(Basic Indent): Add a cross-reference to description of indent-tabs-mode.

[gnu-emacs] / man / programs.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1985,86,87,93,94,95,97,99,2000 Free Software Foundation, Inc.
@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.

  The selective display feature is useful for looking at the overall
structure of a function (@pxref{Selective Display}).  This feature
hides the lines that are indented more than a specified amount.
Programming modes often support Outline minor mode (@pxref{Outline
Mode}).  The Foldout package provides folding-editor features
(@pxref{Foldout}).

  The ``automatic typing'' features may be useful for writing programs.
@xref{Top,,Autotyping, autotype, Autotyping}.

@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.
* Hideshow::            Displaying blocks selectively.
* Glasses::             Making identifiersLikeThis more readable.
* Documentation::       Getting documentation of functions you plan to call.
* Change Log::          Maintaining a change history for your program.
* Authors::             Maintaining an @file{AUTHORS} file.
* Tags::                Go direct to any function in your program in one
                          command.  Tags remembers which file it is in.
* Imenu::               Making buffer indexes as menus.
* Emerge::              A convenient way of merging two versions of a program.
* C Modes::             Special commands of C, C++, Objective-C,
                          Java, and Pike modes.
* Fortran::             Fortran mode and its special features.
* Asm Mode::            Asm mode and its special features.
@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
@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
@cindex Delphi mode
@cindex PostScript mode
  Emacs also has major modes for the programming languages Lisp, Scheme
(a variant of Lisp) and the Scheme-based DSSSL expression language, Ada,
Awk, C, C++, Delphi (Object Pascal), Fortran (free and fixed format),
Icon, IDLWAVE,
Java, Metafont (@TeX{}'s companion for font creation), Modula2,
Objective-C, Octave, Pascal, Perl, Pike, PostScript, Prolog, Simula,
VHDL, CORBA IDL, and Tcl.
There is also a major mode for makefiles, called Makefile
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 @samp{BAT} files.  In a similar fashion to programming
languages, modes are provided for editing various sorts of configuration
files.

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}).

  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-command}.
@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 c-electric-backspace
  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{c-electric-backspace}).  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

@cindex parentheses, moving across
@cindex matching parenthesis and braces, moving to
@cindex braces, moving across
@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.

@cindex deleting parenthesized expressions
@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.

@cindex transposition of parenthesized expressions
@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

@cindex move to beginning or end of function
@cindex function, move to beginning or end
@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}).

@kindex C-M-h @r{(C mode)}
@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}.

@cindex open-parenthesis in leftmost column
@cindex ( in leftmost column
  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-command
@findex indent-line-function
  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{indent-for-tab-command}
in Lisp mode, @code{c-indent-command} 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.

  Normally, lines are indented with tabs and spaces.  If you want Emacs
to use spaces only, see @ref{Just Spaces}.

@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.

   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
@code{c-offsets-alist}.@refill

@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},
@code{ellemtel}, @code{cc-mode}, and @code{user}.

@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.

  The style @code{gnu} defines the formatting recommend by the GNU
Project; it is the default, so as to encourage the indentation we
recommend.  However, if you make changes in variables such as
@code{c-basic-offset} and @code{c-offsets-alist} in your
@file{~/.emacs} file, your changes override the what @code{gnu} style
says.

@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}.

  If @var{use-now} is non-@code{nil}, @code{c-add-style} selects the new
style after defining it.

@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
@cindex highlighting matching parentheses
@findex show-paren-mode
  Show Paren mode provides a more powerful kind of automatic
parenthesis matching.  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}.

@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
@cindex indentation for comments

  The comment commands in this table insert, kill and align comments.
They are described in this section and following sections.

@table @kbd
@item M-;
Insert or realign comment on current line; alternatively, comment or
uncomment the region (@code{comment-dwim}).
@item C-u M-;
Kill comment on current line (@code{comment-kill}).
@item C-x ;
Set comment column (@code{set-comment-column}).
@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

@kindex M-;
@findex comment-dwim
  The command to create or align a comment is @kbd{M-;}
(@code{comment-dwim}).  The word ``dwim'' is an acronym for ``Do What
I Mean''; it indicates that this command can be used for many
different jobs relating to comments, depending on the situation where
you use it.

  If there is no comment already on the line, @kbd{M-;} inserts a new
comment, aligned at a specific column called the @dfn{comment column}.
The new comment begins with the string Emacs thinks comments should
start with (the value of @code{comment-start}; see below).  Point is
after that string, so you can insert the text of the comment right
away.  If the major mode has specified a string to terminate comments,
@kbd{M-;} inserts that too, to keep the syntax valid.

  If the text of the line extends past the comment column, then the
comment start string is indented to a suitable boundary (usually, at
least one space is inserted).

  You can also use @kbd{M-;} to align an existing comment.  If a line
already contains the comment-start string, @kbd{M-;} reindents it to
the conventional alignment and moves point after it.  (Exception:
comments starting in column 0 are not moved.)  Even when an existing
comment is properly aligned, @kbd{M-;} is still useful for moving
directly to the start of the text inside the comment.

@findex comment-kill
@kindex C-u M-;
  @kbd{C-u M-;} kills any comment on the current line, along with the
whitespace before it.  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 M-;} is not a distinct key; it is @kbd{M-;}
(@code{comment-dwim}) with a prefix argument.  That command is
programmed so that when it receives a prefix argument it calls
@code{comment-kill}.  However, @code{comment-kill} is a valid command
in its own right, and you can bind it directly to a key if you wish.

  @kbd{M-;} does two other jobs when used with an active region in
Transient Mark mode (@pxref{Transient Mark}).  Then it either adds or
removes comment delimiters on each line of the region.  (If every line
is a comment, it removes comment delimiters from each; otherwise, it
adds comment delimiters to each.)  If you are not using Transient Mark
mode, then you should use the commands @code{comment-region} and
@code{uncomment-region} to do these jobs (@pxref{Multi-Line Comments}).
A prefix argument used in these circumstances specifies how many
comment delimiters to add or how many to delete.

  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.

@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.

@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.

  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-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.

@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.

@findex check-parens
@cindex unbalanced parentheses and quotes
  You can use @kbd{M-x check-parens} to find any unbalanced
parentheses and unbalanced string quotes in a buffer.

@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 tags-based 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 (Lisp symbols)
@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.)

@node Hideshow
@section Hideshow minor mode

@findex hs-minor-mode
  Hideshow minor mode provides selective display of portions of a
file, known as @dfn{blocks}.  You can use @kbd{M-x hs-minor-mode} to
enable or disable this mode, or add @code{hs-minor-mode} to the mode
hook for certain major modes in order to enable it automatically for
those modes.

  Just what constitutes a block depends on the major mode.  In C mode
or C++ mode, they are delimited by braces, while in Lisp mode and
similar modes they are delimited by parentheses.  Multi-line comments
also count as blocks.

@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 @@ C-h
@kindex C-c @@ C-s
@kindex C-c @@ C-M-h
@kindex C-c @@ C-M-s
@kindex C-c @@ C-r
@kindex C-c @@ C-l
@kindex S-Mouse-2
@table @kbd
@item C-c @@ C-h
Hide the current block (@code{hs-hide-block}).
@item C-c @@ C-s
Show the current block (@code{hs-show-block}).
@item C-c @@ C-c
Either hide or show the current block (@code{hs-toggle-hiding})
@item S-Mouse-2
Either hide or show the block you click on (@code{hs-mouse-toggle-hiding})
@item C-c @@ C-M-h
Hide all top-level blocks (@code{hs-hide-all}).
@item C-c @@ C-M-s
Show everything in the buffer (@code{hs-show-all}).
@item C-c @@ C-l
Hide all blocks @var{n} levels below this block
(@code{hs-hide-level}).
@end table

@vindex hs-hide-comments-when-hiding-all
@vindex hs-show-hidden-short-form
@vindex hs-isearch-open
@vindex hs-special-modes-alist
  These user options exist for customizing Hideshow mode.

@table @code
@item hs-hide-comments-when-hiding-all
Non-@code{nil} says that @kbd{hs-hide-all} should hide comments too.
@item hs-show-hidden-short-form
Non-@code{nil} says to omit the last line in a form (saving screen
space).
@item hs-isearch-open
Specifies what kind of hidden blocks to open in Isearch mode.
@item hs-special-modes-alist
Specifies
Initializes Hideshow variables for different modes.
@end table

@node Glasses
@section Glasses minor mode
@cindex Glasses mode
@cindex identifiers, making long ones readable
@cindex StudlyCaps, making them readable
@findex glasses-mode

  Glasses minor mode makes @samp{unreadableIdentifiersLikeThis}
readable by altering the display.  It can do this in two different
ways: by displaying underscores between an lower-case letter and the
following capital letter, or by emboldening the capital letters.  It
does not alter the buffer text, only the way they display, so you can
use it even on read-only buffers.  You can use the command @kbd{M-x
glasses-mode} to enable or disable the mode; you can also add
@code{glasses-mode} to the mode hook of appropriate programming
language major modes.

@node Documentation
@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
documentation for a file name.  Currently this supports the following
modes: Awk, Autoconf, Bison, C, Emacs Lisp, LaTeX, M4,
Makefile, Octave, Perl, Scheme and Texinfo, provided you have installed
the relevant Info files, which are typically available with the appropriate GNU
package.

@findex manual-entry
@cindex manual pages
  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.

@cindex sections of manual pages
  Man pages are classified into @dfn{sections}; sometimes there are
man pages with the same name in different sections.  To read a man
page from a specific section, type @samp{@var{topic}(@var{section})} or
@samp{@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 @samp{2v}).

  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 man pages that have
the specified name, so 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.

@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.

@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 the job
in Emacs Lisp, so it works on systems such as MS-Windows, where the
@code{man} program and other the programs it needs are not readily
available.  @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.

