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[bpt/emacs.git] / man / mule.texi

@c This is part of the Emacs manual.
@c Copyright (C) 1997, 1999, 2000 Free Software Foundation, Inc.
@c See file emacs.texi for copying conditions.
@node International, Major Modes, Frames, Top
@chapter International Character Set Support
@cindex MULE
@cindex international scripts
@cindex multibyte characters
@cindex encoding of characters

@cindex Celtic
@cindex Chinese
@cindex Cyrillic
@cindex Czech
@cindex Devanagari
@cindex Hindi
@cindex Marathi
@cindex Ethiopic
@cindex German
@cindex Greek
@cindex Hebrew
@cindex IPA
@cindex Japanese
@cindex Korean
@cindex Lao
@cindex Latin
@cindex Polish
@cindex Romanian
@cindex Slovak
@cindex Slovenian
@cindex Thai
@cindex Tibetan
@cindex Turkish
@cindex Vietnamese
  Emacs supports a wide variety of international character sets,
including European variants of the Latin alphabet, as well as Chinese,
Cyrillic, Devanagari (Hindi and Marathi), Ethiopic, Greek, Hebrew, IPA,
Japanese, Korean, Lao, Thai, Tibetan, and Vietnamese scripts.  These features
have been merged from the modified version of Emacs known as MULE (for
``MULti-lingual Enhancement to GNU Emacs'')

@menu
* International Intro::     Basic concepts of multibyte characters.
* Enabling Multibyte::      Controlling whether to use multibyte characters.
* Language Environments::   Setting things up for the language you use.
* Input Methods::           Entering text characters not on your keyboard.
* Select Input Method::     Specifying your choice of input methods.
* Multibyte Conversion::    How single-byte characters convert to multibyte.
* Coding Systems::          Character set conversion when you read and
                              write files, and so on.
* Recognize Coding::        How Emacs figures out which conversion to use.
* Specify Coding::          Various ways to choose which conversion to use.
* Fontsets::                Fontsets are collections of fonts
                              that cover the whole spectrum of characters.
* Defining Fontsets::       Defining a new fontset.
* Single-Byte Character Support::
                            You can pick one European character set
                            to use without multibyte characters.
@end menu

@node International Intro
@section Introduction to International Character Sets

  The users of international character sets and scripts have established
many more-or-less standard coding systems for storing files.  Emacs
internally uses a single multibyte character encoding, so that it can
intermix characters from all these scripts in a single buffer or string.
This encoding represents each non-ASCII character as a sequence of bytes
in the range 0200 through 0377.  Emacs translates between the multibyte
character encoding and various other coding systems when reading and
writing files, when exchanging data with subprocesses, and (in some
cases) in the @kbd{C-q} command (@pxref{Multibyte Conversion}).

@kindex C-h h
@findex view-hello-file
  The command @kbd{C-h h} (@code{view-hello-file}) displays the file
@file{etc/HELLO}, which shows how to say ``hello'' in many languages.
This illustrates various scripts.  If the font you're using doesn't have
characters for all those different languages, you will see some hollow
boxes instead of characters; see @ref{Fontsets}.

@findex list-charset-chars
@cindex characters in a certain charset
  The command @kbd{M-x list-charset-chars} prompts for a name of a
character set, and displays all the characters in that character set.

  Keyboards, even in the countries where these character sets are used,
generally don't have keys for all the characters in them.  So Emacs
supports various @dfn{input methods}, typically one for each script or
language, to make it convenient to type them.

@kindex C-x RET
  The prefix key @kbd{C-x @key{RET}} is used for commands that pertain
to multibyte characters, coding systems, and input methods.

@node Enabling Multibyte
@section Enabling Multibyte Characters

  You can enable or disable multibyte character support, either for
Emacs as a whole, or for a single buffer.  When multibyte characters are
disabled in a buffer, then each byte in that buffer represents a
character, even codes 0200 through 0377.  The old features for
supporting the European character sets, ISO Latin-1 and ISO Latin-2,
work as they did in Emacs 19 and also work for the other ISO 8859
character sets.

  However, there is no need to turn off multibyte character support to
use ISO Latin; the Emacs multibyte character set includes all the
characters in these character sets, and Emacs can translate
automatically to and from the ISO codes.

  To edit a particular file in unibyte representation, visit it using
@code{find-file-literally}.  @xref{Visiting}.  To convert a buffer in
multibyte representation into a single-byte representation of the same
characters, the easiest way is to save the contents in a file, kill the
buffer, and find the file again with @code{find-file-literally}.  You
can also use @kbd{C-x @key{RET} c}
(@code{universal-coding-system-argument}) and specify @samp{raw-text} as
the coding system with which to find or save a file.  @xref{Specify
Coding}.  Finding a file as @samp{raw-text} doesn't disable format
conversion, uncompression and auto mode selection as
@code{find-file-literally} does.

@vindex enable-multibyte-characters
@vindex default-enable-multibyte-characters
  To turn off multibyte character support by default, start Emacs with
the @samp{--unibyte} option (@pxref{Initial Options}), or set the
environment variable @env{EMACS_UNIBYTE}.  You can also customize
@code{enable-multibyte-characters} or, equivalently, directly set the
variable @code{default-enable-multibyte-characters} in your init file to
have basically the same effect as @samp{--unibyte}.

@cindex Lisp files, and multibyte operation
@cindex multibyte operation, and Lisp files
@cindex unibyte operation, and Lisp files
@cindex init file, and non-ASCII characters
@cindex environment variables, and non-ASCII characters
  Multibyte strings are not created during initialization from the
values of environment variables, @file{/etc/passwd} entries etc.@: that
contain non-ASCII 8-bit characters.  However, Lisp files, when they are
loaded for running, and in particular the initialization file
@file{.emacs}, are normally read as multibyte---even with
@samp{--unibyte}.  To avoid multibyte strings being generated by
non-ASCII characters in Lisp files, put @samp{-*-unibyte: t;-*-} in a
comment on the first line, or specify the coding system @samp{raw-text}
with @kbd{C-x @key{RET} c}.  Do the same for initialization files for
packages like Gnus.

