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Emacs has many commands designed to understand the syntax of programming languages such as Lisp and C. These commands can
The commands for words, sentences and paragraphs are very useful in editing code even though their canonical application is for editing human language text. Most symbols contain words (see section Words); sentences can be found in strings and comments (see section Sentences). Paragraphs per se don't exist in code, but the paragraph commands are useful anyway, because programming language major modes define paragraphs to begin and end at blank lines (see section Paragraphs). Judicious use of blank lines to make the program clearer will also provide useful chunks of text for the paragraph commands to work on.
The selective display feature is useful for looking at the overall structure of a function (see section Selective Display). This feature causes only the lines that are indented less than a specified amount to appear on the screen.
Emacs also has major modes for the programming languages Lisp, Scheme (a variant of Lisp), Awk, C, C++, Fortran, Icon, Java, Objective-C, Pascal, Perl, Pike, CORBA IDL, and Tcl. There is also a major mode for makefiles, called Makefile mode. An second alternative mode for Perl is called CPerl 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 major 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.
C-j is normally defined to do RET followed by TAB;
thus, it too indents in a mode-specific fashion.
In most programming languages, indentation is likely to vary from line to
line. So the major modes for those languages rebind DEL to treat a
tab as if it were the equivalent number of spaces (using the command
backward-delete-char-untabify). This makes it possible to rub out
indentation one column at a time without worrying whether it is made up of
spaces or tabs. Use C-b C-d to delete a tab character before point,
in these modes.
Programming language modes define paragraphs to be separated only by blank lines, so that the paragraph commands remain useful. Auto Fill mode, if enabled in a programming language major mode, indents the new lines which it creates.
Turning on a major mode runs a normal hook called the mode hook,
which is the value of a Lisp variable. Each major mode has a mode hook,
and the hook's name is always made from the mode command's name by
adding `-hook'. For example, turning on C mode runs the hook
c-mode-hook, while turning on Lisp mode runs the hook
lisp-mode-hook. See section Hooks.
By convention, Emacs keys for dealing with balanced expressions are usually Control-Meta characters. They tend to be analogous in function to their Control and Meta equivalents. These commands are usually thought of as pertaining to expressions in programming languages, but can be useful with any language in which some sort of parentheses exist (including human languages).
These commands fall into two classes. Some deal only with lists (parenthetical groupings). They see nothing except parentheses, brackets, braces (whichever ones must balance in the language you are working with), and escape characters that might be used to quote those.
The other commands deal with expressions or sexps. The word `sexp' is derived from s-expression, the ancient term for an expression in Lisp. But in Emacs, the notion of `sexp' is not limited to Lisp. It refers to an expression in whatever language your program is written in. Each programming language has its own major mode, which customizes the syntax tables so that expressions in that language count as sexps.
Sexps typically include symbols, numbers, and string constants, as well as anything contained in parentheses, brackets or braces.
In languages that use prefix and infix operators, such as C, it is not possible for all expressions to be sexps. For example, C mode does not recognize `foo + bar' as a sexp, even though it is a C expression; it recognizes `foo' as one sexp and `bar' as another, with the `+' as punctuation between them. This is a fundamental ambiguity: both `foo + bar' and `foo' are legitimate choices for the sexp to move over if point is at the `f'. Note that `(foo + bar)' is a single sexp in C mode.
Some languages have obscure forms of expression syntax that nobody has bothered to make Emacs understand properly.
forward-sexp).
backward-sexp).
kill-sexp).
backward-kill-sexp).
backward-up-list).
down-list).
forward-list).
backward-list).
transpose-sexps).
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 whole sexp can be done with C-M-k (kill-sexp)
or C-M-DEL (backward-kill-sexp). C-M-k kills
the characters that C-M-f would move over, and C-M-DEL
kills the characters that C-M-b would move over.
The list commands move over lists, as the sexp commands do, but skip
blithely over any number of other kinds of sexps (symbols, strings, etc.).
They are C-M-n (forward-list) and C-M-p
(backward-list). The main reason they are useful is that they
usually ignore comments (since the comments usually do not contain any
lists).
C-M-n and C-M-p stay at the same level in parentheses, when
that's possible. To move up one (or n) levels, use C-M-u
(backward-up-list).
C-M-u moves backward up past one unmatched opening delimiter. A
positive argument serves as a repeat count; a negative argument reverses
direction of motion and also requests repetition, so it moves forward and
up one or more levels.
To move down in list structure, use C-M-d
(down-list). In Lisp mode, where `(' is the only opening
delimiter, this is nearly the same as searching for a `('. An
argument specifies the number of levels of parentheses to go down.
A somewhat random-sounding command which is nevertheless handy is
C-M-t (transpose-sexps), which drags the previous sexp
across the next one. An argument serves as a repeat count, and a
negative argument drags backwards (thus canceling out the effect of
C-M-t with a positive argument). An argument of zero, rather than
doing nothing, transposes the sexps ending after point and the mark.
To set the region around the next sexp in the buffer, use C-M-@
(mark-sexp), which sets mark at the same place that C-M-f
would move to. C-M-@ takes arguments like C-M-f. In
particular, a negative argument is useful for putting the mark at the
beginning of the previous sexp.
The list and sexp commands' understanding of syntax is completely controlled by the syntax table. Any character can, for example, be declared to be an opening delimiter and act like an open parenthesis. See section The Syntax Table.
In Emacs, a parenthetical grouping at the top level in the buffer is
called a defun. The name derives from the fact that most top-level
lists in a Lisp file are instances of the special form defun, but
any top-level parenthetical grouping counts as a defun in Emacs parlance
regardless of what its contents are, and regardless of the programming
language in use. For example, in C, the body of a function definition is a
defun.
beginning-of-defun).
end-of-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 c-mark-function, which is almost the
same as mark-defun; the difference is that it backs up over the
argument declarations, function name and returned data type so that the
entire C function is inside the region. 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.
The best way to keep a program properly indented is to use Emacs to reindent it as you change it. Emacs has commands to indent properly either a single line, a specified number of lines, or all of the lines inside a single parenthetical grouping.
Emacs also provides a Lisp pretty-printer in the library pp.
This program reformats a Lisp object with indentation chosen to look nice.
newline-and-indent).
The basic indentation command is TAB, which gives the current line
the correct indentation as determined from the previous lines. The
function that TAB runs depends on the major mode; it is lisp-indent-line
in Lisp mode, c-indent-line in C mode, etc. These functions
understand different syntaxes for different languages, but they all do
about the same thing. TAB in any programming-language major mode
inserts or deletes whitespace at the beginning of the current line,
independent of where point is in the line. If point is inside the
whitespace at the beginning of the line, TAB leaves it at the end of
that whitespace; otherwise, TAB leaves point fixed with respect to
the characters around it.
Use C-q TAB to insert a tab at point.
When entering lines of new code, use C-j (newline-and-indent),
which is equivalent to a RET followed by a TAB. C-j creates
a blank line and then gives it the appropriate indentation.
TAB indents the second and following lines of the body of a parenthetical grouping each under the preceding one; therefore, if you alter one line's indentation to be nonstandard, the lines below will tend to follow it. This behavior is convenient in cases where you have overridden the standard result of TAB because you find it unaesthetic for a particular line.
Remember that an open-parenthesis, open-brace or other opening delimiter at the left margin is assumed by Emacs (including the indentation routines) to be the start of a function. Therefore, you must never have an opening delimiter in column zero that is not the beginning of a function, not even inside a string. This restriction is vital for making the indentation commands fast; you must simply accept it. See section Defuns, for more information on this.
