Creating grid with GTK buttons - c

Question

I want to create a grid with buttons. When a button is clicked I want it to change color, and 0 or 1 get stored in an array depending on the current state of the button.
Now I do this by creating the buttons with two for loops (rows, and columns).
Inside the for loops;
/*Create an ID number for the button being created*/
btn_nr ++;
char btn_nr_str[3];
sprintf(btn_nr_str,"%d",btn_nr); //convert nr to string
/*Create button*/
button = gtk_button_new();
/* When the button is clicked, we call the "callback" function
* with a pointer to the ID */
gtk_signal_connect (GTK_OBJECT (button), "clicked", GTK_SIGNAL_FUNC (callback)(gpointer) btn_nr_str);
/* Insert button into the table */
gtk_table_attach_defaults (GTK_TABLE(table), button, col, col+1, row, row+1);
gtk_widget_show (button);
The callback function;
void callback( GtkWidget *widget, gpointer nr)
{
GdkColor buttonColor;
gdk_color_parse ("black", &buttonColor);
gtk_widget_modify_bg ( GTK_WIDGET(widget), GTK_STATE_NORMAL, &buttonColor);
g_print ("Hello again - %s was pressed\n", (char *) nr);
}
The buttons are created like wanted, and when clicked they turn black.
However, all buttons print the last created button's ID.
How do I pass the right ID on?

Answer 1

You are accessing a local array (btn_nr_str) from outside (the callback) its scope (the for cycle). The idea is correct (using user_data) but the implementation is not.
For your specific case, you can use the type conversion macros provided by GLib. They are meant for exactly this purpose:
/* In the for cycle */
g_signal_connect(button, "clicked", G_CALLBACK(callback), GINT_TO_POINTER(btn_nr);
/* In the callback */
gint btn_nr = GPOINTER_TO_INT(user_data);
P.S.: gtk_signal_connect has been deprecated years ago.