ch06.htm

Web_Programming_with_Perl5,一个不错的Perl语言教程。

<PRE><FONT COLOR="#0066FF">audio/bsic                        au snd

audio/x-aiff                       aif aiff aifc

audio/x-wav                        wav

</FONT></PRE>

<P><B><TT>Video</TT></B> This is a video Content-Type value for transmitting video

or moving image data, possibly with audio as part of the composite video data format:</P>

<PRE><FONT COLOR="#0066FF">video/mpeg                         mpeg mpg mpe

video/quicktime                    qt mov

video/x-msvideo                    avi

video/x-sgi-movie                  movie

</FONT></PRE>

<H4 ALIGN="CENTER"><A NAME="Heading5"></A><FONT COLOR="#000077">The Content-Transfer-Encoding

Header Field</FONT></H4>

<P>The Content-Transfer-Encoding header field is used to specify an auxiliary encoding

that was applied to the data in order to allow it to pass through mail transport

mechanisms that may have data or character set limitations.

<H4 ALIGN="CENTER"><A NAME="Heading6"></A><FONT COLOR="#000077">Additional Header

Fields</FONT></H4>

<P>Two additional header fields can be used to further describe the data in a message

body: the <BR>

Content-ID and Content-Description header fields.

<H3 ALIGN="CENTER"><A NAME="Heading7"></A><FONT COLOR="#000077">How MIME Data Is

Encoded</FONT></H3>

<P>MIME data can be encoded in two different ways. Each encoding method has its advantages

and disadvantages, which are described later. The first method, Q encoding, is recommended

for use when the characters to be encoded are in the ASCII character set; otherwise,

the B encoding should be used. Both encoding/decoding methods are possible using

MIME::Base64 (B) and MIME::QuotedPrint (Q).</P>

<P>Only a subset of the printable ASCII characters may be used in encoded-text. Space

and tab characters are not allowed, so that the beginning and end of an encoded-word

are obvious. The ? character is used within an encoded-word to separate the various

portions of the encoded-word from one another and thus cannot appear in the encoded-text

portion. Other characters are also illegal in certain contexts. For example, an encoded-word

in a &quot;phrase&quot; preceding an address in a From header field may not contain

any of the &quot;specials&quot; defined in RFC 822. Finally, certain other characters

are disallowed in some contexts to ensure reliability for messages that pass through

internetwork mail gateways.</P>

<P>The B encoding automatically meets these requirements. The Q encoding allows a

wide range of printable characters to be used in non-critical locations in the message

header (such as Subject), with fewer characters available for use in other locations.

<H4 ALIGN="CENTER"><A NAME="Heading8"></A><FONT COLOR="#000077">B Base64 Encoding</FONT></H4>

<P>The B encoding is identical to the Base64 encoding defined by RFC 1521.

<H4 ALIGN="CENTER"><A NAME="Heading9"></A><FONT COLOR="#000077">Q Quoted-Printable

Encoding</FONT></H4>

<P>The Q encoding is similar to the Quoted-Printable content-transfer-encoding defined

in RFC 1521. It is designed to allow text containing mostly ASCII characters to be

decipherable on an ASCII terminal without decoding.</P>

<P>For more information on RFC 1521 and specific information about the rules used

to encode data, go to:</P>

<PRE><FONT COLOR="#0066FF">http://www.freesoft.org/Connected/RFC/1521/index.html

</FONT></PRE>

<P>Data encoded in the &quot;Q&quot; or Quoted-Printable method follows these basic

rules:



<OL>

	<LI>Any 8-bit value may be represented by a <TT>=</TT> followed by two hexadecimal

	digits. For example, if the character set in use were ISO-8859-1, the <TT>=</TT>

	character would thus be encoded as <TT>=3D</TT> and a <TT>SPACE</TT> by <TT>=20</TT>.

	(Uppercase should be used for hexadecimal digits <BR>

	A through F.)

	<P>

	<LI>The 8-bit hexadecimal value 20 (for example, ISO-8859-1 SPACE) may be represented

	as <TT>_</TT> (underscore, ASCII 95). (This character may not pass through some internetwork

	mail gateways, but its use will greatly enhance readability of Q encoded data with

	mail readers that do not support this encoding.) Note that the <TT>_</TT> always

	represents hexadecimal 20, even if the <TT>SPACE</TT> character occupies a different

	code position in the character set in use.

