path: root/usr/src/man/man4i/audio.4i

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.Dd July 8, 2018
.Dt AUDIO 4I
.Os
.Sh NAME
.Nm audio
.Nd generic audio device interface
.Sh SYNOPSIS
.In sys/audio.h
.Sh OVERVIEW
An audio device is used to play and/or record a stream of audio data.
Since a
specific audio device may not support all functionality described below, refer
to the device-specific manual pages for a complete description of each hardware
device.
An application can use the
.Dv AUDIO_GETDEV
.Xr ioctl 2
to determine the current audio hardware associated with
.Pa /dev/audio .
.Pp
The audio framework provides a software mixing engine (audio mixer) for all
audio devices, allowing more than one process to play or record audio at the
same time.
.Ss "Backward Compatibility"
It is no longer possible to disable the mixing function.
Applications must not
assume that they have exclusive access to the audio device.
.Ss "Multi-Stream Codecs"
The audio mixer supports multi-stream Codecs.
These devices have DSP engines
that provide sample rate conversion, hardware mixing, and other features.
The use of such hardware features is opaque to applications.
.Sh AUDIO FORMATS
Digital audio data represents a quantized approximation of an analog audio
signal waveform.
In the simplest case, these quantized numbers represent the
amplitude of the input waveform at particular sampling intervals.
To achieve the best approximation of an input signal, the highest possible sampling
frequency and precision should be used.
However, increased accuracy comes at a cost of increased data storage requirements.
For instance, one minute of
monaural audio recorded in \(*m-Law format (pronounced
.Em mew-law )
at 8 KHz requires nearly 0.5 megabytes of storage, while the standard Compact Disc
audio format (stereo 16-bit linear
.Sy PCM
data sampled at 44.1 KHz) requires approximately 10 megabytes per minute.
.Pp
Audio data may be represented in several different formats.
An audio device's
current audio data format can be determined by using the
.Dv AUDIO_GETINFO
.Xr ioctl 2
described below.
.Pp
An audio data format is characterized in the audio driver by four parameters:
Sample Rate, Encoding, Precision, and Channels.
Refer to the device-specific
manual pages for a list of the audio formats that each device supports.
In addition to the formats that the audio device supports directly, other formats
provide higher data compression.
Applications may convert audio data to and
from these formats when playing or recording.
.Ss "Sample Rate"
Sample rate is a number that represents the sampling frequency (in samples per
second) of the audio data.
.Pp
The audio mixer always configures the hardware for the highest possible sample
rate for both play and record.
This ensures that none of the audio streams
require compute-intensive low pass filtering.
The result is that high sample rate audio streams are not degraded by filtering.
.Pp
Sample rate conversion can be a compute-intensive operation, depending on the
number of channels and a device's sample rate.
For example, an 8KHz signal can
be easily converted to 48KHz, requiring a low cost up sampling by 6.
However,
converting from 44.1KHz to 48KHz is compute intensive because it must be up
sampled by 160 and then down sampled by 147.
This is only done using integer multipliers.
.Pp
Applications can greatly reduce the impact of sample rate conversion by
carefully picking the sample rate.
Applications should always use the highest
sample rate the device supports.
An application can also do its own sample rate
conversion (to take advantage of floating point and accelerated instruction or
use small integers for up and down sampling.
.Pp
All modern audio devices run at 48 kHz or a multiple thereof, hence just using
48 kHz may be a reasonable compromise if the application is not prepared to
select higher sample rates.
.Ss "Encodings"
An encoding parameter specifies the audio data representation.
\(*m-Law
encoding corresponds to
.Em CCITT G.711 ,
and is the standard for voice data
used by telephone companies in the United States, Canada, and Japan.
A-Law encoding is also part of
.Em CCITT G.711
and is the standard encoding for telephony elsewhere in the world.
A-Law and \(*m-Law audio data are sampled at
a rate of 8000 samples per second with 12-bit precision, with the data
compressed to 8-bit samples.
The resulting audio data quality is equivalent to
that of standard analog telephone service.
.Pp
Linear Pulse Code Modulation (PCM) is an uncompressed, signed audio format in
which sample values are directly proportional to audio signal voltages.
Each sample is a 2's complement number that represents a positive or negative
amplitude.
.Ss "Precision"
Precision indicates the number of bits used to store each audio sample.
For instance, \(*m-Law and A-Law data are stored with 8-bit precision.
.Sy PCM
data may be stored at various precisions, though 16-bit is the most common.
