There are, in general, many buffers in an Emacs session. At any time, one of them is designated as the current buffer. This is the buffer in which most editing takes place, because most of the primitives for examining or changing text in a buffer operate implicitly on the current buffer (see section Text). Normally the buffer that is displayed on the screen in the selected window is the current buffer, but this is not always so: a Lisp program can temporarily designate any buffer as current in order to operate on its contents, without changing what is displayed on the screen.
The way to designate a current buffer in a Lisp program is by calling
set-buffer. The specified buffer remains current until a new one
is designated.
When an editing command returns to the editor command loop, the
command loop designates the buffer displayed in the selected window as
current, to prevent confusion: the buffer that the cursor is in when
Emacs reads a command is the buffer that the command will apply to.
(See section Command Loop.) Therefore, set-buffer is not the way to
switch visibly to a different buffer so that the user can edit it. For
this, you must use the functions described in section Displaying Buffers in Windows.
However, Lisp functions that change to a different current buffer
should not depend on the command loop to set it back afterwards.
Editing commands written in Emacs Lisp can be called from other programs
as well as from the command loop. It is convenient for the caller if
the subroutine does not change which buffer is current (unless, of
course, that is the subroutine's purpose). Therefore, you should
normally use set-buffer within a save-current-buffer or
save-excursion (see section Excursions) form that will restore the
current buffer when your function is done. Here is an example, the
code for the command append-to-buffer (with the documentation
string abridged):
(defun append-to-buffer (buffer start end)
"Append to specified buffer the text of the region.
..."
(interactive "BAppend to buffer: \nr")
(let ((oldbuf (current-buffer)))
(save-current-buffer
(set-buffer (get-buffer-create buffer))
(insert-buffer-substring oldbuf start end))))
This function binds a local variable to record the current buffer, and
then save-current-buffer arranges to make it current again.
Next, set-buffer makes the specified buffer current. Finally,
insert-buffer-substring copies the string from the original
current buffer to the specified (and now current) buffer.
If the buffer appended to happens to be displayed in some window, the next redisplay will show how its text has changed. Otherwise, you will not see the change immediately on the screen. The buffer becomes current temporarily during the execution of the command, but this does not cause it to be displayed.
If you make local bindings (with let or function arguments) for
a variable that may also have buffer-local bindings, make sure that the
same buffer is current at the beginning and at the end of the local
binding's scope. Otherwise you might bind it in one buffer and unbind
it in another! There are two ways to do this. In simple cases, you may
see that nothing ever changes the current buffer within the scope of the
binding. Otherwise, use save-current-buffer or
save-excursion to make sure that the buffer current at the
beginning is current again whenever the variable is unbound.
It is not reliable to change the current buffer back with
set-buffer, because that won't do the job if a quit happens while
the wrong buffer is current. Here is what not to do:
(let (buffer-read-only
(obuf (current-buffer)))
(set-buffer ...)
...
(set-buffer obuf))
Using save-current-buffer, as shown here, handles quitting,
errors, and throw, as well as ordinary evaluation.
(let (buffer-read-only)
(save-current-buffer
(set-buffer ...)
...))
(current-buffer)
=> #<buffer buffers.texi>
This function returns the buffer identified by buffer-or-name. An error is signaled if buffer-or-name does not identify an existing buffer.
save-current-buffer macro saves the identity of the current
buffer, evaluates the body forms, and finally restores that buffer
as current. The return value is the value of the last form in
body. The current buffer is restored even in case of an abnormal
exit via throw or error (see section Nonlocal Exits).
If the buffer that used to be current has been killed by the time of
exit from save-current-buffer, then it is not made current again,
of course. Instead, whichever buffer was current just before exit
remains current.
with-current-buffer macro saves the identity of the current
buffer, makes buffer current, evaluates the body forms, and
finally restores the buffer. The return value is the value of the last
form in body. The current buffer is restored even in case of an
abnormal exit via throw or error (see section Nonlocal Exits).
with-temp-buffer macro evaluates the body forms
with a temporary buffer as the current buffer. It saves the identity of
the current buffer, creates a temporary buffer and makes it current,
evaluates the body forms, and finally restores the previous
current buffer while killing the temporary buffer.
The return value is the value of the last form in body. You can
return the contents of the temporary buffer by using
(buffer-string) as the last form.
The current buffer is restored even in case of an abnormal exit via
throw or error (see section Nonlocal Exits).
See also with-temp-file in section Writing to Files.
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