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What is lock and significance of lock in producer consumer problem?

What is lock and significance of lock in producer consumer problem?

You can also solve the producer-consumer problem by using a new lock interface and condition variable instead of using the synchronized keyword and wait and notify methods. The lock provides an alternate way to achieve mutual exclusion and synchronization in Java.

What is producer consumer problem with example?

In computing, the producer-consumer problem (also known as the bounded-buffer problem) is a classic example of a multi-process synchronization problem. The problem describes two processes, the producer and the consumer, which share a common, fixed-size buffer used as a queue.

What are the solutions to a producer consumer problem?

The solution for the producer is to either go to sleep or discard data if the buffer is full. The next time the consumer removes an item from the buffer, it notifies the producer, who starts to fill the buffer again. In the same way, the consumer can go to sleep if it finds the buffer empty.

What are the conditional variables in consumer producer example?

A condition variable represents a queue of threads waiting for some condition to be signaled. Example 4-11 has two such queues, one ( less ) for producers waiting for a slot in the buffer, and the other ( more ) for consumers waiting for a buffer slot containing information.

What is producer-consumer problem in multithreading?

The producer and consumer problem is one of the small collection of standard, well-known problems in concurrent programming. A finite-size buffer and two classes of threads, producers and consumers, put items into the buffer (producers) and take items out of the buffer (consumers).

Does condition variable release lock?

Condition variables: used to wait for a particular condition to become true (e.g. characters in buffer). wait(condition, lock): release lock, put thread to sleep until condition is signaled; when thread wakes up again, re-acquire lock before returning.

How can we use semaphores instead of locks and condition variables?

Semaphores and condition variables build on top of the mutual exclusion provide by locks and are used for providing synchronized access to shared resources. They can be used for similar purposes….The protocol then becomes,

  1. acquire mutex.
  2. check condition.
  3. block and release mutex if condition is true, else release mutex.

What does condition variable wait do?

wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs, optionally looping until some predicate is satisfied. The thread will be unblocked when notify_all() or notify_one() is executed.

Why do condition variables need a lock?

10 Answers. It’s just the way that condition variables are (or were originally) implemented. The mutex is used to protect the condition variable itself. That’s why you need it locked before you do a wait.

Which lock type can a condition object be associated with?

Condition Objects. A condition variable is always associated with some kind of lock; this can be passed in or one will be created by default. Passing one in is useful when several condition variables must share the same lock.

Under what situation would a spin lock be preferred over a condition variable?

Under what situation would a spin-lock be preferred over a condition variable? Protection of a short critical section on a single-processor machine.

How do you implement conditional variables?

The implementation of condition variables involves several mutexes….Condition variables support three operations:

  1. wait – add calling thread to the queue and put it to sleep.
  2. signal – remove a thread form the queue and wake it up.
  3. broadcast – remove and wake-up all threads on the queue.

How do conditional variables work?

Condition variables are synchronization primitives that enable threads to wait until a particular condition occurs. Condition variables support operations that “wake one” or “wake all” waiting threads. After a thread is woken, it re-acquires the lock it released when the thread entered the sleeping state.

What is Pthread condition variable?

pthread question: it appears that a condition variable only works if pthread_cond_wait is called before the other thread calls pthread_cond_notify. If notify somehow happens before wait then wait will be stuck. The scheduler can preempt threads and a notify may happen before wait.

What is conditional variable in C?

A Join allows one thread to wait for another to complete but a condition variable allows any number of threads to wait for another thread to signal a condition. To wait on a condition variable you use: pthread_cond_wait(&myConVar , &mymutex); You need to lock the mutex before calling the function.

What is Pthread_cond_initializer?

The PTHREAD_COND_INITIALIZER macro initializes the static condition variable cond, setting its attributes to default values. This macro should only be used for static condition variables, since no error checking is performed.

Does Pthread_cond_wait unlock mutex?

pthread_cond_wait() simultaneously unlocks the mutex and begins waiting for the condition variable to be signalled. thus you must always have ownership of the mutex before invoking it.

Why are condition variables needed?

You need condition variables, to be used with a mutex (each cond. belongs to a mutex) to signal changing states (conditions) from one thread to another one. The idea is that a thread can wait till some condition becomes true.

What is the condition on a variable?

An equation is a condition on a variable. It is expressed by saying that an expression with a variable is equal to a fixed number, e.g. x – 3 = 10.

Are condition variables thread safe?

If multiple threads share a condition variable, the use of notify_one() is safe only if the following conditions are met: All threads must perform the same set of operations after waking up, which means that any thread can be selected to wake up and resume for a single invocation of notify_one() .

How do condition variables avoid the lost wakeup problem?

To avoid lost-wakeups, the common advice is that all notifiers must hold the lock while updating their condition states. However, if you were to guard the WaitForMessage() call with work_to_do_lock you could prevent other signals from waking up the event loop.

What is lost wakeup?

A lost wake-up occurs when all of the following conditions are in effect: A thread calls pthread_cond_signal() or pthread_cond_broadcast() Another thread is between the test of the condition and the call to pthread_cond_wait() No threads are waiting. The signal has no effect, and therefore is lost.

Why do spurious Wakeups happen?

It’s called spurious because the thread has seemingly been awakened for no reason. But spurious wakeups don’t happen for no reason: they usually happen because, in between the time when the condition variable was signaled and when the waiting thread finally ran, another thread ran and changed the condition.

What is std :: Condition_variable?

std::condition_variable The condition_variable class is a synchronization primitive that can be used to block a thread, or multiple threads at the same time, until another thread both modifies a shared variable (the condition), and notifies the condition_variable .

What is the difference between Unique_lock and Lock_guard?

A lock_guard always holds a lock from its construction to its destruction. A unique_lock can be created without immediately locking, can unlock at any point in its existence, and can transfer ownership of the lock from one instance to another.

Does C++ have semaphore?

Semaphores in C++ The C++ standard does not define a semaphore type. You can write your own with an atomic counter, a mutex and a condition variable if you need, but most uses of semaphores are better replaced with mutexes and/or condition variables anyway.

How do I use mutex in CPP?

A mutex is initialized in the beginning of the main function. The same mutex is locked in the ‘trythis()’ function while using the shared resource ‘counter’. At the end of the function ‘trythis()’ the same mutex is unlocked. At the end of the main function when both the threads are done, the mutex is destroyed.

What happens when mutex is locked?

If the mutex is currently locked by another thread, execution of the calling thread is blocked until unlocked by the other thread (other non-locked threads continue their execution). If the mutex is currently locked by the same thread calling this function, it produces a deadlock (with undefined behavior).

Is std :: mutex movable?

By design, std::mutex is not movable nor copyable. This means that a class A holding a mutex won’t receive a default move constructor.

What is difference between semaphore and mutex?

A mutex object allows multiple process threads to access a single shared resource but only one at a time. On the other hand, semaphore allows multiple process threads to access the finite instance of the resource until available. In mutex, the lock can be acquired and released by the same process at a time.