How do I create a socket(?) ?

[tvz at zandtwerk.Princeton.EDU]

I want to create something that is like a file, called, .e.g, foo,
so that writing to foo is equivalent to piping to some other command.
I think this is a socket, but I've spent some time already RTFM, and
am now looking for help. Please e-mail me if you think this is boring
for everybody else. (BTW, I'm running BSD 4.3 compatible Mach on a
NeXT.)

Tim Van Zandt
tvz at princeton.edu