One document matched: draft-ietf-ftpext-intl-ftp-05.txt
Differences from draft-ietf-ftpext-intl-ftp-04.txt
FTPEXT Working Group B. Curtin
INTERNET DRAFT Defense Information Systems Agency
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Internationalization of the File Transfer Protocol
<draft-ietf-ftpext-intl-ftp-05.txt>
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are
working documents of the Internet Engineering Task Force
(IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of
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(Pacific Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu
(US West Coast).
Distribution of this document is unlimited. Please send
comments to the FTP Extension working group (FTPEXT-WG) of
the Internet Engineering Task Force (IETF) at
<ftp-wg@hops.ag.utk.edu>. Subscription address is
<ftp-wg-request@hops.ag.utk.edu>. Discussions of the group
are archived at <URL:ftp://hops.ag.utk.edu/ftp-wg/archives/>.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as
described in RFC 2119 [RFC 2119].
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Abstract
The File Transfer Protocol, as defined in RFC 959 [RFC959]
and RFC 1123 Section 4 [RFC1123], is one of the oldest and
widely used protocols on the Internet. The protocol's primary
character set, 7 bit ASCII, has served the protocol well
through the early growth years of the Internet. However, as
the Internet becomes more global, there is a need to support
character sets beyond 7 bit ASCII.
This document addresses the internationalization (I18n) of
FTP, which includes supporting the multiple character sets
found throughout the Internet community. This is achieved by
extending the FTP specification and giving recommendations
for proper internationalization support.
Table of Contents
1 INTRODUCTION....................................................3
2 INTERNATIONALIZATION............................................3
2.1 International Character Set.................................4
2.2 Transfer Encoding...........................................4
3 CONFORMANCE.....................................................5
3.1 General.....................................................5
3.2 International Servers.......................................7
3.3 International Clients.......................................7
4 SECURITY........................................................8
5 ACKNOWLEDGMENTS.................................................8
6 GLOSSARY........................................................8
7 BIBLIOGRAPHY....................................................9
8 AUTHOR'S ADDRESS...............................................10
APPENDIX A - IMPLEMENTATION CONSIDERATIONS......................A-1
A.1 General Considerations....................................A-1
A.2 Transition Considerations.................................A-2
APPENDIX B - SAMPLE CODE AND EXAMPLES...........................B-1
B.1 Valid UTF-8 check.........................................B-1
B.2 Conversions...............................................B-2
B.2.1 Conversion from local character set to UTF-8............B-2
B.2.2 Conversion from UTF-8 to local character set............B-5
B.2.3 ISO/IEC 8859-8 Example.................................B-7
B.2.4 Vendor Codepage Example.................................B-7
B.3 Pseudo Code for translating servers.......................B-8
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1 Introduction
As the Internet grows throughout the world the requirement to
support character sets outside of the ASCII [ASCII] / Latin-1
[ISO-8859] character set becomes ever more urgent. For FTP,
because of the large installed base, it is paramount that
this be done without breaking existing clients and servers.
This document addresses this need. In doing so it defines a
solution which will still allow the installed base to
interoperate with new international clients and servers.
This document enhances the capabilities of the File Transfer
Protocol by removing the 7-bit restrictions on pathnames used
in client commands and server responses, recommends the use
of a Universal Character Set (UCS) ISO/IEC 10646 [ISO-10646],
and recommends a UCS transformation format (UTF) UTF-8
[UTF-8].
The recommendations made in this document are consistent with
the recommendations expressed by the 29 Feb - 1 Mar 1996 IAB
Character Set Workshop as expressed in RFC 2130 [RFC 2130].
2 Internationalization
The File Transfer Protocol was developed when the predominate
character sets were 7 bit ASCII and 8 bit EBCDIC. Today these
character sets cannot support the wide range of characters
needed by multinational systems. Given that there are a
number of character sets in current use that provide more
characters than 7-bit ASCII, it makes sense to decide on a
convenient way to represent the union of those possibilities.
