OpenBSD-4.6/lib/libsndio/sio_open.3

.\" $OpenBSD: sio_open.3,v 1.19 2009/05/15 11:07:41 ratchov Exp $
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.\" Copyright (c) 2007 Alexandre Ratchov <alex@caoua.org>
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.Dd $Mdocdate: May 15 2009 $
.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_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
.Nd interface to bidirectional audio streams
.Sh SYNOPSIS
.Fd #include <sndio.h>
.Ft "struct sio_hdl *"
.Fn "sio_open" "char *name" "unsigned 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 "size_t"
.Fn "sio_read" "struct sio_hdl *hdl" "void *addr" "size_t nbytes"
.Ft "size_t"
.Fn "sio_write" "struct sio_hdl *hdl" "void *addr" "size_t nbytes"
.Ft "void"
.Fn "sio_onmove" "struct sio_hdl *hdl" "void (*cb)(void *arg, int delta)" "void *arg"
.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 vol"
.Ft "void"
.Fn "sio_onvol" "struct sio_hdl *hdl" "void (*cb)(void *arg, unsigned vol)" "void *arg"
.Ft "void"
.Fn "sio_initpar" "struct sio_par *par"
.\"Fd #define SIO_BPS(bits)
.\"Fd #define SIO_LE_NATIVE
.Sh DESCRIPTION
The
.Nm sndio
library allows user processes to access
.Xr audio 4
hardware and the
.Xr aucat 1
audio server in a uniform way.
It supports full-duplex operation, and when
used with the
.Xr aucat 1
server it supports resampling and format
conversions on the fly.
.Ss Opening and closing an audio stream
First the application must call the
.Fn sio_open
function to obtain a handle representing the newly created stream;
later it will be passed as the
.Ar hdl
argument of most other functions.
The
.Fn sio_open
function first tries to connect to the
.Xr aucat 1
audio server.
If that fails, it then tries to use the
.Xr audio 4
hardware device.
The
.Ar name
parameter gives the name of the
.Xr aucat 1
socket or the path of the
.Xr audio 4
device.
In most cases it should be set to NULL to allow
the user to select it using the
.Ev AUDIODEVICE
environment variable.
.Pp
The
.Ar mode
parameter gives the direction of the stream.
The following are supported:
.Bl -tag -width "SIO_PLAY | SIO_REC"
.It SIO_PLAY
The stream is play-only; data written to the stream will be played
by the hardware.
.It SIO_REC
The stream is record-only; recorded samples by the hardware
must be read from the stream.
.It SIO_PLAY | SIO_REC
The stream 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
.Ar 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 closes the stream and frees all allocated resources
associated with the
.Nm libsndio
handle.
.Ss Negotiating audio parameters
Audio streams always use linear interleaved encoding.
A frame consists of one sample for each channel in the stream.
For example, a 16-bit stereo stream has two samples per frame
and, typically, two bytes per sample (thus 4 bytes per frame).
.Pp
The set of parameters of the stream that can be controlled
is given by the following structure:
.Bd -literal
struct sio_par {
	unsigned bits;		/* bits per sample */
	unsigned bps;		/* bytes per sample */
	unsigned sig;		/* 1 = signed, 0 = unsigned */
	unsigned le;		/* 1 = LE, 0 = BE byte order */
	unsigned msb;		/* 1 = MSB, 0 = LSB aligned */
	unsigned rchan;		/* number channels for recording */
	unsigned pchan;		/* number channels for playback */
	unsigned rate;		/* frames per second */
	unsigned appbufsz;	/* minimum buffer size without xruns */
	unsigned bufsz;		/* end-to-end buffer size (read-only) */
	unsigned 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 xrun;		/* what to do on overrun/underrun */
};
.Ed
.Pp
The parameters are as follows:
.Bl -tag -width "appbufsz"
.It Va bits
Number of bits per sample: must be between 1 and 32.
.It Va 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
.Va bits .
.It Va sig
If set (i.e. non-zero) then the samples are signed, else unsigned.
.It Va le
If set, then the byte order is little endian, else big endian;
it's meaningful only if
.Va bps
\*(Gt 1.
.It Va msb
If set, then the
.Va bits
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
.Va bits
\*(Lt
.Va bps
* 8.
.It Va rchan
The number of recorded channels; meaningful only if
.Va SIO_REC
mode was selected.
.It Va pchan
The number of played channels; meaningful only if
.Va SIO_PLAY
mode was selected.
.It Va rate
The sampling frequency in Hz.
.It Va 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 Va 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 Va 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 Va 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
.Va SIO_IGNORE ,
.Va SIO_SYNC
or
.Va SIO_ERROR
constants.
.El
.Pp
There are two approaches to negotiate parameters of the stream:
.Bl -bullet
.It
Advanced applications may use native parameters of
the audio subsystem.
This is the best approach from a performance point of view
since it involves no extra format conversions.
