Specialized Emacs Features
**************************

This manual describes specialized features of Emacs.

Copyright © 2004, 2005, 2006 Free Software Foundation, Inc.

     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.

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 C-h p, accessing mode documentation using 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.

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 M-x auto-revert-mode) and setting global-auto-revert-non-file-buffers to 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 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 auto-revert-verbose is non-nil.

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

Auto Reverting the Buffer Menu
==============================

If auto-reverting of non-file buffers is enabled, the Buffer Menu automatically reverts every 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 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.

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 all listed information is up to date, you have to manually revert using g, 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 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 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.  *Find* and *Locate* buffers do not auto-revert either.

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 revert-buffer-function.  See Reverting (the Emacs Lisp Reference Manual).

In addition, it must have a buffer-stale-function.

 -- Variable: 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 NOCONFIRM.  The function should return non-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 NOCONFIRM argument is that it should be t if the buffer is going to be reverted without asking the user and 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, NOCONFIRM is t. If the function is only going to be used for auto-reverting, you can ignore the NOCONFIRM argument.

     If you just want to automatically auto-revert every auto-revert-interval seconds, use:

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

     in the buffer's mode function.

     The special return value 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 auto-revert-verbose is non-nil.  This is important, as getting revert messages every 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.

Once the buffer has a revert-buffer-function and a 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 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 revert-buffer-function does not print messages that unnecessarily duplicate Auto Revert's own messages if auto-revert-verbose is t and effectively override a nil value for 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 auto-revert-interval seconds.

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

Finally, you should add a node to this chapter's menu.  This node 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 auto-revert-interval seconds).

Subdirectory Switches in Dired
******************************

You can insert subdirectories with specified ls switches in Dired buffers, using C-u i.  You can change the ls switches of an already inserted subdirectory using 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 dired-undo (usually bound to C-_ and 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 dired-undo does not forget its switches.  When later reinserted using i, it will be reinserted using its old switches.  Using dired-undo to reinsert a subdirectory that was deleted using the regular Dired commands (not 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 C-u i or C-u l.

Dired does not remember the R switch.  Inserting a subdirectory with switches that include the 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 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 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 M-x dired-reset-subdir-switches.  This also reverts the Dired buffer.

Customizing the Calendar and Diary
**********************************

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

Customizing the Calendar
========================

The variable 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 diary-entry-marker specifies how to mark a date that has diary entries.  The calendar creates faces named holiday-face and diary-face for these purposes; those symbols are the default values of these variables.

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

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

The variable 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 calendar-star-date.

     (add-hook 'today-visible-calendar-hook 'calendar-star-date)

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

     (add-hook 'today-visible-calendar-hook 'calendar-mark-today)

The variable 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 calendar-today-face is provided for this purpose; that symbol is the default for this variable.

A similar normal hook, today-invisible-calendar-hook is run if the current date is not visible in the window.

Each of the calendar cursor motion commands runs the hook calendar-move-hook after it moves the cursor.

Customizing the 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 (general-holidays), local holidays (local-holidays), Christian holidays (christian-holidays), Hebrew (Jewish) holidays (hebrew-holidays), Islamic (Muslim) holidays (islamic-holidays), and other holidays (other-holidays).

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

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

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 all-christian-calendar-holidays, all-hebrew-calendar-holidays, or all-islamic-calendar-holidays to t.  If you want to eliminate the religious holidays, set any or all of the corresponding variables christian-holidays, hebrew-holidays, and islamic-holidays to nil.

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

Each of the lists (general-holidays, local-holidays, christian-holidays, hebrew-holidays, islamic-holidays, and other-holidays) is a list of 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 STRING is always the name of the holiday, as a string.

(holiday-fixed MONTH DAY STRING)
     A fixed date on the Gregorian calendar.

(holiday-float MONTH DAYNAME K STRING)
     The Kth DAYNAME in MONTH on the Gregorian calendar (DAYNAME=0 for Sunday, and so on); negative K means count back from the end of the month.

(holiday-hebrew MONTH DAY STRING)
     A fixed date on the Hebrew calendar.

(holiday-islamic MONTH DAY STRING)
     A fixed date on the Islamic calendar.

(holiday-julian MONTH DAY STRING)
     A fixed date on the Julian calendar.

(holiday-sexp SEXP STRING)
     A date calculated by the Lisp expression SEXP.  The expression should use the variable year to compute and return the date of a holiday, or nil if the holiday doesn't happen this year.  The value of SEXP must represent the date as a list of the form (MONTH DAY YEAR).

(if CONDITION HOLIDAY-FORM)
     A holiday that happens only if CONDITION is true.

(FUNCTION [ARGS])
     A list of dates calculated by the function FUNCTION, called with arguments ARGS.

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

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

The holiday form (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:

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

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, -1 the last occurrence, -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,

     (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")))

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 if or the holiday-sexp form.  For example, American presidential elections occur on the first Tuesday after the first Monday in November of years divisible by 4:

     (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")

or

     (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"))

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 (eclipses) to other-holidays and write an Emacs Lisp function 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:

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

Date Display Format
===================

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

     ((if dayname (concat dayname ", ")) monthname " " day ", " year)

while in the European style this value is the default:

     ((if dayname (concat dayname ", ")) day " " monthname " " year)

The ISO standard date representation is this:

     (year "-" month "-" day)

This specifies a typical American format:

     (month "/" day "/" (substring year -2))

Time Display Format
===================

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 am or 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 calendar-time-display-form.  This variable is a list of expressions that can involve the variables 12-hours, 24-hours, and minutes, which are all numbers in string form, and am-pm and time-zone, which are both alphabetic strings.  The default value of calendar-time-display-form is as follows:

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

Here is a value that provides European style times:

     (24-hours ":" minutes
               (if time-zone " (") time-zone (if time-zone ")"))

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.

If the default choice of rules is not appropriate for your location, you can tell Emacs the rules to use by setting the variables calendar-daylight-savings-starts and calendar-daylight-savings-ends.  Their values should be Lisp expressions that refer to the variable year, and evaluate to the Gregorian date on which daylight savings time starts or (respectively) ends, in the form of a list (MONTH DAY YEAR). The values should be 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:

     (calendar-nth-named-day 1 0 4 year)
     (calendar-nth-named-day -1 0 10 year)

i.e., the first 0th day (Sunday) of the fourth month (April) in the year specified by 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 calendar-daylight-savings-starts to this:

     (list 10 1 year)

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

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

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 calendar-daylight-savings-starts and calendar-daylight-savings-ends to nil.

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

The variable calendar-daylight-savings-starts-time and the variable 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.

Customizing the Diary
=====================

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 holidays-in-diary-buffer to nil.

The variable number-of-diary-entries controls the number of days of diary entries to be displayed at one time.  It affects the initial display when view-diary-entries-initially is t, as well as the command 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 [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.

The variable 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 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.

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 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 (see Regular Expressions (the Emacs Lisp Reference Manual)) or the symbols month, day, year, monthname, and 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 * is a word constituent.

The symbols month, day, year, monthname, and 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 *; since * 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 diary-date-forms in the American style is this:

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

The date patterns in the list must be 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 must be 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 backup, the date pattern must absolutely not match more than a portion of the first word of the diary entry.  The default value of diary-date-forms in the European style is this list:

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

Notice the use of 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.

