Mbed OS Reference | ConditionVariable Class Reference

The ConditionVariable class is a synchronization primitive that allows threads to wait until a particular condition occurs. More...

#include <ConditionVariable.h>

Inheritance diagram for ConditionVariable:

Public Member Functions
	ConditionVariable (Mutex &mutex)
	Create and initialize a ConditionVariable object. More...

void	wait ()
	Wait for a notification. More...

bool	wait_until (uint64_t millisec)
	Wait for a notification until the specified time. More...

bool	wait_for (uint32_t millisec)
	Wait for a notification or timeout. More...

void	notify_one ()
	Notify one waiter on this condition variable that a condition changed. More...

void	notify_all ()
	Notify all waiters on this condition variable that a condition changed. More...

	~ConditionVariable ()
	ConditionVariable destructor. More...

Detailed Description

The ConditionVariable class is a synchronization primitive that allows threads to wait until a particular condition occurs.

Use the condition variable in conjunction with a mutex to safely wait for or notify waiters of condition changes to a resource accessible by multiple threads.

The thread that intends to wait on a ConditionVariable must:

Acquire a lock on a mutex.
Execute wait, wait_for or wait_until. While the thread is waiting, the mutex is unlocked.
When the condition variable has been notified, or in the case of wait_for and wait_until the timeout expires, the thread is awakened.

The thread that intends to notify a ConditionVariable must:

Acquire a lock on the mutex used to construct the condition variable.
Execute notify_one or notify_all on the condition variable.

All threads waiting on the condition variable wake when ConditionVariable::notify_all is called. At least one thread waiting on the condition variable wakes when ConditionVariable::notify_one is called.

While a thread is waiting for notification of a ConditionVariable, it releases the lock held on the mutex. The ConditionVariable reacquires the mutex lock before exiting the wait function.

Unspecified behavior

The thread that is unblocked on ConditionVariable::notify_one is unspecified if there are multiple waiters.
When ConditionVariable::notify_one or ConditionVariable::notify_all is called and there are one or more waiters, and one or more threads attempting to acquire the condition variable's mutex, the order in which the mutex is acquired is unspecified.
Spurious notifications (not triggered by the application) can occur.

Undefined behavior

Calling wait if the mutex is not locked by the current thread is undefined behavior.
The order in which waiting threads acquire the condition variable's mutex after ConditionVariable::notify_all is called is undefined.
The behavior of ConditionVariable::wait and ConditionVariable::wait_for is undefined if the condition variable's mutex is locked more than once by the calling thread.

Note: Synchronization level: Thread safe

Example:

#include "mbed.h"
Mutex mutex;
ConditionVariable cv(mutex);
// These variables are protected by locking the mutex.
uint32_t work_count = 0;
bool done = false;
void worker_thread()
{
  // Acquire lock on mutex before accessing protected variables and waiting.
  mutex.lock();
  while (done == false) {
    printf("Worker thread: Count: %lu\r\n", work_count);
    // Wait for main thread to notify the condition variable.
    printf("Worker thread: Waiting\r\n");
    cv.wait();
  }
  printf("Worker: Exiting\r\n");
  // The condition variable acquires the lock when exiting the `wait` function.
  // Unlock mutex when exiting the thread.
  mutex.unlock();
}
int main()
{
  Thread thread;
  thread.start(worker_thread);
  for (int i = 0; i < 5; i++) {
    // Acquire lock on mutex before modifying variables and notifying.
    mutex.lock();
    // Change count and notify waiters.
    work_count++;
    printf("Main thread: Set count to: %lu\r\n", work_count);
    printf("Main thread: Notifying worker thread\r\n");
    cv.notify_all();
    // Mutex must be unlocked before the worker thread can acquire it.
    mutex.unlock();
    wait(1.0);
  }
  // Change done and notify waiters of this.
  mutex.lock();
  done = true;
  cv.notify_all();
  mutex.unlock();
  thread.join();
  printf("Main: Exiting\r\n");
}

Definition at line 152 of file ConditionVariable.h.

Public Member Functions

Detailed Description

Unspecified behavior

Undefined behavior

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