C Preprocessor

February 21, 2023

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 Preprocessor

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.
C Preprocessor working

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:

  1. Expanding macros: The preprocessor replaces occurrences of macro names with their corresponding definitions.
  2. Including files: The preprocessor inserts the contents of specified files into the source code at the point where the #include directive appears.
  3. Conditional compilation: The preprocessor can include or exclude parts of the source code based on specified conditions, using the #if, #ifdef, and #ifndef directives.
  4. Replacing constants: The preprocessor replaces occurrences of constants with their corresponding values, using the #define directive.
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C Preprocessor Directives

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

Some of the commonly used preprocessor directives are listed below:

DirectiveDescription
#defineThis directive substitutes a preprocessor macro.
#includeThis directive inserts a particular header from another file.
#undefThis directive undefines a preprocessor macro.
#ifdefThis directive returns true if this macro is defined.
#ifndefThis directive returns true if this macro is not defined.

Example

Run

#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;
}

Output

Enter the radius: 6
Area = 113.09

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