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Editing Programs

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

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 (see section Words); sentences can be found in strings and comments (see section 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 (see section 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 (see section Selective Display). This feature causes only the lines that are indented less than a specified amount to appear on the screen.

Major Modes for Programming Languages

Emacs also has major modes for the programming languages Lisp, Scheme (a variant of Lisp), Awk, C, C++, Fortran, Icon, Pascal, Perl and Tcl. There is also a major mode for makefiles, called Makefile mode.

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. See section Executing Lisp Expressions.

Each of the programming language modes defines the 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 TAB is bound to c-indent-line. LFD is normally defined to do RET followed by TAB; thus, it too indents in a mode-specific fashion.

In most programming languages, indentation is likely to vary from line to line. So the major modes for those languages rebind DEL to treat a tab as if it were the equivalent number of spaces (using the command backward-delete-char-untabify). This makes it possible to rub out indentation one column at a time without worrying whether it is made up of spaces or tabs. Use 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.

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. See section Hooks.

Lists and Sexps

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).

These commands fall into two classes. Some deal only with 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 sexps. The word `sexp' is derived from 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 `foo + bar' as a sexp, even though it is a C expression; it recognizes `foo' as one sexp and `bar' as another, with the `+' as punctuation between them. This is a fundamental ambiguity: both `foo + bar' and `foo' are legitimate choices for the sexp to move over if point is at the `f'. Note that `(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.

List And Sexp Commands

Move forward over a sexp (forward-sexp).
Move backward over a sexp (backward-sexp).
Kill sexp forward (kill-sexp).
Kill sexp backward (backward-kill-sexp).
Move up and backward in list structure (backward-up-list).
Move down and forward in list structure (down-list).
Move forward over a list (forward-list).
Move backward over a list (backward-list).
Transpose expressions (transpose-sexps).
Put mark after following expression (mark-sexp).

To move forward over a sexp, 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 sexp. The detailed rules are like those above for C-M-f, but with directions reversed. If there are any prefix characters (single-quote, backquote and comma, in Lisp) preceding the sexp, C-M-b moves back over them as well. The sexp 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 sexp at a time 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.

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

C-M-n and C-M-p stay at the same level in parentheses, when that's possible. 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 direction of motion and also requests repetition, so it moves forward and up one or more levels.

To move down in list 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 of parentheses to go down.

A somewhat random-sounding command which is nevertheless handy is C-M-t (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 C-M-t with a positive argument). An argument of zero, rather than doing nothing, transposes the sexps ending after point and the mark.

To set the region around the next sexp 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 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. See section The Syntax Table.


In Emacs, a parenthetical grouping at the top level in the buffer is called a defun. The name derives from the fact that most top-level lists in a Lisp file are instances of the special form 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.

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 wish 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 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, C-M-h runs the function mark-c-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. See section Commands to Mark Textual Objects.

Emacs assumes that any open-parenthesis found in the leftmost column is the start of a defun. Therefore, 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 (`\', in C and Emacs Lisp, `/' 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 `(' (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.

Indentation for Programs

The best way to keep a program properly indented is to use Emacs to re-indent 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.

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

Basic Program Indentation Commands

Adjust indentation of current line.
Equivalent to RET followed by TAB (newline-and-indent).

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 lisp-indent-line in Lisp mode, c-indent-line in C mode, etc. These functions understand different syntaxes for different languages, but they all do about the same thing. 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, TAB leaves it at the end of that whitespace; otherwise, TAB leaves 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 LFD (newline-and-indent), which is equivalent to a RET followed by a TAB. LFD creates a blank line, and then gives it the appropriate indentation.

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 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 Defuns, for more information on this.

Indenting Several Lines

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

Re-indent all the lines within one list (indent-sexp).
Shift an entire list rigidly sideways so that its first line is properly indented.
Re-indent all lines in the region (indent-region).

You can re-indent the contents of a single list by positioning point before the beginning of it and typing C-M-q (indent-sexp in Lisp mode, indent-c-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 TAB before the C-M-q.

