Using Events and Mutexes Together in VS .NET

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Using Events and Mutexes Together
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The next step is to describe how to use mutexes and events together, generalizing Program 8-2, where we had the following situation, which will occur over and over again Note: This discussion applies to CRITICAL_SECTIONs as well as to mutexes
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The mutex and event are both associated with the message block or other data structure The mutex defines the critical code section for accessing the data structure object The event is used to signal the fact that there is a new message Generalizing, the mutex ensures the object's invariants (or safety properties), and the event signals that the object has changed state (eg, a message has been added or removed from a message buffer), possibly being put into a known state (eg, there is at least one message in the message buffer) One thread (the producer in Program 8-2) locks the data structure, changes the object's state by creating a new message, and sets or pulses the event associated with the fact that there is a new message At least one other thread (the consumer in this example) waits on the event for the object to reach the desired state The wait must occur outside the critical section so that the producer can access the object A consumer thread can also lock the mutex, test the object's state (eg, is there a new message in the buffer ), and avoid the event wait if the object is already in the desired state
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The Condition Variable Model
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Now let's combine all of this into a single code fragment that represents what we will call the condition variable model (CV model) with two variations, the signal and broadcast CV models The first examples use the broadcast variation The result is a program model that will occur many times and can be used to solve a wide variety of synchronization problems For convenience, the example is stated in terms of a producer and a consumer The discussion may seem a bit abstract, but once the techniques are understood, we will be able to solve a number of synchronization problems that would be very difficult without a good model The code fragment has several key elements
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A data structure of type STATE_TYPE that contains all the data or state variables such as the messages, checksums, and counters used in Program 8-2 A mutex and one or more events associated with, and usually a part of, the data structure One or more Boolean functions to evaluate the condition variable predicates, which are the conditions (states) on which a thread might wait Examples include "a new message is ready," "there is available space in the buffer," and "the queue is not empty" A distinct event may be associated with each condition variable predicate, or one event may be used to represent simply
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a change of state or a combination (logical "or") of several predicates In the latter case, individual predicate functions must be tested, with the mutex locked, to determine the actual state If the predicate (logical expression) is simple, there is no need for a separate function The following code segment shows a producer and consumer using these principles, with a single event and condition variable predicate (implemented with a function, cvp, that is assumed but not shown) When the producer signals that a desired state has been reached, this example assumes that it is appropriate to release several consumer threadsthat is, the signal should be broadcast to all waiting consumers For instance, the producer may have created several messages, and the state is changed by increasing the message count In many situations, you want to release only a single thread, as will be discussed after the code fragment This code segment is designed to operate under Windows 9x as well as all NT versions SignalObjectAndWait will then be used to simplify the solution Note and caution: This example deliberately and consciously uses PulseEvent, even though some writers and some of the Microsoft documentation warn against its use (see the remarks section in the MSDN entry) The choice will be justified in the ensuing discussion and the examples, and the reader is invited to attempt to solve the problem (correctly) by using SetEvent typedef struct _state_t { HANDLE Guard; /* Mutex to protect the object */ HANDLE CvpSet; /* Manual-reset event -- cvp () holds */ other condition variables /* State structure with counts, checksums, etc */ struct STATE_VAR_TYPE StateVar; } STATE_TYPE State; /* Initialize State, creating the mutex and event */ /* PRODUCER thread that modifies State */ WaitForSingleObject (StateGuard, INFINITE); /* Change state so that the CV predicate holds */ /* Example: one or more messages are now ready */ StateStateVarMsgCount += N; PulseEvent (StateCvpSet); ReleaseMutex (StateGuard); /* End of the interesting part of the producer */ /* CONSUMER thread function waits for a particular state */ WaitForSingleObject (StateGuard, INFINITE); while (!cvp (&State)) { ReleaseMutex (StateGuard); WaitForSingleObject (StateCvpSet, TimeOut); WaitForSingleObject (StateGuard, INFINITE); } /* This thread now owns the mutex and cvp (&State) holds */ /* Take appropriate action, perhaps modifying State */ ReleaseMutex (StateGuard); /* End of the interesting part of the consumer */
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