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42. Markers

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.

42.1 Overview of Markers  The components of a marker, and how it relocates.
42.2 Predicates on Markers  Testing whether an object is a marker.
42.3 Functions That Create Markers  Making empty markers or markers at certain places.
42.4 Information from Markers  Finding the marker's buffer or character position.
42.5 Changing Marker Positions  Moving the marker to a new buffer or position.
42.6 The Mark  How "the mark" is implemented with a marker.
42.7 The Region  How to access "the region".


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42.1 Overview of Markers

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 41. 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 (see section 43.4 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.

Note that you can use extents to achieve the same functionality, and more, as markers. (Markers were defined before extents, which is why they both continue to exist.) A zero-length extent with the detachable property removed is almost identical to a marker. (See section 47.3 Extent Endpoints, for more information on zero-length extents.)

In particular:

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>


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42.2 Predicates on Markers

You can test an object to see whether it is a marker, or whether it is either an integer or a marker or either an integer, a character, or a marker. The latter tests are useful in connection with the arithmetic functions that work with any of markers, integers, or characters.

Function: markerp object
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.

Function: integer-or-marker-p object
This function returns t if object is an integer or a marker, nil otherwise.

Function: integer-char-or-marker-p object
This function returns t if object is an integer, a character, or a marker, nil otherwise.

Function: number-or-marker-p object
This function returns t if object is a number (either kind) or a marker, nil otherwise.

Function: number-char-or-marker-p object
This function returns t if object is a number (either kind), a character, or a marker, nil otherwise.


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42.3 Functions That Create Markers

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.

Function: make-marker
This functions returns a newly created marker that does not point anywhere.

 
(make-marker)
     => #<marker in no buffer>

Function: point-marker &optional dont-copy-p buffer
This function returns a marker that points to the present position of point in buffer, which defaults to the current buffer. See section 41.1 Point. For an example, see copy-marker, below.

Internally, a marker corresponding to point is always maintained. Normally the marker returned by point-marker is a copy; you may modify it with reckless abandon. However, if optional argument dont-copy-p is non-nil, then the real point-marker is returned; modifying the position of this marker will move point. It is illegal to change the buffer of it, or make it point nowhere.

Function: point-min-marker &optional buffer
This function returns a new marker that points to the beginning of the accessible portion of buffer, which defaults to the current buffer. This will be the beginning of the buffer unless narrowing is in effect. See section 41.4 Narrowing.

Function: point-max-marker &optional buffer
This function returns a new marker that points to the end of the accessible portion of buffer, which defaults to the current buffer. This will be the end of the buffer unless narrowing is in effect. See section 41.4 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>

Function: copy-marker marker-or-integer &optional marker-type
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-or-integer is neither a marker nor an integer.

Optional second argument marker-type specifies the insertion type of the new marker; see marker-insertion-type.

 
(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

(point)
     => 2139

(set-marker p 3000)
     => #<marker at 3000 in markers.texi>

(point)
     => 2139

(setq p (point-marker t))
     => #<marker at 2139 in markers.texi>

(set-marker p 3000)
     => #<marker at 3000 in markers.texi>

(point)
     => 3000

(copy-marker 0)
     => #<marker at 1 in markers.texi>

(copy-marker 20000)
     => #<marker at 7572 in markers.texi>


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42.4 Information from Markers

This section describes the functions for accessing the components of a marker object.

Function: marker-position marker
This function returns the position that marker points to, or nil if it points nowhere.

Function: marker-buffer marker
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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42.5 Changing Marker Positions

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.

Function: set-marker marker position &optional buffer
This function moves marker to position in buffer. If buffer is not provided, it defaults to the current buffer.

position can be a marker, an integer or nil. If position is an integer, set-marker moves marker to point before the positionth character in buffer. If position is nil, marker is made to point nowhere. Then it no longer slows down editing in any buffer. If position is less than 1, marker is moved to the beginning of buffer. If position is greater than the size of buffer, marker is moved to the end of buffer.

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>

Function: move-marker marker position &optional buffer
This is another name for set-marker.


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42.6 The Mark

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.

Once the mark "exists" in a buffer, it normally never ceases to exist. However, it may become inactive, and usually does so after each command (other than simple motion commands and some commands that explicitly activate the mark). When the mark is active, the region between point and the mark is called the active region and is highlighted specially.

Many commands are designed so that when called interactively they operate on the text between point and the mark. Such commands work only when an active region exists, i.e. when the mark is active. (The reason for this is to prevent you from accidentally deleting or changing large chunks of your text.) 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, and automatically signals an error if the command is called interactively when no active region exists. See section 25.2.2 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.") However, only one active region can exist at a time. Activating the mark in one buffer automatically deactivates an active mark in any other buffer. Note that the user can explicitly activate a mark at any time by using the command activate-region (normally bound to M-C-z) or by using the command exchange-point-and-mark (normally bound to C-x C-x), which has the side effect of activating the mark.

Some people do not like active regions, so they disable this behavior by setting the variable zmacs-regions to nil. This makes the mark always active (except when a buffer is just created and the mark points nowhere), and turns off the highlighting of the region between point and the mark. Commands that explicitly retrieve the value of the mark should make sure that they behave correctly and consistently irrespective of the setting of zmacs-regions; some primitives are provided to ensure this behavior.

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.

Function: mark &optional force buffer
This function returns buffer's mark position as an integer. buffer defaults to the current buffer if omitted.

If the mark is inactive, mark normally returns nil. However, if force is non-nil, then mark returns the mark position anyway--or nil, if the mark is not yet set for the buffer.

