The sndiod daemon is an intermediate layer between audio or MIDI programs and the hardware. It performs the necessary audio processing to allow any program to work on any supported hardware. By default, sndiod accepts connections from programs running on the same system only; it initializes only when programs are using its services, allowing sndiod to consume a negligible amount of system resources the rest of the time. Systems with no audio hardware can use sndiod to keep hot-pluggable devices usable by default at virtually no cost.

sndiod operates as follows: it exposes at least one sub-device that any number of audio programs can connect to and use as if it was audio hardware. During playback, sndiod receives audio data concurrently from all programs, mixes it and sends the result to the hardware device. Similarly, during recording it duplicates audio data recorded from the device and sends it to all programs. Since audio data flows through the sndiod process, it has the opportunity to process audio data on the fly:

Change the sound encoding to overcome incompatibilities between software and hardware.

Route the sound from one channel to another, join stereo or split mono.

Control the per-application playback volume as well as the master volume.

Monitor the sound being played, allowing one program to record what other programs play.

Processing is configured on a per sub-device basis, meaning that the sound of all programs connected to the same sub-device will be processed according to the same configuration. Multiple sub-devices can be defined, allowing multiple configurations to coexist. The user selects the configuration a given program will use by selecting the sub-device the program uses.

sndiod exposes MIDI thru boxes (hubs), allowing programs to send MIDI messages to each other or to hardware MIDI ports in a uniform way.

Finally, sndiod exposes a control MIDI port usable for:

Volume control.

Common clock source for audio and MIDI programs.

Start, stop and relocate groups of audio programs.

The options are as follows:

-a flag Control whether sndiod opens the audio device or the MIDI port only when needed or keeps it open all the time. If the flag is on then the audio device or MIDI port is kept open all the time, ensuring no other program can steal it. If the flag is off , then it's automatically closed, allowing other programs to have direct access to the audio device, or the device to be disconnected. The default is off . -b nframes The buffer size of the audio device in frames. A frame consists of one sample for each channel in the stream. This is the number of frames that will be buffered before being played and thus controls the playback latency. The default is 7680 or twice the block size ( -z ), if the block size is set. -C min : max , -c min : max The range of channel numbers for recording and playback directions, respectively any client is allowed to use. This is a subset of the audio device channels. The default is 0:1, i.e. stereo. -d Enable debugging to standard error, and do not disassociate from the controlling terminal. Can be specified multiple times to further increase log verbosity. -e enc Attempt to configure the device to use this encoding. The default is s16 . Encoding names use the following scheme: signedness ( s or u ) followed by the precision in bits, the byte-order ( le or be ), the number of bytes per sample, and the alignment ( msb or lsb ). Only the signedness and the precision are mandatory. Examples: u8 , s16le , s24le3 , s24le4lsb . -F device Specify an alternate device to use. If it doesn't work, the one given with the last -f or -F options will be used. For instance, specifying a USB device following a PCI device allows sndiod to use the USB one preferably when it's connected and to fall back to the PCI one when it's disconnected. Alternate devices may be switched with the server.device control of the sndioctl(1) utility. -f device Add this sndio(7) audio device to devices used for playing and/or recording. Preceding per-device options ( -aberwz ) apply to this device. Sub-devices ( -s ) that are applied after will be attached to this device. Device mode and parameters are determined from sub-devices attached to it. If no -f option is used, sndiod will use rsnd/0 , rsnd/1 , ... , rsnd/3 . -j flag Control whether program channels are joined or expanded if the number of channels requested by a program is not equal to the device number of channels. If the flag is off then client channels are routed to the corresponding device channel, possibly discarding channels not present in the device. If the flag is on , then a single client channel may be sent on multiple device channels, or multiple client channels may be sent to a single device channel. For instance, this feature could be used for mono to stereo conversions. The default is on . -L addr Specify a local network address sndiod should listen on; sndiod will listen on TCP port 11025+n, where n is the unit number specified with -U . Without this option, sndiod listens on the UNIX -domain socket only, and is not reachable from any network. If the option argument is ‘-’ then sndiod will accept connections from any address. As the communication is not secure, this option is only suitable for local networks where all hosts and users are trusted. -m mode Set the sub-device mode. Valid modes are play , rec , and mon , corresponding to playback, recording and monitoring. A monitoring stream is a fake recording stream corresponding to the mix of all playback streams. Multiple modes can be specified, separated by commas, but the same sub-device cannot be used for both recording and monitoring. The default is play , rec (i.e. full-duplex). -Q port Specify an alternate MIDI port to use. If it doesn't work, the one given with the last -Q or -q options will be used. For instance, this allows a USB MIDI controller to be replaced without the need to restart programs using it. -q port Expose the given MIDI port. This allows multiple programs to share the port. If no -q option is used, sndiod will use rmidi/0 , rmidi/1 , ... , rmidi/7 . -r rate Attempt to force the device to use this sample rate in Hertz. The default is 48000. -s name Add name to the list of sub-devices to expose. This allows clients to use sndiod instead of the physical audio device for audio input and output in order to share the physical device with other clients. Defining multiple sub-devices allows splitting a physical audio device into sub-devices having different properties (e.g. channel ranges). The given name corresponds to the “option” part of the sndio(7) device name string. -t mode Select the way clients are controlled by MIDI Machine Control (MMC) messages received by sndiod . If the mode is off (the default), then programs are not affected by MMC messages. If the mode is slave , then programs are started synchronously by MMC start messages; additionally, the server clock is exposed as MIDI Time Code (MTC) messages allowing MTC-capable software or hardware to be synchronized to audio programs. -U unit Unit number. Each sndiod server instance has an unique unit number, used in sndio(7) device names. The default is 0. -v volume Software volume attenuation of playback. The value must be between 1 and 127, corresponding to -42dB and -0dB attenuation in 1/3dB steps. Clients inherit this parameter. Reducing the volume in advance allows a client's volume to stay independent from the number of clients as long as their number is small enough. 18 volume units (i.e. -6dB attenuation) allows the number of playback programs to be doubled. The default is 118 i.e. -3dB. -w flag Control sndiod behaviour when the maximum volume of the hardware is reached and a new program starts playing. This happens only when volumes are not properly set using the -v option. If the flag is on , then the master volume is automatically adjusted to avoid clipping. Using off makes sense in the rare situation where all programs lower their volumes. The default is on . -z nframes The audio device block size in frames. This is the number of frames between audio clock ticks, i.e. the clock resolution. If a sub-device is created with the -t option, and MTC is used for synchronization, the clock resolution must be 96, 100 or 120 ticks per second for maximum accuracy. For instance, 100 ticks per second at 48000Hz corresponds to a 480 frame block size. The default is 960 or half of the buffer size ( -b ), if the buffer size is set.

On the command line, per-device parameters ( -aberwz ) must precede the device definition ( -f ), and per-sub-device parameters ( -Ccjmtvx ) must precede the sub-device definition ( -s ). Sub-device definitions ( -s ) must follow the definition of the device ( -f ) to which they are attached.

If no audio devices ( -f ) are specified, settings are applied as if the default device is specified. If no sub-devices ( -s ) are specified for a device, a default sub-device is created attached to it. If a device ( -f ) is defined twice, both definitions are merged: parameters of the first one are used but sub-devices ( -s ) of both definitions are created. The default sndio(7) device used by sndiod is rsnd/0, and the default sub-device exposed by sndiod is snd/0.

If sndiod is sent SIGINT or SIGTERM , it terminates. If sndiod is sent SIGHUP , it reopens all audio devices and MIDI ports.