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[gnu-emacs] / lispref / nonascii.texi

@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1998 Free Software Foundation, Inc. 
@c See the file elisp.texi for copying conditions.
@setfilename ../info/characters
@node Non-ASCII Characters, Searching and Matching, Text, Top
@chapter Non-ASCII Characters
@cindex multibyte characters
@cindex non-ASCII characters

  This chapter covers the special issues relating to non-@sc{ASCII}
characters and how they are stored in strings and buffers.

@menu
* Text Representations::
* Converting Representations::
* Selecting a Representation::
* Character Codes::
* Character Sets::
* Scanning Charsets::
* Chars and Bytes::
* Coding Systems::
* Lisp and Coding System::
* Default Coding Systems::
* Specifying Coding Systems::
* Explicit Encoding::
* MS-DOS File Types::
* MS-DOS Subprocesses::
@end menu

@node Text Representations
@section Text Representations
@cindex text representations

  Emacs has two @dfn{text representations}---two ways to represent text
in a string or buffer.  These are called @dfn{unibyte} and
@dfn{multibyte}.  Each string, and each buffer, uses one of these two
representations.  For most purposes, you can ignore the issue of
representations, because Emacs converts text between them as
appropriate.  Occasionally in Lisp programming you will need to pay
attention to the difference.

@cindex unibyte text
  In unibyte representation, each character occupies one byte and
therefore the possible character codes range from 0 to 255.  Codes 0
through 127 are @sc{ASCII} characters; the codes from 128 through 255
are used for one non-@sc{ASCII} character set (you can choose which
character set by setting the variable @code{nonascii-insert-offset}).

@cindex leading code
@cindex multibyte text
  In multibyte representation, a character may occupy more than one
byte, and as a result, the full range of Emacs character codes can be
stored.  The first byte of a multibyte character is always in the range
128 through 159 (octal 0200 through 0237).  These values are called
@dfn{leading codes}.  The first byte determines which character set the
character belongs to (@pxref{Character Sets}); in particular, it
determines how many bytes long the sequence is.  The second and
subsequent bytes of a multibyte character are always in the range 160
through 255 (octal 0240 through 0377).

  In a buffer, the buffer-local value of the variable
@code{enable-multibyte-characters} specifies the representation used.
The representation for a string is determined based on the string
contents when the string is constructed.

@tindex enable-multibyte-characters
@defvar enable-multibyte-characters
This variable specifies the current buffer's text representation.
If it is non-@code{nil}, the buffer contains multibyte text; otherwise,
it contains unibyte text.

You cannot set this variable directly; instead, use the function
@code{set-buffer-multibyte} to change a buffer's representation.
@end defvar

@tindex default-enable-multibyte-characters
@defvar default-enable-multibyte-characters
This variable`s value is entirely equivalent to @code{(default-value
'enable-multibyte-characters)}, and setting this variable changes that
default value.  Although setting the local binding of
@code{enable-multibyte-characters} in a specific buffer is dangerous,
changing the default value is safe, and it is a reasonable thing to do.

The @samp{--unibyte} command line option does its job by setting the
default value to @code{nil} early in startup.
@end defvar

@tindex multibyte-string-p
@defun multibyte-string-p string
Return @code{t} if @var{string} contains multibyte characters.
@end defun

@node Converting Representations
@section Converting Text Representations

  Emacs can convert unibyte text to multibyte; it can also convert
multibyte text to unibyte, though this conversion loses information.  In
general these conversions happen when inserting text into a buffer, or
when putting text from several strings together in one string.  You can
also explicitly convert a string's contents to either representation.

  Emacs chooses the representation for a string based on the text that
it is constructed from.  The general rule is to convert unibyte text to
multibyte text when combining it with other multibyte text, because the
multibyte representation is more general and can hold whatever
characters the unibyte text has.