If the relative indentation within a list is correct but the indentation of its first line is not, go to that line and type C-u TAB. 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 re-indented is with the region. The command C-M-\ (indent-region) applies TAB to every line whose first character is between point and mark.

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.

The standard pattern is overridden for certain functions. Functions whose names start with def always indent the second line by 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 lisp-indent-function property of the function name. There are four possibilities for this property:

This is the same as no property; the standard indentation pattern is used.
The pattern used for function names that start with def is used for this function also.
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 standard pattern is followed for that line.
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.

Commands for C Indentation

Here are the commands 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. 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.
Reindent the current line according to its syntax; also rigidly reindent any other lines of the expression that starts on the current line. See section Indenting Several Lines.

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.

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.

Step 1 -- Syntactic Analysis

In the first step, the C indentation mechanism looks at the line you are currently indenting and determines the syntactic components of the construct on that line. It builds a list of these syntactic components, where each component on the list contains a syntactic symbol and a relative buffer position. Syntactic symbols describe grammatical elements such as statement, substatement, class-open, class-close, k&r-argdecl, etc.

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 relative 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):

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

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

((statement . 32))

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

((defun-block-intro . 28))

This indicates that the 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.

Here is another example:

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

Typing C-c C-s on line 4 displays this:

((substatement-open . 43))

This says that the brace opens a substatement block. By the way, a substatement indicates the line after an if, else, while, do, switch, and for statements.

After a line has been analyzed syntactically for indentation, the global variable c-syntactic-context contains a list that describes the results. Each element in this list is a cons cell containing a syntactic symbol and (optionally) its corresponding buffer position. There may be more than one element; syntactic symbols have corresponding buffer positions.

Step 2--Indentation Calculation

The C indentation mechanism calculates the indentation for the current line using the list of syntactic components derived from syntactic analysis. Each component contributes to the final total indentation of the line in two ways.

First, it looks up the syntactic symbol in the c-offsets-alist variable, which is an association list of syntactic symbols and the offsets to apply for those symbols. It then adds the offset corresponding to the syntactic symbol to the running total.

Second, if the component has a relative buffer position, it adds the column number of that position to the running total. It computes the final total indentation for the current line by summing up the offsets and columns for each syntactic component on that line.

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

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

Suppose that the point is on line 3 and you type TAB to reindent the line. Remember that the syntactic component list for that line is:

((defun-block-intro . 28))

So first the indentation engine is going to look up defun-block-intro in the c-offsets-alist variable. Suppose that it finds the integer 2; it adds this to the running total (initialized to zero), yielding a running total indentation of 2 spaces.

Next, it goes to buffer position 28 and asks for the current column. Since the brace at buffer position 28 is in column zero, it adds 0 to the running total, and since there is only one syntactic component on the list for this line, the total indentation for the line is 2 spaces.

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

If you type TAB on line 4 in the example above, the same basic process is performed, despite the differences in the syntactic component list. Remember that the list for this line is:

((substatement-open . 43))

Here, the indentation engine first looks up the substatement-open symbol in c-offsets-alist. We assume the offset for this symbol is 2. At this point the running total is 2 (0 + 2 = 2). It then goes to buffer position 43, which is the `i' in if on line 3. This character is in the fourth column on that line, so adding this to the running total yields a total indentation of 4 spaces.

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

Changing Indentation Style

There are two ways to customize the indentation style for the C 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. See section Syntactic Symbols, for a list of all defined syntactic symbols.

M-x c-set-style RET style RET
Select predefined indentation style style.
C-c C-o symbol RET offset RET
Set the indentation offset for syntactic symbol symbol (c-set-offset). The second argument offset specifies the new indentation offset.

The c-offsets-alist variable controls the amount of indentation to give to each syntactic component. Its value is an association list, and each element of the list has the form (syntactic-symbol . offset). By changing the offsets for various syntactic components, you can customize indentation in fine detail.

