On Mon, 2007-01-08 at 15:39 +0000, Andrew Haley wrote:
> Michael Haubenwallner writes:
> > Hi,
> >
> > after switching to gcc-4.1.1 from gcc-3.4.5, I'm facing a problem with
> > this code searching for non-blank in a string (extracted test-case):
> >
> > #include <stdio.h>
> > int check(const void *pvBuf)
> > {
> > for(; *((char*)pvBuf) == ' '; ++ *((char**)&pvBuf));
>
> This is wrong. Try this:
>
> for(; *((char*)pvBuf) == ' '; pvBuf = (char*)pvbuf + 1);
Yep, have my code changed like this, thanks.
>
> > return *((char*)pvBuf);
> > }
> > int main(void)
> > {
> > printf("%d\n", check(" x"));
> > return 0;
> > }
> >
> > When building without optimization, it works as expected.
> > When building with -O2 or higher, the loop hangs.
> > This seems to be platform independant, as it occurs on x86-linux as well
> > as powerpc-aix, hppa-hpux, ia64-hpux, x86-solaris and sparc-solaris.
> >
> > Now the question is:
> > Is this a user-bug only, not looking at the output of "-Wall" (because
> > this code builds and works successfully for 5 years now),
> > or is it a gcc-bug too, producing working code without optimization
> > and non-working code with optimization level >= 2 ?
> >
> > And: can this code be changed without using an intermediate variable ?
>
> This is an aliasing bug. Please see The C standard, Section 6.3.2.3,
> Pointers.
Hmm, a bug in what product - the standard, gcc, or my application ?
And - sorry for that question - where do one read the standard from ?
I've found C89, this seems to have only 5 sections...
And in the C99 draft, Section 6.3.2.3 has to do with aliasing, but I'm
completely lost here - I'll better let interpreting the standard up to
you :)
>
> Alternatively, you may compile your code with -fno-strict-aliasing.
Yes, this works too, thanks.
/haubi/
