33. Non-ASCII Characters

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

33.1 Text Representations

Emacs has two text representations—two ways to represent text in a string or buffer. These are called unibyte and 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.

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

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 leading codes. The second and subsequent bytes of a multibyte character are always in the range 160 through 255 (octal 0240 through 0377); these values are trailing codes.

Some sequences of bytes are not valid in multibyte text: for example, a single isolated byte in the range 128 through 159 is not allowed. But character codes 128 through 159 can appear in multibyte text, represented as two-byte sequences. All the character codes 128 through 255 are possible (though slightly abnormal) in multibyte text; they appear in multibyte buffers and strings when you do explicit encoding and decoding (see section Explicit Encoding and Decoding).

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 text.

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

Variable: default-enable-multibyte-characters

This variable’s value is entirely equivalent to (default-value 'enable-multibyte-characters), and setting this variable changes that default value. Setting the local binding of enable-multibyte-characters in a specific buffer is not allowed, but changing the default value is supported, and it is a reasonable thing to do, because it has no effect on existing buffers.

The ‘--unibyte’ command line option does its job by setting the default value to nil early in startup.

Function: position-bytes position

Return the byte-position corresponding to buffer position position in the current buffer.

Function: byte-to-position byte-position

Return the buffer position corresponding to byte-position byte-position in the current buffer.

Function: multibyte-string-p string

Return t if string is a multibyte string.

33.2 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 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 likewise character codes 128 through 159. It converts the non-ASCII codes 160 through 255 by adding the value nonascii-insert-offset to each character code. By setting this variable, you specify which character set the unibyte characters correspond to (see section Character Sets). For example, if nonascii-insert-offset is 2048, which is (- (make-char 'latin-iso8859-1) 128), then the unibyte non-ASCII characters correspond to Latin 1. If it is 2688, which is (- (make-char 'greek-iso8859-7) 128), then they correspond to Greek letters.

Converting multibyte text to unibyte is simpler: it discards all but the low 8 bits of each character code. If 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.

Variable: nonascii-insert-offset

This variable specifies the amount to add to a non-ASCII character when converting unibyte text to multibyte. It also applies when self-insert-command inserts a character in the unibyte non-ASCII range, 128 through 255. However, the functions insert and insert-char do not perform this conversion.

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

Variable: nonascii-translation-table

This variable provides a more general alternative to 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 char-table, or nil. If this is non-nil, it overrides nonascii-insert-offset.

Function: string-make-unibyte string

This function converts the text of string to unibyte representation, if it isn’t already, and returns the result. If string is a unibyte string, it is returned unchanged. Multibyte character codes are converted to unibyte by using just the low 8 bits.

Function: string-make-multibyte string

This function converts the text of string to multibyte representation, if it isn’t already, and returns the result. If string is a multibyte string, it is returned unchanged. The function unibyte-char-to-multibyte is used to convert each unibyte character to a multibyte character.

33.3 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; a sequence of two bytes which is treated as one character in multibyte representation will count as two characters in unibyte representation. Character codes 128 through 159 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

This function returns a string with the same bytes as string but treating each byte as a character. This means that the value may have more characters than string has.

If string is already a unibyte string, then the value is string itself. Otherwise it is a newly created string, with no text properties. If string is multibyte, any characters it contains of charset eight-bit-control or eight-bit-graphic are converted to the corresponding single byte.

Function: string-as-multibyte string

This function returns a 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 string is already a multibyte string, then the value is string itself. Otherwise it is a newly created string, with no text properties. If string is unibyte and contains any individual 8-bit bytes (i.e. not part of a multibyte form), they are converted to the corresponding multibyte character of charset eight-bit-control or eight-bit-graphic.

33.4 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. The values 128 through 255 are not entirely proper in multibyte text, but they can occur if you do explicit encoding and decoding (see section Explicit Encoding and Decoding). Some other character codes cannot occur at all in multibyte text. Only the ASCII codes 0 through 127 are completely legitimate in both representations.

Function: char-valid-p charcode &optional genericp

This returns t if charcode is valid for either one of the two text representations.

(char-valid-p 65) ⇒ t (char-valid-p 256) ⇒ nil (char-valid-p 2248) ⇒ t

If the optional argument genericp is non-nil, this function returns t if charcode is a generic character (see section Splitting Characters).

33.5 Character Sets

Emacs classifies characters into various 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, latin-iso8859-1 is one character set, greek-iso8859-7 is another, and 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, generally known as Big 5, is divided into two Emacs character sets, chinese-big5-1 and chinese-big5-2.

ASCII characters are in character set ascii. The non-ASCII characters 128 through 159 are in character set eight-bit-control, and codes 160 through 255 are in character set eight-bit-graphic.

