Date: Sun, 03 Mar 2002 18:11:29 -0800 From: Terry Lambert <tlambert2@mindspring.com> To: "Brian T.Schellenberger" <bts@babbleon.org> Cc: Erik Trulsson <ertr1013@student.uu.se>, Ian <freebsd@damnhippie.dyndns.org>, freebsd-hackers <freebsd-hackers@FreeBSD.ORG> Subject: Re: A few questions about a few includes Message-ID: <3C82D7D1.F9AD882C@mindspring.com> References: <XFMail.20020303091938.conrads@cox.net> <B8A7AB05.AAE6%freebsd@damnhippie.dyndns.org> <20020303180029.GA56041@student.uu.se> <20020304012020.A1681BA05@i8k.babbleon.org>
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"Brian T.Schellenberger" wrote:
> I can't even imagine how one *would* write a compiler where this would
> fail--does anybody know the putative risk that led ANSI to "ban" this (IMHO)
> perfectly-reasonable bahvior?
Order of structure elements is undefined. Zero length arrays
are undefined. Also, packing is undefined.
You can basically get arouns all of this by declaring the
array to be 1 byte, e.g.:
struct pargs {
u_int ar_ref; /* Reference count */
u_int ar_length; /* Length */
u_char ar_args[1]; /* Arguments */
};
And then sizing the allocation unit relatively, e.g., dont
use:
sizeof(struct pargs) + byte_count
use instead:
(int)&((struct pargs *)0)->ar_args + byte_count;
To get the size of the elements up to but not including
the one byte declared, regardless of alignment or packing.
-- Terry
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