ftruncate'ing a pipe?

[Peter Blemel]

I need a portable solution to the following problem: I have two processes
communicating via a pipe. I need some way of ftruncating what's in the 
pipe channel. Using ftruncate doesn't seem to clear the pipe (On a 4.3Taho
vax).

Thanks
Peter Blemel
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Note : It's possible that the email address on this post is wrong. It
should be : bbx!yenta!msinc!peter at unmvax.cs.unm.edu