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

From: Jeff King <peff@peff.net>
Date: Mar 13, 2006, 03:31 UTC
Message-ID: <20060313033121.GA14601@coredump.intra.peff.net>
In-Reply-To: <20060312200812.3fb04638.tihirvon@gmail.com>

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.

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

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

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

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

> 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.

> > Because, according to the C and POSIX specs, they're not wrong.
> They didn't think of 64-bit architectures back then, I suppose.

No, they did think of those issues; they intentionally left such sizing up to the implementation to allow C to grow with the hardware. Mostly you don't have to care, but as I said, typing with variadic functions is a pain.

-Peff