26.6.2.2 The #if directive

The #if directive allows you to test the value of an integer 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 so its value can be computed at compile time. 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 requests warnings for any identifier in an #if that is not defined as a macro.

Preprocessing does not know anything about the data types of C. Therefore, sizeof operators are not recognized in #if; sizeof is simply an identifier, and if it is not a macro, it stands for zero. This is likely to make the expression invalid. Preprocessing does not recognize enum constants; they too are simply identifiers, so if they are not macros, they stand for zero.

Preprocessing calculates the value of expression, and carries out all calculations in the widest integer type known to the compiler; on most machines supported by GNU C 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 compiled; otherwise it is skipped.