Re: Null pointer dereference and side effects

[Jeffrey A Law <law at redhat dot com>]

On Sun, 2004-07-18 at 13:27, Archie Cobbs wrote:
> 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 = &ac;
> 	    *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.
-fno-delete-null-pointer-checks.
jeff