NAME
SDL_OpenAudio - Opens the audio device with the desired parameters.
SYNOPSIS
#include "SDL.h"
int SDL_OpenAudio(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);
DESCRIPTION
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.
EXAMPLES
/* 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);
SEE ALSO
SDL_AudioSpec, SDL_LockAudio, SDL_UnlockAudio, SDL_PauseAudio