The #if and #elif directives

The #if and #elif directives compare the value of constant_expression to zero:

#if and #elif directive syntax
 
                                     .----------------.
                                     V                |
>>-#--+-if---+--constant_expression----token_sequence-+--------><
      '-elif-'
 

If the constant expression evaluates to a nonzero value, the lines of code that immediately follow the condition are passed on to the compiler.

If the expression evaluates to zero and the conditional compilation directive contains a preprocessor #elif directive, the source text located between the #elif and the next #elif or preprocessor #else directive is selected by the preprocessor to be passed on to the compiler. The #elif directive cannot appear after the preprocessor #else directive.

All macros are expanded, any defined() expressions are processed and all remaining identifiers are replaced with the token 0.

The constant_expression that is tested must be integer constant expressions with the following properties:

• No casts are performed.
• Arithmetic is performed using long int values.
• The constant_expression can contain defined macros. No other identifiers can appear in the expression.
• The constant_expression can contain the unary operator defined. This operator can be used only with the preprocessor keyword #if or #elif. The following expressions evaluate to 1 if the identifier is defined in the preprocessor, otherwise to 0:

  defined identifier
  defined(identifier)

  For example:

  #if defined(TEST1) || defined(TEST2)

#if TEST >= 1
   printf("i = %d\n", i);
   printf("array[i] = %d\n", array[i]);
#elif TEST < 0
   printf("array subscript out of bounds \n");
#endif