.Ss "Channels"
Multiple channels of audio may be interleaved at sample boundaries.
A sample frame consists of a single sample from each active channel.
For example, a sample frame of stereo 16-bit
.Sy PCM
data consists of two 16-bit samples,
corresponding to the left and right channel data.
.Pp
The audio mixer sets the hardware to the maximum number of channels supported.
If a mono signal is played or recorded, it is mixed on the first two (usually
the left and right) channels only.
Silence is mixed on the remaining channels
.Ss "Supported Formats"
The audio mixer supports the following audio formats:
.Bl -column "Signed Linear PCM" "Precision" "Mono or Stereo" -offset 2n
.It Encoding Ta Precision Ta Channels
.It "Signed Linear PCM" Ta "32-bit" Ta "Mono or Stereo"
.It "Signed Linear PCM" Ta "16-bit" Ta "Mono or Stereo"
.It "Signed Linear PCM" Ta "8-bit" Ta "Mono or Stereo"
.It "\(*m-Law" Ta "8-bit" Ta "Mono or Stereo"
.It "A-Law" Ta "8-bit" Ta "Mono or Stereo"
.El
.Pp
The audio mixer converts all audio streams to 24-bit Linear PCM before mixing.
After mixing, conversion is made to the best possible Codec format.
The
conversion process is not compute intensive and audio applications can choose
the encoding format that best meets their needs.
.Pp
Note that the mixer discards the low order 8 bits of 32-bit Signed Linear PCM
in order to perform mixing.
(This is done to allow for possible overflows to
fit into 32-bits when mixing multiple streams together.)
Hence, the maximum effective precision is 24-bits.
.Sh DESCRIPTION
The device
.Pa /dev/audio
is a device driver that dispatches audio requests
to the appropriate underlying audio hardware.
The audio driver is implemented as a
.Sy STREAMS
driver.
In order to record audio input, applications
.Xr open 2
the
.Pa /dev/audio
device and read data from it using the
.Xr read 2
system call.
Similarly, sound data is queued to the audio output port by using the
.Xr write 2
system call.
Device configuration is performed using the
.Xr ioctl 2
interface.
.Pp
Because some systems may contain more than one audio device, application
writers are encouraged to query the
.Ev AUDIODEV
environment variable.
If this variable is present in the environment, its value should identify the
path name of the default audio device.
.Ss "Opening the Audio Device"
The audio device is not treated as an exclusive resource.
Each process may open the audio device once.
.Pp
Each
.Xr open 2
completes as long as there are channels available to be allocated.
If no channels are available to be allocated:
.Bl -bullet -offset indent
.It
if either the
.Dv O_NDELAY
or
.Dv O_NONBLOCK
flags are set in the
.Xr open 2
.Fa oflag
argument, then -1 is immediately returned, with
.Va errno
set to
.Er EBUSY .
.It
if neither the
.Dv O_NDELAY
nor the
.Dv O_NONBLOCK
flag are set, then
.Xr open 2
hangs until the device is available or a signal is delivered to
the process, in which case a -1 is returned with
.Va errno
set to
.Er EINTR .
.El
.Pp
Upon the initial
.Xr open 2
of the audio channel, the audio mixer sets the data
format of the audio channel to the default state of 8-bit, 8Khz, mono \(*m-Law
data.
.Pp
If the audio device does not support this configuration, it informs the
audio mixer of the initial configuration.
Audio applications should explicitly set the encoding characteristics to match the
audio data requirements, and not depend on the default configuration.
.Ss "Recording Audio Data"
The
.Xr read 2
system call copies data from the system's buffers to the application.
Ordinarily,
.Xr read 2
blocks until the user buffer is filled.
The
.Dv I_NREAD
.Sy ioctl
(see
.Xr streamio 4I )
may be used to determine the amount of data that may be read without blocking.
The device may alternatively be set to a non-blocking mode, in which case
.Xr read 2
completes immediately, but may return fewer bytes than requested.
Refer to the
.Xr read 2
manual page for a complete description of this behavior.
.Pp
When the audio device is opened with read access, the device driver immediately
starts buffering audio input data.
Since this consumes system resources,
processes that do not record audio data should open the device write-only
.Pq Dv O_WRONLY .
.Pp
The transfer of input data to
.Sy STREAMS
buffers may be paused (or resumed) by using the
.Dv AUDIO_SETINFO
.Sy ioctl
to set (or clear) the
.Fa record.pause
flag in the audio information structure (see below).
All unread input data in the
.Sy STREAMS
queue may be discarded by using the
.Dv I_FLUSH
.Sy STREAMS
ioctl.
See
.Xr streamio 4I .
When changing record parameters, the input stream should be paused and flushed
before the change, and resumed afterward.
Otherwise, subsequent reads may return samples
in the old format followed by samples in the new format.
This is particularly important when new parameters result in a changed sample size.
.Pp
Input data can accumulate in
.Sy STREAMS
buffers very quickly.
At a minimum,
