The two functions, buffer-name and buffer-file-name, show
the difference between a file and a buffer. When you evaluate the
following expression, (buffer-name), the name of the buffer
appears in the echo area. When you evaluate (buffer-file-name),
the name of the file to which the buffer refers appears in the echo
area. Usually, the name returned by (buffer-name) is the same as
the name of the file to which it refers, and the name returned by
(buffer-file-name) is the full path-name of the file.
A file and a buffer are two different entities. A file is information recorded permanently in the computer (unless you delete it). A buffer, on the other hand, is information inside of Emacs that will vanish at the end of the editing session (or when you kill the buffer). Usually, a buffer contains information that you have copied from a file; we say the buffer is visiting that file. This copy is what you work on and modify. Changes to the buffer do not change the file, until you save the buffer. When you save the buffer, the buffer is copied to the file and is thus saved permanently.
If you are reading this in Info inside of GNU Emacs, you can evaluate each of the following expressions by positioning the cursor after it and typing C-x C-e.
(buffer-name) (buffer-file-name)
When I do this in Info, the value returned by evaluating
(buffer-name) is "*info*", and the value returned by
evaluating (buffer-file-name) is nil.
On the other hand, while I am writing this document, the value
returned by evaluating (buffer-name) is
"introduction.texinfo", and the value returned by evaluating
(buffer-file-name) is
"/gnu/work/intro/introduction.texinfo".
The former is the name of the buffer and the latter is the name of the
file. In Info, the buffer name is "*info*". Info does not
point to any file, so the result of evaluating
(buffer-file-name) is nil. The symbol nil is
from the Latin word for “nothing”; in this case, it means that the
buffer is not associated with any file. (In Lisp, nil is also
used to mean “false” and is a synonym for the empty list, ().)
When I am writing, the name of my buffer is "introduction.texinfo". The name of the file to which it points is "/gnu/work/intro/introduction.texinfo".
(In the expressions, the parentheses tell the Lisp interpreter to
treat buffer-name and buffer-file-name as
functions; without the parentheses, the interpreter would attempt to
evaluate the symbols as variables. See Variables.)
In spite of the distinction between files and buffers, you will often find that people refer to a file when they mean a buffer and vice versa. Indeed, most people say, “I am editing a file,” rather than saying, “I am editing a buffer which I will soon save to a file.” It is almost always clear from context what people mean. When dealing with computer programs, however, it is important to keep the distinction in mind, since the computer is not as smart as a person.
The word “buffer”, by the way, comes from the meaning of the word as a cushion that deadens the force of a collision. In early computers, a buffer cushioned the interaction between files and the computer’s central processing unit. The drums or tapes that held a file and the central processing unit were pieces of equipment that were very different from each other, working at their own speeds, in spurts. The buffer made it possible for them to work together effectively. Eventually, the buffer grew from being an intermediary, a temporary holding place, to being the place where work is done. This transformation is rather like that of a small seaport that grew into a great city: once it was merely the place where cargo was warehoused temporarily before being loaded onto ships; then it became a business and cultural center in its own right.
Not all buffers are associated with files. For example, a *scratch* buffer does not visit any file. Similarly, a *Help* buffer is not associated with any file.
In the old days, when you lacked a ~/.emacs file and started an
Emacs session by typing the command emacs alone, without naming
any files, Emacs started with the *scratch* buffer visible.
Nowadays, you will see a splash screen. You can follow one of the
commands suggested on the splash screen, visit a file, or press q
to quit the splash screen and reach the *scratch* buffer.
If you switch to the *scratch* buffer, type
(buffer-name), position the cursor after it, and then type
C-x C-e to evaluate the expression. The name "*scratch*"
will be returned and will appear in the echo area. "*scratch*"
is the name of the buffer. When you type (buffer-file-name) in
the *scratch* buffer and evaluate that, nil will appear
in the echo area, just as it does when you evaluate
(buffer-file-name) in Info.
Incidentally, if you are in the *scratch* buffer and want the value returned by an expression to appear in the *scratch* buffer itself rather than in the echo area, type C-u C-x C-e instead of C-x C-e. This causes the value returned to appear after the expression. The buffer will look like this:
(buffer-name)"*scratch*"
You cannot do this in Info since Info is read-only and it will not allow you to change the contents of the buffer. But you can do this in any buffer you can edit; and when you write code or documentation (such as this book), this feature is very useful.