NAME
preconv - convert encoding of input files to something GNU troff
understands
SYNOPSIS
[files ...] -h | --help -v | --version
It is possible to have whitespace between the -e command line option
and its parameter.
DESCRIPTION
preconv reads files and converts its encoding(s) to a form GNU troff(1)
can process, sending the data to standard output. Currently, this
means ASCII characters and ‘\[uXXXX]’ entities, where ‘XXXX’ is a
hexadecimal number with four to six digits, representing a Unicode
input code. Normally, preconv should be invoked with the -k and -K
options of groff.
OPTIONS
-d Emit debugging messages to standard error (mainly the used
encoding).
-Dencoding
Specify default encoding if everything fails (see below).
-eencoding
Specify input encoding explicitly, overriding all other methods.
This corresponds to groff’s -Kencoding option. Without this
switch, preconv uses the algorithm described below to select the
input encoding.
--help -h Print help message.
-r Do not add .lf requests.
--version
-v Print version number.
USAGE
preconv tries to find the input encoding with the following algorithm.
1. If the input encoding has been explicitly specified with option
-e, use it.
2. Otherwise, check whether the input starts with a Byte Order Mark
(BOM, see below). If found, use it.
3. Finally, check whether there is a known coding tag (see below)
in either the first or second input line. If found, use it.
4. 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.
Coding Tags
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 is
used.
BUGS
preconv doesn’t support local variable lists yet. This is a different
syntax form to specify local variables at the end of a file.
SEE ALSO
groff(1)
the GNU Emacs and XEmacs info pages