The
.Fn sio_getcap ,
described below,
can be used to get the list of native parameter sets and then
.Fn sio_initpar
and
.Fn sio_setpar
can be used to select a working set.
.It
Simpler applications that do not have performance constraints may set up
the audio subsystem to use their own parameters.
The
.Va sio_par
structure must be initialized using the
.Fn sio_initpar
function, filled with the desired parameters and
the
.Fn sio_setpar
function must be called.
Finally, the
.Fn sio_getpar
function should be used to ensure that parameters were actually
accepted.
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
If
.Nm libsndio
is used to connect to the
.Xr aucat 1
server, a transparent emulation layer will
automatically be set up, and in this case any
parameters are supported.
.Pp
To ease filling the
.Va sio_par
structure, the
following macros can be used:
.Bl -tag -width "SIO_BPS(bits)"
.It "SIO_BPS(bits)"
Return the smallest value for
.Va bps
that is a power of two and that is large enough to
hold
.Va bits .
.It "SIO_LE_NATIVE"
Can be used to set the
.Va le
parameter when native byte order is required.
.El
.Pp
Note that (once initialized with the
.Fn sio_initpar
function), not all fields of the
.Va sio_par
structure must be filled; it is recommended to fill only
the required parameters, as other ones will default to
reasonable values.
This approach also ensures that if, in the future, newer parameters
are added, then older unaware applications will continue to
behave correctly.
.Ss Getting stream capabilities
Advanced applications can fetch the native
parameters of the audio subsystem and then choose parameters
optimal for both the application and the audio subsystem.
In this case applications must be able to do
the necessary format conversions.
The
.Va sio_cap
structure, filled by the
.Fn sio_getcap
function, 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 bits;
		unsigned bps;
		unsigned sig;
		unsigned le;
		unsigned msb;
	} enc[SIO_NENC];
	unsigned rchan[SIO_NCHAN];	/* allowed rchans */
	unsigned pchan[SIO_NCHAN];	/* allowed pchans */
	unsigned rate[SIO_NRATE];	/* allowed rates */
	unsigned nconf;			/* num. of confs[] */
	struct sio_conf {
		unsigned enc;		/* bitmask of enc[] indexes */
		unsigned rchan;		/* bitmask of rchan[] indexes */
		unsigned pchan;		/* bitmask of pchan[] indexes */
		unsigned rate;		/* bitmask of rate[] indexes */
	} confs[SIO_NCONF];
};
.Ed
.Pp
The parameters are as follows:
.Bl -tag -width "rchan[SIO_NCHAN]"
.It Va enc[SIO_NENC]
Array of supported encodings.
The tuple of
.Va bits ,
.Va bps ,
.Va sig ,
.Va le
and
.Va msb
parameters are usable in the corresponding parameters
of the
.Va sio_par
structure.
.It Va rchan[SIO_NCHAN]
Array of supported channel numbers for recording usable
in the
.Va sio_par
structure.
.It Va pchan[SIO_NCHAN]
Array of supported channel numbers for playback usable
in the
.Va sio_par
structure.
.It Va rate[SIO_NRATE]
Array of supported sample rates usable
in the
.Va sio_par
structure.
.It Va nconf
Number of different configurations available, i.e. number
of filled elements of the
.Va confs[]
array.
.It Va confs[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
.Va rate
is set, in the
.Va sio_conf
structure, then the second element of the
.Va rate[SIO_NRATE]
array of the
.Va sio_cap
structure is valid for this configuration.
.El
.Ss Starting and stopping the stream
The
.Fn sio_start
function puts the stream in a waiting state:
the stream will wait for playback data to be provided
(using the
.Fn sio_write
function).
Once enough data is queued to ensure that play buffers
will not underrun, actual playback is started automatically.
If record mode only is selected, then recording starts
immediately.
In full-duplex mode, playback and recording will start
synchronously as soon as enough data to play is available.
.Pp
The
.Fn sio_stop
function stops playback and recording and puts the audio subsystem
in the same state as after
.Fn sio_open
is called.
Samples in the play buffers are not discarded, and will continue to
be played after
.Fn sio_stop
returns.
.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
.Ar nbytes
bytes at the
.Ar addr
location and return the number of bytes stored.
Unless the
.Ar 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
.Ar nbio_flag
is set,
.Fn sio_write
will block until the requested amount of data is written.
.Ss Non-blocking mode operation
If the
.Ar 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
Note that non-blocking mode must be used on bidirectional streams.
For instance, if recording is blocked in
.Fn sio_read
then, even if samples can be played,
.Fn sio_write
cannot be called and the play buffers may underrun.
.Pp
To avoid busy loops when non-blocking mode is used, the
.Xr poll 2
system call can be used to check if data can be
read from or written to the stream.
The
.Fn sio_pollfd
function fills the array
.Ar pfd
of
.Va pollfd
structures, used by
.Xr poll 2 ,
with
.Ar events ;
the latter is a bit-mask of
.Va POLLIN
and
.Va POLLOUT
constants; refer to
.Xr poll 2
for more details.
