cpp.info: If

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   The `#if' directive allows you to test the value of an arithmetic
expression, rather than the mere existence of one macro.  Its syntax is
     #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.
   If EXPRESSION is not correctly formed, GCC issues an error and
treats the conditional as having failed.