FLTK: XLib error with X window initialization with multiple Fl_Windows - c++

Question

Per design requirements, I am developing a GUI in FLTK that must have a main (parent) window, a child window, and an X Window that is the child of the child window. This is in Ubuntu 16.04.
Running my original project with this setup produces the XLib error BadWindow (invalid Window parameter) (Details: serial 7 error_code 3 request_code 1 minor_code 0).
I have the following test program with only the features relevant to the error that reproduces a similar error:
#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Button.H>
#include <FL/x.H>
#include <iostream>
class ChildWindow : public Fl_Window {
public:
ChildWindow() : Fl_Window(100, 100, 300, 200, "Child"){
this->end();
this->show();
Display * dis = XOpenDisplay(NULL);
std::cout << "Child XID: " << (void *)fl_xid(this) << '\n';
std::cout << "XWindow XID: " << dis << '\n';
Window win = XCreateSimpleWindow(dis, fl_xid(this),
5, 5, 100, 100, 0, 0, 0);
XMapRaised(dis, win);
XSync(dis, false);
}
};
class ParentWindow : public Fl_Window {
public:
static ChildWindow * child;
ParentWindow() : Fl_Window(100, 100, 400, 300, "Parent"){
Fl_Button * b = new Fl_Button(10, 10, 70, 20, "Make Child");
b->callback(CallbackMakeChild, NULL);
this->add(b);
this->end();
this->show();
std::cout << "Parent XID: " << (void *)fl_xid(this) << '\n';
}
static void CallbackMakeChild(Fl_Widget * w, void * o){
child = new ChildWindow();
child->show();
}
};
ChildWindow * ParentWindow::child = NULL;
with the main function of
int main(){
ParentWindow parent;
Fl::run();
}
When run, the above code should create a window with a button. When the button is pressed, a child window will be created and the X Window will be attempted to be created. Here is the console output:
Parent XID: 0x4400002
Child XID: 0x4400005
XWindow XID: 0xe03140
X_CreateWindow: BadWindow (invalid Window parameter) 0x4400005
X_ConfigureWindow: BadWindow (invalid Window parameter) 0x4600001
X_MapWindow: BadWindow (invalid Window parameter) 0x4600001
The child window and X Window by themselves behave fine:
int main(){
ChildWindow child;
Fl::run();
}
This code (creating two non-hierarchical windows) also fails:
int main(){
ChildWindow child1; // This window works
ChildWindow child2; // This causes similar errors as above
Fl::run();
}
I have also tried various combinations of fl_open_display and Fl_Window::make_current without success.
(All code, being in one file, compiles by running ./fltk-config --compile myprog.cpp)
The pattern here appears that X windows initialize fine for the first FLTK window, but do not for windows created after that. My guess is that it has something to do with how FLTK interfaces/initializes with XLib, but I have not been able to find specifics in the documentation.
Does anyone have an explanation/solution for why X Window initialization only works when using the first-created FLTK window as a parent?

Answer 1

The problem was that XOpenDisplay was being called twice. FLTK opens the display as part of the initialization process, and calling it again with initializing the X window messed up the system.
This was solved by removing the line that said dis = XOpenDisplay(NULL) and replacing references to dis, with the FLTK display located at fl_display.