To work globally either requires support of a number of
character sets and to be able to convert between them, or the
use of a single preferred character set. To assure global
interoperability this document RECOMMENDS the latter approach
and defines a single character set, in addition to NVT ASCII
and EBCDIC, which is understandable by all systems. For FTP
this character set SHALL be ISO/IEC 10646:1993. For support
of global compatibility it is STRONGLY RECOMMENDED that
clients and servers use UTF-8 encoding when exchanging
pathnames. Clients and servers are, however, under no
obligation to perform any conversion on the contents of a
file for operations such as STOR or RETR.
The character set used to store files SHALL remain a local
decision and MAY depend on the capability of local operating
systems. Prior to the exchange of pathnames they should be
converted into a ISO/IEC 10646 format and UTF-8 encoded. This
approach, while allowing international exchange of pathnames,
will still allow backward compatibility with older systems
because the code set positions for ASCII characters are
identical to the one byte sequence in UTF-8.
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Sections 2.1 and 2.2 give a brief description of the
international character set and transfer encoding recommended
by this document. A more thorough description of UTF-8,
ISO/IEC 10646, and UNICODE [UNICODE], beyond that given in
this document, can be found in RFC 2279 [RFC2279].
2.1 International Character Set
The character set defined for international support of FTP
SHALL be the Universal Character Set as defined in ISO
10646:1993 as amended. This standard incorporates the
character sets of many existing international, national, and
corporate standards. ISO/IEC 10646 defines two alternate
forms of encoding, UCS-4 and UCS-2. UCS-4 is a four byte (31
bit) encoding containing 2**31 code positions divided into
128 groups of 256 planes. Each plane consists of 256 rows of
256 cells. UCS-2 is a 2 byte (16 bit) character set
consisting of plane zero or the Basic Multilingual Plane
(BMP). Currently, no codesets have been defined outside of
the 2 byte BMP.
The Unicode standard version 2.0 [UNICODE] is consistent with
the UCS-2 subset of ISO/IEC 10646. The Unicode standard
version 2.0 includes the repertoire of IS 10646 characters,
amendments 1-7 of IS 10646, and editorial and technical
corrigenda.
2.2 Transfer Encoding
UCS Transformation Format 8 (UTF-8), in the past referred to
as UTF-2 or UTF-FSS, SHALL be used as a transfer encoding to
transmit the international character set. UTF-8 is a file
safe encoding which avoids the use of byte values that have
special significance during the parsing of pathname character
strings. UTF-8 is an 8 bit encoding of the characters in the
UCS. Some of UTF-8's benefits are that it is compatible with
7 bit ASCII, so it doesn't affect programs that give special
meanings to various ASCII characters; it is immune to
synchronization errors; its encoding rules allow for easy
identification; and it has enough space to support a large
number of character sets.
UTF-8 encoding represents each UCS character as a sequence of
1 to 6 bytes in length. For all sequences of one byte the
most significant bit is ZERO. For all sequences of more than
one byte the number of ONE bits in the first byte, starting
from the most significant bit position, indicates the number
of bytes in the UTF-8 sequence followed by a ZERO bit. For
example, the first byte of a 3 byte UTF-8 sequence would have
1110 as its most significant bits. Each additional bytes
(continuing bytes) in the UTF-8 sequence, contain a ONE bit
followed by a ZERO bit as their most significant bits. The
remaining free bit positions in the continuing bytes are used
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to identify characters in the UCS. The relationship between
UCS and UTF-8 is demonstrated in the following table:
UCS-4 range(hex) UTF-8 byte sequence(binary)
00000000 - 0000007F 0xxxxxxx
00000080 - 000007FF 110xxxxx 10xxxxxx
00000800 - 0000FFFF 1110xxxx 10xxxxxx 10xxxxxx
00010000 - 001FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
00200000 - 03FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx
10xxxxxx
04000000 - 7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx
10xxxxxx 10xxxxxx
A beneficial property of UTF-8 is that its single byte
sequence is consistent with the ASCII character set. This
feature will allow a transition where old ASCII-only clients
can still interoperate with new servers that support the
UTF-8 encoding.
Another feature is that the encoding rules make it very
unlikely that a character sequence from a different character
set will be mistaken for a UTF-8 encoded character sequence.
Clients and servers can use a simple routine to determine if
the character set being exchanged is valid UTF-8. Section B.1
shows a code example of this check.
