Non-ASCII Characters
********************

This chapter covers the special issues relating to characters and how they are stored in strings and buffers.

Text Representations
====================

Emacs buffers and strings support a large repertoire of characters from many different scripts, allowing users to type and display text in almost any known written language.

To support this multitude of characters and scripts, Emacs closely follows the Unicode Standard.  The Unicode Standard assigns a unique number, called a codepoint, to each and every character. The range of codepoints defined by Unicode, or the Unicode codespace, is 0..#x10FFFF (in hexadecimal notation), inclusive.  Emacs extends this range with codepoints in the range #x110000..#x3FFFFF, which it uses for representing characters that are not unified with Unicode and raw 8-bit bytes that cannot be interpreted as characters.  Thus, a character codepoint in Emacs is a 22-bit integer number.

To conserve memory, Emacs does not hold fixed-length 22-bit numbers that are codepoints of text characters within buffers and strings. Rather, Emacs uses a variable-length internal representation of characters, that stores each character as a sequence of 1 to 5 8-bit bytes, depending on the magnitude of its codepoint (This internal representation is based on one of the encodings defined by the Unicode Standard, called UTF-8, for representing any Unicode codepoint, but Emacs extends UTF-8 to represent the additional codepoints it uses for raw 8-bit bytes and characters not unified with Unicode.).  For example, any ASCII character takes up only 1 byte, a Latin-1 character takes up 2 bytes, etc.  We call this representation of text multibyte.

Outside Emacs, characters can be represented in many different encodings, such as ISO-8859-1, GB-2312, Big-5, etc.  Emacs converts between these external encodings and its internal representation, as appropriate, when it reads text into a buffer or a string, or when it writes text to a disk file or passes it to some other process.

Occasionally, Emacs needs to hold and manipulate encoded text or binary non-text data in its buffers or strings.  For example, when Emacs visits a file, it first reads the file's text verbatim into a buffer, and only then converts it to the internal representation. Before the conversion, the buffer holds encoded text.

Encoded text is not really text, as far as Emacs is concerned, but rather a sequence of raw 8-bit bytes.  We call buffers and strings that hold encoded text unibyte buffers and strings, because Emacs treats them as a sequence of individual bytes.  Usually, Emacs displays unibyte buffers and strings as octal codes such as \237.  We recommend that you never use unibyte buffers and strings except for manipulating encoded text or binary non-text data.

In a buffer, the buffer-local value of the variable enable-multibyte-characters specifies the representation used. The representation for a string is determined and recorded in the string when the string is constructed.

 -- Variable: enable-multibyte-characters
     This variable specifies the current buffer's text representation. If it is non-nil, the buffer contains multibyte text; otherwise, it contains unibyte encoded text or binary non-text data.

     You cannot set this variable directly; instead, use the function set-buffer-multibyte to change a buffer's representation.

 -- Function: position-bytes position
     Buffer positions are measured in character units.  This function returns the byte-position corresponding to buffer position POSITION in the current buffer.  This is 1 at the start of the buffer, and counts upward in bytes.  If POSITION is out of range, the value is nil.

 -- Function: byte-to-position byte-position
     Return the buffer position, in character units, corresponding to given BYTE-POSITION in the current buffer.  If BYTE-POSITION is out of range, the value is nil.  In a multibyte buffer, an arbitrary value of BYTE-POSITION can be not at character boundary, but inside a multibyte sequence representing a single character; in this case, this function returns the buffer position of the character whose multibyte sequence includes BYTE-POSITION. In other words, the value does not change for all byte positions that belong to the same character.

 -- Function: multibyte-string-p string
     Return t if STRING is a multibyte string, nil otherwise.

 -- Function: string-bytes string
     This function returns the number of bytes in STRING. If STRING is a multibyte string, this can be greater than (length STRING).

 -- Function: unibyte-string &rest bytes
     This function concatenates all its argument BYTES and makes the result a unibyte string.

Converting Text Representations
===============================

Emacs can convert unibyte text to multibyte; it can also convert multibyte text to unibyte, provided that the multibyte text contains only ASCII and 8-bit raw bytes.  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 from which it is constructed.  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 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 ASCII characters unchanged, and converts bytes with codes 128 through 255 to the multibyte representation of raw eight-bit bytes.

Converting multibyte text to unibyte converts all ASCII and eight-bit characters to their single-byte form, but loses information for non-ASCII characters by discarding all but the low 8 bits of each character's codepoint.  Converting unibyte text to multibyte and back to unibyte reproduces the original unibyte text.

The next two functions either return the argument STRING, or a newly created string with no text properties.

 -- Function: string-to-multibyte string
     This function returns a multibyte string containing the same sequence of characters as STRING.  If STRING is a multibyte string, it is returned unchanged.  The function assumes that STRING includes only ASCII characters and raw 8-bit bytes; the latter are converted to their multibyte representation corresponding to the codepoints #x3FFF80 through #x3FFFFF, inclusive (see codepoints).

 -- Function: string-to-unibyte string
     This function returns a unibyte string containing the same sequence of characters as STRING.  It signals an error if STRING contains a non-ASCII character.  If STRING is a unibyte string, it is returned unchanged.  Use this function for STRING arguments that contain only ASCII and eight-bit characters.

 -- Function: byte-to-string byte
     This function returns a unibyte string containing a single byte of character data, CHARACTER.  It signals an error if CHARACTER is not an integer between 0 and 255.

 -- Function: multibyte-char-to-unibyte char
     This converts the multibyte character CHAR to a unibyte character, and returns that character.  If CHAR is neither ASCII nor eight-bit, the function returns -1.

 -- Function: unibyte-char-to-multibyte char
     This convert the unibyte character CHAR to a multibyte character, assuming CHAR is either ASCII or raw 8-bit byte.

Selecting a Representation
==========================

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

 -- Function: set-buffer-multibyte multibyte
     Set the representation type of the current buffer.  If MULTIBYTE is non-nil, the buffer becomes multibyte.  If MULTIBYTE is 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; for instance, a sequence of three bytes which is treated as one character in multibyte representation will count as three characters in unibyte representation.  Eight-bit characters representing raw bytes are an exception.  They are represented by one byte in a unibyte buffer, but when the buffer is set to multibyte, they are converted to two-byte sequences, and vice versa.

     This function sets 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.

     You cannot use set-buffer-multibyte on an indirect buffer, because indirect buffers always inherit the representation of the base buffer.

 -- Function: string-as-unibyte string
     If STRING is already a unibyte string, this function returns STRING itself.  Otherwise, it returns a new string with the same bytes as STRING, but treating each byte as a separate character (so that the value may have more characters than STRING); as an exception, each eight-bit character representing a raw byte is converted into a single byte.  The newly-created string contains no text properties.

 -- Function: string-as-multibyte string
     If STRING is a multibyte string, this function returns STRING itself.  Otherwise, it returns a new string with the same bytes as STRING, but treating each multibyte sequence as one character.  This means that the value may have fewer characters than STRING has.  If a byte sequence in STRING is invalid as a multibyte representation of a single character, each byte in the sequence is treated as a raw 8-bit byte.  The newly-created string contains no text properties.

