@c -*-texinfo-*- @c This is part of the GNU Emacs Lisp Reference Manual. @c Copyright (C) 1990, 1991, 1992, 1993, 1994 Free Software Foundation, Inc. @c See the file elisp.texi for copying conditions. @setfilename ../info/markers @node Markers, Text, Positions, Top @chapter Markers @cindex markers A @dfn{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. @menu * Overview of Markers:: The components of a marker, and how it relocates. * Predicates on Markers:: Testing whether an object is a marker. * Creating Markers:: Making empty markers or markers at certain places. * Information from Markers:: Finding the marker's buffer or character position. * Changing Markers:: Moving the marker to a new buffer or position. * The Mark:: How ``the mark'' is implemented with a marker. * The Region:: How to access ``the region''. @end menu @node Overview of Markers @section 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. @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. @cindex marker relocation 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 @code{insert-before-markers} (@pxref{Insertion}). @cindex marker garbage collection 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. @cindex markers as numbers Because it is common to perform arithmetic operations on a marker position, most of the arithmetic operations (including @code{+} and @code{-}) 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: @example @group ;; @r{Make a new marker that initially does not point anywhere:} (setq m1 (make-marker)) @result{} # @end group @group ;; @r{Set @code{m1} to point between the 99th and 100th characters} ;; @r{in the current buffer:} (set-marker m1 100) @result{} # @end group @group ;; @r{Now insert one character at the beginning of the buffer:} (goto-char (point-min)) @result{} 1 (insert "Q") @result{} nil @end group @group ;; @r{@code{m1} is updated appropriately.} m1 @result{} # @end group @group ;; @r{Two markers that point to the same position} ;; @r{are not @code{eq}, but they are @code{equal}.} (setq m2 (copy-marker m1)) @result{} # (eq m1 m2) @result{} nil (equal m1 m2) @result{} t @end group @group ;; @r{When you are finished using a marker, make it point nowhere.} (set-marker m1 nil) @result{} # @end group @end example @node Predicates on Markers @section 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. The latter test is useful in connection with the arithmetic functions that work with both markers and integers. @defun markerp object This function returns @code{t} if @var{object} is a marker, @code{nil} otherwise. Note that integers are not markers, even though many functions will accept either a marker or an integer. @end defun @defun integer-or-marker-p object This function returns @code{t} if @var{object} is an integer or a marker, @code{nil} otherwise. @end defun @defun number-or-marker-p object This function returns @code{t} if @var{object} is a number (either kind) or a marker, @code{nil} otherwise. @end defun @node Creating Markers @section 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. @defun make-marker This functions returns a newly created marker that does not point anywhere. @example @group (make-marker) @result{} # @end group @end example @end defun @defun point-marker This function returns a new marker that points to the present position of point in the current buffer. @xref{Point}. For an example, see @code{copy-marker}, below. @end defun @defun point-min-marker 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}. @end defun @defun point-max-marker @cindex end of buffer marker 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 @code{point-min-marker}, shown in a buffer containing a version of the source file for the text of this chapter. @example @group (point-min-marker) @result{} # (point-max-marker) @result{} # @end group @group (narrow-to-region 100 200) @result{} nil @end group @group (point-min-marker) @result{} # @end group @group (point-max-marker) @result{} # @end group @end example @end defun @defun copy-marker marker-or-integer If passed a marker as its argument, @code{copy-marker} returns a new marker that points to the same place and the same buffer as does @var{marker-or-integer}. If passed an integer as its argument, @code{copy-marker} returns a new marker that points to position @var{marker-or-integer} in the current buffer. If passed an integer argument less than 1, @code{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, @code{copy-marker} returns a new marker that points to the end of the buffer. An error is signaled if @var{marker} is neither a marker nor an integer. @example @group (setq p (point-marker)) @result{} # @end group @group (setq q (copy-marker p)) @result{} # @end group @group (eq p q) @result{} nil @end group @group (equal p q) @result{} t @end group @group (copy-marker 0) @result{} # @end group @group (copy-marker 20000) @result{} # @end group @end example @end defun @node Information from Markers @section Information from Markers This section describes the functions for accessing the components of a marker object. @defun marker-position marker This function returns the position that @var{marker} points to, or @code{nil} if it points nowhere. @end defun @defun marker-buffer marker This function returns the buffer that @var{marker} points into, or @code{nil} if it points nowhere. @example @group (setq m (make-marker)) @result{} # @end group @group (marker-position m) @result{} nil @end group @group (marker-buffer m) @result{} nil @end group @group (set-marker m 3770 (current-buffer)) @result{} # @end group @group (marker-buffer m) @result{} # @end group @group (marker-position m) @result{} 3770 @end group @end example @end defun Two distinct markers are considered @code{equal} (even though not @code{eq}) to each other if they have the same position and buffer, or if they both point nowhere. @node Changing Markers @section 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. @defun set-marker marker position &optional buffer This function moves @var{marker} to @var{position} in @var{buffer}. If @var{buffer} is not provided, it defaults to the current buffer. If @var{position} is less than 1, @code{set-marker} moves @var{marker} to the beginning of the buffer. If @var{position} is greater