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310 lines
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310 lines
11 KiB
ReStructuredText
.. SPDX-License-Identifier: CC-BY-SA-4.0
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.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR)
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.. index:: events
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Event Manager
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*************
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Introduction
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============
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The event manager provides a high performance method of intertask communication
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and synchronization. The directives provided by the event manager are:
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- rtems_event_send_ - Send event set to a task
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- rtems_event_receive_ - Receive event condition
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Background
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==========
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.. index:: event flag, definition
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.. index:: event set, definition
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.. index:: rtems_event_set
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Event Sets
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----------
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An event flag is used by a task (or ISR) to inform another task of the
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occurrence of a significant situation. Thirty-two event flags are associated
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with each task. A collection of one or more event flags is referred to as an
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event set. The data type ``rtems_event_set`` is used to manage event sets.
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The application developer should remember the following key characteristics of
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event operations when utilizing the event manager:
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- Events provide a simple synchronization facility.
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- Events are aimed at tasks.
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- Tasks can wait on more than one event simultaneously.
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- Events are independent of one another.
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- Events do not hold or transport data.
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- Events are not queued. In other words, if an event is sent more than once to
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a task before being received, the second and subsequent send operations to
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that same task have no effect.
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An event set is posted when it is directed (or sent) to a task. A pending
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event is an event that has been posted but not received. An event condition is
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used to specify the event set which the task desires to receive and the
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algorithm which will be used to determine when the request is satisfied. An
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event condition is satisfied based upon one of two algorithms which are
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selected by the user. The ``RTEMS_EVENT_ANY`` algorithm states that an event
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condition is satisfied when at least a single requested event is posted. The
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``RTEMS_EVENT_ALL`` algorithm states that an event condition is satisfied when
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every requested event is posted.
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.. index:: event condition, building
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.. index:: event set, building
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Building an Event Set or Condition
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----------------------------------
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An event set or condition is built by a bitwise OR of the desired events. The
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set of valid events is ``RTEMS_EVENT_0`` through ``RTEMS_EVENT_31``. If an
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event is not explicitly specified in the set or condition, then it is not
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present. Events are specifically designed to be mutually exclusive, therefore
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bitwise OR and addition operations are equivalent as long as each event appears
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exactly once in the event set list.
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For example, when sending the event set consisting of ``RTEMS_EVENT_6``,
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``RTEMS_EVENT_15``, and ``RTEMS_EVENT_31``, the event parameter to the
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``rtems_event_send`` directive should be ``RTEMS_EVENT_6 | RTEMS_EVENT_15 |
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RTEMS_EVENT_31``.
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Building an EVENT_RECEIVE Option Set
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------------------------------------
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In general, an option is built by a bitwise OR of the desired option
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components. The set of valid options for the ``rtems_event_receive`` directive
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are listed in the following table:
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.. list-table::
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:class: rtems-table
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* - ``RTEMS_WAIT``
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- task will wait for event (default)
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* - ``RTEMS_NO_WAIT``
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- task should not wait
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* - ``RTEMS_EVENT_ALL``
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- return after all events (default)
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* - ``RTEMS_EVENT_ANY``
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- return after any events
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Option values are specifically designed to be mutually exclusive, therefore
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bitwise OR and addition operations are equivalent as long as each option
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appears exactly once in the component list. An option listed as a default is
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not required to appear in the option list, although it is a good programming
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practice to specify default options. If all defaults are desired, the option
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``RTEMS_DEFAULT_OPTIONS`` should be specified on this call.
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This example demonstrates the option parameter needed to poll for all events in
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a particular event condition to arrive. The option parameter passed to the
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``rtems_event_receive`` directive should be either ``RTEMS_EVENT_ALL |
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RTEMS_NO_WAIT`` or ``RTEMS_NO_WAIT``. The option parameter can be set to
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``RTEMS_NO_WAIT`` because ``RTEMS_EVENT_ALL`` is the default condition for
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``rtems_event_receive``.
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Operations
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==========
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Sending an Event Set
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--------------------
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The ``rtems_event_send`` directive allows a task (or an ISR) to direct an event
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set to a target task. Based upon the state of the target task, one of the
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following situations applies:
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- Target Task is Blocked Waiting for Events
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- If the waiting task's input event condition is satisfied, then the task is
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made ready for execution.
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- If the waiting task's input event condition is not satisfied, then the
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event set is posted but left pending and the task remains blocked.
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- Target Task is Not Waiting for Events
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- The event set is posted and left pending.
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Receiving an Event Set
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----------------------
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The ``rtems_event_receive`` directive is used by tasks to accept a specific
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input event condition. The task also specifies whether the request is
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satisfied when all requested events are available or any single requested event
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is available. If the requested event condition is satisfied by pending events,
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then a successful return code and the satisfying event set are returned
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immediately. If the condition is not satisfied, then one of the following
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situations applies:
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- By default, the calling task will wait forever for the event condition to be
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satisfied.
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- Specifying the ``RTEMS_NO_WAIT`` option forces an immediate return with an
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error status code.
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- Specifying a timeout limits the period the task will wait before returning
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with an error status code.
