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Debugging CGI Applications?><html><head><title>Debugging CGI Applications (CGI Programming with Perl)</title><link href="../style/style1.css" type="text/css" rel="stylesheet" /><meta name="DC.Creator" content="Scott Guelich, Gunther Birznieks and Shishir Gundavaram" /><meta scheme="MIME" content="text/xml" name="DC.Format" /><meta content="en-US" name="DC.Language" /><meta content="O'Reilly & Associates, Inc." name="DC.Publisher" /><meta scheme="ISBN" name="DC.Source" content="1565924193L" /><meta name="DC.Subject.Keyword" content="stuff" /><meta name="DC.Title" content="CGI Programming with Perl" /><meta content="Text.Monograph" name="DC.Type" /></head><body bgcolor="#ffffff"><img src="gifs/smbanner.gif" alt="Book Home" usemap="#banner-map" border="0" /><map name="banner-map"><area alt="CGI Programming with Perl" href="index.htm" coords="0,0,466,65" shape="rect" /><area alt="Search this book" href="jobjects/fsearch.htm" coords="467,0,514,18" shape="rect" /></map><div class="navbar"><table border="0" width="515"><tr><td width="172" valign="top" align="left"><a href="ch14_05.htm"><img src="../gifs/txtpreva.gif" alt="Previous" border="0" /></a></td><td width="171" valign="top" align="center"><a href="index.htm">CGI Programming with Perl</a></td><td width="172" valign="top" align="right"><a href="ch15_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr></table></div><hr align="left" width="515" /><h1 class="chapter">Chapter 15. Debugging CGI Applications</h1><div class="htmltoc"><h4 class="tochead">Contents:</h4><p><a href="ch15_01.htm">Common Errors</a><br><a href="ch15_02.htm">Perl Coding Techniques</a><br><a href="ch15_03.htm">Debugging Tools</a><br></p></div><p>So far, <a name="INDEX-2905" /> <a name="INDEX-2,906" /> <a name="INDEX-2,907" />we'vediscussed numerous CGI applications, ranging from the trivial to thevery complex, but we haven't touched upon the techniques neededto debug them if something goes wrong. Debugging a CGI application isnot much different than debugging any other type of application,because, after all, code is code. However, since a CGI application isrun by a remote user across the network in a special environmentcreated by the web server, it is sometimes difficult to pinpoint theproblems.</p><p>This chapter is all about debugging CGI applications. First,we'll examine some of the common errors that developersgenerally come across when implementing CGI applications. Theseinclude incorrect server configuration, permission problems, andviolations of the HTTP protocol. Then, we'll explore a fewtips, tricks, and tools that will help us track down problems anddevelop better applications.</p><div class="sect1"><a name="ch15-16509" /><h2 class="sect1">15.1. Common Errors</h2><p>This section <a name="INDEX-2908" /><a name="INDEX-2909" /> <a name="INDEX-2,910" />can serve as a checklist that you canuse to diagnose common problems. Here is a list of common sources oferrors:</p><a name="ch15-1-fm2xml" /><table border="1"><tr><th><p>Source of Problem</p></th><th><p>Typical Error Message</p></th></tr><tr><td><p>Application permissions</p></td><td><p>403 Forbidden</p></td></tr><tr><td><p>The pound-bang line</p></td><td><p>403 Forbidden</p></td></tr><tr><td><p>Line endings</p></td><td><p>500 Internal Server Error</p></td></tr><tr><td><p>"Malformed" header</p></td><td><p>500 Internal Server Error</p></td></tr></table><p>Let's look at each of these in more detail.</p><a name="ch15-2-fm2xml" /><div class="sect2"><h3 class="sect2">15.1.1. Application Permissions</h3><p>Typically, <a name="INDEX-2911" /><a name="INDEX-2912" />web servers are configured to runas <em class="emphasis">nobody</em><a name="INDEX-2913" /><a name="INDEX-2914" /><a name="INDEX-2915" /> or another user withminimal access privileges. This is a great preventative step, and onethat can possibly salvage your data in the case of an attack. Sincethe web server process does not have privileges to write to, readfrom, or execute files in directories that don't have"<a name="INDEX-2916" />world" access, most of yourdata will stay intact.