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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 interesting 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++, Perl, Icon, Fortran and Muddle (another variant of Lisp). 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. For example, turning on C mode runs the hook c-mode-hook. Mode hook variables for other programming language modes include lisp-mode-hook, emacs-lisp-mode-hook, lisp-interaction-mode-hook, scheme-mode-hook and muddle-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 syntax for expressions that nobody has bothered to make Emacs understand properly.

## List And Sexp Commands

C-M-f
Move forward over a sexp (forward-sexp).
C-M-b
Move backward over a sexp (backward-sexp).
C-M-k
Kill sexp forward (kill-sexp).
C-M-u
Move up and backward in list structure (backward-up-list).
C-M-d
Move down and forward in list structure (down-list).
C-M-n
Move forward over a list (forward-list).
C-M-p
Move backward over a list (backward-list).
C-M-t
Transpose expressions (transpose-sexps).
C-M-@
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). C-M-k kills the characters that C-M-f 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 make the region be 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.

## Defuns

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.

C-M-a
Move to beginning of current or preceding defun (beginning-of-defun).
C-M-e
Move to end of current or following defun (end-of-defun).
C-M-h
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.

### Basic Program Indentation Commands

TAB
LFD
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 a large amount 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 is the right behavior in cases where the standard result of TAB is unaesthetic.

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.

C-M-q
Re-indent all the lines within one list (indent-sexp).
C-u TAB
Shift an entire list rigidly sideways so that its first line is properly indented.
C-M-\
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 beginning is not, go to the line the list begins on and type C-u TAB. When TAB is given a numeric argument, it moves all the lines in the grouping starting on the current line sideways the same amount that the current line moves. It is clever, though, and does not move 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 point and mark. 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-hook property of the function name. There are four possibilities for this property:

nil
This is the same as no property; the standard indentation pattern is used.
defun
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:
state
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.
pos
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.

### Customizing C Indentation

Two variables control which commands perform C indentation and when.

If c-auto-newline is non-nil, newlines are inserted both before and after braces that you insert, and after colons and semicolons. Correct C indentation is done on all the lines that are made this way.

If c-tab-always-indent is nil, the TAB command in C mode does indentation only if point is at the left margin or within the line's indentation. If there is non-whitespace to the left of point, then TAB just inserts a tab character in the buffer. Normally, this variable is t, and TAB always reindents the current line. The default behavior means that to insert a real tab character you must quote it by typing C-q TAB.

C does not have anything analogous to particular function names for which special forms of indentation are desirable. However, it has a different need for customization facilities: many different styles of C indentation are in common use.

There are six variables you can set to control the style that Emacs C mode uses.

c-indent-level
Indentation of C statements within surrounding block. The surrounding block's indentation is the indentation of the line on which the open-brace appears.
c-continued-statement-offset
Extra indentation given to a substatement, such as the then-clause of an if or body of a while.
c-brace-offset
Extra indentation for line if it starts with an open brace.
c-brace-imaginary-offset
An open brace following other text is treated as if it were this far to the right of the start of its line.
c-argdecl-indent
Indentation level of declarations of C function arguments.
c-label-offset
Extra indentation for line that is a label, or case or default.

The variable c-indent-level controls the indentation for C statements with respect to the surrounding block. In the example

    {
foo ();


the difference in indentation between the lines is c-indent-level. Its standard value is 2.

If the open-brace beginning the compound statement is not at the beginning of its line, the c-indent-level is added to the indentation of the line, not the column of the open-brace. For example,

if (losing) {
do_this ();


One popular indentation style is that which results from setting c-indent-level to 8 and putting open-braces at the end of a line in this way. I prefer to put the open-brace on a separate line.

In fact, the value of the variable c-brace-imaginary-offset is also added to the indentation of such a statement. Normally this variable is zero. Think of this variable as the imaginary position of the open brace, relative to the first nonblank character on the line. By setting this variable to 4 and c-indent-level to 0, you can get this style:

if (x == y) {
do_it ();
}


When c-indent-level is zero, the statements inside most braces will line up right under the open brace. But there is an exception made for braces in column zero, such as surrounding a function's body. The statements just inside it do not go at column zero. Instead, c-brace-offset and c-continued-statement-offset (see below) are added to produce a typical offset between brace levels, and the statements are indented that far.

c-continued-statement-offset controls the extra indentation for a line that starts within a statement (but not within parentheses or brackets). These lines are usually statements that are within other statements, such as the then-clauses of if statements and the bodies of while statements. This parameter is the difference in indentation between the two lines in

if (x == y)
do_it ();


Its standard value is 2. Some popular indentation styles correspond to a value of zero for c-continued-statement-offset.

c-brace-offset is the extra indentation given to a line that starts with an open-brace. Its standard value is zero; compare

if (x == y)
{


with

if (x == y)
do_it ();


if c-brace-offset were set to 4, the first example would become

if (x == y)
{


c-argdecl-indent controls the indentation of declarations of the arguments of a C function. It is absolute: argument declarations receive exactly c-argdecl-indent spaces. The standard value is 5, resulting in code like this:

char *
index (string, c)
char *string;
int c;


c-label-offset is the extra indentation given to a line that contains a label, a case statement, or a default: statement. Its standard value is -2, resulting in code like this

switch (c)
{
case 'x':


If c-label-offset were zero, the same code would be indented as

switch (c)
{
case 'x':


This example assumes that the other variables above also have their standard values.

