lib/libsndio/mio_open.3

.\" $OpenBSD: mio_open.3,v 1.18 2018/12/18 20:37:27 jmc Exp $
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.\" Copyright (c) 2007 Alexandre Ratchov <alex@caoua.org>
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.Dd $Mdocdate: December 18 2018 $
.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 sndio interface to MIDI streams
.Sh SYNOPSIS
.In sndio.h
.Ft "struct mio_hdl *"
.Fn mio_open "const char *name" "unsigned int 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 sndiod 8
MIDI thru boxes and control ports in a uniform way.
.Ss Opening and closing a 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
.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 MIO_PORTANY 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.
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 prepares the array
.Ar pfd
of
.Va pollfd
structures for use with
.Xr poll 2 .
The optimal size of the
.Ar pfd
array, which the caller must pre-allocate, is provided by the
.Fn mio_nfds
function.
.Pp
.Xr poll 2
will sleep until any of the
.Ar events
requested with
.Fn mio_pollfd
have occurred.
Events are represented as a bit-mask of
.Va POLLIN
and
.Va POLLOUT
constants.
The events which woke up
.Xr poll 2
can be obtained with the
.Fn mio_revents
function.
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 requested with
.Fn mio_pollfd .
.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).
.Sh RETURN VALUES
The
.Fn mio_open
function returns the newly created handle on success or NULL
on failure.
.Pp
The
.Fn mio_pollfd
function returns the number of
.Va pollfd
structures filled.
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.
.Pp
The
.Fn mio_revents
function returns the bit-mask set by
.Xr poll 2
in the
.Va pfd
array of
.Va pollfd
structures.
.Pp
The
.Fn mio_read
and
.Fn mio_write
functions return the number of bytes transferred.
.Pp
The
.Fn mio_eof
function returns 0 if there's no pending error, and a non-zero
value if there's an error.
.Sh ENVIRONMENT
.Bl -tag -width "SNDIO_DEBUGXXX" -compact
.It Ev SNDIO_DEBUG
The debug level:
may be a value between 0 and 2.
.El
.Sh SEE ALSO
.Xr poll 2 ,
.Xr midi 4 ,
.Xr sndio 7 ,
.Xr sndiod 8
.Sh HISTORY
These functions first appeared in
.Ox 4.7 .
.Sh AUTHORS
.An Alexandre Ratchov Aq Mt ratchov@openbsd.org