3 Conformance
3.1 General
- The 7-bit restriction for pathnames exchanged is dropped.
- Many operating system allow the use of spaces <SP>,
carriage return <CR>, and line feed <LF> characters as part
of the pathname. The exchange of pathnames with these
special command characters will cause the pathnames to be
parsed improperly. This is because ftp commands associated
with pathnames have the form:
COMMAND <SP> <pathname> <CRLF>.
To allow the exchange of pathnames containing these
characters, the definition of pathname is changed from
<pathname> ::= <string> ; in BNF format
to
pathname = 1*(%x01..%xFF) ; in ABNF format [ABNF]
To avoid mistaking these characters within pathnames as
special command characters the following rules will apply:
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There MUST be only one <SP> between a ftp command and the
pathname. Implementations MUST assume <SP> characters
following the initial <SP> as part of the pathname. For
example the pathname in STOR <SP><SP><SP>foo.bar<CRLF> is
<SP><SP>foo.bar .
Current implementations, which may allow multiple <SP>
characters as separators between the command and
pathname, MUST assure that they comply with this single
<SP> convention. Note: Implementations which treat 3
character commands (e.g. CWD, MKD, etc.) as a fixed 4
character command by padding the command with a trailing
<SP> are in non-compliance to this specification.
When a <CR> character is encountered as part of a pathname
it MUST be padded with a <NUL> character prior to sending
the command. On receipt of a pathname containing a <CR><NUL>
sequence the <NUL> character MUST be stripped away. This
approach is described in the Telnet protocol [RFC854] on
pages 11 and 12. For example, to store a pathname
foo<CR><LF>boo.bar the pathname would become
foo<CR><NUL><LF>boo.bar prior to sending the command STOR
<SP>foo<CR><NUL><LF>boo.bar<CRLF> .
Upon receipt of the altered pathname the <NUL> character
following the <CR> would be stripped away to form the
original pathname.
- Conforming internationalized clients and servers MUST
support UTF-8 for the transfer and receipt of pathnames.
Clients and servers MAY in addition give users a choice of
specifying interpretation of pathnames in another encoding.
Note that configuring clients and servers to use character
sets / encoding other than UTF-8 is outside of the scope of
this document. While it is recognized that in certain
operational scenarios this may be desirable, this is left as
a quality of implementation and operational issue.
- Pathnames are sequences of bytes. The encoding of names
that are valid UTF-8 sequences is assumed to be UTF-8. The
character set of other names is undefined. Clients and
servers, unless otherwise configured to support a specific
native character set, MUST check for a valid UTF-8 byte
sequence to determine if the pathname being presented is
UTF-8.
- To avoid data loss, clients and servers SHOULD use the UTF-
8 encoded pathnames when unable to convert them to a usable
code set.
- There may be cases when the code set / encoding presented
to the server or client cannot be determined. In such cases
the raw bytes SHOULD be used.
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3.2 International Servers
- Servers MUST support the UTF-8 feature in response to the
FEAT command [FEAT]. The UTF-8 feature is a line containing
the exact string "UTF8". This string is not case sensitive,
but SHOULD be transmitted in upper case. The response to a
FEAT command SHOULD be:
C> feat
S> 211- <any descriptive text>
S> ...
S> UTF8
S> ...
S> 211 end
The ellipses indicate placeholders where other features may
be included, and are not required. The one space indentation
of the feature lines is mandatory [FEAT].
- Mirror servers may want to exactly reflect the site that
they are mirroring. In such cases servers MAY store and
present the exact pathname bytes that it received from the
main server.
3.3 International Clients
- Clients which do not require display of pathnames are under
no obligation to do so. Non-display clients do not need to
conform to requirements associated with display.
- Clients, which are presented UTF-8 pathnames by the server,
SHOULD parse UTF-8 correctly and attempt to display the
pathname within the limitation of the resources available.
- Clients MUST support the FEAT command and recognize the
"UTF8" feature (defined in 3.2 above) to determine if a
server supports UTF-8 encoding.
- Character semantics of other names shall remain undefined.
