libsoundio

C library which provides cross-platform audio input and output. The API is suitable for real-time software such as digital audio workstations as well as consumer software such as music players.

This library is an abstraction; however it prioritizes performance and power over API convenience. Features that only exist in some sound backends are exposed.

This library is a work-in-progress.

Features and Limitations

• Supported backends:
  • PulseAudio
  • ALSA
  • Dummy (silence)
  • (planned) JACK
  • (planned) CoreAudio
  • (planned) WASAPI
  • (planned) ASIO
• C library. Depends only on the respective backend API libraries and libc. Does not depend on libstdc++, and does not have exceptions, run-time type information, or setjmp.
• Does not write anything to stdio. I'm looking at you, PortAudio.
• Supports channel layouts (also known as channel maps), important for surround sound applications.
• Ability to monitor devices and get an event when available devices change.
• Detects which input device is default and which output device is default.
• Ability to connect to multiple backends at once. For example you could have an ALSA device open and a JACK device open at the same time.
• Meticulously checks all return codes and memory allocations and uses meaningful error codes.

Synopsis

Complete program to emit a sine wave over the default device using the best backend:

#include <soundio/soundio.h>

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

__attribute__ ((cold))
__attribute__ ((noreturn))
__attribute__ ((format (printf, 1, 2)))
static void panic(const char *format, ...) {
    va_list ap;
    va_start(ap, format);
    vfprintf(stderr, format, ap);
    fprintf(stderr, "\n");
    va_end(ap);
    abort();
}

static const float PI = 3.1415926535f;
static float seconds_offset = 0.0f;
static void write_callback(struct SoundIoOutStream *outstream, int requested_frame_count) {
    float float_sample_rate = outstream->sample_rate;
    float seconds_per_frame = 1.0f / float_sample_rate;
    int err;

    for (;;) {
        int frame_count = requested_frame_count;

        struct SoundIoChannelArea *areas;
        if ((err = soundio_outstream_begin_write(outstream, &areas, &frame_count)))
            panic("%s", soundio_strerror(err));

        if (!frame_count)
            break;

        const struct SoundIoChannelLayout *layout = &outstream->layout;

        float pitch = 440.0f;
        float radians_per_second = pitch * 2.0f * PI;
        for (int frame = 0; frame < frame_count; frame += 1) {
            float sample = sinf((seconds_offset + frame * seconds_per_frame) * radians_per_second);
            for (int channel = 0; channel < layout->channel_count; channel += 1) {
                float *ptr = (float*)(areas[channel].ptr + areas[channel].step * frame);
                *ptr = sample;
            }
        }
        seconds_offset += seconds_per_frame * frame_count;

        if ((err = soundio_outstream_end_write(outstream, frame_count)))
            panic("%s", soundio_strerror(err));

        requested_frame_count -= frame_count;
        if (requested_frame_count <= 0)
            break;
    }
}

int main(int argc, char **argv) {
    struct SoundIo *soundio = soundio_create();
    if (!soundio)
        panic("out of memory");

    if ((err = soundio_connect(soundio)))
        panic("error connecting: %s", soundio_strerror(err));

    int default_out_device_index = soundio_get_default_output_device_index(soundio);
    if (default_out_device_index < 0)
        panic("no output device found");

    struct SoundIoDevice *device = soundio_get_output_device(soundio, default_out_device_index);
    if (!device)
        panic("out of memory");

    fprintf(stderr, "Output device: %s: %s\n", device->name, device->description);

    struct SoundIoOutStream *outstream = soundio_outstream_create(device);
    outstream->format = SoundIoFormatFloat32NE;
    outstream->write_callback = write_callback;

    if ((err = soundio_outstream_open(outstream)))
        panic("unable to open device: %s", soundio_strerror(err));

    if (outstream->layout_error)
        fprintf(stderr, "unable to set channel layout: %s\n", soundio_strerror(outstream->layout_error));

    if ((err = soundio_outstream_start(outstream)))
        panic("unable to start device: %s", soundio_strerror(err));

    for (;;)
        soundio_wait_events(soundio);

    soundio_outstream_destroy(outstream);
    soundio_device_unref(device);
    soundio_destroy(soundio);
    return 0;
}

Backend Priority

When you use soundio_connect, libsoundio tries these backends in order. If unable to connect to that backend, due to the backend not being installed, or the server not running, or the platform is wrong, the next backend is tried.

