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21. Editing Programs

XEmacs provides specialized support for editing source files for many different programming languages. For example it is possible to

The commands available for words, sentences, and paragraphs are useful in editing code even though their canonical application is for editing human language text. Most symbols contain words (see section 20.2 Words); sentences can be found in strings and comments (see section 20.3 Sentences). Paragraphs per se are not present in code, but the paragraph commands are useful anyway, because Lisp mode and C mode define paragraphs to begin and end at blank lines (see section 20.4 Paragraphs). Judicious use of blank lines to make the program clearer also provides 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 11.3 Selective Display). This feature causes only the lines that are indented less than a specified amount to appear on the screen.

21.1 Major Modes for Programming Languages  Major modes for editing programs.
21.2 Lists and Sexps  Expressions with balanced parentheses. There are editing commands to operate on them.
21.3 Defuns  Each program is made up of separate functions. There are editing commands to operate on them.
21.4 Indentation for Programs  Adjusting indentation to show the nesting.
21.5 Automatic Display of Matching Parentheses  Insertion of a close-delimiter flashes matching open.
21.6 Manipulating Comments  Inserting, filling and aligning comments.
21.7 Editing Without Unbalanced Parentheses  Inserting two matching parentheses at once, etc.
21.8 Completion for Lisp Symbols  Completion on symbol names in Lisp code.
21.9 Documentation Commands  Getting documentation of functions you plan to call.
21.10 Change Logs  Maintaining a change history for your program.
21.11 Tags Tables  Go direct to any function in your program in one command. Tags remembers which file it is in.
21.12 Modes for C, C++, Java and similar languages  
21.13 Fortran Mode  Fortran mode and its special features.
21.14 Asm Mode  Asm mode and its special features.


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21.1 Major Modes for Programming Languages

Emacs has several major modes (see section 18. Major Modes) to support programming languages. These major modes will typically understand language syntax, provide automatic indentation features, syntax based highlighting of text, and will often provide interfaces to the programming environment to assist in compiling, executing and debugging programs.

A language mode exist when someone decides to take the trouble to write it. At this time many widely used programming languages are supported by XEmacs. Examples include Ada, Awk, C, C++, CORBA (IDL), Fortran, Java, Lisp, Modula 2, Objective-C, Perl, Pike, Prolog, Python, Ruby, Scheme, Simula, SQL, Tcl, Unix Shell scripts, and VHDL. Some of these language have separate manuals, and some times more than one mode may be available for a language. For example, there are several variants of Lisp mode, which differ in the way they interface to Lisp execution. See section 22.2 Major Modes for Lisp.

Major modes for programming language support are distributed in optional XEmacs packages (see section 22.8 Packages) that must be installed before use. A notable exception to this rule is that a Lisp Mode is integral to XEmacs. The Programming Mode Package (`prog-modes') contains many such modes. Some languages are supported by packages of their own; prominent examples of such packages include `cc-mode' for C, C++, Java, Objective C etc., `python-modes' for Python, and `scheme' for Scheme.

For a language named lang the major mode for the language will typically be named lang-mode. For example, the mode for C is called c-mode, that for Bourne shell scripts is called sh-mode and so on. These modes will invoke the functions listed in the corresponding hook variables as a last step. See section 18.2 Mode Hook Variables.

A mode can be invoked by typing M-x lang-mode RET. However this step is not normally required. If the package for a language mode is installed XEmacs usually knows when to automatically invoke the mode. This is normally done based on examining the file name to determine the language. 18.1 Choosing Major Modes.

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. 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. In these modes, use C-b C-d to delete a tab character before point.

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.


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21.2 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 English).

The commands fall into two classes. Some commands deal only with lists (parenthetical groupings). They see nothing except parentheses, brackets, braces (depending on what 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 term for a symbolic expression in Lisp. In Emacs, the notion of `sexp' is not limited to Lisp. It refers to an expression in the 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 an 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 sexp in C mode.

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

C-M-f
Move forward over an sexp (forward-sexp).
C-M-b
Move backward over an 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 an 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. If the character after point is a closing delimiter, C-M-f just moves past it. (This last is not really moving across an sexp; it is an exception which is included in the definition of C-M-f because it is as useful a behavior as anyone can think of for that situation.)

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, back quote, and comma, in Lisp) preceding the sexp, C-M-b moves back over them as well.