Each offset value in c-offsets-alist can be an integer, a function or variable name, or one of the following symbols: +, -, ++, or --, indicating positive or negative multiples of the variable c-basic-offset. Thus, if you want to change the levels of indentation to be 3 spaces instead of 2 spaces, you can probably get this result by setting 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 cons of the syntactic element symbol and the relative indent point. The function should return an integer offset.

The command C-c C-o (c-set-offset) is the easiest way to set offsets, both interactively or in your `~/.emacs' file. First specify the syntactic symbol, then the offset you want. See section Syntactic Symbols, for a list of valid syntactic symbols and their meanings.

The variable c-offsets-alist-default holds the default settings for offsets of syntactic elements. Do not change this value!

Syntactic Symbols

Here is the list of valid syntactic element symbols for C mode indentation. Normally most of these symbols are assigned offsets in c-offsets-alist.

Inside a multi-line string.
Inside a multi-line C style block comment.
On a brace that opens a function definition.
On a brace that closes a function definition.
In the first line in a top-level defun.
On a brace that opens a class definition.
On a brace that closes a class definition.
On a brace that opens an in-class inline method.
On a brace that closes an in-class inline method.
In the nether region between an ANSI function declaration and the defun opening brace.
On the first line of a K&R C argument declaration.
In one of the subsequent lines in a K&R C argument declaration.
On the first line in a topmost construct definition.
On the topmost definition continuation lines.
On the first line in a member initialization list.
On one of the subsequent member initialization list lines.
On the first line of a multiple inheritance list.
On one of the subsequent multiple inheritance lines.
On a statement block open brace.
On a statement block close brace.
On the opening brace of an enum or static array list.
On the closing brace of an enum or static array list.
On the first line in an enum or static array list.
On one of the subsequent lines in an enum or static array list.
On an ordinary statement.
On a continuation line of a statement.
On the first line in a new statement block.
On the first line in a case "block".
On the first line in a case block starting with brace.
On the first line after an if, while, for, do, or else.
On the brace that opens a substatement block.
On a case or default label.
On a C++ private, protected, or public access label.
On any ordinary label.
On the while that ends a do-while construct.
On the else of an if-else construct.
On a line containing only a comment introduction.
On the first line in an argument list.
On one of the subsequent argument list lines when no arguments follow on the same line as the the arglist opening parenthesis.
On one of the subsequent argument list lines when at least one argument follows on the same line as the arglist opening parenthesis.
On the closing parenthesis of an argument list.
On one of the lines continuing a stream operator construct.
On a construct which is nested inside a class definition.
On the start of a cpp macro.
On a C++ friend declaration.
On the first line of an Objective-C method definition.
On one of the lines continuing an Objective-C method definition.
On one of the lines continuing an Objective-C method call.

Variables for C Indentation

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

Association list of syntactic element symbols and their indentation offsets. See section Changing Indentation Style, for details.
Default settings for offsets of syntactic elements. See section Changing Indentation Style.
Variable for specifying styles of indentation; see below.
Indentation level of declarations of C function arguments.
Amount of basic offset used by + and - symbols in c-offsets-alist.
Imagined indentation of a C open brace that actually follows a statement.
Extra indentation for braces, compared with other text in same context.
Extra indentation for lines not starting new statements.
Extra indentation for substatements that start with open-braces. This is in addition to c-continued-statement-offset.
Offset of C label lines and case statements relative to usual indentation.
If this variable is non-nil, C mode and Objective C mode recognize K&R constructs. This variable is needed because of ambiguities in C syntax that make recognition of K&R constructs problematic and slow. If you always use ANSI C prototype syntax, set this variable to nil to speed up C indentation. This variable is nil by default in C++ mode, and t by default in C mode and Objective C mode.
Level of indentation of C statements with respect to containing block.
Hook for user-defined special indentation adjustments. This hook gets called after a line is indented by the mode.

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

When variable is c-offsets-alist, that is a special case: value is appended to the front of the value of c-offsets-alist instead of replacing that value outright. Therefore, it is not necessary for 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 c-comment-only-line-offset (see section Comments in C Modes).

Automatic Display Of Matching Parentheses

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. See section The Syntax Table.

