[TUHS] sockets (was Re: First appearance of named pipes)

Norman Wilson norman at oclsc.org
Mon Mar 9 09:09:01 AEST 2020

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

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