ch15_03.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

main::(Dev:Pseudo:7):	my $q = new CGI;
  DB&lt;1&gt;</pre></blockquote>


<p>The debugger can be intimidating at first, but it is very powerful.
To help you get going, <a href="ch15_03.htm#ch15-28900">Table 15-1</a> shows a brief
<a name="INDEX-3075" />
<a name="INDEX-3076" />summary of all the basic commands you need
to know to debug a script. You can debug all of your CGI scripts with
just these commands, although there are many more features actually
available. Practice walking through scripts that you know work in
order to learn how to move around within the debugger. The debugger
will not change your files, so you cannot damage a working script by
typing a wrong command.</p>


<p>Complete documentation for the <a name="INDEX-3077" />Perl debugger is available in the
<em class="citetitle">perldebug</em> manpage, and a quick reference for
the complete set of commands is available by typing
<tt class="literal">h</tt> within the debugger.</p>


<a name="ch15-28900" /><h4 class="objtitle">Table 15-1. Basic Perl Debugger Commands </h4><table border="1">



<tr>
<th>
<p>Command</p></th>
<th>
<p>Description</p></th>
</tr>




<tr>
<td>
<p><tt class="literal">s</tt></p></td>
<td>
<p>Step; Perl executes the line listed above the prompt, stepping into
any subroutines; note that a line with multiple commands may take a
few steps to evaluate.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">n</tt></p></td>
<td>
<p>Next; Perl executes the line listed above the prompt, stepping over
any subroutines (they still run; Perl waits for them to finish before
continuing).</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">c</tt></p></td>
<td>
<p>Continue to the end of the program or the next break point, whichever
comes first.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">c 123</tt></p></td>
<td>
<p>Continue up to line 123; line 123 must contain a command (it cannot
be a comment, blank line, the second half of a command, etc.).</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">b</tt></p></td>
<td>
<p>Set a breakpoint at current line; breakpoints halt execution caused
by <tt class="literal">c</tt>.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">b 123</tt></p></td>
<td>
<p>Set a breakpoint at line 123; line 123 must contain a command (it
cannot be a comment, blank line, the second half of a command, etc.).</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">b my_sub</tt></p></td>
<td>
<p>Set a breakpoint at the first executable line of the
<tt class="function">my_sub</tt> sub.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">d</tt></p></td>
<td>
<p>Delete a breakpoint from the current line; takes same arguments as
<tt class="literal">b</tt>.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">D</tt></p></td>
<td>
<p>Deletes all breakpoints.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">x $var</tt></p></td>
<td>
<p>Display the value of <tt class="literal">$var</tt> in list and scalar
contexts; note that it will recurse down complex, nested data
structures.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">r</tt></p></td>
<td>
<p>Return from the current sub; Perl finishes executing the current
subroutine, displays the result, and continues at the next line after
the sub.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">l</tt></p></td>
<td>
<p>List the next 10 lines of your script; this command can be used
successively.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">l 123</tt></p></td>
<td>
<p>List line 123 of your script.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">l 200-300</tt></p></td>
<td>
<p>List lines 200 through 300 of your script.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">l my_sub</tt></p></td>
<td>
<p>List the first 10 lines of the <tt class="function">my_sub</tt> sub.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">q</tt></p></td>
<td>
<p>Quit.</p></td>
</tr>

<tr>
<td>
<p><tt class="literal">R</tt></p></td>
<td>
<p>Restart the script in the debugger.</p></td>
</tr>


</table>
</div>



<a name="ch15-19-fm2xml" /><div class="sect3">
<h3 class="sect3">15.3.5.2. ptkdb</h3>


<p>Another option is
<em class="emphasis">ptkdb</em>
<a name="INDEX-3078" /> <a name="INDEX-3079" /> (see <a href="ch15_03.htm#ch15-65096">Figure 15-1</a>), the Perl/Tk debugger, which is available on
CPAN as <em class="emphasis">Devel-ptkdb</em>. It allows you to debug your
scripts with a graphical interface. It also allows you to debug your
CGI interactively as they are running.</p>


<a name="ch15-65096" /><div class="figure"><img width="481" src="figs/cgi2.1501.gif" height="374" alt="Figure 15-1" /></div><h4 class="objtitle">Figure 15-1. Debugging a CGI script with ptkdb</h4>

<p>In order to use <em class="emphasis">ptkdb</em>, you need two things.
First, you need access to an X Window server;<a href="#FOOTNOTE-21">[21]</a> the
<a name="INDEX-3080" />X Window System is included with most Unix
and compatible systems; commercial versions are available for other
operating systems as well. Second, the web server must have
<a name="INDEX-3081" />
<a name="INDEX-3082" />Tk.pm
module, available on CPAN, which requires Tk. Tk is
a<a name="INDEX-3083" /> <a name="INDEX-3084" /> graphics toolkit that is
typically distributed with the Tcl scripting language. You can obtain
Tcl/Tk from <a href="http://www.scriptics.com/">http://www.scriptics.com/</a>. For more
information on using Perl

