Date: Mon, 11 Nov 2002 11:18:28 -0500 (EST) From: Thomas David Rivers <rivers@dignus.com> To: brandt@fokus.gmd.de, gallatin@cs.duke.edu Cc: FreeBSD-current@FreeBSD.ORG Subject: Re: gcc 3.2.1 optimization bug ? Message-ID: <200211111618.gABGIS155735@lakes.dignus.com> In-Reply-To: <20021111163918.Y13339-100000@beagle.fokus.gmd.de>
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Harti Brandt <brandt@fokus.gmd.de> wrote:
>
> Well, I just had a long discussion with a collegue about the topic. The
> main problem is in the ISO-C standard, section 6.7 point 4 which states:
>
> All declarations in the same scope that refer to the same object or
> function shall specify compatible types.
>
> This makes any fiddeling with a pointer of a type, that is different from
> the type of the variable illegal. It also makes fiddeling with unions and
> structs illegal. As far as I understand types are compatible when they
> have the same type and type qualifiers. Structs are partly compatible when
> they start with fields of the same type. So the only way to legally swap
> integer values of any size is via arithmetic. Of course unions and
> pointers may work with the given compiler version and optimisation level.
> But, nothing in the standard guarantees you anything.
That's it exactly!
>
> It may be possible that 6.5 (7) the last sentence allows you to access your
> 32-bit or 64-bit value character-wise (although I'm not sure).
>
> harti
>
The C99 standard does seem to add an "exception" for access via
character pointers. The C89 standard did not have that clause.
As far as the union question - the answer is that you cannot
"read" from a union from a different member than it was "stored"
to.
For example:
char ch;
union {
int x;
char c;
} my_union;
my_union.x = 10;
ch = my_union.c;
Technically, this is illegal in ANSI C, and not guaranteed to "work". Although
it will likely "work" in just about every reasonable environment.
(For some definition of "work" :-) )
- Dave Rivers -
--
rivers@dignus.com Work: (919) 676-0847
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