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@c This is part of the Emacs manual.
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@c Copyright (C) 1985,86,87,93,94,95,97,99,2000 Free Software Foundation, Inc.
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@c See file emacs.texi for copying conditions.
@node Programs, Building, Text, Top
@chapter Editing Programs
@cindex Lisp editing
@cindex C editing
@cindex program editing

  Emacs has many commands designed to understand the syntax of programming
languages such as Lisp and C.  These commands can

@itemize @bullet
@item
Move over or kill balanced expressions or @dfn{sexps} (@pxref{Lists}).
@item
Move over or mark top-level expressions---@dfn{defuns}, in Lisp;
functions, in C (@pxref{Defuns}).
@item
Show how parentheses balance (@pxref{Matching}).
@item
Insert, kill or align comments (@pxref{Comments}).
@item
Follow the usual indentation conventions of the language
(@pxref{Program Indent}).
@end itemize

  The commands for words, sentences and paragraphs are very useful in
editing code even though their canonical application is for editing
human language text.  Most symbols contain words (@pxref{Words});
sentences can be found in strings and comments (@pxref{Sentences}).
Paragraphs per se don't exist in code, but the paragraph commands are
useful anyway, because programming language major modes define
paragraphs to begin and end at blank lines (@pxref{Paragraphs}).
Judicious use of blank lines to make the program clearer will also
provide useful chunks of text for the paragraph commands to work
on.

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@cindex selective display
@cindex outline
@cindex folding
@findex outline-minor-mode
@cindex outlines
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  The selective display feature is useful for looking at the overall
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structure of a function (@pxref{Selective Display}).  This feature
causes only the lines that are indented less than a specified amount to
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appear on the screen.  Programming modes often support Outline minor
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mode (@pxref{Outline Mode}).  The Foldout package provides
folding-editor features (@pxref{Foldout}).
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  The `automatic typing' features may be useful when writing programs.
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@xref{,Autotyping,, autotype, Autotyping}.
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@menu
* Program Modes::       Major modes for editing programs.
* Lists::	        Expressions with balanced parentheses.
* List Commands::       The commands for working with list and sexps.
* Defuns::	        Each program is made up of separate functions.
			  There are editing commands to operate on them.
* Program Indent::      Adjusting indentation to show the nesting.
* Matching::	        Insertion of a close-delimiter flashes matching open.
* Comments::	        Inserting, killing, and aligning comments.
* Balanced Editing::    Inserting two matching parentheses at once, etc.
* Symbol Completion::   Completion on symbol names of your program or language.
* Which Function::      Which Function mode shows which function you are in.
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* Hideshow::            Displaying blocks selectively.
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* Glasses::             Making identifiersLikeThis more readable.
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* Documentation::       Getting documentation of functions you plan to call.
* Change Log::	        Maintaining a change history for your program.
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* Authors::             Maintaining an @file{AUTHORS} file.
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* Tags::	        Go direct to any function in your program in one
			  command.  Tags remembers which file it is in.
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* Imenu::               Making buffer indexes as menus.
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* Emerge::	        A convenient way of merging two versions of a program.
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* C Modes::             Special commands of C, C++, Objective-C,
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                          Java, and Pike modes.
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* Fortran::             Fortran mode and its special features.
* Asm Mode::            Asm mode and its special features.
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@end menu

@node Program Modes
@section Major Modes for Programming Languages

@cindex modes for programming languages
@cindex Perl mode
@cindex Icon mode
@cindex Awk mode
@cindex Makefile mode
@cindex Tcl mode
@cindex CPerl mode
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@cindex DSSSL mode
@cindex Octave mode
@cindex Metafont mode
@cindex Modula2 mode
@cindex Prolog mode
@cindex Simula mode
@cindex VHDL mode
@cindex M4 mode
@cindex Shell-script mode
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@cindex Delphi mode
@cindex PostScript mode
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  Emacs also has major modes for the programming languages Lisp, Scheme
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(a variant of Lisp) and the Scheme-based DSSSL expression language, Ada,
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Awk, C, C++, Delphi (Object Pascal), Fortran (free and fixed format),
Icon, IDLWAVE,
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Java, Metafont (@TeX{}'s companion for font creation), Modula2,
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Objective-C, Octave, Pascal, Perl, Pike, PostScript, Prolog, Simula,
VHDL, CORBA IDL, and Tcl.
There is also a major mode for makefiles, called Makefile
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mode.  An alternative mode for Perl is called CPerl mode.  Modes
are available for scripts for the common Unix shells, VMS DCL and
MS-DOS/MS-Windows `BAT' files.  In a similar fashion to programming
languages, modes are provided for editing various sorts of configuration
files.

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Separate manuals are available for the modes for Ada (@pxref{Top, , Ada
Mode, ada-mode, Ada Mode}), C/C++/Objective C/Java/Corba IDL
(@pxref{Top, , CC Mode, ccmode, CC Mode}) and the IDLWAVE modes
(@pxref{Top, , IDLWAVE, idlwave, IDLWAVE User Manual}).
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  Ideally, a major mode should be implemented for each programming
language that you might want to edit with Emacs; but often the mode for
one language can serve for other syntactically similar languages.  The
language modes that exist are those that someone decided to take the
trouble to write.

  There are several forms of Lisp mode, which differ in the way they
interface to Lisp execution.  @xref{Executing Lisp}.

  Each of the programming language major modes defines the @key{TAB} key
to run an indentation function that knows the indentation conventions of
that language and updates the current line's indentation accordingly.
For example, in C mode @key{TAB} is bound to @code{c-indent-line}.
@kbd{C-j} is normally defined to do @key{RET} followed by @key{TAB};
thus, it too indents in a mode-specific fashion.

@kindex DEL @r{(programming modes)}
@findex backward-delete-char-untabify
  In most programming languages, indentation is likely to vary from line to
line.  So the major modes for those languages rebind @key{DEL} to treat a
tab as if it were the equivalent number of spaces (using the command
@code{backward-delete-char-untabify}).  This makes it possible to rub out
indentation one column at a time without worrying whether it is made up of
spaces or tabs.  Use @kbd{C-b C-d} to delete a tab character before point,
in these modes.

  Programming language modes define paragraphs to be separated only by
blank lines, so that the paragraph commands remain useful.  Auto Fill mode,
if enabled in a programming language major mode, indents the new lines
which it creates.

@cindex mode hook
@vindex c-mode-hook
@vindex lisp-mode-hook
@vindex emacs-lisp-mode-hook
@vindex lisp-interaction-mode-hook
@vindex scheme-mode-hook
  Turning on a major mode runs a normal hook called the @dfn{mode hook},
which is the value of a Lisp variable.  Each major mode has a mode hook,
and the hook's name is always made from the mode command's name by
adding @samp{-hook}.  For example, turning on C mode runs the hook
@code{c-mode-hook}, while turning on Lisp mode runs the hook
@code{lisp-mode-hook}.  @xref{Hooks}.

@node Lists
@section Lists and Sexps

@cindex Control-Meta
  By convention, Emacs keys for dealing with balanced expressions are
usually Control-Meta characters.  They tend to be analogous in
function to their Control and Meta equivalents.  These commands are
usually thought of as pertaining to expressions in programming
languages, but can be useful with any language in which some sort of
parentheses exist (including human languages).

@cindex list
@cindex sexp
@cindex expression
  These commands fall into two classes.  Some deal only with @dfn{lists}
(parenthetical groupings).  They see nothing except parentheses, brackets,
braces (whichever ones must balance in the language you are working with),
and escape characters that might be used to quote those.

  The other commands deal with expressions or @dfn{sexps}.  The word `sexp'
is derived from @dfn{s-expression}, the ancient term for an expression in
Lisp.  But in Emacs, the notion of `sexp' is not limited to Lisp.  It
refers to an expression in whatever language your program is written in.
Each programming language has its own major mode, which customizes the
syntax tables so that expressions in that language count as sexps.

  Sexps typically include symbols, numbers, and string constants, as well
as anything contained in parentheses, brackets or braces.

  In languages that use prefix and infix operators, such as C, it is not
possible for all expressions to be sexps.  For example, C mode does not
recognize @samp{foo + bar} as a sexp, even though it @emph{is} a C expression;
it recognizes @samp{foo} as one sexp and @samp{bar} as another, with the
@samp{+} as punctuation between them.  This is a fundamental ambiguity:
both @samp{foo + bar} and @samp{foo} are legitimate choices for the sexp to
move over if point is at the @samp{f}.  Note that @samp{(foo + bar)} is a
single sexp in C mode.

  Some languages have obscure forms of expression syntax that nobody
has bothered to make Emacs understand properly.

@node List Commands
@section List And Sexp Commands

@c doublewidecommands
@table @kbd
@item C-M-f
Move forward over a sexp (@code{forward-sexp}).
@item C-M-b
Move backward over a sexp (@code{backward-sexp}).
@item C-M-k
Kill sexp forward (@code{kill-sexp}).
@item C-M-@key{DEL}
Kill sexp backward (@code{backward-kill-sexp}).
@item C-M-u
Move up and backward in list structure (@code{backward-up-list}).
@item C-M-d
Move down and forward in list structure (@code{down-list}).
@item C-M-n
Move forward over a list (@code{forward-list}).
@item C-M-p
Move backward over a list (@code{backward-list}).
@item C-M-t
Transpose expressions (@code{transpose-sexps}).
@item C-M-@@
Put mark after following expression (@code{mark-sexp}).
@end table

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@cindex parentheses, moving across
@cindex matching parenthesis and braces, moving to
@cindex braces, moving across
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@kindex C-M-f
@kindex C-M-b
@findex forward-sexp
@findex backward-sexp
  To move forward over a sexp, use @kbd{C-M-f} (@code{forward-sexp}).  If
the first significant character after point is an opening delimiter
(@samp{(} in Lisp; @samp{(}, @samp{[} or @samp{@{} in C), @kbd{C-M-f}
moves past the matching closing delimiter.  If the character begins a
symbol, string, or number, @kbd{C-M-f} moves over that.

