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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

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.