sio_onvol(3) sndio interface to audio devices

Other Alias

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_initpar


In sndio.h Ft struct sio_hdl * Fn sio_open const char *name unsigned int 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 const 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 int vol Ft int Fn sio_onvol struct sio_hdl *hdl void (*cb)(void *arg, unsigned int vol) void *arg Ft void Fn sio_initpar struct sio_par *par


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 Fn sio_open function to obtain a handle to the device; later it will be passed as the Fa hdl argument of most other functions. The Fa 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 Fa 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 Fa 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.

The Fn sio_close function stops the device as if Fn sio_stop is called and frees the handle. Thus, no samples submitted with Fn 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:

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.
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.
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.
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 Vt sio_par structure using Fn sio_initpar and fill it with the desired parameters. Then call Fn sio_setpar to request the device to use them. Parameters left unset in the Vt sio_par structure will be set to device-specific defaults.
  • Call Fn 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 Fn sio_start has been called, Fn 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 Vt sio_par structure, the following macros can be used:

SIO_BPS (Fa bits )
Return the smallest value for bps that is a power of two and that is large enough to hold Fa bits .
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 Fn sio_getcap function.

The Vt 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:

Array of supported encodings. The tuple of bits bps sig le and msb parameters are usable in the corresponding parameters of the Vt sio_par structure.
rchan Bq SIO_NCHAN
Array of supported channel numbers for recording usable in the Vt sio_par structure.
pchan Bq SIO_NCHAN
Array of supported channel numbers for playback usable in the Vt sio_par structure.
Array of supported sample rates usable in the Vt sio_par structure.
Number of different configurations available, i.e. number of filled elements of the confs[] array.
confs Bq 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 Vt sio_conf structure, then the second element of the rate Bq SIO_NRATE array of the Vt sio_cap structure is valid for this configuration.

Starting and stopping the device

The Fn sio_start function puts the device in a waiting state: the device 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.

The Fn sio_stop function puts the audio subsystem in the same state as before Fn 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 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 Fa nbytes bytes at the Fa addr location and return the number of bytes stored. Unless the Fa 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 Fn sio_write function must be called to provide data to play. Unless the Fa nbio_flag is set, Fn sio_write will block until the requested amount of data is written.

Non-blocking mode operation

If the Fa 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.

The poll(2) system call can be used to check if data can be read from or written to the device. The Fn sio_pollfd function fills the array Fa pfd of Vt pollfd structures, used by poll(2), with Fa events ; the latter is a bit-mask of POLLIN and POLLOUT constants; refer to poll(2) for more details. Fn sio_pollfd returns the number of Vt pollfd structures filled. The Fn sio_revents function returns the bit-mask set by poll(2) in the Fa pfd array of Vt pollfd structures. If POLLIN is set, recorded samples are available in the device buffer and can be read with Fn sio_read . If POLLOUT is set, space is available in the device buffer and new samples to play can be submitted with Fn sio_write . POLLHUP may be set if an error occurs, even if it is not selected with Fn sio_pollfd .

The Fn sio_nfds function returns the number of Vt pollfd structures the caller must preallocate in order to be sure that Fn sio_pollfd will never overrun.

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 Fn sio_onmove function can be used to register the Fn cb callback function called at regular time intervals. The Fa delta argument contains the number of frames the hardware played and/or recorded since the last call of Fn cb . It is called by Fn sio_read , Fn sio_write , and Fn sio_revents . When the first sample is played and/or recorded, right after the device starts, the callback is invoked with a zero Fa delta argument. The value of the Fa 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 Fn sio_start is called) and adding to the current position Fa delta every time Fn 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 Fn 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 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 - 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 Vt sio_par structure, the audio subsystem will behave as follows:

The devices pauses during overruns and underruns, thus the current position (obtained through Fn 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 Fn 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.

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

This mode is mostly useful for testing.

Controlling the volume

The Fn sio_setvol function can be used to set playback attenuation. The Fa 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 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 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, Fn sio_onvol returns 0 and the callback is never invoked and calls to Fn sio_setvol are ignored.

The Fn 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 Fn sio_read on a play-only stream) are considered fatal. Once an error occurs, all functions taking a Fa sio_hdl argument, except Fn sio_close and Fn sio_eof , stop working (i.e. always return 0).

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.


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.


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


The audio(4) driver doesn't drain playback buffers, thus if sndio is used to directly access an audio(4) device, the Fn 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 Fn sio_onmove callback or volume changes may be delayed.

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.