  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.

@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.

  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}.

  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
(@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---that is useful for making log entries for functions that
have been deleted in the current version.

  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.

  @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.

@vindex add-log-keep-changes-together
  When the option @code{add-log-keep-changes-together} is
non-@code{nil}, @kbd{C-x 4 a} adds to any existing entry for the file
rather than starting a new entry.

@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-@code{nil}, @kbd{C-x 4 a} ads the file's version number to the
change log entry.  It finds the version number by searching the first
ten percent of the file, using regular expressions from the variable
@code{change-log-version-number-regexp-list}.

@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.

@findex change-log-merge
  You can use the command @kbd{M-x change-log-merge} to merge other
log files into a buffer in Change Log Mode, preserving the date
ordering of entries.

@findex change-log-redate
@cindex converting change log date style
  Versions of Emacs before 20.1 used a different format for the time of
the change log entry:

@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, to make the file uniform in style.  This is handy when
entries are contributed by many different people, some of whom use old
versions of Emacs.

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

@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.

  Do not assume that this command finds all the contributors; don't
assume that a person not listed in the output was not a contributor.
If you merged in someone's contribution and did not put his name
in the change log, he won't show up in @kbd{M-x authors} either.

@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 file names, functions and subroutines, and may
also include global variables, data types, and anything else
convenient.  Each name recorded is called a @dfn{tag}.

@cindex C++ class browser, tags
@cindex tags, C++
@cindex class browser, C++
@cindex Ebrowse
  See also the Ebrowse facility, which is tailored for C++.
@xref{Top,, Ebrowse, ebrowse, Ebrowse User's Manual}.

@menu
* Tag Syntax::          Tag syntax for various types of code and text files.
* Create Tags Table::   Creating a tags table with @code{etags}.
* Etags Regexps::       Create arbitrary tags using regular expressions.
* Select Tags Table::   How to visit a tags table.
* Find Tag::            Commands to find the definition of a specific tag.
* 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
@code{struct}, @code{union} and @code{enum}.
@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.

You can tag function declarations and external variables in addition
to function definitions by giving the @samp{--declarations} option to
@code{etags}.

@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
@samp{@var{class}::@var{function}}.  @code{operator} definitions have
tag names like @samp{operator+}.

@item
In Java code, tags include all the constructs recognized in C++, plus
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}}.

@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
environment variable @env{TEXTAGS} before invoking @code{etags}.  The
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

@item
In Ada code, functions, procedures, packages, tasks, and types are
tags.  Use the @samp{--packages-only} option to create tags for
packages only.

In Ada, the same name can be used for different kinds of entity
(e.g.@:, for a procedure and for a function).  Also, for things like
packages, procedures and functions, there is the spec (i.e.@: the
interface) and the body (i.e.@: the implementation).  To make it
easier to pick the definition you want, Ada tag name have suffixes
indicating the type of entity:

@table @samp
@item /b
package body.
@item /f
function.
@item /k
task.
@item /p
procedure.
@item /s
package spec.
@item /t
type.
@end table

  Thus, @kbd{M-x find-tag @key{RET} bidule/b @key{RET}} will go
directly to the body of the package @code{bidule}, while @kbd{M-x
find-tag @key{RET} bidule @key{RET}} will just search for any tag
@code{bidule}.

@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 makefiles, targets 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
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.

@item
In PostScript code, the tags are the functions.

@item
In Prolog code, a tag name appears at the left margin.

@item
In Python code, @code{def} or @code{class} at the beginning of a line
generate a tag.
@end itemize

  You can also generate tags based on regexp matching (@pxref{Etags
Regexps}) to handle other formats and languages.

@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
The @code{etags} program reads the specified files, and writes a tags
table named @file{TAGS} in the current working directory.

  If the specified files don't exist, @code{etags} looks for
compressed versions of them and uncompresses them to read them.  Under
MS-DOS, @code{etags} also looks for file names like @file{mycode.cgz}
if it is given @samp{mycode.c} on the command line and @file{mycode.c}
does not exist.

  @code{etags} recognizes the language used in an input file based on
its file name and contents.  You can specify the language with the
@samp{--language=@var{name}} option, described below.

  If the tags table data become outdated due to changes in the files
described in the table, the way to update the tags table is the same
way it was made in the first place.  But it is not necessary to do
this very often.

  If the tags table fails to record a tag, or records it for the wrong
file, then Emacs cannot possibly find its definition.  However, if the
position recorded in the tags table becomes a little bit wrong (due to
some editing in the file that the tag definition is in), the only
consequence is a slight delay in finding the tag.  Even if the stored
position is very wrong, Emacs will still find the tag, but it must
search the entire file for it.

  So you should update a tags table when you define new tags that you want
to have listed, or when you move tag definitions from one file to another,
or when changes become substantial.  Normally there is no need to update
the tags table after each edit, or even every day.

  One tags table can virtually include another.  Specify the included
tags file name with the @samp{--include=@var{file}} option when
creating the file that is to include it.  The latter file then acts as
if it covered all the source files specified in the included file, as
well as the files it directly contains.

  If you specify the source files with relative file names when you run
@code{etags}, the tags file will contain file names relative to the
directory where the tags file was initially written.  This way, you can
move an entire directory tree containing both the tags file and the
source files, and the tags file will still refer correctly to the source
files.

  If you specify absolute file names as arguments to @code{etags}, then
the tags file will contain absolute file names.  This way, the tags file
will still refer to the same files even if you move it, as long as the
source files remain in the same place.  Absolute file names start with
@samp{/}, or with @samp{@var{device}:/} on MS-DOS and MS-Windows.

  When you want to make a tags table from a great number of files, you
may have problems listing them on the command line, because some systems
have a limit on its length.  The simplest way to circumvent this limit
is to tell @code{etags} to read the file names from its standard input,
by typing a dash in place of the file names, like this:

@smallexample
find . -name "*.[chCH]" -print | etags -
@end smallexample

  Use the option @samp{--language=@var{name}} to specify the language
explicitly.  You can intermix these options with file names; each one
applies to the file names that follow it.  Specify
@samp{--language=auto} to tell @code{etags} to resume guessing the
language from the file names and file contents.  Specify
@samp{--language=none} to turn off language-specific processing
entirely; then @code{etags} recognizes tags by regexp matching alone
(@pxref{Etags Regexps}).

  @samp{etags --help} prints the list of the languages @code{etags}
knows, and the file name rules for guessing the language.  It also prints
a list of all the available @code{etags} options, together with a short
explanation.

@node Etags Regexps
@subsection Etags Regexps

  The @samp{--regex} option provides a general way of recognizing tags
based on regexp matching.  You can freely intermix it with file names.
Each @samp{--regex} option adds to the preceding ones, and applies only
to the following files.  The syntax is:

@smallexample
--regex=/@var{tagregexp}[/@var{nameregexp}]/
@end smallexample

@noindent
where @var{tagregexp} is used to match the lines to tag.  It is always
anchored, that is, it behaves as if preceded by @samp{^}.  If you want
to account for indentation, just match any initial number of blanks by
beginning your regular expression with @samp{[ \t]*}.  In the regular
expressions, @samp{\} quotes the next character, and @samp{\t} stands
for the tab character.  Note that @code{etags} does not handle the other
C escape sequences for special characters.

@cindex interval operator (in regexps)
  The syntax of regular expressions in @code{etags} is the same as in
Emacs, augmented with the @dfn{interval operator}, which works as in
@code{grep} and @code{ed}.  The syntax of an interval operator is
@samp{\@{@var{m},@var{n}\@}}, and its meaning is to match the preceding
expression at least @var{m} times and up to @var{n} times.

  You should not match more characters with @var{tagregexp} than that
needed to recognize what you want to tag.  If the match is such that
more characters than needed are unavoidably matched by @var{tagregexp}
(as will usually be the case), you should add a @var{nameregexp}, to
pick out just the tag.  This will enable Emacs to find tags more
accurately and to do completion on tag names more reliably.  You can
find some examples below.

  The option @samp{--ignore-case-regex} (or @samp{-c}) works like
@samp{--regex}, except that matching ignores case.  This is
appropriate for certain programming languages.

  The @samp{-R} option deletes all the regexps defined with
@samp{--regex} options.  It applies to the file names following it, as
you can see from the following example:

@smallexample
etags --regex=/@var{reg1}/ voo.doo --regex=/@var{reg2}/ \
    bar.ber -R --lang=lisp los.er
@end smallexample

@noindent
Here @code{etags} chooses the parsing language for @file{voo.doo} and
@file{bar.ber} according to their contents.  @code{etags} also uses
@var{reg1} to recognize additional tags in @file{voo.doo}, and both
@var{reg1} and @var{reg2} to recognize additional tags in
@file{bar.ber}.  @code{etags} uses the Lisp tags rules, and no regexp
matching, to recognize tags in @file{los.er}.

  You can specify a regular expression for a particular language, by
writing @samp{@{lang@}} in front of it.  Then @code{etags} will use
the regular expression only for files of that language.  (@samp{etags
--help} prints the list of languages recognised by @code{etags}.)  The
following example tags the @code{DEFVAR} macros in the Emacs source
files, for the C language only:

@smallexample
--regex='@{c@}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'
@end smallexample

@noindent
This feature is particularly useful when you store a list of regular
expressions in a file.  The following option syntax instructs
@code{etags} to read two files of regular expressions.  The regular
expressions contained in the second file are matched without regard to
case.

@smallexample
--regex=@@first-file --ignore-case-regex=@@second-file
@end smallexample

@noindent
A regex file contains one regular expressions per line.  Empty lines,
and lines beginning with space or tab are ignored.  When the first
character in a line is @samp{@@}, @code{etags} assumes that the rest
of the line is the name of a file of regular expressions; thus, one
such file can include another file.  All the other lines are taken to
be regular expressions.  If the first non-whitespace text on the line
is @samp{--}, that line is a comment.

