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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. 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).