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Mach ports are [[capabilities]].

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 [[NamingContext]] 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.