condition_variable是同步原语,被使用在std::mutex去阻塞块在不同线程,直到线程修改共享变量并且唤醒条件变量;
线程尝试修改共享变量必须:
1、获得mutex;例如std::lock_guard
2、获得锁后修改共享变量;(即使共享变量是原子量,也要获得锁才能修改)
3、接着调用notify_one或者notify_all;
线程等等待条件变量必须:
1、获得std::unique_lock
2、做其中一种:检查条件;调用wait,wait_for,wait_until;检查条件和如果没有唤醒则重新等待,
实现简单的条件变量:
#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
void worker_thread()
{
// Wait until main() sends data
std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{
return ready;});
// after the wait, we own the lock.
std::cout << "Worker thread is processing data\n";
data += " after processing";
// Send data back to main()
processed = true;
std::cout << "Worker thread signals data processing completed\n";
// Manual unlocking is done before notifying, to avoid waking up
// the waiting thread only to block again (see notify_one for details)
lk.unlock();
cv.notify_one();
}
int main()
{
std::thread worker(worker_thread);
data = "Example data";
// send data to the worker thread
{
std::lock_guard<std::mutex> lk(m);
ready = true;
std::cout << "main() signals data ready for processing\n";
}
cv.notify_one();
// wait for the worker
{
std::unique_lock<std::mutex> lk(m);
cv.wait(lk, []{
return processed;});
}
std::cout << "Back in main(), data = " << data << '\n';
worker.join();
}
参考: