The function strlen determines the length of characters in the string pointed to by sptr.
Arguments
The argument sptr points to a string,
which is an array of characters terminated by a null-character.
Return values
The function strlen returns the number of characters,
not including the terminating null-character,
in the string pointed to by sptr.
Usage
strlen is often used to determine the length of a string
prior to allocating space to hold a copy of the string.
In this case, remember to allocate one more byte than the size
returned by strlen, to hold the null-character.