ch02_01.htm

用perl编写CGI的好书。本书从解释CGI和底层HTTP协议如何工作开始

absolute URL is <em class="emphasis">http://localhost/cgi/script.cgi.</em></p></dd>



<dt><b>Relative URL</b></dt>
<dd><p>URLs without a scheme, host, or port are called
<a name="INDEX-221" /> <a name="INDEX-222" />
<a name="INDEX-223" />relative URLs. These can be further
broken down into full and relative paths:</p>


<dl>
<dt><b>Full paths</b></dt>
<dd><p>Relative URLs with an absolute
<a name="INDEX-224" />path are sometimes referred to
as <em class="firstterm">full paths</em> (even though they can also
include a query string and fragment identifier). Full paths can be
distinguished from URLs with relative paths because they always start
with a forward slash. Note that in all these cases, the paths are
virtual paths, and do not necessarily correspond to a path on the web
server's filesystem. An example of an absolute path is
<em class="emphasis">/index.html</em>.</p></dd>



<dt><b>Relative paths</b></dt>
<dd><p>Relative URLs that begin with a character other than a forward slash
are <em class="firstterm">relative paths</em>. Examples of relative paths
include <em class="emphasis">script.cgi</em> and
<em class="emphasis">../images/photo.jpg.</em></p></dd>

</dl></dd>

</dl>
</div>








<a name="ch02-80730" /><div class="sect2">
<h3 class="sect2">2.1.3. URL Encoding</h3>



<p>Many characters must be <a name="INDEX-225" /> <a name="INDEX-226" />
<a name="INDEX-227" />encoded within a URL for a
variety of reasons. For example, certain characters such as
<tt class="literal">?</tt>, <tt class="literal">#</tt>, and <tt class="literal">/</tt>
have special meaning within URLs and will be misinterpreted unless
encoded. It is possible to name a file
<em class="filename">doc#2.html</em> on some systems, but the URL
<em class="filename">http://localhost/doc#2.html</em> would not point to
this document. It points to the fragment <em class="filename">2.html</em>
in a (possibly nonexistent) file named <em class="filename">doc</em>. We
must encode the <tt class="literal">#</tt> character so the web browser and
server recognize that it is part of the resource name instead.</p>



<p>Characters are encoded by representing them with a
<a name="INDEX-228" />
<a name="INDEX-229" /> <a name="INDEX-230" />
<a name="INDEX-231" />percent sign (<tt class="literal">%</tt>)
followed by the two-digit hexadecimal value for that character based
upon the ISO Latin 1 character set or ASCII character set (these
character sets are the same for the first eight bits). For example,
the <tt class="literal">#</tt> symbol has a hexadecimal value of
<tt class="literal">0x23</tt>, so it is encoded as <tt class="literal">%23</tt>.</p>



<p>The following characters must be encoded:</p>



<ul><li><p>Control characters: ASCII <tt class="literal">0x00</tt> through
<tt class="literal">0x1F</tt> plus <tt class="literal">0x7F</tt></p></li><li><p>Eight-bit characters: ASCII <tt class="literal">0x80</tt> through
<tt class="literal">0xFF</tt></p></li><li><p>Characters given special importance within URLs: <tt class="literal">; / ? : @
&amp; = + $ ,</tt></p></li><li><p>Characters often used to delimit (quote) URLs: <tt class="literal">&lt; &gt; # %
"</tt></p></li><li><p>Characters considered unsafe because they may have special meaning
for other protocols used to transmit URLs (e.g., SMTP): <tt class="literal">{ }
| \ ^ [ ] `</tt></p></li></ul>
<p>Additionally,
<a name="INDEX-232" /> <a name="INDEX-233" /> <a name="INDEX-234" />spaces should be encoded as
<tt class="literal">+</tt> although <tt class="literal">%20</tt> is also allowed.
As you can see, most characters must be encoded; the list of
<a name="INDEX-235" />allowed characters is actually much
shorter:</p>