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:

     (add-hook 'nongregorian-diary-listing-hook 'list-hebrew-diary-entries)
     (add-hook 'nongregorian-diary-marking-hook 'mark-hebrew-diary-entries)

If you want Islamic-date entries, do this:

     (add-hook 'nongregorian-diary-listing-hook 'list-islamic-diary-entries)
     (add-hook 'nongregorian-diary-marking-hook 'mark-islamic-diary-entries)

Hebrew- and Islamic-date diary entries have the same formats as Gregorian-date diary entries, except that H precedes a Hebrew date and 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:

     HHeshvan 25 Happy Hebrew birthday!

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:

     IDhu al-Qada 25 Happy Islamic birthday!

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

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:

i h d
     Add a diary entry for the Hebrew date corresponding to the selected date (insert-hebrew-diary-entry).

i h m
     Add a diary entry for the day of the Hebrew month corresponding to the selected date (insert-monthly-hebrew-diary-entry).  This diary entry matches any date that has the same Hebrew day-within-month as the selected date.

i h y
     Add a diary entry for the day of the Hebrew year corresponding to the selected date (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.

i i d
     Add a diary entry for the Islamic date corresponding to the selected date (insert-islamic-diary-entry).

i i m
     Add a diary entry for the day of the Islamic month corresponding to the selected date (insert-monthly-islamic-diary-entry).

i i y
     Add a diary entry for the day of the Islamic year corresponding to the selected date (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.

Fancy Diary Display
===================

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

     (add-hook 'diary-display-hook 'fancy-diary-display)

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 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 7 d, and then do 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 holidays-in-diary-buffer to nil.

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 diary-list-include-blanks to t.

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

     (add-hook 'list-diary-entries-hook 'sort-diary-entries t)

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:

     #include "FILENAME"

includes the diary entries from the file 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:

     (add-hook 'list-diary-entries-hook 'include-other-diary-files)
     (add-hook 'mark-diary-entries-hook 'mark-included-diary-files)

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

Sexp Entries and the Fancy Diary Display
========================================

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 %d in this dairy entry:

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

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

     Arthur's birthday (42 years old)

If the diary file instead contains this entry:

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

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

     Arthur's 42nd birthday

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

     %%(diary-cyclic 50 1 1 1990) Renew medication (%d%s time)

looks like this:

     Renew medication (5th time)

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

     %%(diary-remind '(diary-anniversary 12 22 1968) 7) Ed's anniversary

and the fancy diary will show

     Ed's anniversary

both on December 15 and on December 22.

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

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

causes the fancy diary to show

     Rake leaves

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

The function 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 MONTH, DAYNAME, and an index N. The entry appears on the Nth DAYNAME of MONTH, where DAYNAME=0 means Sunday, 1 means Monday, and so on.  If N is negative it counts backward from the end of MONTH.  The value of MONTH can be a list of months, a single month, or t to specify all months.  You can also use an optional parameter DAY to specify the Nth DAYNAME of MONTH on or after/before DAY; the value of DAY defaults to 1 if N is positive and to the last day of MONTH if N is negative.  For example,

     %%(diary-float t 1 -1) Pay rent

causes the fancy diary to show

     Pay rent

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-nil, the entry applies to that date; otherwise, it does not.  The expression can use the variable date to find the date being considered; its value is a list (MONTH DAY YEAR) that refers to the Gregorian calendar.

The sexp diary entry applies to a date when the expression's value is non-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 (MARK . STRING); then MARK specifies how to mark the date in the calendar, and STRING is the description of the event.  If MARK is a single-character string, that character appears next to the date in the calendar.  If MARK is a face name, the date is displayed in that face.  If MARK is 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:

     &%%(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

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:

%%(diary-sunrise-sunset)
     Make a diary entry for the local times of today's sunrise and sunset.

%%(diary-phases-of-moon)
     Make a diary entry for the phases (quarters) of the moon.

%%(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.

%%(diary-iso-date)
     Make a diary entry with today's equivalent ISO commercial date.

%%(diary-julian-date)
     Make a diary entry with today's equivalent date on the Julian calendar.

%%(diary-astro-day-number)
     Make a diary entry with today's equivalent astronomical (Julian) day number.

%%(diary-hebrew-date)
     Make a diary entry with today's equivalent date on the Hebrew calendar.

%%(diary-islamic-date)
     Make a diary entry with today's equivalent date on the Islamic calendar.

%%(diary-french-date)
     Make a diary entry with today's equivalent date on the French Revolutionary calendar.

%%(diary-mayan-date)
     Make a diary entry with today's equivalent date on the Mayan calendar.

Thus including the diary entry

     &%%(diary-hebrew-date)

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 &%%(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:

%%(diary-rosh-hodesh)
     Make a diary entry that tells the occurrence and ritual announcement of each new Hebrew month.

%%(diary-parasha)
     Make a Saturday diary entry that tells the weekly synagogue scripture reading.

%%(diary-sabbath-candles)
     Make a Friday diary entry that tells the local time of Sabbath candle lighting.

%%(diary-omer)
     Make a diary entry that gives the omer count, when appropriate.

%%(diary-yahrzeit MONTH DAY YEAR) NAME
     Make a diary entry marking the anniversary of a date of death.  The date is the 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 DAY, MONTH, YEAR.)

All the functions documented above take an optional argument 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 MARK.

Merging Files with Emerge
*************************

It's not unusual for programmers to get their signals crossed and modify the same program in two different directions.  To recover from this confusion, you need to merge the two versions.  Emerge makes this easier.  For other ways to compare files, see Comparing Files (the Emacs Manual) and Ediff (The Ediff Manual).

Overview of Emerge
==================

To start Emerge, run one of these four commands:

M-x emerge-files
     Merge two specified files.

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

M-x emerge-buffers
     Merge two buffers.

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

The Emerge commands compare two files or buffers, and display the comparison in three buffers: one for each input text (the A buffer and the B buffer), and one (the merge buffer) where merging takes place.  The merge buffer shows the full merged text, not just the differences.  Wherever the two input texts differ, you can choose which one of them to include in the merge buffer.

The Emerge commands that take input from existing buffers use only the accessible portions of those buffers, if they are narrowed. See Narrowing (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 with-ancestor commands if you want to specify a common ancestor text.  These commands read three file or buffer names---variant A, variant B, and the common ancestor.

After the comparison is done and the buffers are prepared, the interactive merging starts.  You control the merging by typing special merge commands in the merge buffer (see 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 selected difference.  This difference is marked off in the three buffers like this:

     vvvvvvvvvvvvvvvvvvvv
     TEXT THAT DIFFERS
     ^^^^^^^^^^^^^^^^^^^^

Emerge numbers all the differences sequentially and the mode line always shows the number of the selected difference.

Normally, the merge buffer starts out with the A version of the text. But when the A version of a difference agrees with the common ancestor, then the B version is initially preferred for that difference.

Emerge leaves the merged text in the merge buffer when you exit.  At that point, you can save it in a file with C-x C-w.  If you give a numeric argument to emerge-files or emerge-files-with-ancestor, it reads the name of the output file using the minibuffer.  (This is the last file name those commands read.) Then exiting from Emerge saves the merged text in the output file.

Normally, Emerge commands save the output buffer in its file when you exit.  If you abort Emerge with C-], the Emerge command does not save the output buffer, but you can save it yourself if you wish.

Submodes of Emerge
==================

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

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

Emerge has two additional submodes that affect how particular merge commands work: Auto Advance mode and Skip Prefers mode.