  For example, one can create a file called @samp{emacs.tags} with the
following contents:

@smallexample
        -- This is for GNU Emacs C source files
@{c@}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/\1/
@end smallexample

@noindent
and then use it like this:

@smallexample
etags --regex=@@emacs.tags *.[ch] */*.[ch]
@end smallexample

  Here are some more examples.  The regexps are quoted to protect them
from shell interpretation.

@itemize @bullet

@item
Tag Octave files:

@smallexample
etags --language=none \
      --regex='/[ \t]*function.*=[ \t]*\([^ \t]*\)[ \t]*(/\1/' \
      --regex='/###key \(.*\)/\1/' \
      --regex='/[ \t]*global[ \t].*/' \
      *.m
@end smallexample

@noindent
Note that tags are not generated for scripts, so that you have to add
a line by yourself of the form @samp{###key @var{scriptname}} if you
want to jump to it.

@item
Tag Tcl files:

@smallexample
etags --language=none --regex='/proc[ \t]+\([^ \t]+\)/\1/' *.tcl
@end smallexample

@item
Tag VHDL files:

@smallexample
etags --language=none \
  --regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/' \
  --regex='/[ \t]*\(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\
  \( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'
@end smallexample
@end itemize

@node Select Tags Table
@subsection Selecting a Tags Table

@vindex tags-file-name
@findex visit-tags-table
  Emacs has at any time one @dfn{selected} tags table, and all the commands
for working with tags tables use the selected one.  To select a tags table,
type @kbd{M-x visit-tags-table}, which reads the tags table file name as an
argument.  The name @file{TAGS} in the default directory is used as the
default file name.

  All this command does is store the file name in the variable
@code{tags-file-name}.  Emacs does not actually read in the tags table
contents until you try to use them.  Setting this variable yourself is just
as good as using @code{visit-tags-table}.  The variable's initial value is
@code{nil}; that value tells all the commands for working with tags tables
that they must ask for a tags table file name to use.

  Using @code{visit-tags-table} when a tags table is already loaded
gives you a choice: you can add the new tags table to the current list
of tags tables, or start a new list.  The tags commands use all the tags
tables in the current list.  If you start a new list, the new tags table
is used @emph{instead} of others.  If you add the new table to the
current list, it is used @emph{as well as} the others.  When the tags
commands scan the list of tags tables, they don't always start at the
beginning of the list; they start with the first tags table (if any)
that describes the current file, proceed from there to the end of the
list, and then scan from the beginning of the list until they have
covered all the tables in the list.

@vindex tags-table-list
  You can specify a precise list of tags tables by setting the variable
@code{tags-table-list} to a list of strings, like this:

@c keep this on two lines for formatting in smallbook
@example
@group
(setq tags-table-list
      '("~/emacs" "/usr/local/lib/emacs/src"))
@end group
@end example

@noindent
This tells the tags commands to look at the @file{TAGS} files in your
@file{~/emacs} directory and in the @file{/usr/local/lib/emacs/src}
directory.  The order depends on which file you are in and which tags
table mentions that file, as explained above.

  Do not set both @code{tags-file-name} and @code{tags-table-list}.

@node Find Tag
@subsection Finding a Tag

  The most important thing that a tags table enables you to do is to find
the definition of a specific tag.

@table @kbd
@item M-.@: @var{tag} @key{RET}
Find first definition of @var{tag} (@code{find-tag}).
@item C-u M-.
Find next alternate definition of last tag specified.
@item C-u - M-.
Go back to previous tag found.
@item C-M-. @var{pattern} @key{RET}
Find a tag whose name matches @var{pattern} (@code{find-tag-regexp}).
@item C-u C-M-.
Find the next tag whose name matches the last pattern used.
@item C-x 4 .@: @var{tag} @key{RET}
Find first definition of @var{tag}, but display it in another window
(@code{find-tag-other-window}).
@item C-x 5 .@: @var{tag} @key{RET}
Find first definition of @var{tag}, and create a new frame to select the
buffer (@code{find-tag-other-frame}).
@item M-*
Pop back to where you previously invoked @kbd{M-.} and friends.
@end table

@kindex M-.
@findex find-tag
  @kbd{M-.}@: (@code{find-tag}) is the command to find the definition of
a specified tag.  It searches through the tags table for that tag, as a
string, and then uses the tags table info to determine the file that the
definition is in and the approximate character position in the file of
the definition.  Then @code{find-tag} visits that file, moves point to
the approximate character position, and searches ever-increasing
distances away to find the tag definition.

  If an empty argument is given (just type @key{RET}), the sexp in the
buffer before or around point is used as the @var{tag} argument.
@xref{Lists}, for info on sexps.

  You don't need to give @kbd{M-.} the full name of the tag; a part
will do.  This is because @kbd{M-.} finds tags in the table which
contain @var{tag} as a substring.  However, it prefers an exact match
to a substring match.  To find other tags that match the same
substring, give @code{find-tag} a numeric argument, as in @kbd{C-u
M-.}; this does not read a tag name, but continues searching the tags
table's text for another tag containing the same substring last used.
If you have a real @key{META} key, @kbd{M-0 M-.}@: is an easier
alternative to @kbd{C-u M-.}.

@kindex C-x 4 .
@findex find-tag-other-window
@kindex C-x 5 .
@findex find-tag-other-frame
  Like most commands that can switch buffers, @code{find-tag} has a
variant that displays the new buffer in another window, and one that
makes a new frame for it.  The former is @kbd{C-x 4 .}, which invokes
the command @code{find-tag-other-window}.  The latter is @kbd{C-x 5 .},
which invokes @code{find-tag-other-frame}.

  To move back to places you've found tags recently, use @kbd{C-u -
M-.}; more generally, @kbd{M-.} with a negative numeric argument.  This
command can take you to another buffer.  @kbd{C-x 4 .} with a negative
argument finds the previous tag location in another window.

@kindex M-*
@findex pop-tag-mark
@vindex find-tag-marker-ring-length
  As well as going back to places you've found tags recently, you can go
back to places @emph{from where} you found them.  Use @kbd{M-*}, which
invokes the command @code{pop-tag-mark}, for this.  Typically you would
find and study the definition of something with @kbd{M-.} and then
return to where you were with @kbd{M-*}.

  Both @kbd{C-u - M-.} and @kbd{M-*} allow you to retrace your steps to
a depth determined by the variable @code{find-tag-marker-ring-length}.

@findex find-tag-regexp
@kindex C-M-.
  The command @kbd{C-M-.} (@code{find-tag-regexp}) visits the tags that
match a specified regular expression.  It is just like @kbd{M-.} except
that it does regexp matching instead of substring matching.

@node Tags Search
@subsection Searching and Replacing with Tags Tables
@cindex search and replace in multiple files
@cindex multiple-file search and replace

  The commands in this section visit and search all the files listed in the
selected tags table, one by one.  For these commands, the tags table serves
only to specify a sequence of files to search.

@table @kbd
@item M-x tags-search @key{RET} @var{regexp} @key{RET}
Search for @var{regexp} through the files in the selected tags
table.
@item M-x tags-query-replace @key{RET} @var{regexp} @key{RET} @var{replacement} @key{RET}
Perform a @code{query-replace-regexp} on each file in the selected tags table.
@item M-,
Restart one of the commands above, from the current location of point
(@code{tags-loop-continue}).
@end table

@findex tags-search
  @kbd{M-x tags-search} reads a regexp using the minibuffer, then
searches for matches in all the files in the selected tags table, one
file at a time.  It displays the name of the file being searched so you
can follow its progress.  As soon as it finds an occurrence,
@code{tags-search} returns.

@kindex M-,
@findex tags-loop-continue
  Having found one match, you probably want to find all the rest.  To find
one more match, type @kbd{M-,} (@code{tags-loop-continue}) to resume the
@code{tags-search}.  This searches the rest of the current buffer, followed
by the remaining files of the tags table.@refill

@findex tags-query-replace
  @kbd{M-x tags-query-replace} performs a single
@code{query-replace-regexp} through all the files in the tags table.  It
reads a regexp to search for and a string to replace with, just like
ordinary @kbd{M-x query-replace-regexp}.  It searches much like @kbd{M-x
tags-search}, but repeatedly, processing matches according to your
input.  @xref{Replace}, for more information on query replace.

@vindex tags-case-fold-search
@cindex case-sensitivity and tags search
  You can control the case-sensitivity of tags search commands by
customizing the value of the variable @code{tags-case-fold-search}.  The
default is to use the same setting as the value of
@code{case-fold-search} (@pxref{Search Case}).

  It is possible to get through all the files in the tags table with a
single invocation of @kbd{M-x tags-query-replace}.  But often it is
useful to exit temporarily, which you can do with any input event that
has no special query replace meaning.  You can resume the query replace
subsequently by typing @kbd{M-,}; this command resumes the last tags
search or replace command that you did.

  The commands in this section carry out much broader searches than the
@code{find-tag} family.  The @code{find-tag} commands search only for
definitions of tags that match your substring or regexp.  The commands
@code{tags-search} and @code{tags-query-replace} find every occurrence
of the regexp, as ordinary search commands and replace commands do in
the current buffer.

  These commands create buffers only temporarily for the files that they
have to search (those which are not already visited in Emacs buffers).
Buffers in which no match is found are quickly killed; the others
continue to exist.

  It may have struck you that @code{tags-search} is a lot like
@code{grep}.  You can also run @code{grep} itself as an inferior of
Emacs and have Emacs show you the matching lines one by one.  This works
much like running a compilation; finding the source locations of the
@code{grep} matches works like finding the compilation errors.
@xref{Compilation}.

