FILE *ifp, *ofp;
char *mode = "r";
char outputFilename[] = "out.list";

ifp = fopen("in.list", mode);

if (ifp == NULL) {
  fprintf(stderr, "Can't open input file in.list!\n");
  exit(1);
}

ofp = fopen(outputFilename, "w");

if (ofp == NULL) {
  fprintf(stderr, "Can't open output file %s!\n", outputFilename);
  exit(1);
}