it will accumulate at 8000 bytes per second for 8-bit, 8 KHz, mono, \(*m-Law data.
If the device is configured for 16-bit linear or higher sample rates, it will
accumulate even faster.
If the application that consumes the data cannot keep
up with this data rate, the
.Sy STREAMS
queue may become full.
When this occurs, the
.Fa record.error
flag is set in the audio information structure and input sampling ceases until
there is room in the input queue for additional data.
In such cases, the input data stream contains a discontinuity.
For this reason, audio recording applications should open the audio device when
they are prepared to begin reading data, rather than at the start of extensive
initialization.
.Ss "Playing Audio Data"
The
.Xr write 2
system call copies data from an application's buffer to the
.Sy STREAMS
output queue.
Ordinarily,
.Xr write 2
blocks until the entire user buffer is transferred.
The device may alternatively be set to a non-blocking mode, in which case
.Xr write 2
completes immediately, but may have transferred fewer bytes than requested.
See
.Xr write 2 .
.Pp
Although
.Xr write 2
returns when the data is successfully queued, the actual
completion of audio output may take considerably longer.
The
.Dv AUDIO_DRAIN
ioctl may be issued to allow an application to block until all of the
queued output data has been played.
Alternatively, a process may request
asynchronous notification of output completion by writing a zero-length buffer
(end-of-file record) to the output stream.
When such a buffer has been
processed, the
.Fa play.eof
flag in the audio information structure is incremented.
.Pp
The final
.Xr close 2
of the file descriptor hangs until all of the audio
output has drained.
If a signal interrupts the
.Xr close 2 ,
or if the process exits without closing the device, any remaining data queued
for audio output is flushed and the device is closed immediately.
.Pp
The consumption of output data may be paused (or resumed) by using the
.Dv AUDIO_SETINFO
ioctl to set (or clear) the
.Fa play.pause
flag in the audio information structure.
Queued output data may be discarded by using
the
.Dv I_FLUSH
.Sy STREAMS
ioctl.
(See
.Xr streamio 4I ) .
.Pp
Output data is played from the
.Sy STREAMS
buffers at a default rate of at
least 8000 bytes per second for \(*m-Law, A-Law or 8-bit PCM data (faster for
16-bit linear data or higher sampling rates).
If the output queue becomes
empty, the
.Fa play.error
flag is set in the audio information structure and
output is stopped until additional data is written.
If an application attempts
to write a number of bytes that is not a multiple of the current sample frame
size, an error is generated and the bad data is thrown away.
Additional writes are allowed.
.Ss "Asynchronous I/O"
The
.Dv I_SETSIG
.Sy STREAMS
ioctl
enables asynchronous notification, through the
.Dv SIGPOLL
signal, of input and output ready condition changes.
The
.Dv O_NONBLOCK
flag may be set using the
.Dv F_SETFL
.Xr fcntl 2
to
enable non-blocking
.Xr read 2
and
.Xr write 2
requests.
This is normally
sufficient for applications to maintain an audio stream in the background.
.Ss "Audio Control Pseudo-Device"
It is sometimes convenient to have an application, such as a volume control
panel, modify certain characteristics of the audio device while it is being
used by an unrelated process.
.Pp
The
.Pa /dev/audioctl
pseudo-device is provided for this purpose.
Any number of processes may open
.Pa /dev/audioctl
simultaneously.
However,
.Xr read 2
and
.Xr write 2
system calls are ignored by
.Pa /dev/audioctl .
The
.Dv AUDIO_GETINFO
and
.Dv AUDIO_SETINFO
ioctl commands may be issued to
.Pa /dev/audioctl
to determine the status or alter the behavior of
.Pa /dev/audio .
Note: In general, the audio control device name is
constructed by appending the letters
.Qq Sy ctl
to the path name of the audio device.
.Ss "Audio Status Change Notification"
Applications that open the audio control pseudo-device may request asynchronous
notification of changes in the state of the audio device by setting the
.Dv S_MSG
flag in an
.Dv I_SETSIG
.Sy STREAMS
ioctl.
Such processes receive a
.Dv SIGPOLL
signal when any of the following events occur:
.Bl -bullet -offset indent
.It
An
.Dv AUDIO_SETINFO
ioctl has altered the device state.
.It
An input overflow or output underflow has occurred.
.It
An end-of-file record (zero-length buffer) has been processed on output.
.It
An
.Xr open 2
or
.Xr close 2
of
.Pa /dev/audio
has altered the device state.
.It
An external event (such as speakerbox's volume control) has altered the device
state.
.El
.Sh IOCTLS
.Ss "Audio Information Structure"
The state of the audio device may be polled or modified using the
.Dv AUDIO_GETINFO
and
.Dv AUDIO_SETINFO
ioctl commands.
These commands operate on the
.Vt audio_info
structure as defined, in
.In sys/audio.h ,
as follows:
.Bd -literal -offset 2n
/*
 * This structure contains state information for audio device
 * IO streams
 */

