C #include

The #include preprocessor directive is used to paste the code of given file into the 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.

#include <filename>
#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 the program is running.

#include directive example

Let's see a simple example of the #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 the #include directive, the 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 the only comment after filename otherwise it will give the error.