Example

       #include <stdio.h>
       #include <stdlib.h>

       main(argc, argv)
           int argc;
           char *argv[];
       {
           FILE *fp;

           fp = fopen(argv[1], "r");   /* Open an input file. */
           if (fp == NULL) {

               /* If the fopen call failed, perror prints out a        */
               /* diagnostic:                                          */
               /*                                                      */
               /*  "open: <error message>"                             */
               /*  This error message provides a diagnostic explaining */
               /*  the cause of the failure.                           */

               perror("open");
               exit(EXIT_FAILURE);
           }
           else
               fclose(fd) ;
       }