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 an 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, C-M-n (forward-list) and C-M-p (backward-list), move over lists like the sexp commands but skip over any number of other kinds of sexps (symbols, strings, etc). In some situations, these commands are useful because 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 is 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.

C-M-t (transpose-sexps) 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 the 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 27.6 The Syntax Table.


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21.3 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 Lisp are instances of the special operator defun, but Emacs calls any top-level parenthetical grouping counts a defun regardless of its contents or the programming language. 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).

To operate on the current defun, use C-M-h (mark-defun) which puts point at the beginning and the mark at the end of the current or next defun. This is the easiest way to prepare for moving the defun to a different place. In C mode, C-M-h runs the function mark-c-function, which is almost the same as mark-defun, but which backs up over the argument declarations, function name, and returned data type so that the entire C function is inside the region.

To compile and evaluate the current defun, use M-x compile-defun. This function prints the results in the minibuffer. If you include an argument, it inserts the value in the current buffer after the defun.

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.

The original Emacs found defuns by moving upward a level of parentheses until there were no more levels to go up. This required scanning back to the beginning of the buffer for every function. To speed this up, 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; however, it mandates the convention described above.


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21.4 Indentation for Programs

The best way to keep a program properly indented ("ground") is to use Emacs to re-indent it as you change the program. 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.

21.4.1 Basic Program Indentation Commands  
21.4.2 Indenting Several Lines  Commands to reindent many lines at once.
21.4.3 Customizing Lisp Indentation  Specifying how each Lisp function should be indented.


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21.4.1 Basic Program Indentation Commands

TAB
Adjust indentation of current line.
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, 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 tend to follow it. This is the right behavior in cases where the standard result of TAB does not look good.

Remember that Emacs assumes that an open-parenthesis, open-brace, or other opening delimiter at the left margin (including the indentation routines) is the start of a function. You should therefore 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. See section 21.3 Defuns, for more information on this behavior.


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21.4.2 Indenting Several Lines

Several commands are available to re-indent several lines of code which have been altered or moved to a different level in a list structure.

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).

To re-indent the contents of a single list, position point before the beginning of it and type C-M-q. This key is bound to indent-sexp in Lisp mode, indent-c-exp in C mode, and bound to other suitable functions 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 C-M-q.

If the relative indentation within a list is correct but the indentation of its beginning is not, go to the line on which the list begins and type C-u TAB. When you give TAB a numeric argument, it moves all the lines in the group, starting on the current line, sideways the same amount that the current line moves. The command does not move lines that start inside strings, or C preprocessor lines when in C mode.

Another way to specify a 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.


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21.4.3 Customizing Lisp Indentation

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

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

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

Certain functions override the standard pattern. Functions whose names start with def always indent the second line by lisp-body-indention extra columns beyond the open-parenthesis starting the expression.

Individual functions can override the standard pattern in various ways, according to the lisp-indent-function property of the function name. (Note: lisp-indent-function was formerly called lisp-indent-hook). 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 first element 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 computed by C-M-q; if the value is a number, C-M-q need not recalculate indentation for the following lines until the end of the list.


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21.5 Automatic Display of Matching Parentheses

The Emacs parenthesis-matching feature shows you automatically how parentheses match in the text. Whenever a self-inserting character that is a closing delimiter is typed, the cursor moves momentarily to the location of the matching opening delimiter, provided that is visible on the screen. If it is not on the screen, some text starting with that opening delimiter is displayed in the echo area. Either way, you see the grouping you are closing off.

In Lisp, automatic matching applies only to parentheses. In C, it also applies to braces and brackets. Emacs knows which characters to regard as matching delimiters based on the syntax table set by the major mode. See section 27.6 The Syntax Table.

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

Two variables control parenthesis matching displays. blink-matching-paren turns the feature on or off. The default is t (match display is on); nil turns it off. blink-matching-paren-distance specifies how many characters back Emacs searches to find a matching opening delimiter. If the match is not found in the specified region, scanning stops, and nothing is displayed. This prevents wasting lots of time scanning when there is no match. The default is 4000.


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21.6 Manipulating Comments

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).

The command that creates a comment is Meta-; (indent-for-comment). If there is no comment already on the line, a new comment is created and aligned at a specific column called the comment column. Emacs creates the comment by inserting the string at the value of comment-start; see below. Point is left after that string. If the text of the line extends past the comment column, 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 string is inserted after point, to keep the syntax valid.

