pack_start() packs the child widget from the top or left of the box container. The expand parameter is a boolean value that determines whether the child can expand when the parent container is resized. The second parameter fill is also a boolean value, and determines whether the child should fill the space available to it. The third parameter padding is an integer describing the padding around the child widget, in pixels. The default values are true, true, 0.

Calls to pack_start() can be mixed with calls to pack_end() or even add() , within the same box container.