"Tcl_ServiceModeHook: Notifier not initialized" error with FLTK and c++11 thread - tcl

Question

I'm trying to get FLTK running in my c++ project. I'm creating an OMNeT++ simulation and would like to use the GUI like an emulated host, that is, I can make a send button and the host attached to the GUI will queue up a message to send in the simulation.
The issue I'm having is that, when the FLTK window runs, it waits for a response which causes the whole simulation to freeze until I close the window. My solution was to run the GUI in a separate thread. However, now I'm getting this error:
Tcl_ServiceModeHook: Notifier not initialized
Below is the code for the class containing the thread and the GUI.
#include <Enumerations.H>
#include <Fl.H>
#include <Fl_Box.H>
#include <Fl_Widget.H>
#include <Fl_Window.H>
#include <GUI.h>
void GUI::callThread() {
t = std::thread(&GUI::openWindow, this);
}
int GUI::openWindow() {
Fl_Window *window = new Fl_Window(300,180);
Fl_Box *box = new Fl_Box(20,40,260,100,"Hello, World!");
box->box(FL_UP_BOX);
box->labelsize(36);
box->labelfont(FL_BOLD+FL_ITALIC);
box->labeltype(FL_SHADOW_LABEL);
window->end();
window->show();
return Fl::run();
}
All I do to call it is:
GUI *g = new GUI();
g->callThread();
If I instead just call openWindow() directly, The window opens fine but it waits for an action so the rest of the simulation is not able to continue.
I'm trying to make this cross-platform (Has to work on Mac and Windows). I tried Qt (too complicated to set up) and wxWidgets (make errors on my computer) so FLTK seemed like one of the next best choices.
If anyone knows how to fix this error or has any opinions on a better way to set up my GUI, I'm all ears. This just seemed like the only thing I could do.

Answer 1

OMNeT++ 5.0 and up has Qtenv, which is a QT based runtime (Tkenv is now deprecated), so the whole QT setup, dependency etc. is done for you by OMNeT++. Qtenv will be the default runtime in OMNeT++ 5.1
It's not really clear what you are trying to achieve. If you want to insert events into the event queue whenever you press a button, you definitely have to use a separate thread, but you have to clarify how the simulation and wall clock time is related to each other. If they must be synced, you need to implement a real-time scheduler that syncs the simulation time with the wall clock time. The sockets example in OMNeT++ does this and behaves similarly what you describe except that the external event inserted into event queue is coming from a socket (where you can attach with a browser) isntead of coming from a GUI.
If you don't care about synchronization i.e. you want to use this for some kind of demonstration, then you don't have to use a real-time scheduler, but you should be aware the OMNeT++ is inherently single threaded and your GUI is running in a separate thread, so you have to sync those too. i.e. It's absolutely forbidden to access ANYTHING that OMNET is using from the GUI thread (or bad things will happen).
And the poor man's solution: if you just want to trigger an event (i.e. you really need only a button). Write an application in the host which regularly polls a volatile bool parameter. If it finds the parameter "true" it sends the event and then sets the parameter to "false". Now how you trigger an event? You can browse the parameters of the given node in the property inspector (lower left panel in runtime) and you can change it's value by hand back to true, then allow the simulation to continue. On the next poll, the host's app will detect again that the parameter is true and do the sending again. Obviously it's limited, but it gives a limited interaction without writing too much code (and all the multithreading issues are handled by the runtime itself)