[[license text="""
Copyright © 2007 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.2 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
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"""]]

Mach ports are [[capabilities|capability]].

A Mach port is a kernel queue.  Each port has associated with
it a receive right and one or more send and send-once rights.
A queue can hold a number of messages.  Once the queue is full,
the send blocks until their is space to enqueue the message
(this is interruptible via a timeout mechanism).

A receive right designates a queue and authorizes the holder to
dequeue messages from the queue, and to create send and send-once
rights.

Send and send-once rights designate a queue and authorize the
hold to enqueue messages (in the case of a send-once right,
a single message).  Enqueuing a message is equivalent to
[[invoke|invoking]] a capability.

Send and receive rights are named using local names.  Each
task has associated with it a port [[address_space]].  A ports
are addressed via this table.  Each task thus has its own
private [[naming_context]] for ports.

Ports can be [[delegate]]d in an [[IPC]] message.  When the
receiver dequeues the message, the right is made available
to it.

A [[thread]] can only block receiving on a single port.  To work
around this, the concept of a port set was introduced.  A receive
right can be added to (at most) one port set.  When a thread
receives from a port set, it dequeues from any of the ports that
has a message available.