You can also use Meta-; to align an existing comment. If a line already contains the string that starts comments, 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 happens. To reinsert the comment on another line, move to the end of that line, type first C-y, and then M-; to realign the comment. 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 to call kill-comment when called with a negative argument. However, kill-comment is a valid command which you could bind directly to a key if you wanted to.


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21.6.1 Multiple Lines of Comments

If you are typing a comment and want to continue it on another line, use the command Meta-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. If Auto Fill mode is on and you go past the fill column while typing, the comment is continued in just this fashion. If point is not at the end of the line when you type M-LFD, the text on the rest of the line becomes part of the new comment line.


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21.6.2 Options Controlling Comments

The comment column is stored in the variable comment-column. You can explicitly set it to a number. 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 calls Meta-; 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.

comment-column is a per-buffer variable; altering the variable affects only the current buffer. You can also change the default value. See section 27.3.4 Local Variables. Many major modes initialize this variable for the current buffer.

The comment commands recognize comments based on the regular expression that is the value of the variable comment-start-skip. 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 12.5 Syntax of Regular Expressions.)

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

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 M-LFD terminates the comment on the starting line and starts a new comment on the new following line. If comment-multi-line is not nil, then M-LFD sets up the new following line 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 are legal, the value you choose for this variable is a matter of taste.

The variable comment-indent-hook should contain a function that is called to compute the indentation for a newly inserted comment or for aligning an existing comment. Major modes set this variable differently. 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. The function should return the column in which the comment ought to start. For example, in Lisp mode, the indent hook function bases its decision on the number of semicolons that begin an existing comment and on the code in the preceding lines.


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21.7 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. You can then type M-), which moves past the close parenthesis, deletes any indentation preceding it (in this example there is none), and indents with LFD after it.


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21.8 Completion for Lisp Symbols

Completion usually happens in the minibuffer. An exception is completion for Lisp symbol names, which is available in all buffers.

The command M-TAB (lisp-complete-symbol) takes the partial Lisp symbol before point to be an abbreviation, and compares it against all non-trivial Lisp symbols currently known to Emacs. Any additional characters that they all have in common are inserted at point. Non-trivial symbols are those that have function definitions, values, or properties.

If there is an open-parenthesis immediately before the beginning of the partial symbol, only symbols with function definitions are considered as completions.

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.


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21.9 Documentation Commands

As you edit Lisp code to be run in Emacs, you can use the commands C-h f (describe-function) and C-h v (describe-variable) to print documentation of functions and variables 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.

The M-x manual-entry command gives you access to documentation on Unix commands, system calls, and libraries. The command reads a topic as an argument, and displays the Unix manual page for that topic. manual-entry always searches all 8 sections of the manual and concatenates all the entries it finds. For example, the topic `termcap' finds the description of the termcap library from section 3, followed by the description of the termcap data base from section 5.


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21.10 Change Logs

The Emacs command M-x add-change-log-entry helps you keep a record of when and why you have changed a program. It assumes that you have a file in which you write a chronological sequence of entries describing individual changes. The default is to store the change entries in a file called `ChangeLog' in the same directory as the file you are editing. The same `ChangeLog' file therefore records changes for all the files in a directory.

A change log entry starts with a header line that contains your name and the current date. Except for these header lines, every line in the change log starts with a tab. One entry can describe several changes; each change starts with a line starting with a tab and a star. M-x add-change-log-entry visits the change log file and creates a new entry unless the most recent entry is for today's date and your name. In either case, it adds a new line to start the description of another change just after the header line of the entry. When M-x add-change-log-entry is finished, all is prepared for you to edit in the description of what you changed and how. You must then save the change log file yourself.

The change log file is always visited in Indented Text mode, which means that LFD and auto-filling indent each new line like the previous line. This is convenient for entering the contents of an entry, which must be indented. See section 20.1 Text Mode.

Here is an example of the formatting conventions used in the change log for Emacs:

 
Wed Jun 26 19:29:32 1985  Richard M. Stallman  (rms at mit-prep)

        * xdisp.c (try_window_id):
        If C-k is done at end of next-to-last line,
        this fn updates window_end_vpos and cannot leave
        window_end_pos nonnegative (it is zero, in fact).
        If display is preempted before lines are output,
        this is inconsistent.  Fix by setting
        blank_end_of_window to nonzero.

Tue Jun 25 05:25:33 1985  Richard M. Stallman  (rms at mit-prep)

        * cmds.c (Fnewline):
        Call the auto fill hook if appropriate.