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

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

When using X Windows, you can request a more powerful kind of automatic parenthesis matching by loading the paren library. To load it, type M-x load-library RET paren RET. This library turns off the usual kind of matching parenthesis display and substitutes another: 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.)

Manipulating Comments

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

Comment Commands

The comment commands insert, kill and align comments.

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

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

M-; can also be used to align an existing comment. If a line already contains the string that starts comments, then M-; just moves point after it and re-indents it to the conventional place. Exception: comments starting in column 0 are not moved.

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.

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

C-u - C-x ; (kill-comment) kills the comment on the current line, if there is one. The indentation before the start of the comment is killed as well. If there does not appear to be a comment in the line, nothing is done. To reinsert the comment on another line, move to the end of that line, do C-y, and then do M-; to realign it. Note that C-u - C-x ; is not a distinct key; it is C-x ; (set-comment-column) with a negative argument. That command is programmed so that when it receives a negative argument it calls kill-comment. However, kill-comment is a valid command which you could bind directly to a key if you wanted to.

Multiple Lines of Comments

If you are typing a comment and wish to continue it on another line, you can use the command M-LFD (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 M-LFD 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, if between defuns, and three, if within a defun.

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 ; (set-comment-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. Note that C-u - C-x ; runs the function kill-comment as described above.

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 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. (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 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-multi-line controls how M-LFD (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.

Editing Without Unbalanced Parentheses

Put parentheses around next sexp(s) (insert-parentheses).
Move past next close parenthesis and re-indent (move-over-close-and-reindent).

The commands M-( (insert-parentheses) and M-) (move-over-close-and-reindent) are designed to facilitate a style of editing which keeps parentheses balanced at all times. M-( inserts a pair of parentheses, either together as in `()', or, if given an argument, around the next several sexps. It leaves point after the open parenthesis. The command M-) moves past the close parenthesis, deleting any indentation preceding it (in this example there is none), and indenting with LFD after it.

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

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

Completion for Symbol Names

Usually completion happens 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. 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.

There are two ways of determining the set of legitimate symbol names to complete against. In most major modes, this uses a tags table (see section Tags Tables); the legitimate symbol names are the tag names listed in the tags table file. The command which implements this is complete-tag.

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 Checking and Correcting Spelling.

Documentation Commands

As you edit Lisp code to be run in Emacs, the commands C-h f (describe-function) and C-h v (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. C-h f sets the default to the function called in the innermost list containing point. C-h v uses the symbol name around or adjacent to point as its default.

Documentation on operating system commands, library functions and system calls can be obtained with the M-x manual-entry command. This reads a topic as an argument, and displays the "man page" on that topic. manual-entry starts a background process that formats the manual page, by running the man program. The result goes in a buffer named `*man topic*'. These buffers use a special major mode, Man mode, that facilitates scrolling and examining other manual pages. For details, type C-h m while in a man page buffer.

For a long man page, setting the faces properly can take substantial time. By default, Emacs uses faces in man pages if you are using X Windows. You can turn off use of faces in 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.

Eventually the GNU project hopes to replace most man pages with better-organized manuals that you can browse with Info. See section Other Help Commands. Since this process is only partially completed, it is still useful to read manual pages.

Change Logs

The Emacs command C-x 4 a adds a new entry to the change log file for the file you are editing (add-change-log-entry-other-window).

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 `ChangeLog' in the same directory as the file you are editing, or one of its parent directories. A single `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 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 items, each of which starts with a line starting with whitespace and a star. Here are two entries, each with two items:


Wed May  5 14:11:45 1993  Richard Stallman  <rms@gnu.ai.mit.edu>

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

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

Tue May  4 12:42:19 1993  Richard Stallman  <rms@gnu.ai.mit.edu>

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

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.

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.

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. LFD and auto-fill indent each new line like the previous line; this is convenient for entering the contents of an entry.

Version control systems are another way keep track of changes in your program and keep a change log. See section Log Entries.

