emacs/man/emacs-xtra.texi

\input texinfo    @c -*-texinfo-*-
@comment %**start of header
@setfilename ../info/emacs-xtra
@settitle Specialized Emacs Features
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex ky cp
@comment %**end of header

@copying
This manual describes specialized features of Emacs.

Copyright @copyright{} 2004, 2005, 2006 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover texts being ``A GNU
Manual'', and with the Back-Cover Texts as in (a) below.  A copy of the
license is included in the section entitled ``GNU Free Documentation
License'' in the Emacs manual.

(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify
this GNU Manual, like GNU software.  Copies published by the Free
Software Foundation raise funds for GNU development.''

This document is part of a collection distributed under the GNU Free
Documentation License.  If you want to distribute this document
separately from the collection, you can do so by adding a copy of the
license to the document, as described in section 6 of the license.
@end quotation
@end copying

@dircategory Emacs
@direntry
* Emacs-Xtra: (emacs-xtra).    Specialized Emacs features.
@end direntry

@titlepage
@title Specialized Emacs Features
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top Specialized Emacs Features

@insertcopying

@end ifnottex

@menu
* Introduction::                  What documentation belongs here?
* Autorevert::                    Auto Reverting non-file buffers.
* Subdir switches::               Subdirectory switches in Dired.
* Advanced Calendar/Diary Usage:: Advanced Calendar/Diary customization.
* Emerge::                        A convenient way of merging two versions
                                    of a program.
* Picture Mode::                  Editing pictures made up of characters
                                    using the quarter-plane screen model.
* Fortran::                       Fortran mode and its special features.
* MS-DOG::                        
* Index::
@end menu

@node Introduction
@unnumbered Introduction

This manual contains detailed information about various features that
are too specialized to be included in the Emacs manual.  It is
intended to be readable by anyone having a basic knowledge of Emacs.
However, certain sections may be intended for a more specialized
audience, such as Elisp authors.  This should be clearly pointed out
at the beginning of these sections.

This manual is intended as a complement, rather than an alternative,
to other ways to gain a more detailed knowledge of Emacs than the
Emacs manual can provide, such as browsing packages using @kbd{C-h p},
accessing mode documentation using @kbd{C-h m} and browsing user
options using Custom.  Also, certain packages, or collections of
related features, have their own manuals.  The present manual is
mainly intended to be a collection of smaller specialized features,
too small to get their own manual.

Sections intended specifically for Elisp programmers can follow the
style of the Elisp manual.  Other sections should follow the style of
the Emacs manual.

@node Autorevert
@chapter Auto Reverting non-file Buffers

Normally Global Auto Revert Mode only reverts file buffers.  There are
two ways to auto-revert certain non-file buffers: enabling Auto Revert
Mode in those buffers (using @kbd{M-x auto-revert-mode}) and setting
@code{global-auto-revert-non-file-buffers} to @code{t}.  The latter
enables Auto Reverting for all types of buffers for which it is
implemented, that is, for the types of buffers listed in the menu
below.

Like file buffers, non-file buffers should normally not revert while
you are working on them, or while they contain information that might
get lost after reverting.  Therefore, they do not revert if they are
``modified''.  This can get tricky, because deciding when a non-file
buffer should be marked modified is usually more difficult than for
file buffers.

Another tricky detail is that, for efficiency reasons, Auto Revert
often does not try to detect all possible changes in the buffer, only
changes that are ``major'' or easy to detect.  Hence, enabling
auto-reverting for a non-file buffer does not always guarantee that
all information in the buffer is up to date and does not necessarily
make manual reverts useless.

At the other extreme, certain buffers automatically auto-revert every
@code{auto-revert-interval} seconds.  (This currently only applies to
the Buffer Menu.)  In this case, Auto Revert does not print any
messages while reverting, even when @code{auto-revert-verbose} is
non-@code{nil}.

The details depend on the particular types of buffers and are
explained in the corresponding sections.

@menu
* Auto Reverting the Buffer Menu::
* Auto Reverting Dired::
* Supporting additional buffers::
@end menu

@node Auto Reverting the Buffer Menu
@section Auto Reverting the Buffer Menu

If auto-reverting of non-file buffers is enabled, the Buffer Menu
automatically reverts every @code{auto-revert-interval} seconds,
whether there is a need for it or not.  (It would probably take longer
to check whether there is a need than to actually revert.)

If the Buffer Menu inappropriately gets marked modified, just revert
it manually using @kbd{g} and auto-reverting will resume.  However, if
you marked certain buffers to get deleted or to be displayed, you have
to be careful, because reverting erases all marks.  The fact that
adding marks sets the buffer's modified flag prevents Auto Revert from
automatically erasing the marks.

@node Auto Reverting Dired
@section Auto Reverting Dired buffers

Auto-reverting Dired buffers currently works on GNU or Unix style
operating systems.  It may not work satisfactorily on some other
systems.

Dired buffers only auto-revert when the file list of the buffer's main
directory changes.  They do not auto-revert when information about a
particular file changes or when inserted subdirectories change.  To be
sure that @emph{all} listed information is up to date, you have to
manually revert using @kbd{g}, @emph{even} if auto-reverting is
enabled in the Dired buffer.  Sometimes, you might get the impression
that modifying or saving files listed in the main directory actually
does cause auto-reverting.  This is because making changes to a file,
or saving it, very often causes changes in the directory itself, for
instance, through backup files or auto-save files.  However, this is
not guaranteed.

If the Dired buffer is marked modified and there are no changes you
want to protect, then most of the time you can make auto-reverting
resume by manually reverting the buffer using @kbd{g}.  There is one
exception.  If you flag or mark files, you can safely revert the
buffer.  This will not erase the flags or marks (unless the marked
file has been deleted, of course).  However, the buffer will stay
modified, even after reverting, and auto-reverting will not resume.
This is because, if you flag or mark files, you may be working on the
buffer and you might not want the buffer to change without warning.
If you want auto-reverting to resume in the presence of marks and
flags, mark the buffer non-modified using @kbd{M-~}.  However, adding,
deleting or changing marks or flags will mark it modified again.

Remote Dired buffers are not auto-reverted.  Neither are Dired buffers
for which you used shell wildcards or file arguments to list only some
of the files.  @samp{*Find*} and @samp{*Locate*} buffers do not
auto-revert either.

@node Supporting additional buffers
@section Adding Support for Auto-Reverting additional Buffers.

This section is intended for Elisp programmers who would like to add
support for auto-reverting new types of buffers.

To support auto-reverting the buffer must first of all have a
@code{revert-buffer-function}.  @xref{Definition of
revert-buffer-function,, Reverting, elisp, the Emacs Lisp Reference Manual}.

In addition, it @emph{must} have a @code{buffer-stale-function}.

@defvar buffer-stale-function
The value of this variable is a function to check whether a non-file
buffer needs reverting.  This should be a function with one optional
argument @var{noconfirm}.  The function should return non-@code{nil}
if the buffer should be reverted.  The buffer is current when this
function is called.

While this function is mainly intended for use in auto-reverting, it
could be used for other purposes as well.  For instance, if
auto-reverting is not enabled, it could be used to warn the user that
the buffer needs reverting.  The idea behind the @var{noconfirm}
argument is that it should be @code{t} if the buffer is going to be
reverted without asking the user and @code{nil} if the function is
just going to be used to warn the user that the buffer is out of date.
In particular, for use in auto-reverting, @var{noconfirm} is @code{t}.
If the function is only going to be used for auto-reverting, you can
ignore the @var{noconfirm} argument.

If you just want to automatically auto-revert every
@code{auto-revert-interval} seconds, use:

@example
(set (make-local-variable 'buffer-stale-function)
     #'(lambda (&optional noconfirm) 'fast))
@end example

@noindent
in the buffer's mode function.

The special return value @samp{fast} tells the caller that the need
for reverting was not checked, but that reverting the buffer is fast.
It also tells Auto Revert not to print any revert messages, even if
@code{auto-revert-verbose} is non-@code{nil}.  This is important, as
getting revert messages every @code{auto-revert-interval} seconds can
be very annoying.  The information provided by this return value could
also be useful if the function is consulted for purposes other than
auto-reverting.
@end defvar

Once the buffer has a @code{revert-buffer-function} and a
@code{buffer-stale-function}, several problems usually remain.

The buffer will only auto-revert if it is marked unmodified.  Hence,
you will have to make sure that various functions mark the buffer
modified if and only if either the buffer contains information that
might be lost by reverting or there is reason to believe that the user
might be inconvenienced by auto-reverting, because he is actively
working on the buffer.  The user can always override this by manually
adjusting the modified status of the buffer.  To support this, calling
the @code{revert-buffer-function} on a buffer that is marked
unmodified should always keep the buffer marked unmodified.

It is important to assure that point does not continuously jump around
as a consequence of auto-reverting.  Of course, moving point might be
inevitable if the buffer radically changes.

You should make sure that the @code{revert-buffer-function} does not
print messages that unnecessarily duplicate Auto Revert's own messages
if @code{auto-revert-verbose} is @code{t} and effectively override a
@code{nil} value for @code{auto-revert-verbose}.  Hence, adapting a
mode for auto-reverting often involves getting rid of such messages.
This is especially important for buffers that automatically
auto-revert every @code{auto-revert-interval} seconds.

Also, you may want to update the documentation string of
@code{global-auto-revert-non-file-buffers}.

@ifinfo
Finally, you should add a node to this chapter's menu.  This node
@end ifinfo
@ifnotinfo
Finally, you should add a section to this chapter.  This section
@end ifnotinfo
should at the very least make clear whether enabling auto-reverting
for the buffer reliably assures that all information in the buffer is
completely up to date (or will be after @code{auto-revert-interval}
seconds).

@node Subdir switches
@chapter Subdirectory Switches in Dired

You can insert subdirectories with specified @code{ls} switches in
Dired buffers, using @kbd{C-u i}.  You can change the @code{ls}
switches of an already inserted subdirectory using @kbd{C-u l}.

In Emacs versions 22.1 and later, Dired remembers the switches, so
that reverting the buffer will not change them back to the main
directory's switches.  Deleting a subdirectory forgets about its
switches.

Using @code{dired-undo} (usually bound to @kbd{C-_} and @kbd{C-x u})
to reinsert or delete subdirectories, that were inserted with explicit
switches, can bypass Dired's machinery for remembering (or forgetting)
switches.  Deleting a subdirectory using @code{dired-undo} does not
forget its switches.  When later reinserted using @kbd{i}, it will be
reinserted using its old switches.  Using @code{dired-undo} to
reinsert a subdirectory that was deleted using the regular
Dired commands (not @code{dired-undo}) will originally insert it with
its old switches.  However, reverting the buffer will relist it using
the buffer's default switches.  If any of this yields problems, you
can easily correct the situation using @kbd{C-u i} or @kbd{C-u l}.

Dired does not remember the @code{R} switch.  Inserting a subdirectory
with switches that include the @code{R} switch is equivalent with
inserting each of its subdirectories using all remaining switches.
For instance, updating or killing a subdirectory that was inserted
with the @code{R} switch will not update or kill its subdirectories.

The buffer's default switches do not affect subdirectories that were
inserted using explicitly specified switches.  In particular,
commands such as @kbd{s}, that change the buffer's switches do not
affect such subdirectories.  (They do affect subdirectories without
explicitly assigned switches, however.)

You can make Dired forget about all subdirectory switches and relist
all subdirectories with the buffer's default switches using
@kbd{M-x dired-reset-subdir-switches}.  This also reverts the Dired buffer.


@c Moved here from the Emacs Lisp Reference Manual, 2005-03-26.
@node Advanced Calendar/Diary Usage
@chapter Customizing the Calendar and Diary

  There are many customizations that you can use to make the calendar and
diary suit your personal tastes.

@menu
* Calendar Customizing::   Defaults you can set.
* Holiday Customizing::    Defining your own holidays.
* Date Display Format::    Changing the format.
* Time Display Format::    Changing the format.
* Daylight Savings::       Changing the default.
* Diary Customizing::      Defaults you can set.
* Hebrew/Islamic Entries:: How to obtain them.
* Fancy Diary Display::    Enhancing the diary display, sorting entries,
                             using included diary files.
* Sexp Diary Entries::     Fancy things you can do.
@end menu

@node Calendar Customizing
@section Customizing the Calendar
@vindex calendar-holiday-marker
@vindex diary-entry-marker
  The variable @code{calendar-holiday-marker} specifies how to mark a
date as being a holiday.  Its value may be a single-character string
to insert next to the date, or a face name to use for displaying the
date.  Likewise, the variable @code{diary-entry-marker} specifies how
to mark a date that has diary entries.  The calendar creates faces
named @code{holiday-face} and @code{diary-face} for these purposes;
those symbols are the default values of these variables.

@vindex calendar-load-hook
  The variable @code{calendar-load-hook} is a normal hook run when the
calendar package is first loaded (before actually starting to display
the calendar).

@vindex initial-calendar-window-hook
  Starting the calendar runs the normal hook
@code{initial-calendar-window-hook}.  Recomputation of the calendar
display does not run this hook.  But if you leave the calendar with the
@kbd{q} command and reenter it, the hook runs again.@refill

@vindex today-visible-calendar-hook
  The variable @code{today-visible-calendar-hook} is a normal hook run
after the calendar buffer has been prepared with the calendar when the
current date is visible in the window.  One use of this hook is to
replace today's date with asterisks; to do that, use the hook function
@code{calendar-star-date}.

@findex calendar-star-date
@example
(add-hook 'today-visible-calendar-hook 'calendar-star-date)
@end example

@noindent
Another standard hook function marks the current date, either by
changing its face or by adding an asterisk.  Here's how to use it:

@findex calendar-mark-today
@example
(add-hook 'today-visible-calendar-hook 'calendar-mark-today)
@end example

@noindent
@vindex calendar-today-marker
The variable @code{calendar-today-marker} specifies how to mark
today's date.  Its value should be a single-character string to insert
next to the date or a face name to use for displaying the date.  A
face named @code{calendar-today-face} is provided for this purpose;
that symbol is the default for this variable.

