RE: [Valgrind-users] User error? - Valgrind 3 failing terribly compared to purify

[Yeshurun, M. <mei...@in...>]

The purpose of this little program was to do exactly that. My question
is why isn't the error suppressed by specifying =
--partial-loads-ok=3Dyes.

Meir

-----Original Message-----
From: Brian Crowder [mailto:cr...@fi...]=20
Sent: Monday, October 17, 2005 11:03 PM
To: Yeshurun, Meir
Cc: Tom Hughes; val...@li...
Subject: Re: [Valgrind-users] User error? - Valgrind 3 failing terribly
compared to purify


"XXX" is uninitialized memory.  Ints are assumed to be 4-byte:

a             =3D { '0', '1', '2', '3', '4', '5', 6', '7', '8', '9',   =
0,
XXX, XXX }
((int*)a + 9) =3D { ............................................RRR, =
RRR,
RRR, RRR }

the last two bytes being read in the int dereference are uninitialized
memory.


-- Brian

Yeshurun, Meir wrote:
> The following program generates an invalid read even when explicitly
> specifying --partial-loads-ok=3Dyes. Am I missing something here?
>=20
> #include <cstring>
> #include <iostream>
>=20
> using namespace std;
>=20
> int main()
> {
>     char *a =3D new char[11];
>     strcpy(a, "0123456789");
>     int b =3D *(int *)(a + 9);   =20
> }
>=20
>=20
> Thanks,
>=20
> Meir
>=20
> -----Original Message-----
> From: val...@li...
> [mailto:val...@li...] On Behalf Of Tom
> Hughes
> Sent: Monday, October 17, 2005 9:07 PM
> To: val...@li...
> Subject: RE: [Valgrind-users] User error? - Valgrind 3 failing
terribly
> compared to purify
>=20
> In message
>
<942...@ha...>
>           "Yeshurun, Meir" <mei...@in...> wrote:
>=20
>=20
>>There is one issue though: It looks like Valgrind reports partial
>=20
> loads
>=20
>>as errors by default. I think this shouldn't be the default behavior.
>=20
>=20
> Actually valgrind doesn't report any loads as errors - it only
> reports an error when you use an undefined value in a way that
> would effect the result of the program. In other words when a
> conditional jump depends on it or you use it as a pointer and
> read or write through that pointer.
>=20
> It tracks definedness at bit level, so a partial load will mark
> some bits as defined and leaves others alone. If you then later
> use one of the undefined bits it will complain.
>=20
> There are edge cases where it thinks a bit is used when it isn't
> really but they are rare.
>=20
> I think you need to explain what you mean more fully.
>=20
> Tom
>=20