@node List Tags
@subsection Tags Table Inquiries

@table @kbd
@item M-x list-tags @key{RET} @var{file} @key{RET}
Display a list of the tags defined in the program file @var{file}.
@item M-x tags-apropos @key{RET} @var{regexp} @key{RET}
Display a list of all tags matching @var{regexp}.
@end table

@findex list-tags
  @kbd{M-x list-tags} reads the name of one of the files described by
the selected tags table, and displays a list of all the tags defined in
that file.  The ``file name'' argument is really just a string to
compare against the file names recorded in the tags table; it is read as
a string rather than as a file name.  Therefore, completion and
defaulting are not available, and you must enter the file name the same
way it appears in the tags table.  Do not include a directory as part of
the file name unless the file name recorded in the tags table includes a
directory.

@findex tags-apropos
@vindex tags-apropos-verbose
  @kbd{M-x tags-apropos} is like @code{apropos} for tags
(@pxref{Apropos}).  It finds all the tags in the selected tags table
whose entries match @var{regexp}, and displays them.  If the variable
@code{tags-apropos-verbose} is non-@code{nil}, it displays the names
of the tags files together with the tag names.

@vindex tags-tag-face
@vindex tags-apropos-additional-actions
You can customize the appearance of the output with the face
@code{tags-tag-face}.  You can display additional output with @kbd{M-x
tags-apropos} by customizing the variable
@code{tags-apropos-additional-actions}---see its documentation for
details.

  You can also use the collection of tag names to complete a symbol
name in the buffer.  @xref{Symbol Completion}.

@node Imenu
@section Imenu
@cindex indexes of buffer contents
@cindex buffer content indexes
@cindex tags

  The Imenu facility is another way to find definitions or sections
in a file.  It is similar in spirit to Tags, but operates on a single
buffer only, and works entirely within Emacs with no need for a separate
tags table.

@findex imenu
@findex imenu-add-menu-bar-index
  If you type @kbd{M-x imenu}, it reads the name of a section or
definition in the current buffer, then goes to that section or
definition.  You can use completion to specify the name, and a
complete list of possible names is always displayed.

  Alternatively you can bind the command @code{imenu} to a mouse
click.  Then it displays mouse menus for you to select the section or
definition you want.  You can also add the buffer's index to the menu
bar by calling @code{imenu-add-menu-bar-index}.  If you want to have
this menu bar item available for all buffers in a certain major mode,
you can do this by adding @code{imenu-add-menu-bar-index} to its mode
hook.  But then you will have to wait for the buffer to be searched
for sections and definitions, each time you visit a file which uses
that mode.

@vindex imenu-auto-rescan
  When you change the contents of a buffer, if you add or delete
definitions or sections, you can update the buffer's index to
correspond to the new contents by invoking the @samp{*Rescan*} item in
the menu.  Rescanning happens automatically if
@code{imenu-auto-rescan} is non-@code{nil}.  There is no need to
rescan because of small changes in the text.

@vindex imenu-sort-function
  You can customize the way the menus are sorted via the variable
@code{imenu-sort-function}.  By default names are ordered as they
occur in the buffer; alphabetic sorting is provided as an alternative.

  Imenu provides the information to guide Which Function mode
(@pxref{Which Function}).  The Speedbar can also use it
(@pxref{Speedbar}).

@node Emerge, C Modes, Imenu, Programs
@section Merging Files with Emerge
@cindex Emerge
@cindex merging files

It's not unusual for programmers to get their signals crossed and modify
the same program in two different directions.  To recover from this
confusion, you need to merge the two versions.  Emerge makes this
easier.  See also @ref{Comparing Files}, for commands to compare
in a more manual fashion, and @ref{,Ediff,, ediff, The Ediff Manual}.

@menu
* Overview of Emerge::  How to start Emerge.  Basic concepts.
* Submodes of Emerge::  Fast mode vs. Edit mode.
                          Skip Prefers mode and Auto Advance mode.
* State of Difference:: You do the merge by specifying state A or B
                          for each difference.
* Merge Commands::      Commands for selecting a difference,
                          changing states of differences, etc.
* Exiting Emerge::      What to do when you've finished the merge.
* Combining in Emerge::     How to keep both alternatives for a difference.
* Fine Points of Emerge::   Misc.
@end menu

@node Overview of Emerge
@subsection Overview of Emerge

To start Emerge, run one of these four commands:

@table @kbd
@item M-x emerge-files
@findex emerge-files
Merge two specified files.

@item M-x emerge-files-with-ancestor
@findex emerge-files-with-ancestor
Merge two specified files, with reference to a common ancestor.

@item M-x emerge-buffers
@findex emerge-buffers
Merge two buffers.

@item M-x emerge-buffers-with-ancestor
@findex emerge-buffers-with-ancestor
Merge two buffers with reference to a common ancestor in a third
buffer.
@end table

@cindex merge buffer (Emerge)
@cindex A and B buffers (Emerge)
  The Emerge commands compare two files or buffers, and display the
comparison in three buffers: one for each input text (the @dfn{A buffer}
and the @dfn{B buffer}), and one (the @dfn{merge buffer}) where merging
takes place.  The merge buffer shows the full merged text, not just the
differences.  Wherever the two input texts differ, you can choose which
one of them to include in the merge buffer.

  The Emerge commands that take input from existing buffers use only the
accessible portions of those buffers, if they are narrowed
(@pxref{Narrowing}).

  If a common ancestor version is available, from which the two texts to
be merged were both derived, Emerge can use it to guess which
alternative is right.  Wherever one current version agrees with the
ancestor, Emerge presumes that the other current version is a deliberate
change which should be kept in the merged version.  Use the
@samp{with-ancestor} commands if you want to specify a common ancestor
text.  These commands read three file or buffer names---variant A,
variant B, and the common ancestor.

  After the comparison is done and the buffers are prepared, the
interactive merging starts.  You control the merging by typing special
@dfn{merge commands} in the merge buffer.  The merge buffer shows you a
full merged text, not just differences.  For each run of differences
between the input texts, you can choose which one of them to keep, or
edit them both together.

  The merge buffer uses a special major mode, Emerge mode, with commands
for making these choices.  But you can also edit the buffer with
ordinary Emacs commands.

  At any given time, the attention of Emerge is focused on one
particular difference, called the @dfn{selected} difference.  This
difference is marked off in the three buffers like this:

@example
vvvvvvvvvvvvvvvvvvvv
@var{text that differs}
^^^^^^^^^^^^^^^^^^^^
@end example

@noindent
Emerge numbers all the differences sequentially and the mode
line always shows the number of the selected difference.

  Normally, the merge buffer starts out with the A version of the text.
But when the A version of a difference agrees with the common ancestor,
then the B version is initially preferred for that difference.

  Emerge leaves the merged text in the merge buffer when you exit.  At
that point, you can save it in a file with @kbd{C-x C-w}.  If you give a
numeric argument to @code{emerge-files} or
@code{emerge-files-with-ancestor}, it reads the name of the output file
using the minibuffer.  (This is the last file name those commands read.)
Then exiting from Emerge saves the merged text in the output file.

  Normally, Emerge commands save the output buffer in its file when you
exit.  If you abort Emerge with @kbd{C-]}, the Emerge command does not
save the output buffer, but you can save it yourself if you wish.

@node Submodes of Emerge
@subsection Submodes of Emerge

  You can choose between two modes for giving merge commands: Fast mode
and Edit mode.  In Fast mode, basic merge commands are single
characters, but ordinary Emacs commands are disabled.  This is
convenient if you use only merge commands.  In Edit mode, all merge
commands start with the prefix key @kbd{C-c C-c}, and the normal Emacs
commands are also available.  This allows editing the merge buffer, but
slows down Emerge operations.

  Use @kbd{e} to switch to Edit mode, and @kbd{C-c C-c f} to switch to
Fast mode.  The mode line indicates Edit and Fast modes with @samp{E}
and @samp{F}.

  Emerge has two additional submodes that affect how particular merge
commands work: Auto Advance mode and Skip Prefers mode.

  If Auto Advance mode is in effect, the @kbd{a} and @kbd{b} commands
advance to the next difference.  This lets you go through the merge
faster as long as you simply choose one of the alternatives from the
input.  The mode line indicates Auto Advance mode with @samp{A}.

  If Skip Prefers mode is in effect, the @kbd{n} and @kbd{p} commands
skip over differences in states prefer-A and prefer-B (@pxref{State of
Difference}).  Thus you see only differences for which neither version
is presumed ``correct.''  The mode line indicates Skip Prefers mode with
@samp{S}.

@findex emerge-auto-advance-mode
@findex emerge-skip-prefers-mode
  Use the command @kbd{s a} (@code{emerge-auto-advance-mode}) to set or
clear Auto Advance mode.  Use @kbd{s s}
(@code{emerge-skip-prefers-mode}) to set or clear Skip Prefers mode.
These commands turn on the mode with a positive argument, turns it off
with a negative or zero argument, and toggle the mode with no argument.

@node State of Difference
@subsection State of a Difference

  In the merge buffer, a difference is marked with lines of @samp{v} and
@samp{^} characters.  Each difference has one of these seven states:

@table @asis
@item A
The difference is showing the A version.  The @kbd{a} command always
produces this state; the mode line indicates it with @samp{A}.

@item B
The difference is showing the B version.  The @kbd{b} command always
produces this state; the mode line indicates it with @samp{B}.

@item default-A
@itemx default-B
The difference is showing the A or the B state by default, because you
haven't made a choice.  All differences start in the default-A state
(and thus the merge buffer is a copy of the A buffer), except those for
which one alternative is ``preferred'' (see below).

When you select a difference, its state changes from default-A or
default-B to plain A or B.  Thus, the selected difference never has
state default-A or default-B, and these states are never displayed in
the mode line.

