Text
****

This chapter describes the functions that deal with the text in a buffer.  Most examine, insert, or delete text in the current buffer, often operating at point or on text adjacent to point.  Many are interactive.  All the functions that change the text provide for undoing the changes (see Undo).

Many text-related functions operate on a region of text defined by two buffer positions passed in arguments named START and END. These arguments should be either markers (see Markers) or numeric character positions (see Positions).  The order of these arguments does not matter; it is all right for START to be the end of the region and END the beginning.  For example, (delete-region 1 10) and (delete-region 10 1) are equivalent.  An args-out-of-range error is signaled if either START or END is outside the accessible portion of the buffer.  In an interactive call, point and the mark are used for these arguments.

Throughout this chapter, ``text'' refers to the characters in the buffer, together with their properties (when relevant).  Keep in mind that point is always between two characters, and the cursor appears on the character after point.

Examining Text Near Point
=========================

Many functions are provided to look at the characters around point. Several simple functions are described here.  See also looking-at in Regexp Search.

 -- Function: char-after &optional position
     This function returns the character in the current buffer at (i.e., immediately after) position POSITION.  If POSITION is out of range for this purpose, either before the beginning of the buffer, or at or beyond the end, then the value is nil.  The default for POSITION is point.

     In the following example, assume that the first character in the buffer is @:

          (char-to-string (char-after 1))
               => "@"

 -- Function: char-before &optional position
     This function returns the character in the current buffer immediately before position POSITION.  If POSITION is out of range for this purpose, either before the beginning of the buffer, or at or beyond the end, then the value is nil.  The default for POSITION is point.

 -- Function: following-char
     This function returns the character following point in the current buffer.  This is similar to (char-after (point)).  However, if point is at the end of the buffer, then following-char returns 0.

     Remember that point is always between characters, and the terminal cursor normally appears over the character following point.  Therefore, the character returned by following-char is the character the cursor is over.

     In this example, point is between the a and the c.

          ---------- Buffer: foo ----------
          Gentlemen may cry ``Pea-!-ce! Peace!,''
          but there is no peace.
          ---------- Buffer: foo ----------

          (char-to-string (preceding-char))
               => "a"
          (char-to-string (following-char))
               => "c"

 -- Function: preceding-char
     This function returns the character preceding point in the current buffer.  See above, under following-char, for an example.  If point is at the beginning of the buffer, preceding-char returns 0.

 -- Function: bobp
     This function returns t if point is at the beginning of the buffer.  If narrowing is in effect, this means the beginning of the accessible portion of the text.  See also point-min in Point.

 -- Function: eobp
     This function returns t if point is at the end of the buffer. If narrowing is in effect, this means the end of accessible portion of the text.  See also point-max in see Point.

 -- Function: bolp
     This function returns t if point is at the beginning of a line. See Text Lines.  The beginning of the buffer (or of its accessible portion) always counts as the beginning of a line.

 -- Function: eolp
     This function returns t if point is at the end of a line.  The end of the buffer (or of its accessible portion) is always considered the end of a line.

Examining Buffer Contents
=========================

This section describes two functions that allow a Lisp program to convert any portion of the text in the buffer into a string.

 -- Function: buffer-substring start end
     This function returns a string containing a copy of the text of the region defined by positions START and END in the current buffer.  If the arguments are not positions in the accessible portion of the buffer, buffer-substring signals an args-out-of-range error.

     It is not necessary for START to be less than END; the arguments can be given in either order.  But most often the smaller argument is written first.

     If the text being copied has any text properties, these are copied into the string along with the characters they belong to.  See Text Properties.  However, overlays (see Overlays) in the buffer and their properties are ignored, not copied.

          ---------- Buffer: foo ----------
          This is the contents of buffer foo

          ---------- Buffer: foo ----------

          (buffer-substring 1 10)
          => "This is t"

          (buffer-substring (point-max) 10)
          => "he contents of buffer foo
          "

 -- Function: buffer-substring-no-properties start end
     This is like buffer-substring, except that it does not copy text properties, just the characters themselves.  See Text Properties.

 -- Function: buffer-string
     This function returns the contents of the entire accessible portion of the current buffer as a string.  It is equivalent to

          (buffer-substring (point-min) (point-max))

          ---------- Buffer: foo ----------
          This is the contents of buffer foo

          ---------- Buffer: foo ----------

          (buffer-string)
               => "This is the contents of buffer foo
          "

 -- Function: thing-at-point thing
     Return the THING around or next to point, as a string.

     The argument THING is a symbol which specifies a kind of syntactic entity.  Possibilities include symbol, list, sexp, defun, filename, url, word, sentence, whitespace, line, page, and others.

          ---------- Buffer: foo ----------
          Gentlemen may cry ``Pea-!-ce! Peace!,''
          but there is no peace.
          ---------- Buffer: foo ----------

          (thing-at-point 'word)
               => "Peace"
          (thing-at-point 'line)
               => "Gentlemen may cry ``Peace! Peace!,''\n"
          (thing-at-point 'whitespace)
               => nil

Comparing Text
==============

This function lets you compare portions of the text in a buffer, without copying them into strings first.

 -- Function: compare-buffer-substrings buffer1 start1 end1 buffer2 start2 end2
     This function lets you compare two substrings of the same buffer or two different buffers.  The first three arguments specify one substring, giving a buffer and two positions within the buffer.  The last three arguments specify the other substring in the same way.  You can use nil for BUFFER1, BUFFER2, or both to stand for the current buffer.

     The value is negative if the first substring is less, positive if the first is greater, and zero if they are equal.  The absolute value of the result is one plus the index of the first differing characters within the substrings.

     This function ignores case when comparing characters if case-fold-search is non-nil.  It always ignores text properties.

     Suppose the current buffer contains the text foobarbar haha!rara!; then in this example the two substrings are rbar and rara!.  The value is 2 because the first substring is greater at the second character.

          (compare-buffer-substrings nil 6 11 nil 16 21)
               => 2

Inserting Text
==============

Insertion means adding new text to a buffer.  The inserted text goes at point---between the character before point and the character after point.  Some insertion functions leave point before the inserted text, while other functions leave it after.  We call the former insertion after point and the latter insertion before point.

Insertion relocates markers that point at positions after the insertion point, so that they stay with the surrounding text (see Markers).  When a marker points at the place of insertion, insertion may or may not relocate the marker, depending on the marker's insertion type (see Marker Insertion Types).  Certain special functions such as insert-before-markers relocate all such markers to point after the inserted text, regardless of the markers' insertion type.

Insertion functions signal an error if the current buffer is read-only or if they insert within read-only text.

These functions copy text characters from strings and buffers along with their properties.  The inserted characters have exactly the same properties as the characters they were copied from.  By contrast, characters specified as separate arguments, not part of a string or buffer, inherit their text properties from the neighboring text.

The insertion functions convert text from unibyte to multibyte in order to insert in a multibyte buffer, and vice versa---if the text comes from a string or from a buffer.  However, they do not convert unibyte character codes 128 through 255 to multibyte characters, not even if the current buffer is a multibyte buffer.  See Converting Representations.

 -- Function: insert &rest args
     This function inserts the strings and/or characters ARGS into the current buffer, at point, moving point forward.  In other words, it inserts the text before point.  An error is signaled unless all ARGS are either strings or characters.  The value is nil.

 -- Function: insert-before-markers &rest args
     This function inserts the strings and/or characters ARGS into the current buffer, at point, moving point forward.  An error is signaled unless all ARGS are either strings or characters.  The value is nil.

     This function is unlike the other insertion functions in that it relocates markers initially pointing at the insertion point, to point after the inserted text.  If an overlay begins the insertion point, the inserted text falls outside the overlay; if a nonempty overlay ends at the insertion point, the inserted text falls inside that overlay.

 -- Function: insert-char character &optional count inherit
     This function inserts COUNT instances of CHARACTER into the current buffer before point.  The argument COUNT should be a number (nil means 1), and CHARACTER must be a character. The value is nil.

     This function does not convert unibyte character codes 128 through 255 to multibyte characters, not even if the current buffer is a multibyte buffer.  See Converting Representations.

     If INHERIT is non-nil, then the inserted characters inherit sticky text properties from the two characters before and after the insertion point.  See Sticky Properties.

 -- Function: insert-buffer-substring from-buffer-or-name &optional start end
     This function inserts a portion of buffer FROM-BUFFER-OR-NAME (which must already exist) into the current buffer before point.  The text inserted is the region from START and END.  (These arguments default to the beginning and end of the accessible portion of that buffer.)  This function returns nil.

     In this example, the form is executed with buffer bar as the current buffer.  We assume that buffer bar is initially empty.

          ---------- Buffer: foo ----------
          We hold these truths to be self-evident, that all
          ---------- Buffer: foo ----------

          (insert-buffer-substring "foo" 1 20)
               => nil

          ---------- Buffer: bar ----------
          We hold these truth-!-
          ---------- Buffer: bar ----------

See Sticky Properties, for other insertion functions that inherit text properties from the nearby text in addition to inserting it. Whitespace inserted by indentation functions also inherits text properties.

User-Level Insertion Commands
=============================

This section describes higher-level commands for inserting text, commands intended primarily for the user but useful also in Lisp programs.

 -- Command: insert-buffer from-buffer-or-name
     This command inserts the entire contents of FROM-BUFFER-OR-NAME (which must exist) into the current buffer after point.  It leaves the mark after the inserted text.  The value is nil.

 -- Command: self-insert-command count
     This command inserts the last character typed; it does so COUNT times, before point, and returns nil.  Most printing characters are bound to this command.  In routine use, self-insert-command is the most frequently called function in Emacs, but programs rarely use it except to install it on a keymap.

     In an interactive call, COUNT is the numeric prefix argument.

     This command calls auto-fill-function whenever that is non-nil and the character inserted is in the table auto-fill-chars (see Auto Filling).

     This command performs abbrev expansion if Abbrev mode is enabled and the inserted character does not have word-constituent syntax. (see Abbrevs, and Syntax Class Table.)

     This is also responsible for calling blink-paren-function when the inserted character has close parenthesis syntax (see Blinking).

     Do not try substituting your own definition of self-insert-command for the standard one.  The editor command loop handles this function specially.

 -- Command: newline &optional number-of-newlines
     This command inserts newlines into the current buffer before point. If NUMBER-OF-NEWLINES is supplied, that many newline characters are inserted.

     This function calls auto-fill-function if the current column number is greater than the value of fill-column and NUMBER-OF-NEWLINES is nil.  Typically what auto-fill-function does is insert a newline; thus, the overall result in this case is to insert two newlines at different places: one at point, and another earlier in the line.  newline does not auto-fill if NUMBER-OF-NEWLINES is non-nil.

     This command indents to the left margin if that is not zero. See Margins.

     The value returned is nil.  In an interactive call, COUNT is the numeric prefix argument.

 -- Command: split-line
     This command splits the current line, moving the portion of the line after point down vertically so that it is on the next line directly below where it was before.  Whitespace is inserted as needed at the beginning of the lower line, using the indent-to function. split-line returns the position of point.

     Programs hardly ever use this function.

 -- Variable: overwrite-mode
     This variable controls whether overwrite mode is in effect.  The value should be overwrite-mode-textual, overwrite-mode-binary, or nil.  overwrite-mode-textual specifies textual overwrite mode (treats newlines and tabs specially), and overwrite-mode-binary specifies binary overwrite mode (treats newlines and tabs like any other characters).

Deleting Text
=============

Deletion means removing part of the text in a buffer, without saving it in the kill ring (see The Kill Ring).  Deleted text can't be yanked, but can be reinserted using the undo mechanism (see Undo). Some deletion functions do save text in the kill ring in some special cases.

All of the deletion functions operate on the current buffer, and all return a value of nil.

 -- Command: erase-buffer
     This function deletes the entire text of the current buffer, leaving it empty.  If the buffer is read-only, it signals a buffer-read-only error; if some of the text in it is read-only, it signals a text-read-only error.  Otherwise, it deletes the text without asking for any confirmation.  It returns nil.