than the size of the buffer, @code{set-marker} moves marker to the end of the buffer. If @var{position} is @code{nil} or a marker that points nowhere, then @var{marker} is set to point nowhere. The value returned is @var{marker}. @example @group (setq m (point-marker)) @result{} # @end group @group (set-marker m 55) @result{} # @end group @group (setq b (get-buffer "foo")) @result{} # @end group @group (set-marker m 0 b) @result{} # @end group @end example @end defun @defun move-marker marker position &optional buffer This is another name for @code{set-marker}. @end defun @node The Mark @section The Mark @cindex mark, the @cindex mark ring One special marker in each buffer is designated @dfn{the mark}. It records a position for the user for the sake of commands such as @kbd{C-w} and @kbd{C-x @key{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 @code{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 @code{interactive} with the @samp{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 @dfn{inactive}, if Transient Mark mode is enabled. The variable @code{mark-active}, which is always local in all buffers, indicates whether the mark is active: non-@code{nil} means yes. A command can request deactivation of the mark upon return to the editor command loop by setting @code{deactivate-mark} to a non-@code{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 @dfn{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 @code{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. @defun mark &optional force @cindex current buffer mark This function returns the current buffer's mark position as an integer. If the mark is inactive, @code{mark} normally signals an error. However, if @var{force} is non-@code{nil}, then @code{mark} returns the mark position anyway---or @code{nil}, if the mark is not yet set for this buffer. @end defun @defun mark-marker This function returns the current buffer's mark. This is the very marker that records the mark location inside Emacs, 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. @example @group (setq m (mark-marker)) @result{} # @end group @group (set-marker m 100) @result{} # @end group @group (mark-marker) @result{} # @end group @end example 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. @end defun @ignore @deffn Command set-mark-command jump If @var{jump} is @code{nil}, this command sets the mark to the value of point and pushes the previous value of the mark on the mark ring. The message @samp{Mark set} is also displayed in the echo area. If @var{jump} is not @code{nil}, this command sets point to the value of the mark, and sets the mark to the previous saved mark value, which is popped off the mark ring. This function is @emph{only} intended for interactive use. @end deffn @end ignore @defun set-mark position This function sets the mark to @var{position}, and activates the mark. The old value of the mark is @emph{not} pushed onto the mark ring. @strong{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 @code{mark-ring}. For this reason, most applications should use @code{push-mark} and @code{pop-mark}, not @code{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: @example @group (let ((beg (point))) (forward-line 1) (delete-region beg (point))). @end group @end example @end defun @c for interactive use only @ignore @deffn Command exchange-point-and-mark This function exchanges the positions of point and the mark. It is intended for interactive use. @end deffn @end ignore @defun push-mark &optional position nomsg activate This function sets the current buffer's mark to @var{position}, and pushes a copy of the previous mark onto @code{mark-ring}. If @var{position} is @code{nil}, then the value of point is used. @code{push-mark} returns @code{nil}. The function @code{push-mark} normally @emph{does not} activate the mark. To do that, specify @code{t} for the argument @var{activate}. A @samp{Mark set} message is displayed unless @var{nomsg} is non-@code{nil}. @end defun @defun pop-mark This function pops off the top element of @code{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 @code{mark-ring} is empty. It deactivates the mark. The return value is not meaningful. @end defun @defopt transient-mark-mode @cindex Transient Mark mode This variable if non-@code{nil} enables Transient Mark mode, in which every buffer-modifying primitive sets @code{deactivate-mark}. The consequence of this is that commands that modify the buffer normally make the mark inactive. @end defopt @defvar deactivate-mark If an editor command sets this variable non-@code{nil}, then the editor command loop deactivates the mark after the command returns, but only if Transient Mark mode is enabled. @end defvar @defun deactivate-mark This function deactivates the mark, but only if Transient Mark mode is enabled. @end defun @defvar mark-active The mark is active when this variable is non-@code{nil}. This variable is always local in each buffer. @end defvar @defvar activate-mark-hook @defvarx deactivate-mark-hook These normal hooks are run, respectively, when the mark becomes active and when it becomes inactive. The hook @code{activate-mark-hook} is also run at the end of a command if the mark is active and the region may have changed. @end defvar @defvar mark-ring The value of this buffer-local variable is the list of saved former marks of the current buffer, most recent first. @example @group mark-ring @result{} (# # @dots{}) @end group @end example @end defvar @defopt mark-ring-max The value of this variable is the maximum size of @code{mark-ring}. If more marks than this are pushed onto the @code{mark-ring}, @code{push-mark} discards an old mark when it adds a new one. @end defopt @node The Region @section The Region @cindex region, the The text between point and the mark is known as @dfn{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. @defun region-beginning 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. @end defun @defun region-end 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. @end defun Few programs need to use the @code{region-beginning} and @code{region-end} functions. A command designed to operate on a region should normally use @code{interactive} with the @samp{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}.)