@vindex today-invisible-calendar-hook
@noindent
  A similar normal hook, @code{today-invisible-calendar-hook} is run if
the current date is @emph{not} visible in the window.

@vindex calendar-move-hook
  Each of the calendar cursor motion commands runs the hook
@code{calendar-move-hook} after it moves the cursor.

@node Holiday Customizing
@section Customizing the Holidays

@vindex calendar-holidays
@vindex christian-holidays
@vindex hebrew-holidays
@vindex islamic-holidays
  Emacs knows about holidays defined by entries on one of several lists.
You can customize these lists of holidays to your own needs, adding or
deleting holidays.  The lists of holidays that Emacs uses are for
general holidays (@code{general-holidays}), local holidays
(@code{local-holidays}), Christian holidays (@code{christian-holidays}),
Hebrew (Jewish) holidays (@code{hebrew-holidays}), Islamic (Muslim)
holidays (@code{islamic-holidays}), and other holidays
(@code{other-holidays}).

@vindex general-holidays
  The general holidays are, by default, holidays common throughout the
United States.  To eliminate these holidays, set @code{general-holidays}
to @code{nil}.

@vindex local-holidays
  There are no default local holidays (but sites may supply some).  You
can set the variable @code{local-holidays} to any list of holidays, as
described below.

@vindex all-christian-calendar-holidays
@vindex all-hebrew-calendar-holidays
@vindex all-islamic-calendar-holidays
  By default, Emacs does not include all the holidays of the religions
that it knows, only those commonly found in secular calendars.  For a
more extensive collection of religious holidays, you can set any (or
all) of the variables @code{all-christian-calendar-holidays},
@code{all-hebrew-calendar-holidays}, or
@code{all-islamic-calendar-holidays} to @code{t}.  If you want to
eliminate the religious holidays, set any or all of the corresponding
variables @code{christian-holidays}, @code{hebrew-holidays}, and
@code{islamic-holidays} to @code{nil}.@refill

@vindex other-holidays
  You can set the variable @code{other-holidays} to any list of
holidays.  This list, normally empty, is intended for individual use.

@cindex holiday forms
  Each of the lists (@code{general-holidays}, @code{local-holidays},
@code{christian-holidays}, @code{hebrew-holidays},
@code{islamic-holidays}, and @code{other-holidays}) is a list of
@dfn{holiday forms}, each holiday form describing a holiday (or
sometimes a list of holidays).

  Here is a table of the possible kinds of holiday form.  Day numbers
and month numbers count starting from 1, but ``dayname'' numbers
count Sunday as 0.  The element @var{string} is always the
name of the holiday, as a string.

@table @code
@item (holiday-fixed @var{month} @var{day} @var{string})
A fixed date on the Gregorian calendar.

@item (holiday-float @var{month} @var{dayname} @var{k} @var{string})
The @var{k}th @var{dayname} in @var{month} on the Gregorian calendar
(@var{dayname}=0 for Sunday, and so on); negative @var{k} means count back
from the end of the month.

@item (holiday-hebrew @var{month} @var{day} @var{string})
A fixed date on the Hebrew calendar.

@item (holiday-islamic @var{month} @var{day} @var{string})
A fixed date on the Islamic calendar.

@item (holiday-julian @var{month} @var{day} @var{string})
A fixed date on the Julian calendar.

@item (holiday-sexp @var{sexp} @var{string})
A date calculated by the Lisp expression @var{sexp}.  The expression
should use the variable @code{year} to compute and return the date of a
holiday, or @code{nil} if the holiday doesn't happen this year.  The
value of @var{sexp} must represent the date as a list of the form
@code{(@var{month} @var{day} @var{year})}.

@item (if @var{condition} @var{holiday-form})
A holiday that happens only if @var{condition} is true.

@item (@var{function} @r{[}@var{args}@r{]})
A list of dates calculated by the function @var{function}, called with
arguments @var{args}.
@end table

  For example, suppose you want to add Bastille Day, celebrated in
France on July 14.  You can do this as follows:

@smallexample
(setq other-holidays '((holiday-fixed 7 14 "Bastille Day")))
@end smallexample

@noindent
The holiday form @code{(holiday-fixed 7 14 "Bastille Day")} specifies the
fourteenth day of the seventh month (July).

  Many holidays occur on a specific day of the week, at a specific time
of month.  Here is a holiday form describing Hurricane Supplication Day,
celebrated in the Virgin Islands on the fourth Monday in August:

@smallexample
(holiday-float 8 1 4 "Hurricane Supplication Day")
@end smallexample

@noindent
Here the 8 specifies August, the 1 specifies Monday (Sunday is 0,
Tuesday is 2, and so on), and the 4 specifies the fourth occurrence in
the month (1 specifies the first occurrence, 2 the second occurrence,
@minus{}1 the last occurrence, @minus{}2 the second-to-last occurrence, and
so on).

  You can specify holidays that occur on fixed days of the Hebrew,
Islamic, and Julian calendars too.  For example,

@smallexample
(setq other-holidays
      '((holiday-hebrew 10 2 "Last day of Hanukkah")
        (holiday-islamic 3 12 "Mohammed's Birthday")
        (holiday-julian 4 2 "Jefferson's Birthday")))
@end smallexample

@noindent
adds the last day of Hanukkah (since the Hebrew months are numbered with
1 starting from Nisan), the Islamic feast celebrating Mohammed's
birthday (since the Islamic months are numbered from 1 starting with
Muharram), and Thomas Jefferson's birthday, which is 2 April 1743 on the
Julian calendar.

  To include a holiday conditionally, use either Emacs Lisp's @code{if} or the
@code{holiday-sexp} form.  For example, American presidential elections
occur on the first Tuesday after the first Monday in November of years
divisible by 4:

@smallexample
(holiday-sexp '(if (= 0 (% year 4))
                   (calendar-gregorian-from-absolute
                    (1+ (calendar-dayname-on-or-before
                          1 (+ 6 (calendar-absolute-from-gregorian
                                  (list 11 1 year)))))))
              "US Presidential Election")
@end smallexample

@noindent
or

@smallexample
(if (= 0 (% displayed-year 4))
    (fixed 11
           (extract-calendar-day
             (calendar-gregorian-from-absolute
               (1+ (calendar-dayname-on-or-before
                     1 (+ 6 (calendar-absolute-from-gregorian
                              (list 11 1 displayed-year)))))))
           "US Presidential Election"))
@end smallexample

  Some holidays just don't fit into any of these forms because special
calculations are involved in their determination.  In such cases you
must write a Lisp function to do the calculation.  To include eclipses,
for example, add @code{(eclipses)} to @code{other-holidays}
and write an Emacs Lisp function @code{eclipses} that returns a
(possibly empty) list of the relevant Gregorian dates among the range
visible in the calendar window, with descriptive strings, like this:

@smallexample
(((6 27 1991) "Lunar Eclipse") ((7 11 1991) "Solar Eclipse") ... )
@end smallexample

@node Date Display Format
@section Date Display Format
@vindex calendar-date-display-form

  You can customize the manner of displaying dates in the diary, in mode
lines, and in messages by setting @code{calendar-date-display-form}.
This variable holds a list of expressions that can involve the variables
@code{month}, @code{day}, and @code{year}, which are all numbers in
string form, and @code{monthname} and @code{dayname}, which are both
alphabetic strings.  In the American style, the default value of this
list is as follows:

@smallexample
((if dayname (concat dayname ", ")) monthname " " day ", " year)
@end smallexample

@noindent
while in the European style this value is the default:

@smallexample
((if dayname (concat dayname ", ")) day " " monthname " " year)
@end smallexample

@noindent
The ISO standard date representation is this:

@smallexample
(year "-" month "-" day)
@end smallexample

@noindent
This specifies a typical American format:

@smallexample
(month "/" day "/" (substring year -2))
@end smallexample

@node Time Display Format
@section Time Display Format
@vindex calendar-time-display-form

  The calendar and diary by default display times of day in the
conventional American style with the hours from 1 through 12, minutes,
and either @samp{am} or @samp{pm}.  If you prefer the European style,
also known in the US as military, in which the hours go from 00 to 23,
you can alter the variable @code{calendar-time-display-form}.  This
variable is a list of expressions that can involve the variables
@code{12-hours}, @code{24-hours}, and @code{minutes}, which are all
numbers in string form, and @code{am-pm} and @code{time-zone}, which are
both alphabetic strings.  The default value of
@code{calendar-time-display-form} is as follows:

@smallexample
(12-hours ":" minutes am-pm
          (if time-zone " (") time-zone (if time-zone ")"))
@end smallexample

@noindent
Here is a value that provides European style times:

@smallexample
(24-hours ":" minutes
          (if time-zone " (") time-zone (if time-zone ")"))
@end smallexample

@node Daylight Savings
@section Daylight Savings Time
@cindex daylight savings time

  Emacs understands the difference between standard time and daylight
savings time---the times given for sunrise, sunset, solstices,
equinoxes, and the phases of the moon take that into account.  The rules
for daylight savings time vary from place to place and have also varied
historically from year to year.  To do the job properly, Emacs needs to
know which rules to use.

  Some operating systems keep track of the rules that apply to the place
where you are; on these systems, Emacs gets the information it needs
from the system automatically.  If some or all of this information is
missing, Emacs fills in the gaps with the rules currently used in
Cambridge, Massachusetts, which is the center of GNU's world.


@vindex calendar-daylight-savings-starts
@vindex calendar-daylight-savings-ends
  If the default choice of rules is not appropriate for your location,
you can tell Emacs the rules to use by setting the variables
@code{calendar-daylight-savings-starts} and
@code{calendar-daylight-savings-ends}.  Their values should be Lisp
expressions that refer to the variable @code{year}, and evaluate to the
Gregorian date on which daylight savings time starts or (respectively)
ends, in the form of a list @code{(@var{month} @var{day} @var{year})}.
The values should be @code{nil} if your area does not use daylight
savings time.

  Emacs uses these expressions to determine the start and end dates of
daylight savings time as holidays and for correcting times of day in the
solar and lunar calculations.

  The values for Cambridge, Massachusetts are as follows:

@example
@group
(calendar-nth-named-day 1 0 4 year)
(calendar-nth-named-day -1 0 10 year)
@end group
@end example

@noindent
i.e., the first 0th day (Sunday) of the fourth month (April) in
the year specified by @code{year}, and the last Sunday of the tenth month
(October) of that year.  If daylight savings time were
changed to start on October 1, you would set
@code{calendar-daylight-savings-starts} to this:

@example
(list 10 1 year)
@end example

  For a more complex example, suppose daylight savings time begins on
the first of Nisan on the Hebrew calendar.  You should set
@code{calendar-daylight-savings-starts} to this value:

@example
(calendar-gregorian-from-absolute
  (calendar-absolute-from-hebrew
    (list 1 1 (+ year 3760))))
@end example

@noindent
because Nisan is the first month in the Hebrew calendar and the Hebrew
year differs from the Gregorian year by 3760 at Nisan.

  If there is no daylight savings time at your location, or if you want
all times in standard time, set @code{calendar-daylight-savings-starts}
and @code{calendar-daylight-savings-ends} to @code{nil}.

@vindex calendar-daylight-time-offset
  The variable @code{calendar-daylight-time-offset} specifies the
difference between daylight savings time and standard time, measured in
minutes.  The value for Cambridge is 60.

@vindex calendar-daylight-savings-starts-time
@vindex calendar-daylight-savings-ends-time
  The variable @code{calendar-daylight-savings-starts-time} and the
variable @code{calendar-daylight-savings-ends-time} specify the number
of minutes after midnight local time when the transition to and from
daylight savings time should occur.  For Cambridge, both variables'
values are 120.

@node Diary Customizing
@section Customizing the Diary

@vindex holidays-in-diary-buffer
  Ordinarily, the mode line of the diary buffer window indicates any
holidays that fall on the date of the diary entries.  The process of
checking for holidays can take several seconds, so including holiday
information delays the display of the diary buffer noticeably.  If you'd
prefer to have a faster display of the diary buffer but without the
holiday information, set the variable @code{holidays-in-diary-buffer} to
@code{nil}.@refill

@vindex number-of-diary-entries
  The variable @code{number-of-diary-entries} controls the number of
days of diary entries to be displayed at one time.  It affects the
initial display when @code{view-diary-entries-initially} is @code{t}, as
well as the command @kbd{M-x diary}.  For example, the default value is
1, which says to display only the current day's diary entries.  If the
value is 2, both the current day's and the next day's entries are
displayed.  The value can also be a vector of seven elements: for
example, if the value is @code{[0 2 2 2 2 4 1]} then no diary entries
appear on Sunday, the current date's and the next day's diary entries
appear Monday through Thursday, Friday through Monday's entries appear
on Friday, while on Saturday only that day's entries appear.

@vindex print-diary-entries-hook
@findex print-diary-entries
  The variable @code{print-diary-entries-hook} is a normal hook run
after preparation of a temporary buffer containing just the diary
entries currently visible in the diary buffer.  (The other, irrelevant
diary entries are really absent from the temporary buffer; in the diary
buffer, they are merely hidden.)  The default value of this hook does
the printing with the command @code{lpr-buffer}.  If you want to use a
different command to do the printing, just change the value of this
hook.  Other uses might include, for example, rearranging the lines into
order by day and time.

