<二>线程间互斥-mutex互斥锁和lock

多线程程序

竞态条件:多线程程序执行的结果是一致的,不会随着CPU对线程不同的调用顺序而产生不同的运行结果.

解决？:互斥锁 mutex

经典的卖票问题,三个线程卖100张票

代码1

#include <iostream>

#include <thread>

#include <list>

#include <mutex>

int ticketCount = 100;

std::mutex mtx;//互斥锁

void sellTicket(int window) {

	while (ticketCount > 0) {

		mtx.lock();

		std::cout << "窗口" << window << "销售" << ticketCount << std::endl;

		ticketCount--;

		mtx.unlock();

		std::this_thread::sleep_for(std::chrono::milliseconds(50));	

	}

}//end

int main() {

	std::list<std::thread> tlist;

	for (int i = 0; i < 3; i++) {

		tlist.push_back(std::thread(sellTicket,i));

	}

	for (std::thread & t : tlist) {

		t.join();

	}

	system("pause");

	return 0;

}

上面代码的问题...

while (ticketCount > 0) {

    mtx.lock();

    std::cout << "窗口" << window << "销售" << ticketCount << std::endl;

    ticketCount--;

    mtx.unlock();

    std::this_thread::sleep_for(std::chrono::milliseconds(50));	

}

如果ticketCount =1 ，当前有一个线程A while (ticketCount > 0)为true,线程A还没执行ticketCount--完成时,cpu交给了线程B

线程B while (ticketCount > 0)也为true,进入 循环体内,造成了买0号票,改进如下

代码2

#include <iostream>

#include <thread>

#include <list>

#include <mutex>

int ticketCount = 100;

std::mutex mtx;//互斥锁

void sellTicket(int window) {

	while (ticketCount > 0) {

		mtx.lock();

                if(ticketCount >0){

		  std::cout << "窗口" << window << "销售" << ticketCount << std::endl;

		  ticketCount--;

                }

		mtx.unlock();

		std::this_thread::sleep_for(std::chrono::milliseconds(50));

	}

}//end

int main() {

	std::list<std::thread> tlist;

	for (int i = 0; i < 3; i++) {

		tlist.push_back(std::thread(sellTicket,i));

	}

	for (std::thread & t : tlist) {

		t.join();

	}

	system("pause");

	return 0;

}

代码2还有些问题!! 如下

mtx.lock();

代码

代码

代码

代码

.....

mtx.unlock();	

如果在代码lock()和unlock()之间 非常返回,导致mtx没有正常unlock(),那么出现死锁问题 =》智能指针  lock_gurad unique_lock

看lock_gurad

#include <iostream>

#include <thread>

#include <list>

#include <mutex>

int ticketCount = 100;

std::mutex mtx;//互斥锁

void sellTicket(int window) {

	while (ticketCount > 0) {

		{

			std::lock_guard<std::mutex> lock(mtx);

			std::cout << "窗口" << window << "销售" << ticketCount << std::endl;

			ticketCount--;

			std::this_thread::sleep_for(std::chrono::milliseconds(50));

		}

	}

}//end

int main() {

	std::list<std::thread> tlist;

	for (int i = 0; i < 3; i++) {

		tlist.push_back(std::thread(sellTicket,i));

	}

	for (std::thread & t : tlist) {

		t.join();

	}

	system("pause");

	return 0;

}

上面的图片中我们知道lock_gurad 的拷贝构造函数被关闭了,所以当我们遇到函数调用需要拷贝构造lock_guard的时候,就有障碍了，这个时候可以用unique_lock

unique_lock 转移指针,支持带右值得拷贝赋值,支持参数传递拷贝构造的,他的左值的拷贝构造也是被关闭了看下图

巴特西

<二>线程间互斥-mutex互斥锁和lock_guard

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