Re: [PATCH] Use explicit pointers for execl...() sentinels.

From: Horst von Brand <vonbrand@inf.utfsm.cl>
Date: Mar 13, 2006, 04:12 UTC
Message-ID: <200603130412.k2D4CW1b011631@laptop11.inf.utfsm.cl>
In-Reply-To: <peff@peff.net>

Jeff King <peff@peff.net> wrote:

Show 5 quoted lines

> On Sun, Mar 12, 2006 at 08:08:12PM +0200, Timo Hirvonen wrote:
> > NULL pointer does not point to any data, it just says it's 'empty'.  So
> > it doesn't need to be same type pointer as specified in the function
> > prototype.  Pointers are just addresses, it doesn't matter from to code
> > generation point of view whether it is (char *)0 or (void *)0.

Show 6 quoted lines

> Sorry, but I think you're wrong according to the C standard. Pointers of 
> different types do NOT have to share the same representation (e.g.,
> there have been some platforms where char* and int* were different
> sizes). A void pointer must be capable of representing any type of
> pointer (for example, holding the largest possible type). However, if
> sizeof(void *) == 8 and sizeof(char *) == 4,

Very improbable, they'll be the same normally ("void *" is a way of getting rid of the overloading of the meaning of "char *" for this before ANSI C). Sure, sizeof(int *) might be 4, but I think that is pretty far off.

>                                              you have a problem with
> variadic functions which are expecting to pull 4 byte off the stack.

There are special rules for variadic functions, probably pointers would be cast to/from void * in such a case by the compiler.

> In a non-variadic function, the compiler would do the right implicit
> casting. In a variadic function, it can't.

It sure can. The rules where defined so that it works.

> The real question is, does git want to care about portability to such
> platforms.

Broken platform, on which the compiler fails miserably in doing its job? No, it doesn't.

> If you remain unconvinced, I can try to find chapter and verse of the
> standard.

Please do.

> > sizeof(unsigned long) is sizeof(void *) in real world.

> Are you saying that because it encompasses all of the platforms you've
> worked on, or do you have some evidence that it is largely the case? It
> certainly isn't guaranteed by the C standard.

More because a machine with pointers that are much larger than the largest "normal" integer would be pretty weird (sure, on intel 8086 they where 32 ("far" pointer, segment + offset) and 16 bits, but...

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