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 or string, based on the file’s or string’s data, when they are
decoded or encoded. The coding system prefer-utf-8 is like
undecided, but it prefers to choose utf-8 when possible.
In general, a coding system doesn’t guarantee roundtrip identity: decoding a byte sequence using a 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. (This was the convention used in Classic Mac OS.)
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-unix (so it forces no conversion of end-of-line,
unlike utf-8-emacs, which can decode all 3 kinds of
end-of-line conventions).
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.
This function returns the list of aliases of coding-system.