When you wish to reindent several lines of code which have been altered or moved to a different level in the list structure, you have several commands available.
indent-sexp).
indent-region).
You can reindent the contents of a single list by positioning point
before the beginning of it and typing C-M-q (indent-sexp in
Lisp mode, c-indent-exp in C mode; also bound to other suitable
commands in other modes). The indentation of the line the sexp starts on
is not changed; therefore, only the relative indentation within the list,
and not its position, is changed. To correct the position as well, type a
TAB before the C-M-q.
If the relative indentation within a list is correct but the indentation of its first line is not, go to that line and type C-u TAB. TAB with a numeric argument reindents the current line as usual, then reindents by the same amount all the lines in the grouping starting on the current line. In other words, it reindents the whole grouping rigidly as a unit. It is clever, though, and does not alter lines that start inside strings, or C preprocessor lines when in C mode.
Another way to specify the range to be reindented is with the region.
The command C-M-\ (indent-region) applies TAB to
every line whose first character is between point and mark.
The indentation pattern for a Lisp expression can depend on the function called by the expression. For each Lisp function, you can choose among several predefined patterns of indentation, or define an arbitrary one with a Lisp program.
The standard pattern of indentation is as follows: the second line of the expression is indented under the first argument, if that is on the same line as the beginning of the expression; otherwise, the second line is indented underneath the function name. Each following line is indented under the previous line whose nesting depth is the same.
If the variable lisp-indent-offset is non-nil, it overrides
the usual indentation pattern for the second line of an expression, so that
such lines are always indented lisp-indent-offset more columns than
the containing list.
The standard pattern is overridden for certain functions. Functions
whose names start with def always indent the second line by
lisp-body-indent extra columns beyond the open-parenthesis
starting the expression.
The standard pattern can be overridden in various ways for individual
functions, according to the lisp-indent-function property of the
function name. There are four possibilities for this property:
nil
defun
def is used for
this function also.
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.
parse-partial-sexp (a Lisp primitive for
indentation and nesting computation) when it parses up to the
beginning of this line.
Here are the commands for indentation in C mode and related modes:
C-c C-q
c-indent-defun).
C-M-q
c-indent-exp). A prefix argument inhibits error checking and
warning messages about invalid syntax.
TAB
c-indent-command).
If c-tab-always-indent is t, this command always reindents
the current line and does nothing else. This is the default.
If that variable is nil, this command reindents the current line
only if point is at the left margin or in the line's indentation;
otherwise, it inserts a tab (or the equivalent number of spaces,
if indent-tabs-mode is nil).
Any other value (not nil or t) means always reindent the
line, and also insert a tab if within a comment, a string, or a
preprocessor directive.
C-u TAB
To reindent the whole current buffer, type C-x h C-M-\. This first selects the whole buffer as the region, then reindents that region.
To reindent the current block, use C-M-u C-M-q. This moves to the front of the block and then reindents it all.
C mode and related modes use a simple yet flexible mechanism for customizing indentation. The mechanism works in two steps: first it classifies the line syntactically according to its contents and context; second, it associates each kind of syntactic construct with an indentation offset which you can customize.
In the first step, the C indentation mechanism looks at the line
before the one you are currently indenting and determines the syntactic
components of the construct on that line. It builds a list of these
syntactic components, each of which contains a syntactic symbol
and sometimes also a buffer position. Some syntactic symbols describe
grammatical elements, for example statement and
substatement; others describe locations amidst grammatical
elements, for example class-open and knr-argdecl.
Conceptually, a line of C code is always indented relative to the indentation of some line higher up in the buffer. This is represented by the buffer positions in the syntactic component list.
Here is an example. Suppose we have the following code in a C++ mode buffer (the line numbers don't actually appear in the buffer):
1: void swap (int& a, int& b)
2: {
3: int tmp = a;
4: a = b;
5: b = tmp;
6: }
If you type C-c C-s (which runs the command
c-show-syntactic-information) on line 4, it shows the result of
the indentation mechanism for that line:
((statement . 32))
This indicates that the line is a statement and it is indented
relative to buffer position 32, which happens to be the `i' in
int on line 3. If you move the cursor to line 3 and type
C-c C-s, it displays this:
((defun-block-intro . 28))
This indicates that the int line is the first statement in a
block, and is indented relative to buffer position 28, which is the
brace just after the function header.
Here is another example:
1: int add (int val, int incr, int doit)
2: {
3: if (doit)
4: {
5: return (val + incr);
6: }
7: return (val);
8: }
Typing C-c C-s on line 4 displays this:
((substatement-open . 43))
This says that the brace opens a substatement block. By the
way, a substatement indicates the line after an if,
else, while, do, switch, for,
try, catch, finally, or synchronized
statement.
Within the C indentation commands, after a line has been analyzed
syntactically for indentation, the variable c-syntactic-context
contains a list that describes the results. Each element in this list
is a syntactic component: a cons cell containing a syntactic
symbol and (optionally) its corresponding buffer position. There may be
several elements in a component list; typically only one element has a
buffer position.
The C indentation mechanism calculates the indentation for the current
line using the list of syntactic components, c-syntactic-context,
derived from syntactic analysis. Each component is a cons cell that
contains a syntactic symbol and may also contain a buffer position.
Each component contributes to the final total indentation of the line
in two ways. First, the syntactic symbol identifies an element of
c-offsets-alist, which is an association list mapping syntactic
symbols into indentation offsets. Each syntactic symbol's offset adds
to the total indentation. Second, if the component includes a buffer
position, the column number of that position adds to the indentation.
All these offsets and column numbers, added together, give the total
indentation.
The following examples demonstrate the workings of the C indentation mechanism:
1: void swap (int& a, int& b)
2: {
3: int tmp = a;
4: a = b;
5: b = tmp;
6: }
Suppose that point is on line 3 and you type TAB to reindent the line. As explained above (see section Step 1--Syntactic Analysis), the syntactic component list for that line is:
((defun-block-intro . 28))
In this case, the indentation calculation first looks up
defun-block-intro in the c-offsets-alist alist. Suppose
that it finds the integer 2; it adds this to the running total
(initialized to zero), yielding a updated total indentation of 2 spaces.
The next step is to find the column number of buffer position 28. Since the brace at buffer position 28 is in column zero, this adds 0 to the running total. Since this line has only one syntactic component, the total indentation for the line is 2 spaces.
1: int add (int val, int incr, int doit)
2: {
3: if (doit)
4: {
5: return(val + incr);
6: }
7: return(val);
8: }
If you type TAB on line 4, the same process is performed, but with different data. The syntactic component list for this line is:
((substatement-open . 43))
Here, the indentation calculation's first job is to look up the
symbol substatement-open in c-offsets-alist. Let's assume
that the offset for this symbol is 2. At this point the running total
is 2 (0 + 2 = 2). Then it adds the column number of buffer position 43,
which is the `i' in if on line 3. This character is in
column 2 on that line. Adding this yields a total indentation of 4
spaces.
If a syntactic symbol in the analysis of a line does not appear in
c-offsets-alist, it is ignored; if in addition the variable
c-strict-syntax-p is non-nil, it is an error.
There are two ways to customize the indentation style for the C-like modes. First, you can select one of several predefined styles, each of which specifies offsets for all the syntactic symbols. For more flexibility, you can customize the handling of individual syntactic symbols. See section Syntactic Symbols, for a list of all defined syntactic symbols.
c-set-offset). The second argument offset specifies the
new indentation offset.