  The command @kbd{C-M-b} (@code{backward-sexp}) moves backward over a
sexp.  The detailed rules are like those above for @kbd{C-M-f}, but with
directions reversed.  If there are any prefix characters (single-quote,
backquote and comma, in Lisp) preceding the sexp, @kbd{C-M-b} moves back
over them as well.  The sexp commands move across comments as if they
were whitespace in most modes.

  @kbd{C-M-f} or @kbd{C-M-b} with an argument repeats that operation the
specified number of times; with a negative argument, it moves in the
opposite direction.

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@cindex deleting parenthesized expressions
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@kindex C-M-k
@findex kill-sexp
@kindex C-M-DEL
@findex backward-kill-sexp
  Killing a whole sexp can be done with @kbd{C-M-k} (@code{kill-sexp})
or @kbd{C-M-@key{DEL}} (@code{backward-kill-sexp}).  @kbd{C-M-k} kills
the characters that @kbd{C-M-f} would move over, and @kbd{C-M-@key{DEL}}
kills the characters that @kbd{C-M-b} would move over.

@kindex C-M-n
@kindex C-M-p
@findex forward-list
@findex backward-list
  The @dfn{list commands} move over lists, as the sexp commands do, but skip
blithely over any number of other kinds of sexps (symbols, strings, etc.).
They are @kbd{C-M-n} (@code{forward-list}) and @kbd{C-M-p}
(@code{backward-list}).  The main reason they are useful is that they
usually ignore comments (since the comments usually do not contain any
lists).@refill

@kindex C-M-u
@kindex C-M-d
@findex backward-up-list
@findex down-list
  @kbd{C-M-n} and @kbd{C-M-p} stay at the same level in parentheses, when
that's possible.  To move @emph{up} one (or @var{n}) levels, use @kbd{C-M-u}
(@code{backward-up-list}).
@kbd{C-M-u} moves backward up past one unmatched opening delimiter.  A
positive argument serves as a repeat count; a negative argument reverses
direction of motion and also requests repetition, so it moves forward and
up one or more levels.@refill

  To move @emph{down} in list structure, use @kbd{C-M-d}
(@code{down-list}).  In Lisp mode, where @samp{(} is the only opening
delimiter, this is nearly the same as searching for a @samp{(}.  An
argument specifies the number of levels of parentheses to go down.

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@cindex transposition of parenthesized expressions
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@kindex C-M-t
@findex transpose-sexps
  A somewhat random-sounding command which is nevertheless handy is
@kbd{C-M-t} (@code{transpose-sexps}), which drags the previous sexp
across the next one.  An argument serves as a repeat count, and a
negative argument drags backwards (thus canceling out the effect of
@kbd{C-M-t} with a positive argument).  An argument of zero, rather than
doing nothing, transposes the sexps ending after point and the mark.

@kindex C-M-@@
@findex mark-sexp
  To set the region around the next sexp in the buffer, use @kbd{C-M-@@}
(@code{mark-sexp}), which sets mark at the same place that @kbd{C-M-f}
would move to.  @kbd{C-M-@@} takes arguments like @kbd{C-M-f}.  In
particular, a negative argument is useful for putting the mark at the
beginning of the previous sexp.

  The list and sexp commands' understanding of syntax is completely
controlled by the syntax table.  Any character can, for example, be
declared to be an opening delimiter and act like an open parenthesis.
@xref{Syntax}.

@node Defuns
@section Defuns
@cindex defuns

  In Emacs, a parenthetical grouping at the top level in the buffer is
called a @dfn{defun}.  The name derives from the fact that most top-level
lists in a Lisp file are instances of the special form @code{defun}, but
any top-level parenthetical grouping counts as a defun in Emacs parlance
regardless of what its contents are, and regardless of the programming
language in use.  For example, in C, the body of a function definition is a
defun.

@c doublewidecommands
@table @kbd
@item C-M-a
Move to beginning of current or preceding defun
(@code{beginning-of-defun}).
@item C-M-e
Move to end of current or following defun (@code{end-of-defun}).
@item C-M-h
Put region around whole current or following defun (@code{mark-defun}).
@end table

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@cindex move to beginning or end of function
@cindex function, move to beginning or end
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@kindex C-M-a
@kindex C-M-e
@kindex C-M-h
@findex beginning-of-defun
@findex end-of-defun
@findex mark-defun
  The commands to move to the beginning and end of the current defun are
@kbd{C-M-a} (@code{beginning-of-defun}) and @kbd{C-M-e} (@code{end-of-defun}).

@findex c-mark-function
  If you wish to operate on the current defun, use @kbd{C-M-h}
(@code{mark-defun}) which puts point at the beginning and mark at the end
of the current or next defun.  For example, this is the easiest way to get
ready to move the defun to a different place in the text.  In C mode,
@kbd{C-M-h} runs the function @code{c-mark-function}, which is almost the
same as @code{mark-defun}; the difference is that it backs up over the
argument declarations, function name and returned data type so that the
entire C function is inside the region.  @xref{Marking Objects}.

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@cindex open-parenthesis in leftmost column
@cindex ( in leftmost column
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  Emacs assumes that any open-parenthesis found in the leftmost column
is the start of a defun.  Therefore, @strong{never put an
open-parenthesis at the left margin in a Lisp file unless it is the
start of a top-level list.  Never put an open-brace or other opening
delimiter at the beginning of a line of C code unless it starts the body
of a function.}  The most likely problem case is when you want an
opening delimiter at the start of a line inside a string.  To avoid
trouble, put an escape character (@samp{\}, in C and Emacs Lisp,
@samp{/} in some other Lisp dialects) before the opening delimiter.  It
will not affect the contents of the string.

  In the remotest past, the original Emacs found defuns by moving upward a
level of parentheses until there were no more levels to go up.  This always
required scanning all the way back to the beginning of the buffer, even for
a small function.  To speed up the operation, Emacs was changed to assume
that any @samp{(} (or other character assigned the syntactic class of
opening-delimiter) at the left margin is the start of a defun.  This
heuristic is nearly always right and avoids the costly scan; however,
it mandates the convention described above.

@node Program Indent
@section Indentation for Programs
@cindex indentation for programs

  The best way to keep a program properly indented is to use Emacs to
reindent it as you change it.  Emacs has commands to indent properly
either a single line, a specified number of lines, or all of the lines
inside a single parenthetical grouping.

@menu
* Basic Indent::	Indenting a single line.
* Multi-line Indent::   Commands to reindent many lines at once.
* Lisp Indent::		Specifying how each Lisp function should be indented.
* C Indent::		Extra features for indenting C and related modes.
* Custom C Indent::	Controlling indentation style for C and related modes.
@end menu

  Emacs also provides a Lisp pretty-printer in the library @code{pp}.
This program reformats a Lisp object with indentation chosen to look nice.

@node Basic Indent
@subsection Basic Program Indentation Commands

@c WideCommands
@table @kbd
@item @key{TAB}
Adjust indentation of current line.
@item C-j
Equivalent to @key{RET} followed by @key{TAB} (@code{newline-and-indent}).
@end table

@kindex TAB @r{(programming modes)}
@findex c-indent-line
@findex lisp-indent-line
  The basic indentation command is @key{TAB}, which gives the current line
the correct indentation as determined from the previous lines.  The
function that @key{TAB} runs depends on the major mode; it is @code{lisp-indent-line}
in Lisp mode, @code{c-indent-line} in C mode, etc.  These functions
understand different syntaxes for different languages, but they all do
about the same thing.  @key{TAB} in any programming-language major mode
inserts or deletes whitespace at the beginning of the current line,
independent of where point is in the line.  If point is inside the
whitespace at the beginning of the line, @key{TAB} leaves it at the end of
that whitespace; otherwise, @key{TAB} leaves point fixed with respect to
the characters around it.

  Use @kbd{C-q @key{TAB}} to insert a tab at point.

@kindex C-j
@findex newline-and-indent
  When entering lines of new code, use @kbd{C-j} (@code{newline-and-indent}),
which is equivalent to a @key{RET} followed by a @key{TAB}.  @kbd{C-j} creates
a blank line and then gives it the appropriate indentation.

  @key{TAB} indents the second and following lines of the body of a
parenthetical grouping each under the preceding one; therefore, if you
alter one line's indentation to be nonstandard, the lines below will
tend to follow it.  This behavior is convenient in cases where you have
overridden the standard result of @key{TAB} because you find it
unaesthetic for a particular line.

  Remember that an open-parenthesis, open-brace or other opening delimiter
at the left margin is assumed by Emacs (including the indentation routines)
to be the start of a function.  Therefore, you must never have an opening
delimiter in column zero that is not the beginning of a function, not even
inside a string.  This restriction is vital for making the indentation
commands fast; you must simply accept it.  @xref{Defuns}, for more
information on this.

@node Multi-line Indent
@subsection Indenting Several Lines

  When you wish to reindent several lines of code which have been altered
or moved to a different level in the list structure, you have several
commands available.