  When inserting text into a buffer, Emacs converts the text to the
buffer's representation, as specified by
@code{enable-multibyte-characters} in that buffer.  In particular, when
you insert multibyte text into a unibyte buffer, Emacs converts the text
to unibyte, even though this conversion cannot in general preserve all
the characters that might be in the multibyte text.  The other natural
alternative, to convert the buffer contents to multibyte, is not
acceptable because the buffer's representation is a choice made by the
user that cannot be overridden automatically.

  Converting unibyte text to multibyte text leaves @sc{ASCII} characters
unchanged, and likewise 128 through 159.  It converts the non-@sc{ASCII}
codes 160 through 255 by adding the value @code{nonascii-insert-offset}
to each character code.  By setting this variable, you specify which
character set the unibyte characters correspond to.  For example, if
@code{nonascii-insert-offset} is 2048, which is @code{(- (make-char
'latin-iso8859-1 0) 128)}, then the unibyte non-@sc{ASCII} characters
correspond to Latin 1.  If it is 2688, which is @code{(- (make-char
'greek-iso8859-7 0) 128)}, then they correspond to Greek letters.

  Converting multibyte text to unibyte is simpler: it performs
logical-and of each character code with 255.  If
@code{nonascii-insert-offset} has a reasonable value, corresponding to
the beginning of some character set, this conversion is the inverse of
the other: converting unibyte text to multibyte and back to unibyte
reproduces the original unibyte text.

@tindex nonascii-insert-offset
@defvar nonascii-insert-offset
This variable specifies the amount to add to a non-@sc{ASCII} character
when converting unibyte text to multibyte.  It also applies when
@code{insert-char} or @code{self-insert-command} inserts a character in
the unibyte non-@sc{ASCII} range, 128 through 255.

The right value to use to select character set @var{cs} is @code{(-
(make-char @var{cs} 0) 128)}.  If the value of
@code{nonascii-insert-offset} is zero, then conversion actually uses the
value for the Latin 1 character set, rather than zero.
@end defvar

@tindex nonascii-translate-table
@defvar nonascii-translate-table
This variable provides a more general alternative to
@code{nonascii-insert-offset}.  You can use it to specify independently
how to translate each code in the range of 128 through 255 into a
multibyte character.  The value should be a vector, or @code{nil}.
If this is non-@code{nil}, it overrides @code{nonascii-insert-offset}.
@end defvar

@tindex string-make-unibyte
@defun string-make-unibyte string
This function converts the text of @var{string} to unibyte
representation, if it isn't already, and return the result.  If
@var{string} is a unibyte string, it is returned unchanged.
@end defun

@tindex string-make-multibyte
@defun string-make-multibyte string
This function converts the text of @var{string} to multibyte
representation, if it isn't already, and return the result.  If
@var{string} is a multibyte string, it is returned unchanged.
@end defun

@node Selecting a Representation
@section Selecting a Representation

  Sometimes it is useful to examine an existing buffer or string as
multibyte when it was unibyte, or vice versa.

@tindex set-buffer-multibyte
@defun set-buffer-multibyte multibyte
Set the representation type of the current buffer.  If @var{multibyte}
is non-@code{nil}, the buffer becomes multibyte.  If @var{multibyte}
is @code{nil}, the buffer becomes unibyte.

This function leaves the buffer contents unchanged when viewed as a
sequence of bytes.  As a consequence, it can change the contents viewed
as characters; a sequence of two bytes which is treated as one character
in multibyte representation will count as two characters in unibyte
representation.

This function sets @code{enable-multibyte-characters} to record which
representation is in use.  It also adjusts various data in the buffer
(including overlays, text properties and markers) so that they cover the
same text as they did before.
@end defun

@tindex string-as-unibyte
@defun string-as-unibyte string
This function returns a string with the same bytes as @var{string} but
treating each byte as a character.  This means that the value may have
more characters than @var{string} has.

If @var{string} is unibyte already, then the value is @var{string}
itself.
@end defun

@tindex string-as-multibyte
@defun string-as-multibyte string
This function returns a string with the same bytes as @var{string} but
treating each multibyte sequence as one character.  This means that the
value may have fewer characters than @var{string} has.

