Hi,

Does anyone know what has happened to QApplication:: postEvent and multithreading in Qt 4? If I post an event in another thread than the gui thread, the event is never handled.

Consider this minimal program:
Qt Code: 
  1. #include <Qt/qapplication.h>
  2. #include <Qt/qthread.h>
  3. #include <iostream>
  4.  
  5. struct Terminator : QObject {
  6.   bool event(QEvent* event) {
  7.     std::cerr << "Terminator::event" << std::endl;
  8.     return true;
  9.   }
  10. };
  11.  
  12. int main(int argc, char** argv) {
  13.   std::cerr << "main thread: " << QThread::currentThreadId() << std::endl;
  14.  
  15.   QApplication app(argc, argv);
  16.   QApplication::postEvent(new Terminator,
  17.                           new QEvent(QEvent::User));
  18.   return app.exec();
  19. }
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A custom event that terminates the application is posted in the main loop. When QApplication::exec is called, the event is processed. Everything works as expected and the output is:

main thread: 140735005136720
Terminator::event

But if the event is posted in a different thread, it will not be handled. Here's a minimal thread class and accordingly adjusted main function:
Qt Code: 
  1. struct MyThread : QThread {
  2.   void run() {
  3.     std::cerr << "MyThread: " << currentThreadId() << std::endl;
  4.  
  5.     QApplication::postEvent(new Terminator,
  6.                             new QEvent(QEvent::User));
  7.   }
  8. };
  9.  
  10. int main(int argc, char** argv) {
  11.   std::cerr << "main thread: " << QThread::currentThreadId() << std::endl;
  12.  
  13.   QApplication app(argc, argv);
  14.  
  15.   MyThread mt;
  16.   mt.start();
  17.  
  18.   return app.exec();
  19. }
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The output of the program is:

main thread: 139728441435984
MyThread: 1099274576

which means that Terminator::event is never called.

The system is a standard Fedora (2.6.27.5-41.fc9.x86_64) with pthreads. Everything worked in Qt 3, so somewhere along the way something has changed. Any clues?