MIO_OPEN(3)

NAME
mio_open, mio_close, mio_read, mio_write, mio_nfds, mio_pollfd, mio_revents, mio_eof - 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(1) MIDI thru boxes and control ports in a uniform way.

  Opening and closing an 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 mio_open() function tries to connect to the sndiod(1) MIDI thru box or to use the midi(4) hardware port. 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 and return the number of bytes stored. 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 fills the array pfd of pollfd structures, used by poll(2), with events; the latter is a bit-mask of POLLIN and POLLOUT constants; refer to poll(2) for more details. mio_pollfd() returns the number of pollfd structures filled. The mio_revents() function returns the bit-mask set by poll(2) in the pfd array of pollfd structures. 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 selected with mio_pollfd().

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.

  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).

The 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.

RETURN VALUES
The mio_open() function returns the newly created handle on success or NULL on failure. The mio_pollfd() function returns 1 on success and 0 on failure. The mio_read() and mio_write() functions return the number of bytes transferred.

ENVIRONMENT
SNDIO_DEBUG
    The debug level: may be a value between 0 and 2.

FILES
/tmp/aucat-<uid>/aucat0
    Default path to sndiod(1) socket to connect to.
/dev/rmidiX
    midi(4) devices.

SEE ALSO
sndiod(1), midi(4), sndio(7)