@vindex diary-date-forms
  You can customize the form of dates in your diary file, if neither the
standard American nor European styles suits your needs, by setting the
variable @code{diary-date-forms}.  This variable is a list of patterns
for recognizing a date.  Each date pattern is a list whose elements may
be regular expressions (@pxref{Regular Expressions,,, elisp, the Emacs
Lisp Reference Manual}) or the symbols @code{month}, @code{day},
@code{year}, @code{monthname}, and @code{dayname}.  All these elements
serve as patterns that match certain kinds of text in the diary file.
In order for the date pattern, as a whole, to match, all of its elements
must match consecutively.

  A regular expression in a date pattern matches in its usual fashion,
using the standard syntax table altered so that @samp{*} is a word
constituent.

  The symbols @code{month}, @code{day}, @code{year}, @code{monthname},
and @code{dayname} match the month number, day number, year number,
month name, and day name of the date being considered.  The symbols that
match numbers allow leading zeros; those that match names allow
three-letter abbreviations and capitalization.  All the symbols can
match @samp{*}; since @samp{*} in a diary entry means ``any day'', ``any
month'', and so on, it should match regardless of the date being
considered.

  The default value of @code{diary-date-forms} in the American style is
this:

@example
((month "/" day "[^/0-9]")
 (month "/" day "/" year "[^0-9]")
 (monthname " *" day "[^,0-9]")
 (monthname " *" day ", *" year "[^0-9]")
 (dayname "\\W"))
@end example

  The date patterns in the list must be @emph{mutually exclusive} and
must not match any portion of the diary entry itself, just the date and
one character of whitespace.  If, to be mutually exclusive, the pattern
must match a portion of the diary entry text---beyond the whitespace
that ends the date---then the first element of the date pattern
@emph{must} be @code{backup}.  This causes the date recognizer to back
up to the beginning of the current word of the diary entry, after
finishing the match.  Even if you use @code{backup}, the date pattern
must absolutely not match more than a portion of the first word of the
diary entry.  The default value of @code{diary-date-forms} in the
European style is this list:

@example
((day "/" month "[^/0-9]")
 (day "/" month "/" year "[^0-9]")
 (backup day " *" monthname "\\W+\\<[^*0-9]")
 (day " *" monthname " *" year "[^0-9]")
 (dayname "\\W"))
@end example

@noindent
Notice the use of @code{backup} in the third pattern, because it needs
to match part of a word beyond the date itself to distinguish it from
the fourth pattern.

@node Hebrew/Islamic Entries
@section Hebrew- and Islamic-Date Diary Entries

  Your diary file can have entries based on Hebrew or Islamic dates, as
well as entries based on the world-standard Gregorian calendar.
However, because recognition of such entries is time-consuming and most
people don't use them, you must explicitly enable their use.  If you
want the diary to recognize Hebrew-date diary entries, for example,
you must do this:

@vindex nongregorian-diary-listing-hook
@vindex nongregorian-diary-marking-hook
@findex list-hebrew-diary-entries
@findex mark-hebrew-diary-entries
@smallexample
(add-hook 'nongregorian-diary-listing-hook 'list-hebrew-diary-entries)
(add-hook 'nongregorian-diary-marking-hook 'mark-hebrew-diary-entries)
@end smallexample

@noindent
If you want Islamic-date entries, do this:

@findex list-islamic-diary-entries
@findex mark-islamic-diary-entries
@smallexample
(add-hook 'nongregorian-diary-listing-hook 'list-islamic-diary-entries)
(add-hook 'nongregorian-diary-marking-hook 'mark-islamic-diary-entries)
@end smallexample

  Hebrew- and Islamic-date diary entries have the same formats as
Gregorian-date diary entries, except that @samp{H} precedes a Hebrew
date and @samp{I} precedes an Islamic date.  Moreover, because the
Hebrew and Islamic month names are not uniquely specified by the first
three letters, you may not abbreviate them.  For example, a diary entry
for the Hebrew date Heshvan 25 could look like this:

@smallexample
HHeshvan 25 Happy Hebrew birthday!
@end smallexample

@noindent
and would appear in the diary for any date that corresponds to Heshvan 25
on the Hebrew calendar.  And here is an Islamic-date diary entry that matches
Dhu al-Qada 25:

@smallexample
IDhu al-Qada 25 Happy Islamic birthday!
@end smallexample

  As with Gregorian-date diary entries, Hebrew- and Islamic-date entries
are nonmarking if they are preceded with an ampersand (@samp{&}).

  Here is a table of commands used in the calendar to create diary entries
that match the selected date and other dates that are similar in the Hebrew
or Islamic calendar:

@table @kbd
@item i h d
Add a diary entry for the Hebrew date corresponding to the selected date
(@code{insert-hebrew-diary-entry}).
@item i h m
Add a diary entry for the day of the Hebrew month corresponding to the
selected date (@code{insert-monthly-hebrew-diary-entry}).  This diary
entry matches any date that has the same Hebrew day-within-month as the
selected date.
@item i h y
Add a diary entry for the day of the Hebrew year corresponding to the
selected date (@code{insert-yearly-hebrew-diary-entry}).  This diary
entry matches any date which has the same Hebrew month and day-within-month
as the selected date.
@item i i d
Add a diary entry for the Islamic date corresponding to the selected date
(@code{insert-islamic-diary-entry}).
@item i i m
Add a diary entry for the day of the Islamic month corresponding to the
selected date (@code{insert-monthly-islamic-diary-entry}).
@item i i y
Add a diary entry for the day of the Islamic year corresponding to the
selected date (@code{insert-yearly-islamic-diary-entry}).
@end table

@findex insert-hebrew-diary-entry
@findex insert-monthly-hebrew-diary-entry
@findex insert-yearly-hebrew-diary-entry
@findex insert-islamic-diary-entry
@findex insert-monthly-islamic-diary-entry
@findex insert-yearly-islamic-diary-entry
  These commands work much like the corresponding commands for ordinary
diary entries: they apply to the date that point is on in the calendar
window, and what they do is insert just the date portion of a diary entry
at the end of your diary file.  You must then insert the rest of the
diary entry.

@node Fancy Diary Display
@section Fancy Diary Display
@vindex diary-display-hook
@findex simple-diary-display

  Diary display works by preparing the diary buffer and then running the
hook @code{diary-display-hook}.  The default value of this hook
(@code{simple-diary-display}) hides the irrelevant diary entries and
then displays the buffer.  However, if you specify the hook as follows,

@cindex diary buffer
@findex fancy-diary-display
@example
(add-hook 'diary-display-hook 'fancy-diary-display)
@end example

@noindent
this enables fancy diary display.  It displays diary entries and
holidays by copying them into a special buffer that exists only for the
sake of display.  Copying to a separate buffer provides an opportunity
to change the displayed text to make it prettier---for example, to sort
the entries by the dates they apply to.

  As with simple diary display, you can print a hard copy of the buffer
with @code{print-diary-entries}.  To print a hard copy of a day-by-day
diary for a week, position point on Sunday of that week, type
@kbd{7 d}, and then do @kbd{M-x print-diary-entries}.  As usual, the
inclusion of the holidays slows down the display slightly; you can speed
things up by setting the variable @code{holidays-in-diary-buffer} to
@code{nil}.

@vindex diary-list-include-blanks
  Ordinarily, the fancy diary buffer does not show days for which there are
no diary entries, even if that day is a holiday.  If you want such days to be
shown in the fancy diary buffer, set the variable
@code{diary-list-include-blanks} to @code{t}.@refill

@cindex sorting diary entries
  If you use the fancy diary display, you can use the normal hook
@code{list-diary-entries-hook} to sort each day's diary entries by their
time of day.  Here's how:

@findex sort-diary-entries
@example
(add-hook 'list-diary-entries-hook 'sort-diary-entries t)
@end example

@noindent
For each day, this sorts diary entries that begin with a recognizable
time of day according to their times.  Diary entries without times come
first within each day.

  Fancy diary display also has the ability to process included diary
files.  This permits a group of people to share a diary file for events
that apply to all of them.  Lines in the diary file of this form:

@smallexample
#include "@var{filename}"
@end smallexample

@noindent
includes the diary entries from the file @var{filename} in the fancy
diary buffer.  The include mechanism is recursive, so that included files
can include other files, and so on; you must be careful not to have a
cycle of inclusions, of course.  Here is how to enable the include
facility:

@vindex list-diary-entries-hook
@vindex mark-diary-entries-hook
@findex include-other-diary-files
@findex mark-included-diary-files
@smallexample
(add-hook 'list-diary-entries-hook 'include-other-diary-files)
(add-hook 'mark-diary-entries-hook 'mark-included-diary-files)
@end smallexample

The include mechanism works only with the fancy diary display, because
ordinary diary display shows the entries directly from your diary file.

@node Sexp Diary Entries
@section Sexp Entries and the Fancy Diary Display
@cindex sexp diary entries

  Sexp diary entries allow you to do more than just have complicated
conditions under which a diary entry applies.  If you use the fancy
diary display, sexp entries can generate the text of the entry depending
on the date itself.  For example, an anniversary diary entry can insert
the number of years since the anniversary date into the text of the
diary entry.  Thus the @samp{%d} in this dairy entry:

@findex diary-anniversary
@smallexample
%%(diary-anniversary 10 31 1948) Arthur's birthday (%d years old)
@end smallexample

@noindent
gets replaced by the age, so on October 31, 1990 the entry appears in
the fancy diary buffer like this:

@smallexample
Arthur's birthday (42 years old)
@end smallexample

@noindent
If the diary file instead contains this entry:

@smallexample
%%(diary-anniversary 10 31 1948) Arthur's %d%s birthday
@end smallexample

@noindent
the entry in the fancy diary buffer for October 31, 1990 appears like this:

@smallexample
Arthur's 42nd birthday
@end smallexample

  Similarly, cyclic diary entries can interpolate the number of repetitions
that have occurred:

@findex diary-cyclic
@smallexample
%%(diary-cyclic 50 1 1 1990) Renew medication (%d%s time)
@end smallexample

@noindent
looks like this:

@smallexample
Renew medication (5th time)
@end smallexample

@noindent
in the fancy diary display on September 8, 1990.

  There is an early reminder diary sexp that includes its entry in the
diary not only on the date of occurrence, but also on earlier dates.
For example, if you want a reminder a week before your anniversary, you
can use

@findex diary-remind
@smallexample
%%(diary-remind '(diary-anniversary 12 22 1968) 7) Ed's anniversary
@end smallexample

@noindent
and the fancy diary will show
@smallexample
Ed's anniversary
@end smallexample
@noindent
both on December 15 and on December 22.

@findex diary-date
  The function @code{diary-date} applies to dates described by a month,
day, year combination, each of which can be an integer, a list of
integers, or @code{t}. The value @code{t} means all values.  For
example,

@smallexample
%%(diary-date '(10 11 12) 22 t) Rake leaves
@end smallexample

@noindent
causes the fancy diary to show

@smallexample
Rake leaves
@end smallexample

@noindent
on October 22, November 22, and December 22 of every year.

@findex diary-float
  The function @code{diary-float} allows you to describe diary entries
that apply to dates like the third Friday of November, or the last
Tuesday in April.  The parameters are the @var{month}, @var{dayname},
and an index @var{n}. The entry appears on the @var{n}th @var{dayname}
of @var{month}, where @var{dayname}=0 means Sunday, 1 means Monday, and
so on.  If @var{n} is negative it counts backward from the end of
@var{month}.  The value of @var{month} can be a list of months, a single
month, or @code{t} to specify all months.  You can also use an optional
parameter @var{day} to specify the @var{n}th @var{dayname} of
@var{month} on or after/before @var{day}; the value of @var{day} defaults
to 1 if @var{n} is positive and to the last day of @var{month} if
@var{n} is negative.  For example,

@smallexample
%%(diary-float t 1 -1) Pay rent
@end smallexample

@noindent
causes the fancy diary to show

@smallexample
Pay rent
@end smallexample

@noindent
on the last Monday of every month.

  The generality of sexp diary entries lets you specify any diary
entry that you can describe algorithmically.  A sexp diary entry
contains an expression that computes whether the entry applies to any
given date.  If its value is non-@code{nil}, the entry applies to that
date; otherwise, it does not.  The expression can use the variable
@code{date} to find the date being considered; its value is a list
(@var{month} @var{day} @var{year}) that refers to the Gregorian
calendar.

  The sexp diary entry applies to a date when the expression's value
is non-@code{nil}, but some values have more specific meanings.  If
the value is a string, that string is a description of the event which
occurs on that date.  The value can also have the form
@code{(@var{mark} . @var{string})}; then @var{mark} specifies how to
mark the date in the calendar, and @var{string} is the description of
the event.  If @var{mark} is a single-character string, that character
appears next to the date in the calendar.  If @var{mark} is a face
name, the date is displayed in that face.  If @var{mark} is
@code{nil}, that specifies no particular highlighting for the date.

  Suppose you get paid on the 21st of the month if it is a weekday, and
on the Friday before if the 21st is on a weekend.  Here is how to write
a sexp diary entry that matches those dates:

@smallexample
&%%(let ((dayname (calendar-day-of-week date))
         (day (car (cdr date))))
      (or (and (= day 21) (memq dayname '(1 2 3 4 5)))
          (and (memq day '(19 20)) (= dayname 5)))
         ) Pay check deposited
@end smallexample

  The following sexp diary entries take advantage of the ability (in the fancy
diary display) to concoct diary entries whose text varies based on the date:

@findex diary-sunrise-sunset
@findex diary-phases-of-moon
@findex diary-day-of-year
@findex diary-iso-date
@findex diary-julian-date
@findex diary-astro-day-number
@findex diary-hebrew-date
@findex diary-islamic-date
@findex diary-french-date
@findex diary-mayan-date
@table @code
@item %%(diary-sunrise-sunset)
Make a diary entry for the local times of today's sunrise and sunset.
@item %%(diary-phases-of-moon)
Make a diary entry for the phases (quarters) of the moon.
@item %%(diary-day-of-year)
Make a diary entry with today's day number in the current year and the number
of days remaining in the current year.
@item %%(diary-iso-date)
Make a diary entry with today's equivalent ISO commercial date.
@item %%(diary-julian-date)
Make a diary entry with today's equivalent date on the Julian calendar.
@item %%(diary-astro-day-number)
Make a diary entry with today's equivalent astronomical (Julian) day number.
@item %%(diary-hebrew-date)
Make a diary entry with today's equivalent date on the Hebrew calendar.
@item %%(diary-islamic-date)
Make a diary entry with today's equivalent date on the Islamic calendar.
@item %%(diary-french-date)
Make a diary entry with today's equivalent date on the French Revolutionary
calendar.
@item %%(diary-mayan-date)
Make a diary entry with today's equivalent date on the Mayan calendar.
@end table

@noindent
Thus including the diary entry

@example
&%%(diary-hebrew-date)
@end example

@noindent
causes every day's diary display to contain the equivalent date on the
Hebrew calendar, if you are using the fancy diary display.  (With simple
diary display, the line @samp{&%%(diary-hebrew-date)} appears in the
diary for any date, but does nothing particularly useful.)