If @var{string} is multibyte already, then the value is @var{string}
itself.
@end defun

@node Character Codes
@section Character Codes
@cindex character codes

  The unibyte and multibyte text representations use different character
codes.  The valid character codes for unibyte representation range from
0 to 255---the values that can fit in one byte.  The valid character
codes for multibyte representation range from 0 to 524287, but not all
values in that range are valid.  In particular, the values 128 through
255 are not legitimate in multibyte text (though they can occur in ``raw
bytes''; @pxref{Explicit Encoding}).  Only the @sc{ASCII} codes 0
through 127 are fully legitimate in both representations.

@defun char-valid-p charcode
This returns @code{t} if @var{charcode} is valid for either one of the two
text representations.

@example
(char-valid-p 65)
     @result{} t
(char-valid-p 256)
     @result{} nil
(char-valid-p 2248)
     @result{} t
@end example
@end defun

@node Character Sets
@section Character Sets
@cindex character sets

  Emacs classifies characters into various @dfn{character sets}, each of
which has a name which is a symbol.  Each character belongs to one and
only one character set.

  In general, there is one character set for each distinct script.  For
example, @code{latin-iso8859-1} is one character set,
@code{greek-iso8859-7} is another, and @code{ascii} is another.  An
Emacs character set can hold at most 9025 characters; therefore, in some
cases, characters that would logically be grouped together are split
into several character sets.  For example, one set of Chinese characters
is divided into eight Emacs character sets, @code{chinese-cns11643-1}
through @code{chinese-cns11643-7}.

@tindex charsetp
@defun charsetp object
Return @code{t} if @var{object} is a character set name symbol,
@code{nil} otherwise.
@end defun

@tindex charset-list
@defun charset-list
This function returns a list of all defined character set names.
@end defun

@tindex char-charset
@defun char-charset character
This function returns the the name of the character
set that @var{character} belongs to.
@end defun

@node Scanning Charsets
@section Scanning for Character Sets

  Sometimes it is useful to find out which character sets appear in a
part of a buffer or a string.  One use for this is in determining which
coding systems (@pxref{Coding Systems}) are capable of representing all
of the text in question.

@tindex find-charset-region
@defun find-charset-region beg end &optional unification
This function returns a list of the character sets
that appear in the current buffer between positions @var{beg}
and @var{end}.
@end defun

@tindex find-charset-string
@defun find-charset-string string &optional unification
This function returns a list of the character sets
that appear in the string @var{string}.
@end defun

@node Chars and Bytes
@section Characters and Bytes
@cindex bytes and characters

  In multibyte representation, each character occupies one or more
bytes.  The functions in this section convert between characters and the
byte values used to represent them.  For most purposes, there is no need
to be concerned with the number of bytes used to represent a character
because Emacs translates automatically when necessary.

@tindex char-bytes
@defun char-bytes character
This function returns the number of bytes used to represent the
character @var{character}.  In most cases, this is the same as
@code{(length (split-char @var{character}))}; the only exception is for
ASCII characters and the codes used in unibyte text, which use just one
byte.

@example
(char-bytes 2248)
     @result{} 2
(char-bytes 65)
     @result{} 1
@end example

This function's values are correct for both multibyte and unibyte
representations, because the non-@sc{ASCII} character codes used in
those two representations do not overlap.

@example
(char-bytes 192)
     @result{} 1
@end example
@end defun

@tindex split-char
@defun split-char character
Return a list containing the name of the character set of
@var{character}, followed by one or two byte-values which identify
@var{character} within that character set.

@example
(split-char 2248)
     @result{} (latin-iso8859-1 72)
(split-char 65)
     @result{} (ascii 65)
@end example

Unibyte non-@sc{ASCII} characters are considered as part of
the @code{ascii} character set:

@example
(split-char 192)
     @result{} (ascii 192)
@end example
@end defun

@tindex make-char
@defun make-char charset &rest byte-values
Thus function returns the character in character set @var{charset}
identified by @var{byte-values}.  This is roughly the opposite of
split-char.

