Null pointer dereference and side effects

Archie Cobbs archie@dellroad.org
Sun Jul 18 19:30:00 GMT 2004


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



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