sndseq

sndseq refers to the ALSA sequencer interface, the MIDI sequencing and routing system provided by the ALSA project on Linux. It is accessed through the ALSA library API (snd_seq_ functions) and enables software and hardware MIDI ports to communicate via a central event bus.

The sequencer consists of a kernel-level component that handles event routing and timing and a user-space API that applications use to interact with it. A program opens the sequencer, creates one or more ports representing its input and output endpoints, and connects ports to other clients. It can then send and receive MIDI events such as note on/off, program changes, control changes, aftertouch, and system exclusive messages, as well as time-related events and sysex data. The API supports multiple clients, virtual ports, and dynamic reconfiguration of connections.

Key features include event-driven messaging, queue-based delivery, and time management capabilities. Applications can post events with time stamps, adjust tempo and timing information, and subscribe to or route events between different clients. The sequencer also supports playback and recording workflows, allowing complex routing of MIDI data across software synths, samplers, and external hardware through a unified interface.

Usage context and ecosystem: sndseq is widely used by Linux MIDI software, including digital audio workstations, soft synthesizers, and MIDI routers. It can operate standalone or in conjunction with other Linux audio subsystems, and can be bridged to JACK or other routing layers to integrate with modern audio workflows. As part of ALSA, sndseq relies on the /dev/snd/seq device and integrates with the broader ALSA sound architecture.