GtkCheckMenuItem::set_active

void set_active(bool is_active);

set_active() sets the state of the calling GtkCheckMenuItem to either the GTK_STATE_ACTIVE or the GTK_STATE_NORMAL GtkStateType. When the menu item is in its active state, the toggle indicator is always displayed.

$checkmenuitem->set_active(true); will set the GtkCheckMenuItem's state to active.

The default setting is false.