        * xdisp.c (try_window_id):
        If point is found by compute_motion after xp, record that
        permanently.  If display_text_line sets point position wrong
        (case where line is killed, point is at eob and that line is
        not displayed), set it again in final compute_motion.


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21.11 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.

The Ebrowse is a separate facility tailored for C++, with tags and a class browser. See section `Ebrowse' in Ebrowse User's Manual.

21.11.1 Source File Tag Syntax  Tag syntax for various types of code and text files.
21.11.2 Creating Tags Tables  Creating a tags table with etags.
21.11.3 Etags Regexps  Create arbitrary tags using regular expressions.
21.11.4 Selecting a Tags Table  How to visit a tags table.
21.11.5 Finding a Tag  Commands to find the definition of a specific tag.
21.11.6 Searching and Replacing with Tags Tables  Using a tags table for searching and replacing.
21.11.7 Tags Table Inquiries  Listing and finding tags defined in a file.


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21.11.1 Source File Tag Syntax

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

Several other languages are also supported:

You can also generate tags based on regexp matching (see section 21.11.3 Etags Regexps) to handle other formats and languages.


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21.11.2 Creating Tags Tables

The etags program is used to create a tags table file. It knows the syntax of several languages, as described in 21.11.1 Source File Tag Syntax. 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. You can intermix compressed and plain text source file names. etags knows about the most common compression formats, and does the right thing. So you can compress all your source files and have etags look for compressed versions of its file name arguments, if it does not find uncompressed versions. Under MS-DOS, etags also looks for file names like `mycode.cgz' if it is given `mycode.c' on the command line and `mycode.c' does not exist.

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.

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

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

When you want to make a tags table from a great number of files, you may have problems listing them on the command line, because some systems have a limit on its length. The simplest way to circumvent this limit is to tell 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 (see section 21.11.3 Etags Regexps).

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


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21.11.3 Etags Regexps

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:

 
--regex=/tagregexp[/nameregexp]/

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.

The syntax of regular expressions in etags is the same as in Emacs, augmented with the interval operator, which works as in grep and ed. The syntax of an interval operator is `\{m,n\}', and its meaning is to match the preceding expression at least m times and up to n times.

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 (as will usually be the case), you should add a nameregexp, to pick out just the tag. This will enable Emacs to find tags more accurately and to do completion on tag names more reliably. You can find some examples below.

The option `--ignore-case-regex' (or `-c') is like `--regex', except that the regular expression provided will be matched without regard to case, which is appropriate for various programming languages.

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'.

A regular expression can be bound to a given language, by prepending it with `{lang}'. When you do this, etags will use the regular expression only for files of that language. `etags --help' prints the list of languages recognised by etags. The following example tags the DEFVAR macros in the Emacs source files. etags applies this regular expression to C files only:

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

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

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

A regex file contains one regular expressions per line. Empty lines, and lines beginning with space or tab are ignored. When the first character in a line is `@', etags assumes that the rest of the line is the name of a file of regular expressions. This means that such files can be nested. All the other lines are taken to be regular expressions. For example, one can create a file called `emacs.tags' with the following contents (the first line in the file is a comment):

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

and then use it like this:

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

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


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21.11.4 Selecting a Tags Table

At any time Emacs has one selected tags table, and all the commands for working with tags tables use the selected one. To select a tags table, use the variable tag-table-alist.

The value of tag-table-alist is a list that determines which TAGS files should be active for a given buffer. This is not really an association list, in that all elements are checked. The car of each element of this list is a pattern against which the buffers file name is compared; if it matches, then the cdr of the list should be the name of the tags table to use. If more than one element of this list matches the buffers file name, all of the associated tags tables are used. Earlier ones are searched first.

If the car of elements of this list are strings, they are treated as regular-expressions against which the file is compared (like the auto-mode-alist). If they are not strings, they are evaluated. If they evaluate to non-nil, the current buffer is considered to match.

If the cdr of the elements of this list are strings, they are assumed to name a tags file. If they name a directory, the string `tags' is appended to them to get the file name. If they are not strings, they are evaluated and must return an appropriate string.