	<P>

	<LI>8-bit values that correspond to printable ASCII characters other than <TT>=</TT>,

	<TT>?</TT>, <TT>_</TT> (underscore), and <TT>SPACE</TT> may be represented as those

	characters.

</OL>



<H3 ALIGN="CENTER"><A NAME="Heading10"></A><FONT COLOR="#000077">Encoding and Decoding

MIME with libwww</FONT></H3>

<P>Mechanisms for encoding and decoding MIME messages are provided in the MIME::Base64

and Mime::QuotedPrint classes.

<H4 ALIGN="CENTER"><A NAME="Heading11"></A><FONT COLOR="#000077">Using MIME::Base64</FONT></H4>

<P>Mime::Base64 includes two functions: <TT>encode_base64()</TT> and <TT>decode_base64()</TT>.

To use Mime::Base64 in your script, include the following line near the beginning

of your script:</P>

<PRE><FONT COLOR="#0066FF">use MIME::Base64;

</FONT></PRE>

<P>After called, encoding and decoding strings of MIME is quite simple. Encoding

is handled by sending a string of non-encoded text (which can be stored in a variable)

to the <TT>encode_base64</TT> routine. Here's how it's done:</P>

<PRE><FONT COLOR="#0066FF">$MyEncodedMime = encode_base64($MyPlainText);

</FONT></PRE>

<P>Where, <TT>$MyEncodedMime</TT> will contain the Base64 encoded version of <TT>$MyPlainText</TT>.</P>

<P>Decoding is handled in the same way, using <TT>decode_base64</TT>.</P>

<P>Here's an example:</P>

<PRE><FONT COLOR="#0066FF">$MyDecodedText = decode_base64($MyEncodedMIME);

</FONT></PRE>

<P>Many popular e-mail clients, such as Netscape Mail, have standardized on Base64

method encoding to attach binary files to ASCII text e-mail messages. Let's take

a look at an example of an <BR>

e-mail message generated by Netscape mail that has Base64 encoded data as an attachment

(see Listing 7.1).

<H3 ALIGN="CENTER"><A NAME="Heading12"></A><FONT COLOR="#000077">Listing 7.1. E-mail

with Base64 encoded</FONT></H3>

<H3 ALIGN="CENTER"><A NAME="Heading13"></A><FONT COLOR="#000077">MIME attachment</FONT></H3>

<PRE><FONT COLOR="#0066FF">From - Mon Jan 13 01:50:26 1997

Return-Path: &lt;[e-mail]&gt;

Received: from chris (ts8p1.ro.com [205.216.92.168])

          by sh1.ro.com (8.8.4/8.8.4) with SMTP

  id CAA12517 for &lt;[e-mail]&gt;; Mon, 13 Jan 1997 02:50:08 -0600

Message-ID: &lt;[e-mail]&gt;

Date: Mon, 13 Jan 1997 01:47:47 -0800

From: Chris Kemp &lt;[e-mail]&gt;

Reply-To: [e-mail]

Organization: Silicon Graphics

X-Mailer: Mozilla 3.01 (Win95; I)

MIME-Version: 1.0

To: [e-mail]

Subject: MIME Test

Content-Type: multipart/mixed; boundary=&quot;------------6B797646304&quot;

X-UIDL: 48f2146a82b1f706edbd5e7c350b352a

X-Mozilla-Status: 0001



This is a multi-part message in MIME format.



--------------6B797646304

Content-Type: text/plain; charset=us-ascii

Content-Transfer-Encoding: 7bit



Chris,



I have attached a picture of the Boeing F-22 to this message.



--------------6B797646304

Content-Type: image/jpeg; name=&quot;F22.jpg&quot;

Content-Transfer-Encoding: base64

Content-Disposition: inline; filename=&quot;F22.jpg&quot;



/9j/4AAQSkZJRgABAgEASABIAAD/7QG4UGhvdG9zaG9wIDMuMAA4QklNA+kAAAAAAHgAAwAA

AEgASAAAAAAC2gIo/+H/4gL5AkYDRwUoA/wAAgAAAEgASAAAAAAC2AIoAAEAAABkAAAAAQAD

AwMAAAABJw8AAQABAAAAAAAAAAAAAAAAYAgAGQGQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