  The mode line indicates whether multibyte character support is enabled
in the current buffer.  If it is, there are two or more characters (most
often two dashes) before the colon near the beginning of the mode line.
When multibyte characters are not enabled, just one dash precedes the
colon.

@node Language Environments
@section Language Environments
@cindex language environments

  All supported character sets are supported in Emacs buffers whenever
multibyte characters are enabled; there is no need to select a
particular language in order to display its characters in an Emacs
buffer.  However, it is important to select a @dfn{language environment}
in order to set various defaults.  The language environment really
represents a choice of preferred script (more or less) rather than a
choice of language.

  The language environment controls which coding systems to recognize
when reading text (@pxref{Recognize Coding}).  This applies to files,
incoming mail, netnews, and any other text you read into Emacs.  It may
also specify the default coding system to use when you create a file.
Each language environment also specifies a default input method.

@findex set-language-environment
@vindex current-language-environment
  To select a language environment, customize the option
@code{current-language-environment} or use the command @kbd{M-x
set-language-environment}.  It makes no difference which buffer is
current when you use this command, because the effects apply globally to
the Emacs session.  The supported language environments include:

@cindex euro sign
@quotation
Chinese-BIG5, Chinese-CNS, Chinese-GB, Cyrillic-ALT, Cyrillic-ISO,
Cyrillic-KOI8, Czech, Devanagari, English, Ethiopic, German, Greek,
Hebrew, IPA, Japanese, Korean, Lao, Latin-1, Latin-2, Latin-3, Latin-4,
Latin-5, Latin-8 (Celtic), Latin-9 (updated Latin-1, with the Euro
sign), Polish, Romanian, Slovak, Slovenian, Thai, Tibetan, Turkish, and
Vietnamese.
@end quotation

@findex set-locale-environment
@vindex locale-language-names
@vindex locale-charset-language-names
  Some operating systems let you specify the language you are using by
setting the locale environment variables @env{LC_ALL}, @env{LC_CTYPE},
and @env{LANG}; the first of these which is nonempty specifies your
locale.  Emacs handles this during startup by invoking the
@code{set-locale-environment} function, which matches your locale
against entries in the value of the variable
@code{locale-language-names} and selects the corresponding language
environment if a match is found.  But if your locale also matches an
entry in the variable @code{locale-charset-language-names}, this entry
is preferred if its character set disagrees.  For example, suppose the
locale @samp{en_GB.ISO8859-15} matches @code{"Latin-1"} in
@code{locale-language-names} and @code{"Latin-9"} in
@code{locale-charset-language-names}; since these two language
environments' character sets disagree, Emacs uses @code{"Latin-9"}.

  If all goes well, the @code{set-locale-environment} function selects
the language environment, since language is part of locale.  It also
adjusts the display table and terminal coding system, the locale coding
system, and the preferred coding system as needed for the locale.

  Since the @code{set-locale-environment} function is automatically
invoked during startup, you normally do not need to invoke it yourself.
However, if you modify the @env{LC_ALL}, @env{LC_CTYPE}, or @env{LANG}
environment variables, you may want to invoke the
@code{set-locale-environment} function afterwards.

@findex set-locale-environment
@vindex locale-preferred-coding-systems
  The @code{set-locale-environment} function normally uses the preferred
coding system established by the language environment to decode system
messages.  But if your locale matches an entry in the variable
@code{locale-preferred-coding-systems}, Emacs uses the corresponding
coding system instead.  For example, if the locale @samp{ja_JP.PCK}
matches @code{japanese-shift-jis} in
@code{locale-preferred-coding-systems}, Emacs uses that encoding even
though it might normally use @code{japanese-iso-8bit}.

  The environment chosen from the locale when Emacs starts is
overidden by any explicit use of the command
@code{set-language-environment} or customization of
@code{current-language-environment} in your init file.

@kindex C-h L
@findex describe-language-environment
  To display information about the effects of a certain language
environment @var{lang-env}, use the command @kbd{C-h L @var{lang-env}
@key{RET}} (@code{describe-language-environment}).  This tells you which
languages this language environment is useful for, and lists the
character sets, coding systems, and input methods that go with it.  It
also shows some sample text to illustrate scripts used in this language
environment.  By default, this command describes the chosen language
environment.

@vindex set-language-environment-hook
  You can customize any language environment with the normal hook
@code{set-language-environment-hook}.  The command
@code{set-language-environment} runs that hook after setting up the new
language environment.  The hook functions can test for a specific
language environment by checking the variable
@code{current-language-environment}.

@vindex exit-language-environment-hook
  Before it starts to set up the new language environment,
@code{set-language-environment} first runs the hook
@code{exit-language-environment-hook}.  This hook is useful for undoing
customizations that were made with @code{set-language-environment-hook}.
For instance, if you set up a special key binding in a specific language
environment using @code{set-language-environment-hook}, you should set
up @code{exit-language-environment-hook} to restore the normal binding
for that key.

@node Input Methods
@section Input Methods

@cindex input methods
  An @dfn{input method} is a kind of character conversion designed
specifically for interactive input.  In Emacs, typically each language
has its own input method; sometimes several languages which use the same
characters can share one input method.  A few languages support several
input methods.

  The simplest kind of input method works by mapping ASCII letters into
another alphabet.  This is how the Greek and Russian input methods work.

  A more powerful technique is composition: converting sequences of
characters into one letter.  Many European input methods use composition
to produce a single non-ASCII letter from a sequence that consists of a
letter followed by accent characters (or vice versa).  For example, some
methods convert the sequence @kbd{a'} into a single accented letter.
These input methods have no special commands of their own; all they do
is compose sequences of printing characters.