@example
(make-char 'latin-iso8859-1 72)
     @result{} 2248
@end example
@end defun

@node Coding Systems
@section Coding Systems

@cindex coding system
  When Emacs reads or writes a file, and when Emacs sends text to a
subprocess or receives text from a subprocess, it normally performs
character code conversion and end-of-line conversion as specified
by a particular @dfn{coding system}.

@cindex character code conversion
  @dfn{Character code conversion} involves conversion between the encoding
used inside Emacs and some other encoding.  Emacs supports many
different encodings, in that it can convert to and from them.  For
example, it can convert text to or from encodings such as Latin 1, Latin
2, Latin 3, Latin 4, Latin 5, and several variants of ISO 2022.  In some
cases, Emacs supports several alternative encodings for the same
characters; for example, there are three coding systems for the Cyrillic
(Russian) alphabet: ISO, Alternativnyj, and KOI8.

  Most coding systems specify a particular character code for
conversion, but some of them leave this unspecified---to be chosen
heuristically based on the data.

@cindex end of line conversion
  @dfn{End of line conversion} handles three different conventions used
on various systems for representing end of line in files.  The Unix
convention is to use the linefeed character (also called newline).  The
DOS convention is to use the two character sequence, carriage-return
linefeed, at the end of a line.  The Mac convention is to use just
carriage-return.

@cindex base coding system
@cindex variant coding system
  @dfn{Base coding systems} such as @code{latin-1} leave the end-of-line
conversion unspecified, to be chosen based on the data.  @dfn{Variant
coding systems} such as @code{latin-1-unix}, @code{latin-1-dos} and
@code{latin-1-mac} specify the end-of-line conversion explicitly as
well.  Each base coding system has three corresponding variants whose
names are formed by adding @samp{-unix}, @samp{-dos} and @samp{-mac}.

@node Lisp and Coding Systems
@subsection Coding Systems in Lisp

  Here are Lisp facilities for working with coding systems;

@tindex coding-system-list
@defun coding-system-list &optional base-only
This function returns a list of all coding system names (symbols).  If
@var{base-only} is non-@code{nil}, the value includes only the
base coding systems.  Otherwise, it includes variant coding systems as well.
@end defun

@tindex coding-system-p
@defun coding-system-p object
This function returns @code{t} if @var{object} is a coding system
name.
@end defun

@tindex check-coding-system
@defun check-coding-system coding-system
This function checks the validity of @var{coding-system}.
If that is valid, it returns @var{coding-system}.
Otherwise it signals an error with condition @code{coding-system-error}.
@end defun

@tindex find-safe-coding-system
@defun find-safe-coding-system from to
Return a list of proper coding systems to encode a text between
@var{from} and @var{to}.  All coding systems in the list can safely
encode any multibyte characters in the text.

If the text contains no multibyte characters, return a list of a single
element @code{undecided}.
@end defun

@tindex detect-coding-region
@defun detect-coding-region start end highest
This function chooses a plausible coding system for decoding the text
from @var{start} to @var{end}.  This text should be ``raw bytes''
(@pxref{Explicit Encoding}).

Normally this function returns is a list of coding systems that could
handle decoding the text that was scanned.  They are listed in order of
decreasing priority, based on the priority specified by the user with
@code{prefer-coding-system}.  But if @var{highest} is non-@code{nil},
then the return value is just one coding system, the one that is highest
in priority.
@end defun

@tindex detect-coding-string string highest
@defun detect-coding-string
This function is like @code{detect-coding-region} except that it
operates on the contents of @var{string} instead of bytes in the buffer.
@end defun

@defun find-operation-coding-system operation &rest arguments
This function returns the coding system to use (by default) for
performing @var{operation} with @var{arguments}.  The value has this
form:

@example
(@var{decoding-system} @var{encoding-system})
@end example

The first element, @var{decoding-system}, is the coding system to use
for decoding (in case @var{operation} does decoding), and
@var{encoding-system} is the coding system for encoding (in case
@var{operation} does encoding).