If a client detects that a server is non UTF-8, it SHOULD
change its display appropriately. How a client
implementation handles non UTF-8 is a quality of
implementation issue. It MAY try to assume some other
encoding, give the user a chance to try to assume something,
or save encoding assumptions for a server from one FTP
session to another.
- Glyph rendering is outside the scope of this document. How
a client presents characters it cannot display is a quality
of implementation issue. This document RECOMMENDS that
octets corresponding to non-displayable characters SHOULD be
presented in URL %HH format defined in RFC 1738 [RFC1738].
They MAY, however, display them as question marks, with
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their UCS hexadecimal value, or in any other suitable
fashion.
- Many existing clients interpret 8-bit pathnames as being in
the local character set. They MAY continue to do so for
pathnames that are not valid UTF-8.
4 Security
This document addresses the support of character sets beyond
1 byte. Conformance to this document should not induce a
security threat.
5 Acknowledgments
The following people have contributed to this document:
D. J. Bernstein
Martin J. Duerst
Mark Harris
Paul Hethmon
Alun Jones
James Matthews
Keith Moore
Sandra O'Donnell
Benjamin Riefenstahl
Stephen Tihor
(and others from the FTPEXT working group)
6 Glossary
BIDI - abbreviation for Bi-directional, a reference to mixed
right-to-left and left-to-right text.
Character Set - a collection of characters used to represent
textual information in which each character has a numeric
value
Code Set - (see character set).
Glyph - a character image represented on a display device.
I18N - "I eighteen N", the first and last letters of the word
"internationalization" and the eighteen letters in between.
UCS-2 - the ISO/IEC 10646 two octet Universal Character Set
form.
UCS-4 - the ISO/IEC 10646 four octet Universal Character Set
form.
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UTF-8 - the UCS Transformation Format represented in 8 bits.
UTF-16 - A 16-bit format including the BMP (directly encoded)
and surrogate pairs to represent characters in planes 01-16;
equivalent to Unicode.
7 Bibliography
[ABNF]
D. Crocker, P. Overell, Augmented BNF for Syntax
Specifications: ABNF, RFC 2234, November 1997.
[ASCII]
ANSI X3.4:1986 Coded Character Sets - 7 Bit American
National Standard Code for Information Interchange (7-bit
ASCII)
[FEAT]
R. Elz, P. Hethmon, "Feature Negotiation Mechanism for the
File Transfer Protocol", Work in Progress, <draft-ietf-
ftpext-feat-02.txt> November 1997.
[ISO-8859]
ISO 8859. International standard -- Information processing
-- 8-bit single-byte coded graphic character sets -- Part 1:
Latin alphabet No. 1 (1987) -- Part 2: Latin alphabet No. 2
(1987) -- Part 3: Latin alphabet No. 3 (1988) -- Part 4:
Latin alphabet No. 4 (1988) -- Part 5: Latin/Cyrillic
alphabet (1988) -- Part 6: Latin/Arabic alphabet (1987) --
Part : Latin/Greek alphabet (1987) -- Part 8: Latin/Hebrew
alphabet (1988) -- Part 9: Latin alphabet No. 5 (1989) --
Part10: Latin alphabet No. 6 (1992)
[ISO-10646]
ISO/IEC 10646-1:1993. International standard -- Information
technology -- Universal multiple-octet coded character set
(UCS) -- Part 1: Architecture and basic multilingual plane.
[RFC854]
J. Postel, J Reynolds, "Telnet Protocol Specification", RFC
854, May 1983.
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[RFC959]
J. Postel, J Reynolds, "File Transfer Protocol (FTP)", RFC
959, October 1985.
[RFC1123]
R. Braden, "Requirements for Internet Hosts -- Application
and Support", RFC 1123, October 1989.
[RFC1738]
T. Berners-Lee, L. Masinter, M.McCahill, "Uniform Resource
Locators (URL)", RFC 1738, December 1994.
[RFC2279]
F. Yergeau, "UTF-8, a transformation format of ISO 10646",
RFC 2279, January 1998.
[RFC 2119]
S. Bradner, " Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC 2130]
C. Weider, C. Preston, K.Simonsen, H. Alvestrand, " The
Report of the IAB Character Set Workshop held 29 February -
1 March, 1996", RFC 2130, April, 1997.
[UNICODE]
The Unicode Consortium, "The Unicode Standard - Version
2.0", Addison Westley Developers Press, July 1996.
