debian man page for io_fd

io_fd(3)						     Library Functions Manual							  io_fd(3)

NAME
       io_fd - prepare descriptor for io_wait

SYNTAX
       #include <io.h>

       int io_fd(int64 fd);

DESCRIPTION
       If  you	want to use io_canread() and io_canwrite() on a descriptor, you have to use io_wait() on it first, and io_wait() has to know which
       descriptors you are interested in.  Use io_fd() for this.

       io_pipe and io_socketpair already call io_fd for you.

       Waiting on descriptors only works for sockets, fifos and pipes.	It may also work on devices and TTYs, but that is  platform  dependent	--
       you should not rely on that.  It does not work on files.

RETURN VALUE
       io_fd returns 1 on success, 0 on error.

SEE ALSO
       io_wait(3), io_wantread(3), io_canread(3), io_eagain(3), io_nonblock(3)

																	  io_fd(3)