The argument @var{operation} should be an Emacs I/O primitive:
@code{insert-file-contents}, @code{write-region}, @code{call-process},
@code{call-process-region}, @code{start-process}, or
@code{open-network-stream}.

The remaining arguments should be the same arguments that might be given
to that I/O primitive.  Depending on which primitive, one of those
arguments is selected as the @dfn{target}.  For example, if
@var{operation} does file I/O, whichever argument specifies the file
name is the target.  For subprocess primitives, the process name is the
target.  For @code{open-network-stream}, the target is the service name
or port number.

This function looks up the target in @code{file-coding-system-alist},
@code{process-coding-system-alist}, or
@code{network-coding-system-alist}, depending on @var{operation}.
@xref{Default Coding Systems}.
@end defun

  Here are two functions you can use to let the user specify a coding
system, with completion.  @xref{Completion}.

@tindex read-coding-system
@defun read-coding-system prompt default
This function reads a coding system using the minibuffer, prompting with
string @var{prompt}, and returns the coding system name as a symbol.  If
the user enters null input, @var{default} specifies which coding system
to return.  It should be a symbol or a string.
@end defun

@tindex read-non-nil-coding-system
@defun read-non-nil-coding-system prompt
This function reads a coding system using the minibuffer, prompting with
string @var{prompt},and returns the coding system name as a symbol.  If
the user tries to enter null input, it asks the user to try again.
@xref{Coding Systems}.
@end defun

@node Default Coding Systems
@section Default Coding Systems

  These variable specify which coding system to use by default for
certain files or when running certain subprograms.  The idea of these
variables is that you set them once and for all to the defaults you
want, and then do not change them again.  To specify a particular coding
system for a particular operation in a Lisp program, don't change these
variables; instead, override them using @code{coding-system-for-read}
and @code{coding-system-for-write} (@pxref{Specifying Coding Systems}).

@tindex file-coding-system-alist
@defvar file-coding-system-alist
This variable is an alist that specifies the coding systems to use for
reading and writing particular files.  Each element has the form
@code{(@var{pattern} . @var{coding})}, where @var{pattern} is a regular
expression that matches certain file names.  The element applies to file
names that match @var{pattern}.

The @sc{cdr} of the element, @var{val}, should be either a coding
system, a cons cell containing two coding systems, or a function symbol.
If @var{val} is a coding system, that coding system is used for both
reading the file and writing it.  If @var{val} is a cons cell containing
two coding systems, its @sc{car} specifies the coding system for
decoding, and its @sc{cdr} specifies the coding system for encoding.

If @var{val} is a function symbol, the function must return a coding
system or a cons cell containing two coding systems.  This value is used
as described above.
@end defvar

@tindex process-coding-system-alist
@defvar process-coding-system-alist
This variable is an alist specifying which coding systems to use for a
subprocess, depending on which program is running in the subprocess.  It
works like @code{file-coding-system-alist}, except that @var{pattern} is
matched against the program name used to start the subprocess.  The coding
system or systems specified in this alist are used to initialize the
coding systems used for I/O to the subprocess, but you can specify
other coding systems later using @code{set-process-coding-system}.
@end defvar

@tindex network-coding-system-alist
@defvar network-coding-system-alist
This variable is an alist that specifies the coding system to use for
network streams.  It works much like @code{file-coding-system-alist},
with the difference that the @var{pattern} in an element may be either a
port number or a regular expression.  If it is a regular expression, it
is matched against the network service name used to open the network
stream.
@end defvar

@tindex default-process-coding-system
@defvar default-process-coding-system
This variable specifies the coding systems to use for subprocess (and
network stream) input and output, when nothing else specifies what to
do.

The value should be a cons cell of the form @code{(@var{output-coding}
. @var{input-coding})}.  Here @var{output-coding} applies to output to
the subprocess, and @var{input-coding} applies to input from it.
@end defvar

@node Specifying Coding Systems
@section Specifying a Coding System for One Operation

  You can specify the coding system for a specific operation by binding
the variables @code{coding-system-for-read} and/or
@code{coding-system-for-write}.