  The input methods for syllabic scripts typically use mapping followed
by composition.  The input methods for Thai and Korean work this way.
First, letters are mapped into symbols for particular sounds or tone
marks; then, sequences of these which make up a whole syllable are
mapped into one syllable sign.

  Chinese and Japanese require more complex methods.  In Chinese input
methods, first you enter the phonetic spelling of a Chinese word (in
input method @code{chinese-py}, among others), or a sequence of portions
of the character (input methods @code{chinese-4corner} and
@code{chinese-sw}, and others).  Since one phonetic spelling typically
corresponds to many different Chinese characters, you must select one of
the alternatives using special Emacs commands.  Keys such as @kbd{C-f},
@kbd{C-b}, @kbd{C-n}, @kbd{C-p}, and digits have special definitions in
this situation, used for selecting among the alternatives.  @key{TAB}
displays a buffer showing all the possibilities.

   In Japanese input methods, first you input a whole word using
phonetic spelling; then, after the word is in the buffer, Emacs converts
it into one or more characters using a large dictionary.  One phonetic
spelling corresponds to many differently written Japanese words, so you
must select one of them; use @kbd{C-n} and @kbd{C-p} to cycle through
the alternatives.

  Sometimes it is useful to cut off input method processing so that the
characters you have just entered will not combine with subsequent
characters.  For example, in input method @code{latin-1-postfix}, the
sequence @kbd{e '} combines to form an @samp{e} with an accent.  What if
you want to enter them as separate characters?

  One way is to type the accent twice; that is a special feature for
entering the separate letter and accent.  For example, @kbd{e ' '} gives
you the two characters @samp{e'}.  Another way is to type another letter
after the @kbd{e}---something that won't combine with that---and
immediately delete it.  For example, you could type @kbd{e e @key{DEL}
'} to get separate @samp{e} and @samp{'}.

  Another method, more general but not quite as easy to type, is to use
@kbd{C-\ C-\} between two characters to stop them from combining.  This
is the command @kbd{C-\} (@code{toggle-input-method}) used twice.
@ifinfo
@xref{Select Input Method}.
@end ifinfo

  @kbd{C-\ C-\} is especially useful inside an incremental search,
because it stops waiting for more characters to combine, and starts
searching for what you have already entered.

@vindex input-method-verbose-flag
@vindex input-method-highlight-flag
  The variables @code{input-method-highlight-flag} and
@code{input-method-verbose-flag} control how input methods explain what
is happening.  If @code{input-method-highlight-flag} is non-@code{nil},
the partial sequence is highlighted in the buffer.  If
@code{input-method-verbose-flag} is non-@code{nil}, the list of possible
characters to type next is displayed in the echo area (but not when you
are in the minibuffer).

@cindex Leim package
Input methods are implemented in the separate Leim package, which must
be installed with Emacs.

@node Select Input Method
@section Selecting an Input Method

@table @kbd
@item C-\
Enable or disable use of the selected input method.

@item C-x @key{RET} C-\ @var{method} @key{RET}
Select a new input method for the current buffer.

@item C-h I @var{method} @key{RET}
@itemx C-h C-\ @var{method} @key{RET}
@findex describe-input-method
@kindex C-h I
@kindex C-h C-\
Describe the input method @var{method} (@code{describe-input-method}).
By default, it describes the current input method (if any).  This
description should give you the full details of how to use any
particular input method once you have selected it but before selecting
it once you will typically get only a two-line summary.

@item M-x list-input-methods
Display a list of all the supported input methods.
@end table

@findex set-input-method
@vindex current-input-method
@kindex C-x RET C-\
  To choose an input method for the current buffer, use @kbd{C-x
@key{RET} C-\} (@code{set-input-method}).  This command reads the
input method name with the minibuffer; the name normally starts with the
language environment that it is meant to be used with.  The variable
@code{current-input-method} records which input method is selected.
  
@findex toggle-input-method
@kindex C-\
  Input methods use various sequences of ASCII characters to stand for
non-ASCII characters.  Sometimes it is useful to turn off the input
method temporarily.  To do this, type @kbd{C-\}
(@code{toggle-input-method}).  To reenable the input method, type
@kbd{C-\} again.

  If you type @kbd{C-\} and you have not yet selected an input method,
it prompts for you to specify one.  This has the same effect as using
@kbd{C-x @key{RET} C-\} to specify an input method.

@vindex default-input-method
  Selecting a language environment specifies a default input method for
use in various buffers.  When you have a default input method, you can
select it in the current buffer by typing @kbd{C-\}.  The variable
@code{default-input-method} specifies the default input method
(@code{nil} means there is none).

@findex quail-set-keyboard-layout
  Some input methods for alphabetic scripts work by (in effect)
remapping the keyboard to emulate various keyboard layouts commonly used
for those scripts.  How to do this remapping properly depends on your
actual keyboard layout.  To specify which layout your keyboard has, use
the command @kbd{M-x quail-set-keyboard-layout}.

@findex list-input-methods
  To display a list of all the supported input methods, type @kbd{M-x
list-input-methods}.  The list gives information about each input
method, including the string that stands for it in the mode line.

@node Multibyte Conversion
@section Unibyte and Multibyte Non-ASCII characters

  When multibyte characters are enabled, character codes 0240 (octal)
through 0377 (octal) are not really legitimate in the buffer.  The valid
non-ASCII printing characters have codes that start from 0400.

  If you type a self-inserting character in the invalid range 0240
through 0377, Emacs assumes you intended to use one of the ISO
Latin-@var{n} character sets, and converts it to the Emacs code
representing that Latin-@var{n} character.  You select @emph{which} ISO
Latin character set to use through your choice of language environment
@iftex
(see above).
@end iftex
@ifinfo
(@pxref{Language Environments}).
@end ifinfo
If you do not specify a choice, the default is Latin-1.

  The same thing happens when you use @kbd{C-q} to enter an octal code
in this range.

