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Date: Sat, 12 Jan 2013 01:46:11 -0500
From: Rich Felker <>
Subject: Re: NULL

On Sat, Jan 12, 2013 at 12:32:44AM -0600, Rob Landley wrote:
> On 01/09/2013 07:36:43 AM, John Spencer wrote:
> >>using NULL in the argument of variadic functions is ub both
> >>in c and c++
> >
> >many developers don't care about the standard. they take the stance:
> >"works for me, if you want it patched then do it yourself and
> >we'll eventually merge"
> Why is it UB? The standard says it's a pointer. If you pull %p off
> in printf, feeding NULL in that slot should work fine.

See my other message. NULL is not required to have pointer type. It
can be any null pointer constant, which includes things like 0, 0L,
0ULL, (sizeof 1 - sizeof 2), (void *)(1ULL/2ULL), etc.

The %p specifier, on the other hand, requires an argument of type void
*; passing any other type yields UB.


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