[UTF-8]
ISO/IEC 10646-1:1993 AMENDMENT 2 (1996). UCS Transformation
Format 8 (UTF-8).
8 Author's Address
JIEO
Attn JEBBD (Bill Curtin)
Ft. Monmouth, N.J.
07703-5613
curtinw@ftm.disa.mil
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Annex A - Implementation Considerations
A.1 General Considerations
- Implementers should ensure that their code accounts for
potential problems, such as using a NULL character to
terminate a string or no longer being able to steal the high
order bit for internal use, when supporting the extended
character set.
- Implementers should be aware that there is a chance that
pathnames that are non UTF-8 may be parsed as valid UTF-8.
The probabilities are low for some encoding or statistically
zero to zero for others. A recent non-scientific analysis
found that EUC encoded Japanese words had a 2.7% false
reading; SJIS had a 0.0005% false reading; other encoding
such as ASCII or KOI-8 have a 0% false reading. This
probability is highest for short pathnames and decreases as
pathname size increases. Implementers may want to look for
signs that pathnames which parse as UTF-8 are not valid UTF-
8, such as the existence of multiple local character sets in
short pathnames. Hopefully, as more implementations conform
to UTF-8 transfer encoding there will be a smaller need to
guess at the encoding.
- Client developers should be aware that it will be possible
for pathnames to contain mixed characters (e.g.
/Latin1DirectoryName/HebrewFileName). They should be
prepared to handle the Bi-directional (BIDI) display of
these character sets (i.e. right to left display for the
directory and left to right display for the filename). While
bi-directional display is outside the scope of this document
and more complicated than the above example, an algorithm
for bi-directional display can be found in the UNICODE 2.0
[UNICODE] standard. Also note that pathnames can have
different byte ordering yet be logically and display-wise
equivalent due to the insertion of BIDI control characters
at different points during composition. Also note that mixed
character sets may also present problems with font swapping.
- A server that copies pathnames transparently from a local
filesystem may continue to do so. It is then up to the local
file creators to use UTF-8 pathnames.
- Servers can supports charset labeling of files and/or
directories, such that different pathnames may have
different charsets. The server should attempt to convert all
pathnames to UTF-8, but if it can't then it should leave
that name in its raw form.
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- Some server's OS do not mandate character sets, but allow
administrators to configure it in the FTP server. These
servers should be configured to use a particular mapping
table (either external or built-in). This will allow the
flexibility of defining different charsets for different
directories.
- If the server's OS does not mandate the character set and
the FTP server cannot be configured, the server should
simply use the raw bytes in the file name. They might be
ASCII or UTF-8.
- If the server is a mirror, and wants to look just like the
site it is mirroring, it should store the exact file name
bytes that it received from the main server.
A.2 Transition Considerations
-Clients and servers can transition to UTF-8 by either
converting to/from the local encoding, or the users can
store UTF-8 filenames. The former approach is easier on
tightly controlled file systems (e.g. PCs and MACs). The
latter approach is easier on more free form file systems
(e.g. Unix).
-For interactive use attention should be focused on user
interface and ease of use. Non-interactive use requires a
consistent and controlled behavior.
-There may be many applications which reference files under
their old raw pathname (e.g. linked URLs). Changing the
pathname to UTF-8 will cause access to the old URL to fail.
A solution may be for the server to act as if there was 2
different pathnames associated with the file. This might be
done internal to the server on controlled file systems or by
using symbolic links on free form systems. While this
approach may work for single file transfer non-interactive
use, a non-interactive transfer of all of the files in a
directory will produce duplicates. Interactive users may be
presented with lists of files which are double the actual
number files.
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Annex B - Sample Code and Examples
B.1 Valid UTF-8 check
The following routine checks if a byte sequence is valid UTF-
8. This is done by checking for the proper tagging of the
first and following bytes to make sure they conform to the
UTF-8 format. It then checks to assure that the data part of
the UTF-8 sequence conforms to the proper range allowed by
the encoding. Note: This routine will not detect characters
that have not been assigned and therefore do not exist.
int utf8_valid(const unsigned char *buf, unsigned int len)
{
const unsigned char *endbuf = buf + len;
unsigned char byte2mask=0x00, c;
int trailing = 0; // trailing (continuation)
bytes to follow
while (buf != endbuf)
{
c = *buf++;
if (trailing)
if ((c&0xC0) == 0x80) // Does trailing byte follow UTF-8
format?