     Normally, deleting a large amount of text from a buffer inhibits further auto-saving of that buffer ``because it has shrunk''.  However, erase-buffer does not do this, the idea being that the future text is not really related to the former text, and its size should not be compared with that of the former text.

 -- Command: delete-region start end
     This command deletes the text between positions START and END in the current buffer, and returns nil.  If point was inside the deleted region, its value afterward is START. Otherwise, point relocates with the surrounding text, as markers do.

 -- Function: delete-and-extract-region start end
     This function deletes the text between positions START and END in the current buffer, and returns a string containing the text just deleted.

     If point was inside the deleted region, its value afterward is START.  Otherwise, point relocates with the surrounding text, as markers do.

 -- Command: delete-char count &optional killp
     This command deletes COUNT characters directly after point, or before point if COUNT is negative.  If KILLP is non-nil, then it saves the deleted characters in the kill ring.

     In an interactive call, COUNT is the numeric prefix argument, and KILLP is the unprocessed prefix argument.  Therefore, if a prefix argument is supplied, the text is saved in the kill ring.  If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

     The value returned is always nil.

 -- Command: delete-backward-char count &optional killp
     This command deletes COUNT characters directly before point, or after point if COUNT is negative.  If KILLP is non-nil, then it saves the deleted characters in the kill ring.

     In an interactive call, COUNT is the numeric prefix argument, and KILLP is the unprocessed prefix argument.  Therefore, if a prefix argument is supplied, the text is saved in the kill ring.  If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

     The value returned is always nil.

 -- Command: backward-delete-char-untabify count &optional killp
     This command deletes COUNT characters backward, changing tabs into spaces.  When the next character to be deleted is a tab, it is first replaced with the proper number of spaces to preserve alignment and then one of those spaces is deleted instead of the tab.  If KILLP is non-nil, then the command saves the deleted characters in the kill ring.

     Conversion of tabs to spaces happens only if COUNT is positive. If it is negative, exactly -COUNT characters after point are deleted.

     In an interactive call, COUNT is the numeric prefix argument, and KILLP is the unprocessed prefix argument.  Therefore, if a prefix argument is supplied, the text is saved in the kill ring.  If no prefix argument is supplied, then one character is deleted, but not saved in the kill ring.

     The value returned is always nil.

 -- User Option: backward-delete-char-untabify-method
     This option specifies how backward-delete-char-untabify should deal with whitespace.  Possible values include untabify, the default, meaning convert a tab to many spaces and delete one; hungry, meaning delete all the whitespace characters before point with one command, and nil, meaning do nothing special for whitespace characters.

User-Level Deletion Commands
============================

This section describes higher-level commands for deleting text, commands intended primarily for the user but useful also in Lisp programs.

 -- Command: delete-horizontal-space
     This function deletes all spaces and tabs around point.  It returns nil.

     In the following examples, we call delete-horizontal-space four times, once on each line, with point between the second and third characters on the line each time.

          ---------- Buffer: foo ----------
          I -!-thought
          I -!-     thought
          We-!- thought
          Yo-!-u thought
          ---------- Buffer: foo ----------

          (delete-horizontal-space)   ; Four times.
               => nil

          ---------- Buffer: foo ----------
          Ithought
          Ithought
          Wethought
          You thought
          ---------- Buffer: foo ----------

 -- Command: delete-indentation &optional join-following-p
     This function joins the line point is on to the previous line, deleting any whitespace at the join and in some cases replacing it with one space.  If JOIN-FOLLOWING-P is non-nil, delete-indentation joins this line to the following line instead.  The function returns nil.

     If there is a fill prefix, and the second of the lines being joined starts with the prefix, then delete-indentation deletes the fill prefix before joining the lines.  See Margins.

     In the example below, point is located on the line starting events, and it makes no difference if there are trailing spaces in the preceding line.

          ---------- Buffer: foo ----------
          When in the course of human
          -!-    events, it becomes necessary
          ---------- Buffer: foo ----------

          (delete-indentation)
               => nil

          ---------- Buffer: foo ----------
          When in the course of human-!- events, it becomes necessary
          ---------- Buffer: foo ----------

     After the lines are joined, the function fixup-whitespace is responsible for deciding whether to leave a space at the junction.

 -- Function: fixup-whitespace
     This function replaces all the whitespace surrounding point with either one space or no space, according to the context.  It returns nil.

     At the beginning or end of a line, the appropriate amount of space is none.  Before a character with close parenthesis syntax, or after a character with open parenthesis or expression-prefix syntax, no space is also appropriate.  Otherwise, one space is appropriate.  See Syntax Class Table.

     In the example below, fixup-whitespace is called the first time with point before the word spaces in the first line.  For the second invocation, point is directly after the (.

          ---------- Buffer: foo ----------
          This has too many     -!-spaces
          This has too many spaces at the start of (-!-   this list)
          ---------- Buffer: foo ----------

          (fixup-whitespace)
               => nil
          (fixup-whitespace)
               => nil

          ---------- Buffer: foo ----------
          This has too many spaces
          This has too many spaces at the start of (this list)
          ---------- Buffer: foo ----------

 -- Command: just-one-space
     This command replaces any spaces and tabs around point with a single space.  It returns nil.

 -- Command: delete-blank-lines
     This function deletes blank lines surrounding point.  If point is on a blank line with one or more blank lines before or after it, then all but one of them are deleted.  If point is on an isolated blank line, then it is deleted.  If point is on a nonblank line, the command deletes all blank lines following it.

     A blank line is defined as a line containing only tabs and spaces.

     delete-blank-lines returns nil.

The Kill Ring
=============

Kill functions delete text like the deletion functions, but save it so that the user can reinsert it by yanking.  Most of these functions have kill- in their name.  By contrast, the functions whose names start with delete- normally do not save text for yanking (though they can still be undone); these are ``deletion'' functions.

Most of the kill commands are primarily for interactive use, and are not described here.  What we do describe are the functions provided for use in writing such commands.  You can use these functions to write commands for killing text.  When you need to delete text for internal purposes within a Lisp function, you should normally use deletion functions, so as not to disturb the kill ring contents. See Deletion.

Killed text is saved for later yanking in the kill ring.  This is a list that holds a number of recent kills, not just the last text kill.  We call this a ``ring'' because yanking treats it as having elements in a cyclic order.  The list is kept in the variable kill-ring, and can be operated on with the usual functions for lists; there are also specialized functions, described in this section, that treat it as a ring.

Some people think this use of the word ``kill'' is unfortunate, since it refers to operations that specifically do not destroy the entities ``killed''.  This is in sharp contrast to ordinary life, in which death is permanent and ``killed'' entities do not come back to life.  Therefore, other metaphors have been proposed.  For example, the term ``cut ring'' makes sense to people who, in pre-computer days, used scissors and paste to cut up and rearrange manuscripts.  However, it would be difficult to change the terminology now.

Kill Ring Concepts
------------------

The kill ring records killed text as strings in a list, most recent first.  A short kill ring, for example, might look like this:

     ("some text" "a different piece of text" "even older text")

When the list reaches kill-ring-max entries in length, adding a new entry automatically deletes the last entry.

When kill commands are interwoven with other commands, each kill command makes a new entry in the kill ring.  Multiple kill commands in succession build up a single kill-ring entry, which would be yanked as a unit; the second and subsequent consecutive kill commands add text to the entry made by the first one.

For yanking, one entry in the kill ring is designated the ``front'' of the ring.  Some yank commands ``rotate'' the ring by designating a different element as the ``front.''  But this virtual rotation doesn't change the list itself---the most recent entry always comes first in the list.

Functions for Killing
---------------------

kill-region is the usual subroutine for killing text.  Any command that calls this function is a ``kill command'' (and should probably have kill in its name).  kill-region puts the newly killed text in a new element at the beginning of the kill ring or adds it to the most recent element.  It determines automatically (using last-command) whether the previous command was a kill command, and if so appends the killed text to the most recent entry.

 -- Command: kill-region start end
     This function kills the text in the region defined by START and END.  The text is deleted but saved in the kill ring, along with its text properties.  The value is always nil.

     In an interactive call, START and END are point and the mark.

     If the buffer or text is read-only, kill-region modifies the kill ring just the same, then signals an error without modifying the buffer. This is convenient because it lets the user use a series of kill commands to copy text from a read-only buffer into the kill ring.

 -- User Option: kill-read-only-ok
     If this option is non-nil, kill-region does not signal an error if the buffer or text is read-only.  Instead, it simply returns, updating the kill ring but not changing the buffer.

 -- Command: copy-region-as-kill start end
     This command saves the region defined by START and END on the kill ring (including text properties), but does not delete the text from the buffer.  It returns nil.  It also indicates the extent of the text copied by moving the cursor momentarily, or by displaying a message in the echo area.

     The command does not set this-command to kill-region, so a subsequent kill command does not append to the same kill ring entry.

     Don't call copy-region-as-kill in Lisp programs unless you aim to support Emacs 18.  For newer Emacs versions, it is better to use kill-new or kill-append instead.  See Low-Level Kill Ring.

Functions for Yanking
---------------------

Yanking means reinserting an entry of previously killed text from the kill ring.  The text properties are copied too.

 -- Command: yank &optional arg
     This command inserts before point the text in the first entry in the kill ring.  It positions the mark at the beginning of that text, and point at the end.

     If ARG is a list (which occurs interactively when the user types C-u with no digits), then yank inserts the text as described above, but puts point before the yanked text and puts the mark after it.

     If ARG is a number, then yank inserts the ARGth most recently killed text---the ARGth element of the kill ring list.

     yank does not alter the contents of the kill ring or rotate it. It returns nil.

 -- Command: yank-pop arg
     This command replaces the just-yanked entry from the kill ring with a different entry from the kill ring.

     This is allowed only immediately after a yank or another yank-pop.  At such a time, the region contains text that was just inserted by yanking.  yank-pop deletes that text and inserts in its place a different piece of killed text.  It does not add the deleted text to the kill ring, since it is already in the kill ring somewhere.

     If ARG is nil, then the replacement text is the previous element of the kill ring.  If ARG is numeric, the replacement is the ARGth previous kill.  If ARG is negative, a more recent kill is the replacement.

     The sequence of kills in the kill ring wraps around, so that after the oldest one comes the newest one, and before the newest one goes the oldest.

     The return value is always nil.

Low-Level Kill Ring
-------------------

These functions and variables provide access to the kill ring at a lower level, but still convenient for use in Lisp programs, because they take care of interaction with window system selections (see Window System Selections).

 -- Function: current-kill n &optional do-not-move
     The function current-kill rotates the yanking pointer, which designates the ``front'' of the kill ring, by N places (from newer kills to older ones), and returns the text at that place in the ring.

     If the optional second argument DO-NOT-MOVE is non-nil, then current-kill doesn't alter the yanking pointer; it just returns the Nth kill, counting from the current yanking pointer.

     If N is zero, indicating a request for the latest kill, current-kill calls the value of interprogram-paste-function (documented below) before consulting the kill ring.

 -- Function: kill-new string
     This function puts the text STRING into the kill ring as a new entry at the front of the ring.  It discards the oldest entry if appropriate.  It also invokes the value of interprogram-cut-function (see below).

 -- Function: kill-append string before-p
     This function appends the text STRING to the first entry in the kill ring.  Normally STRING goes at the end of the entry, but if BEFORE-P is non-nil, it goes at the beginning.  This function also invokes the value of interprogram-cut-function (see below).

 -- Variable: interprogram-paste-function
     This variable provides a way of transferring killed text from other programs, when you are using a window system.  Its value should be nil or a function of no arguments.

     If the value is a function, current-kill calls it to get the ``most recent kill''.  If the function returns a non-nil value, then that value is used as the ``most recent kill''.  If it returns nil, then the first element of kill-ring is used.

     The normal use of this hook is to get the window system's primary selection as the most recent kill, even if the selection belongs to another application.  See Window System Selections.

 -- Variable: interprogram-cut-function
     This variable provides a way of communicating killed text to other programs, when you are using a window system.  Its value should be nil or a function of one argument.

     If the value is a function, kill-new and kill-append call it with the new first element of the kill ring as an argument.

     The normal use of this hook is to set the window system's primary selection from the newly killed text.  See Window System Selections.

