SDL2 Squarewave

sdl_squarewave.c

#include <math.h>

#include <SDL2/SDL.h>


struct squarewave {
        float phase_inc;
        float phase;
        float volume;
};

void audio_callback(void *userdata, uint8_t *stream, int len)
{
        union floatbytes {
                float f;
                uint8_t bytes[4]; // Assume little endian
        };

        union floatbytes overlay;

        struct squarewave *swdata = (struct squarewave *)userdata;

        SDL_Log("Audio callback! len: %d\n", len);
        SDL_Log("phase_inc: %f\n", swdata->phase_inc);
        SDL_Log("phase: %f\n", swdata->phase);
        SDL_Log("volume: %f\n", swdata->volume);

        for (int i = 0; i < len; i += 4) {
                overlay.f = swdata->volume;
                if (swdata->phase > 0.5) {
                        overlay.f = -1 * swdata->volume;
                }

                stream[i + 0] = overlay.bytes[0];
                stream[i + 1] = overlay.bytes[1];
                stream[i + 2] = overlay.bytes[2];
                stream[i + 3] = overlay.bytes[3];

                swdata->phase = fmodf((swdata->phase + swdata->phase_inc), (float)1.0);
        }
}

int main()
{
        int err;

        SDL_AudioDeviceID dev;
        SDL_AudioSpec want, have;

        struct squarewave swdata;

        SDL_Log("SDL Squarewave");

        err = SDL_Init(SDL_INIT_AUDIO);
        if (err != 0) {
                SDL_Log("Init error: %s", SDL_GetError());
                return 1;
        }

        swdata.phase_inc = 440.0 / 44100.0;
        swdata.phase = 0.0;
        swdata.volume = 0.25;


        SDL_memset(&want, 0, sizeof(want));
        want.freq = 44100;
        want.format = AUDIO_F32;
        want.channels = 1;
        want.samples = 2048;
        want.userdata = &swdata;
        want.callback = audio_callback;

        dev = SDL_OpenAudioDevice(NULL, 0, &want, &have, SDL_AUDIO_ALLOW_FORMAT_CHANGE);

        SDL_Log("freq: %d, format: %d, channels: %d, samples: %d, size: %d", have.freq, have.format, have.channels, have.samples, have.size);

        if (dev == 0) {
                SDL_Log("Unable to open audio device: %s", SDL_GetError());
        }

        SDL_PauseAudioDevice(dev, 0);
        SDL_Delay(2000);
        SDL_PauseAudioDevice(dev, 1);

        return 0;
}