DSP(4I) 4I DSP(4I)

NAME


dsp - generic audio device interface

SYNOPSIS


#include <sys/soundcard.h>

DESCRIPTION


To record audio input, applications open(2) the appropriate device and read
data from it using the read(2) system call. Similarly, sound data is
queued to the audio output port by using the write(2) system call. Device
configuration is performed using the ioctl(2) interface.

Because some systems can contain more than one audio device, application
writers are encouraged to open the /dev/mixer device and determine the
physical devices present on the system using the SNDCTL_SYSINFO and
SNDCTL_AUDIOINFO ioctls. See mixer(4I). The user should be provided wth
the ability to select a different audio device, or alternatively, an
environment variable such as AUDIODSP can be used. In the absence of any
specific configuration from the user, the generic device file, /dev/dsp,
can be used. This normally points to a reasonably appropriate default
audio device for the system.

Opening the Audio Device


The audio device is not treated as an exclusive resource.

Each open(2) completes as long as there are channels available to be
allocated. If no channels are available to be allocated, the call returns
-1 with the errno set to EBUSY.

Audio applications should explicitly set the encoding characteristics to
match the audio data requirements after opening the device, and not depend
on any default configuration.

Recording Audio Data


The read(2) system call copies data from the system's buffers to the
application. Ordinarily, read(2) blocks until the user buffer is filled.
The poll(2) system call can be used to determine the presence of data that
can be read without blocking. The device can alternatively be set to a
non-blocking mode, in which case read(2) completes immediately, but can
return fewer bytes than requested. Refer to the read(2) manual page for a
complete description of this behavior.

When the audio device is opened with read access, the device driver
allocates resources for recording. Since this consumes system resources,
processes that do not record audio data should open the device write-only
(O_WRONLY).

The recording process can be stopped by using the SNDCTL_DSP_HALT_INPUT
ioctl, which also discards all pending record data in underlying device
FIFOs.

Before changing record parameters, the input should be stopped using the
SNDCTL_DSP_HALT_INPUT ioctl, which also flushes the any underlying device
input FIFOs. (This is not necessary if the process never started recording
by calling read(2). Otherwise, subsequent reads can 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.

Input data can accumulate in device buffers very quickly. At a minimum, it
accumulates at 8000 bytes per second for 8-bit, 8 KHz, mono, <mu>-Law data.
If the device is configured for more channels, higher sample resolution, or
higher sample rates, it accumulates even faster. If the application that
consumes the data cannot keep up with this data rate, the underlying FIFOs
can become full. When this occurs, any new incoming data is lost until the
application makes room available by consuming data. Additionally, a record
overrun is noted, which can be retrieved using the SNDCTL_DSP_GETERROR
ioctl.

Record volume for a stream can be adjusted by issuing the
SNDCTL_DSP_SETRECVOL ioctl. The volume can also be retrieved using the
SNDCTL_DSP_GETRECVOL.

Playing Audio Data


The write(1) system call copies data from an application's buffer to the
device output FIFO. Ordinarily, write(2) blocks until the entire user
buffer is transferred. The device can alternatively be set to a non-
blocking mode, in which case write(2) completes immediately, but might have
transferred fewer bytes than requested. See write(2).

Although write(2) returns when the data is successfully queued, the actual
completion of audio output might take considerably longer. The
SNDCTL_DSP_SYNC ioctl can be issued to allow an application to block until
all of the queued output data has been played.

The final close(2) of the file descriptor waits until all of the audio
output has drained. If a signal interrupts the 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.

The output of playback data can be halted entirely, by calling the
SNDCTL_DSP_HALT_OUTPUT ioctl. This also discards any data that is queued
for playback in device FIFOs.

Before changing playback parameters, the output should be drained using the
SNDCTL_DSP_SYNC ioctl, and then stopped using the SNDCTL_DSP_HALT_OUTPUT
ioctl, which also flushes the any underlying device output FIFOs. This is
not necessary if the process never started playback, such as by calling
write(2). This is particularly important when new parameters result in a
changed sample size.

Output data is played from the playback buffers at a default rate of at
least 8000 bytes per second for <mu>-Law, A-Law or 8-bit PCM data (faster
for 16-bit linear data or higher sampling rates). If the output FIFO
becomes empty, the framework plays silence, resulting in audible stall or
click in the output, until more data is supplied by the application. The
condition is also noted as a play underrun, which can be determined using
the SNDCTL_DSP_GETERROR ioctl.

Playback volume for a stream can be adjusted by issuing the
SNDCTL_DSP_SETPLAYVOL ioctl. The volume can also be retrieved using the
SNDCTL_DSP_GETPLAYVOL.

Asynchronous I/O
The O_NONBLOCK flag can be set using the F_SETFL fcntl(2) to enable non-
blocking read(2) and write(2) requests. This is normally sufficient for
applications to maintain an audio stream in the background.

It is also possible to determine the amount of data that can be transferred
for playback or recording without blocking using the SNDCTL_DSP_GETOSPACE
or SNDCTL_DSP_GETISPACE ioctls, respectively.

