Gtk, Glade and signal handlers in C++

Thu, 05 May 2011 08:00:49 +0000

Much was written about connecting signal handlers to interfaces made with Glade and imported with GtkBuilder. The problem is that everybody uses a different system and/or language. So here is a guide which explains all the magic:

Glade Interface

First of all create your widget in Glade and assign it a signal handler. We are going to create a very simple application which has a single button which exits it. The Glade interface should look something like this. It is basically a Main Window (main_window) widget with a Button (exit_button) inside. We define a handler for the signal “clicked” for the button and call it exit_button_handler.

C++ code

Now let’s create the source file, it looks like this. As you can see our signal handler is just a basic void returning function.

#include <gtk/gtk.h>

extern "C" G_MODULE_EXPORT void exit_button_handler(GtkObject* caller, gpointer data)
{
    gtk_main_quit();
}

int main(int argc, char ** argv)
{
    gtk_init(&amp;argc, &amp;argv);
    GtkBuilder* l_BuilderInterface = gtk_builder_new();
    gtk_builder_add_from_file(l_BuilderInterface, "ui.glade", NULL);
    gtk_builder_connect_signals(l_BuilderInterface, NULL);
    gtk_widget_show(GTK_WIDGET(gtk_builder_get_object(l_BuilderInterface, "main_window")));
    gtk_main();
    return 0;
}

The important thing to notice is that the declaration of the callback function is preceded by extern "C" G_MODULE_EXPORT. These two bits of code ensure that the produced object will have a C-compatible table-entry for this function even though we are using a C++ compiler and that it will be accessible under Windows as well. Indeed, the G_MODULE_EXPORT macro expands to nothing under Linux.

Compilation

Now, we need to compile the whole thing. Let’s say we have called the file above main.cpp. The line to call on Linux would be :

g++ `pkg-config --cflags --libs gtk+-2.0 gmodule-2.0` main.cpp -o test

Let’s look at the output of the pkg-config file for a while :

-pthread -I/usr/include/atk-1.0 -I/usr/include/pango-1.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib/x86_64-linux-gnu/glib-2.0/include -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pixman-1  -pthread -Wl,--export-dynamic -L/usr/lib/x86_64-linux-gnu -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lpangocairo-1.0 -lgdk_pixbuf-2.0 -lm -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 -lgthread-2.0 -lgmodule-2.0 -lrt -lglib-2.0

That’s a lot of stuff! Depending on the system your mileage can vary, but the important bit is : -Wl,--export-dynamic. This piece of code ensures that you will be actually able to find the symbol once it is needed. On Windows, there is no need for the –export-dynamic flag, just be sure to load the gtk library as well as the gmodule. And that’s it, done and done!

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Gtk, Glade and signal handlers in C++

Glade Interface

C++ code

Compilation