Date: Tue, 4 Aug 1998 17:43:34 -0400 (EDT) From: Chuck Robey <chuckr@glue.umd.edu> To: Thomas David Rivers <rivers@dignus.com> Cc: Nicolas.Souchu@prism.uvsq.fr, freebsd-hackers@FreeBSD.ORG Subject: Re: C and static initialization with unions Message-ID: <Pine.BSF.4.00.9808041738180.409-100000@picnic.mat.net> In-Reply-To: <199808042227.SAA16749@lakes.dignus.com>
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On Tue, 4 Aug 1998, Thomas David Rivers wrote:
> Also - you should be aware that ANSI forbids assigning to one element
> of a union and referencing a different element. (Basically, the
> 'type-punning' problem) C compilers are allowed to optimize field
> references based on type; so that:
>
> main()
> {
> union foo_t foo;
> void *vp;
>
> foo.i = 5;
> foo.p = 0;
>
> if(foo.i) {
> printf("true\n");
> } else {
> printf("false\n");
> }
> }
>
> may, depending on the optimizer, print either true or false. Technically,
> this is an invalid ANSI C program.
I never saw that, and I think you may be wrong, so if you have the ansi
standard lying around, give me a quote, ok? Harbison & Steele says, on
page 132-133, that doing what you said above is non-portable (I agree),
but programmers "sometimes do this to 'reach under' C's type system
...". There was more, but I'm sure you know it already.
----------------------------+-----------------------------------------------
Chuck Robey | Interests include any kind of voice or data
chuckr@glue.umd.edu | communications topic, C programming, and Unix.
213 Lakeside Drive Apt T-1 |
Greenbelt, MD 20770 | I run Journey2 and picnic (FreeBSD-current)
(301) 220-2114 | and jaunt (NetBSD).
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