More about refresh and windows

As mentioned above, ETI routines do not update a terminal until refresh is called. Instead, they write to an internal representation of the screen called a window. When refresh is called, all the accumulated output is sent from the window to the current terminal screen.

A window acts a lot like a buffer does when you use a UNIX system editor. When you invoke vi(1), for instance, to edit a file, the changes you make to the contents of the file are reflected in the buffer. The changes become part of the permanent file only when you use the w or ZZ command. Similarly, when you invoke a screen program made up of ETI routines, they change the contents of a window. The changes become part of the current terminal screen only when refresh is called.

ocurses.h supplies a default window named stdscr (standard screen), which is the size of the current terminal's screen, for all programs using ETI routines. The header file defines stdscr to be of the type WINDOW, a pointer to a C structure which you might think of as a two-dimensional array of characters representing a terminal screen. The program always keeps track of what is on the physical screen, as well as what is in stdscr. When refresh is called, it compares the two screen images and sends a stream of characters to the terminal that make the physical screen look like stdscr. An ETI program considers many different ways to do this, taking into account the various capabilities of the terminal and similarities between what is on the screen and what is on the window (stdscr). It optimizes output by printing as few characters as is possible. ``The relationship between stdscr and a terminal screen (sheet 1 of 2)'' and ``The relationship between stdscr and a terminal screen (sheet 2 of 2)'' illustrate what happens when you execute the sample ETI program that prints BullsEye at the center of a terminal screen. Notice in the figure that the terminal screen retains whatever garbage is on it until the first refresh is called. This refresh clears the screen and updates it with the current contents of stdscr.

The relationship between stdscr and a terminal screen (sheet 1 of 2)

The relationship between stdscr and a terminal screen (sheet 2 of 2)

You can create other windows and use them instead of stdscr. Windows are useful for maintaining several different screen images. For example, many data entry and retrieval applications use two windows: one to control input and output and one to print error messages that do not mess up the other window. It is possible to subdivide a screen into many windows, refreshing each one of them as desired. And it is possible to create a window within a window; the smaller window is called a subwindow. See ``ETI windows'' for more information.