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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.
1.1 Overview of Markers | The components of a marker, and how it relocates. | |
1.2 Predicates on Markers | Testing whether an object is a marker. | |
1.3 Functions That Create Markers | Making empty markers or markers at certain places. | |
1.4 Information from Markers | Finding the marker’s buffer or character position. | |
1.5 Changing Marker Positions | Moving the marker to a new buffer or position. | |
1.6 The Mark | How “the mark” is implemented with a marker. | |
1.7 The Region | How to access “the region”. |
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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. @xref{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 that 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
(@pxref{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 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>
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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 kind)
or a marker, nil
otherwise.
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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.
This functions 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. @xref{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. @xref{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. @xref{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 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>
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.
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>
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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 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
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.
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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 less than 1, set-marker
moves marker
to the beginning of the buffer. If 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>
This is another name for set-marker
.
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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. @xref{Interactive Codes}.
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 causes deactivation only 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. @xref{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.
This function returns the current buffer’s mark position as an integer.
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.
This function returns the current buffer’s mark. This is the very marker that records the mark location inside XEmacs, not a copy. Therefore, changing this marker’s position will directly affect the position of the mark. Don’t do it 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. 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.
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.
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
.
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.
The return value is not meaningful.
This variable if non-nil
enables Transient Mark mode, in which
every buffer-modifying primitive sets deactivate-mark
. The
consequence of this is that commands that modify the buffer normally
make the mark inactive.
If an editor command sets this variable non-nil
, then the editor
command loop deactivates the mark after the command returns, but only if
Transient Mark mode is enabled.
This function deactivates the mark, but only if Transient Mark mode is enabled.
The mark is active when this variable is non-nil
. This variable
is always local in each buffer.
These normal hooks are run, respectively, when the mark becomes active
and when it becomes inactive. The hook activate-mark-hook
is also
run at the end of a command if the mark is active and the region may
have changed.
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.
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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.
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.
If the mark does not point anywhere, an error is signaled.
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.
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. (@xref{Interactive
Codes}.)
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