man/man4/midi.4

.\" $OpenBSD: midi.4,v 1.29 2016/08/31 13:54:58 jmc Exp $
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.\" Copyright (c) 2006 Alexandre Ratchov <alex@caoua.org>
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.Dd $Mdocdate: August 31 2016 $
.Dt MIDI 4
.Os
.Sh NAME
.Nm midi
.Nd raw device independent interface to MIDI ports
.Sh SYNOPSIS
.Cd "midi* at autri?"
.Cd "midi* at eap?"
.Cd "midi* at envy?"
.Cd "midi* at mpu?"
.Cd "midi* at sb?"
.Cd "midi* at umidi?"
.Cd "midi* at ym?"
.Sh DESCRIPTION
The
.Nm
driver makes MIDI ports available to user applications.
A
.Nm
device corresponds to 2 MIDI ports: one input port and one
output port.
Data received on the input port is not interpreted and is passed
to the user program as-is.
Similarly, data issued by the user program is sent as-is to the
output port.
.Pp
Only one process may hold open a
.Nm
device at a given time, although file descriptors may be shared
between processes once the first open completes.
If it is opened read-only (write-only) only the input (output)
MIDI port is available.
.Ss Writing to the device
A process can send raw MIDI data to the output port by using the
.Xr write 2
system call.
Data is queued and the system call returns immediately; the data
is sent as fast as possible to the output MIDI port.
However, if the in-kernel buffer is full or the requested amount
is too large, then
.Xr write 2
may block.
The current size of the in-kernel buffer is 1024 bytes, which
ensures that
.Xr write 2
isn't blocking in most situations.
.Ss Reading from the device
Data received from the input MIDI port is stored into the
in-kernel buffer.
A process can retrieve its contents by using
the
.Xr read 2
system call.
If there is less data than the amount requested for reading, then
a shorter amount is returned.
If no data is available, then the
.Xr read 2
system call will block until data is received,
and then return immediately.
.Pp
The MIDI protocol has been designed for real-time performance and
doesn't support flow control.
An application must be able to read the incoming data fast enough
(the MIDI standard's maximum rate is 3125 bytes per second).
The kernel can buffer up to 1024 bytes; once the buffer is full
input will be silently discarded.
.Ss Polling the device
A process can use the
.Xr poll 2
system call to poll for the following events:
.Bl -tag -width POLLOUT
.It Dv POLLIN
The in-kernel input buffer isn't empty, i.e. at least one byte is
available for reading.
A subsequent call to
.Xr read 2
will not be blocking.
.It Dv POLLOUT
The in-kernel output buffer is empty, thus a subsequent call to
.Xr write 2
will not be blocking if a reasonable amount of data is written
(currently less that 1024 bytes).
.El
.Pp
Using the
.Xr poll 2
system call is the recommended way to handle multiple
.Nm
devices in a real-time MIDI application.
.Ss Non-blocking I/O
If the
.Nm
device is opened with the O_NONBLOCK flag (see
.Xr open 2 ) ,
then subsequent calls to
.Xr read 2
or
.Xr write 2
will never block.
The
.Xr write 2
system call may write less bytes than requested, or may return
EAGAIN if no data could be sent or queued.
Similarly, the
.Xr read 2
system call may return EAGAIN if no input is available.
.Pp
Note that even if non-blocking I/O is not selected,
.Xr read 2
and
.Xr write 2
system calls are non-blocking when the kernel buffers permit it.
.Sh FILES
.Bl -tag -width /dev/rmidim -compact
.It Pa /dev/rmidi*
.Nm
devices
.El
.Sh EXAMPLES
The following command could record the memory dump of a
synthesizer in a file:
.Pp
.Dl $ cat -u /dev/rmidi2 >dumpfile
.Pp
A MIDI keyboard could be connected to a synthesizer by the
command:
.Pp
.Dl $ cat -u /dev/rmidi1 >/dev/rmidi2
.Pp
The input port could be connected to the output port by the
command:
.Pp
.Dl $ cat -u <>/dev/rmidi1 >&0
.Pp
The following example reads MIDI timing events from an input
device, MIDI common and voice events from another input device, and
sends the result to a third (output) device.
.Bd -literal -offset indent
#define BUFSIZE		0x100
#define ISTIMING(c)	((c) == 0xf8 || (c) == 0xfa || (c) == 0xfc)
#define ISCOMMON(c)	((c) < 0xf8)

