While developing the logger for GNU/Linux Anywhere USB Controller (https://savannah.nongnu.org/projects/awusb/), I needed to know the length of the UDP packet to be read so that I could do `malloc()' with the appropriate size. Aproposing for `datagram length' and googling for `how to get the length of a UDP datagram' didn't return any result. But, I found the answer when I tried `man 7 udp'.