#include in c

The #include preprocessor directive is used to paste code of given file into current file. It is used include system-defined and user-defined header files. If included file is not found, compiler renders error.

By the use of #include directive, we provide information to the preprocessor where to look for the header files. There are two variants to use #include directive.

  1. #include <filename>
  2. #include “filename”

The #include <filename> tells the compiler to look for the directory where system header files are held. In UNIX, it is \usr\include directory.

The #include “filename” tells the compiler to look in the current directory from where program is running.

#include directive example

Let’s see a simple example of #include directive. In this program, we are including stdio.h file because printf() function is defined in this file.

 
  • #include<stdio.h>  
  •  int main(){    
  •    printf(“Hello C”);      
  •    return 0;  
  •  }   

Output:

Hello C

#include notes:

Note 1: In #include directive, comments are not recognized. So in case of #include <a//b>, a//b is treated as filename.

Note 2: In #include directive, backslash is considered as normal text not escape sequence. So in case of #include <a\nb>, a\nb is treated as filename.

Note 3: You can use only comment after filename otherwise it will give error.

C #undef

The #undef preprocessor directive is used to undefine the constant or macro defined by #define.

Syntax:

 
  • #undef token  

Let’s see a simple example to define and undefine a constant.

 
  • #include <stdio.h>  
  • #define PI 3.14  
  • #undef PI  
  • main() {  
  •    printf(“%f”,PI);  
  • }  

Output:

Compile Time Error: 'PI' undeclared

The #undef directive is used to define the preprocessor constant to a limited scope so that you can declare constant again.

Let’s see an example where we are defining and undefining number variable. But before being undefined, it was used by square variable.

 
  • #include <stdio.h>  
  • #define number 15  
  • int square=number*number;  
  • #undef number  
  • main() {  
  •    printf(“%d”,square);  
  • }  

Output:

225