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Determining the Pending Event Set
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---------------------------------
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A task can determine the pending event set by calling the
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``rtems_event_receive`` directive with a value of ``RTEMS_PENDING_EVENTS`` for
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the input event condition. The pending events are returned to the calling task
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but the event set is left unaltered.
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Receiving all Pending Events
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----------------------------
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A task can receive all of the currently pending events by calling the
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``rtems_event_receive`` directive with a value of ``RTEMS_ALL_EVENTS`` for the
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input event condition and ``RTEMS_NO_WAIT | RTEMS_EVENT_ANY`` for the option
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set. The pending events are returned to the calling task and the event set is
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cleared. If no events are pending then the ``RTEMS_UNSATISFIED`` status code
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will be returned.
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Directives
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==========
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This section details the event manager's directives. A subsection is dedicated
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to each of this manager's directives and describes the calling sequence,
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related constants, usage, and status codes.
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.. raw:: latex
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\clearpage
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.. index:: send event set to a task
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.. index:: rtems_event_send
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.. _rtems_event_send:
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EVENT_SEND - Send event set to a task
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-------------------------------------
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CALLING SEQUENCE:
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.. code-block:: c
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rtems_status_code rtems_event_send (
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rtems_id id,
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rtems_event_set event_in
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);
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DIRECTIVE STATUS CODES:
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.. list-table::
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:class: rtems-table
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* - ``RTEMS_SUCCESSFUL``
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- event set sent successfully
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* - ``RTEMS_INVALID_ID``
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- invalid task id
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DESCRIPTION:
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This directive sends an event set, event_in, to the task specified by id.
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If a blocked task's input event condition is satisfied by this directive,
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then it will be made ready. If its input event condition is not satisfied,
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then the events satisfied are updated and the events not satisfied are left
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pending. If the task specified by id is not blocked waiting for events,
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then the events sent are left pending.
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NOTES:
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Specifying ``RTEMS_SELF`` for id results in the event set being sent to the
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calling task.
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Identical events sent to a task are not queued. In other words, the
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second, and subsequent, posting of an event to a task before it can perform
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an ``rtems_event_receive`` has no effect.
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The calling task will be preempted if it has preemption enabled and a
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higher priority task is unblocked as the result of this directive.
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Sending an event set to a global task which does not reside on the local
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node will generate a request telling the remote node to send the event set
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to the appropriate task.
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.. raw:: latex
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\clearpage
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.. index:: receive event condition
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.. index:: rtems_event_receive
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.. _rtems_event_receive:
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EVENT_RECEIVE - Receive event condition
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---------------------------------------
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CALLING SEQUENCE:
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.. code-block:: c
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rtems_status_code rtems_event_receive (
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rtems_event_set event_in,
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rtems_option option_set,
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rtems_interval ticks,
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rtems_event_set *event_out
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);
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DIRECTIVE STATUS CODES:
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.. list-table::
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:class: rtems-table
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* - ``RTEMS_SUCCESSFUL``
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- event received successfully
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* - ``RTEMS_UNSATISFIED``
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- input event not satisfied (``RTEMS_NO_WAIT``)
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* - ``RTEMS_INVALID_ADDRESS``
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- ``event_out`` is NULL
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* - ``RTEMS_TIMEOUT``
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- timed out waiting for event
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DESCRIPTION:
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This directive attempts to receive the event condition specified in
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event_in. If event_in is set to ``RTEMS_PENDING_EVENTS``, then the current
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pending events are returned in event_out and left pending. The
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``RTEMS_WAIT`` and ``RTEMS_NO_WAIT`` options in the option_set parameter
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are used to specify whether or not the task is willing to wait for the
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event condition to be satisfied. ``RTEMS_EVENT_ANY`` and
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``RTEMS_EVENT_ALL`` are used in the option_set parameter are used to
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specify whether a single event or the complete event set is necessary to
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satisfy the event condition. The event_out parameter is returned to the
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calling task with the value that corresponds to the events in event_in that
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were satisfied.
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If pending events satisfy the event condition, then event_out is set to the
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satisfied events and the pending events in the event condition are cleared.
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If the event condition is not satisfied and ``RTEMS_NO_WAIT`` is specified,
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then event_out is set to the currently satisfied events. If the calling
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task chooses to wait, then it will block waiting for the event condition.
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If the calling task must wait for the event condition to be satisfied, then
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the timeout parameter is used to specify the maximum interval to wait. If
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it is set to ``RTEMS_NO_TIMEOUT``, then the calling task will wait forever.
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NOTES:
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This directive only affects the events specified in event_in. Any pending
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events that do not correspond to any of the events specified in event_in
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will be left pending.
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The following event receive option constants are defined by RTEMS:
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.. list-table::
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:class: rtems-table
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* - ``RTEMS_WAIT``
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- task will wait for event (default)
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* - ``RTEMS_NO_WAIT``
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- task should not wait
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* - ``RTEMS_EVENT_ALL``
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- return after all events (default)
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* - ``RTEMS_EVENT_ANY``
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- return after any events
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A clock tick is required to support the functionality of this directive.
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