Internals of the Kill Ring
--------------------------

The variable kill-ring holds the kill ring contents, in the form of a list of strings.  The most recent kill is always at the front of the list.

The kill-ring-yank-pointer variable points to a link in the kill ring list, whose CAR is the text to yank next.  We say it identifies the ``front'' of the ring.  Moving kill-ring-yank-pointer to a different link is called rotating the kill ring.  We call the kill ring a ``ring'' because the functions that move the yank pointer wrap around from the end of the list to the beginning, or vice-versa.  Rotation of the kill ring is virtual; it does not change the value of kill-ring.

Both kill-ring and kill-ring-yank-pointer are Lisp variables whose values are normally lists.  The word ``pointer'' in the name of the kill-ring-yank-pointer indicates that the variable's purpose is to identify one element of the list for use by the next yank command.

The value of kill-ring-yank-pointer is always eq to one of the links in the kill ring list.  The element it identifies is the CAR of that link.  Kill commands, which change the kill ring, also set this variable to the value of kill-ring.  The effect is to rotate the ring so that the newly killed text is at the front.

Here is a diagram that shows the variable kill-ring-yank-pointer pointing to the second entry in the kill ring ("some text" "a different piece of text" "yet older text").

     kill-ring                  ---- kill-ring-yank-pointer
       |                       |
       |                       v
       |     --- ---          --- ---      --- ---
        --> |   |   |------> |   |   |--> |   |   |--> nil
             --- ---          --- ---      --- ---
              |                |            |
              |                |            |
              |                |             -->"yet older text"
              |                |
              |                 --> "a different piece of text"
              |
               --> "some text"

This state of affairs might occur after C-y (yank) immediately followed by M-y (yank-pop).

 -- Variable: kill-ring
     This variable holds the list of killed text sequences, most recently killed first.

 -- Variable: kill-ring-yank-pointer
     This variable's value indicates which element of the kill ring is at the ``front'' of the ring for yanking.  More precisely, the value is a tail of the value of kill-ring, and its CAR is the kill string that C-y should yank.

 -- User Option: kill-ring-max
     The value of this variable is the maximum length to which the kill ring can grow, before elements are thrown away at the end.  The default value for kill-ring-max is 30.

Undo
====

Most buffers have an undo list, which records all changes made to the buffer's text so that they can be undone.  (The buffers that don't have one are usually special-purpose buffers for which Emacs assumes that undoing is not useful.)  All the primitives that modify the text in the buffer automatically add elements to the front of the undo list, which is in the variable buffer-undo-list.

 -- Variable: buffer-undo-list
     This variable's value is the undo list of the current buffer. A value of t disables the recording of undo information.

Here are the kinds of elements an undo list can have:

POSITION
     This kind of element records a previous value of point; undoing this element moves point to POSITION.  Ordinary cursor motion does not make any sort of undo record, but deletion operations use these entries to record where point was before the command.

(BEG . END)
     This kind of element indicates how to delete text that was inserted. Upon insertion, the text occupied the range BEG--END in the buffer.

(TEXT . POSITION)
     This kind of element indicates how to reinsert text that was deleted. The deleted text itself is the string TEXT.  The place to reinsert it is (abs POSITION).

(t HIGH . LOW)
     This kind of element indicates that an unmodified buffer became modified.  The elements HIGH and LOW are two integers, each recording 16 bits of the visited file's modification time as of when it was previously visited or saved.  primitive-undo uses those values to determine whether to mark the buffer as unmodified once again; it does so only if the file's modification time matches those numbers.

(nil PROPERTY VALUE BEG . END)
     This kind of element records a change in a text property. Here's how you might undo the change:

          (put-text-property BEG END PROPERTY VALUE)

(MARKER . ADJUSTMENT)
     This kind of element records the fact that the marker MARKER was relocated due to deletion of surrounding text, and that it moved ADJUSTMENT character positions.  Undoing this element moves MARKER - ADJUSTMENT characters.

nil
     This element is a boundary.  The elements between two boundaries are called a change group; normally, each change group corresponds to one keyboard command, and undo commands normally undo an entire group as a unit.

 -- Function: undo-boundary
     This function places a boundary element in the undo list.  The undo command stops at such a boundary, and successive undo commands undo to earlier and earlier boundaries.  This function returns nil.

     The editor command loop automatically creates an undo boundary before each key sequence is executed.  Thus, each undo normally undoes the effects of one command.  Self-inserting input characters are an exception.  The command loop makes a boundary for the first such character; the next 19 consecutive self-inserting input characters do not make boundaries, and then the 20th does, and so on as long as self-inserting characters continue.

     All buffer modifications add a boundary whenever the previous undoable change was made in some other buffer.  This is to ensure that each command makes a boundary in each buffer where it makes changes.

     Calling this function explicitly is useful for splitting the effects of a command into more than one unit.  For example, query-replace calls undo-boundary after each replacement, so that the user can undo individual replacements one by one.

 -- Function: primitive-undo count list
     This is the basic function for undoing elements of an undo list. It undoes the first COUNT elements of LIST, returning the rest of LIST.  You could write this function in Lisp, but it is convenient to have it in C.

     primitive-undo adds elements to the buffer's undo list when it changes the buffer.  Undo commands avoid confusion by saving the undo list value at the beginning of a sequence of undo operations.  Then the undo operations use and update the saved value.  The new elements added by undoing are not part of this saved value, so they don't interfere with continuing to undo.

Maintaining Undo Lists
======================

This section describes how to enable and disable undo information for a given buffer.  It also explains how the undo list is truncated automatically so it doesn't get too big.

Recording of undo information in a newly created buffer is normally enabled to start with; but if the buffer name starts with a space, the undo recording is initially disabled.  You can explicitly enable or disable undo recording with the following two functions, or by setting buffer-undo-list yourself.

 -- Command: buffer-enable-undo &optional buffer-or-name
     This command enables recording undo information for buffer BUFFER-OR-NAME, so that subsequent changes can be undone.  If no argument is supplied, then the current buffer is used.  This function does nothing if undo recording is already enabled in the buffer.  It returns nil.

     In an interactive call, BUFFER-OR-NAME is the current buffer. You cannot specify any other buffer.

 -- Command: buffer-disable-undo &optional buffer
 -- Command: buffer-flush-undo &optional buffer
     This function discards the undo list of BUFFER, and disables further recording of undo information.  As a result, it is no longer possible to undo either previous changes or any subsequent changes.  If the undo list of BUFFER is already disabled, this function has no effect.

     This function returns nil.

     The name buffer-flush-undo is not considered obsolete, but the preferred name is buffer-disable-undo.

As editing continues, undo lists get longer and longer.  To prevent them from using up all available memory space, garbage collection trims them back to size limits you can set.  (For this purpose, the ``size'' of an undo list measures the cons cells that make up the list, plus the strings of deleted text.)  Two variables control the range of acceptable sizes: undo-limit and undo-strong-limit.

 -- Variable: undo-limit
     This is the soft limit for the acceptable size of an undo list.  The change group at which this size is exceeded is the last one kept.

 -- Variable: undo-strong-limit
     This is the upper limit for the acceptable size of an undo list.  The change group at which this size is exceeded is discarded itself (along with all older change groups).  There is one exception: the very latest change group is never discarded no matter how big it is.

Filling
=======

Filling means adjusting the lengths of lines (by moving the line breaks) so that they are nearly (but no greater than) a specified maximum width.  Additionally, lines can be justified, which means inserting spaces to make the left and/or right margins line up precisely.  The width is controlled by the variable fill-column. For ease of reading, lines should be no longer than 70 or so columns.

You can use Auto Fill mode (see Auto Filling) to fill text automatically as you insert it, but changes to existing text may leave it improperly filled.  Then you must fill the text explicitly.

Most of the commands in this section return values that are not meaningful.  All the functions that do filling take note of the current left margin, current right margin, and current justification style (see Margins).  If the current justification style is none, the filling functions don't actually do anything.

Several of the filling functions have an argument JUSTIFY. If it is non-nil, that requests some kind of justification.  It can be left, right, full, or center, to request a specific style of justification.  If it is t, that means to use the current justification style for this part of the text (see current-justification, below).  Any other value is treated as full.

When you call the filling functions interactively, using a prefix argument implies the value full for JUSTIFY.

 -- Command: fill-paragraph justify
     This command fills the paragraph at or after point.  If JUSTIFY is non-nil, each line is justified as well. It uses the ordinary paragraph motion commands to find paragraph boundaries.  See Paragraphs (The Emacs Manual).

 -- Command: fill-region start end &optional justify nosqueeze to-eop
     This command fills each of the paragraphs in the region from START to END.  It justifies as well if JUSTIFY is non-nil.

     If NOSQUEEZE is non-nil, that means to leave whitespace other than line breaks untouched.  If TO-EOP is non-nil, that means to keep filling to the end of the paragraph---or the next hard newline, if use-hard-newlines is enabled (see below).

     The variable paragraph-separate controls how to distinguish paragraphs.  See Standard Regexps.

 -- Command: fill-individual-paragraphs start end &optional justify citation-regexp
     This command fills each paragraph in the region according to its individual fill prefix.  Thus, if the lines of a paragraph were indented with spaces, the filled paragraph will remain indented in the same fashion.

     The first two arguments, START and END, are the beginning and end of the region to be filled.  The third and fourth arguments, JUSTIFY and CITATION-REGEXP, are optional.  If JUSTIFY is non-nil, the paragraphs are justified as well as filled.  If CITATION-REGEXP is non-nil, it means the function is operating on a mail message and therefore should not fill the header lines.  If CITATION-REGEXP is a string, it is used as a regular expression; if it matches the beginning of a line, that line is treated as a citation marker.

     Ordinarily, fill-individual-paragraphs regards each change in indentation as starting a new paragraph.  If fill-individual-varying-indent is non-nil, then only separator lines separate paragraphs.  That mode can handle indented paragraphs with additional indentation on the first line.

 -- User Option: fill-individual-varying-indent
     This variable alters the action of fill-individual-paragraphs as described above.

 -- Command: fill-region-as-paragraph start end &optional justify nosqueeze squeeze-after
     This command considers a region of text as a single paragraph and fills it.  If the region was made up of many paragraphs, the blank lines between paragraphs are removed.  This function justifies as well as filling when JUSTIFY is non-nil.

     In an interactive call, any prefix argument requests justification.

     If NOSQUEEZE is non-nil, that means to leave whitespace other than line breaks untouched.  If SQUEEZE-AFTER is non-nil, it specifies a position in the region, and means don't canonicalize spaces before that position.

     In Adaptive Fill mode, this command calls fill-context-prefix to choose a fill prefix by default.  See Adaptive Fill.

 -- Command: justify-current-line &optional how eop nosqueeze
     This command inserts spaces between the words of the current line so that the line ends exactly at fill-column.  It returns nil.

     The argument HOW, if non-nil specifies explicitly the style of justification.  It can be left, right, full, center, or none.  If it is t, that means to do follow specified justification style (see current-justification, below).  nil means to do full justification.

     If EOP is non-nil, that means do left-justification if current-justification specifies full justification.  This is used for the last line of a paragraph; even if the paragraph as a whole is fully justified, the last line should not be.

     If NOSQUEEZE is non-nil, that means do not change interior whitespace.

 -- User Option: default-justification
     This variable's value specifies the style of justification to use for text that doesn't specify a style with a text property.  The possible values are left, right, full, center, or none.  The default value is left.

 -- Function: current-justification
     This function returns the proper justification style to use for filling the text around point.

 -- User Option: sentence-end-double-space
     If this variable is non-nil, a period followed by just one space does not count as the end of a sentence, and the filling functions avoid breaking the line at such a place.

 -- Variable: fill-paragraph-function
     This variable provides a way for major modes to override the filling of paragraphs.  If the value is non-nil, fill-paragraph calls this function to do the work.  If the function returns a non-nil value, fill-paragraph assumes the job is done, and immediately returns that value.

     The usual use of this feature is to fill comments in programming language modes.  If the function needs to fill a paragraph in the usual way, it can do so as follows:

          (let ((fill-paragraph-function nil))
            (fill-paragraph arg))

 -- Variable: use-hard-newlines
     If this variable is non-nil, the filling functions do not delete newlines that have the hard text property.  These ``hard newlines'' act as paragraph separators.