For example:

 
  (setq tag-table-alist
        '(("/usr/src/public/perl/" . "/usr/src/public/perl/perl-3.0/")
          ("\\.el$" . "/usr/local/emacs/src/")
          ("/jbw/gnu/" . "/usr15/degree/stud/jbw/gnu/")
          ("" . "/usr/local/emacs/src/")
          ))

The example defines the tags table alist in the following way:

If you had a file called `/usr/jbw/foo.el', it would use both `TAGS' files,
`/usr/local/emacs/src/TAGS' and `/usr15/degree/stud/jbw/gnu/TAGS' (in that order), because it matches both patterns.

If the buffer-local variable buffer-tag-table is set, it names a tags table that is searched before all others when find-tag is executed from this buffer.

If there is a file called `TAGS' in the same directory as the file in question, then that tags file will always be used as well (after the buffer-tag-table but before the tables specified by this list).

If the variable tags-file-name is set, the `TAGS' file it names will apply to all buffers (for backwards compatibility.) It is searched first.

If the value of the variable tags-always-build-completion-table is t, the tags file will always be added to the completion table without asking first, regardless of the size of the tags file.

The function M-x visit-tags-table, is largely made obsolete by the variable tag-table-alist, tells tags commands to use the tags table file file first. The file should be the name of a file created with the etags program. A directory name is also acceptable; it means the file `TAGS' in that directory. The function only stores the file name you provide in the variable tags-file-name. Emacs does not actually read in the tags table contents until you try to use them. You can set the variable explicitly instead of using visit-tags-table. The value of the variable tags-file-name is the name of the tags table used by all buffers. This is for backward compatibility, and is largely supplanted by the variable tag-table-alist.


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21.11.5 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 &optional other-window
Find first definition of tag (find-tag).
C-u M-.
Find next alternate definition of last tag specified.
C-x 4 . tag
Find first definition of tag, but display it in another window (find-tag-other-window).

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, then uses the tags table information to determine the file in which the definition is used and the approximate character position of the definition in the file. Then find-tag visits the file, moves point to the approximate character position, and starts searching ever-increasing distances away for the text that should appear at the beginning of the definition.

If an empty argument is given (by typing RET), the sexp in the buffer before or around point is used as the name of the tag to find. See section 21.2 Lists and Sexps, for information on sexps.

The argument to find-tag need not be the whole tag name; it can be a substring of a tag name. However, there can be many tag names containing the substring you specify. Since find-tag works by searching the text of the tags table, it finds the first tag in the table that the specified substring appears in. To find other tags that match the 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 your keyboard has a real META key, M-0 M-. is an easier alternative to C-u M-..

If the optional second argument other-window is non-nil, it uses another window to display the tag. Multiple active tags tables and completion are supported.

Variables of note include the following:

tag-table-alist
Controls which tables apply to which buffers.
tags-file-name
Stores a default tags table.
tags-build-completion-table
Controls completion behavior.
buffer-tag-table
Specifies a buffer-local table.
make-tags-files-invisible
Sets whether tags tables should be very hidden.
tag-mark-stack-max
Specifies how many tags-based hops to remember.

Like most commands that can switch buffers, find-tag has another similar command that displays the new buffer in another window. C-x 4 . invokes the function find-tag-other-window. (This key sequence ends with a period.)

Emacs comes with a tags table file `TAGS' (in the directory containing Lisp libraries) 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 look at the source of any Emacs function.


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21.11.6 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. A related command is M-x grep (see section 22.1 Running "make", or Compilers Generally).

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.
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 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 12.7 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 22.1 Running "make", or Compilers Generally.

If you wish to process all the files in a selected tags table, but M-x tags-search and M-x tags-query-replace are not giving you the desired result, you can use M-x next-file.

C-u M-x next-file
With a numeric argument, regardless of its value, 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.


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21.11.7 Tags 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 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 names recorded in the tags table; it is read as a string rather than 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 tags table contains that directory.

M-x tags-apropos is like apropos for tags. 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.


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21.12 Modes for C, C++, Java and similar languages

The recommended means for supporting the "C--like" programming languages in XEmacs is the `cc-mode' package. CC Mode is not included in the basic XEmacs distribution but is available as an optional package. If loading a file whose names ends in the `.cc' extension does not automatically invoke a C++ Mode then the `cc-mode' package is probably not yet installed. See section 22.8 Packages.

CC Mode provides modes for editing source files in Awk (awk-mode), C (c-mode), C++ (c++-mode), CORBA IDL (idl-mode), Java (java-mode), Objective C (objc-mode), and Pike (pike-mode). All these languages are supported with an sophisticated "indentation engine" that is feature rich, customizable and quite efficient.

