man/man4i/dsp.4i

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.Dd July 9, 2018
.Dt DSP 4I
.Os
.Sh NAME
.Nm dsp
.Nd generic audio device interface
.Sh SYNOPSIS
.In sys/soundcard.h
.Sh DESCRIPTION
To record audio input, applications
.Xr open 2
the appropriate 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 can contain more than one audio device, application
writers are encouraged to open the
.Pa /dev/mixer
device and determine the
physical devices present on the system using the
.Dv SNDCTL_SYSINFO
and
.Dv SNDCTL_AUDIOINFO
ioctls.
See
.Xr mixer 4I .
The user should be provided
wth the ability to select a different audio device, or alternatively, an
environment variable such as
.Ev AUDIODSP
can be used.
In the absence of any
specific configuration from the user, the generic device file,
.Pa /dev/dsp ,
can be used.
This normally points to a reasonably appropriate default audio
device for the system.
.Ss "Opening the Audio Device"
The audio device is not treated as an exclusive resource.
.Pp
Each
.Xr open 2
completes as long as there are channels available to be allocated.
If no channels are available to be allocated, the call returns
.Sy -1
with the
.Va errno
set to
.Er EBUSY .
.Pp
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.
.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
.Xr 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
.Xr read 2
completes immediately, but can 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 allocates
resources for recording.
Since this consumes system resources, processes that
do not record audio data should open the device write-only
.Pq Dv O_WRONLY .
.Pp
The recording process can be stopped by using the
.Dv SNDCTL_DSP_HALT_INPUT
ioctl, which also discards all pending record data in underlying device FIFOs.
.Pp
Before changing record parameters, the input should be stopped using the
.Dv 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
.Xr 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.
.Pp
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,
.Sy \(*m-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
.Dv SNDCTL_DSP_GETERROR
ioctl.
.Pp
Record volume for a stream can be adjusted by issuing the
.Dv SNDCTL_DSP_SETRECVOL
ioctl.
The volume can also be retrieved using the
.Dv SNDCTL_DSP_GETRECVOL .
.Ss "Playing Audio Data"
The
.Xr write 1
system call copies data from an application's buffer to the device output FIFO.
Ordinarily,
.Xr write 2
blocks until the entire user
buffer is transferred.
The device can alternatively be set to a non-blocking
mode, in which case
.Xr write 2
completes immediately, but might 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 might take considerably longer.
The
.Dv SNDCTL_DSP_SYNC
ioctl can be issued to allow an application to block
until all of the queued output data has been played.
.Pp
The final
.Xr close 2
of the file descriptor waits 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 output of playback data can be halted entirely, by calling the
.Dv SNDCTL_DSP_HALT_OUTPUT
ioctl.
This also discards any data that is queued for playback in device FIFOs.
.Pp
Before changing playback parameters, the output should be drained using the
.Dv SNDCTL_DSP_SYNC
ioctl, and then stopped using the
.Dv 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
.Xr write 2 .
This is particularly important
when new parameters result in a changed sample size.
.Pp
Output data is played from the playback 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 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
.Dv SNDCTL_DSP_GETERROR
ioctl.
.Pp
Playback volume for a stream can be adjusted by issuing the
.Dv SNDCTL_DSP_SETPLAYVOL
ioctl.
The volume can also be retrieved using the
.Dv SNDCTL_DSP_GETPLAYVOL .
.Ss "Asynchronous I/O"
The
.Dv O_NONBLOCK
flag can 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.
.Pp
It is also possible to determine the amount of data that can be transferred for
playback or recording without blocking using the
.Dv SNDCTL_DSP_GETOSPACE
or
.Dv SNDCTL_DSP_GETISPACE
ioctls, respectively.
.Ss "Mixer Pseudo-Device"
The
.Pa /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.
.Pp
Applications should
.Xr open 2
.Pa /dev/mixer ,
and use the
.Dv SNDCTL_SYSINFO
and
.Dv SNDCTL_AUDIOINFO
ioctls to determine the device node names of audio devices on the system.
See
.Xr mixer 4I
for additional details.
.Sh IOCTLS
.Ss "Information IOCTLs"
The following ioctls are supported on the audio device, as well as the mixer
device.
