RWTPCValBufferBaseGuarded<Type,GuardDecorator>

Module:  Threads   Package:  Interthread Communication

RWTPCValBufferBaseGuarded<Type,GuardDecorator>        
    (virtual) RWTPCValBufferBaseDecorated<Type,GuardDecorator>

• Local Index
• Header File
• Description
• Public Destructor
• Inherited Functions
• Public Member Functions
• See Also

Members

peek() read()

RWTPCValBufferBaseGuarded() tryPeek()

tryRead() tryWrite()

write()

#include <rw/itc/RWTPCValBufferBaseGuarded.h> 

RWTPCValBufferBaseGuarded<Type,GuardDecorator> is the base class for the family of classes that provide buffered producer-consumer synchronization semantics for exchanging guarded values between cooperating threads.

In the producer-consumer synchronization model, reader threads (consumers) are blocked while a buffer is empty, and writer threads (producers) are blocked while a buffer is full. A buffer is considered full when the number of unread entries equals or exceeds some user-specified maximum capacity.

The write operations provided by this class and its subclasses bind additional data items to each value prior to storing that value in an internal buffer. One of these additional data items, or decorations, is a guard functor that is used during read operations to determine whether the associated value is currently eligible for retrieval from the buffer. The template parameter GuardDecorator identifies the class that is used to encapsulate both the value, guard function, and any other decorations as a single object for storage in the internal buffer. The decorator classes used by subclasses to instantiate this class are intended for the module's internal use, and are not documented as part of the public interface.

virtual
~RWTPCValBufferBaseGuarded();

Virtual destructor.

The following superclass functions have been redeclared or redefined to allow them to be overloaded within this class:

bool     tryWrite(const Type& value);
void          write(const Type& value);
RWWaitStatus  write(const Type& value, 
                     unsigned long  milliseconds);

These functions provide the same capabilities and behavior as their counter-parts in this class, except that these functions attach an empty guard functor instance to each value.

The superclass functions that are used for peeking or reading a buffer behave differently when called on instances of this class. These functions provide nearly the same capabilities and behavior as when used in their superclasses, with the exception that they now test the guard functor instance associated with each buffered value to determine whether or not that value is eligible for reading.

The detailed behavioral differences are described below.

Type   peek();
Type   read();

A peek or a read function traverses the buffer, in an order that is determined by the implementation supplied in derived classes, until it finds a value whose guard functor is undefined or evaluates to true, at which point the function copies or removes, then returns that value. If the buffer is open and there are no values whose guard functors evaluate to true, or if the buffer is empty, this function blocks the calling thread until another value is inserted into the buffer, or until the buffer is closed. If the buffer is closed, and is empty or contains no readable entries, this function immediately returns by throwing an RWTHRClosedException.

Each time a new value is written into the buffer, one waiting reader thread is unblocked to give it a chance to re-evaluate the contents of the buffer, including the new entry. Since a write operation only unblocks a single waiting reader thread, any other threads that are waiting to peek or read remain blocked even if the most recent transfer produces a change in program state that would cause one or more guard functors to evaluate to true. This means that developers should use a single thread to read from a guarded buffer; additional threads could potentially be stuck waiting for a write operations even when there are entries eligible for reading.

If the buffer is closed while a thread is blocked within this function, that thread is unblocked, after which this function re-evaluates the guard functors one last time. If it finds a value whose functor evaluates to true, it returns that value, otherwise it returns by throwing an RWTHRClosedException.

RWWaitStatus peek(Type& value,
                  unsigned long milliseconds);
RWWaitStatus read(Type& value,
                  unsigned long milliseconds);

A peek or a read function traverses the buffer, in an order that is determined by the implementation supplied in derived classes, until it finds a value whose guard functor is undefined or evaluates to true. At this point the function copies or removes the value, and then copies it into the instance passed as a reference, and returns RW_THR_COMPLETED to indicate that the read succeeded. If the buffer is open and there are no values whose guard functors evaluate to true, or if the buffer is empty, this function blocks the calling thread until another value is inserted into the buffer, or until the buffer is closed, or until the specified amount of time passes. If no value becomes available within the specified amount of time, the function returns with value of RW_THR_TIMEOUT. If the buffer is closed, and is empty or contains no readable entries, this function immediately returns by throwing an RWTHRClosedException.