The c-offsets-alist variable controls the amount of
indentation to give to each syntactic symbol. Its value is an
association list, and each element of the list has the form
(syntactic-symbol . offset). By changing the offsets
for various syntactic symbols, you can customize indentation in fine
detail. To change this alist, use c-set-offset (see below).
Each offset value in c-offsets-alist can be an integer, a
function or variable name, a list, or one of the following symbols: +,
-, ++, --, *, or /, indicating positive or negative
multiples of the variable c-basic-offset. Thus, if you want to
change the levels of indentation to be 3 spaces instead of 2 spaces, set
c-basic-offset to 3.
Using a function as the offset value provides the ultimate flexibility
in customizing indentation. The function is called with a single
argument containing the cons of the syntactic symbol and
the buffer position, if any. The function should return an integer
offset.
If the offset value is a list, its elements are processed according
to the rules above until a non-nil value is found. That value is
then added to the total indentation in the normal manner. The primary
use for this is to combine the results of several functions.
The command C-c C-o (c-set-offset) is the easiest way to
set offsets, both interactively or in your `~/.emacs' file. First
specify the syntactic symbol, then the offset you want. See section Syntactic Symbols, for a list of valid syntactic symbols and their meanings.
Here is a table of valid syntactic symbols for indentation in C and
related modes, with their syntactic meanings. Normally, most of these
symbols are assigned offsets in c-offsets-alist.
string
c
defun-open
defun-close
defun-block-intro
class-open
class-close
inline-open
inline-close
extern-lang-open
extern-lang-close
func-decl-cont
throws declarations and other things can appear in this context.
knr-argdecl-intro
knr-argdecl
topmost-intro
topmost-intro-cont
member-init-intro
member-init-cont
inher-intro
inher-cont
block-open
block-close
brace-list-open
enum or static array list.
brace-list-close
enum or static array list.
brace-list-intro
enum or static array list.
brace-list-entry
enum or static array
list.
brace-entry-open
enum or static array
list, when the line begins with an open brace.
statement
statement-cont
statement-block-intro
statement-case-intro
case "block."
statement-case-open
case block starting with brace.
inexpr-statement
inexpr-class
substatement
if, while, for,
do, or else.
substatement-open
case-label
case or default label.
access-label
private, protected, or public access label.
label
do-while-closure
while that ends a do-while construct.
else-clause
else of an if-else construct.
catch-clause
catch and finally lines in
try...catch constructs in C++ and Java.
comment-intro
arglist-intro
arglist-cont
arglist-cont-nonempty
arglist-close
stream-op
inclass
inextern-lang
inexpr-statement
({ ... }).
It is also used for the special functions that takes a statement block
as an argument in Pike.
inexpr-class
cpp-macro
friend
friend declaration.
objc-method-intro
objc-method-args-cont
objc-method-call-cont
inlambda
inclass, but used inside lambda (i.e. anonymous) functions. Only
used in Pike.
lambda-intro-cont
lambda keyword and the function body. Only used in Pike.
This section describes additional variables which control the indentation behavior of C mode and related mode.
c-offsets-alist
c-set-offset.
See section Changing Indentation Style, for details.
c-style-alist
c-basic-offset
+ and - symbols in
c-offsets-alist.
c-special-indent-hook
The variable c-style-alist specifies the predefined indentation
styles. Each element has form (name
variable-setting...), where name is the name of the
style. Each variable-setting has the form (variable
. value); variable is one of the customization variables
used by C mode, and value is the value for that variable when
using the selected style.
When variable is c-offsets-alist, that is a special case:
value is appended to the front of the value of c-offsets-alist
instead of replacing that value outright. Therefore, it is not necessary
for value to specify each and every syntactic symbol--only those
for which the style differs from the default.
The indentation of lines containing only comments is also affected by
the variable c-comment-only-line-offset (see section Comments in C Modes).
A C style is a collection of indentation style customizations.
Emacs comes with several predefined indentation styles for C and related
modes, including gnu, k&r, bsd, stroustrup,
linux, python, java, whitesmith,
ellemtel, and cc-mode. The default style is gnu.
To choose the style you want, use the command M-x c-set-style.
Specify a style name as an argument (case is not significant in C style
names). The chosen style only affects newly visited buffers, not those
you are already editing. You can also set the variable
c-default-style to specify the style for various major modes.
Its value should be an alist, in which each element specifies one major
mode and which indentation style to use for it. For example,
(setq c-default-style
'((java-mode . "java") (other . "gnu")))
specifies an explicit choice for Java mode, and the default `gnu' style for the other C-like modes.
To define a new C indentation style, call the function
c-add-style:
(c-add-style name values use-now)
Here name is the name of the new style (a string), and
values is an alist whose elements have the form
(variable . value). The variables you specify should
be among those documented in section Variables for C Indentation.
If use-now is non-nil, c-add-style switches to the
new style after defining it.
The Emacs parenthesis-matching feature is designed to show automatically how parentheses match in the text. Whenever you type a self-inserting character that is a closing delimiter, the cursor moves momentarily to the location of the matching opening delimiter, provided that is on the screen. If it is not on the screen, some text near it is displayed in the echo area. Either way, you can tell what grouping is being closed off.
In Lisp, automatic matching applies only to parentheses. In C, it applies to braces and brackets too. Emacs knows which characters to regard as matching delimiters based on the syntax table, which is set by the major mode. See section The Syntax Table.
If the opening delimiter and closing delimiter are mismatched--such as in `[x)'---a warning message is displayed in the echo area. The correct matches are specified in the syntax table.
Three variables control parenthesis match display.
blink-matching-paren turns the feature on or off; nil
turns it off, but the default is t to turn match display on.
blink-matching-delay says how many seconds to wait; the default
is 1, but on some systems it is useful to specify a fraction of a
second. blink-matching-paren-distance specifies how many
characters back to search to find the matching opening delimiter. If
the match is not found in that far, scanning stops, and nothing is
displayed. This is to prevent scanning for the matching delimiter from
wasting lots of time when there is no match. The default is 12,000.
When using X Windows, you can request a more powerful alternative kind of automatic parenthesis matching by enabling Show Paren mode. This mode turns off the usual kind of matching parenthesis display and instead uses highlighting to show what matches. Whenever point is after a close parenthesis, the close parenthesis and its matching open parenthesis are both highlighted; otherwise, if point is before an open parenthesis, the matching close parenthesis is highlighted. (There is no need to highlight the open parenthesis after point because the cursor appears on top of that character.) Use the command M-x show-paren-mode to enable or disable this mode.
Because comments are such an important part of programming, Emacs provides special commands for editing and inserting comments.
The comment commands insert, kill and align comments.
indent-for-comment).
set-comment-column).
kill-comment).
indent-new-comment-line).
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 reindents 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.
If you are typing a comment and wish to continue it on another line,
you can use the command C-M-j (indent-new-comment-line).
This terminates the comment you are typing, creates a new blank line
afterward, and begins a new comment indented under the old one. When
Auto Fill mode is on, going past the fill column while typing a comment
causes the comment to be continued in just this fashion. If point is
not at the end of the line when C-M-j is typed, the text on
the rest of the line becomes part of the new comment line.
To turn existing lines into comment lines, use the M-x comment-region command. It adds comment delimiters to the lines that start in the region, thus commenting them out. With a negative argument, it does the opposite--it deletes comment delimiters from the lines in the region.
