C ++ 11 threads and thread-safe atomic variables

// threadTest.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <iostream>
#include <thread>
#include <string>
#include <chrono>
#include <mutex>
using namespace std;

const int N = 100000000;
int num = 0;
void run()
{
	for (int i = 0; i < N; i++) {
		num++;
	}
}

int main()
{
	clock_t start = clock();
	thread t1(run);
	thread t2(run);
	t1.join();
	t2.join();
	clock_t end = clock();
	cout << "num=" << num << ", 用时" << end - start << " ms" << endl;
    return 0;
}

operation result:

Analysis: From the above results, the code, num did not turn 2 billion, due to the thread clashed, leading to incorrect results.

Let us join lock mechanism

// threadTest.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <iostream>
#include <thread>
#include <string>
#include <chrono>
#include <mutex>
using namespace std;

const int N = 100000000;
int num = 0;
std::mutex m;

void run()
{
	for (int i = 0; i < N; i++) {
		m.lock();
		num++;
		m.unlock();
	}
}

int main()
{
	clock_t start = clock();
	thread t1(run);
	thread t2(run);
	t1.join();
	t2.join();
	clock_t end = clock();
	cout << "num=" << num << ", 用时" << end - start << " ms" << endl;
    return 0;
}

operation result:

The result is correct, but it takes too long, because the mutex locking and unlocking takes time.

Modify the code again, with atomic variables

// threadTest.cpp : 定义控制台应用程序的入口点。
//
#include "stdafx.h"
#include <iostream>
#include <thread>
#include <string>
#include <chrono>
#include <mutex>
#include <atomic>
using namespace std;

const int N = 100000000;
//int num = 0;
atomic_int num = 0;

void run()
{
	for (int i = 0; i < N; i++) {
		num++;
	}
}

int main()
{
	clock_t start = clock();
	thread t1(run);
	thread t2(run);
	t1.join();
	t2.join();
	clock_t end = clock();
	cout << "num=" << num << ", 用时" << end - start << " ms" << endl;
    return 0;
}

operation result:

The result is correct, consuming better than with some of the mutex