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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.

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 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 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).


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-current