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audio(4) - OpenBSD manual pages
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It provides a uniform programming interface layer above different underlying audio hardware drivers.</p> <p class="Pp">In addition to hardware mixer controls like those documented in <a class="Xr" href="/azalia.4" aria-label="azalia, section 4">azalia(4)</a>, the <code class="Nm">audio</code> driver exposes the <var class="Va">record.enable</var> control. The superuser can change it with <a class="Xr" href="/mixerctl.8" aria-label="mixerctl, section 8">mixerctl(8)</a>. It accepts the following values:</p> <p class="Pp"></p> <div class="Bd-indent"> <dl class="Bl-tag Bl-compact"> <dt id="on"><a class="permalink" href="#on"><code class="Cm">on</code></a></dt> <dd>Recording is enabled.</dd> <dt id="off"><a class="permalink" href="#off"><code class="Cm">off</code></a></dt> <dd>Silence is returned instead of the recorded samples.</dd> <dt id="sysctl"><a class="permalink" href="#sysctl"><code class="Cm">sysctl</code></a></dt> <dd>Behavior is controlled by the <var class="Va">kern.audio.record</var> <a class="Xr" href="/sysctl.2" aria-label="sysctl, section 2">sysctl(2)</a> variable. This is the default.</dd> </dl> </div> <p class="Pp">There are two types of device files available for audio operation:</p> <ul class="Bl-dash Bd-indent"> <li>Audio devices are used for recording or playback of digital samples.</li> <li>Control devices are used to manipulate audio device parameters like volume or recording source. They can also read certain <code class="Nm">audio</code> driver variables while it is in use.</li> </ul> </section> <section class="Sh"> <h2 class="Sh" id="AUDIO_DEVICES"><a class="permalink" href="#AUDIO_DEVICES">AUDIO DEVICES</a></h2> <p class="Pp">When audio devices are opened, they attempt to maintain the previous audio sample format and record/playback mode. In addition, if one is opened read-only (write-only) the device is set to record-only (play-only) mode with recording (playing) unpaused.</p> <p class="Pp">If a writing process does not call <a class="Xr" href="/write.2" aria-label="write, section 2">write(2)</a> frequently enough to provide samples at the pace the hardware consumes them, silence is inserted. If a reading process does not call <a class="Xr" href="/read.2" aria-label="read, section 2">read(2)</a> frequently enough, it will simply miss samples.</p> <p class="Pp">The following <a class="Xr" href="/ioctl.2" aria-label="ioctl, section 2">ioctl(2)</a> commands are supported on the sample devices:</p> <p class="Pp"></p> <dl class="Bl-tag Bl-compact"> <dt id="AUDIO_GETDEV"><a class="permalink" href="#AUDIO_GETDEV"><code class="Dv">AUDIO_GETDEV</code></a> <var class="Fa">audio_device_t *</var></dt> <dd>This command fetches the current hardware device information into the <var class="Vt">audio_device_t *</var> argument. <div class="Bd Pp Li"> <pre>typedef struct audio_device { char name[MAX_AUDIO_DEV_LEN]; char version[MAX_AUDIO_DEV_LEN]; char config[MAX_AUDIO_DEV_LEN]; } audio_device_t;</pre> </div> <p class="Pp"></p> </dd> <dt id="AUDIO_SETPAR"><a class="permalink" href="#AUDIO_SETPAR"><code class="Dv">AUDIO_SETPAR</code></a> <var class="Fa">struct audio_swpar *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_GETPAR"><a class="permalink" href="#AUDIO_GETPAR"><code class="Dv">AUDIO_GETPAR</code></a> <var class="Fa">struct audio_swpar *</var></dt> <dd>Set or get audio parameters as encoded in the <var class="Vt">audio_swpar</var> structure. <div class="Bd Pp Li"> <pre>struct audio_swpar { unsigned int sig; /* if 1, encoding is signed */ unsigned int le; /* if 1, encoding is little-endian */ unsigned int bits; /* bits per sample */ unsigned int bps; /* bytes per sample */ unsigned int msb; /* if 1, bits