The pthread_cond_wait() function blocks the calling thread, waiting for the condition specified by cond to be signaled or broadcast to. When pthread_cond_wait() is called, the calling thread must have mutex locked. You should always associate only one mutex with a condition at a time..
In this way, what is Pthread_cond_t?
DESCRIPTION. The pthread_cond_wait() and pthread_cond_timedwait() functions are used to block on a condition variable. They are called with mutex locked by the calling thread or undefined behaviour will result.
Also, does Pthread_cond_wait unlock mutex? The pthread_cond_wait() function blocks the calling thread on the condition variable cond, and unlocks the associated mutex mutex. The calling thread must have locked mutex before waiting on the condition variable. On return from the function, the mutex is again locked and owned by the calling thread.
Subsequently, question is, why does Pthread_cond_wait need a mutex?
The mutex is used to protect the condition variable itself. That's why you need it locked before you do a wait. Then when the condition variable is signalled or broadcast to, one or more of the threads on the waiting list will be woken up and the mutex will be magically locked again for that thread.
How do you use condition variables?
In typical use, a condition expression is evaluated under the protection of a mutex lock. When the condition expression is false, the thread blocks on the condition variable. The condition variable is then signaled by another thread when it changes the condition value.
Related Question Answers
What is thread condition variable?
Condition variables are synchronization primitives that enable threads to wait until a particular condition occurs. Condition variables are user-mode objects that cannot be shared across processes. Condition variables enable threads to atomically release a lock and enter the sleeping state.What causes spurious wakeup?
Spurious wakeup describes a complication in the use of condition variables as provided by certain multithreading APIs such as POSIX Threads and the Windows API. Even after a condition variable appears to have been signaled from a waiting thread's point of view, the condition that was awaited may still be false.What is Pthread_mutex_t?
pthread_mutex_t is used to declare an object of type mutex. thus: pthread_mutex_t mymutexvariable; You would then use the mutex variable to lock and unlock a mutex.What is conditional variable in C?
Condition Variables. 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.What is Pthread_mutex_lock?
DESCRIPTION. The pthread_mutex_lock() function locks the specified mutex. If the mutex is already locked, the calling thread blocks until the mutex becomes available. This operation returns with the mutex in the locked state with the calling thread as its owner.What does Pthread_create return?
pthread_create Return Values. pthread_create() returns zero when the call completes successfully. Any other return value indicates that an error occurred. When any of the following conditions are detected, pthread_create() fails and returns the corresponding value.Do threads inherit signal handlers?
As mentioned earlier, a thread inherits its signal mask from the thread which creates it. The main() function sets the signal mask to block all signals, so all threads created after this point will have all signals blocked, including the signal-handling thread.What is condition variable in multithreading?
Condition Variable is a kind of Event used for signaling between two or more threads. One or more thread can wait on it to get signaled, while an another thread can signal this.Why is mutex needed?
It ensures that only one thread is executing a key piece of code at a time, which in turns limits access to a data structure. It ensures that the both threads have a full and proper view of that memory irrespective of any CPU reordering. The mutex is an absolute necessity when doing concurrent programming.What is the difference between Semaphore and condition variable?
In summary, the major differences between condition variables and semaphores: A call to the condition variable operation Wait() will always cause the calling thread to block (wait). A semaphore is a numeric value that can be used as a counter in an algorithm as they are used in the producer-consumer problem.How can we use semaphores to implement monitors?
One possible implementation of a monitor uses a semaphore "mutex" to control mutual exclusionary access to the monitor, and a counting semaphore "next" on which processes can suspend themselves after they are already "inside" the monitor ( in conjunction with condition variables, see below. )Why do you have to wait for a condition variable inside of a lock?
2 Answers. Condition variables are generally used to signal a change of state. A mutex is usually needed to make that change, and the following signal, atomic. A condition variable is more primitive, only providing the signal.What is a mutex in OS?
Mutex. Mutex is a mutual exclusion object that synchronizes access to a resource. It is created with a unique name at the start of a program. The Mutex is a locking mechanism that makes sure only one thread can acquire the Mutex at a time and enter the critical section.What is Pthread H?
Pthreads are defined as a set of C language programming types and procedure calls, implemented with a pthread. h header/include file and a thread library - though this library may be part of another library, such as libc, in some implementations.How does condition variable work C++?
A condition variable is an object able to block the calling thread until notified to resume. It uses a unique_lock (over a mutex ) to lock the thread when one of its wait functions is called. 