Each language major mode runs hooks in the conventionally named hook variables (see section 18.2 Mode Hook Variables). In addition to this conventional behavior all the CC Mode major modes will also run hooks in c-mode-common-hook before invoking the major mode specific hook.

CC Mode runs any hooks in c-initialization-hook exactly once when it is first loaded. CC Mode is a very comprehensive and flexible system and full description of its capabilities is beyond the scope of this manual. It is strongly recommended that the reader consult the CC Mode documentation for details once the package has been installed. See section `Top' in The CC Mode Manual.

21.12.1 Older Modes for C and AWK  
21.12.2 Customizing Indentation in CC Mode  An Introduction to Customizing CC Mode.


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21.12.1 Older Modes for C and AWK

XEmacs provides older versions of a C Mode and an AWK Mode in the `prog-modes' package. These older modes do not share the indentation engine in CC Mode have have their own specific means of customizing indentation. To use these modes the `prog-modes' package must be installed.

This older C mode is known simply as the "Old C Mode". It supports only the C language and it lacks many of the features of CC Mode. However the old C mode offers modest space requirements and very fast operation. Old C Mode might be useful in space constrained environments, on slow machines, or for editing very large files. This old C mode is available in the `old-c-mode' library. See section `old-c-mode' in The Programming Modes Package Manual.

The old AWK mode exists for similar reasons. It is available in the `awk-mode' library. See section `awk-mode' in The Programming Modes Package Manual.

Note that the prog-modes package will never automatically invoke these older modes for a user. However installing the `cc-mode' package will make CC Mode's versions available automatically. As a result a user who wants to use these older modes must explicitly load the old libraries to use them.


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21.12.2 Customizing Indentation in CC Mode

A very brief introduction is included here on customizing CC Mode. CC Mode has many features, including useful minor modes, that are completely documented in its own manual.

CC Mode implements several different "styles" for C code (and the other languages supported by CC Mode). If you need to change the indentation style for CC Mode it is recommended that you first see if an existing style meets your requirements. The style chosen will affect the placement of language elements like braces, function declarations and comments. You can choose a style interactively by typing C-c . and pressing the space bar at the prompt to get a list of supported styles. C-c . runs the function c-set-style which applies to all CC Mode language modes though its name might suggest otherwise. A few of the the supported styles are listed below.

The default style in XEmacs is "gnu" except for Java mode where it is the "java" style (this is governed by the variable c-default-style).

The styles included in CC Mode all use a buffer local variable called c-basic-offset as the basic indentation level (this buffer local variable is used in all CC Mode language modes though its name might suggest otherwise). All indentation is, by default, expressed in multiples of c-basic-offset.

Each style defines a default value for c-basic-offset, for the "gnu" style sets it to 2. A very common customization scenario is where a user wants to use an existing style but with a different basic offset value. An easy way to do this is to set c-basic-offset in the language mode hook after selecting the chosen style.

For example, a user might want to follow a local coding convention of using the "k&r" style for C code with indentation in two columns multiples (instead of the five column default provided by the CC Mode "k&r" style). This can be achieved with the following code in the initialization file (see section 27.7 The Init File)

 
(defun my-c-mode-hook ()
  (c-set-style "k&r")
  (setq c-basic-offset 2))
(add-hook 'c-mode-hook 'my-c-mode-hook)

Most customizations for indentation in various CC modes can be accomplished by choosing a style and then choosing value for c-basic-offset that meets the local coding convention. CC Mode has a very customizable indentation engine and a further discussion is really beyond the scope of this manual. See section `Indentation Engine' in The CC Mode Manual.


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21.13 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.

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. Doing so calls the value of fortran-mode-hook as a function of no arguments if that variable has a non-nil value.

21.13.1 Motion Commands  Moving point by statements or subprograms.
21.13.2 Fortran Indentation  Indentation commands for Fortran.
21.13.3 Comments  Inserting and aligning comments.
21.13.4 Columns  Measuring columns for valid Fortran.
21.13.5 Fortran Keyword Abbrevs  Built-in abbrevs for Fortran keywords.

Fortran mode was contributed by Michael Prange.


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21.13.1 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, which is 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
).


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21.13.2 Fortran Indentation

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

21.13.2.1 Fortran Indentation Commands  Commands for indenting Fortran.
21.13.2.2 Line Numbers and Continuation  How line numbers auto-indent.
21.13.2.3 Syntactic Conventions  Conventions you must obey to avoid trouble.
21.13.2.4 Variables for Fortran Indentation  Variables controlling Fortran indent style.


