Windows pthread_mutex_t

However, we can enforce the order of the execution by using semaphore as shown in the next example:. Though we can implement synchronization by controlling thread access to data, condition variables allow threads to synchronize based upon the actual value of data.

While the mutex provides mutual exclusion for accessing the shared variable, the condition variable is used to signal changes the state variable. In this section, we will see how to use a semaphore to achieve this effect. Actually, the example in the previous section showed how. But here, we will start from the very simple one. By using a semaphore, we can achieve the similar effect of join.

In the code, the main thread is waiting for the worker thread to finish its task. Suppose one or more producer threads and one or more consumer threads. Producers produce data items and wish to place them in a buffer. Then, consumers grab data items out of the buffer consume the data in some way. This arrangement occurs in many places within real systems.

For example, in a multithread web server , a producer puts HTTP requests into a work queue i. This example runs two processes concurrently; cat notes. The other end of this pipe is connected to the standard input of the process wc , which simply counts the number of lines in the input stream and prints out the result. Thus, the cat process is the producer, and the wc process is the consumer. Between them is a bounded buffer. Because the bounded buffer is a shared resource , we must of course require synchronized access to it to avoid any race condition.

To understand this problem better, let us examine some actual code:. In this example, we assume that the shared buffer buffer is just an array of integers, and that the fill and use integers are used as indices into the array, and are used to track where to both put data fill and get data use. Suppose, we have just two threads, a producer and a consumer , and that the producer just writes some number of integers into the buffer which the consumer removes from the buffer and prints:.

However, without synchronization, we may not get that. For example, imagine if the consumer thread runs first; it will call get to get data that hasn't even been produced yet, and thus not function as desired.

Things get worse when we add multiple producers or consumers, as there could be race conditions in the update of the use or fill indices. As a first try, we introduces two semaphores, empty and full , which the threads will use to indicate when a buffer entry has been emptied or filled, respectively.

In the code below, the producer first waits for a buffer to become empty in order to put data into it, and the consumer similarly waits for a buffer to become filled before using it. Suppose, there are two threads, a producer and a consumer. Assume the consumer gets to run first.

Then, the producer runs. Thus, empty will be decremented to 0 and the producer will put a data value into the first entry of buffer. In this case, one of two things could happen. This time, however, it would block, as the empty semaphore's value is 0. In either case, we achieve the desired behavior:. Suppose, there are multiple producers and multiple consumers. We now have a problem: a race condition. Imagine two producers both calling into put at the same time. Before the producer gets a chance to increment the fill counter to 1, it is interrupted.

Producer 2 starts to run, and at the same line of code, it also puts its data into the 0th element of buffer, which means that the old data there is overwritten! As we can see, what we've forgotten here is mutual exclusion. The filling of a buffer and incrementing of the index into the buffer is a critical section , and thus must be guarded carefully. So let's use binary semaphore and add some locks. However, it still doesn't work. Suppose, two threads, one producer and one consumer. The consumer gets to run first.

Because there is no data yet, this call causes the consumer to block and thus yield the CPU; importantly, though, the consumer still holds the lock! A producer then runs. It has data to produce and if it were able to run, it would be able to wake the consumer thread and all would be good.

The lock is already held. Hence, the producer is now stuck waiting too. There is a simple cycle here. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Asked 5 years, 8 months ago. Active 2 years, 7 months ago. Viewed 8k times. Improve this question. Jonathon Hill Jonathon Hill 2 2 gold badges 14 14 silver badges 22 22 bronze badges. The std:: api allows you to get a native thread handle for pthread stuff.

Then, adapting the app to e. Some applications might attempt to restore the resource to a consistent state; this example simply returns an error and stops using the mutex. For more information, see Mutex Objects. Skip to main content. This browser is no longer supported. Download Microsoft Edge More info. A condition variable must always be associated with a mutex to avoid a race condition created by one thread preparing to wait and another thread which may signal the condition before the first thread actually waits on it resulting in a deadlock.

The thread will be perpetually waiting for a signal that is never sent. Any mutex can be used, there is no explicit link between the mutex and the condition variable. Everything else is random. The logic conditions the "if" and "while" statements must be chosen to insure that the "signal" is executed if the "wait" is ever processed.

Poor software logic can also lead to a deadlock condition. Note: Race conditions abound with this example because count is used as the condition and can't be locked in the while statement without causing deadlock.