0. JACK
1. PulseAudio
2. ALSA (Linux)
3. CoreAudio (OSX)
4. WASAPI (Windows)
5. ASIO (Windows)
6. Dummy

If you don't like this order, you can use soundio_connect_backend to explicitly choose a backend to connect to. You can use soundio_backend_count and soundio_get_backend to get the list of available backends.

For complete API documentation, see src/soundio.h.

Contributing

libsoundio is programmed in a tiny subset of C++11:

• No STL.
• No new or delete.
• No class. All fields in structs are public.
• No exceptions or run-time type information.
• No references.
• No linking against libstdc++.

Do not be fooled - this is a C library, not a C++ library. We just take advantage of a select few C++11 compiler features such as templates, and then link against libc.

Building

Install the dependencies:

• cmake
• ALSA library (optional)
• libjack2 (optional)
• libpulseaudio (optional)

mkdir build
cd build
cmake ..
make
sudo make install

Building for Windows

You can build libsoundio with mxe. Follow the requirements section to install the packages necessary on your system. Then somewhere on your file system:

git clone https://github.com/mxe/mxe
cd mxe
make gcc

Then in the libsoundio source directory (replace "/path/to/mxe" with the appropriate path):

mkdir build-win
cd build-win
cmake .. -DCMAKE_TOOLCHAIN_FILE=/path/to/mxe/usr/i686-w64-mingw32.static/share/cmake/mxe-conf.cmake
make

Running the Tests

make test

For more detailed output:

make
./unit_tests

To see test coverage, install lcov, run make coverage and then view coverage/index.html in a browser.

Roadmap

0. implement JACK backend, get examples working
1. Steal PulseAudio's default channel maps per channel count
2. Ability to parse PulseAudio's "front-left" "front-right" channel label strings
3. When two soundio clients are talking to each other, use port names to negotiate channel maps.
4. why does pulseaudio microphone use up all the CPU?
5. merge in/out stream structures and functions?
6. implement CoreAudio (OSX) backend, get examples working
7. implement WASAPI (Windows) backend, get examples working
8. implement ASIO (Windows) backend, get examples working
9. Avoid calling soundio_panic in PulseAudio.
10. Figure out a way to test prebuf. I suspect prebuf not working for ALSA which is why we have to pre-fill the ring buffer with silence for the microphone example.
11. PulseAudio: when prebuf gets set to 0 need to pass PA_STREAM_START_CORKED.
12. In ALSA do we need to wake up the poll when destroying the in or out stream?
13. Create a test for clearing the playback buffer.
14. Create a test for pausing and resuming input and output streams.
15. Create a test for the latency / synchronization API.
    • Input is an audio file and some events indexed at particular frame - when listening the events should line up exactly with a beat or visual indicator, even when the latency is large.
    • Play the audio file, have the user press an input right at the beat. Find out what the frame index it thinks the user pressed it at and make sure that is correct.
16. Expose JACK options in jack_client_open
17. Allow calling functions from outside the callbacks as long as they first call lock and then unlock when done.
18. Should pause/resume be callable from outside the callbacks?
19. device.name -> device.id, device.description -> device.name
20. PulseAudio: when opening a device start it corked that way we can get accurate buffer readings
21. clean up API and improve documentation
    • make sure every function which can return an error documents which errors it can return
22. use a documentation generator and host the docs somewhere
23. -fvisibility=hidden and then explicitly export stuff, or explicitly make the unexported stuff private
24. Integrate into libgroove and test with Groove Basin
25. look at microphone example and determine if fewer memcpys can be done with the audio data
    • test that sending the frame count to begin read works with PulseAudio
26. add len arguments to APIs that have char *
27. Test in an app that needs to synchronize video to test the latency/synchronization API.
28. Support PulseAudio proplist properties for main context and streams
29. custom allocator support
30. mlock memory which is accessed in the real time path
31. instead of void *backend_data use a union for better cache locality and smaller mlock requirements
32. Consider testing on FreeBSD
33. make rtprio warning a callback and have existing behavior be the default callback
34. write detailed docs on buffer underflows explaining when they occur, what state changes are related to them, and how to recover from them.

Planned Uses for libsoundio

• Genesis
• libgroove (Groove Basin)