The command @kbd{d a} chooses default-A as the default state, and @kbd{d
b} chooses default-B.  This chosen default applies to all differences
which you haven't ever selected and for which no alternative is preferred.
If you are moving through the merge sequentially, the differences you
haven't selected are those following the selected one.  Thus, while
moving sequentially, you can effectively make the A version the default
for some sections of the merge buffer and the B version the default for
others by using @kbd{d a} and @kbd{d b} between sections.

@item prefer-A
@itemx prefer-B
The difference is showing the A or B state because it is
@dfn{preferred}.  This means that you haven't made an explicit choice,
but one alternative seems likely to be right because the other
alternative agrees with the common ancestor.  Thus, where the A buffer
agrees with the common ancestor, the B version is preferred, because
chances are it is the one that was actually changed.

These two states are displayed in the mode line as @samp{A*} and @samp{B*}.

@item combined
The difference is showing a combination of the A and B states, as a
result of the @kbd{x c} or @kbd{x C} commands.

Once a difference is in this state, the @kbd{a} and @kbd{b} commands
don't do anything to it unless you give them a numeric argument.

The mode line displays this state as @samp{comb}.
@end table

@node Merge Commands
@subsection Merge Commands

  Here are the Merge commands for Fast mode; in Edit mode, precede them
with @kbd{C-c C-c}:

@table @kbd
@item p
Select the previous difference.

@item n
Select the next difference.

@item a
Choose the A version of this difference.

@item b
Choose the B version of this difference.

@item C-u @var{n} j
Select difference number @var{n}.

@item .
Select the difference containing point.  You can use this command in the
merge buffer or in the A or B buffer.

@item q
Quit---finish the merge.

@item C-]
Abort---exit merging and do not save the output.

@item f
Go into Fast mode.  (In Edit mode, this is actually @kbd{C-c C-c f}.)

@item e
Go into Edit mode.

@item l
Recenter (like @kbd{C-l}) all three windows.

@item -
Specify part of a prefix numeric argument.

@item @var{digit}
Also specify part of a prefix numeric argument.

@item d a
Choose the A version as the default from here down in
the merge buffer.

@item d b
Choose the B version as the default from here down in
the merge buffer.

@item c a
Copy the A version of this difference into the kill ring.

@item c b
Copy the B version of this difference into the kill ring.

@item i a
Insert the A version of this difference at point.

@item i b
Insert the B version of this difference at point.

@item m
Put point and mark around the difference.

@item ^
Scroll all three windows down (like @kbd{M-v}).

@item v
Scroll all three windows up (like @kbd{C-v}).

@item <
Scroll all three windows left (like @kbd{C-x <}).

@item >
Scroll all three windows right (like @kbd{C-x >}).

@item |
Reset horizontal scroll on all three windows.

@item x 1
Shrink the merge window to one line.  (Use @kbd{C-u l} to restore it
to full size.)

@item x c
Combine the two versions of this difference (@pxref{Combining in
Emerge}).

@item x f
Show the names of the files/buffers Emerge is operating on, in a Help
window.  (Use @kbd{C-u l} to restore windows.)

@item x j
Join this difference with the following one.
(@kbd{C-u x j} joins this difference with the previous one.)

@item x s
Split this difference into two differences.  Before you use this
command, position point in each of the three buffers at the place where
you want to split the difference.

@item x t
Trim identical lines off the top and bottom of the difference.
Such lines occur when the A and B versions are
identical but differ from the ancestor version.
@end table

@node Exiting Emerge
@subsection Exiting Emerge

  The @kbd{q} command (@code{emerge-quit}) finishes the merge, storing
the results into the output file if you specified one.  It restores the
A and B buffers to their proper contents, or kills them if they were
created by Emerge and you haven't changed them.  It also disables the
Emerge commands in the merge buffer, since executing them later could
damage the contents of the various buffers.

  @kbd{C-]} aborts the merge.  This means exiting without writing the
output file.  If you didn't specify an output file, then there is no
real difference between aborting and finishing the merge.

  If the Emerge command was called from another Lisp program, then its
return value is @code{t} for successful completion, or @code{nil} if you
abort.

@node Combining in Emerge
@subsection Combining the Two Versions

  Sometimes you want to keep @emph{both} alternatives for a particular
difference.  To do this, use @kbd{x c}, which edits the merge buffer
like this:

@example
@group
#ifdef NEW
@var{version from A buffer}
#else /* not NEW */
@var{version from B buffer}
#endif /* not NEW */
@end group
@end example

@noindent
@vindex emerge-combine-versions-template
While this example shows C preprocessor conditionals delimiting the two
alternative versions, you can specify the strings to use by setting
the variable @code{emerge-combine-versions-template} to a string of your
choice.  In the string, @samp{%a} says where to put version A, and
@samp{%b} says where to put version B.  The default setting, which
produces the results shown above, looks like this:

@example
@group
"#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"
@end group
@end example

@node Fine Points of Emerge
@subsection Fine Points of Emerge

  During the merge, you mustn't try to edit the A and B buffers yourself.
Emerge modifies them temporarily, but ultimately puts them back the way
they were.

  You can have any number of merges going at once---just don't use any one
buffer as input to more than one merge at once, since the temporary
changes made in these buffers would get in each other's way.

  Starting Emerge can take a long time because it needs to compare the
files fully.  Emacs can't do anything else until @code{diff} finishes.
Perhaps in the future someone will change Emerge to do the comparison in
the background when the input files are large---then you could keep on
doing other things with Emacs until Emerge is ready to accept
commands.

@vindex emerge-startup-hook
  After setting up the merge, Emerge runs the hook
@code{emerge-startup-hook} (@pxref{Hooks}).

@node C Modes
@section C and Related Modes
@cindex C mode
@cindex Java mode
@cindex Pike mode
@cindex IDL mode
@cindex CORBA IDL mode
@cindex Objective C mode
@cindex C++ mode
@cindex mode, Java
@cindex mode, C
@cindex mode, Objective C
@cindex mode, CORBA IDL
@cindex mode, Pike

  This section gives a brief description of the special features
available in C, C++, Objective-C, Java, CORBA IDL, and Pike modes.
(These are called ``C mode and related modes.'')  @xref{Top, CC Mode,
ccmode, , CC Mode}, for a more extensive description of these modes
and their special features.

@menu
* Motion in C::
* Electric C::
* Hungry Delete::
* Other C Commands::
* Comments in C::
@end menu

@node Motion in C
@subsection C Mode Motion Commands

  This section describes commands for moving point, in C mode and
related modes.

@table @code
@item C-c C-u
@kindex C-c C-u @r{(C mode)}
@findex c-up-conditional
Move point back to the containing preprocessor conditional, leaving the
mark behind.  A prefix argument acts as a repeat count.  With a negative
argument, move point forward to the end of the containing
preprocessor conditional.  When going backwards, @code{#elif} is treated
like @code{#else} followed by @code{#if}.  When going forwards,
@code{#elif} is ignored.@refill

@item C-c C-p
@kindex C-c C-p @r{(C mode)}
@findex c-backward-conditional
Move point back over a preprocessor conditional, leaving the mark
behind.  A prefix argument acts as a repeat count.  With a negative
argument, move forward.

@item C-c C-n
@kindex C-c C-n @r{(C mode)}
@findex c-forward-conditional
Move point forward across a preprocessor conditional, leaving the mark
behind.  A prefix argument acts as a repeat count.  With a negative
argument, move backward.

@item M-a
@kindex ESC a
@findex c-beginning-of-statement
Move point to the beginning of the innermost C statement
(@code{c-beginning-of-statement}).  If point is already at the beginning
of a statement, move to the beginning of the preceding statement.  With
prefix argument @var{n}, move back @var{n} @minus{} 1 statements.

If point is within a string or comment, or next to a comment (only
whitespace between them), this command moves by sentences instead of
statements.

When called from a program, this function takes three optional
arguments: the numeric prefix argument, a buffer position limit
(don't move back before that place), and a flag that controls whether
to do sentence motion when inside of a comment.

@item M-e
@kindex ESC e
@findex c-end-of-statement
Move point to the end of the innermost C statement; like @kbd{M-a}
except that it moves in the other direction (@code{c-end-of-statement}).

@item M-x c-backward-into-nomenclature
@findex c-backward-into-nomenclature
Move point backward to beginning of a C++ nomenclature section or word.
With prefix argument @var{n}, move @var{n} times.  If @var{n} is
negative, move forward.  C++ nomenclature means a symbol name in the
style of NamingSymbolsWithMixedCaseAndNoUnderlines; each capital letter
begins a section or word.

In the GNU project, we recommend using underscores to separate words
within an identifier in C or C++, rather than using case distinctions.

@item M-x c-forward-into-nomenclature
@findex c-forward-into-nomenclature
Move point forward to end of a C++ nomenclature section or word.
With prefix argument @var{n}, move @var{n} times.
@end table

@node Electric C
@subsection Electric C Characters

  In C mode and related modes, certain printing characters are
``electric''---in addition to inserting themselves, they also reindent
the current line and may insert newlines.  This feature is controlled by
the variable @code{c-auto-newline}.  The ``electric'' characters are
@kbd{@{}, @kbd{@}}, @kbd{:}, @kbd{#}, @kbd{;}, @kbd{,}, @kbd{<},
@kbd{>}, @kbd{/}, @kbd{*}, @kbd{(}, and @kbd{)}.

