Crash when hovering menu item

[Mo_Al_]

Hello

This was reported in the fltk-rs repo:
https://github.com/fltk-rs/fltk-rs/issues/1671

But it seems reproducible in C++, although I couldn't reproduce it on my setup, which appears to require a multimonitor setup on Linux. So I wanted to see if any of the devs here can reproduce it.

Although the issue was closed by the dev reporting it, the backtrace seems to point to a null dereference in (Fl_Menu.cxx:1068):

  STARTUP:

    menuwindow& cw = *pp.p[pp.menu_number];

    const Fl_Menu_Item* m = pp.current_item;

    if (!m->activevisible()) { // pointing at inactive item

      cw.set_selected(-1);

      initial_item = 0; // turn off startup code

      continue;

    } This was reported against a relelase which depends on FLTK 1.4.4.
However, looking at the code through git lens, it appears to not have changed for nearly 27 years :)

It might be that some reentrant code is nullifying the `m` variable elsewhere in certain setups.
The backtrace from the issue:

#0  0x00005cd18cb40182 in Fl_Menu_Item::activevisible (this=0x0)

    at /path/to/fltk/src/../FL/Fl_Menu_Item.H:451

#1  0x00005cd18cb4370f in Fl_Menu_Item::pulldown

    (this=0x5cd19bc3a470, X=760, Y=260, W=400, H=35, initial_item=0x0, pbutton=0x5cd19bbbfbb0, title=0x0, menubar=1)

    at /path/to/fltk/src/Fl_Menu.cxx:1068

#2  0x00005cd18cb44dda in Fl_Menu_Bar::handle (this=0x5cd19bbbfbb0, event=1)

    at /path/to/fltk/src/Fl_Menu_Bar.cxx:52

#3  0x00005cd18cb16085 in Widget_Derived<Fl_Sys_Menu_Bar>::handle (this=0x5cd19bbbfbb0, event=1)

    at /path/to/include/cfl_widget.hpp:77

#4  0x00005cd18cb32d62 in send (o=0x5cd19bbbfbb0, event=1)

    at /path/to/fltk/src/Fl_Group.cxx:96

#5  0x00005cd18cb334b7 in Fl_Group::handle (this=0x5cd19bbe6ab0, event=1)

    at /path/to/fltk/src/Fl_Group.cxx:226

#6  0x00005cd18cb138ad in Widget_Derived<Fl_Pack>::handle (this=0x5cd19bbe6ab0, event=1)

    at /path/to/include/cfl_widget.hpp:77

#7  0x00005cd18cb32d62 in send (o=0x5cd19bbe6ab0, event=1)

    at /path/to/fltk/src/Fl_Group.cxx:96

#8  0x00005cd18cb334b7 in Fl_Group::handle (this=0x5cd19bbf0380, event=1)

    at /path/to/fltk/src/Fl_Group.cxx:226

#9  0x00005cd18cb6241e in Fl_Window::handle (this=0x5cd19bbf0380, ev=1)

    at /path/to/fltk/src/Fl_Window.cxx:606

#10 0x00005cd18cb11de3 in Widget_Derived<Fl_Double_Window>::handle (this=0x5cd19bbf0380, event=1)

    at /path/to/include/cfl_widget.hpp:77

#11 0x00005cd18cb1aedc in send_event (event=1, to=0x5cd19bbf0380, window=0x5cd19bbf0380)

    at /path/to/fltk/src/Fl.cxx:1274

#12 0x00005cd18cb1b0fd in Fl::handle_ (e=1, window=0x5cd19bbf0380)

    at /path/to/fltk/src/Fl.cxx:1415

#13 0x00005cd18cb1af87 in Fl::handle (e=1, window=0x5cd19bbf0380)

    at /path/to/fltk/src/Fl.cxx:1364

#14 0x00005cd18cb85466 in fl_handle (thisevent=...)

    at /path/to/fltk/src/Fl_x.cxx:2301

#15 0x00005cd18cb7f3c3 in do_queued_events ()

    at /path/to/fltk/src/Fl_x.cxx:117

#16 0x00005cd18cb7f79e in fd_callback ()

    at /path/to/fltk/src/Fl_x.cxx:311

#17 0x00005cd18cb7ebd1 in Fl_Unix_Screen_Driver::poll_or_select_with_delay (this=0x5cd19bbc12d0, time_to_wait=1e+20)

    at /path/to/fltk/src/drivers/Unix/Fl_Unix_Screen_Driver.cxx:79

#18 0x00005cd18cb7f485 in Fl_X11_Screen_Driver::poll_or_select_with_delay (this=0x5cd19bbc12d0, time_to_wait=1e+20)

    at /path/to/fltk/src/Fl_x.cxx:138

#19 0x00005cd18cc0f03c in Fl_Unix_System_Driver::wait (this=0x5cd19bba69d0, time_to_wait=1e+20)

    at /path/to/fltk/src/drivers/Unix/Fl_Unix_System_Driver.cxx:810

#20 0x00005cd18cb1a18d in Fl::wait (time_to_wait=1e+20)

    at /path/to/fltk/src/Fl.cxx:632

#21 0x00005cd18cb1a1ab in Fl::run ()

    at /path/to/fltk/src/Fl.cxx:652

The C++ repro:

#include <FL/Fl.H>

#include <FL/Fl_Double_Window.H>

#include <FL/Fl_Pack.H>

#include <FL/Fl_Sys_Menu_Bar.H>

#include <FL/Fl_Text_Display.H>

#include <cstdio>

#include <string>

#define APP_VERSION "0.1.0"

int main() {

  Fl::scheme("gtk+");

  Fl::visual(FL_MULTISAMPLE | FL_ALPHA);

  Fl_Double_Window wind(400, 600, (std::string("J1939 Log ") + APP_VERSION).c_str());

  Fl_Pack pack(0, 0, wind.w(), wind.h());

  pack.type(Fl_Pack::VERTICAL);

  Fl_Sys_Menu_Bar menu(0, 0, 100, 35);

  menu.add("&Menu/Crash...\t",

           0,

           [](Fl_Widget*, void*) { fprintf(stderr, "Boom\n"); },

           nullptr,

           0

  );

  Fl_Text_Display text(0, 0, 100, 100);

  pack.resizable(&text);

  pack.end();

  wind.end();

  wind.resizable(&wind);

  wind.show();

  return Fl::run();

}

[Gonzalo Garramuño]

C++ program works fine for me with two monitors under both X11 and Wayland.

El 27/7/25 a las 17:08, Mo_Al_ escribió:

Hello

This was reported in the fltk-rs repo:
https://github.com/fltk-rs/fltk-rs/issues/1671

But it seems reproducible in C++, although I couldn't reproduce it on my setup, which appears to require a multimonitor setup on Linux. So I wanted to see if any of the devs here can reproduce it.

The C++ repro:
...etc..

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[Manolo]

Neither do I get a crash here with the C++ repo program under Wayand and Ubuntu 25.04 with 2 displays.