A marker is a Lisp object used to specify a position in a buffer relative to the surrounding text. A marker changes its offset from the beginning of the buffer automatically whenever text is inserted or deleted, so that it stays with the two characters on either side of it.
A marker specifies a buffer and a position in that buffer. The marker can be used to represent a position in the functions that require one, just as an integer could be used. See section Positions, for a complete description of positions.
A marker has two attributes: the marker position, and the marker buffer. The marker position is an integer which is equivalent (at a given time) to the marker as a position in that buffer. But the marker's position value can change often during the life of the marker. Insertion and deletion of text in the buffer relocate the marker. The idea is that a marker positioned between two characters remains between those two characters despite insertion and deletion elsewhere in the buffer. Relocation changes the integer equivalent of the marker.
Deleting text around a marker's position leaves the marker between the
characters immediately before and after the deleted text. Inserting
text at the position of a marker normally leaves the marker in front of
the new text--unless it is inserted with insert-before-markers
(see section Insertion).
Insertion and deletion in a buffer must check all the markers and relocate them if necessary. This slows processing in a buffer with a large number of markers. For this reason, it is a good idea to make a marker point nowhere if you are sure you don't need it any more. Unreferenced markers are garbage collected eventually, but until then will continue to use time if they do point somewhere.
Because it is common to perform arithmetic operations on a marker
position, most of the arithmetic operations (including + and
-) accept markers as arguments. In such cases, the marker
stands for its current position.
Here are examples of creating markers, setting markers, and moving point to markers:
;; Make a new marker that initially does not point anywhere:
(setq m1 (make-marker))
=> #<marker in no buffer>
;; Set m1 to point between the 99th and 100th characters
;; in the current buffer:
(set-marker m1 100)
=> #<marker at 100 in markers.texi>
;; Now insert one character at the beginning of the buffer:
(goto-char (point-min))
=> 1
(insert "Q")
=> nil
;; m1 is updated appropriately.
m1
=> #<marker at 101 in markers.texi>
;; Two markers that point to the same position
;; are not eq, but they are equal.
(setq m2 (copy-marker m1))
=> #<marker at 101 in markers.texi>
(eq m1 m2)
=> nil
(equal m1 m2)
=> t
;; When you are finished using a marker, make it point nowhere.
(set-marker m1 nil)
=> #<marker in no buffer>
You can test an object to see whether it is a marker, or whether it is either an integer or a marker. The latter test is useful in connection with the arithmetic functions that work with both markers and integers.
t if object is a marker, nil
otherwise. Note that integers are not markers, even though many
functions will accept either a marker or an integer.
t if object is an integer or a marker,
nil otherwise.
t if object is a number (either kind)
or a marker, nil otherwise.
When you create a new marker, you can make it point nowhere, or point to the present position of point, or to the beginning or end of the accessible portion of the buffer, or to the same place as another given marker.
(make-marker)
=> #<marker in no buffer>
copy-marker, below.
Here are examples of this function and point-min-marker, shown in
a buffer containing a version of the source file for the text of this
chapter.
(point-min-marker)
=> #<marker at 1 in markers.texi>
(point-max-marker)
=> #<marker at 15573 in markers.texi>
(narrow-to-region 100 200)
=> nil
(point-min-marker)
=> #<marker at 100 in markers.texi>
(point-max-marker)
=> #<marker at 200 in markers.texi>
copy-marker returns a
new marker that points to the same place and the same buffer as does
marker-or-integer. If passed an integer as its argument,
copy-marker returns a new marker that points to position
marker-or-integer in the current buffer.
If passed an integer argument less than 1, copy-marker returns a
new marker that points to the beginning of the current buffer. If
passed an integer argument greater than the length of the buffer,
copy-marker returns a new marker that points to the end of the
buffer.
An error is signaled if marker is neither a marker nor an integer.
(setq p (point-marker))
=> #<marker at 2139 in markers.texi>
(setq q (copy-marker p))
=> #<marker at 2139 in markers.texi>
(eq p q)
=> nil
(equal p q)
=> t
(copy-marker 0)
=> #<marker at 1 in markers.texi>
(copy-marker 20000)
=> #<marker at 7572 in markers.texi>
This section describes the functions for accessing the components of a marker object.
nil if it points nowhere.
nil if it points nowhere.
(setq m (make-marker))
=> #<marker in no buffer>
(marker-position m)
=> nil
(marker-buffer m)
=> nil
(set-marker m 3770 (current-buffer))
=> #<marker at 3770 in markers.texi>
(marker-buffer m)
=> #<buffer markers.texi>
(marker-position m)
=> 3770
Two distinct markers are considered equal (even though not
eq) to each other if they have the same position and buffer, or
if they both point nowhere.
This section describes how to change the position of an existing marker. When you do this, be sure you know whether the marker is used outside of your program, and, if so, what effects will result from moving it--otherwise, confusing things may happen in other parts of Emacs.
