23.线程锁的使用

不合理的设定临界区域，会让线程的调用失去意义。

1.不应该频繁的使用锁

2.减小锁使用的区域，线程公共资源之外 的资源 尽量不要放到临界区。

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 示例一：（这样使用线程锁，设定的临界区域不合理，会让线程的调用失去意义，线程并没有并发执行）

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

using namespace std;

mutex m_lock;

int sum = 0;

void workFun(int index)
{
    m_lock.lock();
    for (int i = 0; i < 1000; i++)
    {
        sum++;
        cout << "thread:"<<index << " sum=" << sum<<endl;
    }
    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();
    }

    cout << "sum=" << sum << endl;
    cout << "i am main thread " << endl;
    return 0;
}

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示例二：（不用线程）

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

using namespace std;

mutex m_lock;

int sum = 0;

void workFun()
{
    
    for (int i = 0; i < 300000; i++)
    {
        sum++;
    }
    

}//抢占式

int main()
{
    thread t[3];

    workFun();

    cout << "sum=" << sum << endl;
    cout << "i am main thread " << endl;
    return 0;
}

示例三：（使用线程，并且加锁）

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

using namespace std;

mutex m_lock;

int sum = 0;

void workFun(int index)
{

    for (int i = 0; i < 100000; i++)
    {

        m_lock.lock();
        sum++;
        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();
    }

    cout << "sum=" << sum << endl;
    cout << "i am main thread " << endl;
    return 0;
}

示例三的耗时 会比 示例二的耗时大，所以要慎用多线程，慎用锁。设计不好的多线程反而会更耗时。