Mixer Pseudo-Device
The /dev/mixer provides access to global hardware settings such as master
volume settings, etc. It is also the interface used for determining the
hardware configuration on the system.

Applications should open(2) /dev/mixer, and use the SNDCTL_SYSINFO and
SNDCTL_AUDIOINFO ioctls to determine the device node names of audio devices
on the system. See mixer(4I) for additional details.

IOCTLS


Information IOCTLs


The following ioctls are supported on the audio device, as well as the
mixer device. See mixer(4I) for details.

OSS_GETVERSION
SNDCTL_SYSINFO
SNDCTL_AUDIOINFO
SNDCTL_MIXERINFO
SNDCTL_CARDINFO

Audio IOCTLs


The dsp device supports the following ioctl commands:

SNDCTL_DSP_SYNC The argument is ignored. This command suspends
the calling process until the output FIFOs are
empty and all queued samples have been played, or
until a signal is delivered to the calling
process. An implicit SNDCTL_DSP_SYNC is performed
on the final close(2) of the dsp device.

This ioctl should not be used unnecessarily, as if
it is used in the middle of playback it causes a
small click or pause, as the FIFOs are drained.
The correct use of this ioctl is just before
changing sample formats.

SNDCTL_DSP_HALT
SNDCTL_DSP_HALT_INPUT
SNDCTL_DSP_HALT_OUTPUT The argument is ignored. All input or output (or
both) associated with the file is halted, and any
pending data is discarded.

SNDCTL_DSP_SPEED The argument is a pointer to an integer,
indicating the sample rate (in Hz) to be used.
The rate applies to both input and output for the
file descriptor. On return the actual rate, which
can differ from that requested, is stored in the
integer pointed to by the argument. To query the
configured speed without changing it the value 0
can be used by the application.

SNDCTL_DSP_GETFMTS The argument is a pointer to an integer, which
receives a bit mask of encodings supported by the
device. Possible values are:

AFMT_MU_LAW 8-bit unsigned <mu>-Law
AFMT_A_LAW 8-bit unsigned a-Law
AFMT_U8 8-bit unsigned linear PCM
AFMT_S16_LE 16-bit signed little-endian
linear PCM
AFMT_S16_BE 16-bit signed big-endian linear
PCM
AFMT_S16_NE 16-bit signed native-endian
linear PCM
AFMT_U16_LE 16-bit unsigned little-endian
linear PCM
AFMT_U16_BE 16-bit unsigned big-endian
linear PCM
AFMT_U16_NE 16-bit unsigned native-endian
linear PCM
AFMT_S24_LE 24-bit signed little-endian
linear PCM, 32-bit aligned
AFMT_S24_BE 24-bit signed big-endian linear
PCM, 32-bit aligned
AFMT_S24_NE 24-bit signed native-endian
linear PCM, 32-bit aligned
AFMT_S32_LE 32-bit signed little-endian
linear PCM
AFMT_S32_BE 32-bit signed big-endian linear
PCM
AFMT_S32_NE 32-bit signed native-endian
linear PCM
AFMT_S24_PACKED 24-bit signed little-endian
packed linear PCM

Not all devices support all of these encodings.
This implementation uses AFMT_S24_LE or
AFMT_S24_BE, whichever is native, internally.

SNDCTL_DSP_SETFMT The argument is a pointer to an integer, which
indicates the encoding to be used. The same
values as for SNDCTL_DSP_GETFMT can be used, but
the caller can only specify a single option. The
encoding is used for both input and output
performed on the file descriptor.

SNDCTL_DSP_CHANNELS The argument is a pointer to an integer,
indicating the number of channels to be used (1
for mono, 2 for stereo, etc.) The value applies
to both input and output for the file descriptor.
On return the actual channel configuration (which
can differ from that requested) is stored in the
integer pointed to by the argument. To query the
configured channels without changing it the value
0 can be used by the application.

SNDDCTL_DSP_GETCAPS The argument is a pointer to an integer bit mask,
which indicates the capabilities of the device.
The bits returned can include:

PCM_CAP_OUTPUT Device supports playback
PCM_CAP_INPUT Device supports recording
PCM_CAP_DUPLEX Device supports simultaneous
playback and recording

SNDCTL_DSP_GETPLAYVOL
SNDCTL_DSP_GETRECVOL The argument is a pointer to an integer to receive
the volume level for either playback or record.
The value is encoded as a stereo value with the
values for two channels in the least significant
two bytes. The value for each channel thus has a
range of 0-100. In this implementation, only the
low order byte is used, as the value is treated as
a monophonic value, but a stereo value (with both
channel levels being identical) is returned for
compatibility.

SNDCTL_DSP_SETPLAYVOL
SNDCTL_DSP_SETRECVOL The argument is a pointer to an integer indicating
volume level for either playback or record. The
value is encoded as a stereo value with the values
for two channels in the least significant two
bytes. The value for each channel has a range of
0-100. Note that in this implementation, only the
low order byte is used, as the value is treated as
a monophonic value. Portable applications should
assign the same value to both bytes.

