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 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 either in front
of or after the new text, depending on the marker's insertion
type (see section Marker Insertion
Types)---unless the insertion is done with
insert-before-markers
(see section 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. 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> ;; Setm1
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.
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 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.
(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. The new marker's insertion type is specified by the argument insertion-type. See section Marker Insertion Types.
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.
(copy-marker 0) => #<marker at 1 in markers.texi> (copy-marker 20000) => #<marker at 7572 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.
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
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.
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 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
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 kill-region
and
indent-rigidly
. 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 buffer-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. 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.
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.
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.
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.
nil
. This variable is always
buffer-local in each buffer.
activate-mark-hook
is also run at
the end of a command if the mark is active and it is possible that
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.)