Message passing

Operating-system processes communicate by sending and receiving messages. A message-passing mutual exclusion server can centralize ownership of a shared resource: clients request permission, wait for a grant, and release the resource when finished. Correctness depends on an explicit delivery and failure model; reliable delivery and compliant clients suffice for the basic ownership protocol.

Delay to otherwise serviceable work because a preceding operation cannot complete. A timer service using serial synchronous message passing can block on one unready subscriber before delivering notifications to other ready subscribers. Buffered delivery moves the issue to queue capacity and service rate rather than guaranteeing its absence.

A message passing discipline in which a send completes only when a matching receive accepts the message. This rendezvous transfers both data and control dependencies: an unready recipient can block a service and thereby delay its other clients. A reply is a separate send unless the interface supplies synchronous send-and-receive.

A system call that sends a request and commits the caller to receiving the matching reply as one coordinated operation, often named sendrec. It avoids the gap in which a client has completed a send but has not yet entered a receive. It does not make the service nonblocking or eliminate deadlock caused by other circular dependencies.