Just before exiting, that thread increments the semaphore, which unblocks the main thread at that point, the main thread creates a new thread to handle the pending request it had received. Eventually, one of the threads serving content finishes. However, if the maximum number of threads has already been reached, the main thread gets blocked. If the limit has not been reached, a new thread handles the request. Whenever a request is received, the main thread first decrements the semaphore to determine if the limit has been reached. Mutual exclusion is a special case of multiplexing, where the initial value is 1.Ĭode Listing 8.4 illustrates the code framework for a server that launches a new thread each time a request has been received. Once the thread semaphore has been decremented to 0, the limit has been reached and future accesses are blocked. This initial value is the maximum number of concurrent accesses allowed. The key defining feature of multiplexing is to initialize the semaphore’s value to a positive integer greater than 1. Multiplexing allows multiple concurrent accesses up to a given limit additional requests beyond that limit are blocked until more resources become available. The solution in this case is multiplexing. Allowing an unlimited number of connections would exhaust the system resources, such as consuming all of the system’s available memory or trying to read too many files on a single hard drive. For example, network servers (e.g., web or e-mail servers) allow hundreds or thousands of concurrent network connections to serve content to remote users. In many cases, mutual exclusion is too strong of a system requirement, but it is reasonable to place a limit on the number of concurrent accesses. Code Listing 8.3 shows the structure for these two threads. The two threads need to complete reading the two copies before comparing their contents (a mismatch would indicate corruption). To do this, the program uses one thread to retrieve the file from the web site a second thread reads the same file from a location that is known to be secure and correct. To illustrate the use of a rendezvous, assume that a program wants to detect corrupted files on a web server. Then, when the other thread arrives, it unblocks the first by calling sem_post() however, this thread is not blocked by the call to sem_wait(), as the semaphore’s value has already been incremented to 1. At run-time, one thread will arrive at the rendezvous first and get blocked by the call to sem_wait(). The two threads then call sem_post() on one semaphore just prior to calling sem_wait() on the other. To create a rendezvous, two semaphores are initialized to 0. The goal of a rendezvous is to ensure that two threads meet at a pre-defined common point. Turnstiles permanently unblock all current and future threads based on a key event.Ī rendezvous is a mutual signaling between two threads. Number of threads specified by the sem_op argument. Threads that are already waiting, and semop() unblocks a maximum of the Turnstile, but also those that have not arrived yet. TurnstilesĬause the unblocking to propagate to all threads that are waiting at the There is a keyĭifference that distinguishes the intended use of turnstiles, though. Semop() function to increase a semaphore by more than 1. Turnstiles allow POSIX semaphores to behave similar to the broadcast feature ofĬondition variables or using the System V * Code Listing 8.1: The general structure of a program that uses signaling after initialization */ void * initialize ( void * args ) Until it can be guaranteed that all of the initialization is done. But at a certain point, the main thread needs to pause Initialization may be done concurrently with other work that the main thread is Overwriting the default signal handlers, or other such tasks. For instance, this initialization might involve reading inĪ large amount of data from configuration files, allocating request queues, Then, when the other thread calls sem_wait(), it can proceed withoutīlocking because the event has already occurred.Ĭode Listing 8.1 uses a separate thread to perform some sort of Runs first and calls sem_post(), incrementing the semaphore’s value to 1. The other thread runs and calls sem_post(). Since the semaphore is initialized to 0, the thread must block until In one scenario, the thread that calls sem_wait() Not affect the correctness of the results. The key observation with signaling is that the scheduling of the threads does
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