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char buff[25];
char *end_ptr;
long sl;

if (fgets(buff, sizeof(buff), stdin) == NULL) {
  puts("EOF or read error\n");
} else {
  errno = 0;

  sl = strtol(buff, &end_ptr, 10);

  if (ERANGE == errno) {
    puts("number out of range\n");
  }
  else if (end_ptr == buff) {
    puts("not valid numeric input\n");
  }
  else if ('\n' != *end_ptr && '\0' != *end_ptr) {
    puts("extra characters on input line\n");
  }
}