ch05_05.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

this feature in one of three ways:</p>


<ol><li><p>You can pass the

<a name="INDEX-1215" /><em class="emphasis">-unique_headers</em> flag
when you load CGI.pm:</p>


<blockquote><pre class="code">use CGI qw( -unique_headers );</pre></blockquote></li>

<li><p>You can set the
<tt class="literal">$CGI::HEADERS_ONCE</tt>
<a name="INDEX-1216" /> variable to a true value after you use
CGI.pm, but before you create an object:</p>


<blockquote><pre class="code">use CGI;
$CGI::HEADERS_ONCE = 1;

my $q = new CGI;</pre></blockquote></li>

<li><p>Finally, if you know that you always want this feature, you can
enable it globally for all of your scripts by setting
<tt class="literal">$HEADERS_ONCE</tt>
<a name="INDEX-1217" /> to a true value within your copy of
CGI.pm. You can do this just like <tt class="literal">$POST_MAX</tt> and
<tt class="literal">$DISABLE_UPLOADS</tt> variables we discussed at the
beginning of the chapter. You will find
<tt class="literal">$HEADERS_ONCE</tt> is in the same configurable section
of CGI.pm:</p>


<blockquote><pre class="code"># Change this to 1 to suppress redundant HTTP headers
$HEADERS_ONCE = 0;</pre></blockquote></li></ol>

<p>Although adding subroutines to each of your CGI scripts is certainly
an acceptable way to catch errors, it's still not a very
general solution. You will probably want to create your own error
pages that are customized for your site. Once you start including
complex HTML in your subroutines, it will quickly become too
difficult to maintain them. If you build error subroutines that
output error pages according to your site's template, and then
later someone decides they want to change the site's look, you
must go back and update all of your subroutines. Clearly, a much
better option is to create a general error handler that all of your
CGI scripts can access.</p>
</div>




<a name="ch05-31-fm2xml" /><div class="sect2">
<h3 class="sect2">5.5.4. Custom Module</h3>


<p>It is a good idea to create your own <a name="INDEX-1218" />
<a name="INDEX-1219" />Perl module that's specific to
your site. If you host different sites, or have different
applications within your site with different looks and feels, you may
wish to create a module for each. Within this module, you can place
subroutines that you find yourself using across many CGI scripts.
These subroutines will vary depending on your site, but one should
handle errors.</p>


<p>If you have not created your own Perl module before, don't
worry, it's quite simple. <a href="ch05_05.htm#ch05-67677">Example 5-3</a> shows a
very minimal module.</p>



<a name="ch05-67677" /><div class="example">
<h4 class="objtitle">Example 5-3. CGIBook::Error.pm </h4>
<a name="INDEX-1220" />

<blockquote><pre class="code">#!/usr/bin/perl -wT

package CGIBook::Error;

# Export the error subroutine
use Exporter;
@ISA = "Exporter";
@EXPORT = qw( error );

$VERSION = "0.01";

use strict;
use CGI;
use CGI::Carp qw( fatalsToBrowser );

BEGIN {
    sub carp_error {
        my $error_message = shift;
        my $q = new CGI;
        my $discard_this = $q-&gt;header( "text/html" );
        error( $q, $error_message );
    }
    CGI::Carp::set_message( \&amp;carp_error );
}

sub error {
    my( $q, $error_message ) = @_;
    
    print $q-&gt;header( "text/html" ),
          $q-&gt;start_html( "Error" ),
          $q-&gt;h1( "Error" ),
          $q-&gt;p( "Sorry, the following error has occurred: " ),
          $q-&gt;p( $q-&gt;i( $error_message ) ),
          $q-&gt;end_html;
    exit;
}

1;</pre></blockquote>
</div>


<p>The only difference between a <a name="INDEX-1221" /> <a name="INDEX-1,222" />Perl module and a standard Perl script is
that you should save your file with a
<em class="filename">.pm</em>
<a name="INDEX-1223" />
<a name="INDEX-1224" /> extension, declare
the name of module's package with the
<tt class="function">package</tt>
<a name="INDEX-1225" />
function (this should match the file's name except without the
<em class="filename">.pm </em>extension and substituting
<tt class="literal">::</tt> for <tt class="literal">/</tt>),<a href="#FOOTNOTE-7">[7]</a> and make sure
that it returns a true value when evaluated (the reason for the
<tt class="literal">1;</tt> at the bottom).</p><blockquote>

<a name="FOOTNOTE-7" /><p>[7]When
determining the package name, the file's name should be
relative to a library path in <tt class="literal">@INC</tt>. In our
example, we store the file at
<em class="filename">/usr/lib/perl5/site_perl/5.005/CGIBook/Error.pm</em>.
<em class="filename">/usr/lib/perl5/site_perl/5.005 </em>is a library
directory. Thus, the path to the module relative to the library
directory is <em class="filename">CGIBook/Error.pm </em>so the package is
<em class="emphasis">CGIBook::Error</em>.</p>

</blockquote>


<p>It is standard practice to store the
<a name="INDEX-1226" />version of the module in
<tt class="literal">$VERSION</tt>. For the sake of convenience, we also use
the <a name="INDEX-1227" />
<a name="INDEX-1228" />Exporter module to export
the <tt class="function">error</tt> subroutine. This allows us to refer to
it in our scripts<tt class="function"> </tt>as<tt class="function"> error</tt>
instead of <tt class="function">CGIBook::Exporter::error</tt>. Refer to
the Exporter manpage or a primary Perl text, such as
<em class="citetitle">Programming Perl</em>, for details on using
Exporter.</p>


<p>You have a couple options for saving this file. The simplest solution
is to save it within the
<em class="filename">site_perl</em>
<a name="INDEX-1229" />
<a name="INDEX-1230" /> directory
of your <a name="INDEX-1231" />Perl libraries, such as
<em class="filename">/usr/lib/perl5/site_perl/5.005/CGIBook/Error.pm</em>.
The<em class="emphasis"> site_perl</em> directory includes modules that
are site-specific (i.e., not included in Perl's standard
distribution). The paths of your Perl libraries may differ; you can
locate them on your system with the following command:</p>


<blockquote><pre class="code">$ perl -e 'print map "$_\n", @INC'</pre></blockquote>


<p>You probably want to create a subdirectory that is unique to your
organization, as we did with <em class="filename">CGIBook</em>, to hold
all the Perl modules you create.</p>


<p>You can use the module as follows:</p>


<blockquote><pre class="code">#!/usr/bin/perl -wT

use strict;
use CGI;
use CGIBook::Error;

my $q = new CGI;

unless ( check_something_important(  ) ) {
    error( $q, "Something bad happened." );
}</pre></blockquote>


<p>If you do not have the permission to install the module in your Perl
library directory, and if you cannot get your system administrator to
do it, then you can place the module in another location, for
example,
<em class="filename">/usr/local/apache/perl-lib/CGIBook/Error.pm</em>.
Then you must remember to include this directory in the list that
Perl searches for modules. The simplest way to <a name="INDEX-1232" />do this is
<a name="INDEX-1233" />
<a name="INDEX-1234" />with the
<tt class="function">lib</tt> <a name="INDEX-1235" /> <a name="INDEX-1,236" />pragma:</p>


<blockquote><pre class="code">#!/usr/bin/perl -wT

use strict;
use lib "/usr/local/apache/perl-lib";

use CGI;
use CGIBook::Error;
.
.
.</pre></blockquote>
</div>


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