1998-08-13 15:42:56 +00:00
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<TITLE>Advanced Linux Sound Architecture - Library API: Control Interface</TITLE>
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<A HREF="soundapi-2.html">Previous</A>
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<A HREF="soundapi-4.html">Next</A>
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<A HREF="soundapi.html#toc3">Table of Contents</A>
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<HR>
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<H2><A NAME="s3">3. Control Interface</A></H2>
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<P>The control interfaces gives application various information about the
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currently installed sound driver in the system. The interface should be used
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to detect if another sound interface is present for selected soundcard or,
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for example, to create a list of devices (MIXER, PCM etc) from which
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the user can select.</P>
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<H2><A NAME="ss3.1">3.1 Low-Level Layer</A></H2>
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<H3>int snd_cards( void ) </H3>
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<P>Returns the number of soundcards present in the system, if any. Otherwise
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it returns a negative value, which maps to an error code. This function
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will return 0 if no soundcards are detected.</P>
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<H3>unsigned int snd_cards_mask( void ) </H3>
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<P>Returns the bitmap of soundcards present in the system, if any. Otherwise
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it returns a negative value, which maps to an error code. This function
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will return 0 if no soundcards are detected. First soundcard is represented
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with bit 0.</P>
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<H3>int snd_ctl_open( void **handle, int card ) </H3>
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<P>Creates a new handle and opens communication with the kernel sound
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control interface for soundcard number <I>card</I> (0-N). The function
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also checks if protocol is compatible, so as to prevent the use of old
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programs with a new kernel API. Function returns zero if successful,
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otherwise an error code is returned.</P>
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<H3>int snd_ctl_close( void *handle ) </H3>
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<P>Function frees all resources allocated with control handle and
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closes the kernel sound control interface. Function returns zero if
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successful, otherwise it returns an error code.</P>
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<H3>int snd_ctl_file_descriptor( void *handle ) </H3>
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<P>Function returns file descriptor for the kernel sound control interface.
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This function should be used in very special cases. Function returns
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a negative error code if some error was encountered.</P>
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<H3>int snd_ctl_hw_info( void *handle, struct snd_ctl_hw_info *info ) </H3>
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<P>Fills the info structure with data about the sound hardware referenced
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by handle. Function returns zero if successful, otherwise it returns
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an error code.
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<HR>
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<PRE>
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#define SND_CTL_GCAPS_MIDI 0x0000001 /* driver has MIDI interface */
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#define SND_CTL_LCAPS_SYNTH 0x0000001 /* soundcard has synthesizer */
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#define SND_CTL_LCAPS_RAWFM 0x0000002 /* soundcard has RAW FM/OPL3 */
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struct snd_ctl_hw_info {
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unsigned int type; /* type of card - see SND_CARD_TYPE_XXXX */
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unsigned int gcaps; /* see SND_CTL_GCAPS_XXXX */
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unsigned int lcaps; /* see SND_CTL_LCAPS_XXXX */
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unsigned int pcmdevs; /* count of PCM devices (0 to N) */
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unsigned int mixerdevs; /* count of MIXER devices (0 to N) */
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unsigned int mididevs; /* count of raw MIDI devices (0 to N) */
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char id[8]; /* ID of card (user selectable) */
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char name[80]; /* name/info text about soundcard */
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unsigned char reserved[128]; /* reserved for future use */
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};
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</PRE>
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<HR>
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</P>
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<H3>int snd_ctl_pcm_info( void *handle, int dev, snd_pcm_info_t *info ) </H3>
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<P>Fills the *info structure with data about the PCM device. Function returns
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zero if successful, otherwise it returns an error code. Details about
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the snd_pcm_info_t structure are in the <B>Digital Audio (PCM) Interface</B>
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section. The argument <I>dev</I> selects the device number for the
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soundcard referenced by *handle. Its range is 0 to N where N is
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<I>struct snd_ctl_hw_info -> pcmdevs - 1</I>. This function will work if
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the selected PCM device is busy, too. It should be used to collect
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information about PCM devices without exclusive lock.</P>
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<H3>int snd_ctl_pcm_playback_info( void *handle, int dev, snd_pcm_playback_info_t *info ) </H3>
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<P>Fills the *info structure with data about the PCM device and playback direction.
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Function returns zero if successful, otherwise it returns an error code.
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Details about the snd_pcm_playback_info_t structure are in the
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<B>Digital Audio (PCM) Interface</B> section. The argument <I>dev</I>
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selects the device number for the soundcard referenced by *handle. Its
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range is 0 to N where N is <I>struct snd_ctl_hw_info -> pcmdevs - 1</I>.
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This function will work if the selected PCM device is busy, too. It should
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be used to collect information about PCM devices without exclusive lock.</P>
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<H3>int snd_ctl_pcm_record_info( void *handle, int dev, snd_pcm_record_info_t *info ) </H3>
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<P>Fills the *info structure with data about the PCM device and record direction.
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Function returns zero if successful, otherwise it returns an error code.
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Details about the snd_pcm_record_info_t structure are in the
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<B>Digital Audio (PCM) Interface</B> section. The argument <I>dev</I>
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selects the device number for the soundcard referenced by *handle. Its
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range is 0 to N where N is <I>struct snd_ctl_hw_info -> pcmdevs - 1</I>.
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This function will work if the selected PCM device is busy, too. It should
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be used to collect information about PCM devices without exclusive lock.</P>
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<H3>int snd_ctl_mixer_info( void *handle, int dev, snd_mixer_info_t *info ) </H3>
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<P>Fills the *info structure with data about the mixer device. Returns zero
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if successful, otherwise it returns an error code. Details about the
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snd_mixer_info_t structure are in the <B>Mixer Interface</B> section.
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The argument <I>dev</I> specifies the device number for the appropriate
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soundcard. Its range is 0 to N where N found from
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<I>struct snd_ctl_hw_info -> mixerdevs - 1</I>.
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It should be used to collect information about mixer devices.</P>
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<H2><A NAME="ss3.2">3.2 Examples</A></H2>
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<P>The following example shows how all PCM devices can be detected for the first
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soundcard (#0) in the system.</P>
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<P>
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<BLOCKQUOTE><CODE>
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<HR>
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<PRE>
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int card = 0, err;
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void *handle;
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stuct snd_ctl_hw_info info;
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1998-08-31 14:21:06 +00:00
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if ( (err = snd_ctl_open( &handle, card )) < 0 ) {
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1998-08-13 15:42:56 +00:00
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fprintf( stderr, "open failed: %s\n", snd_strerror( err ) );
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return;
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}
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1998-08-31 14:21:06 +00:00
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if ( (err = snd_ctl_hw_info( handle, &info )) < 0 ) {
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1998-08-13 15:42:56 +00:00
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fprintf( stderr, "hw info failed: %s\n", snd_strerror( err ) );
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snd_ctl_close( handle );
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return;
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}
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printf( "Installed PCM devices for card #i: %i\n", card + 1, info.pcmdevs );
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snd_ctl_close( handle );
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</PRE>
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<HR>
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</CODE></BLOCKQUOTE>
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</P>
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<HR>
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<A HREF="soundapi-2.html">Previous</A>
|
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<A HREF="soundapi-4.html">Next</A>
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<A HREF="soundapi.html#toc3">Table of Contents</A>
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</BODY>
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</HTML>
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