Rendezvous (Plan 9)

Rendezvous is a data synchronization mechanism in Plan 9 from Bell Labs. It is a system call that allows two processes to exchange a single datum while synchronizing. 
The rendezvous call takes a "tag" and a "value" as its arguments. The tag is typically an address in memory shared by both processes. Calling rendezvous causes a process to sleep until a second rendezvous call with a matching tag occurs. Then, the values are exchanged and both processes are awakened.
More complex synchronization mechanisms can be created from this primitive operation. See also mutual exclusion.