@tindex coding-system-for-read
@defvar coding-system-for-read
If this variable is non-@code{nil}, it specifies the coding system to
use for reading a file, or for input from a synchronous subprocess.

It also applies to any asynchronous subprocess or network stream, but in
a different way: the value of @code{coding-system-for-read} when you
start the subprocess or open the network stream specifies the input
decoding method for that subprocess or network stream.  It remains in
use for that subprocess or network stream unless and until overridden.

The right way to use this variable is to bind it with @code{let} for a
specific I/O operation.  Its global value is normally @code{nil}, and
you should not globally set it to any other value.  Here is an example
of the right way to use the variable:

@example
;; @r{Read the file with no character code conversion.}
;; @r{Assume @sc{crlf} represents end-of-line.}
(let ((coding-system-for-write 'emacs-mule-dos))
  (insert-file-contents filename))
@end example

When its value is non-@code{nil}, @code{coding-system-for-read} takes
precedence all other methods of specifying a coding system to use for
input, including @code{file-coding-system-alist},
@code{process-coding-system-alist} and
@code{network-coding-system-alist}.
@end defvar

@tindex coding-system-for-write
@defvar coding-system-for-write
This works much like @code{coding-system-for-read}, except that it
applies to output rather than input.  It affects writing to files,
subprocesses, and net connections.

When a single operation does both input and output, as do
@code{call-process-region} and @code{start-process}, both
@code{coding-system-for-read} and @code{coding-system-for-write}
affect it.
@end defvar

@tindex last-coding-system-used
@defvar last-coding-system-used
All I/O operations that use a coding system set this variable
to the coding system name that was used.
@end defvar

@tindex inhibit-eol-conversion
@defvar inhibit-eol-conversion
When this variable is non-@code{nil}, no end-of-line conversion is done,
no matter which coding system is specified.  This applies to all the
Emacs I/O and subprocess primitives, and to the explicit encoding and
decoding functions (@pxref{Explicit Encoding}).
@end defvar

@tindex keyboard-coding-system
@defun keyboard-coding-system
This function returns the coding system that is in use for decoding
keyboard input---or @code{nil} if no coding system is to be used.
@end defun

@tindex set-keyboard-coding-system
@defun set-keyboard-coding-system coding-system
This function specifies @var{coding-system} as the coding system to
use for decoding keyboard input.  If @var{coding-system} is @code{nil},
that means do not decode keyboard input.
@end defun

@tindex terminal-coding-system
@defun terminal-coding-system
This function returns the coding system that is in use for encoding
terminal output---or @code{nil} for no encoding.
@end defun

@tindex set-terminal-coding-system
@defun set-terminal-coding-system coding-system
This function specifies @var{coding-system} as the coding system to use
for encoding terminal output.  If @var{coding-system} is @code{nil},
that means do not encode terminal output.
@end defun

  See also the functions @code{process-coding-system} and
@code{set-process-coding-system}.  @xref{Process Information}.

  See also @code{read-coding-system} in @ref{High-Level Completion}.

@node Explicit Encoding
@section Explicit Encoding and Decoding
@cindex encoding text
@cindex decoding text

  All the operations that transfer text in and out of Emacs have the
ability to use a coding system to encode or decode the text.
You can also explicitly encode and decode text using the functions
in this section.

@cindex raw bytes
  The result of encoding, and the input to decoding, are not ordinary
text.  They are ``raw bytes''---bytes that represent text in the same
way that an external file would.  When a buffer contains raw bytes, it
is most natural to mark that buffer as using unibyte representation,
using @code{set-buffer-multibyte} (@pxref{Selecting a Representation}),
but this is not required.  If the buffer's contents are only temporarily
raw, leave the buffer multibyte, which will be correct after you decode
them.

  The usual way to get raw bytes in a buffer, for explicit decoding, is
to read them from a file with @code{insert-file-contents-literally}
(@pxref{Reading from Files}) or specify a non-@code{nil} @var{rawfile}
argument when visiting a file with @code{find-file-noselect}.