Margins for Filling
===================

 -- User Option: fill-prefix
     This buffer-local variable specifies a string of text that appears at the beginning of normal text lines and should be disregarded when filling them.  Any line that fails to start with the fill prefix is considered the start of a paragraph; so is any line that starts with the fill prefix followed by additional whitespace.  Lines that start with the fill prefix but no additional whitespace are ordinary text lines that can be filled together.  The resulting filled lines also start with the fill prefix.

     The fill prefix follows the left margin whitespace, if any.

 -- User Option: fill-column
     This buffer-local variable specifies the maximum width of filled lines. Its value should be an integer, which is a number of columns.  All the filling, justification, and centering commands are affected by this variable, including Auto Fill mode (see Auto Filling).

     As a practical matter, if you are writing text for other people to read, you should set fill-column to no more than 70.  Otherwise the line will be too long for people to read comfortably, and this can make the text seem clumsy.

 -- Variable: default-fill-column
     The value of this variable is the default value for fill-column in buffers that do not override it.  This is the same as (default-value 'fill-column).

     The default value for default-fill-column is 70.

 -- Command: set-left-margin from to margin
     This sets the left-margin property on the text from FROM to TO to the value MARGIN.  If Auto Fill mode is enabled, this command also refills the region to fit the new margin.

 -- Command: set-right-margin from to margin
     This sets the right-margin property on the text from FROM to TO to the value MARGIN.  If Auto Fill mode is enabled, this command also refills the region to fit the new margin.

 -- Function: current-left-margin
     This function returns the proper left margin value to use for filling the text around point.  The value is the sum of the left-margin property of the character at the start of the current line (or zero if none), and the value of the variable left-margin.

 -- Function: current-fill-column
     This function returns the proper fill column value to use for filling the text around point.  The value is the value of the fill-column variable, minus the value of the right-margin property of the character after point.

 -- Command: move-to-left-margin &optional n force
     This function moves point to the left margin of the current line.  The column moved to is determined by calling the function current-left-margin.  If the argument N is non-nil, move-to-left-margin moves forward N-1 lines first.

     If FORCE is non-nil, that says to fix the line's indentation if that doesn't match the left margin value.

 -- Function: delete-to-left-margin &optional from to
     This function removes left margin indentation from the text between FROM and TO.  The amount of indentation to delete is determined by calling current-left-margin.  In no case does this function delete non-whitespace.  If FROM and TO are omitted, they default to the whole buffer.

 -- Function: indent-to-left-margin
     This is the default indent-line-function, used in Fundamental mode, Text mode, etc.  Its effect is to adjust the indentation at the beginning of the current line to the value specified by the variable left-margin.  This may involve either inserting or deleting whitespace.

 -- Variable: left-margin
     This variable specifies the base left margin column.  In Fundamental mode, C-j indents to this column.  This variable automatically becomes buffer-local when set in any fashion.

 -- Variable: fill-nobreak-predicate
     This variable gives major modes a way to specify not to break a line at certain places.  Its value should be a function.  This function is called during filling, with no arguments and with point located at the place where a break is being considered.  If the function returns non-nil, then the line won't be broken there.

Adaptive Fill Mode
==================

Adaptive Fill mode chooses a fill prefix automatically from the text in each paragraph being filled.

 -- User Option: adaptive-fill-mode
     Adaptive Fill mode is enabled when this variable is non-nil. It is t by default.

 -- Function: fill-context-prefix from to
     This function implements the heart of Adaptive Fill mode; it chooses a fill prefix based on the text between FROM and TO.  It does this by looking at the first two lines of the paragraph, based on the variables described below.

 -- User Option: adaptive-fill-regexp
     This variable holds a regular expression to control Adaptive Fill mode. Adaptive Fill mode matches this regular expression against the text starting after the left margin whitespace (if any) on a line; the characters it matches are that line's candidate for the fill prefix.

 -- User Option: adaptive-fill-first-line-regexp
     In a one-line paragraph, if the candidate fill prefix matches this regular expression, or if it matches comment-start-skip, then it is used---otherwise, spaces amounting to the same width are used instead.

     However, the fill prefix is never taken from a one-line paragraph if it would act as a paragraph starter on subsequent lines.

 -- User Option: adaptive-fill-function
     You can specify more complex ways of choosing a fill prefix automatically by setting this variable to a function.  The function is called when adaptive-fill-regexp does not match, with point after the left margin of a line, and it should return the appropriate fill prefix based on that line.  If it returns nil, that means it sees no fill prefix in that line.

Auto Filling
============

Auto Fill mode is a minor mode that fills lines automatically as text is inserted.  This section describes the hook used by Auto Fill mode. For a description of functions that you can call explicitly to fill and justify existing text, see Filling.

Auto Fill mode also enables the functions that change the margins and justification style to refill portions of the text.  See Margins.

 -- Variable: auto-fill-function
     The value of this variable should be a function (of no arguments) to be called after self-inserting a character from the table auto-fill-chars.  It may be nil, in which case nothing special is done in that case.

     The value of auto-fill-function is do-auto-fill when Auto-Fill mode is enabled.  That is a function whose sole purpose is to implement the usual strategy for breaking a line.

          In older Emacs versions, this variable was named auto-fill-hook, but since it is not called with the standard convention for hooks, it was renamed to auto-fill-function in version 19.

 -- Variable: normal-auto-fill-function
     This variable specifies the function to use for auto-fill-function, if and when Auto Fill is turned on.  Major modes can set buffer-local values for this variable to alter how Auto Fill works.

 -- Variable: auto-fill-chars
     A char table of characters which invoke auto-fill-function when self-inserted---space and newline in most language environments.  They have an entry t in the table.

Sorting Text
============

The sorting functions described in this section all rearrange text in a buffer.  This is in contrast to the function sort, which rearranges the order of the elements of a list (see Rearrangement). The values returned by these functions are not meaningful.

 -- Function: sort-subr reverse nextrecfun endrecfun &optional startkeyfun endkeyfun
     This function is the general text-sorting routine that subdivides a buffer into records and then sorts them.  Most of the commands in this section use this function.

     To understand how sort-subr works, consider the whole accessible portion of the buffer as being divided into disjoint pieces called sort records.  The records may or may not be contiguous, but they must not overlap.  A portion of each sort record (perhaps all of it) is designated as the sort key.  Sorting rearranges the records in order by their sort keys.

     Usually, the records are rearranged in order of ascending sort key. If the first argument to the sort-subr function, REVERSE, is non-nil, the sort records are rearranged in order of descending sort key.

     The next four arguments to sort-subr are functions that are called to move point across a sort record.  They are called many times from within sort-subr.

      1. NEXTRECFUN is called with point at the end of a record.  This function moves point to the start of the next record.  The first record is assumed to start at the position of point when sort-subr is called.  Therefore, you should usually move point to the beginning of the buffer before calling sort-subr.

          This function can indicate there are no more sort records by leaving point at the end of the buffer.

      2. ENDRECFUN is called with point within a record.  It moves point to the end of the record.

      3. STARTKEYFUN is called to move point from the start of a record to the start of the sort key.  This argument is optional; if it is omitted, the whole record is the sort key.  If supplied, the function should either return a non-nil value to be used as the sort key, or return nil to indicate that the sort key is in the buffer starting at point.  In the latter case, ENDKEYFUN is called to find the end of the sort key.

      4. ENDKEYFUN is called to move point from the start of the sort key to the end of the sort key.  This argument is optional.  If STARTKEYFUN returns nil and this argument is omitted (or nil), then the sort key extends to the end of the record.  There is no need for ENDKEYFUN if STARTKEYFUN returns a non-nil value.

     As an example of sort-subr, here is the complete function definition for sort-lines:

          ;; Note that the first two lines of doc string
          ;; are effectively one line when viewed by a user.
          (defun sort-lines (reverse beg end)
            "Sort lines in region alphabetically;\
           argument means descending order.
          Called from a program, there are three arguments:

          REVERSE (non-nil means reverse order),\
           BEG and END (region to sort).
          The variable `sort-fold-case' determines\
           whether alphabetic case affects
          the sort order.

            (interactive "P\nr")
            (save-excursion
              (save-restriction
                (narrow-to-region beg end)
                (goto-char (point-min))
                (sort-subr reverse 'forward-line 'end-of-line))))

     Here forward-line moves point to the start of the next record, and end-of-line moves point to the end of record.  We do not pass the arguments STARTKEYFUN and ENDKEYFUN, because the entire record is used as the sort key.

     The sort-paragraphs function is very much the same, except that its sort-subr call looks like this:

          (sort-subr reverse
                     (function
                       (lambda ()
                         (while (and (not (eobp))
                                (looking-at paragraph-separate))
                           (forward-line 1))))
                     'forward-paragraph)

     Markers pointing into any sort records are left with no useful position after sort-subr returns.

 -- User Option: sort-fold-case
     If this variable is non-nil, sort-subr and the other buffer sorting functions ignore case when comparing strings.

 -- Command: sort-regexp-fields reverse record-regexp key-regexp start end
     This command sorts the region between START and END alphabetically as specified by RECORD-REGEXP and KEY-REGEXP. If REVERSE is a negative integer, then sorting is in reverse order.

     Alphabetical sorting means that two sort keys are compared by comparing the first characters of each, the second characters of each, and so on.  If a mismatch is found, it means that the sort keys are unequal; the sort key whose character is less at the point of first mismatch is the lesser sort key.  The individual characters are compared according to their numerical character codes in the Emacs character set.

     The value of the RECORD-REGEXP argument specifies how to divide the buffer into sort records.  At the end of each record, a search is done for this regular expression, and the text that matches it is taken as the next record.  For example, the regular expression ^.+$, which matches lines with at least one character besides a newline, would make each such line into a sort record.  See Regular Expressions, for a description of the syntax and meaning of regular expressions.

     The value of the KEY-REGEXP argument specifies what part of each record is the sort key.  The KEY-REGEXP could match the whole record, or only a part.  In the latter case, the rest of the record has no effect on the sorted order of records, but it is carried along when the record moves to its new position.

     The KEY-REGEXP argument can refer to the text matched by a subexpression of RECORD-REGEXP, or it can be a regular expression on its own.

     If KEY-REGEXP is:

     \DIGIT
          then the text matched by the DIGITth \(...\) parenthesis grouping in RECORD-REGEXP is the sort key.

     \&
          then the whole record is the sort key.

     a regular expression
          then sort-regexp-fields searches for a match for the regular expression within the record.  If such a match is found, it is the sort key.  If there is no match for KEY-REGEXP within a record then that record is ignored, which means its position in the buffer is not changed.  (The other records may move around it.)

     For example, if you plan to sort all the lines in the region by the first word on each line starting with the letter f, you should set RECORD-REGEXP to ^.*$ and set KEY-REGEXP to \<f\w*\>.  The resulting expression looks like this:

          (sort-regexp-fields nil "^.*$" "\\<f\\w*\\>"
                              (region-beginning)
                              (region-end))

     If you call sort-regexp-fields interactively, it prompts for RECORD-REGEXP and KEY-REGEXP in the minibuffer.

 -- Command: sort-lines reverse start end
     This command alphabetically sorts lines in the region between START and END.  If REVERSE is non-nil, the sort is in reverse order.

 -- Command: sort-paragraphs reverse start end
     This command alphabetically sorts paragraphs in the region between START and END.  If REVERSE is non-nil, the sort is in reverse order.

 -- Command: sort-pages reverse start end
     This command alphabetically sorts pages in the region between START and END.  If REVERSE is non-nil, the sort is in reverse order.

 -- Command: sort-fields field start end
     This command sorts lines in the region between START and END, comparing them alphabetically by the FIELDth field of each line.  Fields are separated by whitespace and numbered starting from 1.  If FIELD is negative, sorting is by the -FIELDth field from the end of the line.  This command is useful for sorting tables.

 -- Command: sort-numeric-fields field start end
     This command sorts lines in the region between START and END, comparing them numerically by the FIELDth field of each line.  The specified field must contain a number in each line of the region.  Fields are separated by whitespace and numbered starting from 1.  If FIELD is negative, sorting is by the -FIELDth field from the end of the line.  This command is useful for sorting tables.

