How to prevent buffer overflow when we recursively ask for input if input is not valid

Have to take a input string of size 5. Also have to check for validity in some unrelated sense. If the input is not valid user is prompted to input again.

Using the scanf("%4s", input); But if the user inputs suppose string of 8 characters, the scanf just scans the last 4 chars still in the buffer before asking for actual user input.

I tried making a buffer variable that stores the input before passing it to the target var.

How to prevent buffer overflow when we recursively ask for input if input is not valid?

1. When seeking an input of up to n characters, (which is n + 1 characters when one counts the Enter or '\n'), read at least a line of at least n + 1 characters. With fgets(), that needs to fit in a buffer of size n + 2. The extra 1 is to store the appended null character.

#define STRING_LENGTH_MAX 5

char buf[STRING_LENGTH_MAX + 2];
for (;;) {
  if (fgets(buf, sizeof buf, stdin) == NULL) {
    // There is no more input from stdin.
    // TBD code - perhaps quit?
  }
  ...

2. Search the string saved for a '\n'. If found, lop it off and continue. If not found and line is long, read the rest of the line. Discard all and loop back (not recurse) to step 1.

  buf[strcspn(buf, "\n")] == '\0'; // Lop off a potential '\n'.
  if (strlen(buf) <= STRING_LENGTH_MAX) {
    // Perform other validation tests on `buf`.
    // Otherwise ...
    break;
  }
  // Read and toss rest of line.
  int ch;
  while ((ch = getchar()) != '\n' && ch != EOF) {
    ;
  }
}

3. Consider putting this all into a helper function.