With a positive argument, comment-region duplicates the last
character of the comment start sequence it adds; the argument specifies
how many copies of the character to insert. Thus, in Lisp mode,
C-u 2 M-x comment-region adds `;;' to each line. Duplicating
the comment delimiter is a way of calling attention to the comment. It
can also affect how the comment is indented. In Lisp, for proper
indentation, you should use an argument of two, if between defuns, and
three, if within a defun.
The variable comment-padding specifies how many spaces
comment-region should insert on each line between the
comment delimiter and the line's original text. The default is 1.
The comment column is stored in the variable comment-column. You
can set it to a number explicitly. Alternatively, the command C-x ;
(set-comment-column) sets the comment column to the column point is
at. C-u C-x ; sets the comment column to match the last comment
before point in the buffer, and then does a M-; to align the
current line's comment under the previous one. Note that C-u - C-x ;
runs the function kill-comment as described above.
The variable comment-column is per-buffer: setting the variable
in the normal fashion affects only the current buffer, but there is a
default value which you can change with setq-default.
See section Local Variables. Many major modes initialize this variable for the
current buffer.
The comment commands recognize comments based on the regular
expression that is the value of the variable comment-start-skip.
Make sure this regexp does not match the null string. It may match more
than the comment starting delimiter in the strictest sense of the word;
for example, in C mode the value of the variable is "/\\*+
*", which matches extra stars and spaces after the `/*' itself.
(Note that `\\' is needed in Lisp syntax to include a `\' in
the string, which is needed to deny the first star its special meaning
in regexp syntax. See section Syntax of Regular Expressions.)
When a comment command makes a new comment, it inserts the value of
comment-start to begin it. The value of comment-end is
inserted after point, so that it will follow the text that you will insert
into the comment. In C mode, comment-start has the value
"/* " and comment-end has the value " */".
The variable comment-multi-line controls how C-M-j
(indent-new-comment-line) behaves when used inside a comment. If
comment-multi-line is nil, as it normally is, then the
comment on the starting line is terminated and a new comment is started
on the new following line. If comment-multi-line is not
nil, then the new following line is set up as part of the same
comment that was found on the starting line. This is done by not
inserting a terminator on the old line, and not inserting a starter on
the new line. In languages where multi-line comments work, the choice
of value for this variable is a matter of taste.
The variable comment-indent-function should contain a function
that will be called to compute the indentation for a newly inserted
comment or for aligning an existing comment. It is set differently by
various major modes. The function is called with no arguments, but with
point at the beginning of the comment, or at the end of a line if a new
comment is to be inserted. It should return the column in which the
comment ought to start. For example, in Lisp mode, the indent hook
function bases its decision on how many semicolons begin an existing
comment, and on the code in the preceding lines.
insert-parentheses).
move-past-close-and-reindent).
The commands M-( (insert-parentheses) and M-)
(move-past-close-and-reindent) are designed to facilitate a style
of editing which keeps parentheses balanced at all times. M-(
inserts a pair of parentheses, either together as in `()', or, if
given an argument, around the next several sexps. It leaves point after
the open parenthesis. The command M-) moves past the close
parenthesis, deleting any indentation preceding it, and indenting with
C-j after it.
For example, instead of typing ( F O O ), you can type M-( F O O, which has the same effect except for leaving the cursor before the close parenthesis.
M-( may insert a space before the open parenthesis, depending on
the syntax class of the preceding character. Set
parens-require-spaces to nil value if you wish to inhibit
this.
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.
In most programming language major modes, M-TAB runs the
command complete-symbol, which provides two kinds of completion.
Normally it does completion based on a tags table (see section Tags Tables); with a
numeric argument (regardless of the value), it does completion based on
the names listed in the Info file indexes for your language. Thus, to
complete the name of a symbol defined in your own program, use
M-TAB with no argument; to complete the name of a standard
library function, use C-u M-TAB. Of course, Info-based
completion works only if there is an Info file for the standard library
functions of your language, and only if it is installed at your site.
In Emacs-Lisp mode, the name space for completion normally consists of
nontrivial symbols present in Emacs--those that have function
definitions, values or properties. However, if there is an
open-parenthesis immediately before the beginning of the partial symbol,
only symbols with function definitions are considered as completions.
The command which implements this is lisp-complete-symbol.
In Text mode and related modes, M-TAB completes words based on the spell-checker's dictionary. See section Checking and Correcting Spelling.
Which Function mode is a minor mode that displays the current function name in the mode line, as you move around in a buffer.
To enable (or disable) Which Function mode, use the command M-x
which-function-mode. This command is global; it applies to all
buffers, both existing ones and those yet to be created. However, this
only affects certain major modes, those listed in the value of
which-func-modes. (If the value is t, then Which Function
mode applies to all major modes that know how to support it--which are
the major modes that support Imenu.)
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.
For Emacs Lisp code, you can also use Eldoc mode. This minor mode constantly displays in the echo area the argument list for the function being called at point. (In other words, it finds the function call that point is contained in, and displays the argument list of that function.) Eldoc mode applies in Emacs Lisp and Lisp Interaction modes only. Use the command M-x eldoc-mode to enable or disable this feature.
For C, Lisp, and other languages, you can use C-h C-i
(info-lookup-symbol) to view the Info documentation for a symbol.
You specify the symbol with the minibuffer; by default, it uses the
symbol that appears in the buffer at point. The major mode determines
where to look for documentation for the symbol--which Info files and
which indices. You can also use M-x info-lookup-file to look for
documentation for a file name.
You can read the "man page" for an operating system command, library
function, or system call, with the M-x manual-entry command. It
runs the man program to format the man page, and runs it
asynchronously if your system permits, so that you can keep on editing
while the page is being formatted. (MS-DOS and MS-Windows 3 do not
permit asynchronous subprocesses, so on these systems you cannot edit
while Emacs waits for man to exit.) The result goes in a buffer
named `*Man topic*'. These buffers use a special major mode,
Man mode, that facilitates scrolling and examining other manual pages.
For details, type C-h m while in a man page buffer.
For a long man page, setting the faces properly can take substantial
time. By default, Emacs uses faces in man pages if Emacs can display
different fonts or colors. You can turn off use of faces in man pages
by setting the variable Man-fontify-manpage-flag to nil.
If you insert the text of a man page into an Emacs buffer in some other fashion, you can use the command M-x Man-fontify-manpage to perform the same conversions that M-x manual-entry does.
Eventually the GNU project hopes to replace most man pages with better-organized manuals that you can browse with Info. See section Other Help Commands. Since this process is only partially completed, it is still useful to read manual pages.
The Emacs command C-x 4 a adds a new entry to the change log
file for the file you are editing
(add-change-log-entry-other-window).
A change log file contains a chronological record of when and why you have changed a program, consisting of a sequence of entries describing individual changes. Normally it is kept in a file called `ChangeLog' in the same directory as the file you are editing, or one of its parent directories. A single `ChangeLog' file can record changes for all the files in its directory and all its subdirectories.
A change log entry starts with a header line that contains your name,
your email address (taken from the variable user-mail-address),
and the current date and time. Aside from these header lines, every
line in the change log starts with a space or a tab. The bulk of the
entry consists of items, each of which starts with a line starting
with whitespace and a star. Here are two entries, both dated in May
1993, each with two items:
@medbreak
1993-05-25 Richard Stallman <rms@gnu.org>
* man.el: Rename symbols `man-*' to `Man-*'.
(manual-entry): Make prompt string clearer.
* simple.el (blink-matching-paren-distance):
Change default to 12,000.
1993-05-24 Richard Stallman <rms@gnu.org>
* vc.el (minor-mode-map-alist): Don't use it if it's void.
(vc-cancel-version): Doc fix.