  Electric characters insert newlines only when the @dfn{auto-newline}
feature is enabled (indicated by @samp{/a} in the mode line after the
mode name).  This feature is controlled by the variable
@code{c-auto-newline}.  You can turn this feature on or off with the
command @kbd{C-c C-a}:

@table @kbd
@item C-c C-a
@kindex C-c C-a @r{(C mode)}
@findex c-toggle-auto-state
Toggle the auto-newline feature (@code{c-toggle-auto-state}).  With a
prefix argument, this command turns the auto-newline feature on if the
argument is positive, and off if it is negative.
@end table

  The colon character is electric because that is appropriate for a
single colon.  But when you want to insert a double colon in C++, the
electric behavior of colon is inconvenient.  You can insert a double
colon with no reindentation or newlines by typing @kbd{C-c :}:

@table @kbd
@item C-c :
@kindex C-c : @r{(C mode)}
@findex c-scope-operator
Insert a double colon scope operator at point, without reindenting the
line or adding any newlines (@code{c-scope-operator}).
@end table

  The electric @kbd{#} key reindents the line if it appears to be the
beginning of a preprocessor directive.  This happens when the value of
@code{c-electric-pound-behavior} is @code{(alignleft)}.  You can turn
this feature off by setting @code{c-electric-pound-behavior} to
@code{nil}.

   The variable @code{c-hanging-braces-alist} controls the insertion of
newlines before and after inserted braces.  It is an association list
with elements of the following form: @code{(@var{syntactic-symbol}
. @var{nl-list})}.  Most of the syntactic symbols that appear in
@code{c-offsets-alist} are meaningful here as well.

   The list @var{nl-list} may contain either of the symbols
@code{before} or @code{after}, or both; or it may be @code{nil}.  When a
brace is inserted, the syntactic context it defines is looked up in
@code{c-hanging-braces-alist}; if it is found, the @var{nl-list} is used
to determine where newlines are inserted: either before the brace,
after, or both.  If not found, the default is to insert a newline both
before and after braces.

   The variable @code{c-hanging-colons-alist} controls the insertion of
newlines before and after inserted colons.  It is an association list
with elements of the following form: @code{(@var{syntactic-symbol}
. @var{nl-list})}.  The list @var{nl-list} may contain either of the
symbols @code{before} or @code{after}, or both; or it may be @code{nil}.

   When a colon is inserted, the syntactic symbol it defines is looked
up in this list, and if found, the @var{nl-list} is used to determine
where newlines are inserted: either before the brace, after, or both.
If the syntactic symbol is not found in this list, no newlines are
inserted.

   Electric characters can also delete newlines automatically when the
auto-newline feature is enabled.  This feature makes auto-newline more
acceptable, by deleting the newlines in the most common cases where you
do not want them.  Emacs can recognize several cases in which deleting a
newline might be desirable; by setting the variable
@code{c-cleanup-list}, you can specify @emph{which} of these cases that
should happen.  The variable's value is a list of symbols, each
describing one case for possible deletion of a newline.  Here are the
meaningful symbols, and their meanings:

@table @code
@item brace-catch-brace
Clean up @samp{@} catch (@var{condition}) @{} constructs by placing the
entire construct on a single line.  The clean-up occurs when you type
the @samp{@{}, if there is nothing between the braces aside from
@code{catch} and @var{condition}.

@item brace-else-brace
Clean up @samp{@} else @{} constructs by placing the entire construct on
a single line.  The clean-up occurs when you type the @samp{@{} after
the @code{else}, but only if there is nothing but white space between
the braces and the @code{else}.

@item brace-elseif-brace
Clean up @samp{@} else if (@dots{}) @{} constructs by placing the entire
construct on a single line.  The clean-up occurs when you type the
@samp{@{}, if there is nothing but white space between the @samp{@}} and
@samp{@{} aside from the keywords and the @code{if}-condition.

@item empty-defun-braces
Clean up empty defun braces by placing the braces on the same
line.  Clean-up occurs when you type the closing brace.

@item defun-close-semi
Clean up the semicolon after a @code{struct} or similar type
declaration, by placing the semicolon on the same line as the closing
brace.  Clean-up occurs when you type the semicolon.

@item list-close-comma
Clean up commas following braces in array and aggregate
initializers.  Clean-up occurs when you type the comma.

@item scope-operator
Clean up double colons which may designate a C++ scope operator, by
placing the colons together.  Clean-up occurs when you type the second
colon, but only when the two colons are separated by nothing but
whitespace.
@end table

@node Hungry Delete
@subsection Hungry Delete Feature in C

  When the @dfn{hungry-delete} feature is enabled (indicated by
@samp{/h} or @samp{/ah} in the mode line after the mode name), a single
@key{DEL} command deletes all preceding whitespace, not just one space.
To turn this feature on or off, use @kbd{C-c C-d}:

@table @kbd
@item C-c C-d
@kindex C-c C-d @r{(C mode)}
@findex c-toggle-hungry-state
Toggle the hungry-delete feature (@code{c-toggle-hungry-state}).  With a
prefix argument, this command turns the hungry-delete feature on if the
argument is positive, and off if it is negative.

@item C-c C-t
@kindex C-c C-t @r{(C mode)}
@findex c-toggle-auto-hungry-state
Toggle the auto-newline and hungry-delete features, both at once
(@code{c-toggle-auto-hungry-state}).
@end table

@vindex c-hungry-delete-key
   The variable @code{c-hungry-delete-key} controls whether the
hungry-delete feature is enabled.

@node Other C Commands
@subsection Other Commands for C Mode

@table @kbd
@item C-M-h
Put mark at the end of a function definition, and put point at the
beginning (@code{c-mark-function}).

@item M-q
@kindex M-q @r{(C mode)}
@findex c-fill-paragraph
Fill a paragraph, handling C and C++ comments (@code{c-fill-paragraph}).
If any part of the current line is a comment or within a comment, this
command fills the comment or the paragraph of it that point is in,
preserving the comment indentation and comment delimiters.

@item C-c C-e
@cindex macro expansion in C
@cindex expansion of C macros
@findex c-macro-expand
@kindex C-c C-e @r{(C mode)}
Run the C preprocessor on the text in the region, and show the result,
which includes the expansion of all the macro calls
(@code{c-macro-expand}).  The buffer text before the region is also
included in preprocessing, for the sake of macros defined there, but the
output from this part isn't shown.

When you are debugging C code that uses macros, sometimes it is hard to
figure out precisely how the macros expand.  With this command, you
don't have to figure it out; you can see the expansions.

@item C-c C-\
@findex c-backslash-region
@kindex C-c C-\ @r{(C mode)}
Insert or align @samp{\} characters at the ends of the lines of the
region (@code{c-backslash-region}).  This is useful after writing or
editing a C macro definition.

If a line already ends in @samp{\}, this command adjusts the amount of
whitespace before it.  Otherwise, it inserts a new @samp{\}.  However,
the last line in the region is treated specially; no @samp{\} is
inserted on that line, and any @samp{\} there is deleted.

@item M-x cpp-highlight-buffer
@cindex preprocessor highlighting
@findex cpp-highlight-buffer
Highlight parts of the text according to its preprocessor conditionals.
This command displays another buffer named @samp{*CPP Edit*}, which
serves as a graphic menu for selecting how to display particular kinds
of conditionals and their contents.  After changing various settings,
click on @samp{[A]pply these settings} (or go to that buffer and type
@kbd{a}) to rehighlight the C mode buffer accordingly.

@item C-c C-s
@findex c-show-syntactic-information
@kindex C-c C-s @r{(C mode)}
Display the syntactic information about the current source line
(@code{c-show-syntactic-information}).  This is the information that
directs how the line is indented.

@item M-x cwarn-mode
@itemx M-x global-cwarn-mode
@findex cwarn-mode
@findex global-cwarn-mode
@cindex CWarn mode
@cindex suspicious constructions in C, C++
CWarn minor mode highlights certain suspicious C and C++ constructions:

@itemize @bullet{}
@item
Assignments inside expressions.
@item
Semicolon following immediately after @samp{if}, @samp{for}, and @samp{while}
(except after a @samp{do @dots{} while} statement);
@item
C++ functions with reference parameters.
@end itemize

@noindent
You can enable the mode for one buffer with the command @kbd{M-x
cwarn-mode}, or for all suitable buffers with the command @kbd{M-x
global-cwarn-mode} or by customizing the variable
@code{global-cwarn-mode}.  You must also enable Font Lock mode to make
it work.

@item M-x hide-ifdef-mode
@findex hide-ifdef-mode
@cindex Hide-ifdef mode
Hide-ifdef minor mode hides selected code within @samp{#if} and
@samp{#ifdef} preprocessor blocks.  See the documentation string of
@code{hide-ifdef-mode} for more information.

@item M-x ff-find-related-file
@cindex related files
@findex ff-find-related-file
@vindex ff-related-file-alist
Find a file ``related'' in a special way to the file visited by the
current buffer.  Typically this will be the header file corresponding
to a C/C++ source file, or vice versa.  The variable
@code{ff-related-file-alist} specifies how to compute related file
names.
@end table

@node Comments in C
@subsection Comments in C Modes

   C mode and related modes use a number of variables for controlling
comment format.

@table @code
@item c-comment-only-line-offset
@vindex c-comment-only-line-offset
Extra offset for line which contains only the start of a comment.  It
can be either an integer or a cons cell of the form
@code{(@var{non-anchored-offset} . @var{anchored-offset})}, where
@var{non-anchored-offset} is the amount of offset given to
non-column-zero anchored comment-only lines, and @var{anchored-offset}
is the amount of offset to give column-zero anchored comment-only lines.
Just an integer as value is equivalent to @code{(@var{val} . 0)}.

@item c-comment-start-regexp
@vindex c-comment-start-regexp
This buffer-local variable specifies how to recognize the start of a comment.

@item c-hanging-comment-ender-p
@vindex c-hanging-comment-ender-p
If this variable is @code{nil}, @code{c-fill-paragraph} leaves the
comment terminator of a block comment on a line by itself.  The default
value is @code{t}, which puts the comment-end delimiter @samp{*/} at the
end of the last line of the comment text.