  These functions can be used to construct sexp diary entries based on
the Hebrew calendar in certain standard ways:

@cindex rosh hodesh
@findex diary-rosh-hodesh
@cindex parasha, weekly
@findex diary-parasha
@cindex candle lighting times
@findex diary-sabbath-candles
@cindex omer count
@findex diary-omer
@cindex yahrzeits
@findex diary-yahrzeit
@table @code
@item %%(diary-rosh-hodesh)
Make a diary entry that tells the occurrence and ritual announcement of each
new Hebrew month.
@item %%(diary-parasha)
Make a Saturday diary entry that tells the weekly synagogue scripture reading.
@item %%(diary-sabbath-candles)
Make a Friday diary entry that tells the @emph{local time} of Sabbath
candle lighting.
@item %%(diary-omer)
Make a diary entry that gives the omer count, when appropriate.
@item %%(diary-yahrzeit @var{month} @var{day} @var{year}) @var{name}
Make a diary entry marking the anniversary of a date of death.  The date
is the @emph{Gregorian} (civil) date of death.  The diary entry appears
on the proper Hebrew calendar anniversary and on the day before.  (In
the European style, the order of the parameters is changed to @var{day},
@var{month}, @var{year}.)
@end table

  All the functions documented above take an optional argument
@var{mark} which specifies how to mark the date in the calendar display.
If one of these functions decides that it applies to a certain date,
it returns a value that contains @var{mark}.

@node Emerge
@chapter Merging Files with Emerge
@cindex Emerge
@cindex merging files

  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.  For other ways to compare files, see @ref{Comparing Files,,,
emacs, the Emacs Manual} and @ref{Top, Ediff,, ediff, The Ediff
Manual}.

@menu
* Overview of Emerge::	How to start Emerge.  Basic concepts.
* Submodes of Emerge::	Fast mode vs. Edit mode.
			  Skip Prefers mode and Auto Advance mode.
* State of Difference::	You do the merge by specifying state A or B
			  for each difference.
* Merge Commands::	Commands for selecting a difference,
			  changing states of differences, etc.
* Exiting Emerge::	What to do when you've finished the merge.
* Combining in Emerge::	    How to keep both alternatives for a difference.
* Fine Points of Emerge::   Misc.
@end menu

@node Overview of Emerge
@section Overview of Emerge

  To start Emerge, run one of these four commands:

@table @kbd
@item M-x emerge-files
@findex emerge-files
Merge two specified files.

@item M-x emerge-files-with-ancestor
@findex emerge-files-with-ancestor
Merge two specified files, with reference to a common ancestor.

@item M-x emerge-buffers
@findex emerge-buffers
Merge two buffers.

@item M-x emerge-buffers-with-ancestor
@findex emerge-buffers-with-ancestor
Merge two buffers with reference to a common ancestor in a third
buffer.
@end table

@cindex merge buffer (Emerge)
@cindex A and B buffers (Emerge)
  The Emerge commands compare two files or buffers, and display the
comparison in three buffers: one for each input text (the @dfn{A buffer}
and the @dfn{B buffer}), and one (the @dfn{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.
@xref{Narrowing,,, emacs, the Emacs Manual}.


  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
@samp{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
@dfn{merge commands} in the merge buffer (@pxref{Merge Commands}).
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 @dfn{selected} difference.  This
difference is marked off in the three buffers like this:

@example
vvvvvvvvvvvvvvvvvvvv
@var{text that differs}
^^^^^^^^^^^^^^^^^^^^
@end example

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

@node Submodes of Emerge
@section Submodes of Emerge

  You can choose between two modes for giving merge commands: Fast mode
and Edit mode.  In Fast mode, basic merge commands are single
characters, but ordinary Emacs commands are disabled.  This is
convenient if you use only merge commands.  In Edit mode, all merge
commands start with the prefix key @kbd{C-c C-c}, and the normal Emacs
commands are also available.  This allows editing the merge buffer, but
slows down Emerge operations.

  Use @kbd{e} to switch to Edit mode, and @kbd{C-c C-c f} to switch to
Fast mode.  The mode line indicates Edit and Fast modes with @samp{E}
and @samp{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 @kbd{a} and @kbd{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 @samp{A}.

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

@findex emerge-auto-advance-mode
@findex emerge-skip-prefers-mode
  Use the command @kbd{s a} (@code{emerge-auto-advance-mode}) to set or
clear Auto Advance mode.  Use @kbd{s s}
(@code{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.

@node State of Difference
@section State of a Difference

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

@table @asis
@item A
The difference is showing the A version.  The @kbd{a} command always
produces this state; the mode line indicates it with @samp{A}.

@item B
The difference is showing the B version.  The @kbd{b} command always
produces this state; the mode line indicates it with @samp{B}.

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

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

The command @kbd{d a} chooses default-A as the default state, and @kbd{d
b} chooses default-B.  This chosen default applies to all differences
which you haven't ever selected and for which no alternative is preferred.
If you are moving through the merge sequentially, the differences you
haven't selected are those following the selected one.  Thus, while
moving sequentially, you can effectively make the A version the default
for some sections of the merge buffer and the B version the default for
others by using @kbd{d a} and @kbd{d b} between sections.

@item prefer-A
@itemx prefer-B
The difference is showing the A or B state because it is
@dfn{preferred}.  This means that you haven't made an explicit choice,
but one alternative seems likely to be right because the other
alternative agrees with the common ancestor.  Thus, where the A buffer
agrees with the common ancestor, the B version is preferred, because
chances are it is the one that was actually changed.

These two states are displayed in the mode line as @samp{A*} and @samp{B*}.

@item combined
The difference is showing a combination of the A and B states, as a
result of the @kbd{x c} or @kbd{x C} commands.

Once a difference is in this state, the @kbd{a} and @kbd{b} commands
don't do anything to it unless you give them a numeric argument.

The mode line displays this state as @samp{comb}.
@end table

@node Merge Commands
@section Merge Commands

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

@table @kbd
@item p
Select the previous difference.

@item n
Select the next difference.

@item a
Choose the A version of this difference.

@item b
Choose the B version of this difference.

@item C-u @var{n} j
Select difference number @var{n}.

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

@item q
Quit---finish the merge.

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

@item f
Go into Fast mode.  (In Edit mode, this is actually @kbd{C-c C-c f}.)

@item e
Go into Edit mode.

@item l
Recenter (like @kbd{C-l}) all three windows.

@item -
Specify part of a prefix numeric argument.

@item @var{digit}
Also specify part of a prefix numeric argument.

@item d a
Choose the A version as the default from here down in
the merge buffer.

@item d b
Choose the B version as the default from here down in
the merge buffer.

@item c a
Copy the A version of this difference into the kill ring.

@item c b
Copy the B version of this difference into the kill ring.

@item i a
Insert the A version of this difference at point.

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

@item m
Put point and mark around the difference.

@item ^
Scroll all three windows down (like @kbd{M-v}).

@item v
Scroll all three windows up (like @kbd{C-v}).

@item <
Scroll all three windows left (like @kbd{C-x <}).

@item >
Scroll all three windows right (like @kbd{C-x >}).

@item |
Reset horizontal scroll on all three windows.

@item x 1
Shrink the merge window to one line.  (Use @kbd{C-u l} to restore it
to full size.)

@item x c
Combine the two versions of this difference (@pxref{Combining in
Emerge}).

@item x f
Show the names of the files/buffers Emerge is operating on, in a Help
window.  (Use @kbd{C-u l} to restore windows.)

@item x j
Join this difference with the following one.
(@kbd{C-u x j} joins this difference with the previous one.)

@item x s
Split this difference into two differences.  Before you use this
command, position point in each of the three buffers at the place where
you want to split the difference.

@item x t
Trim identical lines off the top and bottom of the difference.
Such lines occur when the A and B versions are
identical but differ from the ancestor version.
@end table

@node Exiting Emerge
@section Exiting Emerge

  The @kbd{q} command (@code{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.

  @kbd{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 @code{t} for successful completion, or @code{nil} if you
abort.

@node Combining in Emerge
@section Combining the Two Versions

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

@example
@group
#ifdef NEW
@var{version from A buffer}
#else /* not NEW */
@var{version from B buffer}
#endif /* not NEW */
@end group
@end example

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

@example
@group
"#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"
@end group
@end example

@node Fine Points of Emerge
@section Fine Points of Emerge

  During the merge, you mustn't try to edit the A and B buffers yourself.
Emerge modifies them temporarily, but ultimately puts them back the way
they were.

  You can have any number of merges going at once---just don't use any one
buffer as input to more than one merge at once, since the temporary
changes made in these buffers would get in each other's way.

  Starting Emerge can take a long time because it needs to compare the
files fully.  Emacs can't do anything else until @code{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.

@vindex emerge-startup-hook
  After setting up the merge, Emerge runs the hook
@code{emerge-startup-hook}.  @xref{Hooks,,, emacs, the Emacs Manual}.

@node Picture Mode
@chapter Editing Pictures
@cindex pictures
@cindex making pictures out of text characters
@findex edit-picture

  To edit a picture made out of text characters (for example, a picture
of the division of a register into fields, as a comment in a program),
use the command @kbd{M-x edit-picture} to enter Picture mode.

  In Picture mode, editing is based on the @dfn{quarter-plane} model of
text, according to which the text characters lie studded on an area that
stretches infinitely far to the right and downward.  The concept of the end
of a line does not exist in this model; the most you can say is where the
last nonblank character on the line is found.

  Of course, Emacs really always considers text as a sequence of
characters, and lines really do have ends.  But Picture mode replaces
the most frequently-used commands with variants that simulate the
quarter-plane model of text.  They do this by inserting spaces or by
converting tabs to spaces.

  Most of the basic editing commands of Emacs are redefined by Picture mode
to do essentially the same thing but in a quarter-plane way.  In addition,
Picture mode defines various keys starting with the @kbd{C-c} prefix to
run special picture editing commands.

  One of these keys, @kbd{C-c C-c}, is particularly important.  Often a
picture is part of a larger file that is usually edited in some other
major mode.  @kbd{M-x edit-picture} records the name of the previous
major mode so you can use the @kbd{C-c C-c} command
(@code{picture-mode-exit}) later to go back to that mode.  @kbd{C-c C-c}
also deletes spaces from the ends of lines, unless given a numeric
argument.

  The special commands of Picture mode all work in other modes (provided
the @file{picture} library is loaded), but are not bound to keys except
in Picture mode.  The descriptions below talk of moving ``one column''
and so on, but all the picture mode commands handle numeric arguments as
their normal equivalents do.

@vindex picture-mode-hook
  Turning on Picture mode runs the hook @code{picture-mode-hook}.
Additional extensions to Picture mode can be found in
@file{artist.el}.

@menu
* Basic Picture::         Basic concepts and simple commands of Picture Mode.
* Insert in Picture::     Controlling direction of cursor motion
                            after "self-inserting" characters.
* Tabs in Picture::       Various features for tab stops and indentation.
* Rectangles in Picture:: Clearing and superimposing rectangles.
@end menu

@node Basic Picture
@section Basic Editing in Picture Mode

@findex picture-forward-column
@findex picture-backward-column
@findex picture-move-down
@findex picture-move-up
@cindex editing in Picture mode

  Most keys do the same thing in Picture mode that they usually do, but
do it in a quarter-plane style.  For example, @kbd{C-f} is rebound to
run @code{picture-forward-column}, a command which moves point one
column to the right, inserting a space if necessary so that the actual
end of the line makes no difference.  @kbd{C-b} is rebound to run
@code{picture-backward-column}, which always moves point left one
column, converting a tab to multiple spaces if necessary.  @kbd{C-n} and
@kbd{C-p} are rebound to run @code{picture-move-down} and
@code{picture-move-up}, which can either insert spaces or convert tabs
as necessary to make sure that point stays in exactly the same column.
@kbd{C-e} runs @code{picture-end-of-line}, which moves to after the last
nonblank character on the line.  There is no need to change @kbd{C-a},
as the choice of screen model does not affect beginnings of
lines.

@findex picture-newline
  Insertion of text is adapted to the quarter-plane screen model
through the use of Overwrite mode (@pxref{Minor Modes,,, emacs, the
Emacs Manual}.)  Self-inserting characters replace existing text,
column by column, rather than pushing existing text to the right.
@key{RET} runs @code{picture-newline}, which just moves to the
beginning of the following line so that new text will replace that
line.

@findex picture-backward-clear-column
@findex picture-clear-column
@findex picture-clear-line
  In Picture mode, the commands that normally delete or kill text,
instead erase text (replacing it with spaces).  @key{DEL}
(@code{picture-backward-clear-column}) replaces the preceding
character with a space rather than removing it; this moves point
backwards.  @kbd{C-d} (@code{picture-clear-column}) replaces the next
character or characters with spaces, but does not move point.  (If you
want to clear characters to spaces and move forward over them, use
@key{SPC}.)  @kbd{C-k} (@code{picture-clear-line}) really kills the
contents of lines, but does not delete the newlines from the buffer.

