These functions are exactly like the corresponding libdialog functions, for the unsurprising reason that they need to do much the same as they would in libdialog.
initialize the dialog library; start curses, allocate colors, clear the screen, allocate the helpline, and so on.
After init_dialog(), the screen will be cleared, set to the window color, and (invisibly) broken up into the form window and a one-line helpline window at the bottom of the screen.