If Auto Advance mode is in effect, the a and b commands advance to the next difference.  This lets you go through the merge faster as long as you simply choose one of the alternatives from the input.  The mode line indicates Auto Advance mode with A.

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

Use the command s a (emerge-auto-advance-mode) to set or clear Auto Advance mode.  Use s s (emerge-skip-prefers-mode) to set or clear Skip Prefers mode. These commands turn on the mode with a positive argument, turns it off with a negative or zero argument, and toggle the mode with no argument.

State of a Difference
=====================

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

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

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

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

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

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

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

     These two states are displayed in the mode line as A* and B*.

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

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

     The mode line displays this state as comb.

Merge Commands
==============

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

p
     Select the previous difference.

n
     Select the next difference.

a
     Choose the A version of this difference.

b
     Choose the B version of this difference.

C-u N j
     Select difference number N.

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

q
     Quit---finish the merge.

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

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

e
     Go into Edit mode.

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

-
     Specify part of a prefix numeric argument.

DIGIT
     Also specify part of a prefix numeric argument.

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

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

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

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

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

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

m
     Put point and mark around the difference.

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

v
     Scroll all three windows up (like C-v).

<
     Scroll all three windows left (like C-x <).

>
     Scroll all three windows right (like C-x >).

|
     Reset horizontal scroll on all three windows.

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

x c
     Combine the two versions of this difference (see Combining in Emerge).

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

x j
     Join this difference with the following one. (C-u x j joins this difference with the previous one.)

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

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

Exiting Emerge
==============

The q command (emerge-quit) finishes the merge, storing the results into the output file if you specified one.  It restores the A and B buffers to their proper contents, or kills them if they were created by Emerge and you haven't changed them.  It also disables the Emerge commands in the merge buffer, since executing them later could damage the contents of the various buffers.

C-] aborts the merge.  This means exiting without writing the output file.  If you didn't specify an output file, then there is no real difference between aborting and finishing the merge.

If the Emerge command was called from another Lisp program, then its return value is t for successful completion, or nil if you abort.

Combining the Two Versions
==========================

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

     #ifdef NEW
     VERSION FROM A BUFFER
     #else /* not NEW */
     VERSION FROM B BUFFER
     #endif /* not NEW */

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

     "#ifdef NEW\n%a#else /* not NEW */\n%b#endif /* not NEW */\n"

Fine Points of Emerge
=====================

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

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

Starting Emerge can take a long time because it needs to compare the files fully.  Emacs can't do anything else until diff finishes. Perhaps in the future someone will change Emerge to do the comparison in the background when the input files are large---then you could keep on doing other things with Emacs until Emerge is ready to accept commands.

After setting up the merge, Emerge runs the hook emerge-startup-hook.  See Hooks (the Emacs Manual).

Editing Pictures
****************

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 M-x edit-picture to enter Picture mode.

In Picture mode, editing is based on the 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 C-c prefix to run special picture editing commands.

One of these keys, 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.  M-x edit-picture records the name of the previous major mode so you can use the C-c C-c command (picture-mode-exit) later to go back to that mode.  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 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.

Turning on Picture mode runs the hook picture-mode-hook. Additional extensions to Picture mode can be found in artist.el.

Basic 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, C-f is rebound to run 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.  C-b is rebound to run picture-backward-column, which always moves point left one column, converting a tab to multiple spaces if necessary.  C-n and C-p are rebound to run picture-move-down and picture-move-up, which can either insert spaces or convert tabs as necessary to make sure that point stays in exactly the same column. C-e runs picture-end-of-line, which moves to after the last nonblank character on the line.  There is no need to change C-a, as the choice of screen model does not affect beginnings of lines.

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

In Picture mode, the commands that normally delete or kill text, instead erase text (replacing it with spaces).  DEL (picture-backward-clear-column) replaces the preceding character with a space rather than removing it; this moves point backwards.  C-d (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 SPC.)  C-k (picture-clear-line) really kills the contents of lines, but does not delete the newlines from the buffer.

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

To do actual deletion in Picture mode, use C-w, C-c C-d (which is defined as delete-char, as C-d is in other modes), or one of the picture rectangle commands (see Rectangles in Picture).

Controlling Motion after Insert
===============================

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.

C-c <
C-c LEFT
     Move left after insertion (picture-movement-left).

C-c >
C-c RIGHT
     Move right after insertion (picture-movement-right).

C-c ^
C-c UP
     Move up after insertion (picture-movement-up).

C-c .
C-c DOWN
     Move down after insertion (picture-movement-down).

C-c `
C-c HOME
     Move up and left (``northwest'') after insertion (picture-movement-nw).

C-c '
C-c PAGEUP
     Move up and right (``northeast'') after insertion (picture-movement-ne).

C-c /
C-c END
     Move down and left (``southwest'') after insertion

     (picture-movement-sw).

C-c \
C-c PAGEDOWN
     Move down and right (``southeast'') after insertion

     (picture-movement-se).

Two motion commands move based on the current Picture insertion direction.  The command C-c C-f (picture-motion) moves in the same direction as motion after ``insertion'' currently does, while C-c C-b (picture-motion-reverse) moves in the opposite direction.

Picture Mode Tabs
=================

Two kinds of tab-like action are provided in Picture mode.  Use M-TAB (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 C-u M-TAB, this command moves to the next such interesting character in the current line.  M-TAB does not change the text; it only moves point.  ``Interesting'' characters are defined by the variable picture-tab-chars, which should define a set of characters.  The syntax for this variable is like the syntax used inside of [...] in a regular expression---but without the [ and the ].  Its default value is "!-~".

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

The context-based and tab-stop-based forms of tabbing are brought together by the command C-c TAB (picture-set-tab-stops). This command sets the tab stops to the positions which M-TAB would consider significant in the current line.  The use of this command, together with TAB, can get the effect of context-based tabbing.  But M-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 C-x TAB from messing up the picture.  You can do this by setting the variable indent-tabs-mode to nil.

Picture Mode Rectangle Commands
===============================

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.  See Rectangles (the Emacs Manual).

C-c C-k
     Clear out the region-rectangle with spaces (picture-clear-rectangle).  With argument, delete the text.

C-c C-w R
     Similar, but save rectangle contents in register R first (picture-clear-rectangle-to-register).

C-c C-y
     Copy last killed rectangle into the buffer by overwriting, with upper left corner at point (picture-yank-rectangle).  With argument, insert instead.

C-c C-x R
     Similar, but use the rectangle in register R (picture-yank-rectangle-from-register).

The picture rectangle commands C-c C-k (picture-clear-rectangle) and C-c C-w (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 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. C-c C-k either with or without a numeric argument saves the rectangle for C-c C-y.

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.  C-c C-y (picture-yank-rectangle) inserts (by overwriting) the rectangle that was most recently killed, while C-c C-x (picture-yank-rectangle-from-register) does likewise for the rectangle found in a specified register.

Advanced VC Usage
*****************

Commonly used features of Emacs' version control (VC) support are described in the main Emacs manual (see Version Control (the Emacs Manual)).  This chapter describes more advanced VC usage.

Dired under VC
==============

The VC Dired Mode described here works with all the version control systems that VC supports.  Another more powerful facility, designed specifically for CVS, is called PCL-CVS.  See About PCL-CVS (PCL-CVS --- The Emacs Front-End to CVS).

When you are working on a large program, it is often useful to find out which files have changed within an entire directory tree, or to view the status of all files under version control at once, and to perform version control operations on collections of files.  You can use the command C-x v d (vc-directory) to make a directory listing that includes only files relevant for version control.

