examples/audio/02-simple-playback-callback/simple-playback-callback.c

/*
 * This example code creates an simple audio stream for playing sound, and
 * generates a sine wave sound effect for it to play as time goes on. Unlike
 * the previous example, this uses a callback to generate sound.
 *
 * This might be the path of least resistance if you're moving an SDL2
 * program's audio code to SDL3.
 *
 * This code is public domain. Feel free to use it for any purpose!
 */

#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>

/* We will use this renderer to draw into this window every frame. */
static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;
static SDL_AudioStream *stream = NULL;
static int total_samples_generated = 0;

/* this function will be called (usually in a background thread) when the audio stream is consuming data. */
static void SDLCALL FeedTheAudioStreamMore(void *userdata, SDL_AudioStream *astream, int additional_amount, int total_amount)
{
    /* total_amount is how much data the audio stream is eating right now, additional_amount is how much more it needs
       than what it currently has queued (which might be zero!). You can supply any amount of data here; it will take what
       it needs and use the extra later. If you don't give it enough, it will take everything and then feed silence to the
       hardware for the rest. Ideally, though, we always give it what it needs and no extra, so we aren't buffering more
       than necessary. */
    additional_amount /= sizeof (float);  /* convert from bytes to samples */
    while (additional_amount > 0) {
        float samples[128];  /* this will feed 128 samples each iteration until we have enough. */
        const int total = SDL_min(additional_amount, SDL_arraysize(samples));
        int i;

        for (i = 0; i < total; i++) {
            /* You don't have to care about this math; we're just generating a simple sine wave as we go.
               https://en.wikipedia.org/wiki/Sine_wave */
            const float time = total_samples_generated / 8000.0f;
            const int sine_freq = 500;   /* run the wave at 500Hz */
            samples[i] = SDL_sinf(6.283185f * sine_freq * time);
            total_samples_generated++;
        }

        /* feed the new data to the stream. It will queue at the end, and trickle out as the hardware needs more data. */
        SDL_PutAudioStreamData(astream, samples, total * sizeof (float));
        additional_amount -= total;  /* subtract what we've just fed the stream. */
    }
}

/* This function runs once at startup. */
SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
{
    SDL_AudioSpec spec;

    if (!SDL_Init(SDL_INIT_VIDEO | SDL_INIT_AUDIO)) {
        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't initialize SDL!", SDL_GetError(), NULL);
        return SDL_APP_FAILURE;
    }

    /* we don't _need_ a window for audio-only things but it's good policy to have one. */
    if (!SDL_CreateWindowAndRenderer("examples/audio/simple-playback-callback", 640, 480, 0, &window, &renderer)) {
        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create window/renderer!", SDL_GetError(), NULL);
        return SDL_APP_FAILURE;
    }

    /* We're just playing a single thing here, so we'll use the simplified option.
       We are always going to feed audio in as mono, float32 data at 8000Hz.
       The stream will convert it to whatever the hardware wants on the other side. */
    spec.channels = 1;
    spec.format = SDL_AUDIO_F32;
    spec.freq = 8000;
    stream = SDL_OpenAudioDeviceStream(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, &spec, FeedTheAudioStreamMore, NULL);
    if (!stream) {
        SDL_ShowSimpleMessageBox(SDL_MESSAGEBOX_ERROR, "Couldn't create audio stream!", SDL_GetError(), window);
        return SDL_APP_FAILURE;
    }

    /* SDL_OpenAudioDeviceStream starts the device paused. You have to tell it to start! */
    SDL_ResumeAudioStreamDevice(stream);

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
SDL_AppResult SDL_AppEvent(void *appstate, const SDL_Event *event)
{
    if (event->type == SDL_EVENT_QUIT) {
        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
    }
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once per frame, and is the heart of the program. */
SDL_AppResult SDL_AppIterate(void *appstate)
{
    /* we're not doing anything with the renderer, so just blank it out. */
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);

    /* all the work of feeding the audio stream is happening in a callback in a background thread. */

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once at shutdown. */
void SDL_AppQuit(void *appstate)
{
    /* SDL will clean up the window/renderer for us. */
}