Tags Tables

A 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 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 tags table files. The conventional name for a tags table file is `TAGS'.

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

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

Source File Tag Syntax

Creating Tags Tables

The etags program is used to create a tags table file. It knows the syntax of several languages, as described in the previous section. Here is how to run etags:

etags inputfiles...

The etags program reads the specified files, and writes a tags table named `TAGS' in the current working directory. etags recognizes the language used in an input file based on its file name and contents. You can specify the language with the `--language=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. It is not necessary to do this 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 effectively include another. Specify the included tags file name with the `--include=file' option when creating the file that is to include it. The latter file then acts as if it contained all the files specified in the included file, as well as the files it directly contains.

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 etags to read the file names from its standard input, by typing a dash in place of the file names, like this:

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

Use the option `--language=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 `--language=auto' to tell etags to resume guessing the language from the file names and file contents. Specify `--language=none' to turn off language-specific processing entirely; then etags recognizes tags by regexp matching alone. `etags --help' prints the list of the languages etags knows, and the file name rules for guessing the language.

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


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

You should not match more characters with 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 tagregexp, you may find useful to add a nameregexp, in order to narrow the tag scope. You can find some examples below.

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

etags --regex=/reg1/ voo.doo --regex=/reg2/ \
    bar.ber -R --lang=lisp los.er

Here etags chooses the parsing language for `voo.doo' and `bar.ber' according to their contents. etags also uses reg1 to recognize additional tags in `voo.doo', and both reg1 and reg2 to recognize additional tags in `bar.ber'. etags uses the Lisp tags rules, and no regexp matching, to recognize tags in `los.er'.

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

Tag the DEFVAR macros in the emacs source files:

--regex='/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'

Tag VHDL files (this example is a single long line, broken here for formatting reasons):

--regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/'
\( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'

Tag Cobol files (every label starting in column seven):

--language=none --regex='/.......[a-zA-Z0-9-]+\./'

Tag Postscript files (every label starting in column one):

--language=none --regex='#/[^ \t{]+#/'

Tag TCL files (this last example shows the usage of a nameregexp):

--lang=none --regex='/proc[ \t]+\([^ \t]+\)/\1/'

For a list of the other available etags options, execute etags --help.

Selecting a Tags Table

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

All this command does is store the file name in the variable 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 visit-tags-table. The variable's initial value is 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 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 instead of others. If you add the new table to the current list, it is used 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.

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

(setq tags-table-list
      '("~/emacs" "/usr/local/lib/emacs/src"))

This tells the tags commands to look at the `TAGS' files in your `~/emacs' directory and in the `/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 tags-file-name and tags-table-list.

Finding a Tag

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

M-. tag RET
Find first definition of tag (find-tag).
C-u M-.
Find next alternate definition of last tag specified.
C-u - M-.
Go back to previous tag found.
C-M-. pattern RET
Find a tag whose name matches pattern (find-tag-regexp).
C-u C-M-.
Find the next tag whose name matches the last pattern used.
C-x 4 . tag RET
Find first definition of tag, but display it in another window (find-tag-other-window).
C-x 5 . tag RET
Find first definition of tag, and create a new frame to select the buffer (find-tag-other-frame).

M-. (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 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 RET), the sexp in the buffer before or around point is used as the tag argument. See section Lists and Sexps, for info on sexps.

You don't need to give M-. the full name of the tag; a part will do. This is because M-. finds tags in the table which contain tag as a substring. However, it prefers an exact match to a substring match. To find other tags that match the same substring, give find-tag a numeric argument, as in 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 META key, M-0 M-. is an easier alternative to C-u M-..

Like most commands that can switch buffers, 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 C-x 4 ., which invokes the command find-tag-other-window. The latter is C-x 5 ., which invokes find-tag-other-frame.

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

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

Searching and Replacing with Tags Tables

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.

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

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, tags-search returns.

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

M-x tags-query-replace performs a single 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 M-x query-replace-regexp. It searches much like M-x tags-search, but repeatedly, processing matches according to your input. See section Replacement Commands, for more information on query replace.

