27.2 The Current Buffer

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 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 Command Loop.) Therefore, set-buffer is not the way to switch visibly to a different buffer so that the user can edit it. For that, you must use the functions described in Displaying Buffers.

Warning: 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 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.

Do not rely on using set-buffer to change the current buffer back, 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 ...)
         ...))