Problems with wxThread::Wait in a dll under windows

[Marco Gubernati]

Hi,

I've got a dll that uses some features of wxWidgets. For example
threads. Inside the dll no need to use any GUI functions, so I simply
use wxInitialize/Uninitialize, in order to be able of starting threads.

Under Linux everything works, both if I link the dll(.so) against a
wxWidgets GUI application and a generic application.

Under Windows, if I link the dll against a GUI application not based on
wxWidgets (for example a MFC one) threads
 remain on an infinite loop inside the wxThread::Wait()..
I've inspected the code (wx 2.6.3 and 2.7.2, the following parts are
exactly the same in both version).

first, the wxInitialize creates an application object of class
wxConsoleApp

the wxThread::Wait() executes the following peace of code

    do
    {    [...]

        result = ::MsgWaitForMultipleObjects
                 (
                   1,              // number of objects to wait for
                   &m_hThread,     // the objects
                   false,          // don't wait for all objects
                   INFINITE,       // no timeout
                   QS_ALLINPUT|QS_ALLPOSTMESSAGE   // return as soon as
there are any events
                 );

        switch ( result )
        {
            case 0xFFFFFFFF:
                // error [...]

            case WAIT_OBJECT_0:
                // thread we're waiting for terminated
                break;

            case WAIT_OBJECT_0 + 1:
                // [...]
                if ( wxThread::IsMain() )
                {
                    // it looks that sometimes WAIT_OBJECT_0 + 1 is
                    // returned but there are no messages in the thread
                    // queue -- prevent DoMessageFromThreadWait() from
                    // blocking inside ::GetMessage() forever in this
case
                    ::PostMessage(NULL, WM_NULL, 0, 0);

                    wxAppTraits *traits = wxTheApp ?
wxTheApp->GetTraits()
                                                   : NULL;

                    if ( traits && !traits->DoMessageFromThreadWait() )
                    {
                        // WM_QUIT received: kill the thread
                        Kill();

                        return wxTHREAD_KILLED;
                    }
                }
                break;

            default:
                wxFAIL_MSG(wxT("unexpected result of
MsgWaitForMultipleObject"));
        }
    } while ( result != WAIT_OBJECT_0 );

The problem is that, in the case of a wxConsoleApp, the
DoMessageFromThreadWait() DOES NOTHING, so the thread
continues to return from the MsgWaitForMultipleObjects with code
WAIT_OBJECT_0 + 1 (because the application that links with my dll is a
GUI one that receives messages, but DoMessageFromThreadWait does not
process them)

Any suggestions?? Maybe there's a better way to Initialize/Uninitialize
wxWidgets inside a dll that solves it (remember that under Linux
everything works)... but, apart from this, I don't think it's very
sensible to wait for messages that will not be processed, so, for
example I would modify the wx code as follow

test if it is a wxConsoleApp;

if it is

        result = ::MsgWaitForMultipleObjects
                 (
                   1,              // number of objects to wait for
                   &m_hThread,     // the objects
                   false,          // don't wait for all objects
                   INFINITE,       // no timeout
                   0;
                 );
else

        result = ::MsgWaitForMultipleObjects
                 (
                   1,              // number of objects to wait for
                   &m_hThread,     // the objects
                   false,          // don't wait for all objects
                   INFINITE,       // no timeout
                   QS_ALLINPUT|QS_ALLPOSTMESSAGE   // return as soon as
there are any events
                 );

Comments???? Thanks.

MARCO GUBERNATI

marco.gubernati at erxa.it

http://www.erxa.it
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