@node Coding Systems
@section Coding Systems
@cindex coding systems

  Users of various languages have established many more-or-less standard
coding systems for representing them.  Emacs does not use these coding
systems internally; instead, it converts from various coding systems to
its own system when reading data, and converts the internal coding
system to other coding systems when writing data.  Conversion is
possible in reading or writing files, in sending or receiving from the
terminal, and in exchanging data with subprocesses.

  Emacs assigns a name to each coding system.  Most coding systems are
used for one language, and the name of the coding system starts with the
language name.  Some coding systems are used for several languages;
their names usually start with @samp{iso}.  There are also special
coding systems @code{no-conversion}, @code{raw-text} and
@code{emacs-mule} which do not convert printing characters at all.

@cindex end-of-line conversion
  In addition to converting various representations of non-ASCII
characters, a coding system can perform end-of-line conversion.  Emacs
handles three different conventions for how to separate lines in a file:
newline, carriage-return linefeed, and just carriage-return.

@table @kbd
@item C-h C @var{coding} @key{RET}
Describe coding system @var{coding}.

@item C-h C @key{RET}
Describe the coding systems currently in use.

@item M-x list-coding-systems
Display a list of all the supported coding systems.
@end table

@kindex C-h C
@findex describe-coding-system
  The command @kbd{C-h C} (@code{describe-coding-system}) displays
information about particular coding systems.  You can specify a coding
system name as argument; alternatively, with an empty argument, it
describes the coding systems currently selected for various purposes,
both in the current buffer and as the defaults, and the priority list
for recognizing coding systems (@pxref{Recognize Coding}).

@findex list-coding-systems
  To display a list of all the supported coding systems, type @kbd{M-x
list-coding-systems}.  The list gives information about each coding
system, including the letter that stands for it in the mode line
(@pxref{Mode Line}).

@cindex end-of-line conversion
@cindex MS-DOS end-of-line conversion
@cindex Macintosh end-of-line conversion
  Each of the coding systems that appear in this list---except for
@code{no-conversion}, which means no conversion of any kind---specifies
how and whether to convert printing characters, but leaves the choice of
end-of-line conversion to be decided based on the contents of each file.
For example, if the file appears to use the sequence carriage-return
linefeed to separate lines, DOS end-of-line conversion will be used.

  Each of the listed coding systems has three variants which specify
exactly what to do for end-of-line conversion:

@table @code
@item @dots{}-unix
Don't do any end-of-line conversion; assume the file uses
newline to separate lines.  (This is the convention normally used
on Unix and GNU systems.)

@item @dots{}-dos
Assume the file uses carriage-return linefeed to separate lines, and do
the appropriate conversion.  (This is the convention normally used on
Microsoft systems.@footnote{It is also specified for MIME `text/*'
bodies and in other network transport contexts.  It is different
from the SGML reference syntax record-start/record-end format which
Emacs doesn't support directly.})

@item @dots{}-mac
Assume the file uses carriage-return to separate lines, and do the
appropriate conversion.  (This is the convention normally used on the
Macintosh system.)
@end table

  These variant coding systems are omitted from the
@code{list-coding-systems} display for brevity, since they are entirely
predictable.  For example, the coding system @code{iso-latin-1} has
variants @code{iso-latin-1-unix}, @code{iso-latin-1-dos} and
@code{iso-latin-1-mac}.

  The coding system @code{raw-text} is good for a file which is mainly
ASCII text, but may contain byte values above 127 which are not meant to
encode non-ASCII characters.  With @code{raw-text}, Emacs copies those
byte values unchanged, and sets @code{enable-multibyte-characters} to
@code{nil} in the current buffer so that they will be interpreted
properly.  @code{raw-text} handles end-of-line conversion in the usual
way, based on the data encountered, and has the usual three variants to
specify the kind of end-of-line conversion to use.

  In contrast, the coding system @code{no-conversion} specifies no
character code conversion at all---none for non-ASCII byte values and
none for end of line.  This is useful for reading or writing binary
files, tar files, and other files that must be examined verbatim.  It,
too, sets @code{enable-multibyte-characters} to @code{nil}.

  The easiest way to edit a file with no conversion of any kind is with
the @kbd{M-x find-file-literally} command.  This uses
@code{no-conversion}, and also suppresses other Emacs features that
might convert the file contents before you see them.  @xref{Visiting}.

  The coding system @code{emacs-mule} means that the file contains
non-ASCII characters stored with the internal Emacs encoding.  It
handles end-of-line conversion based on the data encountered, and has
the usual three variants to specify the kind of end-of-line conversion.

@node Recognize Coding
@section Recognizing Coding Systems

  Most of the time, Emacs can recognize which coding system to use for
any given file---once you have specified your preferences.

  Some coding systems can be recognized or distinguished by which byte
sequences appear in the data.  However, there are coding systems that
cannot be distinguished, not even potentially.  For example, there is no
way to distinguish between Latin-1 and Latin-2; they use the same byte
values with different meanings.

  Emacs handles this situation by means of a priority list of coding
systems.  Whenever Emacs reads a file, if you do not specify the coding
system to use, Emacs checks the data against each coding system,
starting with the first in priority and working down the list, until it
finds a coding system that fits the data.  Then it converts the file
contents assuming that they are represented in this coding system.

  The priority list of coding systems depends on the selected language
environment (@pxref{Language Environments}).  For example, if you use
French, you probably want Emacs to prefer Latin-1 to Latin-2; if you use
Czech, you probably want Latin-2 to be preferred.  This is one of the
reasons to specify a language environment.

@findex prefer-coding-system
  However, you can alter the priority list in detail with the command
@kbd{M-x prefer-coding-system}.  This command reads the name of a coding
system from the minibuffer, and adds it to the front of the priority
list, so that it is preferred to all others.  If you use this command
several times, each use adds one element to the front of the priority
list.

