Introduction to C Preprocessor

The Preprocessor used in C is one of the important step which is used in the compilation process but it is not a part of the compiler. In simple terms, a preprocessor is system software or a program which process the source code before the compilation step.

C Program Compilation:

A C Preprocessor performs processing of source code or high-level language (HLL).
The first step in the language processing system is preprocessing. This language processing system converts the high-level language into a language that is easily understood by the machine, i.e., machine-level language.

The intermediate steps involved between writing a C program and it’s execution are shown in the figure above.
A preprocessor is primarily used for performing three tasks on high level language(HLL) code given below:
Expanding macros: The preprocessor replaces occurrences of macro names with their corresponding definitions.
Including files: The preprocessor inserts the contents of specified files into the source code at the point where the #include directive appears.
Conditional compilation: The preprocessor can include or exclude parts of the source code based on specified conditions, using the #if, #ifdef, and #ifndef directives.
Replacing constants: The preprocessor replaces occurrences of constants with their corresponding values, using the #define directive.

C Preprocessor Directives

All preprocessing directives begin with a hash symbol(#).

Some of the commonly used preprocessor directives are listed below:

Directive	Description
#define	This directive substitutes a preprocessor macro.
#include	This directive inserts a particular header from another file.
#undef	This directive undefines a preprocessor macro.
#ifdef	This directive returns true if this macro is defined.
#ifndef	This directive returns true if this macro is not defined.

Example

#include <stdio.h>
#define PI 3.1415

 int main () 
{
  
float r, a;
  
printf ("Enter the radius: ");
  
scanf ("%f", &r);
  
    // Notice, the use of PI
    a = PI * r * r;
  
 
printf ("Area=%.2f", a);
  
return 0;
}