Character Codes
===============

The unibyte and multibyte text representations use different character codes.  The valid character codes for unibyte representation range from 0 to #xFF (255)---the values that can fit in one byte.  The valid character codes for multibyte representation range from 0 to #x3FFFFF.  In this code space, values 0 through #x7F (127) are for ASCII characters, and values #x80 (128) through #x3FFF7F (4194175) are for non-ASCII characters.

Emacs character codes are a superset of the Unicode standard. Values 0 through #x10FFFF (1114111) correspond to Unicode characters of the same codepoint; values #x110000 (1114112) through #x3FFF7F (4194175) represent characters that are not unified with Unicode; and values #x3FFF80 (4194176) through #x3FFFFF (4194303) represent eight-bit raw bytes.

 -- Function: characterp charcode
     This returns t if CHARCODE is a valid character, and nil otherwise.

          (characterp 65)
               => t

          (characterp 4194303)
               => t

          (characterp 4194304)
               => nil

 -- Function: max-char
     This function returns the largest value that a valid character codepoint can have.

          (characterp (max-char))
               => t

          (characterp (1+ (max-char)))
               => nil

 -- Function: get-byte &optional pos string
     This function returns the byte at character position POS in the current buffer.  If the current buffer is unibyte, this is literally the byte at that position.  If the buffer is multibyte, byte values of ASCII characters are the same as character codepoints, whereas eight-bit raw bytes are converted to their 8-bit codes.  The function signals an error if the character at POS is non-ASCII.

     The optional argument STRING means to get a byte value from that string instead of the current buffer.

Character Properties
====================

A character property is a named attribute of a character that specifies how the character behaves and how it should be handled during text processing and display.  Thus, character properties are an important part of specifying the character's semantics.

On the whole, Emacs follows the Unicode Standard in its implementation of character properties.  In particular, Emacs supports the Unicode Character Property Model (http://www.unicode.org/reports/tr23/), and the Emacs character property database is derived from the Unicode Character Database (UCD).  See the Character Properties chapter of the Unicode Standard (http://www.unicode.org/versions/Unicode5.0.0/ch04.pdf), for a detailed description of Unicode character properties and their meaning.  This section assumes you are already familiar with that chapter of the Unicode Standard, and want to apply that knowledge to Emacs Lisp programs.

In Emacs, each property has a name, which is a symbol, and a set of possible values, whose types depend on the property; if a character does not have a certain property, the value is nil.  As a general rule, the names of character properties in Emacs are produced from the corresponding Unicode properties by downcasing them and replacing each _ character with a dash -.  For example, Canonical_Combining_Class becomes canonical-combining-class.  However, sometimes we shorten the names to make their use easier.

Some codepoints are left unassigned by the UCD---they don't correspond to any character.  The Unicode Standard defines default values of properties for such codepoints; they are mentioned below for each property.

Here is the full list of value types for all the character properties that Emacs knows about:

name
     Corresponds to the Name Unicode property.  The value is a string consisting of upper-case Latin letters A to Z, digits, spaces, and hyphen - characters.  For unassigned codepoints, the value is an empty string.

general-category
     Corresponds to the General_Category Unicode property.  The value is a symbol whose name is a 2-letter abbreviation of the character's classification.  For unassigned codepoints, the value is Cn.

canonical-combining-class
     Corresponds to the Canonical_Combining_Class Unicode property. The value is an integer number.  For unassigned codepoints, the value is zero.

bidi-class
     Corresponds to the Unicode Bidi_Class property.  The value is a symbol whose name is the Unicode directional type of the character.  Emacs uses this property when it reorders bidirectional text for display (see Bidirectional Display).  For unassigned codepoints, the value depends on the code blocks to which the codepoint belongs: most unassigned codepoints get the value of L (strong L), but some get values of AL (Arabic letter) or R (strong R).

decomposition
     Corresponds to the Unicode Decomposition_Type and Decomposition_Value properties.  The value is a list, whose first element may be a symbol representing a compatibility formatting tag, such as small (The Unicode specification writes these tag names inside <..> brackets, but the tag names in Emacs do not include the brackets; e.g. Unicode specifies <small> where Emacs uses small.); the other elements are characters that give the compatibility decomposition sequence of this character.  For unassigned codepoints, the value is the character itself.

decimal-digit-value
     Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is Digit.  The value is an integer number.  For unassigned codepoints, the value is nil, which means NaN, or ``not-a-number''.

digit-value
     Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is Decimal.  The value is an integer number.  Examples of such characters include compatibility subscript and superscript digits, for which the value is the corresponding number.  For unassigned codepoints, the value is nil, which means NaN.

numeric-value
     Corresponds to the Unicode Numeric_Value property for characters whose Numeric_Type is Numeric.  The value of this property is an integer or a floating-point number.  Examples of characters that have this property include fractions, subscripts, superscripts, Roman numerals, currency numerators, and encircled numbers.  For example, the value of this property for the character U+2155 (VULGAR FRACTION ONE FIFTH) is 0.2.  For unassigned codepoints, the value is nil, which means NaN.

mirrored
     Corresponds to the Unicode Bidi_Mirrored property.  The value of this property is a symbol, either Y or N.  For unassigned codepoints, the value is N.

mirroring
     Corresponds to the Unicode Bidi_Mirroring_Glyph property.  The value of this property is a character whose glyph represents the mirror image of the character's glyph, or nil if there's no defined mirroring glyph.  All the characters whose mirrored property is N have nil as their mirroring property; however, some characters whose mirrored property is Y also have nil for mirroring, because no appropriate characters exist with mirrored glyphs.  Emacs uses this property to display mirror images of characters when appropriate (see Bidirectional Display).  For unassigned codepoints, the value is nil.

old-name
     Corresponds to the Unicode Unicode_1_Name property.  The value is a string.  For unassigned codepoints, the value is an empty string.

iso-10646-comment
     Corresponds to the Unicode ISO_Comment property.  The value is a string.  For unassigned codepoints, the value is an empty string.

uppercase
     Corresponds to the Unicode Simple_Uppercase_Mapping property. The value of this property is a single character.  For unassigned codepoints, the value is nil, which means the character itself.

lowercase
     Corresponds to the Unicode Simple_Lowercase_Mapping property. The value of this property is a single character.  For unassigned codepoints, the value is nil, which means the character itself.

titlecase
     Corresponds to the Unicode Simple_Titlecase_Mapping property. Title case is a special form of a character used when the first character of a word needs to be capitalized.  The value of this property is a single character.  For unassigned codepoints, the value is nil, which means the character itself.

 -- Function: get-char-code-property char propname
     This function returns the value of CHAR's PROPNAME property.

