C Preprocessor with Examples

C Preprocessor is not a part of the compiler, but is a separate step in the compilation process. The C Preprocessor is just a text substitution tool. It is a micro processor. All preprocessor commands begin with a hash symbol (#). It must be the first nonblank character, and for readability, a preprocessor directive should begin in the first column is knows as C preprocessor.

A list of preprocessor directives.

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	ssues special commands to the compiler, using a standardized method.

Macros

A segment of code which is replace by the value of macro is knows as Macros. Macro is define by #define directive. 

There are two type of Macros

• Object-like Macros
  • The object-like macro is an identifier that is replace by value. It is widely use to represent numeric constants is knows as Object like Macros.
• Function-like Macros
  • The function-like macro looks Just like function call is knows as Function Like Macros.

Predefined Macros in C Programming Language.

ANSI C defines a number of macros. T he predefined macros should not be directly modified.

Macro	Description
_DATE_	represents current date in “MMM DD YYYY” format.
_TIME_	represents current time in “HH:MM:SS” format.
_FILE_	represents current file name.
_LINE_	represents current line number.
_STDC_	Defined as 1 when the compiler complies with the ANSI standard.

Example

Input value

#include <stdio.h>

int main() {

   printf("Date :%s\n", __DATE__ );
   printf("Time :%s\n", __TIME__ );
   printf("ANSI :%d\n", __STDC__ );
   printf("File :%s\n", __FILE__ );
   printf("Line :%d\n", __LINE__ );
   

}

Output value

#include

The #include preprocessor directive is use to paste code of given file into current file. It is use include system define and user define header files is knows as #include.

Example

Input value

#include<stdio.h>  
 int main(){    
   printf("basic Engineer");      
   return 0;  
 }   

Output value

#define

The #define preprocessor directive is use to define constant substitution is knows as #define.

Syntax

• #define token value

Example

Input value

#include <stdio.h>  
#define e 2.71828
main() {  
   printf("%f",e);  
}  

Output value

#underf

The #undef preprocessor directive is use to undefine the constant define by #define is knows as #underf.

Example

Input value

#include <stdio.h>  
#define number 12  
int square=number*number;  
#undef number  
main() {  
   printf("%d",square);  
}  

Output value

#ifdef

The #ifdef preprocessor directive checks if macro is define by #define. If yes, it executes the code otherwise #else code is execute is knows as #ifdef.

Syntax

• #ifdef MACRO  
• //code  
• #endif  

Example

Input value

#include <stdio.h>  
#include <conio.h>  
#define NOINPUT  
void main() {  
int s=0;  
#ifdef NOINPUT  
s=45;  
#else  
printf("Enter s:");  
scanf("%d", &s);  
#endif         
printf("Value of s: %d\n", s);  
getch();  
}  

Output value

#ifndef

The #ifndef preprocessor directive checks if macro is not define by #define. If yes, it executes the code otherwise #else code is execute is knows as #ifndef.

Syntax

• #ifndef MACRO  
• //code  
• #endif  

Example

Input value

#include <stdio.h>  
#include <conio.h>  
void main() {  
int x=0;  
#ifndef INPUT  
x=45;  
#else  
printf("Enter x:");  
scanf("%d", &x);  
#endif         
printf("Value of a: %d\n", x);  
getch();  
}  

Output value

 #pragma

The #pragma preprocessor directive is used to provide additional information to the compiler is know as #pragma

Syntax

#pragma token  

Example

Input value

#include<stdio.h>  
#include<conio.h>  
  
void func() ;  
  
#pragma startup func  
#pragma exit func  
  
void main(){  
printf("\nbasic engineer");  
getch();  
}  
  
void func(){  
printf("\nbasic engineer");  
getch();  
}  

Output value

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To know more about C Programming language please Wikipedia click here .

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