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Preprocessor in c programming

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• One of the most important features of C language is to offer preprocessor directives.
• The preprocessor directives are always preferably initialized at the beginning of the program before the main() function.
• It begins with a # (hash) symbol. It can be placed anywhere, but quite often it is declared at the beginning before the main() function or any particular function. In traditional C, # (hash) must begin at the first column.
• The C Preprocessor is not a part of the compiler, but is a separate step in the compilation process.
• In simple terms, a C Preprocessor is just a text substitution tool and it instructs the compiler to do required pre-processing before the actual compilation. We'll refer to the C Preprocessor as CPP.
• All Preprocessor commands beging with a hash symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column.

 

Directive	Description
#define	Substitutes a preprocessor macro.
#include	Inserts a particular header from another file.
#undef	Undefines a preprocessor macro.
#ifdef	Returns true if this macro is defined.
#ifndef	Returns true if this macro is not defined.
#if	Tests if a compile time condition is true.
#else	The alternative for #if.
#elif	#else and #if in one statement.
#endif	Ends preprocessor conditional.
#error	Prints error message on stderr.
#pragma	Issues special commands to the compiler, using a standardized method.

Example:

#include <stdio.h>
#define PI 3.14
int main()
{
    int radius;
    float area;
    printf("Enter the radius=");
    scanf("%d",& radius);
    area = PI*radius*radius;
    printf("Area =%f",area);
    getch();
    return 0;
}

Output:

Enter the rasius 5
 Area= 78.5

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