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U. Editing Programs

Emacs provides many features to facilitate editing programs. Some of these features can

This chapter describes these features and many more.

U.1 Major Modes for Programming Languages  Major modes for editing programs.
U.2 Top-Level Definitions, or Defuns  Commands to operate on major top-level parts of a program.
U.3 Indentation for Programs  Adjusting indentation to show the nesting.
U.5 Manipulating Comments  Inserting, killing, and aligning comments.
U.4 Commands for Editing with Parentheses  Commands that operate on parentheses.
U.6 Documentation Lookup  Getting documentation of functions you plan to call.
U.7 Hideshow minor mode  Displaying blocks selectively.
U.8 Completion for Symbol Names  Completion on symbol names of your program or language.
U.9 Glasses minor mode  Making identifiersLikeThis more readable.
U.10 Other Features Useful for Editing Programs  Other Emacs features useful for editing programs.
U.11 C and Related Modes  Special commands of C, C++, Objective-C, Java, and Pike modes.
U.12 Fortran Mode  Fortran mode and its special features.
U.13 Asm Mode  Asm mode and its special features.

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U.1 Major Modes for Programming Languages

Emacs has specialized major modes for various programming languages. See section R. Major Modes. A programming language major mode typically specifies the syntax of expressions, the customary rules for indentation, how to do syntax highlighting for the language, and how to find the beginning of a function definition. It often customizes or provides facilities for compiling and debugging programs as well.

Ideally, Emacs should provide a major mode for each programming language that you might want to edit; if it doesn't have a mode for your favorite language, you can contribute one. But often the mode for one language can serve for other syntactically similar languages. The major mode for language l is called l-mode, and you can select it by typing M-x l-mode RET. See section R.1 How Major Modes are Chosen.

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

In most programming languages, indentation should vary from line to line to illustrate the structure of the program. So the major modes for programming languages arrange for TAB to update the indentation of the current line. They also rebind DEL to treat a tab as if it were the equivalent number of spaces; this lets you delete one column of indentation without worrying whether the whitespace consists of spaces or tabs. Use C-b C-d to delete a tab character before point, in these modes.

Separate manuals are available for the modes for Ada (see section `Ada Mode' in Ada Mode), C/C++/Objective C/Java/Corba IDL (see section `CC Mode' in CC Mode) and the IDLWAVE modes (see section `IDLWAVE' in IDLWAVE User Manual).

Turning on a major mode runs a normal hook called the 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 `-hook'. For example, turning on C mode runs the hook c-mode-hook, while turning on Lisp mode runs the hook lisp-mode-hook. The purpose of the mode hook is to give you a place to set up customizations for that major mode. See section AD.2.3 Hooks.

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U.2 Top-Level Definitions, or Defuns

In Emacs, a major definition at the top level in the buffer is called a defun. The name comes from Lisp, but in Emacs we use it for all languages.

In most programming language modes, Emacs assumes that a defun is any pair of parentheses (or braces, if the language uses braces this way) that starts at the left margin. For example, in C, the body of a function definition is normally a defun, because the open-brace that begins it is normally at the left margin. A variable's initializer can also count as a defun, if the open-brace that begins the initializer is at the left margin.

However, some language modes provide their own code for recognizing defuns in a way that suits the language syntax and conventions better.

U.2.1 Left Margin Convention  An open-paren or similar opening delimiter starts a defun if it is at the left margin.
U.2.2 Moving by Defuns  Commands to move over or mark a major definition.
U.2.3 Imenu  Making buffer indexes as menus.
U.2.4 Which Function Mode  Which Function mode shows which function you are in.

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U.2.1 Left Margin Convention

In most major modes, Emacs assumes that any opening delimiter found at the left margin is the start of a top-level definition, or defun. Therefore, never put an opening delimiter at the left margin unless it should have that significance. For instance, 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 is at top level.

If you don't follow this convention, not only will you have trouble when you explicitly use the commands for motion by defuns; other features that use them will also give you trouble. This includes the indentation commands (see section U.3 Indentation for Programs) and Font Lock mode (see section J.2 Font Lock mode).

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 (`\', in C and Emacs Lisp, `/' in some other Lisp dialects) before the opening delimiter. This will not affect the contents of the string, but will prevent that opening delimiter from starting a defun. Here's an example:

  (insert "Foo:

In the earliest days, the original Emacs found defuns by moving upward a level of parentheses or braces 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, we changed Emacs to assume that any opening delimiter at the left margin is the start of a defun. This heuristic is nearly always right, and avoids the need to scan back to the beginning of the buffer. However, it mandates following the convention described above.

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U.2.2 Moving by Defuns

These commands move point or set up the region based on top-level major definitions, also called defuns.

Move to beginning of current or preceding defun (beginning-of-defun).
Move to end of current or following defun (end-of-defun).
Put region around whole current or following defun (mark-defun).

The commands to move to the beginning and end of the current defun are C-M-a (beginning-of-defun) and C-M-e (end-of-defun). If you repeat one of these commands, or use a positive numeric argument, each repetition moves to the next defun in the direction of motion.

