Pour obtenir une clé
d'essai remplissez le formulaire ci-dessous
Demandez des tariffs
Nouvelle licence
Renouvellement de licence
--Sélectionnez la devise--
USD
EUR
RUB
* En cliquant sur ce bouton, vous acceptez notre politique de confidentialité

Free PVS-Studio license for Microsoft MVP specialists
To get the licence for your open-source project, please fill out this form
** En cliquant sur ce bouton, vous acceptez notre politique de confidentialité.

I am interested to try it on the platforms:
** En cliquant sur ce bouton, vous acceptez notre politique de confidentialité.

Votre message a été envoyé.

Nous vous répondrons à


Si vous n'avez toujours pas reçu de réponse, vérifiez votre dossier
Spam/Junk et cliquez sur le bouton "Not Spam".
De cette façon, vous ne manquerez la réponse de notre équipe.

>
>
>
Why A + B != A - (-B)

Why A + B != A - (-B)

21 Déc 2009
Author:

While developing Viva64 analyzer intended for detecting 64-bit errors, I sometimes encounter interesting ways of code behavior. I would like to show you one example that is not very interesting practically but might be helpful in understanding more complicated cases.

char *A = "123456789";
unsigned B = 1;
char *X = A + B; // X: "23456789"
char *Y = A - (-B); // Y: <Bad Ptr>

If we compile the 32-bit version of the code, the expressions "A + B" and "A - (-B)" will be equivalent. In the 32-bit code, the pointers X and Y point to the second item of the array A. To understand it better look at the Figure 1 showing the process of calculating "A - (-B)".

0039_Why_A_+_B_!=_A_-_(-B)/image1.png

But when we compile the 64-bit code, the expressions "A + B" and "A - (-B)" mean absolutely different things. The subexpression "-B" has an unsigned type and equals 0xFFFFFFFFu. And it is this value 0xFFFFFFFFu that is subtracted from the pointer (see Figure 2).

0039_Why_A_+_B_!=_A_-_(-B)/image2.png

The shown error leads to an access outside the array on a 64-bit system. Such errors might occur when working with negative indexes when 32-bit unsigned variables are used to store them. Here is an example:

unsigned Index = -1;
Array[Index] = Z;

Like in the previous case, the expression "Array[Index] = Z;" works well in the 32-bit program but leads to an error in the 64-bit one.

Conclusion:

You should avoid using unsigned data types to store negative values. If the variables used to access array items can take negative values, use only signed data types, for example "int". Or rather use the types size_t and ptrdfiff_t.

Comments (0)

Next comments
Unicorn with delicious cookie
Nous utilisons des cookies pour améliorer votre expérience de navigation. En savoir plus
Accepter