table of contents
MIO_OPEN(3) | Library Functions Manual | MIO_OPEN(3) |
NAME¶
mio_open
,
mio_close
, mio_read
,
mio_write
, mio_nfds
,
mio_pollfd
, mio_revents
,
mio_eof
— sndio interface to
MIDI streams
SYNOPSIS¶
#include
<sndio.h>
struct mio_hdl *
mio_open
(const
char *name, unsigned int
mode, int
nbio_flag);
void
mio_close
(struct
mio_hdl *hdl);
size_t
mio_read
(struct
mio_hdl *hdl, void
*addr, size_t
nbytes);
size_t
mio_write
(struct
mio_hdl *hdl, const void
*addr, size_t
nbytes);
int
mio_nfds
(struct
mio_hdl *hdl);
int
mio_pollfd
(struct
mio_hdl *hdl, struct
pollfd *pfd, int
events);
int
mio_revents
(struct
mio_hdl *hdl, struct
pollfd *pfd);
int
mio_eof
(struct
mio_hdl *hdl);
DESCRIPTION¶
The sndio
library allows user processes to
access midi(4) hardware and sndiod(8)
MIDI thru boxes and control ports in a uniform way.
Opening and closing a MIDI stream¶
First the application must call the
mio_open
()
function to obtain a handle representing the newly created stream; later it
will be passed as the hdl argument of most other
functions. The name parameter gives the device string
discussed in 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 MIDIDEVICE
environment variable.
The mode parameter gives the direction of the stream. The following are supported:
- MIO_OUT
- The stream is output-only; data written to the stream will be sent to the hardware or other programs.
- MIO_IN
- The stream is input-only; received data from the hardware or other programs must be read from the stream.
- MIO_IN | MIO_OUT
- The stream sends and receives data. This mode should be used rather than
calling
mio_open
() twice.
If the nbio_flag argument is
true (i.e. non-zero), then the
mio_read
()
and mio_write
() functions (see below) will be
non-blocking.
The
mio_close
()
function closes the stream and frees all allocated resources associated with
the libsndio
handle.
Sending and receiving data¶
When input mode is selected, the
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 nbytes
bytes at the addr location. Unless the
nbio_flag flag is set, it will block until data
becomes available and will return zero only on error.
When output mode is selected, the
mio_write
()
function can be called to provide data to transmit. Unless the
nbio_flag is set, mio_write
()
will block until the requested amount of data is written.
Non-blocking mode operation¶
If the nbio_flag is set on
mio_open
(), then the
mio_read
() and mio_write
()
functions will never block; if no data is available, they will return zero
immediately.
To avoid busy loops when non-blocking mode is
used, the poll(2) system call can be used to check if data
can be read from or written to the stream. The
mio_pollfd
()
function prepares the array pfd of
pollfd structures for use with
poll(2). The optimal size of the pfd
array, which the caller must pre-allocate, is provided by the
mio_nfds
()
function.
poll(2) will sleep until any
of the events requested with
mio_pollfd
()
have occurred. Events are represented as a bit-mask of
POLLIN and POLLOUT constants.
The events which woke up poll(2) can be obtained with the
mio_revents
()
function. If POLLIN is set,
mio_read
() can be called without blocking. If
POLLOUT is set, mio_write
()
can be called without blocking. POLLHUP may be set if an error occurs, even
if it is not requested with mio_pollfd
().
Error handling¶
Errors related to the MIDI subsystem (like hardware errors or
dropped connections) and programming errors (such as a call to
mio_read
() on a play-only stream) are considered
fatal. Once an error occurs, all functions which take a
mio_hdl argument, except
mio_close
() and
mio_eof
(),
stop working (i.e. always return 0).
RETURN VALUES¶
The mio_open
() function returns the newly
created handle on success or NULL on failure.
The mio_pollfd
() function returns the
number of pollfd structures filled. The
mio_nfds
() function returns the number of
pollfd structures the caller must preallocate in order
to be sure that mio_pollfd
() will never overrun.
The mio_revents
() function returns the
bit-mask set by poll(2) in the pfd
array of pollfd structures.
The mio_read
() and
mio_write
() functions return the number of bytes
transferred.
The mio_eof
() function returns 0 if
there's no pending error, and a non-zero value if there's an error.
ENVIRONMENT¶
SNDIO_DEBUG
- The debug level: may be a value between 0 and 2.
SEE ALSO¶
HISTORY¶
These functions first appeared in OpenBSD 4.7.
AUTHORS¶
Alexandre Ratchov <ratchov@openbsd.org>
November 7, 2024 | Linux 6.4.0-150600.23.25-default |