.TH preconv 1 "16 October 2023" "groff 1.23.0"
.SH Name
preconv \- prepare files for typesetting with
.I groff
.
.
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.\" ====================================================================
.SH Synopsis
.\" ====================================================================
.
.SY preconv
.RB [ \-dr ]
.RB [ \-D\~\c
.IR fallback-encoding ]
.RB [ \-e\~\c
.IR encoding ]
.RI [ file\~ .\|.\|.]
.YS
.
.
.SY preconv
.B \-h
.
.SY preconv
.B \-\-help
.YS
.
.
.SY preconv
.B \-v
.
.SY preconv
.B \-\-version
.YS
.
.
.\" ====================================================================
.SH Description
.\" ====================================================================
.
.I preconv
reads each
.IR file ,
converts its encoded characters to a form
.MR \%troff 1
can interpret,
and sends the result to the standard output stream.
.
Currently,
this means that code points in the range 0\[en]127
(in US-ASCII,
ISO\~8859,
or Unicode)
remain as-is and the remainder are converted to the
.I groff
special character form
.RB \[lq] \[rs][\c
.BI u XXXX ]\c
\[rq],
where
.I XXXX
is a hexadecimal number of four to six digits corresponding to a Unicode
code point.
.
By default,
.I preconv
also inserts a
.I roff
.B .lf
request at the beginning of each
.IR file ,
identifying it for the benefit of later processing
(including diagnostic messages);
the
.B \-r
option suppresses this behavior.
.
.
.PP
In typical usage scenarios,
.I preconv
need not be run directly;
instead it should be invoked with the
.B \-k
or
.B \-K
options of
.IR groff .
.
If no
.I file
operands are given on the command line,
or if
.I file
is
.RB \[lq] \- \[rq],
the standard input stream is read.
.
.
.PP
.I preconv
tries to find the input encoding with the following algorithm,
stopping at the first success.
.
.
.IP 1. 4n
If the input encoding has been explicitly specified with option
.BR \-e ,
use it.
.
.
.IP 2.
If the input starts with a Unicode Byte Order Mark,
determine the encoding as UTF-8,
UTF-16,
or UTF-32 accordingly.
.
.
.IP 3.
If the input stream is seekable,
check the first and second input lines for a recognized GNU\~Emacs
file-local variable identifying the character encoding,
here referred to as the \[lq]coding tag\[rq] for brevity.
.
If found,
use it.
.
.
.IP 4.
If the input stream is seekable,
and if the
.I uchardet
library is available on the system,
use it to try to infer the encoding of the file.
.
.
.IP 5.
If the
.B \-D
option specifies an encoding,
use it.
.
.
.IP 6.
Use the encoding specified by the current locale
.RI ( LC_CTYPE ),
unless the locale is
\[lq]C\[rq],
\[lq]POSIX\[rq],
or empty,
in which case assume Latin-1
(ISO\~8859-1).
.
.
.PP
The coding tag and
.I uchardet
methods in the above procedure rely upon a seekable input stream;
when
.I preconv
reads from a pipe,
the stream is not seekable,
and these detection methods are skipped.
.
If character encoding detection of your input files is unreliable,
arrange for one of the other methods to succeed by using
.IR preconv 's
.B \-D
or
.B \-e
options,
or by configuring your locale appropriately.
.
.I groff
also supports a
.I \%GROFF_ENCODING
environment variable,
which can be overridden by its
.B \-K
option.
.
Valid values for
(or parameters to)
all of these are enumerated in the lists of recognized coding tags in
the next subsection,
and are further influenced by
.I iconv
library support.
.
.
.\" ====================================================================
.SS "Coding tags"
.\" ====================================================================
.
Text editors that 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
heuristics that are reliable for a preponderance of natural language
texts,
they are not absolutely reliable.
.
Heuristics can fail on inputs that are too short or don't represent a
natural language.
.
.
.PP
Consequently,
.I preconv
supports the coding tag convention used by GNU\~Emacs
(with some restrictions).
.
This notation appears in specially marked regions of an input file
designated for \[lq]file-local variables\[rq].
.
.
.PP
.I preconv
interprets the following syntax if it occurs in a
.I roff
comment
in the first or second line of the input file.
.
Both \[lq]\[rs]"\[rq] and \[lq]\[rs]#\[rq] comment forms are recognized,
but the control
(or no-break control)
character must be the default and must begin the line.
