> 	(1) When opening a FIFO with O_RDONLY or O_WRONLY set:
> 	...
> 	    (b) If O_NONBLOCK is clear:
> 		An open() for reading-only shall block until a process
> 		opens the file for writing.  An open() for writing-only
> 		shall block until a process opens the file for reading.

> I claim the behavior you describe is undefined because
> nothing is said about an open() for reading-and-writing but
> I suppose the non blocking behavior can be justified in a
> twisted sort of way.

The behavior is perfectly well defined; "open() opens the named file for
reading and/or writing, as specified by the flags argument".  In the absence
of any special-case description, I'd expect it to just proceed to work as
documented.  (What does it *say* happens if you open a regular file with
O_NDELAY/O_NONBLOCK _clear_?)

The SVID is a good deal less legalistic that (for example) the ANSI C standard
(and the SVVS was a GOOD DEAL less legalistic than the SVID; at least when I
beat my head against it, it tested a good many things that weren't in the SVID
...).