Subject: Re: [PATCH/RFC 4/4] Add interactive mode to git-shell for user-friendliness

From: Thomas Rast <[email-masked]>
Date: Wed, 14 Jul 2010 17:40:37 +0200

[Please don't trim the Cc list without good reason.]

Bernhard R. Link wrote:
> * Kevin P. Fleming <[email-masked]> [100714 15:59]:
> > On 07/14/2010 04:04 AM, Ævar Arnfjörð Bjarmason wrote:
> > > On Wed, Jul 14, 2010 at 03:01, Greg Brockman <[email-masked]> wrote:
> > >> +               execl(prog, prog, (char *) NULL);
> > >
> > > Why the casting of NULL? It's not done in the builtin/help.c code.
> > >
> > > Anyway, if it was cast it should be to (const char *), shouldn't it?
> >
> > When a NULL sentinel is passed to a varargs function that only
> > understands 'char *' arguments, the NULL must be cast specifically,
> > otherwise it will appear in the varargs array as an int or a long.
> 
> To be more specific: If NULL is (void *)0 then it does not need to be
> cast. Sadly the standard allows to define it as 0, and so it is on
> some systems. So to be portable it needs to be cast to be a pointer,
> otherwise the varargs argument is assumed to be an int.

Worse, the pointer representations need not be the same between types,
even though that is a fairly exotic idea:

  http://c-faq.com/null/machexamp.html

So it seems execl() must always have an explicitly-cast (char*)NULL
sentinel.

-- 
Thomas Rast
trast@{inf,student}.ethz.ch