@table @kbd
@item C-M-q
Reindent all the lines within one list (@code{indent-sexp}).
@item C-u @key{TAB}
Shift an entire list rigidly sideways so that its first line
is properly indented.
@item C-M-\
Reindent all lines in the region (@code{indent-region}).
@end table

@kindex C-M-q
@findex indent-sexp
  You can reindent the contents of a single list by positioning point
before the beginning of it and typing @kbd{C-M-q} (@code{indent-sexp} in
Lisp mode, @code{c-indent-exp} in C mode; also bound to other suitable
commands in other modes).  The indentation of the line the sexp starts on
is not changed; therefore, only the relative indentation within the list,
and not its position, is changed.  To correct the position as well, type a
@key{TAB} before the @kbd{C-M-q}.

@kindex C-u TAB
  If the relative indentation within a list is correct but the
indentation of its first line is not, go to that line and type @kbd{C-u
@key{TAB}}.  @key{TAB} with a numeric argument reindents the current
line as usual, then reindents by the same amount all the lines in the
grouping starting on the current line.  In other words, it reindents the
whole grouping rigidly as a unit.  It is clever, though, and does not
alter lines that start inside strings, or C preprocessor lines when in C
mode.

  Another way to specify the range to be reindented is with the region.
The command @kbd{C-M-\} (@code{indent-region}) applies @key{TAB} to
every line whose first character is between point and mark.

@node Lisp Indent
@subsection Customizing Lisp Indentation
@cindex customizing Lisp indentation

  The indentation pattern for a Lisp expression can depend on the function
called by the expression.  For each Lisp function, you can choose among
several predefined patterns of indentation, or define an arbitrary one with
a Lisp program.

  The standard pattern of indentation is as follows: the second line of the
expression is indented under the first argument, if that is on the same
line as the beginning of the expression; otherwise, the second line is
indented underneath the function name.  Each following line is indented
under the previous line whose nesting depth is the same.

@vindex lisp-indent-offset
  If the variable @code{lisp-indent-offset} is non-@code{nil}, it overrides
the usual indentation pattern for the second line of an expression, so that
such lines are always indented @code{lisp-indent-offset} more columns than
the containing list.

@vindex lisp-body-indent
  The standard pattern is overridden for certain functions.  Functions
whose names start with @code{def} always indent the second line by
@code{lisp-body-indent} extra columns beyond the open-parenthesis
starting the expression.

  The standard pattern can be overridden in various ways for individual
functions, according to the @code{lisp-indent-function} property of the
function name.  There are four possibilities for this property:

@table @asis
@item @code{nil}
This is the same as no property; the standard indentation pattern is used.
@item @code{defun}
The pattern used for function names that start with @code{def} is used for
this function also.
@item a number, @var{number}
The first @var{number} arguments of the function are
@dfn{distinguished} arguments; the rest are considered the @dfn{body}
of the expression.  A line in the expression is indented according to
whether the first argument on it is distinguished or not.  If the
argument is part of the body, the line is indented @code{lisp-body-indent}
more columns than the open-parenthesis starting the containing
expression.  If the argument is distinguished and is either the first
or second argument, it is indented @emph{twice} that many extra columns.
If the argument is distinguished and not the first or second argument,
the standard pattern is followed for that line.
@item a symbol, @var{symbol}
@var{symbol} should be a function name; that function is called to
calculate the indentation of a line within this expression.  The
function receives two arguments:
@table @asis
@item @var{state}
The value returned by @code{parse-partial-sexp} (a Lisp primitive for
indentation and nesting computation) when it parses up to the
beginning of this line.
@item @var{pos}
The position at which the line being indented begins.
@end table
@noindent
It should return either a number, which is the number of columns of
indentation for that line, or a list whose car is such a number.  The
difference between returning a number and returning a list is that a
number says that all following lines at the same nesting level should
be indented just like this one; a list says that following lines might
call for different indentations.  This makes a difference when the
indentation is being computed by @kbd{C-M-q}; if the value is a
number, @kbd{C-M-q} need not recalculate indentation for the following
lines until the end of the list.
@end table

@node C Indent
@subsection Commands for C Indentation

  Here are the commands for indentation in C mode and related modes:

@table @code
@item C-c C-q
@kindex C-c C-q @r{(C mode)}
@findex c-indent-defun
Reindent the current top-level function definition or aggregate type
declaration (@code{c-indent-defun}).

@item C-M-q
@kindex C-M-q @r{(C mode)}
@findex c-indent-exp
Reindent each line in the balanced expression that follows point
(@code{c-indent-exp}).  A prefix argument inhibits error checking and
warning messages about invalid syntax.

@item @key{TAB}
@findex c-indent-command
Reindent the current line, and/or in some cases insert a tab character
(@code{c-indent-command}).

If @code{c-tab-always-indent} is @code{t}, this command always reindents
the current line and does nothing else.  This is the default.

If that variable is @code{nil}, this command reindents the current line
only if point is at the left margin or in the line's indentation;
otherwise, it inserts a tab (or the equivalent number of spaces,
if @code{indent-tabs-mode} is @code{nil}).

Any other value (not @code{nil} or @code{t}) means always reindent the
line, and also insert a tab if within a comment, a string, or a
preprocessor directive.

@item C-u @key{TAB}
Reindent the current line according to its syntax; also rigidly reindent
any other lines of the expression that starts on the current line.
@xref{Multi-line Indent}.
@end table

  To reindent the whole current buffer, type @kbd{C-x h C-M-\}.  This
first selects the whole buffer as the region, then reindents that
region.

  To reindent the current block, use @kbd{C-M-u C-M-q}.  This moves
to the front of the block and then reindents it all.

@node Custom C Indent
@subsection Customizing C Indentation

  C mode and related modes use a simple yet flexible mechanism for
customizing indentation.  The mechanism works in two steps: first it
classifies the line syntactically according to its contents and context;
second, it associates each kind of syntactic construct with an
indentation offset which you can customize.

@menu
* Syntactic Analysis::
* Indentation Calculation::
* Changing Indent Style::
* Syntactic Symbols::
* Variables for C Indent::
* C Indent Styles::
@end menu

@node Syntactic Analysis
@subsubsection Step 1---Syntactic Analysis
@cindex syntactic analysis

  In the first step, the C indentation mechanism looks at the line
before the one you are currently indenting and determines the syntactic
components of the construct on that line.  It builds a list of these
syntactic components, each of which contains a @dfn{syntactic symbol}
and sometimes also a buffer position.  Some syntactic symbols describe
grammatical elements, for example @code{statement} and
@code{substatement}; others describe locations amidst grammatical
elements, for example @code{class-open} and @code{knr-argdecl}.

  Conceptually, a line of C code is always indented relative to the
indentation of some line higher up in the buffer.  This is represented
by the buffer positions in the syntactic component list.

  Here is an example.  Suppose we have the following code in a C++ mode
buffer (the line numbers don't actually appear in the buffer):

@example
1: void swap (int& a, int& b)
2: @{
3:   int tmp = a;
4:   a = b;
5:   b = tmp;
6: @}
@end example

  If you type @kbd{C-c C-s} (which runs the command
@code{c-show-syntactic-information}) on line 4, it shows the result of
the indentation mechanism for that line:

@example
((statement . 32))
@end example

  This indicates that the line is a statement and it is indented
relative to buffer position 32, which happens to be the @samp{i} in
@code{int} on line 3.  If you move the cursor to line 3 and type
@kbd{C-c C-s}, it displays this:

@example
((defun-block-intro . 28))
@end example

  This indicates that the @code{int} line is the first statement in a
block, and is indented relative to buffer position 28, which is the
brace just after the function header.

@noindent
Here is another example:

@example
1: int add (int val, int incr, int doit)
2: @{
3:   if (doit)
4:     @{
5:       return (val + incr);
6:     @}
7:   return (val);
8: @}
@end example

@noindent
Typing @kbd{C-c C-s} on line 4 displays this:

@example
((substatement-open . 43))
@end example

  This says that the brace @emph{opens} a substatement block.  By the
way, a @dfn{substatement} indicates the line after an @code{if},
@code{else}, @code{while}, @code{do}, @code{switch}, @code{for},
@code{try}, @code{catch}, @code{finally}, or @code{synchronized}
statement.

@cindex syntactic component
@cindex syntactic symbol
@vindex c-syntactic-context
  Within the C indentation commands, after a line has been analyzed
syntactically for indentation, the variable @code{c-syntactic-context}
contains a list that describes the results.  Each element in this list
is a @dfn{syntactic component}: a cons cell containing a syntactic
symbol and (optionally) its corresponding buffer position.  There may be
several elements in a component list; typically only one element has a
buffer position.

@node Indentation Calculation
@subsubsection  Step 2---Indentation Calculation
@cindex Indentation Calculation

  The C indentation mechanism calculates the indentation for the current
line using the list of syntactic components, @code{c-syntactic-context},
derived from syntactic analysis.  Each component is a cons cell that
contains a syntactic symbol and may also contain a buffer position.

  Each component contributes to the final total indentation of the line
in two ways.  First, the syntactic symbol identifies an element of
@code{c-offsets-alist}, which is an association list mapping syntactic
symbols into indentation offsets.  Each syntactic symbol's offset adds
to the total indentation.  Second, if the component includes a buffer
position, the column number of that position adds to the indentation.
All these offsets and column numbers, added together, give the total
indentation.

