strnlen - determine the length of a fixed-size string
size_t strnlen(const char *s, size_t maxlen);
The strnlen function
returns the number of characters in the string pointed to by s, not including
the terminating '\0' character, but at most maxlen. In doing this, strnlen
looks only at the first maxlen characters at s and never beyond s+maxlen.
The strnlen function returns strlen(s), if that is less than
maxlen, or maxlen if there is no '\0' character among the first maxlen characters
pointed to by s.
This function is a GNU extension.