C-M-a with a negative argument -n moves forward n times to the next beginning of a defun. This is not exactly the same place that C-M-e with argument n would move to; the end of this defun is not usually exactly the same place as the beginning of the following defun. (Whitespace, comments, and perhaps declarations can separate them.) Likewise, C-M-e with a negative argument moves back to an end of a defun, which is not quite the same as C-M-a with a positive argument.

To operate on the current defun, use C-M-h (mark-defun) which puts point at the beginning and mark at the end of the current defun. This is the easiest way to get ready to kill the defun in order to move it to a different place in the file. If you use the command while point is between defuns, it uses the following defun.

In C mode, C-M-h runs the function c-mark-function, which is almost the same as 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. This is an example of how major modes adjust the standard key bindings so that they do their standard jobs in a way better fitting a particular language. Other major modes may replace any or all of these key bindings for that purpose.

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U.2.3 Imenu

The Imenu facility offers a way to find the the major definitions in a file by name. It is also useful in text formatter major modes, where it treats each chapter, section, etc., as a definition. (See section W.2 Tags Tables, for a more powerful feature that handles multiple files together.)

If you type M-x imenu, it reads the name of a definition using the minibuffer, then moves point to that definition. You can use completion to specify the name; the command always displays the whole list of valid names.

Alternatively, you can bind the command imenu to a mouse click. Then it displays mouse menus for you to select a definition name. You can also add the buffer's index to the menu bar by calling imenu-add-menubar-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 imenu-add-menubar-index to its mode hook. But if you have done that, you will have to wait each time you visit a file in that mode, while Emacs finds all the definitions in that buffer.

When you change the contents of a buffer, if you add or delete definitions, you can update the buffer's index based on the new contents by invoking the `*Rescan*' item in the menu. Rescanning happens automatically if you set imenu-auto-rescan to a non-nil value. There is no need to rescan because of small changes in the text.

You can customize the way the menus are sorted by setting the variable imenu-sort-function. By default, names are ordered as they occur in the buffer; if you want alphabetic sorting, use the symbol imenu--sort-by-name as the value. You can also define your own comparison function by writing Lisp code.

Imenu provides the information to guide Which Function mode (see section U.2.4 Which Function Mode). The Speedbar can also use it (see section P.9 Making and Using a Speedbar Frame).

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U.2.4 Which Function Mode

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

To enable (or disable) Which Function mode, use the command M-x which-function-mode. This command is global; it applies to all buffers, both existing ones and those yet to be created. However, it only takes effect in certain major modes, those listed in the value of which-func-modes. If the value is t, then Which Function mode applies to all major modes that know how to support it--in other words, all the major modes that support Imenu.

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U.3 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.

U.3.1 Basic Program Indentation Commands  Indenting a single line.
U.3.2 Indenting Several Lines  Commands to reindent many lines at once.
U.3.3 Customizing Lisp Indentation  Specifying how each Lisp function should be indented.
U.3.4 Commands for C Indentation  Extra features for indenting C and related modes.
U.3.5 Customizing C Indentation  Controlling indentation style for C and related modes.

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

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U.3.1 Basic Program Indentation Commands

The basic indentation commands indent a single line according to the usual conventions of the language you are editing.

Adjust indentation of current line.
Equivalent to RET followed by TAB (newline-and-indent).
This key, if the keyboard has it, is another way to enter C-j.

The basic indentation command is TAB, which gives the current line the correct indentation as determined from the previous lines. The function that TAB runs depends on the major mode; it is indent-for-tab-command in Lisp mode, c-indent-command in C mode, etc. These functions understand the syntax and conventions of different languages, but they all do conceptually the same job: 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 was inside the whitespace at the beginning of the line, TAB puts it at the end of that whitespace; otherwise, TAB keeps point fixed with respect to the characters around it.

Use C-q TAB to insert a tab at point.

When entering lines of new code, use C-j (newline-and-indent), which is equivalent to a RET followed by a TAB. C-j at the end of a line creates a blank line and then gives it the appropriate indentation.

TAB indents lines that start within a parenthetical grouping each under the preceding line (or the text after the parenthesis). Therefore, if you manually give one of these lines a nonstandard indentation, the lines below will tend to follow it. This behavior is convenient in cases where you have overridden the standard result of 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. See section U.2.1 Left Margin Convention, for more information on this.

Normally, lines are indented with tabs and spaces. If you want Emacs to use spaces only, see S.3 Tabs vs. Spaces.

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U.3.2 Indenting Several Lines

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

Reindent all the lines within one parenthetical grouping(indent-sexp).
Reindent all lines in the region (indent-region).
Shift an entire parenthetical grouping rigidly sideways so that its first line is properly indented.
M-x indent-code-rigidly
Shift all the lines in the region rigidly sideways, but do not alter lines that start inside comments and strings.

You can reindent the contents of a single parenthetical grouping by positioning point before the beginning of it and typing C-M-q (indent-sexp in Lisp mode, c-indent-exp in C mode; also bound to other suitable commands in other modes). The indentation of the line where the grouping starts is not changed; therefore, this changes only the relative indentation within the grouping, not its overall indentation. To correct that as well, type TAB first.

