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C Language Programming Library Reference Guide

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Preprocessing Directives -
#define, #undef, #ifdef, #ifndef
The preprocessing directives #define and #undef allow the definition of identifiers which hold a certain value. These identifiers can simply be constants or a macro function. The directives #ifdef and #ifndef allow conditional compiling of certain lines of code based on whether or not an identifier has been defined.

Syntax:
 

#define identifier replacement-code

#undef identifier

#ifdef identifier
#else or #elif
#endif

#ifndef identifier
#else or #elif
#endif

#ifdef identifier is the same is #if defined( identifier).
#ifndef identifier is the same as #if !defined(identifier).
An identifier defined with #define is available anywhere in the source code until a #undef is reached.
A function macro can be defined with #define in the following manner:

#define identifier(parameter-list) (replacement-text)

The values in the parameter-list are replaced in the replacement-text.

 
Examples: 
#define PI 3.141
printf("%f",PI);

#define DEBUG
#ifdef DEBUG
  printf("This is a debug message.");
#endif

#define QUICK(x) printf("%s\n",x);
QUICK("Hi!")

#define ADD(x, y) x + y
z=3 * ADD(5,6)
This evaluates to 21 due to the fact that multiplication takes precedence over addition. 
#define ADD(x,y) (x + y)
z=3 * ADD(5,6)
This evaluates to 33 due to the fact that the summation is encapsulated in parenthesis which takes precedence over multiplication.
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