Passing file descriptors via UNIX domain sockets

[ken lester]

I am reading "The Design and Implementation of the 4.3BSD UNIX Operating 
System", by Leffler, McKusick, Karels, and Quarterman and on page 300 they
claim that file descriptors can passed between processes using UNIX domain
sockets.  I didn't know this was possble.  Can anyone tell me how this is
done?  Any help would be appreciated.  I will try to summarize mail responses
for the benefit of others who are interested in this topic.
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