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MIDI(4)                      Device Drivers Manual                     MIDI(4)

     midi - device-independent MIDI driver layer

     midi* at midibus?
     midi* at pcppi?
     pseudo-device sequencer

     #include <sys/types.h>
     #include <sys/midiio.h>

     The midi driver is the machine independent layer over anything that can
     source or sink a MIDI data stream, whether a physical MIDI IN or MIDI OUT
     jack on a soundcard, cabled to some external synthesizer or input
     controller, an on-board programmable tone generator, or a single jack,
     synthesizer, or controller component within a complex USB or IEEE1394
     MIDI device that has several such components and appears as several MIDI

     One MIDI data stream is a unidirectional stream of MIDI messages, as
     could be carried over one MIDI cable in the MIDI 1.0 specification.  Many
     MIDI messages carry a four-bit channel number, creating up to 16 MIDI
     channels within a single MIDI stream.  There may be multiple consumers of
     a MIDI stream, each configured to react only to messages on specific
     channels; the sets of channels different consumers react to need not be
     disjoint.  Many modern devices such as multitimbral keyboards and tone
     generators listen on all 16 channels, or may be viewed as collections of
     16 independent consumers each listening on one channel.  MIDI defines
     some messages that take no channel number, and apply to all consumers of
     the stream on which they are sent.  For an inbound stream, midi is a
     promiscuous receiver, capturing all messages regardless of channel
     number.  For an outbound stream, the writer can specify a channel number
     per message; there is no notion of binding the stream to one destination
     channel in advance.

     A single midi device instance is the endpoint of one outbound stream, one
     inbound stream, or one of each.  In the third case, the write and read
     sides are independent MIDI streams.  For example, a soundcard driver may
     map its MIDI OUT and MIDI IN jacks to the write and read sides of a
     single device instance, but those jacks can be cabled to completely
     different pieces of gear.  Information from dmesg(8), and a diagram of
     any external MIDI cabling, will help clarify the mapping.

   Underlying drivers and MIDI protocol
     Drivers midi can attach include soundcard drivers, many of which support
     a UART resembling Roland's MPU401 and handled by mpu(4), USB MIDI devices
     via umidi(4), and on-board devices that can make sounds, whether a lowly
     PC speaker or a Yamaha OPL.  Serial port and IEEE1394 connections are
     currently science fiction.

     The MIDI protocol permits some forms of message compression such as
     running status and hidden note-off.  Received messages on inbound streams
     are always canonicalized by midi before presentation to higher layers.
     Messages for transmission are accepted by midi in any valid form.

   Device access
     Access to midi device instances can be through the raw device nodes,
     /dev/rmidiN, or through the sequencer, /dev/music.

   Raw MIDI access
     A /dev/rmidiN device supports read(2), write(2), ioctl(2),
     select(2)/poll(2) and the corresponding kevent(2) filters, and may be
     opened only when it is not already open.  It may be opened in O_RDONLY,
     O_WRONLY, or O_RDWR mode, but a later read(2) or write(2) will return -1
     if the device has no associated input or output stream, respectively.

     Bytes written are passed as quickly as possible to the underlying driver
     as complete MIDI messages; a maximum of two bytes at the end of a
     write(2) may remain buffered if they do not complete a message, until
     completed by a following write(2).

     A read(2) will not block or return EWOULDBLOCK when it could immediately
     return any nonzero count, and MIDI messages received are available to
     read(2) as soon as they are complete, with a maximum of two received
     bytes remaining buffered if they do not complete a message.

     As all MIDI messages are three bytes or fewer except for System
     Exclusive, which can have arbitrary length, these rules imply that System
     Exclusive messages are the only ones of which some bytes can be delivered
     before all are available.

     System Realtime messages are passed with minimum delay in either
     direction, ahead of any possible buffered incomplete message.  As a
     result, they will never interrupt any MIDI message except possibly System

     A read(2) with a buffer large enough to accommodate the first complete
     message available will be satisfied with as many complete messages as
     will fit.  A buffer too small for the first complete message will be
     filled to capacity.  Therefore, an application that reads from an rmidi
     device with buffers of three bytes or larger need never parse across read
     boundaries to assemble a received message, except possibly in the case of
     a System Exclusive message.  However, if the application reads through a
     buffering layer such as fread(3), this property will not be preserved.

