Character Encoding in AOLserver 3.0 and ACS

by Rob Mayoff

Contents

• The Problem
• Terminology
• Database Access
• Configuration Files
• Output from Tcl
• Content Files (not Tcl or ADP Scripts)
• ADP scripts
• ADP - NsTclIncludeCmd
• URL Encoding
• URL Path
• Form Data in application/x-www-form-urlencoded Format
• Form data in multipart/form-data Format
• Cookies
• ns_httpopen / ns_httpget
• References

The Problem

The second case is different. It turns out that Tcl 8.1 and later use Unicode. The interpreter normally stores strings using the UTF-8 encoding (which uses a variable number of bytes per character), and sometimes converts them to UCS-2 encoding (which uses 16-bit "wide characters"). The regsub command is one of those cases where conversions are involved. First, regsub converted the string to UCS-2. Tcl's UTF-8 parser is lenient, so the transformation ended up translating xFC into x00FC. (This happens to be the correct translation because UCS-2 is a superset of ISO-8859-1.) Then regsub did its matching and substitution. Then it converted the UCS-2 representation back to UTF-8. The UTF-8 encoding of x00FC is xC3 xBC. AOLserver does not know anything about UTF-8; it just sends whatever bytes you give it. In ISO-8859-1, xC3 means à and xBC means ¼.

So regsub didn't do anything wrong. We gave it garbage (a non-UTF-8 string), so it gave us garbage. How do we solve this problem? We need to make sure that all of AOLserver's textual input is translated to its UTF-8 representation and that the UTF-8 is translated to the appropriate character encoding on output.

Terminology

Charset is synonymous with "character encoding"; Internet standards use this term.

Tcl 8.1 and later use Unicode and UTF-8 internally and include support for converting between character encodings. The Tcl names for various encodings are different than the Internet standard names. So, in this document, I typically use the term "encoding" when I am referring to Tcl, and "charset" when I am referring to an Internet protocol feature.

Database Access

Configuration Files

Content Files

AOLserver has several APIs for sending the contents of a file directly to the client. All of them send the contents of the file back to the client unmodified - no character encoding translation is performed. This means that it is up to you to ensure that the file's encoding is the same as the encoding the client expects.

The safest thing is to use only US-ASCII bytes in your text files - bytes with the high bit clear. Just about every character encoding you're likely to run across on the Web will be a superset of US-ASCII, so no matter what charset the client is expecting, your content will probably be displayed correctly. If you are sending an HTML (or XML) file, it can still access any Unicode character using the &#nnn; notation. However, if you have non-HTML files, or you don't want to deal with all those character reference entities, you'll have to make sure your client knows what character set you're sending.

The client knows what character set to expect from the Content-Type header. You're probably used to seeing a header like this:

Typically, you determine the content-type to send for a file by calling ns_guesstype on it. ns_guesstype looks up the file extension in AOLserver's file extension table to pick the content-type. The default table is in the AOLserver manual. Some of the default mappings are:

[ns/mimetypes]
.html=text/html; charset=iso-8859-1
.txt=text/plain; charset=iso-8859-1

ns_section ns/mimetypes
ns_param .html "text/html; charset=iso-8859-1"
ns_param .txt "text/plain; charset=iso-8859-1"

[ns/mimetypes]
.html=text/html; charset=iso-8859-1
.txt=text/plain; charset=iso-8859-1
.html_sj=text/html; charset=shift_jis
.txt_sj=text/plain; charset=shift_jis
.html_ej=text/html; charset=euc-jp
.txt_ej=text/plain; charset=euc-jp

ns_section ns/mimetypes
ns_param .html "text/html; charset=iso-8859-1"
ns_param .txt "text/plain; charset=iso-8859-1"
ns_param .html_sj "text/html; charset=shift_jis"
ns_param .txt_sj "text/plain; charset=shift_jis"
ns_param .html_ej "text/html; charset=euc-jp"
ns_param .txt_ej "text/plain; charset=euc-jp"

set fd [open somefile.html_sj r]
fconfigure $fd -encoding shiftjis
set html [read $fd [file size somefile.html_sj]]
close $fd
ns_return 200 "text/html; charset=euc-jp" $html

XXX ACS: ad_serve_html_file

Output from Tcl

• ns_writefp
• ns_connsendfp
• ns_returnfp
• ns_respond
• ns_returnfile
• ns_return (and variants like ns_returnerror)
• ns_write

Tcl stores strings in memory using UTF-8. However, when you send content to the client from Tcl, you may not want the client to receive UTF-8; he may not support it. So AOLserver can translate UTF-8 to a different charset.

