      MIDIPortDispose(this->port);
+      this->port = 0;
+    }
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    unsigned int nBytes = static_cast<unsigned int>(size);
+    if (nBytes == 0)
+    {
+      warning(configuration, "midi_out_core::send_message: no data in message argument!");
+      return;
+    }
+
+    if (message[0] != 0xF0 && nBytes > 3)
+    {
+      warning(
+          configuration,
+          "midi_out_core::send_message: message format problem ... not sysex but "
+          "> 3 bytes?");
+      return;
+    }
+
+    const MIDITimeStamp timestamp = LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME();
+
+    const ByteCount bufsize = nBytes > 65535 ? 65535 : nBytes;
+    Byte buffer[bufsize + 16]; // pad for other struct members
+    ByteCount listSize = sizeof(buffer);
+    MIDIPacketList* packetList = (MIDIPacketList*)buffer;
+
+    ByteCount remainingBytes = nBytes;
+    while (remainingBytes)
+    {
+      MIDIPacket* packet = MIDIPacketListInit(packetList);
+      // A MIDIPacketList can only contain a maximum of 64K of data, so if our message is longer,
+      // break it up into chunks of 64K or less and send out as a MIDIPacketList with only one
+      // MIDIPacket. Here, we reuse the memory allocated above on the stack for all.
+      ByteCount bytesForPacket = remainingBytes > 65535 ? 65535 : remainingBytes;
+      const Byte* dataStartPtr = (const Byte*)&message[nBytes - remainingBytes];
+      packet = MIDIPacketListAdd(
+          packetList, listSize, packet, timestamp, bytesForPacket, dataStartPtr);
+      remainingBytes -= bytesForPacket;
+
+      if (!packet)
+      {
+        error<driver_error>(
+            this->configuration, "midi_out_core::send_message: could not allocate packet list");
+        return;
+      }
+
+      // Send to any destinations that may have connected to us.
+      if (this->endpoint)
+      {
+        auto result = MIDIReceived(this->endpoint, packetList);
+        if (result != noErr)
+        {
+          warning(
+              this->configuration,
+              "midi_out_core::send_message: error sending MIDI to virtual "
+              "destinations.");
+        }
+      }
+
+      // And send to an explicit destination port if we're connected.
+      if (this->destinationId != 0)
+      {
+        auto result = MIDISend(this->port, this->destinationId, packetList);
+        if (result != noErr)
+        {
+          warning(
+              this->configuration,
+              "midi_out_core::send_message: error sending MIDI message to port.");
+        }
+      }
+    }
+  }
+
+  MIDIEndpointRef destinationId{};
+};
+}

include/libremidi/backends/coremidi/observer.hpp

@@ -0,0 +1,175 @@
+#pragma once
+#include <libremidi/backends/coremidi/config.hpp>
+#include <libremidi/backends/coremidi/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <CoreMIDI/CoreMIDI.h>
+
+#include <bit>
+
+namespace libremidi
+{
+class observer_core
+    : public observer_api
+    , public error_handler
+{
+public:
+  struct
+      : observer_configuration
+      , coremidi_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_core(observer_configuration&& conf, coremidi_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (configuration.client_name.empty())
+      configuration.client_name = "libremidi observer";
+
+    if (!configuration.has_callbacks())
+      return;
+
+    auto result = MIDIClientCreate(
+        toCFString(configuration.client_name).get(),
+        +[](const MIDINotification* message, void* ctx) {
+          ((observer_core*)ctx)->notify(message);
+        },
+        this, &client);
+
+    if (result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_core: error creating MIDI client object: " + std::to_string(result));
+      return;
+    }
+
+    if (configuration.on_create_context)
+      configuration.on_create_context(client);
+
+    if (configuration.notify_in_constructor)
+    {
+      if (this->configuration.input_added)
+        for (auto& p : get_input_ports())
+          this->configuration.input_added(p);
+
+      if (this->configuration.output_added)
+        for (auto& p : get_output_ports())
+          this->configuration.output_added(p);
+    }
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI; }
+
+  template <bool Input>
+  auto to_port_info(MIDIObjectRef obj) const noexcept
+      -> std::optional<std::conditional_t<Input, input_port, output_port>>
+  {
+    MIDIEntityRef e{};
+    MIDIEndpointGetEntity(obj, &e);
+    bool physical = bool(e);
+
+    bool ok = false;
+    if (physical && this->configuration.track_hardware)
+      ok = true;
+    else if ((!physical) && this->configuration.track_virtual)
+      ok = true;
+
+    if (!ok)
+      return {};
+
+    return std::conditional_t<Input, input_port, output_port>{
+        {.client = (std::uintptr_t)this->client,
+         .port = std::bit_cast<uint32_t>(get_int_property(obj, kMIDIPropertyUniqueID)),
+         .manufacturer = get_string_property(obj, kMIDIPropertyManufacturer),
+         .device_name = get_string_property(obj, kMIDIPropertyModel),
+         .port_name = get_string_property(obj, kMIDIPropertyName),
+         .display_name = get_string_property(obj, kMIDIPropertyDisplayName)}};
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    std::vector<libremidi::input_port> ret;
+
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+    for (ItemCount i = 0; i < MIDIGetNumberOfSources(); i++)
+    {
+      if (auto p = to_port_info<true>(MIDIGetSource(i)))
+        ret.push_back(std::move(*p));
+    }
+
+    return ret;
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    std::vector<libremidi::output_port> ret;
+
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+    for (ItemCount i = 0; i < MIDIGetNumberOfDestinations(); i++)
+    {
+      if (auto p = to_port_info<false>(MIDIGetDestination(i)))
+        ret.push_back(std::move(*p));
+    }
+    return ret;
+  }
+
+  void notify(const MIDINotification* message)
+  {
+    switch (message->messageID)
+    {
+      case kMIDIMsgObjectAdded: {
+        auto obj = reinterpret_cast<const MIDIObjectAddRemoveNotification*>(message);
+
+        switch (obj->childType)
+        {
+          case kMIDIObjectType_Source:
+            if (auto& cb = configuration.input_added)
+              if (auto p = to_port_info<true>(obj->child))
+                cb(std::move(*p));
+            break;
+          case kMIDIObjectType_Destination:
+            if (auto& cb = configuration.output_added)
+              if (auto p = to_port_info<false>(obj->child))
+                cb(std::move(*p));
+            break;
+          default:
+            break;
+        }
+
+        break;
+      }
+
+      case kMIDIMsgObjectRemoved: {
+        auto obj = reinterpret_cast<const MIDIObjectAddRemoveNotification*>(message);
+
+        switch (obj->childType)
+        {
+          case kMIDIObjectType_Source:
+            if (auto& cb = configuration.input_removed)
+              if (auto p = to_port_info<true>(obj->child))
+                cb(std::move(*p));
+            break;
+          case kMIDIObjectType_Destination:
+            if (auto& cb = configuration.output_removed)
+              if (auto p = to_port_info<false>(obj->child))
+                cb(std::move(*p));
+            break;
+          default:
+            break;
+        }
+
+        break;
+      }
+
+      default:
+        break;
+    }
+  }
+
+  ~observer_core() { MIDIClientDispose(this->client); }
+
+private:
+  MIDIClientRef client{};
+};
+}

include/libremidi/backends/coremidi_ump.hpp

@@ -0,0 +1,22 @@
+#pragma once
+#include <libremidi/backends/coremidi_ump/midi_in.hpp>
+#include <libremidi/backends/coremidi_ump/midi_out.hpp>
+#include <libremidi/backends/coremidi_ump/observer.hpp>
+
+namespace libremidi::coremidi_ump
+{
+struct backend
+{
+  using midi_in = midi_in_impl;
+  using midi_out = midi_out_impl;
+  using midi_observer = observer_impl;
+  using midi_in_configuration = coremidi_input_configuration;
+  using midi_out_configuration = coremidi_output_configuration;
+  using midi_observer_configuration = coremidi_observer_configuration;
+  static const constexpr auto API = libremidi::API::COREMIDI_UMP;
+  static const constexpr auto name = "core_ump";
+  static const constexpr auto display_name = "CoreMIDI UMP";
+
+  static constexpr inline bool available() noexcept { return true; /* todo? */ }
+};
+}

include/libremidi/backends/coremidi_ump/config.hpp

@@ -0,0 +1,9 @@
+#pragma once
+#include <libremidi/backends/coremidi/config.hpp>
+
+namespace libremidi::coremidi_ump
+{
+using input_configuration = coremidi_input_configuration;
+using output_configuration = coremidi_output_configuration;
+using observer_configuration = coremidi_observer_configuration;
+}

include/libremidi/backends/coremidi_ump/helpers.hpp

@@ -0,0 +1,2 @@
+#pragma once
+#include <libremidi/backends/coremidi/helpers.hpp>

include/libremidi/backends/coremidi_ump/midi_in.hpp

@@ -0,0 +1,183 @@
+#pragma once
+#include <libremidi/backends/coremidi_ump/config.hpp>
+#include <libremidi/backends/coremidi_ump/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+
+namespace libremidi::coremidi_ump
+{
+
+class midi_in_impl final
+    : public midi2::in_api
+    , private coremidi_data
+    , public error_handler
+{
+public:
+  struct
+      : ump_input_configuration
+      , coremidi_ump::input_configuration
+  {
+  } configuration;
+
+  midi_in_impl(ump_input_configuration&& conf, coremidi_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (auto result = init_client(configuration); result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_core: error creating MIDI client object: " + std::to_string(result));
+      return;
+    }
+  }
+
+  ~midi_in_impl() override
+  {
+    // Close a connection if it exists.
+    close_port();
+
+    if (this->endpoint)
+      MIDIEndpointDispose(this->endpoint);
+
+    close_client();
+  }
+
+  void close_client()
+  {
+    if (!configuration.context)
+      MIDIClientDispose(this->client);
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_core: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_core: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI_UMP; }
+
+  bool open_port(const input_port& info, std::string_view portName) override
+  {
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+
+    auto source = locate_object(*this, info, kMIDIObjectType_Source);
+    if (source == 0)
+      return false;
+
+    // Create our local sink
+    MIDIPortRef port;
+
+    OSStatus result = MIDIInputPortCreateWithProtocol(
+        this->client, toCFString(portName).get(), kMIDIProtocol_2_0, &port,
+        ^(const MIDIEventList* evtlist, void* __nullable srcConnRefCon) {
+            this->midiInputCallback(evtlist, srcConnRefCon);
+        });
+
+    if (result != noErr)
+    {
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_in_core::open_port: error creating macOS MIDI input port: "
+                                   + std::to_string(result));
+      return false;
+    }
+
+    // Make the connection.
+    if (result = MIDIPortConnectSource(port, source, nullptr); result != noErr)
+    {
+      MIDIPortDispose(port);
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_in_core::open_port: error connecting macOS MIDI input port.");
+      return false;
+    }
+
+    // Save our api-specific port information.
+    this->port = port;
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    // Create a virtual MIDI input destination.
+    MIDIEndpointRef endpoint;
+    OSStatus result = MIDIDestinationCreateWithProtocol(
+        this->client, toCFString(portName).get(), kMIDIProtocol_2_0, &endpoint,
+        ^(const MIDIEventList* evtlist, void* __nullable srcConnRefCon) {
+            this->midiInputCallback(evtlist, srcConnRefCon);
+        });
+
+    if (result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_in_core::open_virtual_port: error creating virtual macOS MIDI "
+          "destination.");
+      return false;
+    }
+
+    // Save our api-specific connection information.
+    this->endpoint = endpoint;
+    return true;
+  }
+
+  void close_port() override
+  {
+    if (this->endpoint)
+    {
+      MIDIEndpointDispose(this->endpoint);
+      this->endpoint = 0;
+    }
+
+    if (this->port)
+    {
+      MIDIPortDispose(this->port);
+      this->port = 0;
+    }
+  }
+
+  void set_timestamp(
+      [[maybe_unused]] const MIDIEventPacket& packet,
+      [[maybe_unused]] libremidi::ump& msg) noexcept
+  {
+  }
+
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return coremidi_data::time_in_nanos(LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME());
+  }
+
+  void midiInputCallback(const MIDIEventList* list, void* /*srcRef*/)
+  {
+    unsigned short nBytes{};
+
+    const MIDIEventPacket* packet = &list->packet[0];
+    for (unsigned int i = 0; i < list->numPackets; ++i)
+    {
+      nBytes = packet->wordCount;
+      if (nBytes == 0)
+      {
+        packet = MIDIEventPacketNext(packet);
+        continue;
+      }
+
+      libremidi::ump msg;
+      coremidi_data::set_timestamp(*this, packet->timeStamp, msg.timestamp);
+
+      if (packet->wordCount <= 4)
+      {
+        std::copy_n(packet->words, packet->wordCount, msg.data);
+        configuration.on_message(std::move(msg));
+      }
+      last_time = time_in_nanos(packet->timeStamp);
+
+      packet = MIDIEventPacketNext(packet);
+    }
+  }
+
+  unsigned long long last_time{};
+};
+
+}

include/libremidi/backends/coremidi_ump/midi_out.hpp

@@ -0,0 +1,185 @@
+#pragma once
+#include <libremidi/backends/coremidi_ump/config.hpp>
+#include <libremidi/backends/coremidi_ump/helpers.hpp>
+#include <libremidi/cmidi2.hpp>
+#include <libremidi/detail/midi_out.hpp>
+#include <libremidi/detail/ump_stream.hpp>
+
+namespace libremidi::coremidi_ump
+{
+class midi_out_impl final
+    : public midi2::out_api
+    , private coremidi_data
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::output_configuration
+      , coremidi_ump::output_configuration
+  {
+  } configuration;
+
+  midi_out_impl(
+      libremidi::output_configuration&& conf, coremidi_ump::output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (auto result = init_client(configuration); result != noErr)
+    {
+      error<driver_error>(
+          this->configuration,
+          "midi_out_impl: error creating MIDI client object: " + std::to_string(result));
+      return;
+    }
+  }
+
+  ~midi_out_impl()
+  {
+    midi_out_impl::close_port();
+
+    if (this->endpoint)
+      MIDIEndpointDispose(this->endpoint);
+
+    close_client();
+  }
+
+  void close_client()
+  {
+    if (!configuration.context)
+      MIDIClientDispose(this->client);
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_impl: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_impl: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI_UMP; }
+
+  bool open_port(const output_port& info, std::string_view portName) override
+  {
+    CFRunLoopRunInMode(kCFRunLoopDefaultMode, 0, false);
+
+    // Find where we want to send
+    auto destination = locate_object(*this, info, kMIDIObjectType_Destination);
+    if (destination == 0)
+      return false;
+
+    // Create our local source
+    MIDIPortRef port;
+    OSStatus result = MIDIOutputPortCreate(this->client, toCFString(portName).get(), &port);
+    if (result != noErr)
+    {
+      close_client();
+      error<driver_error>(
+          this->configuration, "midi_out_impl::open_port: error creating macOS MIDI output port.");
+      return false;
+    }
+
+    // Save our api-specific connection information.
+    this->port = port;
+    this->destinationId = destination;
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    if (this->endpoint)
+    {
+      warning(
+          configuration,
+          "midi_out_impl::open_virtual_port: a virtual output port already "
+          "exists!");
+      return false;
+    }
+
+    // Create a virtual MIDI output source.
+    OSStatus result = MIDISourceCreateWithProtocol(
+        this->client, toCFString(portName).get(), kMIDIProtocol_2_0, &this->endpoint);
+
+    if (result != noErr)
+    {
+      this->endpoint = 0;
+      error<driver_error>(
+          this->configuration,
+          "midi_out_impl::initialize: error creating macOS virtual MIDI source.");
+      return false;
+    }
+
+    return true;
+  }
+
+  void close_port() override
+  {
+    if (this->endpoint)
+    {
+      MIDIEndpointDispose(this->endpoint);
+      this->endpoint = 0;
+    }
+
+    if (this->port)
+    {
+      MIDIPortDispose(this->port);
+      this->port = 0;
+    }
+  }
+
+  void send_ump(const uint32_t* ump_stream, std::size_t count) override
+  {
+    MIDIEventList* eventList = reinterpret_cast<MIDIEventList*>(m_eventListBuffer);
+    MIDIEventPacket* packet = MIDIEventListInit(eventList, kMIDIProtocol_2_0);
+    const MIDITimeStamp ts = LIBREMIDI_AUDIO_GET_CURRENT_HOST_TIME();
+
+    auto write_fun = [ts, &packet, &eventList](const uint32_t* ump, int bytes) {
+      packet = MIDIEventListAdd(eventList, event_list_max_size, packet, ts, bytes / 4, ump);
+      if (packet)
+        return segmentation_error::no_error;
+      else
+        return segmentation_error::need_space;
+    };
+
+    auto realloc_fun = [this, &packet, &eventList] {
+      push_event_list(eventList);
+      packet = MIDIEventListInit(eventList, kMIDIProtocol_2_0);
+    };
+
+    segment_ump_stream(ump_stream, count, write_fun, realloc_fun);
+    push_event_list(eventList);
+  }
+
+  void push_event_list(MIDIEventList* eventList)
+  {
+    if (this->endpoint)
+    {
+      auto result = MIDIReceivedEventList(this->endpoint, eventList);
+      if (result != noErr)
+      {
+        warning(
+            this->configuration,
+            "midi_out_core::send_message: error sending MIDI to virtual "
+            "destinations.");
+      }
+    }
+
+    if (this->destinationId != 0)
+    {
+      auto result = MIDISendEventList(this->port, this->destinationId, eventList);
+      if (result != noErr)
+      {
+        warning(
+            this->configuration,
+            "midi_out_core::send_message: error sending MIDI message to port.");
+      }
+    }
+  }
+
+  MIDIEndpointRef destinationId{};
+
+  static constexpr int event_list_max_size = 65535;
+  unsigned char m_eventListBuffer[sizeof(MIDIEventList) + event_list_max_size];
+};
+}

include/libremidi/backends/coremidi_ump/observer.hpp

@@ -0,0 +1,13 @@
+#pragma once
+#include <libremidi/backends/coremidi/observer.hpp>
+
+namespace libremidi::coremidi_ump
+{
+
+class observer_impl final : public libremidi::observer_core
+{
+  using observer_core::observer_core;
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::COREMIDI_UMP; }
+};
+
+}

include/libremidi/backends/dummy.hpp

@@ -0,0 +1,80 @@
+#pragma once
+#include <libremidi/configurations.hpp>
+#include <libremidi/detail/midi_api.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_out.hpp>
+#include <libremidi/detail/observer.hpp>
+
+namespace libremidi
+{
+class observer_dummy : public observer_api
+{
+public:
+  explicit observer_dummy(const auto& /*configuration*/, const auto&) { }
+
+  ~observer_dummy() { }
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::DUMMY; }
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override { return {}; }
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override { return {}; }
+};
+
+class midi_in_dummy final
+    : public midi1::in_api
+    , public error_handler
+{
+public:
+  explicit midi_in_dummy(const auto& configuration, const auto&)
+  {
+    warning(configuration, "midi_in_dummy: This class provides no functionality.");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::DUMMY; }
+  bool open_port(const input_port& /*pt*/, std::string_view /*local_port_name*/) override
+  {
+    return true;
+  }
+  bool open_virtual_port(std::string_view /*portName*/) override { return true; }
+  void close_port() override { }
+  void set_client_name(std::string_view /*clientName*/) override { }
+  void set_port_name(std::string_view /*portName*/) override { }
+  timestamp absolute_timestamp() const noexcept override { return 0; }
+};
+
+class midi_out_dummy final
+    : public midi1::out_api
+    , public error_handler
+{
+public:
+  explicit midi_out_dummy(const auto& configuration, const auto&)
+  {
+    warning(configuration, "midi_out_dummy: This class provides no functionality.");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::DUMMY; }
+  bool open_port(const output_port& /*pt*/, std::string_view /*local_port_name*/) override
+  {
+    return true;
+  }
+  bool open_virtual_port(std::string_view /*portName*/) override { return true; }
+
+  void close_port() override { }
+  void set_client_name(std::string_view /*clientName*/) override { }
+  void set_port_name(std::string_view /*portName*/) override { }
+  void send_message(const unsigned char* /*message*/, size_t /*size*/) override { }
+};
+
+struct dummy_backend
+{
+  using midi_in = midi_in_dummy;
+  using midi_out = midi_out_dummy;
+  using midi_observer = observer_dummy;
+  using midi_in_configuration = dummy_configuration;
+  using midi_out_configuration = dummy_configuration;
+  using midi_observer_configuration = dummy_configuration;
+  static const constexpr auto API = libremidi::API::DUMMY;
+  static const constexpr auto name = "dummy";
+  static const constexpr auto display_name = "Dummy";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+}

include/libremidi/backends/emscripten.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#if defined(__EMSCRIPTEN__)
+
+  #include <libremidi/backends/emscripten/config.hpp>
+  #include <libremidi/backends/emscripten/midi_in.hpp>
+  #include <libremidi/backends/emscripten/midi_out.hpp>
+  #include <libremidi/backends/emscripten/observer.hpp>
+
+namespace libremidi
+{
+
+struct emscripten_backend
+{
+  using midi_in = midi_in_emscripten;
+  using midi_out = midi_out_emscripten;
+  using midi_observer = observer_emscripten;
+  using midi_in_configuration = emscripten_input_configuration;
+  using midi_out_configuration = emscripten_output_configuration;
+  using midi_observer_configuration = emscripten_observer_configuration;
+  static const constexpr auto API = libremidi::API::WEBMIDI;
+  static const constexpr auto name = "webmidi";
+  static const constexpr auto display_name = "WebMIDI";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+
+}
+
+#endif

include/libremidi/backends/emscripten/config.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+namespace libremidi
+{
+
+struct emscripten_input_configuration
+{
+};
+struct emscripten_output_configuration
+{
+};
+struct emscripten_observer_configuration
+{
+};
+
+}

include/libremidi/backends/emscripten/helpers.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <string>
+
+namespace libremidi
+{
+namespace webmidi_helpers
+{
+struct device_information
+{
+  std::string id;
+  std::string name;
+  bool connected{};
+};
+}
+}

include/libremidi/backends/emscripten/midi_access.cpp

@@ -0,0 +1,22 @@
+#if defined(__EMSCRIPTEN__)
+  #include <libremidi/backends/emscripten/midi_access.hpp>
+
+extern "C" {
+LIBREMIDI_EXPORT
+LIBREMIDI_INLINE
+EMSCRIPTEN_KEEPALIVE
+void libremidi_devices_poll()
+{
+  libremidi::webmidi_helpers::midi_access_emscripten::instance().devices_poll();
+}
+
+LIBREMIDI_EXPORT
+LIBREMIDI_INLINE
+EMSCRIPTEN_KEEPALIVE
+void libremidi_devices_input(int port, double timestamp, int len, char* bytes)
+{
+  libremidi::webmidi_helpers::midi_access_emscripten::instance().devices_input(
+      port, timestamp, len, bytes);
+}
+}
+#endif

include/libremidi/backends/emscripten/midi_access.hpp

@@ -0,0 +1,303 @@
+#pragma once
+#include <libremidi/backends/emscripten/midi_in.hpp>
+#include <libremidi/backends/emscripten/midi_out.hpp>
+#include <libremidi/backends/emscripten/observer.hpp>
+#include <libremidi/libremidi.hpp>
+
+#include <emscripten.h>
+
+#include <map>
+
+extern "C" {
+EMSCRIPTEN_KEEPALIVE
+void libremidi_devices_poll();
+
+EMSCRIPTEN_KEEPALIVE
+void libremidi_devices_input(int port, double timestamp, int len, char* bytes);
+}
+
+namespace libremidi
+{
+namespace webmidi_helpers
+{
+
+class midi_access_emscripten
+{
+public:
+  static midi_access_emscripten& instance() noexcept
+  {
+    static midi_access_emscripten inst;
+    return inst;
+  }
+
+  bool available() const noexcept
+  {
+    return EM_ASM_INT(return typeof globalThis.__libreMidi_access !== undefined;);
+  }
+
+  int input_count() const noexcept
+  {
+    return EM_ASM_INT(return globalThis.__libreMidi_currentInputs.length;);
+  }
+
+  int output_count() const noexcept
+  {
+    return EM_ASM_INT(return globalThis.__libreMidi_currentOutputs.length;);
+  }
+
+  void load_current_infos() noexcept
+  {
+#define get_js_string(variable_to_read, ...) \
+  (char*)EM_ASM_INT(                            \
+        {                                         \
+              var jsstr = variable_to_read;           \
+              var bytes = lengthBytesUTF8(jsstr) + 1; \
+              var str = _malloc(bytes);               \
+              stringToUTF8(jsstr, str, bytes);        \
+              return str;                             \
+        },                                        \
+        __VA_ARGS__);
+
+    EM_ASM_INT({
+      let inputs = [];
+      let outputs = [];
+      for (let inpt of globalThis.__libreMidi_access.inputs.values())
+      {
+        inputs.push(inpt);
+      }
+      for (let outpt of globalThis.__libreMidi_access.outputs.values())
+      {
+        outputs.push(outpt);
+      }
+      globalThis.__libreMidi_currentInputs = inputs;
+      globalThis.__libreMidi_currentOutputs = outputs;
+    });
+    const int inputs = input_count();
+    const int outputs = output_count();
+
+    m_current_inputs.resize(inputs);
+    m_current_outputs.resize(outputs);
+
+    for (int i = 0; i < inputs; i++)
+    {
+      int device_index = -1;
+      char* midi_id = get_js_string(globalThis.__libreMidi_currentInputs[$0].id, i);
+      auto it = m_input_indices.find(midi_id); // TODO transparent comparator, string_view...
+      if (it == m_input_indices.end())
+      {
+        device_index = m_input_indices.size();
+        m_input_indices[midi_id] = device_index;
+      }
+      else
+      {
+        device_index = it->second;
+      }
+
+      char* midi_name = get_js_string(globalThis.__libreMidi_currentInputs[$0].name, i);
+
+      const bool connected
+          = EM_ASM_INT(return globalThis.__libreMidi_currentInputs[$0].state === "connected", i);
+
+      m_current_inputs[device_index].id = midi_id;
+      m_current_inputs[device_index].name = midi_name;
+      m_current_inputs[device_index].connected = connected;
+
+      free(midi_id);
+      free(midi_name);
+    }
+
+    for (int i = 0; i < outputs; i++)
+    {
+      int device_index = -1;
+      char* midi_id = get_js_string(globalThis.__libreMidi_currentOutputs[$0].id, i);
+
+      auto it = m_output_indices.find(midi_id); // TODO transparent comparator, string_view...
+      if (it == m_output_indices.end())
+      {
+        device_index = m_output_indices.size();
+        m_output_indices[midi_id] = device_index;
+      }
+      else
+      {
+        device_index = it->second;
+      }
+
+      char* midi_name = get_js_string(globalThis.__libreMidi_currentOutputs[$0].name, i);
+
+      const bool connected
+          = EM_ASM_INT(return globalThis.__libreMidi_currentOutputs[$0].state === "connected", i);
+
+      m_current_outputs[device_index].id = midi_id;
+      m_current_outputs[device_index].name = midi_name;
+      m_current_outputs[device_index].connected = connected;
+
+      free(midi_id);
+      free(midi_name);
+    }
+
+#undef get_js_string
+  }
+
+  void register_observer(observer_emscripten& obs)
+  {
+    m_observers.push_back(&obs);
+    if (m_observers.size() == 1)
+    {
+      start_observing();
+    }
+  }
+
+  void unregister_observer(observer_emscripten& obs)
+  {
+    if (m_observers.size() == 1)
+    {
+      stop_observing();
+    }
+    auto it = std::find(m_observers.begin(), m_observers.end(), &obs);
+    if (it != m_observers.end())
+    {
+      m_observers.erase(it);
+    }
+  }
+
+  void devices_poll()
+  {
+    load_current_infos();
+
+    for (auto& obs : m_observers)
+    {
+      obs->update(m_current_inputs, m_current_outputs);
+    }
+  }
+
+  void open_input(int port_index, midi_in_emscripten& input)
+  {
+    auto& vec = m_opened_inputs[port_index];
+    vec.push_back(&input);
+    if (vec.size() != 1)
+      return;
+
+    start_stream(port_index);
+  }
+
+  void close_input(int port_index, midi_in_emscripten& input)
+  {
+    auto& vec = m_opened_inputs[port_index];
+    auto it = std::find(vec.begin(), vec.end(), &input);
+    if (it != vec.end())
+    {
+      vec.erase(it);
+    }
+
+    if (vec.empty())
+    {
+      stop_stream(port_index);
+    }
+  }
+
+  void devices_input(int port, double timestamp, int len, char* data)
+  {
+    unsigned char* bytes = reinterpret_cast<unsigned char*>(data);
+    for (auto input : m_opened_inputs[port])
+    {
+      input->on_input(timestamp, bytes, bytes + len);
+    }
+  }
+
+  void send_message(int port_index, const char* bytes, int len)
+  {
+    const auto& id = m_current_outputs[port_index].id;
+    EM_ASM(
+        {
+          let data = HEAPU8.subarray($0, $0 + $1);
+          const id = UTF8ToString($2);
+          let output = globalThis.__libreMidi_access.outputs.get(id);
+          let bytes = HEAPU8.subarray($0, $0 + $1);
+          output.send(Array.from(bytes));
+        },
+        bytes, len, id.c_str());
+  }
+
+  const std::vector<device_information>& inputs() const noexcept { return m_current_inputs; }
+  const std::vector<device_information>& outputs() const noexcept { return m_current_outputs; }
+
+private:
+  midi_access_emscripten() noexcept
+  {
+    EM_ASM(
+        if (navigator.requestMIDIAccess) {
+              navigator.requestMIDIAccess().then(
+                  (midiAccess) => { globalThis.__libreMidi_access = midiAccess; },
+                  () => console.log('MIDI support rejected, MIDI will not be available;'));
+        } else { console.log('WebMIDI is not supported in this browser.'); });
+  }
+
+  ~midi_access_emscripten() { stop_observing(); }
+
+  void start_observing()
+  {
+    EM_ASM(
+      let id = setInterval(Module._libremidi_devices_poll, 100);
+      globalThis.__libreMidi_timer = id;);
+  }
+
+  void stop_observing()
+  {
+    EM_ASM(clearInterval(globalThis.__libreMidi_timer); globalThis.__libreMidi_timer = undefined;);
+  }
+
+  void start_stream(int port_index)
+  {
+    // Isn't life great...
+    // https://github.com/Planeshifter/emscripten-examples/tree/master/01_PassingArrays
+    const auto& id = m_current_inputs[port_index].id;
+    EM_ASM(
+      const port_index = $0;
+      const id = UTF8ToString($1);
+
+      let input = globalThis.__libreMidi_access.inputs.get(id);
+
+      function _arrayToHeap(typedArray){
+                      const numBytes = typedArray.length * typedArray.BYTES_PER_ELEMENT;
+                      const ptr = Module._malloc(numBytes);
+                      const heapBytes = new Uint8Array(Module.HEAPU8.buffer, ptr, numBytes);
+                      heapBytes.set(new Uint8Array(typedArray.buffer));
+                      return heapBytes;
+      }
+
+      function _freeArray(heapBytes){
+                      Module._free(heapBytes.byteOffset);
+      }
+
+      input.onmidimessage = (message) => {
+                          let bytes = message.data;
+                          var heapBytes = _arrayToHeap(bytes);
+                          Module._libremidi_devices_input(port_index, message.timeStamp, bytes.length, heapBytes.byteOffset);
+                          _freeArray(heapBytes);
+      };
+    , port_index
+    , id.c_str()
+    );
+  }
+
+  void stop_stream(int port_index)
+  {
+    const auto& id = m_current_inputs[port_index].id;
+    EM_ASM(const id = UTF8ToString($1);
+
+           let input = globalThis.__libreMidi_access.inputs.get(id);
+           input.onmidimessage = undefined;, id.c_str());
+  }
+
+  std::vector<observer_emscripten*> m_observers;
+  std::vector<device_information> m_current_inputs;
+  std::vector<device_information> m_current_outputs;
+
+  std::map<int, std::vector<midi_in_emscripten*>> m_opened_inputs;
+
+  std::map<std::string, int> m_input_indices;
+  std::map<std::string, int> m_output_indices;
+};
+
+}
+}