 -- Command: sort-columns reverse &optional beg end
     This command sorts the lines in the region between BEG and END, comparing them alphabetically by a certain range of columns. The column positions of BEG and END bound the range of columns to sort on.

     If REVERSE is non-nil, the sort is in reverse order.

     One unusual thing about this command is that the entire line containing position BEG, and the entire line containing position END, are included in the region sorted.

     Note that sort-columns uses the sort utility program, and so cannot work properly on text containing tab characters.  Use M-x untabify to convert tabs to spaces before sorting.

Counting Columns
================

The column functions convert between a character position (counting characters from the beginning of the buffer) and a column position (counting screen characters from the beginning of a line).

These functions count each character according to the number of columns it occupies on the screen.  This means control characters count as occupying 2 or 4 columns, depending upon the value of ctl-arrow, and tabs count as occupying a number of columns that depends on the value of tab-width and on the column where the tab begins.  See Usual Display.

Column number computations ignore the width of the window and the amount of horizontal scrolling.  Consequently, a column value can be arbitrarily high.  The first (or leftmost) column is numbered 0.

 -- Function: current-column
     This function returns the horizontal position of point, measured in columns, counting from 0 at the left margin.  The column position is the sum of the widths of all the displayed representations of the characters between the start of the current line and point.

     For an example of using current-column, see the description of count-lines in Text Lines.

 -- Function: move-to-column column &optional force
     This function moves point to COLUMN in the current line.  The calculation of COLUMN takes into account the widths of the displayed representations of the characters between the start of the line and point.

     If column COLUMN is beyond the end of the line, point moves to the end of the line.  If COLUMN is negative, point moves to the beginning of the line.

     If it is impossible to move to column COLUMN because that is in the middle of a multicolumn character such as a tab, point moves to the end of that character.  However, if FORCE is non-nil, and COLUMN is in the middle of a tab, then move-to-column converts the tab into spaces so that it can move precisely to column COLUMN.  Other multicolumn characters can cause anomalies despite FORCE, since there is no way to split them.

     The argument FORCE also has an effect if the line isn't long enough to reach column COLUMN; if it is t, that means to add whitespace at the end of the line to reach that column.

     If COLUMN is not an integer, an error is signaled.

     The return value is the column number actually moved to.

Indentation
===========

The indentation functions are used to examine, move to, and change whitespace that is at the beginning of a line.  Some of the functions can also change whitespace elsewhere on a line.  Columns and indentation count from zero at the left margin.

Indentation Primitives
----------------------

This section describes the primitive functions used to count and insert indentation.  The functions in the following sections use these primitives.  See Width, for related functions.

 -- Function: current-indentation
     This function returns the indentation of the current line, which is the horizontal position of the first nonblank character.  If the contents are entirely blank, then this is the horizontal position of the end of the line.

 -- Command: indent-to column &optional minimum
     This function indents from point with tabs and spaces until COLUMN is reached.  If MINIMUM is specified and non-nil, then at least that many spaces are inserted even if this requires going beyond COLUMN.  Otherwise the function does nothing if point is already beyond COLUMN.  The value is the column at which the inserted indentation ends.

     The inserted whitespace characters inherit text properties from the surrounding text (usually, from the preceding text only).  See Sticky Properties.

 -- User Option: indent-tabs-mode
     If this variable is non-nil, indentation functions can insert tabs as well as spaces.  Otherwise, they insert only spaces.  Setting this variable automatically makes it buffer-local in the current buffer.

Indentation Controlled by Major Mode
------------------------------------

An important function of each major mode is to customize the TAB key to indent properly for the language being edited.  This section describes the mechanism of the TAB key and how to control it. The functions in this section return unpredictable values.

 -- Variable: indent-line-function
     This variable's value is the function to be used by TAB (and various commands) to indent the current line.  The command indent-according-to-mode does no more than call this function.

     In Lisp mode, the value is the symbol lisp-indent-line; in C mode, c-indent-line; in Fortran mode, fortran-indent-line. In Fundamental mode, Text mode, and many other modes with no standard for indentation, the value is indent-to-left-margin (which is the default value).

 -- Command: indent-according-to-mode
     This command calls the function in indent-line-function to indent the current line in a way appropriate for the current major mode.

 -- Command: indent-for-tab-command
     This command calls the function in indent-line-function to indent the current line; however, if that function is indent-to-left-margin, insert-tab is called instead.  (That is a trivial command that inserts a tab character.)

 -- Command: newline-and-indent
     This function inserts a newline, then indents the new line (the one following the newline just inserted) according to the major mode.

     It does indentation by calling the current indent-line-function. In programming language modes, this is the same thing TAB does, but in some text modes, where TAB inserts a tab, newline-and-indent indents to the column specified by left-margin.

 -- Command: reindent-then-newline-and-indent
     This command reindents the current line, inserts a newline at point, and then indents the new line (the one following the newline just inserted).

     This command does indentation on both lines according to the current major mode, by calling the current value of indent-line-function. In programming language modes, this is the same thing TAB does, but in some text modes, where TAB inserts a tab, reindent-then-newline-and-indent indents to the column specified by left-margin.

Indenting an Entire Region
--------------------------

This section describes commands that indent all the lines in the region.  They return unpredictable values.

 -- Command: indent-region start end to-column
     This command indents each nonblank line starting between START (inclusive) and END (exclusive).  If TO-COLUMN is nil, indent-region indents each nonblank line by calling the current mode's indentation function, the value of indent-line-function.

     If TO-COLUMN is non-nil, it should be an integer specifying the number of columns of indentation; then this function gives each line exactly that much indentation, by either adding or deleting whitespace.

     If there is a fill prefix, indent-region indents each line by making it start with the fill prefix.

 -- Variable: indent-region-function
     The value of this variable is a function that can be used by indent-region as a short cut.  It should take two arguments, the start and end of the region.  You should design the function so that it will produce the same results as indenting the lines of the region one by one, but presumably faster.

     If the value is nil, there is no short cut, and indent-region actually works line by line.

     A short-cut function is useful in modes such as C mode and Lisp mode, where the indent-line-function must scan from the beginning of the function definition: applying it to each line would be quadratic in time.  The short cut can update the scan information as it moves through the lines indenting them; this takes linear time.  In a mode where indenting a line individually is fast, there is no need for a short cut.

     indent-region with a non-nil argument TO-COLUMN has a different meaning and does not use this variable.

 -- Command: indent-rigidly start end count
     This command indents all lines starting between START (inclusive) and END (exclusive) sideways by COUNT columns. This ``preserves the shape'' of the affected region, moving it as a rigid unit.  Consequently, this command is useful not only for indenting regions of unindented text, but also for indenting regions of formatted code.

     For example, if COUNT is 3, this command adds 3 columns of indentation to each of the lines beginning in the region specified.

     In Mail mode, C-c C-y (mail-yank-original) uses indent-rigidly to indent the text copied from the message being replied to.

 -- Function: indent-code-rigidly start end columns &optional nochange-regexp
     This is like indent-rigidly, except that it doesn't alter lines that start within strings or comments.

     In addition, it doesn't alter a line if NOCHANGE-REGEXP matches at the beginning of the line (if NOCHANGE-REGEXP is non-nil).

Indentation Relative to Previous Lines
--------------------------------------

This section describes two commands that indent the current line based on the contents of previous lines.

 -- Command: indent-relative &optional unindented-ok
     This command inserts whitespace at point, extending to the same column as the next indent point of the previous nonblank line.  An indent point is a non-whitespace character following whitespace.  The next indent point is the first one at a column greater than the current column of point.  For example, if point is underneath and to the left of the first non-blank character of a line of text, it moves to that column by inserting whitespace.

     If the previous nonblank line has no next indent point (i.e., none at a great enough column position), indent-relative either does nothing (if UNINDENTED-OK is non-nil) or calls tab-to-tab-stop.  Thus, if point is underneath and to the right of the last column of a short line of text, this command ordinarily moves point to the next tab stop by inserting whitespace.

     The return value of indent-relative is unpredictable.

     In the following example, point is at the beginning of the second line:

                      This line is indented twelve spaces.
          -!-The quick brown fox jumped.

     Evaluation of the expression (indent-relative nil) produces the following:

                      This line is indented twelve spaces.
                      -!-The quick brown fox jumped.

     In this next example, point is between the m and p of jumped:

                      This line is indented twelve spaces.
          The quick brown fox jum-!-ped.

     Evaluation of the expression (indent-relative nil) produces the following:

                      This line is indented twelve spaces.
          The quick brown fox jum  -!-ped.

 -- Command: indent-relative-maybe
     This command indents the current line like the previous nonblank line, by calling indent-relative with t as the UNINDENTED-OK argument.  The return value is unpredictable.

     If the previous nonblank line has no indent points beyond the current column, this command does nothing.

Adjustable ``Tab Stops''
------------------------

This section explains the mechanism for user-specified ``tab stops'' and the mechanisms that use and set them.  The name ``tab stops'' is used because the feature is similar to that of the tab stops on a typewriter.  The feature works by inserting an appropriate number of spaces and tab characters to reach the next tab stop column; it does not affect the display of tab characters in the buffer (see Usual Display).  Note that the TAB character as input uses this tab stop feature only in a few major modes, such as Text mode.

 -- Command: tab-to-tab-stop
     This command inserts spaces or tabs before point, up to the next tab stop column defined by tab-stop-list.  It searches the list for an element greater than the current column number, and uses that element as the column to indent to.  It does nothing if no such element is found.

 -- User Option: tab-stop-list
     This variable is the list of tab stop columns used by tab-to-tab-stops.  The elements should be integers in increasing order.  The tab stop columns need not be evenly spaced.

     Use M-x edit-tab-stops to edit the location of tab stops interactively.

Indentation-Based Motion Commands
---------------------------------

These commands, primarily for interactive use, act based on the indentation in the text.

 -- Command: back-to-indentation
     This command moves point to the first non-whitespace character in the current line (which is the line in which point is located).  It returns nil.

 -- Command: backward-to-indentation arg
     This command moves point backward ARG lines and then to the first nonblank character on that line.  It returns nil.

 -- Command: forward-to-indentation arg
     This command moves point forward ARG lines and then to the first nonblank character on that line.  It returns nil.

Case Changes
============

The case change commands described here work on text in the current buffer.  See Case Conversion, for case conversion functions that work on strings and characters.  See Case Tables, for how to customize which characters are upper or lower case and how to convert them.

 -- Command: capitalize-region start end
     This function capitalizes all words in the region defined by START and END.  To capitalize means to convert each word's first character to upper case and convert the rest of each word to lower case.  The function returns nil.

     If one end of the region is in the middle of a word, the part of the word within the region is treated as an entire word.

     When capitalize-region is called interactively, START and END are point and the mark, with the smallest first.

          ---------- Buffer: foo ----------
          This is the contents of the 5th foo.
          ---------- Buffer: foo ----------

          (capitalize-region 1 44)
          => nil

          ---------- Buffer: foo ----------
          This Is The Contents Of The 5th Foo.
          ---------- Buffer: foo ----------

 -- Command: downcase-region start end
     This function converts all of the letters in the region defined by START and END to lower case.  The function returns nil.

     When downcase-region is called interactively, START and END are point and the mark, with the smallest first.

 -- Command: upcase-region start end
     This function converts all of the letters in the region defined by START and END to upper case.  The function returns nil.

     When upcase-region is called interactively, START and END are point and the mark, with the smallest first.

 -- Command: capitalize-word count
     This function capitalizes COUNT words after point, moving point over as it does.  To capitalize means to convert each word's first character to upper case and convert the rest of each word to lower case. If COUNT is negative, the function capitalizes the -COUNT previous words but does not move point.  The value is nil.

     If point is in the middle of a word, the part of the word before point is ignored when moving forward.  The rest is treated as an entire word.

     When capitalize-word is called interactively, COUNT is set to the numeric prefix argument.

 -- Command: downcase-word count
     This function converts the COUNT words after point to all lower case, moving point over as it does.  If COUNT is negative, it converts the -COUNT previous words but does not move point. The value is nil.