C-x v d creates a buffer which uses VC Dired Mode.  This looks much like an ordinary Dired buffer (see Dired (the Emacs Manual)); however, normally it shows only the noteworthy files (those locked or not up-to-date).  This is called terse display. If you set the variable vc-dired-terse-display to nil, then VC Dired shows all relevant files---those managed under version control, plus all subdirectories (full display).  The command v t in a VC Dired buffer toggles between terse display and full display (see VC Dired Commands).

By default, VC Dired produces a recursive listing of noteworthy or relevant files at or below the given directory.  You can change this by setting the variable vc-dired-recurse to nil; then VC Dired shows only the files in the given directory.

The line for an individual file shows the version control state in the place of the hard link count, owner, group, and size of the file.  If the file is unmodified, in sync with the master file, the version control state shown is blank.  Otherwise it consists of text in parentheses.  Under RCS and SCCS, the name of the user locking the file is shown; under CVS, an abbreviated version of the cvs status output is used.  Here is an example using RCS:

       /home/jim/project:

       -rw-r--r-- (jim)      Apr  2 23:39 file1
       -r--r--r--            Apr  5 20:21 file2

The files file1 and file2 are under version control, file1 is locked by user jim, and file2 is unlocked.

Here is an example using CVS:

       /home/joe/develop:

       -rw-r--r-- (modified) Aug  2  1997 file1.c
       -rw-r--r--            Apr  4 20:09 file2.c
       -rw-r--r-- (merge)    Sep 13  1996 file3.c

Here file1.c is modified with respect to the repository, and file2.c is not.  file3.c is modified, but other changes have also been checked in to the repository---you need to merge them with the work file before you can check it in.

In the above, if the repository were on a remote machine, VC would only contact it when the variable vc-stay-local (or vc-cvs-stay-local) is nil (see CVS Options).  This is because access to the repository may be slow, or you may be working offline and not have access to the repository at all.  As a consequence, VC would not be able to tell you that file3.c is in the ``merge'' state; you would learn that only when you try to check-in your modified copy of the file, or use a command such as C-x v m.

In practice, this is not a problem because CVS handles this case consistently whenever it arises.  In VC, you'll simply get prompted to merge the remote changes into your work file first.  The benefits of less network communication usually outweigh the disadvantage of not seeing remote changes immediately.

When VC Dired displays subdirectories (in the ``full'' display mode), it omits some that should never contain any files under version control. By default, this includes Version Control subdirectories such as RCS and CVS; you can customize this by setting the variable vc-directory-exclusion-list.

You can fine-tune VC Dired's format by typing C-u C-x v d---as in ordinary Dired, that allows you to specify additional switches for the ls command.

VC Dired Commands
=================

All the usual Dired commands work normally in VC Dired mode, except for v, which is redefined as the version control prefix.  You can invoke VC commands such as vc-diff and vc-print-log by typing v =, or v l, and so on.  Most of these commands apply to the file name on the current line.

The command v v (vc-next-action) operates on all the marked files, so that you can lock or check in several files at once. If it operates on more than one file, it handles each file according to its current state; thus, it might lock one file, but check in another file.  This could be confusing; it is up to you to avoid confusing behavior by marking a set of files that are in a similar state.  If no files are marked, v v operates on the file in the current line.

If any files call for check-in, v v reads a single log entry, then uses it for all the files being checked in.  This is convenient for registering or checking in several files at once, as part of the same change.

You can toggle between terse display (only locked files, or files not up-to-date) and full display at any time by typing v t (vc-dired-toggle-terse-mode).  There is also a special command * l (vc-dired-mark-locked), which marks all files currently locked (or, with CVS, all files not up-to-date).  Thus, typing * l t k is another way to delete from the buffer all files except those currently locked.

Remote Repositories
===================

A common way of using CVS is to set up a central CVS repository on some Internet host, then have each developer check out a personal working copy of the files on his local machine.  Committing changes to the repository, and picking up changes from other users into one's own working area, then works by direct interactions with the CVS server.

One difficulty is that access to the CVS server is often slow, and that developers might need to work off-line as well.  VC is designed to reduce the amount of network interaction necessary.

Version Backups
---------------

When VC sees that the CVS repository for a file is on a remote machine, it automatically makes local backups of unmodified versions of the file---automatic version backups.  This means that you can compare the file to the repository version (C-x v =), or revert to that version (C-x v u), without any network interactions.

The local copy of the unmodified file is called a version backup to indicate that it corresponds exactly to a version that is stored in the repository.  Note that version backups are not the same as ordinary Emacs backup files (see Backup (the Emacs Manual)).  But they follow a similar naming convention.

For a file that comes from a remote CVS repository, VC makes a version backup whenever you save the first changes to the file, and removes it after you have committed your modified version to the repository. You can disable the making of automatic version backups by setting vc-cvs-stay-local to nil (see CVS Options).

The name of the automatic version backup for version VERSION of file FILE is FILE.~VERSION.~.  This is almost the same as the name used by C-x v ~ (see Old Versions (the Emacs Manual)), the only difference being the additional dot (.)  after the version number.  This similarity is intentional, because both kinds of files store the same kind of information.  The file made by C-x v ~ acts as a manual version backup.

All the VC commands that operate on old versions of a file can use both kinds of version backups.  For instance, C-x v ~ uses either an automatic or a manual version backup, if possible, to get the contents of the version you request.  Likewise, C-x v = and C-x v u use either an automatic or a manual version backup, if one of them exists, to get the contents of a version to compare or revert to.  If you changed a file outside of Emacs, so that no automatic version backup was created for the previous text, you can create a manual backup of that version using C-x v ~, and thus obtain the benefit of the local copy for Emacs commands.

The only difference in Emacs's handling of manual and automatic version backups, once they exist, is that Emacs deletes automatic version backups when you commit to the repository.  By contrast, manual version backups remain until you delete them.

Local Version Control
---------------------

When you make many changes to a file that comes from a remote repository, it can be convenient to have version control on your local machine as well.  You can then record intermediate versions, revert to a previous state, etc., before you actually commit your changes to the remote server.

VC lets you do this by putting a file under a second, local version control system, so that the file is effectively registered in two systems at the same time.  For the description here, we will assume that the remote system is CVS, and you use RCS locally, although the mechanism works with any combination of version control systems (back ends).

To make it work with other back ends, you must make sure that the ``more local'' back end comes before the ``more remote'' back end in the setting of vc-handled-backends (see Customizing VC).  By default, this variable is set up so that you can use remote CVS and local RCS as described here.

To start using local RCS for a file that comes from a remote CVS server, you must register the file in RCS, by typing C-u C-x v v rcs RET.  (In other words, use vc-next-action with a prefix argument, and specify RCS as the back end.)

You can do this at any time; it does not matter whether you have already modified the file with respect to the version in the CVS repository.  If possible, VC tries to make the RCS master start with the unmodified repository version, then checks in any local changes as a new version.  This works if you have not made any changes yet, or if the unmodified repository version exists locally as a version backup (see Version Backups).  If the unmodified version is not available locally, the RCS master starts with the modified version; the only drawback to this is that you cannot compare your changes locally to what is stored in the repository.

The version number of the RCS master is derived from the current CVS version, starting a branch from it.  For example, if the current CVS version is 1.23, the local RCS branch will be 1.23.1.  Version 1.23 in the RCS master will be identical to version 1.23 under CVS; your first changes are checked in as 1.23.1.1.  (If the unmodified file is not available locally, VC will check in the modified file twice, both as 1.23 and 1.23.1.1, to make the revision numbers consistent.)

