Monitor flashing when running a Windows SendInput API

Question

Well, I certainly should go to python since I did several functions of this type, keyboard event and mouse event, but decide to try to learn the windows api.

My goal is to know when button 1 of the mouse is pressed.

I created this file in a very beginner way, it returns in mouseData only 0. The curious thing is that whenever I run it, it flashes my monitor at short intervals in blinks, but between 1 second with it off. Very strange that, execution is not viable.

Could someone help me understand and try to execute to see if it is only here.

Code:

int main()
{
    DWORD mouseData = 0;

    MOUSEINPUT tagMouse;
    tagMouse.dx = 0;
    tagMouse.dy = 0;
    tagMouse.mouseData = mouseData;
    tagMouse.dwFlags = MOUSEEVENTF_XDOWN;
    tagMouse.dwExtraInfo = 0;

    INPUT tagInput;
    tagInput.type = INPUT_MOUSE;
    tagInput.mi = tagMouse;

    while (true) {
        if (GetAsyncKeyState(VK_DELETE)) break;
        SendInput(1, &tagInput, sizeof(INPUT));
        printf("KEYWORD: %d\n", mouseData);
        Sleep(500);
    }
    system("pause");
    return 0;
}

Answer 1

I can reproduce your reported 'symptoms' - and the effect is really brutal!

Now, while I cannot offer a full explanation, I can offer a fix! You have an uninitialized field in your tagMouse structure (the time member, which is a time-stamp used by the system). Setting this to zero (which tells the system to generate its own time-stamp) fixes the problem. So, just add this line to your other initializer statements:

//...
    tagMouse.dwExtraInfo = 0;
    tagMouse.time = 0; // Adding this line fixes it!
//...

Hope this helps!

Note: I, too, would appreciate a fuller explanation; however, an uninitialized field, to me, smells like undefined behaviour! I have tried a variety of other values (i.e. not zero) for the time field but haven't yet found one that works.

The discussion here on devblogs may help. This quote seems relevant:

And who knows what sort of havoc that will create if a program checks the timestamps and notices that they are either from the future or have traveled back in time.