Handling error in c

It is used to display the error no 

#include <stdio.h>
#include <errno.h>

extern int errno;

int main () {
 FILE * fp;
 fp = fopen ("filedoesnotexist.txt", "rb");
 if (fp == NULL) {
  fprintf(stderr, "Value of errno: %d\n", errno);
 } else {
  fclose (fp);
 }
 return 0;
} 

 To display error message use 

strerror(errno) in printf

or

perror("message to be displayd before error");