{if (byte2mask) // Need to check 2nd byte for
proper range?
if (c&byte2mask) // Are appropriate bits set?
byte2mask=0x00;
else
return 0;
trailing--; }
else
return 0;
else
if ((c&0x80) == 0x00) continue; // valid 1 byte
UTF-8
else if ((c&0xE0) == 0xC0) // valid 2 byte
UTF-8
if (c&0x1E) // Is UTF-8 byte in
proper range?
trailing =1;
else
return 0;
else if ((c&0xF0) == 0xE0) // valid 3 byte
UTF-8
{if (!(c&0x0F)) // Is UTF-8 byte in
proper range?
byte2mask=0x20; // If not set mask
to check next byte
trailing = 2;}
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else if ((c&0xF8) == 0xF0) // valid 4 byte
UTF-8
{if (!(c&0x07)) // Is UTF-8 byte in
proper range?
byte2mask=0x30; // If not set mask
to check next byte
trailing = 3;}
else if ((c&0xFC) == 0xF8) // valid 5 byte
UTF-8
{if (!(c&0x03)) // Is UTF-8 byte in
proper range?
byte2mask=0x38; // If not set mask
to check next byte
trailing = 4;}
else if ((c&0xFE) == 0xFC) // valid 6 byte
UTF-8
{if (!(c&0x01)) // Is UTF-8 byte in
proper range?
byte2mask=0x3C; // If not set mask
to check next byte
trailing = 5;}
else return 0;
}
return trailing == 0;
}
B.2 Conversions
The code examples in this section closely reflect the
algorithm in ISO 10646 and may not present the most efficient
solution for converting to / from UTF-8 encoding. If
efficiency is an issue, implementers should use the
appropriate bitwise operators.
Additional code examples and numerous mapping tables can be
found at the Unicode site, HTTP://www.unicode.org or
FTP://unicode.org.
Note that the conversion examples below assume that the local
character set supported in the operating system is something
other than UCS2/UTF-16. There are some operating systems that
already support UCS2/UTF-16 (notably Plan 9 and Windows NT).
In this case no conversion will be necessary from the local
character set to the UCS.
B.2.1 Conversion from local character set to UTF-8
Conversion from the local filesystem character set to UTF-8
will normally involve a two step process. First convert the
local character set to the UCS; then convert the UCS to
UTF-8.
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The first step in the process can be performed by maintaining
a mapping table that includes the local character set code
and the corresponding UCS code. For instance the ISO/IEC
8859-8 [ISO-8859] code for the Hebrew letter "VAV" is 0xE4.
The corresponding 4 byte ISO/IEC 10646 code is 0x000005D5.
