docs/html/sdlopenaudio.html - third_party/sdl - Git at Google

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 ><H1
 ><A
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 ></A
 >SDL_OpenAudio</H1
 ><DIV
 CLASS="REFNAMEDIV"
 ><A
 NAME="AEN6650"
 ></A
 ><H2
 >Name</H2
 >SDL_OpenAudio&nbsp;--&nbsp;Opens the audio device with the desired parameters.</DIV
 ><DIV
 CLASS="REFSYNOPSISDIV"
 ><A
 NAME="AEN6653"
 ></A
 ><H2
 >Synopsis</H2
 ><DIV
 CLASS="FUNCSYNOPSIS"
 ><A
 NAME="AEN6654"
 ></A
 ><P
 ></P
 ><PRE
 CLASS="FUNCSYNOPSISINFO"
 >#include "SDL.h"</PRE
 ><P
 ><CODE
 ><CODE
 CLASS="FUNCDEF"
 >int <B
 CLASS="FSFUNC"
 >SDL_OpenAudio</B
 ></CODE
 >(SDL_AudioSpec *desired, SDL_AudioSpec *obtained);</CODE
 ></P
 ><P
 ></P
 ></DIV
 ></DIV
 ><DIV
 CLASS="REFSECT1"
 ><A
 NAME="AEN6660"
 ></A
 ><H2
 >Description</H2
 ><P
 >This function opens the audio device with the <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > parameters, and
 returns 0 if successful, placing the actual hardware parameters in the
 structure pointed to by <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 >.  If <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 > 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.</P
 ><P
 >To open the audio device a <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > <A
 HREF="sdlaudiospec.html"
 ><SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 ></A
 > must be created.
 <PRE
 CLASS="PROGRAMLISTING"
 >SDL_AudioSpec *desired;
 .
 .
 desired = malloc(sizeof(SDL_AudioSpec));</PRE
 >
 You must then fill this structure with your desired audio specifications.</P
 ><P
 ></P
 ><DIV
 CLASS="VARIABLELIST"
 ><DL
 ><DT
 ><SPAN
 CLASS="STRUCTNAME"
 >desired</SPAN
 >-&#62;<TT
 CLASS="STRUCTFIELD"
 ><I
 >freq</I
 ></TT
 ></DT
 ><DD
 ><P
 >The desired audio frequency in samples-per-second.</P
 ></DD
 ><DT
 ><SPAN
 CLASS="STRUCTNAME"
 >desired</SPAN
 >-&#62;<TT
 CLASS="STRUCTFIELD"
 ><I
 >format</I
 ></TT
 ></DT
 ><DD
 ><P
 >The desired audio format (see <A
 HREF="sdlaudiospec.html"
 ><SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 ></A
 >)</P
 ></DD
 ><DT
 ><SPAN
 CLASS="STRUCTNAME"
 >desired</SPAN
 >-&#62;<TT
 CLASS="STRUCTFIELD"
 ><I
 >samples</I
 ></TT
 ></DT
 ><DD
 ><P
 >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</P
 ></DD
 ><DT
 ><SPAN
 CLASS="STRUCTNAME"
 >desired</SPAN
 >-&#62;<TT
 CLASS="STRUCTFIELD"
 ><I
 >callback</I
 ></TT
 ></DT
 ><DD
 ><P
 >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 <A
 HREF="sdllockaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_LockAudio</TT
 ></A
 > and <A
 HREF="sdlunlockaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_UnlockAudio</TT
 ></A
 > in your code. The callback prototype is:
 <PRE
 CLASS="PROGRAMLISTING"
 >void callback(void *userdata, Uint8 *stream, int len);</PRE
 >
 <TT
 CLASS="PARAMETER"
 ><I
 >userdata</I
 ></TT
 > is the pointer stored in <TT
 CLASS="STRUCTFIELD"
 ><I
 >userdata</I
 ></TT
 > field of the <SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 >. <TT
 CLASS="PARAMETER"
 ><I
 >stream</I
 ></TT
