On Tue, 2009-12-15 at 11:24 +0100, Andi Kleen wrote:
> John Regehr <reg...@cs.utah.edu> writes:
>
> >> I would only be worried for cases where no warning is issued *and*
> >> unitialized accesses are eliminated.
> >
> > Yeah, it would be excellent if GCC maintained the invariant that for
> > all uses of uninitialized storage, either the compiler or else
> > valgrind will issue a warning.
>
> My understanding was that valgrind's detection of uninitialized
> local variables is not 100% reliable because it cannot track
> all updates of the frames (it's difficult to distingush stack
> reuse from uninitialized stack)
I am not a valgrind expert so, take the following with a grain of salt
but I think that the above statement is wrong: valgrind reliably detects
use of uninitialized variables if you define 'use' as meaning 'affects
control flow of your program' in valgrind.
i.e., try this:
[mlac...@diese ~]$ cat > test.c
int f(void)
{
int x;
return x;
}
int main (int argc, char *argv[])
{
if (f())
{
printf ("something\n");
}
return 0;
}
^C
[mlac...@diese ~]$ gcc ./test.c
./test.c: In function ‘main’:
./test.c:10: warning: incompatible implicit declaration of built-in
function ‘printf’
[mlac...@diese ~]$ valgrind ./a.out
==18933== Memcheck, a memory error detector.
==18933== Copyright (C) 2002-2007, and GNU GPL'd, by Julian Seward et
al.
==18933== Using LibVEX rev 1804, a library for dynamic binary
translation.
==18933== Copyright (C) 2004-2007, and GNU GPL'd, by OpenWorks LLP.
==18933== Using valgrind-3.3.0, a dynamic binary instrumentation
framework.
==18933== Copyright (C) 2000-2007, and GNU GPL'd, by Julian Seward et
al.
==18933== For more details, rerun with: -v
==18933==
==18933== Conditional jump or move depends on uninitialised value(s)
==18933== at 0x80483D7: main (in /home/mlacage/a.out)
something
==18933==
==18933== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 12 from
1)
==18933== malloc/free: in use at exit: 0 bytes in 0 blocks.
==18933== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==18933== For counts of detected errors, rerun with: -v
==18933== All heap blocks were freed -- no leaks are possible.
[mlac...@diese ~]$
