io_nonblock puts UNIX descriptor fd into non-blocking mode. Calling
io_nonblock(fd) before io_fd(fd) makes io_tryread and
io_trywrite faster and more efficient.
Actually, current UNIX kernels do not support non-blocking descriptors; they
support non-blocking open files. Furthermore, many programs will break if they
encounter non-blocking mode. This means that you must not use io_nonblock for a
descriptor inherited from another program.
io_nonblock has no return value; it always succeeds. If d is not the number of
a UNIX descriptor, io_nonblock has no effect.
If io_fd is given a descriptor in blocking mode, io_tryread and io_trywrite go
through the following contortions to avoid blocking:
Stop if poll says that the descriptor is not ready. Otherwise theres a good
chance, but not a guarantee: even if poll says the descriptor is ready, the
descriptor might not be ready a moment later. (Furthermore, poll can fail on
Catch SIGALRM. SIGALRM must not be blocked, and must not be used elsewhere in
Set an interval timer so that any blocking call will be interrupted by SIGALRM
within 10 milliseconds. (Current UNIX kernels do not allow any shorter
interval.) Of course, this may still mean a 10-millisecond delay.
If io_fd is given a descriptor in non-blocking mode (or a descriptor for a
regular disk file), io_tryread and io_trywrite avoid these contortions.