struct audio_prinfo {
     /*
      * The following values describe the
      * audio data encoding
      */
      uint_t sample_rate; /* samples per second */
      uint_t channels;    /* number of interleaved channels */
      uint_t precision;   /* number of bits per sample */
      uint_t encoding;    /* data encoding method */

      /*
       * The following values control audio device
       * configuration
       */
      uint_t gain; /* volume level */
      uint_t port; /* selected I/O port */
      uint_t buffer_size; /* I/O buffer size */

     /*
      * The following values describe the current device
      * state
      */
     uint_t samples; /* number of samples converted */
     uint_t eof;     /* End Of File counter (play only) */
     uchar_t pause;  /* non-zero if paused, zero to resume */
     uchar_t error;  /* non-zero if overflow/underflow */
     uchar_t waiting; /* non-zero if a process wants access */
     uchar_t balance; /* stereo channel balance */

     /*
      * The following values are read-only device state
      * information
      */
      uchar_t open;   /* non-zero if open access granted */
      uchar_t active; /* non-zero if I/O active */
      uint_t avail_ports; /* available I/O ports */
      uint_t mod_ports; /* modifiable I/O ports */
};
typedef struct audio_prinfo audio_prinfo_t;

/*
 * This structure is used in AUDIO_GETINFO and AUDIO_SETINFO ioctl
 * commands
 */
struct audio_info {
    audio_prinfo_t record;/* input status info */
    audio_prinfo_t play; /* output status info */
    uint_t monitor_gain; /* input to output mix */
    uchar_toutput_muted; /* non-zero if output muted */
    uint_t hw_features;  /* supported H/W features */
    uint_t sw_features;  /* supported S/W features */
    uint_t sw_features_enabled;
                /* supported S/W features enabled */
};
typedef struct audio_info audio_info_t;

/* Audio encoding types */
#define AUDIO_ENCODING_ULAW (1) /* u-Law encoding */
#define AUDIO_ENCODING_ALAW (2) /* A-Law encoding */
#define AUDIO_ENCODING_LINEAR (3) /* Signed Linear PCM encoding */

/*
 * These ranges apply to record, play, and
 * monitor gain values
 */
#define AUDIO_MIN_GAIN (0)/* minimum gain value */
#define AUDIO_MAX_GAIN (255) /* maximum gain value */

