[Qt-interest] Running Qt in a child thread on OSX fails; suggestions?

[Penn Taylor]

Summary:
Running Qt in a child thread on OSX fails, but works fine on GNU/Linux. 
Why? And what can be done to make this work?

Details:
My application starts up a Qt "application" in a child thread; that is, 
NOT in the primordial thread. The main entry point, event loop, etc. for 
the Qt application are all started up in this same child thread. I'm 
using a non-Qt parent application (vrjuggler), so no QThreads are 
explicitly involved. This strategy works fine on GNU/Linux (using both 
Qt 4.6.0 and 4.6.3), but on Mac OSX (Qt 4.6.1) I'm getting the following 
warnings before the program seg faults:

QCoreApplication::sendPostedEvents: Cannot send posted events for 
objects in another thread
QCoreApplication::sendPostedEvents: Cannot send posted events for 
objects in another thread
QPixmap: It is not safe to use pixmaps outside the GUI thread
QPixmap: It is not safe to use pixmaps outside the GUI thread

I've researched these and dealt with them before on Linux, but I'm 
baffled why this fails on OSX. Everything *is* happening in the GUI 
Thread, it just happens that the GUI Thread is not the primordial 
thread. The QCoreApplication::sendPostedEvents warning is only given 
when the sending thread and receiving thread do not match, but in this 
case they *should* match because all Qt events are happening in the same 
child thread.

Any clues as to what is going on here? Is there some subtlety to Mac's 
threading model that causes Qt to get a different thread identifier when 
it looks at the same child thread twice?

To replicate this issue, just fire off the following code in a child 
thread of some multi-threaded non-Qt application:

QApplication a( argc, argv );
QPushButton aButton("Test");
aButton.show();
a.exec();