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What are the Pre-processor Commands in C language?
The preprocessor is a program that sends the source code before it passes through the compiler. It operates under preprocessor directives which begin with the symbol #.
Types
The three types of preprocessor commands are as follows −
Macro substitution directives.
File inclusion directives.
Compiler control directives.
Macro substitution directives
It replaces every occurrence of an identifier by a predefined string.
The syntax for defining a macro substitution directive is as follows −
# define identifier string
For example,
#define PI 3.1415 #define f(x) x *x #undef PI
Example
Following is the C program for the macro substitution directive −
#define wait getch( )
main ( ){
clrscr ( );
printf ("Hello");
wait ;
}Output
When the above program is executed, it produces the following result −
Hello
File inclusion directives
An external file containing functions (or) macro definitions can be included by using #include directive.
The syntax for file inclusion directive is as follows −
# include <filename> (or) #include "filename"
Example
Following is the C program for the file inclusion directive −
#include <stdio.h>
main ( ){
printf ("hello");
}Output
When the above program is executed, it produces the following result −
Hello
The definition of the function printf ( ) is present in <stdio.h>header file.
Compiler control directives
C preprocessor offers a feature known as a conditional compilation, which can be used to switch ON (or) OFF a particular line (or) group of lines in a program.
Example
Following is the C program for the compiler control directive −
#if, #else, #endif etc.
#define LINE 1
#include<stdio.h>
main ( ){
#ifdef LINE
printf ("this is line number one");
#else
printf("This is line number two");
#endif
}Output
When the above program is executed, it produces the following result −
This line number one
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