21.多线程-锁与临界区域

不合理的设定临界区域，会让线程的调用失去意义。

代码一：

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

using namespace std;

mutex m_lock;

void workFun(int index)
{

    for (int i = 0; i < 100; i++)
    {
        m_lock.lock();
        //临界区域-开始
        cout << index << " child thread " << i << endl;
        //临界区域-结束
        m_lock.unlock();
    }
}//抢占式

int main()
{
    thread t[3];
    for (int n = 0; n<3; n++)
    {
        t[n] = thread(workFun, n);
    }

    for (int n = 0; n<3; n++)
    {
        t[n].join();
    }

    for (int i = 0; i<4; i++)
        cout << "i am main thread " << i << endl;
    return 0;
}

代码二：

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

using namespace std;

mutex m_lock;

void workFun(int index)
{
    m_lock.lock();
    //临界区域-开始
    for (int i = 0; i < 100; i++)
    {
        cout << index << " child thread " << i << endl;
    }
    //临界区域-结束
    m_lock.unlock();

}//抢占式

int main()
{
    thread t[3];
    for (int n = 0; n<3; n++)
    {
        t[n] = thread(workFun, n);
    }

    for (int n = 0; n<3; n++)
    {
        t[n].join();
    }

    for (int i = 0; i<4; i++)
        cout << "i am main thread " << i << endl;
    return 0;
}

代码二，会让 t[0] t[1]  t[2] 这三个线程不再并行，而是顺序执行。这样就失去了调用线程的意义。