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The #if Directive

The #if directive in its simplest form consists of the following statement.

    #if expression
    controlled text
    #endif /* expression */

The comment following #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 the following.

- Integer constants, which are all regarded as long or unsigned long.
- Character constants, which are interpreted according to the char-acter 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, which, in the expression, are expanded before actual computation of the expression’s value begins.

Note:
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.