.TH PRECONV 1 "7 November 2018" "Groff Version 1.22.3"
.SH NAME
preconv \- convert encoding of input files to something GNU troff understands
.
.
.\" license (copying)
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Copyright \[co] 2006-2014 Free Software Foundation, Inc.

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..
.
.\" --------------------------------------------------------------------
.SH SYNOPSIS
.\" --------------------------------------------------------------------
.
.SY preconv
.OP \-dr
.OP \-e encoding
.RI [ files
.IR .\|.\|. ]
.
.SY preconv
.B \-h
|
.B \-\-help
.
.SY preconv
.B \-v
|
.B \-\-version
.YS
.
.
.PP
It is possible to have whitespace between the
.B \-e
command line option and its parameter.
.
.
.\" --------------------------------------------------------------------
.SH DESCRIPTION
.\" --------------------------------------------------------------------
.
.B preconv
reads
.I files
and converts its encoding(s) to a form GNU
.BR troff (1)
can process, sending the data to standard output.
.
Currently, this means ASCII characters and \[oq]\e[uXXXX]\[cq]
entities, where \[oq]XXXX\[cq] is a hexadecimal number with four to
six digits, representing a Unicode input code.
.
Normally,
.B preconv
should be invoked with the
.B \-k
and
.B \-K
options of
.BR groff .
.
.
.\" --------------------------------------------------------------------
.SH OPTIONS
.\" --------------------------------------------------------------------
.
.TP
.B \-d
Emit debugging messages to standard error (mainly the used encoding).
.
.TP
.BI \-D encoding
Specify default encoding if everything fails (see below).
.
.TP
.BI \-e encoding
Specify input encoding explicitly, overriding all other methods.
.
This corresponds to
.BR groff \[aq]s
.BI \-K encoding
option.
.
Without this switch,
.B preconv
uses the algorithm described below to select the input encoding.
.
.TP
.B \-\-help
.TQ
.B \-h
Print help message.
.
.TP
.B \-r
Do not add \&.lf requests.
.
.TP
.B \-\-version
.TQ
.B \-v
Print version number.
.
.
.\" --------------------------------------------------------------------
.SH USAGE
.\" --------------------------------------------------------------------
.
.B preconv
tries to find the input encoding with the following algorithm.
.
.IP 1.
If the input encoding has been explicitly specified with option
.BR \-e ,
use it.
.
.IP 2.
Otherwise, check whether the input starts with a
.I Byte Order Mark
(BOM, see below).
.
If found, use it.
.
.IP 3.
Finally, check whether there is a known
.I coding tag
(see below) in either the first or second input line.
.
If found, use it.
.
.IP 4.
If everything fails, use a default encoding as given with option
.BR \-D ,
by the current locale, or \[oq]latin1\[cq] if the locale is set to
\[oq]C\[cq], \[oq]POSIX\[cq], or empty (in that order).
.
.
.PP
Note that the
.B groff
program supports a
.B GROFF_ENCODING
environment variable which is eventually expanded to option
.BR \-k .
.
.
.\" --------------------------------------------------------------------
.SS "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 \%\[oq]UTF-16\[cq]
and \%\[oq]UTF-32\[cq] mandate that the data stream starts with U+FEFF.
.
Similarly, the data stream encoded as \%\[oq]UTF-8\[cq] 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
.I not
part of the data but part of the encoding protocol; in other words,
.BR preconv \[aq]s
output doesn\[aq]t contain it.
.
.
.PP
Note that U+FEFF not at the start of the input data actually is
emitted; it has then the meaning of a \[oq]zero width no-break
space\[cq] character \[en] something not needed normally in
.BR groff .
.
.
.\" --------------------------------------------------------------------
.SS "Coding Tags"
.\" --------------------------------------------------------------------
.
Editors which support more than a single character encoding need tags
within the input files to mark the file\[aq]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\[aq]t represent a natural language.
.
.
.PP
For these reasons,
.B preconv
supports the coding tag convention (with some restrictions) as used by
.B "GNU Emacs"
and
.B XEmacs
(and probably other programs too).
.
.
.PP
Coding tags in
.B "GNU Emacs"
and
.B XEmacs
are stored in so-called
.IR "File Variables" .
.
.B preconv
recognizes the following syntax form which must be put into a troff comment
in the first or second line.
.
.RS
.PP
\-*\-
.IR tag1 :
.IR value1 ;
.IR tag2 :
.IR value2 ;
\&.\|.\|.\& \-*\-
.RE
.
.
.PP
The only relevant tag for
.B preconv
is \[oq]coding\[cq] which can take the values listed below.
.
Here an example line which tells
.B Emacs
to edit a file in troff mode, and to use \%latin2 as its encoding.
.
.RS
.PP
.EX
\&.\[rs]" \-*\- mode: troff; coding: latin-2 \-*\-\""
.EE
.RE
.
.
.PP
The following list gives all MIME coding tags (either lowercase or
uppercase) supported by
.BR preconv ;
this list is hard-coded in the source.
.
.RS
.PP
.ad l
\%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
.ad
.RE
.
.
.PP
In addition, the following hard-coded list of other tags is recognized
which eventually map to values from the list above.
.
.RS
.PP
.ad l
\%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
.ad
.RE
.
.
.PP
Those tags are taken from
.B "GNU Emacs"
and
.BR XEmacs ,
together with some aliases.
.
Trailing \%\[oq]-dos\[cq], \%\[oq]-unix\[cq], and \%\[oq]-mac\[cq]
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.
.
.SS "Iconv Issues"
.B preconv
by itself only supports three encodings: \%latin-1, cp1047, and \%UTF-8;
all other encodings are passed to the
.B iconv
library functions.
.
At compile time it is searched and checked for a valid
.B iconv
implementation; a call to \[oq]preconv \-\-version\[cq] shows whether
.B iconv
is used.
.
.
.\" --------------------------------------------------------------------
.SH BUGS
.\" --------------------------------------------------------------------
.
.B preconv
doesn\[aq]t support
.I "local variable lists"
yet.
.
This is a different syntax form to specify local variables at the end of a
file.
.
.
.\" --------------------------------------------------------------------
.SH "SEE ALSO"
.\" --------------------------------------------------------------------
.
.BR groff (1)
.br
the
.B "GNU Emacs"
and
.B XEmacs
info pages
.
.
.\" --------------------------------------------------------------------
.SH COPYING
.\" --------------------------------------------------------------------
.co
.
.
.\" Emacs setting
.\" Local Variables:
.\" mode: nroff
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