(Remember that if zmacs-regions is nil, the mark is always active as long as it exists, and the force argument will have no effect.)

If you are using this in an editing command, you are most likely making a mistake; see the documentation of set-mark below.

Function: mark-marker &optional force buffer
This function returns buffer's mark. buffer defaults to the current buffer if omitted. 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.

If the mark is inactive, mark-marker normally returns nil. However, if force is non-nil, then mark-marker returns the mark anyway.

 
(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.

Function: set-mark position &optional buffer
This function sets buffer's mark to position, and activates the mark. buffer defaults to the current buffer if omitted. 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 XEmacs 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 ((start (point)))
  (forward-line 1)
  (delete-region start (point))).

Command: exchange-point-and-mark &optional dont-activate-region
This function exchanges the positions of point and the mark. It is intended for interactive use. The mark is also activated unless dont-activate-region is non-nil.

Function: push-mark &optional position nomsg activate buffer
This function sets buffer's mark to position, and pushes a copy of the previous mark onto mark-ring. buffer defaults to the current buffer if omitted. If position is nil, then the value of point is used. push-mark returns nil.

If the last global mark pushed was not in buffer, also push position on the global mark ring (see below).

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.

Function: pop-mark
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.

Variable: mark-ring
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>
    ...)

User Option: mark-ring-max
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.

In additional to a per-buffer mark ring, there is a global mark ring. Marks are pushed onto the global mark ring the first time you set a mark after switching buffers.

Variable: global-mark-ring
The value of this variable is the list of saved former global marks, most recent first.

User Option: mark-ring-max
The value of this variable is the maximum size of global-mark-ring. If more marks than this are pushed onto the global-mark-ring, push-mark discards an old mark when it adds a new one.

Command: pop-global-mark
This function pops a mark off the global mark ring and jumps to that location.


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42.7 The Region

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.

When zmacs-regions is non-nil (this is the default), the concept of an active region exists. The region is active when the corresponding mark is active. Note that only one active region at a time can exist--i.e. only one buffer's region is active at a time. See section 42.6 The Mark, for more information about active regions.

User Option: zmacs-regions
If non-nil (the default), active regions are used. See section 42.6 The Mark, for a detailed explanation of what this means.

A number of functions are provided for explicitly determining the bounds of the region and whether it is active. Few programs need to use these functions, however. 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 and automatically respects the user's setting for zmacs-regions. (See section 25.2.2 Code Characters for interactive.)

Function: region-beginning &optional buffer
This function returns the position of the beginning of buffer's region (as an integer). This is the position of either point or the mark, whichever is smaller. buffer defaults to the current buffer if omitted.

If the mark does not point anywhere, an error is signaled. Note that this function ignores whether the region is active.

Function: region-end &optional buffer
This function returns the position of the end of buffer's region (as an integer). This is the position of either point or the mark, whichever is larger. buffer defaults to the current buffer if omitted.

If the mark does not point anywhere, an error is signaled. Note that this function ignores whether the region is active.

Function: region-exists-p
This function is non-nil if the region exists. If active regions are in use (i.e. zmacs-regions is true), this means that the region is active. Otherwise, this means that the user has pushed a mark in this buffer at some point in the past. If this function returns nil, a function that uses the `r' interactive specification will cause an error when called interactively.

Function: region-active-p
If zmacs-regions is true, this is equivalent to region-exists-p. Otherwise, this function always returns false. This function is used by commands such as fill-paragraph-or-region and capitalize-region-or-word, which operate either on the active region or on something else (e.g. the word or paragraph at point).

Variable: zmacs-region-stays
If a command sets this variable to true, the currently active region will remain activated when the command finishes. (Normally the region is deactivated when each command terminates.) If zmacs-regions is false, however, this has no effect. Under normal circumstances, you do not need to set this; use the interactive specification `_' instead, if you want the region to remain active.

Function: zmacs-activate-region
This function activates the region in the current buffer (this is equivalent to activating the current buffer's mark). This will normally also highlight the text in the active region and set zmacs-region-stays to t. (If zmacs-regions is false, however, this function has no effect.)

Function: zmacs-deactivate-region
This function deactivates the region in the current buffer (this is equivalent to deactivating the current buffer's mark). This will normally also unhighlight the text in the active region and set zmacs-region-stays to nil. (If zmacs-regions is false, however, this function has no effect.)

Function: zmacs-update-region
This function updates the active region, if it's currently active. (If there is no active region, this function does nothing.) This has the effect of updating the highlighting on the text in the region; but you should never need to call this except under rather strange circumstances. The command loop automatically calls it when appropriate. Calling this function will call the hook zmacs-update-region-hook, if the region is active.

Variable: zmacs-activate-region-hook
This normal hook is called when a region becomes active. (Usually this happens as a result of a command that activates the region, such as set-mark-command, activate-region, or exchange-point-and-mark.) Note that calling `zmacs-activate-region' will call this hook, even if the region is already active. If zmacs-regions is false, however, this hook will never get called under any circumstances.

Variable: zmacs-deactivate-region-hook
This normal hook is called when an active region becomes inactive. (Calling `zmacs-deactivate-region' when the region is inactive will not cause this hook to be called.) If zmacs-regions is false, this hook will never get called.

Variable: zmacs-update-region-hook
This normal hook is called when an active region is "updated" by zmacs-update-region. This normally gets called at the end of each command that sets zmacs-region-stays to t, indicating that the region should remain activated. The motion commands do this.


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