          (get-char-code-property ?  'general-category)
               => Zs

          (get-char-code-property ?1  'general-category)
               => Nd

          ;; subscript 4
          (get-char-code-property ?\u2084 'digit-value)
               => 4

          ;; one fifth
          (get-char-code-property ?\u2155 'numeric-value)
               => 0.2

          ;; Roman IV
          (get-char-code-property ?\u2163 'numeric-value)
               => 4

 -- Function: char-code-property-description prop value
     This function returns the description string of property PROP's VALUE, or nil if VALUE has no description.

          (char-code-property-description 'general-category 'Zs)
               => "Separator, Space"

          (char-code-property-description 'general-category 'Nd)
               => "Number, Decimal Digit"

          (char-code-property-description 'numeric-value '1/5)
               => nil

 -- Function: put-char-code-property char propname value
     This function stores VALUE as the value of the property PROPNAME for the character CHAR.

 -- Variable: unicode-category-table
     The value of this variable is a char-table (see Char-Tables) that specifies, for each character, its Unicode General_Category property as a symbol.

 -- Variable: char-script-table
     The value of this variable is a char-table that specifies, for each character, a symbol whose name is the script to which the character belongs, according to the Unicode Standard classification of the Unicode code space into script-specific blocks.  This char-table has a single extra slot whose value is the list of all script symbols.

 -- Variable: char-width-table
     The value of this variable is a char-table that specifies the width of each character in columns that it will occupy on the screen.

 -- Variable: printable-chars
     The value of this variable is a char-table that specifies, for each character, whether it is printable or not.  That is, if evaluating (aref printable-chars char) results in t, the character is printable, and if it results in nil, it is not.

Character Sets
==============

An Emacs character set, or charset, is a set of characters in which each character is assigned a numeric code point.  (The Unicode Standard calls this a coded character set.)  Each Emacs charset has a name which is a symbol.  A single character can belong to any number of different character sets, but it will generally have a different code point in each charset.  Examples of character sets include ascii, iso-8859-1, greek-iso8859-7, and windows-1255.  The code point assigned to a character in a charset is usually different from its code point used in Emacs buffers and strings.

Emacs defines several special character sets.  The character set unicode includes all the characters whose Emacs code points are in the range 0..#x10FFFF.  The character set emacs includes all ASCII and non-ASCII characters. Finally, the eight-bit charset includes the 8-bit raw bytes; Emacs uses it to represent raw bytes encountered in text.

 -- Function: charsetp object
     Returns t if OBJECT is a symbol that names a character set, nil otherwise.

 -- Variable: charset-list
     The value is a list of all defined character set names.

 -- Function: charset-priority-list &optional highestp
     This functions returns a list of all defined character sets ordered by their priority.  If HIGHESTP is non-nil, the function returns a single character set of the highest priority.

 -- Function: set-charset-priority &rest charsets
     This function makes CHARSETS the highest priority character sets.

 -- Function: char-charset character &optional restriction
     This function returns the name of the character set of highest priority that CHARACTER belongs to.  ASCII characters are an exception: for them, this function always returns ascii.

     If RESTRICTION is non-nil, it should be a list of charsets to search.  Alternatively, it can be a coding system, in which case the returned charset must be supported by that coding system (see Coding Systems).

 -- Function: charset-plist charset
     This function returns the property list of the character set CHARSET.  Although CHARSET is a symbol, this is not the same as the property list of that symbol.  Charset properties include important information about the charset, such as its documentation string, short name, etc.

 -- Function: put-charset-property charset propname value
     This function sets the PROPNAME property of CHARSET to the given VALUE.

 -- Function: get-charset-property charset propname
     This function returns the value of CHARSETs property PROPNAME.

 -- Command: list-charset-chars charset
     This command displays a list of characters in the character set CHARSET.

Emacs can convert between its internal representation of a character and the character's codepoint in a specific charset.  The following two functions support these conversions.

 -- Function: decode-char charset code-point
     This function decodes a character that is assigned a CODE-POINT in CHARSET, to the corresponding Emacs character, and returns it.  If CHARSET doesn't contain a character of that code point, the value is nil.  If CODE-POINT doesn't fit in a Lisp integer (see most-positive-fixnum), it can be specified as a cons cell (HIGH . LOW), where LOW are the lower 16 bits of the value and HIGH are the high 16 bits.

 -- Function: encode-char char charset
     This function returns the code point assigned to the character CHAR in CHARSET.  If the result does not fit in a Lisp integer, it is returned as a cons cell (HIGH . LOW) that fits the second argument of decode-char above.  If CHARSET doesn't have a codepoint for CHAR, the value is nil.

The following function comes in handy for applying a certain function to all or part of the characters in a charset:

 -- Function: map-charset-chars function charset &optional arg from-code to-code
     Call FUNCTION for characters in CHARSET.  FUNCTION is called with two arguments.  The first one is a cons cell (FROM .  TO), where FROM and TO indicate a range of characters contained in charset.  The second argument passed to FUNCTION is ARG.

     By default, the range of codepoints passed to FUNCTION includes all the characters in CHARSET, but optional arguments FROM-CODE and TO-CODE limit that to the range of characters between these two codepoints of CHARSET.  If either of them is nil, it defaults to the first or last codepoint of CHARSET, respectively.

Scanning for Character Sets
===========================

Sometimes it is useful to find out which character set a particular character belongs to.  One use for this is in determining which coding systems (see Coding Systems) are capable of representing all of the text in question; another is to determine the font(s) for displaying that text.

 -- Function: charset-after &optional pos
     This function returns the charset of highest priority containing the character at position POS in the current buffer.  If POS is omitted or nil, it defaults to the current value of point. If POS is out of range, the value is nil.

 -- Function: find-charset-region beg end &optional translation
     This function returns a list of the character sets of highest priority that contain characters in the current buffer between positions BEG and END.

     The optional argument TRANSLATION specifies a translation table to use for scanning the text (see Translation of Characters).  If it is non-nil, then each character in the region is translated through this table, and the value returned describes the translated characters instead of the characters actually in the buffer.

 -- Function: find-charset-string string &optional translation
     This function returns a list of character sets of highest priority that contain characters in STRING.  It is just like find-charset-region, except that it applies to the contents of STRING instead of part of the current buffer.

Translation of Characters
=========================

A translation table is a char-table (see Char-Tables) that specifies a mapping of characters into characters.  These tables are used in encoding and decoding, and for other purposes.  Some coding systems specify their own particular translation tables; there are also default translation tables which apply to all other coding systems.

A translation table has two extra slots.  The first is either nil or a translation table that performs the reverse translation; the second is the maximum number of characters to look up for translating sequences of characters (see the description of make-translation-table-from-alist below).

 -- Function: make-translation-table &rest translations
     This function returns a translation table based on the argument TRANSLATIONS.  Each element of TRANSLATIONS should be a list of elements of the form (FROM . TO); this says to translate the character FROM into TO.

     The arguments and the forms in each argument are processed in order, and if a previous form already translates TO to some other character, say TO-ALT, FROM is also translated to TO-ALT.

