https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91187
--- Comment #9 from Harald van Dijk <harald at gigawatt dot nl> ---
(In reply to Jonathan Wakely from comment #8)
> (In reply to Albert Astals Cid from comment #0)
> > #define Z_NULL 0
>
> N.B. this is actually a bug. Using Z_NULL where a null pointer is expected
> might cause bugs e.g. execl("foo", "foo", Z_NULL) can pass a 32-bit int
> where a 64-bit pointer is expected, leading to undefined behaviour.
Whether this is a bug in the definition of Z_NULL depends on how Z_NULL is
meant to be used. If it is not supported as a variadic function argument, the
fact that it does not work as a variadic function argument is not a bug.
zlib.h's definition contains a comment:
#define Z_NULL 0 /* for initializing zalloc, zfree, opaque */
This does not include calling variadic functions.
> So maybe the warning should be taken as an opportunity to fix the code (or
> report a bug to the library) to use something safer:
>
> #if __cplusplus
> #define Z_NULL nullptr
> #else
> #define Z_NULL 0L
> #endif
If Z_NULL is meant to be usable as a variadic function argument, this is wrong
too. There is no guarantee that 0L is the same size as a pointer (think W64).
There is not even a guarantee that NULL is the same size as a pointer.
