table of contents
SIO_OPEN(3) | Library Functions Manual | SIO_OPEN(3) |
NAME¶
sio_open
,
sio_close
, sio_setpar
,
sio_getpar
, sio_getcap
,
sio_start
, sio_stop
,
sio_flush
, sio_read
,
sio_write
, sio_onmove
,
sio_nfds
, sio_pollfd
,
sio_revents
, sio_eof
,
sio_setvol
, sio_onvol
,
sio_initpar
, SIO_BPS
— sndio interface to audio devices
SYNOPSIS¶
#include
<sndio.h>
struct sio_hdl *
sio_open
(const
char *name, unsigned int
mode, int
nbio_flag);
void
sio_close
(struct
sio_hdl *hdl);
int
sio_setpar
(struct
sio_hdl *hdl, struct
sio_par *par);
int
sio_getpar
(struct
sio_hdl *hdl, struct
sio_par *par);
int
sio_getcap
(struct
sio_hdl *hdl, struct
sio_cap *cap);
int
sio_start
(struct
sio_hdl *hdl);
int
sio_stop
(struct
sio_hdl *hdl);
int
sio_flush
(struct
sio_hdl *hdl);
size_t
sio_read
(struct
sio_hdl *hdl, void
*addr, size_t
nbytes);
size_t
sio_write
(struct
sio_hdl *hdl, const void
*addr, size_t
nbytes);
void
sio_onmove
(struct sio_hdl *hdl,
void (*cb)(void *arg, int delta), void
*arg);
int
sio_nfds
(struct
sio_hdl *hdl);
int
sio_pollfd
(struct
sio_hdl *hdl, struct
pollfd *pfd, int
events);
int
sio_revents
(struct
sio_hdl *hdl, struct
pollfd *pfd);
int
sio_eof
(struct
sio_hdl *hdl);
int
sio_setvol
(struct
sio_hdl *hdl, unsigned
int vol);
int
sio_onvol
(struct sio_hdl *hdl,
void (*cb)(void *arg, unsigned int vol),
void *arg);
void
sio_initpar
(struct
sio_par *par);
unsigned int
SIO_BPS
(unsigned
int bits);
DESCRIPTION¶
The sndio
library allows user processes to
access audio(4) hardware and the
sndiod(8) audio server in a uniform way.
Opening and closing an audio device¶
First the application must call the
sio_open
()
function to obtain a handle to the device; later it will be passed as the
hdl argument of most other functions. The
name parameter gives the device string discussed in
sndio(7). In most cases it should be set to
SIO_DEVANY
to allow the user to select it using the
AUDIODEVICE
environment variable.
The following values of the mode parameter are supported:
SIO_PLAY
- Play-only mode: data written will be played by the device.
SIO_REC
- Record-only mode: samples are recorded by the device and must be read.
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.
If the nbio_flag argument is
true (i.e. non-zero), then the
sio_read
()
and sio_write
() functions (see below) will be
non-blocking.
The
sio_close
()
function stops the device as if sio_stop
() is called
and frees the handle. Thus, no samples submitted with
sio_write
() are discarded.
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).
The set of parameters of the device that can be controlled is given by the following structure:
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 */ };
The parameters are as follows:
- bits
- Number of bits per sample: must be between 1 and 32.
- 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 bits.
- sig
- If set (i.e. non-zero) then the samples are signed, else unsigned.
- le
- If set, then the byte order is little endian, else big endian; it's meaningful only if bps > 1.
- msb
- If set, then the 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 bits < bps * 8.
- rchan
- The number of recorded channels; meaningful only if
SIO_REC
mode was selected. - pchan
- The number of played channels; meaningful only if
SIO_PLAY
mode was selected. - rate
- The sampling frequency in Hz.
- 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.
- 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
sio_write
() orsio_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. - 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.
- 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
SIO_IGNORE
,SIO_SYNC
, orSIO_ERROR
constants.