     The midi driver itself supports the ioctl(2) operations FIOASYNC,
     FIONBIO, and FIONREAD.  Underlying devices may support others.  The value
     returned for FIONREAD reflects the size in bytes of complete messages (or
     System Exclusive chunks) ready to read.  If the ioctl(2) returns n and a
     read(2) of size n is issued, n bytes will be read, but if a read(2) of
     size m < n is issued, fewer than m bytes may be read if m does not fall
     on a message/chunk boundary.

     Raw MIDI access can be used to receive bulk dumps from synthesizers,
     download bulk data to them, and so on.  Simple patching of one device to
     another can be done at the command line, as with
           $ cat -u 0<>/dev/rmidi0 1>&0
     which will loop all messages received on the input stream of rmidi0 input
     stream  back to its output stream in real time.  However, an attempt to
     record and play back music with
           $ cat /dev/rmidiN >foo; cat foo >/dev/rmidiN
     will be disappointing.  The file foo will contain all of the notes that
     were played, but because MIDI messages carry no explicit timing, the
     `playback' will reproduce them all at once, as fast as they can be
     transmitted.  To preserve timing information, the sequencer device can be

   Active Sensing
     The MIDI protocol includes a keepalive function called Active Sensing.
     In any receiver that has not received at least one Active Sense MIDI
     message, the feature is suppressed and no timeout applies.  If at least
     one such message has been received, the lapse of any subsequent 300 ms
     interval without receipt of any message reflects loss of communication,
     and the receiver should silence any currently sounding notes and return
     to non-Active-Sensing behavior.  A sender using Active Sensing generally
     avoids 300 ms gaps in transmission by sending Active Sense messages
     (which have no other effect) as needed when there is no other traffic to
     send in the interval.  This feature can be important for MIDI, which
     relies on separate Note On and Note Off messages, to avoid notes stuck on
     indefinitely if communication is interrupted before a Note Off message

     This protocol is supported in midi.  An outbound stream will be kept
     alive by sending Active Sense messages as needed, beginning after any
     real traffic is sent on the stream, and continuing until the stream is
     closed.  On an inbound stream, if any Active Sense has been received,
     then a process reading an rmidi device will see an end-of-file indication
     if the input timeout elapses.  The stream remains open, the driver
     reverts to enforcing no timeout, and the process may continue to read for
     more input.  Subsequent receipt of an Active Sense message will re-arm
     the timeout.  As received Active Sense messages are handled by midi, they
     are not included among messages read from the /dev/rmidiN device.

     These rules support end-to-end Active Sensing behavior in simple cases
     without special action in an application.  For example, in
           $ cat -u /dev/rmidi0 >/dev/rmidi1
     if the input stream to rmidi0 is lost, the cat(1) command exits; on the
     close(2) of rmidi1, midi ceases to send Active Sense messages, and the
     receiving device will detect the loss and silence any outstanding notes.

   Access through the sequencer
     To play music using the raw MIDI API would require an application to
     issue many small writes with very precise timing.  The sequencer device,
     /dev/music, can manage the timing of MIDI data in the kernel, to avoid
     such demanding real-time constraints on a user process.

     The /dev/music device can be opened only when it is not already open.
     When opened, the sequencer internally opens all MIDI instances existing
     in the system that are not already open at their raw nodes; any attempts
     to open them at their raw nodes while the sequencer is open will fail.
     All access to the corresponding MIDI streams will then be through the

     Reads and writes of /dev/music pass eight-byte event structures defined
     in <sys/midiio.h> (which see for their documentation and examples of
     use).  Some events correspond to MIDI messages, and carry an integer
     device field to identify one of the MIDI devices opened by the sequencer.
     Other events carry timing information interpreted or generated by the
     sequencer itself.