(Previous Emacs versions used a different format for the date.)
One entry can describe several changes; each change should have its own item. Normally there should be a blank line between items. When items are related (parts of the same change, in different places), group them by leaving no blank line between them. The second entry above contains two items grouped in this way.
C-x 4 a visits the change log file and creates a new entry unless the most recent entry is for today's date and your name. It also creates a new item for the current file. For many languages, it can even guess the name of the function or other object that was changed.
The change log file is visited in Change Log mode. In this major mode, each bunch of grouped items counts as one paragraph, and each entry is considered a page. This facilitates editing the entries. C-j and auto-fill indent each new line like the previous line; this is convenient for entering the contents of an entry.
Version control systems are another way to keep track of changes in your program and keep a change log. See section Features of the Log Entry Buffer.
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.
Here is how tag syntax is defined for the most popular languages:
struct, union and enum. #define macro
definitions and enum constants are also tags, unless you specify
`--no-defines' when making the tags table. Similarly, global
variables are tags, unless you specify `--no-globals'. Use of
`--no-globals' and `--no-defines' can make the tags table file
much smaller.
extends and implements constructs. Tags for variables
and functions in classes are named `class.variable' and
`class.function'.
\chapter,
\section, \subsection, \subsubsection,
\eqno, \label, \ref, \cite, \bibitem,
\part, \appendix, \entry, or \index, is a
tag.
Other commands can make tags as well, if you specify them in the
environment variable TEXTAGS before invoking etags. The
value of this environment variable should be a colon-separated list of
command names. For example,
TEXTAGS="def:newcommand:newenvironment" export TEXTAGSspecifies (using Bourne shell syntax) that the commands `\def', `\newcommand' and `\newenvironment' also define tags.
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.
def or with a
construct whose name starts with `def'. They also include variables
set with set! at top level in the file.
Several other languages are also supported:
sub
keyword.
You can also generate tags based on regexp matching (see section Creating Tags Tables) to handle other formats and languages.
The etags program is used to create a tags table file. It knows
the syntax of several languages, as described in
the previous section.
Here is how to run etags:
etags inputfiles...
The etags program reads the specified files, and writes a tags table
named `TAGS' in the current working directory. etags
recognizes the language used in an input file based on its file name and
contents. You can specify the language with the
`--language=name' option, described below.
If the tags table data become outdated due to changes in the files described in the table, the way to update the tags table is the same way it was made in the first place. It is not necessary to do this often.
If the tags table fails to record a tag, or records it for the wrong file, then Emacs cannot possibly find its definition. However, if the position recorded in the tags table becomes a little bit wrong (due to some editing in the file that the tag definition is in), the only consequence is a slight delay in finding the tag. Even if the stored position is very wrong, Emacs will still find the tag, but it must search the entire file for it.
So you should update a tags table when you define new tags that you want to have listed, or when you move tag definitions from one file to another, or when changes become substantial. Normally there is no need to update the tags table after each edit, or even every day.
One tags table can effectively include another. Specify the included tags file name with the `--include=file' option when creating the file that is to include it. The latter file then acts as if it contained all the files specified in the included file, as well as the files it directly contains.
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.
`etags --help' prints the list of the languages etags knows,
and the file name rules for guessing the language.
The `--regex' option provides a general way of recognizing tags based on regexp matching. You can freely intermix it with file names. Each `--regex' option adds to the preceding ones, and applies only to the following files. The syntax is:
--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, you may find useful to add a nameregexp, in order to narrow the tag scope. You can find some examples below.
The `-R' option deletes all the regexps defined with `--regex' options. It applies to the file names following it, as you can see from the following example:
etags --regex=/reg1/ voo.doo --regex=/reg2/ \
bar.ber -R --lang=lisp los.er
Here etags chooses the parsing language for `voo.doo' and
`bar.ber' according to their contents. etags also uses
reg1 to recognize additional tags in `voo.doo', and both
reg1 and reg2 to recognize additional tags in
`bar.ber'. etags uses the Lisp tags rules, and no regexp
matching, to recognize tags in `los.er'.
Here are some more examples. The regexps are quoted to protect them from shell interpretation.
DEFVAR macros in the emacs source files:
--regex='/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'
--language=none --regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/' --regex='/[ \t]*\(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\ \( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'
--lang=none --regex='/proc[ \t]+\([^ \t]+\)/\1/'
For a list of the other available etags options, execute
etags --help.
Emacs has at any time one selected tags table, and all the commands for working with tags tables use the selected one. To select a tags table, type M-x visit-tags-table, which reads the tags table file name as an argument. The name `TAGS' in the default directory is used as the default file name.
All this command does is store the file name in the variable
tags-file-name. Emacs does not actually read in the tags table
contents until you try to use them. Setting this variable yourself is just
as good as using visit-tags-table. The variable's initial value is
nil; that value tells all the commands for working with tags tables
that they must ask for a tags table file name to use.
Using visit-tags-table when a tags table is already loaded
gives you a choice: you can add the new tags table to the current list
of tags tables, or start a new list. The tags commands use all the tags
tables in the current list. If you start a new list, the new tags table
is used instead of others. If you add the new table to the
current list, it is used as well as the others. When the tags
commands scan the list of tags tables, they don't always start at the
beginning of the list; they start with the first tags table (if any)
that describes the current file, proceed from there to the end of the
list, and then scan from the beginning of the list until they have
covered all the tables in the list.
You can specify a precise list of tags tables by setting the variable
tags-table-list to a list of strings, like this:
(setq tags-table-list
'("~/emacs" "/usr/local/lib/emacs/src"))
This tells the tags commands to look at the `TAGS' files in your `~/emacs' directory and in the `/usr/local/lib/emacs/src' directory. The order depends on which file you are in and which tags table mentions that file, as explained above.
Do not set both tags-file-name and tags-table-list.
The most important thing that a tags table enables you to do is to find the definition of a specific tag.
find-tag).
find-tag-regexp).
find-tag-other-window).
find-tag-other-frame).
M-. (find-tag) is the command to find the definition of
a specified tag. It searches through the tags table for that tag, as a
string, and then uses the tags table info to determine the file that the
definition is in and the approximate character position in the file of
the definition. Then find-tag visits that file, moves point to
the approximate character position, and searches ever-increasing
distances away to find the tag definition.
If an empty argument is given (just type RET), the sexp in the buffer before or around point is used as the tag argument. See section Lists and Sexps, for info on sexps.
You don't need to give M-. the full name of the tag; a part
will do. This is because M-. finds tags in the table which
contain tag as a substring. However, it prefers an exact match
to a substring match. To find other tags that match the same
substring, give find-tag a numeric argument, as in C-u
M-.; this does not read a tag name, but continues searching the tags
table's text for another tag containing the same substring last used.
If you have a real META key, M-0 M-. is an easier
alternative to C-u M-..
Like most commands that can switch buffers, find-tag has a
variant that displays the new buffer in another window, and one that
makes a new frame for it. The former is C-x 4 ., which invokes
the command find-tag-other-window. The latter is C-x 5 .,
which invokes find-tag-other-frame.
To move back to places you've found tags recently, use C-u - M-.; more generally, M-. with a negative numeric argument. This command can take you to another buffer. C-x 4 . with a negative argument finds the previous tag location in another window.
As well as going back to places you've found tags recently, you can go
back to places from where you found them. Use M-*, which
invokes the command pop-tag-mark, for this. Typically you would
find and study the definition of something with M-. and then
return to where you were with M-*.
