Byte of word returns one less than what it should be

Question

My professor is having us make our own version of the functions mystrcat, mystrlen and mystrcopy. I have a problem where the word is returning the wrong amount of bytes. Or, rather, I think it is.

The phrase: "HELLO WORLD!" , if I am not wrong, should return 13 bytes? Like in:

const char char_literal[] =
   { 'H', 'E', 'L', 'L', 'O', ' ', 'W', 'O', 'R', 'L', 'D', '!', '\0' };
  //  1    2    3    4    5    6    7    8    9    10   11   12   13 

I wrote the function myStrLen() that returns ({ 'H', 'E', 'L', 'L', 'O', ' '}) as 6, but because it is alone, there should be a '\0' at the end right?
So should I be returning i + 1 or is 6 correct?

int myStrLen(char stringInput[]){
    for(int i = 0; ; i++){
        if(stringInput[i] == '\0'){
            return i;

Answer 1

If your professor tells you to write a function that behaves exactly as strlen, then it shall not count the string termination character:

std::size_t strlen( const char* str );

Returns the length of the given byte string, that is, the number of characters in a character array whose first element is pointed to by str up to and not including the first null character. ...

So returning i is correct, besides the fact that the "original" strlen takes a const input parameter and returns a size_t...