4.2.2 If

The ‘ #if ’ directive allows you to test the value of an arithmetic expression, rather than the mere existence of one macro. Its syntax is

     #if expression

     
     controlled text

     
     #endif /* expression
 */

expression is a C expression of integer type, subject to stringent restrictions. It may contain

• Integer constants.
• Character constants, which are interpreted as they would be in normal code.
• Arithmetic operators for addition, subtraction, multiplication, division, bitwise operations, shifts, comparisons, and logical operations (&& and || ). The latter two obey the usual short-circuiting rules of standard C.
• Macros. All macros in the expression are expanded before actual computation of the expression's value begins.
• Uses of the defined operator, which lets you check whether macros are defined in the middle of an ‘ #if ’.
• Identifiers that are not macros, which are all considered to be the number zero. This allows you to write #if MACRO instead of #ifdef MACRO , if you know that MACRO, when defined, will always have a nonzero value. Function-like macros used without their function call parentheses are also treated as zero.

  In some contexts this shortcut is undesirable. The -Wundef option causes GCC to warn whenever it encounters an identifier which is not a macro in an ‘ #if ’.

The preprocessor does not know anything about types in the language. Therefore, sizeof operators are not recognized in ‘ #if ’, and neither are enum constants. They will be taken as identifiers which are not macros, and replaced by zero. In the case of sizeof , this is likely to cause the expression to be invalid.

The preprocessor calculates the value of expression . It carries out all calculations in the widest integer type known to the compiler; on most machines supported by GCC this is 64 bits. This is not the same rule as the compiler uses to calculate the value of a constant expression, and may give different results in some cases. If the value comes out to be nonzero, the ‘ #if ’ succeeds and the controlled text is included; otherwise it is skipped.