4.153.48 size_t strnlen_P
size_t strnlen_P(const char *, size_t)
The strnlen_P() function is similar to strnlen(), except that src is a pointer to a string in program space.
Remember:
The strnlen_P function returns strlen_P(src), if that is less than len, or len if there is no '\0' character among the first len characters pointed to by src.