size_t strnlen(const char *, size_t) __ATTR_PURE__
The strnlen function returns the number of characters in the string pointed to by src, not including the terminating '\0' character, but at most len. In doing this, strnlen looks only at the first len characters at src and never beyond src+len.
The strnlen function returns strlen(src), if that is less than len, or len if there is no '\0' character among the first len characters pointed to by src.