The following approach is recommended to negotiate device parameters:
- Initialize a sio_par structure
using
sio_initpar
() and fill it with the desired parameters. Then callsio_setpar
() to request the device to use them. Parameters left unset in the sio_par structure will be set to device-specific defaults. - Call
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.
Parameters cannot be changed after
sio_start
()
has been called, sio_stop
() or
sio_flush
() must be called before parameters can be
changed.
If the device is exposed by the 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.
To ease filling the sio_par structure, the following macros can be used:
SIO_BPS
(bits)- Return the smallest value for bps that is a power of two and that is large enough to hold bits.
SIO_LE_NATIVE
- Can be used to set the le parameter when native byte order is required. It is 1 if the native byte order is little endian or 0 otherwise.
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
sio_getcap
()
function. However, for most new applications it's generally not recommended
to use sio_getcap
(). Instead, follow the
recommendations for negotiating device parameters (see above).
The 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 one of the configurations. Parameters of different configurations are not usable together.
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]; };
The parameters are as follows:
- enc[
SIO_NENC
] - Array of supported encodings. The tuple of bits, bps, sig, le, and msb parameters are usable in the corresponding parameters of the sio_par structure.
- rchan[
SIO_NCHAN
] - Array of supported channel numbers for recording usable in the sio_par structure.
- pchan[
SIO_NCHAN
] - Array of supported channel numbers for playback usable in the sio_par structure.
- rate[
SIO_NRATE
] - Array of supported sample rates usable in the sio_par structure.
- nconf
- Number of different configurations available, i.e. number of filled elements of the confs[] array.
- 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
rate is set, in the sio_conf
structure, then the second element of the
rate[
SIO_NRATE
] array of the sio_cap structure is valid for this configuration. As such, when reading the array elements in the sio_cap structure, the corresponding sio_conf bitmasks should always be used.
Starting and stopping the device¶
The
sio_start
()
function prepares the device to start. Once the play buffer is full, i.e.
sio_par.bufsz samples are queued with
sio_write
(), playback starts automatically. If
record-only mode is selected, then sio_start
()
starts recording immediately. In full-duplex mode, playback and recording
will start synchronously as soon as the play buffer is full.
The
sio_stop
()
function puts the audio subsystem in the same state as before
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.
The
sio_flush
()
function stops playback and recording immediately, possibly discarding play
buffer contents, and puts the audio subsystem in the same state as before
sio_start
() was called.
Playing and recording¶
When record mode is selected, the
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 nbytes
bytes at the addr location and return the number of
bytes stored. Unless the nbio_flag flag is set, it
will block until data becomes available and will return zero only on
error.
Similarly, when play mode is selected, the
sio_write
()
function must be called to provide data to play. Unless the
nbio_flag is set, sio_write
()
will block until the requested amount of data is written.
Non-blocking mode operation¶
If the nbio_flag is set on
sio_open
(), then the
sio_read
() and sio_write
()
functions will never block; if no data is available, they will return zero
immediately.
The poll(2) system call can be
used to check if data can be read from or written to the device. The
sio_pollfd
()
function fills the array pfd of
pollfd structures, used by poll(2),
with events; the latter is a bit-mask of
POLLIN
and POLLOUT
constants; refer to poll(2) for more details. The
sio_revents
()
function returns the bit-mask set by poll(2) in the
pfd array of pollfd structures.
If POLLIN
is set, recorded samples are available in
the device buffer and can be read with sio_read
().
If POLLOUT
is set, space is available in the device
buffer and new samples to play can be submitted with
sio_write
(). POLLHUP
may be
set if an error occurs, even if it is not selected with
sio_pollfd
().
The size of the pfd array,
which the caller must pre-allocate, is provided by the
sio_nfds
()
function.
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.
The
sio_onmove
()
function can be used to register the
cb
()
callback function called at regular time intervals. The
delta argument contains the number of frames the
hardware played and/or recorded since the last call of
cb
(). It is called by
sio_read
(), sio_write
(), and
sio_revents
(). When the first sample is played
and/or recorded, right after the device starts, the callback is invoked with
a zero delta argument. The value of the
arg pointer is passed to the callback and can contain
anything.
