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(cpp-295.info)#if Directive

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The `#if' Directive
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   The `#if' directive in its simplest form consists of

     #if EXPRESSION
     CONTROLLED TEXT
     #endif /* EXPRESSION */

   The comment following the `#endif' is not required, but it is a good
practice because it helps people match the `#endif' to the
corresponding `#if'.  Such comments should always be used, except in
short conditionals that are not nested.  In fact, you can put anything
at all after the `#endif' and it will be ignored by the GNU C
preprocessor, but only comments are acceptable in ANSI Standard C.

   EXPRESSION is a C expression of integer type, subject to stringent
restrictions.  It may contain

   * Integer constants, which are all regarded as `long' or `unsigned
     long'.

   * Character constants, which are interpreted according to the
     character set and conventions of the machine and operating system
     on which the preprocessor is running.  The GNU C preprocessor uses
     the C data type `char' for these character constants; therefore,
     whether some character codes are negative is determined by the C
     compiler used to compile the preprocessor.  If it treats `char' as
     signed, then character codes large enough to set the sign bit will
     be considered negative; otherwise, no character code is considered
     negative.

   * Arithmetic operators for addition, subtraction, multiplication,
     division, bitwise operations, shifts, comparisons, and logical
     operations (`&&' and `||').

   * Identifiers that are not macros, which are all treated as zero(!).

   * Macro calls.  All macro calls in the expression are expanded before
     actual computation of the expression's value begins.

   Note that `sizeof' operators and `enum'-type values are not allowed.
`enum'-type values, like all other identifiers that are not taken as
macro calls and expanded, are treated as zero.

   The CONTROLLED TEXT inside of a conditional can include
preprocessing directives.  Then the directives inside the conditional
are obeyed only if that branch of the conditional succeeds.  The text
can also contain other conditional groups.  However, the `#if' and
`#endif' directives must balance.