\input texinfo        @c -*-texinfo-*-          -*- coding: latin-1 -*-
@c %**start of header
@setfilename recode.info
@settitle The @code{recode} reference manual

@c An index for command-line options
@defcodeindex op
@c Put variable and function names together
@syncodeindex vr fn
@finalout
@c %**end of header

@include version.texi

@dircategory Internationalization and character sets
@direntry
* recode: (recode).     Conversion between character sets and surfaces.
@end direntry

@ifinfo
This file documents the @code{recode} command, which has the purpose of
converting files between various character sets and surfaces.

Copyright (C) 1990, 93, 94, 96, 97, 98, 99, 00 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end ifinfo

@titlepage
@title Free recode, version @value{VERSION}
@subtitle The character set converter
@subtitle Edition @value{EDITION}, @value{UPDATED}
@author Fran@,{c}ois Pinard

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1993, 94, 97, 98, 99, 00 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Foundation.
@end titlepage

@ifnottex
@node Top, Tutorial, (dir), (dir)
@top @code{recode}

@c @item @b{@code{recode}} @value{hfillkludge} (UtilT, SrcCD)
@c
This recoding library converts files between various coded character
sets and surface encodings.  When this cannot be achieved exactly, it
may get rid of the offending characters or fall back on approximations.
The library recognises or produces more than 300 different character sets
and is able to convert files between almost any pair.  Most @w{RFC 1345}
character sets, and all @code{libiconv} character sets, are supported.
The @code{recode} program is a handy front-end to the library.

The current @code{recode} release is @value{VERSION}.

@menu
* Tutorial::            Quick Tutorial
* Introduction::        Terminology and purpose
* Invoking recode::     How to use this program
* Library::             A recoding library
* Universal::           The universal charset
* libiconv::            The @code{iconv} library
* Tabular::             Tabular sources (@w{RFC 1345})
* ASCII misc::          ASCII and some derivatives
* IBM and MS::          Some IBM or Microsoft charsets
* CDC::                 Charsets for CDC machines
* Micros::              Other micro-computer charsets
* Miscellaneous::       Various other charsets
* Surfaces::            All about surfaces
* Internals::           Internal aspects
* Concept Index::       Concept Index
* Option Index::        Option Index
* Library Index::       Library Index
* Charset and Surface Index::  Charset and Surface Index

@detailmenu
 --- The Detailed Node Listing ---

Terminology and purpose

* Charset overview::    Overview of charsets
* Surface overview::    Overview of surfaces
* Contributing::        Contributions and bug reports

How to use this program

* Synopsis::            Synopsis of @code{recode} call
* Requests::            The @var{request} parameter
* Listings::            Asking for various lists
* Recoding::            Controlling how files are recoded
* Reversibility::       Reversibility issues
* Sequencing::          Selecting sequencing methods
* Mixed::               Using mixed charset input
* Emacs::               Using @code{recode} within Emacs
* Debugging::           Debugging considerations

A recoding library

* Outer level::         Outer level functions
* Request level::       Request level functions
* Task level::          Task level functions
* Charset level::       Charset level functions
* Errors::              Handling errors

The universal charset

* UCS-2::               Universal Character Set, 2 bytes
* UCS-4::               Universal Character Set, 4 bytes
* UTF-7::               Universal Transformation Format, 7 bits
* UTF-8::               Universal Transformation Format, 8 bits
* UTF-16::              Universal Transformation Format, 16 bits
* count-characters::    Frequency count of characters
* dump-with-names::     Fully interpreted UCS dump

ASCII and some derivatives

* ASCII::               Usual ASCII
* ISO 8859::            ASCII extended by Latin Alphabets
* ASCII-BS::            ASCII 7-bits, @kbd{BS} to overstrike
* flat::                ASCII without diacritics nor underline

Some IBM or Microsoft charsets

* EBCDIC::              EBCDIC codes
* IBM-PC::              IBM's PC code
* Icon-QNX::            Unisys' Icon code

Charsets for CDC machines

* Display Code::        Control Data's Display Code
* CDC-NOS::             ASCII 6/12 from NOS
* Bang-Bang::           ASCII ``bang bang''

Other micro-computer charsets

* Apple-Mac::           Apple's Macintosh code
* AtariST::             Atari ST code

Various other charsets

* HTML::                World Wide Web representations
* LaTeX::               LaTeX macro calls
* Texinfo::             GNU project documentation files
* Vietnamese::          Vietnamese charsets
* African::             African charsets
* Others::              Cyrillic and other charsets
* Texte::               Easy French conventions
* Mule::                Mule as a multiplexed charset

All about surfaces

* Permutations::        Permuting groups of bytes
* End lines::           Representation for end of lines
* MIME::                MIME contents encodings
* Dump::                Interpreted character dumps
* Test::                Artificial data for testing

Internal aspects

* Main flow::           Overall organisation
* New charsets::        Adding new charsets
* New surfaces::        Adding new surfaces
* Design::              Comments on the library design

@end detailmenu
@end menu

@end ifnottex

@node Tutorial, Introduction, Top, Top
@chapter Quick Tutorial

@cindex @code{recode} use, a tutorial
@cindex tutorial
So, really, you just are in a hurry to use @code{recode}, and do not
feel like studying this manual?  Even reading this paragraph slows you down?
We might have a problem, as you will have to do some guess work, and might
not become very proficient unless you have a very solid intuition@dots{}.

Let me use here, as a quick tutorial, an actual reply of mine to a
@code{recode} user, who writes:

@quotation
My situation is this---I occasionally get email with special characters
in it.  Sometimes this mail is from a user using IBM software and sometimes
it is a user using Mac software.  I myself am on a SPARC Solaris machine.
@end quotation

Your situation is similar to mine, except that I @emph{often} receive
email needing recoding, that is, much more than @emph{occasionally}!
The usual recodings I do are Mac to @w{Latin-1}, IBM page codes to @w{Latin-1},
Easy-French to @w{Latin-1}, remove Quoted-Printable, remove Base64.  These are
so frequent that I made myself a few two-keystroke Emacs commands to filter
the Emacs region.  This is very convenient for me.  I also resort to many
other email conversions, yet more rarely than the frequent cases above.

@quotation
It @emph{seems} like this should be doable using @code{recode}.  However,
when I try something like @samp{grecode mac macfile.txt} I get nothing
out---no error, no output, nothing.
@end quotation

Presuming you are using some recent version of @code{recode}, the command:

@example
recode mac macfile.txt
@end example

@noindent
is a request for recoding @file{macfile.txt} over itself, overwriting the
original, from Macintosh usual character code and Macintosh end of lines,
to @w{Latin-1} and Unix end of lines.  This is overwrite mode.  If you want
to use @code{recode} as a filter, which is probably what you need, rather do:

@example
recode mac
@end example

@noindent
and give your Macintosh file as standard input, you'll get the @w{Latin-1}
file on standard output.  The above command is an abbreviation for any of:

@example
recode mac..
recode mac..l1
recode mac..Latin-1
recode mac/CR..Latin-1/
recode Macintosh..ISO_8859-1
recode Macintosh/CR..ISO_8859-1/
@end example

That is, a @code{CR} surface, encoding newlines with ASCII @key{CR}, is
first to be removed (this is a default surface for @samp{mac}), then the
Macintosh charset is converted to @w{Latin-1} and no surface is added to the
result (there is no default surface for @samp{l1}).  If you want @samp{mac}
code converted, but you know that newlines are already coded the Unix way,
just do:

@example
recode mac/
@end example

@noindent
the slash then overriding the default surface with empty, that is, none.
Here are other easy recipes:

@example
recode pc          to filter IBM-PC code and CR-LF (default) to Latin-1
recode pc/         to filter IBM-PC code to Latin-1
recode 850         to filter code page 850 and CR-LF (default) to Latin-1
recode 850/        to filter code page 850 to Latin-1
recode /qp         to remove quoted printable
@end example

The last one is indeed equivalent to any of:

@example
recode /qp..
recode l1/qp..l1/
recode ISO_8859-1/Quoted-Printable..ISO_8859-1/
@end example

Here are some reverse recipes:

@example
recode ..mac       to filter Latin-1 to Macintosh code and CR (default)
recode ..mac/      to filter Latin-1 to Macintosh code
recode ..pc        to filter Latin-1 to IBM-PC code and CR-LF (default)
recode ..pc/       to filter Latin-1 to IBM-PC code
recode ..850       to filter Latin-1 to code page 850 and CR-LF (default)
recode ..850/      to filter Latin-1 to code page 850
recode ../qp       to force quoted printable
@end example

In all the above calls, replace @samp{recode} by @samp{recode -f} if you
want to proceed despite recoding errors.  If you do not use @samp{-f}
and there is an error, the recoding output will be interrupted after first
error in filter mode, or the file will not be replaced by a recoded copy
in overwrite mode.

You may use @samp{recode -l} to get a list of available charsets and
surfaces, and @samp{recode --help} to get a quick summary of options.
The above output is meant for those having already read this manual, so
let me dare a suggestion: why could not you find a few more minutes in
your schedule to peek further down, right into the following chapters!

@node Introduction, Invoking recode, Tutorial, Top
@chapter Terminology and purpose

A few terms are used over and over in this manual, our wise reader will
learn their meaning right away.  Both ISO (International Organization for
Standardisation) and IETF (Internet Engineering Task Force) have their
own terminology, this document does not try to stick to either one in a
strict way, while it does not want to throw more confusion in the field.
On the other hand, it would not be efficient using paraphrases all the time,
so @code{recode} coins a few short words, which are explained below.

@cindex charset, what it is
A @dfn{charset}, in the context of @code{recode}, is a particular association
between computer codes on one side, and a repertoire of intended characters
on the other side.  Codes are usually taken from a set of consecutive
small integers, starting at 0.  Some characters have a graphical appearance
(glyph) or displayable effect, others have special uses like, for example,
to control devices or to interact with neighbouring codes to specify them
more precisely.  So, a @emph{charset} is roughly one of those tables,
giving a meaning to each of the codes from the set of allowable values.
MIME also uses the term charset with approximately the same meaning.
It does @emph{not} exactly corresponds to what ISO calls a @dfn{coded
character set}, that is, a set of characters with an encoding for them.
An coded character set does not necessarily use all available code positions,
while a MIME charset usually tries to specify them all.  A MIME charset
might be the union of a few disjoint coded character sets.

@cindex surface, what it is
A @dfn{surface} is a term used in @code{recode} only, and is a short for
surface transformation of a charset stream.  This is any kind of mapping,
usually reversible, which associates physical bits in some medium for
a stream of characters taken from one or more charsets (usually one).
A surface is a kind of varnish added over a charset so it fits in actual
bits and bytes.  How end of lines are exactly encoded is not really
pertinent to the charset, and so, there is surface for end of lines.
@code{Base64} is also a surface, as we may encode any charset in it.
Other examples would @code{DES} enciphering, or @code{gzip} compression
(even if @code{recode} does not offer them currently): these are ways to give
a real life to theoretical charsets.  The @dfn{trivial} surface consists
into putting characters into fixed width little chunks of bits, usually
eight such bits per character.  But things are not always that simple.

This @code{recode} library, and the program by that name, have the purpose
of converting files between various charsets and surfaces.  When this
cannot be done in exact ways, as it is often the case, the program may
get rid of the offending characters or fall back on approximations.
This library recognises or produces around 175 such charsets under 500
names, and handle a dozen surfaces.  Since it can convert each charset to
almost any other one, many thousands of different conversions are possible.

The @code{recode} program and library do not usually know how to split and
sort out textual and non-textual information which may be mixed in a single
input file.  For example, there is no surface which currently addresses the
problem of how lines are blocked into physical records, when the blocking
information is added as binary markers or counters within files.  So,
@code{recode} should be given textual streams which are rather @emph{pure}.

This tool pays special attention to superimposition of diacritics for
some French representations.  This orientation is mostly historical, it
does not impair the usefulness, generality or extensibility of the program.
@samp{recode} is both a French and English word.  For those who pay attention
to those things, the proper pronunciation is French (that is, @samp{racud},
with @samp{a} like in @samp{above}, and @samp{u} like in @samp{cut}).

The program @code{recode} has been written by Fran@,{c}ois Pinard.
With time, it got to reuse works from other contributors, and notably,
those of Keld Simonsen and Bruno Haible.

@menu
* Charset overview::    Overview of charsets
* Surface overview::    Overview of surfaces
* Contributing::        Contributions and bug reports
@end menu

@node Charset overview, Surface overview, Introduction, Introduction
@section Overview of charsets

@cindex charsets, overview
Recoding is currently possible between many charsets, the bulk of which is
described by @w{RFC 1345} tables or available in the @code{iconv} library.
@xref{Tabular}, and @pxref{libiconv}.  The @code{recode} library also
handles some charsets in some specialised ways.  These are:

@itemize @bullet
@item
6-bit charsets based on CDC display code: 6/12 code from NOS; bang-bang
code from Universit@'e de Montr@'eal;

@item
7-bit ASCII: without any diacritics, or else: using backspace for
overstriking; Unisys' Icon convention; @TeX{}/La@TeX{} coding; easy
French conventions for electronic mail;

@item
8-bit extensions to ASCII: ISO @w{Latin-1}, Atari ST code, IBM's code for
the PC, Apple's code for the Macintosh;

@item
8-bit non-ASCII codes: three flavours of EBCDIC;

@item
16-bit or 31-bit universal characters, and their transfer encodings.
@end itemize

The introduction of @w{RFC 1345} in @code{recode} has brought with it a few
charsets having the functionality of older ones, but yet being different
in subtle ways.  The effects have not been fully investigated yet, so for
now, clashes are avoided, the old and new charsets are kept well separate.

@cindex unavailable conversions
@cindex conversions, unavailable
@cindex impossible conversions
@cindex unreachable charsets
@cindex exceptions to available conversions
@cindex pseudo-charsets
@tindex flat@r{, not as before charset}
@tindex count-characters@r{, not as before charset}
@tindex dump-with-names@r{, not as before charset}
@tindex data@r{, not with charsets}
@tindex libiconv@r{, not in requests}
Conversion is possible between almost any pair of charsets.  Here is a
list of the exceptions.  One may not recode @emph{from} the @code{flat},
@code{count-characters} or @code{dump-with-names} charsets, nor @emph{from}
or @emph{to} the @code{data}, @code{tree} or @code{:libiconv:} charsets.
Also, if we except the @code{data} and @code{tree} pseudo-charsets, charsets
and surfaces live in disjoint recoding spaces, one cannot really transform
a surface into a charset or vice-versa, as surfaces are only meant to be
applied over charsets, or removed from them.

@node Surface overview, Contributing, Charset overview, Introduction
@section Overview of surfaces

@cindex surfaces, overview
For various practical considerations, it sometimes happens that the codes
making up a text, written in a particular charset, cannot simply be put
out in a file one after another without creating problems or breaking
other things.  Sometimes, 8-bit codes cannot be written on a 7-bit medium,
variable length codes need kind of envelopes, newlines require special
treatment, etc.  We sometimes have to apply @dfn{surfaces} to a stream
of codes, which surfaces are kind of tricks used to fit the charset into
those practical constraints.  Moreover, similar surfaces or tricks may
be useful for many unrelated charsets, and many surfaces can be used at
once over a single charset.

@cindex pure charset
@cindex charset, pure
So, @code{recode} has machinery to describe a combination of a charset with
surfaces used over it in a file.  We would use the expression @dfn{pure
charset} for referring to a charset free of any surface, that is, the
conceptual association between integer codes and character intents.

It is not always clear if some transformation will yield a charset or a
surface, especially for those transformations which are only meaningful
over a single charset.  The @code{recode} library is not overly picky as
identifying surfaces as such: when it is practical to consider a specialised
surface as if it were a charset, this is preferred, and done.

@node Contributing,  , Surface overview, Introduction
@section Contributions and bug reports

@cindex contributing charsets
Even being the @code{recode} author and current maintainer, I am no
specialist in charset standards.  I only made @code{recode} along the
years to solve my own needs, but felt it was applicable for the needs
of others.  Some FSF people liked the program structure and suggested
to make it more widely available.  I often rely on @code{recode} users
suggestions to decide what is best to be done next.

Properly protecting @code{recode} about possible copyright fights is a
pain for me and for contributors, but we cannot avoid addressing the issue
in the long run.  Besides, the Free Software Foundation, which mandates
the GNU project, is very sensible to this matter.  GNU standards suggest
that we stay cautious before looking at copyrighted code.  The safest and
simplest way for me is to gather ideas and reprogram them anew, even if
this might slow me down considerably.  For contributions going beyond a
few lines of code here and there, the FSF definitely requires employer
disclaimers and copyright assignments in writing.

When you contribute something to @code{recode}, @emph{please} explain what
it is about.  Do not take for granted that I know those charsets which
are familiar to you.  Once again, I'm no expert, and you have to help me.
Your explanations could well find their way into this documentation, too.
Also, for contributing new charsets or new surfaces, as much as possible,
please provide good, solid, verifiable references for the tables you
used@footnote{I'm not prone at accepting a charset you just invented,
and which nobody uses yet: convince your friends and community first!}.

Many users contributed to @code{recode} already, I am grateful to them for
their interest and involvement.  Some suggestions can be integrated quickly
while some others have to be delayed, I have to draw a line somewhere when
time comes to make a new release, about what would go in it and what would
go in the next.

@cindex bug reports, where to send
@cindex reporting bugs
Please send suggestions, documentation errors and bug reports to
@email{recode-bugs@@iro.umontreal.ca} or, if you prefer, directly to
@email{pinard@@iro.umontreal.ca}, Fran@,{c}ois Pinard.  Do not be afraid
to report details, because this program is the mere aggregation of
hundreds of details.

@node Invoking recode, Library, Introduction, Top
@chapter How to use this program

With the synopsis of the @code{recode} call, we stress the difference
between using this program as a file filter, or recoding many files
at once.  The first parameter of any call states the recoding request,
and this deserves a section on its own.  Options are then presented,
but somewhat grouped according to the related functionalities they
control.

@menu
* Synopsis::            Synopsis of @code{recode} call
* Requests::            The @var{request} parameter
* Listings::            Asking for various lists
* Recoding::            Controlling how files are recoded
* Reversibility::       Reversibility issues
* Sequencing::          Selecting sequencing methods
* Mixed::               Using mixed charset input
* Emacs::               Using @code{recode} within Emacs
* Debugging::           Debugging considerations
@end menu

@node Synopsis, Requests, Invoking recode, Invoking recode
@section Synopsis of @code{recode} call

@cindex @code{recode}, synopsis of invocation
@cindex invocation of @code{recode}, synopsis
The general format of the program call is one of:

@example
recode [@var{option}]@dots{} [@var{charset} | @var{request} [@var{file}]@dots{} ]
@end example

Some calls are used only to obtain lists produced by @code{recode} itself,
without actually recoding any file.  They are recognised through the
usage of listing options, and these options decide what meaning should
be given to an optional @var{charset} parameter.  @xref{Listings}.

In other calls, the first parameter (@var{request}) always explains which
transformations are expected on the files.  There are many variations to
the aspect of this parameter.  We will discuss more complex situations
later (@pxref{Requests}), but for many simple cases, this parameter
merely looks like this@footnote{In previous versions or @code{recode}, a single
colon @samp{:} was used instead of the two dots @samp{..} for separating
charsets, but this was creating problems because colons are allowed in
official charset names.  The old request syntax is still recognised for
compatibility purposes, but is deprecated.}:

@example
@var{before}..@var{after}
@end example

@noindent
where @var{before} and @var{after} each gives the name of a charset.  Each
@var{file} will be read assuming it is coded with charset @var{before}, it
will be recoded over itself so to use the charset @var{after}.  If there
is no @var{file} on the @code{recode} command, the program rather acts
as a Unix filter and transforms standard input onto standard output.
@cindex filter operation
@cindex @code{recode}, operation as filter

The capability of recoding many files at once is very convenient.
For example, one could easily prepare a distribution from @w{Latin-1} to MSDOS,
this way:

@example
mkdir package
cp -p Makefile *.[ch] package
recode Latin-1..MSDOS package/*
zoo ah package.zoo package/*
rm -rf package
@end example

@noindent
(In this example, the non-mandatory @samp{-p} option to @code{cp} is for
preserving timestamps, and the @code{zoo} program is an archiver from
Rahul Dhesi which once was quite popular.)

The filter operation is especially useful when the input files should
not be altered.  Let us make an example to illustrate this point.
Suppose that someone has a file named @file{datum.txt}, which is almost
a @TeX{} file, except that diacriticised characters are written using
@w{Latin-1}.  To complete the recoding of the diacriticised characters
@emph{only} and produce a file @file{datum.tex}, without destroying
the original, one could do:

@example
cp -p datum.txt datum.tex
recode -d l1..tex datum.tex
@end example

However, using @code{recode} as a filter will achieve the same goal more
neatly:

@example
recode -d l1..tex <datum.txt >datum.tex
@end example

This example also shows that @code{l1} could be used instead of
@code{Latin-1}; charset names often have such aliases.

@node Requests, Listings, Synopsis, Invoking recode
@section The @var{request} parameter

In the case where the @var{request} is merely written as
@var{before}..@var{after}, then @var{before} and @var{after} specify the
start charset and the goal charset for the recoding.

@cindex charset names, valid characters
@cindex valid characters in charset names
For @code{recode}, charset names may contain any character, besides a
comma, a forward slash, or two periods in a row.  But in practice, charset
names are currently limited to alphabetic letters (upper or lower case),
digits, hyphens, underlines, periods, colons or round parentheses.

@cindex request, syntax
@cindex @code{recode} request syntax
The complete syntax for a valid @var{request} allows for unusual
things, which might surprise at first.  (Do not pay too much attention
to these facilities on first reading.)  For example, @var{request}
may also contain intermediate charsets, like in the following example:

@example
@var{before}..@var{interim1}..@var{interim2}..@var{after}
@end example

@noindent
@cindex intermediate charsets
@cindex chaining of charsets in a request
@cindex charsets, chaining in a request
meaning that @code{recode} should internally produce the @var{interim1}
charset from the start charset, then work out of this @var{interim1}
charset to internally produce @var{interim2}, and from there towards the
goal charset.  In fact, @code{recode} internally combines recipes and
automatically uses interim charsets, when there is no direct recipe for
transforming @var{before} into @var{after}.  But there might be many ways
to do it.  When many routes are possible, the above @dfn{chaining} syntax
may be used to more precisely force the program towards a particular route,
which it might not have naturally selected otherwise.  On the other hand,
because @code{recode} tries to choose good routes, chaining is only needed
to achieve some rare, unusual effects.

Moreover, many such requests (sub-requests, more precisely) may be
separated with commas (but no spaces at all), indicating a sequence
of recodings, where the output of one has to serve as the input of the
following one.  For example, the two following requests are equivalent:

@example
@var{before}..@var{interim1}..@var{interim2}..@var{after}
@var{before}..@var{interim1},@var{interim1}..@var{interim2},@var{interim2}..@var{after}
@end example

@noindent
In this example, the charset input for any recoding sub-request is identical
to the charset output by the preceding sub-request.  But it does not have
to be so in the general case.  One might wonder what would be the meaning
of declaring the charset input for a recoding sub-request of being of
different nature than the charset output by a preceding sub-request, when
recodings are chained in this way.  Such a strange usage might have a
meaning and be useful for the @code{recode} expert, but they are quite
uncommon in practice.