If desired, the application can maintain the
current position by starting from zero (when
sio_start
()
is called) and adding to the current position delta
every time
cb
()
is called.
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 bufsz parameter (see
the sections above). There's a phase during which
sio_write
() only queues data; once there's enough
data, actual playback starts. During this phase talking about latency is
meaningless.
In any cases, at most 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.
The recording latency is obtained similarly, by subtracting the number of frames read from the current position.
Note that
sio_write
()
might block even if there is buffer space left; using the buffer usage to
guess if sio_write
() would block is false and leads
to unreliable programs – consider using poll(2) for
this.
Handling buffer overruns and underruns¶
When the application cannot accept recorded data fast enough, the record buffer (of size 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 xrun parameter of the sio_par structure, the audio subsystem will behave as follows:
SIO_IGNORE
- The device pauses during overruns and underruns, thus the current position
(obtained through
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.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.
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
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 bufsz.This mode is suitable for applications, like music production, where time is important and where underruns or overruns are short and rare.
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
sio_close
() will succeed.This mode is mostly useful for testing.
Controlling the volume¶
The
sio_setvol
()
function can be used to set playback attenuation. The
vol parameter takes a value between 0 (maximum
attenuation) and 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
mixerctl(8) interface should be used for that purpose
instead.
An application can use the
sio_onvol
()
function to register a callback function that will be called each time the
volume is changed, including when sio_setvol
() is
used. The callback is always invoked when
sio_onvol
() is called in order to provide the
initial volume. An application can safely assume that once
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, sio_onvol
()
returns 0 and the callback is never invoked and calls to
sio_setvol
() are ignored.
The
sio_onvol
()
function can be called with a NULL
argument to check
whether a volume knob is available.
Error handling¶
Errors related to the audio subsystem (like hardware errors,
dropped connections) and programming errors (e.g. call to
sio_read
() on a play-only stream) are considered
fatal. Once an error occurs, all functions taking a
sio_hdl argument, except
sio_close
() and
sio_eof
(),
stop working (i.e. always return 0). The sio_eof
()
function can be used at any stage.
RETURN VALUES¶
The sio_open
() function returns the newly
created handle on success or NULL
on failure.
The sio_setpar
(),
sio_getpar
(), sio_getcap
(),
sio_start
(), sio_stop
(),
sio_flush
(), and
sio_setvol
() functions return 1 on success and 0 on
failure.
The sio_pollfd
() function returns the
number of pollfd structures filled. The
sio_nfds
() function returns the number of
pollfd structures the caller must preallocate in order
to be sure that sio_pollfd
() will never overrun.
The sio_read
() and
sio_write
() functions return the number of bytes
transferred.
The sio_eof
() function returns 0 if
there's no pending error, and a non-zero value if there's an error.
ENVIRONMENT¶
AUDIODEVICE
- Device to use if
sio_open
() is called withSIO_DEVANY
as the name argument. SNDIO_DEBUG
- The debug level: may be a value between 0 and 2.
SEE ALSO¶
mio_open(3), sioctl_open(3), audio(4), sndio(7), sndiod(8), audio(9)
HISTORY¶
These functions first appeared in OpenBSD 4.5.
AUTHORS¶
Alexandre Ratchov <ratchov@openbsd.org>
BUGS¶
The audio(4) driver doesn't drain playback
buffers, thus if sndio is used to directly access an
audio(4) device, the sio_stop
()
function will stop playback immediately.
If the application doesn't consume recorded data fast enough then
“control messages” from the sndiod(8) server
are delayed and consequently sio_onmove
() callback
or volume changes may be delayed.
The sio_open
(),
sio_setpar
(), sio_getpar
(),
sio_getcap
(), sio_start
(),
sio_stop
(), and 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.
October 14, 2024 | Linux 6.4.0-150600.23.17-default |