Simple sound wave generator with SDL in c++

Question

i am having problems understanding how the audio part of the sdl library works now, i know that when you initialize it, you have to specify the frequency and a >>callback<

Answer 1

A boiled-down variant of the beeper-example, reduced to the bare minimum (with error-handling).

#include 
#include 
#include 

const int AMPLITUDE = 28000;
const int SAMPLE_RATE = 44100;

void audio_callback(void *user_data, Uint8 *raw_buffer, int bytes)
{
    Sint16 *buffer = (Sint16*)raw_buffer;
    int length = bytes / 2; // 2 bytes per sample for AUDIO_S16SYS
    int &sample_nr(*(int*)user_data);

    for(int i = 0; i < length; i++, sample_nr++)
    {
        double time = (double)sample_nr / (double)SAMPLE_RATE;
        buffer[i] = (Sint16)(AMPLITUDE * sin(2.0f * M_PI * 441.0f * time)); // render 441 HZ sine wave
    }
}

int main(int argc, char *argv[])
{
    if(SDL_Init(SDL_INIT_AUDIO) != 0) SDL_Log("Failed to initialize SDL: %s", SDL_GetError());

    int sample_nr = 0;

    SDL_AudioSpec want;
    want.freq = SAMPLE_RATE; // number of samples per second
    want.format = AUDIO_S16SYS; // sample type (here: signed short i.e. 16 bit)
    want.channels = 1; // only one channel
    want.samples = 2048; // buffer-size
    want.callback = audio_callback; // function SDL calls periodically to refill the buffer
    want.userdata = &sample_nr; // counter, keeping track of current sample number

    SDL_AudioSpec have;
    if(SDL_OpenAudio(&want, &have) != 0) SDL_LogError(SDL_LOG_CATEGORY_AUDIO, "Failed to open audio: %s", SDL_GetError());
    if(want.format != have.format) SDL_LogError(SDL_LOG_CATEGORY_AUDIO, "Failed to get the desired AudioSpec");

    SDL_PauseAudio(0); // start playing sound
    SDL_Delay(1000); // wait while sound is playing
    SDL_PauseAudio(1); // stop playing sound

    SDL_CloseAudio();

    return 0;
}