  The following examples demonstrate the workings of the C indentation
mechanism:

@example
1: void swap (int& a, int& b)
2: @{
3:   int tmp = a;
4:   a = b;
5:   b = tmp;
6: @}
@end example

  Suppose that point is on line 3 and you type @key{TAB} to reindent the
line.  As explained above (@pxref{Syntactic Analysis}), the syntactic
component list for that line is:

@example
((defun-block-intro . 28))
@end example

  In this case, the indentation calculation first looks up
@code{defun-block-intro} in the @code{c-offsets-alist} alist.  Suppose
that it finds the integer 2; it adds this to the running total
(initialized to zero), yielding a updated total indentation of 2 spaces.

  The next step is to find the column number of buffer position 28.
Since the brace at buffer position 28 is in column zero, this adds 0 to
the running total.  Since this line has only one syntactic component,
the total indentation for the line is 2 spaces.

@example
1: int add (int val, int incr, int doit)
2: @{
3:   if (doit)
4:     @{
5:       return(val + incr);
6:     @}
7:   return(val);
8: @}
@end example

  If you type @key{TAB} on line 4, the same process is performed, but
with different data.  The syntactic component list for this line is:

@example
((substatement-open . 43))
@end example

   Here, the indentation calculation's first job is to look up the
symbol @code{substatement-open} in @code{c-offsets-alist}.  Let's assume
that the offset for this symbol is 2.  At this point the running total
is 2 (0 + 2 = 2).  Then it adds the column number of buffer position 43,
which is the @samp{i} in @code{if} on line 3.  This character is in
column 2 on that line.  Adding this yields a total indentation of 4
spaces.

@vindex c-strict-syntax-p
   If a syntactic symbol in the analysis of a line does not appear in
@code{c-offsets-alist}, it is ignored; if in addition the variable
@code{c-strict-syntax-p} is non-@code{nil}, it is an error.

@node Changing Indent Style
@subsubsection Changing Indentation Style

   There are two ways to customize the indentation style for the C-like
modes.  First, you can select one of several predefined styles, each of
which specifies offsets for all the syntactic symbols.  For more
flexibility, you can customize the handling of individual syntactic
symbols.  @xref{Syntactic Symbols}, for a list of all defined syntactic
symbols.

@table @kbd
@item M-x c-set-style @key{RET} @var{style} @key{RET}
Select predefined indentation style @var{style}.  Type @kbd{?} when
entering @var{style} to see a list of supported styles; to find out what
a style looks like, select it and reindent some C code.

@item C-c C-o @var{symbol} @key{RET} @var{offset} @key{RET}
Set the indentation offset for syntactic symbol @var{symbol}
(@code{c-set-offset}).  The second argument @var{offset} specifies the
new indentation offset.
@end table

   The @code{c-offsets-alist} variable controls the amount of
indentation to give to each syntactic symbol.  Its value is an
association list, and each element of the list has the form
@code{(@var{syntactic-symbol} . @var{offset})}.  By changing the offsets
for various syntactic symbols, you can customize indentation in fine
detail.  To change this alist, use @code{c-set-offset} (see below).

   Each offset value in @code{c-offsets-alist} can be an integer, a
function or variable name, a list, or one of the following symbols: @code{+},
@code{-}, @code{++}, @code{--}, @code{*}, or @code{/}, indicating positive or negative
multiples of the variable @code{c-basic-offset}.  Thus, if you want to
change the levels of indentation to be 3 spaces instead of 2 spaces, set
@code{c-basic-offset} to 3.

   Using a function as the offset value provides the ultimate flexibility
in customizing indentation.  The function is called with a single
argument containing the @code{cons} of the syntactic symbol and
the buffer position, if any.  The function should return an integer
offset.
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   If the offset value is a list, its elements are processed according
to the rules above until a non-@code{nil} value is found.  That value is
then added to the total indentation in the normal manner.  The primary
use for this is to combine the results of several functions.

@kindex C-c C-o @r{(C mode)}
@findex c-set-offset
   The command @kbd{C-c C-o} (@code{c-set-offset}) is the easiest way to
set offsets, both interactively or in your @file{~/.emacs} file.  First
specify the syntactic symbol, then the offset you want.  @xref{Syntactic
Symbols}, for a list of valid syntactic symbols and their meanings.

@node Syntactic Symbols
@subsubsection Syntactic Symbols

   Here is a table of valid syntactic symbols for indentation in C and
related modes, with their syntactic meanings.  Normally, most of these
symbols are assigned offsets in @code{c-offsets-alist}.

@table @code
@item string
Inside a multi-line string.

@item c
Inside a multi-line C style block comment.

@item defun-open
On a brace that opens a function definition.

@item defun-close
On a brace that closes a function definition.

@item defun-block-intro
In the first line in a top-level defun.

@item class-open
On a brace that opens a class definition.

@item class-close
On a brace that closes a class definition.

@item inline-open
On a brace that opens an in-class inline method.

@item inline-close
On a brace that closes an in-class inline method.

@item extern-lang-open
On a brace that opens an external language block.

@item extern-lang-close
On a brace that closes an external language block.

@item func-decl-cont
The region between a function definition's argument list and the defun
opening brace (excluding K&R function definitions).  In C, you cannot
put anything but whitespace and comments between them; in C++ and Java,
@code{throws} declarations and other things can appear in this context.

@item knr-argdecl-intro
On the first line of a K&R C argument declaration.

@item knr-argdecl
In one of the subsequent lines in a K&R C argument declaration.

@item topmost-intro
On the first line in a topmost construct definition.

@item topmost-intro-cont
On the topmost definition continuation lines.

@item member-init-intro
On the first line in a member initialization list.

@item member-init-cont
On one of the subsequent member initialization list lines.

@item inher-intro
On the first line of a multiple inheritance list.

@item inher-cont
On one of the subsequent multiple inheritance lines.

@item block-open
On a statement block open brace.

@item block-close
On a statement block close brace.

@item brace-list-open
On the opening brace of an @code{enum} or @code{static} array list.

@item brace-list-close
On the closing brace of an @code{enum} or @code{static} array list.

@item brace-list-intro
On the first line in an @code{enum} or @code{static} array list.

@item brace-list-entry
On one of the subsequent lines in an @code{enum} or @code{static} array
list.

@item brace-entry-open
On one of the subsequent lines in an @code{enum} or @code{static} array
list, when the line begins with an open brace.

@item statement
On an ordinary statement.

@item statement-cont
On a continuation line of a statement.

@item statement-block-intro
On the first line in a new statement block.

@item statement-case-intro
On the first line in a @code{case} ``block.''

@item statement-case-open
On the first line in a @code{case} block starting with brace.

@item inexpr-statement
On a statement block inside an expression.  This is used for a GNU
extension to the C language, and for Pike special functions that take a
statement block as an argument.

@item inexpr-class
On a class definition inside an expression.  This is used for anonymous
classes and anonymous array initializers in Java.

@item substatement
On the first line after an @code{if}, @code{while}, @code{for},
@code{do}, or @code{else}.

@item substatement-open
On the brace that opens a substatement block.

@item case-label
On a @code{case} or @code{default} label.

@item access-label
On a C++ @code{private}, @code{protected}, or @code{public} access label.

@item label
On any ordinary label.

@item do-while-closure
On the @code{while} that ends a @code{do}-@code{while} construct.

@item else-clause
On the @code{else} of an @code{if}-@code{else} construct.

@item catch-clause
On the @code{catch} and @code{finally} lines in
@code{try}@dots{}@code{catch} constructs in C++ and Java.

@item comment-intro
On a line containing only a comment introduction.

@item arglist-intro
On the first line in an argument list.

@item arglist-cont
On one of the subsequent argument list lines when no arguments follow on
the same line as the arglist opening parenthesis.

@item arglist-cont-nonempty
On one of the subsequent argument list lines when at least one argument
follows on the same line as the arglist opening parenthesis.

@item arglist-close
On the closing parenthesis of an argument list.

@item stream-op
On one of the lines continuing a stream operator construct.

@item inclass
On a construct that is nested inside a class definition.  The
indentation is relative to the open brace of the class definition.

@item inextern-lang
On a construct that is nested inside an external language block.

@item inexpr-statement
On the first line of statement block inside an expression.  This is used
for the GCC extension to C that uses the syntax @code{(@{ @dots{} @})}.
It is also used for the special functions that takes a statement block
as an argument in Pike.

@item inexpr-class
On the first line of a class definition inside an expression.  This is
used for anonymous classes and anonymous array initializers in Java.

@item cpp-macro
On the start of a cpp macro.

@item friend
On a C++ @code{friend} declaration.

@item objc-method-intro
On the first line of an Objective-C method definition.

@item objc-method-args-cont
On one of the lines continuing an Objective-C method definition.

@item objc-method-call-cont
On one of the lines continuing an Objective-C method call.

@item inlambda
Like @code{inclass}, but used inside lambda (i.e. anonymous) functions.  Only
used in Pike.

@item lambda-intro-cont
On a line continuing the header of a lambda function, between the
@code{lambda} keyword and the function body.  Only used in Pike.
@end table

@node Variables for C Indent
@subsubsection Variables for C Indentation

  This section describes additional variables which control the
indentation behavior of C mode and related mode.

@table @code
@item c-offsets-alist
@vindex c-offsets-alist
Association list of syntactic symbols and their indentation offsets.
You should not set this directly, only with @code{c-set-offset}.
@xref{Changing Indent Style}, for details.

