ConditionVariable
ConditionVariable class hierarchy
The ConditionVariable class provides a mechanism to safely wait for or signal state changes. A common scenario when writing multithreaded code is to protect shared resources with a mutex and then release that mutex to wait for a change of that data. If you do not do this carefully, this can lead to a race condition in the code. A condition variable provides a safe solution to this problem by handling the wait for a state change, along with releasing and acquiring the mutex automatically during this waiting period. Note that you cannot make wait or notify calls on a condition variable from ISR context. Unlike EventFlags, ConditionVariable does not let you wait on multiple events at the same time.
ConditionVariable class reference
| Public Member Functions | |
| ConditionVariable (Mutex &mutex) | |
| Create and initialize a ConditionVariable object. More... | |
| void | wait () |
| Wait for a notification. More... | |
| template<typename Predicate > | |
| void | wait (Predicate pred) |
| Wait for a predicate. More... | |
| bool | wait_until (uint64_t millisec) |
| Wait for a notification until the specified time. More... | |
| cv_status | wait_until (Kernel::Clock::time_point abs_time) |
| Wait for a notification until the specified time. More... | |
| template<class Predicate > | |
| bool | wait_until (Kernel::Clock::time_point abs_time, Predicate pred) |
| Wait for a predicate until the specified time. More... | |
| bool | wait_for (uint32_t millisec) |
| Wait for a notification or timeout. More... | |
| cv_status | wait_for (Kernel::Clock::duration_u32 rel_time) |
| Wait for a notification or timeout. More... | |
| template<class Predicate > | |
| bool | wait_for (Kernel::Clock::duration rel_time, Predicate pred) |
| Wait for a predicate 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... | |
ConditionVariable example
Below is an example of ConditionVariable usage, where one thread is generating events every 1s, and the second thread is waiting for the events and executing an action.
/*
* Copyright (c) 2018-2020 Arm Limited and affiliates.
* SPDX-License-Identifier: Apache-2.0
*/
#include "mbed.h"
Mutex mutex;
ConditionVariable cond(mutex);
// These variables are protected by locking mutex
uint32_t counter = 0;
bool done = false;
void worker_thread()
{
mutex.lock();
do {
printf("Worker: Count %lu\r\n", counter);
// Wait for a condition to change
cond.wait();
} while (!done);
printf("Worker: Exiting\r\n");
mutex.unlock();
}
int main()
{
Thread thread;
thread.start(worker_thread);
for (int i = 0; i < 5; i++) {
mutex.lock();
// Change count and signal this
counter++;
printf("Main: Set count to %lu\r\n", counter);
cond.notify_all();
mutex.unlock();
ThisThread::sleep_for(1000);
}
mutex.lock();
// Change done and signal this
done = true;
printf("Main: Set done\r\n");
cond.notify_all();
mutex.unlock();
thread.join();
}