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 latter two obey the usual short-circuiting rules of standard C.
definedoperator, which lets you check whether macros are defined in the middle of an ‘ #if ’.
#if MACROinstead 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,
operators are not recognized in ‘
’, and neither are
constants. They will be taken as identifiers which are not macros, and replaced by zero. In the case of
, 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.