are msb-aligned */ unsigned int rate; /* common play & rec sample rate */ unsigned int pchan; /* play channels */ unsigned int rchan; /* rec channels */ unsigned int nblks; /* number of blocks in play buffer */ unsigned int round; /* common frames per block */ };</pre> </div> <p class="Pp">When setting the device parameters with <code class="Dv">AUDIO_SETPAR</code>, the <var class="Vt">audio_swpar</var> structure should first be initialized with</p> <div class="Bd Pp Li"> <pre>struct audio_swpar ap; AUDIO_INITPAR(&ap);</pre> </div> <p class="Pp">and then only the values to be changed should be set. This ensures that the software will work with future versions of the driver. The driver will attempt to set the given parameters; if the device doesn't support them, it will choose other parameters. Then the software must call <code class="Dv">AUDIO_GETPAR</code> to obtain the parameters in use.</p> <p class="Pp">The parameters are as follows:</p> <dl class="Bl-tag"> <dt id="bits"><var class="Va">bits</var></dt> <dd>Number of bits per sample: must be between 1 and 32.</dd> <dt id="bps"><var class="Va">bps</var></dt> <dd>Bytes per sample; if specified, it must be large enough to hold all bits. By default it's set to the smallest power of two large enough to hold <var class="Va">bits</var>.</dd> <dt id="sig"><var class="Va">sig</var></dt> <dd>If set (i.e. non-zero) then the samples are signed; otherwise they are unsigned.</dd> <dt id="le"><var class="Va">le</var></dt> <dd>If set then the byte order is little endian; if not, it is big endian. It is meaningful only if <var class="Va">bps</var> > 1.</dd> <dt id="msb"><var class="Va">msb</var></dt> <dd>If set, then the <var class="Va">bits</var> are aligned in the packet to the most significant bit (i.e. lower bits are padded), otherwise to the least significant bit (i.e. higher bits are padded). It's meaningful only if <var class="Va">bits</var> < <var class="Va">bps</var> * 8.</dd> <dt id="rchan"><var class="Va">rchan</var></dt> <dd>The number of recorded channels; meaningful only if the device is opened for reading.</dd> <dt id="pchan"><var class="Va">pchan</var></dt> <dd>The number of channels playing; meaningful only if the device is opened for writing.</dd> <dt id="rate"><var class="Va">rate</var></dt> <dd>The sampling frequency in Hz.</dd> <dt id="nblks"><var class="Va">nblks</var></dt> <dd>The number of blocks in the play buffer.</dd> <dt id="round"><var class="Va">round</var></dt> <dd>The audio block size.</dd> </dl> <p class="Pp"></p> </dd> <dt id="AUDIO_START"><a class="permalink" href="#AUDIO_START"><code class="Dv">AUDIO_START</code></a></dt> <dd>Start playback and/or recording immediately. If the device is open for writing (playback), then the play buffer must be filled with the <a class="Xr" href="/write.2" aria-label="write, section 2">write(2)</a> syscall. The buffer size is obtained by multiplying the <var class="Va">nblks</var>, <var class="Va">round</var>, and <var class="Va">bps</var> parameters obtained with <code class="Dv">AUDIO_GETPAR</code>. <p class="Pp"></p> </dd> <dt id="AUDIO_STOP"><a class="permalink" href="#AUDIO_STOP"><code class="Dv">AUDIO_STOP</code></a></dt> <dd>Stop playback and recording immediately. <p class="Pp"></p> </dd> <dt id="AUDIO_GETPOS"><a class="permalink" href="#AUDIO_GETPOS"><code class="Dv">AUDIO_GETPOS</code></a> <var class="Fa">struct audio_pos *</var></dt> <dd>Fetch an atomic snapshot of device timing information in the <var class="Vt">audio_pos</var> structure. <div class="Bd Pp Li"> <pre>struct audio_pos { unsigned int play_pos; /* total bytes played */ unsigned int play_xrun; /* bytes of silence inserted */ unsigned int rec_pos; /* total bytes recorded */ unsigned int rec_xrun; /* bytes dropped */ };</pre> </div> <p class="Pp">The properties have the following meaning:</p> <dl class="Bl-tag"> <dt id="play_pos"><var class="Va">play_pos</var></dt> <dd>Total number of bytes played by the device since playback started (a.k.a the device wall clock).