During decoding, the translation table's translations are applied to the characters that result from ordinary decoding.  If a coding system has the property :decode-translation-table, that specifies the translation table to use, or a list of translation tables to apply in sequence.  (This is a property of the coding system, as returned by coding-system-get, not a property of the symbol that is the coding system's name.  See Basic Concepts of Coding Systems.)  Finally, if standard-translation-table-for-decode is non-nil, the resulting characters are translated by that table.

During encoding, the translation table's translations are applied to the characters in the buffer, and the result of translation is actually encoded.  If a coding system has property :encode-translation-table, that specifies the translation table to use, or a list of translation tables to apply in sequence.  In addition, if the variable standard-translation-table-for-encode is non-nil, it specifies the translation table to use for translating the result.

 -- Variable: standard-translation-table-for-decode
     This is the default translation table for decoding.  If a coding systems specifies its own translation tables, the table that is the value of this variable, if non-nil, is applied after them.

 -- Variable: standard-translation-table-for-encode
     This is the default translation table for encoding.  If a coding systems specifies its own translation tables, the table that is the value of this variable, if non-nil, is applied after them.

 -- Variable: translation-table-for-input
     Self-inserting characters are translated through this translation table before they are inserted.  Search commands also translate their input through this table, so they can compare more reliably with what's in the buffer.

     This variable automatically becomes buffer-local when set.

 -- Function: make-translation-table-from-vector vec
     This function returns a translation table made from VEC that is an array of 256 elements to map bytes (values 0 through #xFF) to characters.  Elements may be nil for untranslated bytes.  The returned table has a translation table for reverse mapping in the first extra slot, and the value 1 in the second extra slot.

     This function provides an easy way to make a private coding system that maps each byte to a specific character.  You can specify the returned table and the reverse translation table using the properties :decode-translation-table and :encode-translation-table respectively in the PROPS argument to define-coding-system.

 -- Function: make-translation-table-from-alist alist
     This function is similar to make-translation-table but returns a complex translation table rather than a simple one-to-one mapping. Each element of ALIST is of the form (FROM . TO), where FROM and TO are either characters or vectors specifying a sequence of characters.  If FROM is a character, that character is translated to TO (i.e. to a character or a character sequence).  If FROM is a vector of characters, that sequence is translated to TO.  The returned table has a translation table for reverse mapping in the first extra slot, and the maximum length of all the FROM character sequences in the second extra slot.

Coding Systems
==============

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 coding system.

How to define a coding system is an arcane matter, and is not documented here.

Basic Concepts of Coding Systems
--------------------------------

Character code conversion involves conversion between the internal representation of characters 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.

Every coding system specifies a particular set of character code conversions, but the coding system undecided is special: it leaves the choice unspecified, to be chosen heuristically for each file, based on the file's data.

In general, a coding system doesn't guarantee roundtrip identity: decoding a byte sequence using coding system, then encoding the resulting text in the same coding system, can produce a different byte sequence.  But some coding systems do guarantee that the byte sequence will be the same as what you originally decoded.  Here are a few examples:

     iso-8859-1, utf-8, big5, shift_jis, euc-jp

Encoding buffer text and then decoding the result can also fail to reproduce the original text.  For instance, if you encode a character with a coding system which does not support that character, the result is unpredictable, and thus decoding it using the same coding system may produce a different text.  Currently, Emacs can't report errors that result from encoding unsupported characters.

End of line conversion handles three different conventions used on various systems for representing end of line in files.  The Unix convention, used on GNU and Unix systems, is to use the linefeed character (also called newline).  The DOS convention, used on MS-Windows and MS-DOS systems, is to use a carriage-return and a linefeed at the end of a line.  The Mac convention is to use just carriage-return.

Base coding systems such as latin-1 leave the end-of-line conversion unspecified, to be chosen based on the data.  Variant coding systems such as latin-1-unix, latin-1-dos and latin-1-mac specify the end-of-line conversion explicitly as well.  Most base coding systems have three corresponding variants whose names are formed by adding -unix, -dos and -mac.

The coding system raw-text is special in that it prevents character code conversion, and causes the buffer visited with this coding system to be a unibyte buffer.  For historical reasons, you can save both unibyte and multibyte text with this coding system.  When you use raw-text to encode multibyte text, it does perform one character code conversion: it converts eight-bit characters to their single-byte external representation.  raw-text does not specify the end-of-line conversion, allowing that to be determined as usual by the data, and has the usual three variants which specify the end-of-line conversion.

no-conversion (and its alias binary) is equivalent to raw-text-unix: it specifies no conversion of either character codes or end-of-line.

The coding system utf-8-emacs specifies that the data is represented in the internal Emacs encoding (see Text Representations).  This is like raw-text in that no code conversion happens, but different in that the result is multibyte data.  The name emacs-internal is an alias for utf-8-emacs.

 -- Function: coding-system-get coding-system property
     This function returns the specified property of the coding system CODING-SYSTEM.  Most coding system properties exist for internal purposes, but one that you might find useful is :mime-charset. That property's value is the name used in MIME for the character coding which this coding system can read and write.  Examples:

          (coding-system-get 'iso-latin-1 :mime-charset)
               => iso-8859-1
          (coding-system-get 'iso-2022-cn :mime-charset)
               => iso-2022-cn
          (coding-system-get 'cyrillic-koi8 :mime-charset)
               => koi8-r

     The value of the :mime-charset property is also defined as an alias for the coding system.

 -- Function: coding-system-aliases coding-system
     This function returns the list of aliases of CODING-SYSTEM.

Encoding and I/O
----------------

The principal purpose of coding systems is for use in reading and writing files.  The function insert-file-contents uses a coding system to decode the file data, and write-region uses one to encode the buffer contents.

You can specify the coding system to use either explicitly (see Specifying Coding Systems), or implicitly using a default mechanism (see Default Coding Systems).  But these methods may not completely specify what to do.  For example, they may choose a coding system such as undefined which leaves the character code conversion to be determined from the data.  In these cases, the I/O operation finishes the job of choosing a coding system.  Very often you will want to find out afterwards which coding system was chosen.

 -- Variable: buffer-file-coding-system
     This buffer-local variable records the coding system used for saving the buffer and for writing part of the buffer with write-region.  If the text to be written cannot be safely encoded using the coding system specified by this variable, these operations select an alternative encoding by calling the function select-safe-coding-system (see User-Chosen Coding Systems).  If selecting a different encoding requires to ask the user to specify a coding system, buffer-file-coding-system is updated to the newly selected coding system.

     buffer-file-coding-system does not affect sending text to a subprocess.

 -- Variable: save-buffer-coding-system
     This variable specifies the coding system for saving the buffer (by overriding buffer-file-coding-system).  Note that it is not used for write-region.

     When a command to save the buffer starts out to use buffer-file-coding-system (or save-buffer-coding-system), and that coding system cannot handle the actual text in the buffer, the command asks the user to choose another coding system (by calling select-safe-coding-system). After that happens, the command also updates buffer-file-coding-system to represent the coding system that the user specified.

