src/lib/libsndio/sio_open.3

.\" $OpenBSD: sio_open.3,v 1.56 2022/05/30 10:31:59 op Exp $
.\"
.\" Copyright (c) 2007 Alexandre Ratchov <alex@caoua.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
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.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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.Dd $Mdocdate: May 30 2022 $
.Dt SIO_OPEN 3
.Os
.Sh NAME
.Nm sio_open ,
.Nm sio_close ,
.Nm sio_setpar ,
.Nm sio_getpar ,
.Nm sio_getcap ,
.Nm sio_start ,
.Nm sio_stop ,
.Nm sio_flush ,
.Nm sio_read ,
.Nm sio_write ,
.Nm sio_onmove ,
.Nm sio_nfds ,
.Nm sio_pollfd ,
.Nm sio_revents ,
.Nm sio_eof ,
.Nm sio_setvol ,
.Nm sio_onvol ,
.Nm sio_initpar ,
.Nm SIO_BPS
.Nd sndio interface to audio devices
.Sh SYNOPSIS
.In sndio.h
.Ft "struct sio_hdl *"
.Fn sio_open "const char *name" "unsigned int mode" "int nbio_flag"
.Ft "void"
.Fn sio_close "struct sio_hdl *hdl"
.Ft "int"
.Fn sio_setpar "struct sio_hdl *hdl" "struct sio_par *par"
.Ft "int"
.Fn sio_getpar "struct sio_hdl *hdl" "struct sio_par *par"
.Ft "int"
.Fn sio_getcap "struct sio_hdl *hdl" "struct sio_cap *cap"
.Ft "int"
.Fn sio_start "struct sio_hdl *hdl"
.Ft "int"
.Fn sio_stop "struct sio_hdl *hdl"
.Ft "int"
.Fn sio_flush "struct sio_hdl *hdl"
.Ft "size_t"
.Fn sio_read "struct sio_hdl *hdl" "void *addr" "size_t nbytes"
.Ft "size_t"
.Fn sio_write "struct sio_hdl *hdl" "const void *addr" "size_t nbytes"
.Ft "void"
.Fo sio_onmove
.Fa "struct sio_hdl *hdl"
.Fa "void (*cb)(void *arg, int delta)"
.Fa "void *arg"
.Fc
.Ft "int"
.Fn sio_nfds "struct sio_hdl *hdl"
.Ft "int"
.Fn sio_pollfd "struct sio_hdl *hdl" "struct pollfd *pfd" "int events"
.Ft "int"
.Fn sio_revents "struct sio_hdl *hdl" "struct pollfd *pfd"
.Ft "int"
.Fn sio_eof "struct sio_hdl *hdl"
.Ft "int"
.Fn sio_setvol "struct sio_hdl *hdl" "unsigned int vol"
.Ft "int"
.Fo sio_onvol
.Fa "struct sio_hdl *hdl"
.Fa "void (*cb)(void *arg, unsigned int vol)"
.Fa "void *arg"
.Fc
.Ft "void"
.Fn sio_initpar "struct sio_par *par"
.Ft unsigned int
.Fn SIO_BPS "unsigned int bits"
.Sh DESCRIPTION
The
.Nm sndio
library allows user processes to access
.Xr audio 4
hardware and the
.Xr sndiod 8
audio server in a uniform way.
.Ss Opening and closing an audio device
First the application must call the
.Fn sio_open
function to obtain a handle to the device;
later it will be passed as the
.Fa hdl
argument of most other functions.
The
.Fa name
parameter gives the device string discussed in
.Xr sndio 7 .
In most cases it should be set to
.Dv SIO_DEVANY
to allow the user to select it using the
.Ev AUDIODEVICE
environment variable.
.Pp
The following values of the
.Fa mode
parameter are supported:
.Bl -tag -width "SIO_PLAY | SIO_REC"
.It Dv SIO_PLAY
Play-only mode: data written will be played by the device.