@cindex surfaces, syntax
More useful is the distinction between the concept of charset, and
the concept of surfaces.  An encoded charset is represented by:

@example
@var{pure-charset}/@var{surface1}/@var{surface2}@dots{}
@end example

@noindent
@cindex surfaces, commutativity
@cindex commutativity of surfaces
using slashes to introduce surfaces, if any.  The order of application
of surfaces is usually important, they cannot be freely commuted.  In the
given example, @var{surface1} is first applied over the @var{pure-charset},
then @var{surface2} is applied over the result.  Given this request:

@example
@var{before}/@var{surface1}/@var{surface2}..@var{after}/@var{surface3}
@end example

@noindent
the @code{recode} program will understand that the input files should
have @var{surface2} removed first (because it was applied last), then
@var{surface1} should be removed.  The next step will be to translate the
codes from charset @var{before} to charset @var{after}, prior to applying
@var{surface3} over the result.

@cindex implied surfaces
@cindex surfaces, implied
@tindex IBM-PC charset, and CR-LF surface
Some charsets have one or more @emph{implied} surfaces.  In this case, the
implied surfaces are automatically handled merely by naming the charset,
without any explicit surface to qualify it.  Let's take an example to
illustrate this feature.  The request @samp{pc..l1} will indeed decode MS-DOS
end of lines prior to converting IBM-PC codes to @w{Latin-1}, because @samp{pc}
is the name of a charset@footnote{More precisely, @code{pc} is an alias for
the charset @code{IBM-PC}.} which has @code{CR-LF} for its usual surface.
The request @samp{pc/..l1} will @emph{not} decode end of lines, since
the slash introduces surfaces, and even if the surface list is empty, it
effectively defeats the automatic removal of surfaces for this charset.
So, empty surfaces are useful, indeed!

@cindex aliases
@cindex alternate names for charsets and surfaces
@cindex charsets, aliases
@cindex surfaces, aliases
Both charsets and surfaces may have predefined alternate names, or aliases.
However, and this is rather important to understand, implied surfaces
are attached to individual aliases rather than on genuine charsets.
Consequently, the official charset name and all of its aliases do not
necessarily share the same implied surfaces.  The charset and all its
aliases may each have its own different set of implied surfaces.

@cindex abbreviated names for charsets and surfaces
@cindex names of charsets and surfaces, abbreviation
Charset names, surface names, or their aliases may always be abbreviated
to any unambiguous prefix.  Internally in @code{recode}, disambiguating
tables are kept separate for charset names and surface names.

@cindex letter case, in charset and surface names
While recognising a charset name or a surface name (or aliases thereof),
@code{recode} ignores all characters besides letters and digits, so for
example, the hyphens and underlines being part of an official charset
name may safely be omitted (no need to un-confuse them!).  There is also
no distinction between upper and lower case for charset or surface names.

One of the @var{before} or @var{after} keywords may be omitted.  If the
double dot separator is omitted too, then the charset is interpreted as
the @var{before} charset.@footnote{Both @var{before} and @var{after} may
be omitted, in which case the double dot separator is mandatory.  This is
not very useful, as the recoding reduces to a mere copy in that case.}

@cindex default charset
@cindex charset, default
@vindex DEFAULT_CHARSET
When a charset name is omitted or left empty, the value of the
@code{DEFAULT_CHARSET} variable in the environment is used instead.  If this
variable is not defined, the @code{recode} library uses the current locale's
encoding. On POSIX compliant systems, this depends on the first non-empty
value among the environment variables LC_ALL, LC_CTYPE, LANG, and can be
determined through the command @samp{locale charmap}.

If the charset name is omitted but followed by surfaces, the surfaces
then qualify the usual or default charset.  For example, the request
@samp{../x} is sufficient for applying an hexadecimal surface to the input
text@footnote{MS-DOS is one of those systems for which the default charset
has implied surfaces, @code{CR-LF} here.  Such surfaces are automatically
removed or applied whenever the default charset is read or written,
exactly as it would go for any other charset.  In the example above, on
such systems, the hexadecimal surface would then @emph{replace} the implied
surfaces.  For @emph{adding} an hexadecimal surface without removing any,
one should write the request as @samp{/../x}.}.

The allowable values for @var{before} or @var{after} charsets, and various
surfaces, are described in the remainder of this document.

@node Listings, Recoding, Requests, Invoking recode
@section Asking for various lists

Many options control listing output generated by @code{recode} itself,
they are not meant to accompany actual file recodings.  These options are:

@table @samp

@item --version
@opindex --version
@cindex @code{recode} version, printing
The program merely prints its version numbers on standard output, and
exits without doing anything else.

@item --help
@opindex --help
@cindex help page, printing
The program merely prints a page of help on standard output, and exits
without doing any recoding.

@item -C
@itemx --copyright
@opindex -C
@opindex --copyright
@cindex copyright conditions, printing
Given this option, all other parameters and options are ignored.  The
program prints briefly the copyright and copying conditions.  See the
file @file{COPYING} in the distribution for full statement of the
Copyright and copying conditions.

@item -h[@var{language}/][@var{name}]
@itemx --header[=[@var{language}/][@var{name}]]
@opindex -h
@opindex --header
@cindex source file generation
@cindex programming language support
@cindex languages, programming
@cindex supported programming languages
Instead of recoding files, @code{recode} writes a @var{language} source
file on standard output and exits.  This source is meant to be included
in a regular program written in the same programming @var{language}:
its purpose is to declare and initialise an array, named @var{name},
which represents the requested recoding.  The only acceptable values for
@var{language} are @samp{c} or @samp{perl}, and may may be abbreviated.
If @var{language} is not specified, @samp{c} is assumed.  If @var{name}
is not specified, then it defaults to @samp{@var{before}_@var{after}}.
Strings @var{before} and @var{after} are cleaned before being used according
to the syntax of @var{language}.

Even if @code{recode} tries its best, this option does not always succeed in
producing the requested source table.  It will however, provided the recoding
can be internally represented by only one step after the optimisation phase,
and if this merged step conveys a one-to-one or a one-to-many explicit
table.  Also, when attempting to produce sources tables, @code{recode}
relaxes its checking a tiny bit: it ignores the algorithmic part of some
tabular recodings, it also avoids the processing of implied surfaces.
But this is all fairly technical.  Better try and see!

Beware that other options might affect the produced source tables, these
are: @samp{-d}, @samp{-g} and, particularly, @samp{-s}.

@item -k @var{pairs}
@itemx --known=@var{pairs}
@opindex -k
@opindex --known=
@cindex unknown charsets
@cindex guessing charsets
@cindex charsets, guessing
This particular option is meant to help identifying an unknown charset,
using as hints some already identified characters of the charset.  Some
examples will help introducing the idea.

Let's presume here that @code{recode} is run in an ISO-8859-1 locale, and
that @code{DEFAULT_CHARSET} is unset in the environment.
Suppose you have guessed that code 130 (decimal) of the unknown charset
represents a lower case @samp{e} with an acute accent.  That is to say
that this code should map to code 233 (decimal) in the usual charset.
By executing:

@example
recode -k 130:233
@end example

@noindent
you should obtain a listing similar to:

@example
AtariST atarist
CWI cphu cwi cwi2
IBM437 437 cp437 ibm437
IBM850 850 cp850 ibm850
IBM851 851 cp851 ibm851
IBM852 852 cp852 ibm852
IBM857 857 cp857 ibm857
IBM860 860 cp860 ibm860
IBM861 861 cp861 cpis ibm861
IBM863 863 cp863 ibm863
IBM865 865 cp865 ibm865
@end example

You can give more than one clue at once, to restrict the list further.
Suppose you have @emph{also} guessed that code 211 of the unknown
charset represents an upper case @samp{E} with diaeresis, that is, code
203 in the usual charset.  By requesting:

@example
recode -k 130:233,211:203
@end example

@noindent
you should obtain:

@example
IBM850 850 cp850 ibm850
IBM852 852 cp852 ibm852
IBM857 857 cp857 ibm857
@end example

The usual charset may be overridden by specifying one non-option argument.
For example, to request the list of charsets for which code 130 maps to
code 142 for the Macintosh, you may ask:

@example
recode -k 130:142 mac
@end example

@noindent
and get:

@example
AtariST atarist
CWI cphu cwi cwi2
IBM437 437 cp437 ibm437
IBM850 850 cp850 ibm850
IBM851 851 cp851 ibm851
IBM852 852 cp852 ibm852
IBM857 857 cp857 ibm857
IBM860 860 cp860 ibm860
IBM861 861 cp861 cpis ibm861
IBM863 863 cp863 ibm863
IBM865 865 cp865 ibm865
@end example

@noindent
which, of course, is identical to the result of the first example, since
the code 142 for the Macintosh is a small @samp{e} with acute.

More formally, option @samp{-k} lists all possible @emph{before}
charsets for the @emph{after} charset given as the sole non-option
argument to @code{recode}, but subject to restrictions given in
@var{pairs}.  If there is no non-option argument, the @emph{after}
charset is taken to be the default charset for this @code{recode}.

The restrictions are given as a comma separated list of pairs, each pair
consisting of two numbers separated by a colon.  The numbers are taken
as decimal when the initial digit is between @samp{1} and @samp{9};
@samp{0x} starts an hexadecimal number, or else @samp{0} starts an
octal number.  The first number is a code in any @emph{before} charset,
while the second number is a code in the specified @emph{after} charset.
If the first number would not be transformed into the second number by
recoding from some @emph{before} charset to the @emph{after} charset,
then this @emph{before} charset is rejected.  A @emph{before} charset is
listed only if it is not rejected by any pair.  The program will only test
those @emph{before} charsets having a tabular style internal description
(@pxref{Tabular}), so should be the selected @emph{after} charset.

The produced list is in fact a subset of the list produced by the
option @samp{-l}.  As for option @samp{-l}, the non-option argument
is interpreted as a charset name, possibly abbreviated to any non
ambiguous prefix.

@item -l[@var{format}]
@itemx --list[=@var{format}]
@opindex -l
@opindex --list
@cindex listing charsets
@cindex information about charsets
This option asks for information about all charsets, or about one
particular charset.  No file will be recoded.

If there is no non-option arguments, @code{recode} ignores the @var{format}
value of the option, it writes a sorted list of charset names on standard
output, one per line.  When a charset name have aliases or synonyms,
they follow the true charset name on its line, sorted from left to right.
Each charset or alias is followed by its implied surfaces, if any.  This list
is over two hundred lines.  It is best used with @samp{grep -i}, as in:

@example
recode -l | grep -i greek
@end example

There might be one non-option argument, in which case it is interpreted
as a charset name, possibly abbreviated to any non ambiguous prefix.
This particular usage of the @samp{-l} option is obeyed @emph{only} for
charsets having a tabular style internal description (@pxref{Tabular}).
Even if most charsets have this property, some do not, and the option
@samp{-l} cannot be used to detail these particular charsets.  For knowing
if a particular charset can be listed this way, you should merely try
and see if this works.  The @var{format} value of the option is a keyword
from the following list.  Keywords may be abbreviated by dropping suffix
letters, and even reduced to the first letter only:

@table @samp
@item decimal
This format asks for the production on standard output of a concise
tabular display of the charset, in which character code values are
expressed in decimal.

@item octal
This format uses octal instead of decimal in the concise tabular display
of the charset.

@item hexadecimal
This format uses hexadecimal instead of decimal in the concise tabular
display of the charset.

@item full
This format requests an extensive display of the charset on standard output,
using one line per character showing its decimal, hexadecimal, octal and
@code{UCS-2} code values, and also a descriptive comment which should be
the 10646 name for the character.

@vindex LANGUAGE@r{, when listing charsets}
@vindex LANG@r{, when listing charsets}
@cindex French description of charsets
The descriptive comment is given in English and ASCII, yet if the English
description is not available but a French one is, then the French description
is given instead, using @w{Latin-1}.  However, if the @code{LANGUAGE}
or @code{LANG} environment variable begins with the letters @samp{fr},
then listing preference goes to French when both descriptions are available.
@end table

When option @samp{-l} is used together with a @var{charset} argument,
the @var{format} defaults to @code{decimal}.

@item -T
@itemx --find-subsets
@opindex -T
@opindex --find-subsets
@cindex identifying subsets in charsets
@cindex subsets in charsets
This option is a maintainer tool for evaluating the redundancy of those
charsets, in @code{recode}, which are internally represented by an @code{UCS-2}
data table.  After the listing has been produced, the program exits
without doing any recoding.  The output is meant to be sorted, like
this: @w{@samp{recode -T | sort}}.  The option triggers @code{recode} into
comparing all pairs of charsets, seeking those which are subsets of others.
The concept and results are better explained through a few examples.
Consider these three sample lines from @samp{-T} output:

@example
[  0] IBM891 == IBM903
[  1] IBM1004 < CP1252
[ 12] INVARIANT < CSA_Z243.4-1985-1
@end example

@noindent
The first line means that @code{IBM891} and @code{IBM903} are completely
identical as far as @code{recode} is concerned, so one is fully redundant
to the other.  The second line says that @code{IBM1004} is wholly
contained within @code{CP1252}, yet there is a single character which is
in @code{CP1252} without being in @code{IBM1004}.  The third line says
that @code{INVARIANT} is wholly contained within @code{CSA_Z243.4-1985-1},
but twelve characters are in @code{CSA_Z243.4-1985-1} without being in
@code{INVARIANT}.  The whole output might most probably be reduced and
made more significant through a transitivity study.
@end table

@node Recoding, Reversibility, Listings, Invoking recode
@section Controlling how files are recoded

The following options have the purpose of giving the user some fine
grain control over the recoding operation themselves.

@table @samp

@item -c
@itemx --colons
@opindex -c
@opindex --colons
@cindex diaeresis
With @code{Texte} Easy French conventions, use the column @kbd{:}
instead of the double-quote @kbd{"} for marking diaeresis.
@xref{Texte}.

@item -g
@itemx --graphics
@opindex -g
@opindex --graphics
@cindex IBM graphics characters
@cindex box-drawing characters
This option is only meaningful while getting @emph{out} of the
@code{IBM-PC} charset.  In this charset, characters 176 to 223 are used
for constructing rulers and boxes, using simple or double horizontal or
vertical lines.  This option forces the automatic selection of ASCII
characters for approximating these rulers and boxes, at cost of making
the transformation irreversible.  Option @samp{-g} implies @samp{-f}.

@item -t
@itemx --touch
@opindex -t
@opindex --touch
@cindex time stamps of files
@cindex file time stamps
The @emph{touch} option is meaningful only when files are recoded over
themselves.  Without it, the time-stamps associated with files are
preserved, to reflect the fact that changing the code of a file does not
really alter its informational contents.  When the user wants the
recoded files to be time-stamped at the recoding time, this option
inhibits the automatic protection of the time-stamps.

@item -v
@itemx --verbose
@opindex -v
@opindex --verbose
@cindex verbose operation
@cindex details about recoding
@cindex recoding details
@cindex quality of recoding
Before doing any recoding, the program will first print on the @code{stderr}
stream the list of all intermediate charsets planned for recoding, starting
with the @var{before} charset and ending with the @var{after} charset.
It also prints an indication of the recoding quality, as one of the word
@samp{reversible}, @samp{one to one}, @samp{one to many}, @samp{many to
one} or @samp{many to many}.

This information will appear once or twice.  It is shown a second time
only when the optimisation and step merging phase succeeds in replacing
many single steps by a new one.

This option also has a second effect.  The program will print on
@code{stderr} one message per recoded @var{file}, so as to keep the user
informed of the progress of its command.

An easy way to know beforehand the sequence or quality of a recoding is
by using the command such as:

@example
recode -v @var{before}..@var{after} < /dev/null
@end example

@noindent
using the fact that, in @code{recode}, an empty input file produces
an empty output file.

@item -x @var{charset}
@itemx --ignore=@var{charset}
@opindex -x
@opindex --ignore
@cindex ignore charsets
@cindex recoding path, rejection
This option tells the program to ignore any recoding path through the
specified @var{charset}, so disabling any single step using this charset
as a start or end point.  This may be used when the user wants to force
@code{recode} into using an alternate recoding path (yet using chained
requests offers a finer control, @pxref{Requests}).

@var{charset} may be abbreviated to any unambiguous prefix.
@end table

@node Reversibility, Sequencing, Recoding, Invoking recode
@section Reversibility issues

The following options are somewhat related to reversibility issues:

@table @samp
@item -f
@itemx --force
@opindex -f
@opindex --force
@cindex force recoding
@cindex irreversible recoding
With this option, irreversible or otherwise erroneous recodings are run
to completion, and @code{recode} does not exit with a non-zero status if
it would be only because irreversibility matters.  @xref{Reversibility}.

Without this option, @code{recode} tries to protect you against recoding
a file irreversibly over itself@footnote{There are still some cases of
ambiguous output which are rather difficult to detect, and for which
the protection is not active.}.  Whenever an irreversible recoding is
met, or any other recoding error, @code{recode} produces a warning on
standard error.  The current input file does not get replaced by its
recoded version, and @code{recode} then proceeds with the recoding of
the next file.

When the program is merely used as a filter, standard output will have
received a partially recoded copy of standard input, up to the first
error point.  After all recodings have been done or attempted, and if
some recoding has been aborted, @code{recode} exits with a non-zero status.

In releases of @code{recode} prior to version 3.5, this option was always
selected, so it was rather meaningless.  Nevertheless, users were invited
to start using @samp{-f} right away in scripts calling @code{recode}
whenever convenient, in preparation for the current behaviour.

@item -q
@itemx --quiet
@itemx --silent
@opindex -q
@opindex --quiet
@opindex --silent
@cindex suppressing diagnostic messages
@cindex error messages, suppressing
@cindex silent operation
This option has the sole purpose of inhibiting warning messages about
irreversible recodings, and other such diagnostics.  It has no other
effect, in particular, it does @emph{not} prevent recodings to be aborted
or @code{recode} to return a non-zero exit status when irreversible
recodings are met.

This option is set automatically for the children processes, when recode
splits itself in many collaborating copies.  Doing so, the diagnostic is
issued only once by the parent.  See option @samp{-p}.

@item -s
@itemx --strict
@opindex -s
@opindex --strict
@cindex strict operation
@cindex map filling, disable
@cindex disable map filling
By using this option, the user requests that @code{recode} be very strict
while recoding a file, merely losing in the transformation any character
which is not explicitly mapped from a charset to another.  Such a loss is
not reversible and so, will bring @code{recode} to fail, unless the option
@samp{-f} is also given as a kind of counter-measure.

Using @samp{-s} without @samp{-f} might render the @code{recode} program
very susceptible to the slighest file abnormalities.  Despite the fact
that it might be
irritating to some users, such paranoia is sometimes wanted and useful.
@end table

@cindex reversibility of recoding
Even if @code{recode} tries hard to keep the recodings reversible,
you should not develop an unconditional confidence in its ability to
do so.  You @emph{ought} to keep only reasonable expectations about
reverse recodings.  In particular, consider:

@itemize @bullet
@item
Most transformations are fully reversible for all inputs, but lose this
property whenever @samp{-s} is specified.

@item
A few transformations are not meant to be reversible, by design.

@item
Reversibility sometimes depends on actual file contents and cannot
be ascertained beforehand, without reading the file.

@item
Reversibility is never absolute across successive versions of this
program.  Even correcting a small bug in a mapping could induce slight
discrepancies later.

@item
Reversibility is easily lost by merging.  This is best explained through
an example.  If you reversibly recode a file from charset @var{A} to
charset @var{B}, then you reversibly recode the result from charset
@var{B} to charset @var{C}, you cannot expect to recover the original
file by merely recoding from charset @var{C} directly to charset @var{A}.
You will instead have to recode from charset @var{C} back to charset
@var{B}, and only then from charset @var{B} to charset @var{A}.

@item
Faulty files create a particular problem.  Consider an example, recoding
from @code{IBM-PC} to @code{Latin-1}.  End of lines are represented as
@samp{\r\n} in @code{IBM-PC} and as @samp{\n} in @code{Latin-1}.  There
is no way by which a faulty @code{IBM-PC} file containing a @samp{\n}
not preceded by @samp{\r} be translated into a @code{Latin-1} file, and
then back.

@item
There is another difficulty arising from code equivalences.  For
example, in a @code{LaTeX} charset file, the string @samp{\^\i@{@}}
could be recoded back and forth through another charset and become
@samp{\^@{\i@}}.  Even if the resulting file is equivalent to the
original one, it is not identical.
@end itemize

@cindex map filling
Unless option @samp{-s} is used, @code{recode} automatically tries to
fill mappings with invented correspondences, often making them fully
reversible.  This filling is not made at random.  The algorithm tries to
stick to the identity mapping and, when this is not possible, it prefers
generating many small permutation cycles, each involving only a few
codes.

For example, here is how @code{IBM-PC} code 186 gets translated to
@kbd{control-U} in @code{Latin-1}.  @kbd{Control-U} is 21.  Code 21 is the
@code{IBM-PC} section sign, which is 167 in @code{Latin-1}.  @code{recode}
cannot reciprocate 167 to 21, because 167 is the masculine ordinal indicator
within @code{IBM-PC}, which is 186 in @code{Latin-1}.  Code 186 within
@code{IBM-PC} has no @code{Latin-1} equivalent; by assigning it back to 21,
@code{recode} closes this short permutation loop.

As a consequence of this map filling, @code{recode} may sometimes produce
@emph{funny} characters.  They may look annoying, they are nevertheless
helpful when one changes his (her) mind and wants to revert to the prior
recoding.  If you cannot stand these, use option @samp{-s}, which asks
for a very strict recoding.

This map filling sometimes has a few surprising consequences, which
some users wrongly interpreted as bugs.  Here are two examples.

@enumerate
@item
In some cases, @code{recode} seems to copy a file without recoding it.
But in fact, it does.  Consider a request:

@example
recode l1..us < File-Latin1 > File-ASCII
cmp File-Latin1 File-ASCII
@end example

@noindent
then @code{cmp} will not report any difference.  This is quite normal.
@w{@code{Latin-1}} gets correctly recoded to ASCII for charsets commonalities
(which are the first 128 characters, in this case).  The remaining last
128 @w{@code{Latin-1}} characters have no ASCII correspondent.  Instead
of losing
them, @code{recode} elects to map them to unspecified characters of ASCII, so
making the recoding reversible.  The simplest way of achieving this is
merely to keep those last 128 characters unchanged.  The overall effect
is copying the file verbatim.

If you feel this behaviour is too generous and if you do not wish to
care about reversibility, simply use option @samp{-s}.  By doing so,
@code{recode} will strictly map only those @w{@code{Latin-1}} characters
which have
an ASCII equivalent, and will merely drop those which do not.  Then,
there is more chance that you will observe a difference between the
input and the output file.

@item
Recoding the wrong way could sometimes give the false impression that
recoding has @emph{almost} been done properly.  Consider the requests:

@example
recode 437..l1 < File-Latin1 > Temp1
recode 437..l1 < Temp1 > Temp2
@end example

@noindent
so declaring wrongly @file{File-Latin1} to be an IBM-PC file, and
recoding to @code{Latin-1}.  This is surely ill defined and not meaningful.
Yet, if you repeat this step a second time, you might notice that
many (not all) characters in @file{Temp2} are identical to those in
@file{File-Latin1}.  Sometimes, people try to discover how @code{recode}
works by experimenting a little at random, rather than reading and
understanding the documentation; results such as this are surely confusing,
as they provide those people with a false feeling that they understood
something.

