Another example, a little server that allows connections on a single port 443 for https and ssh. Ideally after reading the "GET" or ssh banner, it can just exec whichever server is needed (or fork and exec something like netcat). but in fact due to this "already read some data" problem, it has to stay alive and copy the data in and out from the other server.
It shouldn't be too difficult to write a device that allows file descriptors to be passed from one process to another.
The functionality is quite useful. BSD has supported this since the dawn of time (SCM_RIGHTS), and I have used it in a few commercial network server products over the years. (Later System Vs have it as well, and Solaris supports it through their "doors" API. Stevens Vol. 2 describes the various APIs.)
--lyndon
