lib/libsndio/mio_open.3

.\" $OpenBSD: mio_open.3,v 1.4 2011/04/16 10:52:22 ratchov Exp $
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.\" Copyright (c) 2007 Alexandre Ratchov <alex@caoua.org>
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.Dd $Mdocdate: April 16 2011 $
.Dt MIO_OPEN 3
.Os
.Sh NAME
.Nm mio_open ,
.Nm mio_close ,
.Nm mio_read ,
.Nm mio_write ,
.Nm mio_nfds ,
.Nm mio_pollfd ,
.Nm mio_revents ,
.Nm mio_eof
.Nd interface to MIDI streams
.Sh SYNOPSIS
.Fd #include <sndio.h>
.Ft "struct mio_hdl *"
.Fn "mio_open" "const char *name" "unsigned mode" "int nbio_flag"
.Ft "void"
.Fn "mio_close" "struct mio_hdl *hdl"
.Ft "size_t"
.Fn "mio_read" "struct mio_hdl *hdl" "void *addr" "size_t nbytes"
.Ft "size_t"
.Fn "mio_write" "struct mio_hdl *hdl" "const void *addr" "size_t nbytes"
.Ft "int"
.Fn "mio_nfds" "struct mio_hdl *hdl"
.Ft "int"
.Fn "mio_pollfd" "struct mio_hdl *hdl" "struct pollfd *pfd" "int events"
.Ft "int"
.Fn "mio_revents" "struct mio_hdl *hdl" "struct pollfd *pfd"
.Ft "int"
.Fn "mio_eof" "struct mio_hdl *hdl"
.Sh DESCRIPTION
The
.Nm sndio
library allows user processes to access
.Xr midi 4
hardware and
.Xr midicat 1
MIDI thru boxes in a uniform way.
.Ss Opening and closing an MIDI stream
First the application must call the
.Fn mio_open
function to obtain a handle representing the newly created stream;
later it will be passed as the
.Ar hdl
argument of most other functions.
The
.Fn mio_open
function tries to connect to the
.Xr midicat 1
software MIDI thru box or to use the
.Xr midi 4
hardware device.
The
.Ar name
parameter gives the device string discussed in
.Xr sndio 7 .
If the program is using a single device and is providing no device chooser,
it should be set to NULL to allow the user to select it using the
.Ev MIDIDEVICE
environment variable.
.Pp
The
.Ar mode
parameter gives the direction of the stream.
The following are supported:
.Bl -tag -width "MIO_OUT | MIO_IN"
.It MIO_OUT
The stream is output-only; data written to the stream will be sent
to the hardware or other programs.
.It MIO_IN
The stream is input-only; received data from the hardware or
other programs must be read from the stream.
.It MIO_IN | MIO_OUT
The stream sends and receives data.
This mode should be used rather than calling
.Fn mio_open
twice.
.El
.Pp
If the
.Ar nbio_flag
argument is true (i.e. non-zero), then the
.Fn mio_read
and
.Fn mio_write
functions (see below) will be non-blocking.
.Pp
The
.Fn mio_close
function closes the stream and frees all allocated resources
associated with the
.Nm libsndio
handle.
.Ss Sending and receiving data
When input mode is selected, the
.Fn mio_read
function must be called to retrieve received data; it must be called
often enough to ensure that internal buffers will not overrun.
It will store at most
.Ar nbytes
bytes at the
.Ar addr
location and return the number of bytes stored.
Unless the
.Ar nbio_flag
flag is set, it will block until data becomes available and
will return zero only on error.
.Pp
When output mode is selected, the
.Fn mio_write
function can be called to provide data to transmit.
Unless the
.Ar nbio_flag
is set,
.Fn mio_write
will block until the requested amount of data is written.
.Ss Non-blocking mode operation
If the
.Ar nbio_flag
is set on
.Fn mio_open ,
then the
.Fn mio_read
and
.Fn mio_write
functions will never block; if no data is available, they will
return zero immediately.
.Pp
To avoid busy loops when non-blocking mode is used, the
.Xr poll 2
system call can be used to check if data can be
read from or written to the stream.
The
.Fn mio_pollfd
function fills the array
.Ar pfd
of
.Va pollfd
structures, used by
.Xr poll 2 ,
with
.Ar events ;
the latter is a bit-mask of
.Va POLLIN
and
.Va POLLOUT
constants; refer to
.Xr poll 2
for more details.
.Fn mio_pollfd
returns the number of
.Va pollfd
structures filled.
The
.Fn mio_revents
function returns the bit-mask set by
.Xr poll 2
in the
.Va pfd
array of
.Va pollfd
structures.
If
.Va POLLIN
is set,
.Fn mio_read
can be called without blocking.
If
.Va POLLOUT
is set,
.Fn mio_write
can be called without blocking.
POLLHUP may be set if an error occurs, even if
it is not selected with
.Fn mio_pollfd .
.Pp
The
.Fn mio_nfds
function returns the number of
.Va pollfd
structures the caller must preallocate in order to be sure
that
.Fn mio_pollfd
will never overrun.
.Ss Error handling
Errors related to the MIDI subsystem
(like hardware errors or dropped connections) and
programming errors (such as a call to
.Fn mio_read
on a play-only stream) are considered fatal.
Once an error occurs, all functions which take a
.Va mio_hdl
argument, except
.Fn mio_close
and
.Fn mio_eof ,
stop working (i.e. always return 0).
.Pp
The
.Fn mio_eof
function can be used at any stage;
it returns 0 if there's no pending error, and a non-zero
value if there's an error.
.Sh RETURN VALUES
The
.Fn mio_open
function returns the newly created handle on success or NULL
on failure.
The
.Fn mio_pollfd
function returns 1 on success and 0 on failure.
The
.Fn mio_read
and
.Fn mio_write
functions return the number of bytes transferred.
.Sh ENVIRONMENT
.Bl -tag -width "MIO_DEBUGXXX" -compact
.It Ev SNDIO_DEBUG
The debug level:
may be a value between 0 and 2.
.El
.Sh FILES
.Bl -tag -width "/tmp/aucat-<uid>/midithru0" -compact
.It Pa /tmp/aucat-<uid>/midithru0
Default path to
.Xr midicat 1
socket to connect to.
.It Pa /dev/rmidiX
.Xr midi 4
devices.
.El
.Sh SEE ALSO
.Xr midicat 1 ,
.Xr midi 4 ,
.Xr sndio 7