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DDS::WaitSet Class Reference

<<interface>> Allows an application to wait until one or more of the attached DDS::Condition objects has a trigger_value of true or else until the timeout expires. More...

#include <managed_infrastructure.h>

Public Member Functions

void wait (ConditionSeq^ active_conditions, Duration_t timeout)
 Allows an application thread to wait for the occurrence of certain conditions.
 
void attach_condition (Condition^ cond)
 Attaches a DDS::Condition to the DDS::WaitSet.
 
void detach_condition (Condition^ cond)
 Detaches a DDS::Condition from the DDS::WaitSet.
 
void get_conditions (ConditionSeq^ attached_conditions)
 Retrieves the list of attached DDS::Condition (s).
 
void set_property (WaitSetProperty_t prop)
 <<extension>> Sets the DDS::WaitSetProperty_t, to configure the associated DDS::WaitSet to return after one or more trigger events have occurred.
 
void get_property (WaitSetProperty_t% prop)
 <<extension>> Retrieves the DDS::WaitSetProperty_t configuration of the associated DDS::WaitSet.
 
virtual ~WaitSet ()
 Destructor.
 
 WaitSet ()
 Default no-argument constructor.
 
 WaitSet (WaitSetProperty_t% prop)
 <<extension>> Constructor for a DDS::WaitSet that may delay for more while specifying that will be woken up after the given number of events or delay period, whichever happens first
 

Detailed Description

<<interface>> Allows an application to wait until one or more of the attached DDS::Condition objects has a trigger_value of true or else until the timeout expires.

Usage

DDS::Condition (s) (in conjunction with wait-sets) provide an alternative mechanism to allow the middleware to communicate communication status changes (including arrival of data) to the application.

DDSWaitSetConditions.png
::DDS::WaitSet and ::DDS::Condition (s)

This mechanism is wait-based. Its general use pattern is as follows:

Usually the first step is done in an initialization phase, while the others are put in the application main loop.

As there is no extra information passed from the middleware to the application when a wait returns (only the list of triggered DDS::Condition objects), DDS::Condition objects are meant to embed all that is needed to react properly when enabled. In particular, DDS::Entity-related conditions are related to exactly one DDS::Entity and cannot be shared.

The blocking behavior of the DDS::WaitSet is illustrated below.

DDSWaitSetBlocking.png
::DDS::WaitSet blocking behavior

The result of a DDS::WaitSet::wait operation depends on the state of the DDS::WaitSet, which in turn depends on whether at least one attached DDS::Condition has a trigger_value of true. If the wait operation is called on DDS::WaitSet with state BLOCKED, it will block the calling thread. If wait is called on a DDS::WaitSet with state UNBLOCKED, it will return immediately. In addition, when the DDS::WaitSet transitions from BLOCKED to UNBLOCKED it wakes up any threads that had called wait on it.

A key aspect of the Condition and WaitSet mechanism is the setting of the trigger_value of each DDS::Condition.

Trigger State of a ::DDS::StatusCondition

The trigger_value of a DDS::StatusCondition is the boolean OR of the ChangedStatusFlag of all the communication statuses (see Status Kinds) to which it is sensitive. That is, trigger_value == false only if all the values of the ChangedStatusFlags are false.

The sensitivity of the DDS::StatusCondition to a particular communication status is controlled by the list of enabled_statuses set on the condition by means of the DDS::StatusCondition::set_enabled_statuses operation.

Once the trigger_value of a StatusCondition becomes true, it remains true until the status that changed is reset. To reset a status, call the related get_*_status() operation. Or, in the case of the data available status, call read(), take(), or one of their variants. Therefore, if you are using a DDS::StatusCondition on a DDS::WaitSet to be notified of events, your thread will wake up when one of the statuses associated with the StatusCondition becomes true. If you do not reset the status, the StatusCondition trigger_value remains true and your WaitSet will not block again; it will immediately wake up when you call DDS::WaitSet::wait.

Trigger State of a ::DDS::ReadCondition

Similar to the DDS::StatusCondition, a DDS::ReadCondition also has a trigger_value that determines whether the attached DDS::WaitSet is BLOCKED or UNBLOCKED. However, unlike the DDS::StatusCondition, the trigger_value of the DDS::ReadCondition is tied to the presence of at least a sample managed by RTI Connext with DDS::SampleStateKind and DDS::ViewStateKind matching those of the DDS::ReadCondition. Furthermore, for the DDS::QueryCondition to have a trigger_value == true, the data associated with the sample must be such that the query_expression evaluates to true.

The fact that the trigger_value of a DDS::ReadCondition depends on the presence of samples on the associated DDS::DataReader implies that a single take operation can potentially change the trigger_value of several DDS::ReadCondition or DDS::QueryCondition conditions. For example, if all samples are taken, any DDS::ReadCondition and DDS::QueryCondition conditions associated with the DDS::DataReader that had their trigger_value==TRUE before will see the trigger_value change to FALSE. Note that this does not guarantee that DDS::WaitSet objects that were separately attached to those conditions will not be woken up. Once we have trigger_value==TRUE on a condition, it may wake up the attached DDS::WaitSet, the condition transitioning to trigger_value==FALSE does not necessarily 'unwakeup' the WaitSet as 'unwakening' may not be possible in general.

The consequence is that an application blocked on a DDS::WaitSet may return from the wait with a list of conditions, some of which are not no longer 'active'. This is unavoidable if multiple threads are concurrently waiting on separate DDS::WaitSet objects and taking data associated with the same DDS::DataReader entity.

