c++ - 为什么线程似乎随机执行？

Question

我在理解c++线程时遇到了麻烦。
说我有以下代码，

#include <cstdlib>
#include <iostream>
#include <string>
#include <vector>
#include <ctime>
#include <numeric>
#include <cmath>
#include <sstream>
#include <thread>
#include <chrono>
#include <ctime>
#include <mutex>

int GetRandom(int max){
    srand(time(NULL));
    return rand() % max;
}
std::string GetTime(){
    auto nowTime = std::chrono::system_clock::now();
    std::time_t sleepTime = 
            std::chrono::system_clock::to_time_t(nowTime);
    return std::ctime(&sleepTime);
}

double acctBalance = 100;

// Protects shared data from being accessed at the
// same time
std::mutex acctLock;

void GetMoney(int id,
        double withdrawal){

    // The exception safe way to protect access
    // to code within its scope. The lock is released 
    // after execution leaves this scope
    std::lock_guard<std::mutex> lock(acctLock);

    // Blocks access between lock and unlock
    // until execution completes
    // This isn't good to use however if an error 
    // occurs between lock and unlock
    // acctLock.lock();

    std::this_thread::sleep_for(std::chrono::seconds(3));

    std::cout << id << 
            " tries to withdrawal $" <<
            withdrawal << " on " <<
            GetTime() << "\n";

    if((acctBalance - withdrawal) >= 0){
        acctBalance -= withdrawal;
        std::cout << "New Account Balance is $" <<
                acctBalance << "\n";
    } else {
        std::cout << "Not Enough Money in Account\n";
        std::cout << "Current Balance is $" <<
                acctBalance << "\n";
    }
    // acctLock.unlock();
}
int main()
{
    /* ----- SIMPLE THREAD EXAMPLE -----
    // Create a thread and pass a parameter
    // to the function
    std::thread th1 (ExecuteThread, 1);

    // Join the thread to the main thread
    // meaning main waits for this thread to
    // stop executing before continuing execution
    // of code in main
    th1.join();

    std::thread th2 (ExecuteThread, 2);
    th2.join();
    ----- END SIMPLE THREAD EXAMPLE ----- */

    // We will create a pool of threads that
    // will access a bank account in no particular
    // order
    std::thread threads[10];

    for(int i = 0; i < 10; ++i){
        threads[i] = std::thread(GetMoney, i, 15);
    }

    for(int i = 0; i < 10; ++i){
        threads[i].join();
    }



    return 0;
}

如果要监视输出，则可以看到未按调用顺序访问线程。
总的来说，有一个for循环遍历从0到9的线程，但是为什么线程以不同的顺序打印(2,1,0,3,6 ... etc)。
我想念什么？

Answer 1

您什么都不会错过。创建std::thread会告诉操作系统创建一个线程，然后该线程将以任意顺序执行(无论操作系统何时运行它)。由于大多数OS调度程序都是不确定的，因此预期的行为基本上是随机的。您所看到的是预期的行为。

例如，请参见 std::thread::thread 的底部。