include/libremidi/backends/emscripten/midi_in.cpp

@@ -0,0 +1,91 @@
+#if defined(__EMSCRIPTEN__)
+  #include <libremidi/backends/emscripten/midi_access.hpp>
+  #include <libremidi/backends/emscripten/midi_in.hpp>
+  #include <libremidi/detail/midi_stream_decoder.hpp>
+
+  #include <chrono>
+
+namespace libremidi
+{
+LIBREMIDI_INLINE midi_in_emscripten::midi_in_emscripten(
+    input_configuration&& conf, emscripten_input_configuration&& apiconf)
+    : configuration{std::move(conf), std::move(apiconf)}
+{
+}
+
+LIBREMIDI_INLINE midi_in_emscripten::~midi_in_emscripten()
+{
+  // Close a connection if it exists.
+  midi_in_emscripten::close_port();
+}
+
+LIBREMIDI_INLINE libremidi::API midi_in_emscripten::get_current_api() const noexcept
+{
+  return libremidi::API::WEBMIDI;
+}
+
+LIBREMIDI_INLINE bool midi_in_emscripten::open_port(int portNumber, std::string_view)
+{
+  auto& midi = webmidi_helpers::midi_access_emscripten::instance();
+
+  if (portNumber < 0 || portNumber >= midi.input_count())
+  {
+    error<no_devices_found_error>(
+        this->configuration, "midi_in_emscripten::open_port: no MIDI output sources found.");
+    return false;
+  }
+
+  midi.open_input(portNumber, *this);
+  portNumber_ = portNumber;
+  return true;
+}
+
+LIBREMIDI_INLINE bool
+midi_in_emscripten::open_port(const libremidi::input_port& p, std::string_view nm)
+{
+  return open_port(p.port, nm);
+}
+
+LIBREMIDI_INLINE bool midi_in_emscripten::open_virtual_port(std::string_view)
+{
+  warning(configuration, "midi_in_emscripten::open_virtual_port: unsupported.");
+  return false;
+}
+
+LIBREMIDI_INLINE void midi_in_emscripten::close_port()
+{
+  auto& midi = webmidi_helpers::midi_access_emscripten::instance();
+
+  midi.close_input(portNumber_, *this);
+}
+
+LIBREMIDI_INLINE void midi_in_emscripten::set_client_name(std::string_view)
+{
+  warning(configuration, "midi_in_emscripten::set_client_name: unsupported.");
+}
+
+LIBREMIDI_INLINE void midi_in_emscripten::set_port_name(std::string_view)
+{
+  warning(configuration, "midi_in_emscripten::set_port_name: unsupported.");
+}
+
+LIBREMIDI_INLINE int64_t midi_in_emscripten::absolute_timestamp() const noexcept
+{
+  return system_ns();
+}
+
+LIBREMIDI_INLINE void
+midi_in_emscripten::on_input(double ts, unsigned char* begin, unsigned char* end)
+{
+  static constexpr timestamp_backend_info timestamp_info{
+      .has_absolute_timestamps = true,
+      .absolute_is_monotonic = true,
+      .has_samples = false,
+  };
+  const auto to_ns = [=] { return 1e6 * ts; };
+
+  m_processing.on_bytes({begin, end}, m_processing.timestamp<timestamp_info>(to_ns, 0));
+}
+
+}
+#endif

include/libremidi/backends/emscripten/midi_in.hpp

@@ -0,0 +1,41 @@
+#pragma once
+#include <libremidi/backends/emscripten/config.hpp>
+#include <libremidi/backends/emscripten/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+namespace libremidi
+{
+class midi_in_emscripten final
+    : public midi1::in_api
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , emscripten_input_configuration
+  {
+  } configuration;
+
+  midi_in_emscripten(input_configuration&& conf, emscripten_input_configuration&& apiconf);
+  ~midi_in_emscripten() override;
+
+  libremidi::API get_current_api() const noexcept override;
+
+  bool open_port(int portNumber, std::string_view);
+  bool open_port(const input_port& p, std::string_view) override;
+  bool open_virtual_port(std::string_view) override;
+  void close_port() override;
+
+  void set_client_name(std::string_view clientName) override;
+  void set_port_name(std::string_view portName) override;
+  timestamp absolute_timestamp() const noexcept override;
+
+  void on_input(double ts, unsigned char* begin, unsigned char* end);
+
+private:
+  int portNumber_{};
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+}

include/libremidi/backends/emscripten/midi_out.cpp

@@ -0,0 +1,74 @@
+#if defined(__EMSCRIPTEN__)
+  #include <libremidi/backends/emscripten/midi_access.hpp>
+  #include <libremidi/backends/emscripten/midi_out.hpp>
+
+namespace libremidi
+{
+LIBREMIDI_INLINE midi_out_emscripten::midi_out_emscripten(
+    output_configuration&& conf, emscripten_output_configuration&& apiconf)
+    : configuration{std::move(conf), std::move(apiconf)}
+{
+}
+
+LIBREMIDI_INLINE midi_out_emscripten::~midi_out_emscripten()
+{
+  // Close a connection if it exists.
+  midi_out_emscripten::close_port();
+}
+
+LIBREMIDI_INLINE libremidi::API midi_out_emscripten::get_current_api() const noexcept
+{
+  return libremidi::API::WEBMIDI;
+}
+
+LIBREMIDI_INLINE bool midi_out_emscripten::open_port(unsigned int portNumber, std::string_view)
+{
+  auto& midi = webmidi_helpers::midi_access_emscripten::instance();
+
+  if (portNumber >= midi.output_count())
+  {
+    error<no_devices_found_error>(
+        this->configuration, "midi_out_emscripten::open_port: no MIDI output sources found.");
+    return false;
+  }
+
+  portNumber_ = portNumber;
+  return true;
+}
+
+LIBREMIDI_INLINE bool midi_out_emscripten::open_port(const output_port& p, std::string_view nm)
+{
+  return open_port(p.port, nm);
+}
+
+LIBREMIDI_INLINE void midi_out_emscripten::close_port() { }
+
+LIBREMIDI_INLINE void midi_out_emscripten::set_client_name(std::string_view clientName)
+{
+  warning(configuration, "midi_out_emscripten::set_client_name: unsupported.");
+}
+
+LIBREMIDI_INLINE void midi_out_emscripten::set_port_name(std::string_view portName)
+{
+  warning(configuration, "midi_out_emscripten::set_port_name: unsupported.");
+}
+
+LIBREMIDI_INLINE bool midi_out_emscripten::open_virtual_port(std::string_view)
+{
+  warning(configuration, "midi_in_emscripten::open_virtual_port: unsupported.");
+  return false;
+}
+
+LIBREMIDI_INLINE void midi_out_emscripten::send_message(const unsigned char* message, size_t size)
+{
+  if (portNumber_ < 0)
+    error<invalid_use_error>(
+        this->configuration,
+        "midi_out_emscripten::send_message: trying to send a message without an open "
+        "port.");
+
+  webmidi_helpers::midi_access_emscripten::instance().send_message(
+      portNumber_, reinterpret_cast<const char*>(message), size);
+}
+}
+#endif

include/libremidi/backends/emscripten/midi_out.hpp

@@ -0,0 +1,37 @@
+#pragma once
+#include <libremidi/backends/emscripten/config.hpp>
+#include <libremidi/backends/emscripten/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi
+{
+class midi_out_emscripten final
+    : public midi1::out_api
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , emscripten_output_configuration
+  {
+  } configuration;
+
+  midi_out_emscripten(output_configuration&& conf, emscripten_output_configuration&& apiconf);
+  ~midi_out_emscripten() override;
+
+  libremidi::API get_current_api() const noexcept override;
+
+  bool open_port(unsigned int portNumber, std::string_view);
+  bool open_port(const output_port& p, std::string_view) override;
+  bool open_virtual_port(std::string_view) override;
+  void close_port() override;
+
+  void set_client_name(std::string_view clientName) override;
+  void set_port_name(std::string_view portName) override;
+
+  void send_message(const unsigned char* message, size_t size) override;
+
+private:
+  int portNumber_{-1};
+};
+}

include/libremidi/backends/emscripten/observer.cpp

@@ -0,0 +1,105 @@
+#if defined(__EMSCRIPTEN__)
+  #include <libremidi/backends/emscripten/midi_access.hpp>
+  #include <libremidi/backends/emscripten/observer.hpp>
+
+  #include <cassert>
+
+namespace libremidi
+{
+LIBREMIDI_INLINE observer_emscripten::observer_emscripten(
+    observer_configuration&& conf, emscripten_observer_configuration&& apiconf)
+    : configuration{std::move(conf), std::move(apiconf)}
+{
+  if (!configuration.has_callbacks())
+    return;
+
+  auto& webmidi = webmidi_helpers::midi_access_emscripten::instance();
+  webmidi.register_observer(*this);
+
+  // Trigger an initial notification
+  webmidi.load_current_infos();
+
+  if (configuration.notify_in_constructor)
+    update(webmidi.inputs(), webmidi.outputs());
+}
+
+LIBREMIDI_INLINE observer_emscripten::~observer_emscripten()
+{
+  if (!configuration.has_callbacks())
+    return;
+
+  webmidi_helpers::midi_access_emscripten::instance().unregister_observer(*this);
+}
+
+LIBREMIDI_INLINE
+libremidi::API observer_emscripten::get_current_api() const noexcept
+{
+  return libremidi::API::WEBMIDI;
+}
+
+template <bool Input>
+static auto to_port_info(int index, const webmidi_helpers::device_information& dev)
+    -> std::conditional_t<Input, input_port, output_port>
+{
+  return {
+      {.client = 0,
+       .port = (uint64_t)index,
+       .manufacturer = "",
+       .device_name = "",
+       .port_name = dev.name,
+       .display_name = dev.name}};
+}
+
+LIBREMIDI_INLINE
+std::vector<libremidi::input_port> observer_emscripten::get_input_ports() const noexcept
+{
+  std::vector<libremidi::input_port> ret;
+  auto& webmidi = webmidi_helpers::midi_access_emscripten::instance();
+  webmidi.load_current_infos();
+
+  for (std::size_t i = 0, n = webmidi.input_count(); i < n; i++)
+  {
+    ret.push_back(to_port_info<true>(i, webmidi.inputs()[i]));
+  }
+
+  return ret;
+}
+
+LIBREMIDI_INLINE
+std::vector<libremidi::output_port> observer_emscripten::get_output_ports() const noexcept
+{
+  std::vector<libremidi::output_port> ret;
+  auto& webmidi = webmidi_helpers::midi_access_emscripten::instance();
+  webmidi.load_current_infos();
+
+  for (std::size_t i = 0, n = webmidi.output_count(); i < n; i++)
+  {
+    ret.push_back(to_port_info<false>(i, webmidi.outputs()[i]));
+  }
+
+  return ret;
+}
+
+LIBREMIDI_INLINE void observer_emscripten::update(
+    const std::vector<observer_emscripten::device>& current_inputs,
+    const std::vector<observer_emscripten::device>& current_outputs)
+{
+  // WebMIDI never remove inputs, it just marks them as disconnected.
+  // At least in known browsers...
+  assert(current_inputs.size() >= m_known_inputs.size());
+  assert(current_outputs.size() >= m_known_outputs.size());
+
+  for (std::size_t i = m_known_inputs.size(); i < current_inputs.size(); i++)
+  {
+    m_known_inputs.push_back(current_inputs[i]);
+    configuration.input_added(to_port_info<true>(i, m_known_inputs[i]));
+  }
+
+  for (std::size_t i = m_known_outputs.size(); i < current_outputs.size(); i++)
+  {
+    m_known_outputs.push_back(current_outputs[i]);
+    configuration.output_added(to_port_info<false>(i, m_known_outputs[i]));
+  }
+}
+}
+#endif

include/libremidi/backends/emscripten/observer.hpp

@@ -0,0 +1,37 @@
+#pragma once
+#include <libremidi/backends/emscripten/config.hpp>
+#include <libremidi/backends/emscripten/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <vector>
+
+namespace libremidi
+{
+class observer_emscripten final : public observer_api
+{
+  using device = webmidi_helpers::device_information;
+
+public:
+  struct
+      : observer_configuration
+      , emscripten_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_emscripten(
+      observer_configuration&& conf, emscripten_observer_configuration&& apiconf);
+  ~observer_emscripten();
+
+  void
+  update(const std::vector<device>& current_inputs, const std::vector<device>& current_outputs);
+
+  libremidi::API get_current_api() const noexcept override;
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override;
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override;
+
+private:
+  std::vector<device> m_known_inputs;
+  std::vector<device> m_known_outputs;
+};
+}

include/libremidi/backends/jack.hpp

@@ -0,0 +1,37 @@
+#pragma once
+#include <libremidi/backends/jack/midi_in.hpp>
+#include <libremidi/backends/jack/midi_out.hpp>
+#include <libremidi/backends/jack/observer.hpp>
+
+//*********************************************************************//
+//  API: UNIX JACK
+//
+//  Written primarily by Alexander Svetalkin, with updates for delta
+//  time by Gary Scavone, April 2011.
+//
+//  *********************************************************************//
+
+namespace libremidi
+{
+struct jack_backend
+{
+  using midi_in = midi_in_jack;
+  using midi_out = midi_out_jack;
+  using midi_observer = observer_jack;
+  using midi_in_configuration = jack_input_configuration;
+  using midi_out_configuration = jack_output_configuration;
+  using midi_observer_configuration = jack_observer_configuration;
+  static const constexpr auto API = libremidi::API::JACK_MIDI;
+  static const constexpr auto name = "jack";
+  static const constexpr auto display_name = "JACK";
+
+  static inline bool available() noexcept
+  {
+#if LIBREMIDI_WEAKJACK
+    return WeakJack::instance().available() == 0;
+#else
+    return true;
+#endif
+  }
+};
+}

include/libremidi/backends/jack/config.hpp

@@ -0,0 +1,50 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <cinttypes>
+#include <cstdint>
+#include <functional>
+#include <string>
+
+extern "C" {
+typedef struct _jack_client jack_client_t;
+typedef uint32_t jack_nframes_t;
+typedef int (*JackProcessCallback)(jack_nframes_t nframes, void* arg);
+}
+
+namespace libremidi
+{
+using jack_callback_function = std::function<void(int nframes)>;
+struct jack_callback
+{
+  int64_t token;
+  std::function<void(int nframes)> callback;
+};
+struct jack_input_configuration
+{
+  std::string client_name = "libremidi client";
+
+  jack_client_t* context{};
+  std::function<void(jack_callback)> set_process_func;
+  std::function<void(int64_t)> clear_process_func;
+};
+
+struct jack_output_configuration
+{
+  std::string client_name = "libremidi client";
+
+  jack_client_t* context{};
+  std::function<void(jack_callback)> set_process_func;
+  std::function<void(int64_t)> clear_process_func;
+
+  int32_t ringbuffer_size = 16384;
+  bool direct = false;
+};
+
+struct jack_observer_configuration
+{
+  std::string client_name = "libremidi client";
+  jack_client_t* context{};
+};
+
+}

include/libremidi/backends/jack/helpers.hpp

@@ -0,0 +1,241 @@
+#pragma once
+
+#if __has_include(<weakjack/weak_libjack.h>)
+  #include <weakjack/weak_libjack.h>
+#elif __has_include(<weak_libjack.h>)
+  #include <weak_libjack.h>
+#elif __has_include(<jack/jack.h> )
+  #include <jack/jack.h>
+  #include <jack/midiport.h>
+  #include <jack/ringbuffer.h>
+#endif
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/semaphore.hpp>
+
+#include <atomic>
+#include <semaphore>
+
+namespace libremidi
+{
+struct jack_client
+{
+  jack_client_t* client{};
+
+  static std::string get_port_display_name(jack_port_t* port)
+  {
+    auto p1 = std::make_unique<char[]>(jack_port_name_size());
+    auto p2 = std::make_unique<char[]>(jack_port_name_size());
+    char* aliases[3] = {p1.get(), p2.get(), nullptr};
+    int n = jack_port_get_aliases(port, aliases);
+    if (n > 1)
+    {
+      return aliases[1];
+    }
+    else if (n > 0)
+    {
+      std::string str = aliases[0];
+      if (str.starts_with("alsa_pcm:"))
+        str.erase(0, strlen("alsa_pcm:"));
+      return str;
+    }
+    else
+    {
+      const auto short_name = jack_port_short_name(port);
+      if (short_name && strlen(short_name) > 0)
+        return short_name;
+      return jack_port_name(port);
+    }
+  }
+
+  template <bool Input>
+  static auto to_port_info(jack_client_t* client, jack_port_t* port)
+      -> std::conditional_t<Input, input_port, output_port>
+  {
+    return {{
+        .client = reinterpret_cast<std::uintptr_t>(client),
+        .port = 0,
+        .manufacturer = "",
+        .device_name = "",
+        .port_name = jack_port_name(port),
+        .display_name = get_port_display_name(port),
+    }};
+  }
+
+  template <bool Input>
+  static auto
+  get_ports(jack_client_t* client, const char* pattern, const JackPortFlags flags) noexcept
+      -> std::vector<std::conditional_t<Input, input_port, output_port>>
+  {
+    std::vector<std::conditional_t<Input, input_port, output_port>> ret;
+
+    if (!client)
+      return {};
+
+    const char** ports = jack_get_ports(client, pattern, JACK_DEFAULT_MIDI_TYPE, flags);
+
+    if (ports == nullptr)
+      return {};
+
+    int i = 0;
+    while (ports[i] != nullptr)
+    {
+      // FIXME this does not take into account filtering sw / hw ports
+      auto port = jack_port_by_name(client, ports[i]);
+      ret.push_back(to_port_info<Input>(client, port));
+      i++;
+    }
+
+    jack_free(ports);
+
+    return ret;
+  }
+};
+
+struct jack_helpers : jack_client
+{
+  struct port_handle
+  {
+    port_handle& operator=(jack_port_t* p)
+    {
+      impl.get()->store(p);
+      return *this;
+    }
+
+    operator jack_port_t*() const noexcept
+    {
+      if (impl)
+        return impl.get()->load();
+      return {};
+    }
+
+    std::shared_ptr<std::atomic<jack_port_t*>> impl
+        = std::make_shared<std::atomic<jack_port_t*>>(nullptr);
+  } port;
+
+  int64_t this_instance{};
+
+  semaphore_pair_lock thread_lock;
+
+  jack_helpers()
+  {
+    static std::atomic_int64_t instance{};
+    this_instance = ++instance;
+  }
+
+  template <typename Self>
+  jack_status_t connect(Self& self)
+  {
+    auto& configuration = self.configuration;
+
+    if (this->client)
+      return jack_status_t{};
+
+    // Initialize JACK client
+    if (configuration.context)
+    {
+      if (!configuration.set_process_func)
+        return JackFailure;
+
+      configuration.set_process_func(
+          {.token = this_instance,
+           .callback = [&self, p = std::weak_ptr{this->port.impl}](jack_nframes_t nf) -> int {
+             if (auto pt = p.lock())
+               if (auto ppt = pt->load())
+                 self.process(nf);
+
+             self.thread_lock.check_client_released();
+             return 0;
+           }});
+
+      this->client = configuration.context;
+      return jack_status_t{};
+    }
+    else
+    {
+      jack_status_t status{};
+      this->client
+          = jack_client_open(configuration.client_name.c_str(), JackNoStartServer, &status);
+      if (this->client != nullptr)
+      {
+        jack_set_process_callback(
+            this->client,
+            +[](jack_nframes_t nf, void* ctx) -> int {
+              auto& self = *static_cast<Self*>(ctx);
+              jack_port_t* port = self.port;
+
+              // Is port created?
+              if (port == nullptr)
+                return 0;
+
+              self.process(nf);
+
+              self.thread_lock.check_client_released();
+              return 0;
+            },
+            &self);
+        jack_activate(this->client);
+      }
+      return status;
+    }
+  }
+
+  template <typename Self>
+  void disconnect(Self& self)
+  {
+    if (self.configuration.context)
+    {
+      if (self.configuration.clear_process_func)
+      {
+        self.configuration.clear_process_func(this_instance);
+      }
+    }
+
+    if (this->client && !self.configuration.context)
+      jack_client_close(this->client);
+  }
+
+  bool create_local_port(const auto& self, std::string_view portName, JackPortFlags flags)
+  {
+    // full name: "client_name:port_name\0"
+    if (portName.empty())
+      portName = flags & JackPortIsInput ? "i" : "o";
+
+    if (self.configuration.client_name.size() + portName.size() + 2u
+        >= static_cast<size_t>(jack_port_name_size()))
+    {
+      self.template error<invalid_use_error>(
+          self.configuration, "JACK: port name length limit exceeded");
+      return false;
+    }
+
+    if (!this->port)
+    {
+      this->port
+          = jack_port_register(this->client, portName.data(), JACK_DEFAULT_MIDI_TYPE, flags, 0);
+    }
+
+    if (!this->port)
+    {
+      self.template error<driver_error>(self.configuration, "JACK: error creating port");
+      return false;
+    }
+    return true;
+  }
+
+  void do_close_port()
+  {
+    if (this->port == nullptr)
+      return;
+
+    // 1. Ensure that the next time the cycle runs it sees the port as nullptr
+    jack_port_t* port_ptr = this->port.impl->load();
+    this->port = nullptr;
+
+    // 2. Signal through the semaphore and wait for the signal return
+    this->thread_lock.prepare_release_client();
+
+    // 3. Now we are sure that the client is not going to use the port anymore
+    jack_port_unregister(this->client, port_ptr);
+  }
+};
+}

include/libremidi/backends/jack/midi_in.hpp

@@ -0,0 +1,114 @@
+#pragma once
+#include <libremidi/backends/jack/config.hpp>
+#include <libremidi/backends/jack/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+#include <chrono>
+
+namespace libremidi
+{
+class midi_in_jack final
+    : public midi1::in_api
+    , public jack_helpers
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , jack_input_configuration
+  {
+  } configuration;
+
+  explicit midi_in_jack(input_configuration&& conf, jack_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    auto status = connect(*this);
+    if (status != jack_status_t{})
+      warning(configuration, "midi_in_jack: " + std::to_string((int)jack_status_t{}));
+  }
+
+  ~midi_in_jack() override
+  {
+    midi_in_jack::close_port();
+
+    disconnect(*this);
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_jack: set_client_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::JACK_MIDI; }
+
+  bool open_port(const input_port& port, std::string_view portName) override
+  {
+    if (!create_local_port(*this, portName, JackPortIsInput))
+      return false;
+
+    if (auto ret = jack_connect(this->client, port.port_name.c_str(), jack_port_name(this->port));
+        ret != 0)
+    {
+      error<invalid_parameter_error>(
+          configuration, "JACK: could not connect to port: " + port.port_name + " -> "
+                             + jack_port_name(this->port));
+      return false;
+    }
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    return create_local_port(*this, portName, JackPortIsInput);
+  }
+
+  void close_port() override { return do_close_port(); }
+
+  void set_port_name(std::string_view portName) override
+  {
+    jack_port_rename(this->client, this->port, portName.data());
+  }
+
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return 1000 * jack_frames_to_time(client, jack_frame_time(client));
+  }
+
+  int process(jack_nframes_t nframes)
+  {
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = true,
+        .has_samples = true,
+    };
+    void* buff = jack_port_get_buffer(this->port, nframes);
+
+    // Timing
+    jack_nframes_t current_frames{};
+    jack_time_t current_usecs{}; // roughly CLOCK_MONOTONIC
+    jack_time_t next_usecs{};
+    float period_usecs{};
+    jack_get_cycle_times(
+        this->client, &current_frames, &current_usecs, &next_usecs, &period_usecs);
+
+    // We have midi events in buffer
+    uint32_t evCount = jack_midi_get_event_count(buff);
+    for (uint32_t j = 0; j < evCount; j++)
+    {
+      jack_midi_event_t event{};
+      jack_midi_event_get(&event, buff, j);
+      const auto to_ns
+          = [=, this] { return 1000 * jack_frames_to_time(client, current_frames + event.time); };
+
+      m_processing.on_bytes(
+          {event.buffer, event.buffer + event.size},
+          m_processing.timestamp<timestamp_info>(to_ns, event.time));
+    }
+
+    return 0;
+  }
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+}

