A Note About Arithmetic Overflow

In Fig. 4.7, line 54

added each grade entered by the user to the total. Even this simple statement has a potential problem—adding the integers could result in a value that’s too large to store in an int variable. This is known as arithmetic overflow and causes undefined behavior, which can lead to unintended results (en.wikipedia.org/wiki/Integer_overflow#Security_ramifications). Figure 2.5’s addition program had the same issue in line 19, which calculated the sum of two int values entered by the user:

The maximum and minimum values that can be stored in an int variable are represented by the constants INT_MAX and INT_MIN, respectively, which are defined in the header <climits>. There are similar constants for the other integral types and for floating-point types. You can see your platform’s values for these constants by opening the headers <climits> and <cfloat> in a text editor (you can search your file system for these files).

It’s considered a good practice to ensure that before you perform arithmetic calculations like the ones in line 54 of Fig. 4.7 and line 19 of Fig. 2.5, they will not overflow. The code for doing this is shown on the CERT website www.securecoding.cert.org—just search for guideline “INT32-CPP.” The code uses the && (logical AND) and || (logical OR) operators, which are introduced in Chapter 5. In industrial-strength code, you should perform checks like these for all calculations.