.
Similarly,
the escape character must be the default.
.
.
.RS
.EX
.B \-*\- \c
.RB [.\|.\|. ; ]\~\c
.B coding: \c
.I encoding\c
.RB [ ;\~ .\|.\|.\&]\~\c
.B \-*\-
.EE
.RE
.
.
.PP
The only variable
.I preconv
interprets is \[lq]coding\[rq],
which can take the values listed below.
.
.
.PP
The following list comprises all MIME \[lq]charset\[rq] parameter values
recognized,
case-insensitively,
by
.IR preconv .
.
.RS
\%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
.RE
.
.
.PP
In addition,
the following list of other coding tags is recognized,
each of which is mapped to an appropriate value from the list above.
.
.RS
\%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
.RE
.
.
.PP
Trailing
\[lq]\-dos\[rq],
\[lq]\-unix\[rq],
and
\[lq]\-mac\[rq]
suffixes on coding tags
(which indicate the end-of-line convention used in the file)
are disregarded for the purpose of comparison with the above tags.
.
.
.\" ====================================================================
.SS "\f[I]iconv\f[] support"
.\" ====================================================================
.
While
.I preconv
recognizes all of the coding tags listed above,
it is capable on its own of interpreting only three encodings:
Latin-1,
code page 1047,
and UTF-8.
.
If
.I iconv
support is configured at compile time and available at run time,
all others are passed to
.I iconv
library functions,
which may recognize many additional encoding strings.
.
The command
.RB \[lq] preconv\~\-v \[rq]
discloses whether
.I iconv
support is configured.
.
.
.PP
The use of
.I iconv
means that characters in the input that encode invalid code points for
that encoding may be dropped from the output stream or mapped to the
Unicode replacement character
(U+FFFD).
.
Compare the following examples using the input \[lq]caf\['e]\[rq]
(note the \[lq]e\[rq] with an acute accent),
which due to its short length challenges inference of the encoding used.
.
.RS
.EX
printf \[aq]caf\[rs]351\[rs]n\[aq] | LC_ALL=en_US.UTF\-8 preconv
printf \[aq]caf\[rs]351\[rs]n\[aq] | preconv \-e us\-ascii
printf \[aq]caf\[rs]351\[rs]n\[aq] | preconv \-e latin\-1
.EE
.RE
.
The fate of the accented \[lq]e\[rq] differs in each case.
.
In the first,
.I uchardet
fails to detect an encoding
(though the library on your system may behave differently)
and
.I preconv
falls back to the locale settings,
where octal 351 starts an incomplete UTF-8 sequence and results in the
Unicode replacement character.
.
In the second,
it is not a representable character in the declared input encoding of
US-ASCII and is discarded by
.IR iconv .
.
In the last,
it is correctly detected and mapped.
.
.
.\" ====================================================================
.SS Limitations
.\" ====================================================================
.
.I preconv
cannot perform any transformation on input that it cannot see.
.
Examples include files that are interpolated by preprocessors that run
subsequently,
including
.MR \%soelim 1 ;
files included by
.I \%troff
itself through
.RB \[lq] so \[rq]
and similar requests;
and string definitions passed to
.I \%troff
through its
.B \-d
command-line option.
.
.
.P
.I preconv
assumes that its input uses the default escape character,
a backslash
.BR \[rs] ,
and writes special character escape sequences accordingly.
.
.
.\" ====================================================================
.SH Options
.\" ====================================================================
.
.B \-h
and
.B \-\-help
display a usage message,
while
.B \-v
and
.B \-\-version
show version information;
all exit afterward.
.
.
.TP
.B \-d
Emit debugging messages to the standard error stream.
.
.
.TP
.BI \-D\~ fallback-encoding
Report
.I fallback-encoding
if all detection methods fail.
.
.
.TP
.BI \-e\~ encoding
Skip detection and assume
.IR encoding ;
see
.IR groff 's
.B \-K
option.
.
.
.TP
.B \-r
Write files \[lq]raw\[rq];
do not add
.B .lf
requests.
.
.
.\" ====================================================================
.SH "See also"
.\" ====================================================================
.
.MR groff 1 ,
.MR iconv 3 ,
.MR locale 7
.
.
.\" Restore compatibility mode (for, e.g., Solaris 10/11).
.cp \n[*groff_preconv_1_man_C]
.do rr *groff_preconv_1_man_C
.
.
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