If you do not use locking under CVS (the default), locking is also disabled for RCS, so that editing under RCS works exactly as under CVS.

When you are done with local editing, you can commit the final version back to the CVS repository by typing C-u C-x v v cvs RET. This initializes the log entry buffer (see Log Buffer (the Emacs Manual)) to contain all the log entries you have recorded in the RCS master; you can edit them as you wish, and then commit in CVS by typing C-c C-c.  If the commit is successful, VC removes the RCS master, so that the file is once again registered under CVS only. (The RCS master is not actually deleted, just renamed by appending ~ to the name, so that you can refer to it later if you wish.)

While using local RCS, you can pick up recent changes from the CVS repository into your local file, or commit some of your changes back to CVS, without terminating local RCS version control.  To do this, switch to the CVS back end temporarily, with the C-x v b command:

C-x v b
     Switch to another back end that the current file is registered under (vc-switch-backend).

C-u C-x v b BACKEND RET
     Switch to BACKEND for the current file.

C-x v b does not change the buffer contents, or any files; it only changes VC's perspective on how to handle the file.  Any subsequent VC commands for that file will operate on the back end that is currently selected.

If the current file is registered in more than one back end, typing C-x v b ``cycles'' through all of these back ends.  With a prefix argument, it asks for the back end to use in the minibuffer.

Thus, if you are using local RCS, and you want to pick up some recent changes in the file from remote CVS, first visit the file, then type C-x v b to switch to CVS, and finally use C-x v m RET to merge the news (see Merging (the Emacs Manual)).  You can then switch back to RCS by typing C-x v b again, and continue to edit locally.

But if you do this, the revision numbers in the RCS master no longer correspond to those of CVS.  Technically, this is not a problem, but it can become difficult to keep track of what is in the CVS repository and what is not.  So we suggest that you return from time to time to CVS-only operation, by committing your local changes back to the repository using C-u C-x v v cvs RET.

Snapshots
=========

A snapshot is a named set of file versions (one for each registered file) that you can treat as a unit.  One important kind of snapshot is a release, a (theoretically) stable version of the system that is ready for distribution to users.

Making and Using Snapshots
--------------------------

There are two basic commands for snapshots; one makes a snapshot with a given name, the other retrieves a named snapshot.

C-x v s NAME RET
     Define the last saved versions of every registered file in or under the current directory as a snapshot named NAME (vc-create-snapshot).

C-x v r NAME RET
     For all registered files at or below the current directory level, select whatever versions correspond to the snapshot NAME (vc-retrieve-snapshot).

     This command reports an error if any files are locked at or below the current directory, without changing anything; this is to avoid overwriting work in progress.

A snapshot uses a very small amount of resources---just enough to record the list of file names and which version belongs to the snapshot.  Thus, you need not hesitate to create snapshots whenever they are useful.

You can give a snapshot name as an argument to C-x v = or C-x v ~ (see Old Versions (the Emacs Manual)). Thus, you can use it to compare a snapshot against the current files, or two snapshots against each other, or a snapshot against a named version.

Snapshot Caveats
----------------

VC's snapshot facilities are modeled on RCS's named-configuration support.  They use RCS's native facilities for this, so snapshots made using RCS through VC are visible even when you bypass VC.

With CVS, Meta-CVS, and Subversion, VC also uses the native mechanism provided by that back end to make snapshots and retrieve them (tags for CVS and Meta-CVS, copies for Subversion).

For SCCS, VC implements snapshots itself.  The files it uses contain name/file/version-number triples.  These snapshots are visible only through VC.

There is no support for VC snapshots using GNU Arch yet.

A snapshot is a set of checked-in versions.  So make sure that all the files are checked in and not locked when you make a snapshot.

File renaming and deletion can create some difficulties with snapshots. This is not a VC-specific problem, but a general design issue in version control systems that no one has solved very well yet.

If you rename a registered file, you need to rename its master along with it (the command vc-rename-file does this automatically).  If you are using SCCS, you must also update the records of the snapshot, to mention the file by its new name (vc-rename-file does this, too).  An old snapshot that refers to a master file that no longer exists under the recorded name is invalid; VC can no longer retrieve it.  It would be beyond the scope of this manual to explain enough about RCS and SCCS to explain how to update the snapshots by hand.

Using vc-rename-file makes the snapshot remain valid for retrieval, but it does not solve all problems.  For example, some of the files in your program probably refer to others by name.  At the very least, the makefile probably mentions the file that you renamed.  If you retrieve an old snapshot, the renamed file is retrieved under its new name, which is not the name that the makefile expects.  So the program won't really work as retrieved.

Miscellaneous Commands and Features of VC
=========================================

This section explains the less-frequently-used features of VC.

Change Logs and VC
------------------

If you use RCS or CVS for a program and also maintain a change log file for it (see Change Log (the Emacs Manual)), you can generate change log entries automatically from the version control log entries:

C-x v a
     Visit the current directory's change log file and, for registered files in that directory, create new entries for versions checked in since the most recent entry in the change log file. (vc-update-change-log).

     This command works with RCS or CVS only, not with any of the other back ends.

C-u C-x v a
     As above, but only find entries for the current buffer's file.

M-1 C-x v a
     As above, but find entries for all the currently visited files that are maintained with version control.  This works only with RCS, and it puts all entries in the log for the default directory, which may not be appropriate.

For example, suppose the first line of ChangeLog is dated 1999-04-10, and that the only check-in since then was by Nathaniel Bowditch to rcs2log on 1999-05-22 with log text Ignore log messages that start with `#'..  Then C-x v a visits ChangeLog and inserts text like this:

     1999-05-22  Nathaniel Bowditch  <nat@apn.org>

             * rcs2log: Ignore log messages that start with `#'.

You can then edit the new change log entry further as you wish.

Some of the new change log entries may duplicate what's already in ChangeLog.  You will have to remove these duplicates by hand.

Normally, the log entry for file foo is displayed as * foo: TEXT OF LOG ENTRY.  The : after foo is omitted if the text of the log entry starts with (FUNCTIONNAME): .  For example, if the log entry for vc.el is (vc-do-command): Check call-process status., then the text in ChangeLog looks like this:

     1999-05-06  Nathaniel Bowditch  <nat@apn.org>

             * vc.el (vc-do-command): Check call-process status.

When C-x v a adds several change log entries at once, it groups related log entries together if they all are checked in by the same author at nearly the same time.  If the log entries for several such files all have the same text, it coalesces them into a single entry. For example, suppose the most recent check-ins have the following log entries:

* For vc.texinfo: Fix expansion typos.
* For vc.el: Don't call expand-file-name.
* For vc-hooks.el: Don't call expand-file-name.

They appear like this in ChangeLog:

     1999-04-01  Nathaniel Bowditch  <nat@apn.org>

             * vc.texinfo: Fix expansion typos.

             * vc.el, vc-hooks.el: Don't call expand-file-name.

Normally, C-x v a separates log entries by a blank line, but you can mark several related log entries to be clumped together (without an intervening blank line) by starting the text of each related log entry with a label of the form {CLUMPNAME} .  The label itself is not copied to ChangeLog.  For example, suppose the log entries are:

* For vc.texinfo: {expand} Fix expansion typos.
* For vc.el: {expand} Don't call expand-file-name.
* For vc-hooks.el: {expand} Don't call expand-file-name.

