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SNDIO(7) Miscellaneous Information Manual SNDIO(7)

sndio
interface to audio and MIDI

TABLE OF CONTENTS

Programs access audio and MIDI hardware using the sndio library. It allows both access through the sndiod(8) server and raw access to the hardware. The audio device or MIDI port, as well as the access method, are designated by the sndio descriptor. It is provided by the user with the program device selection method, or with the AUDIODEVICE and MIDIDEVICE environment variables if there's no device selection method.

Usually, programs access the hardware via the sndiod(8) server, because raw access to the hardware is exclusive and requires additional privileges. The sndiod(8) server supports multiple connections at a time, allowing multiple programs to use the hardware concurrently. It performs the necessary audio processing on the fly to overcome any incompatibility between software and hardware. Connections to sndiod(8) may also be established through the network, including from virtual machines.

The sndiod(8) server exposes MIDI thru ports, allowing one program to send MIDI data to other programs, for instance to allow a sequencer to send events to a synthesizer.

Additionally, sndiod(8) exposes a MIDI port used to control audio programs using standard MIDI Machine Control (MMC), MIDI Time Code (MTC), and master volume messages.

From the user's perspective every audio device or MIDI port has a descriptor of the form:
type[@hostname][,servnum]/devnum[.option]

This information is used by programs to determine how to access the audio device or MIDI port.

type
The type of the audio device or MIDI port. Possible values are:

Raw audio(4) device.
Raw midi(4) port.
Audio device exposed by sndiod(8).
MIDI thru port created with sndiod(8).
MIDI port exposed by sndiod(8).
Default audio device or MIDI port (see below).
hostname
The hostname or address where the remote sndiod(8) server to connect to is running.
servnum
The number of the sndiod(8) server to connect to, corresponding to the integer specified using the -U option of sndiod(8). Useful only if multiple sndiod(8) servers are running on the same system.
devnum
Device number. For hardware audio or MIDI ports, this corresponds to the character device minor number. For audio devices or MIDI ports created with sndiod(8) it corresponds to the number of the corresponding -fq option on the command line.
option
Corresponds to the sub-device string registered using the -s option of sndiod(8).

For example:

Raw access to first audio device.
Raw access to MIDI port number 5.
Audio device referred by first -f option of sndiod(8).
Sub-device registered with -s rear.
First MIDI thru port created with sndiod(8).

If default is used as the audio device, the program will use the one specified in the AUDIODEVICE environment variable. If it is not set, the program first tries to connect to snd/0. If that fails, it then tries to use rsnd/0. This allows the sndiod(8) audio server to be used by default and the raw hardware as fallback; programs don't have to be reconfigured when sndiod(8) is started or stopped.

Similarly, if no MIDI descriptor is provided to a program or when the reserved word default is passed as the device descriptor, the program uses the one specified in the MIDIDEVICE environment variable. If it is not set, the program first tries to connect to midithru/0. If that fails, it then tries to use rmidi/0. As long as sndiod(8) is running, this allows programs to exchange MIDI data on machines with no MIDI hardware by default, e.g. a MIDI player could use a software synthesizer with no manual configuration required.

For privacy reasons only one user may have connections to sndiod(8) at a given time. Users are identified by their session cookie, which is automatically generated by audio or MIDI programs upon the first connection to the server. The cookie is stored in $HOME/.sndio/cookie and contains 128 bits of raw random data.

If a session needs to be shared between multiple users, they can connect to the server using the same cookie.

Audio device descriptor to use when no descriptor is explicitly specified to a program.
MIDI port descriptor to use when no descriptor is explicitly specified to a program.

These environment variables are ignored by sndio if the program has the set-user-ID or set-group-ID bits set.

~/.sndio/cookie
User's session authentication cookie.
/dev/audioN
Audio devices.
/dev/rmidiN
MIDI ports.

aucat(1), midicat(1), sndioctl(1), mio_open(3), sio_open(3), sioctl_open(3), audio(4), midi(4), sndiod(8)
September 27, 2020 x86_64