  If you use a coding system that specifies the end-of-line conversion
type, such as @code{iso-8859-1-dos}, what that means is that Emacs
should attempt to recognize @code{iso-8859-1} with priority, and should
use DOS end-of-line conversion in case it recognizes @code{iso-8859-1}.

@vindex file-coding-system-alist
  Sometimes a file name indicates which coding system to use for the
file.  The variable @code{file-coding-system-alist} specifies this
correspondence.  There is a special function
@code{modify-coding-system-alist} for adding elements to this list.  For
example, to read and write all @samp{.txt} files using the coding system
@code{china-iso-8bit}, you can execute this Lisp expression:

@smallexample
(modify-coding-system-alist 'file "\\.txt\\'" 'china-iso-8bit)
@end smallexample

@noindent
The first argument should be @code{file}, the second argument should be
a regular expression that determines which files this applies to, and
the third argument says which coding system to use for these files.

@vindex inhibit-eol-conversion
@cindex DOS-style end-of-line display
  Emacs recognizes which kind of end-of-line conversion to use based on
the contents of the file: if it sees only carriage-returns, or only
carriage-return linefeed sequences, then it chooses the end-of-line
conversion accordingly.  You can inhibit the automatic use of
end-of-line conversion by setting the variable @code{inhibit-eol-conversion}
to non-@code{nil}.

@vindex inhibit-iso-escape-detection
@cindex escape sequences in files
  By default, the automatic detection of coding system is sensitive to
escape sequences.  If Emacs sees a sequence of characters that begin
with an @key{ESC} character, and the sequence is valid as an ISO-2022
code, the code is determined as one of ISO-2022 encoding, and the file
is decoded by the corresponding coding system
(e.g. @code{iso-2022-7bit}).

  However, there may be cases that you want to read escape sequences in
a file as is.  In such a case, you can set th variable
@code{inhibit-iso-escape-detection} to non-@code{nil}.  Then the code
detection will ignore any escape sequences, and so no file is detected
as being encoded in some of ISO-2022 encoding.  The result is that all
escape sequences become visible in a buffer.

  The default value of @code{inhibit-iso-escape-detection} is
@code{nil}, and it is strongly recommended not to change it.  That's
because many Emacs Lisp source files that contain non-ASCII characters
are encoded in the coding system @code{iso-2022-7bit} in the Emacs
distribution, and they won't be decoded correctly when you visit those
files if you suppress the escape sequence detection.

@vindex coding
  You can specify the coding system for a particular file using the
@samp{-*-@dots{}-*-} construct at the beginning of a file, or a local
variables list at the end (@pxref{File Variables}).  You do this by
defining a value for the ``variable'' named @code{coding}.  Emacs does
not really have a variable @code{coding}; instead of setting a variable,
it uses the specified coding system for the file.  For example,
@samp{-*-mode: C; coding: latin-1;-*-} specifies use of the Latin-1
coding system, as well as C mode.  If you specify the coding explicitly
in the file, that overrides @code{file-coding-system-alist}.

@vindex auto-coding-alist
  The variable @code{auto-coding-alist} is the strongest way to specify
the coding system for certain patterns of file names; this variable even
overrides @samp{-*-coding:-*-} tags in the file itself.  Emacs uses this
feature for tar and archive files, to prevent Emacs from being confused
by a @samp{-*-coding:-*-} tag in a member of the archive and thinking it
applies to the archive file as a whole.

@vindex buffer-file-coding-system
  Once Emacs has chosen a coding system for a buffer, it stores that
coding system in @code{buffer-file-coding-system} and uses that coding
system, by default, for operations that write from this buffer into a
file.  This includes the commands @code{save-buffer} and
@code{write-region}.  If you want to write files from this buffer using
a different coding system, you can specify a different coding system for
the buffer using @code{set-buffer-file-coding-system} (@pxref{Specify
Coding}).

  While editing a file, you will sometimes insert characters which
cannot be encoded with the coding system stored in
@code{buffer-file-coding-system}.  For example, suppose you start with
an ASCII file and insert a few Latin-1 characters into it.  Or you could
edit a text file in Polish encoded in @code{iso-8859-2} and add to it
translations of several Polish words into Russian.  When you save the
buffer, Emacs can no longer use the previous value of the buffer's
coding system, because the characters you added cannot be encoded by
that coding system.

  When that happens, Emacs tries the most-preferred coding system (set
by @kbd{M-x prefer-coding-system} or @kbd{M-x
set-language-environment}), and if that coding system can safely encode
all of the characters in the buffer, Emacs uses it, and stores its value
in @code{buffer-file-coding-system}.  Otherwise, Emacs pops up a window
with a list of coding systems suitable for encoding the buffer, and
prompts you to choose one of those coding systems.

  If you insert characters which cannot be encoded by the buffer's
coding system while editing a mail message, Emacs behaves a bit
differently.  It additionally checks whether the most-preferred coding
system is recommended for use in MIME messages; if it isn't, Emacs tells
you that the most-preferred coding system is not recommended and prompts
you for another coding system.  This is so you won't inadvertently send
a message encoded in a way that your recipient's mail software will have
difficulty decoding.  (If you do want to use the most-preferred coding
system, you can type its name to Emacs prompt anyway.)

@vindex sendmail-coding-system
  When you send a message with Mail mode (@pxref{Sending Mail}), Emacs has
four different ways to determine the coding system to use for encoding
the message text.  It tries the buffer's own value of
@code{buffer-file-coding-system}, if that is non-@code{nil}.  Otherwise,
it uses the value of @code{sendmail-coding-system}, if that is
non-@code{nil}.  The third way is to use the default coding system for
new files, which is controlled by your choice of language environment,
if that is non-@code{nil}.  If all of these three values are @code{nil},
Emacs encodes outgoing mail using the Latin-1 coding system.