.It Dv SIO_REC
Record-only mode: samples are recorded by the device and must be read.
.It Dv SIO_PLAY | SIO_REC
The device plays and records synchronously; this means that
the n-th recorded sample was physically sampled exactly when
the n-th played sample was actually played.
.El
.Pp
If the
.Fa nbio_flag
argument is true (i.e. non-zero), then the
.Fn sio_read
and
.Fn sio_write
functions (see below) will be non-blocking.
.Pp
The
.Fn sio_close
function stops the device as if
.Fn sio_stop
is called and frees the handle.
Thus, no samples submitted with
.Fn sio_write
are discarded.
.Ss Negotiating audio parameters
Audio samples are interleaved.
A frame consists of one sample for each channel.
For example, a 16-bit stereo encoding has two samples per frame
and, two bytes per sample (thus 4 bytes per frame).
.Pp
The set of parameters of the device that can be controlled
is given by the following structure:
.Bd -literal
struct sio_par {
	unsigned int bits;	/* bits per sample */
	unsigned int bps;	/* bytes per sample */
	unsigned int sig;	/* 1 = signed, 0 = unsigned int */
	unsigned int le;	/* 1 = LE, 0 = BE byte order */
	unsigned int msb;	/* 1 = MSB, 0 = LSB aligned */
	unsigned int rchan;	/* number channels for recording */
	unsigned int pchan;	/* number channels for playback */
	unsigned int rate;	/* frames per second */
	unsigned int appbufsz;	/* minimum buffer size without xruns */
	unsigned int bufsz;	/* end-to-end buffer size (read-only) */
	unsigned int round;	/* optimal buffer size divisor */
#define SIO_IGNORE	0	/* pause during xrun */
#define SIO_SYNC	1	/* resync after xrun */
#define SIO_ERROR	2	/* terminate on xrun */
	unsigned int xrun;	/* what to do on overrun/underrun */
};
.Ed
.Pp
The parameters are as follows:
.Bl -tag -width "appbufsz"
.It Fa bits
Number of bits per sample: must be between 1 and 32.
.It Fa bps
Bytes per samples; if specified, it must be large enough to hold all bits.
By default it's set to the smallest power of two large enough to hold
.Fa bits .
.It Fa sig
If set (i.e. non-zero) then the samples are signed, else unsigned.
.It Fa le
If set, then the byte order is little endian, else big endian;
it's meaningful only if
.Fa bps No > 1 .
.It Fa msb
If set, then the
.Fa bits
are aligned in the packet to the most significant bit
(i.e. lower bits are padded),
else to the least significant bit
(i.e. higher bits are padded);
it's meaningful only if
.Fa bits No < Fa bps No * 8 .
.It Fa rchan
The number of recorded channels; meaningful only if
.Dv SIO_REC
mode was selected.
.It Fa pchan
The number of played channels; meaningful only if
.Dv SIO_PLAY
mode was selected.
.It Fa rate
The sampling frequency in Hz.
.It Fa appbufsz
Size of the buffer in frames the application must maintain non-empty
(on the play end) or non-full (on the record end) by calling
.Fn sio_write
or
.Fn sio_read
fast enough to avoid overrun or underrun conditions.
The audio subsystem may use additional buffering, thus this
parameter cannot be used for latency calculations.
.It Fa bufsz
The maximum number of frames that may be buffered.
This parameter takes into account any buffers, and
can be used for latency calculations.
It is read-only.
.It Fa round
Optimal number of frames that the application buffers
should be a multiple of, to get best performance.
Applications can use this parameter to round their block size.
.It Fa xrun
The action when the client doesn't accept
recorded data or doesn't provide data to play fast enough;
it can be set to one of the
.Dv SIO_IGNORE ,
.Dv SIO_SYNC ,
or
.Dv SIO_ERROR
constants.
.El
.Pp
The following approach is recommended to negotiate device parameters:
.Bl -bullet
.It
Initialize a
.Vt sio_par
structure using
.Fn sio_initpar
and fill it with
the desired parameters.
