Tribblix: manual page: dsp.4i

DSP(4I) Ioctl Requests 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