GTK gtk_label_set_text segmentation fault - c

Question

I am learning GTK+ and this simple application crashes every time I run it.
It creates a label in the main window, and every time a button is clicked (the key_press_event) the label and the title should swap.
If I comment out the gtk_label_set_text in the change_title function the title alternates correctly and the app doesn't crash. Why does gtk_label_set_text crash my app?
#include <gtk/gtk.h>
#include <string.h>
const gchar first[]="FIRST";
const gchar last[]="LAST";
static void destroy(GtkWidget *window,gpointer data)
{
gtk_main_quit();
}
static gboolean change_title(GtkWidget *widget,GtkLabel *data)
{
if(strcmp(last,gtk_window_get_title(GTK_WINDOW(widget)))){
gtk_window_set_title(GTK_WINDOW(widget),last);
gtk_label_set_text(data,first);
} else {
gtk_window_set_title(GTK_WINDOW(widget),first);
gtk_label_set_text(data,last);
}
return FALSE;
}
int main(int argc,char **argv)
{
GtkWidget *window, *label;
gtk_init(&argc,&argv);
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_title(GTK_WINDOW(window),last);
gtk_widget_set_size_request(window,300,100);
g_signal_connect(window,"destroy_event",G_CALLBACK(destroy),NULL);
label = gtk_label_new("caasdasdjadnjadjahadjad");
gtk_container_add(GTK_CONTAINER(window),label);
g_signal_connect(window,"key_press_event",G_CALLBACK(change_title),GTK_LABEL(label));
gtk_widget_show_all(window);
gtk_main();
return 0;
}
EDIT: I found the problem using GDB, the label pointer isn't passed correctly to the change_title function. I don't know why. (Ex: in main() label = 0xb6406608 , in change_title() label = 0x807bda8)

Answer 1

After doing a simple Google search on key_press_event I saw that the callback to that event have another argument between the widget and the user-data pointer. The prototype is this:
gboolean key_event_handler(GtkWidget *widget,GdkEventKey *event, gpointer data);
So simple change your function to this:
static gboolean change_title(GtkWidget *widget, GdkEventKey *event, GtkLabel *data)
and it should work.

Answer 2

Your change_title function has the wrong prototype.
See the documentation for the proper prototype. Most *-event signals pass the actual event as an argument in the handler function, since the handler typically needs to inspect the event in order to execute. For instance, here the GdkEventKey event will contain information about which key was pressed (or released).