AUDIO(4)                     Device Drivers Manual                    AUDIO(4)


     audio, mixer - device-independent audio driver layer


     audio* at ...

     #include <sys/types.h>
     #include <sys/ioctl.h>
     #include <sys/audioio.h>
     #include <string.h>


     The audio driver provides support for various audio peripherals.  It
     provides a uniform programming interface layer above different underlying
     audio hardware drivers.  The audio layer provides full-duplex operation
     if the underlying hardware configuration supports it.

     There are four device files available for audio operation: /dev/audio,
     /dev/sound, /dev/audioctl, and /dev/mixer.  /dev/audio and /dev/sound are
     used for recording or playback of digital samples.  /dev/mixer is used to
     manipulate volume, recording source, or other audio mixer functions.
     /dev/audioctl accepts the same ioctl(2) operations as /dev/sound, but no
     other operations.  In contrast to /dev/sound, which has the exclusive
     open property, /dev/audioctl can be opened at any time and can be used to
     manipulate the audio device while it is in use.


     When /dev/audio or /dev/sound is opened, it attempts to maintain the
     previous audio sample format and record/playback mode.  In addition, if
     it is opened read-only (write-only) the device is set to half-duplex
     record (play) mode with recording (playing) unpaused.  In all respects
     /dev/audio and /dev/sound are identical.

     Only one process may hold open a sampling device at a given time
     (although file descriptors may be shared between processes once the first
     open completes).

     On a half-duplex device, either reads or writes are allowed, but not
     both.  On a full-duplex device, reads and writes may operate concurrently
     without interference.

     If a writing process does not call write(2) frequently enough to provide
     samples at the pace the hardware consumes them silence is inserted.  If a
     reading process does not call read(2) frequently enough, it will simply
     miss samples.

     The audio device is accessed with read(2) or write(2).

     The audio device, like most devices, can be used in poll(2),

     The following ioctl(2) commands are supported on the sample devices:

     AUDIO_RERROR int *
     AUDIO_PERROR int *
             Obsolete.  These commands fetch the count of dropped input or
             output samples into the int * argument, respectively.  There is
             no information regarding when in the sample stream they were

     AUDIO_GETDEV audio_device_t *
             This command fetches the current hardware device information into
             the audio_device_t * argument.

             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;

     AUDIO_GETFD int *
             This command returns 1 if in full-duplex mode, else 0.

     AUDIO_GETENC audio_encoding_t *
             This command is used iteratively to fetch sample encoding names
             and format_ids into the input/output audio_encoding_t * argument.

             typedef struct audio_encoding {
                     int index;      /* input: nth encoding */
                     char name[MAX_AUDIO_DEV_LEN]; /* name of encoding */
                     int encoding;   /* value for encoding parameter */
                     int precision;  /* value for precision parameter */
                     int bps;        /* value for bps parameter */
                     int msb;        /* value for msb parameter */
             } audio_encoding_t;

             To query all the supported encodings, start with an index field
             of 0 and continue with successive encodings (1, 2, ...) until the
             command returns an error.

     AUDIO_SETFD int *
             Does nothing, left for compatibility; argument must point to a
             non-zero integer if the device is opened in read-write mode.

             This command gets a bit set of hardware properties.  If the
             hardware has a certain property, the corresponding bit is set,
             otherwise it is not.  The properties can have the following

             AUDIO_PROP_FULLDUPLEX   The device admits full-duplex operation.
             AUDIO_PROP_INDEPENDENT  The device can set playing and recording
                                     channel counts independently.

     AUDIO_GETIOFFS audio_offset_t *
     AUDIO_GETOOFFS audio_offset_t *
             Obsolete.  These commands fetch the number of bytes played or
             recorded.  The information is returned in the audio_offset

             typedef struct audio_offset {
                     u_int   samples;   /* Total number of bytes transferred */
             } audio_offset_t;

     AUDIO_GETINFO audio_info_t *
     AUDIO_SETINFO audio_info_t *
             Get or set audio information as encoded in the audio_info

             typedef struct audio_info {
                     struct  audio_prinfo play;   /* info for play (output) side */
                     struct  audio_prinfo record; /* info for record (input) side */
                     u_int   hiwat;          /* blocks count in play buffer */
                     u_int   mode;           /* current device mode */
             #define AUMODE_PLAY     0x01
             #define AUMODE_RECORD   0x02
             } audio_info_t;

             When setting the current state with AUDIO_SETINFO, the audio_info
             structure should first be initialized with


             and then the particular values to be changed should be set.  This
             allows the audio driver to only set those things that you wish to
             change and eliminates the need to query the device with
             AUDIO_GETINFO first.