AAAAAAAAAAA4QklNA+0AAAAAABAASAAAAAEAAQBIAAAAAQABOEJJTQPzAAAAAAAIAAAAAAAA

AAA4QklNJxAAAAAAAAoAAQAAAAAAAAACOEJJTQP1AAAAAABIAC9mZgABAGxmZgAGAAAAAAAB

AC9mZgABAKGZmgAGAAAAAAABADIAAAABAFoAAAAGAAAAAAABADUAAAABAC0AAAAGAAAAAAAB

OEJJTQP4AAAAAABwAAD/////////////////////////////A+gAAAAA////////////////

/////////////wPoAAAAAP////////////////////////////8D6AAAAAD/////////////

////////////////A+gAADhCSU0EBgAAAAAAAgAG/+4ADkFkb2JlAGQAAAAAAf/bAIQAAgIC

AgICAgICAgMCAgIDBAMCAgMEBQQEBAQEBQYFBQUFBQUGBgcHCAcHBgkJCgoJCQwMDAwMDAwM

DAwMDAwMDAEDAwMFBAUJBgYJDQoJCg0PDg4ODg8PDAwMDAwPDwwMDAwMDA8MDAwMDAwMDAwM

DAwMDAwMDAwMDAwMDAwMDAwM/8AAEQgCWAMYAwERAAIRAQMRAf/EAaIAAAAHAQEBAQEAAAAA

AAAAAAQFAwIGAQAHCAkKCwEAAgIDAQEBAQEAAAAAAAAAAQACAwQFBgcICQoLEAACAQMDAgQC

BgcDBAIGAnMBAgMRBAAFIRIxQVEGE2EicYEUMpGhBxWxQiPBUtHhMxZi8CRygvElQzRTkqKy

Y3PCNUQnk6OzNhdUZHTD0uIIJoMJChgZhJRFRqS0VtNVKBry4/PE1OT0ZXWFlaW1xdXl9WZ2

hpamtsbW5vY3R1dnd4eXp7fH1+f3OEhYaHiImKi4yNjo+Ck5SVlpeYmZqbnJ2en5KjpKWmp6

ipqqusra6voRAAICAQIDBQUEBQYECAMDbQEAAhEDBCESMUEFURNhIgZxgZEyobHwFMHR4SNC

FVJicvEzJDRDghaSUyWiY7LCB3PSNeJEgxdUkwgJChgZJjZFGidkdFU38qOzwygp0+PzhJSk

tMTU5PRldYWVpbXF1eX1RlZmdoaWprbG1ub2R1dnd4eXp7fH1+f3OEhYaHiImKi4yNjo+DlJ

WWl5iZmpucnZ6fkqOkpaanqKmqq6ytrq+v/dAAQAY//aAAwDAQACEQMRAD8A/P8A4q7FXYq7



&lt;a few hundred K of encoded data removed&gt;

sS+TZPzi8xpdeS/Jmoana6tf6VYteahqd8v1ixlmto4bi91KSRI45P8AX4tir9CMVdirsVdi

rsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirsVdirs

VdirsVdirsVdirsVeRfn5y/5UX+dHH7X+BPMVP8AuGXGLCX0v//Z

--------------6B797646304--

</FONT></PRE>

<P>Notice the MIME content type declarations throughout the message. A practical

application of the Mime::Base64 decode_base64 function would be extracting the encoded

data to a file. Using this method, you could allow users to submit pictures via e-mail

of items that could be integrated into a Web page.

<H4 ALIGN="CENTER"><A NAME="Heading15"></A><FONT COLOR="#000077">Using MIME::QuotedPrint</FONT></H4>

<P>Mime::QuotedPrint includes the functions <TT>encode_qp</TT> and <TT>decode_qp</TT>

that function in the same way as their Base64 counterparts. The syntax is as follows:</P>

<PRE><FONT COLOR="#0066FF">$encoded = encode_yp($decoded);

$decoded = decode_yp($encoded);

</FONT></PRE>

<P>MIME is significant to the Web programmer in two main ways. MIME and its different

data types define standard types of data that can be transmitted as part of a document.

MIME headers are used in every HTTP transaction, thus a familiarity with them is

essential in advanced applications where your program must generate headers. MIME

encoding schemes allow binary data to be included in text messages that can be transported

over older e-mail (not HTTP) gateways.<BR>



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