 -- Variable: last-coding-system-used
     I/O operations for files and subprocesses set this variable to the coding system name that was used.  The explicit encoding and decoding functions (see Explicit Encoding) set it too.

     Warning: Since receiving subprocess output sets this variable, it can change whenever Emacs waits; therefore, you should copy the value shortly after the function call that stores the value you are interested in.

The variable selection-coding-system specifies how to encode selections for the window system.  See Window System Selections.

 -- Variable: file-name-coding-system
     The variable file-name-coding-system specifies the coding system to use for encoding file names.  Emacs encodes file names using that coding system for all file operations.  If file-name-coding-system is 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.

Warning: if you change 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 are handled 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 C-x C-w to specify a new file name for that buffer.

Coding Systems in Lisp
----------------------

Here are the Lisp facilities for working with coding systems:

 -- Function: coding-system-list &optional base-only
     This function returns a list of all coding system names (symbols).  If BASE-ONLY is non-nil, the value includes only the base coding systems.  Otherwise, it includes alias and variant coding systems as well.

 -- Function: coding-system-p object
     This function returns t if OBJECT is a coding system name or nil.

 -- Function: check-coding-system coding-system
     This function checks the validity of CODING-SYSTEM.  If that is valid, it returns CODING-SYSTEM.  If CODING-SYSTEM is nil, the function return nil.  For any other values, it signals an error whose error-symbol is coding-system-error (see signal).

 -- Function: coding-system-eol-type coding-system
     This function returns the type of end-of-line (a.k.a. eol) conversion used by CODING-SYSTEM.  If CODING-SYSTEM specifies a certain eol conversion, the return value is an integer 0, 1, or 2, standing for unix, dos, and mac, respectively.  If CODING-SYSTEM doesn't specify eol conversion explicitly, the return value is a vector of coding systems, each one with one of the possible eol conversion types, like this:

          (coding-system-eol-type 'latin-1)
               => [latin-1-unix latin-1-dos latin-1-mac]

     If this function returns a vector, Emacs will decide, as part of the text encoding or decoding process, what eol conversion to use.  For decoding, the end-of-line format of the text is auto-detected, and the eol conversion is set to match it (e.g., DOS-style CRLF format will imply dos eol conversion).  For encoding, the eol conversion is taken from the appropriate default coding system (e.g., default value of buffer-file-coding-system for buffer-file-coding-system), or from the default eol conversion appropriate for the underlying platform.

 -- Function: coding-system-change-eol-conversion coding-system eol-type
     This function returns a coding system which is like CODING-SYSTEM except for its eol conversion, which is specified by eol-type. EOL-TYPE should be unix, dos, mac, or nil.  If it is nil, the returned coding system determines the end-of-line conversion from the data.

     EOL-TYPE may also be 0, 1 or 2, standing for unix, dos and mac, respectively.

 -- Function: coding-system-change-text-conversion eol-coding text-coding
     This function returns a coding system which uses the end-of-line conversion of EOL-CODING, and the text conversion of TEXT-CODING.  If TEXT-CODING is nil, it returns undecided, or one of its variants according to EOL-CODING.

 -- Function: find-coding-systems-region from to
     This function returns a list of coding systems that could be used to encode a text between FROM and TO.  All coding systems in the list can safely encode any multibyte characters in that portion of the text.

     If the text contains no multibyte characters, the function returns the list (undecided).

 -- Function: find-coding-systems-string string
     This function returns a list of coding systems that could be used to encode the text of STRING.  All coding systems in the list can safely encode any multibyte characters in STRING.  If the text contains no multibyte characters, this returns the list (undecided).

 -- Function: find-coding-systems-for-charsets charsets
     This function returns a list of coding systems that could be used to encode all the character sets in the list CHARSETS.

 -- Function: check-coding-systems-region start end coding-system-list
     This function checks whether coding systems in the list coding-system-list can encode all the characters in the region between START and END.  If all of the coding systems in the list can encode the specified text, the function returns nil.  If some coding systems cannot encode some of the characters, the value is an alist, each element of which has the form (CODING-SYSTEM1 POS1 POS2 ...), meaning that CODING-SYSTEM1 cannot encode characters at buffer positions POS1, POS2, ....

     START may be a string, in which case END is ignored and the returned value references string indices instead of buffer positions.

 -- Function: detect-coding-region start end &optional highest
     This function chooses a plausible coding system for decoding the text from START to END.  This text should be a byte sequence, i.e. unibyte text or multibyte text with only ASCII and eight-bit characters (see Explicit Encoding).

     Normally this function returns a list of coding systems that could handle decoding the text that was scanned.  They are listed in order of decreasing priority.  But if HIGHEST is non-nil, then the return value is just one coding system, the one that is highest in priority.

     If the region contains only ASCII characters except for such ISO-2022 control characters ISO-2022 as ESC, the value is undecided or (undecided), or a variant specifying end-of-line conversion, if that can be deduced from the text.

     If the region contains null bytes, the value is no-conversion, even if the region contains text encoded in some coding system.

 -- Function: detect-coding-string string &optional highest
     This function is like detect-coding-region except that it operates on the contents of STRING instead of bytes in the buffer.

 -- Variable: inhibit-null-byte-detection
     If this variable has a non-nil value, null bytes are ignored when detecting the encoding of a region or a string.  This allows to correctly detect the encoding of text that contains null bytes, such as Info files with Index nodes.

 -- Variable: inhibit-iso-escape-detection
     If this variable has a non-nil value, ISO-2022 escape sequences are ignored when detecting the encoding of a region or a string.  The result is that no text is ever detected as encoded in some ISO-2022 encoding, and all escape sequences become visible in a buffer. Warning: Use this variable with extreme caution, because many files in the Emacs distribution use ISO-2022 encoding.

 -- Function: coding-system-charset-list coding-system
     This function returns the list of character sets (see Character Sets) supported by CODING-SYSTEM.  Some coding systems that support too many character sets to list them all yield special values:

       * If CODING-SYSTEM supports all the ISO-2022 charsets, the value is iso-2022.

       * If CODING-SYSTEM supports all Emacs characters, the value is (emacs).

       * If CODING-SYSTEM supports all emacs-mule characters, the value is emacs-mule.

       * If CODING-SYSTEM supports all Unicode characters, the value is (unicode).

See Process Information, in particular the description of the functions process-coding-system and set-process-coding-system, for how to examine or set the coding systems used for I/O to a subprocess.

User-Chosen Coding Systems
--------------------------

 -- Function: select-safe-coding-system from to &optional default-coding-system accept-default-p file
     This function selects a coding system for encoding specified text, asking the user to choose if necessary.  Normally the specified text is the text in the current buffer between FROM and TO.  If FROM is a string, the string specifies the text to encode, and TO is ignored.

     If the specified text includes raw bytes (see Text Representations), select-safe-coding-system suggests raw-text for its encoding.

