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.
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.
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