Segmentation Fault in FLTK

[SterlingM]

Hi, I am getting a segmentation fault in my FLTK code. It only happens on my callback function for a Fl_Choice or Fl_Menu_Button. Here is relevant code -

in the constructor of my window -

whichSensor = new Fl_Menu_Button(300, 315, 180, 25, "Sensor");
whichSensor->when(FL_WHEN_RELEASE);
whichSensor->callback(cb_choice);
setSensorItems();

void GUIWindow::setSensorItems() {


    whichSensor->add("Bump", 0, 0, (void*)1);
    whichSensor->add("Wheel_Drop", 0, 0, (void*)1);
    whichSensor->add("Wall", 0, 0, (void*)1);
    whichSensor->add("Cliff_Left", 0, 0, (void*)1);
    whichSensor->add("Cliff_Front_Left", 0, 0, (void*)1);
    whichSensor->add("Cliff_Front_Right", 0, 0, (void*)1);
    whichSensor->add("Cliff_Right", 0, 0, (void*)1);
    whichSensor->add("Virtual_Wall", 0, 0, (void*)1);
    whichSensor->add("Low_Side_Driver", 0, 0, (void*)1);
}

void GUIWindow::cb_choice(Fl_Widget* o, void* v) {
    GUIWindow* w = (GUIWindow*)v;
    w->cb_choice_i();
}

void GUIWindow::cb_choice_i() {
    robot->setCurrentSensor(BUMP);
}

If I leave the cb_choice_i empty or just have a cout statement in there, everything works fine. But as soon as I try to access a member of the window class, I get a segmentation fault. I have also tried changing v in GUIWindow* w = (GUIWindow*)v; to o, but that also gave me faults. The line in cb_choice_i is just an example; ideally I want to set it to whatever item the user chooses.

I have done all of my other callback functions this and they all work fine. Here is another for example -

void GUIWindow::cb_toggleSensorStream(Fl_Widget* o, void* v) {
    GUIWindow* w = (GUIWindow*)v;
    w->cb_toggleSensorStream_i();
}

void GUIWindow::cb_toggleSensorStream_i() {
    robot->toggleSensorStream();
}

So I don't understand why I would be getting segmentation faults for the other. My only assumption is that it has something to do with the Fl_Menu_Button, but I don't know what. This problem has been stumping me for some time now so any help at all would be greatly appreciated.

[Paul McKenzie]

If I leave the cb_choice_i empty or just have a cout statement in there, everything works fine. But as soon as I try to access a member of the window class, I get a segmentation fault.

The pointer is pointing to a bogus object, or the function you're calling through the object is giving you the segmentation fault.

It's either one or the other. So determine if that pointer is a valid pointer, and if it isn't why it isn't. If it is a valid pointer, then why that function you're calling (setCurrentSensor) crashes.

Maybe the object it was pointing to no longer exists? Maybe the pointer is not a GUIWindow, but you're assuming it is? You're using a C-style cast, and there is no check to see if that pointer (which is just a void) is actually pointing to a GUIWindow object.

I don't know what FLTK is, but it still has to follow certain norms, and the norm is that if you get a segmentation fault on a dereferenced pointer, only a few scenarios can cause the issue.

Edit:
To add, just because the others work doesn't mean they are correct. If the pointer is indeed invalid, anything can happen, including seemingly "work".

Regards,

Paul McKenzie

[JohnW@Wessex]

If the pointer is indeed invalid, anything can happen, including seemingly "work".

To illustrate Paul's point, consider this bit of code.

#include <iostream>

using namespace std;

class C
{
public:
    void F1()
    {
        cout << "F1\n";
    }

    void F2()
    {
        cout << "F2\n";
        i = 0;
    }

private:

    int i;
};

int main()
{
    C *p_c = 0;

    p_c->F1(); // Works!
    p_c->F2(); // Crash!
}

'p_c' does not point to a valid object, but as F1 does not access any member variables, it appears to function correctly.
F2, on the other hand, tries to access 'i' and fails.

(Compiled and run under Visual Studio 2008)

[MrViggy]

To illustrate Paul's point, consider this bit of code.

#include <iostream>

using namespace std;

class C
{
public:
    void F1()
    {
        cout << "F1\n";
    }

    void F2()
    {
        cout << "F2\n";
        i = 0;
    }

private:

    int i;
};

int main()
{
    C *p_c = 0;

    p_c->F1(); // Works!
    p_c->F2(); // Crash!
}

'p_c' does not point to a valid object, but as F1 does not access any member variables, it appears to function correctly.
F2, on the other hand, tries to access 'i' and fails.

(Compiled and run under Visual Studio 2008)

Just to add, while "p_c -> F1()" 'works', it is still undefined behavior and may not work in the future.

Viggy

[SterlingM]

Yes, the void pointer v is null when I enter the function. That is what is causing the segmentation fault (however, I don't know why it would be null). Thank you for the help and replies.

EDIT: Okay the problem was in

whichSensor->add("Bump", 0, 0, (void*)1);

Instead of (void*)1, it should be (void*)this. (void*)1 passes no user_data where (void*)this does.

Thanks again for the help.

[JohnW@Wessex]

Are you sure that you really need void*? Since I started programming in C++ I've very rarely had need for one. It's often a C idiom used to emulate polymorphism in C++.

[SterlingM]

The example I went by used it and it worked so I just went with it.