reading and writing to another process

[Mark Horton]

It's worth pointing that the pipeline method posted by Dave
Sherman, while the ONLY portable way to do it, only works if
the two processes in question have a common ancestor and knew
they wanted a pipeline before they separated into two processes.
So it works fine for the shell, or for applications that want to
start out as a single command and break into processes.  But it
does not work for arbitrary processes to talk to each other, as
with a user/server model.

There are many methods to implement arbitrary IPC.  None of them
will work in every version of UNIX.  In fact, very few of them
will work in more than one version of UNIX.  These include (but
are not limited to)
	Named pipes		System III and V
	Messages		System V
	Sockets			4.2BSD
	Semaphores		various, all incompatible.