If you use ns_return or ns_respond to send a Tcl string to the client, AOLserver determines what character set to use by examining the content type you specify:

1. If your content-type includes a charset parameter, then AOLserver translates the string to that charset.
2. Otherwise, if your content-type is text/anything, then AOLserver translates the string to the charset specified in the config file by ns/parameters/OutputCharset (iso-8859-1 by default).
3. Otherwise, AOLserver sends the string unmodified.

In the second instance, where AOLserver uses the ns/parameters/OutputCharset, if ns/parameters/HackContentType is also set to true, then AOLserver will modify the Content-Type header to include the charset parameter. HackContentType is set by default, and I strongly recommend leaving it set, because it's always safer to tell the client explicitly what charset you are sending.

For example, the default configuration is equivalent to this:

[ns/parameters]
OutputCharset=iso-8859-1
HackContentType=true

If you write the headers to the client with ns_write instead of letting AOLserver do it (via ns_return or ns_respond), then AOLserver does not parse the content-type. You must explicitly tell it what charset to use immediately after you write the headers, by calling ns_startcontent in one of these forms:

ns_startcontent

Tells AOLserver that you have written the headers and do not wish the content to be translated.

ns_startcontent -charset charset

Tells AOLserver that you have written the headers and wish the following content to be translated to the specified charset.

ns_startcontent -type content-type

Tells AOLserver that you have written the headers and wish the following content to be translated to the charset specified by content-type, which should be the same value you sent to the client in the Content-Type header. If content-type does not contain a charset parameter, AOLserver translates to ISO-8859-1.

The ns_choosecharset algorithm:

1. Set preferred-charsets to the list of charsets specified by the -preference flag. If that flag was not given, use the config parameter ns/parameters/PreferredCharsets. If the config parameter is missing, use {utf-8 iso-8859-1}. The list order is significant.
2. Set acceptable-charsets to the intersection of the Accept-Charset charsets and the charsets supported by AOLserver.
3. If acceptable-charsets is empty, return the charset specified by config parameter ns/parameters/DefaultCharset, or iso-8859-1 by default.
4. Choose the first charset from preferred-charsets that also appears in acceptable-charsets. Return that charset.
5. If no charset in preferred-charsets also appears in acceptable-charsets, then choose the first charset listed in Accept-Charsets that also appears in acceptable-charsets. Return that charset.

(Note: the last step will always return a charset because acceptable-charsets can only contain charsets listed by Accept-Charsets.)

Example:

# Assume japanesetext.html_sj is stored in Shift-JIS encoding.
set fd [open japanesetext.html_sj r]
fconfigure $fd -encoding shiftjis
set html [read $fd [file size japanesetext.html_sj]]
close $fd

set charset [ns_choosecharset -preference {utf-8 shift-jis euc-jp iso-2022-jp}]
set type "text/html; charset=$charset"
ns_write "HTTP/1.0 200 OK
Content-Type: $type
\n"
ns_startcontent -type $type
ns_write $html

URL Encoding

In URL encoding, one byte may be encoded as three bytes which in US-ASCII represent a percent character ("%") followed by two hexadecimal digits.

After a URL is decoded, any bytes less that x80 represent US-ASCII characters. The problem with URLs and URL encoding is that historically, no standard defined what bytes larger than x80 represent. Various proposals such as IURI Internet-Draft propose using UTF-8 exclusively as the character encoding in URLs, but existing software does not work that way.

AOLserver's ns_urlencode and ns_urldecode choose the character encoding to use in one of three ways:

1. If the command was invoked with a -charset flag, use that charset. For example:
   ns_urlencode -charset shift_jis "\u304b"
   Unicode character U+304B is HIRAGANA LETTER KA. In Shift-JIS this is encoded as x82 xA9, so the command returns the string "%82%A9".
2. If no -charset flag was given, then the ns_urlcharset command determines what encoding is used. The ns_urlcharset sets the default charset for the ns_urlencode and ns_urldecode commands for one connection. For example, these commands have the same result as the preceding example:
   ns_urlcharset shiftjis
   ns_urlencode "\u304b"
   The ns_urlcharset command is only valid when called from a connection thread. Do not call it from an ns_schedule_proc thread.
3. If neither of the preceding steps specified a charset, then the AOLserver config parameter ns/parameters/URLCharset determines the charset. The default value for the parameter is "iso-8859-1".

A URL, as seen by AOLserver in an HTTP request, consists of two parts, the path and the query. For example:

/register/user-new.tcl path

?

first_names=Rob&last_name=Mayoff query

We will consider the path part and the query part separately.

