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MIO_OPEN(3) Library Functions Manual MIO_OPEN(3)

mio_open, mio_close, mio_read, mio_write, mio_nfds, mio_pollfd, mio_revents, mio_eofsndio interface to MIDI streams

#include <sndio.h>

struct mio_hdl *
mio_open(const char *name, unsigned int mode, int nbio_flag);

mio_close(struct mio_hdl *hdl);

mio_read(struct mio_hdl *hdl, void *addr, size_t nbytes);

mio_write(struct mio_hdl *hdl, const void *addr, size_t nbytes);

mio_nfds(struct mio_hdl *hdl);

mio_pollfd(struct mio_hdl *hdl, struct pollfd *pfd, int events);

mio_revents(struct mio_hdl *hdl, struct pollfd *pfd);

mio_eof(struct mio_hdl *hdl);

The sndio library allows user processes to access midi(4) hardware and sndiod(8) MIDI thru boxes and control ports in a uniform way.

First the application must call the () 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:

The stream is output-only; data written to the stream will be sent to the hardware or other programs.
The stream is input-only; received data from the hardware or other programs must be read from the stream.
The stream sends and receives data. This mode should be used rather than calling () twice.

If the nbio_flag argument is true (i.e. non-zero), then the () and mio_write() functions (see below) will be non-blocking.

The () function closes the stream and frees all allocated resources associated with the libsndio handle.

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 () 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.

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 () 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 () function.

poll(2) will sleep until any of the events requested with () 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 () 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().

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 (), stop working (i.e. always return 0).

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.

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

poll(2), midi(4), sndio(7), sndiod(8)

January 7, 2016 OpenBSD-6.1