@vindex rmail-decode-mime-charset
  When you get new mail in Rmail, each message is translated
automatically from the coding system it is written in---as if it were a
separate file.  This uses the priority list of coding systems that you
have specified.  If a MIME message specifies a character set, Rmail
obeys that specification, unless @code{rmail-decode-mime-charset} is
@code{nil}.

@vindex rmail-file-coding-system
  For reading and saving Rmail files themselves, Emacs uses the coding
system specified by the variable @code{rmail-file-coding-system}.  The
default value is @code{nil}, which means that Rmail files are not
translated (they are read and written in the Emacs internal character
code).

@node Specify Coding
@section Specifying a Coding System

  In cases where Emacs does not automatically choose the right coding
system, you can use these commands to specify one:

@table @kbd
@item C-x @key{RET} f @var{coding} @key{RET}
Use coding system @var{coding} for the visited file
in the current buffer.

@item C-x @key{RET} c @var{coding} @key{RET}
Specify coding system @var{coding} for the immediately following
command.

@item C-x @key{RET} k @var{coding} @key{RET}
Use coding system @var{coding} for keyboard input.

@item C-x @key{RET} t @var{coding} @key{RET}
Use coding system @var{coding} for terminal output.

@item C-x @key{RET} p @var{input-coding} @key{RET} @var{output-coding} @key{RET}
Use coding systems @var{input-coding} and @var{output-coding} for
subprocess input and output in the current buffer.

@item C-x @key{RET} x @var{coding} @key{RET}
Use coding system @var{coding} for transferring selections to and from
other programs through the window system.

@item C-x @key{RET} X @var{coding} @key{RET}
Use coding system @var{coding} for transferring @emph{one}
selection---the next one---to or from the window system.
@end table

@kindex C-x RET f
@findex set-buffer-file-coding-system
  The command @kbd{C-x @key{RET} f} (@code{set-buffer-file-coding-system})
specifies the file coding system for the current buffer---in other
words, which coding system to use when saving or rereading the visited
file.  You specify which coding system using the minibuffer.  Since this
command applies to a file you have already visited, it affects only the
way the file is saved.

@kindex C-x RET c
@findex universal-coding-system-argument
  Another way to specify the coding system for a file is when you visit
the file.  First use the command @kbd{C-x @key{RET} c}
(@code{universal-coding-system-argument}); this command uses the
minibuffer to read a coding system name.  After you exit the minibuffer,
the specified coding system is used for @emph{the immediately following
command}.

  So if the immediately following command is @kbd{C-x C-f}, for example,
it reads the file using that coding system (and records the coding
system for when the file is saved).  Or if the immediately following
command is @kbd{C-x C-w}, it writes the file using that coding system.
Other file commands affected by a specified coding system include
@kbd{C-x C-i} and @kbd{C-x C-v}, as well as the other-window variants of
@kbd{C-x C-f}.

  @kbd{C-x @key{RET} c} also affects commands that start subprocesses,
including @kbd{M-x shell} (@pxref{Shell}).

  However, if the immediately following command does not use the coding
system, then @kbd{C-x @key{RET} c} ultimately has no effect.

  An easy way to visit a file with no conversion is with the @kbd{M-x
find-file-literally} command.  @xref{Visiting}.

@vindex default-buffer-file-coding-system
  The variable @code{default-buffer-file-coding-system} specifies the
choice of coding system to use when you create a new file.  It applies
when you find a new file, and when you create a buffer and then save it
in a file.  Selecting a language environment typically sets this
variable to a good choice of default coding system for that language
environment.

@kindex C-x RET t
@findex set-terminal-coding-system
  The command @kbd{C-x @key{RET} t} (@code{set-terminal-coding-system})
specifies the coding system for terminal output.  If you specify a
character code for terminal output, all characters output to the
terminal are translated into that coding system.

  This feature is useful for certain character-only terminals built to
support specific languages or character sets---for example, European
terminals that support one of the ISO Latin character sets.  You need to
specify the terminal coding system when using multibyte text, so that
Emacs knows which characters the terminal can actually handle.

  By default, output to the terminal is not translated at all, unless
Emacs can deduce the proper coding system from your terminal type.

@kindex C-x RET k
@findex set-keyboard-coding-system
  The command @kbd{C-x @key{RET} k} (@code{set-keyboard-coding-system})
specifies the coding system for keyboard input.  Character-code
translation of keyboard input is useful for terminals with keys that
send non-ASCII graphic characters---for example, some terminals designed
for ISO Latin-1 or subsets of it.

  By default, keyboard input is not translated at all.

  There is a similarity between using a coding system translation for
keyboard input, and using an input method: both define sequences of
keyboard input that translate into single characters.  However, input
methods are designed to be convenient for interactive use by humans, and
the sequences that are translated are typically sequences of ASCII
printing characters.  Coding systems typically translate sequences of
non-graphic characters.

@kindex C-x RET x
@kindex C-x RET X
@findex set-selection-coding-system
@findex set-next-selection-coding-system
  The command @kbd{C-x @key{RET} x} (@code{set-selection-coding-system})
specifies the coding system for sending selected text to the window
system, and for receiving the text of selections made in other
applications.  This command applies to all subsequent selections, until
you override it by using the command again.  The command @kbd{C-x
@key{RET} X} (@code{set-next-selection-coding-system}) specifies the
coding system for the next selection made in Emacs or read by Emacs.

@kindex C-x RET p
@findex set-buffer-process-coding-system
  The command @kbd{C-x @key{RET} p} (@code{set-buffer-process-coding-system})
specifies the coding system for input and output to a subprocess.  This
command applies to the current buffer; normally, each subprocess has its
own buffer, and thus you can use this command to specify translation to
and from a particular subprocess by giving the command in the
corresponding buffer.

  The default for translation of process input and output depends on the
current language environment.