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

If you like the relative indentation within a grouping, but not the indentation of its first line, you can type C-u TAB to reindent the whole grouping as a rigid unit. (This works in Lisp modes and C and related modes.) TAB with a numeric argument reindents the current line as usual, then reindents by the same amount all the lines in the parenthetical grouping starting on the current line. It is clever, though, and does not alter lines that start inside strings, or C preprocessor lines when in C mode.

You can also perform this operation on the region, using the command M-x indent-code-rigidly. It rigidly shifts all the lines in the region sideways, like indent-rigidly does (see section S.1 Indentation Commands and Techniques). It doesn't alter the indentation of lines that start inside a comment or a string, unless the region starts inside that comment or string.

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U.3.3 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.

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

Certain functions override the standard pattern. Functions whose names start with def treat the the second lines as the start of a body, by indenting the second line lisp-body-indent additional columns beyond the open-parenthesis that starts the expression.

You can override the standard pattern in various ways for individual functions, according to the lisp-indent-function property of the function name. There are four possibilities for this property:

This is the same as no property--use the standard indentation pattern.
Handle this function like a `def' construct: treat the second line as the start of a body.
a number, number
The first number arguments of the function are distinguished arguments; the rest are considered the 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 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 twice that many extra columns. If the argument is distinguished and not the first or second argument, the line uses the standard pattern.
a symbol, symbol
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:
The value returned by parse-partial-sexp (a Lisp primitive for indentation and nesting computation) when it parses up to the beginning of this line.
The position at which the line being indented begins.
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 C-M-q; if the value is a number, C-M-q need not recalculate indentation for the following lines until the end of the list.

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U.3.4 Commands for C Indentation

Here are special features for indentation in C mode and related modes:

C-c C-q
Reindent the current top-level function definition or aggregate type declaration (c-indent-defun).

Reindent each line in the balanced expression that follows point (c-indent-exp). A prefix argument inhibits error checking and warning messages about invalid syntax.

Reindent the current line, and/or in some cases insert a tab character (c-indent-command).

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

If that variable is 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 indent-tabs-mode is nil).

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

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

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

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U.3.5 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 based on your selected style.

M-x c-set-style RET style RET
Select predefined indentation style style.

A style is a named collection of indentation customizations that can be used in C mode and the related modes. Emacs comes with several predefined styles, including gnu, k&r, bsd, stroustrup, linux, python, java, whitesmith, ellemtel, cc-mode, and user. Some of these styles are primarily intended for one language, but any of them can be used with any of the languages supported by these modes. To find out what a style looks like, select it and reindent some code, e.g., by typing C-M-q at the start of a function definition.

To choose a style for the current buffer, use the command M-x c-set-style. Specify a style name as an argument (case is not significant). This command affects the current buffer only, and it affects only future invocations of the indentation commands; it does not reindent the code in the buffer. To reindent the whole buffer in the new style, you can type C-x h C-M-\.

You can also set the variable c-default-style to specify the default 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,

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

specifies an explicit choice for Java mode, and the default `gnu' style for the other C-like modes. This variable takes effect when you select one of the C-like major modes; thus, if you specify a new default style for Java mode, you can make it take effect in an existing Java mode buffer by typing M-x java-mode there.

The gnu style specifies the formatting recommended by the GNU Project for C; it is the default, so as to encourage use of our recommended style.

See section `Customizing Indentation' in the CC Mode Manual, for more information on customizing indentation for C and related modes, including how to override parts of an existing style and how to define your own styles.

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U.4 Commands for Editing with Parentheses

This section describes the commands and features that take advantage of the parenthesis structure in a program, or help you keep it balanced.

When talking about these facilities, the term "parenthesis" also includes braces, brackets, or whatever delimiters are defined to match in pairs. The major mode controls which delimiters are significant, through the syntax table (see section AD.6 The Syntax Table). In Lisp, only parentheses count; in C, these commands apply to braces and brackets too.

You can use M-x check-parens to find any unbalanced parentheses and unbalanced string quotes in the buffer.

U.4.1 Expressions with Balanced Parentheses  Expressions with balanced parentheses.
U.4.2 Moving in the Parenthesis Structure  Commands for moving up, down and across in the structure of parentheses.
U.4.3 Automatic Display Of Matching Parentheses  Insertion of a close-delimiter flashes matching open.

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U.4.1 Expressions with Balanced Parentheses

These commands deal with balanced expressions, also called sexps(9).

Move forward over a balanced expression (forward-sexp).
Move backward over a balanced expression(backward-sexp).
Kill balanced expression forward (kill-sexp).
Kill balanced expression backward (backward-kill-sexp).
Transpose expressions (transpose-sexps).
Put mark after following expression (mark-sexp).

Each programming language major mode customizes the definition of balanced expressions to suit that language. Balanced expressions typically include symbols, numbers, and string constants, as well as any pair of matching delimiters and their contents. Some languages have obscure forms of expression syntax that nobody has bothered to implement in Emacs.

By convention, the keys for these commands are all Control-Meta characters. They usually act on expressions just as the corresponding Meta characters act on words. For instance, the command C-M-b moves backward over a balanced expression, just as M-b moves back over a word.

