Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!usenet.ins.cwru.edu!hal!ncoast!allbery
From: allbery@NCoast.ORG (Brandon S. Allbery)
Newsgroups: comp.unix.i386
Subject: Re: Message queues.  Possibly dumb mistake.
Message-ID: <1989Dec8.234510.8542@NCoast.ORG>
Date: 8 Dec 89 23:45:10 GMT
References: <1989Dec7.150205.12104@antel.uucp>
Reply-To: allbery@ncoast.ORG (Brandon S. Allbery)
Followup-To: comp.unix.i386
Organization: North Coast Public Access UN*X, Cleveland, OH
Lines: 38

As quoted from <1989Dec7.150205.12104@antel.uucp> by geoff@antel.uucp (Geoff Vona):
+---------------
| struct msgbuf {
| long mtype;
| char mtext[1];
| };
+---------------

"char xxx[1];" is a convention for an "extensible" structure; you can't declare
an array of size 0, so you declare it of size 1 and allocate it as

	struct msgbuf *p = malloc(sizeof *p + msglen - 1);

This, BTW, is a rather miserable -- and potentially non-portable -- way of
doing things; a better design would have been to pass mtype as an argument
on msgsnd() and return it from msgrcv(), and pass the message itself as a
(char *) (or (void *) for ANSI C folks).  But they didn't ask my opinion
before designing it.

+---------------
| struct msgbuf {
| long mtype;
| char mtext[];
| };
+---------------

This is illegal C; it does, however, express that the array mtext extends
immediately after the structure.

This topic comes up every so often, so I've decided to post.

++Brandon
-- 
Brandon S. Allbery    allbery@NCoast.ORG, BALLBERY (MCI Mail), ALLBERY (Delphi)
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