@item c-style-alist
@vindex c-style-alist
Variable for defining indentation styles; see below.

@item c-basic-offset
@vindex c-basic-offset
Amount of basic offset used by @code{+} and @code{-} symbols in
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@item c-special-indent-hook
@vindex c-special-indent-hook
Hook for user-defined special indentation adjustments.  This hook is
called after a line is indented by C mode and related modes.
@end table

  The variable @code{c-style-alist} specifies the predefined indentation
styles.  Each element has form @code{(@var{name}
@var{variable-setting}@dots{})}, where @var{name} is the name of the
style.  Each @var{variable-setting} has the form @code{(@var{variable}
. @var{value})}; @var{variable} is one of the customization variables
used by C mode, and @var{value} is the value for that variable when
using the selected style.

  When @var{variable} is @code{c-offsets-alist}, that is a special case:
@var{value} is appended to the front of the value of @code{c-offsets-alist}
instead of replacing that value outright.  Therefore, it is not necessary
for @var{value} to specify each and every syntactic symbol---only those
for which the style differs from the default.

  The indentation of lines containing only comments is also affected by
the variable @code{c-comment-only-line-offset} (@pxref{Comments in C}).

@node C Indent Styles
@subsubsection C Indentation Styles
@cindex c indentation styles

  A @dfn{C style} is a collection of indentation style customizations.
Emacs comes with several predefined indentation styles for C and related
modes, including @code{gnu}, @code{k&r}, @code{bsd}, @code{stroustrup},
@code{linux}, @code{python}, @code{java}, @code{whitesmith},
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@code{ellemtel}, @code{cc-mode}, and @code{user}.
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@findex c-set-style
@vindex c-default-style
  To choose the style you want, use the command @kbd{M-x c-set-style}.
Specify a style name as an argument (case is not significant in C style
names).  The chosen style only affects newly visited buffers, not those
you are already editing.  You can also set the variable
@code{c-default-style} to specify the style for various major modes.
Its value should be an alist, in which each element specifies one major
mode and which indentation style to use for it.  For example,

@example
(setq c-default-style
      '((java-mode . "java") (other . "gnu")))
@end example

@noindent
specifies an explicit choice for Java mode, and the default @samp{gnu}
style for the other C-like modes.

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  The style @code{gnu} defines the formatting recommend by the GNU
Project; it is the default, so as to encourage the indentation we
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recommend. If you make changes in variables such as
@code{c-basic-offset} and @code{c-offsets-alist} in your @file{~/.emacs}
file, they will however take precedence.
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@findex c-add-style
  To define a new C indentation style, call the function
@code{c-add-style}:

@example
(c-add-style @var{name} @var{values} @var{use-now})
@end example

@noindent
Here @var{name} is the name of the new style (a string), and
@var{values} is an alist whose elements have the form
@code{(@var{variable} . @var{value})}.  The variables you specify should
be among those documented in @ref{Variables for C Indent}.

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  If @var{use-now} is non-@code{nil}, @code{c-add-style} selects the new
style after defining it.
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@node Matching
@section Automatic Display Of Matching Parentheses
@cindex matching parentheses
@cindex parentheses, displaying matches

  The Emacs parenthesis-matching feature is designed to show
automatically how parentheses match in the text.  Whenever you type a
self-inserting character that is a closing delimiter, the cursor moves
momentarily to the location of the matching opening delimiter, provided
that is on the screen.  If it is not on the screen, some text near it is
displayed in the echo area.  Either way, you can tell what grouping is
being closed off.

  In Lisp, automatic matching applies only to parentheses.  In C, it
applies to braces and brackets too.  Emacs knows which characters to regard
as matching delimiters based on the syntax table, which is set by the major
mode.  @xref{Syntax}.

  If the opening delimiter and closing delimiter are mismatched---such as
in @samp{[x)}---a warning message is displayed in the echo area.  The
correct matches are specified in the syntax table.

@vindex blink-matching-paren
@vindex blink-matching-paren-distance
@vindex blink-matching-delay
  Three variables control parenthesis match display.
@code{blink-matching-paren} turns the feature on or off; @code{nil}
turns it off, but the default is @code{t} to turn match display on.
@code{blink-matching-delay} says how many seconds to wait; the default
is 1, but on some systems it is useful to specify a fraction of a
second.  @code{blink-matching-paren-distance} specifies how many
characters back to search to find the matching opening delimiter.  If
the match is not found in that far, scanning stops, and nothing is
displayed.  This is to prevent scanning for the matching delimiter from
wasting lots of time when there is no match.  The default is 12,000.

@cindex Show Paren mode
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@cindex highlighting matching parentheses
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@findex show-paren-mode
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  You can also request a more powerful alternative kind of automatic
parenthesis matching by enabling Show Paren mode.  This mode turns off
the usual kind of matching parenthesis display and instead uses
highlighting to show what matches.  Whenever point is after a close
parenthesis, the close parenthesis and its matching open parenthesis are
both highlighted; otherwise, if point is before an open parenthesis, the
matching close parenthesis is highlighted.  (There is no need to
highlight the open parenthesis after point because the cursor appears on
top of that character.)  Use the command @kbd{M-x show-paren-mode} to
enable or disable this mode.

  By default, @code{show-paren-mode} uses colors to highlight the
parentheses.  However, if your display doesn't support colors, you can
customize the faces @code{show-paren-match-face} and
@code{show-paren-mismatch-face} to use other attributes, such as bold or
underline.  @xref{Face Customization}.
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@node Comments
@section Manipulating Comments
@cindex comments

  Because comments are such an important part of programming, Emacs
provides special commands for editing and inserting comments.

@menu
* Comment Commands::
* Multi-Line Comments::
* Options for Comments::
@end menu

@node Comment Commands
@subsection Comment Commands

@kindex M-;
@cindex indentation for comments
@findex indent-for-comment
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@findex comment-dwim
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  The comment commands insert, kill and align comments.

@c WideCommands
@table @kbd
@item M-;
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Call the comment command that is appropriate for the context
(@code{comment-dwim}).
@item M-x indent-for-comment
Insert or align comment.
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@item C-x ;
Set comment column (@code{set-comment-column}).
@item C-u - C-x ;
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Kill comment on current line (@code{comment-kill}).
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@item C-M-j
Like @key{RET} followed by inserting and aligning a comment
(@code{indent-new-comment-line}).
@item M-x comment-region
Add or remove comment delimiters on all the lines in the region.
@end table

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  The command that creates a comment is @kbd{M-x indent-for-comment}.
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If there is no comment already on the line, a new comment is created,
aligned at a specific column called the @dfn{comment column}.  The comment
is created by inserting the string Emacs thinks comments should start with
(the value of @code{comment-start}; see below).  Point is left after that
string.  If the text of the line extends past the comment column, then the
indentation is done to a suitable boundary (usually, at least one space is
inserted).  If the major mode has specified a string to terminate comments,
that is inserted after point, to keep the syntax valid.

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  @kbd{M-x indent-for-comment} can also be used to align an existing
comment.  If a line already contains the string that starts comments,
then @kbd{M-x indent-for-comment} just moves point after it and
reindents it to the conventional place.  Exception: comments starting in
column 0 are not moved.

  @kbd{M-;} (@code{comment-dwim}) conveniently combines
@code{indent-for-comment} with @code{comment-region} and
@code{uncomment-region}, described below in @ref{Multi-Line Comments},
as appropriate for the current context.  If the region is active and the
Transient Mark mode is on (@pxref{Transient Mark}), @kbd{M-;} invokes
@code{comment-region}, unless the region consists only of comments, in
which case it invokes @code{uncomment-region}.  Otherwise, if the
current line is empty, @kbd{M-;} inserts a comment and indents it.  If
the current line is not empty, @kbd{M-;} invokes @code{comment-kill} if
a numeric argument was given, else it reindents the comment on the
current line.  (The @dfn{dwim} in @code{comment-dwim} is an acronym for
``Do What I Mean''.)
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  Some major modes have special rules for indenting certain kinds of
comments in certain contexts.  For example, in Lisp code, comments which
start with two semicolons are indented as if they were lines of code,
instead of at the comment column.  Comments which start with three
semicolons are supposed to start at the left margin.  Emacs understands
these conventions by indenting a double-semicolon comment using @key{TAB},
and by not changing the indentation of a triple-semicolon comment at all.

@example
;; This function is just an example
;;; Here either two or three semicolons are appropriate.
(defun foo (x)
;;; And now, the first part of the function:
  ;; The following line adds one.
  (1+ x))           ; This line adds one.
@end example

  In C code, a comment preceded on its line by nothing but whitespace
is indented like a line of code.

  Even when an existing comment is properly aligned, @kbd{M-;} is still
useful for moving directly to the start of the comment.

@kindex C-u - C-x ;
@findex kill-comment
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@findex comment-kill
  @kbd{C-u - C-x ;} (@code{comment-kill}) kills the comment on the current line,
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if there is one.  The indentation before the start of the comment is killed
as well.  If there does not appear to be a comment in the line, nothing is
done.  To reinsert the comment on another line, move to the end of that
line, do @kbd{C-y}, and then do @kbd{M-;} to realign it.  Note that
@kbd{C-u - C-x ;} is not a distinct key; it is @kbd{C-x ;} (@code{set-comment-column})
with a negative argument.  That command is programmed so that when it
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receives a negative argument it calls @code{comment-kill}.  However,
@code{comment-kill} is a valid command which you could bind directly to a
key if you wanted to.  (For compatibility with previous versions,
@code{kill-comment} is provided as an alias to @code{comment-kill}.)
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@node Multi-Line Comments
@subsection Multiple Lines of Comments

@kindex C-M-j
@cindex blank lines in programs
@findex indent-new-comment-line
  If you are typing a comment and wish to continue it on another line,
you can use the command @kbd{C-M-j} (@code{indent-new-comment-line}).
This terminates the comment you are typing, creates a new blank line
afterward, and begins a new comment indented under the old one.  When
Auto Fill mode is on, going past the fill column while typing a comment
causes the comment to be continued in just this fashion.  If point is
not at the end of the line when @kbd{C-M-j} is typed, the text on
the rest of the line becomes part of the new comment line.

