path: root/doc/user/event.t

@c
@c  COPYRIGHT (c) 1988-2002.
@c  On-Line Applications Research Corporation (OAR).
@c  All rights reserved.
@c
@c  $Id$
@c

@chapter Event Manager

@cindex events

@section Introduction

The event manager provides a high performance method
of intertask communication and synchronization.  The directives
provided by the event manager are:

@itemize @bullet
@item @code{@value{DIRPREFIX}event_send} - Send event set to a task
@item @code{@value{DIRPREFIX}event_receive} - Receive event condition
@end itemize

@section Background

@subsection Event Sets

@cindex event flag, definition
@cindex event set, definition
@findex rtems_event_set

An event flag is used by a task (or ISR) to inform
another task of the occurrence of a significant situation.
Thirty-two event flags are associated with each task.  A
collection of one or more event flags is referred to as an event
set.  The data type @code{@value{DIRPREFIX}event_set} is used to manage
event sets.

The application developer should remember the following
key characteristics of event operations when utilizing the event
manager:

@itemize @bullet
@item Events provide a simple synchronization facility.

@item Events are aimed at tasks.

@item Tasks can wait on more than one event simultaneously.

@item Events are independent of one another.

@item Events do not hold or transport data.

@item Events are not queued.  In other words, if an event is
sent more than once to a task before being received, the second and
subsequent send operations to that same task have no effect.  
@end itemize

An event set is posted when it is directed (or sent) to a task.  A
pending event is an event that has been posted but not received.  An event
condition is used to specify the event set which the task desires to receive
and the algorithm which will be used to determine when the request is
satisfied. An event condition is satisfied based upon one of two
algorithms which are selected by the user.  The
@code{@value{RPREFIX}EVENT_ANY} algorithm states that an event condition
is satisfied when at least a single requested event is posted.  The
@code{@value{RPREFIX}EVENT_ALL} algorithm states that an event condition
is satisfied when every requested event is posted.

@subsection Building an Event Set or Condition

@cindex event condition, building
@cindex event set, building

An event set or condition is built by a bitwise OR of
the desired events.  The set of valid events is @code{@value{RPREFIX}EVENT_0} through
@code{@value{RPREFIX}EVENT_31}.  If an event is not explicitly specified in the set or
condition, then it is not present.  Events are specifically
designed to be mutually exclusive, therefore bitwise OR and
addition operations are equivalent as long as each event appears
exactly once in the event set list.

For example, when sending the event set consisting of
@code{@value{RPREFIX}EVENT_6}, @code{@value{RPREFIX}EVENT_15}, and @code{@value{RPREFIX}EVENT_31},
the event parameter to the @code{@value{DIRPREFIX}event_send}
directive should be @code{@value{RPREFIX}EVENT_6 @value{OR}
@value{RPREFIX}EVENT_15 @value{OR} @value{RPREFIX}EVENT_31}.

@subsection Building an EVENT_RECEIVE Option Set

In general, an option is built by a bitwise OR of the
desired option components.  The set of valid options for the
@code{@value{DIRPREFIX}event_receive} directive are listed
in the following table:

@itemize @bullet
@item @code{@value{RPREFIX}WAIT} - task will wait for event (default)
@item @code{@value{RPREFIX}NO_WAIT} - task should not wait
@item @code{@value{RPREFIX}EVENT_ALL} - return after all events (default)
@item @code{@value{RPREFIX}EVENT_ANY} - return after any events
@end itemize

Option values are specifically designed to be
mutually exclusive, therefore bitwise OR and addition operations
are equivalent as long as each option appears exactly once in
the component list.  An option listed as a default is not
required to appear in the option list, although it is a good
programming practice to specify default options.  If all
defaults are desired, the option @code{@value{RPREFIX}DEFAULT_OPTIONS} should be
specified on this call.

This example demonstrates the option parameter needed
to poll for all events in a particular event condition to
arrive.  The option parameter passed to the
@code{@value{DIRPREFIX}event_receive} directive should be either
@code{@value{RPREFIX}EVENT_ALL @value{OR} @value{RPREFIX}NO_WAIT}
or @code{@value{RPREFIX}NO_WAIT}.  The option parameter can be set to
@code{@value{RPREFIX}NO_WAIT} because @code{@value{RPREFIX}EVENT_ALL} is the
default condition for @code{@value{DIRPREFIX}event_receive}.

@section Operations

@subsection Sending an Event Set

The @code{@value{DIRPREFIX}event_send} directive allows a task (or an ISR) to
direct an event set to a target task.  Based upon the state of
the target task, one of the following situations applies:

@itemize @bullet
@item Target Task is Blocked Waiting for Events

@itemize -

@item If the waiting task's input event condition is
satisfied, then the task is made ready for execution.

@item If the waiting task's input event condition is not
satisfied, then the event set is posted but left pending and the
task remains blocked.

@end itemize

@item Target Task is Not Waiting for Events

@itemize -
@item The event set is posted and left pending.
@end itemize

@end itemize

@subsection Receiving an Event Set

The @code{@value{DIRPREFIX}event_receive} directive is used by tasks to
accept a specific input event condition.  The task also
specifies whether the request is satisfied when all requested
events are available or any single requested event is available.
If the requested event condition is satisfied by pending
events, then a successful return code and the satisfying event
set are returned immediately.  If the condition is not
satisfied, then one of the following situations applies:

@itemize @bullet
@item By default, the calling task will wait forever for the
event condition to be satisfied.