int ofd;
struct pollfd ifd[2];
unsigned char ibuf[BUFSIZE], obuf[2 * BUFSIZE];
ssize_t iused, oused, i;

ifd[0].events = ifd[1].events = POLLIN;
for (;;) {
	oused = 0;
	if (poll(ifd, 2, -1) == -1)
		errx(1, "poll");
	if (ifd[0].revents & POLLIN) {
		if ((iused = read(ifd[0].fd, ibuf, BUFSIZE)) == -1)
			errx(1, "read");
		for (i = 0; i < iused; i++)
			if (ISTIMING(ibuf[i]))
				obuf[oused++] = ibuf[i];
	}
	if (ifd[1].revents & POLLIN) {
		if ((iused = read(ifd[1].fd, ibuf, BUFSIZE)) == -1)
			errx(1, "read");
		for (i = 0; i < iused; i++)
			if (ISCOMMON(ibuf[i]))
				obuf[oused++] = ibuf[i];
	}
	if (write(ofd, obuf, oused) == -1)
		errx(1, "write");
}
.Ed
.Pp
In the above example, unless kernel buffers are full, processing
is done in real-time without any noticeable latency; as expected,
the only blocking system call is
.Xr poll 2 .
.Sh ERRORS
If
.Xr open 2 ,
.Xr read 2 ,
.Xr write 2 ,
or
.Xr poll 2
fail then
.Xr errno 2
may be set to one of:
.Bl -tag -width Er
.It Bq Er ENXIO
The device is opened read-only (write-only) but
.Xr write 2
.Pf ( Xr read 2 )
was called.
.It Bq Er EIO
The device is being detached while a process has been trying to
read or write (for instance an
.Xr umidi 4
device has been unplugged).
.It Bq Er EAGAIN
Non-blocking I/O was selected and the output buffer is full (on
writing) or the input buffer is empty (on reading).
.It Bq Er EBUSY
The device is already open by another process.
.El
.Sh SEE ALSO
.Xr autri 4 ,
.Xr eap 4 ,
.Xr envy 4 ,
.Xr mpu 4 ,
.Xr sb 4 ,
.Xr umidi 4
.Sh HISTORY
The
.Nm
driver first appeared in
.Ox 2.5 .
.Sh AUTHORS
.An -nosplit
The
.Nm
driver was originally written by
.An Lennart Augustsson
and later largely rewritten by
.An Alexandre Ratchov .
.Sh CAVEATS
MIDI hardware was designed for real time performance and software
using such hardware must be able to process MIDI events without
any noticeable latency (typically no more than 5ms, which
is the time it takes for sound to propagate 1.75 meters).
.Pp
The
.Ox
.Nm
driver processes data fast enough, however if a MIDI application
tries to write data faster than the hardware is able to process it
(typically 3125 bytes per second), then kernel buffers may become
full and the application may be blocked.
.Pp
The other common reason for MIDI data being delayed is the system
load.
Processes cannot be preempted while running in kernel mode.
If there are too much processes running concurrently (especially
if they are running a lot of expensive system calls) then the
scheduling of a real-time MIDI application may be delayed.
Even on low-end machines this delay hardly reaches a few
milliseconds provided that the system load is reasonable.
.Pp
A real-time MIDI application can avoid being swapped by locking
its memory (see
.Xr mlock 2
and
.Xr mlockall 2 ) .
.Sh BUGS
For a given device, even if the physical MIDI input and output
ports are independent, there is no way for one process to use the
input MIDI port and for another process to use the output MIDI
port at the same time.