  The usual way to use the raw bytes that result from explicitly
encoding text is to copy them to a file or process---for example, to
write them with @code{write-region} (@pxref{Writing to Files}), and
suppress encoding for that @code{write-region} call by binding
@code{coding-system-for-write} to @code{no-conversion}.

@tindex encode-coding-region
@defun encode-coding-region start end coding-system
This function encodes the text from @var{start} to @var{end} according
to coding system @var{coding-system}.  The encoded text replaces the
original text in the buffer.  The result of encoding is ``raw bytes,''
but the buffer remains multibyte if it was multibyte before.
@end defun

@tindex encode-coding-string
@defun encode-coding-string string coding-system
This function encodes the text in @var{string} according to coding
system @var{coding-system}.  It returns a new string containing the
encoded text.  The result of encoding is a unibyte string of ``raw bytes.''
@end defun

@tindex decode-coding-region
@defun decode-coding-region start end coding-system
This function decodes the text from @var{start} to @var{end} according
to coding system @var{coding-system}.  The decoded text replaces the
original text in the buffer.  To make explicit decoding useful, the text
before decoding ought to be ``raw bytes.''
@end defun

@tindex decode-coding-string
@defun decode-coding-string string coding-system
This function decodes the text in @var{string} according to coding
system @var{coding-system}.  It returns a new string containing the
decoded text.  To make explicit decoding useful, the contents of
@var{string} ought to be ``raw bytes.''
@end defun

@node MS-DOS File Types
@section MS-DOS File Types
@cindex DOS file types
@cindex MS-DOS file types
@cindex Windows file types
@cindex file types on MS-DOS and Windows
@cindex text files and binary files
@cindex binary files and text files

  Emacs on MS-DOS and on MS-Windows recognizes certain file names as
text files or binary files.  For a text file, Emacs always uses DOS
end-of-line conversion.  For a binary file, Emacs does no end-of-line
conversion and no character code conversion.

@defvar buffer-file-type
This variable, automatically buffer-local in each buffer, records the
file type of the buffer's visited file.  The value is @code{nil} for
text, @code{t} for binary.  When a buffer does not specify a coding
system with @code{buffer-file-coding-system}, this variable is used by
the function @code{find-buffer-file-type-coding-system} to determine
which coding system to use when writing the contents of the buffer.
@end defvar

@defopt file-name-buffer-file-type-alist
This variable holds an alist for recognizing text and binary files.
Each element has the form (@var{regexp} . @var{type}), where
@var{regexp} is matched against the file name, and @var{type} may be
@code{nil} for text, @code{t} for binary, or a function to call to
compute which.  If it is a function, then it is called with a single
argument (the file name) and should return @code{t} or @code{nil}.

Emacs when running on MS-DOS or MS-Windows checks this alist to decide
which coding system to use when reading a file.  For a text file,
@code{undecided-dos} is used.  For a binary file, @code{no-conversion}
is used.

If no element in this alist matches a given file name, then
@code{default-buffer-file-type} says how to treat the file.
@end defopt

@defopt default-buffer-file-type
This variable says how to handle files for which
@code{file-name-buffer-file-type-alist} says nothing about the type.

If this variable is non-@code{nil}, then these files are treated as
binary.  Otherwise, nothing special is done for them---the coding system
is deduced solely from the file contents, in the usual Emacs fashion.
@end defopt

@node MS-DOS Subprocesses
@section MS-DOS Subprocesses

  On Microsoft operating systems, these variables provide an alternative
way to specify the kind of end-of-line conversion to use for input and
output.  The variable @code{binary-process-input} applies to input sent
to the subprocess, and @code{binary-process-output} applies to output
received from it.  A non-@code{nil} value means the data is ``binary,''
and @code{nil} means the data is text.

@defvar binary-process-input
If this variable is @code{nil}, convert newlines to @sc{crlf} sequences in
the input to a synchronous subprocess.
@end defvar

@defvar binary-process-output
If this variable is @code{nil}, convert @sc{crlf} sequences to newlines in
the output from a synchronous subprocess.
@end defvar