@item Specifying the @code{@value{RPREFIX}NO_WAIT} option forces an immediate return
with an error status code.

@item Specifying a timeout limits the period the task will
wait before returning with an error status code.
@end itemize

@subsection Determining the Pending Event Set

A task can determine the pending event set by calling
the @code{@value{DIRPREFIX}event_receive} directive with a value of
@code{@value{RPREFIX}PENDING_EVENTS} for the input event condition.  
The pending events are returned to the calling task but the event
set is left unaltered.

@subsection Receiving all Pending Events

A task can receive all of the currently pending
events by calling the @code{@value{DIRPREFIX}event_receive}
directive with a value of @code{@value{RPREFIX}ALL_EVENTS}
for the input event condition and
@code{@value{RPREFIX}NO_WAIT @value{OR} @value{RPREFIX}EVENT_ANY}
for the option set.  The pending events are returned to the
calling task and the event set is cleared.  If no events are
pending then the @code{@value{RPREFIX}UNSATISFIED} status code will be returned.

@section Directives

This section details the event manager's directives.
A subsection is dedicated to each of this manager's directives
and describes the calling sequence, related constants, usage,
and status codes.

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@page
@subsection EVENT_SEND - Send event set to a task

@cindex send event set to a task

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_event_send
@example
rtems_status_code rtems_event_send (
  rtems_id         id,
  rtems_event_set  event_in
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Event_Send (
   ID       : in     RTEMS.ID;
   Event_In : in     RTEMS.Event_Set;
   Result   :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - event set sent successfully@*
@code{@value{RPREFIX}INVALID_ID} - invalid task id

@subheading DESCRIPTION:

This directive sends an event set, event_in, to the
task specified by id.  If a blocked task's input event condition
is satisfied by this directive, then it will be made ready.  If
its input event condition is not satisfied, then the events
satisfied are updated and the events not satisfied are left
pending.  If the task specified by id is not blocked waiting for
events, then the events sent are left pending.

@subheading NOTES:

Specifying @code{@value{RPREFIX}SELF} for id results in the event set being
sent to the calling task.

Identical events sent to a task are not queued.  In
other words, the second, and subsequent, posting of an event to
a task before it can perform an @code{@value{DIRPREFIX}event_receive}
has no effect.

The calling task will be preempted if it has
preemption enabled and a higher priority task is unblocked as
the result of this directive.

Sending an event set to a global task which does not
reside on the local node will generate a request telling the
remote node to send the event set to the appropriate task.

@c
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@page
@subsection EVENT_RECEIVE - Receive event condition

@cindex receive event condition

@subheading CALLING SEQUENCE:

@ifset is-C
@findex rtems_event_receive
@example 
rtems_status_code rtems_event_receive (
  rtems_event_set  event_in,
  rtems_option     option_set,
  rtems_interval   ticks,
  rtems_event_set *event_out
);
@end example
@end ifset

@ifset is-Ada
@example
procedure Event_Receive (
   Event_In   : in     RTEMS.Event_Set;
   Option_Set : in     RTEMS.Option;
   Ticks      : in     RTEMS.Interval;
   Event_Out  :    out RTEMS.Event_Set;
   Result     :    out RTEMS.Status_Codes
);
@end example
@end ifset

@subheading DIRECTIVE STATUS CODES:
@code{@value{RPREFIX}SUCCESSFUL} - event received successfully@*
@code{@value{RPREFIX}UNSATISFIED} - input event not satisfied (@code{@value{RPREFIX}NO_WAIT})@*
@code{@value{RPREFIX}INVALID_ADDRESS} - @code{event_out} is NULL@*
@code{@value{RPREFIX}TIMEOUT} - timed out waiting for event

@subheading DESCRIPTION:

This directive attempts to receive the event
condition specified in event_in.  If event_in is set to
@code{@value{RPREFIX}PENDING_EVENTS}, then the current pending events are returned in
event_out and left pending.  The @code{@value{RPREFIX}WAIT} and @code{@value{RPREFIX}NO_WAIT} options in the
option_set parameter are used to specify whether or not the task
is willing to wait for the event condition to be satisfied.
@code{@value{RPREFIX}EVENT_ANY} and @code{@value{RPREFIX}EVENT_ALL} are used in the option_set parameter are
used to specify whether a single event or the complete event set
is necessary to satisfy the event condition.  The event_out
parameter is returned to the calling task with the value that
corresponds to the events in event_in that were satisfied.

If pending events satisfy the event condition, then
event_out is set to the satisfied events and the pending events
in the event condition are cleared.  If the event condition is
not satisfied and @code{@value{RPREFIX}NO_WAIT} is specified, then event_out is set to
the currently satisfied events.  If the calling task chooses to
wait, then it will block waiting for the event condition.

If the calling task must wait for the event condition
to be satisfied, then the timeout parameter is used to specify
the maximum interval to wait.  If it is set to @code{@value{RPREFIX}NO_TIMEOUT}, then
the calling task will wait forever.

@subheading NOTES:

This directive only affects the events specified in
event_in.  Any pending events that do not correspond to any of
the events specified in event_in will be left pending.

The following event receive option constants are defined by
RTEMS:

@itemize @bullet
@item @code{@value{RPREFIX}WAIT} 	task will wait for event (default)
@item @code{@value{RPREFIX}NO_WAIT} 	task should not wait
@item @code{@value{RPREFIX}EVENT_ALL} 	return after all events (default)
@item @code{@value{RPREFIX}EVENT_ANY} 	return after any events
@end itemize

A clock tick is required to support the functionality of this directive.