Provided by: groff-base_1.23.0-3build2_amd64 
Name
preconv - prepare files for typesetting with groff
Synopsis
preconv [-dr] [-D fallback-encoding] [-e encoding] [file ...]
preconv -h
preconv --help
preconv -v
preconv --version
Description
preconv reads each file, converts its encoded characters to a form troff(1) can interpret, and sends the
result to the standard output stream. Currently, this means that code points in the range 0–127 (in US-
ASCII, ISO 8859, or Unicode) remain as-is and the remainder are converted to the groff special character
form “\[uXXXX]”, where XXXX is a hexadecimal number of four to six digits corresponding to a Unicode code
point. By default, preconv also inserts a roff .lf request at the beginning of each file, identifying it
for the benefit of later processing (including diagnostic messages); the -r option suppresses this
behavior.
In typical usage scenarios, preconv need not be run directly; instead it should be invoked with the -k or
-K options of groff. If no file operands are given on the command line, or if file is “-”, the standard
input stream is read.
preconv tries to find the input encoding with the following algorithm, stopping at the first success.
1. If the input encoding has been explicitly specified with option -e, use it.
2. If the input starts with a Unicode Byte Order Mark, determine the encoding as UTF-8, UTF-16, or
UTF-32 accordingly.
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 “coding tag” for
brevity. If found, use it.
4. If the input stream is seekable, and if the uchardet library is available on the system, use it to
try to infer the encoding of the file.
5. If the -D option specifies an encoding, use it.
6. Use the encoding specified by the current locale (LC_CTYPE), unless the locale is “C”, “POSIX”, or
empty, in which case assume Latin-1 (ISO 8859-1).
The coding tag and uchardet methods in the above procedure rely upon a seekable input stream; when
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 preconv's -D or -e options, or by configuring your locale appropriately. groff also
supports a GROFF_ENCODING environment variable, which can be overridden by its -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 iconv library support.
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.
Consequently, 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 “file-local variables”.
preconv interprets the following syntax if it occurs in a roff comment in the first or second line of the
input file. Both “\"” and “\#” 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.
-*- [...;] coding: encoding[; ...] -*-
The only variable preconv interprets is “coding”, which can take the values listed below.
The following list comprises all MIME “charset” parameter values recognized, case-insensitively, by
preconv.
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 list of other coding tags is recognized, each of which is mapped to an
appropriate value 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
Trailing “-dos”, “-unix”, and “-mac” 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.
iconv support
While 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 iconv support is configured at compile time
and available at run time, all others are passed to iconv library functions, which may recognize many
additional encoding strings. The command “preconv -v” discloses whether iconv support is configured.
The use of 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 “café” (note the “e” with an acute accent), which due to its short
length challenges inference of the encoding used.
printf 'caf\351\n' | LC_ALL=en_US.UTF-8 preconv
printf 'caf\351\n' | preconv -e us-ascii
printf 'caf\351\n' | preconv -e latin-1
The fate of the accented “e” differs in each case. In the first, uchardet fails to detect an encoding
(though the library on your system may behave differently) and 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 iconv. In the last, it is correctly detected and mapped.
Limitations
preconv cannot perform any transformation on input that it cannot see. Examples include files that are
interpolated by preprocessors that run subsequently, including soelim(1); files included by troff itself
through “so” and similar requests; and string definitions passed to troff through its -d command-line
option.
preconv assumes that its input uses the default escape character, a backslash \, and writes special
character escape sequences accordingly.
Options
-h and --help display a usage message, while -v and --version show version information; all exit
afterward.
-d Emit debugging messages to the standard error stream.
-D fallback-encoding
Report fallback-encoding if all detection methods fail.
-e encoding
Skip detection and assume encoding; see groff's -K option.
-r Write files “raw”; do not add .lf requests.
See also
groff(1), iconv(3), locale(7)
groff 1.23.0 31 March 2024 preconv(1)