It is possible to get through all the files in the tags table with a single invocation of 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 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 find-tag family. The find-tag commands search only for definitions of tags that match your substring or regexp. The commands tags-search and 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 tags-search is a lot like grep. You can also run 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 grep matches works like finding the compilation errors. See section Running Compilations under Emacs.

Stepping Through a Tags Table

If you wish to process all the files in the selected tags table, but not in the specific ways that M-x tags-search and M-x tags-query-replace do, you can use M-x next-file to visit the files one by one.

C-u M-x next-file
Visit the first file in the tags table, and prepare to advance sequentially by files.
M-x next-file
Visit the next file in the selected tags table.

Tags Table Inquiries

M-x list-tags RET file RET
Display a list of the tags defined in the program file `file'.
M-x tags-apropos RET regexp RET
Display a list of all tags matching regexp.

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.

M-x tags-apropos is like apropos for tags (see section Apropos). It reads a regexp, then finds all the tags in the selected tags table whose entries match that regexp, and displays the tag names found.

You can also perform completion in the buffer on the name space of tag names in the current tags tables. See section Completion for Symbol Names.

Merging Files with Emerge

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 section Comparing Files.

Overview of Emerge

To start Emerge, run one of these four commands:

M-x emerge-files
Merge two specified files.
M-x emerge-files-with-ancestor
Merge two specified files, with reference to a common ancestor.
M-x emerge-buffers
Merge two buffers.
M-x emerge-buffers-with-ancestor
Merge two buffers with reference to a common ancestor in a third buffer.

The Emerge commands compare two files or buffers, and display the comparison in three buffers: one for each input text (the A buffer and the B buffer), and one (the 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 (see section 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 `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 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 selected difference. This difference is marked off in the three buffers like this:

text that differs

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 C-x C-w. If you give a numeric argument to emerge-files or 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 C-], the Emerge command does not save the output buffer, but you can save it yourself if you wish.

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 C-c C-c, and the normal Emacs commands are also available. This allows editing the merge buffer, but slows down Emerge operations.

Use e to switch to Edit mode, and C-c C-c f to switch to Fast mode. The mode line indicates Edit and Fast modes with `E' and `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 a and 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 `A'.

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

Use the command s a (emerge-auto-advance-mode) to set or clear Auto Advance mode. Use s s (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.

State of a Difference

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

The difference is showing the A version. The a command always produces this state; the mode line indicates it with `A'.
The difference is showing the B version. The b command always produces this state; the mode line indicates it with `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 d a chooses default-A as the default state, and 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 d a and d b between sections.
The difference is showing the A or B state because it is 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 `A*' and `B*'.
The difference is showing a combination of the A and B states, as a result of the x c or x C commands. Once a difference is in this state, the a and b commands don't do anything to it unless you give them a numeric argument. The mode line displays this state as `comb'.

Merge Commands

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

Select the previous difference.
Select the next difference.
Choose the A version of this difference.
Choose the B version of this difference.
C-u n j
Select difference number n.
Select the difference containing point. You can use this command in the merge buffer or in the A or B buffer.
Quit--finish the merge.
Abort--exit merging and do not save the output.
Go into Fast mode. (In Edit mode, this is actually C-c C-c f.)
Go into Edit mode.
Recenter (like C-l) all three windows.
Specify part of a prefix numeric argument.
Also specify part of a prefix numeric argument.
d a
Choose the A version as the default from here down in the merge buffer.
d b
Choose the B version as the default from here down in the merge buffer.
c a
Copy the A version of this difference into the kill ring.
c b
Copy the B version of this difference into the kill ring.
i a
Insert the A version of this difference at point.
i b
Insert the B version of this difference at point.
Put point and mark around the difference.
Scroll all three windows down (like M-v).
Scroll all three windows up (like C-v).
Scroll all three windows left (like C-x <).
Scroll all three windows right (like C-x >).
Reset horizontal scroll on all three windows.
x 1
Shrink the merge window to one line. (Use C-u l to restore it to full size.)
x c
Combine the two versions of this difference (see section Combining the Two Versions).
x f
Show the names of the files/buffers Emerge is operating on, in a Help window. (Use C-u l to restore windows.)
x j
Join this difference with the following one. (C-u x j joins this difference with the previous one.)
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.
x t
Trim identical lines off top and bottom of the difference. Such lines occur when the A and B versions are identical but differ from the ancestor version.