Each time a new value is written into the buffer, one waiting reader thread is unblocked to give it a chance to re-evaluate the contents of the buffer, including the new entry. Since a write operation only unblocks a single waiting reader thread, any other threads that are waiting to peek or read remain blocked even if the most recent transfer produces a change in program state that would cause one or more guard functors to evaluate to true. This means that developers should use a single thread to read from a guarded buffer; additional threads could potentially be stuck waiting for a write operations even when there are entries eligible for reading.

If the buffer is closed while a thread is blocked within this function, that thread is unblocked, after which, this function re-evaluates the guard functors one last time. If it finds a value whose functor evaluates to true, it returns that value, otherwise it returns by throwing an RWTHRClosedException.

While these functions guarantee that they will wait for a period of time equal to or greater than the specified number of milliseconds, they cannot guarantee the thread will actually be scheduled for execution at the end of the time-out period, and cannot prevent the individual guard functors from blocking the caller for indefinite periods of time.

bool    tryPeek(Type& value);
bool    tryRead(Type& value);

A tryPeek or a tryRead function traverses the buffer, in an order that is determined by the implementation supplied in derived classes, until it finds a value whose guard functor is undefined or evaluates to true. At this point the function copies or removes the value, and then copies it into the instance passed as a reference, and returns true to indicate that the read succeeded. If the buffer is open and there are no values whose guard functors evaluate to true, or if the buffer is empty, this function immediately returns false to indicate that the read was unsuccessful. If the buffer is closed, and is empty or contains no readable values, this function immediately returns by throwing an RWTHRClosedException.

While these functions do not directly block the caller if the buffer is empty or contains no readable values, they cannot prevent the individual guard functors from indirectly blocking the caller.

bool 
tryWrite(const Type& value, 
         const RWTFunctorR0<bool>& guard);

Inserts a guarded value into the buffer at a position that is determined by the implementation supplied in derived classes, but only if the value may be written immediately; in other words, only if there is sufficient free capacity.

If the buffer is open and has sufficient capacity to hold another value, this function inserts the value, and returns a value of true to indicate that the write succeeded. If the buffer is open and full, this function immediately returns a value of false to indicate that the write was unsuccessful. If the buffer is closed, this function immediately returns by throwing an RWTHRClosedException.

Calling this function on a full buffer may result in the invocation of the on-full callback functor, if a valid one was registered. The on-full callback functor is guaranteed to execute only within a writer thread. Calling this function on a full buffer will result in the invocation of an on-full callback functor under either of these scenarios:

• The calling thread is waiting on an full buffer when an on-full callback is registered.

• The calling thread is the first to attempt writing to a full buffer in which an on-full callback was previously registered.

Repeated attempts to write to a full buffer will not result in repeated invocations of the callback functor. Once the full buffer is read from, however, it resets the callback trigger; should the buffer again become full, the first thread to attempt to write will cause another invocation of the on-full callback.

While this function guarantees not to block the caller if the buffer is full, it cannot prevent the on-full callback functor, if executed, from indirectly blocking the caller.

This function takes two parameters. The parameter value is a const reference to an instance of the type used to instantiate this template class. The parameter guard is a reference to an RWTFunctorR0<bool> functor instance that is invoked during read and peek operations to determine whether the associated value is currently eligible for reading. An empty functor handle indicates that the entry is always eligible for reading. The functor must not attempt to access the buffer instance, as such access results in deadlock.

void 
write(const Type& value, const RWTFunctorR0<bool>& guard);

Inserts a guarded value into the buffer at a position that is determined by the implementation supplied in derived classes.

