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. A marker can be used to represent a position in functions that require one, just as an integer could be used. In that case, the marker’s buffer is normally ignored. Of course, a marker used in this way usually points to a position in the buffer that the function operates on, but that is entirely the programmer’s responsibility. See Positions, for a complete description of positions.
A marker has three attributes: the marker position, the marker buffer, and the insertion type. The marker position is an integer that is equivalent (at a given time) to the marker as a position in that buffer. But the marker’s position value can change during the life of the marker, and often does. 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 either in
front of or after the new text, depending on the marker’s insertion
type (see Marker Insertion Types)—unless the insertion is done
with insert-before-markers (see Inserting Text).
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. Markers that can no longer be accessed are eventually removed (see Garbage Collection).
Because it is common to perform arithmetic operations on a marker
position, most of these 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 noteq, but they areequal. (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.
This function returns 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.
This function returns t if object is an integer or a marker,
nil otherwise.
This function returns t if object is a number (either
integer or floating point) 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.
The next four functions all return markers with insertion type
nil. See Marker Insertion Types.
This function returns a newly created marker that does not point anywhere.
(make-marker)
⇒ #<marker in no buffer>
This function returns a new marker that points to the present position
of point in the current buffer. See Point. For an example, see
copy-marker, below.
This function returns a new marker that points to the beginning of the accessible portion of the buffer. This will be the beginning of the buffer unless narrowing is in effect. See Narrowing.
This function returns a new marker that points to the end of the accessible portion of the buffer. This will be the end of the buffer unless narrowing is in effect. See Narrowing.
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 24080 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>
If passed a marker as its argument, 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.
The new marker’s insertion type is specified by the argument insertion-type. See Marker Insertion Types.
(copy-marker 0)
⇒ #<marker at 1 in markers.texi>
(copy-marker 90000)
⇒ #<marker at 24080 in markers.texi>
An error is signaled if marker is neither a marker nor an integer.
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.
(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
This section describes the functions for accessing the components of a marker object.
This function returns the position that marker points to, or
nil if it points nowhere.
This function returns the last known position of marker in its
buffer. It behaves like marker-position with one exception: if
the buffer of marker has been killed, it returns the last position
of marker in that buffer before the buffer was killed, instead of
returning nil.
This function returns the buffer that marker points into, or
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
When you insert text directly at the place where a marker points,
there are two possible ways to relocate that marker: it can point before
the inserted text, or point after it. You can specify which one a given
marker should do by setting its insertion type. Note that use of
insert-before-markers ignores markers’ insertion types, always
relocating a marker to point after the inserted text.
This function sets the insertion type of marker marker to
type. If type is t, marker will advance when
text is inserted at its position. If type is nil,
marker does not advance when text is inserted there.
This function reports the current insertion type of marker.
All functions that create markers without accepting an argument that
specifies the insertion type, create them with insertion type
nil (see Functions that Create Markers). Also, the mark has, by
default, insertion type nil.
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.
This function moves marker to position in buffer. If buffer is not provided, it defaults to the current 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>
This is another name for set-marker.
Each buffer has a special marker, which is designated the mark. When a buffer is newly created, this marker exists but does not point anywhere; this means that the mark doesn’t exist in that buffer yet. Subsequent commands can set the mark.
The mark specifies a position to bound a range of text for many
commands, such as kill-region and indent-rigidly. These
commands typically act on the text between point and the mark, which
is called the region. If you are writing a command that
operates on the region, 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 Code Characters for interactive.
Some commands set the mark as a side-effect. Commands should do
this only if it has 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.
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 buffer-local variable mark-active, if
non-nil, means that the mark is active. A command can call the
function deactivate-mark to deactivate the mark directly, or it
can request deactivation of the mark upon return to the editor command
loop by setting the variable deactivate-mark to a
non-nil value.
If Transient Mark mode is enabled, certain editing commands that normally apply to text near point, apply instead to the region when the mark is active. This is the main motivation for using Transient Mark mode. (Another is that this enables highlighting of the region when the mark is active. See 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.
There is also a separate global mark ring, but that is used only in a few particular user-level commands, and is not relevant to Lisp programming. So we do not describe it here.
This function returns the current buffer’s mark position as an integer,
or nil if no mark has ever been set in this buffer.
If Transient Mark mode is enabled, and mark-even-if-inactive is
nil, mark signals an error if the mark is inactive.
However, if force is non-nil, then mark disregards
inactivity of the mark, and returns the mark position (or nil)
anyway.
This function returns the marker that represents the current buffer’s mark. It is not a copy, it is the marker used internally. Therefore, changing this marker’s position will directly affect the buffer’s mark. Don’t do that unless that is the effect you want.
(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. If you make it point at any buffer other than the one of which it is the mark, it will yield perfectly consistent, but rather odd, results. We recommend that you not do it!
This function sets the mark to position, and activates the mark. The old value of the mark is not pushed onto the mark ring.
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))).