/*
 * These values apply to the balance field to adjust channel
 * gain values
 */
#define AUDIO_LEFT_BALANCE   (0) /* left channel only */
#define AUDIO_MID_BALANCE   (32) /* equal left/right balance */
#define AUDIO_RIGHT_BALANCE (64) /* right channel only */

/*
 * Define some convenient audio port names
 * (for port, avail_ports and mod_ports)
 */

/* output ports (several might be enabled at once) */
#define AUDIO_SPEAKER   (0x01) /* built-in speaker */
#define AUDIO_HEADPHONE (0x02) /* headphone jack */
#define AUDIO_LINE_OUT  (0x04) /* line out */
#define AUDIO_SPDIF_OUT (0x08) /* SPDIF port */
#define AUDIO_AUX1_OUT  (0x10) /* aux1 out */
#define AUDIO_AUX2_OUT  (0x20) /* aux2 out */

/*
 * input ports (usually only one may be enabled at a time)
 */
#define AUDIO_MICROPHONE (0x01) /* microphone */
#define AUDIO_LINE_IN  (0x02) /* line in */
#define AUDIO_CD       (0x04) /* on-board CD inputs */
#define AUDIO_SPDIF_IN (0x08) /* SPDIF port */
#define AUDIO_AUX1_IN  (0x10) /* aux1 in */
#define AUDIO_AUX2_IN  (0x20) /* aux2 in */
#define AUDIO_CODEC_LOOPB_IN (0x40) /* Codec inter. loopback */

/* These defines are for hardware features */
#define AUDIO_HWFEATURE_DUPLEX (0x00000001u)
/* simult. play & cap. supported */

#define AUDIO_HWFEATURE_MSCODEC (0x00000002u)
/* multi-stream Codec */

/* These defines are for software features *
#define AUDIO_SWFEATURE_MIXER (0x00000001u)
/* audio mixer audio pers. mod. */

