I run Ubuntu 18.04 through Oracle VirtualBox. In the terminal man commands for recvfrom, sendto, bind do not show any manual entries as shown below.
rishi@rishi-VirtualBox:~$ man recvfrom
No manual entry for recvfrom
rishi@rishi-VirtualBox:~$ man sendto
No manual entry for sendto
rishi@rishi-VirtualBox:~$ man bind
No manual entry for bind
/usr/share/man/man2/recv.2.gz
,/usr/share/man/man2/send.2.gz
and/usr/share/man/man2/bind.2.gz
. What's the value of your$MANPATH
?