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// 'empty', 'full', and 'mutex' are all semaphore objects empty = n // n = the maximum number of items // permitted to be waiting to be consumed full = 0 mutex = 1 /* Producer: */ while( true ) { // while( true ) means continue looping forever . . . produce an item . . . P(empty); P(mutex); . . . send item to consumer . . . V(mutex); V(full); } /* Consumer: */ while( true ) { P( full ) P( mutex ); . . . consume an item . . . V( mutex ); V( empty ); . . . } When the producer has an item ready to give to the consumer, the producer executes a P operation on the empty semaphore. As long as there are fewer than the maximum permitted items waiting to be consumed, the producer does not block. Then the producer executes a P operation on the mutex semaphore. The mutex semaphore is there to make sure that the producer and consumer cannot simultaneously add and consume items, for that would lead to elusive errors of synchronization. When the producer succeeds and produces a new item, the producer executes a V operation on the mutex semaphore to release control of the group of items to be consumed, and a V operation on the full semaphore. The full semaphore keeps a count of the items waiting to be consumed, and the consumer will test that semaphore with a P operation before trying to consume an item. When the consumer is ready to do something with the items provided by the producer, the consumer executes a P operation on the full semaphore to be sure that something is there to consume. If so, the consumer does not block, and instead goes on to test the mutex semaphore using the P operation. If the P operation on the mutex semaphore does not block, it means that the consumer has exclusive access to the set of items ready to be consumed. After the consumer removes an item from the set, the consumer releases the mutex semaphore by executing a V operation on the mutex. The consumer then increments the count of additional items permitted to be in the set of items to be consumed, by executing a V operation on the empty semaphore. Semaphores are simple and powerful, but it can be difficult to design more complex interactions between processes using semaphores. For instance, suppose a number of threads share read and write access to a file.
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Perhaps the designer wants to avoid the situation where a writer changes something while a reader is reading. It s fairly easy to use three semaphores to design the application so that as long as any reader is active with the file or waiting to be active, no writer is permitted to make any changes. On the other hand, suppose the designer wants to avoid having a writer change the file contents while a reader is reading, but also wants to give a writer priority over waiting readers so that readers will get the latest information. To implement this writers-first policy requires five semaphores and very careful thinking about how to use them. MONITORS In the mid 1970s, researchers Hoare and Brinch Hansen proposed the monitor idea. A properly written monitor can simplify the coordination of processes or threads. Monitors are either built into the programming language, as with Java, or if the application language does not provide monitors, they are specially written. A monitor is a special object, or abstract data type, that can insure that only one process or thread is active in it at one time. A process enters the monitor by calling one of the monitor s methods. Once a process enters the monitor, no other process can enter until the first process exits the monitor. If another process calls one of the monitor s methods while a process is active within the monitor, the second process blocks, waiting for the first process to exit the monitor. Since the monitor insures mutual exclusion by virtue of the fact that only one process may be active within it, the programmer using the monitor can dispense with worrying about semaphores or other mechanisms to assure mutual exclusion. In fact, the first lines of code in a monitor s method may use a semaphore internally to enforce the mutual exclusion, but the programmer need not be concerned with the mechanism. In addition to insuring mutual exclusion, a monitor can be used to synchronize access by different threads or processes by using condition variables to allow a process to wait, if some condition necessary for success is absent at the time it is active in the monitor. Condition variables also allow the waiting process to be restarted when the condition changes. The two operations defined for condition variables are usually called wait and signal (to reactivate). For instance, suppose a consumer process using a monitor finds there is nothing to consume at this moment. The monitor would provide a condition variable, perhaps called itemsReady, on which the process could wait. When a process is waiting on a condition variable within the monitor, another process may enter. In fact, if another process has been waiting for the monitor, as soon as the active process in the monitor waits for a condition variable, the process that has been waiting will gain access to the monitor and become the monitor s active process. At some point, some cooperating process will change the condition on which another process has been waiting. For instance, a producer process will create an item that the consumer can process. When a process makes such a change, the active process will signal the condition variable. A signal will cause the monitor to reactivate one of the processes waiting (there may be more than one process waiting) on that condition variable. Signals are not counters, as semaphores are. If a signal occurs and no process is waiting on that condition variable, nothing happens. The signal is like the old tree falling in the forest. The noise makes no difference if no one is there to hear it. Suppose that a monitor called PC (for Producer Consumer) is available to support a set of processes that cooperate producing items (e.g., product orders) and consuming them (e.g., making entries in shipment schedules for different distribution centers). The monitor has two methods, addOrder and retrieveOrder; addOrder takes an Order as an argument, and retrieveOrder returns an Order. The application code becomes very simple: Producer: while( true ) { Order newOrder = createOrder( orderNumber ); PC.addOrder( newOrder ); }
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