     When downcase-word is called interactively, COUNT is set to the numeric prefix argument.

 -- Command: upcase-word count
     This function converts the COUNT words after point to all upper case, moving point over as it does.  If COUNT is negative, it converts the -COUNT previous words but does not move point. The value is nil.

     When upcase-word is called interactively, COUNT is set to the numeric prefix argument.

Text Properties
===============

Each character position in a buffer or a string can have a text property list, much like the property list of a symbol (see Property Lists).  The properties belong to a particular character at a particular place, such as, the letter T at the beginning of this sentence or the first o in foo---if the same character occurs in two different places, the two occurrences generally have different properties.

Each property has a name and a value.  Both of these can be any Lisp object, but the name is normally a symbol.  The usual way to access the property list is to specify a name and ask what value corresponds to it.

If a character has a category property, we call it the category of the character.  It should be a symbol.  The properties of the symbol serve as defaults for the properties of the character.

Copying text between strings and buffers preserves the properties along with the characters; this includes such diverse functions as substring, insert, and buffer-substring.

Examining Text Properties
-------------------------

The simplest way to examine text properties is to ask for the value of a particular property of a particular character.  For that, use get-text-property.  Use text-properties-at to get the entire property list of a character.  See Property Search, for functions to examine the properties of a number of characters at once.

These functions handle both strings and buffers.  Keep in mind that positions in a string start from 0, whereas positions in a buffer start from 1.

 -- Function: get-text-property pos prop &optional object
     This function returns the value of the PROP property of the character after position POS in OBJECT (a buffer or string).  The argument OBJECT is optional and defaults to the current buffer.

     If there is no PROP property strictly speaking, but the character has a category that is a symbol, then get-text-property returns the PROP property of that symbol.

 -- Function: get-char-property pos prop &optional object
     This function is like get-text-property, except that it checks overlays first and then text properties.  See Overlays.

     The argument OBJECT may be a string, a buffer, or a window.  If it is a window, then the buffer displayed in that window is used for text properties and overlays, but only the overlays active for that window are considered.  If OBJECT is a buffer, then all overlays in that buffer are considered, as well as text properties.  If OBJECT is a string, only text properties are considered, since strings never have overlays.

 -- Function: text-properties-at position &optional object
     This function returns the entire property list of the character at POSITION in the string or buffer OBJECT.  If OBJECT is nil, it defaults to the current buffer.

 -- Variable: default-text-properties
     This variable holds a property list giving default values for text properties.  Whenever a character does not specify a value for a property, neither directly nor through a category symbol, the value stored in this list is used instead.  Here is an example:

          (setq default-text-properties '(foo 69))
          ;; Make sure character 1 has no properties of its own.
          (set-text-properties 1 2 nil)
          ;; What we get, when we ask, is the default value.
          (get-text-property 1 'foo)
               => 69

Changing Text Properties
------------------------

The primitives for changing properties apply to a specified range of text in a buffer or string.  The function set-text-properties (see end of section) sets the entire property list of the text in that range; more often, it is useful to add, change, or delete just certain properties specified by name.

Since text properties are considered part of the contents of the buffer (or string), and can affect how a buffer looks on the screen, any change in buffer text properties marks the buffer as modified.  Buffer text property changes are undoable also (see Undo).

 -- Function: put-text-property start end prop value &optional object
     This function sets the PROP property to VALUE for the text between START and END in the string or buffer OBJECT. If OBJECT is nil, it defaults to the current buffer.

 -- Function: add-text-properties start end props &optional object
     This function adds or overrides text properties for the text between START and END in the string or buffer OBJECT.  If OBJECT is nil, it defaults to the current buffer.

     The argument PROPS specifies which properties to add.  It should have the form of a property list (see Property Lists): a list whose elements include the property names followed alternately by the corresponding values.

     The return value is t if the function actually changed some property's value; nil otherwise (if PROPS is nil or its values agree with those in the text).

     For example, here is how to set the comment and face properties of a range of text:

          (add-text-properties START END
                               '(comment t face highlight))

 -- Function: remove-text-properties start end props &optional object
     This function deletes specified text properties from the text between START and END in the string or buffer OBJECT.  If OBJECT is nil, it defaults to the current buffer.

     The argument PROPS specifies which properties to delete.  It should have the form of a property list (see Property Lists): a list whose elements are property names alternating with corresponding values. But only the names matter---the values that accompany them are ignored. For example, here's how to remove the face property.

          (remove-text-properties START END '(face nil))

     The return value is t if the function actually changed some property's value; nil otherwise (if PROPS is nil or if no character in the specified text had any of those properties).

     To remove all text properties from certain text, use set-text-properties and specify nil for the new property list.

 -- Function: set-text-properties start end props &optional object
     This function completely replaces the text property list for the text between START and END in the string or buffer OBJECT. If OBJECT is nil, it defaults to the current buffer.

     The argument PROPS is the new property list.  It should be a list whose elements are property names alternating with corresponding values.

     After set-text-properties returns, all the characters in the specified range have identical properties.

     If PROPS is nil, the effect is to get rid of all properties from the specified range of text.  Here's an example:

          (set-text-properties START END nil)

The easiest way to make a string with text properties is with propertize:

 -- Function: propertize string &rest properties
     This function returns a copy of STRING which has the text properties PROPERTIES.  These properties apply to all the characters in the string that is returned.  Here is an example that constructs a string with a face property and a mouse-face property:

          (propertize "foo" 'face 'italic
                      'mouse-face 'bold-italic)
               => #("foo" 0 3 (mouse-face bold-italic face italic))

     To put different properties on various parts of a string, you can construct each part with propertize and then combine them with concat:

          (concat
           (propertize "foo" 'face 'italic
                       'mouse-face 'bold-italic)
           " and "
           (propertize "bar" 'face 'italic
                       'mouse-face 'bold-italic))
               => #("foo and bar"
                           0 3 (face italic mouse-face bold-italic)
                           3 8 nil
                           8 11 (face italic mouse-face bold-italic))

See also the function buffer-substring-no-properties (see Buffer Contents) which copies text from the buffer but does not copy its properties.

Text Property Search Functions
------------------------------

In typical use of text properties, most of the time several or many consecutive characters have the same value for a property.  Rather than writing your programs to examine characters one by one, it is much faster to process chunks of text that have the same property value.

Here are functions you can use to do this.  They use eq for comparing property values.  In all cases, OBJECT defaults to the current buffer.

For high performance, it's very important to use the LIMIT argument to these functions, especially the ones that search for a single property---otherwise, they may spend a long time scanning to the end of the buffer, if the property you are interested in does not change.

These functions do not move point; instead, they return a position (or nil).  Remember that a position is always between two characters; the position returned by these functions is between two characters with different properties.

 -- Function: next-property-change pos &optional object limit
     The function scans the text forward from position POS in the string or buffer OBJECT till it finds a change in some text property, then returns the position of the change.  In other words, it returns the position of the first character beyond POS whose properties are not identical to those of the character just after POS.

     If LIMIT is non-nil, then the scan ends at position LIMIT.  If there is no property change before that point, next-property-change returns LIMIT.

     The value is nil if the properties remain unchanged all the way to the end of OBJECT and LIMIT is nil.  If the value is non-nil, it is a position greater than or equal to POS. The value equals POS only when LIMIT equals POS.

     Here is an example of how to scan the buffer by chunks of text within which all properties are constant:

          (while (not (eobp))
            (let ((plist (text-properties-at (point)))
                  (next-change
                   (or (next-property-change (point) (current-buffer))
                       (point-max))))
              Process text from point to NEXT-CHANGE...
              (goto-char next-change)))

 -- Function: next-single-property-change pos prop &optional object limit
     The function scans the text forward from position POS in the string or buffer OBJECT till it finds a change in the PROP property, then returns the position of the change.  In other words, it returns the position of the first character beyond POS whose PROP property differs from that of the character just after POS.

     If LIMIT is non-nil, then the scan ends at position LIMIT.  If there is no property change before that point, next-single-property-change returns LIMIT.

     The value is nil if the property remains unchanged all the way to the end of OBJECT and LIMIT is nil.  If the value is non-nil, it is a position greater than or equal to POS; it equals POS only if LIMIT equals POS.

 -- Function: previous-property-change pos &optional object limit
     This is like next-property-change, but scans back from POS instead of forward.  If the value is non-nil, it is a position less than or equal to POS; it equals POS only if LIMIT equals POS.

 -- Function: previous-single-property-change pos prop &optional object limit
     This is like next-single-property-change, but scans back from POS instead of forward.  If the value is non-nil, it is a position less than or equal to POS; it equals POS only if LIMIT equals POS.

 -- Function: next-char-property-change position &optional limit
     This is like next-property-change except that it considers overlay properties as well as text properties.  There is no OBJECT operand because this function operates only on the current buffer.  It returns the next address at which either kind of property changes.

 -- Function: previous-char-property-change position &optional limit
     This is like next-char-property-change, but scans back from POSITION instead of forward.

 -- Function: text-property-any start end prop value &optional object
     This function returns non-nil if at least one character between START and END has a property PROP whose value is VALUE.  More precisely, it returns the position of the first such character.  Otherwise, it returns nil.

     The optional fifth argument, OBJECT, specifies the string or buffer to scan.  Positions are relative to OBJECT.  The default for OBJECT is the current buffer.

 -- Function: text-property-not-all start end prop value &optional object
     This function returns non-nil if at least one character between START and END does not have a property PROP with value VALUE.  More precisely, it returns the position of the first such character.  Otherwise, it returns nil.

     The optional fifth argument, OBJECT, specifies the string or buffer to scan.  Positions are relative to OBJECT.  The default for OBJECT is the current buffer.

Properties with Special Meanings
--------------------------------

Here is a table of text property names that have special built-in meanings.  The following sections list a few additional special property names that control filling and property inheritance.  All other names have no standard meaning, and you can use them as you like.

category
     If a character has a category property, we call it the category of the character.  It should be a symbol.  The properties of the symbol serve as defaults for the properties of the character.

face
     You can use the property face to control the font and color of text.  See Faces, for more information.

     In the simplest case, the value is a face name.  It can also be a list; then each element can be any of these possibilities;

       * A face name (a symbol or string).

       * Starting in Emacs 21, a property list of face attributes.  This has the form (KEYWORD VALUE ...), where each KEYWORD is a face attribute name and VALUE is a meaningful value for that attribute.  With this feature, you do not need to create a face each time you want to specify a particular attribute for certain text. See Face Attributes.

       * A cons cell of the form (foreground-color . COLOR-NAME) or (background-color . COLOR-NAME).  These elements specify just the foreground color or just the background color.

          (foreground-color . COLOR-NAME) is equivalent to (:foreground COLOR-NAME), and likewise for the background.

     See Font Lock Mode, for information on how to update face properties automatically based on the contents of the text.

mouse-face
     The property mouse-face is used instead of face when the mouse is on or near the character.  For this purpose, ``near'' means that all text between the character and where the mouse is have the same mouse-face property value.

fontified
     This property, if non-nil, says that text in the buffer has had faces assigned automatically by a feature such as Font-Lock mode. See Auto Faces.

display
     This property activates various features that change the way text is displayed.  For example, it can make text appear taller or shorter, higher or lower, wider or narrow, or replaced with an image. See Display Property.

help-echo
     If text has a string as its help-echo property, then when you move the mouse onto that text, Emacs displays that string in the echo area, or in the tooltip window.  This feature is used in the mode line. It is available starting in Emacs 21.

local-map
     You can specify a different keymap for some of the text in a buffer by means of the local-map property.  The property's value for the character after point, if non-nil, is used for key lookup instead of the buffer's local map.  If the property value is a symbol, the symbol's function definition is used as the keymap.  See Active Keymaps.

syntax-table
     The syntax-table property overrides what the syntax table says about this particular character.  See Syntax Properties.

read-only
     If a character has the property read-only, then modifying that character is not allowed.  Any command that would do so gets an error, text-read-only.

     Insertion next to a read-only character is an error if inserting ordinary text there would inherit the read-only property due to stickiness.  Thus, you can control permission to insert next to read-only text by controlling the stickiness.  See Sticky Properties.

