Gets the file descriptor for a cancellable job. This can be used to
implement cancellable operations on Unix systems. The returned fd will
turn readable when
cancellable is cancelled.
You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with g_cancellable_reset().
After a successful return from this function, you should use
g_cancellable_release_fd() to free up resources allocated for
the returned file descriptor.
See also g_cancellable_make_pollfd().