To move forward over a balanced expression, use C-M-f (forward-sexp). If the first significant character after point is an opening delimiter (`(' in Lisp; `(', `[' or `{' in C), C-M-f moves past the matching closing delimiter. If the character begins a symbol, string, or number, C-M-f moves over that.

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

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

Killing a whole balanced expression can be done with C-M-k (kill-sexp) or C-M-DEL (backward-kill-sexp). C-M-k kills the characters that C-M-f would move over, and C-M-DEL kills the characters that C-M-b would move over. On some machines, C-M-DEL typed on the console is a command to reboot; when that is so, you cannot use it as an Emacs command. This conflict is rare, though: usually the DEL key for Emacs is really BACKSPACE, and the reboot command is C-M-DELETE, so there is no conflict.

A somewhat random-sounding command which is nevertheless handy is C-M-t (transpose-sexps), which drags the previous balanced expression across the next one. An argument serves as a repeat count, and a negative argument drags the previous balanced expression backwards across those before it (thus canceling out the effect of C-M-t with a positive argument). An argument of zero, rather than doing nothing, transposes the balanced expressions ending at or after point and the mark.

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

In languages that use infix operators, such as C, it is not possible to recognize all balanced expressions as such because there can be multiple possibilities at a given position. For example, C mode does not treat `foo + bar' as a single expression, even though it is one C expression; instead, it recognizes `foo' as one expression and `bar' as another, with the `+' as punctuation between them. Both `foo + bar' and `foo' are legitimate choices for "the expression following point" when point is at the `f', so the expression commands must perforce choose one or the other to operate on. Note that `(foo + bar)' is recognized as a single expression in C mode, because of the parentheses.

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U.4.2 Moving in the Parenthesis Structure

The Emacs commands for handling parenthetical groupings see nothing except parentheses (or whatever characters must balance in the language you are working with), and the escape characters that might be used to quote those. They are mainly intended for editing programs, but can be useful for editing any text that has parentheses. They are sometimes called "list" commands because in Lisp these groupings are lists.

Move forward over a parenthetical group (forward-list).
Move backward over a parenthetical group(backward-list).
Move up in parenthesis structure (backward-up-list).
Move down in parenthesis structure (down-list).

The "list" commands C-M-n (forward-list) and C-M-p (backward-list) move over one (or n) parenthetical groupings, skipping blithely over any amount of text that doesn't include meaningful parentheses (symbols, strings, etc.).

C-M-n and C-M-p try to stay at the same level in the parenthesis structure. To move up one (or n) levels, use C-M-u (backward-up-list). C-M-u moves backward up past one unmatched opening delimiter. A positive argument serves as a repeat count; a negative argument reverses the direction of motion, so that the command moves forward and up one or more levels.

To move down in the parenthesis structure, use C-M-d (down-list). In Lisp mode, where `(' is the only opening delimiter, this is nearly the same as searching for a `('. An argument specifies the number of levels to go down.

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U.4.3 Automatic Display Of Matching Parentheses

The Emacs parenthesis-matching feature is designed to show automatically how parentheses (and other matching delimiters) 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, Emacs displays some of the text near it in the echo area. Either way, you can tell which grouping you are closing off.

If the opening delimiter and closing delimiter are mismatched--such as in `[x)'---a warning message is displayed in the echo area.

Three variables control parenthesis match display. blink-matching-paren turns the feature on or off: nil disables it, but the default is t to enable match display.

blink-matching-delay says how many seconds to leave the cursor on the matching opening delimiter, before bringing it back to the real location of point; the default is 1, but on some systems it is useful to specify a fraction of a second.

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 distance, scanning stops, and nothing is displayed. This is to prevent the scan for the matching delimiter from wasting lots of time when there is no match. The default is 25600.

Show Paren mode provides a more powerful kind of automatic matching. Whenever point is after a closing delimiter, that delimiter and its matching opening delimiter are both highlighted; otherwise, if point is before an opening delimiter, the matching closing delimiter is highlighted. (There is no need to highlight the opening delimiter in that case, because the cursor appears on top of that character.) Use the command M-x show-paren-mode to enable or disable this mode.

By default, show-paren-mode uses colors to highlight the parentheses. However, if your display doesn't support colors, you can customize the faces show-paren-match-face and show-paren-mismatch-face to use other attributes, such as bold or underline. See section AD.2.2.3 Customizing Faces.

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U.5 Manipulating Comments

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

U.5.1 Comment Commands  Inserting, killing, and indenting comments.
U.5.2 Multiple Lines of Comments  Commands for adding and editing multi-line comments.
U.5.3 Options Controlling Comments  Customizing the comment features.

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U.5.1 Comment Commands

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

Insert or realign comment on current line; alternatively, comment or uncomment the region (comment-dwim).
C-u M-;
Kill comment on current line (comment-kill).
C-x ;
Set comment column (comment-set-column).
Like RET followed by inserting and aligning a comment (comment-indent-new-line).
M-x comment-region
Add or remove comment delimiters on all the lines in the region.

The command to create or align a comment is M-; (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, M-; inserts a new comment, aligned at a specific column called the comment column. The new comment begins with the string Emacs thinks comments should start with (the value of 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, 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 M-; to align an existing comment. If a line already contains the comment-start string, 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, M-; is still useful for moving directly to the start of the text inside the comment.