     If DEFAULT-CODING-SYSTEM is non-nil, that is the first coding system to try; if that can handle the text, select-safe-coding-system returns that coding system.  It can also be a list of coding systems; then the function tries each of them one by one.  After trying all of them, it next tries the current buffer's value of buffer-file-coding-system (if it is not undecided), then the default value of buffer-file-coding-system and finally the user's most preferred coding system, which the user can set using the command prefer-coding-system (see Recognizing Coding Systems (The GNU Emacs Manual)).

     If one of those coding systems can safely encode all the specified text, select-safe-coding-system chooses it and returns it. Otherwise, it asks the user to choose from a list of coding systems which can encode all the text, and returns the user's choice.

     DEFAULT-CODING-SYSTEM can also be a list whose first element is t and whose other elements are coding systems.  Then, if no coding system in the list can handle the text, select-safe-coding-system queries the user immediately, without trying any of the three alternatives described above.

     The optional argument ACCEPT-DEFAULT-P, if non-nil, should be a function to determine whether a coding system selected without user interaction is acceptable. select-safe-coding-system calls this function with one argument, the base coding system of the selected coding system.  If ACCEPT-DEFAULT-P returns nil, select-safe-coding-system rejects the silently selected coding system, and asks the user to select a coding system from a list of possible candidates.

     If the variable select-safe-coding-system-accept-default-p is non-nil, it should be a function taking a single argument. It is used in place of ACCEPT-DEFAULT-P, overriding any value supplied for this argument.

     As a final step, before returning the chosen coding system, select-safe-coding-system checks whether that coding system is consistent with what would be selected if the contents of the region were read from a file.  (If not, this could lead to data corruption in a file subsequently re-visited and edited.)  Normally, select-safe-coding-system uses buffer-file-name as the file for this purpose, but if FILE is non-nil, it uses that file instead (this can be relevant for write-region and similar functions).  If it detects an apparent inconsistency, select-safe-coding-system queries the user before selecting the coding system.

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

 -- Function: read-coding-system prompt &optional default
     This function reads a coding system using the minibuffer, prompting with string PROMPT, and returns the coding system name as a symbol.  If the user enters null input, DEFAULT specifies which coding system to return.  It should be a symbol or a string.

 -- Function: read-non-nil-coding-system prompt
     This function reads a coding system using the minibuffer, prompting with string 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. See Coding Systems.

Default Coding Systems
----------------------

This section describes variables that specify the default coding system for certain files or when running certain subprograms, and the function that I/O operations use to access them.

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 coding-system-for-read and coding-system-for-write (see Specifying Coding Systems).

 -- User Option: auto-coding-regexp-alist
     This variable is an alist of text patterns and corresponding coding systems. Each element has the form (REGEXP . CODING-SYSTEM); a file whose first few kilobytes match REGEXP is decoded with CODING-SYSTEM when its contents are read into a buffer.  The settings in this alist take priority over coding: tags in the files and the contents of file-coding-system-alist (see below).  The default value is set so that Emacs automatically recognizes mail files in Babyl format and reads them with no code conversions.

 -- User Option: 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 (PATTERN . CODING), where PATTERN is a regular expression that matches certain file names.  The element applies to file names that match PATTERN.

     The CDR of the element, CODING, should be either a coding system, a cons cell containing two coding systems, or a function name (a symbol with a function definition).  If CODING is a coding system, that coding system is used for both reading the file and writing it.  If CODING is a cons cell containing two coding systems, its CAR specifies the coding system for decoding, and its CDR specifies the coding system for encoding.

     If CODING is a function name, the function should take one argument, a list of all arguments passed to find-operation-coding-system.  It must return a coding system or a cons cell containing two coding systems.  This value has the same meaning as described above.

     If CODING (or what returned by the above function) is undecided, the normal code-detection is performed.

 -- User Option: auto-coding-alist
     This variable is an alist that specifies the coding systems to use for reading and writing particular files.  Its form is like that of file-coding-system-alist, but, unlike the latter, this variable takes priority over any coding: tags in the file.

 -- Variable: 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 file-coding-system-alist, except that 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 set-process-coding-system.

Warning: Coding systems such as undecided, which determine the coding system from the data, do not work entirely reliably with asynchronous subprocess output.  This is because Emacs handles asynchronous subprocess output in batches, as it arrives.  If the coding system leaves the character code conversion unspecified, or leaves the end-of-line conversion unspecified, Emacs must try to detect the proper conversion from one batch at a time, and this does not always work.

Therefore, with an asynchronous subprocess, if at all possible, use a coding system which determines both the character code conversion and the end of line conversion---that is, one like latin-1-unix, rather than undecided or latin-1.

 -- Variable: network-coding-system-alist
     This variable is an alist that specifies the coding system to use for network streams.  It works much like file-coding-system-alist, with the difference that the 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.

 -- Variable: 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 (INPUT-CODING . OUTPUT-CODING).  Here INPUT-CODING applies to input from the subprocess, and OUTPUT-CODING applies to output to it.

 -- User Option: auto-coding-functions
     This variable holds a list of functions that try to determine a coding system for a file based on its undecoded contents.

     Each function in this list should be written to look at text in the current buffer, but should not modify it in any way.  The buffer will contain undecoded text of parts of the file.  Each function should take one argument, SIZE, which tells it how many characters to look at, starting from point.  If the function succeeds in determining a coding system for the file, it should return that coding system. Otherwise, it should return nil.

     If a file has a coding: tag, that takes precedence, so these functions won't be called.

 -- Function: find-auto-coding filename size
     This function tries to determine a suitable coding system for FILENAME.  It examines the buffer visiting the named file, using the variables documented above in sequence, until it finds a match for one of the rules specified by these variables.  It then returns a cons cell of the form (CODING . SOURCE), where CODING is the coding system to use and SOURCE is a symbol, one of auto-coding-alist, auto-coding-regexp-alist, :coding, or auto-coding-functions, indicating which one supplied the matching rule.  The value :coding means the coding system was specified by the coding: tag in the file (see coding tag (The GNU Emacs Manual)). The order of looking for a matching rule is auto-coding-alist first, then auto-coding-regexp-alist, then the coding: tag, and lastly auto-coding-functions.  If no matching rule was found, the function returns nil.

     The second argument SIZE is the size of text, in characters, following point.  The function examines text only within SIZE characters after point.  Normally, the buffer should be positioned at the beginning when this function is called, because one of the places for the coding: tag is the first one or two lines of the file; in that case, SIZE should be the size of the buffer.

 -- Function: set-auto-coding filename size
     This function returns a suitable coding system for file FILENAME.  It uses find-auto-coding to find the coding system.  If no coding system could be determined, the function returns nil.  The meaning of the argument SIZE is like in find-auto-coding.

 -- Function: find-operation-coding-system operation &rest arguments
     This function returns the coding system to use (by default) for performing OPERATION with ARGUMENTS.  The value has this form:

          (DECODING-SYSTEM . ENCODING-SYSTEM)

     The first element, DECODING-SYSTEM, is the coding system to use for decoding (in case OPERATION does decoding), and ENCODING-SYSTEM is the coding system for encoding (in case OPERATION does encoding).