@findex comment-region
  To turn existing lines into comment lines, use the @kbd{M-x
comment-region} command.  It adds comment delimiters to the lines that start
in the region, thus commenting them out.  With a negative argument, it
does the opposite---it deletes comment delimiters from the lines in the
region.

  With a positive argument, @code{comment-region} duplicates the last
character of the comment start sequence it adds; the argument specifies
how many copies of the character to insert.  Thus, in Lisp mode,
@kbd{C-u 2 M-x comment-region} adds @samp{;;} to each line.  Duplicating
the comment delimiter is a way of calling attention to the comment.  It
can also affect how the comment is indented.  In Lisp, for proper
indentation, you should use an argument of two, if between defuns, and
three, if within a defun.

@vindex comment-padding
  The variable @code{comment-padding} specifies how many spaces
@code{comment-region} should insert on each line between the
comment delimiter and the line's original text.  The default is 1.

@node Options for Comments
@subsection Options Controlling Comments

@vindex comment-column
@kindex C-x ;
@findex set-comment-column
  The comment column is stored in the variable @code{comment-column}.  You
can set it to a number explicitly.  Alternatively, the command @kbd{C-x ;}
(@code{set-comment-column}) sets the comment column to the column point is
at.  @kbd{C-u C-x ;} sets the comment column to match the last comment
before point in the buffer, and then does a @kbd{M-;} to align the
current line's comment under the previous one.  Note that @kbd{C-u - C-x ;}
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runs the function @code{comment-kill} as described above.
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  The variable @code{comment-column} is per-buffer: setting the variable
in the normal fashion affects only the current buffer, but there is a
default value which you can change with @code{setq-default}.
@xref{Locals}.  Many major modes initialize this variable for the
current buffer.

@vindex comment-start-skip
  The comment commands recognize comments based on the regular
expression that is the value of the variable @code{comment-start-skip}.
Make sure this regexp does not match the null string.  It may match more
than the comment starting delimiter in the strictest sense of the word;
for example, in C mode the value of the variable is @code{@t{"/\\*+
*"}}, which matches extra stars and spaces after the @samp{/*} itself.
(Note that @samp{\\} is needed in Lisp syntax to include a @samp{\} in
the string, which is needed to deny the first star its special meaning
in regexp syntax.  @xref{Regexps}.)

@vindex comment-start
@vindex comment-end
  When a comment command makes a new comment, it inserts the value of
@code{comment-start} to begin it.  The value of @code{comment-end} is
inserted after point, so that it will follow the text that you will insert
into the comment.  In C mode, @code{comment-start} has the value
@w{@code{"/* "}} and @code{comment-end} has the value @w{@code{" */"}}.

@vindex comment-multi-line
  The variable @code{comment-multi-line} controls how @kbd{C-M-j}
(@code{indent-new-comment-line}) behaves when used inside a comment.  If
@code{comment-multi-line} is @code{nil}, as it normally is, then the
comment on the starting line is terminated and a new comment is started
on the new following line.  If @code{comment-multi-line} is not
@code{nil}, then the new following line is set up as part of the same
comment that was found on the starting line.  This is done by not
inserting a terminator on the old line, and not inserting a starter on
the new line.  In languages where multi-line comments work, the choice
of value for this variable is a matter of taste.

@vindex comment-indent-function
  The variable @code{comment-indent-function} should contain a function
that will be called to compute the indentation for a newly inserted
comment or for aligning an existing comment.  It is set differently by
various major modes.  The function is called with no arguments, but with
point at the beginning of the comment, or at the end of a line if a new
comment is to be inserted.  It should return the column in which the
comment ought to start.  For example, in Lisp mode, the indent hook
function bases its decision on how many semicolons begin an existing
comment, and on the code in the preceding lines.

@node Balanced Editing
@section Editing Without Unbalanced Parentheses

@table @kbd
@item M-(
Put parentheses around next sexp(s) (@code{insert-parentheses}).
@item M-)
Move past next close parenthesis and reindent
(@code{move-past-close-and-reindent}).
@end table

@kindex M-(
@kindex M-)
@findex insert-parentheses
@findex move-past-close-and-reindent
  The commands @kbd{M-(} (@code{insert-parentheses}) and @kbd{M-)}
(@code{move-past-close-and-reindent}) are designed to facilitate a style
of editing which keeps parentheses balanced at all times.  @kbd{M-(}
inserts a pair of parentheses, either together as in @samp{()}, or, if
given an argument, around the next several sexps.  It leaves point after
the open parenthesis.  The command @kbd{M-)} moves past the close
parenthesis, deleting any indentation preceding it, and indenting with
@kbd{C-j} after it.

  For example, instead of typing @kbd{( F O O )}, you can type @kbd{M-(
F O O}, which has the same effect except for leaving the cursor before
the close parenthesis.

@vindex parens-require-spaces
  @kbd{M-(} may insert a space before the open parenthesis, depending on
the syntax class of the preceding character.  Set
@code{parens-require-spaces} to @code{nil} value if you wish to inhibit
this.

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@findex check-parens
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@cindex unbalanced parentheses and quotes
You can use @kbd{M-x check-parens} to find any unbalanced parentheses
and unbalanced quotes in strings in a buffer.
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@node Symbol Completion
@section Completion for Symbol Names
@cindex completion (symbol names)

  Usually completion happens in the minibuffer.  But one kind of completion
is available in all buffers: completion for symbol names.

@kindex M-TAB
  The character @kbd{M-@key{TAB}} runs a command to complete the partial
symbol before point against the set of meaningful symbol names.  Any
additional characters determined by the partial name are inserted at
point.

  If the partial name in the buffer has more than one possible completion
and they have no additional characters in common, a list of all possible
completions is displayed in another window.

@cindex completion using tags
@cindex tags completion
@cindex Info index completion
@findex complete-symbol
  In most programming language major modes, @kbd{M-@key{TAB}} runs the
command @code{complete-symbol}, which provides two kinds of completion.
Normally it does completion based on a tags table (@pxref{Tags}); with a
numeric argument (regardless of the value), it does completion based on
the names listed in the Info file indexes for your language.  Thus, to
complete the name of a symbol defined in your own program, use
@kbd{M-@key{TAB}} with no argument; to complete the name of a standard
library function, use @kbd{C-u M-@key{TAB}}.  Of course, Info-based
completion works only if there is an Info file for the standard library
functions of your language, and only if it is installed at your site.

@cindex Lisp symbol completion
@cindex completion in Lisp
@findex lisp-complete-symbol
  In Emacs-Lisp mode, the name space for completion normally consists of
nontrivial symbols present in Emacs---those that have function
definitions, values or properties.  However, if there is an
open-parenthesis immediately before the beginning of the partial symbol,
only symbols with function definitions are considered as completions.
The command which implements this is @code{lisp-complete-symbol}.

  In Text mode and related modes, @kbd{M-@key{TAB}} completes words
based on the spell-checker's dictionary.  @xref{Spelling}.

@node Which Function
@section Which Function Mode

  Which Function mode is a minor mode that displays the current function
name in the mode line, as you move around in a buffer.

@findex which-function-mode
@vindex which-func-modes
  To enable (or disable) Which Function mode, use the command @kbd{M-x
which-function-mode}.  This command is global; it applies to all
buffers, both existing ones and those yet to be created.  However, this
only affects certain major modes, those listed in the value of
@code{which-func-modes}.  (If the value is @code{t}, then Which Function
mode applies to all major modes that know how to support it---which are
the major modes that support Imenu.)

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@node Hideshow
@section Hideshow minor mode

@findex hs-minor-mode
Hideshow minor mode provides selective display of blocks.  Use @kbd{M-x
hs-minor-mode} to toggle the mode or add @code{hs-minor-mode} to the
hook for major modes with which you want to use it and which support it.

Blocks are defined dependent on the mode.  In C mode or C++ mode, they
are delimited by braces, while in Lisp-ish modes they are delimited by
parens.  Multi-line comments can also be hidden.

@findex hs-hide-all
@findex hs-hide-block
@findex hs-show-all
@findex hs-show-block
@findex hs-show-region
@findex hs-hide-level
@findex hs-minor-mode
@kindex C-c h
@kindex C-c s
@kindex C-c H
@kindex C-c S
@kindex C-c R
@kindex C-c L
@kindex S-mouse-2
The mode provides the commands @kbd{C-c h} (@kbd{M-x hs-hide-all}),
@kbd{C-c s} (@kbd{M-x hs-hide-block}), @kbd{C-c H} (@kbd{M-x
hs-show-all}), @kbd{C-c S} (@kbd{M-x hs-show-block}), @kbd{C-c R}
(@kbd{M-x hs-show-region}) and @kbd{C-c L} (@kbd{M-x hs-hide-level})
with obvious functions and @kbd{S-mouse-2} toggles hiding of a block
with the mouse.