Both C-u - M-. and M-* allow you to retrace your steps to
a depth determined by the variable find-tag-marker-ring-length.
The command C-M-. (find-tag-regexp) visits the tags that
match a specified regular expression. It is just like M-. except
that it does regexp matching instead of substring matching.
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.
query-replace-regexp on each file in the selected tags table.
tags-loop-continue).
M-x tags-search reads a regexp using the minibuffer, then
searches for matches in all the files in the selected tags table, one
file at a time. It displays the name of the file being searched so you
can follow its progress. As soon as it finds an occurrence,
tags-search returns.
Having found one match, you probably want to find all the rest. To find
one more match, type M-, (tags-loop-continue) to resume the
tags-search. This searches the rest of the current buffer, followed
by the remaining files of the tags table.
M-x tags-query-replace performs a single
query-replace-regexp through all the files in the tags table. It
reads a regexp to search for and a string to replace with, just like
ordinary M-x query-replace-regexp. It searches much like M-x
tags-search, but repeatedly, processing matches according to your
input. See section Replacement Commands, for more information on query replace.
It is possible to get through all the files in the tags table with a single invocation of M-x tags-query-replace. But often it is useful to exit temporarily, which you can do with any input event that has no special query replace meaning. You can resume the query replace subsequently by typing M-,; this command resumes the last tags search or replace command that you did.
The commands in this section carry out much broader searches than the
find-tag family. The find-tag commands search only for
definitions of tags that match your substring or regexp. The commands
tags-search and tags-query-replace find every occurrence
of the regexp, as ordinary search commands and replace commands do in
the current buffer.
These commands create buffers only temporarily for the files that they have to search (those which are not already visited in Emacs buffers). Buffers in which no match is found are quickly killed; the others continue to exist.
It may have struck you that tags-search is a lot like
grep. You can also run grep itself as an inferior of
Emacs and have Emacs show you the matching lines one by one. This works
much like running a compilation; finding the source locations of the
grep matches works like finding the compilation errors.
See section Running Compilations under Emacs.
M-x list-tags reads the name of one of the files described by the selected tags table, and displays a list of all the tags defined in that file. The "file name" argument is really just a string to compare against the file names recorded in the tags table; it is read as a string rather than as a file name. Therefore, completion and defaulting are not available, and you must enter the file name the same way it appears in the tags table. Do not include a directory as part of the file name unless the file name recorded in the tags table includes a directory.
M-x tags-apropos is like apropos for tags
(see section Apropos). It reads a regexp, then finds all the tags in the
selected tags table whose entries match that regexp, and displays the
tag names found.
You can also perform completion in the buffer on the name space of tag names in the current tags tables. See section Completion for Symbol Names.
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, for commands to compare in a more manual fashion, and section `Emerge' in The Ediff Manual.
To start Emerge, run one of these four commands:
The Emerge commands compare two files or buffers, and display the comparison in three buffers: one for each input text (the A buffer and the B buffer), and one (the merge buffer) where merging takes place. The merge buffer shows the full merged text, not just the differences. Wherever the two input texts differ, you can choose which one of them to include in the merge buffer.
The Emerge commands that take input from existing buffers use only the accessible portions of those buffers, if they are narrowed (see section Narrowing).
If a common ancestor version is available, from which the two texts to be merged were both derived, Emerge can use it to guess which alternative is right. Wherever one current version agrees with the ancestor, Emerge presumes that the other current version is a deliberate change which should be kept in the merged version. Use the `with-ancestor' commands if you want to specify a common ancestor text. These commands read three file or buffer names--variant A, variant B, and the common ancestor.
After the comparison is done and the buffers are prepared, the interactive merging starts. You control the merging by typing special merge commands in the merge buffer. The merge buffer shows you a full merged text, not just differences. For each run of differences between the input texts, you can choose which one of them to keep, or edit them both together.
The merge buffer uses a special major mode, Emerge mode, with commands for making these choices. But you can also edit the buffer with ordinary Emacs commands.
At any given time, the attention of Emerge is focused on one particular difference, called the selected difference. This difference is marked off in the three buffers like this:
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 difference agrees with the common ancestor, then the B version is initially preferred for that difference.
Emerge leaves the merged text in the merge buffer when you exit. At
that point, you can save it in a file with C-x C-w. If you give a
numeric argument to emerge-files or
emerge-files-with-ancestor, it reads the name of the output file
using the minibuffer. (This is the last file name those commands read.)
Then exiting from Emerge saves the merged text in the output file.
Normally, Emerge commands save the output buffer in its file when you exit. If you abort Emerge with C-], the Emerge command does not save the output buffer, but you can save it yourself if you wish.
You can choose between two modes for giving merge commands: Fast mode and Edit mode. In Fast mode, basic merge commands are single characters, but ordinary Emacs commands are disabled. This is convenient if you use only merge commands. In Edit mode, all merge commands start with the prefix key C-c C-c, and the normal Emacs commands are also available. This allows editing the merge buffer, but slows down Emerge operations.
Use e to switch to Edit mode, and C-c C-c f to switch to Fast mode. The mode line indicates Edit and Fast modes with `E' and `F'.
Emerge has two additional submodes that affect how particular merge commands work: Auto Advance mode and Skip Prefers mode.
If Auto Advance mode is in effect, the a and b commands advance to the next difference. This lets you go through the merge faster as long as you simply choose one of the alternatives from the input. The mode line indicates Auto Advance mode with `A'.
If Skip Prefers mode is in effect, the n and p commands skip over differences in states prefer-A and prefer-B (see section State of a Difference). Thus you see only differences for which neither version is presumed "correct." The mode line indicates Skip Prefers mode with `S'.
Use the command s a (emerge-auto-advance-mode) to set or
clear Auto Advance mode. Use s s
(emerge-skip-prefers-mode) to set or clear Skip Prefers mode.
These commands turn on the mode with a positive argument, turns it off
with a negative or zero argument, and toggle the mode with no argument.
In the merge buffer, a difference is marked with lines of `v' and `^' characters. Each difference has one of these seven states:
Here are the Merge commands for Fast mode; in Edit mode, precede them with C-c C-c:
The q command (emerge-quit) finishes the merge, storing
the results into the output file if you specified one. It restores the
A and B buffers to their proper contents, or kills them if they were
created by Emerge and you haven't changed them. It also disables the
Emerge commands in the merge buffer, since executing them later could
damage the contents of the various buffers.
C-] aborts the merge. This means exiting without writing the output file. If you didn't specify an output file, then there is no real difference between aborting and finishing the merge.
If the Emerge command was called from another Lisp program, then its
return value is t for successful completion, or nil if you
abort.
Sometimes you want to keep both alternatives for a particular difference. To do this, use x c, which edits the merge buffer like this:
#ifdef NEW version from A buffer #else /* not NEW */ version from B buffer #endif /* not NEW */
While this example shows C preprocessor conditionals delimiting the two
alternative versions, you can specify the strings to use by setting
the variable emerge-combine-versions-template to a string of your
choice. In the string, `%a' says where to put version A, and
`%b' says where to put version B. The default setting, which
produces the results shown above, looks like this:
"#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"
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 is ready to accept
commands.
After setting up the merge, Emerge runs the hook
emerge-startup-hook (see section Hooks).
This section describes special features available in C, C++, Objective-C, Java, CORBA IDL, and Pike modes. When we say "C mode and related modes," those are the modes we mean.
This section describes commands for moving point, in C mode and related modes.
C-c C-u
#elif is treated
like #else followed by #if. When going forwards,
#elif is ignored.