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 C-y, and then do M-; to realign it.

Note that C-u M-; is not a distinct key; it is M-; (comment-dwim) with a prefix argument. That command is programmed so that when it receives a prefix argument it calls comment-kill. However, comment-kill is a valid command in its own right, and you can bind it directly to a key if you wish.

M-; does two other jobs when used with an active region in Transient Mark mode (see section H.2 Transient Mark Mode). 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 comment-region and uncomment-region to do these jobs (see section U.5.2 Multiple Lines of 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 TAB, and by not changing the indentation of a triple-semicolon comment at all.

;; 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.

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

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U.5.2 Multiple Lines of Comments

If you are typing a comment and wish to continue it on another line, you can use the command C-M-j (comment-indent-new-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 C-M-j is typed, the text on the rest of the line becomes part of the new comment line.

To turn existing lines into comment lines, use the 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, 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, C-u 2 M-x comment-region adds `;;' 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 or three, if between defuns; if within a defun, it must be three.

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U.5.3 Options Controlling Comments

The comment column is stored in the variable comment-column. You can set it to a number explicitly. Alternatively, the command C-x ; (comment-set-column) sets the comment column to the column point is at. C-u C-x ; sets the comment column to match the last comment before point in the buffer, and then does a M-; to align the current line's comment under the previous one.

The variable 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 setq-default. See section AD.2.4 Local Variables. Many major modes initialize this variable for the current buffer.

The comment commands recognize comments based on the regular expression that is the value of the variable 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 "/\\*+ *\\|//+ *"", which matches extra stars and spaces after the `/*' itself, and accepts C++ style comments also. (Note that `\\' is needed in Lisp syntax to include a `\' in the string, which is needed to deny the first star its special meaning in regexp syntax. See section K.5 Syntax of Regular Expressions.)

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

The variable comment-padding specifies how many spaces comment-region should insert on each line between the comment delimiter and the line's original text. The default is 1, to insert one space.

The variable comment-multi-line controls how C-M-j (indent-new-comment-line) behaves when used inside a comment. If comment-multi-line is 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 comment-multi-line is not 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.

The variable 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.

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U.6 Documentation Lookup

Emacs provides several features you can use to look up the documentation of functions, variables and commands that you plan to use in your program.

U.6.1 Info Documentation Lookup  Looking up library functions and commands in Info files.
U.6.2 Man Page Lookup  Looking up man pages of library functions and commands.
U.6.3 Emacs Lisp Documentation Lookup  Looking up Emacs Lisp functions, etc.

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U.6.1 Info Documentation Lookup

For C, Lisp, and other languages that have documentation in Info, you can use C-h C-i (info-lookup-symbol) to view the Info documentation for a symbol. You specify the symbol with the minibuffer; the default is the symbol appearing in the buffer at point.

The major mode determines where to look for documentation for the symbol--which Info files to look in, and which indices to search. You can also use M-x info-lookup-file to look for documentation for a file name.

This feature currently supports the 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.

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U.6.2 Man Page Lookup

On Unix, the main form of on-line documentation was the manual page or man page. In the GNU operating system, we hope to replace man pages with better-organized manuals that you can browse with Info (see section G.8 Other Help Commands). This process is not finished, so it is still useful to read manual pages.

You can read the man page for an operating system command, library function, or system call, with the M-x manual-entry command. It runs the man program to format the man page; if the system permits, it runs man asynchronously, so that you can keep on editing while the page is being formatted. (On MS-DOS and MS-Windows 3, you cannot edit while Emacs waits for man to finish.) The result goes in a buffer named `*Man topic*'. These buffers use a special major mode, Man mode, that facilitates scrolling and jumping to other manual pages. For details, type C-h m while in a man page buffer.

Each man page belongs to one of ten or more sections, each named by a digit or by a digit and a letter. Sometimes there are multiple man pages with the same name in different sections. To read a man page from a specific section, type `topic(section)' or `section topic' when M-x manual-entry prompts for the topic. For example, to read the man page for the C library function chmod (as opposed to a command of the same name), type M-x manual-entry RET chmod(2) RET (chmod is a system call, so it is in section `2').

If you do not specify a section, the results depend on how the man program 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 move between them with the M-n and M-p keys(10). The mode line shows how many manual pages are present in the Man buffer.

By default, Emacs highlights the text in man pages. For a long man page, highlighting can take substantial time. You can turn off highlighting of man pages by setting the variable Man-fontify-manpage-flag to nil.

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

An alternative way of reading manual pages is the M-x woman command(11). Unlike 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 man program (and the other programs it uses) are not generally available.

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 woman. The word at point in the current buffer is used to suggest the default for the name the manual page.

With a numeric argument, 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 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.

By default, M-x woman looks for manual pages in the directories specified in the MANPATH environment variable. (If MANPATH is not set, woman uses a suitable default value, which can be customized.) More precisely, woman looks for subdirectories that match the shell wildcard pattern `man*' in each one of these directories, and tries to find the manual pages in those subdirectories. When first invoked, M-x woman converts the value of MANPATH to a list of directory names and stores that list in the woman-manpath variable. Changing the value of this variable is another way to control the list of directories used.