@vindex hs-hide-comments-when-hiding-all
@vindex hs-show-hidden-short-form
@vindex hs-isearch-open
@vindex hs-special-modes-alist
Hideshow is customized by the variables
@table @code
@item hs-hide-comments-when-hiding-all
Specifies whether @kbd{hs-hide-all} should hide comments too.
@item hs-show-hidden-short-form
Specifies whether or not the last line in a form is omitted (saving
screen space).
@item hs-isearch-open
Specifies what kind of hidden blocks to open in Isearch mode.
@item hs-special-modes-alist
Initializes Hideshow variables for different modes.
@end table

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@node Glasses
@section Glasses minor mode
@cindex Glasses mode
@cindex identifiers, unreadable
@cindex StudlyCaps
@findex glasses-mode

Glasses minor mode makes @samp{unreadableIdentifiersLikeThis} readable
by displaying underscores between all the pairs of lower and upper
English letters or by emboldening the capitals.  The text is not
altered, only the display, so that you can use this mode on code written
with such a convention for separating words in identifiers without
modifying the code.  It can be customized under the group
@samp{glasses}.  You can use it by adding @code{glasses-mode} to the
mode hook of appropriate programming modes.


@node Documentation
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@section Documentation Commands

  As you edit Lisp code to be run in Emacs, the commands @kbd{C-h f}
(@code{describe-function}) and @kbd{C-h v} (@code{describe-variable}) can
be used to print documentation of functions and variables that you want to
call.  These commands use the minibuffer to read the name of a function or
variable to document, and display the documentation in a window.

  For extra convenience, these commands provide default arguments based on
the code in the neighborhood of point.  @kbd{C-h f} sets the default to the
function called in the innermost list containing point.  @kbd{C-h v} uses
the symbol name around or adjacent to point as its default.

@cindex Eldoc mode
@findex eldoc-mode
  For Emacs Lisp code, you can also use Eldoc mode.  This minor mode
constantly displays in the echo area the argument list for the function
being called at point.  (In other words, it finds the function call that
point is contained in, and displays the argument list of that function.)
Eldoc mode applies in Emacs Lisp and Lisp Interaction modes only.  Use
the command @kbd{M-x eldoc-mode} to enable or disable this feature.

@findex info-lookup-symbol
@findex info-lookup-file
@kindex C-h C-i
  For C, Lisp, and other languages, you can use @kbd{C-h C-i}
(@code{info-lookup-symbol}) to view the Info documentation for a symbol.
You specify the symbol with the minibuffer; by default, it uses the
symbol that appears in the buffer at point.  The major mode determines
where to look for documentation for the symbol---which Info files and
which indices.  You can also use @kbd{M-x info-lookup-file} to look for
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documentation for a file name.  Currently the modes supported by
Info-lookup are: Awk, Autoconf, Bison, C, Emacs Lisp, LaTeX, M4,
Makefile, Octave, Perl, Scheme and Texinfo.  The relevant Info files
mostly must be obtained separately, typically from the appropriate GNU
package.
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@findex manual-entry
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@cindex manual pages
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  You can read the ``man page'' for an operating system command, library
function, or system call, with the @kbd{M-x manual-entry} command.  It
runs the @code{man} program to format the man page, and runs it
asynchronously if your system permits, so that you can keep on editing
while the page is being formatted.  (MS-DOS and MS-Windows 3 do not
permit asynchronous subprocesses, so on these systems you cannot edit
while Emacs waits for @code{man} to exit.)  The result goes in a buffer
named @samp{*Man @var{topic}*}.  These buffers use a special major mode,
Man mode, that facilitates scrolling and examining other manual pages.
For details, type @kbd{C-h m} while in a man page buffer.

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@cindex sections of manual pages
  Man pages are subdivided into @dfn{sections}, and some man pages have
identical names, but belong to different sections.  To read a man page
from a certain section, type @kbd{@var{topic}(@var{section})} or
@kbd{@var{section} @var{topic}} when @kbd{M-x manual-entry} prompts for
the topic.  For example, to read the man page for the C library function
@code{chmod} (as opposed to a command by the same name), type @kbd{M-x
manual-entry @key{RET} chmod(2v) @key{RET}} (assuming @code{chmod} is in
section @code{2v}).

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  If you do not specify a section, the results depend on how the
@code{man} command works on your system.  Some of them display only the
first man page they find, others display all the man pages, and you can
page between them with the @kbd{M-n} and @kbd{M-p} keys.  The mode line
shows how many manual pages are available in the Man buffer.

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@vindex Man-fontify-manpage-flag
  For a long man page, setting the faces properly can take substantial
time.  By default, Emacs uses faces in man pages if Emacs can display
different fonts or colors.  You can turn off use of faces in man pages
by setting the variable @code{Man-fontify-manpage-flag} to @code{nil}.

@findex Man-fontify-manpage
  If you insert the text of a man page into an Emacs buffer in some
other fashion, you can use the command @kbd{M-x Man-fontify-manpage} to
perform the same conversions that @kbd{M-x manual-entry} does.

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@findex woman
@cindex manual pages, on MS-DOS/MS-Windows
  An alternative way of reading manual pages is the @kbd{M-x woman}
command@footnote{The name of the command, @code{woman}, is an acronym
for ``w/o (without) man'', since it doesn't use the @code{man}
program.}.  Unlike @kbd{M-x man}, it does not run any external programs
to format and display the man pages, instead it does that entirely in
Emacs Lisp.  Thus, it is useful on systems such as MS-Windows, where the
@code{man} program and the programs it runs are not readily available.
When invoked, @kbd{M-x woman} prompts for a name of a manual page and
provides completion based on the list of manual pages that are installed
on your machine; the list of available manual pages is computed
automatically the first time you invoke @code{woman}.  The word at point
in the current buffer is used to suggest the default name of the manual
page.

  With a numeric argument, @kbd{M-x woman} recomputes the list of the
manual pages used for completion.  This is useful if you add or delete
manual pages.

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  If you type a name of a manual page and @kbd{M-x woman} finds that
several manual pages by the same name exist in different sections, it
pops up a window with possible candidates asking you to choose one of
them.

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@vindex woman-manpath
  By default, @kbd{M-x woman} looks up the manual pages in directories
listed by the @code{MANPATH} environment variable.  (If @code{MANPATH}
is not set, @code{woman} uses a suitable default value, which can be
customized.)  More precisely, @code{woman} looks for subdirectories that
match the shell wildcard @file{man*} in each one of these directories,
and tries to find the manual pages in those subdirectories.  When first
invoked, @kbd{M-x woman} converts the value of @code{MANPATH} to a list
of directory names and stores that list in the @code{woman-manpath}
variable.  By changing the value of this variable, you can customize the
list of directories where @code{woman} looks for manual pages.

@vindex woman-path
  In addition, you can augment the list of directories searched by
@code{woman} by setting the value of the @code{woman-path} variable.
This variable should hold a list of specific directories which
@code{woman} should search, in addition to those in
@code{woman-manpath}.  Unlike @code{woman-manpath}, the directories in
@code{woman-path} are searched for the manual pages, not for @file{man*}
subdirectories.

@findex woman-find-file
  Occasionally, you might need to display manual pages that are not in
any of the directories listed by @code{woman-manpath} and
@code{woman-path}.  The @kbd{M-x woman-find-file} command prompts for a
name of a manual page file, with completion, and then formats and
displays that file like @kbd{M-x woman} does.

@vindex woman-dired-keys
  First time you invoke @kbd{M-x woman}, it defines the Dired @kbd{W}
key to run the @code{woman-find-file} command on the current line's
file.  You can disable this by setting the variable
@code{woman-dired-keys} to @code{nil}.  @xref{Dired}.  In addition, the
Tar-mode @kbd{w} key is bound to @code{woman-find-file} on the current
line's archive member.

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  For more information about setting up and using @kbd{M-x woman}, see
@ref{Top, WoMan, Browse UN*X Manual Pages WithOut Man, woman, The WoMan
Manual}.

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  Eventually the GNU project hopes to replace most man pages with
better-organized manuals that you can browse with Info.  @xref{Misc
Help}.  Since this process is only partially completed, it is still
useful to read manual pages.

@node Change Log
@section Change Logs

@cindex change log
@kindex C-x 4 a
@findex add-change-log-entry-other-window
  The Emacs command @kbd{C-x 4 a} adds a new entry to the change log
file for the file you are editing
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(@code{add-change-log-entry-other-window}).  If that file is actually a
backup file, it makes an entry appropriate for the file's parent.  This
is useful for making log entries by comparing a version with deleted
functions.
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  A change log file contains a chronological record of when and why you
have changed a program, consisting of a sequence of entries describing
individual changes.  Normally it is kept in a file called
@file{ChangeLog} in the same directory as the file you are editing, or
one of its parent directories.  A single @file{ChangeLog} file can
record changes for all the files in its directory and all its
subdirectories.

  A change log entry starts with a header line that contains your name,
your email address (taken from the variable @code{user-mail-address}),
and the current date and time.  Aside from these header lines, every
line in the change log starts with a space or a tab.  The bulk of the
entry consists of @dfn{items}, each of which starts with a line starting
with whitespace and a star.  Here are two entries, both dated in May
1993, each with two items:

@iftex
@medbreak
@end iftex
@smallexample
1993-05-25  Richard Stallman  <rms@@gnu.org>

        * man.el: Rename symbols `man-*' to `Man-*'.
        (manual-entry): Make prompt string clearer.