/*
 * Parameter for the AUDIO_GETDEV ioctl
 * to determine current audio devices
 */
#define MAX_AUDIO_DEV_LEN (16)
struct audio_device {
    char name[MAX_AUDIO_DEV_LEN];
    char version[MAX_AUDIO_DEV_LEN];
    char config[MAX_AUDIO_DEV_LEN];
};
typedef struct audio_device audio_device_t;
.Ed
.Pp
The
.Fa play.gain
and
.Fa record.gain
fields specify the output and input volume levels.
A value of
.Dv AUDIO_MAX_GAIN
indicates maximum volume.
Audio output may also be temporarily muted by setting a non-zero value in the
.Fa output_muted
field.
Clearing this field restores audio output to the normal state.
.Pp
The
.Va monitor_gain
field is present for compatibility, and is no longer supported.
See
.Xr dsp 4I
for more detail.
.Pp
Likewise, the
.Fa play.port ,
.Fa play.ports ,
.Fa play.mod_ports ,
.Fa record.port ,
.Fa record.ports ,
and
.Fa record.mod_ports
are no longer supported.
See
.Xr dsp 4I
for more detail.
.Pp
The
.Fa play.balance
and
.Fa record.balance
fields are fixed to
.Dv AUDIO_MID_BALANCE .
Changes to volume levels for different channels can be
made using the interfaces in
.Xr dsp 4I .
.Pp
The
.Fa play.pause
and
.Fa record.pause
flags may be used to pause and
resume the transfer of data between the audio device and the
.Sy STREAMS
buffers.
The
.Fa play.error
and
.Fa record.error
flags indicate that data underflow or overflow has occurred.
The
.Fa play.active
and
.Fa record.active
flags indicate that data transfer is currently active in the corresponding
direction.
.Pp
The
.Fa play.open
and
.Fa record.open
flags indicate that the device is
currently open with the corresponding access permission.
The
.Fa play.waiting
and
.Fa record.waiting
flags provide an indication that a process may be waiting to access the device.
These flags are set automatically when a process blocks on
.Xr open 2 ,
though they may also be set using the
.Dv AUDIO_SETINFO
ioctl command.
They are cleared only when a process relinquishes access by closing the device.
.Pp
The
.Fa play.samples
and
.Fa record.samples
fields are zeroed at
.Xr open 2
and are incremented each time a data sample is copied to or from
the associated
.Sy STREAMS
queue.
Some audio drivers may be limited to counting
buffers of samples, instead of single samples for their samples accounting.
For this reason, applications should not assume that the samples fields contain a
perfectly accurate count.
The
.Fa play.eof
field increments whenever a zero-length output buffer is synchronously processed.
Applications may use this
field to detect the completion of particular segments of audio output.
.Pp
The
.Fa record.buffer_size
field controls the amount of input data that is
buffered in the device driver during record operations.
Applications that have
particular requirements for low latency should set the value appropriately.
Note however that smaller input buffer sizes may result in higher system
overhead.
The value of this field is specified in bytes and drivers will
constrain it to be a multiple of the current sample frame size.
Some drivers may place other requirements on the value of this field.
Refer to the audio device-specific manual page for more details.
If an application changes the
format of the audio device and does not modify the
.Fa record.buffer_size
field, the device driver may use a default value to compensate for the new data
rate.
Therefore, if an application is going to modify this field, it should
modify it during or after the format change itself, not before.
When changing
the
.Fa record.buffer_size
parameters, the input stream should be paused and
flushed before the change, and resumed afterward.
Otherwise, subsequent reads
may return samples in the old format followed by samples in the new format.
This is particularly important when new parameters result in a changed sample
size.
If you change the
.Fa record.buffer_size
for the first packet, this protocol must be followed or the first buffer will
be the default buffer size for the device, followed by packets of the requested
change size.
.Pp
The
.Fa record.buffer_size
field may be modified only on the
.Pa /dev/audio
device by processes that have it opened for reading.
.Pp
The
.Fa play.buffer_size
field is currently not supported.
.Pp
The audio data format is indicated by the
.Fa sample_rate ,
.Fa channels ,
.Fa precision ,
and
.Fa encoding
fields.
The values of these fields correspond to the
descriptions in the
.Sx "AUDIO FORMATS"
section of this man page.
Refer to the
audio device-specific manual pages for a list of supported data format
combinations.
.Pp
The data format fields can be modified only on the
.Pa /dev/audio
device.
.Pp
If the parameter changes requested by an
.Dv AUDIO_SETINFO
ioctl cannot all be accommodated,
.Xr ioctl 2
returns with
.Va errno
set to
.Er EINVAL
and no changes are made to the device state.
.Ss "Streamio IOCTLS"
All of the
.Xr streamio 4I
.Xr ioctl 2
commands may be issued for the
.Pa /dev/audio
device.
Because the
.Pa /dev/audioctl
device has its own
.Sy STREAMS
queues, most of these commands neither modify nor report the
state of
.Pa /dev/audio
if issued for the
.Pa /dev/audioctl
device.
The
.Dv I_SETSIG
ioctl may be issued for
.Pa /dev/audioctl
to enable the notification of audio status changes, as described above.
.Ss "Audio IOCTLS"
The audio device additionally supports the following
.Xr ioctl 2
commands:
.Bl -tag -width "AUDIO_GETINFO"
