CODEWITHSUNDEEP
c++windowsmultithreadingwinapi
Zhang
Zhang

Reputation: 3346

WaitOnAddress doesn't wake after the variable changed

According to WaitOnAddress doc, WaitOnAddress should block until the value at the given address changes.

#include <synchapi.h>
#include <thread>
#include <chrono>
using namespace std::chrono_literals;

#pragma comment(lib,"Synchronization.lib")

namespace __monitorVaribleChangeTest
{

    void startAThreadToWaitAVaribleChange(int& a)
    {
        std::thread th([&a]() {
            auto b = a;
            cout << "WaitOnAddress start\n";
            WaitOnAddress(&a, &b, sizeof(a), INFINITE);
            cout << "WaitOnAddress passed\n";
            });
        th.detach();
    }

    void main()
    {
        int a = 100;
        startAThreadToWaitAVaribleChange(a);

        std::this_thread::sleep_for(3000ms);
        a = 99;
        std::this_thread::sleep_for(3000ms);
    }
}

So, WaitOnAddress passed is supposed to be shown after a = 99; – but it never showed.

enter image description here

Upvotes: 1

Views: 1185

Answers (1)

Adrian Mole
Adrian Mole

Reputation: 51835

According to the documentation that you linked, you will need to signal that the value at the given address has changed, by calling either WakeByAddressSingle or WakeByAddressAll:

The address on which to wait. If the value at Address differs from the value at CompareAddress, the function returns immediately. If the values are the same, the function does not return until another thread in the same process signals that the value at Address has changed by calling WakeByAddressSingle or WakeByAddressAll or the timeout elapses, whichever comes first.

The following version of your main does what (I think) you expect:

    void main()
    {
        int a = 100;
        startAThreadToWaitAVaribleChange(a);

        std::this_thread::sleep_for(3000ms);
        a = 99;
        WakeByAddressSingle(&a);
        std::this_thread::sleep_for(3000ms);
    }

Upvotes: 2

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