You can also augment the list of directories searched by woman by setting the value of the woman-path variable. This variable should hold a list of specific directories which woman should search, in addition to those in woman-manpath. Unlike woman-manpath, the directories in woman-path are searched for the manual pages, not for `man*' subdirectories.

Occasionally, you might need to display manual pages that are not in any of the directories listed by woman-manpath and woman-path. The 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 M-x woman does.

The first time you invoke M-x woman, it defines the Dired W key to run the woman-find-file command on the current line's file. You can disable this by setting the variable woman-dired-keys to nil. See section AB. Dired, the Directory Editor. In addition, the Tar-mode w key is define to invoke woman-find-file on the current line's archive member.

For more information about setting up and using M-x woman, see section `Browse UN*X Manual Pages WithOut Man' in The WoMan Manual.

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U.6.3 Emacs Lisp Documentation Lookup

As you edit Lisp code to be run in Emacs, you can use the commands C-h f (describe-function) and C-h v (describe-variable) to view documentation of functions and variables that you want to use. These commands use the minibuffer to read the name of a function or variable to document, and display the documentation in a window. Their default arguments are based on the code in the neighborhood of point. For C-h f, the default is the function called in the innermost list containing point. C-h v uses the symbol name around or adjacent to point as its default.

A more automatic but less powerful method is 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 M-x eldoc-mode to enable or disable this feature.

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U.7 Hideshow minor mode

Hideshow minor mode provides selective display of portions of a program, known as blocks. You can use M-x hs-minor-mode to enable or disable this mode, or add 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.

C-c @ C-h
Hide the current block (hs-hide-block).
C-c @ C-s
Show the current block (hs-show-block).
C-c @ C-c
Either hide or show the current block (hs-toggle-hiding)
Either hide or show the block you click on (hs-mouse-toggle-hiding)
C-c @ C-M-h
Hide all top-level blocks (hs-hide-all).
C-c @ C-M-s
Show everything in the buffer (hs-show-all).
C-c @ C-l
Hide all blocks n levels below this block (hs-hide-level).

These user options exist for customizing Hideshow mode.

Non-nil says that hs-hide-all should hide comments too.

Specifies what kind of hidden blocks to open in Isearch mode. The value should be one of these four symbols.

Open only code blocks.
Open only comments.
Open both code blocks and comments.
Open neither code blocks nor comments.

A list of elements, each specifying how to initialize Hideshow variables for one major mode. See the variable's documentation string for more information.

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U.8 Completion for Symbol Names

In Emacs, completion is something you normally do in the minibuffer. But one kind of completion is available in all buffers: completion for symbol names.

The character M-TAB runs a command to complete the partial symbol before point against the set of meaningful symbol names. This command inserts at point any additional characters that it can determine from the partial name.

If the partial name in the buffer has multiple possible completions that differ in the very next character, so that it is impossible to complete even one more character, M-TAB displays a list of all possible completions in another window.

In most programming language major modes, M-TAB runs the command complete-symbol, which provides two kinds of completion. Normally it does completion based on a tags table (see section W.2 Tags Tables); 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 M-TAB with no argument; to complete the name of a standard library function, use C-u M-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.

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 lisp-complete-symbol.

In Text mode and related modes, M-TAB completes words based on the spell-checker's dictionary. See section L.4 Checking and Correcting Spelling.

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U.9 Glasses minor mode

Glasses minor mode makes `unreadableIdentifiersLikeThis' readable by altering the way they display. It knows two different ways to do this: by displaying underscores between a lower-case letter and the following capital letter, and 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 M-x glasses-mode to enable or disable the mode in the current buffer; you can also add glasses-mode to the mode hook of the programming language major modes in which you normally want to use Glasses mode.

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U.10 Other Features Useful for Editing Programs

A number of Emacs commands that aren't designed specifically for editing programs are useful for that nonetheless.

The Emacs commands that operate on words, sentences and paragraphs are useful for editing code. Most symbols names contain words (see section T.1 Words); sentences can be found in strings and comments (see section T.2 Sentences). Paragraphs in the strict sense can be found in program code (in long comments), but the paragraph commands are useful in other places too, because programming language major modes define paragraphs to begin and end at blank lines (see section T.3 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. Auto Fill mode, if enabled in a programming language major mode, indents the new lines which it creates.

The selective display feature is useful for looking at the overall structure of a function (see section J.9 Selective Display). This feature hides the lines that are indented more than a specified amount. Programming modes often support Outline minor mode (see section T.8 Outline Mode). The Foldout package provides folding-editor features (see section T.8.5 Folding Editing).

The "automatic typing" features may be useful for writing programs. See section `Autotyping' in Autotyping.

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U.11 C and Related Modes

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.") See section `ccmode' in CC Mode, for a more extensive description of these modes and their special features.

U.11.1 C Mode Motion Commands  Commands to move by C statements, etc.
U.11.2 Electric C Characters  Colon and other chars can automatically reindent.
U.11.3 Hungry Delete Feature in C  A more powerful DEL command.
U.11.4 Other Commands for C Mode  Filling comments, viewing expansion of macros, and other neat features.
U.11.5 Comments in C Modes  Options for customizing comment style.

