preconv tries to find the input encoding with the following algorithm.
- If the input encoding has been explicitly specified with option -e, use it.
- Otherwise, check whether the input starts with a Byte Order Mark (BOM, see below). If found, use it.
- Finally, check whether there is a known coding tag (see below) in either the first or second input line. If found, use it.
- If everything fails, use a default encoding as given with option -D, by the current locale, or 'latin1' if the locale is set to 'C', 'POSIX', or empty (in that order).
Note that the groff program supports a GROFF_ENCODING environment variable which is eventually expanded to option -k.
Byte Order Mark
The Unicode Standard defines character U+FEFF as the Byte Order Mark (BOM). On the other hand, value U+FFFE is guaranteed not be a Unicode character at all. This allows to detect the byte order within the data stream (either big-endian or lower-endian), and the MIME encodings 'UTF-16' and 'UTF-32' mandate that the data stream starts with U+FEFF. Similarly, the data stream encoded as 'UTF-8' might start with a BOM (to ease the conversion from and to UTF-16 and UTF-32). In all cases, the byte order mark is not
part of the data but part of the encoding protocol; in other words, preconv
's output doesn't contain it.
Note that U+FEFF not at the start of the input data actually is emitted; it has then the meaning of a 'zero width no-break space' character - something not needed normally in groff.
Editors which support more than a single character encoding need tags within the input files to mark the file's encoding. While it is possible to guess the right input encoding with the help of heuristic algorithms for data which represents a greater amount of a natural language, it is still just a guess. Additionally, all algorithms fail easily for input which is either too short or doesn't represent a natural language.
For these reasons, preconv supports the coding tag convention (with some restrictions) as used by GNU Emacs and XEmacs (and probably other programs too).
Coding tags in GNU Emacs and XEmacs are stored in so-called File Variables. preconv recognizes the following syntax form which must be put into a troff comment in the first or second line.
-*- tag1: value1; tag2: value2; ... -*-
The only relevant tag for preconv is 'coding' which can take the values listed below. Here an example line which tells Emacs to edit a file in troff mode, and to use latin2 as its encoding.
.\" -*- mode: troff; coding: latin-2 -*-
The following list gives all MIME coding tags (either lowercase or uppercase) supported by preconv; this list is hard-coded in the source.
big5, cp1047, euc-jp, euc-kr, gb2312, iso-8859-1, iso-8859-2, iso-8859-5, iso-8859-7, iso-8859-9, iso-8859-13, iso-8859-15, koi8-r, us-ascii, utf-8, utf-16, utf-16be, utf-16le
In addition, the following hard-coded list of other tags is recognized which eventually map to values from the list above.
ascii, chinese-big5, chinese-euc, chinese-iso-8bit, cn-big5, cn-gb, cn-gb-2312, cp878, csascii, csisolatin1, cyrillic-iso-8bit, cyrillic-koi8, euc-china, euc-cn, euc-japan, euc-japan-1990, euc-korea, greek-iso-8bit, iso-10646/utf8, iso-10646/utf-8, iso-latin-1, iso-latin-2, iso-latin-5, iso-latin-7, iso-latin-9, japanese-euc, japanese-iso-8bit, jis8, koi8, korean-euc, korean-iso-8bit, latin-0, latin1, latin-1, latin-2, latin-5, latin-7, latin-9, mule-utf-8, mule-utf-16, mule-utf-16be, mule-utf-16-be, mule-utf-16be-with-signature, mule-utf-16le, mule-utf-16-le, mule-utf-16le-with-signature, utf8, utf-16-be, utf-16-be-with-signature, utf-16be-with-signature, utf-16-le, utf-16-le-with-signature, utf-16le-with-signature
Those tags are taken from GNU Emacs and XEmacs, together with some aliases. Trailing '-dos', '-unix', and '-mac' suffixes of coding tags (which give the end-of-line convention used in the file) are stripped off before the comparison with the above tags happens.
Iconv Issues preconv
by itself only supports three encodings: latin-1, cp1047, and UTF-8; all other encodings are passed to the iconv
library functions. At compile time it is searched and checked for a valid iconv
implementation; a call to 'preconv --version' shows whether iconv
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