</dd> <dt id="play_xrun"><var class="Va">play_xrun</var></dt> <dd>The number of bytes corresponding to silence played because <a class="Xr" href="/write.2" aria-label="write, section 2">write(2)</a> wasn't called fast enough.</dd> <dt id="rec_pos"><var class="Va">rec_pos</var></dt> <dd>Total number of bytes recorded by the device since recording started (a.k.a the device wall clock).</dd> <dt id="rec_xrun"><var class="Va">rec_xrun</var></dt> <dd>The number of bytes dropped because <a class="Xr" href="/read.2" aria-label="read, section 2">read(2)</a> wasn't called fast enough.</dd> </dl> <p class="Pp"></p> </dd> <dt id="AUDIO_GETSTATUS"><a class="permalink" href="#AUDIO_GETSTATUS"><code class="Dv">AUDIO_GETSTATUS</code></a> <var class="Fa">struct audio_status *</var></dt> <dd>Fetch the current device status from the audio driver in the <var class="Vt">audio_status</var> structure. This <a class="Xr" href="/ioctl.2" aria-label="ioctl, section 2">ioctl(2)</a> is intended for use with diagnostic tools and is of no use to audio programs. <div class="Bd Pp Li"> <pre>struct audio_status { #define AUMODE_PLAY 0x01 #define AUMODE_RECORD 0x02 int mode; /* current mode */ int pause; /* not started yet */ int active; /* playing/recording in progress */ };</pre> </div> <p class="Pp">The properties have the following meaning:</p> <dl class="Bl-tag"> <dt id="mode"><var class="Va">mode</var></dt> <dd>The current mode determined by <a class="Xr" href="/open.2" aria-label="open, section 2">open(2)</a> flags.</dd> <dt id="pause"><var class="Va">pause</var></dt> <dd>If set, indicates that <code class="Dv">AUDIO_STOP</code> was called, and the device is not attempting to start.</dd> <dt id="active"><var class="Va">active</var></dt> <dd>If set, indicates that the device is playing and/or recording.</dd> </dl> </dd> </dl> </section> <section class="Sh"> <h2 class="Sh" id="CONTROL_DEVICES"><a class="permalink" href="#CONTROL_DEVICES">CONTROL DEVICES</a></h2> <p class="Pp">Control devices support the following <a class="Xr" href="/ioctl.2" aria-label="ioctl, section 2">ioctl(2)</a> commands:</p> <p class="Pp"></p> <dl class="Bl-tag Bl-compact"> <dt id="AUDIO_GETDEV~2"><a class="permalink" href="#AUDIO_GETDEV~2"><code class="Dv">AUDIO_GETDEV</code></a> <var class="Fa">audio_device_t *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_GETPOS~2"><a class="permalink" href="#AUDIO_GETPOS~2"><code class="Dv">AUDIO_GETPOS</code></a> <var class="Fa">struct audio_pos *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_GETSTATUS~2"><a class="permalink" href="#AUDIO_GETSTATUS~2"><code class="Dv">AUDIO_GETSTATUS</code></a> <var class="Fa">struct audio_status *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_GETPAR~2"><a class="permalink" href="#AUDIO_GETPAR~2"><code class="Dv">AUDIO_GETPAR</code></a> <var class="Fa">struct audio_swpar *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_SETPAR~2"><a class="permalink" href="#AUDIO_SETPAR~2"><code class="Dv">AUDIO_SETPAR</code></a> <var class="Fa">struct audio_swpar *</var></dt> <dd>These commands are the same as described above for the audio devices. While the audio device is open, <code class="Dv">AUDIO_SETPAR</code> may not be used. <p class="Pp"></p> </dd> <dt id="AUDIO_MIXER_READ"><a class="permalink" href="#AUDIO_MIXER_READ"><code class="Dv">AUDIO_MIXER_READ</code></a> <var class="Fa">mixer_ctrl_t *</var></dt> <dd style="width: auto;"> </dd> <dt id="AUDIO_MIXER_WRITE"><a class="permalink" href="#AUDIO_MIXER_WRITE"><code class="Dv">AUDIO_MIXER_WRITE</code></a> <var class="Fa">mixer_ctrl_t *</var></dt> <dd>These commands read the current mixer