#include

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
#include "myheader.h"

include a file into your c sourcecode

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
int main(void){
   printf("the answer to life the universe and everything is %d \n", 42 );
}

  user@host:~$ ./include 
  the answer to life the universe and everything is 42