Then the text in ChangeLog looks like this:

     1999-04-01  Nathaniel Bowditch  <nat@apn.org>

             * vc.texinfo: Fix expansion typos.
             * vc.el, vc-hooks.el: Don't call expand-file-name.

A log entry whose text begins with # is not copied to ChangeLog.  For example, if you merely fix some misspellings in comments, you can log the change with an entry beginning with # to avoid putting such trivia into ChangeLog.

Renaming VC Work Files and Master Files
---------------------------------------

When you rename a registered file, you must also rename its master file correspondingly to get proper results.  Use vc-rename-file to rename the source file as you specify, and rename its master file accordingly.  It also updates any snapshots (see Snapshots) that mention the file, so that they use the new name; despite this, the snapshot thus modified may not completely work (see Snapshot Caveats).

Some back ends do not provide an explicit rename operation to their repositories.  After issuing vc-rename-file, use C-x v v on the original and renamed buffers and provide the necessary edit log.

You cannot use vc-rename-file on a file that is locked by someone else.

Inserting Version Control Headers
---------------------------------

Sometimes it is convenient to put version identification strings directly into working files.  Certain special strings called version headers are replaced in each successive version by the number of that version, the name of the user who created it, and other relevant information.  All of the back ends that VC supports have such a mechanism, except GNU Arch.

VC does not normally use the information contained in these headers. The exception is RCS---with RCS, version headers are sometimes more reliable than the master file to determine which version of the file you are editing.  Note that in a multi-branch environment, version headers are necessary to make VC behave correctly (see Multi-User Branching (the Emacs Manual)).

Searching for RCS version headers is controlled by the variable vc-consult-headers.  If it is non-nil (the default), Emacs searches for headers to determine the version number you are editing.  Setting it to nil disables this feature.

Note that although CVS uses the same kind of version headers as RCS does, VC never searches for these headers if you are using CVS, regardless of the above setting.

You can use the C-x v h command (vc-insert-headers) to insert a suitable header string.

C-x v h
     Insert headers in a file for use with your version-control system.

The default header string is $Id$ for RCS and %W% for SCCS.  You can specify other headers to insert by setting the variables vc-BACKEND-header where BACKEND is rcs or sccs.

Instead of a single string, you can specify a list of strings; then each string in the list is inserted as a separate header on a line of its own.

It may be necessary to use apparently-superfluous backslashes when writing the strings that you put in this variable.  For instance, you might write "$Id\$" rather than "$Id$".  The extra backslash prevents the string constant from being interpreted as a header, if the Emacs Lisp file containing it is maintained with version control.

Each header is inserted surrounded by tabs, inside comment delimiters, on a new line at point.  Normally the ordinary comment start and comment end strings of the current mode are used, but for certain modes, there are special comment delimiters for this purpose; the variable vc-comment-alist specifies them.  Each element of this list has the form (MODE STARTER ENDER).

The variable vc-static-header-alist specifies further strings to add based on the name of the buffer.  Its value should be a list of elements of the form (REGEXP . FORMAT).  Whenever REGEXP matches the buffer name, FORMAT is inserted as part of the header.  A header line is inserted for each element that matches the buffer name, and for each string specified by vc-BACKEND-header.  The header line is made by processing the string from vc-BACKEND-header with the format taken from the element.  The default value for vc-static-header-alist is as follows:

     (("\\.c$" .
       "\n#ifndef lint\nstatic char vcid[] = \"\%s\";\n\
     #endif /* lint */\n"))

It specifies insertion of text of this form:

     #ifndef lint
     static char vcid[] = "STRING";
     #endif /* lint */

Note that the text above starts with a blank line.

If you use more than one version header in a file, put them close together in the file.  The mechanism in revert-buffer that preserves markers may not handle markers positioned between two version headers.

Customizing VC
==============

The variable vc-handled-backends determines which version control systems VC should handle.  The default value is (RCS CVS SVN SCCS Arch MCVS), so it contains all six version systems that are currently supported.  If you want VC to ignore one or more of these systems, exclude its name from the list.  To disable VC entirely, set this variable to nil.

The order of systems in the list is significant: when you visit a file registered in more than one system (see Local Version Control), VC uses the system that comes first in vc-handled-backends by default.  The order is also significant when you register a file for the first time, see Registering (the Emacs Manual) for details.

General Options
---------------

Emacs normally does not save backup files for source files that are maintained with version control.  If you want to make backup files even for files that use version control, set the variable vc-make-backup-files to a non-nil value.

Normally the work file exists all the time, whether it is locked or not.  If you set vc-keep-workfiles to nil, then checking in a new version with C-x v v deletes the work file; but any attempt to visit the file with Emacs creates it again.  (With CVS, work files are always kept.)

Editing a version-controlled file through a symbolic link can be dangerous.  It bypasses the version control system---you can edit the file without locking it, and fail to check your changes in.  Also, your changes might overwrite those of another user.  To protect against this, VC checks each symbolic link that you visit, to see if it points to a file under version control.

The variable vc-follow-symlinks controls what to do when a symbolic link points to a version-controlled file.  If it is nil, VC only displays a warning message.  If it is t, VC automatically follows the link, and visits the real file instead, telling you about this in the echo area.  If the value is ask (the default), VC asks you each time whether to follow the link.

If vc-suppress-confirm is non-nil, then C-x v v and C-x v i can save the current buffer without asking, and C-x v u also operates without asking for confirmation.  (This variable does not affect C-x v c; that operation is so drastic that it should always ask for confirmation.)

VC mode does much of its work by running the shell commands for RCS, CVS and SCCS.  If vc-command-messages is non-nil, VC displays messages to indicate which shell commands it runs, and additional messages when the commands finish.

You can specify additional directories to search for version control programs by setting the variable vc-path.  These directories are searched before the usual search path.  It is rarely necessary to set this variable, because VC normally finds the proper files automatically.

Options for RCS and SCCS
------------------------

By default, RCS uses locking to coordinate the activities of several users, but there is a mode called non-strict locking in which you can check-in changes without locking the file first.  Use rcs -U to switch to non-strict locking for a particular file, see the rcs manual page for details.

When deducing the version control state of an RCS file, VC first looks for an RCS version header string in the file (see Version Headers).  If there is no header string, VC normally looks at the file permissions of the work file; this is fast.  But there might be situations when the file permissions cannot be trusted.  In this case the master file has to be consulted, which is rather expensive.  Also the master file can only tell you if there's any lock on the file, but not whether your work file really contains that locked version.

You can tell VC not to use version headers to determine the file status by setting vc-consult-headers to nil.  VC then always uses the file permissions (if it is supposed to trust them), or else checks the master file.

You can specify the criterion for whether to trust the file permissions by setting the variable vc-mistrust-permissions. Its value can be t (always mistrust the file permissions and check the master file), nil (always trust the file permissions), or a function of one argument which makes the decision. The argument is the directory name of the RCS subdirectory.  A non-nil value from the function says to mistrust the file permissions.  If you find that the file permissions of work files are changed erroneously, set vc-mistrust-permissions to t. Then VC always checks the master file to determine the file's status.

VC determines the version control state of files under SCCS much as with RCS.  It does not consider SCCS version headers, though.  Thus, the variable vc-mistrust-permissions affects SCCS use, but vc-consult-headers does not.

Options specific for CVS
------------------------

By default, CVS does not use locking to coordinate the activities of several users; anyone can change a work file at any time.  However, there are ways to restrict this, resulting in behavior that resembles locking.

