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From: tony@artecon.UUCP
Newsgroups: comp.unix.questions
Subject: System V.2 IPC (msgop - message passing) ?
Message-ID: <419@artecon.artecon.UUCP>
Date: Fri, 10-Apr-87 16:31:11 EST
Article-I.D.: artecon.419
Posted: Fri Apr 10 16:31:11 1987
Date-Received: Sat, 11-Apr-87 21:09:56 EST
Organization: Artecon Inc., San Diego
Lines: 49
Keywords: Include file definitions


Question about System V message passing (IPC):

In the man pages for msgop(2), it shows the message to be passed
as type *msgbuf.  The man pages say the struct is:

struct msgbuf{
	int	mtype;	/* type of message (arbitrary) */
	char	mtext[];	/* body of message */
	}

Now this implies that the pointer mtext can be assigned to an
array or can be malloc'd to. (which makes sense)

However, looking at the include file when this is defined <sys/msg.h>,
it defines the structure like this:

struct msgbuf {
	int	mtype;
	char	mtext[1];	/* note the '1' */
	}

What is going on here, I can't redefine this pointer, and I certainly
can't pass descent messages in *1* byte.

Of course the work around is obvious, define the structure myself as:

struct whatever {
	int	mtype;
	char	mtext[100];	/* or  char *mtype  */
	}

This works just fine, however, it is no longer portible code.
What if some versions of sys V (maybe some in the future) throw
in another flag.  Then my code is F*CKED.

So, anyone know the story??  This is not an isolated case, as both
HP-UX (based on sys V) and even Sun's System V compatibility mode
use this same definition (char mtext[1]).

Thanx in advance (as usual),
				    Tony

 
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*  Tony Parkhurst -- {hplabs|sdcsvax|ncr-sd|hpfcla|ihnp4}!hp-sdd!artecon!adp  *
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