GTK+ Forums

clinisbut · Familiar Face Joined: 10 Apr 2008 Posts: 19

I just starting with GTK+ but I have some time experience with c and c++.

I'm having a problem executing a simple app I made. Here is the code simplified:

Code: (C)

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int main( int argc, char *argv[] )
{
GtkWidget *window;
GtkWidget* vbox; //Contenedor

gtk_init (&argc, &argv);

window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
build_toolbarMenu( window, vbox );

GtkWidget* button;
button = gtk_button_new_with_label("Hi!");
gtk_box_pack_start( GTK_BOX(vbox), button, TRUE, TRUE, 3 );

gtk_widget_show_all( window );
gtk_main();
}

void build_toolbarMenu( GtkWidget* window, GtkWidget* vbox )
{
vbox = gtk_vbox_new( FALSE, 0 );
gtk_container_add( GTK_CONTAINER(window), vbox );
// gtk_box_pack_start( GTK_BOX(vbox), SOME_WIDGETS, FALSE, FALSE, 3 );
}

This code execution results in no error compiling, but an error at executing time, just before any windows shows.
Debugging with codeblocks, I got this message:

"Program received signal SIGSEGV, Segmentation fault.
Do you want to view the backtrace?"

If I click on "YES" I see this:

#0 62760DC7 ??() (C:\Archivos de programa\Archivos comunes\GTK\2.0\bin\libgobject-2.0-0.dll:??)
#1 00000000 ??() (??:??)

The line that causes that error is

Code: (C)
1 2 3	gtk_box_pack_start( GTK_BOX(vbox), button, TRUE, TRUE, 3 );

in main function. If I comment this line there is no error.

However if I change the vbox declaration:

Code: (C)

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int main( int argc, char *argv[] )
{
GtkWidget *window;
GtkWidget* vbox; //Contenedor

gtk_init (&argc, &argv);

window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
vbox = gtk_vbox_new( FALSE, 0 );
gtk_container_add( GTK_CONTAINER(window), vbox );

//Configuracion ventana
set_main_window( window );

//Colocamos el menú
build_toolbarMenu( window, vbox );

GtkWidget* button;
button = gtk_button_new_with_label("Hi!");
gtk_box_pack_start( GTK_BOX(vbox), button, TRUE, TRUE, 3 );

gtk_widget_show_all( window );
gtk_main();
return 0;
}

There is no error!
What's wrong with the first example?[/code]

openldev · Posted: Fri Apr 11, 2008 1:35 pm Post subject:

It comes down to how C passes variables. You will want to do the following:

Code: (Plaintext)

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int main( int argc, char *argv[] )
{
GtkWidget *window;
GtkWidget* vbox; //Contenedor

gtk_init (&argc, &argv);

window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
build_toolbarMenu( window, vbox );

GtkWidget* button;
button = gtk_button_new_with_label("Hi!");
gtk_box_pack_start( GTK_BOX(vbox), button, TRUE, TRUE, 3 );

gtk_widget_show_all( window );
gtk_main();
}

void build_toolbarMenu( GtkWidget* window, GtkWidget** vbox )
{
&vbox = gtk_vbox_new( FALSE, 0 );
gtk_container_add( GTK_CONTAINER(window), &vbox );
// gtk_box_pack_start( GTK_BOX(vbox), SOME_WIDGETS, FALSE, FALSE, 3 );
}

C is passing a copy of the vbox pointer to your function in the original code, so it never actually updates the copy of the pointer in your main function. Therefore, the vbox memory is actually leaked.

The code I just provided passes a pointer to the vbox pointer. When you dereference it, the new GtkVBox widget is actually placed at the correct memory location instead of a copy.

clinisbut · Familiar Face Joined: 10 Apr 2008 Posts: 19

I see but I don't understand why C is passing a copy of pointer.

I thought that the meaning of pointer was exactly to avoid this kind of things.
Passing a variable (not its pointer) passes a copy of that variable, but when I pass a pointer I'm passing the memory address of that variable

openldev · Posted: Mon Apr 14, 2008 2:13 pm Post subject:

Think of it this way. When you pass an integer to a function, if you update that integer within the function, it would not be reflected in the variable that was copied in the function call. In C, if you want it to be updated, you would have to pass a pointer to the integer.

With your code, you are passing a pointer to a function. If you update that pointer within the function, it will not be reflected in the variable that was copied in the function call. (Note that GTK+ is updating the pointer address.) In C, if you want it to be updated, you would have to pass a pointer to the pointer.

I realize that I just repeated myself, but it was to show the logic behind how C works with function parameters.

clinisbut · Familiar Face Joined: 10 Apr 2008 Posts: 19

Now I understand.
gtk_vbox_new() changes the address of the pointer, not the value of the actual address.
Thank you.