Null pointer dereference and side effects

Alexandre Oliva aoliva@redhat.com
Tue Jul 20 09:55:00 GMT 2004


Hi, Archie!  Long time...

On Jul 18, 2004, Archie Cobbs <archie@dellroad.org> wrote:

> 	    r2 = (void *)0;	/* or (void *)1, etc. */
> 	    if (*r1 != *r2)
> 		    return 1;
> 	    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?

If you insert an `asm ("" : "+X" (r2));' statement after the
assignment to r2, such that the compiler will no longer have any idea
that r2 is NULL, and won't optimize away the dereference.

-- 
Alexandre Oliva             http://www.ic.unicamp.br/~oliva/
Red Hat Compiler Engineer   aoliva@{redhat.com, gcc.gnu.org}
Free Software Evangelist  oliva@{lsd.ic.unicamp.br, gnu.org}



More information about the Gcc-help mailing list