I strongly recommend that you try out the indentation style produced by the standard settings of these variables, together with putting open braces on separate lines. You can see how it looks in all the C source files of GNU Emacs.

## 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.

Two 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-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.)

The comment commands insert, kill and align comments.

M-;
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).
M-LFD
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.

The M-x comment-region command 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 adds comment delimiters and duplicates the last character of the comment start sequence as many times as the argument specifies. 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.

If you are typing a comment and find that you wish to continue it on another line, you can use the command M-LFD (indent-new-comment-line), which 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.

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. This regexp should 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

M-(
Put parentheses around next sexp(s) (insert-parentheses).
M-)
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, and leaves point after the open parenthesis. 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. Then you can type M-), which moves past the close parenthesis, deleting any indentation preceding it (in this example there is none), and indenting with LFD after it.

M-( may insert a space before the open parenthesis, depending on the syntax class or 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 tag table (see section Tag Tables); the legitimate symbol names are the tag names listed in the tag 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.

## 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 Unix commands, system calls and libraries can be obtained with the M-x manual-entry command. This reads a topic as an argument, and displays the text on that topic from the Unix manual. 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 have a special major mode that facilitates scrolling and examining other manual pages.

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 and the current date. 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@mole.gnu.ai.mit.edu)

* man.el: Rename functions and variables 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@mole.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. 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.

## Tag Tables

A tag 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 you can use to find the definition of a function without having to know which of the files it is in.

Tag tables are stored in files called tag table files. The conventional name for a tag table file is TAGS'.

Each entry in the tag 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 tag 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

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

In Scheme code, tags include anything defined with def or with a construct whose name starts with def'. They also include variables set with set! at top level in the file.

In C code, any C function is a tag, and so is any typedef if -t is specified when the tag table is constructed.

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

In Fortran code, functions and subroutines are tags.

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

In assembler code, labels appearing at the beginning of a line, followed by a colon, are tags.

In LaTeX text, the argument of any of the commands \chapter, \section, \subsection, \subsubsection, \eqno, \label, \ref, \cite, \bibitem and \typeout is a tag.

### Creating Tag Tables

The etags program is used to create a tag 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 tag table named TAGS' in the current working directory. etags recognizes the language used in an input file based on its file name and contents; there are no switches for specifying the language. The -t switch tells etags to record typedefs in C code as tags.

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

If the tag 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 tag 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 tag 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 tag table after each edit, or even every day.

### Selecting a Tag Table

Emacs has at any time one selected tag table, and all the commands for working with tag tables use the selected one. To select a tag table, type M-x visit-tags-table, which reads the tag 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 tag 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 tag tables that they must ask for a tag table file name to use.

Using visit-tags-table to load a new tag table does not discard the other tables previously loaded. The other tags commands use all the tag tables that are loaded; the first one they use is the one that mentions the current visited file.

You can specify a precise list of tag 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.

### Finding a Tag

The most important thing that a tag 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.
M-x find-tag-regexp RET pattern RET
Find a tag whose name matches pattern.
C-u M-x find-tag-regexp
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 tag table for that tag, as a string, and then uses the tag 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 tag 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 new command M-x 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.

Emacs comes with a tag table file src/TAGS' that includes all the Lisp libraries and all the C sources of Emacs. By specifying this file with visit-tags-table and then using M-. you can quickly find the source for any Emacs function.

### Searching and Replacing with Tag Tables

The commands in this section visit and search all the files listed in the selected tag table, one by one. For these commands, the tag table serves only to specify a sequence of files to search. A related command is M-x grep (see section Running make', or Compilers Generally).

M-x tags-search
Search for the specified regexp through the files in the selected tag table.
M-x tags-query-replace
Perform a query-replace on each file in the selected tag table.
M-,
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 tag 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 tag table.

M-x tags-query-replace performs a single query-replace through all the files in the tag 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 tag table with a single invocation of M-x tags-query-replace. But since any unrecognized character causes the command to exit, you may need to continue where you left off. M-, can be used for this. It resumes the last tags search or replace command that you did.

The commands in this section carry out much broader searches than the find-tags family. The find-tags 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 mostly the same as running a compilation and having Emacs show you where the errors were. See section Running make', or Compilers Generally.