Reversible codings have this property that, if applied several times
in the same direction, they will eventually bring any character back
to its original value.  Since @code{recode} seeks small permutation
cycles when creating reversible codings, besides characters unchanged
by the recoding, most permutation cycles will be of length 2, and
fewer of length 3, etc.  So, it is just expectable that applying the
recoding twice in the same direction will recover most characters,
but will fail to recover those participating in permutation cycles of
length 3.  On the other end, recoding six times in the same direction
would recover all characters in cycles of length 1, 2, 3 or 6.
@end enumerate

@node Sequencing, Mixed, Reversibility, Invoking recode
@section Selecting sequencing methods

@cindex sequencing
This program uses a few techniques when it is discovered that many
passes are needed to comply with the @var{request}.  For example,
suppose that four elementary steps were selected at recoding path
optimisation time.  Then @code{recode} will split itself into four
different interconnected tasks, logically equivalent to:

@example
@var{step1} <@var{input} | @var{step2} | @var{step3} | @var{step4} >@var{output}
@end example

The splitting into subtasks is often done using Unix pipes.
But the splitting may also be completely avoided, and rather
simulated by using memory buffer, or intermediate files.  The various
@samp{--sequence=@var{strategy}} options gives you control over the flow
methods, by replacing @var{strategy} with @samp{memory}, @samp{pipe}
or @samp{files}.  So, these options may be used to override the default
behaviour, which is also explained below.

@table @samp
@item --sequence=memory
@opindex --sequence
@cindex memory sequencing
When the recoding requires a combination of two or more elementary
recoding steps, this option forces many passes over the data, using
in-memory buffers to hold all intermediary results.
@c This should be the default behaviour when
@c files to be recoded are @emph{small} enough.

@item -i
@itemx --sequence=files
@opindex -i
@cindex file sequencing
When the recoding requires a combination of two or more elementary
recoding steps, this option forces many passes over the data, using
intermediate files between passes.  This is the default behaviour when
files are recoded over themselves.  If this option is selected in filter
mode, that is, when the program reads standard input and writes standard
output, it might take longer for programs further down the pipe chain to
start receiving some recoded data.

@item -p
@itemx --sequence=pipe
@opindex -p
@cindex pipe sequencing
When the recoding requires a combination of two or more elementary
recoding steps, this option forces the program to fork itself into a few
copies interconnected with pipes, using the @code{pipe(2)} system call.
All copies of the program operate in parallel.  This is the default
behaviour in filter mode.  If this option is used when files are recoded
over themselves, this should also save disk space because some temporary
files might not be needed, at the cost of more system overhead.

If, at installation time, the @code{pipe(2)} call is said to be
unavailable, selecting option @samp{-p} is equivalent to selecting
option @samp{-i}.  (This happens, for example, on MS-DOS systems.)
@end table

@node Mixed, Emacs, Sequencing, Invoking recode
@section Using mixed charset input

In real life and practice, textual files are often made up of many charsets
at once.  Some parts of the file encode one charset, while other parts
encode another charset, and so forth.  Usually, a file does not toggle
between more than two or three charsets.  The means to distinguish
which charsets are encoded at various places is not always available.
The @code{recode} program is able to handle only a few simple cases
of mixed input.

The default @code{recode} behaviour is to expect pure charset files, to
be recoded as other pure charset files.  However, the following options
allow for a few precise kinds of mixed charset files.

@ignore
Some notes on transliteration and substitution.

Transliteration is still much study, discussion and work to come, but
when generic transliteration will be added in @code{recode}, it will be
added @emph{through} the @code{recode} library.

However, I agree that it might be *convenient* that the `latin1..fi'
conversion works by letting all ASCII characters through, but then, the
result would be a mix of ASCII and `fi', it would not be pure `fi' anymore.
It would be convenient because, in practice, people might write programs in
ASCII, keeping comments or strings directly in `fi', all in the same file.
The original files are indeed mixed, and people sometimes expect that
`recode' will do mixed conversions.

A conversion does not become *right* because it is altered to be more
convenient.  And recode is not *wrong* because it does not offer some
conveniences people would like to have.  As long as `recode' main job is
producing `fi', than '[' is just not representable in `fi', and recode is
rather right in not letting `[' through.  It has to do something special
about it.  The character might be thrown away, transliterated or replaced
by a substitute, or mapped to some other code for reversibility purposes.

Transliteration or substitution are currently not implemented in `recode',
yet for the last few years, I've been saving documentation about these
phenomena.  The transliteration which you are asking for, here, is that the
'[' character in @w{Latin-1}, for example, be transliterated to A-umlaut in
`fi', which is a bit non-meaningful.  Remember, there is no `[' in `fi'.
@end ignore

@table @samp
@item -d
@itemx --diacritics
@opindex -d
@opindex --diacritics
@cindex convert a subset of characters
@cindex partial conversion
While converting to or from one of @code{HTML} or @code{LaTeX}
charset, limit conversion to some subset of all characters.
For @code{HTML}, limit conversion to the subset of all non-ASCII
characters.  For @code{LaTeX}, limit conversion to the subset of all
non-English letters.  This is particularly useful, for example, when
people create what would be valid @code{HTML}, @TeX{} or La@TeX{}
files, if only they were using provided sequences for applying
diacritics instead of using the diacriticised characters directly
from the underlying character set.

While converting to @code{HTML} or @code{LaTeX} charset, this option
assumes that characters not in the said subset are properly coded
or protected already, @code{recode} then transmit them literally.
While converting the other way, this option prevents translating back
coded or protected versions of characters not in the said subset.
@xref{HTML}.  @xref{LaTeX}.

@ignore
@item -M
@itemx --message
@opindex -M
@opindex --message
Option @samp{-M} would be for messages, it would ideally process @w{RFC
1522} inserts
in ASCII headers, converting them to the goal code, rewriting some MIME
header line too, and stopping its special work at the first empty line.
A special combination of both capabilities would be for the recoding of
PO files, in which the header, and @code{msgid} and @code{msgstr} strings, might
all use different charsets.  Recoding some PO files currently looks like
a nightmare, which I would like @code{recode} to repair.
@end ignore

@item -S[@var{language}]
@itemx --source[=@var{language}]
@opindex -S
@opindex --source
@cindex convert strings and comments
@cindex string and comments conversion
The bulk of the input file is expected to be written in @code{ASCII},
except for parts, like comments and string constants, which are written
using another charset than @code{ASCII}.  When @var{language} is @samp{c},
the recoding will proceed only with the contents of comments or strings,
while everything else will be copied without recoding.  When @var{language}
is @samp{po}, the recoding will proceed only within translator comments
(those having whitespace immediately following the initial @samp{#})
and with the contents of @code{msgstr} strings.

For the above things to work, the non-@code{ASCII} encoding of the comment
or string should be such that an @code{ASCII} scan will successfully find
where the comment or string ends.

Even if @code{ASCII} is the usual charset for writing programs, some
compilers are able to directly read other charsets, like @code{UTF-8}, say.
There is currently no provision in @code{recode} for reading mixed charset
sources which are not based on @code{ASCII}.  It is probable that the need
for mixed recoding is not as pressing in such cases.

For example, after one does:

@example
recode -Spo pc/..u8 < @var{input}.po > @var{output}.po
@end example

@noindent
file @file{@var{output}.po} holds a copy of @file{@var{input}.po} in which
@emph{only} translator comments and the contents of @code{msgstr} strings
have been recoded from the @code{IBM-PC} charset to pure @code{UTF-8},
without attempting conversion of end-of-lines.  Machine generated comments
and original @code{msgid} strings are not to be touched by this recoding.

If @var{language} is not specified, @samp{c} is assumed.
@end table

@node Emacs, Debugging, Mixed, Invoking recode
@section Using @code{recode} within Emacs

The fact @code{recode} is a filter makes it quite easy to use from
within GNU Emacs.  For example, recoding the whole buffer from
the @code{IBM-PC} charset to current charset (@w{@code{Latin-1}} on
Unix) is easily done with:

@example
C-x h C-u M-| recode ibmpc RET
@end example

@noindent
@samp{C-x h} selects the whole buffer, and @samp{C-u M-|} filters and
replaces the current region through the given shell command.  Here is
another example, binding the keys @w{@samp{C-c T}} to the recoding of
the current region from Easy French to @w{@code{Latin-1}} (on Unix) and the key
@w{@samp{C-u C-c T}} from @w{@code{Latin-1}} (on Unix) to Easy French:

@example
(global-set-key "\C-cT" 'recode-texte)

(defun recode-texte (flag)
  (interactive "P")
  (shell-command-on-region
   (region-beginning) (region-end)
   (concat "recode " (if flag "..txte" "txte")) t)
  (exchange-point-and-mark))
@end example

@node Debugging,  , Emacs, Invoking recode
@section Debugging considerations

It is our experience that when @code{recode} does not provide satisfying
results, either @code{recode} was not called properly, correct results
raised some doubts nevertheless, or files to recode were somewhat mangled.
Genuine bugs are surely possible.

Unless you already are a @code{recode} expert, it might be a good idea to
quickly revisit the tutorial (@pxref{Tutorial}) or the prior sections in this
chapter, to make sure that you properly formatted your recoding request.
In the case you intended to use @code{recode} as a filter, make sure that you
did not forget to redirect your standard input (through using the @kbd{<}
symbol in the shell, say).  Some @code{recode} false mysteries are also
easily explained, @xref{Reversibility}.

For the other cases, some investigation is needed.  To illustrate how to
proceed, let's presume that you want to recode the @file{nicepage} file,
coded @code{UTF-8}, into @code{HTML}.  The problem is that the command
@samp{recode u8..h nicepage} yields:

@example
recode: Invalid input in step `UTF-8..ISO-10646-UCS-2'
@end example

One good trick is to use @code{recode} in filter mode instead of in file
replacement mode, @xref{Synopsis}.  Another good trick is to use the
@samp{-v} option asking for a verbose description of the recoding steps.
We could rewrite our recoding call as @samp{recode -v u8..h <nicepage},
to get something like:

@example
Request: UTF-8..:libiconv:..ISO-10646-UCS-2..HTML_4.0
Shrunk to: UTF-8..ISO-10646-UCS-2..HTML_4.0
[@dots{}@var{some output}@dots{}]
recode: Invalid input in step `UTF-8..ISO-10646-UCS-2'
@end example

This might help you to better understand what the diagnostic means.  The
recoding request is achieved in two steps, the first recodes @code{UTF-8}
into @code{UCS-2}, the second recodes @code{UCS-2} into @code{HTML}.
The problem occurs within the first of these two steps, and since, the
input of this step is the input file given to @code{recode}, this is
this overall input file which seems to be invalid.  Also, when used in
filter mode, @code{recode} processes as much input as possible before the
error occurs and sends the result of this processing to standard output.
Since the standard output has not been redirected to a file, it is merely
displayed on the user screen.  By inspecting near the end of the resulting
@code{HTML} output, that is, what was recoding a bit before the recoding
was interrupted, you may infer about where the error stands in the real
@code{UTF-8} input file.

If you have the proper tools to examine the intermediate recoding data,
you might also prefer to reduce the problem to a single step to better
study it.  This is what I usually do.  For example, the last @code{recode}
call above is more or less equivalent to:

@example
recode -v UTF-8..ISO_10646-UCS-2 <nicepage >temporary
recode -v ISO_10646-UCS-2..HTML_4.0 <temporary
rm temporary
@end example

If you know that the problem is within the first step, you might prefer to
concentrate on using the first @code{recode} line.  If you know that the
problem is within the second step, you might execute the first @code{recode}
line once and for all, and then play with the second @code{recode} call,
repeatedly using the @file{temporary} file created once by the first call.

Note that the @samp{-f} switch may be used to force the production of
@code{HTML} output despite invalid input, it might be satisfying enough
for you, and easier than repairing the input file.  That depends on how
strict you would like to be about the precision of the recoding process.

If you later see that your HTML file begins with @samp{@@lt;html@@gt;} when
you expected @samp{<html>}, then @code{recode} might have done a bit more
that you wanted.  In this case, your input file was half-@code{UTF-8},
half-@code{HTML} already, that is, a mixed file (@pxref{Mixed}).  There is a
special @code{-d} switch for this case.  So, your might be end up calling
@samp{recode -fd nicepage}.  Until you are quite sure that you accept
overwriting your input file whatever what, I recommend that you stick with
filter mode.

If, after such experiments, you seriously think that the @code{recode}
program does not behave properly, there might be a genuine bug in the
program itself, in which case I invite you to to contribute a bug report,
@xref{Contributing}.

@node Library, Universal, Invoking recode, Top
@chapter A recoding library

@cindex recoding library
The program named @code{recode} is just an application of its recoding
library.  The recoding library is available separately for other C
programs.  A good way to acquire some familiarity with the recoding
library is to get acquainted with the @code{recode} program itself.

To use the recoding library once it is installed, a C program needs to
have a line:

@example
#include <recode.h>
@end example

@noindent
near its beginning, and the user should have @samp{-lrecode} on the
linking call, so modules from the recoding library are found.

The library is still under development.  As it stands, it contains four
identifiable sets of routines: the outer level functions, the request
level functions, the task level functions and the charset level functions.
There are discussed in separate sections.

For effectively using the recoding library in most applications, it should
be rarely needed to study anything beyond the main initialisation function
at outer level, and then, various functions at request level.

@menu
* Outer level::         Outer level functions
* Request level::       Request level functions
* Task level::          Task level functions
* Charset level::       Charset level functions
* Errors::              Handling errors
@end menu

@node Outer level, Request level, Library, Library
@section Outer level functions

@cindex outer level functions
The outer level functions mainly prepare the whole recoding library for
use, or do actions which are unrelated to specific recodings.  Here is
an example of a program which does not really make anything useful.

@example
@group
#include <stdbool.h>
#include <recode.h>

const char *program_name;

int
main (int argc, char *const *argv)
@{
  program_name = argv[0];
  RECODE_OUTER outer = recode_new_outer (true);

  recode_delete_outer (outer);
  exit (0);
@}
@end group
@end example

@vindex RECODE_OUTER structure
The header file @code{<recode.h>} declares an opaque @code{RECODE_OUTER}
structure, which the programmer should use for allocating a variable in
his program (let's assume the programmer is a male, here, no prejudice
intended).  This @samp{outer} variable is given as a first argument to
all outer level functions.

@cindex @code{stdbool.h} header
@cindex @code{bool} data type
The @code{<recode.h>} header file uses the Boolean type setup by the
system header file @code{<stdbool.h>}.  But this header file is still
fairly new in C standards, and likely does not exist everywhere.  If you
system does not offer this system header file yet, the proper compilation
of the @code{<recode.h>} file could be guaranteed through the replacement
of the inclusion line by:

@example
typedef enum @{false = 0, true = 1@} bool;
@end example

People wanting wider portability, or Autoconf lovers, might arrange their
@file{configure.in} for being able to write something more general, like:

@example
@group
#if STDC_HEADERS
# include <stdlib.h>
#endif

/* Some systems do not define EXIT_*, even with STDC_HEADERS.  */
#ifndef EXIT_SUCCESS
# define EXIT_SUCCESS 0
#endif
#ifndef EXIT_FAILURE
# define EXIT_FAILURE 1
#endif
/* The following test is to work around the gross typo in systems like Sony
   NEWS-OS Release 4.0C, whereby EXIT_FAILURE is defined to 0, not 1.  */
#if !EXIT_FAILURE
# undef EXIT_FAILURE
# define EXIT_FAILURE 1
#endif

#if HAVE_STDBOOL_H
# include <stdbool.h>
#else
typedef enum @{false = 0, true = 1@} bool;
#endif

#include <recode.h>

const char *program_name;

int
main (int argc, char *const *argv)
@{
  program_name = argv[0];
  RECODE_OUTER outer = recode_new_outer (true);

  recode_term_outer (outer);
  exit (EXIT_SUCCESS);
@}
@end group
@end example

@noindent
but we will not insist on such details in the examples to come.

@itemize @bullet
@item Initialisation functions
@cindex initialisation functions, outer

@example
RECODE_OUTER recode_new_outer (@var{auto_abort});
bool recode_delete_outer (@var{outer});
@end example

@findex recode_new_outer
@findex recode_delete_outer
The recoding library absolutely needs to be initialised before being used,
and @code{recode_new_outer} has to be called once, first.  Besides the
@var{outer} it is meant to initialise, the function accepts a Boolean
argument whether or not the library should automatically issue diagnostics
on standard and abort the whole program on errors.  When @var{auto_abort}
is @code{true}, the library later conveniently issues diagnostics itself,
and aborts the calling program on errors.  This is merely a convenience,
because if this parameter was @code{false}, the calling program should always
take care of checking the return value of all other calls to the recoding
library functions, and when any error is detected, issue a diagnostic and
abort processing itself.

Regardless of the setting of @var{auto_abort}, all recoding library
functions return a success status.  Most functions are geared for returning
@code{false} for an error, and @code{true} if everything went fine.
Functions returning structures or strings return @code{NULL} instead
of the result, when the result cannot be produced.  If @var{auto_abort}
is selected, functions either return @code{true}, or do not return at all.

As in the example above, @code{recode_new_outer} is called only once in
most cases.  Calling @code{recode_new_outer} implies some overhead, so
calling it more than once should preferably be avoided.

The termination function @code{recode_delete_outer} reclaims the memory
allocated by @code{recode_new_outer} for a given @var{outer} variable.
Calling @code{recode_delete_outer} prior to program termination is more
aesthetic then useful, as all memory resources are automatically reclaimed
when the program ends.  You may spare this terminating call if you prefer.

@item The @code{program_name} declaration

@cindex @code{program_name} variable
As we just explained, the user may set the @code{recode} library so that,
in case of problems error, it issues the diagnostic itself and aborts the
whole processing.  This capability may be quite convenient.  When this
feature is used, the aborting routine includes the name of the running
program in the diagnostic.  On the other hand, when this feature is not
used, the library merely return error codes, giving the library user fuller
control over all this.  This behaviour is more like what usual libraries
do: they return codes and never abort.  However, I would rather not force
library users to necessarily check all return codes themselves, by leaving
no other choice.  In most simple applications, letting the library diagnose
and abort is much easier, and quite welcome.  This is precisely because
both possibilities exist that the @code{program_name} variable is needed: it
may be used by the library @emph{when} the user sets it to diagnose itself.
@end itemize

@node Request level, Task level, Outer level, Library
@section Request level functions

@cindex request level functions
The request level functions are meant to cover most recoding needs
programmers may have; they should provide all usual functionality.
Their API is almost stable by now.  To get started with request level
functions, here is a full example of a program which sole job is to filter
@code{ibmpc} code on its standard input into @code{latin1} code on its
standard output.

@example
@group
#include <stdio.h>
#include <stdbool.h>
#include <recode.h>

const char *program_name;

int
main (int argc, char *const *argv)
@{
  program_name = argv[0];
  RECODE_OUTER outer = recode_new_outer (true);
  RECODE_REQUEST request = recode_new_request (outer);
  bool success;

  recode_scan_request (request, "ibmpc..latin1");

  success = recode_file_to_file (request, stdin, stdout);

  recode_delete_request (request);
  recode_delete_outer (outer);

  exit (success ? 0 : 1);
@}
@end group
@end example

@vindex RECODE_REQUEST structure
The header file @code{<recode.h>} declares a @code{RECODE_REQUEST} structure,
which the programmer should use for allocating a variable in his program.
This @var{request} variable is given as a first argument to all request
level functions, and in most cases, may be considered as opaque.

@itemize @bullet
@item Initialisation functions
@cindex initialisation functions, request

@example
RECODE_REQUEST recode_new_request (@var{outer});
bool recode_delete_request (@var{request});
@end example

@findex recode_new_request
@findex recode_delete_request
No @var{request} variable may not be used in other request level
functions of the recoding library before having been initialised by
@code{recode_new_request}.  There may be many such @var{request}
variables, in which case, they are independent of one another and
they all need to be initialised separately.  To avoid memory leaks, a
@var{request} variable should not be initialised a second time without
calling @code{recode_delete_request} to ``un-initialise'' it.

Like for @code{recode_delete_outer}, calling @code{recode_delete_request}
prior to program termination, in the example above, may be left out.

@item Fields of @code{struct recode_request}
@vindex recode_request structure

Here are the fields of a @code{struct recode_request} which may be
meaningfully changed, once a @var{request} has been initialised by
@code{recode_new_request}, but before it gets used.  It is not very frequent,
in practice, that these fields need to be changed.  To access the fields,
you need to include @file{recodext.h} @emph{instead} of @file{recode.h},
in which case there also is a greater chance that you need to recompile
your programs if a new version of the recoding library gets installed.

@table @code
@item verbose_flag
@vindex verbose_flag
This field is initially @code{false}.  When set to @code{true}, the
library will echo to stderr the sequence of elementary recoding steps
needed to achieve the requested recoding.

@item diaeresis_char
@vindex diaeresis_char
This field is initially the ASCII value of a double quote @kbd{"},
but it may also be the ASCII value of a colon @kbd{:}.  In @code{texte}
charset, some countries use double quotes to mark diaeresis, while other
countries prefer colons.  This field contains the diaeresis character
for the @code{texte} charset.

@item make_header_flag
@vindex make_header_flag
This field is initially @code{false}.  When set to @code{true}, it
indicates that the program is merely trying to produce a recoding table in
source form rather than completing any actual recoding.  In such a case,
the optimisation of step sequence can be attempted much more aggressively.
If the step sequence cannot be reduced to a single step, table production
will fail.

@item diacritics_only
@vindex diacritics_only
This field is initially @code{false}.  For @code{HTML} and @code{LaTeX}
charset, it is often convenient to recode the diacriticized characters
only, while just not recoding other HTML code using ampersands or angular
brackets, or La@TeX{} code using backslashes.  Set the field to @code{true}
for getting this behaviour.  In the other charset, one can edit text as
well as HTML or La@TeX{} directives.

@item ascii_graphics
@vindex ascii_graphics
This field is initially @code{false}, and relate to characters 176 to
223 in the @code{ibmpc} charset, which are use to draw boxes.  When set
to @code{true}, while getting out of @code{ibmpc}, ASCII characters are
selected so to graphically approximate these boxes.
@end table

@item Study of request strings

@example
bool recode_scan_request (@var{request}, "@var{string}");
@end example

@findex recode_scan_request
The main role of a @var{request} variable is to describe a set of
recoding transformations.  Function @code{recode_scan_request} studies
the given @var{string}, and stores an internal representation of it into
@var{request}.  Note that @var{string} may be a full-fledged @code{recode}
request, possibly including surfaces specifications, intermediary
charsets, sequences, aliases or abbreviations (@pxref{Requests}).

The internal representation automatically receives some pre-conditioning
and optimisation, so the @var{request} may then later be used many times
to achieve many actual recodings.  It would not be efficient calling
@code{recode_scan_request} many times with the same @var{string}, it is
better having many @var{request} variables instead.

@item Actual recoding jobs

Once the @var{request} variable holds the description of a recoding
transformation, a few functions use it for achieving an actual recoding.
Either input or output of a recoding may be string, an in-memory buffer,
or a file.

Functions with names like
@code{recode_@var{input-type}_to_@var{output-type}} request an actual
recoding, and are described below.  It is easy to remember which arguments
each function accepts, once grasped some simple principles for each
possible @var{type}.  However, one of the recoding function escapes these
principles and is discussed separately, first.

@example
recode_string (@var{request}, @var{string});
@end example

@findex recode_string
The function @code{recode_string} recodes @var{string} according
to @var{request}, and directly returns the resulting recoded string
freshly allocated, or @code{NULL} if the recoding could not succeed for
some reason.  When this function is used, it is the responsibility of
the programmer to ensure that the memory used by the returned string is
later reclaimed.