@findex picture-open-line
  To do actual insertion, you must use special commands.  @kbd{C-o}
(@code{picture-open-line}) creates a blank line after the current
line; it never splits a line.  @kbd{C-M-o} (@code{split-line}) makes
sense in Picture mode, so it is not changed.  @kbd{C-j}
(@code{picture-duplicate-line}) inserts another line with the same
contents below the current line.

@kindex C-c C-d @r{(Picture mode)}
   To do actual deletion in Picture mode, use @kbd{C-w}, @kbd{C-c C-d}
(which is defined as @code{delete-char}, as @kbd{C-d} is in other
modes), or one of the picture rectangle commands (@pxref{Rectangles in
Picture}).

@node Insert in Picture
@section Controlling Motion after Insert

@findex picture-movement-up
@findex picture-movement-down
@findex picture-movement-left
@findex picture-movement-right
@findex picture-movement-nw
@findex picture-movement-ne
@findex picture-movement-sw
@findex picture-movement-se
@kindex C-c < @r{(Picture mode)}
@kindex C-c > @r{(Picture mode)}
@kindex C-c ^ @r{(Picture mode)}
@kindex C-c . @r{(Picture mode)}
@kindex C-c ` @r{(Picture mode)}
@kindex C-c ' @r{(Picture mode)}
@kindex C-c / @r{(Picture mode)}
@kindex C-c \ @r{(Picture mode)}
  Since ``self-inserting'' characters in Picture mode overwrite and move
point, there is no essential restriction on how point should be moved.
Normally point moves right, but you can specify any of the eight
orthogonal or diagonal directions for motion after a ``self-inserting''
character.  This is useful for drawing lines in the buffer.

@table @kbd
@item C-c <
@itemx C-c @key{LEFT}
Move left after insertion (@code{picture-movement-left}).
@item C-c >
@itemx C-c @key{RIGHT}
Move right after insertion (@code{picture-movement-right}).
@item C-c ^
@itemx C-c @key{UP}
Move up after insertion (@code{picture-movement-up}).
@item C-c .
@itemx C-c @key{DOWN}
Move down after insertion (@code{picture-movement-down}).
@item C-c `
@itemx C-c @key{HOME}
Move up and left (``northwest'') after insertion (@code{picture-movement-nw}).
@item C-c '
@itemx C-c @key{PAGEUP}
Move up and right (``northeast'') after insertion
(@code{picture-movement-ne}).
@item C-c /
@itemx C-c @key{END}
Move down and left (``southwest'') after insertion
@*(@code{picture-movement-sw}).
@item C-c \
@itemx C-c @key{PAGEDOWN}
Move down and right (``southeast'') after insertion
@*(@code{picture-movement-se}).
@end table

@kindex C-c C-f @r{(Picture mode)}
@kindex C-c C-b @r{(Picture mode)}
@findex picture-motion
@findex picture-motion-reverse
  Two motion commands move based on the current Picture insertion
direction.  The command @kbd{C-c C-f} (@code{picture-motion}) moves in the
same direction as motion after ``insertion'' currently does, while @kbd{C-c
C-b} (@code{picture-motion-reverse}) moves in the opposite direction.

@node Tabs in Picture
@section Picture Mode Tabs

@kindex M-TAB @r{(Picture mode)}
@findex picture-tab-search
@vindex picture-tab-chars
  Two kinds of tab-like action are provided in Picture mode.  Use
@kbd{M-@key{TAB}} (@code{picture-tab-search}) for context-based tabbing.
With no argument, it moves to a point underneath the next
``interesting'' character that follows whitespace in the previous
nonblank line.  ``Next'' here means ``appearing at a horizontal position
greater than the one point starts out at.''  With an argument, as in
@kbd{C-u M-@key{TAB}}, this command moves to the next such interesting
character in the current line.  @kbd{M-@key{TAB}} does not change the
text; it only moves point.  ``Interesting'' characters are defined by
the variable @code{picture-tab-chars}, which should define a set of
characters.  The syntax for this variable is like the syntax used inside
of @samp{[@dots{}]} in a regular expression---but without the @samp{[}
and the @samp{]}.  Its default value is @code{"!-~"}.

@findex picture-tab
  @key{TAB} itself runs @code{picture-tab}, which operates based on the
current tab stop settings; it is the Picture mode equivalent of
@code{tab-to-tab-stop}.  Normally it just moves point, but with a numeric
argument it clears the text that it moves over.

@kindex C-c TAB @r{(Picture mode)}
@findex picture-set-tab-stops
  The context-based and tab-stop-based forms of tabbing are brought
together by the command @kbd{C-c @key{TAB}} (@code{picture-set-tab-stops}).
This command sets the tab stops to the positions which @kbd{M-@key{TAB}}
would consider significant in the current line.  The use of this command,
together with @key{TAB}, can get the effect of context-based tabbing.  But
@kbd{M-@key{TAB}} is more convenient in the cases where it is sufficient.

  It may be convenient to prevent use of actual tab characters in
pictures.  For example, this prevents @kbd{C-x @key{TAB}} from messing
up the picture.  You can do this by setting the variable
@code{indent-tabs-mode} to @code{nil}.

@node Rectangles in Picture
@section Picture Mode Rectangle Commands
@cindex rectangles and Picture mode
@cindex Picture mode and rectangles

  Picture mode defines commands for working on rectangular pieces of
the text in ways that fit with the quarter-plane model.  The standard
rectangle commands may also be useful.  @xref{Rectangles,,, emacs, the
Emacs Manual}.

@table @kbd
@item C-c C-k
Clear out the region-rectangle with spaces
(@code{picture-clear-rectangle}).  With argument, delete the text.
@item C-c C-w @var{r}
Similar, but save rectangle contents in register @var{r} first
(@code{picture-clear-rectangle-to-register}).
@item C-c C-y
Copy last killed rectangle into the buffer by overwriting, with upper
left corner at point (@code{picture-yank-rectangle}).  With argument,
insert instead.
@item C-c C-x @var{r}
Similar, but use the rectangle in register @var{r}
(@code{picture-yank-rectangle-from-register}).
@end table

@kindex C-c C-k @r{(Picture mode)}
@kindex C-c C-w @r{(Picture mode)}
@findex picture-clear-rectangle
@findex picture-clear-rectangle-to-register
  The picture rectangle commands @kbd{C-c C-k}
(@code{picture-clear-rectangle}) and @kbd{C-c C-w}
(@code{picture-clear-rectangle-to-register}) differ from the standard
rectangle commands in that they normally clear the rectangle instead of
deleting it; this is analogous with the way @kbd{C-d} is changed in Picture
mode.

  However, deletion of rectangles can be useful in Picture mode, so
these commands delete the rectangle if given a numeric argument.
@kbd{C-c C-k} either with or without a numeric argument saves the
rectangle for @kbd{C-c C-y}.

@kindex C-c C-y @r{(Picture mode)}
@kindex C-c C-x @r{(Picture mode)}
@findex picture-yank-rectangle
@findex picture-yank-rectangle-from-register
  The Picture mode commands for yanking rectangles differ from the
standard ones in that they overwrite instead of inserting.  This is
the same way that Picture mode insertion of other text differs from
other modes.  @kbd{C-c C-y} (@code{picture-yank-rectangle}) inserts
(by overwriting) the rectangle that was most recently killed, while
@kbd{C-c C-x} (@code{picture-yank-rectangle-from-register}) does
likewise for the rectangle found in a specified register.

@node Fortran
@chapter Fortran Mode
@cindex Fortran mode
@cindex mode, Fortran

  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 support for 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 @kbd{M-x fortran-mode} to switch to this major mode.  This
command runs the hook @code{fortran-mode-hook}.  @xref{Hooks,,, emacs,
the Emacs Manual}.

@cindex Fortran77 and Fortran90
@findex f90-mode
@findex fortran-mode
  Fortran mode is meant for editing Fortran77 ``fixed format'' (and also
``tab format'') source code.  For editing the modern Fortran90 or
Fortran95 ``free format'' source code, use F90 mode (@code{f90-mode}).
Emacs normally uses Fortran mode for files with extension @samp{.f},
@samp{.F} or @samp{.for}, and F90 mode for the extension @samp{.f90} and
@samp{.f95}.  GNU Fortran supports both kinds of format.

@menu
* Motion: Fortran Motion.	 Moving point by statements or subprograms.
* Indent: Fortran Indent.	 Indentation commands for Fortran.
* Comments: Fortran Comments.	 Inserting and aligning comments.
* Autofill: Fortran Autofill.	 Auto fill support for Fortran.
* Columns: Fortran Columns.	 Measuring columns for valid Fortran.
* Abbrev: Fortran Abbrev.	 Built-in abbrevs for Fortran keywords.
@end menu

@node Fortran Motion
@section Motion Commands

  In addition to the normal commands for moving by and operating on
``defuns'' (Fortran subprograms---functions and subroutines, as well as
modules for F90 mode), Fortran mode provides special commands to move by
statements and other program units.

@table @kbd
@kindex C-c C-n @r{(Fortran mode)}
@findex fortran-next-statement
@findex f90-next-statement
@item C-c C-n
Move to the beginning of the next statement
(@code{fortran-next-statement}/@code{f90-next-statement}).

@kindex C-c C-p @r{(Fortran mode)}
@findex fortran-previous-statement
@findex f90-previous-statement
@item C-c C-p
Move to the beginning of the previous statement
(@code{fortran-previous-statement}/@code{f90-previous-statement}).
If there is no previous statement (i.e. if called from the first
statement in the buffer), move to the start of the buffer.

@kindex C-c C-e @r{(F90 mode)}
@findex f90-next-block
@item C-c C-e
Move point forward to the start of the next code block
(@code{f90-next-block}).  A code block is a subroutine,
@code{if}--@code{endif} statement, and so forth.  This command exists
for F90 mode only, not Fortran mode.  With a numeric argument, this
moves forward that many blocks.

@kindex C-c C-a @r{(F90 mode)}
@findex f90-previous-block
@item C-c C-a
Move point backward to the previous code block
(@code{f90-previous-block}).  This is like @code{f90-next-block}, but
moves backwards.

@kindex C-M-n @r{(Fortran mode)}
@findex fortran-end-of-block
@findex f90-end-of-block
@item C-M-n
Move to the end of the current code block
(@code{fortran-end-of-block}/@code{f90-end-of-block}).  With a numeric
agument, move forward that number of blocks.  The mark is set before
moving point.  The F90 mode version of this command checks for
consistency of block types and labels (if present), but it does not
check the outermost block since that may be incomplete.

@kindex C-M-p @r{(Fortran mode)}
@findex fortran-beginning-of-block
@findex f90-beginning-of-block
@item C-M-p
Move to the start of the current code block
(@code{fortran-beginning-of-block}/@code{f90-beginning-of-block}). This
is like @code{fortran-end-of-block}, but moves backwards.
@end table

@node Fortran Indent
@section Fortran Indentation

  Special commands and features are needed for indenting Fortran code in
order to make sure various syntactic entities (line numbers, comment line
indicators and continuation line flags) appear in the columns that are
required for standard, fixed (or tab) format Fortran.

@menu
* Commands: ForIndent Commands.  Commands for indenting and filling Fortran.
* Contline: ForIndent Cont.      How continuation lines indent.
* Numbers:  ForIndent Num.       How line numbers auto-indent.
* Conv:     ForIndent Conv.      Conventions you must obey to avoid trouble.
* Vars:     ForIndent Vars.      Variables controlling Fortran indent style.
@end menu

@node ForIndent Commands
@subsection Fortran Indentation and Filling Commands

@table @kbd
@item C-M-j
Break the current line at point and set up a continuation line
(@code{fortran-split-line}).
@item M-^
Join this line to the previous line (@code{fortran-join-line}).
@item C-M-q
Indent all the lines of the subprogram point is in
(@code{fortran-indent-subprogram}).
@item M-q
Fill a comment block or statement.
@end table

@kindex C-M-q @r{(Fortran mode)}
@findex fortran-indent-subprogram
  The key @kbd{C-M-q} runs @code{fortran-indent-subprogram}, a command
to reindent all the lines of the Fortran subprogram (function or
subroutine) containing point.

@kindex C-M-j @r{(Fortran mode)}
@findex fortran-split-line
  The key @kbd{C-M-j} runs @code{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.

@kindex M-^ @r{(Fortran mode)}
@kindex C-c C-d @r{(Fortran mode)}
@findex fortran-join-line
  @kbd{M-^} or @kbd{C-c C-d} runs the command @code{fortran-join-line},
which joins a continuation line back to the previous line, roughly as
the inverse of @code{fortran-split-line}.  The point must be on a
continuation line when this command is invoked.

@kindex M-q @r{(Fortran mode)}
@kbd{M-q} in Fortran mode fills the comment block or statement that
point is in.  This removes any excess statement continuations.

@node ForIndent Cont
@subsection Continuation Lines
@cindex Fortran continuation lines

@vindex fortran-continuation-string
  Most Fortran77 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 @dfn{fixed
format}.  (In GNU Emacs we always count columns from 0; but note that
the Fortran standard counts from 1.)  The variable
@code{fortran-continuation-string} specifies what character to put in
column 5.  A line that starts with a tab character followed by any digit
except @samp{0} is also a continuation line.  We call this style of
continuation @dfn{tab format}.  (Fortran90 introduced ``free format'',
with another style of continuation lines).