     The argument OPERATION is a symbol; it should be one of write-region, start-process, call-process, call-process-region, insert-file-contents, or open-network-stream.  These are the names of the Emacs I/O primitives that can do character code and eol conversion.

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

     Depending on OPERATION, this function looks up the target in file-coding-system-alist, process-coding-system-alist, or network-coding-system-alist.  If the target is found in the alist, find-operation-coding-system returns its association in the alist; otherwise it returns nil.

     If OPERATION is insert-file-contents, the argument corresponding to the target may be a cons cell of the form (FILENAME . BUFFER)).  In that case, FILENAME is a file name to look up in file-coding-system-alist, and BUFFER is a buffer that contains the file's contents (not yet decoded).  If file-coding-system-alist specifies a function to call for this file, and that function needs to examine the file's contents (as it usually does), it should examine the contents of BUFFER instead of reading the file.

Specifying a Coding System for One Operation
--------------------------------------------

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

 -- Variable: coding-system-for-read
     If this variable is non-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 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 let for a specific I/O operation.  Its global value is normally nil, and you should not globally set it to any other value.  Here is an example of the right way to use the variable:

          ;; Read the file with no character code conversion.
          ;; Assume crlf represents end-of-line.
          (let ((coding-system-for-read 'emacs-mule-dos))
            (insert-file-contents filename))

     When its value is non-nil, this variable takes precedence over all other methods of specifying a coding system to use for input, including file-coding-system-alist, process-coding-system-alist and network-coding-system-alist.

 -- Variable: coding-system-for-write
     This works much like coding-system-for-read, except that it applies to output rather than input.  It affects writing to files, as well as sending output to subprocesses and net connections.

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

 -- User Option: inhibit-eol-conversion
     When this variable is non-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 (see Explicit Encoding).

Sometimes, you need to prefer several coding systems for some operation, rather than fix a single one.  Emacs lets you specify a priority order for using coding systems.  This ordering affects the sorting of lists of coding systems returned by functions such as find-coding-systems-region (see Lisp and Coding Systems).

 -- Function: coding-system-priority-list &optional highestp
     This function returns the list of coding systems in the order of their current priorities.  Optional argument HIGHESTP, if non-nil, means return only the highest priority coding system.

 -- Function: set-coding-system-priority &rest coding-systems
     This function puts CODING-SYSTEMS at the beginning of the priority list for coding systems, thus making their priority higher than all the rest.

 -- Macro: with-coding-priority coding-systems &rest body...
     This macro execute BODY, like progn does (see progn), with CODING-SYSTEMS at the front of the priority list for coding systems.  CODING-SYSTEMS should be a list of coding systems to prefer during execution of BODY.

Explicit Encoding and Decoding
------------------------------

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.

The result of encoding, and the input to decoding, are not ordinary text.  They logically consist of a series of byte values; that is, a series of ASCII and eight-bit characters.  In unibyte buffers and strings, these characters have codes in the range 0 through #xFF (255).  In a multibyte buffer or string, eight-bit characters have character codes higher than #xFF (see Text Representations), but Emacs transparently converts them to their single-byte values when you encode or decode such text.

The usual way to read a file into a buffer as a sequence of bytes, so you can decode the contents explicitly, is with insert-file-contents-literally (see Reading from Files); alternatively, specify a non-nil RAWFILE argument when visiting a file with find-file-noselect.  These methods result in a unibyte buffer.

The usual way to use the byte sequence that results from explicitly encoding text is to copy it to a file or process---for example, to write it with write-region (see Writing to Files), and suppress encoding by binding coding-system-for-write to no-conversion.

Here are the functions to perform explicit encoding or decoding.  The encoding functions produce sequences of bytes; the decoding functions are meant to operate on sequences of bytes.  All of these functions discard text properties.  They also set last-coding-system-used to the precise coding system they used.

 -- Command: encode-coding-region start end coding-system &optional destination
     This command encodes the text from START to END according to coding system CODING-SYSTEM.  Normally, the encoded text replaces the original text in the buffer, but the optional argument DESTINATION can change that.  If DESTINATION is a buffer, the encoded text is inserted in that buffer after point (point does not move); if it is t, the command returns the encoded text as a unibyte string without inserting it.

     If encoded text is inserted in some buffer, this command returns the length of the encoded text.

     The result of encoding is logically a sequence of bytes, but the buffer remains multibyte if it was multibyte before, and any 8-bit bytes are converted to their multibyte representation (see Text Representations).

     Do not use undecided for CODING-SYSTEM when encoding text, since that may lead to unexpected results.  Instead, use select-safe-coding-system (see select-safe-coding-system) to suggest a suitable encoding, if there's no obvious pertinent value for CODING-SYSTEM.

 -- Function: encode-coding-string string coding-system &optional nocopy buffer
     This function encodes the text in STRING according to coding system CODING-SYSTEM.  It returns a new string containing the encoded text, except when NOCOPY is non-nil, in which case the function may return STRING itself if the encoding operation is trivial.  The result of encoding is a unibyte string.

 -- Command: decode-coding-region start end coding-system &optional destination
     This command decodes the text from START to END according to coding system CODING-SYSTEM.  To make explicit decoding useful, the text before decoding ought to be a sequence of byte values, but both multibyte and unibyte buffers are acceptable (in the multibyte case, the raw byte values should be represented as eight-bit characters).  Normally, the decoded text replaces the original text in the buffer, but the optional argument DESTINATION can change that.  If DESTINATION is a buffer, the decoded text is inserted in that buffer after point (point does not move); if it is t, the command returns the decoded text as a multibyte string without inserting it.

     If decoded text is inserted in some buffer, this command returns the length of the decoded text.

     This command puts a charset text property on the decoded text. The value of the property states the character set used to decode the original text.

 -- Function: decode-coding-string string coding-system &optional nocopy buffer
     This function decodes the text in STRING according to CODING-SYSTEM.  It returns a new string containing the decoded text, except when NOCOPY is non-nil, in which case the function may return STRING itself if the decoding operation is trivial.  To make explicit decoding useful, the contents of STRING ought to be a unibyte string with a sequence of byte values, but a multibyte string is also acceptable (assuming it contains 8-bit bytes in their multibyte form).

     If optional argument BUFFER specifies a buffer, the decoded text is inserted in that buffer after point (point does not move).  In this case, the return value is the length of the decoded text.

     This function puts a charset text property on the decoded text. The value of the property states the character set used to decode the original text:

          (decode-coding-string "Gr\374ss Gott" 'latin-1)
               => #("Grüss Gott" 0 9 (charset iso-8859-1))

 -- Function: decode-coding-inserted-region from to filename &optional visit beg end replace
     This function decodes the text from FROM to TO as if it were being read from file FILENAME using insert-file-contents using the rest of the arguments provided.