@findex recode_string_to_buffer
@findex recode_string_to_file
@findex recode_buffer_to_buffer
@findex recode_buffer_to_file
@findex recode_file_to_buffer
@findex recode_file_to_file
@example
char *recode_string_to_buffer (@var{request},
  @var{input_string},
  &@var{output_buffer}, &@var{output_length}, &@var{output_allocated});
bool recode_string_to_file (@var{request},
  @var{input_file},
  @var{output_file});
bool recode_buffer_to_buffer (@var{request},
  @var{input_buffer}, @var{input_length},
  &@var{output_buffer}, &@var{output_length}, &@var{output_allocated});
bool recode_buffer_to_file (@var{request},
  @var{input_buffer}, @var{input_length},
  @var{output_file});
bool recode_file_to_buffer (@var{request},
  @var{input_file},
  &@var{output_buffer}, &@var{output_length}, &@var{output_allocated});
bool recode_file_to_file (@var{request},
  @var{input_file},
  @var{output_file});
@end example

All these functions return a @code{bool} result, @code{false} meaning that
the recoding was not successful, often because of reversibility issues.
The name of the function well indicates on which types it reads and which
type it produces.  Let's discuss these three types in turn.

@table @asis
@item string

A string is merely an in-memory buffer which is terminated by a @code{NUL}
character (using as many bytes as needed), instead of being described
by a byte length.  For input, a pointer to the buffer is given through
one argument.

It is notable that there is no @code{to_string} functions.  Only one
function recodes into a string, and it is @code{recode_string}, which
has already been discussed separately, above.

@item buffer

A buffer is a sequence of bytes held in computer memory.  For input, two
arguments provide a pointer to the start of the buffer and its byte size.
Note that for charsets using many bytes per character, the size is given
in bytes, not in characters.

For output, three arguments provide the address of three variables, which
will receive the buffer pointer, the used buffer size in bytes, and the
allocated buffer size in bytes.  If at the time of the call, the buffer
pointer is @code{NULL}, then the allocated buffer size should also be zero,
and the buffer will be allocated afresh by the recoding functions.  However,
if the buffer pointer is not @code{NULL}, it should be already allocated,
the allocated buffer size then gives its size.  If the allocated size
gets exceeded while the recoding goes, the buffer will be automatically
reallocated bigger, probably elsewhere, and the allocated buffer size will
be adjusted accordingly.

The second variable, giving the in-memory buffer size, will receive the
exact byte size which was needed for the recoding.  A @code{NUL} character
is guaranteed at the end of the produced buffer, but is not counted in the
byte size of the recoding.  Beyond that @code{NUL}, there might be some
extra space after the recoded data, extending to the allocated buffer size.

@item file

@findex recode_filter_open@r{, not available}
@findex recode_filter_close@r{, not available}
A file is a sequence of bytes held outside computer memory, but
buffered through it.  For input, one argument provides a pointer to a
file already opened for read.  The file is then read and recoded from its
current position until the end of the file, effectively swallowing it in
memory if the destination of the recoding is a buffer.  For reading a file
filtered through the recoding library, but only a little bit at a time, one
should rather use @code{recode_filter_open} and @code{recode_filter_close}
(these two functions are not yet available).

For output, one argument provides a pointer to a file already opened
for write.  The result of the recoding is written to that file starting
at its current position.
@end table
@end itemize

@findex recode_format_table
The following special function is still subject to change:

@example
void recode_format_table (@var{request}, @var{language}, "@var{name}");
@end example

@noindent
and is not documented anymore for now.

@node Task level, Charset level, Request level, Library
@section Task level functions
@cindex task level functions

The task level functions are used internally by the request level
functions, they allow more explicit control over files and memory
buffers holding input and output to recoding processes.  The interface
specification of task level functions is still subject to change a bit.

To get started with task level functions, here is a full example of a
program which sole job is to filter @code{ibmpc} code on its standard input
into @code{latin1} code on its standard output.  That is, this program has
the same goal as the one from the previous section, but does its things
a bit differently.

@example
@group
#include <stdio.h>
#include <stdbool.h>
#include <recodext.h>

const char *program_name;

int
main (int argc, char *const *argv)
@{
  program_name = argv[0];
  RECODE_OUTER outer = recode_new_outer (false);
  RECODE_REQUEST request = recode_new_request (outer);
  RECODE_TASK task;
  bool success;

  recode_scan_request (request, "ibmpc..latin1");

  task = recode_new_task (request);
  task->input.file = "";
  task->output.file = "";
  success = recode_perform_task (task);

  recode_delete_task (task);
  recode_delete_request (request);
  recode_delete_outer (outer);

  exit (success ? 0 : 1);
@}
@end group
@end example

@vindex RECODE_TASK structure
The header file @code{<recode.h>} declares a @code{RECODE_TASK}
structure, which the programmer should use for allocating a variable in
his program.  This @code{task} variable is given as a first argument to
all task level functions.  The programmer ought to change and possibly
consult a few fields in this structure, using special functions.

@itemize @bullet
@item Initialisation functions
@cindex initialisation functions, task

@findex recode_new_task
@findex recode_delete_task
@example
RECODE_TASK recode_new_task (@var{request});
bool recode_delete_task (@var{task});
@end example

No @var{task} variable may be used in other task level functions
of the recoding library without having first been initialised with
@code{recode_new_task}.  There may be many such @var{task} variables,
in which case, they are independent of one another and they all need to be
initialised separately.  To avoid memory leaks, a @var{task} variable should
not be initialised a second time without calling @code{recode_delete_task} to
``un-initialise'' it.  This function also accepts a @var{request} argument
and associates the request to the task.  In fact, a task is essentially
a set of recoding transformations with the specification for its current
input and its current output.

The @var{request} variable may be scanned before or after the call to
@code{recode_new_task}, it does not matter so far.  Immediately after
initialisation, before further changes, the @var{task} variable associates
@var{request} empty in-memory buffers for both input and output.
The output buffer will later get allocated automatically on the fly,
as needed, by various task processors.

Even if a call to @code{recode_delete_task} is not strictly mandatory
before ending the program, it is cleaner to always include it.  Moreover,
in some future version of the recoding library, it might become required.

@item Fields of @code{struct task_request}
@vindex task_request structure

Here are the fields of a @code{struct task_request} which may be meaningfully
changed, once a @var{task} has been initialised by @code{recode_new_task}.
In fact, fields are expected to change.  Once again, to access the fields,
you need to include @file{recodext.h} @emph{instead} of @file{recode.h},
in which case there also is a greater chance that you need to recompile
your programs if a new version of the recoding library gets installed.

@table @code
@item request

The field @code{request} points to the current recoding request, but may
be changed as needed between recoding calls, for example when there is
a need to achieve the construction of a resulting text made up of many
pieces, each being recoded differently.

@item input.name
@itemx input.file

If @code{input.name} is not @code{NULL} at start of a recoding, this is
a request that a file by that name be first opened for reading and later
automatically closed once the whole file has been read. If the file name is
not @code{NULL} but an empty string, it means that standard input is to
be used.  The opened file pointer is then held into @code{input.file}.

If @code{input.name} is @code{NULL} and @code{input.file} is not, than
@code{input.file} should point to a file already opened for read, which
is meant to be recoded.

@item input.buffer
@itemx input.cursor
@itemx input.limit

When both @code{input.name} and @code{input.file} are @code{NULL}, three
pointers describe an in-memory buffer containing the text to be recoded.
The buffer extends from @code{input.buffer} to @code{input.limit},
yet the text to be recoded only extends from @code{input.cursor} to
@code{input.limit}.  In most situations, @code{input.cursor} starts with
the value that @code{input.buffer} has.  (Its value will internally advance
as the recoding goes, until it reaches the value of @code{input.limit}.)

@item output.name
@itemx output.file

If @code{output.name} is not @code{NULL} at start of a recoding, this
is a request that a file by that name be opened for write and later
automatically closed after the recoding is done.  If the file name is
not @code{NULL} but an empty string, it means that standard output is to
be used.  The opened file pointer is then held into @code{output.file}.
If several passes with intermediate files are needed to produce the
recoding, the @code{output.name} file is opened only for the final pass.

If @code{output.name} is @code{NULL} and @code{output.file} is not, then
@code{output.file} should point to a file already opened for write, which
will receive the result of the recoding.

@item output.buffer
@itemx output.cursor
@itemx output.limit

When both @code{output.name} and @code{output.file} are @code{NULL}, three
pointers describe an in-memory buffer meant to receive the text, once it
is recoded.  The buffer is already allocated from @code{output.buffer}
to @code{output.limit}.  In most situations, @code{output.cursor} starts
with the value that @code{output.buffer} has.  Once the recoding is done,
@code{output.cursor} will point at the next free byte in the buffer,
just after the recoded text, so another recoding could be called without
changing any of these three pointers, for appending new information to it.
The number of recoded bytes in the buffer is the difference between
@code{output.cursor} and @code{output.buffer}.

Each time @code{output.cursor} reaches @code{output.limit}, the buffer
is reallocated bigger, possibly at a different location in memory, always
held up-to-date in @code{output.buffer}.  It is still possible to call a
task level function with no output buffer at all to start with, in which
case all three fields should have @code{NULL} as a value.  This is the
situation immediately after a call to @code{recode_new_task}.

@item strategy
@vindex strategy
@vindex RECODE_STRATEGY_UNDECIDED
This field, which is of type @code{enum recode_sequence_strategy}, tells
how various recoding steps (passes) will be interconnected.  Its initial
value is @code{RECODE_STRATEGY_UNDECIDED}, which is a constant defined in
the header file @file{<recodext.h>}.  Other possible values are:

@table @code
@item RECODE_SEQUENCE_IN_MEMORY
@vindex RECODE_SEQUENCE_IN_MEMORY
Keep intermediate recodings in memory.
@item RECODE_SEQUENCE_WITH_FILES
@vindex RECODE_SEQUENCE_WITH_FILES
Do not fork, use intermediate files.
@item RECODE_SEQUENCE_WITH_PIPE
@vindex RECODE_SEQUENCE_WITH_PIPE
Fork processes connected with @code{pipe(2)}.
@end table

@c FIXME
The best for now is to leave this field alone, and let the recoding
library decide its strategy, as many combinations have not been tested yet.

@item byte_order_mark
@vindex byte_order_mark
This field, which is preset to @code{true}, indicates that a byte order
mark is to be expected at the beginning of any canonical @code{UCS-2}
or @code{UTF-16} text, and that such a byte order mark should be also
produced for these charsets.

@item fail_level
@vindex fail_level
This field, which is of type @code{enum recode_error} (@pxref{Errors}),
sets the error level at which task level functions should report a failure.
If an error being detected is equal or greater than @code{fail_level},
the function will eventually return @code{false} instead of @code{true}.
The preset value for this field is @code{RECODE_NOT_CANONICAL}, that means
that if not reset to another value, the library will report failure on
@emph{any} error.

@item abort_level
@vindex abort_level
@vindex RECODE_MAXIMUM_ERROR
This field, which is of type @code{enum recode_error} (@pxref{Errors}), sets
the error level at which task level functions should immediately interrupt
their processing.  If an error being detected is equal or greater than
@code{abort_level}, the function returns immediately, but the returned
value (@code{true} or @code{false}) is still is decided from the setting
of @code{fail_level}, not @code{abort_level}.  The preset value for this
field is @code{RECODE_MAXIMUM_ERROR}, that means that is not reset to
another value, the library will never interrupt a recoding task.

@item error_so_far
@vindex error_so_far
This field, which is of type @code{enum recode_error} (@pxref{Errors}),
maintains the maximum error level met so far while the recoding task
was proceeding.  The preset value is @code{RECODE_NO_ERROR}.
@end table

@item Task execution
@cindex task execution

@findex recode_perform_task
@findex recode_filter_open
@findex recode_filter_close
@example
recode_perform_task (@var{task});
recode_filter_open (@var{task}, @var{file});
recode_filter_close (@var{task});
@end example

The function @code{recode_perform_task} reads as much input as possible,
and recode all of it on prescribed output, given a properly initialised
@var{task}.

Functions @code{recode_filter_open} and @code{recode_filter_close} are
only planned for now.  They are meant to read input in piecemeal ways.
Even if functionality already exists informally in the library, it has
not been made available yet through such interface functions.
@end itemize

@node Charset level, Errors, Task level, Library
@section Charset level functions
@cindex charset level functions

@cindex internal functions
Many functions are internal to the recoding library.  Some of them
have been made external and available, for the @code{recode} program
had to retain all its previous functionality while being transformed
into a mere application of the recoding library.  These functions are
not really documented here for the time being, as we hope that many of
them will vanish over time.  When this set of routines will stabilise,
it would be convenient to document them as an API for handling charset
names and contents.

@findex find_charset
@findex list_all_charsets
@findex list_concise_charset
@findex list_full_charset
@example
RECODE_CHARSET find_charset (@var{name}, @var{cleaning-type});
bool list_all_charsets (@var{charset});
bool list_concise_charset (@var{charset}, @var{list-format});
bool list_full_charset (@var{charset});
@end example

@node Errors,  , Charset level, Library
@section Handling errors
@cindex error handling
@cindex handling errors

@cindex error messages
The @code{recode} program, while using the @code{recode} library, needs to
control whether recoding problems are reported or not, and then reflect
these in the exit status.  The program should also instruct the library
whether the recoding should be abruptly interrupted when an error is
met (so sparing processing when it is known in advance that a wrong
result would be discarded anyway), or if it should proceed nevertheless.
Here is how the library groups errors into levels, listed here in order
of increasing severity.

@table @code
@item RECODE_NO_ERROR
@vindex RECODE_NO_ERROR

No error was met on previous library calls.

@item RECODE_NOT_CANONICAL
@vindex RECODE_NOT_CANONICAL
@cindex non canonical input, error message

The input text was using one of the many alternative codings for some
phenomenon, but not the one @code{recode} would have canonically generated.
So, if the reverse recoding is later attempted, it would produce a text
having the same @emph{meaning} as the original text, yet not being byte
identical.

For example, a @code{Base64} block in which end-of-lines appear elsewhere
that at every 76 characters is not canonical.  An e-circumflex in @TeX{}
which is coded as @samp{\^@{e@}} instead of @samp{\^e} is not canonical.

@item RECODE_AMBIGUOUS_OUTPUT
@vindex RECODE_AMBIGUOUS_OUTPUT
@cindex ambiguous output, error message

It has been discovered that if the reverse recoding was attempted on
the text output by this recoding, we would not obtain the original text,
only because an ambiguity was generated by accident in the output text.
This ambiguity would then cause the wrong interpretation to be taken.

Here are a few examples.  If the @code{Latin-1} sequence @samp{e^}
is converted to Easy French and back, the result will be interpreted
as e-circumflex and so, will not reflect the intent of the original two
characters.  Recoding an @code{IBM-PC} text to @code{Latin-1} and back,
where the input text contained an isolated @kbd{LF}, will have a spurious
@kbd{CR} inserted before the @kbd{LF}.

Currently, there are many cases in the library where the production of
ambiguous output is not properly detected, as it is sometimes a difficult
problem to accomplish this detection, or to do it speedily.

@item RECODE_UNTRANSLATABLE
@vindex RECODE_UNTRANSLATABLE
@cindex untranslatable input, error message

One or more input character could not be recoded, because there is just
no representation for this character in the output charset.

Here are a few examples.  Non-strict mode often allows @code{recode} to
compute on-the-fly mappings for unrepresentable characters, but strict
mode prohibits such attribution of reversible translations: so strict
mode might often trigger such an error.  Most @code{UCS-2} codes used to
represent Asian characters cannot be expressed in various Latin charsets.

@item RECODE_INVALID_INPUT
@vindex RECODE_INVALID_INPUT
@cindex invalid input, error message

The input text does not comply with the coding it is declared to hold.  So,
there is no way by which a reverse recoding would reproduce this text,
because @code{recode} should never produce invalid output.

Here are a few examples.  In strict mode, @code{ASCII} text is not allowed
to contain characters with the eight bit set.  @code{UTF-8} encodings
ought to be minimal@footnote{The minimality of an @code{UTF-8} encoding
is guaranteed on output, but currently, it is not checked on input.}.

@item RECODE_SYSTEM_ERROR
@vindex RECODE_SYSTEM_ERROR
@cindex system detected problem, error message

The underlying system reported an error while the recoding was going on,
likely an input/output error.
(This error symbol is currently unused in the library.)

@item RECODE_USER_ERROR
@vindex RECODE_USER_ERROR
@cindex misuse of recoding library, error message

The programmer or user requested something the recoding library is unable
to provide, or used the API wrongly.
(This error symbol is currently unused in the library.)

@item RECODE_INTERNAL_ERROR
@vindex RECODE_INTERNAL_ERROR
@cindex internal recoding bug, error message

Something really wrong, which should normally never happen, was detected
within the recoding library.  This might be due to genuine bugs in the
library, or maybe due to un-initialised or overwritten arguments to
the API.
(This error symbol is currently unused in the library.)

@item RECODE_MAXIMUM_ERROR
@vindex RECODE_MAXIMUM_ERROR

This error code should never be returned, it is only internally used as
a sentinel for the list of all possible error codes.
@end table

@cindex error level threshold
@cindex threshold for error reporting
One should be able to set the error level threshold for returning failure
at end of recoding, and also the threshold for immediate interruption.
If many errors occur while the recoding proceed, which are not severe
enough to interrupt the recoding, then the most severe error is retained,
while others are forgotten@footnote{Another approach would have been
to define the level symbols as masks instead, and to give masks to
threshold setting routines, and to retain all errors---yet I never
met myself such a need in practice, and so I fear it would be overkill.
On the other hand, it might be interesting to maintain counters about
how many times each kind of error occurred.}.  So, in case of an error,
the possible actions currently are:

@itemize @bullet
@item do nothing and let go, returning success at end of recoding,
@item just let go for now, but return failure at end of recoding,
@item interrupt recoding right away and return failure now.
@end itemize

@noindent
@xref{Task level}, and particularly the description of the fields
@code{fail_level}, @code{abort_level} and @code{error_so_far}, for more
information about how errors are handled.

@ignore
@c FIXME: Take a look at these matters, indeed.

A last topic around errors is where the error related fields are kept.
To work nicely with threads, my feeling is that the main API levels (based
on either of @code{struct recode_outer}, @code{struct recode_request}
or @code{struct recode_task}) should each have their error thresholds,
values, and last explicit message strings.  Thresholds would be inherited
by requests from outers, and by tasks from requests.  Error values and
strings would be automatically propagated out from tasks to requests,
for these request level routines which internally set up and use recoding
tasks.

One simple way to avoid locking while sparing the initialisation of many
identical requests, a programmer could prepare the common request before
splitting threads, and merely @emph{copy} the @code{struct recode_request}
so each thread has its own copy---either using a mere assignment or
@code{memcpy}.  The same could be said for @code{struct recode_outer}
or @code{struct recode_task} blocks, yet it makes less sense to me to do
so in practice.
@end ignore

@node Universal, libiconv, Library, Top
@chapter The universal charset

@cindex ISO 10646
Standard @w{ISO 10646} defines a universal character set, intended to encompass
in the long run all languages written on this planet.  It is based on
wide characters, and offer possibilities for two billion characters
(@math{2^31}).

@tindex UCS
This charset was to become available in @code{recode} under the name
@code{UCS}, with many external surfaces for it.  But in the current
version, only surfaces of @code{UCS} are offered, each presented as a
genuine charset rather than a surface.  Such surfaces are only meaningful
for the @code{UCS} charset, so it is not that useful to draw a line
between the surfaces and the only charset to which they may apply.

@tindex UTF-1
@code{UCS} stands for Universal Character Set.  @code{UCS-2} and
@code{UCS-4} are fixed length encodings, using two or four bytes per
character respectively.  @code{UTF} stands for @code{UCS} Transformation
Format, and are variable length encodings dedicated to @code{UCS}.
@code{UTF-1} was based on @w{ISO 2022}, it did not succeed@footnote{It is not
probable that @code{recode} will ever support @code{UTF-1}.}.  @code{UTF-2}
replaced it, it has been called @code{UTF-FSS} (File System Safe) in
Unicode or Plan9 context, but is better known today as @code{UTF-8}.
To complete the picture, there is @code{UTF-16} based on 16 bits bytes,
and @code{UTF-7} which is meant for transmissions limited to 7-bit bytes.
Most often, one might see @code{UTF-8} used for external storage, and
@code{UCS-2} used for internal storage.

@c FIXME: the manual never explains what the U+NNNN notation means!
When @code{recode} is producing any representation of @code{UCS},
it uses the replacement character @code{U+FFFD} for any @emph{valid}
character which is not representable in the goal charset@footnote{This
is when the goal charset allows for 16-bits.  For shorter charsets,
the @samp{--strict} (@samp{-s}) option decides what happens: either the
character is dropped, or a reversible mapping is produced on the fly.}.
This happens, for example, when @code{UCS-2} is not capable to echo a
wide @code{UCS-4} character, or for a similar reason, an @code{UTF-8}
sequence using more than three bytes.  The replacement character is
meant to represent an existing character.  So, it is never produced to
represent an invalid sequence or ill-formed character in the input text.
In such cases, @code{recode} just gets rid of the noise, while taking note
of the error in its usual ways.

Even if @code{UTF-8} is an encoding, really, it is the encoding of a single
character set, and nothing else.  It is useful to distinguish between an
encoding (a @emph{surface} within @code{recode}) and a charset, but only
when the surface may be applied to several charsets.  Specifying a charset
is a bit simpler than specifying a surface in a @code{recode} request.
There would not be a practical advantage at imposing a more complex syntax
to @code{recode} users, when it is simple to assimilate @code{UTF-8} to
a charset.  Similar considerations apply for @code{UCS-2}, @code{UCS-4},
@code{UTF-16} and @code{UTF-7}.  These are all considered to be charsets.

@menu
* UCS-2::               Universal Character Set, 2 bytes
* UCS-4::               Universal Character Set, 4 bytes
* UTF-7::               Universal Transformation Format, 7 bits
* UTF-8::               Universal Transformation Format, 8 bits
* UTF-16::              Universal Transformation Format, 16 bits
* count-characters::    Frequency count of characters
* dump-with-names::     Fully interpreted UCS dump
@end menu

@node UCS-2, UCS-4, Universal, Universal
@section Universal Character Set, 2 bytes

@tindex UCS-2
@cindex Unicode
One surface of @code{UCS} is usable for the subset defined by its first
sixty thousand characters (in fact, @math{31 * 2^11} codes), and uses
exactly two bytes per character.  It is a mere dump of the internal
memory representation which is @emph{natural} for this subset and as such,
conveys with it endianness problems.

@cindex byte order mark
A non-empty @code{UCS-2} file normally begins with a so called @dfn{byte
order mark}, having value @code{0xFEFF}.  The value @code{0xFFFE} is not an
@code{UCS} character, so if this value is seen at the beginning of a file,
@code{recode} reacts by swapping all pairs of bytes.  The library also
properly reacts to other occurrences of @code{0xFEFF} or @code{0xFFFE}
elsewhere than at the beginning, because concatenation of @code{UCS-2}
files should stay a simple matter, but it might trigger a diagnostic
about non canonical input.

By default, when producing an @code{UCS-2} file, @code{recode} always
outputs the high order byte before the low order byte.  But this could be
easily overridden through the @code{21-Permutation} surface
(@pxref{Permutations}).  For example, the command:

@example
recode u8..u2/21 < @var{input} > @var{output}
@end example

@noindent
asks for an @code{UTF-8} to @code{UCS-2} conversion, with swapped byte
output.

@tindex ISO-10646-UCS-2, and aliases
@tindex BMP
@tindex rune
@tindex u2
Use @code{UCS-2} as a genuine charset.  This charset is available in
@code{recode} under the name @code{ISO-10646-UCS-2}.  Accepted aliases
are @code{UCS-2}, @code{BMP}, @code{rune} and @code{u2}.

