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SIO_OPEN(3) Library Functions Manual SIO_OPEN(3)

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_BPSsndio interface to audio devices

#include <sndio.h>

struct sio_hdl *
sio_open(const char *name, unsigned int mode, int nbio_flag);

sio_close(struct sio_hdl *hdl);

sio_setpar(struct sio_hdl *hdl, struct sio_par *par);

sio_getpar(struct sio_hdl *hdl, struct sio_par *par);

sio_getcap(struct sio_hdl *hdl, struct sio_cap *cap);

sio_start(struct sio_hdl *hdl);

sio_stop(struct sio_hdl *hdl);

sio_flush(struct sio_hdl *hdl);

sio_read(struct sio_hdl *hdl, void *addr, size_t nbytes);

sio_write(struct sio_hdl *hdl, const void *addr, size_t nbytes);

sio_onmove(struct sio_hdl *hdl, void (*cb)(void *arg, int delta), void *arg);

sio_nfds(struct sio_hdl *hdl);

sio_pollfd(struct sio_hdl *hdl, struct pollfd *pfd, int events);

sio_revents(struct sio_hdl *hdl, struct pollfd *pfd);

sio_eof(struct sio_hdl *hdl);

sio_setvol(struct sio_hdl *hdl, unsigned int vol);

sio_onvol(struct sio_hdl *hdl, void (*cb)(void *arg, unsigned int vol), void *arg);

sio_initpar(struct sio_par *par);

unsigned int
SIO_BPS(unsigned int bits);

The sndio library allows user processes to access audio(4) hardware and the sndiod(8) audio server in a uniform way.

First the application must call the () 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:

Play-only mode: data written will be played by the device.
Record-only mode: samples are recorded by the device and must be read.
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 () and sio_write() functions (see below) will be non-blocking.

The () function stops the device as if sio_stop() is called and frees the handle. Thus, no samples submitted with sio_write() are discarded.

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:

Number of bits per sample: must be between 1 and 32.
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.
If set (i.e. non-zero) then the samples are signed, else unsigned.
If set, then the byte order is little endian, else big endian; it's meaningful only if bps > 1.
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.
The number of recorded channels; meaningful only if SIO_REC mode was selected.
The number of played channels; meaningful only if SIO_PLAY mode was selected.
The sampling frequency in Hz.
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 () 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.
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.
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.
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 () and fill it with the desired parameters. Then call () to request the device to use them. Parameters left unset in the sio_par structure will be set to device-specific defaults.
  • Call () 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 () 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:

Return the smallest value for bps that is a power of two and that is large enough to hold bits.
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.

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 () 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 of the configurations. Parameters of different configurations 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:

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.
Array of supported channel numbers for recording usable in the sio_par structure.
Array of supported channel numbers for playback usable in the sio_par structure.
Array of supported sample rates usable in the sio_par structure.
Number of different configurations available, i.e. number of filled elements of the confs[] array.
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.

The () 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 () 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 () 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.

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 () 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.

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 () 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 () 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 () function.

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 () function can be used to register the () 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 () is called) and adding to the current position delta every time () is called.

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 () 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.

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:

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.

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 ()) 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.

With this mode, on the first play buffer underrun or record buffer overrun, playback and/or recording is terminated and no other function than () will succeed.

This mode is mostly useful for testing.

The () 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 () 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 () function can be called with a NULL argument to check whether a volume knob is available.

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 (), stop working (i.e. always return 0). The sio_eof() function can be used at any stage.


If the sndio library is used in combination with pledge(2), then the sio_open() function needs the stdio, rpath, wpath, cpath, inet, unix, dns, and audio pledge(2) promises.

  • , wpath, and cpath are needed to read, write or create the authentication cookie ~/.sndio/cookie.
  • , wpath, and audio are needed when the device is a local raw device.
  • is needed when the device is a local sndiod(8) sub-device.
  • and dns are needed when the device is a remote sndiod(8) sub-device.

Once no further calls to () will be made, all these pledge(2) promises may be dropped, except for the audio promise.

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.

Device to use if sio_open() is called with SIO_DEVANY as the name argument.
The debug level: may be a value between 0 and 2.

pledge(2), mio_open(3), sioctl_open(3), audio(4), sndio(7), sndiod(8), audio(9)

These functions first appeared in OpenBSD 4.5.

Alexandre Ratchov <>

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.

May 30, 2022 OpenBSD-current