     The normal way to use this function is after reading text from a file without decoding, if you decide you would rather have decoded it. Instead of deleting the text and reading it again, this time with decoding, you can call this function.

Terminal I/O Encoding
---------------------

Emacs can decode keyboard input using a coding system, and encode terminal output.  This is useful for terminals that transmit or display text using a particular encoding such as Latin-1.  Emacs does not set last-coding-system-used for encoding or decoding of terminal I/O.

 -- Function: keyboard-coding-system &optional terminal
     This function returns the coding system that is in use for decoding keyboard input from TERMINAL---or nil if no coding system is to be used for that terminal.  If TERMINAL is omitted or nil, it means the selected frame's terminal.  See Multiple Terminals.

 -- Command: set-keyboard-coding-system coding-system &optional terminal
     This command specifies CODING-SYSTEM as the coding system to use for decoding keyboard input from TERMINAL.  If CODING-SYSTEM is nil, that means do not decode keyboard input.  If TERMINAL is a frame, it means that frame's terminal; if it is nil, that means the currently selected frame's terminal.  See Multiple Terminals.

 -- Function: terminal-coding-system &optional terminal
     This function returns the coding system that is in use for encoding terminal output from TERMINAL---or nil if the output is not encoded.  If TERMINAL is a frame, it means that frame's terminal; if it is nil, that means the currently selected frame's terminal.

 -- Command: set-terminal-coding-system coding-system &optional terminal
     This command specifies CODING-SYSTEM as the coding system to use for encoding terminal output from TERMINAL.  If CODING-SYSTEM is nil, terminal output is not encoded.  If TERMINAL is a frame, it means that frame's terminal; if it is nil, that means the currently selected frame's terminal.

MS-DOS File Types
-----------------

On MS-DOS and Microsoft Windows, Emacs guesses the appropriate end-of-line conversion for a file by looking at the file's name.  This feature classifies files as text files and binary files.  By ``binary file'' we mean a file of literal byte values that are not necessarily meant to be characters; Emacs does no end-of-line conversion and no character code conversion for them.  On the other hand, the bytes in a text file are intended to represent characters; when you create a new file whose name implies that it is a text file, Emacs uses DOS end-of-line conversion.

 -- Variable: buffer-file-type
     This variable, automatically buffer-local in each buffer, records the file type of the buffer's visited file.  When a buffer does not specify a coding system with buffer-file-coding-system, this variable is used to determine which coding system to use when writing the contents of the buffer.  It should be nil for text, t for binary. If it is t, the coding system is no-conversion. Otherwise, undecided-dos is used.

     Normally this variable is set by visiting a file; it is set to nil if the file was visited without any actual conversion.

     Its default value is used to decide how to handle files for which file-name-buffer-file-type-alist says nothing about the type: If the default value is non-nil, then these files are treated as binary: the coding system no-conversion is used.  Otherwise, nothing special is done for them---the coding system is deduced solely from the file contents, in the usual Emacs fashion.

 -- User Option: file-name-buffer-file-type-alist
     This variable holds an alist for recognizing text and binary files. Each element has the form (REGEXP . TYPE), where REGEXP is matched against the file name, and TYPE may be nil for text, 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 t or nil.

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

     If no element in this alist matches a given file name, then the default value of buffer-file-type says how to treat the file.

Input Methods
=============

Input methods provide convenient ways of entering non-ASCII characters from the keyboard.  Unlike coding systems, which translate non-ASCII characters to and from encodings meant to be read by programs, input methods provide human-friendly commands.  (see Input Methods (The GNU Emacs Manual), for information on how users use input methods to enter text.)  How to define input methods is not yet documented in this manual, but here we describe how to use them.

Each input method has a name, which is currently a string; in the future, symbols may also be usable as input method names.

 -- Variable: current-input-method
     This variable holds the name of the input method now active in the current buffer.  (It automatically becomes local in each buffer when set in any fashion.)  It is nil if no input method is active in the buffer now.

 -- User Option: default-input-method
     This variable holds the default input method for commands that choose an input method.  Unlike current-input-method, this variable is normally global.

 -- Command: set-input-method input-method
     This command activates input method INPUT-METHOD for the current buffer.  It also sets default-input-method to INPUT-METHOD. If INPUT-METHOD is nil, this command deactivates any input method for the current buffer.

 -- Function: read-input-method-name prompt &optional default inhibit-null
     This function reads an input method name with the minibuffer, prompting with PROMPT.  If DEFAULT is non-nil, that is returned by default, if the user enters empty input.  However, if INHIBIT-NULL is non-nil, empty input signals an error.

     The returned value is a string.

 -- Variable: input-method-alist
     This variable defines all the supported input methods. Each element defines one input method, and should have the form:

          (INPUT-METHOD LANGUAGE-ENV ACTIVATE-FUNC
           TITLE DESCRIPTION ARGS...)

     Here INPUT-METHOD is the input method name, a string; LANGUAGE-ENV is another string, the name of the language environment this input method is recommended for.  (That serves only for documentation purposes.)

     ACTIVATE-FUNC is a function to call to activate this method.  The ARGS, if any, are passed as arguments to ACTIVATE-FUNC.  All told, the arguments to ACTIVATE-FUNC are INPUT-METHOD and the ARGS.

     TITLE is a string to display in the mode line while this method is active.  DESCRIPTION is a string describing this method and what it is good for.

The fundamental interface to input methods is through the variable input-method-function.  See Reading One Event, and Invoking the Input Method.

Locales
=======

POSIX defines a concept of ``locales'' which control which language to use in language-related features.  These Emacs variables control how Emacs interacts with these features.

 -- Variable: locale-coding-system
     This variable specifies the coding system to use for decoding system error messages and---on X Window system only---keyboard input, for encoding the format argument to format-time-string, and for decoding the return value of format-time-string.

 -- Variable: system-messages-locale
     This variable specifies the locale to use for generating system error messages.  Changing the locale can cause messages to come out in a different language or in a different orthography.  If the variable is nil, the locale is specified by environment variables in the usual POSIX fashion.

 -- Variable: system-time-locale
     This variable specifies the locale to use for formatting time values. Changing the locale can cause messages to appear according to the conventions of a different language.  If the variable is nil, the locale is specified by environment variables in the usual POSIX fashion.

 -- Function: locale-info item
     This function returns locale data ITEM for the current POSIX locale, if available.  ITEM should be one of these symbols:

     codeset
          Return the character set as a string (locale item CODESET).

     days
          Return a 7-element vector of day names (locale items DAY_1 through DAY_7);

     months
          Return a 12-element vector of month names (locale items MON_1 through MON_12).

     paper
          Return a list (WIDTH HEIGHT) for the default paper size measured in millimeters (locale items PAPER_WIDTH and PAPER_HEIGHT).

     If the system can't provide the requested information, or if ITEM is not one of those symbols, the value is nil.  All strings in the return value are decoded using locale-coding-system.  See Locales (The GNU Libc Manual), for more information about locales and locale items.