Manpages - SDL_OpenAudio.3

SDL_OpenAudio - Opens the audio device with the desired parameters.

#include “SDL.h”

int SDL_OpenAudio*(*SDL_AudioSpec *desired, SDL_AudioSpec *obtained);

This function opens the audio device with the desired parameters, and returns 0 if successful, placing the actual hardware parameters in the structure pointed to by obtained. If obtained is NULL, the audio data passed to the callback function will be guaranteed to be in the requested format, and will be automatically converted to the hardware audio format if necessary. This function returns -1 if it failed to open the audio device, or couldn’t set up the audio thread.

To open the audio device a desired SDL_AudioSpec must be created.

  SDL_AudioSpec *desired;
  .
  .
  desired=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));

You must then fill this structure with your desired audio specifications.

desired->freq

desired->format

desired->samples

desired->callback

  void callback(void *userdata, Uint8 *stream, int len);

userdata is the pointer stored in userdata field of the SDL_AudioSpec. stream is a pointer to the audio buffer you want to fill with information and len is the length of the audio buffer in bytes.

desired->userdata

SDL_OpenAudio reads these fields from the desired SDL_AudioSpec structure pass to the function and attempts to find an audio configuration matching your desired. As mentioned above, if the obtained parameter is NULL then SDL with convert from your desired audio settings to the hardware settings as it plays.

If obtained is NULL then the desired SDL_AudioSpec is your working specification, otherwise the obtained SDL_AudioSpec becomes the working specification and the desirec specification can be deleted. The data in the working specification is used when building SDL_AudioCVT’s for converting loaded data to the hardware format.

SDL_OpenAudio calculates the size and silence fields for both the desired and obtained specifications. The size field stores the total size of the audio buffer in bytes, while the silence stores the value used to represent silence in the audio buffer

The audio device starts out playing silence when it’s opened, and should be enabled for playing by calling SDL_PauseAudio*/(/*0) when you are ready for your audio callback function to be called. Since the audio driver may modify the requested size of the audio buffer, you should allocate any local mixing buffers after you open the audio device.

  /* Prototype of our callback function */
  void my_audio_callback(void *userdata, Uint8 *stream, int len);

  /* Open the audio device */
  SDL_AudioSpec *desired, *obtained;
  SDL_AudioSpec *hardware_spec;

  /* Allocate a desired SDL_AudioSpec */
  desired=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));

  /* Allocate space for the obtained SDL_AudioSpec */
  obtained=(SDL_AudioSpec *)malloc(sizeof(SDL_AudioSpec));

  /* 22050Hz - FM Radio quality */
  desired->freq=22050;

  /* 16-bit signed audio */
  desired->format=AUDIO_S16LSB;

  /* Mono */
  desired->channels=0;

  /* Large audio buffer reduces risk of dropouts but increases response time */
  desired->samples=8192;

  /* Our callback function */
  desired->callback=my_audio_callback;

  desired->userdata=NULL;

  /* Open the audio device */
  if ( SDL_OpenAudio(desired, obtained) < 0 ){
    fprintf(stderr, "Couldn't open audio: %s
  ", SDL_GetError());
    exit(-1);
  }
  /* desired spec is no longer needed */
  free(desired);
  hardware_spec=obtained;
  .
  .
  /* Prepare callback for playing */
  .
  .
  .
  /* Start playing */
  SDL_PauseAudio(0);

SDL_AudioSpec, SDL_LockAudio, SDL_UnlockAudio, SDL_PauseAudio