Then call
.Fn sio_setpar
to request the device to use them.
Parameters left unset in the
.Vt sio_par
structure will be set to device-specific defaults.
.It
Call
.Fn sio_getpar
to retrieve the actual parameters of the device
and check that they are usable.
If they are not, then fail or set up a conversion layer.
Sometimes the rate set can be slightly different to what was requested.
A difference of about 0.5% is not audible and should be ignored.
.El
.Pp
Parameters cannot be changed after
.Fn sio_start
has been called,
.Fn sio_stop
or
.Fn sio_flush
must be called before parameters can be changed.
.Pp
If the device is exposed by the
.Xr sndiod 8
server, which is the default configuration,
a transparent emulation layer will
automatically be set up, and in this case any combination of
rate, encoding and numbers of channels is supported.
.Pp
To ease filling the
.Vt sio_par
structure, the
following macros can be used:
.Bl -tag -width "SIO_BPS(bits)"
.It Fn SIO_BPS bits
Return the smallest value for
.Fa bps
that is a power of two and that is large enough to
hold
.Fa bits .
.It Dv SIO_LE_NATIVE
Can be used to set the
.Fa le
parameter when native byte order is required.
It is 1 if the native byte order is little endian or 0 otherwise.
.El
.Ss Getting device capabilities
There's no way to get an exhaustive list of all parameter
combinations the device supports.
Applications that need to have a set of working
parameter combinations in advance can use the
.Fn sio_getcap
function.
However, for most new applications it's generally
not recommended to use
.Fn sio_getcap .
Instead, follow the recommendations for negotiating
device parameters (see above).
.Pp
The
.Vt sio_cap
structure contains the list of parameter configurations.
Each configuration contains multiple parameter sets.
The application must examine all configurations, and
choose its parameter set from
.Em one
of the configurations.
Parameters of different configurations
.Em are not
usable together.
.Bd -literal
struct sio_cap {
	struct sio_enc {		/* allowed encodings */
		unsigned int bits;
		unsigned int bps;
		unsigned int sig;
		unsigned int le;
		unsigned int msb;
	} enc[SIO_NENC];
	unsigned int rchan[SIO_NCHAN];	/* allowed rchans */
	unsigned int pchan[SIO_NCHAN];	/* allowed pchans */
	unsigned int rate[SIO_NRATE];	/* allowed rates */
	unsigned int nconf;		/* num. of confs[] */
	struct sio_conf {
		unsigned int enc;	/* bitmask of enc[] indexes */
		unsigned int rchan;	/* bitmask of rchan[] indexes */
		unsigned int pchan;	/* bitmask of pchan[] indexes */
		unsigned int rate;	/* bitmask of rate[] indexes */
	} confs[SIO_NCONF];
};
.Ed
.Pp
The parameters are as follows:
.Bl -tag -width "rchan[SIO_NCHAN]"
.It Fa enc Ns Bq Dv SIO_NENC
Array of supported encodings.
The tuple of
.Fa bits ,
.Fa bps ,
.Fa sig ,
.Fa le ,
and
.Fa msb
parameters are usable in the corresponding parameters
of the
.Vt sio_par
structure.
.It Fa rchan Ns Bq Dv SIO_NCHAN
Array of supported channel numbers for recording usable
in the
.Vt sio_par
structure.
.It Fa pchan Ns Bq Dv SIO_NCHAN
Array of supported channel numbers for playback usable
in the
.Vt sio_par
structure.
.It Fa rate Ns Bq Dv SIO_NRATE
Array of supported sample rates usable
in the
.Vt sio_par
structure.
.It Fa nconf
Number of different configurations available, i.e. number
of filled elements of the
.Fa confs Ns Bq
array.
.It Fa confs Ns Bq Dv SIO_NCONF
Array of available configurations.
Each configuration contains bitmasks indicating which
elements of the above parameter arrays are valid for the
given configuration.