@item c-hanging-comment-starter-p
@vindex c-hanging-comment-starter-p
If this variable is @code{nil}, @code{c-fill-paragraph} leaves the
starting delimiter of a block comment on a line by itself.  The default
value is @code{t}, which puts the comment-start delimiter @samp{/*} at
the beginning of the first line of the comment text.
@end table

@node Fortran
@section Fortran Mode
@cindex Fortran mode
@cindex mode, Fortran

  Fortran mode provides special motion commands for Fortran statements and
subprograms, and indentation commands that understand Fortran conventions
of nesting, line numbers and continuation statements.  Fortran mode has
its own Auto Fill mode that breaks long lines into proper Fortran
continuation lines.

  Special commands for comments are provided because Fortran comments
are unlike those of other languages.  Built-in abbrevs optionally save
typing when you insert Fortran keywords.

  Use @kbd{M-x fortran-mode} to switch to this major mode.  This command
runs the hook @code{fortran-mode-hook} (@pxref{Hooks}).

@cindex Fortran77 and Fortran90
@findex f90-mode
@findex fortran-mode
  Fortan mode is meant for editing Fortran77 ``fixed format'' source
code.  For editing the modern Fortran90 ``free format'' source code,
use F90 mode (@code{f90-mode}).  Emacs normally uses Fortran mode for
files with extension @samp{.f}, @samp{.F} or @samp{.for}, and F90 mode
for the extension @samp{.f90}.  GNU Fortran supports both kinds of
format.

@menu
* Motion: Fortran Motion.        Moving point by statements or subprograms.
* Indent: Fortran Indent.        Indentation commands for Fortran.
* Comments: Fortran Comments.    Inserting and aligning comments.
* Autofill: Fortran Autofill.    Auto fill minor mode for Fortran.
* Columns: Fortran Columns.      Measuring columns for valid Fortran.
* Abbrev: Fortran Abbrev.        Built-in abbrevs for Fortran keywords.
@end menu

@node Fortran Motion
@subsection Motion Commands

  In addition to the normal commands for moving by and operating on
``defuns'' (Fortran subprograms---functions and subroutines), Fortran
mode provides special commands to move by statements.

@table @kbd
@kindex C-c C-n @r{(Fortran mode)}
@findex fortran-next-statement
@item C-c C-n
Move to beginning of current or next statement
(@code{fortran-next-statement}).

@kindex C-c C-p @r{(Fortran mode)}
@findex fortran-previous-statement
@item C-c C-p
Move to beginning of current or previous statement
(@code{fortran-previous-statement}).
@end table

@node Fortran Indent
@subsection Fortran Indentation

  Special commands and features are needed for indenting Fortran code in
order to make sure various syntactic entities (line numbers, comment line
indicators and continuation line flags) appear in the columns that are
required for standard Fortran.

@menu
* Commands: ForIndent Commands.  Commands for indenting and filling Fortran.
* Contline: ForIndent Cont.      How continuation lines indent.
* Numbers:  ForIndent Num.       How line numbers auto-indent.
* Conv:     ForIndent Conv.      Conventions you must obey to avoid trouble.
* Vars:     ForIndent Vars.      Variables controlling Fortran indent style.
@end menu

@node ForIndent Commands
@subsubsection Fortran Indentation and Filling Commands

@table @kbd
@item C-M-j
Break the current line and set up a continuation line
(@code{fortran-split-line}).
@item M-^
Join this line to the previous line (@code{fortran-join-line}).
@item C-M-q
Indent all the lines of the subprogram point is in
(@code{fortran-indent-subprogram}).
@item M-q
Fill a comment block or statement.
@end table

@kindex C-M-q @r{(Fortran mode)}
@findex fortran-indent-subprogram
  The key @kbd{C-M-q} runs @code{fortran-indent-subprogram}, a command
to reindent all the lines of the Fortran subprogram (function or
subroutine) containing point.

@kindex C-M-j @r{(Fortran mode)}
@findex fortran-split-line
  The key @kbd{C-M-j} runs @code{fortran-split-line}, which splits
a line in the appropriate fashion for Fortran.  In a non-comment line,
the second half becomes a continuation line and is indented
accordingly.  In a comment line, both halves become separate comment
lines.

@kindex M-^ @r{(Fortran mode)}
@kindex C-c C-d @r{(Fortran mode)}
@findex fortran-join-line
  @kbd{M-^} or @kbd{C-c C-d} runs the command @code{fortran-join-line},
which joins a continuation line back to the previous line, roughly as
the inverse of @code{fortran-split-line}.  The point must be on a
continuation line when this command is invoked.

@kindex M-q @r{(Fortran mode)}
@kbd{M-q} in Fortran mode fills the comment block or statement that
point is in.  This removes any excess statement continuations.

@node ForIndent Cont
@subsubsection Continuation Lines
@cindex Fortran continuation lines

@vindex fortran-continuation-string
  Most modern Fortran compilers allow two ways of writing continuation
lines.  If the first non-space character on a line is in column 5, then
that line is a continuation of the previous line.  We call this
@dfn{fixed format}.  (In GNU Emacs we always count columns from 0.)  The
variable @code{fortran-continuation-string} specifies what character to
put on column 5.  A line that starts with a tab character followed by
any digit except @samp{0} is also a continuation line.  We call this
style of continuation @dfn{tab format}.

@vindex indent-tabs-mode @r{(Fortran mode)}
  Fortran mode can make either style of continuation line, but you
must specify which one you prefer.  The value of the variable
@code{indent-tabs-mode} controls the choice: @code{nil} for fixed
format, and non-@code{nil} for tab format.  You can tell which style
is presently in effect by the presence or absence of the string
@samp{Tab} in the mode line.

  If the text on a line starts with the conventional Fortran
continuation marker @samp{$}, or if it begins with any non-whitespace
character in column 5, Fortran mode treats it as a continuation line.
When you indent a continuation line with @key{TAB}, it converts the line
to the current continuation style.  When you split a Fortran statement
with @kbd{C-M-j}, the continuation marker on the newline is created
according to the continuation style.

  The setting of continuation style affects several other aspects of
editing in Fortran mode.  In fixed format mode, the minimum column
number for the body of a statement is 6.  Lines inside of Fortran
blocks that are indented to larger column numbers always use only the
space character for whitespace.  In tab format mode, the minimum
column number for the statement body is 8, and the whitespace before
column 8 must always consist of one tab character.

@vindex fortran-tab-mode-default
@vindex fortran-analyze-depth
  When you enter Fortran mode for an existing file, it tries to deduce the
proper continuation style automatically from the file contents.  The first
line that begins with either a tab character or six spaces determines the
choice.  The variable @code{fortran-analyze-depth} specifies how many lines
to consider (at the beginning of the file); if none of those lines
indicates a style, then the variable @code{fortran-tab-mode-default}
specifies the style.  If it is @code{nil}, that specifies fixed format, and
non-@code{nil} specifies tab format.

@node ForIndent Num
@subsubsection Line Numbers

  If a number is the first non-whitespace in the line, Fortran
indentation assumes it is a line number and moves it to columns 0
through 4.  (Columns always count from 0 in GNU Emacs.)

@vindex fortran-line-number-indent
  Line numbers of four digits or less are normally indented one space.
The variable @code{fortran-line-number-indent} controls this; it
specifies the maximum indentation a line number can have.  Line numbers
are indented to right-justify them to end in column 4 unless that would
require more than this maximum indentation.  The default value of the
variable is 1.

@vindex fortran-electric-line-number
  Simply inserting a line number is enough to indent it according to
these rules.  As each digit is inserted, the indentation is recomputed.
To turn off this feature, set the variable
@code{fortran-electric-line-number} to @code{nil}.  Then inserting line
numbers is like inserting anything else.

@node ForIndent Conv
@subsubsection Syntactic Conventions

  Fortran mode assumes that you follow certain conventions that simplify
the task of understanding a Fortran program well enough to indent it
properly:

@itemize @bullet
@item
Two nested @samp{do} loops never share a @samp{continue} statement.

@item
Fortran keywords such as @samp{if}, @samp{else}, @samp{then}, @samp{do}
and others are written without embedded whitespace or line breaks.

Fortran compilers generally ignore whitespace outside of string
constants, but Fortran mode does not recognize these keywords if they
are not contiguous.  Constructs such as @samp{else if} or @samp{end do}
are acceptable, but the second word should be on the same line as the
first and not on a continuation line.
@end itemize

@noindent
If you fail to follow these conventions, the indentation commands may
indent some lines unaesthetically.  However, a correct Fortran program
retains its meaning when reindented even if the conventions are not
followed.

@node ForIndent Vars
@subsubsection Variables for Fortran Indentation

@vindex fortran-do-indent
@vindex fortran-if-indent
@vindex fortran-structure-indent
@vindex fortran-continuation-indent
@vindex fortran-check-all-num@dots{}
@vindex fortran-minimum-statement-indent@dots{}
  Several additional variables control how Fortran indentation works:

@table @code
@item fortran-do-indent
Extra indentation within each level of @samp{do} statement (default 3).

@item fortran-if-indent
Extra indentation within each level of @samp{if} statement (default 3).
This value is also used for extra indentation within each level of the
Fortran 90 @samp{where} statement.

@item fortran-structure-indent
Extra indentation within each level of @samp{structure}, @samp{union}, or
@samp{map} statements (default 3).

@item fortran-continuation-indent
Extra indentation for bodies of continuation lines (default 5).

@item fortran-check-all-num-for-matching-do
If this is @code{nil}, indentation assumes that each @samp{do} statement
ends on a @samp{continue} statement.  Therefore, when computing
indentation for a statement other than @samp{continue}, it can save time
by not checking for a @samp{do} statement ending there.  If this is
non-@code{nil}, indenting any numbered statement must check for a
@samp{do} that ends there.  The default is @code{nil}.

@item fortran-blink-matching-if
If this is @code{t}, indenting an @samp{endif} statement moves the
cursor momentarily to the matching @samp{if} statement to show where it
is.  The default is @code{nil}.