 > is a pointer to the audio buffer you want to fill with information and <TT
 CLASS="PARAMETER"
 ><I
 >len</I
 ></TT
 > is the length of the audio buffer in bytes.</P
 ></DD
 ><DT
 ><SPAN
 CLASS="STRUCTNAME"
 >desired</SPAN
 >-&#62;<TT
 CLASS="STRUCTFIELD"
 ><I
 >userdata</I
 ></TT
 ></DT
 ><DD
 ><P
 >This pointer is passed as the first parameter to the <TT
 CLASS="FUNCTION"
 >callback</TT
 > function.</P
 ></DD
 ></DL
 ></DIV
 ><P
 ><TT
 CLASS="FUNCTION"
 >SDL_OpenAudio</TT
 > reads these fields from the <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > <SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 > structure pass to the function and attempts to find an audio configuration matching your <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 >. As mentioned above, if the <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 > parameter is <TT
 CLASS="LITERAL"
 >NULL</TT
 > then SDL with convert from your <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > audio settings to the hardware settings as it plays.</P
 ><P
 >If <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 > is <TT
 CLASS="LITERAL"
 >NULL</TT
 > then the <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > <SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 > is your working specification, otherwise the <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 > <SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioSpec</SPAN
 > becomes the working specification and the <TT
 CLASS="PARAMETER"
 ><I
 >desirec</I
 ></TT
 > specification can be deleted. The data in the working specification is used when building <SPAN
 CLASS="STRUCTNAME"
 >SDL_AudioCVT</SPAN
 >'s for converting loaded data to the hardware format.</P
 ><P
 ><TT
 CLASS="FUNCTION"
 >SDL_OpenAudio</TT
 > calculates the <TT
 CLASS="STRUCTFIELD"
 ><I
 >size</I
 ></TT
 > and <TT
 CLASS="STRUCTFIELD"
 ><I
 >silence</I
 ></TT
 > fields for both the <TT
 CLASS="PARAMETER"
 ><I
 >desired</I
 ></TT
 > and <TT
 CLASS="PARAMETER"
 ><I
 >obtained</I
 ></TT
 > specifications. The <TT
 CLASS="STRUCTFIELD"
 ><I
 >size</I
 ></TT
 > field stores the total size of the audio buffer in bytes, while the <TT
 CLASS="STRUCTFIELD"
 ><I
 >silence</I
 ></TT
 > stores the value used to represent silence in the audio buffer</P
 ><P
 >The audio device starts out playing <TT
 CLASS="STRUCTFIELD"
 ><I
 >silence</I
 ></TT
 > when it's opened, and should be enabled for playing by calling <A
 HREF="sdlpauseaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_PauseAudio</TT
 >(<TT
 CLASS="PARAMETER"
 ><I
 >0</I
 ></TT
 >)</A
 > when you are ready for your audio <TT
 CLASS="STRUCTFIELD"
 ><I
 >callback</I
 ></TT
 > function to be called.  Since the audio driver may modify the requested <TT
 CLASS="STRUCTFIELD"
 ><I
 >size</I
 ></TT
 > of the audio buffer, you should allocate any local mixing buffers after you open the audio device.</P
 ></DIV
 ><DIV
 CLASS="REFSECT1"
 ><A
 NAME="AEN6747"
 ></A
 ><H2
 >Examples</H2
 ><PRE
 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 = malloc(sizeof(SDL_AudioSpec));