@tindex combined-UCS-2
@cindex combining characters
The @code{recode} library is able to combine @code{UCS-2} some sequences
of codes into single code characters, to represent a few diacriticized
characters, ligatures or diphtongs which have been included to ease
mapping with other existing charsets.  It is also able to explode
such single code characters into the corresponding sequence of codes.
The request syntax for triggering such operations is rudimentary and
temporary.  The @code{combined-UCS-2} pseudo character set is a special
form of @code{UCS-2} in which known combinings have been replaced by the
simpler code.  Using @code{combined-UCS-2} instead of @code{UCS-2} in an
@emph{after} position of a request forces a combining step, while using
@code{combined-UCS-2} instead of @code{UCS-2} in a @emph{before} position
of a request forces an exploding step.  For the time being, one has to
resort to advanced request syntax to achieve other effects.  For example:

@example
recode u8..co,u2..u8 < @var{input} > @var{output}
@end example

@noindent
copies an @code{UTF-8} @var{input} over @var{output}, still to be in
@code{UTF-8}, yet merging combining characters into single codes whenever
possible.

@node UCS-4, UTF-7, UCS-2, Universal
@section Universal Character Set, 4 bytes

@tindex UCS-4
Another surface of @code{UCS} uses exactly four bytes per character, and is
a mere dump of the internal memory representation which is @emph{natural}
for the whole charset and as such, conveys with it endianness problems.

@tindex ISO-10646-UCS-4, and aliases
@tindex ISO_10646
@tindex 10646
@tindex u4
Use it as a genuine charset.  This charset is available in @code{recode}
under the name @code{ISO-10646-UCS-4}.  Accepted aliases are @code{UCS},
@code{UCS-4}, @code{ISO_10646}, @code{10646} and @code{u4}.

@node UTF-7, UTF-8, UCS-4, Universal
@section Universal Transformation Format, 7 bits

@tindex UTF-7
@code{UTF-7} comes from IETF rather than ISO, and is described by @w{RFC
2152}, in the MIME series.  The @code{UTF-7} encoding is meant to fit
@code{UCS-2} over channels limited to seven bits per byte.  It proceeds
from a mix between the spirit of @code{Quoted-Printable} and methods of
@code{Base64}, adapted to Unicode contexts.

@tindex UNICODE-1-1-UTF-7, and aliases
@tindex TF-7
@tindex u7
This charset is available in @code{recode} under the name
@code{UNICODE-1-1-UTF-7}.  Accepted aliases are @code{UTF-7}, @code{TF-7}
and @code{u7}.

@node UTF-8, UTF-16, UTF-7, Universal
@section Universal Transformation Format, 8 bits

@tindex UTF-8
Even if @code{UTF-8} does not originally come from IETF, there is now
@w{RFC 2279} to describe it.  In letters sent on 1995-01-21 and 1995-04-20,
Markus Kuhn writes:

@quotation
@code{UTF-8} is an @code{ASCII} compatible multi-byte encoding of the @w{ISO
10646} universal character set (@code{UCS}).  @code{UCS} is a 31-bit superset
of all other character set standards.  The first 256 characters of @code{UCS}
are identical to those of @w{ISO 8859-1} (@w{Latin-1}).  The @code{UCS-2}
encoding of UCS is a sequence of bigendian 16-bit words, the @code{UCS-4}
encoding is a sequence of bigendian 32-bit words.  The @code{UCS-2} subset
of @w{ISO 10646} is also known as ``Unicode''.  As both @code{UCS-2}
and @code{UCS-4} require heavy modifications to traditional @code{ASCII}
oriented system designs (e.g. Unix), the @code{UTF-8} encoding has been
designed for these applications.

In @code{UTF-8}, only @code{ASCII} characters are encoded using bytes
below 128.  All other non-ASCII characters are encoded as multi-byte
sequences consisting only of bytes in the range 128-253.  This avoids
critical bytes like @kbd{NUL} and @kbd{/} in @code{UTF-8} strings, which
makes the @code{UTF-8} encoding suitable for being handled by the standard
C string library and being used in Unix file names.  Other properties
include the preserved lexical sorting order and that @code{UTF-8} allows
easy self-synchronisation of software receiving @code{UTF-8} strings.
@end quotation

@code{UTF-8} is the most common external surface of @code{UCS}, each
character uses from one to six bytes, and is able to encode all @math{2^31}
characters of the @code{UCS}.  It is implemented as a charset, with the
following properties:

@itemize @bullet
@item
Strict 7-bit @code{ASCII} is completely invariant under @code{UTF-8},
and those are the only one-byte characters.  @code{UCS} values and
@code{ASCII} values coincide.  No multi-byte characters ever contain bytes
less than 128.  @code{NUL} @emph{is} @code{NUL}.  A multi-byte character
always starts with a byte of 192 or more, and is always followed by a
number of bytes between 128 to 191.  That means that you may read at
random on disk or memory, and easily discover the start of the current,
next or previous character.  You can count, skip or extract characters
with this only knowledge.

@item
If you read the first byte of a multi-byte character in binary, it contains
many @samp{1} bits in successions starting with the most significant one
(from the left), at least two.  The length of this @samp{1} sequence equals
the byte size of the character.  All succeeding bytes start by @samp{10}.
This is a lot of redundancy, making it fairly easy to guess that a file
is valid @code{UTF-8}, or to safely state that it is not.

@item
In a multi-byte character, if you remove all leading @samp{1} bits of the
first byte of a multi-byte character, and the initial @samp{10} bits of
all remaining bytes (so keeping 6 bits per byte for those), the remaining
bits concatenated are the UCS value.
@end itemize

@noindent
These properties also have a few nice consequences:

@itemize @bullet
@item
Conversion to/from values is algorithmically simple, and reasonably speedy.

@item
A sequence of @var{N} bytes can hold characters needing up to 2 + 5@var{N}
bits in their @code{UCS} representation.  Here, @var{N} is a number between
1 and 6.  So, @code{UTF-8} is most economical when mapping ASCII (1 byte),
followed by @code{UCS-2} (1 to 3 bytes) and @code{UCS-4} (1 to 6 bytes).

@item
The lexicographic sorting order of @code{UCS} strings is preserved.

@item
Bytes with value 254 or 255 never appear, and because of that, these are
sometimes used when escape mechanisms are needed.
@end itemize

In some case, when little processing is done on a lot of strings, one may
choose for efficiency reasons to handle @code{UTF-8} strings directly even
if variable length, as it is easy to get start of characters.  Character
insertion or replacement might require moving the remainder of the string
in either direction.  In most cases, it is faster and easier to convert
from @code{UTF-8} to @code{UCS-2} or @code{UCS-4} prior to processing.

@tindex UTF-8, aliases
@tindex UTF-FSS
@tindex FSS_UTF
@tindex TF-8
@tindex u8
This charset is available in @code{recode} under the name @code{UTF-8}.
Accepted aliases are @code{UTF-2}, @code{UTF-FSS}, @code{FSS_UTF},
@code{TF-8} and @code{u8}.

@node UTF-16, count-characters, UTF-8, Universal
@section Universal Transformation Format, 16 bits

@tindex UTF-16, and aliases
Another external surface of @code{UCS} is also variable length, each
character using either two or four bytes.  It is usable for the subset
defined by the first million characters (@math{17 * 2^16}) of @code{UCS}.

Martin J. D@"urst writes (to @uref{comp.std.internat}, on 1995-03-28):

@quotation
@code{UTF-16} is another method that reserves two times 1024 codepoints in
Unicode and uses them to index around one million additional characters.
@code{UTF-16} is a little bit like former multibyte codes, but quite
not so, as both the first and the second 16-bit code clearly show what
they are.  The idea is that one million codepoints should be enough for
all the rare Chinese ideograms and historical scripts that do not fit
into the Base Multilingual Plane of @w{ISO 10646} (with just about 63,000
positions available, now that 2,000 are gone).
@end quotation

@tindex Unicode, an alias for UTF-16
@tindex TF-16
@tindex u6
This charset is available in @code{recode} under the name @code{UTF-16}.
Accepted aliases are @code{Unicode}, @code{TF-16} and @code{u6}.

@node count-characters, dump-with-names, UTF-16, Universal
@section Frequency count of characters

@tindex count-characters
@cindex counting characters
A device may be used to obtain a list of characters in a file, and how many
times each character appears.  Each count is followed by the @code{UCS-2}
value of the character and, when known, the @w{RFC 1345} mnemonic for that
character.

This charset is available in @code{recode} under the name
@code{count-characters}.

This @code{count} feature has been implemented as a charset.  This may
change in some later version, as it would sometimes be convenient to count
original bytes, instead of their @code{UCS-2} equivalent.

@node dump-with-names,  , count-characters, Universal
@section Fully interpreted UCS dump

@tindex dump-with-names
@cindex dumping characters, with description
@cindex character streams, description
@cindex description of individual characters
Another device may be used to get fully interpreted dumps of an @code{UCS-2}
stream of characters, with one @code{UCS-2} character displayed on a full
output line.  Each line receives the @w{RFC 1345} mnemonic for the character
if it exists, the @code{UCS-2} value of the character, and a descriptive
comment for that character.  As each input character produces its own
output line, beware that the output file from this conversion may be much,
much bigger than the input file.

This charset is available in @code{recode} under the name
@code{dump-with-names}.

This @code{dump-with-names} feature has been implemented as a charset rather
than a surface.  This is surely debatable.  The current implementation
allows for dumping charsets other than @code{UCS-2}.  For example, the
command @w{@samp{recode l2..full < @var{input}}} implies a necessary
conversion from @code{Latin-2} to @code{UCS-2}, as @code{dump-with-names}
is only connected out from @code{UCS-2}.  In such cases, @code{recode}
does not display the original @code{Latin-2} codes in the dump, only the
corresponding @code{UCS-2} values.  To give a simpler example, the command

@example
echo 'Hello, world!' | recode us..dump
@end example

@noindent
produces the following output:

@example
UCS2   Mne   Description

0048   H     latin capital letter h
0065   e     latin small letter e
006C   l     latin small letter l
006C   l     latin small letter l
006F   o     latin small letter o
002C   ,     comma
0020   SP    space
0077   w     latin small letter w
006F   o     latin small letter o
0072   r     latin small letter r
006C   l     latin small letter l
0064   d     latin small letter d
0021   !     exclamation mark
000A   LF    line feed (lf)
@end example

The descriptive comment is given in English and @code{ASCII},
yet if the English description is not available but a French one is, then
the French description is given instead, using @code{Latin-1}.  However,
if the @code{LANGUAGE} or @code{LANG} environment variable begins with
the letters @samp{fr}, then listing preference goes to French when both
descriptions are available.

Here is another example.  To get the long description of the code 237 in
@w{Latin-5} table, one may use the following command.

@example
echo -n 237 | recode l5/d..dump
@end example

@noindent
If your @code{echo} does not grok @samp{-n}, use @samp{echo 237\c} instead.
Here is how to see what Unicode @code{U+03C6} means, while getting rid of
the title lines.

@example
echo -n 0x03C6 | recode u2/x2..dump | tail +3
@end example

@node libiconv, Tabular, Universal, Top
@chapter The @code{iconv} library

@cindex @code{iconv} library
@cindex library, @code{iconv}
@cindex @code{libiconv}
@cindex interface, with @code{iconv} library
@cindex Haible, Bruno
The @code{recode} library itself contains most code and tables from the
portable @code{iconv} library, written by Bruno Haible.  In fact, many
capabilities of the @code{recode} library are duplicated because of this
merging, as the older @code{recode} and @code{iconv} libraries share many
charsets.  We discuss, here, the issues related to this duplication, and
other peculiarities specific to the @code{iconv} library.  The plan is to
remove duplications and better merge specificities, as @code{recode} evolves.

As implemented, if a recoding request can be satisfied by the @code{recode}
library both with and without its @code{iconv} library part, it is likely
that the @code{iconv} library will be used.  To sort out if the @code{iconv}
is indeed used of not, just use the @samp{-v} or @samp{--verbose} option,
@pxref{Recoding}.

@tindex libiconv
The @code{:libiconv:} charset represents a conceptual pivot charset
within the @code{iconv} part of the @code{recode} library (in fact,
this pivot exists, but is not directly reachable).  This charset has a
mere @code{:} (a colon) for an alias.  It is not allowed to recode from
or to this charset directly.  But when this charset is selected as an
intermediate, usually by automatic means, then the @code{iconv} part
of the @code{recode} library is called to handle the transformations.
By using an @samp{--ignore=:libiconv:} option on the @code{recode} call
or equivalently, but more simply, @samp{-x:}, @code{recode} is instructed
to fully avoid this charset as an intermediate, with the consequence that
the @code{iconv} part of the library is defeated.  Consider these two calls:

@example
recode l1..1250 < @var{input} > @var{output}
recode -x: l1..1250 < @var{input} > @var{output}
@end example

@noindent
Both should transform @var{input} from @code{ISO-8859-1} to @code{CP1250}
on @var{output}.  The first call uses the @code{iconv} part of the library,
while the second call avoids it.  Whatever the path used, the results should
normally be identical.  However, there might be observable differences.
Most of them might result from reversibility issues, as the @code{iconv}
engine, which the @code{recode} library directly uses for the time being,
does not address reversibility.  Even if much less likely, some differences
might result from slight errors in the tables used, such differences should
then be reported as bugs.

Other irregularities might be seen in the area of error detection and
recovery.  The @code{recode} library usually tries to detect canonicity
errors in input, and production of ambiguous output, but the @code{iconv}
part of the library currently does not.  Input is always validated, however.
The @code{recode} library may not always react properly when its @code{iconv}
part has no translation for a given character.

Within a collection of names for a single charset, the @code{recode}
library distinguishes one of them as being the genuine charset name,
while the others are said to be aliases.  When @code{recode} lists all
charsets, for example with the @samp{-l} or @samp{--list} option, the list
integrates all @code{iconv} library charsets.  The selection of one of the
aliases as the genuine charset name is an artifact added by @code{recode},
it does not come from @code{iconv}.  Moreover, the @code{recode} library
dynamically resolves some conflicts when it initialises itself at runtime.
This might explain some discrepancies in the table below, as for what is
the genuine charset name.

@include libiconv.texi

@node Tabular, ASCII misc, libiconv, Top
@chapter Tabular sources (@w{RFC 1345})

@cindex RFC 1345
@cindex character mnemonics, documentation
@cindex @code{chset} tools
An important part of the tabular charset knowledge in @code{recode}
comes from @w{RFC 1345} or, alternatively, from the @code{chset} tools,
both maintained by Keld Simonsen.  The @w{RFC 1345} document:

@quotation
``Character Mnemonics & Character Sets'', K. Simonsen, Request for
Comments no. 1345, Network Working Group, June 1992.
@end quotation

@noindent
@cindex deviations from RFC 1345
defines many character mnemonics and character sets.  The @code{recode}
library implements most of @w{RFC 1345}, however:

@itemize @bullet
@item
@tindex dk-us@r{, not recognised by }recode
@tindex us-dk@r{, not recognised by }recode
It does not recognise those charsets which overload character positions:
@code{dk-us} and @code{us-dk}.  However, @xref{Mixed}.

@item
@tindex ANSI_X3.110-1983@r{, not recognised by }recode
@tindex ISO_6937-2-add@r{, not recognised by }recode
@tindex T.101-G2@r{, not recognised by }recode
@tindex T.61-8bit@r{, not recognised by }recode
@tindex iso-ir-90@r{, not recognised by }recode
It does not recognise those charsets which combine two characters for
representing a third: @code{ANSI_X3.110-1983}, @code{ISO_6937-2-add},
@code{T.101-G2}, @code{T.61-8bit}, @code{iso-ir-90} and
@code{videotex-suppl}.

@item
@tindex GB_2312-80@r{, not recognised by }recode
@tindex JIS_C6226-1978@r{, not recognised by }recode
@tindex JIS_X0212-1990@r{, not recognised by }recode
@tindex KS_C_5601-1987@r{, not recognised by }recode
It does not recognise 16-bits charsets: @code{GB_2312-80},
@code{JIS_C6226-1978}, @code{JIS_C6226-1983}, @code{JIS_X0212-1990} and
@code{KS_C_5601-1987}.

@item
@tindex isoir91
@tindex isoir92
It interprets the charset @code{isoir91} as @code{NATS-DANO} (alias
@code{iso-ir-9-1}), @emph{not} as @code{JIS_C6229-1984-a} (alias
@code{iso-ir-91}).  It also interprets the charset @code{isoir92}
as @code{NATS-DANO-ADD} (alias @code{iso-ir-9-2}), @emph{not} as
@code{JIS_C6229-1984-b} (alias @code{iso-ir-92}).  It might be better
just avoiding these two alias names.
@end itemize

Keld Simonsen @email{keld@@dkuug.dk} did most of @w{RFC 1345} himself, with
some funding from Danish Standards and Nordic standards (INSTA) project.
He also did the character set design work, with substantial input from
Olle Jaernefors.  Keld typed in almost all of the tables, some have been
contributed.  A number of people have checked the tables in various
ways.  The RFC lists a number of people who helped.

@cindex @code{recode}, and RFC 1345
Keld and the @code{recode} maintainer have an arrangement by which any new
discovered information submitted by @code{recode} users, about tabular
charsets, is forwarded to Keld, eventually merged into Keld's work,
and only then, reimported into @code{recode}.  Neither the @code{recode}
program nor its library try to compete, nor even establish themselves as
an alternate or diverging reference: @w{RFC 1345} and its new drafts stay the
genuine source for most tabular information conveyed by @code{recode}.
Keld has been more than collaborative so far, so there is no reason that
we act otherwise.  In a word, @code{recode} should be perceived as the
application of external references, but not as a reference in itself.

@tindex RFC1345@r{, a charset, and its aliases}
@tindex 1345
@tindex mnemonic@r{, an alias for RFC1345 charset}
Internally, @w{RFC 1345} associates which each character an unambiguous
mnemonic of a few characters, taken from @w{ISO 646}, which is a minimal
ASCII subset of 83 characters.  The charset made up by these mnemonics
is available in @code{recode} under the name @code{RFC1345}.  It has
@code{mnemonic} and @code{1345} for aliases.  As implemened, this charset
exactly corresponds to @code{mnemonic+ascii+38}, using @w{RFC 1345}
nomenclature.  Roughly said, @w{ISO 646} characters represent themselves,
except for the ampersand (@kbd{&}) which appears doubled.  A prefix of a
single ampersand introduces a mnemonic.  For mnemonics using two characters,
the prefix is immediately by the mnemonic.  For longer mnemonics, the prefix
is followed by an underline (@kbd{_}), the mmemonic, and another underline.
Conversions to this charset are usually reversible.

Currently, @code{recode} does not offer any of the many other possible
variations of this family of representations.  They will likely be
implemented in some future version, however.

@table @code
@include rfc1345.texi
@end table

@node ASCII misc, IBM and MS, Tabular, Top
@chapter ASCII and some derivatives

@menu
* ASCII::               Usual ASCII
* ISO 8859::            ASCII extended by Latin Alphabets
* ASCII-BS::            ASCII 7-bits, @kbd{BS} to overstrike
* flat::                ASCII without diacritics nor underline
@end menu

@node ASCII, ISO 8859, ASCII misc, ASCII misc
@section Usual ASCII

@tindex ASCII@r{, an alias for the }ANSI_X3.4-1968@r{ charset}
@tindex ANSI_X3.4-1968@r{, and its aliases}
@tindex IBM367
@tindex US-ASCII
@tindex cp367
@tindex iso-ir-6
@tindex us
This charset is available in @code{recode} under the name @code{ASCII}.
In fact, it's true name is @code{ANSI_X3.4-1968} as per @w{RFC 1345},
accepted aliases being @code{ANSI_X3.4-1986}, @code{ASCII},
@code{IBM367}, @code{ISO646-US}, @code{ISO_646.irv:1991},
@code{US-ASCII}, @code{cp367}, @code{iso-ir-6} and @code{us}.  The
shortest way of specifying it in @code{recode} is @code{us}.

@cindex ASCII table, recreating with @code{recode}
This documentation used to include ASCII tables.  They have been removed
since the @code{recode} program can now recreate these easily:

@example
recode -lf us                   for commented ASCII
recode -ld us                   for concise decimal table
recode -lo us                   for concise octal table
recode -lh us                   for concise hexadecimal table
@end example

@node ISO 8859, ASCII-BS, ASCII, ASCII misc
@section ASCII extended by Latin Alphabets

@cindex Latin charsets
There are many Latin charsets.  The following has been written by Tim
Lasko @email{lasko@@video.dec.com}, a long while ago:

@quotation
ISO @w{Latin-1}, or more completely ISO Latin Alphabet No 1, is now an
international standard as of February 1987 (IS 8859, Part 1).  For those
American USEnet'rs that care, the 8-bit ASCII standard, which is essentially
the same code, is going through the final administrative processes prior
to publication.  ISO @w{Latin-1} (IS 8859/1) is actually one of an entire
family of eight-bit one-byte character sets, all having ASCII on the left
hand side, and with varying repertoires on the right hand side:

@itemize @bullet
@item
Latin Alphabet No 1 (caters to Western Europe - now approved).
@item
Latin Alphabet No 2 (caters to Eastern Europe - now approved).
@item
Latin Alphabet No 3 (caters to SE Europe + others - in draft ballot).
@item
Latin Alphabet No 4 (caters to Northern Europe - in draft ballot).
@item
Latin-Cyrillic alphabet (right half all Cyrillic - processing currently
suspended pending USSR input).
@item
Latin-Arabic alphabet (right half all Arabic - now approved).
@item
Latin-Greek alphabet (right half Greek + symbols - in draft ballot).
@item
Latin-Hebrew alphabet (right half Hebrew + symbols - proposed).
@end itemize
@end quotation

@tindex Latin-1
The ISO Latin Alphabet 1 is available as a charset in @code{recode} under
the name @code{Latin-1}.  In fact, it's true name is @code{ISO_8859-1:1987}
as per @w{RFC 1345}, accepted aliases being @code{CP819}, @code{IBM819},
@code{ISO-8859-1}, @code{ISO_8859-1}, @code{iso-ir-100}, @code{l1}
and @code{Latin-1}.  The shortest way of specifying it in @code{recode}
is @code{l1}.

@cindex Latin-1 table, recreating with @code{recode}
It is an eight-bit code which coincides with ASCII for the lower half.
This documentation used to include @w{Latin-1} tables.  They have been removed
since the @code{recode} program can now recreate these easily:

@example
recode -lf l1                   for commented ISO Latin-1
recode -ld l1                   for concise decimal table
recode -lo l1                   for concise octal table
recode -lh l1                   for concise hexadecimal table
@end example

@node ASCII-BS, flat, ISO 8859, ASCII misc
@section ASCII 7-bits, @kbd{BS} to overstrike

@tindex ASCII-BS@r{, and its aliases}
@tindex BS@r{, an alias for }ASCII-BS@r{ charset}
This charset is available in @code{recode} under the name
@code{ASCII-BS}, with @code{BS} as an acceptable alias.

@cindex diacritics, with @code{ASCII-BS} charset
The file is straight ASCII, seven bits only.  According to the definition
of ASCII, diacritics are applied by a sequence of three characters: the
letter, one @kbd{BS}, the diacritic mark.  We deviate slightly from this
by exchanging the diacritic mark and the letter so, on a screen device, the
diacritic will disappear and let the letter alone.  At recognition time,
both methods are acceptable.

The French quotes are coded by the sequences: @w{@kbd{< BS "}} or @w{@kbd{"
BS <}} for the opening quote and @w{@kbd{> BS "}} or @w{@kbd{" BS >}}
for the closing quote.  This artificial convention was inherited in
straight @code{ASCII-BS} from habits around @code{Bang-Bang} entry, and
is not well known.  But we decided to stick to it so that @code{ASCII-BS}
charset will not lose French quotes.