C-c C-p
C-c C-n
M-a
c-beginning-of-statement). If point is already at the beginning
of a statement, move to the beginning of the preceding statement. With
prefix argument n, move back n - 1 statements.
If point is within a string or comment, or next to a comment (only
whitespace between them), this command moves by sentences instead of
statements.
When called from a program, this function takes three optional
arguments: the numeric prefix argument, a buffer position limit
(don't move back before that place), and a flag that controls whether
to do sentence motion when inside of a comment.
M-e
c-end-of-statement).
M-x c-backward-into-nomenclature
M-x c-forward-into-nomenclature
In C mode and related modes, certain printing characters are
"electric"---in addition to inserting themselves, they also reindent
the current line and may insert newlines. This feature is controlled by
the variable c-auto-newline. The "electric" characters are
{, }, :, #, ;, ,, <,
>, /, *, (, and ).
Electric characters insert newlines only when the auto-newline
feature is enabled (indicated by `/a' in the mode line after the
mode name). This feature is controlled by the variable
c-auto-newline. You can turn this feature on or off with the
command C-c C-a:
c-toggle-auto-state). With a
prefix argument, this command turns the auto-newline feature on if the
argument is positive, and off if it is negative.
The colon character is electric because that is appropriate for a single colon. But when you want to insert a double colon in C++, the electric behavior of colon is inconvenient. You can insert a double colon with no reindentation or newlines by typing C-c ::
c-scope-operator).
The electric # key reindents the line if it appears to be the
beginning of a preprocessor directive. This happens when the value of
c-electric-pound-behavior is (alignleft). You can turn
this feature off by setting c-electric-pound-behavior to
nil.
The variable c-hanging-braces-alist controls the insertion of
newlines before and after inserted braces. It is an association list
with elements of the following form: (syntactic-symbol
. nl-list). Most of the syntactic symbols that appear in
c-offsets-alist are meaningful here as well.
The list nl-list may contain either of the symbols
before or after, or both; or it may be nil. When a
brace is inserted, the syntactic context it defines is looked up in
c-hanging-braces-alist; if it is found, the nl-list is used
to determine where newlines are inserted: either before the brace,
after, or both. If not found, the default is to insert a newline both
before and after braces.
The variable c-hanging-colons-alist controls the insertion of
newlines before and after inserted colons. It is an association list
with elements of the following form: (syntactic-symbol
. nl-list). The list nl-list may contain either of the
symbols before or after, or both; or it may be nil.
When a colon is inserted, the syntactic symbol it defines is looked up in this list, and if found, the nl-list is used to determine where newlines are inserted: either before the brace, after, or both. If the syntactic symbol is not found in this list, no newlines are inserted.
Electric characters can also delete newlines automatically when the
auto-newline feature is enabled. This feature makes auto-newline more
acceptable, by deleting the newlines in the most common cases where you
do not want them. Emacs can recognize several cases in which deleting a
newline might be desirable; by setting the variable
c-cleanup-list, you can specify which of these cases that
should happen. The variable's value is a list of symbols, each
describing one case for possible deletion of a newline. Here are the
meaningful symbols, and their meanings:
brace-catch-brace
catch and condition.
brace-else-brace
else, but only if there is nothing but white space between
the braces and the else.
brace-elseif-brace
if-condition.
empty-defun-braces
defun-close-semi
struct or similar type
declaration, by placing the semicolon on the same line as the closing
brace. Clean-up occurs when you type the semicolon.
list-close-comma
scope-operator
When the hungry-delete feature is enabled (indicated by `/h' or `/ah' in the mode line after the mode name), a single DEL command deletes all preceding whitespace, not just one space. To turn this feature on or off, use C-c C-d:
c-toggle-hungry-state). With a
prefix argument, this command turns the hungry-delete feature on if the
argument is positive, and off if it is negative.
c-toggle-auto-hungry-state).
The variable c-hungry-delete-key controls whether the
hungry-delete feature is enabled.
c-mark-function).
c-fill-paragraph).
If any part of the current line is a comment or within a comment, this
command fills the comment or the paragraph of it that point is in,
preserving the comment indentation and comment delimiters.
c-macro-expand). The buffer text before the region is also
included in preprocessing, for the sake of macros defined there, but the
output from this part isn't shown.
When you are debugging C code that uses macros, sometimes it is hard to
figure out precisely how the macros expand. With this command, you
don't have to figure it out; you can see the expansions.
c-backslash-region). This is useful after writing or
editing a C macro definition.
If a line already ends in `\', this command adjusts the amount of
whitespace before it. Otherwise, it inserts a new `\'. However,
the last line in the region is treated specially; no `\' is
inserted on that line, and any `\' there is deleted.
c-show-syntactic-information). This is the information that
directs how the line is indented.
C mode and related modes use a number of variables for controlling comment format.
c-comment-only-line-offset
(non-anchored-offset . anchored-offset), where
non-anchored-offset is the amount of offset given to
non-column-zero anchored comment-only lines, and anchored-offset
is the amount of offset to give column-zero anchored comment-only lines.
Just an integer as value is equivalent to (val . 0).
c-comment-start-regexp
c-hanging-comment-ender-p
nil, c-fill-paragraph leaves the
comment terminator of a block comment on a line by itself. The default
value is t, which puts the comment-end delimiter `*/' at the
end of the last line of the comment text.
c-hanging-comment-starter-p
nil, c-fill-paragraph leaves the
starting delimiter of a block comment on a line by itself. The default
value is t, which puts the comment-start delimiter `/*' at
the beginning of the first line of the comment text.
Fortran mode provides special motion commands for Fortran statements and subprograms, and indentation commands that understand Fortran conventions of nesting, line numbers and continuation statements. Fortran mode has its own Auto Fill mode that breaks long lines into proper Fortran continuation lines.
Special commands for comments are provided because Fortran comments are unlike those of other languages. Built-in abbrevs optionally save typing when you insert Fortran keywords.
Use M-x fortran-mode to switch to this major mode. This command
runs the hook fortran-mode-hook (see section Hooks).
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.
beginning-of-fortran-subprogram).
end-of-fortran-subprogram).
mark-fortran-subprogram).
fortran-next-statement).
fortran-previous-statement).
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-indent-line).
fortran-indent-new-line).
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 C-j 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 C-M-j runs fortran-split-line, which splits
a line in the appropriate fashion for Fortran. In a non-comment line,
the second half becomes a continuation line and is indented
accordingly. In a comment line, both halves become separate comment
lines.
M-^ runs the command fortran-join-line, which is more or
less the inverse of fortran-split-line. It joins the current
line to the previous line in a suitable way for Fortran code.
Most modern Fortran compilers allow two ways of writing continuation
lines. If the first non-space character on a line is in column 5, then
that line is a continuation of the previous line. We call this
fixed format. (In GNU Emacs we always count columns from 0.) The
variable fortran-continuation-string specifies what character to
put on column 5. A line that starts with a tab character followed by
any digit except `0' is also a continuation line. We call this
style of continuation tab format.
Fortran mode can make either style of continuation line, but you
must specify which one you prefer. The value of the variable
indent-tabs-mode controls the choice: nil for fixed
format, and non-nil for tab format. You can tell which style
is presently in effect by the presence or absence of the string
`Tab' in the mode line.
If the text on a line starts with the conventional Fortran continuation marker `$', or if it begins with any non-whitespace character in column 5, Fortran mode treats it as a continuation line. When you indent a continuation line with TAB, it converts the line to the current continuation style. When you split a Fortran statement with C-M-j, the continuation marker on the newline is created according to the continuation style.
