Derek Fawcus: Yeah - I always found that a bit weird, having to use socketpair() to get a bidirectional "pipe". In the Research system, pipes became bidirectional when they became streams. That happened slightly before my time, but so far as I know it broke absolutely nothing. Some time in the late 1980s, the System V people wanted to allow pipes to be streams, but were worried about the bidirectionality. They proposed to have a new system call to make a bidirectional pipe. I attended a meeting with the relevant programmers and program manager to find out why they thought pipes couldn't just be bi-directional, as they had been without fuss in the Research system for some years. They agreed with me that that was how it ought to be; the trouble was that System V releases all had to pass an official System V Verification Suite (reasonable enough), and that suite checked not only that you could read the one pipe file descriptor and write the other, but that you couldn't do it the wrong way. Wait a minute, I said. I'm pretty sure that's not how the official System V Interface Description reads. Anyone got a current copy handy? We found one, and we looked, and sure enough, the official verification suite was wrong. The specification said that data written to fd[1] must be readable from fd[0], but nothing about the other direction: full-duplex pipes were not required but neither were they outlawed! The programming group was delighted: I'd given them the ammo they needed to do it right (make pipes streams, and make them full-duplex by default). I believe that is how it came out, though the only reference I have is Solaris 10, where the manual page specifically says that what pipe(2) makes is full-duplex (and a stream). I wish POSIX and Linux and the BSDs would catch up; that was only 30 years ago. Norman Wilson Toronto ON