@vindex file-name-coding-system
  The variable @code{file-name-coding-system} specifies a coding system
to use for encoding file names.  If you set the variable to a coding
system name (as a Lisp symbol or a string), Emacs encodes file names
using that coding system for all file operations.  This makes it
possible to use non-ASCII characters in file names---or, at least, those
non-ASCII characters which the specified coding system can encode.

  If @code{file-name-coding-system} is @code{nil}, Emacs uses a default
coding system determined by the selected language environment.  In the
default language environment, any non-ASCII characters in file names are
not encoded specially; they appear in the file system using the internal
Emacs representation.

  @strong{Warning:} if you change @code{file-name-coding-system} (or the
language environment) in the middle of an Emacs session, problems can
result if you have already visited files whose names were encoded using
the earlier coding system and cannot be encoded (or are encoded
differently) under the new coding system.  If you try to save one of
these buffers under the visited file name, saving may use the wrong file
name, or it may get an error.  If such a problem happens, use @kbd{C-x
C-w} to specify a new file name for that buffer.

@vindex locale-coding-system
  The variable @code{locale-coding-system} specifies a coding system to
use when encoding and decoding system strings such as system error
messages and @code{format-time-string} formats and time stamps.  This
coding system should be compatible with the underlying system's coding
system, which is normally specified by the first environment variable in
the list @env{LC_ALL}, @env{LC_CTYPE}, @env{LANG} whose value is
nonempty.

@node Fontsets
@section Fontsets
@cindex fontsets

  A font for X Windows typically defines shapes for one alphabet or
script.  Therefore, displaying the entire range of scripts that Emacs
supports requires a collection of many fonts.  In Emacs, such a
collection is called a @dfn{fontset}.  A fontset is defined by a list of
fonts, each assigned to handle a range of character codes. 

  Each fontset has a name, like a font.  The available X fonts are
defined by the X server; fontsets, however, are defined within Emacs
itself.  Once you have defined a fontset, you can use it within Emacs by
specifying its name, anywhere that you could use a single font.  Of
course, Emacs fontsets can use only the fonts that the X server
supports; if certain characters appear on the screen as hollow boxes,
this means that the fontset in use for them has no font for those
characters.

  Emacs creates two fontsets automatically: the @dfn{standard fontset}
and the @dfn{startup fontset}.  The standard fontset is most likely to
have fonts for a wide variety of non-ASCII characters; however, this is
not the default for Emacs to use.  (By default, Emacs tries to find a
font which has bold and italic variants.)  You can specify use of the
standard fontset with the @samp{-fn} option, or with the @samp{Font} X
resource (@pxref{Font X}).  For example,

@example
emacs -fn fontset-standard
@end example

  A fontset does not necessarily specify a font for every character
code.  If a fontset specifies no font for a certain character, or if it
specifies a font that does not exist on your system, then it cannot
display that character properly.  It will display that character as an
empty box instead.

@vindex highlight-wrong-size-font
  The fontset height and width are determined by the ASCII characters
(that is, by the font used for ASCII characters in that fontset).  If
another font in the fontset has a different height, or a different
width, then characters assigned to that font are clipped to the
fontset's size.  If @code{highlight-wrong-size-font} is non-@code{nil},
a box is displayed around these wrong-size characters as well.

@node Defining Fontsets
@section Defining fontsets

@vindex standard-fontset-spec
@cindex standard fontset
  Emacs creates a standard fontset automatically according to the value
of @code{standard-fontset-spec}.  This fontset's name is

@example
-*-fixed-medium-r-normal-*-16-*-*-*-*-*-fontset-standard
@end example

@noindent
or just @samp{fontset-standard} for short.

  Bold, italic, and bold-italic variants of the standard fontset are
created automatically.  Their names have @samp{bold} instead of
@samp{medium}, or @samp{i} instead of @samp{r}, or both.

@cindex startup fontset
  If you specify a default ASCII font with the @samp{Font} resource or
the @samp{-fn} argument, Emacs generates a fontset from it
automatically.  This is the @dfn{startup fontset} and its name is
@code{fontset-startup}.  It does this by replacing the @var{foundry},
@var{family}, @var{add_style}, and @var{average_width} fields of the
font name with @samp{*}, replacing @var{charset_registry} field with
@samp{fontset}, and replacing @var{charset_encoding} field with
@samp{startup}, then using the resulting string to specify a fontset.

  For instance, if you start Emacs this way,

@example
emacs -fn "*courier-medium-r-normal--14-140-*-iso8859-1"
@end example

@noindent
Emacs generates the following fontset and uses it for the initial X
window frame:

@example
-*-*-medium-r-normal-*-14-140-*-*-*-*-fontset-startup
@end example

  With the X resource @samp{Emacs.Font}, you can specify a fontset name
just like an actual font name.  But be careful not to specify a fontset
name in a wildcard resource like @samp{Emacs*Font}---that wildcard
specification applies to various other purposes, such as menus, and
menus cannot handle fontsets.

  You can specify additional fontsets using X resources named
@samp{Fontset-@var{n}}, where @var{n} is an integer starting from 0.
The resource value should have this form:

@smallexample
@var{fontpattern}, @r{[}@var{charsetname}:@var{fontname}@r{]@dots{}}
@end smallexample

@noindent
@var{fontpattern} should have the form of a standard X font name, except
for the last two fields.  They should have the form
@samp{fontset-@var{alias}}.

  The fontset has two names, one long and one short.  The long name is
@var{fontpattern}.  The short name is @samp{fontset-@var{alias}}.  You
can refer to the fontset by either name.

  The construct @samp{@var{charset}:@var{font}} specifies which font to
use (in this fontset) for one particular character set.  Here,
@var{charset} is the name of a character set, and @var{font} is the
font to use for that character set.  You can use this construct any
number of times in defining one fontset.

  For the other character sets, Emacs chooses a font based on
@var{fontpattern}.  It replaces @samp{fontset-@var{alias}} with values
that describe the character set.  For the ASCII character font,
@samp{fontset-@var{alias}} is replaced with @samp{ISO8859-1}.

