GtkSpinButton::set_snap_to_ticks
void set_snap_to_ticks(bool snap_to_ticks); 
    
     set_snap_to_ticks() , if set
     to true, will restrict input values to the order of
     the step increment, taking the nearest value to the increment as the
     input value.  For example, if you have set the step increment to 
     0.5 in a range of 0.0 to 
     2.5 and the user inputs 1.4, the
     input will be reset to 1.5 - the nearest incremental
     value.
    
    
     The default setting is false, which allows the user
     to key in anything within range to the given number of decimal points.
    
    
     See also:  set_digits() .