<a name="INDEX-3085" />with Tk via Tk.pm,
refer to <em class="citetitle">Learning Perl/Tk</em> by Nancy Walsh (O'Reilly &amp; Associates,
Inc.).</p><blockquote>

<a name="FOOTNOTE-21" /><p>[21]In the
X Window System, you run an X Window <em class="emphasis">server
</em>locally, which displays programs that you may execute
remotely. The use of "server" in this context is
sometimes confusing, since you typically use a <em class="emphasis">client
</em>to interact with remote systems.</p>

</blockquote>


<p>In order to debug a CGI script with <em class="emphasis">ptkdb</em>, begin
your CGI scripts as follows:</p>


<blockquote><pre class="code">#! /usr/bin/perl -d:ptkdb

sub BEGIN {
    $ENV{DISPLAY} = "your.machine.hostname:0.0" ;
}</pre></blockquote>


<p>You should replace <tt class="literal">your.machine.hostname</tt> with the
hostname or IP address of your machine. You can use
<tt class="literal">localhost</tt> if you are running an X Window session
on the web server.</p>


<p>You also need to allow the web server to display programs on your X
Window server. On Unix and compatible systems, you do so by adding
the registering the hostname or IP address of the webserver with the
<tt class="command">xhost</tt>
<a name="INDEX-3086" />
<a name="INDEX-3087" /> command:</p>


<blockquote><pre class="code">$ xhost www.webserver.hostname
www.webserver.hostname being added to access control list</pre></blockquote>


<p>You can then access your CGI script via a browser, which should open
a debugging window on your system. Note that your web browser may
time out if you spend much time interacting with the debugger without
your script producing output.</p>
</div>



<a name="ch15-20-fm2xml" /><div class="sect3">
<h3 class="sect3">15.3.5.3. ActiveState Perl debugger</h3>


<p>The final option is available only to <a name="INDEX-3088" />
<a name="INDEX-3089" />
<a name="INDEX-3090" /> <a name="INDEX-3091" />Win32 users.
ActiveState distributes a graphical Perl debugger with their Perl
Development Kit (PDK), shown in <a href="ch15_03.htm#ch15-43713">Figure 15-2</a>.</p>


<a name="ch15-43713" /><div class="figure"><img width="481" src="figs/cgi2.1502.gif" height="441" alt="Figure 15-2" /></div><h4 class="objtitle">Figure 15-2. Debugging a CGI script with the ActiveState Perl debugger</h4>

<p>Once installed, using the
<em class="emphasis">-d</em>
<a name="INDEX-3093" /> flag with <tt class="command">perl</tt>
invokes this debugger instead of the standard Perl debugger. It can
also be invoked when running CGI scripts if you are logged into the
web server.</p>


<p>You can obtain the <a name="INDEX-3094" />
<a name="INDEX-3095" />PDK and corresponding
documentation from ActiveState's web site at <a href="http://www.activestate.com/">http://www.activestate.com/</a>. The PDK is a
commercial product, but as of the time this book was written,
ActiveState <a name="INDEX-3096" /> <a name="INDEX-3097" />offers a free <a name="INDEX-3098" /> <a name="INDEX-3099" /> <a name="INDEX-3100" />seven-day trial.</p>
</div>
</div>


<hr align="left" width="515" />
<div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch15_02.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm"><img src="../gifs/txthome.gif" alt="Home" border="0" /></a></td><td width="172" valign="top" align="right"><a href="ch16_01.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">15.2. Perl Coding Techniques</td><td width="171" valign="top" align="center"><a href="index/index.htm"><img src="../gifs/index.gif" alt="Book Index" border="0" /></a></td><td width="172" valign="top" align="right">16. Guidelines for Better CGI Applications</td></tr></table></div>
<hr align="left" width="515" />

<img src="../gifs/navbar.gif" alt="Library Navigation Links" usemap="#library-map" border="0" />
<p><font size="-1"><a href="copyrght.htm">Copyright &copy; 2001</a> O'Reilly &amp; Associates. All rights reserved.</font></p>

<map name="library-map"><area href="../index.htm" coords="1,1,83,102" shape="rect" /><area href="../lnut/index.htm" coords="81,0,152,95" shape="rect" /><area href="../run/index.htm" coords="172,2,252,105" shape="rect" /><area href="../apache/index.htm" coords="238,2,334,95" shape="rect" /><area href="../sql/index.htm" coords="336,0,412,104" shape="rect" /><area href="../dbi/index.htm" coords="415,0,507,101" shape="rect" /><area href="../cgi/index.htm" coords="511,0,601,99" shape="rect" /></map>

</body></html>