int main(int argc, char *argv[]) {
  FILE *fp1, *fp2;

  if(argc!=3) {
    printf("Usage: compare <file 1> <file 2>\n");
    exit(1);
  }

  if((fp1 = fopen(argv[1], "rb"))==NULL) {
    printf("Cannot open first file.\n");
    exit(1);
  }

  if((fp2 = fopen(argv [2], "rb"))==NULL) {
    printf("Cannot open second file.\n");
    exit(1);
  }
.....................
}