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.