include/libremidi/backends/jack/midi_out.hpp

@@ -0,0 +1,231 @@
+#pragma once
+#include <libremidi/backends/jack/config.hpp>
+#include <libremidi/backends/jack/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+#include <semaphore>
+
+namespace libremidi
+{
+struct jack_queue
+{
+public:
+  static constexpr auto size_sz = sizeof(int32_t);
+
+  jack_queue() = default;
+  jack_queue(const jack_queue&) = delete;
+  jack_queue(jack_queue&&) = delete;
+  jack_queue& operator=(const jack_queue&) = delete;
+
+  jack_queue& operator=(jack_queue&& other) noexcept
+  {
+    ringbuffer = other.ringbuffer;
+    ringbuffer_space = other.ringbuffer_space;
+    other.ringbuffer = nullptr;
+    return *this;
+  }
+
+  explicit jack_queue(int64_t sz) noexcept
+  {
+    ringbuffer = jack_ringbuffer_create(sz);
+    ringbuffer_space = jack_ringbuffer_write_space(ringbuffer);
+  }
+
+  ~jack_queue() noexcept
+  {
+    if (ringbuffer)
+      jack_ringbuffer_free(ringbuffer);
+  }
+
+  void write(const unsigned char* data, int64_t sz) const noexcept
+  {
+    if (static_cast<std::size_t>(sz + size_sz) > ringbuffer_space)
+      return;
+
+    while (jack_ringbuffer_write_space(ringbuffer) < sz + size_sz)
+      sched_yield();
+
+    jack_ringbuffer_write(ringbuffer, reinterpret_cast<char*>(&sz), size_sz);
+    jack_ringbuffer_write(ringbuffer, reinterpret_cast<const char*>(data), sz);
+  }
+
+  void read(void* jack_events) const noexcept
+  {
+    int32_t sz;
+    while (jack_ringbuffer_peek(ringbuffer, reinterpret_cast<char*>(&sz), size_sz) == size_sz
+           && jack_ringbuffer_read_space(ringbuffer) >= size_sz + sz)
+    {
+      jack_ringbuffer_read_advance(ringbuffer, size_sz);
+
+      if (auto midi = jack_midi_event_reserve(jack_events, 0, sz))
+        jack_ringbuffer_read(ringbuffer, reinterpret_cast<char*>(midi), sz);
+      else
+        jack_ringbuffer_read_advance(ringbuffer, sz);
+    }
+  }
+
+  jack_ringbuffer_t* ringbuffer{};
+  std::size_t ringbuffer_space{}; // actual writable size, usually 1 less than ringbuffer
+};
+
+class midi_out_jack
+    : public midi1::out_api
+    , public jack_helpers
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , jack_output_configuration
+  {
+  } configuration;
+
+  midi_out_jack(output_configuration&& conf, jack_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+  }
+
+  ~midi_out_jack() override { }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_jack: set_client_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::JACK_MIDI; }
+
+  bool open_port(const output_port& port, std::string_view portName) override
+  {
+    if (!create_local_port(*this, portName, JackPortIsOutput))
+      return false;
+
+    // Connecting to the output
+    if (jack_connect(this->client, jack_port_name(this->port), port.port_name.c_str()) != 0)
+    {
+      error<invalid_parameter_error>(
+          configuration, "JACK: could not connect to port" + port.port_name);
+      return false;
+    }
+
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view portName) override
+  {
+    return create_local_port(*this, portName, JackPortIsOutput);
+  }
+
+  void close_port() override { return do_close_port(); }
+
+  void set_port_name(std::string_view portName) override
+  {
+    jack_port_rename(this->client, this->port, portName.data());
+  }
+};
+
+class midi_out_jack_queued final : public midi_out_jack
+{
+public:
+  midi_out_jack_queued(output_configuration&& conf, jack_output_configuration&& apiconf)
+      : midi_out_jack{std::move(conf), std::move(apiconf)}
+      , queue{configuration.ringbuffer_size}
+  {
+    auto status = connect(*this);
+    if (status != jack_status_t{})
+      warning(configuration, "midi_out_jack_queued: " + std::to_string((int)jack_status_t{}));
+  }
+
+  ~midi_out_jack_queued() override
+  {
+    midi_out_jack::close_port();
+
+    disconnect(*this);
+  }
+
+  void send_message(const unsigned char* message, std::size_t size) override
+  {
+    queue.write(message, size);
+  }
+
+  int process(jack_nframes_t nframes)
+  {
+    void* buff = jack_port_get_buffer(this->port, nframes);
+    jack_midi_clear_buffer(buff);
+
+    this->queue.read(buff);
+
+    return 0;
+  }
+
+private:
+  jack_queue queue;
+};
+
+class midi_out_jack_direct final : public midi_out_jack
+{
+public:
+  midi_out_jack_direct(output_configuration&& conf, jack_output_configuration&& apiconf)
+      : midi_out_jack{std::move(conf), std::move(apiconf)}
+  {
+    auto status = connect(*this);
+    if (status != jack_status_t{})
+      warning(configuration, "midi_out_jack_direct: " + std::to_string((int)jack_status_t{}));
+
+    buffer_size = jack_get_buffer_size(this->client);
+  }
+
+  ~midi_out_jack_direct() override
+  {
+    midi_out_jack::close_port();
+
+    disconnect(*this);
+  }
+
+  int process(jack_nframes_t nframes)
+  {
+    void* buff = jack_port_get_buffer(this->port, nframes);
+    jack_midi_clear_buffer(buff);
+    return 0;
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    void* buff = jack_port_get_buffer(this->port, buffer_size);
+    jack_midi_event_write(buff, 0, message, size);
+  }
+
+  int convert_timestamp(int64_t user) const noexcept
+  {
+    switch (configuration.timestamps)
+    {
+      case timestamp_mode::AudioFrame:
+        return static_cast<int>(user);
+
+      default:
+        // TODO
+        return 0;
+    }
+  }
+
+  void schedule_message(int64_t ts, const unsigned char* message, size_t size) override
+  {
+    void* buff = jack_port_get_buffer(this->port, buffer_size);
+    jack_midi_event_write(buff, convert_timestamp(ts), message, size);
+  }
+
+  int buffer_size{};
+};
+}
+
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<midi_out_api> make<midi_out_jack>(
+    libremidi::output_configuration&& conf, libremidi::jack_output_configuration&& api)
+{
+  if (api.direct)
+    return std::make_unique<midi_out_jack_direct>(std::move(conf), std::move(api));
+  else
+    return std::make_unique<midi_out_jack_queued>(std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/jack/observer.hpp

@@ -0,0 +1,224 @@
+#pragma once
+#include <libremidi/backends/jack/config.hpp>
+#include <libremidi/backends/jack/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <unordered_set>
+
+namespace libremidi
+{
+class observer_jack final
+    : public observer_api
+    , private jack_client
+    , private error_handler
+{
+public:
+  struct
+      : observer_configuration
+      , jack_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_jack(observer_configuration&& conf, jack_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    // Initialize JACK client
+    if (configuration.context)
+    {
+      this->client = configuration.context;
+      set_callbacks();
+    }
+    else
+    {
+      jack_status_t status{};
+      this->client
+          = jack_client_open(configuration.client_name.c_str(), JackNoStartServer, &status);
+      if (status != jack_status_t{})
+        warning(configuration, "observer_jack: " + std::to_string((int)jack_status_t{}));
+
+      if (this->client != nullptr)
+      {
+        set_callbacks();
+
+        jack_activate(this->client);
+      }
+    }
+  }
+
+  void initial_callback()
+  {
+    {
+      const char** ports
+          = jack_get_ports(client, nullptr, JACK_DEFAULT_MIDI_TYPE, JackPortIsOutput);
+
+      if (ports != nullptr)
+      {
+        int i = 0;
+        while (ports[i] != nullptr)
+        {
+          auto port = jack_port_by_name(client, ports[i]);
+          auto flags = jack_port_flags(port);
+
+          bool physical = flags & JackPortIsPhysical;
+          bool ok = configuration.track_any;
+          if (configuration.track_hardware)
+            ok |= physical;
+          if (configuration.track_virtual)
+            ok |= !physical;
+
+          if (ok)
+          {
+            seen_input_ports.insert(ports[i]);
+            if (this->configuration.input_added && configuration.notify_in_constructor)
+              this->configuration.input_added(to_port_info<true>(client, port));
+          }
+          i++;
+        }
+      }
+
+      jack_free(ports);
+    }
+
+    {
+      const char** ports
+          = jack_get_ports(client, nullptr, JACK_DEFAULT_MIDI_TYPE, JackPortIsInput);
+
+      if (ports != nullptr)
+      {
+        int i = 0;
+        while (ports[i] != nullptr)
+        {
+          auto port = jack_port_by_name(client, ports[i]);
+          auto flags = jack_port_flags(port);
+
+          bool physical = flags & JackPortIsPhysical;
+          bool ok = configuration.track_any;
+          if (configuration.track_hardware)
+            ok |= physical;
+          if (configuration.track_virtual)
+            ok |= !physical;
+
+          if (ok)
+          {
+            seen_output_ports.insert(ports[i]);
+            if (this->configuration.output_added && configuration.notify_in_constructor)
+              this->configuration.output_added(to_port_info<false>(client, port));
+          }
+          i++;
+        }
+      }
+
+      jack_free(ports);
+    }
+  }
+
+  void on_port_callback(jack_port_t* port, bool reg)
+  {
+    auto flags = jack_port_flags(port);
+    std::string name = jack_port_name(port);
+    if (reg)
+    {
+      std::string_view type = jack_port_type(port);
+      if (type != JACK_DEFAULT_MIDI_TYPE)
+        return;
+
+      bool physical = flags & JackPortIsPhysical;
+      bool ok = configuration.track_any;
+      if (configuration.track_hardware)
+        ok |= physical;
+      if (configuration.track_virtual)
+        ok |= !physical;
+      if (!ok)
+        return;
+
+      // Note: we keep track of the ports as
+      // when disconnecting, jack_port_type and jack_port_flags aren't correctly
+      // set anymore.
+
+      if (flags & JackPortIsOutput)
+      {
+        seen_input_ports.insert(name);
+        if (this->configuration.input_added)
+          this->configuration.input_added(to_port_info<true>(client, port));
+      }
+      else if (flags & JackPortIsInput)
+      {
+        seen_output_ports.insert(name);
+        if (this->configuration.output_added)
+          this->configuration.output_added(to_port_info<false>(client, port));
+      }
+    }
+    else
+    {
+      if (auto it = seen_input_ports.find(name); it != seen_input_ports.end())
+      {
+        if (this->configuration.input_removed)
+          this->configuration.input_removed(to_port_info<true>(client, port));
+        seen_input_ports.erase(it);
+      }
+      if (auto it = seen_output_ports.find(name); it != seen_output_ports.end())
+      {
+        if (this->configuration.output_removed)
+          this->configuration.output_removed(to_port_info<false>(client, port));
+        seen_output_ports.erase(it);
+      }
+    }
+  }
+
+  void set_callbacks()
+  {
+    initial_callback();
+
+    if (!configuration.has_callbacks())
+      return;
+
+    jack_set_port_registration_callback(
+        this->client,
+        +[](jack_port_id_t p, int r, void* arg) {
+          auto& self = *(observer_jack*)arg;
+          if (auto port = jack_port_by_id(self.client, p))
+          {
+            self.on_port_callback(port, r != 0);
+          }
+        },
+        this);
+
+    jack_set_port_rename_callback(
+        this->client,
+        +[](jack_port_id_t p, const char* /*old_name*/, const char* /*new_name*/, void* arg) {
+          const auto& self = *static_cast<observer_jack*>(arg);
+
+          auto port = jack_port_by_id(self.client, p);
+          if (!port)
+            return;
+        },
+        this);
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::JACK_MIDI; }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    return get_ports<true>(this->client, nullptr, JackPortIsOutput);
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    return get_ports<false>(this->client, nullptr, JackPortIsInput);
+  }
+
+  ~observer_jack()
+  {
+    if (client && !configuration.context)
+    {
+      // If we own the client, deactivate it
+      jack_deactivate(this->client);
+      jack_client_close(this->client);
+      this->client = nullptr;
+    }
+  }
+
+  std::unordered_set<std::string> seen_input_ports;
+  std::unordered_set<std::string> seen_output_ports;
+};
+}

include/libremidi/backends/jack/shared_handler.hpp

@@ -0,0 +1,153 @@
+#pragma once
+#if __has_include(<boost/lockfree/spsc_queue.hpp>)
+  #include <libremidi/backends/jack/config.hpp>
+  #include <libremidi/backends/jack/helpers.hpp>
+  #include <libremidi/shared_context.hpp>
+
+  #include <boost/lockfree/spsc_queue.hpp>
+
+  #include <variant>
+
+namespace libremidi::jack
+{
+
+// Create a JACK client which will be shared across objects
+struct shared_handler : public libremidi::shared_context
+{
+  explicit shared_handler(std::string_view v)
+  {
+    midiin_callbacks.reserve(64);
+    midiout_callbacks.reserve(64);
+
+    jack_status_t status{};
+    client = jack_client_open(v.data(), JackNoStartServer, &status);
+    assert(client);
+    assert(status == 0);
+    jack_set_process_callback(
+        client,
+        +[](jack_nframes_t cnt, void* ctx) -> int {
+          ((shared_handler*)ctx)->jack_callback(cnt);
+          return 0;
+        },
+        this);
+  }
+
+  virtual void start_processing() override { jack_activate(client); }
+  virtual void stop_processing() override { jack_deactivate(client); }
+
+  static shared_configurations make(std::string_view client_name)
+  {
+    auto clt = std::make_shared<shared_handler>(client_name);
+    auto add_in_cb = [client = std::weak_ptr{clt}](libremidi::jack_callback cb) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::in_callback_added, std::move(cb)});
+    };
+    auto clear_in_cb = [client = std::weak_ptr{clt}](int64_t index) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::in_callback_removed, index});
+    };
+    auto add_out_cb = [client = std::weak_ptr{clt}](libremidi::jack_callback cb) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::out_callback_added, std::move(cb)});
+    };
+    auto clear_out_cb = [client = std::weak_ptr{clt}](int64_t index) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::out_callback_removed, index});
+    };
+    return {
+        .context = clt,
+        .observer = jack_observer_configuration{.context = clt->client},
+        .in
+        = jack_input_configuration{.context = clt->client, .set_process_func = add_in_cb, .clear_process_func = clear_in_cb},
+        .out
+        = jack_output_configuration{.context = clt->client, .set_process_func = add_out_cb, .clear_process_func = clear_out_cb},
+    };
+  }
+
+  int jack_callback(jack_nframes_t cnt)
+  {
+    // 1. Process the events that will change the callback list
+    event ev;
+    while (events.pop(ev))
+    {
+      switch (ev.type)
+      {
+        case in_callback_added:
+          midiin_callbacks.push_back(
+              std::move(*std::get_if<libremidi::jack_callback>(&ev.payload)));
+          break;
+        case in_callback_removed: {
+          auto idx = *std::get_if<int64_t>(&ev.payload);
+          for (auto it = midiin_callbacks.begin(); it != midiin_callbacks.end();)
+          {
+            if (it->token == idx)
+            {
+              midiin_callbacks.erase(it);
+              break;
+            }
+            else
+            {
+              ++it;
+            }
+          }
+          break;
+        }
+        case out_callback_added:
+          midiout_callbacks.push_back(
+              std::move(*std::get_if<libremidi::jack_callback>(&ev.payload)));
+          break;
+        case out_callback_removed:
+          auto idx = *std::get_if<int64_t>(&ev.payload);
+          for (auto it = midiout_callbacks.begin(); it != midiout_callbacks.end();)
+          {
+            if (it->token == idx)
+            {
+              midiout_callbacks.erase(it);
+              break;
+            }
+            else
+            {
+              ++it;
+            }
+          }
+          break;
+      }
+    }
+
+    for (auto& cb : midiin_callbacks)
+      cb.callback(cnt);
+
+    for (auto& cb : midiout_callbacks)
+      cb.callback(cnt);
+
+    return 0;
+  }
+
+  ~shared_handler()
+  {
+    jack_deactivate(client);
+    jack_client_close(client);
+  }
+
+  jack_client_t* client{};
+
+  enum event_type
+  {
+    in_callback_added,
+    in_callback_removed,
+    out_callback_added,
+    out_callback_removed,
+  };
+  struct event
+  {
+    event_type type;
+    std::variant<libremidi::jack_callback, int64_t> payload;
+  };
+
+  boost::lockfree::spsc_queue<event> events{16};
+
+  std::vector<libremidi::jack_callback> midiin_callbacks;
+  std::vector<libremidi::jack_callback> midiout_callbacks;
+};
+}
+#endif

include/libremidi/backends/linux/alsa.hpp

@@ -0,0 +1,459 @@
+#pragma once
+
+#include <libremidi/backends/linux/dylib_loader.hpp>
+
+#include <alsa/asoundlib.h>
+
+#if defined(SND_LIB_VERSION)
+  #if __has_include(<alsa/rawmidi.h>) && SND_LIB_VERSION >= ((1 << 16) | (2 << 8) | 6)
+    #define LIBREMIDI_ALSA_HAS_RAMWIDI 1
+    #define LIBREMIDI_ALSA_HAS_RAWMIDI_TREAD 1
+  #endif
+
+  #if __has_include(<alsa/ump.h>) && SND_LIB_VERSION >= ((1 << 16) | (2 << 8) | 10)
+    #define LIBREMIDI_ALSA_HAS_UMP 1
+  #endif
+#endif
+
+namespace libremidi
+{
+
+struct libasound
+{
+  // Useful one-liner:
+  // nm -A * | grep ' snd_' | grep -v '@' | cut -f 2 -d 'U' | sort | uniq  | sed 's/ snd_//' | sed 's/_/, /' | awk ' { print "LIBREMIDI_SYMBOL_DEF(snd_"$1 " " $2 ");" }'
+
+  explicit libasound()
+      : library{"libasound.so.2"}
+  {
+    if (!library)
+    {
+      available = false;
+      return;
+    }
+
+    strerror = library.symbol<decltype(&::snd_strerror)>("snd_strerror");
+    if (!strerror)
+      available = false;
+  }
+
+  static const libasound& instance()
+  {
+    static const libasound self;
+    return self;
+  }
+
+  dylib_loader library;
+  decltype(&::snd_strerror) strerror{};
+  bool available{true};
+
+  struct card_t
+  {
+    explicit card_t(const dylib_loader& library)
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+
+      LIBREMIDI_SYMBOL_INIT(snd_card, get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_card, next);
+    }
+    bool available{true};
+
+    LIBREMIDI_SYMBOL_DEF(snd_card, get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_card, next);
+  } card{library};
+
+  struct ctl_t
+  {
+    explicit ctl_t(const dylib_loader& library)
+        : rawmidi{library}
+#if LIBREMIDI_ALSA_HAS_UMP
+        , ump{library}
+#endif
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+
+      LIBREMIDI_SYMBOL_INIT(snd_ctl, close);
+      LIBREMIDI_SYMBOL_INIT(snd_ctl, open);
+    }
+    bool available{true};
+
+    LIBREMIDI_SYMBOL_DEF(snd_ctl, close);
+    LIBREMIDI_SYMBOL_DEF(snd_ctl, open);
+
+    struct rawmidi_t
+    {
+      explicit rawmidi_t(const dylib_loader& library)
+      {
+        if (!library)
+        {
+          available = false;
+          return;
+        }
+
+        LIBREMIDI_SYMBOL_INIT(snd_ctl_rawmidi, info);
+        LIBREMIDI_SYMBOL_INIT(snd_ctl_rawmidi, next_device);
+      }
+      bool available{true};
+      LIBREMIDI_SYMBOL_DEF(snd_ctl_rawmidi, info);
+      LIBREMIDI_SYMBOL_DEF(snd_ctl_rawmidi, next_device);
+    } rawmidi;
+
+#if LIBREMIDI_ALSA_HAS_UMP
+    struct ump_t
+    {
+      explicit ump_t(const dylib_loader& library)
+      {
+        if (!library)
+        {
+          available = false;
+          return;
+        }
+
+        LIBREMIDI_SYMBOL_INIT(snd_ctl_ump, block_info);
+        LIBREMIDI_SYMBOL_INIT(snd_ctl_ump, endpoint_info);
+        LIBREMIDI_SYMBOL_INIT(snd_ctl_ump, next_device);
+      }
+      bool available{true};
+      LIBREMIDI_SYMBOL_DEF(snd_ctl_ump, block_info);
+      LIBREMIDI_SYMBOL_DEF(snd_ctl_ump, endpoint_info);
+      LIBREMIDI_SYMBOL_DEF(snd_ctl_ump, next_device);
+    } ump;
+#endif
+  } ctl{library};
+
+  struct midi_t
+  {
+    explicit midi_t(const dylib_loader& library)
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_decode);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_encode);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_free);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_init);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_new);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_no_status);
+      LIBREMIDI_SYMBOL_INIT(snd_midi, event_resize_buffer);
+    }
+
+    bool available{true};
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_decode);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_encode);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_free);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_init);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_new);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_no_status);
+    LIBREMIDI_SYMBOL_DEF(snd_midi, event_resize_buffer);
+  } midi{library};
+
+#if LIBREMIDI_ALSA_HAS_RAMWIDI
+  struct rawmidi_t
+  {
+    explicit rawmidi_t(const dylib_loader& library)
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, close);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_get_subdevice_name);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_get_subdevices_count);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_set_device);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_set_stream);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_set_subdevice);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, info_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, open);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_current);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_get_buffer_size);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_set_clock_type);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_set_no_active_sensing);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_set_read_mode);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, params_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, poll_descriptors);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, poll_descriptors_count);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, poll_descriptors_revents);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, read);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, status);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, status_get_avail);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, status_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, tread);
+      LIBREMIDI_SYMBOL_INIT(snd_rawmidi, write);
+    }
+
+    bool available{true};
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, close);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_get_subdevice_name);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_get_subdevices_count);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_set_device);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_set_stream);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_set_subdevice);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, info_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, open);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_current);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_get_buffer_size);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_set_clock_type);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_set_no_active_sensing);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_set_read_mode);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, params_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, poll_descriptors);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, poll_descriptors_count);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, poll_descriptors_revents);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, read);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, status);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, status_get_avail);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, status_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, tread);
+    LIBREMIDI_SYMBOL_DEF(snd_rawmidi, write);
+  } rawmidi{library};
+#endif
+
+  struct seq_t
+  {
+    explicit seq_t(const dylib_loader& library)
+#if LIBREMIDI_ALSA_HAS_UMP
+        : ump{library}
+#endif
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+
+      LIBREMIDI_SYMBOL_INIT(snd_seq, alloc_queue);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, client_id);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, client_info_get_client);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, client_info_get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, client_info_set_client);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, client_info_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, close);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, connect_from);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, control_queue);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, create_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, delete_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, drain_output);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, event_input);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, event_input_pending);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, event_output);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, free_event);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, free_queue);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, get_any_client_info);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, get_any_port_info);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, get_port_info);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, open);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, poll_descriptors);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, poll_descriptors_count);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_get_addr);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_get_capability);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_get_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_get_type);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_capability);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_client);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_midi_channels);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_name);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_timestamping);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_timestamp_queue);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_timestamp_real);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_set_type);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_info_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_free);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_malloc);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_set_dest);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_set_sender);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_set_time_real);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, port_subscribe_set_time_update);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, query_next_client);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, query_next_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, queue_tempo_set_ppq);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, queue_tempo_set_tempo);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, queue_tempo_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, set_client_name);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, set_port_info);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, set_queue_tempo);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, subscribe_port);
+      LIBREMIDI_SYMBOL_INIT(snd_seq, unsubscribe_port);
+    }
+
+    bool available{true};
+    LIBREMIDI_SYMBOL_DEF(snd_seq, alloc_queue);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, client_id);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, client_info_get_client);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, client_info_get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, client_info_set_client);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, client_info_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, close);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, connect_from);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, control_queue);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, create_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, delete_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, drain_output);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, event_input);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, event_input_pending);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, event_output);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, free_event);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, free_queue);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, get_any_client_info);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, get_any_port_info);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, get_port_info);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, open);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, poll_descriptors);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, poll_descriptors_count);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_get_addr);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_get_capability);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_get_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_get_type);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_capability);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_client);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_midi_channels);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_name);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_timestamping);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_timestamp_queue);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_timestamp_real);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_set_type);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_info_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_free);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_malloc);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_set_dest);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_set_sender);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_set_time_real);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, port_subscribe_set_time_update);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, query_next_client);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, query_next_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, queue_tempo_set_ppq);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, queue_tempo_set_tempo);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, queue_tempo_sizeof);
+
+    LIBREMIDI_SYMBOL_DEF(snd_seq, set_client_name);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, set_port_info);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, set_queue_tempo);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, subscribe_port);
+    LIBREMIDI_SYMBOL_DEF(snd_seq, unsubscribe_port);
+
+#if LIBREMIDI_ALSA_HAS_UMP
+    struct ump_t
+    {
+      explicit ump_t(const dylib_loader& library)
+      {
+        if (!library)
+        {
+          available = false;
+          return;
+        }
+        LIBREMIDI_SYMBOL_INIT(snd_seq, set_client_midi_version);
+        LIBREMIDI_SYMBOL_INIT(snd_seq_ump, event_input);
+        LIBREMIDI_SYMBOL_INIT(snd_seq_ump, event_output);
+        LIBREMIDI_SYMBOL_INIT(snd_seq_ump, event_output_direct);
+      }
+
+      bool available{true};
+
+      LIBREMIDI_SYMBOL_DEF(snd_seq, set_client_midi_version);
+      LIBREMIDI_SYMBOL_DEF(snd_seq_ump, event_input);
+      LIBREMIDI_SYMBOL_DEF(snd_seq_ump, event_output);
+      LIBREMIDI_SYMBOL_DEF(snd_seq_ump, event_output_direct);
+    } ump;
+#endif
+  } seq{library};
+
+#if LIBREMIDI_ALSA_HAS_UMP
+  struct ump_t
+  {
+    explicit ump_t(const dylib_loader& library)
+    {
+      if (!library)
+      {
+        available = false;
+        return;
+      }
+
+      LIBREMIDI_SYMBOL_INIT(snd_ump, block_info_get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, block_info_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, close);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, endpoint_info_get_name);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, endpoint_info_sizeof);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, open);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, poll_descriptors);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, poll_descriptors_count);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, poll_descriptors_revents);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, rawmidi);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, rawmidi_params);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, rawmidi_params_current);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, read);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, tread);
+      LIBREMIDI_SYMBOL_INIT(snd_ump, write);
+    }
+
+    bool available{true};
+    LIBREMIDI_SYMBOL_DEF(snd_ump, block_info_get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, block_info_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, close);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, endpoint_info_get_name);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, endpoint_info_sizeof);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, open);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, poll_descriptors);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, poll_descriptors_count);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, poll_descriptors_revents);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, rawmidi);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, rawmidi_params);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, rawmidi_params_current);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, read);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, tread);
+    LIBREMIDI_SYMBOL_DEF(snd_ump, write);
+  } ump{library};
+#endif
+};
+
+#undef snd_dylib_alloca
+#define snd_dylib_alloca(ptr, access, type)                                \
+  {                                                                        \
+    *ptr = (snd_##access##_##type##_t*)alloca(snd.access.type##_sizeof()); \
+    memset(*ptr, 0, snd.access.type##_sizeof());                           \
+  }
+#define snd_dylib_alloca2(ptr, access1, access2, type)                                         \
+  {                                                                                            \
+    *ptr = (snd_##access1##_access2##_##type##_t*)alloca(snd.access1.access2.type##_sizeof()); \
+    memset(*ptr, 0, snd.access1.access2.type##_sizeof());                                      \
+  }
+
+#undef snd_rawmidi_info_alloca
+#define snd_rawmidi_info_alloca(ptr) snd_dylib_alloca(ptr, rawmidi, info)
+#undef snd_rawmidi_params_alloca
+#define snd_rawmidi_params_alloca(ptr) snd_dylib_alloca(ptr, rawmidi, params)
+#undef snd_rawmidi_status_alloca
+#define snd_rawmidi_status_alloca(ptr) snd_dylib_alloca(ptr, rawmidi, status)
+
+#undef snd_seq_client_info_alloca
+#define snd_seq_client_info_alloca(ptr) snd_dylib_alloca(ptr, seq, client_info)
+#undef snd_seq_port_info_alloca
+#define snd_seq_port_info_alloca(ptr) snd_dylib_alloca(ptr, seq, port_info)
+#undef snd_seq_queue_tempo_alloca
+#define snd_seq_queue_tempo_alloca(ptr) snd_dylib_alloca(ptr, seq, queue_tempo)
+
+#if LIBREMIDI_ALSA_HAS_UMP
+  #undef snd_ump_block_info_alloca
+  #define snd_ump_block_info_alloca(ptr) snd_dylib_alloca(ptr, ump, block_info)
+  #undef snd_ump_endpoint_info_alloca
+  #define snd_ump_endpoint_info_alloca(ptr) snd_dylib_alloca(ptr, ump, endpoint_info)
+#endif
+}

