segmentation fault?

kalleanka · 11-20-2006, 05:33 AM

hi,

I got a segmentation fault when I click the button. I think its has to do with the memory allocation of the struct. Could someone please explain why I get the fault so I wont do the same mistake again.

I fixed the codetag and I saw that I get a null-pointer in data. I wounder why?

Code:

#include <gtk/gtk.h>
typedef enum { APPLE, BANANA } Fruit_Type;

typedef struct Fruit {Fruit_Type ft;} Fruit;


void PealAFruit(Fruit_Type fruit)
{
	g_message ("PealAFruit called");
}

static gboolean button_clicked( GtkWidget      *widget, 
                              GdkEventButton *event,
                              gpointer        data)   /* null here why?*/
{
	Fruit *afruit = (Fruit*)data; 

	PealAFruit(afruit->ft);   /* segmentationfault here */
	return TRUE;
}

int main( int   argc, char *argv[] )
{
    g_message ("start"); 
    GtkWidget *window;
    GtkWidget *button;
    Fruit     *afruit;

    gtk_init (&argc, &argv);

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    button = gtk_button_new_with_label ("Hello World");
 
    if ( (afruit = (Fruit*)g_malloc(sizeof(Fruit))) != NULL) 
	{
		afruit->ft = APPLE;
		g_signal_connect (G_OBJECT (button), "clicked",
		      G_CALLBACK (button_clicked), afruit);
	}

    gtk_container_add (GTK_CONTAINER (window), button);

    gtk_widget_show (button);
    gtk_widget_show (window);
    gtk_main ();
    return 0;
}

paulsm4 · 11-20-2006, 10:09 AM

Hi -

C++ has this really neat operator called "new". You might want to read about it ;-)

kalleanka · 11-20-2006, 01:02 PM

well this is c.

exvor · 11-20-2006, 01:59 PM

Never mind Ive removed my post due to its extream retardedness

dmail · 11-20-2006, 02:03 PM

I've never used gtk...

Code:

typedef struct Fruit {Fruit_Type ft;} Fruit;

It's a while since I used C, can the tag and the name be the same?

Code:

if ( (afruit = (Fruit*)g_malloc(sizeof(Fruit))) != NULL)

Casting the return of malloc, this is not required and could be at fault.

Why bit machine are you using ? 64?

Code:

g_signal_connect (G_OBJECT (button), "clicked",
		      G_CALLBACK (button_clicked), afruit);

Every example I have seen of gtk casts the data to gpointer.

paulsm4 · 11-20-2006, 02:14 PM

Sorry - It looked like you were calling GTK+ from C++ and you neglected to initialize the "fruit" variable.

This code works fine for me:

Code:

#include <stdio.h>
#include <string.h>
#include <gtk/gtk.h>

typedef enum
{
  APPLE, BANANA
}
Fruit_Type;

struct Fruit
{
  Fruit_Type ft;
};


void PeelAFruit(Fruit_Type type)
{
  g_message ("PeelAFruit (%d) called", type);
}

static gboolean button_clicked( GtkWidget *widget, gpointer data)
{
    struct Fruit *afruit = (struct Fruit *)data;
    PeelAFruit(afruit->ft);
    return TRUE;
}

int 
main( int argc, char *argv[])
{
    g_message ("start");
    GtkWidget *window;
    GtkWidget *button;
    struct Fruit     *afruit;

    gtk_init (&argc, &argv);

    window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    button = gtk_button_new_with_label ("Hello World");

    /*
     * NOTE:
     * Per Harmaa Kettu, checking the error status of "g_malloc()" is redundant.
     * Personally, I think if your code might cause a program abort, it would be
     * nice to be explicit about it.  Hence back to standard "malloc()".
     */
    afruit = (struct Fruit *)malloc(sizeof(struct Fruit));
    if (afruit == NULL)
    {
      perror ("Unable to alloc a fruit!");
      abort ();
    }
    afruit->ft = APPLE;
    g_signal_connect (G_OBJECT (button), "clicked",
      G_CALLBACK (button_clicked), afruit);

    gtk_container_add (GTK_CONTAINER (window), button);

    gtk_widget_show (button);
    gtk_widget_show (window);
    gtk_main ();
    return 0;
}

Quote:

gcc -g -I/opt/gnome/include/gtk-2.0 -I/opt/gnome/include/glib-2.0 -I/opt/gnome/lib/gtk-2.0/include -I/opt/gnome/lib/glib-2.0/include -I/opt/gnome/include/pango-1.0 -I/opt/gnome/include/atk-1.0 x.c -o x -L/opt/gnome/lib -L/usr/X11R6/lib -lgtk-x11-2.0 -lgdk-x11-2.0 -lX11 -lXext -lm

** Message: start
** Message: PeelAFruit (1) called

Harmaa Kettu · 11-20-2006, 02:39 PM

From GtkButton documentation:

Code:

"clicked"   void        user_function      (GtkButton *button,
                                            gpointer   user_data)      : Run first / Action

Your callback function has an extra parameter, the GdkButtonEvent shouldn't be there.
Also, you don't need to check the return value of g_malloc(), because g_malloc() aborts the program if there isn't enough free memory. That is the whole point of using it instead of normal malloc().

exvor · 11-20-2006, 03:09 PM

Nevermind.......im just going to stop posting on this topic.

kalleanka · 11-20-2006, 06:30 PM

thanks a lot for all the tips like:

an extra parameter

and that I do not need to test g_malloc

and the code that works.

Kitos and thanks.

I will work with it tomorrow

bigearsbilly · 11-22-2006, 05:42 AM

finding seg faults:

one way to do it:

use gdb,
compile your program with -g debugger.
run the program and you dump a 'core' file.

then
gdb program core
and type 'where' it will tell where it was when it failed.