17.1 Introduction to Buffers

A buffer is logically just a Lisp object that holds some text. In this, it is like a string, but a buffer is optimized for frequent insertion and deletion, while a string is not. Furthermore:

1. Buffers are permanent objects, i.e. once you create them, they remain around, and need to be explicitly deleted before they go away.
2. Each buffer has a unique name, which is a string. Buffers are normally referred to by name. In this respect, they are like symbols.
3. Buffers have a default insertion position, called point. Inserting text (unless you explicitly give a position) goes at point, and moves point forward past the text. This is what is going on when you type text into Emacs.
4. Buffers have lots of extra properties associated with them.
5. Buffers can be displayed. What this means is that there exist a number of windows, which are objects that correspond to some visible section of your display, and each window has an associated buffer, and the current contents of the buffer are shown in that section of the display. The redisplay mechanism (which takes care of doing this) knows how to look at the text of a buffer and come up with some reasonable way of displaying this. Many of the properties of a buffer control how the buffer’s text is displayed.
6. One buffer is distinguished and called the current buffer. It is stored in the variable current_buffer. Buffer operations operate on this buffer by default. When you are typing text into a buffer, the buffer you are typing into is always current_buffer. Switching to a different window changes the current buffer. Note that Lisp code can temporarily change the current buffer using set-buffer (often enclosed in a save-excursion so that the former current buffer gets restored when the code is finished). However, calling set-buffer will NOT cause a permanent change in the current buffer. The reason for this is that the top-level event loop sets current_buffer to the buffer of the selected window, each time it finishes executing a user command.

Make sure you understand the distinction between current buffer and buffer of the selected window, and the distinction between point of the current buffer and window-point of the selected window. (This latter distinction is explained in detail in the section on windows.)