include/libremidi/backends/linux/dylib_loader.hpp

@@ -0,0 +1,85 @@
+#pragma once
+#if __has_include(<dlfcn.h>)
+  #include <dlfcn.h>
+
+  #include <cassert>
+
+namespace libremidi
+{
+class dylib_loader
+{
+public:
+  explicit dylib_loader(const char* const so)
+  {
+    impl = dlopen(so, RTLD_LAZY | RTLD_LOCAL | RTLD_NODELETE);
+  }
+
+  dylib_loader(const dylib_loader&) noexcept = delete;
+  dylib_loader& operator=(const dylib_loader&) noexcept = delete;
+  dylib_loader(dylib_loader&& other) noexcept
+  {
+    impl = other.impl;
+    other.impl = nullptr;
+  }
+
+  dylib_loader& operator=(dylib_loader&& other) noexcept
+  {
+    impl = other.impl;
+    other.impl = nullptr;
+    return *this;
+  }
+
+  ~dylib_loader()
+  {
+    if (impl)
+    {
+      dlclose(impl);
+    }
+  }
+
+  template <typename T>
+  T symbol(const char* const sym) const noexcept
+  {
+    assert(impl);
+    return reinterpret_cast<T>(dlsym(impl, sym));
+  }
+
+  operator bool() const noexcept { return bool(impl); }
+
+private:
+  void* impl{};
+};
+
+}
+
+  #define LIBREMIDI_SYMBOL_NAME_S(prefix, name) #prefix "_" #name
+  #define LIBREMIDI_SYMBOL_NAME(prefix, name) prefix##_##name
+  #define LIBREMIDI_SYMBOL_DEF(prefix, name) \
+    decltype(&::LIBREMIDI_SYMBOL_NAME(prefix, name)) name{};
+  #define LIBREMIDI_SYMBOL_INIT(prefix, name)                                  \
+    {                                                                          \
+      name = library.symbol<decltype(&::LIBREMIDI_SYMBOL_NAME(prefix, name))>( \
+          LIBREMIDI_SYMBOL_NAME_S(prefix, name));                              \
+      if (!name)                                                               \
+      {                                                                        \
+        available = false;                                                     \
+        return;                                                                \
+      }                                                                        \
+    }
+
+// Because some libs have names that are C++ keywords, e.g. udev_new:
+  #define LIBREMIDI_SYMBOL_NAME2_S(prefix, name, varname) #prefix "_" #name
+  #define LIBREMIDI_SYMBOL_NAME2(prefix, name, varname) prefix##_##name
+  #define LIBREMIDI_SYMBOL_DEF2(prefix, name, varname) \
+    decltype(&::LIBREMIDI_SYMBOL_NAME2(prefix, name, varname)) varname{};
+  #define LIBREMIDI_SYMBOL_INIT2(prefix, name, varname)                                     \
+    {                                                                                       \
+      varname = library.symbol<decltype(&::LIBREMIDI_SYMBOL_NAME2(prefix, name, varname))>( \
+          LIBREMIDI_SYMBOL_NAME2_S(prefix, name, varname));                                 \
+      if (!varname)                                                                         \
+      {                                                                                     \
+        available = false;                                                                  \
+        return;                                                                             \
+      }                                                                                     \
+    }
+#endif

include/libremidi/backends/linux/helpers.hpp

@@ -0,0 +1,77 @@
+#pragma once
+#include <sys/eventfd.h>
+#include <sys/timerfd.h>
+
+#include <poll.h>
+#include <unistd.h>
+
+#include <iostream>
+
+namespace libremidi
+{
+struct eventfd_notifier
+{
+  eventfd_notifier(bool semaphore = true)
+  {
+    if (semaphore)
+      this->fd = eventfd(0, EFD_SEMAPHORE | EFD_NONBLOCK);
+    else
+      this->fd = eventfd(0, EFD_NONBLOCK);
+  }
+  ~eventfd_notifier() { close(this->fd); }
+
+  eventfd_notifier(const eventfd_notifier&) = delete;
+  eventfd_notifier(eventfd_notifier&&) = delete;
+  eventfd_notifier& operator=(const eventfd_notifier&) = delete;
+  eventfd_notifier& operator=(eventfd_notifier&&) = delete;
+
+  void notify() noexcept { eventfd_write(fd, 1); }
+  static bool ready(pollfd res) noexcept { return res.revents & POLLIN; }
+  eventfd_t consume() noexcept
+  {
+    eventfd_t val;
+    eventfd_read(fd, &val);
+    return val;
+  }
+
+  operator int() const noexcept { return fd; }
+  operator pollfd() const noexcept { return {.fd = fd, .events = POLLIN, .revents = 0}; }
+  int fd{-1};
+};
+
+struct timerfd_timer
+{
+  timerfd_timer() { this->fd = timerfd_create(CLOCK_MONOTONIC, 0); }
+  ~timerfd_timer() { close(this->fd); }
+
+  timerfd_timer(const timerfd_timer&) = delete;
+  timerfd_timer(timerfd_timer&&) = delete;
+  timerfd_timer& operator=(const timerfd_timer&) = delete;
+  timerfd_timer& operator=(timerfd_timer&&) = delete;
+
+  void oneshot(int64_t nsec)
+  {
+    itimerspec t{};
+    t.it_value.tv_nsec = nsec;
+    timerfd_settime(this->fd, 0, &t, nullptr);
+  }
+
+  void restart(int64_t nsec)
+  {
+    itimerspec t{};
+    t.it_value.tv_nsec = nsec;
+    t.it_interval.tv_nsec = nsec;
+    timerfd_settime(this->fd, 0, &t, nullptr);
+  }
+
+  void cancel()
+  {
+    itimerspec t{};
+    timerfd_settime(this->fd, 0, &t, nullptr);
+  }
+
+  operator int() const noexcept { return fd; }
+  operator pollfd() const noexcept { return {.fd = fd, .events = POLLIN, .revents = 0}; }
+  int fd{-1};
+};
+}

include/libremidi/backends/linux/pipewire.hpp

@@ -0,0 +1,169 @@
+#pragma once
+
+#include <libremidi/backends/linux/dylib_loader.hpp>
+
+#include <pipewire/pipewire.h>
+
+namespace libremidi
+{
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+class libpipewire
+{
+public:
+  decltype(&::pw_init) init{};
+  decltype(&::pw_deinit) deinit{};
+
+  decltype(&::pw_context_new) context_new{};
+  decltype(&::pw_context_connect) context_connect{};
+  decltype(&::pw_context_destroy) context_destroy{};
+
+  decltype(&::pw_core_disconnect) core_disconnect{};
+
+  decltype(&::pw_proxy_add_listener) proxy_add_listener{};
+  decltype(&::pw_proxy_destroy) proxy_destroy{};
+
+  decltype(&::pw_main_loop_new) main_loop_new{};
+  decltype(&::pw_main_loop_destroy) main_loop_destroy{};
+  decltype(&::pw_main_loop_quit) main_loop_quit{};
+  decltype(&::pw_main_loop_run) main_loop_run{};
+  decltype(&::pw_main_loop_get_loop) main_loop_get_loop{};
+  /*
+  decltype(&::pw_thread_loop_new) thread_loop_new{};
+  decltype(&::pw_thread_loop_destroy) thread_loop_destroy{};
+  decltype(&::pw_thread_loop_wait) thread_loop_wait{};
+  decltype(&::pw_thread_loop_start) thread_loop_start{};
+  decltype(&::pw_thread_loop_stop) thread_loop_stop;
+  decltype(&::pw_thread_loop_lock) thread_loop_lock{};
+  decltype(&::pw_thread_loop_unlock) thread_loop_unlock;
+  decltype(&::pw_thread_loop_get_loop) thread_loop_get_loop{};
+*/
+  decltype(&::pw_properties_new) properties_new{};
+  decltype(&::pw_properties_free) properties_free{};
+  decltype(&::pw_properties_get) properties_get{};
+
+  decltype(&::pw_filter_new_simple) filter_new_simple{};
+  decltype(&::pw_filter_get_node_id) filter_get_node_id{};
+  decltype(&::pw_filter_get_properties) filter_get_properties{};
+  decltype(&::pw_filter_add_port) filter_add_port{};
+  decltype(&::pw_filter_remove_port) filter_remove_port{};
+  decltype(&::pw_filter_update_properties) filter_update_properties{};
+  decltype(&::pw_filter_update_params) filter_update_params{};
+  decltype(&::pw_filter_get_time) filter_get_time{};
+  decltype(&::pw_filter_destroy) filter_destroy{};
+  decltype(&::pw_filter_connect) filter_connect{};
+  decltype(&::pw_filter_get_dsp_buffer) filter_get_dsp_buffer{};
+  decltype(&::pw_filter_queue_buffer) filter_queue_buffer{};
+  decltype(&::pw_filter_dequeue_buffer) filter_dequeue_buffer{};
+  decltype(&::pw_filter_flush) filter_flush{};
+
+  static const libpipewire& instance()
+  {
+    static const libpipewire self;
+    return self;
+  }
+
+  bool available{true};
+
+private:
+  dylib_loader library;
+
+  libpipewire()
+      : library("libpipewire-0.3.so.0")
+  {
+    if (!library)
+    {
+      available = false;
+      return;
+    }
+
+    // in terms of regex:
+    // decltype\(&::([a-z_]+)\) [a-z_]+{};
+    // \1 = library.symbol<decltype(&::\1)>("\1");
+    init = library.symbol<decltype(&::pw_init)>("pw_init");
+    deinit = library.symbol<decltype(&::pw_deinit)>("pw_deinit");
+
+    context_new = library.symbol<decltype(&::pw_context_new)>("pw_context_new");
+    context_connect = library.symbol<decltype(&::pw_context_connect)>("pw_context_connect");
+    context_destroy = library.symbol<decltype(&::pw_context_destroy)>("pw_context_destroy");
+
+    core_disconnect = library.symbol<decltype(&::pw_core_disconnect)>("pw_core_disconnect");
+
+    proxy_add_listener
+        = library.symbol<decltype(&::pw_proxy_add_listener)>("pw_proxy_add_listener");
+    proxy_destroy = library.symbol<decltype(&::pw_proxy_destroy)>("pw_proxy_destroy");
+
+    main_loop_new = library.symbol<decltype(&::pw_main_loop_new)>("pw_main_loop_new");
+    main_loop_destroy = library.symbol<decltype(&::pw_main_loop_destroy)>("pw_main_loop_destroy");
+    main_loop_quit = library.symbol<decltype(&::pw_main_loop_quit)>("pw_main_loop_quit");
+    main_loop_run = library.symbol<decltype(&::pw_main_loop_run)>("pw_main_loop_run");
+    main_loop_get_loop
+        = library.symbol<decltype(&::pw_main_loop_get_loop)>("pw_main_loop_get_loop");
+
+    properties_new = library.symbol<decltype(&::pw_properties_new)>("pw_properties_new");
+    properties_free = library.symbol<decltype(&::pw_properties_free)>("pw_properties_free");
+    properties_get = library.symbol<decltype(&::pw_properties_get)>("pw_properties_get");
+
+    filter_new_simple = library.symbol<decltype(&::pw_filter_new_simple)>("pw_filter_new_simple");
+    filter_get_node_id
+        = library.symbol<decltype(&::pw_filter_get_node_id)>("pw_filter_get_node_id");
+    filter_get_properties
+        = library.symbol<decltype(&::pw_filter_get_properties)>("pw_filter_get_properties");
+    filter_add_port = library.symbol<decltype(&::pw_filter_add_port)>("pw_filter_add_port");
+    filter_remove_port
+        = library.symbol<decltype(&::pw_filter_remove_port)>("pw_filter_remove_port");
+    filter_update_properties
+        = library.symbol<decltype(&::pw_filter_update_properties)>("pw_filter_update_properties");
+    filter_update_params
+        = library.symbol<decltype(&::pw_filter_update_params)>("pw_filter_update_params");
+    filter_get_time = library.symbol<decltype(&::pw_filter_get_time)>("pw_filter_get_time");
+    filter_destroy = library.symbol<decltype(&::pw_filter_destroy)>("pw_filter_destroy");
+    filter_connect = library.symbol<decltype(&::pw_filter_connect)>("pw_filter_connect");
+    filter_get_dsp_buffer
+        = library.symbol<decltype(&::pw_filter_get_dsp_buffer)>("pw_filter_get_dsp_buffer");
+    filter_dequeue_buffer
+        = library.symbol<decltype(&::pw_filter_dequeue_buffer)>("pw_filter_dequeue_buffer");
+    filter_queue_buffer
+        = library.symbol<decltype(&::pw_filter_queue_buffer)>("pw_filter_queue_buffer");
+    filter_flush = library.symbol<decltype(&::pw_filter_flush)>("pw_filter_flush");
+
+    assert(init);
+    assert(deinit);
+
+    assert(context_new);
+    assert(context_connect);
+    assert(context_destroy);
+
+    assert(core_disconnect);
+
+    assert(proxy_destroy);
+
+    assert(main_loop_new);
+    assert(main_loop_destroy);
+    assert(main_loop_quit);
+    assert(main_loop_run);
+    assert(main_loop_get_loop);
+
+    assert(properties_new);
+    assert(properties_free);
+    assert(properties_get);
+
+    assert(filter_new_simple);
+    assert(filter_get_node_id);
+    assert(filter_get_properties);
+    assert(filter_add_port);
+    assert(filter_remove_port);
+    assert(filter_update_properties);
+    assert(filter_update_params);
+    assert(filter_get_time);
+    assert(filter_destroy);
+    assert(filter_connect);
+    assert(filter_get_dsp_buffer);
+    assert(filter_dequeue_buffer);
+    assert(filter_queue_buffer);
+    assert(filter_flush);
+  }
+};
+#pragma GCC diagnostic pop
+}

include/libremidi/backends/linux/udev.hpp

@@ -0,0 +1,92 @@
+#pragma once
+#include <libremidi/backends/linux/dylib_loader.hpp>
+#include <libremidi/backends/linux/helpers.hpp>
+
+#include <libudev.h>
+
+#include <cassert>
+
+namespace libremidi
+{
+
+struct libudev
+{
+  // Useful one-liner:
+  // nm -A * | grep ' udev_'  | grep -v '@' | cut -f 2 -d 'U' | sort | uniq   | sed 's/ udev_//'| awk ' { print "LIBREMIDI_SYMBOL_DEF(udev, "$1");" }'
+
+  explicit libudev()
+      : library{"libudev.so.1"}
+  {
+    if (!library)
+    {
+      available = false;
+      return;
+    }
+
+    LIBREMIDI_SYMBOL_INIT(udev, device_get_action);
+    LIBREMIDI_SYMBOL_INIT(udev, device_get_subsystem);
+    LIBREMIDI_SYMBOL_INIT(udev, device_unref);
+    LIBREMIDI_SYMBOL_INIT(udev, monitor_enable_receiving);
+    LIBREMIDI_SYMBOL_INIT(udev, monitor_get_fd);
+    LIBREMIDI_SYMBOL_INIT(udev, monitor_new_from_netlink);
+    LIBREMIDI_SYMBOL_INIT(udev, monitor_receive_device);
+    LIBREMIDI_SYMBOL_INIT(udev, monitor_unref);
+    LIBREMIDI_SYMBOL_INIT2(udev, new, create);
+    LIBREMIDI_SYMBOL_INIT(udev, unref);
+  }
+
+  static const libudev& instance()
+  {
+    static const libudev self;
+    return self;
+  }
+
+  dylib_loader library;
+  bool available{true};
+
+  LIBREMIDI_SYMBOL_DEF(udev, device_get_action);
+  LIBREMIDI_SYMBOL_DEF(udev, device_get_subsystem);
+  LIBREMIDI_SYMBOL_DEF(udev, device_unref);
+  LIBREMIDI_SYMBOL_DEF(udev, monitor_enable_receiving);
+  LIBREMIDI_SYMBOL_DEF(udev, monitor_get_fd);
+  LIBREMIDI_SYMBOL_DEF(udev, monitor_new_from_netlink);
+  LIBREMIDI_SYMBOL_DEF(udev, monitor_receive_device);
+  LIBREMIDI_SYMBOL_DEF(udev, monitor_unref);
+  LIBREMIDI_SYMBOL_DEF2(udev, new, create);
+  LIBREMIDI_SYMBOL_DEF(udev, unref);
+};
+
+struct udev_helper
+{
+  udev_helper()
+  {
+    instance = udev.create();
+    assert(instance);
+
+    monitor = udev.monitor_new_from_netlink(instance, "udev");
+    assert(monitor);
+    udev.monitor_enable_receiving(monitor);
+  }
+
+  ~udev_helper()
+  {
+    udev.monitor_unref(monitor);
+    udev.unref(instance);
+  }
+
+  udev_helper(const udev_helper&) = delete;
+  udev_helper(udev_helper&&) = delete;
+  udev_helper& operator=(const udev_helper&) = delete;
+  udev_helper& operator=(udev_helper&&) = delete;
+
+  operator pollfd() const noexcept
+  {
+    return {.fd = udev.monitor_get_fd(monitor), .events = POLLIN, .revents = 0};
+  }
+
+  const libudev& udev = libudev::instance();
+  struct udev* instance{};
+  udev_monitor* monitor{};
+};
+
+}

include/libremidi/backends/pipewire.hpp

@@ -0,0 +1,30 @@
+#pragma once
+#include <libremidi/backends/dummy.hpp>
+#include <libremidi/backends/linux/pipewire.hpp>
+#include <libremidi/backends/pipewire/config.hpp>
+#include <libremidi/backends/pipewire/helpers.hpp>
+#include <libremidi/backends/pipewire/midi_in.hpp>
+#include <libremidi/backends/pipewire/midi_out.hpp>
+#include <libremidi/backends/pipewire/observer.hpp>
+
+namespace libremidi::pipewire
+{
+struct backend
+{
+  using midi_in = midi_in_pipewire;
+  using midi_out = midi_out_pipewire;
+  using midi_observer = observer_pipewire;
+  using midi_in_configuration = pipewire_input_configuration;
+  using midi_out_configuration = pipewire_output_configuration;
+  using midi_observer_configuration = pipewire_observer_configuration;
+  static const constexpr auto API = libremidi::API::PIPEWIRE;
+  static const constexpr auto name = "pipewire";
+  static const constexpr auto display_name = "PipeWire";
+
+  static inline bool available() noexcept
+  {
+    static const libpipewire& pw = libpipewire::instance();
+    return pw.available;
+  }
+};
+}

include/libremidi/backends/pipewire/config.hpp

@@ -0,0 +1,53 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <cinttypes>
+#include <cstdint>
+#include <functional>
+#include <string>
+
+extern "C" {
+struct pw_main_loop;
+struct pw_filter;
+struct spa_io_position;
+}
+
+namespace libremidi
+{
+using pipewire_callback_function = std::function<void(spa_io_position*)>;
+struct pipewire_callback
+{
+  int64_t token;
+  pipewire_callback_function callback;
+};
+
+struct pipewire_input_configuration
+{
+  std::string client_name = "libremidi client";
+
+  pw_main_loop* context{};
+  pw_filter* filter{};
+  std::function<void(pipewire_callback)> set_process_func;
+  std::function<void(int64_t)> clear_process_func;
+};
+
+struct pipewire_output_configuration
+{
+  std::string client_name = "libremidi client";
+
+  pw_main_loop* context{};
+  pw_filter* filter{};
+  std::function<void(pipewire_callback)> set_process_func;
+  std::function<void(int64_t)> clear_process_func;
+
+  int64_t output_buffer_size{65536};
+};
+
+struct pipewire_observer_configuration
+{
+  std::string client_name = "libremidi client";
+
+  pw_main_loop* context{};
+};
+
+}

