Provided by: libsndio-dev_1.5.0-3_amd64 
NAME
sio_open, sio_close, sio_setpar, sio_getpar, sio_getcap, sio_start, sio_stop, sio_read, sio_write,
sio_onmove, sio_nfds, sio_pollfd, sio_revents, sio_eof, sio_setvol, sio_onvol, sio_initpar — 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);
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);
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() 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.
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.
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.
Starting and stopping the device
The sio_start() function puts the device in a waiting state: the device will wait for playback data to be
provided (using the 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.
The sio_stop() function puts the audio subsystem in the same state as before sio_start() is 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.
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 devices 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(1) 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(), 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
audio(4), sndio(7), sndiod(8), audio(9)
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(), and 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.
Debian $Mdocdate$ SIO_OPEN(3)