        * simple.el (blink-matching-paren-distance):
        Change default to 12,000.

1993-05-24  Richard Stallman  <rms@@gnu.org>

        * vc.el (minor-mode-map-alist): Don't use it if it's void.
        (vc-cancel-version): Doc fix.
@end smallexample

  One entry can describe several changes; each change should have its
own item.  Normally there should be a blank line between items.  When
items are related (parts of the same change, in different places), group
them by leaving no blank line between them.  The second entry above
contains two items grouped in this way.

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@vindex add-log-keep-changes-together
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  @kbd{C-x 4 a} visits the change log file and creates a new entry
unless the most recent entry is for today's date and your name.  It also
creates a new item for the current file.  For many languages, it can
even guess the name of the function or other object that was changed.
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When the option @code{add-log-keep-changes-together} is set, @kbd{C-x 4
a} adds to any existing entry for the file rather than starting a new
entry.
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@vindex change-log-version-info-enabled
@vindex change-log-version-number-regexp-list
@cindex file version in change log entries
  If the value of the variable @code{change-log-version-info-enabled} is
non-nil, the file's version number is automatically added to change log
entries.  The search for the file's version number is performed based on
regular expressions from the variable
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@code{change-log-version-number-regexp-list}, which can be customized
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(versions of files that are under version control systems are known to
Emacs through the version-control interface).

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@cindex Change Log mode
@findex change-log-mode
  The change log file is visited in Change Log mode.  In this major
mode, each bunch of grouped items counts as one paragraph, and each
entry is considered a page.  This facilitates editing the entries.
@kbd{C-j} and auto-fill indent each new line like the previous line;
this is convenient for entering the contents of an entry.

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@findex change-log-merge
The command @kbd{M-x change-log-merge} can be used to merge other log
files into a buffer in Change Log Mode, preserving the date ordering
of entries with either the current or old-style date formats.

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@findex change-log-redate
@cindex converting change log date style
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  Versions of Emacs before 20.1 used a different format for the time of
the change log entry:
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@smallexample
Fri May 25 11:23:23 1993 Richard Stallman  <rms@@gnu.org>
@end smallexample

@noindent
The @kbd{M-x change-log-redate} command converts all the old-style date
entries in the change log file visited in the current buffer to the new
format, so that all entries are kept in unified format.  This is handy
when the entries are contributed by many different people some of whom
still use old versions of Emacs.

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  Version control systems are another way to keep track of changes in your
program and keep a change log.  @xref{Log Buffer}.

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@node Authors
@section @file{AUTHORS} files
@cindex @file{AUTHORS} file

  Programs which have many contributors usually include a file named
@file{AUTHORS} in their distribution, which lists the individual
contributions.  Emacs has a special command for maintaining the
@file{AUTHORS} file that is part of the Emacs distribution.

@findex authors
  The @kbd{M-x authors} command prompts for the name of the root of the
Emacs source directory.  It then scans @file{ChageLog} files and Lisp
source files under that directory for information about authors of
individual packages and people who made changes in source files, and
puts the information it gleans into a buffer named @samp{*Authors*}.
You can then edit the contents of that buffer and merge it with the
exisiting @file{AUTHORS} file.

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@node Tags
@section Tags Tables
@cindex tags table

  A @dfn{tags table} is a description of how a multi-file program is
broken up into files.  It lists the names of the component files and the
names and positions of the functions (or other named subunits) in each
file.  Grouping the related files makes it possible to search or replace
through all the files with one command.  Recording the function names
and positions makes possible the @kbd{M-.} command which finds the
definition of a function by looking up which of the files it is in.

  Tags tables are stored in files called @dfn{tags table files}.  The
conventional name for a tags table file is @file{TAGS}.

  Each entry in the tags table records the name of one tag, the name of the
file that the tag is defined in (implicitly), and the position in that file
of the tag's definition.

  Just what names from the described files are recorded in the tags table
depends on the programming language of the described file.  They
normally include all functions and subroutines, and may also include
global variables, data types, and anything else convenient.  Each name
recorded is called a @dfn{tag}.

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@cindex C++ class browser, tags
@cindex tags, C++
@cindex class browser, C++
@cindex Ebrowse
The Ebrowse is a separate facility tailored for C++, with tags and a
class browser.  @xref{,,, ebrowse, Ebrowse User's Manual}.

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@menu
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* Tag Syntax::		Tag syntax for various types of code and text files.
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* Create Tags Table::	Creating a tags table with @code{etags}.
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* Etags Regexps::       Create arbitrary tags using regular expressions.
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* Select Tags Table::	How to visit a tags table.
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* Find Tag::		Commands to find the definition of a specific tag.
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* Tags Search::		Using a tags table for searching and replacing.
* List Tags::		Listing and finding tags defined in a file.
@end menu

@node Tag Syntax
@subsection Source File Tag Syntax

  Here is how tag syntax is defined for the most popular languages:

@itemize @bullet
@item
In C code, any C function or typedef is a tag, and so are definitions of
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@code{struct}, @code{union} and @code{enum}.  You can tag function
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declarations and external variables in addition to function definitions
by giving the @samp{--declarations} option to @code{etags}.
@code{#define} macro definitions and @code{enum} constants are also
tags, unless you specify @samp{--no-defines} when making the tags table.
Similarly, global variables are tags, unless you specify
@samp{--no-globals}.  Use of @samp{--no-globals} and @samp{--no-defines}
can make the tags table file much smaller.
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@item
In C++ code, in addition to all the tag constructs of C code, member
functions are also recognized, and optionally member variables if you
use the @samp{--members} option.  Tags for variables and functions in
classes are named @samp{@var{class}::@var{variable}} and
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@samp{@var{class}::@var{function}}.  @code{operator} functions tags are
named, for example @samp{operator+}.
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@item
In Java code, tags include all the constructs recognized in C++, plus
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the @code{interface}, @code{extends} and @code{implements} constructs.
Tags for variables and functions in classes are named
@samp{@var{class}.@var{variable}} and @samp{@var{class}.@var{function}}.
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@item
In La@TeX{} text, the argument of any of the commands @code{\chapter},
@code{\section}, @code{\subsection}, @code{\subsubsection},
@code{\eqno}, @code{\label}, @code{\ref}, @code{\cite}, @code{\bibitem},
@code{\part}, @code{\appendix}, @code{\entry}, or @code{\index}, is a
tag.@refill

Other commands can make tags as well, if you specify them in the
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environment variable @env{TEXTAGS} before invoking @code{etags}.  The
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value of this environment variable should be a colon-separated list of
command names.  For example,

@example
TEXTAGS="def:newcommand:newenvironment"
export TEXTAGS
@end example

@noindent
specifies (using Bourne shell syntax) that the commands @samp{\def},
@samp{\newcommand} and @samp{\newenvironment} also define tags.

@item
In Lisp code, any function defined with @code{defun}, any variable
defined with @code{defvar} or @code{defconst}, and in general the first
argument of any expression that starts with @samp{(def} in column zero, is
a tag.

@item
In Scheme code, tags include anything defined with @code{def} or with a
construct whose name starts with @samp{def}.  They also include variables
set with @code{set!} at top level in the file.
@end itemize

  Several other languages are also supported:

@itemize @bullet
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@item
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In Ada code, functions, procedures, packages, tasks, and types are
tags.  Use the @samp{--packages-only} option to create tags for packages
only.
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@item
In assembler code, labels appearing at the beginning of a line,
followed by a colon, are tags.

@item
In Bison or Yacc input files, each rule defines as a tag the nonterminal
it constructs.  The portions of the file that contain C code are parsed
as C code.

@item
In Cobol code, tags are paragraph names; that is, any word starting in
column 8 and followed by a period.

@item
In Erlang code, the tags are the functions, records, and macros defined
in the file.

@item
In Fortran code, functions, subroutines and blockdata are tags.

@item
In Objective C code, tags include Objective C definitions for classes,
class categories, methods, and protocols.

@item
In Pascal code, the tags are the functions and procedures defined in
the file.

@item
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In Perl code, the tags are the procedures defined by the @code{sub},
@code{my} and @code{local} keywords.  Use @samp{--globals} if you want
to tag global variables.
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@item
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In PostScript code, the tags are the functions.
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@item
In Prolog code, a tag name appears at the left margin.

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@item
In Python code, @code{def} or @code{class} at the beginning of a line
generate a tag.
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@end itemize
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  You can also generate tags based on regexp matching (@pxref{Etags
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Regexps}) to handle other formats and languages.
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@node Create Tags Table
@subsection Creating Tags Tables
@cindex @code{etags} program

  The @code{etags} program is used to create a tags table file.  It knows
the syntax of several languages, as described in
@iftex
the previous section.
@end iftex
@ifinfo
@ref{Tag Syntax}.
@end ifinfo
Here is how to run @code{etags}:

@example
etags @var{inputfiles}@dots{}
@end example

@noindent
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The @code{etags} program reads the specified files, and writes a tags
table named @file{TAGS} in the current working directory.  You can
intermix compressed and plain text source file names.  @code{etags}
knows about the most common compression formats, and does the right
thing.  So you can compress all your source files and have @code{etags}
look for compressed versions of its file name arguments, if it does not
find uncompressed versions.  Under MS-DOS, @code{etags} also looks for
file names like @samp{mycode.cgz} if it is given @samp{mycode.c} on the
command line and @samp{mycode.c} does not exist.