The next step is to convert the UCS character code to the
UTF-8 encoding. The following routine can be used to
determine and encode the correct number of bytes based on the
UCS-4 character code:
unsigned int ucs4_to_utf8 (unsigned long *ucs4_buf, unsigned int
ucs4_len, unsigned char *utf8_buf)
{
const unsigned long *ucs4_endbuf = ucs4_buf + ucs4_len;
unsigned int utf8_len = 0; // return value for UTF8 size
unsigned char *t_utf8_buf = utf8_buf; // Temporary pointer
// to load UTF8 values
while (ucs4_buf != ucs4_endbuf)
{
if ( *ucs4_buf <= 0x7F) // ASCII chars no conversion needed
{
*t_utf8_buf++ = (unsigned char) *ucs4_buf;
utf8_len++;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x07FF ) // In the 2 byte utf-8 range
{
*t_utf8_buf++= (unsigned char) (0xC0 + (*ucs4_buf/0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=2;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0xFFFF ) /* In the 3 byte utf-8 range. The
values 0x0000FFFE, 0x0000FFFF
and 0x0000D800 - 0x0000DFFF do
not occur in UCS-4 */
{
*t_utf8_buf++= (unsigned char) (0xE0 +
(*ucs4_buf/0x1000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=3;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x1FFFFF ) //In the 4 byte utf-8 range
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{
*t_utf8_buf++= (unsigned char) (0xF0 +
(*ucs4_buf/0x040000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x10000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 + (*ucs4_buf%0x40));
utf8_len+=4;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x03FFFFFF )//In the 5 byte utf-8 range
{
*t_utf8_buf++= (unsigned char) (0xF8 +
(*ucs4_buf/0x01000000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x040000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x1000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
(*ucs4_buf%0x40));
utf8_len+=5;
ucs4_buf++;
}
else
if ( *ucs4_buf <= 0x7FFFFFFF )//In the 6 byte utf-8 range
{
*t_utf8_buf++= (unsigned char)
(0xF8 +(*ucs4_buf/0x40000000));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x01000000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x040000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x1000)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
((*ucs4_buf/0x40)%0x40));
*t_utf8_buf++= (unsigned char) (0x80 +
(*ucs4_buf%0x40));
utf8_len+=6;
ucs4_buf++;
}
}
return (utf8_len);
}
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B.2.2 Conversion from UTF-8 to local character set
When moving from UTF-8 encoding to the local character set
the reverse procedure is used. First the UTF-8 encoding is
transformed into the UCS-4 character set. The UCS-4 is then
converted to the local character set from a mapping table
(i.e. the opposite of the table used to form the UCS-4
character code).
To convert from UTF-8 to UCS-4 the free bits (those that do
not define UTF-8 sequence size or signify continuation bytes)
in a UTF-8 sequence are concatenated as a bit string. The
bits are then distributed into a four-byte sequence starting
from the least significant bits. Those bits not assigned a
bit in the four-byte sequence are padded with ZERO bits. The
following routine converts the UTF-8 encoding to UCS-4
character codes:
int utf8_to_ucs4 (unsigned long *ucs4_buf, unsigned int utf8_len,
unsigned char *utf8_buf)
{
const unsigned char *utf8_endbuf = utf8_buf + utf8_len;
unsigned int ucs_len=0;
while (utf8_buf != utf8_endbuf)
{
if ((*utf8_buf & 0x80) == 0x00) /*ASCII chars no conversion
needed */
{
*ucs4_buf++ = (unsigned long) *utf8_buf;
utf8_buf++;
ucs_len++;
}
else
if ((*utf8_buf & 0xE0)== 0xC0) //In the 2 byte utf-8 range
{
*ucs4_buf++ = (unsigned long) (((*utf8_buf - 0xC0) * 0x40)
+ ( *(utf8_buf+1) - 0x80));
utf8_buf += 2;
ucs_len++;
}
else
if ( (*utf8_buf & 0xF0) == 0xE0 ) /*In the 3 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long) (((*utf8_buf - 0xE0) * 0x1000)
+ (( *(utf8_buf+1) - 0x80) * 0x40)
+ ( *(utf8_buf+2) - 0x80));
utf8_buf+=3;
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ucs_len++;
}
else
if ((*utf8_buf & 0xF8) == 0xF0) /* In the 4 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xF0) * 0x040000)
+ (( *(utf8_buf+1) - 0x80) * 0x1000)
+ (( *(utf8_buf+2) - 0x80) * 0x40)
+ ( *(utf8_buf+3) - 0x80));
utf8_buf+=4;
ucs_len++;
}
else
if ((*utf8_buf & 0xFC) == 0xF8) /* In the 5 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xF8) * 0x01000000)
+ ((*(utf8_buf+1) - 0x80) * 0x040000)
+ (( *(utf8_buf+2) - 0x80) * 0x1000)
+ (( *(utf8_buf+3) - 0x80) * 0x40)
+ ( *(utf8_buf+4) - 0x80));
utf8_buf+=5;
ucs_len++;
}
else
if ((*utf8_buf & 0xFE) == 0xFC) /* In the 6 byte utf-8
range */
{
*ucs4_buf++ = (unsigned long)
(((*utf8_buf - 0xFC) * 0x40000000)
+ ((*(utf8_buf+1) - 0x80) * 0x010000000)
+ ((*(utf8_buf+2) - 0x80) * 0x040000)
+ (( *(utf8_buf+3) - 0x80) * 0x1000)
+ (( *(utf8_buf+4) - 0x80) * 0x40)
+ ( *(utf8_buf+5) - 0x80));
utf8_buf+=6;
ucs_len++;
}
}
return (ucs_len);
}
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B.2.3 ISO/IEC 8859-8 Example
This example demonstrates mapping ISO/IEC 8859-8 character
set to UTF-8 and back to ISO/IEC 8859-8. As noted earlier,
the Hebrew letter "VAV" is convertd from the ISO/IEC 8859-8
character code 0xE4 to the corresponding 4 byte ISO/IEC 10646
code of 0x000005D5 by a simple lookup of a conversion/mapping
file.