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

 /* 22050Hz - FM Radio quality */
 desired-&#62;freq=22050;

 /* 16-bit signed audio */
 desired-&#62;format=AUDIO_S16LSB;

 /* Mono */
 desired-&#62;channels=0;

 /* Large audio buffer reduces risk of dropouts but increases response time */
 desired-&#62;samples=8192;

 /* Our callback function */
 desired-&#62;callback=my_audio_callback;

 desired-&#62;userdata=NULL;

 /* Open the audio device */
 if ( SDL_OpenAudio(desired, obtained) &#60; 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);</PRE
 ></DIV
 ><DIV
 CLASS="REFSECT1"
 ><A
 NAME="AEN6750"
 ></A
 ><H2
 >See Also</H2
 ><P
 ><A
 HREF="sdlaudiospec.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_AudioSpec</TT
 ></A
 >,
 <A
 HREF="sdllockaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_LockAudio</TT
 ></A
 >,
 <A
 HREF="sdlunlockaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_UnlockAudio</TT
 ></A
 >,
 <A
 HREF="sdlpauseaudio.html"
 ><TT
 CLASS="FUNCTION"
 >SDL_PauseAudio</TT
 ></A
 ></P
 ></DIV
 ><DIV
 CLASS="NAVFOOTER"
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	<HTML
	><HEAD
	><TITLE
	>SDL_OpenAudio</TITLE
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	><H1
	><A
	NAME="SDLOPENAUDIO"
	></A
	>SDL_OpenAudio</H1
	><DIV
	CLASS="REFNAMEDIV"
	><A
	NAME="AEN6650"
	></A
	><H2
	>Name</H2
	>SDL_OpenAudio -- Opens the audio device with the desired parameters.</DIV
	><DIV
	CLASS="REFSYNOPSISDIV"
	><A
	NAME="AEN6653"
	></A
	><H2
	>Synopsis</H2
	><DIV
	CLASS="FUNCSYNOPSIS"
	><A
	NAME="AEN6654"
	></A
	><P
	></P
	><PRE
	CLASS="FUNCSYNOPSISINFO"
	>#include "SDL.h"</PRE
	><P
	><CODE
	><CODE
	CLASS="FUNCDEF"
	>int <B
	CLASS="FSFUNC"
	>SDL_OpenAudio</B
	></CODE
	>(SDL_AudioSpec desired, SDL_AudioSpec obtained);</CODE
	></P
	><P
	></P
	></DIV
	></DIV
	><DIV
	CLASS="REFSECT1"
	><A
	NAME="AEN6660"
	></A
	><H2
	>Description</H2
	><P
	>This function opens the audio device with the <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> parameters, and
	returns 0 if successful, placing the actual hardware parameters in the
	structure pointed to by <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	>. If <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	> 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.</P
	><P
	>To open the audio device a <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> <A
	HREF="sdlaudiospec.html"
	><SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	></A
	> must be created.
	<PRE
	CLASS="PROGRAMLISTING"
	>SDL_AudioSpec *desired;
	.
	.
	desired = malloc(sizeof(SDL_AudioSpec));</PRE
	>
	You must then fill this structure with your desired audio specifications.</P
	><P
	></P
	><DIV
	CLASS="VARIABLELIST"
	><DL
	><DT
	><SPAN
	CLASS="STRUCTNAME"
	>desired</SPAN
	>-><TT
	CLASS="STRUCTFIELD"
	><I
	>freq</I
	></TT
	></DT
	><DD
	><P
	>The desired audio frequency in samples-per-second.</P
	></DD
	><DT
	><SPAN
	CLASS="STRUCTNAME"
	>desired</SPAN
	>-><TT
	CLASS="STRUCTFIELD"
	><I
	>format</I
	></TT
	></DT
	><DD
	><P
	>The desired audio format (see <A
	HREF="sdlaudiospec.html"
	><SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	></A
	>)</P
	></DD
	><DT
	><SPAN
	CLASS="STRUCTNAME"
	>desired</SPAN
	>-><TT
	CLASS="STRUCTFIELD"
	><I
	>samples</I
	></TT
	></DT
	><DD
	><P
	>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</P
	></DD
	><DT
	><SPAN
	CLASS="STRUCTNAME"
	>desired</SPAN
	>-><TT
	CLASS="STRUCTFIELD"
	><I
	>callback</I
	></TT
	></DT
	><DD
	><P
	>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 <A
	HREF="sdllockaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_LockAudio</TT
	></A
	> and <A
	HREF="sdlunlockaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_UnlockAudio</TT
	></A
	> in your code. The callback prototype is:
	<PRE
	CLASS="PROGRAMLISTING"
	>void callback(void userdata, Uint8 stream, int len);</PRE
	>
	<TT
	CLASS="PARAMETER"
	><I
	>userdata</I
	></TT
	> is the pointer stored in <TT
