thread-01

Posted 2020-10-08 Lucky、Dog

// 8:15 AM/09/28/2017
#pragma once
#include <iostream>       // std::cout
#include <thread>         // std::thread
#include <mutex>          // std::mutex
#include <chrono>
using namespace std;
volatile int counter(0);
volatile int counter2(0);
mutex mtx;//This mutex class is synchronization primitive that
//can be used to protect shared data from being simultaneously accessed by multiple threads.
// mutex类是一个同步原语，用来保护共享数据，阻止多线程同时访问
mutex mtx2;

void run()
{
    for (int i = 0; i < 100; ++i)
    {
        mtx.lock();// lock mtx,blocks if mtx is not available
        // the word block means that when the mtx is avaiable,it will lock mtx and the following code will being executed
        ++counter;
        cout << this_thread::get_id() << "==> " << counter << endl;

        mtx.unlock();// this function will make mtx is available,
        //and other threads that is being blocked will detect the mtx is available
        // but the others don‘t mean that all of them can detect the mtx is available because if one detect it and it will lock it.
        // only the one thread will own the mtx 
        //here the function unlock is necessary
        //一般不直接使用mutex 而用 std::unique_lock, std::lock_guard等
        //mutex is usually not accessed directly
 

    }
}

void run2()
{
    for (int i = 0; i < 100; i++)
    {
        if (mtx2.try_lock())
            //It differs from the function lock.Here,it will not block and if mtx2 is available,it will be lock and return ture.
        {
            ++counter2;
            cout << this_thread::get_id() << "==> " << counter2 << endl;
            mtx2.unlock();
        }
    }
}

int main(int argc, const char* argv[])
{
    thread ts[10];
    for (int i = 0; i < 10; ++i)
    {
        ts[i] = thread(run);
    }
    for (auto& t : ts) t.join();

    std::this_thread::sleep_for(std::chrono::seconds(2));
    // sleep for 2s

    thread ts2[10];
    for (int i = 0; i < 10; ++i)
    {
        ts2[i] = thread(run2);
    }
    for (auto& t : ts2)t.join();
}
//We see that the results of counter and counter2 are not same,and we convincingly konw the counter is equal
//to 1000 because of the function lock.The counter2,however,may not have a unique result owing to the function
// try_lock without blocking.