C++并发及互斥保护示例-编程知识

最近要写一个多线程的并发数据库，主要是希望使用读写锁实现库的并发访问，同时考虑到其他平台(如Iar)没有C++的读写锁，需要操作系统提供，就将读写锁封装起来。整个过程还是比较曲折的，碰到了不少问题，在此就简单分析总结下并发和互斥吧。

首先，先贴上一部分源代码：

#include <shared_mutex>
#include <iostream>
#include <windows.h>
#include <synchapi.h>using cegn_mutex = std::shared_mutex;
cegn_mutex g_cegn_mutex;
void cegn_mutex_unique_lck(cegn_mutex& testmutex)	//独占锁，写数据
{std::unique_lock<cegn_mutex> cegn_lock(testmutex);
}void cegn_mutex_share_lck(cegn_mutex& Dbmutex)	//共享锁，读数据
{std::shared_lock<cegn_mutex> cegn_lock(Dbmutex);
}void cegn_mutex_unlck(cegn_mutex& Dbmutex)
{;	//vc读写锁离开作用域自动释放
}int g_dwVal = 0;
void FastWriteData(int i)
{while (1){cegn_mutex_unique_lck(g_cegn_mutex);g_dwVal++;std::cout << "FastWriteData " << "  Set dwVal= " << g_dwVal << "\n";Sleep(1000);cegn_mutex_unlck(g_cegn_mutex);}
}void SlowWriteData(int i)
{while (1){cegn_mutex_unique_lck(g_cegn_mutex);g_dwVal++;std::cout << "SlowWriteData " << " Set dwVal= " << g_dwVal << "\n";Sleep(5000);cegn_mutex_unlck(g_cegn_mutex);}
}void ReadData(int i)
{while (1){cegn_mutex_share_lck(g_cegn_mutex);std::cout << "ReadData " << " Get dwVal= " << g_dwVal << "\n";Sleep(500);cegn_mutex_unlck(g_cegn_mutex);}
}int main()
{std::cout << "main start !!" << std::endl;std::thread thread1 = std::thread(FastWriteData, 0);std::thread thread2 = std::thread(SlowWriteData, 0);thread1.join();thread2.join();getchar();return 1;
}

代码不长，逻辑也挺清晰的，但结果不正确：

似乎就没有互斥保护，因为FastWriteData和SlowWriteData中都独占了cegn_mutex_unique_lck(g_cegn_mutex);

且在while(1)中，不存在释放写锁的情况，那就不应该两个写线程交替出现。

如上让chatgpt分析下，它认为没啥问题，我尝试修改回标准读写锁接口，如下：

void FastWriteData(int i)
{while (1){
//		cegn_mutex_unique_lck(g_cegn_mutex);std::unique_lock<cegn_mutex> lck(g_cegn_mutex);g_dwVal++;std::cout << "FastWriteData " << "  Set dwVal= " << g_dwVal << "\n";Sleep(1000);cegn_mutex_unlck(g_cegn_mutex);}
}void SlowWriteData(int i)
{while (1){
//		cegn_mutex_unique_lck(g_cegn_mutex);std::unique_lock<cegn_mutex> lck(g_cegn_mutex);g_dwVal++;std::cout << "SlowWriteData " << " Set dwVal= " << g_dwVal << "\n";Sleep(5000);cegn_mutex_unlck(g_cegn_mutex);}
}

如上，代码运行就是正常了

main start !!
FastWriteData   Set dwVal= 1
FastWriteData   Set dwVal= 2
FastWriteData   Set dwVal= 3
FastWriteData   Set dwVal= 4
FastWriteData   Set dwVal= 5
FastWriteData   Set dwVal= 6
FastWriteData   Set dwVal= 7
FastWriteData   Set dwVal= 8
FastWriteData   Set dwVal= 9
FastWriteData   Set dwVal= 10
FastWriteData   Set dwVal= 11
FastWriteData   Set dwVal= 12
FastWriteData   Set dwVal= 13
FastWriteData   Set dwVal= 14

现在FastWriteData就独占了互斥量，导致SlowWriteData无法运行。为啥使用接口：

void cegn_mutex_unique_lck(cegn_mutex& testmutex) //独占锁，写数据
{
std::unique_lock<cegn_mutex> cegn_lock(testmutex);
}

就不行了？

修改成直接调用：

using cegn_mutex = std::shared_mutex;
cegn_mutex g_cegn_mutex;
void cegn_mutex_unique_lck(cegn_mutex& testmutex)	//独占锁，写数据
{
//	std::unique_lock<cegn_mutex> cegn_lock(testmutex);std::unique_lock<cegn_mutex> cegn_lock(g_cegn_mutex);
}

还是不能正确互斥，修改如下也一样：

void cegn_mutex_unique_lck(cegn_mutex& testmutex)	//独占锁，写数据
{
//	std::unique_lock<cegn_mutex> cegn_lock(testmutex);std::unique_lock<std::shared_mutex> cegn_lock(g_cegn_mutex);
}

经过分析，问题是：

void cegn_mutex_unique_lck(cegn_mutex& testmutex)

函数中定义了一个互斥量cegn_lock ：

std::unique_lock<cegn_mutex> cegn_lock(testmutex);

该互斥量在函数退出的时候，生命周期就结束了，所以自动销毁，最终导致无法互斥，那是在想要封装，如何实现呢，可以自己协议个类封装：

完整的简单代码如下：

#include <iostream>
#include <thread>
#include <mutex>
#include <windows.h>class MutexWrapper {
public:MutexWrapper(std::mutex& mutex) : m_mutex(mutex) {m_mutex.lock();}~MutexWrapper() {m_mutex.unlock();}private:std::mutex& m_mutex;
};std::mutex g_mutex_test;
int g_dwVal = 0;void FastWriteData(int i) {while (1) {MutexWrapper lock(g_mutex_test);g_dwVal++;std::cout << "FastWriteData " << "  Set dwVal= " << g_dwVal << "\n";Sleep(1000);}
}void SlowWriteData(int i) {while (1) {MutexWrapper lock(g_mutex_test);g_dwVal++;std::cout << "SlowWriteData " << " Set dwVal= " << g_dwVal << "\n";Sleep(3000);}
}int main() {std::cout << "main start !!" << std::endl;std::thread thread1 = std::thread(FastWriteData, 0);std::thread thread2 = std::thread(SlowWriteData, 0);thread1.join();thread2.join();getchar();return 1;
}

如此，运行正常了

修改下例程，让两个进程都整行跑

void FastWriteData(int i) {while (1) {{MutexWrapper lock(g_mutex_test);g_dwVal++;std::cout << "FastWriteData " << "  Set dwVal= " << g_dwVal << "\n";}Sleep(1000);}
}void SlowWriteData(int i) {while (1) {{MutexWrapper lock(g_mutex_test);g_dwVal++;std::cout << "SlowWriteData " << " Set dwVal= " << g_dwVal << "\n";}Sleep(3000);}
}

如上，代码就基本都正常了。

当然，也可以将互斥锁修改为读写锁，如下：

class MutexWrapper {
public:MutexWrapper(std::shared_mutex& mutex) : m_mutex(mutex) {m_mutex.lock();}~MutexWrapper() {m_mutex.unlock();}private:std::shared_mutex& m_mutex;
};std::shared_mutex g_mutex_test;

代码也运行正常了。

综上：

1：基于RAII，C++的很多变量生命周期有限，必须特别注意智能变量的生命周期。

2：如果需要封装读写锁，不能简单函数分装，实在不行，就用一个类封装吧

3：要熟练掌握std::thread，std::shared_mutex，std::mutex的用法，这个是变法互斥基本要求