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From: thorinn@diku.UUCP (Lars Henrik Mathiesen)
Newsgroups: comp.lang.c
Subject: Re: Re: Arrays of Unknown Length in Structures (and sizeof)
Message-ID: <3453@diku.UUCP>
Date: Wed, 7-Oct-87 10:36:23 EDT
Article-I.D.: diku.3453
Posted: Wed Oct  7 10:36:23 1987
Date-Received: Wed, 14-Oct-87 04:50:20 EDT
References: <243@mit-prep.ARPA> <1044@ius1.cs.cmu.edu> <668@its63b.ed.ac.uk>
Organization: DIKU, U of Copenhagen, DK
Lines: 43

In article <668@its63b.ed.ac.uk> xsimon@its63b.ed.ac.uk (Simon Brown) writes:
>Umm, but what about the System V "msgbuf" type, used for the msg IPC system 
>calls? This has type
>	struct msgbuf {
>		long mtype;
>		char mtext[1];
>	}
>and is intended to be used as a template type, by malloc()'ing space of whatever
>message buffer size is required, with something like
>	msg = (struct msgbuf *)malloc(sizeof(struct msgbuf) + NBYTES);
>which results in a message with bytes mtext[0] ... mtext[NBYTES].

Just to note (again) that this may allocate space for up to
			NBYTES + sizeof(long) bytes:
The structure size will be rounded to the alignment of a long for use in
arrays - on most machines this will be stricter than sizeof(char), typically
sizeof(long). If you really care about those "wasted" bytes, you can use a
"portable" trick like this:
	struct msgbuf {
		long mtype;
		union mu {
			char mt[1];
			double manon;
		} mu;
	};
#define mtext mu.mt
#define MSGBUFBASE (sizeof(struct msgbuf) - sizeof(double))
	msg = (struct msgbuf *)malloc(MSGBUFBASE + NBYTES);

But given a structure like
	struct msgbuf { long mtype; char mflags, mtext[1] };
the former trick would force a stricter alignment on mtext than "necessary",
so you would "have to" be unportable and write something like

#define MSGBUFBASE (sizeof(msgbuf) - LONGALIGN + CHARARRAYALIGN)

By the way, is there anything in the various standards that says whether
the alignment of a double (or float) has to be the at least as strict as
that of any other datatype? Ditto for sizeof? If not, the first trick has
to be fixed.
--
Lars Mathiesen, DIKU, U of Copenhagen, Denmark      [uunet!]mcvax!diku!thorinn
Institute of Datalogy -- we're scientists, not engineers.