To elaborate further, consider the following example: A DDS::ReadCondition that has a sample_state_mask = {DDS::SampleStateKind::NOT_READ_SAMPLE_STATE} will have trigger_value of true whenever a new sample arrives and will transition to false as soon as all the newly-arrived samples are either read (so their sample state changes to READ) or taken (so they are no longer managed by RTI Connext). However if the same DDS::ReadCondition had a sample_state_mask = { DDS::SampleStateKind::READ_SAMPLE_STATE, DDS::SampleStateKind::NOT_READ_SAMPLE_STATE }, then the trigger_value would only become false once all the newly-arrived samples are taken (it is not sufficient to read them as that would only change the sample state to READ), which overlaps the mask on the DDS::ReadCondition.

Trigger State of a ::DDS::GuardCondition

The trigger_value of a DDS::GuardCondition is completely controlled by the application via the operation DDS::GuardCondition::set_trigger_value.

See Also
Status Kinds
DDS::StatusCondition, DDS::GuardCondition
DDS::Listener

Constructor & Destructor Documentation

virtual DDS::WaitSet::~WaitSet ( )
virtual

Destructor.

Releases the resources asociated with this DDS::WaitSet.

Calling this method multiple times on the same DDS::WaitSet is safe; subsequent deletions will have no effect.

DDS::WaitSet::WaitSet ( )

Default no-argument constructor.

Construct a new DDS::WaitSet.

Returns
A new DDS::WaitSet or null if one could not be allocated.
DDS::WaitSet::WaitSet ( WaitSetProperty_t prop)

<<extension>> Constructor for a DDS::WaitSet that may delay for more while specifying that will be woken up after the given number of events or delay period, whichever happens first

Constructs a new DDS::WaitSet.

Returns
A new DDS::WaitSet or null if one could not be allocated.

Member Function Documentation

void DDS::WaitSet::wait ( ConditionSeq active_conditions,
Duration_t  timeout 
)

Allows an application thread to wait for the occurrence of certain conditions.

If none of the conditions attached to the DDS::WaitSet have a trigger_value of true, the wait operation will block, suspending the calling thread.

The result of the wait operation is the list of all the attached conditions that have a trigger_value of true (i.e., the conditions that unblocked the wait).

Note: The resolution of the timeout period is constrained by the resolution of the system clock.

The wait operation takes a timeout argument that specifies the maximum duration for the wait. If this duration is exceeded and none of the attached DDS::Condition objects are true, wait will return with the return code DDS::Retcode_Timeout. In this case, the resulting list of conditions will be empty.

Note: The resolution of the timeout period is constrained by the resolution of the system clock.

DDS::Retcode_Timeout will not be returned when the timeout duration is exceeded if attached DDS::Condition objects are true, or in the case of a DDS::WaitSet waiting for more than one trigger event, if one or more trigger events have occurred.

It is not allowable for more than one application thread to be waiting on the same DDS::WaitSet. If the wait operation is invoked on a DDS::WaitSet that already has a thread blocking on it, the operation will return immediately with the value DDS::Retcode_PreconditionNotMet.

Parameters
active_conditions<<inout>> a valid non-null DDS::ConditionSeq object. Note that RTI Connext will not allocate a new object if active_conditions is null; the method will return DDS::Retcode_PreconditionNotMet.
timeout<<in>> a wait timeout
Exceptions
Oneof the Standard Return Codes or DDS::Retcode_PreconditionNotMet or DDS::Retcode_Timeout.
void DDS::WaitSet::attach_condition ( Condition cond)

Attaches a DDS::Condition to the DDS::WaitSet.

It is possible to attach a DDS::Condition on a DDS::WaitSet that is currently being waited upon (via the wait operation). In this case, if the DDS::Condition has a trigger_value of true, then attaching the condition will unblock the DDS::WaitSet.

Parameters
cond<<in>> Condition to be attached.
Exceptions
Oneof the Standard Return Codes, or DDS::Retcode_OutOfResources.
void DDS::WaitSet::detach_condition ( Condition cond)

Detaches a DDS::Condition from the DDS::WaitSet.

If the DDS::Condition was not attached to the DDS::WaitSet the operation will return DDS::Retcode_BadParameter.

Parameters
cond<<in>> Condition to be detached.
Exceptions
Oneof the Standard Return Codes, or DDS::Retcode_PreconditionNotMet.
void DDS::WaitSet::get_conditions ( ConditionSeq attached_conditions)

Retrieves the list of attached DDS::Condition (s).

Parameters
attached_conditions<<inout>> a DDS::ConditionSeq object where the list of attached conditions will be returned
Exceptions
Oneof the Standard Return Codes, or DDS::Retcode_PreconditionNotMet.
void DDS::WaitSet::set_property ( WaitSetProperty_t  prop)

<<extension>> Sets the DDS::WaitSetProperty_t, to configure the associated DDS::WaitSet to return after one or more trigger events have occurred.

Parameters
prop<<in>>
Exceptions
Oneof the Standard Return Codes
void DDS::WaitSet::get_property ( WaitSetProperty_t prop)

<<extension>> Retrieves the DDS::WaitSetProperty_t configuration of the associated DDS::WaitSet.

Parameters
prop<<out>>
Exceptions
Oneof the Standard Return Codes

RTI Connext .Net APIs Version 5.2.0 Copyright © Sun Jun 21 2015 Real-Time Innovations, Inc