Exiting Emerge

The q command (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.

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 t for successful completion, or nil if you abort.

Combining the Two Versions

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

#ifdef NEW
version from A buffer
#else /* NEW */
version from B buffer
#endif /* NEW */

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

"#ifdef NEW\n%a#else /* NEW */\n%b#endif /* NEW */\n"

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 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 gets ready to accept commands.

After setting up the merge, Emerge runs the hook emerge-startup-hook (see section Hooks).

C Mode

This section describes special features available in C mode, C++ mode and Objective C mode.

C Mode Motion Commands

This section 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 the 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. If point is already at the beginning of a statement, it moves 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 two optional arguments: the numeric prefix argument, and a buffer position limit (don't move back before that place).
Move point to the end of the innermost C statement. If point is at the end of a statement, move to the end of the next statement. With prefix argument n, move forward 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 two optional arguments: the numeric prefix argument, and a buffer position limit (don't move past that place).
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.
M-x c-forward-into-nomenclature
Move the point forward to end of a C++ nomenclature section or word. With prefix argument n, move n times.

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. 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 language element that it defines is looked up in this list, and if found, the nl-list is used to determine where newlines are inserted. If the language element for the colon 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 `} else {' constructs by placing 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 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.

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 auto-newline 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.

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 results.
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.

Comments in C Modes

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

This variable specifies how to reindent block comments. The C modes support five styles of block comments:
 style 1:    style 2 (GNU):    style 3:     style 4:     style 5:
 /*          /* Blah           /*           /*           /*
    blah        blah.  */       * blah      ** blah      blah
    blah                        * blah      ** blah      blah
    */                          */          */           */
For the styles 1 through 4, c-block-comments-indent-p should be nil (the default). If you want to use style 5, set c-block-comments-indent-p to t. This variable has no effect on the indentation of the comment-start itself or on insertion of asteristks when auto-filling C comments.
Extra offset for line which contains only the start of a comment. It can contain be 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 comments begin.
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 always puts the comment-end delimiter `*/' at the end of the last line of the comment text.

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 Hooks).

Motion Commands

Fortran mode provides special commands to move by subprograms (functions and subroutines) and by statements. There is also a command to put the region around one subprogram, convenient for killing it or moving it.

Move to beginning of subprogram (beginning-of-fortran-subprogram).
Move to end of subprogram (end-of-fortran-subprogram).
Put point at beginning of subprogram and mark at end (mark-fortran-subprogram).
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).

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.

Fortran Indentation Commands

Indent the current line (fortran-indent-line).
Indent the current and start a new indented line (fortran-indent-new-line).
Break the current line and set up a continuation line.
Indent all the lines of the subprogram point is in (fortran-indent-subprogram).

Fortran mode redefines TAB to reindent the current line for Fortran (fortran-indent-line). This command indents Line numbers and continuation markers to their required columns, and independently indents the body of the statement based on its nesting in the program.

The key LFD runs the command fortran-indent-new-line, which reindents the current line then makes and indents a new line. This command is useful to reindent the closing statement of `do' loops and other blocks before starting a new line.

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 M-LFD 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.

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 M-LFD, 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.

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.

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.

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.

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 a nonstandard comment syntax where comments start with `!' and can follow other text. Because only some Fortran 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 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 columns 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.

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 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 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 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 Hooks.

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. This may help you avoid making lines longer than the 72 character limit that some fortran compilers impose (fortran-window-create-momentarily).

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.

For even more help, use C-c C-w (fortran-window-create), a command which splits the current window horizontally, making a window 72 columns wide. By editing in this window you can immediately see when you make a line too wide to be correct Fortran.

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 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.

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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