thread

介绍

之前有讨论过 pthread 这个库，这个库是 POSIX 系统的标准库，而 thread 是 C++11 的标准库，因此在多数能编译 C++ 的平台上直接运行（需要系统的支持）。

示例

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mtx;
int counter = 0;

void increaseCounterA(int& counter, std::mutex& mtx)
{
    while(1)
    {
        std::chrono::seconds sleepTime(2);
        std::this_thread::sleep_for(sleepTime);
        mtx.lock();
        ++counter;
        std::cout << "A Counter value: " << counter << std::endl;
        mtx.unlock();
    }
}

void increaseCounterB(int& counter, std::mutex& mtx)
{
    while(1)
    {
        std::chrono::seconds sleepTime(2);
        std::this_thread::sleep_for(sleepTime);
        mtx.lock();
        ++counter;
        std::cout << "B Counter value: " << counter << std::endl;
        mtx.unlock();
    }
}

int main()
{
    std::thread t1{increaseCounterA, std::ref(counter), std::ref(mtx)};
    std::thread t2{increaseCounterB, std::ref(counter), std::ref(mtx)};

    t1.join();
    t2.join();

    return 0;
}

上面的示例展示了 thread 库的接口，可以观察到几点：

1. 共享的资源通过线程的构造函数参数传入，线程回调函数通过引用接收；
2. 锁也通过参数传入

有多把锁的时候使用 std::lock_guard 或者 std::unique_lock

unique_lock

#include <iostream>
#include <thread>
#include <mutex>

std::mutex mutex;

void workerThread()
{
    std::unique_lock<std::mutex> lock(mutex);
    std::cout << "Worker thread is running." << std::endl;
    lock.unlock();
}

int main()
{
    std::thread worker(workerThread);

    std::unique_lock<std::mutex> lock(mutex);
    std::cout << "Main thread is running." << std::endl;
    lock.unlock();

    worker.join();
    return 0;
}

条件变量

解决死锁问题的，同 pthread 笔记

#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>

std::mutex mutex;
std::condition_variable condVar;
bool ready = false;

void workerThread()
{
    // 模拟一些耗时的操作
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::unique_lock<std::mutex> lock(mutex);
    ready = true;
    lock.unlock();
    condVar.notify_one();
}

int main()
{
    std::thread worker(workerThread);

    std::unique_lock<std::mutex> lock(mutex);
    while (!ready) {
        condVar.wait(lock);
    }

    std::cout << "Worker thread is ready." << std::endl;

    worker.join();
    return 0;
}

这里是主线程先释放锁进行等待