MFC main UI thread workings and modal dialogs

Question

First let me say that I have roughly searched stackoverflow for this, but couldn't find a specific answer.

My question is rather theoretical and there is no problem with the running of any code. Please consider a simple MFC application with a timer event and a button (attached to an OnClick event).

void SampleDlg::OnTimer(UINT_PTR nIDEvent)
{
    CString msg;
    msg.Format("time: %lld, tid: %d", (int64_t)time(0), GetCurrentThreadId());

    SetWindowText(msg);
}

My intuition suggests that if I sleep inside the OnClick event, main UI thread should hang and timer events shouldn't kick in.

void SampleDlg::OnClick()
{
    Sleep(10000);
}

That is fine, however if I show a new modal dialog inside the OnClick, the timer events still happen. What is different here?

void SampleDlg::OnClick()
{
    CString msg;
    msg.Format("tid: %d is waiting...", GetCurrentThreadId());

    ::MessageBox(GetSafeHwnd(), msg, "Msg", 0);
    // at this point msgbox tells us that thread with tid is waiting

    // thread with tid wont reach this line until msgbox is closed
}

Edit: I have included GetCurrentThreadId() calls to make what i want to ask more clear.

When I run the code above, both msgbox and the window title gives me the same thread-id: 22012 (for example). My question is, what is the value of PC/IP (program counter or instruction pointer) of thread 22012 when msgbox is being shown?

Answer 1

The MessageBox has it's own message loop, like all modal dialogs.

So it uses PeekMessage/GetMessage. WM_TIMER and WM_PAINT messages are pseudo messages that are generated when the message loop executes.

This means that MessageBox internally calls GetMessage or PeekMessage and this causes the effect that the thread still executes MessageBox, but new messages are delivered to you windows.