The UCS-4 character code is transformed into UTF-8 using the
ucs4_to_utf8 routine described earlier by:
1. Because the UCS-4 character is between 0x80 and 0x07FF it
will map to a 2 byte UTF-8 sequence.
2. The first byte is defined by (0xC0 + (0x000005D5 / 0x40))
= 0xD7.
3. The second byte is defined by (0x80 + (0x000005D5 %
0x40)) = 0x95.
The UTF-8 encoding is transferred back to UCS-4 by using the
utf8_to_ucs4 routine described earlier by:
1. Because the first byte of the sequence, when the '&'
operator with a value of 0xE0 is applied, will produce
0xC0 (0xD7 & 0xE0 = 0xC0) the UTF-8 is a 2 byte sequence.
2. The four byte UCS-4 character code is produced by
(((0xD7 - 0xC0) * 0x40) + (0x95 -0x80)) = 0x000005D5.
Finally, the UCS-4 character code is converted to ISO/IEC
8859-8 character code (using the mapping table which matches
ISO/IEC 8859-8 to UCS-4 ) to produce the original 0xE4 code
for the Hebrew letter "VAV".
B.2.4 Vendor Codepage Example
This example demonstrates the mapping of a codepage to UTF-8
and back to a vendor codepage. Mapping between vendor
codepages can be done in a very similar manner as described
above. For instance both the PC and Mac codepages reflect the
character set from the Thai standard TIS 620-2533. The
character code on both platforms for the Thai letter "SO SO"
is 0xAB. This character can then be mapped into the UCS-4 by
way of a conversion/mapping file to produce the UCS-4 code of
0x0E0B.
The UCS-4 character code is transformed into UTF-8 using the
ucs4_to_utf8 routine described earlier by:
1. Because the UCS-4 character is between 0x0800 and 0xFFFF
it will map to a 3 byte UTF-8 sequence.
2. The first byte is defined by (0xE0 + (0x00000E0B /
0x1000) = 0xE0.
3. The second byte is defined by (0x80 + ((0x00000E0B /
0x40) % 0x40))) = 0xB8.
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4. The third byte is defined by (0x80 + (0x00000E0B % 0x40))
= 0x8B.
The UTF-8 encoding is transferred back to UCS-4 by using the
utf8_to_ucs4 routine described earlier by:
1. Because the first byte of the sequence, when the '&'
operator with a value of 0xF0 is applied, will produce
0xE0 (0xE0 & 0xF0 = 0xE0) the UTF-8 is a 3 byte sequence.
2. The four byte UCS-4 character code is produced by
(((0xE0 - 0xE0) * 0x1000) + ((0xB8 - 0x80) * 0x40) +
(0x8B -0x80) = 0x0000E0B.
Finally, the UCS-4 character code is converted to either the
PC or MAC codepage character code (using the mapping table
which matches codepage to UCS-4 ) to produce the original
0xAB code for the Thai letter "SO SO".
B.3 Pseudo Code for a high-quality translating server
if utf8_valid(fn)
{
attempt to convert fn to the local charset, producing localfn
if (conversion fails temporarily) return error
if (conversion succeeds)
{
attempt to open localfn
if (open fails temporarily) return error
if (open succeeds) return success
}
}
attempt to open fn
if (open fails temporarily) return error
if (open succeeds) return success
return permanent error
Expires 01 December 1998 [Page B-8 ]
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