Re: C standard compliance?

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Jan 29, 2014, 23:33 UTC
Message-ID: <20140129233321.GC91854@vauxhall.crustytoothpaste.net>
In-Reply-To: <87eh3q5x42.fsf@fencepost.gnu.org>

On Wed, Jan 29, 2014 at 09:52:45PM +0100, David Kastrup wrote:

Show 9 quoted lines

> Junio C Hamano <gitster@pobox.com> writes:
> > Hmm... if you were to implement a set of pointers in such a way that
> > you can cheaply tell if an unknown pointer belongs to that set, you
> > would use a hashtable, keyed with something that is derived from the
> > value of the pointer casted to uintptr_t, I would think.
> 
> The types intptr_t and uintptr_t are optional in ISO/IEC 9899:1999
> (C99).  So it would seem that I'd be covering fewer cases rather than
> more in that manner.

I think we already use uintptr_t in the codebase, and if it's not present, we typedef it to unsigned long. So I think it should be fine (and well-defined) if instead of doing

  void *p, *q;
  ...
  if (p < q)
    ...

you do:

  void *p, *q;
  ...
  if ((uintptr_t)p < (uintptr_t)q)
    ...

Then on those systems where the compiler has some bizarre undefined behavior checking, the code will work. On systems that don't have uintptr_t, the compiler is probably not smart enough to perform such a check anyway.

-- 
brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | http://www.crustytoothpaste.net/~bmc | My opinion only
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