Next: , Previous: , Up: Conditional Syntax   [Contents][Index] 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

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.

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