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NAME="SDLOPENAUDIO"

>SDL_OpenAudio

>

>

CLASS="REFNAMEDIV"

>

NAME="AEN6103"

>

>

>Name

>SDL_OpenAudio -- Opens the audio device with the desired parameters.

>

CLASS="REFSYNOPSISDIV"

>

NAME="AEN6106"

>

>

>Synopsis

>

CLASS="FUNCSYNOPSIS"

>

NAME="AEN6107"

>

>

>

>

CLASS="FUNCSYNOPSISINFO"

>#include "SDL.h"

>

>

>

CLASS="FUNCDEF"

>int 

CLASS="FSFUNC"

>SDL_OpenAudio

>

>(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);

>

>

>

>

>

>

CLASS="REFSECT1"

>

NAME="AEN6113"

>

>

>Description

>

>This function opens the audio device with the 

CLASS="PARAMETER"

>

>desired

>

> parameters, and

returns 0 if successful, placing the actual hardware parameters in the

structure pointed to by 

CLASS="PARAMETER"

>

>obtained

>

>.  If 

CLASS="PARAMETER"

>

>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 

CLASS="PARAMETER"

>

>desired

>

> 

HREF="sdlaudiospec.html"

>

CLASS="STRUCTNAME"

>SDL_AudioSpec

>

> must be created.

CLASS="PROGRAMLISTING"

>SDL_AudioSpec *desired;

.

.

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

>

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

>

>

>

CLASS="VARIABLELIST"

>

>

>

CLASS="STRUCTNAME"

>desired

>->

CLASS="STRUCTFIELD"

>

>freq

>

>

>

>

>The desired audio frequency in samples-per-second.

>

>

>

CLASS="STRUCTNAME"

>desired

>->

CLASS="STRUCTFIELD"

>

>format

>

>

>

>

>The desired audio format (see 

HREF="sdlaudiospec.html"

>

CLASS="STRUCTNAME"

>SDL_AudioSpec

>

>)

>

>

>

CLASS="STRUCTNAME"

>desired

>->

CLASS="STRUCTFIELD"

>

>samples

>

>

>

>

>The desired size of the audio buffer in samples. This number should be a power of two, and may be adjusted by the audio driver to a value more suitable for the hardware.  Good values seem to range between 512 and 8192 inclusive, depending on the application and CPU speed.  Smaller values yield faster response time, but can lead to underflow if the application is doing heavy processing and cannot fill the audio buffer in time.  A stereo sample consists of both right and left channels in LR ordering.  Note that the number of samples is directly related to time by the following formula:  ms = (samples*1000)/freq

>

>

>

CLASS="STRUCTNAME"

>desired

>->

CLASS="STRUCTFIELD"

>

>callback

>

>

>

>

>This should be set to a function that will be called when the audio device is ready for more data.  It is passed a pointer to the audio buffer, and the length in bytes of the audio buffer. This function usually runs in a separate thread, and so you should protect data structures that it accesses by calling 

HREF="sdllockaudio.html"

>

CLASS="FUNCTION"

>SDL_LockAudio

>

> and 

HREF="sdlunlockaudio.html"

>

CLASS="FUNCTION"

>SDL_UnlockAudio

>

> in your code. The callback prototype is:

CLASS="PROGRAMLISTING"

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

>

CLASS="PARAMETER"

>

>userdata

>

> is the pointer stored in 

CLASS="STRUCTFIELD"

>

>userdata

>

> field of the 

CLASS="STRUCTNAME"

>SDL_AudioSpec

>. 

CLASS="PARAMETER"

>

>stream

>

> is a pointer to the audio buffer you want to fill with information and 

CLASS="PARAMETER"

>

>len

>

> is the length of the audio buffer in bytes.

>

>

>

CLASS="STRUCTNAME"

>desired

>->

CLASS="STRUCTFIELD"

>

>userdata

>

>

>

>

>This pointer is passed as the first parameter to the 

CLASS="FUNCTION"

>callback

> function.

>

>

>

>

>

CLASS="FUNCTION"

>SDL_OpenAudio

> reads these fields from the 

CLASS="PARAMETER"

>

>desired

>

> 

CLASS="STRUCTNAME"

>SDL_AudioSpec

> structure pass to the function and attempts to find an audio configuration matching your 

CLASS="PARAMETER"

>

>desired

>

>. As mentioned above, if the 

CLASS="PARAMETER"

>

>obtained

>

> parameter is 

CLASS="LITERAL"

>NULL

> then SDL with convert from your 

CLASS="PARAMETER"

>

>desired

>

> audio settings to the hardware settings as it plays.

>

>If 

CLASS="PARAMETER"

>

>obtained

>

> is 

CLASS="LITERAL"

>NULL

> then the 

CLASS="PARAMETER"

>

>desired

>

> 

CLASS="STRUCTNAME"

>SDL_AudioSpec

> is your working specification, otherwise the 

CLASS="PARAMETER"

>

>obtained

>

> 

CLASS="STRUCTNAME"

>SDL_AudioSpec

> becomes the working specification and the 

CLASS="PARAMETER"

>

>desirec

>

> specification can be deleted. The data in the working specification is used when building 

CLASS="STRUCTNAME"

>SDL_AudioCVT

>'s for converting loaded data to the hardware format.

>

>

CLASS="FUNCTION"

>SDL_OpenAudio

> calculates the 

CLASS="STRUCTFIELD"

>

>size

>

> and 

CLASS="STRUCTFIELD"

>

>silence

>

> fields for both the 

CLASS="PARAMETER"

>

>desired

>

> and 

CLASS="PARAMETER"

>

>obtained

>

> specifications. The 

CLASS="STRUCTFIELD"

>

>size

>

> field stores the total size of the audio buffer in bytes, while the 

CLASS="STRUCTFIELD"

>

>silence

>

> stores the value used to represent silence in the audio buffer

>

>The audio device starts out playing 

CLASS="STRUCTFIELD"

>

>silence

>

> when it's opened, and should be enabled for playing by calling 

HREF="sdlpauseaudio.html"

>

CLASS="FUNCTION"

>SDL_PauseAudio

>(

CLASS="PARAMETER"

>

>0

>

>)

> when you are ready for your audio 

CLASS="STRUCTFIELD"

>

>callback

>

> function to be called.  Since the audio driver may modify the requested 

CLASS="STRUCTFIELD"

>

>size

>

> of the audio buffer, you should allocate any local mixing buffers after you open the audio device.

>

>

CLASS="REFSECT1"

>

NAME="AEN6200"

>

>

>Examples

>

CLASS="PROGRAMLISTING"

>/* 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;

/* 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\n", SDL_GetError());

  exit(-1);

}

/* desired spec is no longer needed */

free(desired);

hardware_spec=obtained;

.

.

/* Prepare callback for playing */

.

.

.

/* Start playing */

SDL_PauseAudio(0);

>

>

CLASS="REFSECT1"

>

NAME="AEN6203"

>

>

>See Also

>

>

HREF="sdlaudiospec.html"

>

CLASS="FUNCTION"

>SDL_AudioSpec

>

>,

HREF="sdllockaudio.html"

>

CLASS="FUNCTION"

>SDL_LockAudio

>

>,

HREF="sdlunlockaudio.html"

>

CLASS="FUNCTION"

>SDL_UnlockAudio

>

>,

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>

CLASS="FUNCTION"

>SDL_PauseAudio

>

>

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