lesson_audio.c

#include <SDL2/SDL.h>
#include <math.h>

#define FREQ 44100

struct note_to_play
{
    Uint8 *data;
    Uint32 len;
    Uint32 offset;
};

static void fill_note(float note_frequency, float *data, int len)
{
    float duration = 1.0;
    float period = duration / note_frequency;

    int number_of_samples_per_period = period * FREQ;
    int i = 0;
    float value = 0;
    for (i = 0; i < FREQ; i++)
    {
        // note the pointer math data[0] is at address 0, data[1] is at address 4, data[2] is at address 8
        data[i] = sinf(value);
        value += (M_PI * 2) / (float)number_of_samples_per_period;
    }
}

static void fill_audio(void *user_data, Uint8 *stream, int len)
{
    struct note_to_play *note = (struct note_to_play *) user_data;

    SDL_Log("callback wants %d bytes", len);
    // silence
    //SDL_memset(stream, 0, len);
    if (note->offset + len < note->len)
    {
        SDL_memcpy(stream, note->data + note->offset, len);
        note->offset += len;
    }
}

int main(int argc, char **argv)
{
    if (SDL_Init(SDL_INIT_AUDIO))
    {
        SDL_Log("init error: %s", SDL_GetError());
        return -1;
    }

    // get the number of audio devices in the system
    int device_num = SDL_GetNumAudioDevices(0);
    SDL_Log("found %d sound devices", device_num);

    // print the names of audio devices
    int i;
    for (i = 0; i < device_num; i++)
    {
        SDL_Log("[%d] %s", i, SDL_GetAudioDeviceName(i, 0));
    }  

    // create a note sample with frequency 880
    float *note_buffer = SDL_malloc(sizeof(float) * FREQ);
    fill_note(880, note_buffer, FREQ);

    // convert from float to sint16
    Sint16 *note_s16 = SDL_malloc(sizeof(Sint16) * FREQ);
    for (i = 0; i < FREQ; i++)
    {
        // pointer math note_s16[1] is at address 2, note_buffer[1] is at address 4
        note_s16[i] = note_buffer[i] * SDL_MAX_SINT16;
    }

    SDL_AudioSpec wanted, received;
    SDL_memset(&wanted, 0, sizeof(SDL_AudioSpec));

    struct note_to_play note;
    note.data = (Uint8 *)note_s16;
    note.offset = 0;
    note.len = sizeof(Sint16) * FREQ;

    wanted.channels = 1;
    wanted.format = AUDIO_S16;
    wanted.freq = FREQ;
    wanted.samples = 4096;
    wanted.callback = fill_audio;
    wanted.userdata = &note;

    SDL_AudioDeviceID dev = SDL_OpenAudioDevice(NULL, 0, &wanted, &received, 0);
    if (!dev)
    {
        SDL_Log("open device error: %s", SDL_GetError());
        return -1;
    }

    SDL_PauseAudioDevice(dev, 0);

    for (;;)
    {
    }

    SDL_Quit();
    return 0;
}