### Stepping Through a Tag Table

If you wish to process all the files in the selected tag 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 tag table, and prepare to advance sequentially by files.
M-x next-file
Visit the next file in the selected tag table.

@break

### Tag Table Inquiries

M-x list-tags
Display a list of the tags defined in a specific program file.
M-x tags-apropos
Display a list of all tags matching a specified regexp.

M-x list-tags reads the name of one of the files described by the selected tag 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 names recorded in the tag 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 string the same way it appears in the tag table. Do not include a directory as part of the file name unless the file name recorded in the tag table includes a directory.

M-x tags-apropos is like apropos for tags. It reads a regexp, then finds all the tags in the selected tag 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 tag 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 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:

vvvvvvvvvvvvvvvvvvvv
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 part of the buffer agrees with the common ancestor, then the B version is preferred for that part.

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 prefix 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.

If you abort Emerge with C-], the output is not saved.

### Submodes of Emerge

You can choose between two modes for giving merge commands: Fast mode and Edit mode. In Fast mode, basic Emerge commands are single characters, but ordinary Emacs commands are disabled. This is convenient if you use only Emerge commands.

In Edit mode, all Emerge 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. 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:

A
The difference is showing the A version. The a command always produces this state; the mode line indicates it with A'.

B
The difference is showing the B version. The b command always produces this state; the mode line indicates it with B'.

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

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

The command 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 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 at the end of each section.

prefer-A
prefer-B
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*'.

combined
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 prefix 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:

p
Select the previous difference.

n
Select the next difference.

a
Choose the A version of this difference.

b
Choose the B version of this difference.

j
Select a particular difference; specify the sequence number of that difference as a prefix argument.

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

q
Quit--finish the merge.

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

f
Go into Fast mode. (In Edit mode, this is actually C-c C-c f.)

e
Go into Edit mode.

l
Recenter (like C-l) all three windows.

-
Specify part of a prefix numeric argument.

digit
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 the point.

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

m
Put the point and mark around the difference region.

^
Scroll all three windows down (like M-v).

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.

x f
Show the files/buffers Emerge is operating on in 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 to 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 Emerge 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 locus. To do this, use x c, which edits the merge buffer like this:

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


While this example shows C preprocessor conditionals delimiting the two alternative versions, you can specify the strings you want 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

In addition to the facilities of typical programming language major modes (see section Major Modes for Programming Languages), C mode has various special facilities.

M-a
M-e
In C mode, M-a and M-e now move by complete C statements (c-beginning-of-statement and c-end-of-statement). These commands do ordinary, textual sentence motion when in or next to a comment.

M-q
M-q in C mode runs c-fill-paragraph, which is designed for filling C comments. (We assume you don't want to fill the actual C code in a C program.)

C-c C-u
Move back to the containing preprocessor conditional, setting the mark at the starting point (c-up-conditional).

A prefix argument acts as a repeat count. With a negative argument, this command moves forward to the end of the containing preprocessor conditional. When going backwards, #elif' acts like #else' followed by #if'. When going forwards, #elif' is ignored.

C-c C-n
Move forward across the next preprocessor conditional, setting the mark at the starting point (c-forward-conditional).

C-c C-p
Move backward across the previous preprocessor conditional, setting the at the starting point (c-backward-conditional).

M-x c-macro-expand
When you are debugging C code that uses macros, sometimes it is hard to figure out precisely how the macros expand. The command M-x c-macro-expand runs the C preprocessor and shows you what expansion results from the region. The portion of the buffer before the region is also included in preprocessing, for the sake of macros defined there, but the output from this part isn't shown.

M-x c-backslash-region
Insert or align \' characters at the ends of the lines of the region, except for the last such line. 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 \'.

C++ mode is like C mode, except that it understands C++ comment syntax and certain other differences between C and C++. It also has a command M-x fill-c++-comment, which fills a paragraph made of C++ comment lines.

The command comment-region is useful in C++ mode for commenting out several consecutive lines, or removing the commenting out of such lines. (You don't need this command with C comment syntax because you don't need to put comment delimiters on each line.) See section Manipulating Comments.

## 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 it's 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).

Fortran mode was contributed by Michael Prange. It has been updated by Stephen A. Wood who has collated the contributions and suggestions of many users.

### 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.

C-M-a
Move to beginning of subprogram (beginning-of-fortran-subprogram).
C-M-e
Move to end of subprogram (end-of-fortran-subprogram).
C-M-h
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

TAB
Indent the current line (fortran-indent-line).
LFD
Indent the current and start a new indented line (fortran-indent-new-line).
M-LFD
Break the current line and set up a continuation line.
C-M-q
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.) 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.

### 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. 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.

### Columns

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 is the value of the variable fortran-comment-ruler. By changing this variable, you can change the 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:

TAB
tab-to-tab-stop.
LFD
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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