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-nul-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 the encoding of text that contains null bytes to be correctly detected, 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 Emacs characters, the value is (emacs).
If coding-system supports all Unicode characters, the value is (unicode).
If coding-system supports all ISO-2022 charsets, the value is iso-2022.
If coding-system supports all the characters in the internal coding system used by Emacs version 21 (prior to the implementation of internal Unicode support), the value is emacs-mule.

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.