@vindex indent-tabs-mode @r{(Fortran mode)}
@vindex fortran-analyze-depth
@vindex fortran-tab-mode-default
  Fortran mode can use either style of continuation line.  When you
enter Fortran mode, it tries to deduce the proper continuation style
automatically from the buffer contents.  It does this by scanning up to
@code{fortran-analyze-depth} (default 100) lines from the start of the
buffer.  The first line that begins with either a tab character or six
spaces determines the choice.  If the scan fails (for example, if the
buffer is new and therefore empty), the value of
@code{fortran-tab-mode-default} (@code{nil} for fixed format, and
non-@code{nil} for tab format) is used.  @samp{/t} in the mode line
indicates tab format is selected.  Fortran mode sets the value of
@code{indent-tabs-mode} accordingly.

  If the text on a line starts with the Fortran continuation marker
@samp{$}, 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 @key{TAB}, it converts the line to the current
continuation style.  When you split a Fortran statement with
@kbd{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.

@node ForIndent Num
@subsection Line Numbers

  If a number is the first non-whitespace in the line, Fortran
indentation assumes it is a line number and moves it to columns 0
through 4.  (Columns always count from 0 in GNU Emacs.)

@vindex fortran-line-number-indent
  Line numbers of four digits or less are normally indented one space.
The variable @code{fortran-line-number-indent} controls this; it
specifies the maximum indentation a line number can have.  The default
value of the variable is 1.  Fortran mode tries to prevent line number
digits passing column 4, reducing the indentation below the specified
maximum if necessary.  If @code{fortran-line-number-indent} has the
value 5, line numbers are right-justified to end in column 4.

@vindex fortran-electric-line-number
  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
@code{fortran-electric-line-number} to @code{nil}.


@node ForIndent Conv
@subsection 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:

@itemize @bullet
@item
Two nested @samp{do} loops never share a @samp{continue} statement.

@item
Fortran keywords such as @samp{if}, @samp{else}, @samp{then}, @samp{do}
and others are written without embedded whitespace or line breaks.

Fortran compilers generally ignore whitespace outside of string
constants, but Fortran mode does not recognize these keywords if they
are not contiguous.  Constructs such as @samp{else if} or @samp{end do}
are acceptable, but the second word should be on the same line as the
first and not on a continuation line.
@end itemize

@noindent
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.

@node ForIndent Vars
@subsection Variables for Fortran Indentation

@vindex fortran-do-indent
@vindex fortran-if-indent
@vindex fortran-structure-indent
@vindex fortran-continuation-indent
@vindex fortran-check-all-num@dots{}
@vindex fortran-minimum-statement-indent@dots{}
  Several additional variables control how Fortran indentation works:

@table @code
@item fortran-do-indent
Extra indentation within each level of @samp{do} statement (default 3).

@item fortran-if-indent
Extra indentation within each level of @samp{if}, @samp{select case}, or
@samp{where} statements (default 3).

@item fortran-structure-indent
Extra indentation within each level of @samp{structure}, @samp{union},
@samp{map}, or @samp{interface} statements (default 3).

@item fortran-continuation-indent
Extra indentation for bodies of continuation lines (default 5).

@item fortran-check-all-num-for-matching-do
In Fortran77, a numbered @samp{do} statement is ended by any statement
with a matching line number.  It is common (but not compulsory) to use a
@samp{continue} statement for this purpose.  If this variable has a
non-@code{nil} value, indenting any numbered statement must check for a
@samp{do} that ends there.  If you always end @samp{do} statements with
a @samp{continue} line (or if you use the more modern @samp{enddo}),
then you can speed up indentation by setting this variable to
@code{nil}.  The default is @code{nil}.

@item fortran-blink-matching-if
If this is @code{t}, indenting an @samp{endif} (or @samp{enddo}
statement moves the cursor momentarily to the matching @samp{if} (or
@samp{do}) statement to show where it is.  The default is @code{nil}.

@item fortran-minimum-statement-indent-fixed
Minimum indentation for Fortran statements when using fixed format
continuation line style.  Statement bodies are never indented less than
this much.  The default is 6.

@item fortran-minimum-statement-indent-tab
Minimum indentation for Fortran statements for tab format continuation line
style.  Statement bodies are never indented less than this much.  The
default is 8.
@end table

The variables controlling the indentation of comments are described in
the following section.

@node Fortran Comments
@section Fortran Comments

  The usual Emacs comment commands assume that a comment can follow a
line of code.  In Fortran77, 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.

@vindex fortran-comment-line-start
  Fortran mode can also handle the Fortran90 comment syntax where comments
start with @samp{!} and can follow other text.  Because only some Fortran77
compilers accept this syntax, Fortran mode will not insert such comments
unless you have said in advance to do so.  To do this, set the variable
@code{fortran-comment-line-start} to @samp{"!"}.

@table @kbd
@item M-;
Align comment or insert new comment (@code{fortran-indent-comment}).

@item C-x ;
Applies to nonstandard @samp{!} comments only.

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

@findex fortran-indent-comment
  @kbd{M-;} in Fortran mode is redefined as the command
@code{fortran-indent-comment}.  Like the usual @kbd{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 @samp{!}
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 @samp{!} 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
@code{fortran-comment-indent-style} to one of these values:

@vindex fortran-comment-indent-style
@vindex fortran-comment-line-extra-indent
@table @code
@item fixed
Align the text at a fixed column, which is the sum of
@code{fortran-comment-line-extra-indent} and the minimum statement
indentation.  This is the default.

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

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

@item nil
Don't move text in full-line comments automatically.
@end table

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

@vindex fortran-directive-re
  Compiler directive lines, or preprocessor lines, have much the same
appearance as comment lines.  It is important, though, that such lines
never be indented at all, no matter what the value of
@code{fortran-comment-indent-style}.  The variable
@code{fortran-directive-re} is a regular expression that specifies which
lines are directives.  Matching lines are never indented, and receive
distinctive font-locking.

  The normal Emacs comment command @kbd{C-x ;} has not been redefined.  If
you use @samp{!} comments, this command can be used with them.  Otherwise
it is useless in Fortran mode.

@kindex C-c ; @r{(Fortran mode)}
@findex fortran-comment-region
@vindex fortran-comment-region
  The command @kbd{C-c ;} (@code{fortran-comment-region}) turns all the
lines of the region into comments by inserting the string @samp{C$$$} at
the front of each one.  With a numeric argument, it turns the region
back into live code by deleting @samp{C$$$} from the front of each line
in it.  The string used for these comments can be controlled by setting
the variable @code{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.

@node Fortran Autofill
@section Auto Fill in Fortran Mode

  Fortran mode has specialized support for Auto Fill mode, which is a
minor mode that automatically splits statements as you insert them
when they become too wide.  Splitting a statement involves making
continuation lines using @code{fortran-continuation-string}
(@pxref{ForIndent Cont}).  This splitting happens when you type
@key{SPC}, @key{RET}, or @key{TAB}, and also in the Fortran
indentation commands.  You activate Auto Fill in Fortran mode in the
normal way.  @xref{Auto Fill,,, emacs, the Emacs Manual}.

@vindex fortran-break-before-delimiters
   Auto Fill breaks lines at spaces or delimiters when the lines get
longer than the desired width (the value of @code{fill-column}).  The
delimiters (besides whitespace) that Auto Fill can break at are
@samp{+}, @samp{-}, @samp{/}, @samp{*}, @samp{=}, @samp{<}, @samp{>},
and @samp{,}.  The line break comes after the delimiter if the
variable @code{fortran-break-before-delimiters} is @code{nil}.
Otherwise (and by default), the break comes before the delimiter.

  To enable Auto Fill in all Fortran buffers, add
@code{turn-on-auto-fill} to @code{fortran-mode-hook}.  @xref{Hooks,,,
emacs, the Emacs Manual}.

@node Fortran Columns
@section Checking Columns in Fortran

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

@kindex C-c C-r @r{(Fortran mode)}
@findex fortran-column-ruler
  The command @kbd{C-c C-r} (@code{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.

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

@kindex C-c C-w @r{(Fortran mode)}
@findex fortran-window-create-momentarily
  @kbd{C-c C-w} (@code{fortran-window-create-momentarily}) temporarily
splits the current window horizontally, making a window 72 columns
wide, so you can see any lines that are too long.  Type a space to
restore the normal width.

@kindex C-u C-c C-w @r{(Fortran mode)}
@findex fortran-window-create
  You can also split the window horizontally and continue editing with
the split in place.  To do this, use @kbd{C-u C-c C-w} (@code{M-x
fortran-window-create}).  By editing in this window you can
immediately see when you make a line too wide to be correct Fortran.

@findex fortran-strip-sequence-nos
  The command @kbd{M-x fortran-strip-sequence-nos} deletes all text in
column 72 and beyond, on all lines in the current buffer.  This is the
easiest way to get rid of old sequence numbers.

@node Fortran Abbrev
@section Fortran Keyword Abbrevs

  Fortran mode provides many built-in abbrevs for common keywords and
declarations.  These are the same sort of abbrev that you can define
yourself.  To use them, you must turn on Abbrev mode.
@xref{Abbrevs,,, emacs, the Emacs Manual}.

  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 @samp{;c} for
@samp{continue}.  If you insert @samp{;c} and then insert a punctuation
character such as a space or a newline, the @samp{;c} expands automatically
to @samp{continue}, provided Abbrev mode is enabled.@refill

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


@node MS-DOG
@chapter Emacs and MS-DOS
@cindex MS-DOG
@cindex MS-DOS peculiarities

  This section briefly describes the peculiarities of using Emacs on
the MS-DOS ``operating system'' (also known as ``MS-DOG'').
Information about Emacs and Microsoft's current operating system
Windows (also known as ``Losedows) is in the main Emacs manual
(@pxref{Emacs and Microsoft Systems,,, emacs, the Emacs Manual}).

  If you build Emacs for MS-DOS, the binary will also run on Windows
3.X, Windows NT, Windows 9X/ME, Windows 2000, or OS/2 as a DOS
application; all of this chapter applies for all of those systems, if
you use an Emacs that was built for MS-DOS.

  @xref{Text and Binary,,,emacs, the Emacs Manual}, for information
about Emacs' special handling of text files under MS-DOS (and
Windows).

@menu
* Keyboard: MS-DOS Keyboard.   Keyboard conventions on MS-DOS.
* Mouse: MS-DOS Mouse.         Mouse conventions on MS-DOS.
* Display: MS-DOS Display.     Fonts, frames and display size on MS-DOS.
* Files: MS-DOS File Names.    File name conventions on MS-DOS.
* Printing: MS-DOS Printing.   How to specify the printer on MS-DOS.
* I18N: MS-DOS and MULE.       Support for internationalization on MS-DOS.
* Processes: MS-DOS Processes. Running subprocesses on MS-DOS.
@end menu

@node MS-DOS Keyboard
@section Keyboard Usage on MS-DOS

@kindex DEL @r{(MS-DOS)}
@kindex BS @r{(MS-DOS)}
  The key that is called @key{DEL} in Emacs (because that's how it is
designated on most workstations) is known as @key{BS} (backspace) on a
PC.  That is why the PC-specific terminal initialization remaps the
@key{BS} key to act as @key{DEL}; the @key{DELETE} key is remapped to act
as @kbd{C-d} for the same reasons.

@kindex C-g @r{(MS-DOS)}
@kindex C-BREAK @r{(MS-DOS)}
@cindex quitting on MS-DOS
  Emacs built for MS-DOS recognizes @kbd{C-@key{BREAK}} as a quit
character, just like @kbd{C-g}.  This is because Emacs cannot detect
that you have typed @kbd{C-g} until it is ready for more input.  As a
consequence, you cannot use @kbd{C-g} to stop a running command
(@pxref{Quitting,,,emacs, the Emacs Manual}).  By contrast,
@kbd{C-@key{BREAK}} @emph{is} detected as soon as you type it (as
@kbd{C-g} is on other systems), so it can be used to stop a running
command and for emergency escape (@pxref{Emergency Escape,,,emacs, the
Emacs Manual}).

@cindex Meta (under MS-DOS)
@cindex Hyper (under MS-DOS)
@cindex Super (under MS-DOS)
@vindex dos-super-key
@vindex dos-hyper-key
  The PC keyboard maps use the left @key{ALT} key as the @key{META} key.
You have two choices for emulating the @key{SUPER} and @key{HYPER} keys:
choose either the right @key{CTRL} key or the right @key{ALT} key by
setting the variables @code{dos-hyper-key} and @code{dos-super-key} to 1
or 2 respectively.  If neither @code{dos-super-key} nor
@code{dos-hyper-key} is 1, then by default the right @key{ALT} key is
also mapped to the @key{META} key.  However, if the MS-DOS international
keyboard support program @file{KEYB.COM} is installed, Emacs will
@emph{not} map the right @key{ALT} to @key{META}, since it is used for
accessing characters like @kbd{~} and @kbd{@@} on non-US keyboard
layouts; in this case, you may only use the left @key{ALT} as @key{META}
key.

@kindex C-j @r{(MS-DOS)}
@vindex dos-keypad-mode
  The variable @code{dos-keypad-mode} is a flag variable that controls
what key codes are returned by keys in the numeric keypad.  You can also
define the keypad @key{ENTER} key to act like @kbd{C-j}, by putting the
following line into your @file{_emacs} file:

@smallexample
;; @r{Make the @key{ENTER} key from the numeric keypad act as @kbd{C-j}.}
(define-key function-key-map [kp-enter] [?\C-j])
@end smallexample

@node MS-DOS Mouse
@section Mouse Usage on MS-DOS

@cindex mouse support under MS-DOS
  Emacs on MS-DOS supports a mouse (on the default terminal only).
The mouse commands work as documented, including those that use menus
and the menu bar (@pxref{Menu Bar,,,emacs, the Emacs Manual}).  Scroll
bars don't work in MS-DOS Emacs.  PC mice usually have only two
buttons; these act as @kbd{Mouse-1} and @kbd{Mouse-2}, but if you
press both of them together, that has the effect of @kbd{Mouse-3}.  If
the mouse does have 3 buttons, Emacs detects that at startup, and all
the 3 buttons function normally, as on X.