See
.Xr mixer 4I
for details.
.Bd -literal -offset 2n
.Dv OSS_GETVERSION
.Dv SNDCTL_SYSINFO
.Dv SNDCTL_AUDIOINFO
.Dv SNDCTL_MIXERINFO
.Dv SNDCTL_CARDINFO
.Ed
.Ss "Audio IOCTLs"
The
.Nm
device supports the following ioctl commands:
.Pp
.\" A compact list is used so that items with multiple tags don't have
.\" vertical whitespace.  However that means explicit .Pp statements must
.\" be added.
.Bl -tag -width "SNDCTL_DSP_CURRENT_IPTR" -compact
.It Dv 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
.Dv SNDCTL_DSP_SYNC
is performed on the final
.Xr close 2
of the
.Nm
device.
.Pp
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.
.Pp
.It Dv SNDCTL_DSP_HALT
.It Dv SNDCTL_DSP_HALT_INPUT
.It Dv 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.
.Pp
.It Dv 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.
.Pp
.It Dv 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:
.Pp
.Bl -tag -width "AFMT_S24_PACKED" -compact -offset 2n
.It Dv AFMT_MU_LAW
8-bit unsigned \(*m-Law
.It Dv AFMT_A_LAW
8-bit unsigned a-Law
.It Dv AFMT_U8
8-bit unsigned linear PCM
.It Dv AFMT_S16_LE
16-bit signed little-endian linear PCM
.It Dv AFMT_S16_BE
16-bit signed big-endian linear PCM
.It Dv AFMT_S16_NE
16-bit signed native-endian linear PCM
.It Dv AFMT_U16_LE
16-bit unsigned little-endian linear PCM
.It Dv AFMT_U16_BE
16-bit unsigned big-endian linear PCM
.It Dv AFMT_U16_NE
16-bit unsigned native-endian linear PCM
.It Dv AFMT_S24_LE
24-bit signed little-endian linear PCM, 32-bit aligned
.It Dv AFMT_S24_BE
24-bit signed big-endian linear PCM, 32-bit aligned
.It Dv AFMT_S24_NE
24-bit signed native-endian linear PCM, 32-bit aligned
.It Dv AFMT_S32_LE
32-bit signed little-endian linear PCM
.It Dv AFMT_S32_BE
32-bit signed big-endian linear PCM
.It Dv AFMT_S32_NE
32-bit signed native-endian linear PCM
.It Dv AFMT_S24_PACKED
24-bit signed little-endian packed linear PCM
.El
.Pp
Not all devices support all of these encodings.
This implementation uses
.Dv AFMT_S24_LE
or
.Dv AFMT_S24_BE ,
whichever is native, internally.
.Pp
.It Dv SNDCTL_DSP_SETFMT
The argument is a pointer to an integer, which indicates the encoding to be
used.
The same values as for
.Dv 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.
.Pp
.It Dv 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.
.Pp
.It Dv 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:
.Pp
.Bl -tag -width "PCM_CAP_DUPLEX" -compact -offset 2n
.It Dv PCM_CAP_OUTPUT
Device supports playback
.It Dv PCM_CAP_INPUT
Device supports recording
.It Dv PCM_CAP_DUPLEX
Device supports simultaneous playback and recording
.El
.Pp
.It Dv SNDCTL_DSP_GETPLAYVOL
.It Dv 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.
.Pp
.It Dv SNDCTL_DSP_SETPLAYVOL
.It Dv 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.
.Pp
.It Dv SNDCTL_DSP_GETISPACE
.It Dv SNDCTL_DSP_GETOSPACE
The argument is a pointer to a
.Vt struct audio_buf_info ,
which has the following structure:
.Bd -literal -offset 2n
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;
.Ed
.Pp
The fields
.Fa fragments ,
.Fa fragstotal ,
and
.Fa fragsize
are intended for use with compatible applications (and in the future with
.Xr 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.