     A message received on an input stream is wrapped in a sequencer event
     along with the device index of the stream it arrived on, and queued for
     the reader of /dev/music.  If a measurable time interval passed since the
     last preceding message, a timing event that represents a delay for that
     interval is queued ahead of the received event.  The sequencer handles
     output events by interpreting any timing event, and routing any MIDI
     message event at the proper time to an underlying output stream according
     to its device index.  Therefore
           $ cat /dev/music >foo; cat foo >/dev/music
     can be expected to capture and reproduce an input performance including

     The process of playing back a complex MIDI file is illustrated below.
     The file may contain several tracks--four, in this example--of MIDI
     events, each marked with a device index and a time stamp, that may
     overlap in time.  In the example, a, b, and c are device indices of the
     three output MIDI streams; the left-hand digit in each input event
     represents a MIDI channel on the selected stream, and the right-hand
     digit represents a time for the event's occurrence.  As illustrated, the
     input tracks are not firmly associated with output streams; any track may
     contain events for any stream.

          |      |     a2|4     |
        a0|3     |     c1|3   c0|3
          |    b0|2    b1|2     |
          |    b1|1      |    c0|1
        a0|0     |     b0|0     |
          v      v       v      v
       | merge to 1 ordered stream |
       | user code, eg midiplay(1) |
              | /dev/music  |     kernel
              | (sequencer) |
                |    1    0
          +-----'    |    '-----.
          0          0          |
          v          v          v
       +-------+ +--------+ +---------+
       |midi(4)| |midi(4) | |midi(4)  |
       |rmidia | |rmidib  | |rmidic   |
       +-------+ +--------+ +---------+
       | mpu(4)| |umidi(4)| |midisyn  |
       +-------+ +--------+ +---------+
       |  HW   |     |      | opl(4)  |
       | MIDI  |     U      +---------+
       | UART  |      S     | internal|
       +-------+       B    |   tone  |
           |           |    |generator|
           v           |    +---------+
        external       v
       MIDI device  external
                   MIDI device

     A user process must merge the tracks into a single stream of sequencer
     MIDI and timing events in order by desired timing.  The sequencer obeys
     the timing events and distributes the MIDI events to the three
     destinations, in this case two external devices connected to a sound card
     UART and a USB interface, and an OPL tone generator on a sound card.

     Use of select(2)/poll(2) with the sequencer is supported, however, there
     is no guarantee that a write(2) will not block or return EWOULDBLOCK if
     it begins with a timer-wait event, even if select(2)/poll(2) reported the
     sequencer writable.

     The delivery of a realtime message ahead of buffered bytes of an
     incomplete message may cause the realtime message to seem, in a saved
     byte stream, to have arrived up to 640 us earlier than it really did, at
     MIDI 1.0 data rates.  Higher data rates make the effect less significant.

     Another sequencer device, /dev/sequencer, is provided only for backward
     compatibility with an obsolete OSS interface in which some sequencer
     events were four-byte records.  It is not further documented here, and
     the /dev/music API should be used in new code.  The /dev/sequencer
     emulation is implemented only for writing, and that might not be

     Some hardware devices supporting midi lack transmit-ready interrupts, and
     some have the capability in hardware but currently lack driver support.
     They can be recognized by the annotation (CPU-intensive output) in
     dmesg(8).  While suitable for music playback, they may have an
     objectionable impact on system responsiveness during bulk transmission
     such as patch downloads, and are best avoided for that purpose if other
     suitable devices are present.

     Buffer space in midi itself is adequate for about 200 ms of traffic at
     MIDI 1.0 data rates, per stream.

     Event counters record bytes and messages discarded because of protocol
     errors or buffer overruns, and can be viewed with vmstat -e.  They can be
     useful in diagnosing flaky cables and other communication problems.

     A raw sound generator uses the midisyn layer to present a MIDI message-
     driven interface attachable by midi.

     While midi accepts messages for transmission in any valid mixture of
     compressed or canonical form, they are always presented to an underlying
     driver in the form it prefers.  Drivers for simple UART-like devices
     register their preference for a compressed byte stream, while those like
     umidi(4), which uses a packet protocol, or midisyn, which interprets
     complete messages, register for intact canonical messages.  This design
     eliminates the need for compression and canonicalization logic from all
     layers above and below midi itself.