	CLASS="STRUCTFIELD"
	><I
	>userdata</I
	></TT
	> field of the <SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	>. <TT
	CLASS="PARAMETER"
	><I
	>stream</I
	></TT
	> is a pointer to the audio buffer you want to fill with information and <TT
	CLASS="PARAMETER"
	><I
	>len</I
	></TT
	> is the length of the audio buffer in bytes.</P
	></DD
	><DT
	><SPAN
	CLASS="STRUCTNAME"
	>desired</SPAN
	>-><TT
	CLASS="STRUCTFIELD"
	><I
	>userdata</I
	></TT
	></DT
	><DD
	><P
	>This pointer is passed as the first parameter to the <TT
	CLASS="FUNCTION"
	>callback</TT
	> function.</P
	></DD
	></DL
	></DIV
	><P
	><TT
	CLASS="FUNCTION"
	>SDL_OpenAudio</TT
	> reads these fields from the <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> <SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	> structure pass to the function and attempts to find an audio configuration matching your <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	>. As mentioned above, if the <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	> parameter is <TT
	CLASS="LITERAL"
	>NULL</TT
	> then SDL with convert from your <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> audio settings to the hardware settings as it plays.</P
	><P
	>If <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	> is <TT
	CLASS="LITERAL"
	>NULL</TT
	> then the <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> <SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	> is your working specification, otherwise the <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	> <SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioSpec</SPAN
	> becomes the working specification and the <TT
	CLASS="PARAMETER"
	><I
	>desirec</I
	></TT
	> specification can be deleted. The data in the working specification is used when building <SPAN
	CLASS="STRUCTNAME"
	>SDL_AudioCVT</SPAN
	>'s for converting loaded data to the hardware format.</P
	><P
	><TT
	CLASS="FUNCTION"
	>SDL_OpenAudio</TT
	> calculates the <TT
	CLASS="STRUCTFIELD"
	><I
	>size</I
	></TT
	> and <TT
	CLASS="STRUCTFIELD"
	><I
	>silence</I
	></TT
	> fields for both the <TT
	CLASS="PARAMETER"
	><I
	>desired</I
	></TT
	> and <TT
	CLASS="PARAMETER"
	><I
	>obtained</I
	></TT
	> specifications. The <TT
	CLASS="STRUCTFIELD"
	><I
	>size</I
	></TT
	> field stores the total size of the audio buffer in bytes, while the <TT
	CLASS="STRUCTFIELD"
	><I
	>silence</I
	></TT
	> stores the value used to represent silence in the audio buffer</P
	><P
	>The audio device starts out playing <TT
	CLASS="STRUCTFIELD"
	><I
	>silence</I
	></TT
	> when it's opened, and should be enabled for playing by calling <A
	HREF="sdlpauseaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_PauseAudio</TT
	>(<TT
	CLASS="PARAMETER"
	><I
	>0</I
	></TT
	>)</A
	> when you are ready for your audio <TT
	CLASS="STRUCTFIELD"
	><I
	>callback</I
	></TT
	> function to be called. Since the audio driver may modify the requested <TT
	CLASS="STRUCTFIELD"
	><I
	>size</I
	></TT
	> of the audio buffer, you should allocate any local mixing buffers after you open the audio device.</P
	></DIV
	><DIV
	CLASS="REFSECT1"
	><A
	NAME="AEN6747"
	></A
	><H2
	>Examples</H2
	><PRE
	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 = malloc(sizeof(SDL_AudioSpec));

	/* Allocate space for the obtained SDL_AudioSpec */
	obtained = 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\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);</PRE
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	>See Also</H2
	><P
	><A
	HREF="sdlaudiospec.html"
	><TT
	CLASS="FUNCTION"
	>SDL_AudioSpec</TT
	></A
	>,
	<A
	HREF="sdllockaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_LockAudio</TT
	></A
	>,
	<A
	HREF="sdlunlockaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_UnlockAudio</TT
	></A
	>,
	<A
	HREF="sdlpauseaudio.html"
	><TT
	CLASS="FUNCTION"
	>SDL_PauseAudio</TT
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