The setting of continuation style affects several other aspects of editing in Fortran mode. In fixed format mode, the minimum column number for the body of a statement is 6. Lines inside of Fortran blocks that are indented to larger column numbers always use only the space character for whitespace. In tab format mode, the minimum column number for the statement body is 8, and the whitespace before column 8 must always consist of one tab character.
When you enter Fortran mode for an existing file, it tries to deduce the
proper continuation style automatically from the file contents. The first
line that begins with either a tab character or six spaces determines the
choice. The variable fortran-analyze-depth specifies how many lines
to consider (at the beginning of the file); if none of those lines
indicates a style, then the variable fortran-tab-mode-default
specifies the style. If it is nil, that specifies fixed format, and
non-nil specifies tab format.
If a number is the first non-whitespace in the line, Fortran indentation assumes it is a line number and moves it to columns 0 through 4. (Columns always count from 0 in GNU Emacs.)
Line numbers of four digits or less are normally indented one space.
The variable fortran-line-number-indent controls this; it
specifies the maximum indentation a line number can have. Line numbers
are indented to right-justify them to end in column 4 unless that would
require more than this maximum indentation. The default value of the
variable is 1.
Simply inserting a line number is enough to indent it according to
these rules. As each digit is inserted, the indentation is recomputed.
To turn off this feature, set the variable
fortran-electric-line-number to nil. Then inserting line
numbers is like inserting anything else.
Fortran mode assumes that you follow certain conventions that simplify the task of understanding a Fortran program well enough to indent it properly:
If you fail to follow these conventions, the indentation commands may indent some lines unaesthetically. However, a correct Fortran program retains its meaning when reindented even if the conventions are not followed.
Several additional variables control how Fortran indentation works:
fortran-do-indent
fortran-if-indent
fortran-structure-indent
fortran-continuation-indent
fortran-check-all-num-for-matching-do
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-blink-matching-if
t, indenting an `endif' statement moves the
cursor momentarily to the matching `if' statement to show where it
is. The default is nil.
fortran-minimum-statement-indent-fixed
fortran-minimum-statement-indent-tab
The usual Emacs comment commands assume that a comment can follow a line of code. In Fortran, the standard comment syntax requires an entire line to be just a comment. Therefore, Fortran mode replaces the standard Emacs comment commands and defines some new variables.
Fortran mode can also handle a nonstandard comment syntax where comments
start with `!' and can follow other text. Because only some Fortran
compilers accept this syntax, Fortran mode will not insert such comments
unless you have said in advance to do so. To do this, set the variable
comment-start to `"!"' (see section Variables).
fortran-comment-indent).
fortran-comment-region).
M-; in Fortran mode is redefined as the command
fortran-comment-indent. Like the usual M-; command, this
recognizes any kind of existing comment and aligns its text appropriately;
if there is no existing comment, a comment is inserted and aligned. But
inserting and aligning comments are not the same in Fortran mode as in
other modes.
When a new comment must be inserted, if the current line is blank, a full-line comment is inserted. On a non-blank line, a nonstandard `!' comment is inserted if you have said you want to use them. Otherwise a full-line comment is inserted on a new line before the current line.
Nonstandard `!' comments are aligned like comments in other
languages, but full-line comments are different. In a standard full-line
comment, the comment delimiter itself must always appear in column zero.
What can be aligned is the text within the comment. You can choose from
three styles of alignment by setting the variable
fortran-comment-indent-style to one of these values:
fixed
fortran-comment-line-extra-indent and the minimum statement
indentation. This is the default.
The minimum statement indentation is
fortran-minimum-statement-indent-fixed for fixed format
continuation line style and fortran-minimum-statement-indent-tab
for tab format style.
relative
fortran-comment-line-extra-indent columns of indentation.
nil
In addition, you can specify the character to be used to indent within
full-line comments by setting the variable
fortran-comment-indent-char to the single-character string you want
to use.
Fortran mode introduces two variables comment-line-start and
comment-line-start-skip, which play for full-line comments the same
roles played by comment-start and comment-start-skip for
ordinary text-following comments. Normally these are set properly by
Fortran mode, so you do not need to change them.
The normal Emacs comment command C-x ; has not been redefined. If you use `!' comments, this command can be used with them. Otherwise it is useless in Fortran mode.
The command C-c ; (fortran-comment-region) turns all the
lines of the region into comments by inserting the string `C$$$' at
the front of each one. With a numeric argument, it turns the region
back into live code by deleting `C$$$' from the front of each line
in it. The string used for these comments can be controlled by setting
the variable fortran-comment-region. Note that here we have an
example of a command and a variable with the same name; these two uses
of the name never conflict because in Lisp and in Emacs it is always
clear from the context which one is meant.
Fortran Auto Fill mode is a minor mode which automatically splits
Fortran statements as you insert them when they become too wide.
Splitting a statement involves making continuation lines using
fortran-continuation-string (see section Continuation Lines). This
splitting happens when you type SPC, RET, or TAB, and
also in the Fortran indentation commands.
M-x fortran-auto-fill-mode turns Fortran Auto Fill mode on if it was off, or off if it was on. This command works the same as M-x auto-fill-mode does for normal Auto Fill mode (see section Filling Text). A positive numeric argument turns Fortran Auto Fill mode on, and a negative argument turns it off. You can see when Fortran Auto Fill mode is in effect by the presence of the word `Fill' in the mode line, inside the parentheses. Fortran Auto Fill mode is a minor mode, turned on or off for each buffer individually. See section Minor Modes.
Fortran Auto Fill mode breaks lines at spaces or delimiters when the
lines get longer than the desired width (the value of fill-column).
The delimiters that Fortran Auto Fill mode may break at are `,',
`'', `+', `-', `/', `*', `=', and `)'.
The line break comes after the delimiter if the variable
fortran-break-before-delimiters is nil. Otherwise (and by
default), the break comes before the delimiter.
By default, Fortran Auto Fill mode is not enabled. If you want this
feature turned on permanently, add a hook function to
fortran-mode-hook to execute (fortran-auto-fill-mode 1).
See section Hooks.
fortran-column-ruler).
fortran-window-create-momentarily).
The command C-c C-r (fortran-column-ruler) shows a column
ruler momentarily above the current line. The comment ruler is two lines
of text that show you the locations of columns with special significance in
Fortran programs. Square brackets show the limits of the columns for line
numbers, and curly brackets show the limits of the columns for the
statement body. Column numbers appear above them.
Note that the column numbers count from zero, as always in GNU Emacs. As a result, the numbers may be one less than those you are familiar with; but the positions they indicate in the line are standard for Fortran.
The text used to display the column ruler depends on the value of
the variable indent-tabs-mode. If indent-tabs-mode is
nil, then the value of the variable
fortran-column-ruler-fixed is used as the column ruler.
Otherwise, the variable fortran-column-ruler-tab is displayed.
By changing these variables, you can change the column ruler display.
For even more help, use C-c C-w (fortran-window-create), a
command which splits the current window horizontally, making a window 72
columns wide. By editing in this window you can immediately see when you
make a line too wide to be correct Fortran.
Fortran 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.
Fortran mode redefines the key C-x n d to run the command
fortran-narrow-to-subprogram, which is the Fortran analogue
of the key's usual definition. It narrows the buffer to the subprogram
containing point.
Asm mode is a major mode for editing files of assembler code. It defines these commands:
tab-to-tab-stop.
tab-to-tab-stop.
tab-to-tab-stop.
The variable asm-comment-char specifies which character
starts comments in assembler syntax.
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