include/libremidi/backends/pipewire/context.hpp

@@ -0,0 +1,600 @@
+#pragma once
+#include <libremidi/backends/linux/pipewire.hpp>
+#include <libremidi/detail/memory.hpp>
+
+#include <pipewire/filter.h>
+#include <pipewire/pipewire.h>
+#include <spa/control/control.h>
+#include <spa/param/props.h>
+#include <spa/utils/defs.h>
+#include <spa/utils/result.h>
+
+#include <algorithm>
+#include <array>
+#include <atomic>
+#include <functional>
+#include <iostream>
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include <spa/param/audio/format-utils.h>
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-field-initializers"
+namespace libremidi
+{
+template <typename K, typename V>
+using hash_map = std::unordered_map<K, V>;
+
+struct pipewire_instance
+{
+  const libpipewire& pw = libpipewire::instance();
+  pipewire_instance()
+  {
+    /// Initialize the PipeWire main loop, context, etc.
+    int argc = 0;
+    char* argv[] = {NULL};
+    char** aa = argv;
+    pw.init(&argc, &aa);
+  }
+
+  ~pipewire_instance() { pw.deinit(); }
+};
+
+struct pipewire_context
+{
+  struct listened_port
+  {
+    uint32_t id{};
+    pw_port* port{};
+    std::unique_ptr<spa_hook> listener;
+  };
+
+  struct port_info
+  {
+    uint32_t id{};
+
+    std::string format;
+    std::string port_name;
+    std::string port_alias;
+    std::string object_path;
+    std::string node_id;
+    std::string port_id;
+
+    bool physical{};
+    bool terminal{};
+    bool monitor{};
+    pw_direction direction{};
+  };
+
+  struct node
+  {
+    std::vector<port_info> inputs;
+    std::vector<port_info> outputs;
+  };
+
+  struct graph
+  {
+    mutable std::mutex mtx;
+    libremidi::hash_map<uint32_t, node> physical_audio;
+    libremidi::hash_map<uint32_t, node> physical_midi;
+    libremidi::hash_map<uint32_t, node> software_audio;
+    libremidi::hash_map<uint32_t, node> software_midi;
+    libremidi::hash_map<uint32_t, port_info> port_cache;
+
+    void for_each_port(auto func)
+    {
+      for (auto& map : {physical_audio, physical_midi, software_audio, software_midi})
+      {
+        for (auto& [id, node] : map)
+        {
+          for (auto& port : node.inputs)
+            func(port);
+          for (auto& port : node.outputs)
+            func(port);
+        }
+      }
+    }
+
+    void remove_port(uint32_t id)
+    {
+      port_cache.erase(id);
+      for (auto map : {&physical_audio, &physical_midi, &software_audio, &software_midi})
+      {
+        for (auto& [_, node] : *map)
+        {
+          std::erase_if(node.inputs, [id](const port_info& p) { return p.id == id; });
+          std::erase_if(node.outputs, [id](const port_info& p) { return p.id == id; });
+        }
+      }
+    }
+  } current_graph;
+
+  explicit pipewire_context(pw_main_loop* inst)
+      : main_loop{inst}
+      , owns_main_loop{false}
+  {
+    assert(main_loop);
+
+    initialize();
+  }
+
+  explicit pipewire_context(std::shared_ptr<pipewire_instance> inst)
+      : global_instance{inst}
+      , owns_main_loop{true}
+  {
+    this->main_loop = pw.main_loop_new(nullptr);
+    if (!this->main_loop)
+    {
+      // libremidi::logger().error("PipeWire: main_loop_new failed!");
+      return;
+    }
+    initialize();
+  }
+
+  void initialize()
+  {
+    this->lp = pw.main_loop_get_loop(this->main_loop);
+    if (!lp)
+    {
+      // libremidi::logger().error("PipeWire: main_loop_get_loop failed!");
+      return;
+    }
+
+    this->context = pw.context_new(lp, nullptr, 0);
+    if (!this->context)
+    {
+      // libremidi::logger().error("PipeWire: context_new failed!");
+      return;
+    }
+
+    this->core = pw.context_connect(this->context, nullptr, 0);
+    if (!this->core)
+    {
+      // libremidi::logger().error("PipeWire: context_connect failed!");
+      return;
+    }
+
+    this->registry = pw_core_get_registry(this->core, PW_VERSION_REGISTRY, 0);
+    if (!this->registry)
+    {
+      // libremidi::logger().error("PipeWire: core_get_registry failed!");
+      return;
+    }
+
+    initialize_observation();
+
+    synchronize();
+
+    // Add a manual 1ms event loop iteration at the end of
+    // ctor to ensure synchronous clients will still see the ports
+    pw_loop_iterate(this->lp, 1);
+  }
+
+  void initialize_observation()
+  {
+    // Register a listener which will listen on when ports are added / removed
+    spa_zero(registry_listener);
+
+    static constexpr const struct pw_registry_events registry_events = {
+        .version = PW_VERSION_REGISTRY_EVENTS,
+        .global =
+            [](void* object, uint32_t id, uint32_t /*permissions*/, const char* type,
+               uint32_t /*version*/, const struct spa_dict* /*props*/) {
+      pipewire_context& self = *(pipewire_context*)object;
+      if (strcmp(type, PW_TYPE_INTERFACE_Port) == 0)
+        self.register_port(id, type);
+        },
+        .global_remove =
+            [](void* object, uint32_t id) {
+      pipewire_context& self = *(pipewire_context*)object;
+      self.unregister_port(id);
+        },
+        };
+
+    // Start listening
+    pw_registry_add_listener(this->registry, &this->registry_listener, &registry_events, this);
+  }
+
+  void register_port(uint32_t id, const char* type)
+  {
+    auto port = (pw_port*)pw_registry_bind(registry, id, type, PW_VERSION_PORT, 0);
+    port_listener.push_back({id, port, std::make_unique<spa_hook>()});
+    auto& l = port_listener.back();
+
+    static constexpr const struct pw_port_events port_events = {
+        .version = PW_VERSION_PORT_EVENTS,
+        .info
+        = [](void* object,
+             const pw_port_info* info) { ((pipewire_context*)object)->update_port_info(info); },
+    };
+    pw_port_add_listener(l.port, l.listener.get(), &port_events, this);
+  }
+
+  void unregister_port(uint32_t id)
+  {
+    // When a port is removed:
+    // Notify
+    std::unique_lock _{current_graph.mtx, std::defer_lock};
+    if (on_port_removed)
+    {
+      _.lock();
+      if (auto it = current_graph.port_cache.find(id); it != current_graph.port_cache.end())
+      {
+        auto copy = it->second;
+        _.unlock();
+        on_port_removed(copy);
+      }
+      else
+      {
+        _.unlock();
+      }
+    }
+
+    // Remove from the graph
+    {
+      _.lock();
+      current_graph.remove_port(id);
+      _.unlock();
+    }
+
+    // Remove from the listeners
+    auto it
+        = std::find_if(port_listener.begin(), port_listener.end(), [&](const listened_port& l) {
+            return l.id == id;
+          });
+    if (it != port_listener.end())
+    {
+      pw.proxy_destroy((pw_proxy*)it->port);
+      port_listener.erase(it);
+    }
+  }
+
+  void synchronize()
+  {
+    pending = 0;
+    done = 0;
+
+    if (!core)
+      return;
+
+    spa_hook core_listener;
+
+    static constexpr struct pw_core_events core_events = {
+        .version = PW_VERSION_CORE_EVENTS,
+        .done =
+        [](void* object, uint32_t id, int seq) {
+      auto& self = *(pipewire_context*)object;
+      if(id == PW_ID_CORE && seq == self.pending)
+      {
+        self.done = 1;
+        libpipewire::instance().main_loop_quit(self.main_loop);
+      }
+        },
+    };
+
+    spa_zero(core_listener);
+    pw_core_add_listener(core, &core_listener, &core_events, this);
+
+    pending = pw_core_sync(core, PW_ID_CORE, 0);
+    while (!done)
+    {
+      pw.main_loop_run(this->main_loop);
+    }
+    spa_hook_remove(&core_listener);
+  }
+
+  [[nodiscard]] pw_proxy* link_ports(uint32_t out_port, uint32_t in_port)
+  {
+    auto props = pw.properties_new(
+        PW_KEY_LINK_OUTPUT_PORT, std::to_string(out_port).c_str(), PW_KEY_LINK_INPUT_PORT,
+        std::to_string(in_port).c_str(), nullptr);
+
+    auto proxy = (pw_proxy*)pw_core_create_object(
+        this->core, "link-factory", PW_TYPE_INTERFACE_Link, PW_VERSION_LINK, &props->dict, 0);
+
+    if (!proxy)
+    {
+      std::cerr << "PipeWire: could not allocate link\n";
+      pw.properties_free(props);
+      return nullptr;
+    }
+
+    synchronize();
+    pw.properties_free(props);
+    return proxy;
+  }
+
+  void unlink_ports(pw_proxy* link) { pw.proxy_destroy(link); }
+
+  void update_port_info(const pw_port_info* info)
+  {
+    const spa_dict_item* item{};
+
+    port_info p;
+    p.id = info->id;
+
+    spa_dict_for_each(item, info->props)
+    {
+      std::string_view k{item->key}, v{item->value};
+      if (k == "format.dsp")
+        p.format = v;
+      else if (k == "port.name")
+        p.port_name = v;
+      else if (k == "port.alias")
+        p.port_alias = v;
+      else if (k == "object.path")
+        p.object_path = v;
+      else if (k == "port.id")
+        p.port_id = v;
+      else if (k == "node.id")
+        p.node_id = v;
+      else if (k == "port.physical" && v == "true")
+        p.physical = true;
+      else if (k == "port.terminal" && v == "true")
+        p.terminal = true;
+      else if (k == "port.monitor" && v == "true")
+        p.monitor = true;
+      else if (k == "port.direction")
+      {
+        if (v == "out")
+        {
+          p.direction = pw_direction::SPA_DIRECTION_OUTPUT;
+        }
+        else
+        {
+          p.direction = pw_direction::SPA_DIRECTION_INPUT;
+        }
+      }
+    }
+
+    if (p.node_id.empty())
+      return;
+
+    const auto nid = std::stoul(p.node_id);
+    auto get_node = [&]() -> node* {
+      if (p.physical)
+      {
+        if (p.format.find("audio") != p.format.npos)
+          return &this->current_graph.physical_audio[nid];
+        else if (p.format.find("midi") != p.format.npos)
+          return &this->current_graph.physical_midi[nid];
+      }
+      else
+      {
+        if (p.format.find("audio") != p.format.npos)
+          return &this->current_graph.software_audio[nid];
+        else if (p.format.find("midi") != p.format.npos)
+          return &this->current_graph.software_midi[nid];
+      }
+      return nullptr;
+    };
+
+    {
+      std::lock_guard _{current_graph.mtx};
+      current_graph.port_cache[p.id] = p;
+      if (auto node = get_node())
+      {
+        if (p.direction == pw_direction::SPA_DIRECTION_OUTPUT)
+          node->outputs.push_back(p);
+        else
+          node->inputs.push_back(p);
+      }
+    }
+
+    if (on_port_added)
+      on_port_added(p);
+  }
+
+  int get_fd() const noexcept
+  {
+    if (!this->lp)
+      return -1;
+
+    auto spa_callbacks = this->lp->control->iface.cb;
+    auto spa_loop_methods = (const spa_loop_control_methods*)spa_callbacks.funcs;
+    if (spa_loop_methods->get_fd)
+      return spa_loop_methods->get_fd(spa_callbacks.data);
+    else
+      return -1;
+  }
+
+  ~pipewire_context()
+  {
+    if (this->registry)
+      pw.proxy_destroy((pw_proxy*)this->registry);
+    for (auto& [id, p, l] : this->port_listener)
+      if (l)
+        pw.proxy_destroy((pw_proxy*)p);
+    if (this->core)
+      pw.core_disconnect(this->core);
+    if (this->context)
+      pw.context_destroy(this->context);
+    if (owns_main_loop && this->main_loop)
+      pw.main_loop_destroy(this->main_loop);
+  }
+
+  friend struct pipewire_filter;
+  const libpipewire& pw = libpipewire::instance();
+  std::shared_ptr<pipewire_instance> global_instance;
+
+  pw_main_loop* main_loop{};
+  pw_loop* lp{};
+
+  pw_context* context{};
+  pw_core* core{};
+
+  pw_registry* registry{};
+  spa_hook registry_listener{};
+
+  std::function<void(const port_info&)> on_port_added;
+  std::function<void(const port_info&)> on_port_removed;
+
+  std::vector<listened_port> port_listener{};
+
+  std::atomic<int> pending{};
+  std::atomic<int> done{};
+  bool owns_main_loop{true};
+  int sync{};
+};
+
+struct pipewire_filter
+{
+  const libpipewire& pw = libpipewire::instance();
+  std::shared_ptr<pipewire_context> loop{};
+  pw_filter* filter{};
+  std::vector<pw_proxy*> links{};
+
+  struct port
+  {
+    void* data;
+  }* port{};
+
+  explicit pipewire_filter(std::shared_ptr<pipewire_context> loop)
+      : loop{loop}
+  {
+  }
+
+  explicit pipewire_filter(std::shared_ptr<pipewire_context> loop, pw_filter* filter)
+      : loop{loop}
+      , filter{filter}
+  {
+  }
+
+  void create_filter(std::string_view filter_name, const pw_filter_events& events, void* context)
+  {
+    assert(!filter);
+
+    auto& pw = libpipewire::instance();
+    // clang-format off
+    this->filter = pw.filter_new_simple(
+        loop->lp,
+        filter_name.data(),
+        pw.properties_new(
+            PW_KEY_MEDIA_TYPE, "Midi",
+            PW_KEY_MEDIA_CATEGORY, "Filter",
+            PW_KEY_MEDIA_ROLE, "DSP",
+            PW_KEY_MEDIA_NAME, "libremidi",
+#if defined(PW_KEY_NODE_LOCK_RATE)
+            PW_KEY_NODE_LOCK_RATE, "true",
+#endif
+            PW_KEY_NODE_ALWAYS_PROCESS, "true",
+            PW_KEY_NODE_PAUSE_ON_IDLE, "false",
+#if defined(PW_KEY_NODE_SUSPEND_ON_IDLE)
+            PW_KEY_NODE_SUSPEND_ON_IDLE, "false",
+#endif
+            nullptr),
+        &events,
+        context);
+    // clang-format on
+    assert(filter);
+  }
+
+  void destroy()
+  {
+    if (this->filter)
+      pw.filter_destroy(this->filter);
+  }
+
+  void create_local_port(std::string_view port_name, spa_direction direction)
+  {
+    // clang-format off
+    this->port = (struct port*)pw.filter_add_port(
+        this->filter,
+        direction,
+        PW_FILTER_PORT_FLAG_MAP_BUFFERS,
+        sizeof(struct port),
+        pw.properties_new(
+            PW_KEY_FORMAT_DSP, "8 bit raw midi",
+            PW_KEY_PORT_NAME, port_name.data(),
+            nullptr),
+        nullptr, 0);
+    // clang-format on
+    assert(port);
+  }
+
+  void set_port_buffer(int bytes)
+  {
+    uint8_t buffer[1024];
+    struct spa_pod_builder builder;
+    spa_pod_builder_init(&builder, buffer, sizeof(buffer));
+
+    // clang-format off
+    const struct spa_pod* params[1] = {
+      (spa_pod*) spa_pod_builder_add_object(
+        &builder,
+        SPA_TYPE_OBJECT_ParamBuffers, SPA_PARAM_Buffers,
+        SPA_PARAM_BUFFERS_buffers, SPA_POD_CHOICE_RANGE_Int(1, 1, 32),
+        SPA_PARAM_BUFFERS_blocks, SPA_POD_Int(1),
+        SPA_PARAM_BUFFERS_size, SPA_POD_CHOICE_RANGE_Int(bytes, 4096, INT32_MAX),
+        SPA_PARAM_BUFFERS_stride, SPA_POD_Int(1)
+      )
+    };
+    // clang-format on
+
+    pw.filter_update_params(this->filter, this->port, params, 1);
+  }
+
+  void remove_port()
+  {
+    assert(this->port);
+    pw.filter_remove_port(this->port);
+    this->port = nullptr;
+  }
+
+  void rename_port(std::string_view port_name)
+  {
+    assert(this->port);
+    spa_dict_item items[1] = {
+        SPA_DICT_ITEM_INIT(PW_KEY_PORT_NAME, port_name.data()),
+    };
+
+    auto properties = SPA_DICT_INIT(items, 1);
+    pw.filter_update_properties(this->filter, this->port, &properties);
+    this->port = nullptr;
+  }
+
+  void start_filter()
+  {
+    if (pw.filter_connect(this->filter, PW_FILTER_FLAG_RT_PROCESS, NULL, 0) < 0)
+    {
+      std::cerr << "can't connect\n";
+      return;
+    }
+  }
+
+  uint32_t filter_node_id() { return this->loop->pw.filter_get_node_id(this->filter); }
+
+  void synchronize_node()
+  {
+    this->loop->synchronize();
+    int k = 0;
+    auto node_id = filter_node_id();
+    while (node_id == 4294967295)
+    {
+      this->loop->synchronize();
+      node_id = filter_node_id();
+
+      if (k++; k > 100)
+        return;
+    }
+  }
+  void synchronize_ports(const pipewire_context::node& this_node)
+  {
+    // Leave some time to resolve the ports
+    int k = 0;
+    const auto num_local_ins = 1;
+    const auto num_local_outs = 0;
+    while (this_node.inputs.size() < num_local_ins || this_node.outputs.size() < num_local_outs)
+    {
+      this->loop->synchronize();
+      if (k++; k > 100)
+        return;
+    }
+  }
+};
+}
+
+#pragma GCC diagnostic pop

include/libremidi/backends/pipewire/helpers.hpp

@@ -0,0 +1,464 @@
+#pragma once
+
+#include <libremidi/backends/linux/helpers.hpp>
+#include <libremidi/backends/linux/pipewire.hpp>
+#include <libremidi/backends/pipewire/context.hpp>
+#include <libremidi/detail/memory.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/semaphore.hpp>
+
+#include <atomic>
+#include <semaphore>
+#include <stop_token>
+#include <thread>
+
+namespace libremidi
+{
+struct pipewire_helpers
+{
+  struct port
+  {
+    void* data{};
+  };
+
+  // All pipewire operations have to happen in the same thread
+  // - and pipewire checks that internally.
+  std::jthread main_loop_thread;
+  const libpipewire& pw = libpipewire::instance();
+  std::shared_ptr<pipewire_instance> global_instance;
+  std::shared_ptr<pipewire_context> global_context;
+  std::unique_ptr<pipewire_filter> filter;
+  pw_proxy* link{};
+
+  int64_t this_instance{};
+
+  eventfd_notifier termination_event{};
+  pollfd fds[2]{};
+
+  semaphore_pair_lock thread_lock;
+  std::shared_ptr<void> canary = std::make_shared<int>();
+
+  enum poll_state
+  {
+    start_poll,
+    in_poll,
+    not_in_poll
+  };
+  std::atomic<poll_state> current_state{not_in_poll};
+
+  pipewire_helpers()
+  {
+    static std::atomic_int64_t instance{};
+    this_instance = ++instance;
+
+    fds[1] = termination_event;
+  }
+
+  template <typename Self>
+  void create_filter(Self& self)
+  {
+    if (this->filter)
+      return;
+
+    auto& configuration = self.configuration;
+    if (configuration.context && configuration.filter && configuration.set_process_func)
+    {
+      this->filter = std::make_unique<pipewire_filter>(this->global_context, configuration.filter);
+
+      pipewire_callback cbs{
+          .token = this_instance,
+          .callback = [&self, p = std::weak_ptr{canary}](spa_io_position* nf) -> void {
+            if (auto pt = p.lock())
+              self.process(nf);
+
+            self.thread_lock.check_client_released();
+          }};
+      configuration.set_process_func(cbs);
+    }
+    else
+    {
+      this->filter = std::make_unique<pipewire_filter>(this->global_context);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wmissing-field-initializers"
+      static constexpr struct pw_filter_events filter_events
+          = {.version = PW_VERSION_FILTER_EVENTS,
+             .process = +[](void* _data, struct spa_io_position* position) -> void {
+               // FIXME likely we need the thread_lock check here too
+               Self& self = *static_cast<Self*>(_data);
+               self.process(position);
+             }};
+#pragma GCC diagnostic pop
+
+      this->filter->create_filter(self.configuration.client_name, filter_events, &self);
+      this->filter->start_filter();
+    }
+  }
+
+  template <typename Self>
+  void destroy_filter(Self& self)
+  {
+    assert(global_context);
+    if (!global_context->owns_main_loop)
+    {
+      if (self.configuration.clear_process_func)
+      {
+        self.configuration.clear_process_func(this_instance);
+      }
+    }
+    else
+    {
+      if (this->filter)
+      {
+        this->filter->destroy();
+      }
+    }
+
+    this->filter.reset();
+  }
+
+  template <typename Self>
+  int create_context(Self& self)
+  {
+    if (this->global_context)
+      return 0;
+
+    // Initialize PipeWire client
+    auto& configuration = self.configuration;
+    if (configuration.context)
+    {
+      this->global_context = std::make_shared<pipewire_context>(configuration.context);
+    }
+    else
+    {
+      this->global_instance = std::make_shared<pipewire_instance>();
+      this->global_context = std::make_shared<pipewire_context>(this->global_instance);
+    }
+    return 0;
+  }
+
+  void destroy_context()
+  {
+    assert(this->global_context);
+    this->global_context.reset();
+    this->global_instance.reset();
+  }
+
+  void run_poll_loop()
+  try
+  {
+    // Note: called from a std::jthread.
+    assert(this->global_context);
+    if (int fd = this->global_context->get_fd(); fd != -1)
+    {
+      fds[0] = {.fd = fd, .events = POLLIN, .revents = 0};
+      current_state = poll_state::in_poll;
+
+      for (;;)
+      {
+        if (int err = poll(fds, 2, -1); err < 0)
+        {
+          if (err == -EAGAIN)
+            continue;
+          else
+            break;
+        }
+
+        // Check pipewire fd:
+        if (fds[0].revents & POLLIN)
+        {
+          if (auto lp = this->global_context->lp)
+          {
+            int result = pw_loop_iterate(lp, 0);
+            if (result < 0)
+              std::cerr << "pw_loop_iterate: " << spa_strerror(result) << "\n";
+          }
+          fds[0].revents = 0;
+        }
+
+        // Check exit fd:
+        if (fds[1].revents & POLLIN)
+        {
+          break;
+        }
+      }
+    }
+    current_state = poll_state::not_in_poll;
+  }
+  catch (...)
+  {
+    current_state = poll_state::not_in_poll;
+  }
+
+  template <typename Self>
+  bool create_local_port(Self& self, std::string_view portName, spa_direction direction)
+  {
+    assert(this->global_context);
+    assert(this->filter);
+
+    if (portName.empty())
+      portName = direction == SPA_DIRECTION_INPUT ? "i" : "o";
+
+    if (!this->filter->port)
+    {
+      this->filter->create_local_port(portName.data(), direction);
+    }
+
+    if (!this->filter->port)
+    {
+      self.template error<driver_error>(self.configuration, "PipeWire: error creating port");
+      return false;
+    }
+    return true;
+  }
+
+  void add_callbacks(const observer_configuration& conf)
+  {
+    assert(global_context);
+    global_context->on_port_added = [&conf](const pipewire_context::port_info& port) {
+      if (port.format.find("midi") == std::string::npos)
+        return;
+
+      bool unfiltered = conf.track_any;
+      unfiltered |= (port.physical && conf.track_hardware);
+      unfiltered |= (!port.physical && conf.track_virtual);
+      if (unfiltered)
+      {
+        if (port.direction == SPA_DIRECTION_INPUT)
+        {
+          if (conf.output_added)
+            conf.output_added(to_port_info<SPA_DIRECTION_INPUT>(port));
+        }
+        else
+        {
+          if (conf.input_added)
+            conf.input_added(to_port_info<SPA_DIRECTION_OUTPUT>(port));
+        }
+      }
+    };
+
+    global_context->on_port_removed = [&conf](const pipewire_context::port_info& port) {
+      if (port.format.find("midi") == std::string::npos)
+        return;
+
+      bool unfiltered = conf.track_any;
+      unfiltered |= (port.physical && conf.track_hardware);
+      unfiltered |= (!port.physical && conf.track_virtual);
+      if (unfiltered)
+      {
+        if (port.direction == SPA_DIRECTION_INPUT)
+        {
+          if (conf.output_removed)
+            conf.output_removed(to_port_info<SPA_DIRECTION_INPUT>(port));
+        }
+        else
+        {
+          if (conf.input_removed)
+            conf.input_removed(to_port_info<SPA_DIRECTION_OUTPUT>(port));
+        }
+      }
+    };
+  }
+
+  void start_thread()
+  {
+    if (!this->global_context->owns_main_loop)
+      return;
+
+    current_state = poll_state::start_poll;
+    main_loop_thread = std::jthread{[this]() { run_poll_loop(); }};
+  }
+
+  void stop_thread()
+  {
+    assert(this->global_context);
+    if (!this->global_context->owns_main_loop)
+      return;
+
+    if (main_loop_thread.joinable() || current_state != poll_state::not_in_poll)
+    {
+      termination_event.notify();
+      main_loop_thread.request_stop();
+
+      termination_event.notify();
+      for (int i = 0; i < 100; i++)
+      {
+        if (current_state == poll_state::not_in_poll)
+          break;
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
+        termination_event.notify();
+      }
+
+      if (main_loop_thread.joinable())
+        main_loop_thread.join();
+    }
+  }
+
+  void do_close_port()
+  {
+    if (!this->filter)
+      return;
+    if (!this->filter->port)
+      return;
+
+    if (!this->global_context->owns_main_loop)
+    {
+      this->canary.reset();
+      this->thread_lock.prepare_release_client();
+    }
+
+    unlink_ports();
+    this->filter->remove_port();
+  }
+
+  void rename_port(std::string_view port_name)
+  {
+    if (this->filter)
+      this->filter->rename_port(port_name);
+  }
+
+  void unlink_ports()
+  {
+    if (link)
+    {
+      this->global_context->unlink_ports(link);
+      link = nullptr;
+    }
+  }
+
+  bool link_ports(auto& self, const input_port& in_port)
+  {
+    // Wait for the pipewire server to send us back our node's info
+    for (int i = 0; i < 1000; i++)
+      this->filter->synchronize_node();
+
+    auto this_node = this->filter->filter_node_id();
+    auto& midi = this->global_context->current_graph.software_midi;
+    auto node_it = midi.find(this_node);
+    if (node_it == midi.end())
+    {
+      std::cerr << "Node " << this_node << " not found! \n";
+      return false;
+    }
+
+    // Wait for the pipewire server to send us back our node's ports
+    this->filter->synchronize_ports(node_it->second);
+
+    if (node_it->second.inputs.empty())
+    {
+      std::cerr << "Node " << this_node << " has no ports! \n";
+      return false;
+    }
+
+    // Link ports
+    const auto& p = node_it->second.inputs.front();
+    link = this->global_context->link_ports(in_port.port, p.id);
+    pw_loop_iterate(this->global_context->lp, 1);
+    if (!link)
+    {
+      self.template error<invalid_parameter_error>(
+          self.configuration,
+          "PipeWire: could not connect to port: " + in_port.port_name + " -> " + p.port_name);
+      return false;
+    }
+
+    return true;
+  }
+
+  bool link_ports(auto& self, const output_port& out_port)
+  {
+    // Wait for the pipewire server to send us back our node's info
+    for (int i = 0; i < 1000; i++)
+      this->filter->synchronize_node();
+
+    auto this_node = this->filter->filter_node_id();
+    auto& midi = this->global_context->current_graph.software_midi;
+    auto node_it = midi.find(this_node);
+    if (node_it == midi.end())
+    {
+      std::cerr << "Node " << this_node << " not found! \n";
+      return false;
+    }
+
+    // Wait for the pipewire server to send us back our node's ports
+    this->filter->synchronize_ports(node_it->second);
+
+    if (node_it->second.outputs.empty())
+    {
+      std::cerr << "Node " << this_node << " has no ports! \n";
+      return false;
+    }
+
+    // Link ports
+    const auto& p = node_it->second.outputs.front();
+    link = this->global_context->link_ports(p.id, out_port.port);
+    pw_loop_iterate(this->global_context->lp, 1);
+    if (!link)
+    {
+      self.template error<invalid_parameter_error>(
+          self.configuration,
+          "PipeWire: could not connect to port: " + p.port_name + " -> " + out_port.port_name);
+      return false;
+    }
+
+    return true;
+  }
+
+  template <spa_direction Direction>
+  static auto to_port_info(const pipewire_context::port_info& port)
+      -> std::conditional_t<Direction == SPA_DIRECTION_OUTPUT, input_port, output_port>
+  {
+    std::string device_name, port_name;
+    auto name_colon = port.port_alias.find(':');
+    if (name_colon != std::string::npos)
+    {
+      device_name = port.port_alias.substr(0, name_colon);
+      port_name = port.port_alias.substr(name_colon + 1);
+    }
+    else
+    {
+      port_name = port.port_alias;
+    }
+
+    return {{
+        .client = 0,
+        .port = port.id,
+        .manufacturer = "",
+        .device_name = device_name,
+        .port_name = port.port_name,
+        .display_name = port_name,
+    }};
+  }
+
+  // Note: keep in mind that an "input" port for us (e.g. a keyboard that goes to the computer)
+  // is an "output" port from the point of view of pipewire as data will come out of it
+  template <spa_direction Direction>
+  static auto get_ports(const pipewire_context& ctx) noexcept -> std::vector<
+      std::conditional_t<Direction == SPA_DIRECTION_OUTPUT, input_port, output_port>>
+  {
+    std::vector<std::conditional_t<Direction == SPA_DIRECTION_OUTPUT, input_port, output_port>>
+        ret;
+
+    {
+      std::lock_guard _{ctx.current_graph.mtx};
+      for (auto& node : ctx.current_graph.physical_midi)
+      {
+        for (auto& p :
+             (Direction == SPA_DIRECTION_INPUT ? node.second.inputs : node.second.outputs))
+        {
+          ret.push_back(to_port_info<Direction>(p));
+        }
+      }
+      for (auto& node : ctx.current_graph.software_midi)
+      {
+        for (auto& p :
+             (Direction == SPA_DIRECTION_INPUT ? node.second.inputs : node.second.outputs))
+        {
+          ret.push_back(to_port_info<Direction>(p));
+        }
+      }
+    }
+
+    return ret;
+  }
+};
+}

include/libremidi/backends/pipewire/midi_in.hpp

@@ -0,0 +1,125 @@
+#pragma once
+#include <libremidi/backends/pipewire/config.hpp>
+#include <libremidi/backends/pipewire/helpers.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+#include <chrono>
+
+namespace libremidi
+{
+class midi_in_pipewire final
+    : public midi1::in_api
+    , public pipewire_helpers
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , pipewire_input_configuration
+  {
+  } configuration;
+
+  explicit midi_in_pipewire(input_configuration&& conf, pipewire_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    create_context(*this);
+    create_filter(*this);
+  }
+
+  ~midi_in_pipewire() override
+  {
+    stop_thread();
+    do_close_port();
+    destroy_filter(*this);
+    destroy_context();
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_pipewire: set_client_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::PIPEWIRE; }
+
+  bool open_port(const input_port& in_port, std::string_view name) override
+  {
+    if (!create_local_port(*this, name, SPA_DIRECTION_INPUT))
+      return false;
+
+    if (!link_ports(*this, in_port))
+      return false;
+
+    start_thread();
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view name) override
+  {
+    if (!create_local_port(*this, name, SPA_DIRECTION_INPUT))
+      return false;
+
+    start_thread();
+    return true;
+  }
+
+  void close_port() override
+  {
+    stop_thread();
+    do_close_port();
+  }
+
+  void set_port_name(std::string_view port_name) override { rename_port(port_name); }
+
+  timestamp absolute_timestamp() const noexcept override { return system_ns(); }
+
+  void process(struct spa_io_position* position)
+  {
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = true,
+        .has_samples = true,
+    };
+
+    assert(this->filter);
+    assert(this->filter->port);
+    const auto b = pw.filter_dequeue_buffer(this->filter->port);
+    if (!b)
+      return;
+
+    const auto buf = b->buffer;
+    const auto d = &buf->datas[0];
+
+    if (d->data == nullptr)
+      return;
+
+    const auto pod
+        = (spa_pod*)spa_pod_from_data(d->data, d->maxsize, d->chunk->offset, d->chunk->size);
+    if (!pod)
+      return;
+    if (!spa_pod_is_sequence(pod))
+      return;
+
+    struct spa_pod_control* c{};
+    SPA_POD_SEQUENCE_FOREACH((struct spa_pod_sequence*)pod, c)
+    {
+      if (c->type != SPA_CONTROL_Midi)
+        continue;
+
+      auto data = (uint8_t*)SPA_POD_BODY(&c->value);
+      auto size = SPA_POD_BODY_SIZE(&c->value);
+
+      const auto to_ns = [=, clk = position->clock] {
+        return 1e9 * ((clk.position + c->offset) / (double)clk.rate.denom);
+      };
+
+      m_processing.on_bytes(
+          {data, data + size}, m_processing.timestamp<timestamp_info>(to_ns, c->offset));
+    }
+
+    pw.filter_queue_buffer(this->filter->port, b);
+  }
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+}

include/libremidi/backends/pipewire/midi_out.hpp

@@ -0,0 +1,171 @@
+#pragma once
+#include <libremidi/backends/pipewire/config.hpp>
+#include <libremidi/backends/pipewire/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+#include <readerwriterqueue.h>
+
+#include <semaphore>
+
+namespace libremidi
+{
+class midi_out_pipewire
+    : public midi1::out_api
+    , public pipewire_helpers
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , pipewire_output_configuration
+  {
+  } configuration;
+
+  midi_out_pipewire(output_configuration&& conf, pipewire_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    create_context(*this);
+    create_filter(*this);
+  }
+
+  ~midi_out_pipewire() override
+  {
+    stop_thread();
+    do_close_port();
+    destroy_filter(*this);
+    destroy_context();
+  }
+
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_pipewire: set_client_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::PIPEWIRE; }
+
+  bool open_port(const output_port& out_port, std::string_view name) override
+  {
+    if (!create_local_port(*this, name, SPA_DIRECTION_OUTPUT))
+      return false;
+
+    this->filter->set_port_buffer(configuration.output_buffer_size);
+
+    if (!link_ports(*this, out_port))
+      return false;
+
+    start_thread();
+    return true;
+  }
+
+  bool open_virtual_port(std::string_view name) override
+  {
+    if (!create_local_port(*this, name, SPA_DIRECTION_OUTPUT))
+      return false;
+
+    this->filter->set_port_buffer(configuration.output_buffer_size);
+
+    start_thread();
+    return true;
+  }
+
+  void close_port() override
+  {
+    stop_thread();
+    do_close_port();
+  }
+
+  void set_port_name(std::string_view port_name) override { rename_port(port_name); }
+
+  int process(spa_io_position* pos)
+  {
+    m_process_clock.store(pos->clock.nsec, std::memory_order_relaxed);
+    const auto b = pw.filter_dequeue_buffer(this->filter->port);
+    if (!b)
+      return 1;
+
+    const auto buf = b->buffer;
+    const auto d = &buf->datas[0];
+
+    if (d->data == nullptr)
+      return 1;
+
+    spa_pod_builder build;
+    spa_zero(build);
+    spa_pod_builder_init(&build, d->data, d->maxsize);
+
+    spa_pod_frame f;
+    spa_pod_builder_push_sequence(&build, &f, 0);
+
+    // for all events
+    while (auto m_ptr = m_queue.peek())
+    {
+      auto& m = *m_ptr;
+      if (m.empty())
+      {
+        m_queue.pop();
+        continue;
+      }
+
+      // TODO why
+      if (m.bytes[0] == 0xff)
+      {
+        m_queue.pop();
+        continue;
+      }
+
+      spa_pod_builder_control(&build, m.timestamp, SPA_CONTROL_Midi);
+      int res = spa_pod_builder_bytes(&build, m.bytes.data(), m.bytes.size());
+
+      // Try again next buffer
+      if (res == -ENOSPC)
+        break;
+
+      m_queue.pop();
+    }
+    spa_pod_builder_pop(&build, &f);
+
+    int n_fill_frames = build.state.offset;
+    if (n_fill_frames > 0)
+    {
+      d->chunk->offset = 0;
+      d->chunk->stride = 1;
+      d->chunk->size = n_fill_frames;
+      b->size = n_fill_frames;
+
+      pw.filter_queue_buffer(this->filter->port, b);
+      return 0;
+    }
+
+    pw.filter_flush(this->filter->filter, true);
+
+    return 0;
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    m_queue.enqueue(libremidi::message(midi_bytes{message, message + size}, 0));
+  }
+
+  int convert_timestamp(int64_t user) const noexcept
+  {
+    switch (configuration.timestamps)
+    {
+      case timestamp_mode::AudioFrame:
+        return static_cast<int>(user);
+
+      default:
+        // TODO
+        return 0;
+    }
+  }
+
+  void schedule_message(int64_t ts, const unsigned char* message, size_t size) override
+  {
+    m_queue.enqueue(
+        libremidi::message(midi_bytes{message, message + size}, convert_timestamp(ts)));
+  }
+
+  moodycamel::ReaderWriterQueue<libremidi::message> m_queue;
+  std::atomic_int64_t m_process_clock = 0;
+};
+}

