On Wed, 02 Nov 2011 08:41:25 +0100, Peter Toft wrote:
> On Wed,
02 Nov 2011 00:32:44 +0000, Tom Hughes wrote:
>
>> On 01/11/11 22:33,
Peter Toft wrote:
>>
>>> Try to find the errors in this C/C++ snippet
using valgrind: #include /* Save as code.c */ int main(void) { int
i=-1,a[2],b[2],c[2]; a[0] = 1; a[1] = 2; b[0] = 3; b[1] = 4; c[0] = 5;
c[1] = 6; printf("%i %in",b[i],a[i]); return 0; } Compile using "gcc -o
bla code.c -Wall" and check the code using "valgrind ./bla". Valgrind
finds nothing even though I index a[-1] and b[-1] - not good...
>>
That's because you're using the wrong tool. You're using the default
memcheck tool, which will tell you about any use of uninitialised data,
but because you are dealing with stack variables you have no gaps
between your variables (with heap memory valgrind would ensure there was
a gap) so an out of bounds access is likely to just access an adjacent
(and defined) variable. This should all be explained in the manual,
which you probably want to read to understand what memcheck will and
won't find. The tool that might find this problem is exp-ptrcheck (in
the released version) or exp-sgcheck (in the svn code). That
specifically looks for out of bounds access to stack variables by using
the debug information to discover the bounds. Tom
>
> He he - I knew
perfectly well, that I could not find this problem with the memcheck
tool, but wanted your input on how to cope with those problems.
>
>
Actually in the future I would wish that memcheck could be extended so
it could catch it - even if it would cost compile-time changes.
>
>
Valgrind is a great tool, but its user-value would increase quite a bit,
if it could catch a bit more (e.g. like my example).
>
> Would it
possible to make this?
>
> I will check exp-sgcheck in a sec - thanx
Tom!
I cannot see exp-sgcheck catching anything (compiled from the SVN
version) :/
Best
/pto
--
Peter Toft, PhD
http://petertoft.dk
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