HP C/iX Reference Manual (31506-90011)

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108 Chapter7

Preprocessing Directives

Conditional Compilation

#ifdef max

#ifndef min

The #if preprocessing directive has the form:

#if

constant-expression

Use #if to test an expression. The compiler evaluates the expression in the directive. If it

is true (a nonzero value), the code following the directive is included. If the expression

evaluates to false (a zero value), the compiler ignores the code up to the next #else,

#endif, or #elif directive.

All macro identiﬁers that appear in the

constant-expression

are replaced by their

current replacement lists before the expression is evaluated. All defined expressions are

replaced with either 1 or 0 depending on their operands.

Whichever directive you use to begin the condition (#if, #ifdef, or #ifndef), you must

use #endif to end the if-section.

The following preprocessing directives are used to test for a deﬁnition:

#ifdef

identifier

#ifndef

identifier

They behave like the #if directive but are considered true if the

identifier

was

previously deﬁned using a #define directive.

You can nest these constructions. Delimit portions of the source program using conditional

directives at the same level of nesting, or with a -D option on the command line.

Use the #else directive to specify an alternative section of code to be compiled if the #if,

#ifdef, or #ifndef conditions fail. The code after the #else directive is compiled if the

code following any of the if directives does not compile.

The #elif constant-expression directive tests whether a condition of the previous #if,

#ifdef,or#ifndef was false. #elif is syntactically the same as the #if directive and can

be used in place of an #else directive.

Examples

The following are examples of valid combinations of these conditional compilation

directives:

#ifdef SWITCH

/* compiled if SWITCH is defined */

#else

/* compiled if SWITCH is undefined */

#endif /* end of if */

#if defined(THING)

/* compiled if THING is defined */

#endif /* end of if */

#if A>47

/* compiled if A evaluates > 47 */

#else

#ifA<20