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.

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++ );
  printf("a, b = %d, %d.\n", a, b);  /* prints a, b = 14, 4. */
  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.

Priority: P4 Level: L3

If the object really is constant, the compiler may have put it in ROM or write-protected memory. Trying to modify such an object may lead to a program crash. This could allow an attacker to mount a denial-of-service attack.

References

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