For instance, if the second bit of
.Fa rate
is set, in the
.Vt sio_conf
structure, then the second element of the
.Fa rate Ns Bq Dv SIO_NRATE
array of the
.Vt sio_cap
structure is valid for this configuration.
As such, when reading the array elements in the
.Vt sio_cap
structure, the corresponding
.Vt sio_conf
bitmasks should always be used.
.El
.Ss Starting and stopping the device
The
.Fn sio_start
function prepares the device to start.
Once the play buffer is full, i.e.\&
.Fa sio_par.bufsz
samples are queued with
.Fn sio_write ,
playback starts automatically.
If record-only mode is selected, then
.Fn sio_start
starts recording immediately.
In full-duplex mode, playback and recording will start
synchronously as soon as the play buffer is full.
.Pp
The
.Fn sio_stop
function puts the audio subsystem
in the same state as before
.Fn sio_start
was called.
It stops recording, drains the play buffer and then stops playback.
If samples to play are queued but playback hasn't started yet
then playback is forced immediately; playback will actually stop
once the buffer is drained.
In no case are samples in the play buffer discarded.
.Pp
The
.Fn sio_flush
function stops playback and recording immediately,
possibly discarding play buffer contents, and puts the audio subsystem
in the same state as before
.Fn sio_start
was called.
.Ss Playing and recording
When record mode is selected, the
.Fn sio_read
function must be called to retrieve recorded data; it must be called
often enough to ensure that internal buffers will not overrun.
It will store at most
.Fa nbytes
bytes at the
.Fa addr
location and return the number of bytes stored.
Unless the
.Fa nbio_flag
flag is set, it will block until data becomes available and
will return zero only on error.
.Pp
Similarly, when play mode is selected, the
.Fn sio_write
function must be called to provide data to play.
Unless the
.Fa nbio_flag
is set,
.Fn sio_write
will block until the requested amount of data is written.
.Ss Non-blocking mode operation
If the
.Fa nbio_flag
is set on
.Fn sio_open ,
then the
.Fn sio_read
and
.Fn sio_write
functions will never block; if no data is available, they will
return zero immediately.
.Pp
The
.Xr poll 2
system call can be used to check if data can be
read from or written to the device.
The
.Fn sio_pollfd
function fills the array
.Fa pfd
of
.Vt pollfd
structures, used by
.Xr poll 2 ,
with
.Fa events ;
the latter is a bit-mask of
.Dv POLLIN
and
.Dv POLLOUT
constants; refer to
.Xr poll 2
for more details.
The
.Fn sio_revents
function returns the bit-mask set by
.Xr poll 2
in the
.Fa pfd
array of
.Vt pollfd
structures.
If
.Dv POLLIN
is set, recorded samples are available in the device buffer
and can be read with
.Fn sio_read .
If
.Dv POLLOUT
is set, space is available in the device buffer and new samples
to play can be submitted with
.Fn sio_write .
.Dv POLLHUP
may be set if an error occurs, even if
it is not selected with
.Fn sio_pollfd .
.Pp
The size of the
.Fa pfd
array, which the caller must pre-allocate, is provided by the
.Fn sio_nfds
function.
.Ss Synchronizing non-audio events to the audio stream in real-time
In order to perform actions at precise positions of the audio stream,
such as displaying video in sync with the audio stream,
the application must be notified in real-time of the exact
position in the stream the hardware is processing.
.Pp
The
.Fn sio_onmove
function can be used to register the
.Fn cb
callback function called at regular time intervals.
The
.Fa delta
argument contains the number of frames the hardware played and/or recorded
since the last call of
.Fn cb .
It is called by
.Fn sio_read ,
.Fn sio_write ,
and
.Fn sio_revents .
When the first sample is played and/or recorded, right after the device starts,
the callback is invoked with a zero
.Fa delta
argument.
The value of the
.Fa arg
pointer is passed to the callback and can contain anything.
