We create a GLUI button with a call to the function

     add_button_to_panel

The function requires the following four parameters. The first parameter is the pointer to the panel where we are adding the button -- mainPanel for our example. The second parameter is a string that appears on the button -- "Close Program" is used here since that is the purpose of our button. The third parameter is an integer that serves as the identifier for the button and is passed to the button callback handler. The fourth and last parameter is the pointer to (name of) our button handler.