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diff --git a/doc/user/event.t b/doc/user/event.t new file mode 100644 index 0000000000..13d5af980f --- /dev/null +++ b/doc/user/event.t @@ -0,0 +1,343 @@ +@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. + +@c +@c +@c +@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 +@c +@c +@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. |