Render basic waveform using Core Audio on macOS

synth.c

// Compile this with:
// $ gcc synth.c -o synth -framework AudioToolbox

// Based on the CH07_AUGraphSineWave example from the book
// "Learning Core Audio: A Hands-On Guide to Audio Programming
// for Mac and iOS" by Chris Adamson and Kevin Avila

#include <AudioToolbox/AudioToolbox.h>

typedef struct
{
    AudioUnit outputUnit;
    double sampleRate;
    double phase;
}
Player;

OSStatus renderAudio(
    void *inRefCon,
    AudioUnitRenderActionFlags *ioActionFlags,
    const AudioTimeStamp *inTimeStamp,
    UInt32 inBusNumber,
    UInt32 inNumberFrames,
    AudioBufferList *ioData)
{
    Player *player = (Player *)inRefCon;

    double sineFrequency = 880.0;
    double cycleLength = player->sampleRate / sineFrequency;

    double phase = player->phase;

    float *dataL = (float *)ioData->mBuffers[0].mData;
    float *dataR = (float *)ioData->mBuffers[1].mData;

    for (UInt32 frame = 0; frame < inNumberFrames; ++frame)
    {
        dataL[frame] = sin(2 * M_PI * phase / cycleLength);
        dataR[frame] = sin(2 * M_PI * phase / cycleLength);

        phase += 1.0;
        if (phase > cycleLength) {
            phase -= cycleLength;
        }
    }

    player->phase = phase;
    return noErr;
}

int	main(int argc, const char *argv[])
{
    Player player = { 0 };

    AudioComponentDescription desc = { 0 };
    desc.componentType = kAudioUnitType_Output;
    desc.componentSubType = kAudioUnitSubType_DefaultOutput;
    desc.componentManufacturer = kAudioUnitManufacturer_Apple;

    AudioComponent comp = AudioComponentFindNext(NULL, &desc);
    if (comp == NULL) {
        fprintf(stderr, "Couldn't get output AudioUnit\n");
        exit(1);
    }

    if (noErr != AudioComponentInstanceNew(comp, &player.outputUnit)) {
        fprintf(stderr, "Couldn't instantiate component for output AudioUnit\n");
        exit(1);
    }

    AURenderCallbackStruct input;
    input.inputProc = renderAudio;
    input.inputProcRefCon = &player;
    if (noErr != AudioUnitSetProperty(player.outputUnit,
                                      kAudioUnitProperty_SetRenderCallback,
                                      kAudioUnitScope_Input,
                                      0,
                                      &input,
                                      sizeof(input))) {
        fprintf(stderr, "Couldn't set render callback on output AudioUnit\n");
        exit(1);
    }

    if (noErr != AudioUnitInitialize(player.outputUnit)) {
        fprintf(stderr, "Couldn't initialize output AudioUnit\n");
        exit(1);
    }

    UInt32 size = sizeof(double);
    if (noErr != AudioUnitGetProperty(player.outputUnit,
                                      kAudioUnitProperty_SampleRate,
                                      kAudioUnitScope_Output,
                                      0,
                                      &player.sampleRate,
                                      &size)) {
        fprintf(stderr, "Couldn't get sample rate\n");
        exit(1);
    }

    if (noErr != AudioOutputUnitStart(player.outputUnit)) {
        fprintf(stderr, "Couldn't start output AudioUnit\n");
        exit(1);
    }

    sleep(5);   // play for 5 seconds

    AudioOutputUnitStop(player.outputUnit);
    AudioUnitUninitialize(player.outputUnit);
    AudioComponentInstanceDispose(player.outputUnit);

    return 0;
}