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U.11.1 C Mode Motion Commands

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

C-c C-u
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, #elif is treated like #else followed by #if. When going forwards, #elif is ignored.

C-c C-p
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.

C-c C-n
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.

Move point to the beginning of the innermost C statement (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 n, move back n - 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.

Move point to the end of the innermost C statement; like M-a except that it moves in the other direction (c-end-of-statement).

M-x c-backward-into-nomenclature
Move point backward to beginning of a C++ nomenclature section or word. With prefix argument n, move n times. If 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.

M-x c-forward-into-nomenclature
Move point forward to end of a C++ nomenclature section or word. With prefix argument n, move n times.

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U.11.2 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 c-auto-newline. The "electric" characters are {, }, :, #, ;, ,, <, >, /, *, (, and ).

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

C-c C-a
Toggle the auto-newline feature (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.

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 C-c ::

C-c :
Insert a double colon scope operator at point, without reindenting the line or adding any newlines (c-scope-operator).

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

The variable 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: (syntactic-symbol . nl-list). Most of the syntactic symbols that appear in c-offsets-alist are meaningful here as well.

The list nl-list may contain either of the symbols before or after, or both; or it may be nil. When a brace is inserted, the syntactic context it defines is looked up in c-hanging-braces-alist; if it is found, the 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 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: (syntactic-symbol . nl-list). The list nl-list may contain either of the symbols before or after, or both; or it may be nil.

When a colon is inserted, the syntactic symbol it defines is looked up in this list, and if found, the 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 c-cleanup-list, you can specify 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:

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

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

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

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

Clean up the semicolon after a 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.

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

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.

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U.11.3 Hungry Delete Feature in C

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

C-c C-d
Toggle the hungry-delete feature (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.

C-c C-t
Toggle the auto-newline and hungry-delete features, both at once (c-toggle-auto-hungry-state).

The variable c-hungry-delete-key controls whether the hungry-delete feature is enabled.

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U.11.4 Other Commands for C Mode

Put mark at the end of a function definition, and put point at the beginning (c-mark-function).

Fill a paragraph, handling C and C++ comments (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.

C-c C-e
Run the C preprocessor on the text in the region, and show the result, which includes the expansion of all the macro calls (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.

C-c C-\
Insert or align `\' characters at the ends of the lines of the region (c-backslash-region). This is useful after writing or editing a C macro definition.

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

M-x cpp-highlight-buffer
Highlight parts of the text according to its preprocessor conditionals. This command displays another buffer named `*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 `[A]pply these settings' (or go to that buffer and type a) to rehighlight the C mode buffer accordingly.

C-c C-s
Display the syntactic information about the current source line (c-show-syntactic-information). This is the information that directs how the line is indented.

M-x cwarn-mode
M-x global-cwarn-mode
CWarn minor mode highlights certain suspicious C and C++ constructions:

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

M-x hide-ifdef-mode
Hide-ifdef minor mode hides selected code within `#if' and `#ifdef' preprocessor blocks. See the documentation string of hide-ifdef-mode for more information.

M-x ff-find-related-file
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 ff-related-file-alist specifies how to compute related file names.

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U.11.5 Comments in C Modes

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

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 (non-anchored-offset . anchored-offset), where non-anchored-offset is the amount of offset given to non-column-zero anchored comment-only lines, and anchored-offset is the amount of offset to give column-zero anchored comment-only lines. Just an integer as value is equivalent to (val . 0).

This buffer-local variable specifies how to recognize the start of a comment.

If this variable is nil, c-fill-paragraph leaves the comment terminator of a block comment on a line by itself. The default value is t, which puts the comment-end delimiter `*/' at the end of the last line of the comment text.

If this variable is nil, c-fill-paragraph leaves the starting delimiter of a block comment on a line by itself. The default value is t, which puts the comment-start delimiter `/*' at the beginning of the first line of the comment text.

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U.12 Fortran Mode

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 M-x fortran-mode to switch to this major mode. This command runs the hook fortran-mode-hook (see section AD.2.3 Hooks).

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

U.12.1 Motion Commands  Moving point by statements or subprograms.
U.12.2 Fortran Indentation  Indentation commands for Fortran.
U.12.3 Fortran Comments  Inserting and aligning comments.
U.12.4 Fortran Auto Fill Mode  Auto fill minor mode for Fortran.
U.12.5 Checking Columns in Fortran  Measuring columns for valid Fortran.
U.12.6 Fortran Keyword Abbrevs  Built-in abbrevs for Fortran keywords.

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U.12.1 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.

C-c C-n
Move to beginning of current or next statement (fortran-next-statement).

C-c C-p
Move to beginning of current or previous statement (fortran-previous-statement).

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U.12.2 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.

U.12.2.1 Fortran Indentation and Filling Commands  Commands for indenting and filling Fortran.
U.12.2.2 Continuation Lines  How continuation lines indent.
U.12.2.3 Line Numbers  How line numbers auto-indent.
U.12.2.4 Syntactic Conventions  Conventions you must obey to avoid trouble.
U.12.2.5 Variables for Fortran Indentation  Variables controlling Fortran indent style.