.Pp
If desired, the application can maintain the current position by
starting from zero (when
.Fn sio_start
is called) and adding to the current position
.Fa delta
every time
.Fn cb
is called.
.Ss Measuring the latency and buffers usage
The playback latency is the delay it will take for the
frame just written to become audible, expressed in number of frames.
The exact playback
latency can be obtained by subtracting the current position
from the number of frames written.
Once playback is actually started (first sample audible),
the latency will never exceed the
.Fa bufsz
parameter (see the sections above).
There's a phase during which
.Fn sio_write
only queues data;
once there's enough data, actual playback starts.
During this phase talking about latency is meaningless.
.Pp
In any cases, at most
.Fa bufsz
frames are buffered.
This value takes into account all buffers.
The number of frames stored is equal to the number of frames
written minus the current position.
.Pp
The recording latency is obtained similarly, by subtracting
the number of frames read from the current position.
.Pp
Note that
.Fn sio_write
might block even if there is buffer space left;
using the buffer usage to guess if
.Fn sio_write
would block is false and leads to unreliable programs \(en consider using
.Xr poll 2
for this.
.Ss Handling buffer overruns and underruns
When the application cannot accept recorded data fast enough,
the record buffer (of size
.Fa appbufsz )
might overrun; in this case recorded data is lost.
Similarly if the application cannot provide data to play
fast enough, the play buffer underruns and silence is played
instead.
Depending on the
.Fa xrun
parameter of the
.Vt sio_par
structure, the audio subsystem will behave as follows:
.Bl -tag -width "SIO_IGNORE"
.It Dv SIO_IGNORE
The device pauses during overruns and underruns,
thus the current position (obtained through
.Fn sio_onmove )
stops being incremented.
Once the overrun and/or underrun condition is gone, the device resumes;
play and record are always kept in sync.
With this mode, the application cannot notice
underruns and/or overruns and shouldn't care about them.
.Pp
This mode is the default.
It's suitable for applications,
like audio players and telephony, where time
is not important and overruns or underruns are not short.
.It Dv SIO_SYNC
If the play buffer underruns, then silence is played,
but in order to reach the right position in time,
the same amount of written samples will be
discarded once the application is unblocked.
Similarly, if the record buffer overruns, then
samples are discarded, but the same amount of silence will be
returned later.
The current position (obtained through
.Fn sio_onmove )
is still incremented.
When the play buffer underruns the play latency might become negative;
when the record buffer overruns, the record latency might become
larger than
.Fa bufsz .
.Pp
This mode is suitable for applications, like music production,
where time is important and where underruns or overruns are
short and rare.
.It Dv SIO_ERROR
With this mode, on the first play buffer underrun or
record buffer overrun, playback and/or recording is terminated and
no other function than
.Fn sio_close
will succeed.
.Pp
This mode is mostly useful for testing.
.El
.Ss Controlling the volume
The
.Fn sio_setvol
function can be used to set playback attenuation.
The
.Fa vol
parameter takes a value between 0 (maximum attenuation)
and
.Dv SIO_MAXVOL
(no attenuation).
It specifies the weight the audio subsystem will
give to this stream.
It is not meant to control hardware parameters like
speaker gain; the
.Xr mixerctl 8
interface should be used for that purpose instead.
.Pp
An application can use the
.Fn sio_onvol
function to register a callback function that
will be called each time the volume is changed,
including when
.Fn sio_setvol
is used.
The callback is always invoked when
.Fn sio_onvol
is called in order to provide the initial volume.
An application can safely assume that once
.Fn sio_onvol
has returned a non-zero value,
the callback has been invoked and thus
the current volume is available.
If there's no volume setting available,
.Fn sio_onvol
returns 0 and the callback is never invoked and calls to
.Fn sio_setvol
are ignored.
.Pp
The
.Fn sio_onvol
function can be called with a
.Dv NULL
argument to check whether a volume knob is available.