state or set new mixer state for the specified device <var class="Va">dev</var>. <var class="Va">type</var> identifies which type of value is supplied in the <var class="Vt">mixer_ctrl_t *</var> argument. <div class="Bd Pp Li"> <pre>#define AUDIO_MIXER_CLASS 0 #define AUDIO_MIXER_ENUM 1 #define AUDIO_MIXER_SET 2 #define AUDIO_MIXER_VALUE 3 typedef struct mixer_ctrl { int dev; /* input: nth device */ int type; union { int ord; /* enum */ int mask; /* set */ mixer_level_t value; /* value */ } un; } mixer_ctrl_t; #define AUDIO_MIN_GAIN 0 #define AUDIO_MAX_GAIN 255 typedef struct mixer_level { int num_channels; u_char level[8]; /* [num_channels] */ } mixer_level_t; #define AUDIO_MIXER_LEVEL_MONO 0 #define AUDIO_MIXER_LEVEL_LEFT 0 #define AUDIO_MIXER_LEVEL_RIGHT 1</pre> </div> <p class="Pp">For a mixer value, the <var class="Va">value</var> field specifies both the number of channels and the values for each channel. If the channel count does not match the current channel count, the attempt to change the setting may fail (depending on the hardware device driver implementation). For an enumeration value, the <var class="Va">ord</var> field should be set to one of the possible values as returned by a prior <code class="Dv">AUDIO_MIXER_DEVINFO</code> command. The type <code class="Dv">AUDIO_MIXER_CLASS</code> is only used for classifying particular <code class="Nm">mixer</code> device types and is not used for <code class="Dv">AUDIO_MIXER_READ</code> or <code class="Dv">AUDIO_MIXER_WRITE</code>.</p> <p class="Pp"></p> </dd> <dt id="AUDIO_MIXER_DEVINFO"><a class="permalink" href="#AUDIO_MIXER_DEVINFO"><code class="Dv">AUDIO_MIXER_DEVINFO</code></a> <var class="Fa">mixer_devinfo_t *</var></dt> <dd>This command is used iteratively to fetch audio <code class="Nm">mixer</code> device information into the input/output <var class="Vt">mixer_devinfo_t *</var> argument. To query all the supported devices, start with an index field of 0 and continue with successive devices (1, 2, ...) until the command returns an error. <div class="Bd Pp Li"> <pre>typedef struct mixer_devinfo { int index; /* input: nth mixer device */ audio_mixer_name_t label; int type; int mixer_class; int next, prev; #define AUDIO_MIXER_LAST -1 union { struct audio_mixer_enum { int num_mem; struct { audio_mixer_name_t label; int ord; } member[32]; } e; struct audio_mixer_set { int num_mem; struct { audio_mixer_name_t label; int mask; } member[32]; } s; struct audio_mixer_value { audio_mixer_name_t units; int num_channels; int delta; } v; } un; } mixer_devinfo_t;</pre> </div> <p class="Pp">The <var class="Va">label</var> field identifies the name of this particular mixer control. The <var class="Va">index</var> field may be used as the <var class="Va">dev</var> field in <code class="Dv">AUDIO_MIXER_READ</code> and <code class="Dv">AUDIO_MIXER_WRITE</code> commands. The <var class="Va">type</var> field identifies the type of this mixer control. Enumeration types are typically used for on/off style controls (e.g., a mute control) or for input/output device selection (e.g., select recording input source from CD, line in, or microphone). Set types are similar to enumeration types but any combination of the mask bits can be used.</p> <p class="Pp">The <var class="Va">mixer_class</var> field identifies what class of control this is. This value is set to the index value used to query the class itself. The (arbitrary) value set by the hardware driver may be determined by examining the <var class="Va">mixer_class</var> field of the class itself, a mixer of type <code class="Dv">AUDIO_MIXER_CLASS</code>. For example, a mixer level controlling the input gain on the “line in” circuit would have a <var class="Va">mixer_class</var> that matches an input class device with the name “inputs” (<code class="Dv">AudioCinputs</code>) and would have a <var class="Va">label</var> of “line” (<code class="Dv">AudioNline</code>). Mixer controls which control audio circuitry for a particular audio source (e.g., line-in, CD in, DAC output) are collected under the input class, while those which control all audio sources (e.g., master volume, equalization controls) are under the output class. Hardware devices capable of recording typically also have a record class, for controls that only affect recording, and also a monitor class.