  Help strings for menu-bar and pop-up menus are displayed in the echo
area when the mouse pointer moves across the menu items.  Highlighting
of mouse-sensitive text (@pxref{Mouse References,,,emacs, the Emacs
Manual}) is also supported.

@cindex mouse, set number of buttons
@findex msdos-set-mouse-buttons
  Some versions of mouse drivers don't report the number of mouse
buttons correctly.  For example, mice with a wheel report that they
have 3 buttons, but only 2 of them are passed to Emacs; the clicks on
the wheel, which serves as the middle button, are not passed.  In
these cases, you can use the @kbd{M-x msdos-set-mouse-buttons} command
to tell Emacs how many mouse buttons to expect.  You could make such a
setting permanent by adding this fragment to your @file{_emacs} init
file:

@example
;; @r{Treat the mouse like a 2-button mouse.}
(msdos-set-mouse-buttons 2)
@end example

@cindex Windows clipboard support
  Emacs built for MS-DOS supports clipboard operations when it runs on
Windows.  Commands that put text on the kill ring, or yank text from
the ring, check the Windows clipboard first, just as Emacs does on the
X Window System (@pxref{Mouse Commands,,,emacs, the Emacs Manual}).
Only the primary selection and the cut buffer are supported by MS-DOS
Emacs on Windows; the secondary selection always appears as empty.

  Due to the way clipboard access is implemented by Windows, the
length of text you can put into the clipboard is limited by the amount
of free DOS memory that is available to Emacs.  Usually, up to 620KB of
text can be put into the clipboard, but this limit depends on the system
configuration and is lower if you run Emacs as a subprocess of
another program.  If the killed text does not fit, Emacs outputs a
message saying so, and does not put the text into the clipboard.

  Null characters also cannot be put into the Windows clipboard.  If the
killed text includes null characters, Emacs does not put such text into
the clipboard, and displays in the echo area a message to that effect.

@vindex dos-display-scancodes
  The variable @code{dos-display-scancodes}, when non-@code{nil},
directs Emacs to display the @acronym{ASCII} value and the keyboard scan code of
each keystroke; this feature serves as a complement to the
@code{view-lossage} command, for debugging.

@node MS-DOS Display
@section Display on MS-DOS
@cindex faces under MS-DOS
@cindex fonts, emulating under MS-DOS

  Display on MS-DOS cannot use font variants, like bold or italic, but
it does support multiple faces, each of which can specify a foreground
and a background color.  Therefore, you can get the full functionality
of Emacs packages that use fonts (such as @code{font-lock}, Enriched
Text mode, and others) by defining the relevant faces to use different
colors.  Use the @code{list-colors-display} command (@pxref{Frame
Parameters,,,emacs, the Emacs Manual}) and the
@code{list-faces-display} command (@pxref{Faces,,,emacs, the Emacs
Manual}) to see what colors and faces are available and what they look
like.

  @xref{MS-DOS and MULE}, later in this chapter, for information on
how Emacs displays glyphs and characters that aren't supported by the
native font built into the DOS display.

@cindex cursor shape on MS-DOS
  When Emacs starts, it changes the cursor shape to a solid box.  This
is for compatibility with other systems, where the box cursor is the
default in Emacs.  This default shape can be changed to a bar by
specifying the @code{cursor-type} parameter in the variable
@code{default-frame-alist} (@pxref{Creating Frames,,,emacs, the Emacs
Manual}).  The MS-DOS terminal doesn't support a vertical-bar cursor,
so the bar cursor is horizontal, and the @code{@var{width}} parameter,
if specified by the frame parameters, actually determines its height.
For this reason, the @code{bar} and @code{hbar} cursor types produce
the same effect on MS-DOS.  As an extension, the bar cursor
specification can include the starting scan line of the cursor as well
as its width, like this:

@example
 '(cursor-type bar @var{width} . @var{start})
@end example

@noindent
In addition, if the @var{width} parameter is negative, the cursor bar
begins at the top of the character cell.

@cindex frames on MS-DOS
  The MS-DOS terminal can only display a single frame at a time.  The
Emacs frame facilities work on MS-DOS much as they do on text-only
terminals (@pxref{Frames,,,emacs, the Emacs Manual}).  When you run
Emacs from a DOS window on MS-Windows, you can make the visible frame
smaller than the full screen, but Emacs still cannot display more than
a single frame at a time.

@cindex frame size under MS-DOS
@findex mode4350
@findex mode25
  The @code{mode4350} command switches the display to 43 or 50
lines, depending on your hardware; the @code{mode25} command switches
to the default 80x25 screen size.

  By default, Emacs only knows how to set screen sizes of 80 columns by
25, 28, 35, 40, 43 or 50 rows.  However, if your video adapter has
special video modes that will switch the display to other sizes, you can
have Emacs support those too.  When you ask Emacs to switch the frame to
@var{n} rows by @var{m} columns dimensions, it checks if there is a
variable called @code{screen-dimensions-@var{n}x@var{m}}, and if so,
uses its value (which must be an integer) as the video mode to switch
to.  (Emacs switches to that video mode by calling the BIOS @code{Set
Video Mode} function with the value of
@code{screen-dimensions-@var{n}x@var{m}} in the @code{AL} register.)
For example, suppose your adapter will switch to 66x80 dimensions when
put into video mode 85.  Then you can make Emacs support this screen
size by putting the following into your @file{_emacs} file:

@example
(setq screen-dimensions-66x80 85)
@end example

  Since Emacs on MS-DOS can only set the frame size to specific
supported dimensions, it cannot honor every possible frame resizing
request.  When an unsupported size is requested, Emacs chooses the next
larger supported size beyond the specified size.  For example, if you
ask for 36x80 frame, you will get 40x80 instead.

  The variables @code{screen-dimensions-@var{n}x@var{m}} are used only
when they exactly match the specified size; the search for the next
larger supported size ignores them.  In the above example, even if your
VGA supports 38x80 dimensions and you define a variable
@code{screen-dimensions-38x80} with a suitable value, you will still get
40x80 screen when you ask for a 36x80 frame.  If you want to get the
38x80 size in this case, you can do it by setting the variable named
@code{screen-dimensions-36x80} with the same video mode value as
@code{screen-dimensions-38x80}.

  Changing frame dimensions on MS-DOS has the effect of changing all the
other frames to the new dimensions.

@node MS-DOS File Names
@section File Names on MS-DOS
@cindex file names under MS-DOS
@cindex init file, default name under MS-DOS

  MS-DOS normally uses a backslash, @samp{\}, to separate name units
within a file name, instead of the slash used on other systems.  Emacs
on MS-DOS permits use of either slash or backslash, and also knows
about drive letters in file names.

  On MS-DOS, file names are case-insensitive and limited to eight
characters, plus optionally a period and three more characters.  Emacs
knows enough about these limitations to handle file names that were
meant for other operating systems.  For instance, leading dots
@samp{.}  in file names are invalid in MS-DOS, so Emacs transparently
converts them to underscores @samp{_}; thus your default init file
(@pxref{Init File,,,emacs, the Emacs Manual}) is called @file{_emacs}
on MS-DOS.  Excess characters before or after the period are generally
ignored by MS-DOS itself; thus, if you visit the file
@file{LongFileName.EvenLongerExtension}, you will silently get
@file{longfile.eve}, but Emacs will still display the long file name
on the mode line.  Other than that, it's up to you to specify file
names which are valid under MS-DOS; the transparent conversion as
described above only works on file names built into Emacs.

@cindex backup file names on MS-DOS
  The above restrictions on the file names on MS-DOS make it almost
impossible to construct the name of a backup file (@pxref{Backup
Names,,,emacs, the Emacs Manual}) without losing some of the original
file name characters.  For example, the name of a backup file for
@file{docs.txt} is @file{docs.tx~} even if single backup is used.

@cindex file names under Windows 95/NT
@cindex long file names in DOS box under Windows 95/NT
  If you run Emacs as a DOS application under Windows 9X, Windows ME, or
Windows 2000, you can turn on support for long file names.  If you do
that, Emacs doesn't truncate file names or convert them to lower case;
instead, it uses the file names that you specify, verbatim.  To enable
long file name support, set the environment variable @env{LFN} to
@samp{y} before starting Emacs.  Unfortunately, Windows NT doesn't allow
DOS programs to access long file names, so Emacs built for MS-DOS will
only see their short 8+3 aliases.

@cindex @env{HOME} directory under MS-DOS
  MS-DOS has no notion of home directory, so Emacs on MS-DOS pretends
that the directory where it is installed is the value of the @env{HOME}
environment variable.  That is, if your Emacs binary,
@file{emacs.exe}, is in the directory @file{c:/utils/emacs/bin}, then
Emacs acts as if @env{HOME} were set to @samp{c:/utils/emacs}.  In
particular, that is where Emacs looks for the init file @file{_emacs}.
With this in mind, you can use @samp{~} in file names as an alias for
the home directory, as you would on GNU or Unix.  You can also set
@env{HOME} variable in the environment before starting Emacs; its
value will then override the above default behavior.

  Emacs on MS-DOS handles the directory name @file{/dev} specially,
because of a feature in the emulator libraries of DJGPP that pretends
I/O devices have names in that directory.  We recommend that you avoid
using an actual directory named @file{/dev} on any disk.

@node MS-DOS Printing
@section Printing and MS-DOS

  Printing commands, such as @code{lpr-buffer}
(@pxref{Printing,,,emacs, the Emacs Manual }) and
@code{ps-print-buffer} (@pxref{PostScript,,,emacs, the Emacs Manual})
can work in MS-DOS and MS-Windows by sending the output to one of the
printer ports, if a Posix-style @code{lpr} program is unavailable.
The same Emacs variables control printing on all systems, but in some
cases they have different default values on MS-DOS and MS-Windows.

@vindex printer-name @r{(MS-DOS)}
  If you want to use your local printer, printing on it in the usual DOS
manner, then set the Lisp variable @code{lpr-command} to @code{""} (its
default value) and @code{printer-name} to the name of the printer
port---for example, @code{"PRN"}, the usual local printer port (that's
the default), or @code{"LPT2"}, or @code{"COM1"} for a serial printer.
You can also set @code{printer-name} to a file name, in which case
``printed'' output is actually appended to that file.  If you set
@code{printer-name} to @code{"NUL"}, printed output is silently
discarded (sent to the system null device).

  On MS-Windows, when the Windows network software is installed, you can
also use a printer shared by another machine by setting
@code{printer-name} to the UNC share name for that printer---for example,
@code{"//joes_pc/hp4si"}.  (It doesn't matter whether you use forward
slashes or backslashes here.)  To find out the names of shared printers,
run the command @samp{net view} at a DOS command prompt to obtain a list
of servers, and @samp{net view @var{server-name}} to see the names of printers
(and directories) shared by that server.  Alternatively, click the
@samp{Network Neighborhood} icon on your desktop, and look for machines
which share their printers via the network.

@cindex @samp{net use}, and printing on MS-Windows
@cindex networked printers (MS-Windows)
  If the printer doesn't appear in the output of @samp{net view}, or
if setting @code{printer-name} to the UNC share name doesn't produce a
hardcopy on that printer, you can use the @samp{net use} command to
connect a local print port such as @code{"LPT2"} to the networked
printer.  For example, typing @kbd{net use LPT2:
\\joes_pc\hp4si}@footnote{
Note that the @samp{net use} command requires the UNC share name to be
typed with the Windows-style backslashes, while the value of
@code{printer-name} can be set with either forward- or backslashes.}
causes Windows to @dfn{capture} the LPT2 port and redirect the printed
material to the printer connected to the machine @code{joes_pc}.
After this command, setting @code{printer-name} to @code{"LPT2"}
should produce the hardcopy on the networked printer.

  With some varieties of Windows network software, you can instruct
Windows to capture a specific printer port such as @code{"LPT2"}, and
redirect it to a networked printer via the @w{@code{Control
Panel->Printers}} applet instead of @samp{net use}.

  Some printers expect DOS codepage encoding of non-@acronym{ASCII} text, even
though they are connected to a Windows machine which uses a different
encoding for the same locale.  For example, in the Latin-1 locale, DOS
uses codepage 850 whereas Windows uses codepage 1252.  @xref{MS-DOS and
MULE}.  When you print to such printers from Windows, you can use the
@kbd{C-x RET c} (@code{universal-coding-system-argument}) command before
@kbd{M-x lpr-buffer}; Emacs will then convert the text to the DOS
codepage that you specify.  For example, @kbd{C-x RET c cp850-dos RET
M-x lpr-region RET} will print the region while converting it to the
codepage 850 encoding.  You may need to create the @code{cp@var{nnn}}
coding system with @kbd{M-x codepage-setup}.

  If you set @code{printer-name} to a file name, it's best to use an
absolute file name.  Emacs changes the working directory according to
the default directory of the current buffer, so if the file name in
@code{printer-name} is relative, you will end up with several such
files, each one in the directory of the buffer from which the printing
was done.

@findex print-buffer @r{(MS-DOS)}
@findex print-region @r{(MS-DOS)}
@vindex lpr-headers-switches @r{(MS-DOS)}
  The commands @code{print-buffer} and @code{print-region} call the
@code{pr} program, or use special switches to the @code{lpr} program, to
produce headers on each printed page.  MS-DOS and MS-Windows don't
normally have these programs, so by default, the variable
@code{lpr-headers-switches} is set so that the requests to print page
headers are silently ignored.  Thus, @code{print-buffer} and
@code{print-region} produce the same output as @code{lpr-buffer} and
@code{lpr-region}, respectively.  If you do have a suitable @code{pr}
program (for example, from GNU Textutils), set
@code{lpr-headers-switches} to @code{nil}; Emacs will then call
@code{pr} to produce the page headers, and print the resulting output as
specified by @code{printer-name}.