Function: charsetp object

Returns t if object is a symbol that names a character set, nil otherwise.

Function: charset-list

This function returns a list of all defined character set names.

Function: char-charset character

This function returns the name of the character set that character belongs to.

Function: charset-plist charset

This function returns the charset 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 are used for special purposes within Emacs; for example, preferred-coding-system helps determine which coding system to use to encode characters in a charset.

33.6 Characters and Bytes

In multibyte representation, each character occupies one or more bytes. Each character set has an introduction sequence, which is normally one or two bytes long. (Exception: the ASCII character set and the EIGHT-BIT-GRAPHIC character set have a zero-length introduction sequence.) The introduction sequence is the beginning of the byte sequence for any character in the character set. The rest of the character’s bytes distinguish it from the other characters in the same character set. Depending on the character set, there are either one or two distinguishing bytes; the number of such bytes is called the dimension of the character set.

Function: charset-dimension charset

This function returns the dimension of charset; at present, the dimension is always 1 or 2.

Function: charset-bytes charset

This function returns the number of bytes used to represent a character in character set charset.

This is the simplest way to determine the byte length of a character set’s introduction sequence:

(- (charset-bytes charset)
   (charset-dimension charset))

33.7 Splitting Characters

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 sequence of bytes used to represent a character, because Emacs translates automatically when necessary.

Function: split-char character

Return a list containing the name of the character set of character, followed by one or two byte values (integers) which identify character within that character set. The number of byte values is the character set’s dimension.

(split-char 2248) ⇒ (latin-iso8859-1 72) (split-char 65) ⇒ (ascii 65) (split-char 128) ⇒ (eight-bit-control 128)

Function: make-char charset &optional code1 code2

This function returns the character in character set charset whose position codes are code1 and code2. This is roughly the inverse of split-char. Normally, you should specify either one or both of code1 and code2 according to the dimension of charset. For example,

(make-char 'latin-iso8859-1 72) ⇒ 2248

If you call make-char with no byte-values, the result is a generic character which stands for charset. A generic character is an integer, but it is not valid for insertion in the buffer as a character. It can be used in char-table-range to refer to the whole character set (see section Char-Tables). char-valid-p returns nil for generic characters. For example:

(make-char 'latin-iso8859-1)
     ⇒ 2176
(char-valid-p 2176)
     ⇒ nil
(char-valid-p 2176 t)
     ⇒ t
(split-char 2176)
     ⇒ (latin-iso8859-1 0)

The character sets ASCII, EIGHT-BIT-CONTROL, and EIGHT-BIT-GRAPHIC don’t have corresponding generic characters. If charset is one of them and you don’t supply code1, make-char returns the character code corresponding to the smallest code in charset.

33.8 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 (see section Coding Systems) are capable of representing all of the text in question.

Function: find-charset-region beg end &optional translation

This function returns a list of the character sets that appear in the current buffer between positions beg and end.

The optional argument translation specifies a translation table to be used in scanning the text (see section 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 the character sets that appear in the string string. It is just like find-charset-region, except that it applies to the contents of string instead of part of the current buffer.

33.9 Translation of Characters

A translation table 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.

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.

You can also map one whole character set into another character set with the same dimension. To do this, you specify a generic character (which designates a character set) for from (see section Splitting Characters). In this case, to should also be a generic character, for another character set of the same dimension. Then the translation table translates each character of from’s character set into the corresponding character of to’s character set.

In decoding, the translation table’s translations are applied to the characters that result from ordinary decoding. If a coding system has property character-translation-table-for-decode, that specifies the translation table to use. Otherwise, if standard-translation-table-for-decode is non-nil, decoding uses that table.

In 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 character-translation-table-for-encode, that specifies the translation table to use. Otherwise the variable standard-translation-table-for-encode specifies the translation table.

Variable: standard-translation-table-for-decode

This is the default translation table for decoding, for coding systems that don’t specify any other translation table.

Variable: standard-translation-table-for-encode

This is the default translation table for encoding, for coding systems that don’t specify any other translation table.

33.10 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.

33.10.1 Basic Concepts of Coding Systems

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 the choice unspecified—to be chosen heuristically for each file, based on the data.

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 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 that coding system to be a unibyte buffer. It 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 is equivalent to raw-text-unix: it specifies no conversion of either character codes or end-of-line.

The coding system emacs-mule specifies that the data is represented in the internal Emacs encoding. This is like raw-text in that no code conversion happens, but different in that the result is multibyte data.

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.

33.10.2 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 for decoding the file data, and write-region uses one to encode the buffer contents.