</p> <p class="Pp">The <var class="Va">next</var> and <var class="Va">prev</var> may be used by the hardware device driver to provide hints for the next and previous devices in a related set (for example, the line in level control would have the line in mute as its “next” value). If there is no relevant next or previous value, <code class="Dv">AUDIO_MIXER_LAST</code> is specified.</p> <p class="Pp">For <code class="Dv">AUDIO_MIXER_ENUM</code> mixer control types, the enumeration values and their corresponding names are filled in. For example, a mute control would return appropriate values paired with <code class="Dv">AudioNon</code> and <code class="Dv">AudioNoff</code>. For the <code class="Dv">AUDIO_MIXER_VALUE</code> and <code class="Dv">AUDIO_MIXER_SET</code> mixer control types, the channel count is returned; the units name specifies what the level controls (typical values are <code class="Dv">AudioNvolume</code>, <code class="Dv">AudioNtreble</code>, and <code class="Dv">AudioNbass</code>).</p> </dd> </dl> <p class="Pp">A process may read the control device to get notifications about mixer changes. Whenever a control changes, the <a class="Xr" href="/read.2" aria-label="read, section 2">read(2)</a> function fetches an integer identifying the control. It may be used in the <var class="Va">dev</var> field of the <var class="Va">mixer_ctrl</var> structure to call <code class="Dv">AUDIO_MIXER_READ</code>.</p> <p class="Pp">In contrast to audio devices, which have the exclusive open property, control devices can be opened at any time in write-only mode. Only one reader is allowed at a time.</p> </section> <section class="Sh"> <h2 class="Sh" id="FILES"><a class="permalink" href="#FILES">FILES</a></h2> <dl class="Bl-tag Bl-compact"> <dt><span class="Pa">/dev/audio</span><var class="Ar">N</var></dt> <dd>Audio device.</dd> <dt><span class="Pa">/dev/audioctl</span><var class="Ar">N</var></dt> <dd>Control device.</dd> </dl> </section> <section class="Sh"> <h2 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE ALSO</a></h2> <p class="Pp"><a class="Xr" href="/aucat.1" aria-label="aucat, section 1">aucat(1)</a>, <a class="Xr" href="/cdio.1" aria-label="cdio, section 1">cdio(1)</a>, <a class="Xr" href="/sndioctl.1" aria-label="sndioctl, section 1">sndioctl(1)</a>, <a class="Xr" href="/ioctl.2" aria-label="ioctl, section 2">ioctl(2)</a>, <a class="Xr" href="/sio_open.3" aria-label="sio_open, section 3">sio_open(3)</a>, <a class="Xr" href="/sioctl_open.3" aria-label="sioctl_open, section 3">sioctl_open(3)</a>, <a class="Xr" href="/ac97.4" aria-label="ac97, section 4">ac97(4)</a>, <a class="Xr" href="/uaudio.4" aria-label="uaudio, section 4">uaudio(4)</a>, <a class="Xr" href="/sndio.7" aria-label="sndio, section 7">sndio(7)</a>, <a class="Xr" href="/audioctl.8" aria-label="audioctl, section 8">audioctl(8)</a>, <a class="Xr" href="/mixerctl.8" aria-label="mixerctl, section 8">mixerctl(8)</a>, <a class="Xr" href="/sndiod.8" aria-label="sndiod, section 8">sndiod(8)</a>, <a class="Xr" href="/audio.9" aria-label="audio, section 9">audio(9)</a></p> </section> </main> <div class="foot" role="doc-pagefooter" aria-label="Manual footer line"><span class="foot-left"></span><span class="foot-date">March 31, 2022</span> <span class="foot-os">OpenBSD-current</span></div> </body> </html>