             The mode field is read-only and set to AUMODE_PLAY,
             AUMODE_RECORD, or a bitwise OR combination of the three.  Only
             full-duplex audio devices support simultaneous record and

             hiwat contains the number of blocks in the kernel play buffer.
             Writes to the audio devices will queue blocks until the play
             buffer is full, at which point any more write calls will block
             until space for at least one byte is available.

             struct audio_prinfo {
                     u_int   sample_rate;    /* sample rate in bit/s */
                     u_int   channels;       /* number of channels, usually 1 or 2 */
                     u_int   precision;      /* number of bits/sample */
                     u_int   bps;            /* number of bytes/sample */
                     u_int   msb;            /* data alignment */
                     u_int   encoding;       /* data encoding (AUDIO_ENCODING_* below) */
                     u_int   block_size;     /* size a block */
                     /* Current state of device: */
                     u_char  pause;          /* non-zero if paused, zero to resume */
                     u_char  active;         /* non-zero if I/O is currently active */

             The audio driver requires identical playback and recording sample
             rates, sample encodings, and block durations.

             The encoding parameter can have the following values:

             AUDIO_ENCODING_ULAW        mu-law encoding, 8 bits/sample
             AUDIO_ENCODING_ALAW        A-law encoding, 8 bits/sample
             AUDIO_ENCODING_SLINEAR_LE  two's complement signed linear
                                        encoding with little endian byte order
             AUDIO_ENCODING_SLINEAR_BE  two's complement signed linear
                                        encoding with big endian byte order
             AUDIO_ENCODING_ULINEAR_LE  unsigned linear encoding with little
                                        endian byte order
             AUDIO_ENCODING_ULINEAR_BE  unsigned linear encoding with big
                                        endian byte order

             The precision parameter describes the number of bits of audio
             data per sample.  The bps parameter describes the number of bytes
             of audio data per sample.  The msb parameter describes the
             alignment of the data in the sample.  It is only meaningful when
             precision / NBBY < bps.  A value of 1 means the data is aligned
             to the most significant bit.

             block_size is the block size in bytes, which determines the
             frequency at which blocking read(2), write(2), or poll(2), wake
             up.  The generic audio driver layer and the hardware driver have
             the opportunity to adjust this block size to get it within
             implementation-required limits.  Normally the block_size is
             recalculated when other parameters changes.

             It is recommended to set block_size at the same time as, or
             after, all other parameters have been set.

             pause returns the current pause/unpause state for recording or
             playback.  For AUDIO_SETINFO, if the pause value is specified it
             will either pause or unpause the particular direction.  In full-
             duplex the pause values for both directions must be equal.

     AUDIO_GETPOS struct audio_pos *
             Fetch an atomic snapshot of device timing information in the
             audio_pos structure.

             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 */

             The properties have the following meaning:

             play_pos   Total number of bytes played by the device since
                        playback started (a.k.a the device wall clock).

             play_xrun  The number of bytes corresponding to silence played
                        because write(2) wasn't called fast enough.

             rec_pos    Total number of bytes recorded by the device since
                        recording started (a.k.a the device wall clock).

             rec_xrun   The number of bytes dropped because read(2) wasn't
                        called fast enough.


     The mixer device, /dev/mixer, may be manipulated with ioctl(2) but does
     not support read(2) or write(2).  It supports the following ioctl(2)

     AUDIO_GETDEV audio_device_t *
             This command is the same as described above for the sampling

     AUDIO_MIXER_READ mixer_ctrl_t *
     AUDIO_MIXER_WRITE mixer_ctrl_t *
             These commands read the current mixer state or set new mixer
             state for the specified device dev.  type identifies which type
             of value is supplied in the mixer_ctrl_t * argument.

             #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

             For a mixer value, the value 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 ord field should
             be set to one of the possible values as returned by a prior
             AUDIO_MIXER_DEVINFO command.  The type AUDIO_MIXER_CLASS is only
             used for classifying particular mixer device types and is not
             used for AUDIO_MIXER_READ or AUDIO_MIXER_WRITE.

     AUDIO_MIXER_DEVINFO mixer_devinfo_t *
             This command is used iteratively to fetch audio mixer device
             information into the input/output mixer_devinfo_t * 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.

             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;

             The label field identifies the name of this particular mixer
             control.  The index field may be used as the dev field in
             AUDIO_MIXER_READ and AUDIO_MIXER_WRITE commands.  The type 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.

             The mixer_class 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 mixer_class field of the class
             itself, a mixer of type AUDIO_MIXER_CLASS.  For example, a mixer
             level controlling the input gain on the ``line in'' circuit would
             have a mixer_class that matches an input class device with the
             name ``inputs'' (AudioCinputs) and would have a label of ``line''
             (AudioNline).  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.

             The next and prev 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, AUDIO_MIXER_LAST is specified.

             For AUDIO_MIXER_ENUM mixer control types, the enumeration values
             and their corresponding names are filled in.  For example, a mute
             control would return appropriate values paired with AudioNon and
             AudioNoff.  For the AUDIO_MIXER_VALUE and AUDIO_MIXER_SET mixer
             control types, the channel count is returned; the units name
             specifies what the level controls (typical values are
             AudioNvolume, AudioNtreble, and AudioNbass).

     By convention, all the mixer devices can be distinguished from other
     mixer controls because they use a name from one of the AudioC* string




     aucat(1), audioctl(1), cdio(1), mixerctl(1), ioctl(2), sio_open(3),
     ac97(4), uaudio(4), audio(9)

OpenBSD 5.9                      July 28, 2015                     OpenBSD 5.9

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