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.
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.
/dev/rmidiN
,
or through the sequencer,
/dev/music
.
/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 Exclusive.
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 used.
This protocol is supported in
.
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
,
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.
/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
sequencer.
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
timing.
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) |
+---------------------------+
b1|2
b0|2
c0|1
b1|1
b0|0
a0|0
v
_______+-------------+_______user
| /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.
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 complete.
(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 .
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.
/dev/rmidiN
/dev/music
/dev/sequencer
EPROTO
can be returned if the bytes to be written on a raw
midi
device would violate
MIDI
protocol.
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 . 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.