You can specify the coding system to use either explicitly (see section Specifying a Coding System for One Operation), or implicitly using the defaulting mechanism (see section 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 variable records the coding system that was used for visiting the current buffer. It is used for saving the buffer, and for writing part of the buffer with write-region. When those operations ask the user to specify a different coding system, buffer-file-coding-system is updated to the coding system specified.

However, 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. 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 section Explicit Encoding and Decoding) 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 section Window System Selections.

33.10.3 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.

Function: check-coding-system coding-system

This function checks the validity of coding-system. If that is valid, it returns coding-system. Otherwise it signals an error with condition coding-system-error.

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.

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: 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 (see section Explicit Encoding and Decoding).

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, the value is undecided or (undecided).

Function: detect-coding-string string highest

This function is like detect-coding-region except that it operates on the contents of string instead of bytes in the buffer.

See section Process Information, for how to examine or set the coding systems used for I/O to a subprocess.

33.10.4 User-Chosen Coding Systems

Function: select-safe-coding-system from to &optional default-coding-system accept-default-p

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, defaulting to the whole buffer if they are nil. If from is a string, the string is the specified text, and to is ignored.

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 user’s most preferred coding system (see prefer-coding-system: (emacs)Recognize Coding section ‘the description of prefer-coding-system’ in GNU Emacs Manual), and after that the current buffer’s value of buffer-file-coding-system (if it is not undecided).

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.

The optional argument accept-default-p, if non-nil, should be a function to determine whether the coding system selected without user interaction is acceptable. If this function returns nil, the silently selected coding system is rejected, and the user is asked to select a coding system from a list of possible candidates.

If the variable select-safe-coding-system-accept-default-p is non-nil, its value overrides the value of accept-default-p.

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

33.10.5 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 section Specifying a Coding System for One Operation).

Variable: 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.

Variable: 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 must return a coding system or a cons cell containing two coding systems. This value is used as described above.

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.

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 should be a symbol, one of insert-file-contents, write-region, call-process, call-process-region, start-process, or open-network-stream. These are the names of the Emacs I/O primitives that can do coding system conversion.

The remaining arguments should be the same arguments that might be given to that 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.

This function looks up the target in file-coding-system-alist, process-coding-system-alist, or network-coding-system-alist, depending on operation. See section Default Coding Systems.

33.10.6 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-write 'emacs-mule-dos)) (insert-file-contents filename))

When its value is non-nil, coding-system-for-read 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.

Variable: 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 section Explicit Encoding and Decoding).

33.10.7 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 characters whose codes are in the range 0 through 255. In a multibyte buffer or string, character codes 128 through 159 are represented by multibyte sequences, but this is invisible to Lisp programs.

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 section 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 section 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 decoding functions produce sequences of bytes; the encoding functions are meant to operate on sequences of bytes. All of these functions discard text properties.

Function: encode-coding-region start end coding-system

This function encodes the text from start to end according to coding system coding-system. The encoded text replaces the original text in the buffer. The result of encoding is logically a sequence of bytes, but the buffer remains multibyte if it was multibyte before.

Function: encode-coding-string string coding-system

This function encodes the text in string according to coding system coding-system. It returns a new string containing the encoded text. The result of encoding is a unibyte string.

Function: decode-coding-region start end coding-system

This function decodes the text from start to end according to coding system coding-system. The decoded text replaces the original text in the buffer. 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.

Function: decode-coding-string string coding-system

This function decodes the text in string according to coding system coding-system. It returns a new string containing the decoded text. To make explicit decoding useful, the contents of string ought to be a sequence of byte values, but a multibyte string is acceptable.

33.10.8 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 for the terminal.

Function: keyboard-coding-system

This function returns the coding system that is in use for decoding keyboard input—or nil if no coding system is to be used.

Function: set-keyboard-coding-system coding-system

This function specifies coding-system as the coding system to use for decoding keyboard input. If coding-system is nil, that means do not decode keyboard input.

Function: terminal-coding-system

This function returns the coding system that is in use for encoding terminal output—or nil for no encoding.

Function: set-terminal-coding-system coding-system

This function specifies coding-system as the coding system to use for encoding terminal output. If coding-system is nil, that means do not encode terminal output.

33.10.9 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.

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 default-buffer-file-type says how to treat the file.

User Option: default-buffer-file-type

This variable says how to handle files for which file-name-buffer-file-type-alist says nothing about the type.

If this variable 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.

33.11 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 (emacs)Input Methods section ‘Input Methods’ in 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.

Variable: 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.

Function: set-input-method input-method

This function activates input method input-method for the current buffer. It also sets default-input-method to input-method. If input-method is nil, this function 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 section Reading One Event.

This document was generated by Yasutaka SHINDOH on April 18, 2010 using texi2html 1.82.