.It Dv AUDIO_DRAIN
The argument is ignored.
This command suspends the calling process until the
output STREAMS queue is empty and all queued samples have been played, or until
a signal is delivered to the calling process.
It may not be issued for the
.Pa /dev/audioctl
device.
An implicit
.Dv AUDIO_DRAIN
is performed on the final
.Xr close 2
of
.Pa /dev/audio .
.It Dv AUDIO_GETDEV
The argument is a pointer to an
.Vt audio_device_t
structure.
This command may be issued for either
.Pa /dev/audio
or
.Pa /dev/audioctl .
The returned
value in the name field will be a string that will identify the current
.Pa /dev/audio
hardware device, the value in version will be a string
indicating the current version of the hardware, and
.Fa config
will be a device-specific string identifying the properties of the audio stream
associated with that file descriptor.
Refer to the audio device-specific manual
pages to determine the actual strings returned by the device driver.
.It Dv AUDIO_GETINFO
The argument is a pointer to an
.Vt audio_info_t
structure.
This command may be issued for either
.Pa /dev/audio
or
.Pa /dev/audioctl .
The current state of the
.Pa /dev/audio
device is returned in the structure.
Values return pertain to a logical view of the device as seen by and private to
the process, and do not necessarily reflect the actual hardware device itself.
.It Dv AUDIO_SETINFO
The argument is a pointer to an
.Vt audio_info_t
structure.
This command may be issued for either the
.Pa /dev/audio
or the
.Pa /dev/audioctl
device with some restrictions.
This command configures the audio device according to
the supplied structure and overwrites the existing structure with the new state
of the device.
Note: The
.Fa play.samples ,
.Fa record.samples ,
.Fa play.error ,
.Fa record.error ,
and
.Fa play.eof
fields are modified to reflect the state of the device when the
.Dv AUDIO_SETINFO
is issued.
This allows programs to automatically modify these fields while retrieving the
previous value.
.Pp
As with
.Dv AUDIO_SETINFO ,
the settings managed by this ioctl deal with a
logical view of the device which is private to the process, and don't
necessarily have any impact on the hardware device itself.
.El
.Pp
Certain fields in the audio information structure, such as the pause flags, are
treated as read-only when
.Pa /dev/audio
is not open with the corresponding access permission.
Other fields, such as the gain levels and encoding
information, may have a restricted set of acceptable values.
Applications that
attempt to modify such fields should check the returned values to be sure that
the corresponding change took effect.
The
.Fa sample_rate ,
.Fa channels ,
.Fa precision ,
and
.Fa encoding
fields treated as read-only for
.Pa /dev/audioctl ,
so that applications can be guaranteed that the existing
audio format will stay in place until they relinquish the audio device.
.Dv AUDIO_SETINFO
will return
.Er EINVAL
when the desired configuration is not possible, or
.Er EBUSY
when another process has control of the audio device.
.Pp
All of the logical device state is reset when the corresponding I/O stream of
.Pa /dev/audio
is closed.
.Pp
The
.Vt audio_info_t
structure may be initialized through the use of the
.Dv AUDIO_INITINFO
macro.
This macro sets all fields in the structure to
values that are ignored by the
.Dv AUDIO_SETINFO
command.
For instance, the following code switches the output port from the built-in
speaker to the headphone jack without modifying any other audio parameters:
.Bd -literal -offset 2n
audio_info_t info;
AUDIO_INITINFO();
info.play.port = AUDIO_HEADPHONE;
err = ioctl(audio_fd, AUDIO_SETINFO, );
.Ed
.Pp
This technique eliminates problems associated with using a sequence of
.Dv AUDIO_GETINFO
followed by
.Dv AUDIO_SETINFO .
.Sh FILES
The physical audio device names are system dependent and are rarely used by
programmers.
Programmers should use the following generic device names:
.Bl -tag -width "/usr/share/audio/samples"
.It Pa /dev/audio
Symbolic link to the system's primary audio device
.It Pa /dev/audioctl
Symbolic link to the control device for
.Pa /dev/audio
.It Pa /dev/sound/0
First audio device in the system
.It Pa /dev/sound/0ctl
Audio control device for
.Pa /dev/sound/0
.It Pa /usr/share/audio/samples
Audio files
.El
.Sh ERRORS
An
.Xr open 2
call will fail if:
.Bl -tag -width "EINTR"
.It Er EBUSY
The requested play or record access is busy and either the
.Dv O_NDELAY
or
.Dv O_NONBLOCK
flag was set in the
.Xr open 2
request.
.It Er EINTR
The requested play or record access is busy and a signal interrupted the
.Xr open 2
request.
.El
.Pp
An
.Xr ioctl 2
call will fail if:
.Bl -tag -width "EINVAL"
.It Er EINVAL
The parameter changes requested in the
.Dv AUDIO_SETINFO
ioctl are invalid or are not supported by the device.
.El
.Sh ARCHITECTURE
SPARC
X86
.Sh INTERFACE STABILITY
Obsolete Uncommitted
.Sh SEE ALSO
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr ioctl 2 ,
.Xr open 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr dsp 4I ,
.Xr streamio 4I ,
.Xr attributes 7
.Sh BUGS
Due to a feature of the
.Sy STREAMS
implementation, programs that are terminated or
exit without closing the audio device may hang for a short period while audio
output drains.
In general, programs that produce audio output should catch the
.Dv SIGINT
signal and flush the output stream before exiting.