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:

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).