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From: Szabolcs Nagy <nsz@port70.net>
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Subject: Re: NULL
Date: Wed, 9 Jan 2013 22:53:27 +0100
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* Rob <robpilling@gmail.com> [2013-01-09 21:11:28 +0000]:
> On Wed, Jan 09, 2013 at 10:36:30AM -0500, Rich Felker wrote:
> > 	char s[1][1+(int)NULL];
> > 	int i = 0;
> > 	return sizeof s[i++], i;
> 
> Magic... is `s' a VLA here? My mind is boggled because
> __builtin_constant_p(1+(int)NULL) returns 1, and I can't think of any
> reason why the sizeof is evaluated.
> 
> Also, seeing that clang and tcc return 0 in all cases, is this a bug in
> both of them?

sizeof evaluates its argument if and only if it is a vla
(c11 6.5.3.4p2)

in c99 (and c11) vla is created if the size in the array
declarator is not an "integer constant expression"
(c11 6.7.6.2p4)

eg '1 + (int)(void*)0' is not an integer constant expression
because of the pointer cast, but '1 + (int)0' is
(c11 6.6p6)

hence sizeof s[i++] evaluates the argument if NULL has a pointer
cast in it