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21.13.2.1 Fortran Indentation Commands

TAB
Indent the current line (fortran-indent-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).

TAB is redefined by Fortran mode to reindent the current line for Fortran (fortran-indent-line). Line numbers and continuation markers are indented to their required columns, and the body of the statement is independently indented, based on its nesting in the program.

The key C-M-q is redefined as fortran-indent-subprogram, a command that reindents all the lines of the Fortran subprogram (function or subroutine) containing point.

The key M-LFD is redefined as fortran-split-line, a command to split 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.


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21.13.2.2 Line Numbers and Continuation

If a number is the first non-whitespace in the line, it is assumed to be a line number and is moved to columns 0 through 4. (Columns are always counted from 0 in XEmacs.) If the text on the line starts with the conventional Fortran continuation marker `$', it is moved to column 5. If the text begins with any non whitespace character in column 5, it is assumed to be an unconventional continuation marker and remains in column 5.

Line numbers of four digits or less are normally indented one space. This amount is controlled by the variable fortran-line-number-indent, which is 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 the maximum indentation. The default value of the variable is 1.

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


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21.13.2.3 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 will retain its meaning when reindented even if the conventions are not followed.


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21.13.2.4 Variables for Fortran Indentation

Several additional variables control how Fortran indentation works.

fortran-do-indent
Extra indentation within each level of `do' statement (the default is 3).

fortran-if-indent
Extra indentation within each level of `if' statement (the default is 3).

fortran-continuation-indent
Extra indentation for bodies of continuation lines (the default is 5).

fortran-check-all-num-for-matching-do
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.

fortran-minimum-statement-indent
Minimum indentation for Fortran statements. For standard Fortran, this is 6. Statement bodies are always indented at least this much.


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21.13.3 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 non-standard 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 specified to do so in advance by setting the variable comment-start to `"!"' (see section 27.3 Variables).

M-;
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 arg) 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, it recognizes an existing comment and aligns its text appropriately. If there is no existing comment, a comment is inserted and aligned.

Inserting and aligning comments is not the same in Fortran mode as in other modes. When a new comment must be inserted, a full-line comment is inserted if the current line is blank. On a non-blank line, a non-standard `!' comment is inserted if you previously specified you wanted to use them. Otherwise a full-line comment is inserted on a new line before the current line.

Non-standard `!' comments are aligned like comments in other languages, but full-line comments are aligned differently. 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:

fixed
The text is aligned at a fixed column, which is the value of fortran-comment-line-column. This is the default.
relative
The text is aligned as if it were a line of code, but with an additional fortran-comment-line-column columns of indentation.
nil
Text in full-line columns is not moved automatically.

You can also specify the character to be used to indent within full-line comments by setting the variable fortran-comment-indent-char to the character you want to use.

Fortran mode introduces two variables comment-line-start and comment-line-start-skip, which do for full-line comments what comment-start and comment-start-skip do 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. It can therefore be used if you use `!' comments, but is useless in Fortran mode otherwise.

The command C-c ; (fortran-comment-region) turns all the lines of the region into comments by inserting the string `C$$$' at the front of each one. With a numeric arg, the region is turned back into live code by deleting `C$$$' from the front of each line. You can control the string used for the comments by setting the variable fortran-comment-region. Note that here we have an example of a command and a variable with the same name; the two uses of the name never conflict because in Lisp and in Emacs it is always clear from the context which one is referred to.


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21.13.4 Columns

C-c C-r
Displays a "column ruler" momentarily above the current line (fortran-column-ruler).
C-c C-w
Splits the current window horizontally so that it is 72 columns wide. This may help you avoid going over that limit (fortran-window-create).

The command C-c C-r (fortran-column-ruler) shows a column ruler above the current line. The comment ruler consists of 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 XEmacs. As a result, the numbers may not be those you are familiar with; but the actual positions in the line are standard 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, resulting in a window 72 columns wide. When you edit in this window, you can immediately see when a line gets too wide to be correct Fortran.


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21.13.5 Fortran Keyword Abbrevs

Fortran mode provides many built-in abbrevs for common keywords and declarations. These are the same sort of abbrevs that you can define yourself. To use them, you must turn on Abbrev mode. see section 23. 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' changes automatically to `continue', provided Abbrev mode is enabled.

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


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21.14 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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