SNDCTL_DSP_GETISPACE
SNDCTL_DSP_GETOSPACE The argument is a pointer to a struct
audio_buf_info, which has the following structure:

typedef struct audio_buf_info {
int fragments; /* # of available fragments */
int fragstotal;
/* Total # of fragments allocated */
int fragsize;
/* Size of a fragment in bytes */
int bytes;
/* Available space in bytes */
/*
* Note! 'bytes' could be more than
* fragments*fragsize
*/
} audio_buf_info;

The fields fragments, fragstotal, and fragsize are
intended for use with compatible applications (and
in the future with mmap(2)) only, and need not be
used by typical applications. On successful
return the bytes member contains the number of
bytes that can be transferred without blocking.

SNDCTL_DSP_CURRENT_IPTR
SNDCTL_DSP_CURRENT_OPTR The argument is a pointer to an oss_count_t, which
has the following definition:

typedef struct {
long long samples;
/* Total # of samples */
int fifo_samples;
/* Samples in device FIFO */
int filler[32]; /* For future use */
} oss_count_t;

The samples field contains the total number of
samples transferred by the device so far. The
fifo_samples is the depth of any hardware FIFO.
This structure can be useful for accurate stream
positioning and latency calculations.

SNDCTL_DSP_GETIPTR
SNDCTL_DSP_GETOPTR The argument is a pointer to a struct count_info,
which has the following definition:

typedef struct count_info {
unsigned int bytes;
/* Total # of bytes processed */
int blocks;
/*
* # of fragment transitions since
* last time
*/
int ptr; /* Current DMA pointer value */
} count_info;

These ioctls are primarily supplied for
compatibility, and should not be used by most
applications.

SNDCTL_DSP_GETODELAY The argument is a pointer to an integer. On
return, the integer contains the number of bytes
still to be played before the next byte written
are played. This can be used for accurate
determination of device latency. The result can
differ from actual value by up the depth of the
internal device FIFO, which is typically 64 bytes.

SNDCTL_DSP_GETERROR The argument is a pointer to a struct
audio_errinfo, defined as follows:

typedef struct audio_errinfo {
int play_underruns;
int rec_overruns;
unsigned int play_ptradjust;
unsigned int rec_ptradjust;
int play_errorcount;
int rec_errorcount;
int play_lasterror;
int rec_lasterror;
int play_errorparm;
int rec_errorparm;
int filler[16];
} audio_errinfo;

For this implementation, only the play_underruns
and rec_overruns values are significant. No other
fields are used in this implementation.

These fields are reset to zero each time their
value is retrieved using this ioctl.

Compatibility IOCTLS


These ioctls are supplied exclusively for compatibility with existing
applications. Their use is not recommended, and they are not documented
here. Many of these are implemented as simple no-ops.

SNDCTL_DSP_POST
SNDCTL_DSP_STEREO
SNDCTL_DSP_SETDUPLEX
SNDCTL_DSP_LOW_WATER
SNDCTL_DSP_PROFILE
SNDCTL_DSP_GETBLKSIZE
SNDCTL_DSP_SUBDIVIDE
SNDCTL_DSP_SETFRAGMENT
SNDCTL_DSP_COOKEDMODE
SNDCTL_DSP_READCTL
SNDCTL_DSP_WRITECTL
SNDCTL_DSP_SILENCE
SNDCTL_DSP_SKIP
SNDCTL_DSP_POST
SNDCTL_DSP_GET_RECSRC
SNDCTL_DSP_SET_RECSRC
SNDCTL_DSP_SET_RECSRC_NAMES
SNDCTL_DSP_GET_PLAYTGT
SNDCTL_DSP_SET_PLAYTGT
SNDCTL_DSP_SET_PLAYTGT_NAMES
SNDCTL_DSP_GETTRIGGER
SNDCTL_DSP_SETTRIGGER
SNDCTL_AUDIOINFO_EX
SNDCTL_ENGINEINFO

FILES


The physical audio device names are system dependent and are rarely used by
programmers. Programmers should use the generic device names listed below.

/dev/dsp Symbolic link to the system's primary audio
device

/dev/mixer Symbolic link to the pseudo mixer device for the
system

/dev/sndstat Symbolic link to the pseudo mixer device for the
system

/usr/share/audio/samples Audio files

ERRORS


An open(2) call fails if:

EBUSY The requested play or record access is busy and either the O_NDELAY
or O_NONBLOCK flag was set in the open(2) request.

EINTR The requested play or record access is busy and a signal
interrupted the open(2) request.

EINVAL The device cannot support the requested play or record access.

An ioctl(2) call fails if:

EINVAL The parameter changes requested in the ioctl are invalid or are not
supported by the device.

ARCHITECTURE


SPARC X86

INTERFACE STABILITY


Uncommitted

SEE ALSO


close(2), fcntl(2), ioctl(2), mmap(2), open(2), poll(2), read(2), write(2),
audio(4D), mixer(4I), attributes(7)

OmniOS July 9, 2018 OmniOS