C++ thread类多线程编程

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https://blog.csdn.net/dcrmg/article/details/53912941

多线程操作的thread类，简单多线程示例：

#include <iostream>
#include <thread>
#include <Windows.h>
 
using namespace std;
 
void thread01()
{
	for (int i = 0; i < 5; i++)
	{
		cout << "Thread 01 is working ！" << endl;
		Sleep(100);
	}
}
void thread02()
{
	for (int i = 0; i < 5; i++)
	{
		cout << "Thread 02 is working ！" << endl;
		Sleep(200);
	}
}
 
int main()
{
	thread task01(thread01);
	thread task02(thread02);
	task01.join();
	task02.join();
 
	for (int i = 0; i < 5; i++)
	{
		cout << "Main thread is working ！" << endl;
		Sleep(200);
	}
	system("pause");
}

输出：

Thread 02 is working ！Thread 01 is working ！

Thread 01 is working ！
Thread 02 is working ！
Thread 01 is working ！
Thread 01 is working ！
Thread 02 is working ！
Thread 01 is working ！
Thread 02 is working ！
Thread 02 is working ！
Main thread is working ！
Main thread is working ！
Main thread is working ！
Main thread is working ！
Main thread is working ！
请按任意键继续. . .

两个子线程并行执行，join函数会阻塞主流程，所以子线程都执行完成之后才继续执行主线程。可以使用detach将子线程从主流程中分离，独立运行，不会阻塞主线程：

#include <iostream>
#include <thread>
#include <Windows.h>
 
using namespace std;
 
void thread01()
{
	for (int i = 0; i < 5; i++)
	{
		cout << "Thread 01 is working ！" << endl;
		Sleep(100);
	}
}
void thread02()
{
	for (int i = 0; i < 5; i++)
	{
		cout << "Thread 02 is working ！" << endl;
		Sleep(200);
	}
}
 
int main()
{
	thread task01(thread01);
	thread task02(thread02);
	task01.detach();
	task02.detach();
 
	for (int i = 0; i < 5; i++)
	{
		cout << "Main thread is working ！" << endl;
		Sleep(200);
	}
	system("pause");
}

输出：

Main thread is working ！
Thread 01 is working ！Thread 02 is working ！

Thread 01 is working ！
Thread 02 is working ！Main thread is working ！

Thread 01 is working ！
Thread 01 is working ！
Thread 02 is working ！Main thread is working ！

Thread 01 is working ！
Main thread is working ！Thread 02 is working ！

Main thread is working ！Thread 02 is working ！

请按任意键继续. . .

用detach的主线程和两个子线程并行执行。

在绑定的时候也可以同时给带参数的线程传入参数：

#include <iostream>
#include <thread>
#include <Windows.h>
 
using namespace std;
 
//定义带参数子线程
void thread01(int num)
{
	for (int i = 0; i < num; i++)
	{
		cout << "Thread 01 is working ！" << endl;
		Sleep(100);
	}
}
void thread02(int num)
{
	for (int i = 0; i < num; i++)
	{
		cout << "Thread 02 is working ！" << endl;
		Sleep(200);
	}
}
 
int main()
{
	thread task01(thread01, 5);  //带参数子线程
	thread task02(thread02, 5);
	task01.detach();
	task02.detach();
 
	for (int i = 0; i < 5; i++)
	{
		cout << "Main thread is working ！" << endl;
		Sleep(200);
	}
	system("pause");
}

输出跟上例输出一样

多线程数据竞争

多个线程同时对同一变量进行操作的时候，如果不对变量做一些保护处理，有可能导致处理结果异常：

#include <iostream>
#include <thread>
#include <Windows.h>
 
using namespace std;
 
int totalNum = 100;
 
void thread01()
{
	while (totalNum > 0)
	{
		cout << totalNum << endl;
		totalNum--;
		Sleep(100);
	}
}
void thread02()
{
	while (totalNum > 0)
	{
		cout << totalNum << endl;
		totalNum--;
		Sleep(100);
	}
}
 
int main()
{
	thread task01(thread01);
	thread task02(thread02);
	task01.detach();
	task02.detach();
	system("pause");
}

有两个问题，一是有很多变量被重复输出了，而有的变量没有被输出；二是正常情况下每个线程输出的数据后应该紧跟一个换行符，但这里大部分却是另一个线程的输出。

这是由于第一个线程对变量操作的过程中，第二个线程也对同一个变量进行各操作，导致第一个线程处理完后的输出有可能是线程二操作的结果。针对这种数据竞争的情况，可以使用线程互斥对象mutex保持数据同步。

mutex类的使用需要包含头文件mutex：

#include <iostream>
#include <thread>
#include <Windows.h>
#include <mutex>
 
using namespace std;
 
mutex mu;  //线程互斥对象
 
int totalNum = 100;
 
void thread01()
{
	while (totalNum > 0)
	{
		mu.lock(); //同步数据锁
		cout << totalNum << endl;
		totalNum--;
		Sleep(100);
		mu.unlock();  //解除锁定
	}
}
void thread02()
{
	while (totalNum > 0)
	{
		mu.lock();
		cout << totalNum << endl;
		totalNum--;
		Sleep(100);
		mu.unlock();
	}
}
 
int main()
{
	thread task01(thread01);
	thread task02(thread02);
	task01.detach();
	task02.detach();
	system("pause");
}

多线程中加入mutex互斥对象之后输出正常。

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