.Fn sio_pollfd
returns the number of
.Va pollfd
structures filled.
The
.Fn sio_revents
function returns the bit-mask set by
.Xr poll 2
in the
.Va pfd
array of
.Va pollfd
structures.
If
.Va POLLIN
is set,
.Fn sio_read
can be called without blocking.
If
.Va POLLOUT
is set,
.Fn sio_write
can be called without blocking.
POLLHUP may be set if an error occurs, even if
it is not selected with
.Fn sio_pollfd .
.Pp
The
.Fn sio_nfds
function returns the number of
.Va pollfd
structures the caller must preallocate in order to be sure
that
.Fn sio_pollfd
will never overrun.
.Ss Synchronizing non-audio events to the stream in real-time
In order to perform actions at precise positions of the 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
.Va cb
callback function that will be called by the
.Nm sndio
library at regular time intervals to notify the application
the position in the stream changed.
The
.Va delta
argument contains the number of frames the hardware moved in the stream
since the last call of
.Va cb .
The value of the
.Va 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
.Va delta
every time
.Fn cb
is called.
Note that at the beginning the current position might be
negative indicating that the stream is being buffered,
but has not reached the hardware yet.
.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
.Va 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 the current position is negative and
the latency might be longer than
.Va bufsz .
.Pp
In any cases, at most
.Va bufsz
frames are buffered.
This value takes into account all buffers,
including device, kernel and socket buffers.
During the buffering phase, the number of frames stored
is equal to the number of frames written.
Once playback is started, it 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
It is strongly discouraged to use the latency and/or the buffer
usage for anything but monitoring.
Especially, 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
.Va 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
.Va xrun
parameter of the
.Va sio_par
structure, the audio subsystem will behave as follows:
.Bl -tag -width "SIO_IGNORE"
.It SIO_IGNORE
The stream is paused during overruns and underruns,
thus the current position (obtained through
.Va sio_onmove )
stops being incremented.
Once the overrun and/or underrun condition is gone, the stream is unpaused;
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 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
.Va 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
.Va 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 SIO_ERROR
With this mode, on the first play buffer underrun or
record buffer overrun, the stream is terminated and
no other function than
.Fn sio_close
will succeed.
.Pp
This mode is mostly useful for testing; portable
applications shouldn't depend on it, since it's not available
on other systems.
.El
.Ss Controlling the volume
The
.Fn sio_setvol
function can be used to set playback attenuation.
The
.Va 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 1
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
returns, the callback has already been invoked and thus
the current volume 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
.Va sio_hdl
argument, except
.Fn sio_close
and
.Fn sio_eof ,
stop working (i.e. always return 0).
.Pp
The
.Fn sio_eof
function can be used at any stage;
it returns 0 if there's no pending error, and a non-zero
value if there's an error.
.Sh RETURN VALUES
The
.Fn sio_open
function returns the newly created handle on success or NULL
on failure.
The
.Fn sio_setpar ,
.Fn sio_getpar ,
.Fn sio_getcap ,
.Fn sio_start ,
.Fn sio_stop ,
.Fn sio_pollfd
and
.Fn sio_setvol
functions return 1 on success and 0 on failure.
The
.Fn sio_read
and
.Fn sio_write
functions return the number of bytes transferred.
.Sh ENVIRONMENT
.Bl -tag -width "AUDIODEVICEXXX" -compact
.It Ev AUDIODEVICE
Name of the
.Xr aucat 1
socket to connect to, or path to the
.Xr audio 4
device to use.
.It Ev SIO_DEBUG
The debug level:
may be a value between 0 and 2.
.El
.Sh FILES
.Bl -tag -width "/tmp/aucat-<uid>/default" -compact
.It Pa /tmp/aucat-<uid>/default
Default path to
.Xr aucat 1
socket to connect to.
.It Pa /dev/audio
Default
.Xr audio 4
device to use.
.El
.\".Sh EXAMPLES
.\".Bd -literal -offset indent
.\".Ed
.Sh SEE ALSO
.Xr aucat 1 ,
.Xr audio 4 ,
.Xr audio 9
.Sh BUGS
The
.Xr audio 4
driver cannot drain playback buffers in the background, thus if
.Nm libsndio
is used to directly access an
.Xr audio 4
device,
the
.Fn sio_stop
function will stop playback immediately.
.Pp
The
.Xr aucat 1
server doesn't implement flow control (for performance reasons).
If the application doesn't consume recorded data fast enough then
.Dq "control messages"
are delayed (or lost) and consequently
overruns and underruns stay unnoticed by the application in the
.Va SIO_SYNC
mode (overruns and underruns are handled on the server side, so
synchronization is never lost).
.Pp
The
.Fn sio_open ,
.Fn sio_setpar ,
.Fn sio_getpar ,
.Fn sio_getcap ,
.Fn sio_start
and
.Fn sio_stop
functions may block for a very short period of time, thus they should
be avoided in code sections where blocking is not desirable.