This function sets the current buffer’s mark to position, and
pushes a copy of the previous mark onto mark-ring. If
position is nil, then the value of point is used.
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.
This function pops off the top element of 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.
This variable, if non-nil, enables Transient Mark mode. In
Transient Mark mode, every buffer-modifying primitive sets
deactivate-mark. As a consequence, most commands that modify
the buffer also deactivate the mark.
When Transient Mark mode is enabled and the mark is active, many
commands that normally apply to the text near point instead apply to
the region. Such commands should use the function use-region-p
to test whether they should operate on the region. See The Region.
Lisp programs can set transient-mark-mode to non-nil,
non-t values to enable Transient Mark mode temporarily. If the
value is lambda, Transient Mark mode is automatically turned
off after any action, such as buffer modification, that would normally
deactivate the mark. If the value is (only . oldval),
then transient-mark-mode is set to the value oldval after
any subsequent command that moves point and is not shift-translated
(see shift-translation), or after any other
action that would normally deactivate the mark. (Marking a region
with the mouse will temporarily enable transient-mark-mode in
this way.)
If this is non-nil, Lisp programs and the Emacs user can use the
mark even when it is inactive. This option affects the behavior of
Transient Mark mode. When the option is non-nil, deactivation of
the mark turns off region highlighting, but commands that use the mark
behave as if the mark were still active.
If an editor command sets this variable non-nil, then the editor
command loop deactivates the mark after the command returns (if
Transient Mark mode is enabled). All the primitives that change the
buffer set deactivate-mark, to deactivate the mark when the
command is finished. Setting this variable makes it buffer-local.
To write Lisp code that modifies the buffer without causing
deactivation of the mark at the end of the command, bind
deactivate-mark to nil around the code that does the
modification. For example:
(let (deactivate-mark) (insert " "))
If Transient Mark mode is enabled or force is non-nil,
this function deactivates the mark and runs the normal hook
deactivate-mark-hook. Otherwise, it does nothing.
The mark is active when this variable is non-nil. This
variable is always buffer-local in each buffer. Do not use the
value of this variable to decide whether a command that normally
operates on text near point should operate on the region instead. Use
the function use-region-p for that (see The Region).
These normal hooks are run, respectively, when the mark becomes active
and when it becomes inactive. The hook activate-mark-hook is
also run when the region is reactivated, for instance after using a
command that switches back to a buffer that has an active mark.
This function implements the shift-selection behavior of
point-motion commands. See Shift Selection in The GNU Emacs
Manual. It is called automatically by the Emacs command loop
whenever a command with a ‘^’ character in its interactive
spec is invoked, before the command itself is executed
(see ^).
If shift-select-mode is non-nil and the current command
was invoked via shift translation (see shift-translation), this function sets the mark and temporarily
activates the region, unless the region was already temporarily
activated in this way. Otherwise, if the region has been activated
temporarily, it deactivates the mark and restores the variable
transient-mark-mode to its earlier value.
The value of this buffer-local variable is the list of saved former marks of the current buffer, most recent first.
mark-ring
⇒ (#<marker at 11050 in markers.texi>
#<marker at 10832 in markers.texi>
...)
The value of this variable is the maximum size of 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.
When Delete Selection mode (see Delete Selection in The GNU Emacs Manual) is enabled, commands that operate on the
active region (a.k.a. “selection”) behave slightly differently.
This works by adding the function delete-selection-pre-hook to
the pre-command-hook (see Command Loop Overview). That function
calls delete-selection-helper to delete the selection as
appropriate for the command. If you want to adapt a command to Delete
Selection mode, put the delete-selection property on the
function’s symbol (see Accessing Symbol Properties); commands that don’t have
this property on their symbol won’t delete the selection. This
property can have one of several values to tailor the behavior to what
the command is supposed to do; see the doc strings of
delete-selection-pre-hook and delete-selection-helper
for the details.
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.
The next two functions signal an error if the mark does not point
anywhere. If Transient Mark mode is enabled and
mark-even-if-inactive is nil, they also signal an error
if the mark is inactive.
This function returns the position of the beginning of the region (as an integer). This is the position of either point or the mark, whichever is smaller.
This function returns the position of the end of the region (as an integer). This is the position of either point or the mark, whichever is larger.
Instead of using region-beginning and region-end, a
command designed to operate on a region should normally use
interactive with either a ‘r’ or ‘R’ code letter to
find the beginning and end of the region. This lets other Lisp programs
specify the bounds explicitly as arguments. See Code Characters for interactive.
This function returns t if Transient Mark mode is enabled, the
mark is active, and there is a valid region in the buffer. This
function is intended to be used by commands that operate on the
region, instead of on text near point, when the mark is active.
A region is valid if it has a non-zero size, or if the user option
use-empty-active-region is non-nil (by default, it is
nil). The function region-active-p is similar to
use-region-p, but considers all regions as valid. In most
cases, you should not use region-active-p, since if the region
is empty it is often more appropriate to operate on point.