How Do ncurses et. al. Work?

There are several libraries like ncurses that assist in making command-line GUIs.

Simply put, how do they work?

My first thought was that ncurses intercepts all keyboard input, and draws ea开发者_StackOverflow中文版ch "frame" by outputting it line-by-line normally. Closer inspection, however, reveals that each new frame overwrites the previous one. How does it modify lines that have already been outputted? Furthermore, how does it handle color?

EDIT: The same question applies to anything with a "fancy" interface, like vim and emacs.

Text terminals have command sequences that do things like move the cursor to a particular position on the screen, insert characters, delete lines etc.

Each terminal type is different and has its own set of command sequences. ncurses has a databse (see terminfo for details)

Internally ncurses maintains 2 views of the screen: the current contents and what the screen should look like after the current pending changes are applied. Once the program requests a screen redraw, ncurses calculates an efficient way to update the screen to look like the desired view. The exact characters/command sequences output depend on what terminal type is in use.

curses (and ncurses, too, I think) works by moving the cursor around on the screen. There are control sequences to do such things. Take a look at the code again and you'll see them. These sequences are not ASCII control characters, they are strings starting with (umm...) ESC, maybe. Have a look here for a higher-level explanation.