The @code{ASCII-BS} charset is independent of @code{ASCII}, and
different.  The following examples demonstrate this, knowing at advance
that @samp{!2} is the @code{Bang-Bang} way of representing an @kbd{e}
with an acute accent.  Compare:

@example
% echo \!2 | recode -v bang..l1/d
Request: Bang-Bang..ISO-8859-1/Decimal-1
233,  10
@end example

@noindent
with:

@example
% echo \!2 | recode -v bang..bs/d
Request: Bang-Bang..ISO-8859-1..ASCII-BS/Decimal-1
 39,   8, 101,  10
@end example

In the first case, the @kbd{e} with an acute accent is merely
transmitted by the @code{Latin-1..ASCII} mapping, not having a special
recoding rule for it.  In the @code{Latin-1..ASCII-BS} case, the acute
accent is applied over the @kbd{e} with a backspace: diacriticised
characters have special rules.  For the @code{ASCII-BS} charset,
reversibility is still possible, but there might be difficult cases.

@node flat,  , ASCII-BS, ASCII misc
@section ASCII without diacritics nor underline
@tindex flat@r{, a charset}

This charset is available in @code{recode} under the name @code{flat}.

@cindex diacritics and underlines, removing
@cindex removing diacritics and underlines
This code is ASCII expunged of all diacritics and underlines, as long as
they are applied using three character sequences, with @kbd{BS} in the
middle.  Also, despite slightly unrelated, each control character is
represented by a sequence of two or three graphic characters.  The newline
character, however, keeps its functionality and is not represented.

Note that charset @code{flat} is a terminal charset.  We can convert
@emph{to} @code{flat}, but not @emph{from} it.

@node IBM and MS, CDC, ASCII misc, Top
@chapter Some IBM or Microsoft charsets

@cindex IBM codepages
@cindex codepages
The @code{recode} program provides various IBM or Microsoft code pages
(@pxref{Tabular}).  An easy way to find them all at once out of the
@code{recode} program itself is through the command:

@example
recode -l | egrep -i '(CP|IBM)[0-9]'
@end example

@noindent
But also, see few special charsets presented in the incoming sections.

@menu
* EBCDIC::              EBCDIC codes
* IBM-PC::              IBM's PC code
* Icon-QNX::            Unisys' Icon code
@end menu

@node EBCDIC, IBM-PC, IBM and MS, IBM and MS
@section EBCDIC code

@cindex EBCDIC charsets
This charset is the IBM's External Binary Coded Decimal for Interchange
Coding.  This is an eight bits code.  The following three variants were
implemented in @code{recode} independently of @w{RFC 1345}:

@table @code
@item EBCDIC
@tindex EBCDIC@r{, a charset}
In @code{recode}, the @code{us..ebcdic} conversion is identical to @samp{dd
conv=ebcdic} conversion, and @code{recode} @code{ebcdic..us} conversion is
identical to @samp{dd conv=ascii} conversion.  This charset also represents
the way Control Data Corporation relates EBCDIC to 8-bits ASCII.

@item EBCDIC-CCC
@tindex EBCDIC-CCC
In @code{recode}, the @code{us..ebcdic-ccc} or @code{ebcdic-ccc..us}
conversions represent the way Concurrent Computer Corporation (formerly
Perkin Elmer) relates EBCDIC to 8-bits ASCII.

@item EBCDIC-IBM
@tindex EBCDIC-IBM
In @code{recode}, the @code{us..ebcdic-ibm} conversion is @emph{almost}
identical to the GNU @samp{dd conv=ibm} conversion.  Given the exact
@samp{dd conv=ibm} conversion table, @code{recode} once said:

@example
Codes  91 and 213 both recode to 173
Codes  93 and 229 both recode to 189
No character recodes to  74
No character recodes to 106
@end example

So I arbitrarily chose to recode 213 by 74 and 229 by 106.  This makes the
@code{EBCDIC-IBM} recoding reversible, but this is not necessarily the best
correction.  In any case, I think that GNU @code{dd} should be amended.
@code{dd} and @code{recode} should ideally agree on the same correction.
So, this table might change once again.
@end table

@w{RFC 1345} brings into @code{recode} 15 other EBCDIC charsets, and 21 other
charsets having EBCDIC in at least one of their alias names.  You can
get a list of all these by executing:

@example
recode -l | grep -i ebcdic
@end example

Note that @code{recode} may convert a pure stream of EBCDIC characters,
but it does not know how to handle binary data between records which
is sometimes used to delimit them and build physical blocks.  If end of
lines are not marked, fixed record size may produce something readable,
but @code{VB} or @code{VBS} blocking is likely to yield some garbage in
the converted results.

@node IBM-PC, Icon-QNX, EBCDIC, IBM and MS
@section IBM's PC code

@tindex IBM-PC
@cindex MS-DOS charsets
@tindex MSDOS
@tindex dos
@tindex pc
This charset is available in @code{recode} under the name @code{IBM-PC},
with @code{dos}, @code{MSDOS} and @code{pc} as acceptable aliases.
The shortest way of specifying it in @code{recode} is @code{pc}.

The charset is aimed towards a PC microcomputer from IBM or any compatible.
This is an eight-bit code.  This charset is fairly old in @code{recode},
its tables were produced a long while ago by mere inspection of a printed
chart of the IBM-PC codes and glyph.

It has @code{CR-LF} as its implied surface.  This means that, if the original
end of lines have to be preserved while going out of @code{IBM-PC}, they
should currently be added back through the usage of a surface on the other
charset, or better, just never removed.  Here are examples for both cases:

@example
recode pc..l2/cl < @var{input} > @var{output}
recode pc/..l2 < @var{input} > @var{output}
@end example

@w{RFC 1345} brings into @code{recode} 44 @samp{IBM} charsets or code pages,
and also 8 other code pages.  You can get a list of these all these by
executing:@footnote{On DOS/Windows, stock shells do not know that apostrophes
quote special characters like @kbd{|}, so one need to use double quotes
instead of apostrophes.}

@example
recode -l | egrep -i '(CP|IBM)[0-9]'
@end example

@noindent
@cindex CR-LF surface, in IBM-PC charsets
@tindex IBM819@r{, and CR-LF surface}
All charset or aliases beginning with letters @samp{CP} or @samp{IBM}
also have @code{CR-LF} as their implied surface.  The same is true for a
purely numeric alias in the same family.  For example, all of @code{819},
@code{CP819} and @code{IBM819} imply @code{CR-LF} as a surface.  Note that
@code{ISO-8859-1} does @emph{not} imply a surface, despite it shares the
same tabular data as @code{819}.

@tindex ibm437
There are a few discrepancies between this @code{IBM-PC} charset and the
very similar @w{RFC 1345} charset @code{ibm437}, which have not been analysed
yet, so the charsets are being kept separate for now.  This might change in
the future, and the @code{IBM-PC} charset might disappear.  Wizards would
be interested in comparing the output of these two commands:

@example
recode -vh IBM-PC..Latin-1
recode -vh IBM437..Latin-1
@end example

@noindent
The first command uses the charset prior to @w{RFC 1345} introduction.
Both methods give different recodings.  These differences are annoying,
the fuzziness will have to be explained and settle down one day.

@node Icon-QNX,  , IBM-PC, IBM and MS
@section Unisys' Icon code

@tindex Icon-QNX@r{, and aliases}
@tindex QNX@r{, an alias for a charset}
This charset is available in @code{recode} under the name
@code{Icon-QNX}, with @code{QNX} as an acceptable alias.

The file is using Unisys' Icon way to represent diacritics with code 25
escape sequences, under the system QNX.  This is a seven-bit code, even
if eight-bit codes can flow through as part of IBM-PC charset.

@node CDC, Micros, IBM and MS, Top
@chapter Charsets for CDC machines

@cindex CDC charsets
@cindex charsets for CDC machines
What is now @code{recode} evolved out, through many transformations
really, from a set of programs which were originally written in
@dfn{COMPASS}, Control Data Corporation's assembler, with bits in FORTRAN,
and later rewritten in CDC 6000 Pascal.  The CDC heritage shows by the
fact some old CDC charsets are still supported.

The @code{recode} author used to be familiar with CDC Scope-NOS/BE and
Kronos-NOS, and many CDC formats.  Reading CDC tapes directly on other
machines is often a challenge, and @code{recode} does not always solve
it.  It helps having tapes created in coded mode instead of binary mode,
and using @code{S} (Stranger) tapes instead of @code{I} (Internal) tapes.
ANSI labels and multi-file tapes might be the source of trouble.  There are
ways to handle a few Cyber Record Manager formats, but some of them might
be quite difficult to decode properly after the transfer is done.

The @code{recode} program is usable only for a small subset of NOS text
formats, and surely not with binary textual formats, like @code{UPDATE}
or @code{MODIFY} sources, for example.  @code{recode} is not especially
suited for reading 8/12 or 56/60 packing, yet this could easily arranged
if there was a demand for it.  It does not have the ability to translate
Display Code directly, as the ASCII conversion implied by tape drivers
or FTP does the initial approximation.  @code{recode} can decode 6/12
caret notation over Display Code already mapped to ASCII.

@menu
* Display Code::        Control Data's Display Code
* CDC-NOS::             ASCII 6/12 from NOS
* Bang-Bang::           ASCII ``bang bang''
@end menu

@node Display Code, CDC-NOS, CDC, CDC
@section Control Data's Display Code

@cindex CDC Display Code, a table
This code is not available in @code{recode}, but repeated here for
reference.  This is a 6-bit code used on CDC mainframes.

@example
Octal display code to graphic       Octal display code to octal ASCII

00  :    20  P    40  5   60  #     00 072  20 120  40 065  60 043
01  A    21  Q    41  6   61  [     01 101  21 121  41 066  61 133
02  B    22  R    42  7   62  ]     02 102  22 122  42 067  62 135
03  C    23  S    43  8   63  %     03 103  23 123  43 070  63 045
04  D    24  T    44  9   64  "     04 104  24 124  44 071  64 042
05  E    25  U    45  +   65  _     05 105  25 125  45 053  65 137
06  F    26  V    46  -   66  !     06 106  26 126  46 055  66 041
07  G    27  W    47  *   67  &     07 107  27 127  47 052  67 046
10  H    30  X    50  /   70  '     10 110  30 130  50 057  70 047
11  I    31  Y    51  (   71  ?     11 111  31 131  51 050  71 077
12  J    32  Z    52  )   72  <     12 112  32 132  52 051  72 074
13  K    33  0    53  $   73  >     13 113  33 060  53 044  73 076
14  L    34  1    54  =   74  @@     14 114  34 061  54 075  74 100
15  M    35  2    55      75  \     15 115  35 062  55 040  75 134
16  N    36  3    56  ,   76  ^     16 116  36 063  56 054  76 136
17  O    37  4    57  .   77  ;     17 117  37 064  57 056  77 073
@end example

In older times, @kbd{:} used octal 63, and octal 0 was not a character.
The table above shows the ASCII glyph interpretation of codes 60 to 77,
yet these 16 codes were once defined differently.

There is no explicit end of line in Display Code, and the Cyber Record
Manager introduced many new ways to represent them, the traditional end of
lines being reachable by setting @code{RT} to @samp{Z}.  If 6-bit bytes
in a file are sequentially counted from 1, a traditional end of line
does exist if bytes 10*@var{n}+9 and 10@var{n}+10 are both zero for a
given @var{n}, in which case these two bytes are not to be interpreted as
@kbd{::}.  Also, up to 9 immediately preceeding zero bytes, going backward,
are to be considered as part of the end of line and not interpreted as
@kbd{:}@footnote{This convention replaced an older one saying that up to 4
immediately preceeding @emph{pairs} of zero bytes, going backward, are to
be considered as part of the end of line and not interpreted as @kbd{::}.}.

@node CDC-NOS, Bang-Bang, Display Code, CDC
@section ASCII 6/12 from NOS

@tindex CDC-NOS@r{, and its aliases}
@tindex NOS
This charset is available in @code{recode} under the name
@code{CDC-NOS}, with @code{NOS} as an acceptable alias.

@cindex NOS 6/12 code
@cindex caret ASCII code
This is one of the charsets in use on CDC Cyber NOS systems to represent
ASCII, sometimes named @dfn{NOS 6/12} code for coding ASCII.  This code is
also known as @dfn{caret ASCII}.  It is based on a six bits character set
in which small letters and control characters are coded using a @kbd{^}
escape and, sometimes, a @kbd{@@} escape.

The routines given here presume that the six bits code is already expressed
in ASCII by the communication channel, with embedded ASCII @kbd{^} and
@kbd{@@} escapes.

Here is a table showing which characters are being used to encode each
ASCII character.

@example
000  ^5  020  ^#  040     060  0  100 @@A  120  P  140  @@G  160  ^P
001  ^6  021  ^[  041  !  061  1  101  A  121  Q  141  ^A  161  ^Q
002  ^7  022  ^]  042  "  062  2  102  B  122  R  142  ^B  162  ^R
003  ^8  023  ^%  043  #  063  3  103  C  123  S  143  ^C  163  ^S
004  ^9  024  ^"  044  $  064  4  104  D  124  T  144  ^D  164  ^T
005  ^+  025  ^_  045  %  065  5  105  E  125  U  145  ^E  165  ^U
006  ^-  026  ^!  046  &  066  6  106  F  126  V  146  ^F  166  ^V
007  ^*  027  ^&  047  '  067  7  107  G  127  W  147  ^G  167  ^W
010  ^/  030  ^'  050  (  070  8  110  H  130  X  150  ^H  170  ^X
011  ^(  031  ^?  051  )  071  9  111  I  131  Y  151  ^I  171  ^Y
012  ^)  032  ^<  052  *  072 @@D  112  J  132  Z  152  ^J  172  ^Z
013  ^$  033  ^>  053  +  073  ;  113  K  133  [  153  ^K  173  ^0
014  ^=  034  ^@@  054  ,  074  <  114  L  134  \  154  ^L  174  ^1
015  ^   035  ^\  055  -  075  =  115  M  135  ]  155  ^M  175  ^2
016  ^,  036  ^^  056  .  076  >  116  N  136 @@B  156  ^N  176  ^3
017  ^.  037  ^;  057  /  077  ?  117  O  137  _  157  ^O  177  ^4
@end example

@node Bang-Bang,  , CDC-NOS, CDC
@section ASCII ``bang bang''

@tindex Bang-Bang
This charset is available in @code{recode} under the name @code{Bang-Bang}.

This code, in use on Cybers at Universit@'e de Montr@'eal mainly, served
to code a lot of French texts.  The original name of this charset is
@dfn{ASCII cod@'e Display}.  This code is also known as @dfn{Bang-bang}.
It is based on a six bits character set in which capitals, French
diacritics and a few others are coded using an @kbd{!} escape followed
by a single character, and control characters using a double @kbd{!}
escape followed by a single character.

The routines given here presume that the six bits code is already expressed
in ASCII by the communication channel, with embedded ASCII @kbd{!}
escapes.

Here is a table showing which characters are being used to encode each
ASCII character.

@example
000 !!@@  020 !!P  040    060 0  100 @@   120 !P  140 !@@ 160 P
001 !!A  021 !!Q  041 !" 061 1  101 !A  121 !Q  141 A  161 Q
002 !!B  022 !!R  042 "  062 2  102 !B  122 !R  142 B  162 R
003 !!C  023 !!S  043 #  063 3  103 !C  123 !S  143 C  163 S
004 !!D  024 !!T  044 $  064 4  104 !D  124 !T  144 D  164 T
005 !!E  025 !!U  045 %  065 5  105 !E  125 !U  145 E  165 U
006 !!F  026 !!V  046 &  066 6  106 !F  126 !V  146 F  166 V
007 !!G  027 !!W  047 '  067 7  107 !G  127 !W  147 G  167 W
010 !!H  030 !!X  050 (  070 8  110 !H  130 !X  150 H  170 X
011 !!I  031 !!Y  051 )  071 9  111 !I  131 !Y  151 I  171 Y
012 !!J  032 !!Z  052 *  072 :  112 !J  132 !Z  152 J  172 Z
013 !!K  033 !![  053 +  073 ;  113 !K  133 [   153 K  173 ![
014 !!L  034 !!\  054 ,  074 <  114 !L  134 \   154 L  174 !\
015 !!M  035 !!]  055 -  075 =  115 !M  135 ]   155 M  175 !]
016 !!N  036 !!^  056 .  076 >  116 !N  136 ^   156 N  176 !^
017 !!O  037 !!_  057 /  077 ?  117 !O  137 _   157 O  177 !_
@end example

@node Micros, Miscellaneous, CDC, Top
@chapter Other micro-computer charsets

@cindex NeXT charsets
The @code{NeXT} charset, which used to be especially provided in releases of
@code{recode} before 3.5, has been integrated since as one @w{RFC 1345} table.

@menu
* Apple-Mac::           Apple's Macintosh code
* AtariST::             Atari ST code
@end menu

@node Apple-Mac, AtariST, Micros, Micros
@section Apple's Macintosh code

@tindex Apple-Mac
@cindex Macintosh charset
This charset is available in @code{recode} under the name @code{Apple-Mac}.
The shortest way of specifying it in @code{recode} is @code{ap}.

The charset is aimed towards a Macintosh micro-computer from Apple.
This is an eight bit code.  The file is the data fork only.  This charset
is fairly old in @code{recode}, its tables were produced a long while ago
by mere inspection of a printed chart of the Macintosh codes and glyph.

@cindex CR surface, in Macintosh charsets
It has @code{CR} as its implied surface.  This means that, if the original
end of lines have to be preserved while going out of @code{Apple-Mac}, they
should currently be added back through the usage of a surface on the other
charset, or better, just never removed.  Here are examples for both cases:

@example
recode ap..l2/cr < @var{input} > @var{output}
recode ap/..l2 < @var{input} > @var{output}
@end example

@w{RFC 1345} brings into @code{recode} 2 other Macintosh charsets.  You can
discover them by using @code{grep} over the output of @samp{recode -l}:

@example
recode -l | grep -i mac
@end example

@noindent
@tindex macintosh@r{, a charset, and its aliases}
@tindex macintosh_ce@r{, and its aliases}
@tindex mac
@tindex macce
Charsets @code{macintosh} and @code{macintosh_ce}, as well as their aliases
@code{mac} and @code{macce} also have @code{CR} as their implied surface.

There are a few discrepancies between the @code{Apple-Mac} charset and
the very similar @w{RFC 1345} charset @code{macintosh}, which have not been
analysed yet, so the charsets are being kept separate for now.  This might
change in the future, and the @code{Apple-Mac} charset might disappear.
Wizards would be interested in comparing the output of these two commands:

@example
recode -vh Apple-Mac..Latin-1
recode -vh macintosh..Latin-1
@end example

@noindent
The first command use the charset prior to @w{RFC 1345} introduction.
Both methods give different recodings.  These differences are annoying,
the fuzziness will have to be explained and settle down one day.

@cindex @code{recode}, a Macintosh port
As a side note, some people ask if there is a Macintosh port of the
@code{recode} program.  I'm not aware of any.  I presume that if the tool
fills a need for Macintosh users, someone will port it one of these days?

@node AtariST,  , Apple-Mac, Micros
@section Atari ST code

@tindex AtariST
This charset is available in @code{recode} under the name @code{AtariST}.

This is the character set used on the Atari ST/TT/Falcon.  This is similar
to @code{IBM-PC}, but differs in some details: it includes some more accented
characters, the graphic characters are mostly replaced by Hebrew characters,
and there is a true German @kbd{sharp s} different from Greek @kbd{beta}.

About the end-of-line conversions: the canonical end-of-line on the
Atari is @samp{\r\n}, but unlike @code{IBM-PC}, the OS makes no
difference between text and binary input/output; it is up to the
application how to interpret the data.  In fact, most of the libraries
that come with compilers can grok both @samp{\r\n} and @samp{\n} as end
of lines.  Many of the users who also have access to Unix systems prefer
@samp{\n} to ease porting Unix utilities.  So, for easing reversibility,
@code{recode} tries to let @samp{\r} undisturbed through recodings.

@node Miscellaneous, Surfaces, Micros, Top
@chapter Various other charsets

Even if these charsets were originally added to @code{recode} for
handling texts written in French, they find other uses.  We did use them
a lot for writing French diacriticised texts in the past, so @code{recode}
knows how to handle these particularly well for French texts.

@menu
* HTML::                World Wide Web representations
* LaTeX::               LaTeX macro calls
* Texinfo::             GNU project documentation files
* Vietnamese::
* African::             African charsets
* Others::
* Texte::               Easy French conventions
* Mule::                Mule as a multiplexed charset
@end menu

@node HTML, LaTeX, Miscellaneous, Miscellaneous
@section World Wide Web representations

@cindex HTML
@cindex SGML
@cindex XML
@cindex Web
@cindex World Wide Web
@cindex WWW
@cindex markup language
@cindex entities
@cindex character entities
@cindex character entity references
@cindex numeric character references
Character entities have been introduced by SGML and made widely popular
through HTML, the markup language in use for the World Wide Web, or Web or
WWW for short.  For representing @emph{unusual} characters, HTML texts use
special sequences, beginning with an ampersand @kbd{&} and ending with a
semicolon @kbd{;}.  The sequence may itself start with a number sigh @kbd{#}
and be followed by digits, so forming a @dfn{numeric character reference},
or else be an alphabetic identifier, so forming a @dfn{character entity
reference}.

The HTML standards have been revised into different HTML levels over time,
and the list of allowable character entities differ in them.  The later XML,
meant to simplify many things, has an option (@samp{standalone=yes}) which
much restricts that list.  The @code{recode} library is able to convert
character references between their mnemonic form and their numeric form,
depending on aimed HTML standard level.  It also can, of course, convert
between HTML and various other charsets.

Here is a list of those HTML variants which @code{recode} supports.
Some notes have been provided by François Yergeau @email{yergeau@@alis.com}.

@table @code
@item XML-standalone
@tindex h0
@tindex XML-standalone
This charset is available in @code{recode} under the name
@code{XML-standalone}, with @code{h0} as an acceptable alias.  It is
documented in section 4.1 of @uref{http://www.w3.org/TR/REC-xml}.
It only knows @samp{&amp;}, @samp{&gt;}, @samp{&lt;}, @samp{&quot;}
and @samp{&apos;}.

@item HTML_1.1
@tindex HTML_1.1
@tindex h1
This charset is available in @code{recode} under the name @code{HTML_1.1},
with @code{h1} as an acceptable alias.  HTML 1.0 was never really documented.

@item HTML_2.0
@tindex HTML_2.0
@tindex RFC1866
@tindex 1866
@tindex h2
This charset is available in @code{recode} under the name @code{HTML_2.0},
and has @code{RFC1866}, @code{1866} and @code{h2} for aliases.  HTML 2.0
entities are listed in @w{RFC 1866}.  Basically, there is an entity for
each @emph{alphabetical} character in the right part of @w{ISO 8859-1}.
In addition, there are four entities for syntax-significant ASCII characters:
@samp{&amp;}, @samp{&gt;}, @samp{&lt;} and @samp{&quot;}.

@item HTML-i18n
@tindex HTML-i18n
@tindex RFC2070
@tindex 2070
This charset is available in @code{recode} under the name
@code{HTML-i18n}, and has @code{RFC2070} and @code{2070} for
aliases.  @w{RFC 2070} added entities to cover the whole right
part of @w{ISO 8859-1}.  The list is conveniently accessible at
@uref{http://www.alis.com:8085/ietf/html/html-latin1.sgml}.  In addition,
four i18n-related entities were added: @samp{&zwnj;} (@samp{&#8204;}),
@samp{&zwj;} (@samp{&#8205;}), @samp{&lrm;} (@samp{&#8206}) and @samp{&rlm;}
(@samp{&#8207;}).

