Passing file descriptors through Unix-domain datagram sockets
This caused me no end of trouble, and Google wasn't much help:
The example consists of two files.
socketmaster.c opens a file you pass it and starts listening on a Unix-domain datagram socket.
socketslave.c sends a single-character request to socketmaster's datagram socket. Upon receipt of the request, socketmaster sends the file descriptor of the open file to socketslave's datagram socket. Socketslave reads a small amount of data from the file, displays it, and exits. Run socketslave again and you get the next portion of the file, proving that the file pointer is being shared between the (persistent) socketmaster and the (transient) socketslave.