EXP06-A. Operands to the sizeof operator should not contain side effects

The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. If the type of the operand is not a variable length array type the operand is not evaluated.

Providing an expression that appears to produce side effects may be misleading to programmers who are not aware that these expressions are not evaluated. As a result, programmers may make invalid assumptions about program state leading to errors and possible software vulnerabilities.

Non-Compliant Code Example

In this example, the variable a will still have a value 14 after b has been initialized.

int main(void) {
  int a = 14;
  int b = sizeof(a++);
  ...
  return 0;
}

The expression a++ is not evaluated. Consequently, side effects in the expression are not executed.

Implementation Specific Details

This example compiles cleanly under Microsoft Visual Studio 2005 Version 8.0, with the /W4 option.

Compliant Solution

In this compliant solution, the variable a is incremented.

int main(void) {
  int a = 14;
  int b = sizeof(a);
  a++;
  ...
  return 0;
}

Implementation Specific Details

This example compiles cleanly under Microsoft Visual Studio 2005 Version 8.0, with the /W4 option.

Priority: P3 Level: L3

Operands to the sizeof operator which contain side effects are unlikely to result in software vulnerabilties, but can also be easily remediated.

References

• ISO/IEC 9899-1999 Section 6.5.3.4 The sizeof operator