为什么要对多线程进行加锁操作呢

一.如果不加锁,会怎么样?

可能会发生数据竞争,造成数据错乱.

例子:

本来想要的结果n=0,但是执行发现n的值不为0,而且有多种取值.究其原因,是因为多个线程之间会发生数据竞争,导致CPU线程调度时出现问题,不能够保证线程内执行代码的原子操作.我发现string str = "hello";这一句是必要的,不然它就不会出现n不为0的现象.(有待进一步研究)

注意,如果要所有子线程执行完毕后,再执行主线程.要有WaitForMultipleObjects操作.

#include "windows.h"
#include <iostream>using namespace std;const int ThreadNum = 50;
int g_Num = 0;
DWORD WINAPI ThreadFun(void * param)
{
    for (int i = 0; i < 100; i++)
    {
        g_Num = g_Num+1;
        string str = "hello";
        g_Num = g_Num-1;
    }
    return 0;
}
int main()
{
    DWORD lpThread[ThreadNum];
    HANDLE h[ThreadNum];
    for (int i = 0; i <ThreadNum; i++)
    {
         h[i] = CreateThread(NULL, 0, ThreadFun, NULL, 0, &lpThread[i]);
    }
    WaitForMultipleObjects(ThreadNum, h, TRUE, INFINITE); //等待所有的子线程执行完毕.
    int n = g_Num;
    cout << "n:" << n << endl;
    return 0;
}

二.加锁后的情形

WaitForMultipleObjects是必须的,一开始忘记了使用这个,导致在EnterCriticalSection处报错,猜测是主线程没有等子线程执行完毕就结束了程序.为了验证猜想,将WaitForMultipleObjects注释掉,
在子函数中打印,发现确实是这个原因.
加锁后每次n打印出来的都是0了.

#include "windows.h"
#include <iostream>
using namespace std;
const int ThreadNum = 50;

CRITICAL_SECTION g_Cs;
int g_Num = 0;
DWORD WINAPI ThreadFun(void * param)
{
    for (int i = 0; i < 100; i++)
    {
        EnterCriticalSection(&g_Cs);
        g_Num = g_Num+1;
        string str = "hello";
        g_Num = g_Num-1;
        LeaveCriticalSection(&g_Cs);
    }
    return 0;
}

int main()
{
    InitializeCriticalSection(&g_Cs);
    DWORD lpThread[ThreadNum];
    HANDLE h[ThreadNum];
    for (int i = 0; i <ThreadNum; i++)
    {
        h[i] = CreateThread(NULL, 0, ThreadFun, NULL, 0, &lpThread[i]);
    }
    WaitForMultipleObjects(ThreadNum, h, TRUE, INFINITE); //等待所有的子线程执行完毕.
    int n = g_Num;
    cout << "n:" << n << endl;
    DeleteCriticalSection(&g_Cs);
    return 0;
}