include/libremidi/backends/pipewire/observer.hpp

@@ -0,0 +1,74 @@
+#pragma once
+#include <libremidi/backends/pipewire/config.hpp>
+#include <libremidi/backends/pipewire/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <unordered_set>
+
+namespace libremidi
+{
+class observer_pipewire final
+    : public observer_api
+    , private pipewire_helpers
+    , private error_handler
+{
+public:
+  struct
+      : observer_configuration
+      , pipewire_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_pipewire(observer_configuration&& conf, pipewire_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    create_context(*this);
+
+    // FIXME notify_in_constructor
+    // FIXME port rename callback
+#if 0
+    // Initialize PipeWire client
+    if (configuration.context)
+    {
+      this->client = configuration.context;
+      set_callbacks();
+    }
+    else
+#endif
+    {
+      this->add_callbacks(configuration);
+      this->start_thread();
+    }
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::PIPEWIRE; }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    return get_ports<SPA_DIRECTION_OUTPUT>(*this->global_context);
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    return get_ports<SPA_DIRECTION_INPUT>(*this->global_context);
+  }
+
+  ~observer_pipewire()
+  {
+    stop_thread();
+    destroy_context();
+#if 0
+    if (client && !configuration.context)
+    {
+      // If we own the client, deactivate it
+      pipewire_deactivate(this->client);
+      pipewire_client_close(this->client);
+      this->client = nullptr;
+    }
+#endif
+  }
+
+  std::unordered_set<std::string> seen_input_ports;
+  std::unordered_set<std::string> seen_output_ports;
+};
+}

include/libremidi/backends/pipewire/shared_handler.hpp

@@ -0,0 +1,153 @@
+#pragma once
+#if __has_include(<boost/lockfree/spsc_queue.hpp>)
+  #include <libremidi/backends/pipewire/config.hpp>
+  #include <libremidi/backends/pipewire/helpers.hpp>
+  #include <libremidi/shared_context.hpp>
+
+  #include <boost/lockfree/spsc_queue.hpp>
+
+  #include <variant>
+
+namespace libremidi::pipewire
+{
+
+// Create a PipeWire client which will be shared across objects
+struct shared_handler : public libremidi::shared_context
+{
+  explicit shared_handler(std::string_view v)
+  {
+    midiin_callbacks.reserve(64);
+    midiout_callbacks.reserve(64);
+
+    pipewire_status_t status{};
+    client = pipewire_client_open(v.data(), PipewireNoStartServer, &status);
+    assert(client);
+    assert(status == 0);
+    pipewire_set_process_callback(
+        client,
+        +[](pipewire_nframes_t cnt, void* ctx) -> int {
+          ((shared_handler*)ctx)->pipewire_callback(cnt);
+          return 0;
+        },
+        this);
+  }
+
+  virtual void start_processing() override { pipewire_activate(client); }
+  virtual void stop_processing() override { pipewire_deactivate(client); }
+
+  static shared_configurations make(std::string_view client_name)
+  {
+    auto clt = std::make_shared<shared_handler>(client_name);
+    auto add_in_cb = [client = std::weak_ptr{clt}](libremidi::pipewire_callback cb) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::in_callback_added, std::move(cb)});
+    };
+    auto clear_in_cb = [client = std::weak_ptr{clt}](int64_t index) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::in_callback_removed, index});
+    };
+    auto add_out_cb = [client = std::weak_ptr{clt}](libremidi::pipewire_callback cb) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::out_callback_added, std::move(cb)});
+    };
+    auto clear_out_cb = [client = std::weak_ptr{clt}](int64_t index) {
+      if (auto clt = client.lock())
+        clt->events.push({shared_handler::event_type::out_callback_removed, index});
+    };
+    return {
+        .context = clt,
+        .observer = pipewire_observer_configuration{.context = clt->client},
+        .in
+        = pipewire_input_configuration{.context = clt->client, .set_process_func = add_in_cb, .clear_process_func = clear_in_cb},
+        .out
+        = pipewire_output_configuration{.context = clt->client, .set_process_func = add_out_cb, .clear_process_func = clear_out_cb},
+    };
+  }
+
+  int pipewire_callback(pipewire_nframes_t cnt)
+  {
+    // 1. Process the events that will change the callback list
+    event ev;
+    while (events.pop(ev))
+    {
+      switch (ev.type)
+      {
+        case in_callback_added:
+          midiin_callbacks.push_back(
+              std::move(*std::get_if<libremidi::pipewire_callback>(&ev.payload)));
+          break;
+        case in_callback_removed: {
+          auto idx = *std::get_if<int64_t>(&ev.payload);
+          for (auto it = midiin_callbacks.begin(); it != midiin_callbacks.end();)
+          {
+            if (it->token == idx)
+            {
+              midiin_callbacks.erase(it);
+              break;
+            }
+            else
+            {
+              ++it;
+            }
+          }
+          break;
+        }
+        case out_callback_added:
+          midiout_callbacks.push_back(
+              std::move(*std::get_if<libremidi::pipewire_callback>(&ev.payload)));
+          break;
+        case out_callback_removed:
+          auto idx = *std::get_if<int64_t>(&ev.payload);
+          for (auto it = midiout_callbacks.begin(); it != midiout_callbacks.end();)
+          {
+            if (it->token == idx)
+            {
+              midiout_callbacks.erase(it);
+              break;
+            }
+            else
+            {
+              ++it;
+            }
+          }
+          break;
+      }
+    }
+
+    for (auto& cb : midiin_callbacks)
+      cb.callback(cnt);
+
+    for (auto& cb : midiout_callbacks)
+      cb.callback(cnt);
+
+    return 0;
+  }
+
+  ~shared_handler()
+  {
+    pipewire_deactivate(client);
+    pipewire_client_close(client);
+  }
+
+  pipewire_client_t* client{};
+
+  enum event_type
+  {
+    in_callback_added,
+    in_callback_removed,
+    out_callback_added,
+    out_callback_removed,
+  };
+  struct event
+  {
+    event_type type;
+    std::variant<libremidi::pipewire_callback, int64_t> payload;
+  };
+
+  boost::lockfree::spsc_queue<event> events{16};
+
+  std::vector<libremidi::pipewire_callback> midiin_callbacks;
+  std::vector<libremidi::pipewire_callback> midiout_callbacks;
+};
+}
+#endif

include/libremidi/backends/winmidi.hpp

@@ -0,0 +1,22 @@
+#pragma once
+#include <libremidi/backends/winmidi/midi_in.hpp>
+#include <libremidi/backends/winmidi/midi_out.hpp>
+#include <libremidi/backends/winmidi/observer.hpp>
+
+namespace libremidi::winmidi
+{
+struct backend
+{
+  using midi_in = midi_in_impl;
+  using midi_out = midi_out_impl;
+  using midi_observer = observer_impl;
+  using midi_in_configuration = winmidi::input_configuration;
+  using midi_out_configuration = winmidi::output_configuration;
+  using midi_observer_configuration = winmidi::observer_configuration;
+  static const constexpr auto API = libremidi::API::WINDOWS_MIDI_SERVICES;
+  static const constexpr auto name = "winmidi";
+  static const constexpr auto display_name = "Windows MIDI Services";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+} // namespace libremidi

include/libremidi/backends/winmidi/config.hpp

@@ -0,0 +1,19 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+namespace libremidi::winmidi
+{
+
+struct input_configuration
+{
+};
+
+struct output_configuration
+{
+};
+
+struct observer_configuration
+{
+};
+
+}

include/libremidi/backends/winmidi/helpers.hpp

@@ -0,0 +1,27 @@
+#pragma once
+// clang-format off
+#define NOMINMAX 1
+#define WIN32_LEAN_AND_MEAN 1
+#include <libremidi/detail/midi_api.hpp>
+
+#include <mutex>
+#include <string>
+#include <thread>
+#include <vector>
+#include <guiddef.h>
+#include <winrt/Windows.Foundation.h>
+#include <winrt/Windows.Foundation.Collections.h>
+#include <winrt/Windows.Devices.Enumeration.h>
+
+#include <winrt/Microsoft.Devices.Midi2.h>
+// clang-format on
+
+namespace libremidi::winmidi
+{
+using namespace winrt;
+using namespace winrt::Windows::Foundation;
+using namespace winrt::Windows::Devices::Enumeration;
+using namespace winrt::Windows::Storage::Streams;
+using namespace Microsoft::Devices::Midi2;
+using namespace Windows::Devices::Enumeration;
+}

include/libremidi/backends/winmidi/midi_in.hpp

@@ -0,0 +1,94 @@
+#pragma once
+#include <libremidi/backends/winmidi/config.hpp>
+#include <libremidi/backends/winmidi/helpers.hpp>
+#include <libremidi/backends/winmidi/observer.hpp>
+#include <libremidi/detail/midi_in.hpp>
+
+namespace libremidi::winmidi
+{
+
+class midi_in_impl final
+    : public midi2::in_api
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::ump_input_configuration
+      , winmidi::input_configuration
+  {
+  } configuration;
+
+  explicit midi_in_impl(
+      libremidi::ump_input_configuration&& conf, winmidi::input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+  }
+
+  ~midi_in_impl() override { close_port(); }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmidi: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmidi: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmidi: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override
+  {
+    return libremidi::API::WINDOWS_MIDI_SERVICES;
+  }
+
+  bool open_port(const input_port& port, std::string_view) override
+  {
+#if 0
+    const auto id = winrt::to_hstring(port.port_name);
+    if (id.empty())
+      return false;
+
+    port_ = get(MidiInPort::FromIdAsync(id));
+    if (!port_)
+      return false;
+    port_.MessageReceived(
+        [=](const winrt::Windows::Devices::Midi::IMidiInPort& inputPort,
+            const winrt::Windows::Devices::Midi::MidiMessageReceivedEventArgs& args) {
+      this->process_message(args.Message());
+    });
+
+#endif
+    return true;
+  }
+
+#if 0
+  void process_message(const winrt::Windows::Devices::Midi::IMidiMessage& msg)
+  {
+    auto reader = DataReader::FromBuffer(msg.RawData());
+    auto begin = msg.RawData().data();
+    auto end = begin + msg.RawData().Length();
+
+    auto t = msg.Timestamp().count();
+    this->configuration.on_message(libremidi::message{{begin, end}, t});
+  }
+#endif
+
+  void close_port() override
+  {
+#if 0
+    if (port_)
+    {
+      port_.Close();
+      port_ = nullptr;
+    }
+#endif
+  }
+
+private:
+  // winrt::Microsoft::Devices::Midi2::IMidiInPort port_{nullptr};
+};
+}

include/libremidi/backends/winmidi/midi_out.hpp

@@ -0,0 +1,91 @@
+#pragma once
+#include <libremidi/backends/winmidi/config.hpp>
+#include <libremidi/backends/winmidi/helpers.hpp>
+#include <libremidi/backends/winmidi/observer.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi::winmidi
+{
+
+class midi_out_impl final
+    : public midi2::out_api
+    , public error_handler
+{
+public:
+  struct
+      : libremidi::output_configuration
+      , winmidi::output_configuration
+  {
+  } configuration;
+
+  midi_out_impl(libremidi::output_configuration&& conf, winmidi::output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+  }
+
+  ~midi_out_impl() override { close_port(); }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmidi: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmidi: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmidi: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override
+  {
+    return libremidi::API::WINDOWS_MIDI_SERVICES;
+  }
+
+  bool open_port(const output_port& port, std::string_view) override
+  {
+#if 0
+    const auto id = winrt::to_hstring(port.port_name);
+    if (id.empty())
+      return false;
+
+    port_ = get(MidiOutPort::FromIdAsync(id));
+    return bool(port_);
+#endif
+    return true;
+  }
+
+  void close_port() override
+  {
+#if 0
+    if (port_)
+    {
+      port_.Close();
+      port_ = {};
+    }
+#endif
+  }
+
+  void send_ump(const uint32_t* message, size_t size) override
+  {
+#if 0
+    if (!port_)
+      return;
+
+    InMemoryRandomAccessStream str;
+    DataWriter rb(str);
+    rb.WriteBytes(
+        winrt::array_view<const uint8_t>{(const uint8_t*)message, (const uint8_t*)message + size});
+    port_.SendBuffer(rb.DetachBuffer());
+#endif
+  }
+
+private:
+#if 0
+winrt::Windows::Devices::Midi2::IMidiOutPort port_{nullptr};
+#endif
+};
+
+}

include/libremidi/backends/winmidi/observer.hpp

@@ -0,0 +1,151 @@
+#pragma once
+#include <libremidi/backends/winmidi/config.hpp>
+#include <libremidi/backends/winmidi/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+;
+namespace libremidi::winmidi
+{
+struct port_info
+{
+  hstring id;
+  hstring name;
+};
+
+class observer_impl final : public observer_api
+{
+public:
+  struct
+      : libremidi::observer_configuration
+      , winmidi::observer_configuration
+  {
+  } configuration;
+
+  MidiSession session;
+
+  explicit observer_impl(
+      libremidi::observer_configuration&& conf, winmidi::observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+      , session{
+            MidiSession::CreateNewSession(L"libremidi session", MidiSessionSettings::Default())}
+  {
+    if (!configuration.has_callbacks())
+      return;
+
+    if (configuration.notify_in_constructor)
+    {
+      if (configuration.input_added)
+        for (const auto& p : get_input_ports())
+          configuration.input_added(p);
+      if (configuration.output_added)
+        for (const auto& p : get_output_ports())
+          configuration.output_added(p);
+    }
+
+    /*
+    evTokenOnInputAdded_
+        = internalInPortObserver_.PortAdded([this](const port_info& p) { on_input_added(p); });
+    evTokenOnInputRemoved_
+        = internalInPortObserver_.PortRemoved([this](const port_info& p) { on_input_removed(p); });
+    evTokenOnOutputAdded_
+        = internalOutPortObserver_.PortAdded([this](const port_info& p) { on_output_added(p); });
+    evTokenOnOutputRemoved_ = internalOutPortObserver_.PortRemoved(
+        [this](const port_info& p) { on_output_removed(p); });
+*/
+  }
+
+  ~observer_impl()
+  {
+    if (!configuration.has_callbacks())
+      return;
+    // internalInPortObserver_.PortAdded(evTokenOnInputAdded_);
+    // internalInPortObserver_.PortRemoved(evTokenOnInputRemoved_);
+    // internalOutPortObserver_.PortAdded(evTokenOnOutputAdded_);
+    // internalOutPortObserver_.PortRemoved(evTokenOnOutputRemoved_);
+  }
+
+  libremidi::API get_current_api() const noexcept override
+  {
+    return libremidi::API::WINDOWS_MIDI_SERVICES;
+  }
+
+  template <bool Input>
+  auto to_port_info(const DeviceInformation& p) const noexcept
+      -> std::conditional_t<Input, input_port, output_port>
+  {
+    return {
+        {.client = 0,
+         .port = 0,
+         .manufacturer = "",
+         .device_name = "",
+         .port_name = to_string(p.Id()),
+         .display_name = to_string(p.Name())}};
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    std::vector<libremidi::input_port> ret;
+
+    auto deviceSelector = MidiEndpointConnection::GetDeviceSelector();
+    auto endpointDevices = DeviceInformation::FindAllAsync(deviceSelector).get();
+    for (const auto& selectedEndpointInformation : endpointDevices)
+    {
+      // FIXME if(has input...)
+      ret.emplace_back(to_port_info(selectedEndpointInformation));
+    }
+
+    return ret;
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    std::vector<libremidi::output_port> ret;
+
+    auto deviceSelector = MidiEndpointConnection::GetDeviceSelector();
+    auto endpointDevices = DeviceInformation::FindAllAsync(deviceSelector).get();
+    for (const auto& selectedEndpointInformation : endpointDevices)
+    {
+      // FIXME if(has output...)
+      ret.emplace_back(to_port_info(selectedEndpointInformation));
+    }
+
+    return ret;
+  }
+
+  void on_input_added(const DeviceInformation& name)
+  {
+    if (configuration.input_added)
+      configuration.input_added(to_port_info(name));
+  }
+
+  void on_input_removed(const DeviceInformation& name)
+  {
+    if (configuration.input_removed)
+      configuration.input_removed(to_port_info(name));
+  }
+
+  void on_output_added(const DeviceInformation& name)
+  {
+    if (configuration.output_added)
+      configuration.output_added(to_port_info(name));
+  }
+
+  void on_output_removed(const DeviceInformation& name)
+  {
+    if (configuration.output_removed)
+      configuration.output_removed(to_port_info(name));
+  }
+
+private:
+#if 0
+  static inline observer_winmidi_internal internalInPortObserver_{MidiInPort::GetDeviceSelector()};
+  static inline observer_winmidi_internal internalOutPortObserver_{
+      MidiOutPort::GetDeviceSelector()};
+
+  int evTokenOnInputAdded_{-1};
+  int evTokenOnInputRemoved_{-1};
+  int evTokenOnOutputAdded_{-1};
+  int evTokenOnOutputRemoved_{-1};
+#endif
+};
+
+}

include/libremidi/backends/winmm.hpp

@@ -0,0 +1,36 @@
+#pragma once
+#include <libremidi/backends/winmm/midi_in.hpp>
+#include <libremidi/backends/winmm/midi_out.hpp>
+#include <libremidi/backends/winmm/observer.hpp>
+
+// Default for Windows is to add an identifier to the port names; this
+// flag can be defined (e.g. in your project file) to disable this behaviour.
+// #define LIBREMIDI_DO_NOT_ENSURE_UNIQUE_PORTNAMES
+
+//*********************************************************************//
+//  API: Windows Multimedia Library (MM)
+//*********************************************************************//
+
+// API information deciphered from:
+//  -
+//  http://msdn.microsoft.com/library/default.asp?url=/library/en-us/multimed/htm/_win32_midi_reference.asp
+
+// Thanks to Jean-Baptiste Berruchon for the sysex code.
+namespace libremidi
+{
+
+struct winmm_backend
+{
+  using midi_in = midi_in_winmm;
+  using midi_out = midi_out_winmm;
+  using midi_observer = observer_winmm;
+  using midi_in_configuration = winmm_input_configuration;
+  using midi_out_configuration = winmm_output_configuration;
+  using midi_observer_configuration = winmm_observer_configuration;
+  static const constexpr auto API = libremidi::API::WINDOWS_MM;
+  static const constexpr auto name = "winmm";
+  static const constexpr auto display_name = "Windows Multimedia";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+}

include/libremidi/backends/winmm/config.hpp

@@ -0,0 +1,31 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+#include <chrono>
+#include <functional>
+
+namespace libremidi
+{
+
+struct winmm_input_configuration
+{
+  int sysex_buffer_size = 1024;
+  int sysex_buffer_count = 4;
+};
+
+struct winmm_output_configuration
+{
+};
+
+struct poll_parameters
+{
+  std::function<void()> callback;
+};
+
+struct winmm_observer_configuration
+{
+  std::chrono::milliseconds poll_period{100};
+  std::function<void(const poll_parameters&)> manual_poll;
+};
+
+}

include/libremidi/backends/winmm/helpers.hpp

@@ -0,0 +1,81 @@
+#pragma once
+#define NOMINMAX 1
+#define WIN32_LEAN_AND_MEAN 1
+#include <libremidi/detail/midi_api.hpp>
+
+#include <algorithm>
+
+// clang-format off
+#include <windows.h>
+#include <mmsystem.h>
+// clang-format on
+
+namespace libremidi
+{
+
+// Convert a nullptr-terminated wide string or ANSI-encoded string to UTF-8.
+inline std::string ConvertToUTF8(const TCHAR* str)
+{
+  std::string u8str;
+  const WCHAR* wstr = L"";
+#if defined(UNICODE) || defined(_UNICODE)
+  wstr = str;
+#else
+  // Convert from ANSI encoding to wide string
+  int wlength = MultiByteToWideChar(CP_ACP, 0, str, -1, nullptr, 0);
+  std::wstring wstrtemp;
+  if (wlength)
+  {
+    wstrtemp.assign(wlength - 1, 0);
+    MultiByteToWideChar(CP_ACP, 0, str, -1, &wstrtemp[0], wlength);
+    wstr = &wstrtemp[0];
+  }
+#endif
+  // Convert from wide string to UTF-8
+  int length = WideCharToMultiByte(CP_UTF8, 0, wstr, -1, nullptr, 0, nullptr, nullptr);
+  if (length)
+  {
+    u8str.assign(static_cast<std::string::size_type>(length - 1), 0);
+    /*length =*/WideCharToMultiByte(CP_UTF8, 0, wstr, -1, &u8str[0], length, nullptr, nullptr);
+  }
+  return u8str;
+}
+
+// Next functions add the portNumber to the name so that
+// the device's names are sure to be listed with individual names
+// even when they have the same brand name
+inline void MakeUniqueInPortName(std::string& deviceName, unsigned int portNumber)
+{
+  int x = 1;
+  for (unsigned int i = 0; i < portNumber; i++)
+  {
+    MIDIINCAPS deviceCaps;
+    midiInGetDevCaps(i, &deviceCaps, sizeof(MIDIINCAPS));
+    auto stringName = ConvertToUTF8(deviceCaps.szPname);
+    if (deviceName == stringName)
+    {
+      x++;
+    }
+  }
+  deviceName += " ";
+  deviceName += std::to_string(x);
+}
+
+inline void MakeUniqueOutPortName(std::string& deviceName, unsigned int portNumber)
+{
+  int x = 1;
+  for (unsigned int i = 0; i < portNumber; i++)
+  {
+    MIDIOUTCAPS deviceCaps;
+    midiOutGetDevCaps(i, &deviceCaps, sizeof(MIDIOUTCAPS));
+    auto stringName = ConvertToUTF8(deviceCaps.szPname);
+    if (deviceName == stringName)
+    {
+      x++;
+    }
+  }
+  deviceName += " ";
+  deviceName += std::to_string(x);
+}
+
+}