     Since changing properties counts as modifying the buffer, it is not possible to remove a read-only property unless you know the special trick: bind inhibit-read-only to a non-nil value and then remove the property.  See Read Only Buffers.

invisible
     A non-nil invisible property can make a character invisible on the screen.  See Invisible Text, for details.

intangible
     If a group of consecutive characters have equal and non-nil intangible properties, then you cannot place point between them. If you try to move point forward into the group, point actually moves to the end of the group.  If you try to move point backward into the group, point actually moves to the start of the group.

     When the variable inhibit-point-motion-hooks is non-nil, the intangible property is ignored.

field
     Consecutive characters with the same field property constitute a field.  Some motion functions including forward-word and beginning-of-line stop moving at a field boundary. See Fields.

modification-hooks
     If a character has the property modification-hooks, then its value should be a list of functions; modifying that character calls all of those functions.  Each function receives two arguments: the beginning and end of the part of the buffer being modified.  Note that if a particular modification hook function appears on several characters being modified by a single primitive, you can't predict how many times the function will be called.

insert-in-front-hooks
insert-behind-hooks
     The operation of inserting text in a buffer also calls the functions listed in the insert-in-front-hooks property of the following character and in the insert-behind-hooks property of the preceding character.  These functions receive two arguments, the beginning and end of the inserted text.  The functions are called after the actual insertion takes place.

     See also Change Hooks, for other hooks that are called when you change text in a buffer.

point-entered
point-left
     The special properties point-entered and point-left record hook functions that report motion of point.  Each time point moves, Emacs compares these two property values:

       * the point-left property of the character after the old location, and

       * the point-entered property of the character after the new location.

     If these two values differ, each of them is called (if not nil) with two arguments: the old value of point, and the new one.

     The same comparison is made for the characters before the old and new locations.  The result may be to execute two point-left functions (which may be the same function) and/or two point-entered functions (which may be the same function).  In any case, all the point-left functions are called first, followed by all the point-entered functions.

     It is possible using char-after to examine characters at various positions without moving point to those positions.  Only an actual change in the value of point runs these hook functions.

 -- Variable: inhibit-point-motion-hooks
     When this variable is non-nil, point-left and point-entered hooks are not run, and the intangible property has no effect.  Do not set this variable globally; bind it with let.

Formatted Text Properties
-------------------------

These text properties affect the behavior of the fill commands.  They are used for representing formatted text.  See Filling, and Margins.

hard
     If a newline character has this property, it is a ``hard'' newline. The fill commands do not alter hard newlines and do not move words across them.  However, this property takes effect only if the variable use-hard-newlines is non-nil.

right-margin
     This property specifies an extra right margin for filling this part of the text.

left-margin
     This property specifies an extra left margin for filling this part of the text.

justification
     This property specifies the style of justification for filling this part of the text.

Stickiness of Text Properties
-----------------------------

Self-inserting characters normally take on the same properties as the preceding character.  This is called inheritance of properties.

In a Lisp program, you can do insertion with inheritance or without, depending on your choice of insertion primitive.  The ordinary text insertion functions such as insert do not inherit any properties. They insert text with precisely the properties of the string being inserted, and no others.  This is correct for programs that copy text from one context to another---for example, into or out of the kill ring. To insert with inheritance, use the special primitives described in this section.  Self-inserting characters inherit properties because they work using these primitives.

When you do insertion with inheritance, which properties are inherited, and from where, depends on which properties are sticky. Insertion after a character inherits those of its properties that are rear-sticky.  Insertion before a character inherits those of its properties that are front-sticky.  When both sides offer different sticky values for the same property, the previous character's value takes precedence.

By default, a text property is rear-sticky but not front-sticky; thus, the default is to inherit all the properties of the preceding character, and nothing from the following character.

You can control the stickiness of various text properties with two specific text properties, front-sticky and rear-nonsticky, and with the variable text-property-default-nonsticky.  You can use the variable to specify a different default for a given property. You can use those two text properties to make any specific properties sticky or nonsticky in any particular part of the text.

If a character's front-sticky property is t, then all its properties are front-sticky.  If the front-sticky property is a list, then the sticky properties of the character are those whose names are in the list.  For example, if a character has a front-sticky property whose value is (face read-only), then insertion before the character can inherit its face property and its read-only property, but no others.

The rear-nonsticky property works the opposite way.  Most properties are rear-sticky by default, so the rear-nonsticky property says which properties are not rear-sticky.  If a character's rear-nonsticky property is t, then none of its properties are rear-sticky.  If the rear-nonsticky property is a list, properties are rear-sticky unless their names are in the list.

 -- Variable: text-property-default-nonsticky
     This variable holds an alist which defines the default rear-stickiness of various text properties.  Each element has the form (PROPERTY . NONSTICKINESS), and it defines the stickiness of a particular text property, PROPERTY.

     If NONSTICKINESS is non-nil, this means that the property PROPERTY is rear-nonsticky by default.  Since all properties are front-nonsticky by default, this makes PROPERTY nonsticky in both directions by default.

     The text properties front-sticky and rear-nonsticky, when used, take precedence over the default NONSTICKINESS specifed in text-property-default-nonsticky.

Here are the functions that insert text with inheritance of properties:

 -- Function: insert-and-inherit &rest strings
     Insert the strings STRINGS, just like the function insert, but inherit any sticky properties from the adjoining text.

 -- Function: insert-before-markers-and-inherit &rest strings
     Insert the strings STRINGS, just like the function insert-before-markers, but inherit any sticky properties from the adjoining text.

See Insertion, for the ordinary insertion functions which do not inherit.

Saving Text Properties in Files
-------------------------------

You can save text properties in files (along with the text itself), and restore the same text properties when visiting or inserting the files, using these two hooks:

 -- Variable: write-region-annotate-functions
     This variable's value is a list of functions for write-region to run to encode text properties in some fashion as annotations to the text being written in the file.  See Writing to Files.

     Each function in the list is called with two arguments: the start and end of the region to be written.  These functions should not alter the contents of the buffer.  Instead, they should return lists indicating annotations to write in the file in addition to the text in the buffer.

     Each function should return a list of elements of the form (POSITION . STRING), where POSITION is an integer specifying the relative position within the text to be written, and STRING is the annotation to add there.

     Each list returned by one of these functions must be already sorted in increasing order by POSITION.  If there is more than one function, write-region merges the lists destructively into one sorted list.

     When write-region actually writes the text from the buffer to the file, it intermixes the specified annotations at the corresponding positions.  All this takes place without modifying the buffer.

 -- Variable: after-insert-file-functions
     This variable holds a list of functions for insert-file-contents to call after inserting a file's contents.  These functions should scan the inserted text for annotations, and convert them to the text properties they stand for.

     Each function receives one argument, the length of the inserted text; point indicates the start of that text.  The function should scan that text for annotations, delete them, and create the text properties that the annotations specify.  The function should return the updated length of the inserted text, as it stands after those changes.  The value returned by one function becomes the argument to the next function.

     These functions should always return with point at the beginning of the inserted text.

     The intended use of after-insert-file-functions is for converting some sort of textual annotations into actual text properties.  But other uses may be possible.

We invite users to write Lisp programs to store and retrieve text properties in files, using these hooks, and thus to experiment with various data formats and find good ones.  Eventually we hope users will produce good, general extensions we can install in Emacs.

We suggest not trying to handle arbitrary Lisp objects as text property names or values---because a program that general is probably difficult to write, and slow.  Instead, choose a set of possible data types that are reasonably flexible, and not too hard to encode.

See Format Conversion, for a related feature.

Lazy Computation of Text Properties
-----------------------------------

Instead of computing text properties for all the text in the buffer, you can arrange to compute the text properties for parts of the text when and if something depends on them.

The primitive that extracts text from the buffer along with its properties is buffer-substring.  Before examining the properties, this function runs the abnormal hook buffer-access-fontify-functions.

 -- Variable: buffer-access-fontify-functions
     This variable holds a list of functions for computing text properties. Before buffer-substring copies the text and text properties for a portion of the buffer, it calls all the functions in this list.  Each of the functions receives two arguments that specify the range of the buffer being accessed.  (The buffer itself is always the current buffer.)

The function buffer-substring-no-properties does not call these functions, since it ignores text properties anyway.

In order to prevent the hook functions from being called more than once for the same part of the buffer, you can use the variable buffer-access-fontified-property.

 -- Variable: buffer-access-fontified-property
     If this value's variable is non-nil, it is a symbol which is used as a text property name.  A non-nil value for that text property means, ``the other text properties for this character have already been computed.''

     If all the characters in the range specified for buffer-substring have a non-nil value for this property, buffer-substring does not call the buffer-access-fontify-functions functions.  It assumes these characters already have the right text properties, and just copies the properties they already have.

     The normal way to use this feature is that the buffer-access-fontify-functions functions add this property, as well as others, to the characters they operate on.  That way, they avoid being called over and over for the same text.

Defining Clickable Text
-----------------------

There are two ways to set up clickable text in a buffer. There are typically two parts of this: to make the text highlight when the mouse is over it, and to make a mouse button do something when you click it on that part of the text.

Highlighting is done with the mouse-face text property. Here is an example of how Dired does it:

     (condition-case nil
         (if (dired-move-to-filename)
             (put-text-property (point)
                                (save-excursion
                                  (dired-move-to-end-of-filename)
                                  (point))
                                'mouse-face 'highlight))
       (error nil))

The first two arguments to put-text-property specify the beginning and end of the text.

The usual way to make the mouse do something when you click it on this text is to define mouse-2 in the major mode's keymap.  The job of checking whether the click was on clickable text is done by the command definition.  Here is how Dired does it:

     (defun dired-mouse-find-file-other-window (event)
       "In dired, visit the file or directory name you click on."
       (interactive "e")
       (let (file)
         (save-excursion
           (set-buffer (window-buffer (posn-window (event-end event))))
           (save-excursion
             (goto-char (posn-point (event-end event)))
             (setq file (dired-get-filename))))
         (select-window (posn-window (event-end event)))
         (find-file-other-window (file-name-sans-versions file t))))

The reason for the outer save-excursion construct is to avoid changing the current buffer; the reason for the inner one is to avoid permanently altering point in the buffer you click on.  In this case, Dired uses the function dired-get-filename to determine which file to visit, based on the position found in the event.

Instead of defining a mouse command for the major mode, you can define a key binding for the clickable text itself, using the local-map text property:

     (let ((map (make-sparse-keymap)))
       (define-key-binding map [mouse-2] 'operate-this-button)
       (put-text-property (point)
                          (save-excursion
                            (dired-move-to-end-of-filename)
                            (point))
                          'local-map map))

This method makes it possible to define different commands for various clickable pieces of text.  Also, the major mode definition (or the global definition) remains available for the rest of the text in the buffer.

Defining and Using Fields
-------------------------

A field is a range of consecutive characters in the buffer that are identified by having the same value (comparing with eq) of the field property.  This section describes special functions that are available for operating on fields.

You specify a field with a buffer position, POS.  We think of each field as containing a range of buffer positions, so the position you specify stands for the field containing that position.

When the characters before and after POS are part of the same field, there is no doubt which field contains POS: the one those characters both belong to.  When POS is at a boundary between fields, which field it belongs to depends on the stickiness of the field properties of the two surrounding characters (see Sticky Properties).  The field whose property would be inherited by text inserted at POS is the field that contains POS.

There is an anomalous case where newly inserted text at POS would not inherit the field property from either side.  This happens if the previous character's field property is not rear-sticky, and the following character's field property is not front-sticky.  In this case, POS belongs to neither the preceding field nor the following field; the field functions treat it as belonging to an empty field whose beginning and end are both at POS.

In all of these functions, if POS is omitted or nil, the value of point is used by default.

 -- Function: field-beginning &optional pos escape-from-edge
     This function returns the beginning of the field specified by POS.

     If POS is at the end of a field, and ESCAPE-FROM-EDGE is non-nil, then the return value is always the beginning of the field that ends at POS, regardless of the stickiness of the field properties around POS.

 -- Function: field-end &optional pos escape-from-edge
     This function returns the end of the field specified by POS.