  In addition, when several consecutive fields are wildcards, Emacs
collapses them into a single wildcard.  This is to prevent use of
auto-scaled fonts.  Fonts made by scaling larger fonts are not usable
for editing, and scaling a smaller font is not useful because it is
better to use the smaller font in its own size, which Emacs does.

  Thus if @var{fontpattern} is this,

@example
-*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24
@end example

@noindent
the font specification for ASCII characters would be this:

@example
-*-fixed-medium-r-normal-*-24-*-ISO8859-1
@end example

@noindent
and the font specification for Chinese GB2312 characters would be this:

@example
-*-fixed-medium-r-normal-*-24-*-gb2312*-*
@end example

  You may not have any Chinese font matching the above font
specification.  Most X distributions include only Chinese fonts that
have @samp{song ti} or @samp{fangsong ti} in @var{family} field.  In
such a case, @samp{Fontset-@var{n}} can be specified as below:

@smallexample
Emacs.Fontset-0: -*-fixed-medium-r-normal-*-24-*-*-*-*-*-fontset-24,\
        chinese-gb2312:-*-*-medium-r-normal-*-24-*-gb2312*-*
@end smallexample

@noindent
Then, the font specifications for all but Chinese GB2312 characters have
@samp{fixed} in the @var{family} field, and the font specification for
Chinese GB2312 characters has a wild card @samp{*} in the @var{family}
field.

@findex create-fontset-from-fontset-spec
  The function that processes the fontset resource value to create the
fontset is called @code{create-fontset-from-fontset-spec}.  You can also
call this function explicitly to create a fontset.

  @xref{Font X}, for more information about font naming in X.

@node Single-Byte Character Support
@section Single-byte Character Set Support

@cindex European character sets
@cindex accented characters
@cindex ISO Latin character sets
@cindex Unibyte operation
@vindex enable-multibyte-characters
  The ISO 8859 Latin-@var{n} character sets define character codes in
the range 160 to 255 to handle the accented letters and punctuation
needed by various European languages (and some non-European ones).
If you disable multibyte
characters, Emacs can still handle @emph{one} of these character codes
at a time.  To specify @emph{which} of these codes to use, invoke
@kbd{M-x set-language-environment} and specify a suitable language
environment such as @samp{Latin-@var{n}}.

  For more information about unibyte operation, see @ref{Enabling
Multibyte}.  Note particularly that you probably want to ensure that
your initialization files are read as unibyte if they contain non-ASCII
characters.

@vindex unibyte-display-via-language-environment
  Emacs can also display those characters, provided the terminal or font
in use supports them.  This works automatically.  Alternatively, if you
are using a window system, Emacs can also display single-byte characters
through fontsets, in effect by displaying the equivalent multibyte
characters according to the current language environment.  To request
this, set the variable @code{unibyte-display-via-language-environment}
to a non-@code{nil} value.

@cindex @code{iso-ascii} library
  If your terminal does not support display of the Latin-1 character
set, Emacs can display these characters as ASCII sequences which at
least give you a clear idea of what the characters are.  To do this,
load the library @code{iso-ascii}.  Similar libraries for other
Latin-@var{n} character sets could be implemented, but we don't have
them yet.

@findex standard-display-8bit
@cindex 8-bit display
  Normally non-ISO-8859 characters (between characters 128 and 159
inclusive) are displayed as octal escapes.  You can change this for
non-standard `extended' versions of ISO-8859 character sets by using the
function @code{standard-display-8bit} in the @code{disp-table} library.

  There are several ways you can input single-byte non-ASCII
characters:

@itemize @bullet
@cindex 8-bit input
@item
If your keyboard can generate character codes 128 and up, representing
non-ASCII characters, you can execute the following expression to enable
Emacs to understand them:

@example
(set-input-mode (car (current-input-mode))
                (nth 1 (current-input-mode))
                0)
@end example

It is not necessary to do this under a window system which can
distinguish 8-bit characters and Meta keys.  If you do this on a normal
terminal, you will probably need to use @kbd{ESC} to type Meta
characters.@footnote{In some cases, such as the Linux console and
@code{xterm}, you can arrange for Meta to be converted to @kbd{ESC} and
still be able type 8-bit characters present directly on the keyboard or
using @kbd{Compose} or @kbd{AltGr} keys.}  @xref{User Input}.

@item
You can use an input method for the selected language environment.
@xref{Input Methods}.  When you use an input method in a unibyte buffer,
the non-ASCII character you specify with it is converted to unibyte.

@kindex C-x 8
@cindex @code{iso-transl} library
@cindex compose character
@cindex dead character
@item
For Latin-1 only, you can use the
key @kbd{C-x 8} as a ``compose character'' prefix for entry of
non-ASCII Latin-1 printing characters.  @kbd{C-x 8} is good for
insertion (in the minibuffer as well as other buffers), for searching,
and in any other context where a key sequence is allowed.

@kbd{C-x 8} works by loading the @code{iso-transl} library.  Once that
library is loaded, the @key{ALT} modifier key, if you have one, serves
the same purpose as @kbd{C-x 8}; use @key{ALT} together with an accent
character to modify the following letter.  In addition, if you have keys
for the Latin-1 ``dead accent characters'', they too are defined to
compose with the following character, once @code{iso-transl} is loaded.
Use @kbd{C-x 8 C-h} to list the available translations as mnemonic
command names.

@item
@cindex @code{iso-acc} library
@cindex ISO Accents mode
@findex iso-accents-mode
@cindex Latin-1 input mode
For Latin-1, Latin-2 and Latin-3, @kbd{M-x iso-accents-mode} installs a
minor mode which provides a facility like the @code{latin-1-prefix}
input method but independent of the Leim package.  This mode is
buffer-local.  It can be customized for various languages with @kbd{M-x
iso-accents-customize}.}
@end itemize