     In addition to other errors documented for the write(2) family of system
     calls, EPROTO can be returned if the bytes to be written on a raw midi
     device would violate MIDI protocol.

     midiplay(1), midirecord(1), ioctl(2), ossaudio(3), audio(4), mpu(4),
     opl(4), umidi(4)

     For ports using the ISA bus: cms(4), pcppi(4), sb(4)

     For ports using the PCI bus: autri(4), clcs(4), eap(4)

     The midi driver first appeared in NetBSD 1.4.  It was overhauled and this
     manual page rewritten for NetBSD 4.0.

     Some OSS sequencer events and ioctl(2) operations are unimplemented, as
     <sys/midiio.h> notes.

     OSS source-compatible sequencer macros should be added to
     <sys/soundcard.h>, implemented with the NetBSD ones in <sys/midiio.h>, so
     sources written for OSS can be easily compiled.

     The sequencer blocks (or returns EWOULDBLOCK) only when its buffer
     physically fills, which can represent an arbitrary latency because of
     buffered timing events.  As a result, interrupting a process writing the
     sequencer may not interrupt music playback for a considerable time.  The
     sequencer could enforce a reasonable latency bound by examining timing
     events as they are enqueued and blocking appropriately.

     FIOASYNC enables signal delivery to the calling process only; FIOSETOWN
     is not supported.

     The sequencer can only be a timing master, but does not send timing
     messages to synchronize any slave device; it cannot be slaved to timing
     messages received on any interface (which would presumably require a PLL
     algorithm similar to NTP's, and expertise in that area to implement it).
     The sequencer ignores timing messages received on any interface and does
     not pass them along to the reading process, and the OSS operations to
     change that behavior are unimplemented.

     The SEQUENCER_TMR_TIMEBASE ioctl(2) will report successfully setting any
     timebase up to ridiculously high resolutions, though the actual
     resolution, and therefore jitter, is constrained by hz(9).  Comparable
     sequencer implementations typically allow a selection from available
     sources of time interrupts that may be programmable.

     The device number in a sequencer event is treated on write(2) as index
     into the array of MIDI devices the sequencer has opened, but on read(2)
     as the unit number of the source MIDI device; these are usually the same
     if the sequencer has opened all the MIDI devices (that is, none was
     already open at its raw node when the sequencer was opened), but might
     not be the same otherwise.

     There is at present no way to make reception nonpromiscuous, should
     anyone have a reason to want to.

     There should be ways to override default Active Sense behavior.  As one
     obvious case, if an application is seen to send Active Sense explicitly,
     midi should refrain from adding its own.  On receive, there should be an
     option to pass Active Sense through rather than interpreting it, for apps
     that wish to handle or ignore it themselves and never see EOF.

     When a midi stream is open by the sequencer, Active Sense messages
     received on the stream are passed to the sequencer and not interpreted by
     midi.  The sequencer at present neither does anything itself with Active
     Sense messages received, nor supports the OSS API for making them
     available to the user process.

     System Exclusive messages can be received by reading a raw device, but
     not by reading the sequencer; they are discarded on receipt when the
     stream is open by the sequencer, rather than being presented as the OSS-
     defined sequencer events.

     midisyn is too rudimentary at present to get satisfactory results from
     any onboard synth.  It lacks the required special interpretation of the
     General MIDI percussion channel in GM mode.  More devices should be
     supported; some sound cards with synthesis capability have NetBSD drivers
     that implement the audio(4) but not the midisyn interface.  Voice
     stealing algorithm does not follow the General MIDI Developer Guidelines.

     ALSA sequencer compatibility is lacking, but becoming important to
     applications.  It would require the function of merging multiple tracks
     into a single ordered stream to be moved from user space into the
     sequencer.  Assuming the sequencer driven by periodic interrupts, timing
     wheels could be used as in hardclock(9) itself.  Similar functionality
     will be in OSS4; with the right infrastructure it should be possible to
     support both.  When merging MIDI streams, a notion of transaction is
     needed to group critical message sequences.  If ALSA or OSS4 have no such
     notion, it should be provided as an upward-compatible extension.

     I would rather have open(2) itself return an error (by the POSIX
     description ENODEV looks most appropriate) if a read or write mode is
     requested that is not supported by the instance, rather than letting
     open(2) succeed and read(2) or write(2) return -1, but so help me, the
     latter seems the more common UNIX practice.

NetBSD 10.99                    April 28, 2017                    NetBSD 10.99