For one thing, you can set the CVSREAD environment variable (the value you use makes no difference).  If this variable is defined, CVS makes your work files read-only by default.  In Emacs, you must type C-x v v to make the file writable, so that editing works in fact similar as if locking was used.  Note however, that no actual locking is performed, so several users can make their files writable at the same time.  When setting CVSREAD for the first time, make sure to check out all your modules anew, so that the file protections are set correctly.

Another way to achieve something similar to locking is to use the watch feature of CVS.  If a file is being watched, CVS makes it read-only by default, and you must also use C-x v v in Emacs to make it writable.  VC calls cvs edit to make the file writable, and CVS takes care to notify other developers of the fact that you intend to change the file.  See the CVS documentation for details on using the watch feature.

When a file's repository is on a remote machine, VC tries to keep network interactions to a minimum.  This is controlled by the variable vc-cvs-stay-local.  There is another variable, vc-stay-local, which enables the feature also for other back ends that support it, including CVS.  In the following, we will talk only about vc-cvs-stay-local, but everything applies to vc-stay-local as well.

If vc-cvs-stay-local is t (the default), then VC uses only the entry in the local CVS subdirectory to determine the file's state (and possibly information returned by previous CVS commands). One consequence of this is that when you have modified a file, and somebody else has already checked in other changes to the file, you are not notified of it until you actually try to commit.  (But you can try to pick up any recent changes from the repository first, using C-x v m RET, see Merging (the Emacs Manual)).

When vc-cvs-stay-local is t, VC also makes local version backups, so that simple diff and revert operations are completely local (see Version Backups).

On the other hand, if you set vc-cvs-stay-local to nil, then VC queries the remote repository before it decides what to do in vc-next-action (C-x v v), just as it does for local repositories.  It also does not make any version backups.

You can also set vc-cvs-stay-local to a regular expression that is matched against the repository host name; VC then stays local only for repositories from hosts that match the pattern.

You can specify additional command line options to pass to all CVS operations in the variable vc-cvs-global-switches.  These switches are inserted immediately after the cvs command, before the name of the operation to invoke.

Fortran Mode
************

Fortran mode provides special motion commands for Fortran statements and subprograms, and indentation commands that understand Fortran conventions of nesting, line numbers and continuation statements. Fortran mode has 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 M-x fortran-mode to switch to this major mode.  This command runs the hook fortran-mode-hook.  See Hooks (the Emacs Manual).

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 (f90-mode). Emacs normally uses Fortran mode for files with extension .f, .F or .for, and F90 mode for the extension .f90 and .f95.  GNU Fortran supports both kinds of format.

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.

C-c C-n
     Move to the beginning of the next statement (fortran-next-statement/f90-next-statement).

C-c C-p
     Move to the beginning of the previous statement (fortran-previous-statement/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.

C-c C-e
     Move point forward to the start of the next code block (f90-next-block).  A code block is a subroutine, if--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.

C-c C-a
     Move point backward to the previous code block (f90-previous-block).  This is like f90-next-block, but moves backwards.

C-M-n
     Move to the end of the current code block (fortran-end-of-block/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.

C-M-p
     Move to the start of the current code block (fortran-beginning-of-block/f90-beginning-of-block). This is like fortran-end-of-block, but moves backwards.

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.

Fortran Indentation and Filling Commands
----------------------------------------

C-M-j
     Break the current line at point and set up a continuation line (fortran-split-line).

M-^
     Join this line to the previous line (fortran-join-line).

C-M-q
     Indent all the lines of the subprogram point is in (fortran-indent-subprogram).

M-q
     Fill a comment block or statement.

The key C-M-q runs fortran-indent-subprogram, a command to reindent all the lines of the Fortran subprogram (function or subroutine) containing point.

The key C-M-j runs fortran-split-line, which splits a line in the appropriate fashion for Fortran.  In a non-comment line, the second half becomes a continuation line and is indented accordingly.  In a comment line, both halves become separate comment lines.

M-^ or C-c C-d runs the command fortran-join-line, which joins a continuation line back to the previous line, roughly as the inverse of fortran-split-line.  The point must be on a continuation line when this command is invoked.

M-q in Fortran mode fills the comment block or statement that point is in.  This removes any excess statement continuations.

Continuation Lines
------------------

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

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 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 fortran-tab-mode-default (nil for fixed format, and non-nil for tab format) is used.  /t in the mode line indicates tab format is selected.  Fortran mode sets the value of indent-tabs-mode accordingly.

If the text on a line starts with the Fortran continuation marker $, or if it begins with any non-whitespace character in column 5, Fortran mode treats it as a continuation line.  When you indent a continuation line with TAB, it converts the line to the current continuation style.  When you split a Fortran statement with C-M-j, the continuation marker on the newline is created according to the continuation style.

The setting of continuation style affects several other aspects of editing in Fortran mode.  In fixed format mode, the minimum column number for the body of a statement is 6.  Lines inside of Fortran blocks that are indented to larger column numbers always use only the space character for whitespace.  In tab format mode, the minimum column number for the statement body is 8, and the whitespace before column 8 must always consist of one tab character.

Line Numbers
------------

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

Line numbers of four digits or less are normally indented one space. The variable fortran-line-number-indent controls this; it specifies the maximum indentation a line number can have.  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 fortran-line-number-indent has the value 5, line numbers are right-justified to end in column 4.

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

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:

  * Two nested do loops never share a continue statement.

  * Fortran keywords such as if, else, then, 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 else if or end do are acceptable, but the second word should be on the same line as the first and not on a continuation line.

If you fail to follow these conventions, the indentation commands may indent some lines unaesthetically.  However, a correct Fortran program retains its meaning when reindented even if the conventions are not followed.

Variables for Fortran Indentation
---------------------------------

Several additional variables control how Fortran indentation works:

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

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

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

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

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

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

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.

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.

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

Fortran mode can also handle the Fortran90 comment syntax where comments start with ! 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 fortran-comment-line-start to "!".

M-;
     Align comment or insert new comment (fortran-indent-comment).

C-x ;
     Applies to nonstandard ! comments only.

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

M-; in Fortran mode is redefined as the command fortran-indent-comment.  Like the usual M-; command, this recognizes any kind of existing comment and aligns its text appropriately; if there is no existing comment, a comment is inserted and aligned.  But inserting and aligning comments are not the same in Fortran mode as in other modes.

When a new comment must be inserted, if the current line is blank, a full-line comment is inserted.  On a non-blank line, a nonstandard ! comment is inserted if you have said you want to use them.  Otherwise a full-line comment is inserted on a new line before the current line.

Nonstandard ! comments are aligned like comments in other languages, but full-line comments are different.  In a standard full-line comment, the comment delimiter itself must always appear in column zero. What can be aligned is the text within the comment.  You can choose from three styles of alignment by setting the variable fortran-comment-indent-style to one of these values:

fixed
     Align the text at a fixed column, which is the sum of fortran-comment-line-extra-indent and the minimum statement indentation.  This is the default.

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

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

nil
     Don't move text in full-line comments automatically.

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

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 fortran-comment-indent-style.  The variable 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 C-x ; has not been redefined.  If you use ! comments, this command can be used with them.  Otherwise it is useless in Fortran mode.

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

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 fortran-continuation-string (see ForIndent Cont).  This splitting happens when you type SPC, RET, or TAB, and also in the Fortran indentation commands.  You activate Auto Fill in Fortran mode in the normal way.  See Auto Fill (the Emacs Manual).