@vindex print-region-function @r{(MS-DOS)}
@cindex lpr usage under MS-DOS
@vindex lpr-command @r{(MS-DOS)}
@vindex lpr-switches @r{(MS-DOS)}
  Finally, if you do have an @code{lpr} work-alike, you can set the
variable @code{lpr-command} to @code{"lpr"}.  Then Emacs will use
@code{lpr} for printing, as on other systems.  (If the name of the
program isn't @code{lpr}, set @code{lpr-command} to specify where to
find it.)  The variable @code{lpr-switches} has its standard meaning
when @code{lpr-command} is not @code{""}.  If the variable
@code{printer-name} has a string value, it is used as the value for the
@code{-P} option to @code{lpr}, as on Unix.

@findex ps-print-buffer @r{(MS-DOS)}
@findex ps-spool-buffer @r{(MS-DOS)}
@vindex ps-printer-name @r{(MS-DOS)}
@vindex ps-lpr-command @r{(MS-DOS)}
@vindex ps-lpr-switches @r{(MS-DOS)}
  A parallel set of variables, @code{ps-lpr-command},
@code{ps-lpr-switches}, and @code{ps-printer-name} (@pxref{PostScript
Variables,,,emacs, the Emacs Manual}), defines how PostScript files
should be printed.  These variables are used in the same way as the
corresponding variables described above for non-PostScript printing.
Thus, the value of @code{ps-printer-name} is used as the name of the
device (or file) to which PostScript output is sent, just as
@code{printer-name} is used for non-PostScript printing.  (There are
two distinct sets of variables in case you have two printers attached
to two different ports, and only one of them is a PostScript printer.)

  The default value of the variable @code{ps-lpr-command} is @code{""},
which causes PostScript output to be sent to the printer port specified
by @code{ps-printer-name}, but @code{ps-lpr-command} can also be set to
the name of a program which will accept PostScript files.  Thus, if you
have a non-PostScript printer, you can set this variable to the name of
a PostScript interpreter program (such as Ghostscript).  Any switches
that need to be passed to the interpreter program are specified using
@code{ps-lpr-switches}.  (If the value of @code{ps-printer-name} is a
string, it will be added to the list of switches as the value for the
@code{-P} option.  This is probably only useful if you are using
@code{lpr}, so when using an interpreter typically you would set
@code{ps-printer-name} to something other than a string so it is
ignored.)

  For example, to use Ghostscript for printing on an Epson printer
connected to the @samp{LPT2} port, put this in your @file{_emacs} file:

@example
(setq ps-printer-name t)  ; Ghostscript doesn't understand -P
(setq ps-lpr-command "c:/gs/gs386")
(setq ps-lpr-switches '("-q" "-dNOPAUSE"
			"-sDEVICE=epson"
			"-r240x72"
			"-sOutputFile=LPT2"
			"-Ic:/gs"))
@end example

@noindent
(This assumes that Ghostscript is installed in the @file{"c:/gs"}
directory.)

@vindex dos-printer
@vindex dos-ps-printer
  For backwards compatibility, the value of @code{dos-printer}
(@code{dos-ps-printer}), if it has a value, overrides the value of
@code{printer-name} (@code{ps-printer-name}), on MS-DOS and MS-Windows
only.


@node MS-DOS and MULE
@section International Support on MS-DOS
@cindex international support @r{(MS-DOS)}

  Emacs on MS-DOS supports the same international character sets as it
does on GNU, Unix and other platforms (@pxref{International,,,emacs,
the Emacs Manual}), including coding systems for converting between
the different character sets.  However, due to incompatibilities
between MS-DOS/MS-Windows and other systems, there are several
DOS-specific aspects of this support that you should be aware of.
This section describes these aspects.

  The description below is largely specific to the MS-DOS port of
Emacs, especially where it talks about practical implications for
Emacs users.  For other operating systems, see the @file{code-pages.el}
package, which implements support for MS-DOS- and MS-Windows-specific
encodings for all platforms other than MS-DOS.

@table @kbd
@item M-x dos-codepage-setup
Set up Emacs display and coding systems as appropriate for the current
DOS codepage.

@item M-x codepage-setup
Create a coding system for a certain DOS codepage.
@end table

@cindex codepage, MS-DOS
@cindex DOS codepages
  MS-DOS is designed to support one character set of 256 characters at
any given time, but gives you a variety of character sets to choose
from.  The alternative character sets are known as @dfn{DOS codepages}.
Each codepage includes all 128 @acronym{ASCII} characters, but the other 128
characters (codes 128 through 255) vary from one codepage to another.
Each DOS codepage is identified by a 3-digit number, such as 850, 862,
etc.

  In contrast to X, which lets you use several fonts at the same time,
MS-DOS normally doesn't allow use of several codepages in a single
session.  MS-DOS was designed to load a single codepage at system
startup, and require you to reboot in order to change
it@footnote{Normally, one particular codepage is burnt into the
display memory, while other codepages can be installed by modifying
system configuration files, such as @file{CONFIG.SYS}, and rebooting.
While there is third-party software that allows changing the codepage
without rebooting, we describe here how a stock MS-DOS system
behaves.}.  Much the same limitation applies when you run DOS
executables on other systems such as MS-Windows.

@cindex unibyte operation @r{(MS-DOS)}
  If you invoke Emacs on MS-DOS with the @samp{--unibyte} option
(@pxref{Initial Options,,,emacs, the Emacs Manual}), Emacs does not
perform any conversion of non-@acronym{ASCII} characters.  Instead, it
reads and writes any non-@acronym{ASCII} characters verbatim, and
sends their 8-bit codes to the display verbatim.  Thus, unibyte Emacs
on MS-DOS supports the current codepage, whatever it may be, but
cannot even represent any other characters.

@vindex dos-codepage
  For multibyte operation on MS-DOS, Emacs needs to know which
characters the chosen DOS codepage can display.  So it queries the
system shortly after startup to get the chosen codepage number, and
stores the number in the variable @code{dos-codepage}.  Some systems
return the default value 437 for the current codepage, even though the
actual codepage is different.  (This typically happens when you use the
codepage built into the display hardware.)  You can specify a different
codepage for Emacs to use by setting the variable @code{dos-codepage} in
your init file.

@cindex language environment, automatic selection on @r{MS-DOS}
  Multibyte Emacs supports only certain DOS codepages: those which can
display Far-Eastern scripts, like the Japanese codepage 932, and those
that encode a single ISO 8859 character set.

  The Far-Eastern codepages can directly display one of the MULE
character sets for these countries, so Emacs simply sets up to use the
appropriate terminal coding system that is supported by the codepage.
The special features described in the rest of this section mostly
pertain to codepages that encode ISO 8859 character sets.

  For the codepages which correspond to one of the ISO character sets,
Emacs knows the character set name based on the codepage number.  Emacs
automatically creates a coding system to support reading and writing
files that use the current codepage, and uses this coding system by
default.  The name of this coding system is @code{cp@var{nnn}}, where
@var{nnn} is the codepage number.@footnote{The standard Emacs coding
systems for ISO 8859 are not quite right for the purpose, because
typically the DOS codepage does not match the standard ISO character
codes.  For example, the letter @samp{@,{c}} (@samp{c} with cedilla) has
code 231 in the standard Latin-1 character set, but the corresponding
DOS codepage 850 uses code 135 for this glyph.}

@cindex mode line @r{(MS-DOS)}
  All the @code{cp@var{nnn}} coding systems use the letter @samp{D}
(for ``DOS'') as their mode-line mnemonic.  Since both the terminal
coding system and the default coding system for file I/O are set to
the proper @code{cp@var{nnn}} coding system at startup, it is normal
for the mode line on MS-DOS to begin with @samp{-DD\-}.  @xref{Mode
Line,,,emacs, the Emacs Manual}.  Far-Eastern DOS terminals do not use
the @code{cp@var{nnn}} coding systems, and thus their initial mode
line looks like the Emacs default.

  Since the codepage number also indicates which script you are using,
Emacs automatically runs @code{set-language-environment} to select the
language environment for that script (@pxref{Language
Environments,,,emacs, the Emacs Manual}).

  If a buffer contains a character belonging to some other ISO 8859
character set, not the one that the chosen DOS codepage supports, Emacs
displays it using a sequence of @acronym{ASCII} characters.  For example, if the
current codepage doesn't have a glyph for the letter @samp{@`o} (small
@samp{o} with a grave accent), it is displayed as @samp{@{`o@}}, where
the braces serve as a visual indication that this is a single character.
(This may look awkward for some non-Latin characters, such as those from
Greek or Hebrew alphabets, but it is still readable by a person who
knows the language.)  Even though the character may occupy several
columns on the screen, it is really still just a single character, and
all Emacs commands treat it as one.

@cindex IBM graphics characters (MS-DOS)
@cindex box-drawing characters (MS-DOS)
@cindex line-drawing characters (MS-DOS)
  Not all characters in DOS codepages correspond to ISO 8859
characters---some are used for other purposes, such as box-drawing
characters and other graphics.  Emacs maps these characters to two
special character sets called @code{eight-bit-control} and
@code{eight-bit-graphic}, and displays them as their IBM glyphs.
However, you should be aware that other systems might display these
characters differently, so you should avoid them in text that might be
copied to a different operating system, or even to another DOS machine
that uses a different codepage.

@vindex dos-unsupported-character-glyph
  Emacs supports many other characters sets aside from ISO 8859, but it
cannot display them on MS-DOS.  So if one of these multibyte characters
appears in a buffer, Emacs on MS-DOS displays them as specified by the
@code{dos-unsupported-character-glyph} variable; by default, this glyph
is an empty triangle.  Use the @kbd{C-u C-x =} command to display the
actual code and character set of such characters.  @xref{Position
Info,,,emacs, the Emacs Manual}.

@findex codepage-setup
  By default, Emacs defines a coding system to support the current
codepage.  To define a coding system for some other codepage (e.g., to
visit a file written on a DOS machine in another country), use the
@kbd{M-x codepage-setup} command.  It prompts for the 3-digit code of
the codepage, with completion, then creates the coding system for the
specified codepage.  You can then use the new coding system to read and
write files, but you must specify it explicitly for the file command
when you want to use it (@pxref{Text Coding,,,emacs, the Emacs Manual}).

  These coding systems are also useful for visiting a file encoded using
a DOS codepage, using Emacs running on some other operating system.

@cindex MS-Windows codepages
  MS-Windows provides its own codepages, which are different from the
DOS codepages for the same locale.  For example, DOS codepage 850
supports the same character set as Windows codepage 1252; DOS codepage
855 supports the same character set as Windows codepage 1251, etc.
The MS-Windows version of Emacs uses the current codepage for display
when invoked with the @samp{-nw} option.  Support for codepages in the
Windows port of Emacs is part of the @file{code-pages.el} package.

@node MS-DOS Processes
@section Subprocesses on MS-DOS

@cindex compilation under MS-DOS
@cindex inferior processes under MS-DOS
@findex compile @r{(MS-DOS)}
@findex grep @r{(MS-DOS)}
  Because MS-DOS is a single-process ``operating system,''
asynchronous subprocesses are not available.  In particular, Shell
mode and its variants do not work.  Most Emacs features that use
asynchronous subprocesses also don't work on MS-DOS, including
Shell mode and GUD.  When in doubt, try and see; commands that
don't work output an error message saying that asynchronous processes
aren't supported.

  Compilation under Emacs with @kbd{M-x compile}, searching files with
@kbd{M-x grep} and displaying differences between files with @kbd{M-x
diff} do work, by running the inferior processes synchronously.  This
means you cannot do any more editing until the inferior process
finishes.

  Spell checking also works, by means of special support for synchronous
invocation of the @code{ispell} program.  This is slower than the
asynchronous invocation on other platforms

  Instead of the Shell mode, which doesn't work on MS-DOS, you can use
the @kbd{M-x eshell} command.  This invokes the Eshell package that
implements a Posix-like shell entirely in Emacs Lisp.

  By contrast, Emacs compiled as a native Windows application
@strong{does} support asynchronous subprocesses.  @xref{Windows
Processes,,,emacs, the Emacs Manual}.

@cindex printing under MS-DOS
  Printing commands, such as @code{lpr-buffer}
(@pxref{Printing,,,emacs, the Emacs Manual}) and
@code{ps-print-buffer} (@pxref{PostScript,,,emacs, the Emacs Manual}),
work in MS-DOS by sending the output to one of the printer ports.
@xref{MS-DOS Printing,,,emacs, the Emacs Manual}.

  When you run a subprocess synchronously on MS-DOS, make sure the
program terminates and does not try to read keyboard input.  If the
program does not terminate on its own, you will be unable to terminate
it, because MS-DOS provides no general way to terminate a process.
Pressing @kbd{C-c} or @kbd{C-@key{BREAK}} might sometimes help in these
cases.

  Accessing files on other machines is not supported on MS-DOS.  Other
network-oriented commands such as sending mail, Web browsing, remote
login, etc., don't work either, unless network access is built into
MS-DOS with some network redirector.

@cindex directory listing on MS-DOS
@vindex dired-listing-switches @r{(MS-DOS)}
  Dired on MS-DOS uses the @code{ls-lisp} package where other
platforms use the system @code{ls} command.  Therefore, Dired on
MS-DOS supports only some of the possible options you can mention in
the @code{dired-listing-switches} variable.  The options that work are
@samp{-A}, @samp{-a}, @samp{-c}, @samp{-i}, @samp{-r}, @samp{-S},
@samp{-s}, @samp{-t}, and @samp{-u}.


@node Index
@unnumbered Index

@printindex cp

@bye

@ignore
   arch-tag: 75c33f13-32c6-41b6-9537-847a312e2e49
@end ignore