If position is less than 1, set-marker moves marker
to the beginning of the buffer. If the value of position is
greater than the size of the buffer, set-marker moves marker to
the end of the buffer. If position is nil or a marker that
points nowhere, then marker is set to point nowhere.
The value returned is marker.
(setq m (point-marker))
=> #<marker at 4714 in markers.texi>
(set-marker m 55)
=> #<marker at 55 in markers.texi>
(setq b (get-buffer "foo"))
=> #<buffer foo>
(set-marker m 0 b)
=> #<marker at 1 in foo>
set-marker.
One special marker in each buffer is designated the mark. It
records a position for the user for the sake of commands such as
C-w and C-x TAB. Lisp programs should set the mark
only to values that have a potential use to the user, and never for
their own internal purposes. For example, the replace-regexp
command sets the mark to the value of point before doing any
replacements, because this enables the user to move back there
conveniently after the replace is finished.
Many commands are designed so that when called interactively they
operate on the text between point and the mark. If you are writing such
a command, don't examine the mark directly; instead, use
interactive with the `r' specification. This provides the
values of point and the mark as arguments to the command in an
interactive call, but permits other Lisp programs to specify arguments
explicitly. See section Code Characters for interactive.
Each buffer has its own value of the mark that is independent of the value of the mark in other buffers. When a buffer is created, the mark exists but does not point anywhere. We consider this state as "the absence of a mark in that buffer".
Once the mark "exists" in a buffer, it normally never ceases to
exist. However, it may become inactive, if Transient Mark mode is
enabled. The variable mark-active, which is always local in all
buffers, indicates whether the mark is active: non-nil means
yes. A command can request deactivation of the mark upon return to the
editor command loop by setting deactivate-mark to a
non-nil value (but this deactivation only follows if Transient
Mark mode is enabled).
The main motivation for using Transient Mark mode is that this mode also enables highlighting of the region when the mark is active. See section Emacs Display.
In addition to the mark, each buffer has a mark ring which is a
list of markers containing previous values of the mark. When editing
commands change the mark, they should normally save the old value of the
mark on the mark ring. The variable mark-ring-max specifies the
maximum number of entries in the mark ring; once the list becomes this
long, adding a new element deletes the last element.
If the mark is inactive, mark normally signals an error.
However, if force is non-nil, then mark returns the
mark position anyway--or nil, if the mark is not yet set for
this buffer.
(setq m (mark-marker))
=> #<marker at 3420 in markers.texi>
(set-marker m 100)
=> #<marker at 100 in markers.texi>
(mark-marker)
=> #<marker at 100 in markers.texi>
Like any marker, this marker can be set to point at any buffer you like. We don't recommend that you make it point at any buffer other than the one of which it is the mark. If you do, it will yield perfectly consistent, but rather odd, results.
Please note: use this function only if you want the user to
see that the mark has moved, and you want the previous mark position to
be lost. Normally, when a new mark is set, the old one should go on the
mark-ring. For this reason, most applications should use
push-mark and pop-mark, not set-mark.
Novice Emacs Lisp programmers often try to use the mark for the wrong purposes. The mark saves a location for the user's convenience. An editing command should not alter the mark unless altering the mark is part of the user-level functionality of the command. (And, in that case, this effect should be documented.) To remember a location for internal use in the Lisp program, store it in a Lisp variable. For example:
(let ((beg (point))) (forward-line 1) (delete-region beg (point))).
mark-ring. If
position is nil, then the value of point is used.
push-mark returns nil.
The function push-mark normally does not activate the
mark. To do that, specify t for the argument activate.
A `Mark set' message is displayed unless nomsg is
non-nil.
mark-ring and makes
that mark become the buffer's actual mark. This does not move point in
the buffer, and it does nothing if mark-ring is empty. It
deactivates the mark.
The return value is not meaningful.
deactivate-mark. The consequence
of this is that commands that modify the buffer normally make the mark
inactive.
nil, then the editor
command loop deactivates the mark after the command returns.
nil. This variable
is always local in each buffer.
activate-mark-hook is also
run at the end of a command if the mark is active and the region may
have changed.
mark-ring
=> (#<marker at 11050 in markers.texi>
#<marker at 10832 in markers.texi>
...)
mark-ring. If
more marks than this are pushed onto the mark-ring,
push-mark discards an old mark when it adds a new one.
The text between point and the mark is known as the region. Various functions operate on text delimited by point and the mark, but only those functions specifically related to the region itself are described here.
If the mark does not point anywhere, an error is signaled.
If the mark does not point anywhere, an error is signaled.
Few programs need to use the region-beginning and
region-end functions. A command designed to operate on a region
should normally use interactive with the `r' specification
to find the beginning and end of the region. This lets other Lisp
programs specify the bounds explicitly as arguments. (See section Code Characters for interactive.)