include/libremidi/backends/winmm/midi_in.hpp

@@ -0,0 +1,309 @@
+#pragma once
+#include <libremidi/backends/winmm/config.hpp>
+#include <libremidi/backends/winmm/helpers.hpp>
+#include <libremidi/backends/winmm/observer.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+namespace libremidi
+{
+
+class midi_in_winmm final
+    : public midi1::in_api
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , winmm_input_configuration
+  {
+  } configuration;
+
+  explicit midi_in_winmm(input_configuration&& conf, winmm_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    // We'll issue a warning here if no devices are available but not
+    // throw an error since the user can plugin something later.
+    if (midiInGetNumDevs() == 0)
+    {
+      warning(
+          configuration, "midi_in_winmm::initialize: no MIDI input devices currently available.");
+    }
+
+    if (!InitializeCriticalSectionAndSpinCount(&(this->_mutex), 0x00000400))
+    {
+      warning(
+          configuration,
+          "midi_in_winmm::initialize: InitializeCriticalSectionAndSpinCount failed.");
+    }
+  }
+
+  ~midi_in_winmm() override
+  {
+    // Close a connection if it exists.
+    midi_in_winmm::close_port();
+
+    DeleteCriticalSection(&(this->_mutex));
+  }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmm: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmm: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winmm: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_MM; }
+
+  bool do_open(unsigned int portNumber)
+  {
+    MMRESULT result = midiInOpen(
+        &this->inHandle, portNumber, std::bit_cast<DWORD_PTR>(&midiInputCallback),
+        std::bit_cast<DWORD_PTR>(this), CALLBACK_FUNCTION);
+    if (result != MMSYSERR_NOERROR)
+    {
+      error<driver_error>(
+          configuration, "midi_in_winmm::open_port: error creating Windows MM MIDI input port.");
+      return false;
+    }
+
+    // Allocate and init the sysex buffers.
+    const auto bufferCount = static_cast<std::size_t>(configuration.sysex_buffer_count);
+    this->sysexBuffer.resize(bufferCount);
+    for (std::size_t i = 0; i < bufferCount; ++i)
+    {
+      this->sysexBuffer[i] = new MIDIHDR;
+      this->sysexBuffer[i]->lpData = new char[configuration.sysex_buffer_size];
+      this->sysexBuffer[i]->dwBufferLength = static_cast<DWORD>(configuration.sysex_buffer_size);
+      this->sysexBuffer[i]->dwUser = i; // We use the dwUser parameter as buffer indicator
+      this->sysexBuffer[i]->dwFlags = 0;
+
+      result = midiInPrepareHeader(this->inHandle, this->sysexBuffer[i], sizeof(MIDIHDR));
+      if (result != MMSYSERR_NOERROR)
+      {
+        midiInClose(this->inHandle);
+        this->inHandle = nullptr;
+        error<driver_error>(
+            configuration,
+            "midi_in_winmm::open_port: error starting Windows MM MIDI input port "
+            "(PrepareHeader).");
+        return false;
+      }
+
+      // Register the buffer.
+      result = midiInAddBuffer(this->inHandle, this->sysexBuffer[i], sizeof(MIDIHDR));
+      if (result != MMSYSERR_NOERROR)
+      {
+        midiInClose(this->inHandle);
+        this->inHandle = nullptr;
+        error<driver_error>(
+            configuration,
+            "midi_in_winmm::open_port: error starting Windows MM MIDI input port "
+            "(AddBuffer).");
+        return false;
+      }
+    }
+
+    result = midiInStart(this->inHandle);
+    midi_start_timestamp = std::chrono::steady_clock::now();
+    if (result != MMSYSERR_NOERROR)
+    {
+      midiInClose(this->inHandle);
+      this->inHandle = nullptr;
+      error<driver_error>(
+          configuration, "midi_in_winmm::open_port: error starting Windows MM MIDI input port.");
+      return false;
+    }
+
+    return true;
+  }
+
+  bool open_port(const input_port& p, std::string_view) override
+  {
+    observer_winmm obs{{}, winmm_observer_configuration{}};
+    auto ports = obs.get_input_ports();
+
+    // First check with the display name, e.g. MIDI KEYBOARD 2 will match MIDI KEYBOARD 2
+    for (auto& port : ports)
+    {
+      if (p.display_name == port.display_name)
+        return do_open(port.port);
+    }
+    // If nothing is found, try to check with the raw name
+    for (auto& port : ports)
+    {
+      if (p.port_name == port.port_name)
+        return do_open(port.port);
+    }
+    error<invalid_parameter_error>(
+        configuration, "midi_in_winmm::open_port: port not found: " + p.port_name);
+    return false;
+  }
+
+  void close_port() override
+  {
+    if (connected_)
+    {
+      EnterCriticalSection(&(this->_mutex));
+      midiInReset(this->inHandle);
+      midiInStop(this->inHandle);
+
+      for (std::size_t i = 0; i < static_cast<std::size_t>(configuration.sysex_buffer_count); ++i)
+      {
+        MMRESULT res{};
+
+        int wait_count = 5;
+        while (
+            ((res = midiInUnprepareHeader(this->inHandle, this->sysexBuffer[i], sizeof(MIDIHDR)))
+             == MIDIERR_STILLPLAYING)
+            && wait_count-- >= 0)
+        {
+          Sleep(1);
+        }
+
+        if (res != MMSYSERR_NOERROR)
+        {
+          warning(
+              configuration,
+              "midi_in_winmm::open_port: error closing Windows MM MIDI input "
+              "port (midiInUnprepareHeader).");
+          continue;
+        }
+        else
+        {
+          delete[] this->sysexBuffer[i]->lpData;
+          delete[] this->sysexBuffer[i];
+        }
+      }
+
+      midiInClose(this->inHandle);
+      this->inHandle = nullptr;
+      LeaveCriticalSection(&(this->_mutex));
+    }
+  }
+
+private:
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return std::chrono::duration_cast<std::chrono::nanoseconds>(
+               std::chrono::steady_clock::now() - midi_start_timestamp)
+        .count();
+  }
+
+  static constexpr int bytes_for_message(uint8_t status)
+  {
+    if (status < 0xC0)
+      return 3;
+    else if (status < 0xE0)
+      return 2;
+    else if (status < 0xF0)
+      return 3;
+    else if (status == 0xF1)
+      return 2;
+    else if (status == 0xF2)
+      return 3;
+    else if (status == 0xF3)
+      return 2;
+    else if (status == 0xF8)
+      return 1;
+    else if (status == 0xFE)
+      return 1;
+    else
+      return 0;
+  }
+
+  static void CALLBACK midiInputCallback(
+      HMIDIIN /*hmin*/, UINT inputStatus, DWORD_PTR instancePtr, DWORD_PTR midiMessage,
+      DWORD_PTR timestamp)
+  {
+    if (inputStatus != MIM_DATA && inputStatus != MIM_LONGDATA && inputStatus != MIM_LONGERROR)
+      return;
+
+    auto& self = *reinterpret_cast<midi_in_winmm*>(instancePtr);
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = false,
+        .has_samples = false,
+    };
+
+    const auto to_ns = [timestamp] { return timestamp * 1'000'000; };
+
+    if (inputStatus == MIM_DATA)
+    {
+      // Channel or system message
+      uint8_t message[sizeof(DWORD_PTR)];
+      memcpy(message, &midiMessage, sizeof(DWORD_PTR));
+
+      // Make sure the first byte is a status byte.
+      if (message[0] & 0x80)
+      {
+        self.m_processing.on_bytes(
+            {message, message + bytes_for_message(message[0])},
+            self.m_processing.timestamp<timestamp_info>(to_ns, 0));
+      }
+    }
+    else
+    {
+      // Sysex message ( MIM_LONGDATA or MIM_LONGERROR )
+      const auto* sysex = reinterpret_cast<MIDIHDR*>(midiMessage);
+      if(inputStatus == MIM_LONGERROR)
+      {
+        self.m_processing.message.bytes.clear();
+        self.m_processing.state = self.m_processing.main;
+      }
+      else if (!self.configuration.ignore_sysex)
+      {
+        if(sysex->dwBytesRecorded > 0)
+        {
+          const auto sysex_bytes = reinterpret_cast<uint8_t*>(sysex->lpData);
+
+          self.m_processing.on_bytes(
+              {sysex_bytes, sysex_bytes + sysex->dwBytesRecorded},
+              self.m_processing.timestamp<timestamp_info>(to_ns, 0));
+        }
+      }
+
+      // The WinMM API requires that the sysex buffer be requeued after
+      // input of each sysex message.  Even if we are ignoring sysex
+      // messages, we still need to requeue the buffer in case the user
+      // decides to not ignore sysex messages in the future.  However,
+      // it seems that WinMM calls this function with an empty sysex
+      // buffer when an application closes and in this case, we should
+      // avoid requeueing it, else the computer suddenly reboots after
+      // one or two minutes.
+      if (self.sysexBuffer[sysex->dwUser]->dwBytesRecorded > 0)
+      {
+        EnterCriticalSection(&(self._mutex));
+        MMRESULT result
+            = midiInAddBuffer(self.inHandle, self.sysexBuffer[sysex->dwUser], sizeof(MIDIHDR));
+        LeaveCriticalSection(&(self._mutex));
+        if (result != MMSYSERR_NOERROR)
+        {
+#if defined(__LIBREMIDI_DEBUG__)
+          std::cerr << "\nmidi_in::midiInputCallback: error sending sysex to "
+                       "Midi device!!\n\n";
+#endif
+        }
+      }
+    }
+  }
+
+  HMIDIIN inHandle; // Handle to Midi Input Device
+
+  std::vector<LPMIDIHDR> sysexBuffer;
+  // [Patrice] see
+  // https://groups.google.com/forum/#!topic/mididev/6OUjHutMpEo
+  CRITICAL_SECTION _mutex;
+  std::chrono::steady_clock::time_point midi_start_timestamp;
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+
+}

include/libremidi/backends/winmm/midi_out.hpp

@@ -0,0 +1,181 @@
+#pragma once
+#include <libremidi/backends/winmm/config.hpp>
+#include <libremidi/backends/winmm/helpers.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi
+{
+
+class midi_out_winmm final
+    : public midi1::out_api
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , winmm_output_configuration
+  {
+  } configuration;
+
+  midi_out_winmm(output_configuration&& conf, winmm_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    // We'll issue a warning here if no devices are available but not
+    // throw an error since the user can plug something in later.
+    if (midiOutGetNumDevs() == 0)
+    {
+      warning(
+          configuration,
+          "midi_out_winmm::initialize: no MIDI output devices currently "
+          "available.");
+    }
+  }
+
+  ~midi_out_winmm() override
+  {
+    // Close a connection if it exists.
+    midi_out_winmm::close_port();
+  }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmm: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmm: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winmm: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_MM; }
+
+  [[nodiscard]] bool do_open(unsigned int portNumber)
+  {
+    MMRESULT result = midiOutOpen(&this->outHandle, portNumber, 0, 0, CALLBACK_NULL);
+    if (result != MMSYSERR_NOERROR)
+    {
+      error<driver_error>(
+          configuration,
+          "midi_out_winmm::open_port: error creating Windows MM MIDI output "
+          "port.");
+      return false;
+    }
+
+    return true;
+  }
+
+  bool open_port(const output_port& p, std::string_view) override
+  {
+    observer_winmm obs{{}, winmm_observer_configuration{}};
+    auto ports = obs.get_output_ports();
+
+    // First check with the display name, e.g. MIDI KEYBOARD 2 will match MIDI KEYBOARD 2
+    for (auto& port : ports)
+    {
+      if (p.display_name == port.display_name)
+        return do_open(port.port);
+    }
+    // If nothing is found, try to check with the raw name
+    for (auto& port : ports)
+    {
+      if (p.port_name == port.port_name)
+        return do_open(port.port);
+    }
+    error<invalid_parameter_error>(
+        configuration, "midi_out_winmm::open_port: port not found: " + p.port_name);
+    return false;
+  }
+
+  void close_port() override
+  {
+    if (this->outHandle)
+      midiOutClose(this->outHandle);
+
+    this->outHandle = nullptr;
+    connected_ = false;
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    if (!connected_)
+      return;
+
+    if (size == 0)
+    {
+      warning(configuration, "midi_out_winmm::send_message: message argument is empty!");
+      return;
+    }
+
+    if (message[0] == 0xF0)
+    { // Sysex message
+
+      buffer.assign(message, message + size);
+
+      // FIXME this can be made asynchronous... see Chrome source.
+      // But need to know whe buffers are freed.
+
+      // Create and prepare MIDIHDR structure.
+      MIDIHDR sysex{};
+      sysex.lpData = (LPSTR)buffer.data();
+      sysex.dwBufferLength = size;
+      sysex.dwFlags = 0;
+      auto result = midiOutPrepareHeader(this->outHandle, &sysex, sizeof(MIDIHDR));
+      if (result != MMSYSERR_NOERROR)
+      {
+        error<driver_error>(
+            configuration, "midi_out_winmm::send_message: error preparing sysex header.");
+        return;
+      }
+
+      // Send the message.
+      result = midiOutLongMsg(this->outHandle, &sysex, sizeof(MIDIHDR));
+      if (result != MMSYSERR_NOERROR)
+      {
+        error<driver_error>(
+            configuration, "midi_out_winmm::send_message: error sending sysex message.");
+        return;
+      }
+
+      // Unprepare the buffer and MIDIHDR.
+      // FIXME yuck
+      while (MIDIERR_STILLPLAYING
+             == midiOutUnprepareHeader(this->outHandle, &sysex, sizeof(MIDIHDR)))
+        Sleep(1);
+    }
+    else
+    { // Channel or system message.
+
+      // Make sure the message size isn't too big.
+      if (size > 3)
+      {
+        warning(
+            configuration,
+            "midi_out_winmm::send_message: message size is greater than 3 bytes "
+            "(and not sysex)!");
+        return;
+      }
+
+      // Pack MIDI bytes into double word.
+      DWORD packet;
+      std::copy_n(message, size, (unsigned char*)&packet);
+
+      // Send the message immediately.
+      auto result = midiOutShortMsg(this->outHandle, packet);
+      if (result != MMSYSERR_NOERROR)
+      {
+        error<driver_error>(
+            configuration, "midi_out_winmm::send_message: error sending MIDI message.");
+      }
+    }
+  }
+
+private:
+  HMIDIOUT outHandle; // Handle to Midi Output Device
+  std::vector<char> buffer;
+};
+
+}

include/libremidi/backends/winmm/observer.hpp

@@ -0,0 +1,271 @@
+#pragma once
+#include <libremidi/backends/winmm/config.hpp>
+#include <libremidi/backends/winmm/helpers.hpp>
+#include <libremidi/detail/observer.hpp>
+
+#include <condition_variable>
+#include <mutex>
+#include <ranges>
+#include <thread>
+
+namespace libremidi
+{
+
+class observer_winmm : public observer_api
+{
+public:
+  struct
+      : observer_configuration
+      , winmm_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_winmm(observer_configuration&& conf, winmm_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (!configuration.has_callbacks())
+      return;
+
+    if (configuration.notify_in_constructor)
+      check_new_ports<true>();
+    else
+      check_new_ports<false>();
+  }
+
+  ~observer_winmm() { }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_MM; }
+
+  std::vector<input_port> get_input_ports() const noexcept override
+  {
+    return get_port_list<true>();
+  }
+
+  std::vector<output_port> get_output_ports() const noexcept override
+  {
+    return get_port_list<false>();
+  }
+
+protected:
+  template <bool Notify>
+  void check_new_ports()
+  {
+    auto currInputPortList = get_port_list<true>();
+
+    if constexpr (Notify)
+    {
+      compare_port_lists_and_notify_clients(
+          inputPortList, currInputPortList, configuration.input_added,
+          configuration.input_removed);
+    }
+    inputPortList = std::move(currInputPortList);
+
+    auto currOutputPortList = get_port_list<false>();
+
+    if constexpr (Notify)
+    {
+      compare_port_lists_and_notify_clients(
+          outputPortList, currOutputPortList, configuration.output_added,
+          configuration.output_removed);
+    }
+    outputPortList = std::move(currOutputPortList);
+  }
+
+  void compare_port_lists_and_notify_clients(
+      const auto& prevList, const auto& currList, const auto& portAddedFunc,
+      const auto& portRemovedFunc)
+  {
+    if (portAddedFunc)
+    {
+      for (const auto& port : currList)
+      {
+        auto iter
+            = std::ranges::find(prevList, port.display_name, &port_information::display_name);
+        if (iter == prevList.end())
+          portAddedFunc(port);
+      }
+    }
+
+    if (portRemovedFunc)
+    {
+      for (const auto& port : prevList)
+      {
+        auto iter
+            = std::ranges::find(currList, port.display_name, &port_information::display_name);
+        if (iter == currList.end())
+          portRemovedFunc(port);
+      }
+    }
+  }
+
+  input_port to_in_port_info(std::size_t index) const noexcept
+  {
+    MIDIINCAPS deviceCaps;
+    midiInGetDevCaps(index, &deviceCaps, sizeof(MIDIINCAPS));
+
+    auto rawName = ConvertToUTF8(deviceCaps.szPname);
+    auto portName = rawName;
+    MakeUniqueInPortName(portName, index);
+    return {
+        {.client = 0,
+         .port = index,
+         .manufacturer = "",
+         .device_name = "",
+         .port_name = rawName,
+         .display_name = portName}};
+  }
+
+  output_port to_out_port_info(std::size_t index) const noexcept
+  {
+    MIDIOUTCAPS deviceCaps;
+    midiOutGetDevCaps(index, &deviceCaps, sizeof(MIDIOUTCAPS));
+
+    auto rawName = ConvertToUTF8(deviceCaps.szPname);
+    auto portName = rawName;
+    MakeUniqueOutPortName(portName, index);
+    return {
+        {.client = 0,
+         .port = index,
+         .manufacturer = "",
+         .device_name = "",
+         .port_name = rawName,
+         .display_name = portName}};
+  }
+
+  template <bool Input>
+  auto get_port_list() const noexcept
+      -> std::vector<std::conditional_t<Input, input_port, output_port>>
+  {
+    std::vector<std::conditional_t<Input, input_port, output_port>> portList;
+
+    if constexpr (Input)
+    {
+      std::size_t nDevices = midiInGetNumDevs();
+      for (std::size_t i = 0; i < nDevices; ++i)
+      {
+        portList.push_back(to_in_port_info(i));
+      }
+    }
+    else
+    {
+      std::size_t nDevices = midiOutGetNumDevs();
+      for (std::size_t i = 0; i < nDevices; ++i)
+      {
+        portList.push_back(to_out_port_info(i));
+      }
+    }
+    return portList;
+  }
+
+  static constexpr bool INPUT = true;
+  static constexpr bool OUTPUT = false;
+
+  std::vector<input_port> inputPortList;
+  std::vector<output_port> outputPortList;
+};
+}
+
+#if __has_include(<stop_token>) && __cpp_lib_jthread >= 201911L
+  #include <stop_token>
+namespace libremidi::winmm
+{
+class observer_threaded final : public observer_winmm
+{
+public:
+  struct
+      : observer_configuration
+      , winmm_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_threaded(observer_configuration&& conf, winmm_observer_configuration&& apiconf)
+      : observer_winmm{std::move(conf), std::move(apiconf)}
+  {
+    thread = std::jthread([this](std::stop_token tk) {
+      while (!tk.stop_requested())
+      {
+        check_new_ports<true>();
+        std::this_thread::sleep_for(this->configuration.poll_period);
+      }
+    });
+  }
+
+private:
+  std::jthread thread;
+};
+}
+#else
+  #include <atomic>
+  #include <semaphore>
+namespace libremidi::winmm
+{
+class observer_threaded final : public observer_winmm
+{
+public:
+  struct
+      : observer_configuration
+      , winmm_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_threaded(observer_configuration&& conf, winmm_observer_configuration&& apiconf)
+      : observer_winmm{std::move(conf), std::move(apiconf)}
+      , sema{0}
+  {
+    thread = std::thread([this] {
+      while (!stop_flag.test(std::memory_order_acquire))
+      {
+        check_new_ports<true>();
+        std::this_thread::sleep_for(this->configuration.poll_period);
+      }
+      sema.release();
+    });
+  }
+
+  ~observer_threaded()
+  {
+    stop_flag.test_and_set();
+    sema.acquire();
+    thread.join();
+  }
+
+private:
+  std::thread thread;
+  std::atomic_flag stop_flag = ATOMIC_FLAG_INIT;
+  std::binary_semaphore sema;
+};
+}
+#endif
+
+namespace libremidi::winmm
+{
+class observer_manual final : public observer_winmm
+{
+public:
+  struct
+      : observer_configuration
+      , winmm_observer_configuration
+  {
+  } configuration;
+
+  explicit observer_manual(observer_configuration&& conf, winmm_observer_configuration&& apiconf)
+      : observer_winmm{std::move(conf), std::move(apiconf)}
+  {
+    this->configuration.manual_poll({.callback = [this] { this->check_new_ports<true>(); }});
+  }
+
+  ~observer_manual() { }
+};
+}
+namespace libremidi
+{
+template <>
+inline std::unique_ptr<observer_api> make<observer_winmm>(
+    libremidi::observer_configuration&& conf, libremidi::winmm_observer_configuration&& api)
+{
+  if (api.manual_poll)
+    return std::make_unique<winmm::observer_manual>(std::move(conf), std::move(api));
+  else
+    return std::make_unique<winmm::observer_threaded>(std::move(conf), std::move(api));
+}
+}

include/libremidi/backends/winuwp.hpp

@@ -0,0 +1,22 @@
+#pragma once
+#include <libremidi/backends/winuwp/midi_in.hpp>
+#include <libremidi/backends/winuwp/midi_out.hpp>
+#include <libremidi/backends/winuwp/observer.hpp>
+
+namespace libremidi
+{
+struct winuwp_backend
+{
+  using midi_in = midi_in_winuwp;
+  using midi_out = midi_out_winuwp;
+  using midi_observer = observer_winuwp;
+  using midi_in_configuration = winuwp_input_configuration;
+  using midi_out_configuration = winuwp_output_configuration;
+  using midi_observer_configuration = winuwp_observer_configuration;
+  static const constexpr auto API = libremidi::API::WINDOWS_UWP;
+  static const constexpr auto name = "winuwp";
+  static const constexpr auto display_name = "Windows UWP";
+
+  static constexpr inline bool available() noexcept { return true; }
+};
+} // namespace libremidi

include/libremidi/backends/winuwp/config.hpp

@@ -0,0 +1,19 @@
+#pragma once
+#include <libremidi/config.hpp>
+
+namespace libremidi
+{
+
+struct winuwp_input_configuration
+{
+};
+
+struct winuwp_output_configuration
+{
+};
+
+struct winuwp_observer_configuration
+{
+};
+
+}

include/libremidi/backends/winuwp/helpers.hpp

@@ -0,0 +1,63 @@
+#pragma once
+#define NOMINMAX 1
+#define WIN32_LEAN_AND_MEAN 1
+#include <libremidi/detail/midi_api.hpp>
+
+#include <mutex>
+#include <string>
+#include <thread>
+#include <vector>
+
+#include <winrt/Windows.Devices.Enumeration.h>
+#include <winrt/Windows.Devices.Midi.h>
+#include <winrt/Windows.Foundation.h>
+#include <winrt/Windows.Storage.Streams.h>
+
+namespace libremidi
+{
+inline void winrt_init()
+{
+  // init_apartment should only be called on the threads we own.
+  // Since we're the library we don't own the threads we are called from,
+  // so we should not perform this initialization ourselves.
+  // winrt::init_apartment();
+}
+
+namespace
+{
+using namespace winrt;
+using namespace winrt::Windows::Foundation;
+using namespace winrt::Windows::Devices::Midi;
+using namespace winrt::Windows::Devices::Enumeration;
+using namespace winrt::Windows::Storage::Streams;
+
+// Helper function to allow waiting for aynchronous operation completion
+// from the thread in STA. The only benefit from it compared to the
+// get() function from winrt is that we avoid an assertion if waiting
+// from the STA thread.
+template <typename T>
+auto get(T const& async)
+{
+  if (async.Status() != AsyncStatus::Completed)
+  {
+    slim_mutex m;
+    slim_condition_variable cv;
+    bool completed = false;
+
+    async.Completed([&](auto&&, auto&&) {
+      {
+        slim_lock_guard const guard(m);
+        completed = true;
+      }
+
+      cv.notify_one();
+    });
+
+    slim_lock_guard guard(m);
+    cv.wait(m, [&] { return completed; });
+  }
+
+  return async.GetResults();
+}
+}
+}

include/libremidi/backends/winuwp/midi_in.hpp

@@ -0,0 +1,106 @@
+#pragma once
+#include <libremidi/backends/winuwp/config.hpp>
+#include <libremidi/backends/winuwp/helpers.hpp>
+#include <libremidi/backends/winuwp/observer.hpp>
+#include <libremidi/detail/midi_in.hpp>
+#include <libremidi/detail/midi_stream_decoder.hpp>
+
+namespace libremidi
+{
+
+class midi_in_winuwp final
+    : public midi1::in_api
+    , public error_handler
+{
+public:
+  struct
+      : input_configuration
+      , winuwp_input_configuration
+  {
+  } configuration;
+
+  explicit midi_in_winuwp(input_configuration&& conf, winuwp_input_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+
+  {
+    winrt_init();
+  }
+
+  ~midi_in_winuwp() override { close_port(); }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_in_winuwp: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winuwp: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_in_winuwp: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_UWP; }
+
+  bool open_port(const input_port& port, std::string_view) override
+  {
+    const auto id = winrt::to_hstring(port.port_name);
+    if (id.empty())
+      return false;
+
+    port_ = get(MidiInPort::FromIdAsync(id));
+    if (!port_)
+      return false;
+
+    midi_start_timestamp = std::chrono::steady_clock::now();
+
+    port_.MessageReceived(
+        [=](const winrt::Windows::Devices::Midi::IMidiInPort& inputPort,
+            const winrt::Windows::Devices::Midi::MidiMessageReceivedEventArgs& args) {
+      this->process_message(args.Message());
+    });
+
+    return true;
+  }
+
+  void process_message(const winrt::Windows::Devices::Midi::IMidiMessage& msg)
+  {
+    static constexpr timestamp_backend_info timestamp_info{
+        .has_absolute_timestamps = true,
+        .absolute_is_monotonic = false,
+        .has_samples = false,
+    };
+
+    auto reader = DataReader::FromBuffer(msg.RawData());
+    auto begin = msg.RawData().data();
+    auto end = begin + msg.RawData().Length();
+
+    const auto to_ns = [&msg] { return msg.Timestamp().count() * 1'000'000; };
+    m_processing.on_bytes({begin, end}, m_processing.timestamp<timestamp_info>(to_ns, 0));
+  }
+
+  void close_port() override
+  {
+    if (port_)
+    {
+      port_.Close();
+      port_ = nullptr;
+    }
+  }
+
+  timestamp absolute_timestamp() const noexcept override
+  {
+    return std::chrono::duration_cast<std::chrono::nanoseconds>(
+               std::chrono::steady_clock::now() - midi_start_timestamp)
+        .count();
+  }
+
+private:
+  winrt::Windows::Devices::Midi::IMidiInPort port_{nullptr};
+  std::chrono::steady_clock::time_point midi_start_timestamp;
+
+  midi1::input_state_machine m_processing{this->configuration};
+};
+}

include/libremidi/backends/winuwp/midi_out.hpp

@@ -0,0 +1,80 @@
+#pragma once
+#include <libremidi/backends/winuwp/config.hpp>
+#include <libremidi/backends/winuwp/helpers.hpp>
+#include <libremidi/backends/winuwp/observer.hpp>
+#include <libremidi/detail/midi_out.hpp>
+
+namespace libremidi
+{
+
+class midi_out_winuwp final
+    : public midi1::out_api
+    , public error_handler
+{
+public:
+  struct
+      : output_configuration
+      , winuwp_output_configuration
+  {
+  } configuration;
+
+  midi_out_winuwp(output_configuration&& conf, winuwp_output_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    winrt_init();
+  }
+
+  ~midi_out_winuwp() override { close_port(); }
+
+  bool open_virtual_port(std::string_view) override
+  {
+    warning(configuration, "midi_out_winuwp: open_virtual_port unsupported");
+    return false;
+  }
+  void set_client_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winuwp: set_client_name unsupported");
+  }
+  void set_port_name(std::string_view) override
+  {
+    warning(configuration, "midi_out_winuwp: set_port_name unsupported");
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_UWP; }
+
+  bool open_port(const output_port& port, std::string_view) override
+  {
+    const auto id = winrt::to_hstring(port.port_name);
+    if (id.empty())
+      return false;
+
+    port_ = get(MidiOutPort::FromIdAsync(id));
+    return bool(port_);
+  }
+
+  void close_port() override
+  {
+    if (port_)
+    {
+      port_.Close();
+      port_ = {};
+    }
+  }
+
+  void send_message(const unsigned char* message, size_t size) override
+  {
+    if (!port_)
+      return;
+
+    InMemoryRandomAccessStream str;
+    DataWriter rb(str);
+    rb.WriteBytes(
+        winrt::array_view<const uint8_t>{(const uint8_t*)message, (const uint8_t*)message + size});
+    port_.SendBuffer(rb.DetachBuffer());
+  }
+
+private:
+  winrt::Windows::Devices::Midi::IMidiOutPort port_{nullptr};
+};
+
+}