URL Path

However, other people might link to you from their servers and might be careless about the character encodings. So the safest practice is to use only US-ASCII characters in your URL paths if you possibly can.

Form Data in application/x-www-form-urlencoded Format

• In an HTTP GET request, the query data is the part of the request URL following the first x3F byte (the first question mark). This data is always in application/x-www-form-urlencoded format.

  Okay, it could be raw data from an <ISINDEX> page, but that tag is deprecated in HTML 4.0. Let's simplify our lives by pretending it doesn't exist.

• In an HTTP POST request, the query data is the request contents, following the request header. By default this data is in application/x-www-form-urlencoded format. The other format is covered under POST Data in multipart/form-data Format.

If you always send data in a single charset, and you always specify the charset in the Content-Type header, then it is safe to assume that form data is always encoded using that charset. Just make that your ns/parameters/URLCharset and don't worry about it.

If you cannot limit yourself to a single charset, then you need to use some other technique. No matter how you do it, you must call ns_urlcharset before calling ns_conn form or ns_getform. If you call ns_urlcharset after you've asked AOLserver for the form, it will not work retroactively.

Here are two ways you could determine the charset:

• Include a hidden field in all your forms, to indicate the charset. Example:

  # myform.tcl
  set _charset [ns_choosecharset]
  ns_return 200 "text/html; charset=$_charset" "
      <form action='myform-2.tcl'>
  	<input type='hidden' name='_charset' value='$_charset'>
  	First Names: <input type='text' name='first_names'><br>
  	Last Name: <input type='text' name='last_name'><br>
  	<input type='submit' name='submit' value='Submit'>
      </form>
  "
  

  The chicken-and-egg problem here is that you need the contents of a form field in order to decode the form. Fortunately, all charset names use only US-ASCII characters, so you can extract the _charset field from the query string without decoding it. The predefined command ns_formfieldcharset will do this for you:

  # myform-2.tcl
  ns_formfieldcharset _charset
  set form [ns_conn form]
  set first_names [ns_set get $form first_names]
  set last_name [ns_set get $form last_name]
  etc.
  

  ns_formfieldcharset calls ns_urlcharset, so this will affect all further use of ns_urlencode and ns_urldecode for that connection, unless you call ns_urlcharset again.

• Use a cookie to store the last charset you sent to the user. Example:

  # anotherform.tcl
  set _charset [ns_choosecharset]
  ns_set put [ns_conn outputheaders] Set-Cookie _charset=$_charset
  ns_return 200 "text/html; charset=$_charset" "
      <form action='anotherform-2.tcl'>
  	First Names: <input type='text' name='first_names'><br>
  	Last Name: <input type='text' name='last_name'><br>
  	<input type='submit' name='submit' value='Submit'>
      </form>
  "
  

  There is no chicken-and-egg problem here, but AOLserver still provides the predefined command ns_cookiecharset to set the URL encoding from a cookie:

  # myform-2.tcl
  ns_cookiefield _charset
  set form [ns_conn form]
  set first_names [ns_set get $form first_names]
  set last_name [ns_set get $form last_name]
  etc.
  

  Using a cookie has the big drawback that a cookie is not associated with a single web page. So if the user uses his back button, or has a page cached, or has multiple windows open, the wrong cookie value might be sent back to us.

Form Data in multipart/form-data Format

Alas, multipart/form-data format is no better than application/x-www-form-urlencoded format as far as character encoding issues are concerned. The MIME multipart format allows each form field to include its own Content-Type header with a charset parameter, but in practice clients do not send any indication of the charset used. So we must resort to the same tricks to decide what charset the data is in: always use the same charset, or use a hidden field or a cookie to determine the charset.

The ns_formfieldcharset and ns_cookiecharset commands work for fields in multipart/form-data format except file upload fields. We cannot know what character set the user stores his files in, so we don't know how to translate an uploaded file to utf-8 (assuming the uploaded file is even a text file). So the temporary files created by ns_getform contain the exact bytes sent by the client.

Cookies

ns_httpopen / ns_httpget

References

• How to Use Tcl 8.1 Internationalization Features
• RFC 2617: Hypertext Transfer Protocol -- HTTP/1.1
• RFC 2070: Internationalization of the Hypertext Markup Language
• IANA Registered Charsets
• RFC 2396: Uniform Resource Identifiers (URI): Generic Syntax
• RFC 1345: Character Mnemonics & Character Sets
• RFC 2130: The Report of the IAB Character Set Workshop
• CJKV Information Processing