     If POS is at the beginning of a field, and ESCAPE-FROM-EDGE is non-nil, then the return value is always the end of the field that begins at POS, regardless of the stickiness of the field properties around POS.

 -- Function: field-string &optional pos
     This function returns the contents of the field specified by POS, as a string.

 -- Function: field-string-no-properties &optional pos
     This function returns the contents of the field specified by POS, as a string, discarding text properties.

 -- Function: delete-field &optional pos
     This function deletes the text of the field specified by POS.

 -- beginning-of-line-or-field: &optional count
     Like beginning-of-line, except that this function does not move across a field boundary (see Fields), unless it moves to another line beyond the one that contains the field boundary.  Therefore, if COUNT is zero, and point is initially at a field boundary, point does not move.

 -- end-of-line-or-field: &optional count
     Like end-of-line, except that this function does not move across a field boundary (see Fields), unless it moves to another line beyond the one that contains the field boundary.

 -- Function: constrain-to-field new-pos old-pos &optional escape-from-edge only-in-line
     This function ``constrains'' NEW-POS to the field that OLD-POS belongs to---in other words, it returns the position closest to NEW-POS that is in the same field as OLD-POS.

     If NEW-POS is nil, then constrain-to-field uses the value of point instead, and moves point to the resulting position.

     If OLD-POS is at the boundary of two fields, then the acceptable positions for NEW-POS depend on the value of the optional argument ESCAPE-FROM-EDGE.  If ESCAPE-FROM-EDGE is nil, then NEW-POS is constrained to the field that has the same field text-property that new characters inserted at OLD-POS would get. (This depends on the stickiness of the field property for the characters before and after OLD-POS.)  If ESCAPE-FROM-EDGE is non-nil, NEW-POS is constrained to the union of the two adjacent fields.

     If the optional argument ONLY-IN-LINE is non-nil, and constraining NEW-POS in the usual way would move it to a different line, NEW-POS is returned unconstrained.  This used in commands that move by line, such as next-line and beginning-of-line, so that they respect field boundaries only in the case where they can still move to the right line.

Why Text Properties are not Intervals
-------------------------------------

Some editors that support adding attributes to text in the buffer do so by letting the user specify ``intervals'' within the text, and adding the properties to the intervals.  Those editors permit the user or the programmer to determine where individual intervals start and end.  We deliberately provided a different sort of interface in Emacs Lisp to avoid certain paradoxical behavior associated with text modification.

If the actual subdivision into intervals is meaningful, that means you can distinguish between a buffer that is just one interval with a certain property, and a buffer containing the same text subdivided into two intervals, both of which have that property.

Suppose you take the buffer with just one interval and kill part of the text.  The text remaining in the buffer is one interval, and the copy in the kill ring (and the undo list) becomes a separate interval. Then if you yank back the killed text, you get two intervals with the same properties.  Thus, editing does not preserve the distinction between one interval and two.

Suppose we ``fix'' this problem by coalescing the two intervals when the text is inserted.  That works fine if the buffer originally was a single interval.  But suppose instead that we have two adjacent intervals with the same properties, and we kill the text of one interval and yank it back.  The same interval-coalescence feature that rescues the other case causes trouble in this one: after yanking, we have just one interval.  One again, editing does not preserve the distinction between one interval and two.

Insertion of text at the border between intervals also raises questions that have no satisfactory answer.

However, it is easy to arrange for editing to behave consistently for questions of the form, ``What are the properties of this character?'' So we have decided these are the only questions that make sense; we have not implemented asking questions about where intervals start or end.

In practice, you can usually use the text property search functions in place of explicit interval boundaries.  You can think of them as finding the boundaries of intervals, assuming that intervals are always coalesced whenever possible.  See Property Search.

Emacs also provides explicit intervals as a presentation feature; see Overlays.

Substituting for a Character Code
=================================

The following functions replace characters within a specified region based on their character codes.

 -- Function: subst-char-in-region start end old-char new-char &optional noundo
     This function replaces all occurrences of the character OLD-CHAR with the character NEW-CHAR in the region of the current buffer defined by START and END.

     If NOUNDO is non-nil, then subst-char-in-region does not record the change for undo and does not mark the buffer as modified. This was useful for controlling the old selective display feature (see Selective Display).

     subst-char-in-region does not move point and returns nil.

          ---------- Buffer: foo ----------
          This is the contents of the buffer before.
          ---------- Buffer: foo ----------

          (subst-char-in-region 1 20 ?i ?X)
               => nil

          ---------- Buffer: foo ----------
          ThXs Xs the contents of the buffer before.
          ---------- Buffer: foo ----------

 -- Function: translate-region start end table
     This function applies a translation table to the characters in the buffer between positions START and END.

     The translation table TABLE is a string; (aref TABLE OCHAR) gives the translated character corresponding to OCHAR.  If the length of TABLE is less than 256, any characters with codes larger than the length of TABLE are not altered by the translation.

     The return value of translate-region is the number of characters that were actually changed by the translation.  This does not count characters that were mapped into themselves in the translation table.

Registers
=========

A register is a sort of variable used in Emacs editing that can hold a variety of different kinds of values.  Each register is named by a single character.  All ASCII characters and their meta variants (but with the exception of C-g) can be used to name registers. Thus, there are 255 possible registers.  A register is designated in Emacs Lisp by the character that is its name.

 -- Variable: register-alist
     This variable is an alist of elements of the form (NAME . CONTENTS).  Normally, there is one element for each Emacs register that has been used.

     The object NAME is a character (an integer) identifying the register.

The CONTENTS of a register can have several possible types:

a number
     A number stands for itself.  If insert-register finds a number in the register, it converts the number to decimal.

a marker
     A marker represents a buffer position to jump to.

a string
     A string is text saved in the register.

a rectangle
     A rectangle is represented by a list of strings.

(WINDOW-CONFIGURATION POSITION)
     This represents a window configuration to restore in one frame, and a position to jump to in the current buffer.

(FRAME-CONFIGURATION POSITION)
     This represents a frame configuration to restore, and a position to jump to in the current buffer.

(file FILENAME)
     This represents a file to visit; jumping to this value visits file FILENAME.

(file-query FILENAME POSITION)
     This represents a file to visit and a position in it; jumping to this value visits file FILENAME and goes to buffer position POSITION.  Restoring this type of position asks the user for confirmation first.

The functions in this section return unpredictable values unless otherwise stated.

 -- Function: get-register reg
     This function returns the contents of the register REG, or nil if it has no contents.

 -- Function: set-register reg value
     This function sets the contents of register REG to VALUE. A register can be set to any value, but the other register functions expect only certain data types.  The return value is VALUE.

 -- Command: view-register reg
     This command displays what is contained in register REG.

 -- Command: insert-register reg &optional beforep
     This command inserts contents of register REG into the current buffer.

     Normally, this command puts point before the inserted text, and the mark after it.  However, if the optional second argument BEFOREP is non-nil, it puts the mark before and point after. You can pass a non-nil second argument BEFOREP to this function interactively by supplying any prefix argument.

     If the register contains a rectangle, then the rectangle is inserted with its upper left corner at point.  This means that text is inserted in the current line and underneath it on successive lines.

     If the register contains something other than saved text (a string) or a rectangle (a list), currently useless things happen.  This may be changed in the future.

Transposition of Text
=====================

This subroutine is used by the transposition commands.

 -- Function: transpose-regions start1 end1 start2 end2 &optional leave-markers
     This function exchanges two nonoverlapping portions of the buffer. Arguments START1 and END1 specify the bounds of one portion and arguments START2 and END2 specify the bounds of the other portion.

     Normally, transpose-regions relocates markers with the transposed text; a marker previously positioned within one of the two transposed portions moves along with that portion, thus remaining between the same two characters in their new position.  However, if LEAVE-MARKERS is non-nil, transpose-regions does not do this---it leaves all markers unrelocated.

Base 64 Encoding
================

Base 64 code is used in email to encode a sequence of 8-bit bytes as a longer sequence of ASCII graphic characters.  This section describes the functions for converting to and from this code.

 -- Function: base64-encode-region beg end &optional no-line-break
     This function converts the region from BEG to END into base 64 code.  It returns the length of the encoded text.

     Normally, this function inserts newline characters into the encoded text, to avoid overlong lines.  However, if the optional argument NO-LINE-BREAK is non-nil, these newlines are not added, so the output is just one long line.

 -- Function: base64-encode-string string &optional no-line-break
     This function converts the string STRING into base 64 code.  It returns a string containing the encoded text.

     Normally, this function inserts newline characters into the encoded text, to avoid overlong lines.  However, if the optional argument NO-LINE-BREAK is non-nil, these newlines are not added, so the result string is just one long line.

 -- Function: base64-decode-region beg end
     This function converts the region from BEG to END from base 64 code into the corresponding decoded text.  It returns the length of the decoded text.

     The decoding functions ignore newline characters in the encoded text.

 -- Function: base64-decode-string string
     This function converts the string STRING from base 64 code into the corresponding decoded text.  It returns a string containing the decoded text.

     The decoding functions ignore newline characters in the encoded text.

Change Hooks
============

These hook variables let you arrange to take notice of all changes in all buffers (or in a particular buffer, if you make them buffer-local). See also Special Properties, for how to detect changes to specific parts of the text.

The functions you use in these hooks should save and restore the match data if they do anything that uses regular expressions; otherwise, they will interfere in bizarre ways with the editing operations that call them.

 -- Variable: before-change-functions
     This variable holds a list of functions to call before any buffer modification.  Each function gets two arguments, the beginning and end of the region that is about to change, represented as integers.  The buffer that is about to change is always the current buffer.

 -- Variable: after-change-functions
     This variable holds a list of functions to call after any buffer modification.  Each function receives three arguments: the beginning and end of the region just changed, and the length of the text that existed before the change.  All three arguments are integers.  The buffer that's about to change is always the current buffer.

     The length of the old text is the difference between the buffer positions before and after that text as it was before the change.  As for the changed text, its length is simply the difference between the first two arguments.

 -- Macro: combine-after-change-calls body...
     The macro executes BODY normally, but arranges to call the after-change functions just once for a series of several changes---if that seems safe.

     If a program makes several text changes in the same area of the buffer, using the macro combine-after-change-calls around that part of the program can make it run considerably faster when after-change hooks are in use.  When the after-change hooks are ultimately called, the arguments specify a portion of the buffer including all of the changes made within the combine-after-change-calls body.

     Warning: You must not alter the values of after-change-functions and after-change-function within the body of a combine-after-change-calls form.

     Note: If the changes you combine occur in widely scattered parts of the buffer, this will still work, but it is not advisable, because it may lead to inefficient behavior for some change hook functions.

 -- Variable: before-change-function
     This obsolete variable holds one function to call before any buffer modification (or nil for no function).  It is called just like the functions in before-change-functions.

 -- Variable: after-change-function
     This obsolete variable holds one function to call after any buffer modification (or nil for no function).  It is called just like the functions in after-change-functions.

The four variables above are temporarily bound to nil during the time that any of these functions is running.  This means that if one of these functions changes the buffer, that change won't run these functions.  If you do want a hook function to make changes that run these functions, make it bind these variables back to their usual values.

One inconvenient result of this protective feature is that you cannot have a function in after-change-functions or before-change-functions which changes the value of that variable. But that's not a real limitation.  If you want those functions to change the list of functions to run, simply add one fixed function to the hook, and code that function to look in another variable for other functions to call.  Here is an example:

     (setq my-own-after-change-functions nil)
     (defun indirect-after-change-function (beg end len)
       (let ((list my-own-after-change-functions))
         (while list
           (funcall (car list) beg end len)
           (setq list (cdr list)))))

     (add-hooks 'after-change-functions
                'indirect-after-change-function)

 -- Variable: first-change-hook
     This variable is a normal hook that is run whenever a buffer is changed that was previously in the unmodified state.

 -- Variable: inhibit-modification-hooks
     If this variable is non-nil, all of the change hooks are disabled; none of them run.  This affects all the hook variables described above in this section, as well as the hooks attached to certain special text properties (see Special Properties) and overlay properties (see Overlay Properties).

     This variable is available starting in Emacs 21.