return filename from G_CALLBACK in GTK - c

Question

Hey guys so I would like to return value of filename from function (or pointer) but Im to sure how to do it. My filename stores path to file name /home/username/file. So here is my main
gchar *filename = NULL;
button = GTK_WIDGET( gtk_builder_get_object( builder, "button1" ) );
g_signal_connect (button, "clicked", G_CALLBACK (show_dialog), &filename);
And show dialog look like this
static void
show_dialog ( GtkWidget *button,
gint response_id,
gpointer user_data )
/* Init filechoosedialog, builder etc */
{
gchar *filename = gtk_file_chooser_get_filename (GTK_FILE_CHOOSER (filechoosedialog));
}
Do I need to change static void show_dialog to gchar show_dialog and at end do I need to do return filename; and what else do I need to declare because Im getting compile error
warning: return makes integer from pointer without a cast
So I would like to add code like this to my main and to get value of filename
if filename != NULL
g_print ("Selected filename: %s\n", filename);

Answer 1

This sounds like a misunderstanding of event driven programming. You don't do things like this in main(), you do it in the event handler. In your example you would call g_print() from show_dialog().
If you want to do something more complex you'll need to know some application state: that's what the g_signal_connect() userdata pointer is for. As an example, if you want to set the filename as the text of a label, you would set a pointer to the label as the user data pointer and can then set the label text in the event handler.
More generically, the userdata pointer can be anything: e.g. a data structure that you can use to store the filename string. Typical usage is an application data struct that is allocated in main: The struct contains all "application global" variables (like filename) and a pointer to the struct is given to all GTK+ callbacks so all functions can access and modify the struct contents.
If the problem was using the userdata pointer, here's an example using just a pointer to the filename as userdata:
g_signal_connect (button, "clicked", G_CALLBACK (button_clicked_cb), &filename);
static void
button_clicked_cb (GtkWidget *button,
gint response_id,
gpointer user_data)
{
char **filename_ptr = (char**)user_data;
*filename_ptr = g_strdup ("newly allocated string that the filename pointer in main() stack will now point to");
}