Provided by: libsndio-dev_1.9.0-0.3_amd64 
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() or 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.
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, or SIO_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 call sio_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 with SIO_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.