24 Using Multiple Buffers
The text you are editing in Emacs resides in an object called a
buffer. Each time you visit a file, a buffer is created to hold the
file's text. Each time you invoke Dired, a buffer is created to hold the
directory listing. If you send a message with C-x m, a buffer named
‘*mail*’ is used to hold the text of the message. When you ask for a
command's documentation, that appears in a buffer called ‘*Help*’.
At any time, one and only one buffer is current. It is also
called the selected buffer. Often we say that a command operates on
“the buffer” as if there were only one; but really this means that the
command operates on the current buffer (most commands do).
When Emacs has multiple windows, each window has its own chosen
buffer and displays it; at any time, only one of the windows is
selected, and its chosen buffer is the current buffer. Each window's
mode line normally displays the name of the window's chosen buffer
(see Windows).
Each buffer has a name, which can be of any length, and you can select
any buffer by giving its name. Most buffers are made by visiting files,
and their names are derived from the files' names. But you can also create
an empty buffer with any name you want. A newly started Emacs has a buffer
named ‘*scratch*’ which can be used for evaluating Lisp expressions in
Emacs. The distinction between upper and lower case matters in buffer
names.
Each buffer records individually what file it is visiting, whether it is
modified, and what major mode and minor modes are in effect in it
(see Major Modes). Any Emacs variable can be made local to a
particular buffer, meaning its value in that buffer can be different from
the value in other buffers. See Locals.
A buffer's size cannot be larger than some maximum, which is defined
by the largest buffer position representable by the Emacs integer
data type. This is because Emacs tracks buffer positions using that
data type. For 32-bit machines, the largest buffer size is 256
megabytes.
