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Null pointer dereference and side effects
- From: Archie Cobbs <archie at dellroad dot org>
- To: gcc-help at gcc dot gnu dot org
- Date: Sun, 18 Jul 2004 14:27:31 -0500 (CDT)
- Subject: Null pointer dereference and side effects
I have a question about GCC optimizations and null pointers.
Consider this program:
#include <stdlib.h>
int
main(int ac, char **av)
{
int *r1;
int *r2;
r1 = ∾
*r1 = 0;
r2 = (void *)0; /* or (void *)1, etc. */
if (*r1 != *r2)
return 1;
return 1;
}
When run & compiled with gcc -O2 (gcc 3.2.2) it does not segfault.
Clearly this is because the optimizer realizes that *r1 need not be
read in order to know that the function should return 1.
However, in my application I'm relying on the side effects of
dereferencing a null pointer (ie., SEGV signal), so that causes
bugs for me.
I understand that according to the C standard dereferencing a null
pointer results in "undefined" behavior, so gcc is not behaving
incorrectly.
However, is there some way to get the desired semantics? It appears
omitting the -O2 does it, but I'd prefer to not do that.
Thanks,
-Archie
__________________________________________________________________________
Archie Cobbs * CTO, Awarix * http://www.awarix.com