.Pp
.It Dv SNDCTL_DSP_CURRENT_IPTR
.It Dv SNDCTL_DSP_CURRENT_OPTR
The argument is a pointer to an
.Vt oss_count_t ,
which has the following definition:
.Bd -literal -offset 2n
typedef struct {
    long long samples;
       /* Total # of samples */
    int fifo_samples;
       /* Samples in device FIFO */
    int filler[32]; /* For future use */
} oss_count_t;
.Ed
.Pp
The
.Fa samples
field contains the total number of samples transferred by the
device so far.
The
.Fa fifo_samples
is the depth of any hardware FIFO.
This structure can be useful for accurate stream positioning and latency
calculations.
.Pp
.It Dv SNDCTL_DSP_GETIPTR
.It Dv SNDCTL_DSP_GETOPTR
The argument is a pointer to a
.Vt struct count_info ,
which has the following definition:
.Bd -literal -offset 2n
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;
.Ed
.Pp
These ioctls are primarily supplied for compatibility, and should not be used
by most applications.
.Pp
.It Dv 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.
.Pp
.It Dv SNDCTL_DSP_GETERROR
The argument is a pointer to a
.Vt struct audio_errinfo ,
defined as follows:
.Bd -literal -offset 2n
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;
.Ed
.Pp
For this implementation, only the
.Fa play_underruns
and
.Fa rec_overruns
values are significant.
No other fields are used in this implementation.
.Pp
These fields are reset to zero each time their value is retrieved using this
ioctl.
.El
.Ss "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.
.Pp
.Bl -tag -width "Dv" -offset 2n -compact
.It Dv SNDCTL_DSP_POST
.It Dv SNDCTL_DSP_STEREO
.It Dv SNDCTL_DSP_SETDUPLEX
.It Dv SNDCTL_DSP_LOW_WATER
.It Dv SNDCTL_DSP_PROFILE
.It Dv SNDCTL_DSP_GETBLKSIZE
.It Dv SNDCTL_DSP_SUBDIVIDE
.It Dv SNDCTL_DSP_SETFRAGMENT
.It Dv SNDCTL_DSP_COOKEDMODE
.It Dv SNDCTL_DSP_READCTL
.It Dv SNDCTL_DSP_WRITECTL
.It Dv SNDCTL_DSP_SILENCE
.It Dv SNDCTL_DSP_SKIP
.It Dv SNDCTL_DSP_POST
.It Dv SNDCTL_DSP_GET_RECSRC
.It Dv SNDCTL_DSP_SET_RECSRC
.It Dv SNDCTL_DSP_SET_RECSRC_NAMES
.It Dv SNDCTL_DSP_GET_PLAYTGT
.It Dv SNDCTL_DSP_SET_PLAYTGT
.It Dv SNDCTL_DSP_SET_PLAYTGT_NAMES
.It Dv SNDCTL_DSP_GETTRIGGER
.It Dv SNDCTL_DSP_SETTRIGGER
.It Dv SNDCTL_AUDIOINFO_EX
.It Dv SNDCTL_ENGINEINFO
.El
.Sh FILES
The physical audio device names are system dependent and are rarely used by
programmers.
Programmers should use the generic device names listed below.
.Bl -tag -width "/usr/share/audio/samples"
.It Pa /dev/dsp
Symbolic link to the system's primary audio device
.It Pa /dev/mixer
Symbolic link to the pseudo mixer device for the system
.It Pa /dev/sndstat
Symbolic link to the pseudo mixer device for the system
.It Pa /usr/share/audio/samples
Audio files
.El
.Sh ERRORS
An
.Xr open 2
call fails if:
.Bl -tag -width "EINVAL"
.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.
.It Er EINVAL
The device cannot support the requested play or record access.
.El
.Pp
An
.Xr ioctl 2
call fails if:
.Bl -tag -width "EINVAL"
.It Er EINVAL
The parameter changes requested in the ioctl are invalid or are not supported
by the device.
.El
.Sh ARCHITECTURE
SPARC
X86
.Sh INTERFACE STABILITY
Uncommitted
.Sh SEE ALSO
.Xr close 2 ,
.Xr fcntl 2 ,
.Xr ioctl 2 ,
.Xr mmap 2 ,
.Xr open 2 ,
.Xr poll 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr audio 4D ,
.Xr mixer 4I ,
.Xr attributes 7