/usr/share/man/man3/strnlen.3avr.gz is in avr-libc 1:1.8.0+Atmel3.4.4-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/avr_string.3avr.gz
/usr/share/man/man3/avr_string.3avr.gz