<ul><li><p>Letters: <tt class="literal">a-z</tt> and <tt class="literal">A-Z</tt></p></li><li><p>Digits: <tt class="literal">0-9</tt></p></li><li><p>The following characters: <tt class="literal">- _ . ! ~ * ' ( )</tt></p></li></ul>
<p>It is actually permissible and not uncommon for any of the allowed
characters to also be encoded by some software. Thus, any application
that decodes a URL must decode every occurrence of a percentage sign
followed by any two hexadecimal digits.</p>



<p>The following <a name="INDEX-236" />
<a name="INDEX-237" />code encodes text for URLs:</p>



<blockquote><pre class="code">sub url_encode {
    my $text = shift;
    $text =~ s/([^a-z0-9_.!~*'(  ) -])/sprintf "%%%02X", ord($1)/ei;
    $text =~ tr/ /+/;
    return $text;
}</pre></blockquote>



<p>Any character not in the allowed set is replaced by a percentage sign
and its two-digit hexadecimal equivalent. The three percentage signs
are necessary because percentage signs indicate format codes for
<tt class="function">sprintf</tt>, and literal percentage signs must be
indicated by two percentage signs. Our format code thus includes a
percentage sign, <tt class="literal">%%</tt>, plus the format code for two
hexadecimal digits, <tt class="literal">%02X</tt>.</p>



<p>Code to decode URL encoded text looks like this:</p>



<blockquote><pre class="code">sub url_decode {
    my $text = shift;
    $text =~ tr/\+/ /;
    $text =~ s/%([a-f0-9][a-f0-9])/chr( hex( $1 ) )/ei;
    return $text;
}</pre></blockquote>



<p>Here we first translate any plus signs to spaces. Then we scan for a
percentage sign followed by two hexadecimal digits and use
<a name="INDEX-238" />
<a name="INDEX-239" />Perl's <tt class="function">chr
</tt>function to convert the hexadecimal value into a
character.</p>



<p>Neither the <a name="INDEX-240" />encoding nor the decoding operations
can be safely repeated on the same text. Text encoded twice differs
from text encoded once because the percentage signs introduced in the
first step would themselves be encoded in the second. Likewise, you
cannot encode or decode entire URLs. If you were to decode a URL, you
could no longer reliably parse it, for you may have introduced
characters that would be misinterpreted such as <tt class="literal">/</tt>
or <tt class="literal">?</tt>. You should always parse a URL to get the
components you want before decoding them; likewise, encode components
before building them into a full URL.</p>



<p>Note that it's good to understand how a wheel works but
reinventing it would be pointless. Even though you have just seen how
to encode and decode text for URLs, you shouldn't do so
yourself. The <a name="INDEX-241" />
<a name="INDEX-242" />
<a name="INDEX-243" />
<a name="INDEX-244" />URI::URL module (actually it is a
collection of modules), available on CPAN (see <a href="appb_01.htm">Appendix B, "Perl Modules"</a>), provides many URL-related modules and
functions. One of the included modules, URI::Escape, provides the
<tt class="function">url_escape</tt>
<a name="INDEX-245" />
<a name="INDEX-246" />
<a name="INDEX-247" /> and
<tt class="function">url_unescape</tt> functions. Use them. The
subroutines in these modules have been vigorously tested, and future
versions will reflect any changes to HTTP as it evolves.<a href="#FOOTNOTE-2">[2]</a>
Using standard subroutines will also make your code much clearer to
those who may have to maintain your code later (this includes you).</p><blockquote>


<a name="FOOTNOTE-2" /><p>[2]Don't think this could happen? What if we told you the
tilde character (<tt class="literal">~</tt>) was not always allowed in
URLs? This restriction was removed after it became common practice
for some web servers to accept a tilde plus username in the path to
indicate a user's personal web directory.</p>


</blockquote>



<p>If, despite these warnings, you still insist on writing your own
decoding code yourself, at least place it in appropriately named
subroutines. Granted, some of these actions take only a line or two
of code, but the code is quite cryptic, and these operations
<a name="INDEX-248" />
<a name="INDEX-249" />
<a name="INDEX-250" />should be

<a name="INDEX-251" /> <a name="INDEX-252" />clearly labeled.</p>
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