If the buffer is open and has sufficient capacity to hold another value, this function will insert the value. If the buffer is open and full, this function blocks the calling thread until the buffer is no longer full (because another thread or threads have read values or have changed the maximum capacity), or until the buffer is closed.

If the buffer is closed while the calling thread is blocked and waiting within this function for room to write, the calling thread is unblocked and the function returns by throwing an RWTHRClosedException. If the buffer is already closed when this function is called, the function immediately returns by throwing an RWTHRClosedException.

Calling this function on a full buffer may result in the invocation of the on-full callback functor, if a valid one was registered. The on-full callback functor is guaranteed to execute only within a writer thread. Calling this function on a full buffer will result in the invocation of an on-full callback functor under either of these scenarios:

• The calling thread is waiting on an full buffer when an on-full callback is registered.

• The calling thread is the first to attempt writing to a full buffer in which an on-full callback was previously registered.

Repeated attempts to write to a full buffer will not result in repeated invocations of the callback functor. Once the full buffer is read from, however, it resets the callback trigger; should the buffer again become full, the first thread to attempt to write will cause another invocation of the on-full callback.

This function takes two parameters. The parameter value is a const reference to an instance of the type used to instantiate this template class. The parameter guard is a reference to an RWTFunctorR0<bool> functor instance that is invoked during read and peek operations to determine whether the associated value is currently eligible for reading. An empty functor handle indicates that the entry is always eligible for reading. The functor must not attempt to access the buffer instance as such access results in deadlock.

RWWaitStatus
write(const Type& value,
      const RWTFunctorR0<bool>& guard,
      unsigned long milliseconds);

Inserts a guarded value into the buffer at a position that is determined by the implementation supplied in derived classes.

If the buffer is open and has sufficient capacity to hold another value, this function inserts the value and returns RW_THR_COMPLETED to indicate that the write succeeded. If the buffer is open and full, this function blocks the calling thread until the buffer is no longer full (because another thread or threads have read values or have changed the maximum capacity), or until the buffer is closed, or until the specified amount of time has passed. If sufficient space does not become available within the specified amount of time, the function returns a value of RW_THR_TIMEOUT.

If the buffer is closed while the calling thread is blocked and waiting within this function for room to write, the calling thread is unblocked and the function returns by throwing an RWTHRClosedException. If the buffer is already closed when this function is called, it immediately returns by throwing an RWTHRClosedException.

Calling this function on a full buffer may result in the invocation of the on-full callback functor, if a valid one was registered. The on-full callback functor is guaranteed to execute only within a writer thread. Calling this function on a full buffer will result in the invocation of an on-full callback functor under either of these scenarios:

• The calling thread is waiting on an full buffer when an on-full callback is registered.

• The calling thread is the first to attempt writing to a full buffer in which an on-full callback was previously registered.

Repeated attempts to write to a full buffer will not result in repeated invocations of the callback functor. Once the full buffer is read from, however, it resets the callback trigger; should the buffer again become full, the first thread to attempt to write will cause another invocation of the on-full callback.

While this function guarantees to wait, if necessary, for a period time equal to or greater than the specified number of milliseconds, it cannot guarantee that the thread is actually scheduled for execution at the end of the time-out period. It cannot prevent the on-full callback functor from indirectly blocking the caller for an indefinite period of time.

This function takes three parameters:

• The parameter value is a const reference to an instance of the type used to instantiate this template class.

• The parameter guard is a reference to an RWTFunctorR0<bool> functor instance that is invoked during read and peek operations to determine whether the associated value is currently eligible for reading. An empty functor handle indicates that the entry is always eligible for reading. The functor must not attempt to access the buffer instance as such access results in deadlock.

• The parameter milliseconds is an unsigned long value that specifies the maximum number of milliseconds to wait for the operation to complete.

RWTPCValBufferBaseDecorated<Type,Decorator>, RWTPCValQueueGuarded<Type>, RWTPCValStackGuarded<Type>, RWTPCValBufferBaseGuardedPrioritized<Type,GuardAndPriorityDecorator>

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