@item HTML_3.2
@tindex HTML_3.2
@tindex h3
This charset is available in @code{recode} under the name
@code{HTML_3.2}, with @code{h3} as an acceptable alias.
@uref{http://www.w3.org/TR/REC-html32.html, HTML 3.2} took up the full
@w{Latin-1} list but not the i18n-related entities from @w{RFC 2070}.

@item HTML_4.0
@tindex h4
@tindex h
This charset is available in @code{recode} under the name @code{HTML_4.0},
and has @code{h4} and @code{h} for aliases.  Beware that the particular
alias @code{h} is not @emph{tied} to HTML 4.0, but to the highest HTML
level supported by @code{recode}; so it might later represent HTML level
5 if this is ever created.  @uref{http://www.w3.org/TR/REC-html40/,
HTML 4.0} has the whole @w{Latin-1} list, a set of entities for
symbols, mathematical symbols, and Greek letters, and another set for
markup-significant and internationalization characters comprising the
4 ASCII entities, the 4 i18n-related from @w{RFC 2070} plus some more.
See @uref{http://www.w3.org/TR/REC-html40/sgml/entities.html}.

@end table

Printable characters from @w{Latin-1} may be used directly in an HTML text.
However, partly because people have deficient keyboards, partly because
people want to transmit HTML texts over non 8-bit clean channels while not
using MIME, it is common (yet debatable) to use character entity references
even for @w{Latin-1} characters, when they fall outside ASCII (that is,
when they have the 8th bit set).

When you recode from another charset to @code{HTML}, beware that all
occurrences of double quotes, ampersands, and left or right angle brackets
are translated into special sequences.  However, in practice, people often
use ampersands and angle brackets in the other charset for introducing
HTML commands, compromising it: it is not pure HTML, not it is pure
other charset.  These particular translations can be rather inconvenient,
they may be specifically inhibited through the command option @samp{-d}
(@pxref{Mixed}).

Codes not having a mnemonic entity are output by @code{recode} using the
@samp{&#@var{nnn};} notation, where @var{nnn} is a decimal representation
of the UCS code value.  When there is an entity name for a character, it
is always preferred over a numeric character reference.  ASCII printable
characters are always generated directly.  So is the newline.  While reading
HTML, @code{recode} supports numeric character reference as alternate
writings, even when written as hexadecimal numbers, as in @samp{&#xfffd}.
This is documented in:

@example
http://www.w3.org/TR/REC-html40/intro/sgmltut.html#h-3.2.3
@end example

When @code{recode} translates to HTML, the translation occurs according to
the HTML level as selected by the goal charset.  When translating @emph{from}
HTML, @code{recode} not only accepts the character entity references known at
that level, but also those of all other levels, as well as a few alternative
special sequences, to be forgiving to files using other HTML standards.

@cindex normilise an HTML file
@cindex HTML normalization
The @code{recode} program can be used to @emph{normalise} an HTML file using
oldish conventions.  For example, it accepts @samp{&AE;}, as this once was a
valid writing, somewhere.  However, it should always produce @samp{&AElig;}
instead of @samp{&AE;}.  Yet, this is not completely true.  If one does:

@example
recode h3..h3 < @var{input}
@end example

@noindent
the operation will be optimised into a mere copy, and you can get @samp{&AE;}
this way, if you had some in your input file.  But if you explicitly defeat
the optimisation, like this maybe:

@example
recode h3..u2,u2..h3 < @var{input}
@end example

@noindent
then @samp{&AE;} should be normalised into @samp{&AElig;} by the operation.

@node LaTeX, Texinfo, HTML, Miscellaneous
@section La@TeX{} macro calls

@tindex LaTeX@r{, a charset}
@tindex ltex
@cindex La@TeX{} files
@cindex @TeX{} files
This charset is available in @code{recode} under the name @code{LaTeX}
and has @code{ltex} as an alias.  It is used for ASCII files coded to be
read by La@TeX{} or, in certain cases, by @TeX{}.

Whenever you recode from another charset to @code{LaTeX}, beware that all
occurrences of backslashes @kbd{\} are translated into the string
@samp{\backslash@{@}}.  However, in practice, people often use backslashes
in the other charset for introducing @TeX{} commands, compromising it:
it is not pure @TeX{}, nor it is pure other charset.  This translation
of backslashes into @samp{\backslash@{@}} can be rather inconvenient,
it may be inhibited through the command option @samp{-d} (@pxref{Mixed}).

@node Texinfo, Vietnamese, LaTeX, Miscellaneous
@section GNU project documentation files

@tindex Texinfo@r{, a charset}
@tindex texi
@tindex ti
@cindex Texinfo files
This charset is available in @code{recode} under the name @code{Texinfo}
and has @code{texi} and @code{ti} for aliases.  It is used by the GNU
project for its documentation.  Texinfo files may be converted into Info
files by the @code{makeinfo} program and into nice printed manuals by
the @TeX{} system.

Even if @code{recode} may transform other charsets to Texinfo, it may
not read Texinfo files yet.  In these times, usages are also changing
between versions of Texinfo, and @code{recode} only partially succeeds
in correctly following these changes.  So, for now, Texinfo support in
@code{recode} should be considered as work still in progress (!).

@node Vietnamese, African, Texinfo, Miscellaneous
@section Vietnamese charsets

@cindex Vietnamese charsets
We are currently experimenting the implementation, in @code{recode}, of a few
character sets and transliterated forms to handle the Vietnamese language.
They are quite briefly summarised, here.

@table @code
@item TCVN
@tindex TCVN@r{, for Vienamese}
@tindex VN1@r{, maybe not available}
@tindex VN2@r{, maybe not available}
@tindex VN3@r{, maybe not available}
The TCVN charset has an incomplete name.  It might be one of the three
charset @code{VN1}, @code{VN2} or @code{VN3}.  Yes @code{VN2} might be a
second version of @code{VISCII}.  To be clarified.

@item VISCII
@tindex VISCII
This is an 8-bit character set which seems to be rather popular for
writing Vietnamese.

@item VPS
@tindex VPS
This is an 8-bit character set for Vietnamese.  No much reference.

@item VIQR
@tindex VIQR
The VIQR convention is a 7-bit, @code{ASCII} transliteration for Vietnamese.

@item VNI
@tindex VNI
The VNI convention is a 8-bit, @code{Latin-1} transliteration for Vietnamese.
@end table

@tindex 1129@r{, not available}
@tindex CP1129@r{, not available}
@tindex 1258@r{, not available}
@tindex CP1258@r{, not available}
Still lacking for Vietnamese in @code{recode}, are the charsets @code{CP1129}
and @code{CP1258}.

@node African, Others, Vietnamese, Miscellaneous
@section African charsets

@cindex African charsets
Some African character sets are available for a few languages, when these
are heavily used in countries where French is also currently spoken.

@tindex AFRFUL-102-BPI_OCIL@r{, and aliases}
@tindex bambara
@tindex bra
@tindex ewondo
@tindex fulfude
@tindex AFRFUL-103-BPI_OCIL@r{, and aliases}
@tindex t-bambara
@tindex t-bra
@tindex t-ewondo
@tindex t-fulfude
One African charset is usable for Bambara, Ewondo and Fulfude, as well
as for French.  This charset is available in @code{recode} under the name
@code{AFRFUL-102-BPI_OCIL}.  Accepted aliases are @code{bambara}, @code{bra},
@code{ewondo} and @code{fulfude}.  Transliterated forms of the same are
available under the name @code{AFRFUL-103-BPI_OCIL}.  Accepted aliases
are @code{t-bambara}, @code{t-bra}, @code{t-ewondo} and @code{t-fulfude}.

@tindex AFRLIN-104-BPI_OCIL
@tindex lingala
@tindex lin
@tindex sango
@tindex wolof
@tindex AFRLIN-105-BPI_OCIL
@tindex t-lingala
@tindex t-lin
@tindex t-sango
@tindex t-wolof
Another African charset is usable for Lingala, Sango and Wolof, as well
as for French.  This charset is available in @code{recode} under the
name @code{AFRLIN-104-BPI_OCIL}.  Accepted aliases are @code{lingala},
@code{lin}, @code{sango} and @code{wolof}.  Transliterated forms of the same
are available under the name @code{AFRLIN-105-BPI_OCIL}.  Accepted aliases
are @code{t-lingala}, @code{t-lin}, @code{t-sango} and @code{t-wolof}.

@tindex AFRL1-101-BPI_OCIL
@tindex t-francais
@tindex t-fra
To ease exchange with @code{ISO-8859-1}, there is a charset conveying
transliterated forms for @w{Latin-1} in a way which is compatible with the other
African charsets in this series.  This charset is available in @code{recode}
under the name @code{AFRL1-101-BPI_OCIL}.  Accepted aliases are @code{t-fra}
and @code{t-francais}.

@node Others, Texte, African, Miscellaneous
@section Cyrillic and other charsets

@cindex Cyrillic charsets
The following Cyrillic charsets are already available in @code{recode}
through @w{RFC 1345} tables: @code{CP1251} with aliases @code{1251}, @code{
ms-cyrl} and @code{windows-1251}; @code{CSN_369103} with aliases
@code{ISO-IR-139} and @code{KOI8_L2}; @code{ECMA-cyrillic} with aliases
@code{ECMA-113}, @code{ECMA-113:1986} and @code{iso-ir-111}, @code{IBM880}
with aliases @code{880}, @code{CP880} and @code{EBCDIC-Cyrillic};
@code{INIS-cyrillic} with alias @code{iso-ir-51}; @code{ISO-8859-5} with
aliases @code{cyrillic}, @code{ ISO-8859-5:1988} and @code{iso-ir-144};
@code{KOI-7}; @code{KOI-8} with alias @code{GOST_19768-74}; @code{KOI8-R};
@code{KOI8-RU} and finally @code{KOI8-U}.

There seems to remain some confusion in Roman charsets for Cyrillic
languages, and because a few users requested it repeatedly, @code{recode}
now offers special services in that area.  Consider these charsets as
experimental and debatable, as the extraneous tables describing them are
still a bit fuzzy or non-standard.  Hopefully, in the long run, these
charsets will be covered in Keld Simonsen's works to the satisfaction of
everybody, and this section will merely disappear.

@table @code
@item KEYBCS2
@tindex KEYBCS2
@tindex Kamenicky
This charset is available under the name @code{KEYBCS2}, with
@code{Kamenicky} as an accepted alias.

@item CORK
@tindex CORK
@tindex T1
This charset is available under the name @code{CORK}, with @code{T1}
as an accepted alias.

@item KOI-8_CS2
@tindex KOI-8_CS2
This charset is available under the name @code{KOI-8_CS2}.
@end table

@node Texte, Mule, Others, Miscellaneous
@section Easy French conventions

@tindex Texte
@tindex txte
This charset is available in @code{recode} under the name @code{Texte}
and has @code{txte} for an alias.  It is a seven bits code, identical
to @code{ASCII-BS}, save for French diacritics which are noted using a
slightly different convention.

At text entry time, these conventions provide a little speed up.  At read
time, they slightly improve the readability over a few alternate ways
of coding diacritics.  Of course, it would better to have a specialised
keyboard to make direct eight bits entries and fonts for immediately
displaying eight bit ISO @w{Latin-1} characters.  But not everybody is so
fortunate.  In a few mailing environments, and sadly enough, it still
happens that the eight bit is often willing-fully destroyed.

@cindex Easy French
Easy French has been in use in France for a while.  I only slightly
adapted it (the diaeresis option) to make it more comfortable to several
usages in Qu@'ebec originating from Universit@'e de Montr@'eal.  In fact,
the main problem for me was not to necessarily to invent Easy French, but
to recognise the ``best'' convention to use, (best is not being defined,
here) and to try to solve the main pitfalls associated with the selected
convention.  Shortly said, we have:

@table @kbd
@item e'
for @kbd{e} (and some other vowels) with an acute accent,
@item e`
for @kbd{e} (and some other vowels) with a grave accent,
@item e^
for @kbd{e} (and some other vowels) with a circumflex accent,
@item e"
for @kbd{e} (and some other vowels) with a diaeresis,
@item c,
for @kbd{c} with a cedilla.
@end table

@noindent
There is no attempt at expressing the @kbd{ae} and @kbd{oe} diphthongs.
French also uses tildes over @kbd{n} and @kbd{a}, but seldomly, and this
is not represented either.  In some countries, @kbd{:} is used instead
of @kbd{"} to mark diaeresis.  @code{recode} supports only one convention
per call, depending on the @samp{-c} option of the @code{recode} command.
French quotes (sometimes called ``angle quotes'') are noted the same way
English quotes are noted in @TeX{}, @emph{id est} by @kbd{``} and @kbd{''}.
No effort has been put to preserve Latin ligatures (@kbd{@ae{}}, @kbd{@oe{}})
which are representable in several other charsets.  So, these ligatures
may be lost through Easy French conventions.

The convention is prone to losing information, because the diacritic
meaning overloads some characters that already have other uses.  To
alleviate this, some knowledge of the French language is boosted into
the recognition routines.  So, the following subtleties are systematically
obeyed by the various recognisers.

@enumerate
@item
A comma which follows a @kbd{c} is interpreted as a cedilla only if it is
followed by one of the vowels @kbd{a}, @kbd{o} or @kbd{u}.

@item
A single quote which follows a @kbd{e} does not necessarily means an acute
accent if it is followed by a single other one.  For example:

@table @kbd
@item e'
will give an @kbd{e} with an acute accent.
@item e''
will give a simple @kbd{e}, with a closing quotation mark.
@item e'''
will give an @kbd{e} with an acute accent, followed by a closing quotation
mark.
@end table

There is a problem induced by this convention if there are English
quotations with a French text.  In sentences like:

@example
There's a meeting at Archie's restaurant.
@end example

the single quotes will be mistaken twice for acute accents.  So English
contractions and suffix possessives could be mangled.

@item
A double quote or colon, depending on @samp{-c} option, which follows a
vowel is interpreted as diaeresis only if it is followed by another letter.
But there are in French several words that @emph{end} with a diaeresis,
and the @code{recode} library is aware of them.  There are words ending in
``igue'', either feminine words without a relative masculine (besaigu@"e
and cigu@"e), or feminine words with a relative masculine@footnote{There
are supposed to be seven words in this case.  So, one is missing.}
(aigu@"e, ambigu@"e, contigu@"e, exigu@"e, subaigu@"e and suraigu@"e).
There are also words not ending in ``igue'', but instead, either ending by
``i''@footnote{Look at one of the following sentences (the second has to
be interpreted with the @samp{-c} option):

@example
"Ai"e!  Voici le proble`me que j'ai"
Ai:e!  Voici le proble`me que j'ai:
@end example

There is an ambiguity between an
@tex
a\"\i,
@end tex
@ifinfo
ai",
@end ifinfo
@c FIXME: why not use @dotless{} here?  It works, AFAIK.
@ignore
a@"{@dotless{i}},
@end ignore
the small animal, and the indicative future of @emph{avoir} (first person
singular), when followed by what could be a diaeresis mark.  Hopefully,
the case is solved by the fact that an apostrophe always precedes the
verb and almost never the animal.}
@tex
(a\"\i, conga\"\i, go\"\i, ha\"\i ka\"\i, inou\"\i, sa\"\i, samura\"\i,
tha\"\i{} and toka\"\i),
@end tex
@ifinfo
(ai", congai", goi", hai"kai", inoui", sai", samurai", thai" and tokai"),
@end ifinfo
@ignore
(a@"{@dotless{i}}, conga@"{@dotless{i}}, go@"{@dotless{i}},
ha@"{@dotless{i}}ka@"{@dotless{i}}, inou@"{@dotless{i}}, sa@"{@dotless{i}},
samura@"{@dotless{i}}, tha@"{@dotless{i}} and toka@"{@dotless{i}}),
@end ignore
ending by ``e'' (cano@"e) or ending by ``u''@footnote{I did not pay
attention to proper nouns, but this one showed up as being fairly evident.}
(Esa@"u).

Just to complete this topic, note that it would be wrong to make a rule
for all words ending in ``igue'' as needing a diaerisis, as there are
counter-examples (becfigue, b@`esigue, bigue, bordigue, bourdigue, brigue,
contre-digue, digue, d'intrigue, fatigue, figue, garrigue, gigue, igue,
intrigue, ligue, prodigue, sarigue and zigue).
@end enumerate

@node Mule,  , Texte, Miscellaneous
@section Mule as a multiplexed charset

@tindex Mule@r{, a charset}
@cindex multiplexed charsets
@cindex super-charsets
This version of @code{recode} barely starts supporting multiplexed or
super-charsets, that is, those encoding methods by which a single text
stream may contain a combination of more than one constituent charset.
The only multiplexed charset in @code{recode} is @code{Mule}, and even
then, it is only very partially implemented: the only correspondence
available is with @code{Latin-1}.  The author fastly implemented this
only because he needed this for himself.  However, it is intended that
Mule support to become more real in subsequent releases of @code{recode}.

Multiplexed charsets are not to be confused with mixed charset texts
(@pxref{Mixed}).  For mixed charset input, the rules allowing to distinguish
which charset is current, at any given place, are kind of informal, and
driven from the semantics of what the file contains.  On the other side,
multiplexed charsets are @emph{designed} to be interpreted fairly precisely,
and quite independently of any informational context.

@cindex MULE, in Emacs
The spelling @code{Mule} originally stands for @cite{@emph{mul}tilingual
@emph{e}nhancement to GNU Emacs}, it is the result of a collective
effort orchestrated by Handa Ken'ichi since 1993.  When @code{Mule} got
rewritten in the main development stream of GNU Emacs 20, the FSF renamed
it @code{MULE}, meaning @cite{@emph{mul}tilingual @emph{e}nvironment
in GNU Emacs}.  Even if the charset @code{Mule} is meant to stay
internal to GNU Emacs, it sometimes breaks loose in external files,
and as a consequence, a recoding tool is sometimes needed.  Within Emacs,
@code{Mule} comes with @code{Leim}, which stands for @cite{@emph{l}ibraries
of @emph{e}macs @emph{i}nput @emph{m}ethods}.  One of these libraries is
named @code{quail}@footnote{Usually, quail means quail egg in Japanese,
while egg alone is usually chicken egg.  Both quail egg and chicken
egg are popular food in Japan.  The @code{quail} input system has
been named because it is smaller that the previous @code{EGG} system.
As for @code{EGG}, it is the translation of @code{TAMAGO}.  This word
comes from the Japanese sentence @cite{@emph{ta}kusan @emph{ma}tasete
@emph{go}mennasai}, meaning @cite{sorry to have let you wait so long}.
Of course, the publication of @code{EGG} has been delayed many times@dots{}
(Story by Takahashi Naoto)}.

@node Surfaces, Internals, Miscellaneous, Top
@chapter All about surfaces
@cindex surface, what it is

@cindex trivial surface
The @dfn{trivial surface} consists of using a fixed number of bits
(often eight) for each character, the bits together hold the integer
value of the index for the character in its charset table.  There are
many kinds of surfaces, beyond the trivial one, all having the purpose
of increasing selected qualities for the storage or transmission.
For example, surfaces might increase the resistance to channel limits
(@code{Base64}), the transmission speed (@code{gzip}), the information
privacy (@code{DES}), the conformance to operating system conventions
(@code{CR-LF}), the blocking into records (@code{VB}), and surely other
things as well@footnote{These are mere examples to explain the concept,
@code{recode} only has @code{Base64} and @code{CR-LF}, actually.}.
Many surfaces may be applied to a stream of characters from a charset,
the order of application of surfaces is important, and surfaces
should be removed in the reverse order of their application.

Even if surfaces may generally be applied to various charsets, some
surfaces were specifically designed for a particular charset, and would
not make much sense if applied to other charsets.  In such cases, these
conceptual surfaces have been implemented as @code{recode} charsets,
instead of as surfaces.  This choice yields to cleaner syntax
and usage.  @xref{Universal}.

@cindex surfaces, implementation in @code{recode}
@tindex data@r{, a special charset}
@tindex tree@r{, a special charset}
Surfaces are implemented within @code{recode} as special charsets
which may only transform to or from the @code{data} or @code{tree}
special charsets.  Clever users may use this knowledge for writing
surface names in requests exactly as if they were pure charsets, when
the only need is to change surfaces without any kind of recoding between
real charsets.  In such contexts, either @code{data} or @code{tree} may
also be used as if it were some kind of generic, anonymous charset: the
request @samp{data..@var{surface}} merely adds the given @var{surface},
while the request @samp{@var{surface}..data} removes it.

@cindex structural surfaces
@cindex surfaces, structural
@cindex surfaces, trees
The @code{recode} library distinguishes between mere data surfaces, and
structural surfaces, also called tree surfaces for short.  Structural
surfaces might allow, in the long run, transformations between a few
specialised representations of structural information like MIME parts,
Perl or Python initialisers, LISP S-expressions, XML, Emacs outlines, etc.

We are still experimenting with surfaces in @code{recode}.  The concept opens
the doors to many avenues; it is not clear yet which ones are worth pursuing,
and which should be abandoned.  In particular, implementation of structural
surfaces is barely starting, there is not even a commitment that tree
surfaces will stay in @code{recode}, if they do prove to be more cumbersome
than useful.  This chapter presents all surfaces currently available.

@menu
* Permutations::        Permuting groups of bytes
* End lines::           Representation for end of lines
* MIME::                MIME contents encodings
* Dump::                Interpreted character dumps
* Test::                Artificial data for testing
@end menu

@node Permutations, End lines, Surfaces, Surfaces
@section Permuting groups of bytes
@cindex permutations of groups of bytes

@cindex byte order swapping
@cindex endiannes, changing
A permutation is a surface transformation which reorders groups of
eight-bit bytes.  A @emph{21} permutation exchanges pairs of successive
bytes.  If the text contains an odd number of bytes, the last byte is
merely copied.  An @emph{4321} permutation inverts the order of quadruples
of bytes.  If the text does not contains a multiple of four bytes, the
remaining bytes are nevertheless permuted as @emph{321} if there are
three bytes, @emph{21} if there are two bytes, or merely copied otherwise.

@table @code
@item 21
@tindex 21-Permutation
@tindex swabytes
This surface is available in @code{recode} under the name
@code{21-Permutation} and has @code{swabytes} for an alias.

@item 4321
@tindex 4321-Permutation
This surface is available in @code{recode} under the name
@code{4321-Permutation}.
@end table

@node End lines, MIME, Permutations, Surfaces
@section Representation for end of lines
@cindex end of line format

The same charset might slightly differ, from one system to another, for
the single fact that end of lines are not represented identically on all
systems.  The representation for an end of line within @code{recode}
is the @code{ASCII} or @code{UCS} code with value 10, or @kbd{LF}.  Other
conventions for representing end of lines are available through surfaces.

@table @code
@item CR
@tindex CR@r{, a surface}
This convention is popular on Apple's Macintosh machines.  When this
surface is applied, each line is terminated by @kbd{CR}, which has
@code{ASCII} value 13.  Unless the library is operating in strict mode,
adding or removing the surface will in fact @emph{exchange} @kbd{CR} and
@kbd{LF}, for better reversibility.  However, in strict mode, the exchange
does not happen, any @kbd{CR} will be copied verbatim while applying
the surface, and any @kbd{LF} will be copied verbatim while removing it.

This surface is available in @code{recode} under the name @code{CR},
it does not have any aliases.  This is the implied surface for the Apple
Macintosh related charsets.

@item CR-LF
@tindex CR-LF@r{, a surface}
This convention is popular on Microsoft systems running on IBM PCs and
compatible.  When this surface is applied, each line is terminated by
a sequence of two characters: one @kbd{CR} followed by one @kbd{LF},
in that order.

@cindex Ctrl-Z, discarding
For compatibility with oldish MS-DOS systems, removing a @code{CR-LF}
surface will discard the first encountered @kbd{C-z}, which has
@code{ASCII} value 26, and everything following it in the text.
Adding this surface will not, however, append a @kbd{C-z} to the result.

