[Bug regression/40665] dereferencing type-punned pointer warnings cannot be disabled

mikulas at artax dot karlin dot mff dot cuni dot cz gcc-bugzilla@gcc.gnu.org
Tue Jul 7 16:32:00 GMT 2009



------- Comment #7 from mikulas at artax dot karlin dot mff dot cuni dot cz  2009-07-07 16:31 -------
> > extern int c;
> > int a(void)
> > {
> >        return *(short *)(void *)&c;
> > }
>
> This is a very bad example of a false positive as you are acessing an
> int as a short; that is undefined. I will look at your code later on,
> my laptop for home is currently broken.

Whether it is a false positive or not depends on the context.
For example, if I call function a() from function b():
int b(void)
{
        c = 0x12345678;
        __asm__ volatile ("":::"memory");
        return a();
}
the code is valid and the function b() must return a fixed value depending on
the endianity of the machine. That __asm__ statement works as a barrier that
prevents the compiler from reordering two accesses to "c".

So I am not against the warning. The warning is good. The problem is that there
is no way to shut up the warning.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665



More information about the Gcc-bugs mailing list