[Qt-interest] Correct way to manage the main event loop without QApplication::exec()

[Nikos Chantziaras]

I am using a library in my Qt program that has its own exec()-like 
function.  In other words, its own event loop.  That function never 
returns, but it calls callbacks I provide.  Those callbacks are the only 
way to keep Qt's event loop rolling and to implement the GUI.  Note that 
the library in question is not a GUI library and does not process any 
system events; it does not perform any kind of system calls.

Question is, how is this done correctly?  What problems can be expected 
if I never call app->exec()?