Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!att!chinet!les
From: les@chinet.chi.il.us (Leslie Mikesell)
Newsgroups: comp.unix.wizards
Subject: Re: preserving message boundaries on named pipes - System V
Message-ID: <8486@chinet.chi.il.us>
Date: 18 May 89 20:07:34 GMT
References: <571@lehi3b15.csee.Lehigh.EDU>
Reply-To: les@chinet.chi.il.us (Leslie Mikesell)
Organization: Chinet - Public Access Unix
Lines: 24

In article <571@lehi3b15.csee.Lehigh.EDU> murrey@lehi3b15.csee.Lehigh.EDU (Erik Murrey) writes:

>System V.3.1 Unix

>Is it possible to preserve message boundaries on named pipes under
>System V?  For instance, if a process sends two separate messages (via
>two write()'s) down a named pipe, how can the receiver read those as
>two separate messages?  It seems that right now, if the reading
>process unblocks after both writes have completed, then it gets *both*
>messages in one read.

>I really don't wan't to muck with terminating null characters, and
>the like...

The easy way is to use fixed length write()'s and matching read()'s.
If the input is sufficiently varied that this approach would impose
a lot of overhead you might precede the data with a length value.  This
should be done as a single write() if multiple processes are writing
to the same FIFO to avoid the possibility of interleaving between the
length and data.  The reader would do a fixed length read() to find
out the size the next read() should be for the data.


Les Mikesell