.Ss Error handling
Errors related to the audio subsystem
(like hardware errors, dropped connections) and
programming errors (e.g. call to
.Fn sio_read
on a play-only stream) are considered fatal.
Once an error occurs, all functions taking a
.Fa sio_hdl
argument, except
.Fn sio_close
and
.Fn sio_eof ,
stop working (i.e. always return 0).
The
.Fn sio_eof
function can be used at any stage.
.Ss Use with Xr pledge 2
If the
.Nm sndio
library is used in combination with
.Xr pledge 2 ,
then the
.Fn sio_open
function needs the
.Cm stdio ,
.Cm rpath ,
.Cm wpath ,
.Cm cpath ,
.Cm inet ,
.Cm unix ,
.Cm dns ,
and
.Cm audio
.Xr pledge 2
promises.
.Bl -bullet
.It
.Cm rpath ,
.Cm wpath ,
and
.Cm cpath
are needed to read, write or create the authentication cookie
.Pa ~/.sndio/cookie .
.It
.Cm rpath ,
.Cm wpath ,
and
.Cm audio
are needed when the device is a local raw device.
.It
.Cm unix
is needed when the device is a local
.Xr sndiod 8
sub-device.
.It
.Cm inet
and
.Cm dns
are needed when the device is a remote
.Xr sndiod 8
sub-device.
.El
.Pp
Once no further calls to
.Fn sio_open
will be made, all these
.Xr pledge 2
promises may be dropped, except for the
.Cm audio
promise.
.Sh RETURN VALUES
The
.Fn sio_open
function returns the newly created handle on success or
.Dv NULL
on failure.
.Pp
The
.Fn sio_setpar ,
.Fn sio_getpar ,
.Fn sio_getcap ,
.Fn sio_start ,
.Fn sio_stop ,
.Fn sio_flush ,
and
.Fn sio_setvol
functions return 1 on success and 0 on failure.
.Pp
The
.Fn sio_pollfd
function returns the number of
.Vt pollfd
structures filled.
The
.Fn sio_nfds
function returns the number of
.Vt pollfd
structures the caller must preallocate in order to be sure
that
.Fn sio_pollfd
will never overrun.
.Pp
The
.Fn sio_read
and
.Fn sio_write
functions return the number of bytes transferred.
.Pp
The
.Fn sio_eof
function returns 0 if there's no pending error, and a non-zero
value if there's an error.
.Sh ENVIRONMENT
.Bl -tag -width "SNDIO_DEBUGXXX" -compact
.It Ev AUDIODEVICE
Device to use if
.Fn sio_open
is called with
.Dv SIO_DEVANY
as the
.Fa name
argument.
.It Ev SNDIO_DEBUG
The debug level:
may be a value between 0 and 2.
.El
.Sh SEE ALSO
.Xr pledge 2 ,
.Xr mio_open 3 ,
.Xr sioctl_open 3 ,
.Xr audio 4 ,
.Xr sndio 7 ,
.Xr sndiod 8 ,
.Xr audio 9
.Sh HISTORY
These functions first appeared in
.Ox 4.5 .
.Sh AUTHORS
.An Alexandre Ratchov Aq Mt ratchov@openbsd.org
.Sh BUGS
The
.Xr audio 4
driver doesn't drain playback buffers, thus if sndio
is used to directly access an
.Xr audio 4
device,
the
.Fn sio_stop
function will stop playback immediately.
.Pp
If the application doesn't consume recorded data fast enough then
.Dq "control messages"
from the
.Xr sndiod 8
server are delayed and consequently
.Fn sio_onmove
callback or volume changes may be delayed.
.Pp
The
.Fn sio_open ,
.Fn sio_setpar ,
.Fn sio_getpar ,
.Fn sio_getcap ,
.Fn sio_start ,
.Fn sio_stop ,
and
.Fn sio_flush
functions may block for a very short period of time, thus they should
be avoided in code sections where blocking is not desirable.