@item fortran-minimum-statement-indent-fixed
Minimum indentation for fortran statements when using fixed format
continuation line style.  Statement bodies are never indented less than
this much.  The default is 6.

@item fortran-minimum-statement-indent-tab
Minimum indentation for fortran statements for tab format continuation line
style.  Statement bodies are never indented less than this much.  The
default is 8.
@end table

@node Fortran Comments
@subsection Fortran Comments

  The usual Emacs comment commands assume that a comment can follow a line
of code.  In Fortran, the standard comment syntax requires an entire line
to be just a comment.  Therefore, Fortran mode replaces the standard Emacs
comment commands and defines some new variables.

  Fortran mode can also handle the Fortran90 comment syntax where comments
start with @samp{!} and can follow other text.  Because only some Fortran77
compilers accept this syntax, Fortran mode will not insert such comments
unless you have said in advance to do so.  To do this, set the variable
@code{comment-start} to @samp{"!"} (@pxref{Variables}).

@table @kbd
@item M-;
Align comment or insert new comment (@code{fortran-comment-indent}).

@item C-x ;
Applies to nonstandard @samp{!} comments only.

@item C-c ;
Turn all lines of the region into comments, or (with argument) turn them back
into real code (@code{fortran-comment-region}).
@end table

  @kbd{M-;} in Fortran mode is redefined as the command
@code{fortran-comment-indent}.  Like the usual @kbd{M-;} command, this
recognizes any kind of existing comment and aligns its text appropriately;
if there is no existing comment, a comment is inserted and aligned.  But
inserting and aligning comments are not the same in Fortran mode as in
other modes.

  When a new comment must be inserted, if the current line is blank, a
full-line comment is inserted.  On a non-blank line, a nonstandard @samp{!}
comment is inserted if you have said you want to use them.  Otherwise a
full-line comment is inserted on a new line before the current line.

  Nonstandard @samp{!} comments are aligned like comments in other
languages, but full-line comments are different.  In a standard full-line
comment, the comment delimiter itself must always appear in column zero.
What can be aligned is the text within the comment.  You can choose from
three styles of alignment by setting the variable
@code{fortran-comment-indent-style} to one of these values:

@vindex fortran-comment-indent-style
@vindex fortran-comment-line-extra-indent
@table @code
@item fixed
Align the text at a fixed column, which is the sum of
@code{fortran-comment-line-extra-indent} and the minimum statement
indentation.  This is the default.

The minimum statement indentation is
@code{fortran-minimum-statement-indent-fixed} for fixed format
continuation line style and @code{fortran-minimum-statement-indent-tab}
for tab format style.

@item relative
Align the text as if it were a line of code, but with an additional
@code{fortran-comment-line-extra-indent} columns of indentation.

@item nil
Don't move text in full-line comments automatically at all.
@end table

@vindex fortran-comment-indent-char
  In addition, you can specify the character to be used to indent within
full-line comments by setting the variable
@code{fortran-comment-indent-char} to the single-character string you want
to use.

@vindex comment-line-start
@vindex comment-line-start-skip
  Fortran mode introduces two variables @code{comment-line-start} and
@code{comment-line-start-skip}, which play for full-line comments the same
roles played by @code{comment-start} and @code{comment-start-skip} for
ordinary text-following comments.  Normally these are set properly by
Fortran mode, so you do not need to change them.

  The normal Emacs comment command @kbd{C-x ;} has not been redefined.  If
you use @samp{!} comments, this command can be used with them.  Otherwise
it is useless in Fortran mode.

@kindex C-c ; @r{(Fortran mode)}
@findex fortran-comment-region
@vindex fortran-comment-region
  The command @kbd{C-c ;} (@code{fortran-comment-region}) turns all the
lines of the region into comments by inserting the string @samp{C$$$} at
the front of each one.  With a numeric argument, it turns the region
back into live code by deleting @samp{C$$$} from the front of each line
in it.  The string used for these comments can be controlled by setting
the variable @code{fortran-comment-region}.  Note that here we have an
example of a command and a variable with the same name; these two uses
of the name never conflict because in Lisp and in Emacs it is always
clear from the context which one is meant.

@node Fortran Autofill
@subsection Fortran Auto Fill Mode

  Fortran Auto Fill mode is a minor mode which automatically splits
Fortran statements as you insert them when they become too wide.
Splitting a statement involves making continuation lines using
@code{fortran-continuation-string} (@pxref{ForIndent Cont}).  This
splitting happens when you type @key{SPC}, @key{RET}, or @key{TAB}, and
also in the Fortran indentation commands.

@findex fortran-auto-fill-mode
  @kbd{M-x fortran-auto-fill-mode} turns Fortran Auto Fill mode on if it
was off, or off if it was on.  This command works the same as @kbd{M-x
auto-fill-mode} does for normal Auto Fill mode (@pxref{Filling}).  A
positive numeric argument turns Fortran Auto Fill mode on, and a
negative argument turns it off.  You can see when Fortran Auto Fill mode
is in effect by the presence of the word @samp{Fill} in the mode line,
inside the parentheses.  Fortran Auto Fill mode is a minor mode, turned
on or off for each buffer individually.  @xref{Minor Modes}.

@vindex fortran-break-before-delimiters
   Fortran Auto Fill mode breaks lines at spaces or delimiters when the
lines get longer than the desired width (the value of @code{fill-column}).
The delimiters that Fortran Auto Fill mode may break at are @samp{,},
@samp{'}, @samp{+}, @samp{-}, @samp{/}, @samp{*}, @samp{=}, and @samp{)}.
The line break comes after the delimiter if the variable
@code{fortran-break-before-delimiters} is @code{nil}.  Otherwise (and by
default), the break comes before the delimiter.

  By default, Fortran Auto Fill mode is not enabled.  If you want this
feature turned on permanently, add a hook function to
@code{fortran-mode-hook} to execute @code{(fortran-auto-fill-mode 1)}.
@xref{Hooks}.

@node Fortran Columns
@subsection Checking Columns in Fortran

@table @kbd
@item C-c C-r
Display a ``column ruler'' momentarily above the current line
(@code{fortran-column-ruler}).
@item C-c C-w
Split the current window horizontally temporarily so that it is 72
columns wide (@code{fortran-window-create-momentarily}).  This may
help you avoid making lines longer than the 72-character limit that
some Fortran compilers impose.
@item C-u C-c C-w
Split the current window horizontally so that it is 72 columns wide
(@code{fortran-window-create}).  You can then continue editing.
@item M-x fortran-strip-sequence-nos
Delete all text in column 72 and beyond.
@end table

@kindex C-c C-r @r{(Fortran mode)}
@findex fortran-column-ruler
  The command @kbd{C-c C-r} (@code{fortran-column-ruler}) shows a column
ruler momentarily above the current line.  The comment ruler is two lines
of text that show you the locations of columns with special significance in
Fortran programs.  Square brackets show the limits of the columns for line
numbers, and curly brackets show the limits of the columns for the
statement body.  Column numbers appear above them.

  Note that the column numbers count from zero, as always in GNU Emacs.
As a result, the numbers may be one less than those you are familiar
with; but the positions they indicate in the line are standard for
Fortran.

@vindex fortran-column-ruler-fixed
@vindex fortran-column-ruler-tabs
  The text used to display the column ruler depends on the value of
the variable @code{indent-tabs-mode}.  If @code{indent-tabs-mode} is
@code{nil}, then the value of the variable
@code{fortran-column-ruler-fixed} is used as the column ruler.
Otherwise, the variable @code{fortran-column-ruler-tab} is displayed.
By changing these variables, you can change the column ruler display.

@kindex C-c C-w @r{(Fortran mode)}
@findex fortran-window-create-momentarily
  @kbd{C-c C-w} (@code{fortran-window-create-momentarily}) temporarily
splits the current window horizontally, making a window 72 columns
wide, so you can see which lines that is too long.  Type a space to
restore the normal width.

@kindex C-u C-c C-w @r{(Fortran mode)}
@findex fortran-window-create
  You can also split the window horizontally and continue editing with
the split in place.  To do this, use @kbd{C-u C-c C-w} (@code{M-x
fortran-window-create}).  By editing in this window you can
immediately see when you make a line too wide to be correct Fortran.

@findex fortran-strip-sequence-nos
  The command @kbd{M-x fortran-strip-sequence-nos} deletes all text in
column 72 and beyond, on all lines in the current buffer.  This is the
easiest way to get rid of old sequence numbers.

@node Fortran Abbrev
@subsection Fortran Keyword Abbrevs

  Fortran mode provides many built-in abbrevs for common keywords and
declarations.  These are the same sort of abbrev that you can define
yourself.  To use them, you must turn on Abbrev mode.  @xref{Abbrevs}.

  The built-in abbrevs are unusual in one way: they all start with a
semicolon.  You cannot normally use semicolon in an abbrev, but Fortran
mode makes this possible by changing the syntax of semicolon to ``word
constituent.''

  For example, one built-in Fortran abbrev is @samp{;c} for
@samp{continue}.  If you insert @samp{;c} and then insert a punctuation
character such as a space or a newline, the @samp{;c} expands automatically
to @samp{continue}, provided Abbrev mode is enabled.@refill

  Type @samp{;?} or @samp{;C-h} to display a list of all the built-in
Fortran abbrevs and what they stand for.

@node Asm Mode
@section Asm Mode

@cindex Asm mode
@cindex assembler mode
Asm mode is a major mode for editing files of assembler code.  It
defines these commands:

@table @kbd
@item @key{TAB}
@code{tab-to-tab-stop}.
@item C-j
Insert a newline and then indent using @code{tab-to-tab-stop}.
@item :
Insert a colon and then remove the indentation from before the label
preceding colon.  Then do @code{tab-to-tab-stop}.
@item ;
Insert or align a comment.
@end table

  The variable @code{asm-comment-char} specifies which character
starts comments in assembler syntax.