@tindex cl
This surface is available in @code{recode} under the name @code{CR-LF}
and has @code{cl} for an alias.  This is the implied surface for the IBM
or Microsoft related charsets or code pages.
@end table

Some other charsets might have their own representation for an end of
line, which is different from @kbd{LF}.  For example, this is the case
of various @code{EBCDIC} charsets, or @code{Icon-QNX}.  The recoding of
end of lines is intimately tied into such charsets, it is not available
separately as surfaces.

@node MIME, Dump, End lines, Surfaces
@section MIME contents encodings
@cindex MIME encodings

@cindex RFC 2045
@w{RFC 2045} defines two 7-bit surfaces, meant to prepare 8-bit messages for
transmission.  Base64 is especially usable for binary entities, while
Quoted-Printable is especially usable for text entities, in those case
the lower 128 characters of the underlying charset coincide with ASCII.

@table @code
@tindex Base64
@tindex b64
@tindex 64
@item Base64
This surface is available in @code{recode} under the name @code{Base64},
with @code{b64} and @code{64} as acceptable aliases.

@item Quoted-Printable
@tindex Quoted-Printable
@tindex quote-printable
@tindex QP
This surface is available in @code{recode} under the name
@code{Quoted-Printable}, with @code{quote-printable} and @code{QP} as
acceptable aliases.
@end table

Note that @code{UTF-7}, which may be also considered as a MIME surface,
is provided as a genuine charset instead, as it necessary relates to
@code{UCS-2} and nothing else.  @xref{UTF-7}.

A little historical note, also showing the three levels of acceptance of
Internet standards.  MIME changed from a ``Proposed Standard'' (@w{RFC
1341--1344}, 1992) to a ``Draft Standard'' (@w{RFC 1521--1523}) in 1993,
and was @emph{recycled} as a ``Draft Standard'' in 1996-11.  It is not yet a
``Full Standard''.

@node Dump, Test, MIME, Surfaces
@section Interpreted character dumps

@cindex dumping characters
Dumps are surfaces meant to express, in ways which are a bit more readable,
the bit patterns used to represent characters.  They allow the inspection
or debugging of character streams, but also, they may assist a bit the
production of C source code which, once compiled, would hold in memory a
copy of the original coding.  However, @code{recode} does not attempt, in
any way, to produce complete C source files in dumps.  User hand editing
or @file{Makefile} trickery is still needed for adding missing lines.
Dumps may be given in decimal, hexadecimal and octal, and be based over
chunks of either one, two or four eight-bit bytes.  Formatting has been
chosen to respect the C language syntax for number constants, with commas
and newlines inserted appropriately.

However, when dumping two or four byte chunks, the last chunk may be
incomplete.  This is observable through the usage of narrower expression
for that last chunk only.  Such a shorter chunk would not be compiled
properly within a C initialiser, as all members of an array share a single
type, and so, have identical sizes.

@table @code
@item Octal-1
@tindex Octal-1
@tindex o1
This surface corresponds to an octal expression of each input byte.

It is available in @code{recode} under the name @code{Octal-1},
with @code{o1} and @code{o} as acceptable aliases.

@item Octal-2
@tindex Octal-2
@tindex o2
This surface corresponds to an octal expression of each pair of
input bytes, except for the last pair, which may be short.

It is available in @code{recode} under the name @code{Octal-2}
and has @code{o2} for an alias.

@item Octal-4
@tindex Octal-4
@tindex o4
This surface corresponds to an octal expression of each quadruple of
input bytes, except for the last quadruple, which may be short.

It is available in @code{recode} under the name @code{Octal-4}
and has @code{o4} for an alias.

@item Decimal-1
@tindex Decimal-1
@tindex d1
This surface corresponds to an decimal expression of each input byte.

It is available in @code{recode} under the name @code{Decimal-1},
with @code{d1} and @code{d} as acceptable aliases.

@item Decimal-2
@tindex Decimal-2
@tindex d2
This surface corresponds to an decimal expression of each pair of
input bytes, except for the last pair, which may be short.

It is available in @code{recode} under the name @code{Decimal-2}
and has @code{d2} for an alias.

@item Decimal-4
@tindex Decimal-4
@tindex d4
This surface corresponds to an decimal expression of each quadruple of
input bytes, except for the last quadruple, which may be short.

It is available in @code{recode} under the name @code{Decimal-4}
and has @code{d4} for an alias.

@item Hexadecimal-1
@tindex Hexadecimal-1
@tindex x1
This surface corresponds to an hexadecimal expression of each input byte.

It is available in @code{recode} under the name @code{Hexadecimal-1},
with @code{x1} and @code{x} as acceptable aliases.

@item Hexadecimal-2
@tindex Hexadecimal-2
@tindex x2
This surface corresponds to an hexadecimal expression of each pair of
input bytes, except for the last pair, which may be short.

It is available in @code{recode} under the name @code{Hexadecimal-2},
with @code{x2} for an alias.

@item Hexadecimal-4
@tindex Hexadecimal-4
@tindex x4
This surface corresponds to an hexadecimal expression of each quadruple of
input bytes, except for the last quadruple, which may be short.

It is available in @code{recode} under the name @code{Hexadecimal-4},
with @code{x4} for an alias.
@end table

When removing a dump surface, that is, when reading a dump results back
into a sequence of bytes, the narrower expression for a short last chunk
is recognised, so dumping is a fully reversible operation.  However, in
case you want to produce dumps by other means than through @code{recode},
beware that for decimal dumps, the library has to rely on the number of
spaces to establish the original byte size of the chunk.

Although the library might report reversibility errors, removing a dump
surface is a rather forgiving process: one may mix bases, group a variable
number of data per source line, or use shorter chunks in places other
than at the
far end.  Also, source lines not beginning with a number are skipped.  So,
@code{recode} should often be able to read a whole C header file, wrapping
the results of a previous dump, and regenerate the original byte string.

@node Test,  , Dump, Surfaces
@section Artificial data for testing

A few pseudo-surfaces exist to generate debugging data out of thin air.
These surfaces are only meant for the expert @code{recode} user, and are
only useful in a few contexts, like for generating binary permutations
from the recoding or acting on them.

@cindex debugging surfaces
Debugging surfaces, @emph{when removed}, insert their generated data
at the beginning of the output stream, and copy all the input stream
after the generated data, unchanged.  This strange removal constraint
comes from the fact that debugging surfaces are usually specified in the
@emph{before} position instead of the @emph{after} position within a request.
With debugging surfaces, one often recodes file @file{/dev/null} in filter
mode.  Specifying many debugging surfaces at once has an accumulation
effect on the output, and since surfaces are removed from right to left,
each generating its data at the beginning of previous output, the net
effect is an @emph{impression} that debugging surfaces are generated from
left to right, each appending to the result of the previous.  In any case,
any real input data gets appended after what was generated.

@table @code
@item test7
@tindex test7
When removed, this surface produces 128 single bytes, the first having
value 0, the second having value 1, and so forth until all 128 values have
been generated.

@item test8
@tindex test8
When removed, this surface produces 256 single bytes, the first having
value 0, the second having value 1, and so forth until all 256 values have
been generated.

@item test15
@tindex test15
When removed, this surface produces 64509 double bytes, the first having
value 0, the second having value 1, and so forth until all values have been
generated, but excluding risky @code{UCS-2} values, like all codes from
the surrogate @code{UCS-2} area (for @code{UTF-16}), the byte order mark,
and values known as invalid @code{UCS-2}.

@item test16
@tindex test16
When removed, this surface produces 65536 double bytes, the first having
value 0, the second having value 1, and so forth until all 65536 values
have been generated.
@end table

As an example, the command @samp{recode l5/test8..dump < /dev/null} is a
convoluted way to produce an output similar to @samp{recode -lf l5}.  It says
to generate all possible 256 bytes and interpret them as @code{ISO-8859-9}
codes, while converting them to @code{UCS-2}.  Resulting @code{UCS-2}
characters are dumped one per line, accompanied with their explicative name.

@node Internals, Concept Index, Surfaces, Top
@chapter Internal aspects

@cindex @code{recode} internals
@cindex internals
The incoming explanations of the internals of @code{recode} should
help people who want to dive into @code{recode} sources for adding new
charsets.  Adding new charsets does not require much knowledge about
the overall organisation of @code{recode}.  You can rather concentrate
of your new charset, letting the remainder of the @code{recode}
mechanics take care of interconnecting it with all others charsets.

If you intend to play seriously at modifying @code{recode}, beware that
you may need some other GNU tools which were not required when you first
installing @code{recode}.  If you modify or create any @file{.l} file,
then you need Flex, and some better @code{awk} like @code{mawk},
GNU @code{awk}, or @code{nawk}.  If you modify the documentation (and
you should!), you need @code{makeinfo}.  If you are really audacious,
you may also want Perl for modifying tabular processing, then @code{m4},
Autoconf, Automake and @code{libtool} for adjusting configuration matters.

@menu
* Main flow::           Overall organisation
* New charsets::        Adding new charsets
* New surfaces::        Adding new surfaces
* Design::              Comments on the library design
@end menu

@node Main flow, New charsets, Internals, Internals
@section Overall organisation
@cindex @code{recode}, main flow of operation

The @code{recode} mechanics slowly evolved for many years, and it
would be tedious to explain all problems I met and mistakes I did all
along, yielding the current behaviour.  Surely, one of the key choices
was to stop trying to do all conversions in memory, one line or one
buffer at a time.  It has been fruitful to use the character stream
paradigm, and the elementary recoding steps now convert a whole stream
to another.  Most of the control complexity in @code{recode} exists
so that each elementary recoding step stays simple, making easier
to add new ones.  The whole point of @code{recode}, as I see it, is
providing a comfortable nest for growing new charset conversions.

@cindex single step
The main @code{recode} driver constructs, while initialising all
conversion modules, a table giving all the conversion routines
available (@dfn{single step}s) and for each, the starting charset and
the ending charset.  If we consider these charsets as being the nodes
of a directed graph, each single step may be considered as oriented
arc from one node to the other.  A cost is attributed to each arc:
for example, a high penalty is given to single steps which are prone
to losing characters, a lower penalty is given to those which need
studying more than one input character for producing an output
character, etc.

Given a starting code and a goal code, @code{recode} computes the most
economical route through the elementary recodings, that is, the best
sequence of conversions that will transform the input charset into the
final charset.  To speed up execution, @code{recode} looks for
subsequences of conversions which are simple enough to be merged, and
then dynamically creates new single steps to represent these mergings.

@cindex double step
A @dfn{double step} in @code{recode} is a special concept representing a
sequence of two single steps, the output of the first single step being the
special charset @code{UCS-2}, the input of the second single step being
also @code{UCS-2}.  Special @code{recode} machinery dynamically produces
efficient, reversible, merge-able single steps out of these double steps.

@cindex recoding steps, statistics
@cindex average number of recoding steps
I made some statistics about how many internal recoding steps are required
between any two charsets chosen at random.  The initial recoding layout,
before optimisation, always uses between 1 and 5 steps.  Optimisation could
sometimes produce mere copies, which are counted as no steps at all.
In other cases, optimisation is unable to save any step.  The number of
steps after optimisation is currently between 0 and 5 steps.  Of course,
the @emph{expected} number of steps is affected by optimisation: it drops
from 2.8 to 1.8.  This means that @code{recode} uses a theoretical average
of a bit less than one step per recoding job.  This looks good.  This was
computed using reversible recodings.  In strict mode, optimisation might
be defeated somewhat.  Number of steps run between 1 and 6, both before
and after optimisation, and the expected number of steps decreases by a
lesser amount, going from 2.2 to 1.3.  This is still manageable.

@node New charsets, New surfaces, Main flow, Internals
@section Adding new charsets
@cindex adding new charsets
@cindex new charsets, how to add

The main part of @code{recode} is written in C, as are most single
steps.  A few single steps need to recognise sequences of multiple
characters, they are often better written in Flex.  It is easy for a
programmer to add a new charset to @code{recode}.  All it requires
is making a few functions kept in a single @file{.c} file,
adjusting @file{Makefile.am} and remaking @code{recode}.

One of the function should convert from any previous charset to the
new one.  Any previous charset will do, but try to select it so you will
not lose too much information while converting.  The other function should
convert from the new charset to any older one.  You do not have to select
the same old charset than what you selected for the previous routine.
Once again, select any charset for which you will not lose too much
information while converting.

If, for any of these two functions, you have to read multiple bytes of the
old charset before recognising the character to produce, you might prefer
programming it in Flex in a separate @file{.l} file.  Prototype your
C or Flex files after one of those which exist already, so to keep the
sources uniform.  Besides, at @code{make} time, all @file{.l} files are
automatically merged into a single big one by the script @file{mergelex.awk}.

There are a few hidden rules about how to write new @code{recode}
modules, for allowing the automatic creation of @file{decsteps.h}
and @file{initsteps.h} at @code{make} time, or the proper merging of
all Flex files.  Mimetism is a simple approach which relieves me of
explaining all these rules!  Start with a module closely resembling
what you intend to do.  Here is some advice for picking up a model.
First decide if your new charset module is to be be driven by algorithms
rather than by tables.  For algorithmic recodings, see @file{iconqnx.c} for
C code, or @file{txtelat1.l} for Flex code.  For table driven recodings,
see @file{ebcdic.c} for one-to-one style recodings, @file{lat1html.c}
for one-to-many style recodings, or @file{atarist.c} for double-step
style recodings.  Just select an example from the style that better fits
your application.

Each of your source files should have its own initialisation function,
named @code{module_@var{charset}}, which is meant to be executed
@emph{quickly} once, prior to any recoding.  It should declare the
name of your charsets and the single steps (or elementary recodings)
you provide, by calling @code{declare_step} one or more times.
Besides the charset names, @code{declare_step} expects a description
of the recoding quality (see @file{recodext.h}) and two functions you
also provide.

The first such function has the purpose of allocating structures,
pre-conditioning conversion tables, etc.  It is also the way of further
modifying the @code{STEP} structure.  This function is executed if and
only if the single step is retained in an actual recoding sequence.
If you do not need such delayed initialisation, merely use @code{NULL}
for the function argument.

The second function executes the elementary recoding on a whole file.
There are a few cases when you can spare writing this function:

@c FIXME: functions file_one_to_one and file_one_to_many don't exist!
@itemize @bullet
@item
@findex file_one_to_one
Some single steps do nothing else than a pure copy of the input onto the
output, in this case, you can use the predefined function
@code{file_one_to_one}, while having a delayed initialisation for
presetting the @code{STEP} field @code{one_to_one} to the predefined
value @code{one_to_same}.

@item
Some single steps are driven by a table which recodes one character into
another; if the recoding does nothing else, you can use the predefined
function @code{file_one_to_one}, while having a delayed initialisation
for presetting the @code{STEP} field @code{one_to_one} with your table.

@item
@findex file_one_to_many
Some single steps are driven by a table which recodes one character into
a string; if the recoding does nothing else, you can use the predefined
function @code{file_one_to_many}, while having a delayed initialisation
for presetting the @code{STEP} field @code{one_to_many} with your table.
@end itemize

If you have a recoding table handy in a suitable format but do not use
one of the predefined recoding functions, it is still a good idea to use
a delayed initialisation to save it anyway, because @code{recode} option
@samp{-h} will take advantage of this information when available.

Finally, edit @file{Makefile.am} to add the source file name of your routines
to the @code{C_STEPS} or @code{L_STEPS} macro definition, depending on
the fact your routines is written in C or in Flex.

@node New surfaces, Design, New charsets, Internals
@section Adding new surfaces
@cindex adding new surfaces
@cindex new surfaces, how to add

Adding a new surface is technically quite similar to adding a new charset.
@xref{New charsets}.  A surface is provided as a set of two transformations:
one from the predefined special charset @code{data} or @code{tree} to the
new surface, meant to apply the surface, the other from the new surface
to the predefined special charset @code{data} or @code{tree}, meant to
remove the surface.

@findex declare_step
Internally in @code{recode}, function @code{declare_step} especially
recognises when a charset is so related to @code{data} or @code{tree},
and then takes appropriate actions so that charset gets indeed installed
as a surface.

@node Design,  , New surfaces, Internals
@section Comments on the library design

@itemize @bullet
@item Why a shared library?
@cindex shared library implementation

There are many different approaches to reduce system requirements to
handle all tables needed in the @code{recode} library.  One of them is to
have the tables in an external format and only read them in on demand.
After having pondered this for a while, I finally decided against it,
mainly because it involves its own kind of installation complexity, and
it is not clear to me that it would be as interesting as I first imagined.

It looks more efficient to see all tables and algorithms already mapped
into virtual memory from the start of the execution, yet not loaded in
actual memory, than to go through many disk accesses for opening various
data files once the program is already started, as this would be needed
with other solutions.  Using a shared library also has the indirect effect
of making various algorithms handily available, right in the same modules
providing the tables.  This alleviates much the burden of the maintenance.

Of course, I would like to later make an exception for only a few tables,
built locally by users for their own particular needs once @code{recode}
is installed.  @code{recode} should just go and fetch them.  But I do not
perceive this as very urgent, yet useful enough to be worth implementing.

Currently, all tables needed for recoding are precompiled into binaries,
and all these binaries are then made into a shared library.  As an initial
step, I turned @code{recode} into a main program and a non-shared library,
this allowed me to tidy up the API, get rid of all global variables, etc.
It required a surprising amount of program source massaging.  But once
this cleaned enough, it was easy to use Gordon Matzigkeit's @code{libtool}
package, and take advantage of the Automake interface to neatly turn the
non-shared library into a shared one.

Sites linking with the @code{recode} library, whose system does not
support any form of shared libraries, might end up with bulky executables.
Surely, the @code{recode} library will have to be used statically, and
might not very nicely usable on such systems.  It seems that progress
has a price for those being slow at it.

There is a locality problem I did not address yet.  Currently, the
@code{recode} library takes many cycles to initialise itself, calling
each module in turn for it to set up associated knowledge about charsets,
aliases, elementary steps, recoding weights, etc.  @emph{Then}, the
recoding sequence is decided out of the command given.  I would not be
surprised if initialisation was taking a perceivable fraction of a second
on slower machines.  One thing to do, most probably not right in version
3.5, but the version after, would have @code{recode} to pre-load all tables
and dump them at installation time.  The result would then be compiled and
added to the library.  This would spare many initialisation cycles, but more
importantly, would avoid calling all library modules, scattered through the
virtual memory, and so, possibly causing many spurious page exceptions each
time the initialisation is requested, at least once per program execution.

@item Why not a central charset?

It would be simpler, and I would like, if something like @w{ISO 10646} was
used as a turning template for all charsets in @code{recode}.  Even if
I think it could help to a certain extent, I'm still not fully sure it
would be sufficient in all cases.  Moreover, some people disagree about
using @w{ISO 10646} as the central charset, to the point I cannot totally
ignore them, and surely, @code{recode} is not a mean for me to force my
own opinions on people.  I would like that @code{recode} be practical
more than dogmatic, and reflect usage more than religions.

Currently, if you ask @code{recode} to go from @var{charset1} to
@var{charset2} chosen at random, it is highly probable that the best path
will be quickly found as:

@example
@var{charset1}..@code{UCS-2}..@var{charset2}
@end example

That is, it will almost always use the @code{UCS} as a trampoline between
charsets.  However, @code{UCS-2} will be immediately be optimised out,
and @var{charset1}..@var{charset2} will often be performed in a single
step through a permutation table generated on the fly for the circumstance
@footnote{If strict mapping is requested, another efficient device will
be used instead of a permutation.}.

In those few cases where @code{UCS-2} is not selected as a conceptual
intermediate, I plan to study if it could be made so.  But I guess some cases
will remain where @code{UCS-2} is not a proper choice.  Even if @code{UCS} is
often the good choice, I do not intend to forcefully restrain @code{recode}
around @code{UCS-2} (nor @code{UCS-4}) for now.  We might come to that
one day, but it will come out of the natural evolution of @code{recode}.
It will then reflect a fact, rather than a preset dogma.

@item Why not @code{iconv}?

@cindex @code{iconv}
The @code{iconv} routine and library allows for converting characters
from an input buffer to an input buffer, synchronously advancing both
buffer cursors.  If the output buffer is not big enough to receive
all of the conversion, the routine returns with the input cursor set at
the position where the conversion could later be resumed, and the output
cursor set to indicate until where the output buffer has been filled.
Despite this scheme is simple and nice, the @code{recode} library does
not offer it currently.  Why not?

When long sequences of decodings, stepwise recodings, and re-encodings
are involved, as it happens in true life, synchronising the input buffer
back to where it should have stopped, when the output buffer becomes full,
is a difficult problem.  Oh, we could make it simpler at the expense of
loosing space or speed: by inserting markers between each input character
and counting them at the output end; by processing only one character in a
time through the whole sequence; by repeatedly attempting to recode various
subsets of the input buffer, binary searching on their length until the
output just fits.  The overhead of such solutions looks fully prohibitive
to me, and the gain very minimal.  I do not see a real advantage, nowadays,
imposing a fixed length to an output buffer.  It makes things so much
simpler and efficient to just let the output buffer size float a bit.

Of course, if the above problem was solved, the @code{iconv} library
should be easily emulated, given that @code{recode} has similar knowledge
about charsets, of course.  This either solved or not, the @code{iconv}
program remains trivial (given similar knowledge about charsets).
I also presume that the @code{genxlt} program would be easy too, but
I do not have enough detailed specifications of it to be sure.

A lot of years ago, @code{recode} was using a similar scheme, and I found
it rather hard to manage for some cases.  I rethought the overall structure
of @code{recode} for getting away from that scheme, and never regretted it.
I perceive @code{iconv} as an artificial solution which surely has some
elegances and virtues, but I do not find it really useful as it stands: one
always has to wrap @code{iconv} into something more refined, extending it
for real cases.  From past experience, I think it is unduly hard to fully
implement this scheme.  It would be awkward that we do contortions for
the sole purpose of implementing exactly its specification, without real,
fairly sounded reasons (other then the fact some people once thought it
was worth standardising).  It is much better to immediately aim for the
refinement we need, without uselessly forcing us into the dubious detour
@code{iconv} represents.

Some may argue that if @code{recode} was using a comprehensive charset
as a turning template, as discussed in a previous point, this would make
@code{iconv} easier to implement.  Some may be tempted to say that the
cases which are hard to handle are not really needed, nor interesting,
anyway.  I feel and fear a bit some pressure wanting that @code{recode}
be split into the part that well fits the @code{iconv} model, and the part
that does not fit, considering this second part less important, with the
idea of dropping it one of these days, maybe.  My guess is that users of
the @code{recode} library, whatever its form, would not like to have such
arbitrary limitations.  In the long run, we should not have to explain
to our users that some recodings may not be made available just because
they do not fit the simple model we had in mind when we did it.  Instead,
we should try to stay opened to the difficulties of real life.  There is
still a lot of complex needs for Asian people, say, that @code{recode}
does not currently address, while it should.  Not only the doors should
stay open, but we should force them wider!
@end itemize

@node Concept Index, Option Index, Internals, Top
@unnumbered Concept Index

@printindex cp

@node Option Index, Library Index, Concept Index, Top
@unnumbered Option Index

This is an alphabetical list of all command-line options accepted by
@code{recode}.

@printindex op

@node Library Index, Charset and Surface Index, Option Index, Top
@unnumbered Library Index

This is an alphabetical index of important functions, data structures,
and variables in the @code{recode} library.

@printindex fn

@node Charset and Surface Index,  , Library Index, Top
@unnumbered Charset and Surface Index

This is an alphabetical list of all the charsets and surfaces supported
by @code{recode}, and their aliases.

@printindex tp

@contents
@bye

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