include/libremidi/backends/winuwp/observer.hpp

@@ -0,0 +1,296 @@
+#pragma once
+#include <libremidi/backends/winuwp/config.hpp>
+#include <libremidi/detail/observer.hpp>
+
+namespace libremidi
+{
+class observer_winuwp_internal
+{
+public:
+  struct port_info
+  {
+    hstring id;
+    hstring name;
+  };
+  struct callback
+  {
+    int token{};
+    std::function<void(const port_info&)> function;
+  };
+  struct callbacks
+  {
+    std::vector<callback> cbs;
+    int current_token{};
+    void operator()(const port_info& p)
+    {
+      for (auto& cb : cbs)
+        cb.function(p);
+    }
+
+    int add(std::function<void(const port_info&)> f)
+    {
+      int tk = current_token++;
+      cbs.emplace_back(tk, f);
+      return tk;
+    }
+    void remove(int tk)
+    {
+      auto it = std::remove_if(
+          cbs.begin(), cbs.end(), [tk](const callback& c) { return c.token == tk; });
+      auto r = std::distance(it, cbs.end());
+      cbs.erase(it, cbs.end());
+    }
+  };
+
+  explicit observer_winuwp_internal(hstring deviceSelector) { initialize(deviceSelector); }
+  ~observer_winuwp_internal() { terminate(); }
+
+  std::vector<port_info> get_ports() const
+  {
+    std::lock_guard<std::mutex> lock(portListMutex_);
+    return portList_;
+  }
+
+  unsigned int get_port_count() const
+  {
+    std::lock_guard<std::mutex> lock(portListMutex_);
+    return static_cast<unsigned int>(portList_.size());
+  }
+
+  bool get_port_info(unsigned int portNumber, port_info& portInfo) const
+  {
+    std::lock_guard<std::mutex> lock(portListMutex_);
+    if (portNumber >= portList_.size())
+      return false;
+    portInfo = portList_[portNumber];
+    return true;
+  }
+
+  hstring get_port_id(unsigned int portNumber) const
+  {
+    std::lock_guard<std::mutex> lock(portListMutex_);
+    return portNumber < portList_.size() ? portList_[portNumber].id : hstring{};
+  }
+
+  std::string get_port_name(unsigned int portNumber) const
+  {
+    std::lock_guard<std::mutex> lock(portListMutex_);
+    return portNumber < portList_.size() ? to_string(portList_[portNumber].name) : std::string{};
+  }
+
+  int PortAdded(const std::function<void(port_info)>& handler)
+  {
+    return portAddedEvent_.add(handler);
+  }
+
+  void PortAdded(int token) noexcept { portAddedEvent_.remove(token); }
+
+  int PortRemoved(const std::function<void(port_info)>& handler)
+  {
+    return portRemovedEvent_.add(handler);
+  }
+
+  void PortRemoved(int token) noexcept { portRemovedEvent_.remove(token); }
+
+private:
+  observer_winuwp_internal(const observer_winuwp_internal&) = delete;
+  observer_winuwp_internal& operator=(const observer_winuwp_internal&) = delete;
+
+private:
+  void initialize(hstring deviceSelector)
+  {
+    deviceWatcher_ = DeviceInformation::CreateWatcher(deviceSelector);
+
+    evTokenOnDeviceAdded_
+        = deviceWatcher_.Added({this, &observer_winuwp_internal::on_device_added});
+    evTokenOnDeviceRemoved_
+        = deviceWatcher_.Removed({this, &observer_winuwp_internal::on_device_removed});
+    evTokenOnDeviceUpdated_
+        = deviceWatcher_.Updated({this, &observer_winuwp_internal::on_device_updated});
+    evTokenOnDeviceEnumerationCompleted_ = deviceWatcher_.EnumerationCompleted(
+        {this, &observer_winuwp_internal::on_device_enumeration_completed});
+
+    deviceWatcher_.Start();
+  }
+
+  void terminate()
+  {
+    deviceWatcher_.Stop();
+    deviceWatcher_.EnumerationCompleted(evTokenOnDeviceEnumerationCompleted_);
+    deviceWatcher_.Updated(evTokenOnDeviceUpdated_);
+    deviceWatcher_.Removed(evTokenOnDeviceRemoved_);
+    deviceWatcher_.Added(evTokenOnDeviceAdded_);
+  }
+
+  void on_device_added(DeviceWatcher sender, DeviceInformation deviceInfo)
+  {
+    port_info p;
+    {
+      std::lock_guard<std::mutex> lock(portListMutex_);
+      p = port_info{deviceInfo.Id(), deviceInfo.Name()};
+      portList_.push_back(p);
+    }
+    portAddedEvent_(p);
+  }
+
+  void on_device_removed(DeviceWatcher sender, DeviceInformationUpdate deviceUpdate)
+  {
+    const auto id = deviceUpdate.Id();
+    auto pred = [&id](const port_info& portInfo) { return portInfo.id == id; };
+    std::optional<port_info> p;
+    hstring name;
+    {
+      std::lock_guard<std::mutex> lock(portListMutex_);
+      auto iter = std::find_if(portList_.begin(), portList_.end(), pred);
+      if (iter != portList_.end())
+      {
+        p = *iter;
+        portList_.erase(iter);
+      }
+    }
+    if (p)
+      portRemovedEvent_(*p);
+  }
+
+  void on_device_updated(DeviceWatcher sender, DeviceInformationUpdate deviceUpdate) { }
+
+  void on_device_enumeration_completed(DeviceWatcher sender, IInspectable const&) { }
+
+private:
+  std::vector<port_info> portList_;
+  mutable std::mutex portListMutex_;
+
+  DeviceWatcher deviceWatcher_{nullptr};
+  event_token evTokenOnDeviceAdded_;
+  event_token evTokenOnDeviceRemoved_;
+  event_token evTokenOnDeviceUpdated_;
+  event_token evTokenOnDeviceEnumerationCompleted_;
+
+  callbacks portAddedEvent_;
+  callbacks portRemovedEvent_;
+};
+
+class observer_winuwp final : public observer_api
+{
+public:
+  struct
+      : observer_configuration
+      , winuwp_observer_configuration
+  {
+  } configuration;
+
+  using port_info = observer_winuwp_internal::port_info;
+  explicit observer_winuwp(observer_configuration&& conf, winuwp_observer_configuration&& apiconf)
+      : configuration{std::move(conf), std::move(apiconf)}
+  {
+    if (!configuration.has_callbacks())
+      return;
+
+    if (configuration.notify_in_constructor)
+    {
+      if (configuration.input_added)
+        for (const auto& p : get_input_ports())
+          configuration.input_added(p);
+
+      if (configuration.output_added)
+        for (const auto& p : get_output_ports())
+          configuration.output_added(p);
+    }
+
+    evTokenOnInputAdded_
+        = internalInPortObserver_.PortAdded([this](const port_info& p) { on_input_added(p); });
+    evTokenOnInputRemoved_
+        = internalInPortObserver_.PortRemoved([this](const port_info& p) { on_input_removed(p); });
+    evTokenOnOutputAdded_
+        = internalOutPortObserver_.PortAdded([this](const port_info& p) { on_output_added(p); });
+    evTokenOnOutputRemoved_ = internalOutPortObserver_.PortRemoved(
+        [this](const port_info& p) { on_output_removed(p); });
+  }
+
+  ~observer_winuwp()
+  {
+    if (!configuration.has_callbacks())
+      return;
+    internalInPortObserver_.PortAdded(evTokenOnInputAdded_);
+    internalInPortObserver_.PortRemoved(evTokenOnInputRemoved_);
+    internalOutPortObserver_.PortAdded(evTokenOnOutputAdded_);
+    internalOutPortObserver_.PortRemoved(evTokenOnOutputRemoved_);
+  }
+
+  libremidi::API get_current_api() const noexcept override { return libremidi::API::WINDOWS_UWP; }
+
+  template <bool Input>
+  auto to_port_info(const observer_winuwp_internal::port_info& p) const noexcept
+      -> std::conditional_t<Input, input_port, output_port>
+  {
+    return {
+        {.client = 0,
+         .port = 0,
+         .manufacturer = "",
+         .device_name = "",
+         .port_name = to_string(p.id),
+         .display_name = to_string(p.name)}};
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept override
+  {
+    std::vector<libremidi::input_port> ret;
+    for (auto& port : internalInPortObserver_.get_ports())
+      ret.push_back(to_port_info<true>(port));
+    return ret;
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept override
+  {
+    std::vector<libremidi::output_port> ret;
+    for (auto& port : internalOutPortObserver_.get_ports())
+      ret.push_back(to_port_info<false>(port));
+    return ret;
+  }
+
+  static observer_winuwp_internal& get_internal_in_port_observer()
+  {
+    return internalInPortObserver_;
+  }
+
+  static observer_winuwp_internal& get_internal_out_port_observer()
+  {
+    return internalOutPortObserver_;
+  }
+
+  void on_input_added(const observer_winuwp_internal::port_info& name)
+  {
+    if (configuration.input_added)
+      configuration.input_added(to_port_info<true>(name));
+  }
+
+  void on_input_removed(const observer_winuwp_internal::port_info& name)
+  {
+    if (configuration.input_removed)
+      configuration.input_removed(to_port_info<true>(name));
+  }
+
+  void on_output_added(const observer_winuwp_internal::port_info& name)
+  {
+    if (configuration.output_added)
+      configuration.output_added(to_port_info<false>(name));
+  }
+
+  void on_output_removed(const observer_winuwp_internal::port_info& name)
+  {
+    if (configuration.output_removed)
+      configuration.output_removed(to_port_info<false>(name));
+  }
+
+private:
+  static inline observer_winuwp_internal internalInPortObserver_{MidiInPort::GetDeviceSelector()};
+  static inline observer_winuwp_internal internalOutPortObserver_{
+      MidiOutPort::GetDeviceSelector()};
+
+  int evTokenOnInputAdded_{-1};
+  int evTokenOnInputRemoved_{-1};
+  int evTokenOnOutputAdded_{-1};
+  int evTokenOnOutputRemoved_{-1};
+};
+
+}

include/libremidi/client.cpp

@@ -0,0 +1,47 @@
+#if !defined(LIBREMIDI_HEADER_ONLY)
+  #include "client.hpp"
+#endif
+#include <libremidi/backends.hpp>
+#include <libremidi/shared_context.hpp>
+
+#ifdef LIBREMIDI_ALSA
+  #include <libremidi/backends/alsa_seq/shared_handler.hpp>
+#endif
+#ifdef LIBREMIDI_JACK
+  #include <libremidi/backends/jack/shared_handler.hpp>
+#endif
+namespace libremidi
+{
+LIBREMIDI_INLINE
+shared_configurations
+create_shared_context(const libremidi::API api, [[maybe_unused]] std::string_view client_name)
+{
+  switch (api)
+  {
+#if __has_include(<boost/lockfree/spsc_queue.hpp>)
+  #if defined(LIBREMIDI_ALSA)
+    case libremidi::API::ALSA_SEQ:
+      return alsa_seq::shared_handler::make(client_name);
+  #endif
+
+  #if defined(LIBREMIDI_JACK)
+    case libremidi::API::JACK_MIDI:
+      return jack::shared_handler::make(client_name);
+  #endif
+#endif
+    case libremidi::API::COREMIDI:
+    case libremidi::API::COREMIDI_UMP:
+      // TODO
+
+    case libremidi::API::WINDOWS_MIDI_SERVICES:
+      // TODO
+
+    default:
+      return {
+          .context = {},
+          .observer = observer_configuration_for(api),
+          .in = midi_in_configuration_for(api),
+          .out = midi_out_configuration_for(api)};
+  }
+}
+}

include/libremidi/client.hpp

@@ -0,0 +1,196 @@
+#pragma once
+#include <libremidi/configurations.hpp>
+#include <libremidi/libremidi.hpp>
+#include <libremidi/shared_context.hpp>
+
+#include <map>
+
+namespace libremidi::midi1
+{
+struct client_configuration
+{
+  libremidi::API api = libremidi::midi1::default_api();
+
+  //! A client name, if the backend allows it (e.g. JACK, ALSA SEQ...)
+  std::string_view client_name = "libremidi client";
+
+  //! Set a callback function to be invoked for incoming MIDI messages.
+  //! Mandatory!
+  std::function<void(const libremidi::input_port&, message&&)> on_message
+      = [](const libremidi::input_port& /*port*/, libremidi::message&&) {};
+
+  //! Observation callbacks for when ports are added or removed
+  input_port_callback input_added;
+  input_port_callback input_removed;
+  output_port_callback output_added;
+  output_port_callback output_removed;
+
+  //! Error callback function to be invoked when an error has occured.
+  /*!
+    The callback function will be called whenever an error has occured. It is
+    best to set the error callback function before opening a port.
+  */
+  midi_error_callback on_error{};
+  midi_error_callback on_warning{};
+
+  //! Poll period for observation polling operations, if relevant to the backend
+  std::chrono::milliseconds poll_period{100};
+
+  //! Specify whether certain MIDI message types should be queued or ignored
+  //! during input.
+  /*!
+    By default, MIDI timing and active sensing messages are ignored
+    during message input because of their relative high data rates.
+    MIDI sysex messages are ignored by default as well.  Variable
+    values of "true" imply that the respective message type will be
+    ignored.
+  */
+  uint32_t ignore_sysex : 1 = true;
+  uint32_t ignore_timing : 1 = true;
+  uint32_t ignore_sensing : 1 = true;
+  uint32_t timestamps : 3 = timestamp_mode::Absolute;
+
+  //! Observe hardware ports
+  uint32_t track_hardware : 1 = true;
+
+  //! Observe software (virtual) ports if the API provides it
+  uint32_t track_virtual : 1 = false;
+};
+
+class client
+{
+public:
+  explicit client(const client_configuration& conf)
+      : client{conf, create_shared_context(conf.api, conf.client_name)}
+  {
+  }
+
+  explicit client(const client_configuration& conf, shared_configurations ctx)
+      : configuration{conf}
+      , context{ctx}
+      , m_observer{
+            observer_configuration{
+                .on_error = conf.on_error,
+                .on_warning = conf.on_warning,
+
+                .input_added = conf.input_added,
+                .input_removed = conf.input_removed,
+                .output_added = conf.output_added,
+                .output_removed = conf.output_removed,
+
+                .track_hardware = conf.track_hardware,
+                .track_virtual = conf.track_virtual,
+                .notify_in_constructor = false},
+            context.observer}
+  {
+    if (context.context)
+      context.context->start_processing();
+  }
+
+  ~client()
+  {
+    m_inputs.clear();
+    m_outputs.clear();
+
+    if (context.context)
+      context.context->stop_processing();
+  }
+
+  std::vector<libremidi::input_port> get_input_ports() const noexcept
+  {
+    return m_observer.get_input_ports();
+  }
+
+  std::vector<libremidi::output_port> get_output_ports() const noexcept
+  {
+    return m_observer.get_output_ports();
+  }
+
+  void add_input(const input_port& port, std::string_view name)
+  {
+    if (m_inputs.find(port) != m_inputs.end())
+      return;
+
+    auto res = m_inputs.try_emplace(
+        port,
+        input_configuration{
+            .on_message
+            = [this, port](libremidi::message&& m) {
+      configuration.on_message(port, std::move(m));
+            },
+            .get_timestamp = {},
+
+            .on_error = configuration.on_error,
+            .on_warning = configuration.on_warning,
+
+            .ignore_sysex = configuration.ignore_sysex,
+            .ignore_timing = configuration.ignore_timing,
+            .ignore_sensing = configuration.ignore_sensing,
+
+            .timestamps = configuration.timestamps},
+        context.in);
+
+    res.first->second.open_port(port, name);
+  }
+
+  void add_output(const output_port& port, std::string_view name)
+  {
+    if (m_outputs.find(port) != m_outputs.end())
+      return;
+
+    auto res = m_outputs.try_emplace(
+        port,
+        output_configuration{
+            .on_error = configuration.on_error,
+            .on_warning = configuration.on_warning,
+
+            .timestamps = configuration.timestamps},
+        context.out);
+
+    res.first->second.open_port(port, name);
+  }
+
+  void remove_input(const input_port& port) { m_inputs.erase(port); }
+  void remove_output(const output_port& port) { m_outputs.erase(port); }
+
+  void send_message(const unsigned char* message, size_t size)
+  {
+    for (auto& [_, out] : m_outputs)
+    {
+      out.send_message(message, size);
+    }
+  }
+
+  void send_ump(const uint32_t* message, size_t size)
+  {
+    for (auto& [_, out] : m_outputs)
+    {
+      out.send_ump(message, size);
+    }
+  }
+
+  void send_message(const output_port& port, const unsigned char* message, size_t size)
+  {
+    if (auto it = m_outputs.find(port); it != m_outputs.end())
+      it->second.send_message(message, size);
+  }
+
+  void send_ump(const output_port& port, const uint32_t* message, size_t size)
+  {
+    if (auto it = m_outputs.find(port); it != m_outputs.end())
+      it->second.send_ump(message, size);
+  }
+
+private:
+  client_configuration configuration;
+  shared_configurations context;
+
+  std::map<input_port, midi_in> m_inputs;
+  std::map<output_port, midi_out> m_outputs;
+
+  observer m_observer;
+};
+}
+#if defined(LIBREMIDI_HEADER_ONLY)
+  #include "client.cpp"
+#endif

include/libremidi/config.hpp

@@ -0,0 +1,68 @@
+#pragma once
+
+#if defined(MSC_VER)
+  #define NOMINMAX 1
+  #define WIN32_LEAN_AND_MEAN
+#endif
+#include <algorithm>
+#include <cinttypes>
+#include <memory>
+#include <stdexcept>
+#include <vector>
+
+#if defined(LIBREMIDI_EXPORTS)
+  #if defined(_MSC_VER)
+    #define LIBREMIDI_EXPORT __declspec(dllexport)
+  #elif defined(__GNUC__) || defined(__clang__)
+    #define LIBREMIDI_EXPORT __attribute__((visibility("default")))
+  #endif
+#else
+  #define LIBREMIDI_EXPORT
+#endif
+
+#define LIBREMIDI_VERSION "4.5.0"
+
+#if defined(LIBREMIDI_USE_BOOST)
+  #if !__has_include(<boost/container/small_vector.hpp>)
+    #error \
+        "Boost was used for building libremidi but is not found when using it. Add Boost to your include paths."
+  #endif
+
+  #if defined(LIBREMIDI_NO_BOOST)
+    #error "Boost was used for building libremidi but LIBREMIDI_NO_BOOST is defined."
+  #endif
+#endif
+
+#if __has_include(<boost/container/small_vector.hpp>) && !defined(LIBREMIDI_NO_BOOST)
+
+  #if LIBREMIDI_SLIM_MESSAGE > 0
+    #include <boost/container/static_vector.hpp>
+namespace libremidi
+{
+using midi_bytes = boost::container::static_vector<unsigned char, LIBREMIDI_SLIM_MESSAGE>;
+}
+  #else
+    #include <boost/container/small_vector.hpp>
+namespace libremidi
+{
+static constexpr int small_vector_minimum_size
+    = sizeof(boost::container::small_vector<unsigned char, 1>);
+using midi_bytes = boost::container::small_vector<unsigned char, small_vector_minimum_size>;
+}
+  #endif
+#else
+namespace libremidi
+{
+using midi_bytes = std::vector<unsigned char>;
+}
+#endif
+
+#if __has_include(<midi/universal_packet.h>) && defined(LIBREMIDI_USE_NI_MIDI2)
+  #define LIBREMIDI_NI_MIDI2_COMPAT 1
+#endif
+
+#if defined(LIBREMIDI_HEADER_ONLY)
+  #define LIBREMIDI_INLINE inline
+#else
+  #define LIBREMIDI_INLINE
+#endif

include/libremidi/configurations.hpp

@@ -0,0 +1,35 @@
+#pragma once
+
+#if defined(__linux__)
+  #include <libremidi/backends/alsa_raw/config.hpp>
+  #include <libremidi/backends/alsa_raw_ump/config.hpp>
+  #include <libremidi/backends/alsa_seq/config.hpp>
+  #include <libremidi/backends/alsa_seq_ump/config.hpp>
+  #include <libremidi/backends/pipewire/config.hpp>
+#endif
+
+#if defined(__APPLE__)
+  #include <libremidi/backends/coremidi/config.hpp>
+  #include <libremidi/backends/coremidi_ump/config.hpp>
+#endif
+
+#if defined(_WIN32)
+  #include <libremidi/backends/winmidi/config.hpp>
+  #include <libremidi/backends/winmm/config.hpp>
+  #include <libremidi/backends/winuwp/config.hpp>
+#endif
+
+#if defined(__EMSCRIPTEN__)
+  #include <libremidi/backends/emscripten/config.hpp>
+#endif
+
+#include <libremidi/backends/jack/config.hpp>
+
+namespace libremidi
+{
+
+struct dummy_configuration
+{
+};
+
+}

include/libremidi/defaults.hpp

@@ -0,0 +1,98 @@
+#pragma once
+#include <libremidi/api.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/input_configuration.hpp>
+#include <libremidi/observer_configuration.hpp>
+#include <libremidi/output_configuration.hpp>
+
+#include <any>
+#include <optional>
+
+namespace libremidi
+{
+class midi_in;
+class midi_out;
+class observer;
+
+//! Get the default midi in configuration object for a given API
+LIBREMIDI_EXPORT
+std::any midi_in_configuration_for(libremidi::API);
+
+//! Get the default midi out configuration object for a given API
+LIBREMIDI_EXPORT
+std::any midi_out_configuration_for(libremidi::API);
+
+//! Get the default observer configuration object for a given API
+LIBREMIDI_EXPORT
+std::any observer_configuration_for(libremidi::API);
+
+//! Get a matching midi in configuration object for a given observer instance.
+LIBREMIDI_EXPORT
+std::any midi_in_configuration_for(const libremidi::observer&);
+//! Get a matching midi out configuration object for a given observer instance.
+LIBREMIDI_EXPORT
+std::any midi_out_configuration_for(const libremidi::observer&);
+
+//! Get the default port for midi input (if any) in a given API.
+//! Note that this will only return a hardware / physical port.
+LIBREMIDI_EXPORT
+std::optional<input_port> in_default_port(libremidi::API api) noexcept;
+
+//! Get the default port for midi output (if any) in a given API.
+//! Note that this will only return a hardware / physical port.
+LIBREMIDI_EXPORT
+std::optional<output_port> out_default_port(libremidi::API api) noexcept;
+
+namespace midi1
+{
+//! Get a default configuration for creating a MIDI 1 input
+LIBREMIDI_EXPORT
+std::any in_default_configuration();
+
+//! Get a default configuration for creating a MIDI 1 output
+LIBREMIDI_EXPORT
+std::any out_default_configuration();
+
+//! Get a default configuration for creating a MIDI 1 observer
+LIBREMIDI_EXPORT
+std::any observer_default_configuration();
+
+//! Get a default MIDI 1 input port for the default API of the system
+inline std::optional<input_port> in_default_port() noexcept
+{
+  return libremidi::in_default_port(default_api());
+}
+
+//! Get a default MIDI 1 output port for the default API of the system
+inline std::optional<output_port> out_default_port() noexcept
+{
+  return libremidi::out_default_port(default_api());
+}
+}
+
+namespace midi2
+{
+//! Get a default configuration for creating a MIDI 2 input
+LIBREMIDI_EXPORT
+std::any in_default_configuration();
+
+//! Get a default configuration for creating a MIDI 2 output
+LIBREMIDI_EXPORT
+std::any out_default_configuration();
+
+//! Get a default configuration for creating a MIDI 2 observer
+LIBREMIDI_EXPORT
+std::any observer_default_configuration();
+
+//! Get a default MIDI 2 input port for the default API of the system
+inline std::optional<input_port> in_default_port() noexcept
+{
+  return libremidi::in_default_port(default_api());
+}
+//! Get a default MIDI 2 output port for the default API of the system
+inline std::optional<output_port> out_default_port() noexcept
+{
+  return libremidi::out_default_port(default_api());
+}
+}
+}

include/libremidi/detail/memory.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <memory>
+#include <mutex>
+
+namespace libremidi
+{
+template <auto func>
+struct deleter
+{
+  template <typename U>
+  void operator()(U* x) noexcept(noexcept(func(x)))
+  {
+    func(x);
+  }
+};
+
+template <typename T, auto func>
+using unique_handle = std::unique_ptr<T, deleter<func>>;
+
+template <typename T>
+std::shared_ptr<T> instance()
+{
+  static std::mutex mut;
+  static std::weak_ptr<T> cache;
+
+  std::lock_guard _{mut};
+
+  if (auto ptr = cache.lock())
+  {
+    return ptr;
+  }
+  else
+  {
+    auto shared = std::make_shared<T>();
+    cache = shared;
+    return shared;
+  }
+}
+}

include/libremidi/detail/midi_api.hpp

@@ -0,0 +1,87 @@
+#pragma once
+#include <libremidi/api.hpp>
+#include <libremidi/config.hpp>
+#include <libremidi/error.hpp>
+#include <libremidi/observer_configuration.hpp>
+
+#include <iostream>
+#include <string_view>
+
+namespace libremidi
+{
+struct error_handler
+{
+  //! Error reporting function for libremidi classes. Throws.
+  template <typename Error_T>
+  void error(auto& configuration, std::string_view errorString) const
+  {
+    if (configuration.on_error)
+    {
+      if (first_error)
+        return;
+
+      first_error = true;
+      configuration.on_error(Error_T::code, errorString);
+      first_error = false;
+    }
+    else
+    {
+#if defined(__LIBREMIDI_DEBUG__)
+      std::cerr << '\n' << errorString << "\n\n";
+#endif
+      throw Error_T{errorString.data()};
+    }
+  }
+
+  //! Warning reporting function for libremidi classes.
+  void warning(auto& configuration, std::string_view errorString) const
+  {
+    if (configuration.on_warning)
+    {
+      if (first_warning)
+        return;
+
+      first_warning = true;
+      configuration.on_warning(midi_error::WARNING, errorString);
+      first_warning = false;
+      return;
+    }
+
+#if defined(__LIBREMIDI_DEBUG__)
+    std::cerr << '\n' << errorString << "\n\n";
+#endif
+  }
+
+  // To prevent infinite error loops
+  mutable bool first_error{};
+  mutable bool first_warning{};
+};
+
+class midi_api
+{
+public:
+  midi_api() = default;
+  virtual ~midi_api() = default;
+  midi_api(const midi_api&) = delete;
+  midi_api(midi_api&&) = delete;
+  midi_api& operator=(const midi_api&) = delete;
+  midi_api& operator=(midi_api&&) = delete;
+
+  [[nodiscard]] virtual libremidi::API get_current_api() const noexcept = 0;
+
+  [[nodiscard]] virtual bool open_virtual_port(std::string_view) = 0;
+
+  virtual void close_port() = 0;
+  virtual void set_client_name(std::string_view) = 0;
+  virtual void set_port_name(std::string_view) = 0;
+
+  bool is_port_open() const noexcept { return bool(port_open_); }
+  bool is_port_connected() const noexcept { return bool(connected_); }
+
+protected:
+  friend class midi_in;
+  friend class midi_out;
+  bool port_open_{};
+  bool connected_{};
+};
+}

include/libremidi/detail/midi_in.hpp

@@ -0,0 +1,52 @@
+#pragma once
+#include <libremidi/detail/midi_api.hpp>
+#include <libremidi/input_configuration.hpp>
+
+namespace libremidi
+{
+class midi_in_api : public midi_api
+{
+public:
+  midi_in_api() = default;
+  ~midi_in_api() override = default;
+  midi_in_api(const midi_in_api&) = delete;
+  midi_in_api(midi_in_api&&) = delete;
+  midi_in_api& operator=(const midi_in_api&) = delete;
+  midi_in_api& operator=(midi_in_api&&) = delete;
+
+  [[nodiscard]] virtual bool open_port(const input_port& pt, std::string_view local_port_name) = 0;
+  [[nodiscard]] virtual timestamp absolute_timestamp() const noexcept = 0;
+};
+
+namespace midi1
+{
+class in_api : public midi_in_api
+{
+public:
+  using midi_in_api::midi_in_api;
+};
+}
+
+namespace midi2
+{
+class in_api : public midi_in_api
+{
+public:
+  using midi_in_api::midi_in_api;
+
+  bool firstMessage{true};
+};
+}
+
+template <typename T, typename Arg>
+std::unique_ptr<midi_in_api> make(libremidi::input_configuration&& conf, Arg&& arg)
+{
+  return std::make_unique<T>(std::move(conf), std::move(arg));
+}
+
+template <typename T, typename Arg>
+std::unique_ptr<midi_in_api> make(libremidi::ump_input_configuration&& conf, Arg&& arg)
+{
+  return std::make_unique<T>(std::move(conf), std::move(arg));
+}
+}

include/libremidi/detail/midi_out.hpp

@@ -0,0 +1,95 @@
+#pragma once
+#include <libremidi/cmidi2.hpp>
+#include <libremidi/detail/midi_api.hpp>
+#include <libremidi/output_configuration.hpp>
+
+#include <string_view>
+
+namespace libremidi
+{
+
+class midi_out_api : public midi_api
+{
+public:
+  midi_out_api() = default;
+  ~midi_out_api() override = default;
+  midi_out_api(const midi_out_api&) = delete;
+  midi_out_api(midi_out_api&&) = delete;
+  midi_out_api& operator=(const midi_out_api&) = delete;
+  midi_out_api& operator=(midi_out_api&&) = delete;
+
+  [[nodiscard]] virtual bool open_port(const output_port& pt, std::string_view local_port_name)
+      = 0;
+
+  [[nodiscard]] virtual int64_t current_time() const noexcept { return 0; }
+
+  virtual void send_message(const unsigned char* message, std::size_t size) = 0;
+  virtual void schedule_message(int64_t /*ts*/, const unsigned char* message, std::size_t size)
+  {
+    return send_message(message, size);
+  }
+
+  virtual void send_ump(const uint32_t* message, std::size_t size) = 0;
+  virtual void schedule_ump(int64_t /*ts*/, const uint32_t* ump, std::size_t size)
+  {
+    return send_ump(ump, size);
+  }
+};
+
+namespace midi1
+{
+class out_api : public midi_out_api
+{
+  friend struct midi_stream_decoder;
+
+public:
+  using midi_out_api::midi_out_api;
+
+  void send_ump(const uint32_t* message, std::size_t /*size*/)
+  {
+    uint8_t midi[65536];
+    const auto n
+        = cmidi2_convert_single_ump_to_midi1(midi, sizeof(midi), const_cast<uint32_t*>(message));
+    if (n > 0)
+      send_message(midi, n);
+  }
+};
+}
+
+namespace midi2
+{
+class out_api : public midi_out_api
+{
+  friend struct midi_stream_decoder;
+
+public:
+  using midi_out_api::midi_out_api;
+
+  void send_message(const unsigned char* message, std::size_t size)
+  {
+    cmidi2_midi_conversion_context context{};
+    cmidi2_midi_conversion_context_initialize(&context);
+
+    uint32_t ump[65536 / 4];
+
+    context.midi1 = const_cast<unsigned char*>(message);
+    context.midi1_num_bytes = size;
+    context.midi1_proceeded_bytes = 0;
+    context.ump = ump;
+    context.ump_num_bytes = sizeof(ump);
+    context.ump_proceeded_bytes = 0;
+
+    if (auto res = cmidi2_convert_midi1_to_ump(&context); res != CMIDI2_CONVERSION_RESULT_OK)
+      return;
+
+    send_ump(context.ump, context.ump_proceeded_bytes / 4);
+  }
+};
+}
+
+template <typename T, typename Arg>
+std::unique_ptr<midi_out_api> make(libremidi::output_configuration&& conf, Arg&& arg)
+{
+  return std::make_unique<T>(std::move(conf), std::forward<Arg>(arg));
+}
+}