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U.12.2.1 Fortran Indentation and Filling Commands

Break the current line and set up a continuation line (fortran-split-line).
Join this line to the previous line (fortran-join-line).
Indent all the lines of the subprogram point is in (fortran-indent-subprogram).
Fill a comment block or statement.

The key C-M-q runs fortran-indent-subprogram, a command to reindent all the lines of the Fortran subprogram (function or subroutine) containing point.

The key C-M-j runs 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.

M-^ or C-c C-d runs the command fortran-join-line, which joins a continuation line back to the previous line, roughly as the inverse of fortran-split-line. The point must be on a continuation line when this command is invoked.

M-q in Fortran mode fills the comment block or statement that point is in. This removes any excess statement continuations.

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U.12.2.2 Continuation Lines

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 fixed format. (In GNU Emacs we always count columns from 0.) The variable fortran-continuation-string specifies what character to put on column 5. A line that starts with a tab character followed by any digit except `0' is also a continuation line. We call this style of continuation tab format.

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

If the text on a line starts with the conventional Fortran continuation marker `$', 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 TAB, it converts the line to the current continuation style. When you split a Fortran statement with 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.

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 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 fortran-tab-mode-default specifies the style. If it is nil, that specifies fixed format, and non-nil specifies tab format.

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U.12.2.3 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.)

Line numbers of four digits or less are normally indented one space. The variable 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.

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 fortran-electric-line-number to nil. Then inserting line numbers is like inserting anything else.

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U.12.2.4 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:

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.

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U.12.2.5 Variables for Fortran Indentation

Several additional variables control how Fortran indentation works:

Extra indentation within each level of `do' statement (default 3).

Extra indentation within each level of `if' statement (default 3). This value is also used for extra indentation within each level of the Fortran 90 `where' statement.

Extra indentation within each level of `structure', `union', or `map' statements (default 3).

Extra indentation for bodies of continuation lines (default 5).

If this is nil, indentation assumes that each `do' statement ends on a `continue' statement. Therefore, when computing indentation for a statement other than `continue', it can save time by not checking for a `do' statement ending there. If this is non-nil, indenting any numbered statement must check for a `do' that ends there. The default is nil.

If this is t, indenting an `endif' statement moves the cursor momentarily to the matching `if' statement to show where it is. The default is nil.

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.

Minimum indentation for fortran statements for tab format continuation line style. Statement bodies are never indented less than this much. The default is 8.

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U.12.3 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 `!' 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 comment-start to `"!"' (see section AD.2 Variables).

Align comment or insert new comment (fortran-comment-indent).

C-x ;
Applies to nonstandard `!' comments only.

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

M-; in Fortran mode is redefined as the command fortran-comment-indent. Like the usual 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 `!' 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 `!' 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 fortran-comment-indent-style to one of these values:

Align the text at a fixed column, which is the sum of fortran-comment-line-extra-indent and the minimum statement indentation. This is the default.

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

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

Don't move text in full-line comments automatically at all.

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

Fortran mode introduces two variables comment-line-start and comment-line-start-skip, which play for full-line comments the same roles played by comment-start and 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 C-x ; has not been redefined. If you use `!' comments, this command can be used with them. Otherwise it is useless in Fortran mode.

The command C-c ; (fortran-comment-region) turns all the lines of the region into comments by inserting the string `C$$$' at the front of each one. With a numeric argument, it turns the region back into live code by deleting `C$$$' from the front of each line in it. The string used for these comments can be controlled by setting the variable 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.

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U.12.4 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 fortran-continuation-string (see section U.12.2.2 Continuation Lines). This splitting happens when you type SPC, RET, or TAB, and also in the Fortran indentation commands.

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 M-x auto-fill-mode does for normal Auto Fill mode (see section T.5 Filling Text). 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 `Fill' in the mode line, inside the parentheses. Fortran Auto Fill mode is a minor mode, turned on or off for each buffer individually. See section AD.1 Minor Modes.

Fortran Auto Fill mode breaks lines at spaces or delimiters when the lines get longer than the desired width (the value of fill-column). The delimiters that Fortran Auto Fill mode may break at are `,', `'', `+', `-', `/', `*', `=', and `)'. The line break comes after the delimiter if the variable fortran-break-before-delimiters is 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 fortran-mode-hook to execute (fortran-auto-fill-mode 1). See section AD.2.3 Hooks.

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U.12.5 Checking Columns in Fortran

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

The command C-c C-r (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.

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

C-c C-w (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.

You can also split the window horizontally and continue editing with the split in place. To do this, use C-u C-c C-w (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.

The command 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.

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U.12.6 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. See section X. 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 `;c' for `continue'. If you insert `;c' and then insert a punctuation character such as a space or a newline, the `;c' expands automatically to `continue', provided Abbrev mode is enabled.

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

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U.13 Asm Mode

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

Insert a newline and then indent using tab-to-tab-stop.
Insert a colon and then remove the indentation from before the label preceding colon. Then do tab-to-tab-stop.
Insert or align a comment.

The variable asm-comment-char specifies which character starts comments in assembler syntax.

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