preconv - Online in the Cloud

PROGRAM:

NAME

preconv - convert encoding of input files to something GNU troff understands

SYNOPSIS

preconv [-dr] [-e encoding] [files ...]
preconv -h | --help
preconv -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.

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