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

To enable Auto Fill in all Fortran buffers, add turn-on-auto-fill to fortran-mode-hook.  See Hooks (the Emacs Manual).

Checking Columns in Fortran
===========================

C-c C-r
     Display a ``column ruler'' momentarily above the current line (fortran-column-ruler).

C-c C-w
     Split the current window horizontally temporarily so that it is 72 columns wide (fortran-window-create-momentarily).  This may help you avoid making lines longer than the 72-character limit that some Fortran compilers impose.

C-u C-c C-w
     Split the current window horizontally so that it is 72 columns wide (fortran-window-create).  You can then continue editing.

M-x fortran-strip-sequence-nos
     Delete all text in column 72 and beyond.

The command C-c C-r (fortran-column-ruler) shows a column ruler momentarily above the current line.  The comment ruler is two lines of text that show you the locations of columns with special significance in Fortran programs.  Square brackets show the limits of the columns for line numbers, and curly brackets show the limits of the columns for the statement body.  Column numbers appear above them.

Note that the column numbers count from zero, as always in GNU Emacs. As a result, the numbers may be one less than those you are familiar with; but the positions they indicate in the line are standard for Fortran.

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

C-c C-w (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.

You can also split the window horizontally and continue editing with the split in place.  To do this, use C-u C-c C-w (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.

The command 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.

Fortran Keyword Abbrevs
=======================

Fortran mode provides many built-in abbrevs for common keywords and declarations.  These are the same sort of abbrev that you can define yourself.  To use them, you must turn on Abbrev mode. See Abbrevs (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 ;c for continue.  If you insert ;c and then insert a punctuation character such as a space or a newline, the ;c expands automatically to continue, provided Abbrev mode is enabled.

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

Emacs and MS-DOS
****************

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 (see Microsoft Systems (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.

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

Keyboard Usage on MS-DOS
========================

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

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

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

The variable 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 ENTER key to act like C-j, by putting the following line into your _emacs file:

     ;; Make the ENTER key from the numeric keypad act as C-j.
     (define-key function-key-map [kp-enter] [?\C-j])

Mouse Usage on 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 (see Menu Bar (the Emacs Manual)).  Scroll bars don't work in MS-DOS Emacs.  PC mice usually have only two buttons; these act as Mouse-1 and Mouse-2, but if you press both of them together, that has the effect of 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 (see Mouse References (the Emacs Manual)) is also supported.

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 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 _emacs init file:

     ;; Treat the mouse like a 2-button mouse.
     (msdos-set-mouse-buttons 2)

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 (see Mouse Commands (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.

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

Display on 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 font-lock, Enriched Text mode, and others) by defining the relevant faces to use different colors.  Use the list-colors-display command (see Frame Parameters (the Emacs Manual)) and the list-faces-display command (see Faces (the Emacs Manual)) to see what colors and faces are available and what they look like.

See 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.

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 cursor-type parameter in the variable default-frame-alist (see Creating Frames (the Emacs Manual)).  The MS-DOS terminal doesn't support a vertical-bar cursor, so the bar cursor is horizontal, and the WIDTH parameter, if specified by the frame parameters, actually determines its height. For this reason, the bar and 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:

      '(cursor-type bar WIDTH . START)

In addition, if the WIDTH parameter is negative, the cursor bar begins at the top of the character cell.

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 (see Frames (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.

The mode4350 command switches the display to 43 or 50 lines, depending on your hardware; the 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 N rows by M columns dimensions, it checks if there is a variable called screen-dimensions-NxM, 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 Set Video Mode function with the value of screen-dimensions-NxM in the 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 _emacs file:

     (setq screen-dimensions-66x80 85)

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 screen-dimensions-NxM 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 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 screen-dimensions-36x80 with the same video mode value as screen-dimensions-38x80.

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

File Names on MS-DOS
====================

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 .  in file names are invalid in MS-DOS, so Emacs transparently converts them to underscores _; thus your default init file (see Init File (the Emacs Manual)) is called _emacs on MS-DOS.  Excess characters before or after the period are generally ignored by MS-DOS itself; thus, if you visit the file LongFileName.EvenLongerExtension, you will silently get 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.

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

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 LFN to 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.

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 HOME environment variable.  That is, if your Emacs binary, emacs.exe, is in the directory c:/utils/emacs/bin, then Emacs acts as if HOME were set to c:/utils/emacs.  In particular, that is where Emacs looks for the init file _emacs. With this in mind, you can use ~ in file names as an alias for the home directory, as you would on GNU or Unix.  You can also set 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 /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 /dev on any disk.

Printing and MS-DOS
===================

Printing commands, such as lpr-buffer (see Printing (the Emacs Manual)) and ps-print-buffer (see PostScript (the Emacs Manual)) can work on MS-DOS by sending the output to one of the printer ports, if a Posix-style 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.

See MS-Windows Printing (the Emacs Manual), for details.

Some printers expect DOS codepage encoding of non-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.  See MS-DOS and MULE.  When you print to such printers from Windows, you can use the C-x RET c (universal-coding-system-argument) command before M-x lpr-buffer; Emacs will then convert the text to the DOS codepage that you specify.  For example, 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 cpNNN coding system with M-x codepage-setup.

For backwards compatibility, the value of dos-printer (dos-ps-printer), if it has a value, overrides the value of printer-name (ps-printer-name), on MS-DOS.

International Support on MS-DOS
===============================

Emacs on MS-DOS supports the same international character sets as it does on GNU, Unix and other platforms (see International (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 code-pages.el package, which implements support for MS-DOS- and MS-Windows-specific encodings for all platforms other than MS-DOS.

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

M-x codepage-setup
     Create a coding system for a certain DOS codepage.

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 DOS codepages. Each codepage includes all 128 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 (Normally, one particular codepage is burnt into the display memory, while other codepages can be installed by modifying system configuration files, such as 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.

If you invoke Emacs on MS-DOS with the --unibyte option (see Initial Options (the Emacs Manual)), Emacs does not perform any conversion of non-ASCII characters.  Instead, it reads and writes any non-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.

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 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 dos-codepage in your init file.

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 cpNNN, where NNN is the codepage number. (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 ç (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.)

All the cpNNN coding systems use the letter 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 cpNNN coding system at startup, it is normal for the mode line on MS-DOS to begin with -DD\-.  See Mode Line (the Emacs Manual).  Far-Eastern DOS terminals do not use the cpNNN 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 set-language-environment to select the language environment for that script (see Language Environments (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 ASCII characters.  For example, if the current codepage doesn't have a glyph for the letter ò (small o with a grave accent), it is displayed as {`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.

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 eight-bit-control and 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.

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 dos-unsupported-character-glyph variable; by default, this glyph is an empty triangle.  Use the C-u C-x = command to display the actual code and character set of such characters.  See Position Info (the Emacs Manual).

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 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 (see Text Coding (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.

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 -nw option.  Support for codepages in the Windows port of Emacs is part of the code-pages.el package.

Subprocesses on 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 M-x compile, searching files with M-x grep and displaying differences between files with 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 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 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 does support asynchronous subprocesses.  See Windows Processes (the Emacs Manual).

Printing commands, such as lpr-buffer (see Printing (the Emacs Manual)) and ps-print-buffer (see PostScript (the Emacs Manual)), work in MS-DOS by sending the output to one of the printer ports. See MS-DOS Printing (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 C-c or C-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.

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