</p><p>However, this <a name="INDEX-2917" /> <a name="INDEX-2,918" />alsocreate a few problems for us. First and foremost, we need to set theworld execute bit on the CGI applications, so the server can executethem. Here's how you can check the<a name="INDEX-2919" /><a name="INDEX-2920" />permissions of your applications:</p><blockquote><pre class="code">$ ls -l /usr/local/apache/cgi-bin/clock-rwx------ 1 shishir 3624 Oct 17 17:59 clock</pre></blockquote><p>The first field lists the permissions for the file. This field isdivided into three parts: the privileges for the owner, the group,and the world (from left to right), with the first letter indicatingthe type of the file: either a regular file, or a directory. In thisexample, the owner has sole permission to read, write, and executethe program.</p><p>If you want the server to be able to execute this application, youhave to issue the following<a name="INDEX-2921" /><a name="INDEX-2922" /> <a name="INDEX-2,923" /><a name="INDEX-2924" />command:</p><blockquote><pre class="code">$ chmod 711 clock-rwx--x--x 1 shishir 3624 Oct 17 17:59 clock*</pre></blockquote><p>The <tt class="command">chmod</tt> command (change mode) modifies thepermissions for the file. The octal code of 711 indicates read (octal4), write (octal 2), and execute (octal 1) permissions for the owner,and execute permissions for everyone else.</p><p>That's not the end of our permission woes. We could run intoother problems dealing with file permissions, most notably, theinability to create or update files. We will discuss this in <a href="ch15_02.htm#ch15-21836">Section 15.2, "Perl Coding Techniques"</a> later in this chapter.</p><p>Despite configuring the server to recognize CGI applications andsetting the execute permissions, our applications can still fail toexecute, as you'll see next.</p></div><a name="ch15-3-fm2xml" /><div class="sect2"><h3 class="sect2">15.1.2. The Pound-Bang</h3><p>If a <a name="INDEX-2925" /><a name="INDEX-2926" /><a name="INDEX-2927" />CGI application is written in Perl,Python, Tcl, or another<a name="INDEX-2928" />interpreted scripting language, thenit must have a line at the very top that begins with a pound-bang, or<tt class="literal">#!</tt>, like this:</p><blockquote><pre class="code">#!/usr/bin/perl -wT</pre></blockquote><p>We've seen this above every script throughout this book. Whenthe web server recognizes a request for a CGI application, it callsthe <em class="emphasis">exec</em><a name="INDEX-2929" /><a name="INDEX-2930" /> <a name="INDEX-2,931" /> systemfunction to execute the application. If the application is a compiledexecutable, the operating system will go ahead and execute it.However, if our application is a script of some sort, then theoperating system will look at the first line to see what interpreterto use.</p><p>If your scripts are missing the pound-bang line, or if the<a name="INDEX-2932" /><a name="INDEX-2933" />path you specify is invalid, then youwill get an error. On some systems, for example,<tt class="command">perl</tt><a name="INDEX-2934" /> <a name="INDEX-2,935" /> is found at<em class="filename">/usr/bin/perl</em>, while on others it is found at<em class="filename">/usr/local/bin/perl</em>. On <a name="INDEX-2936" /><a name="INDEX-2937" />Unix systems, you can use either ofthe following commands to locate <em class="emphasis">perl</em> (dependingon your shell):</p><blockquote><pre class="code">$ which perl$ whence perl</pre></blockquote><p>If neither of these commands work, then look for<tt class="command">perl5</tt> instead of <tt class="command">perl</tt>. If youstill cannot locate <tt class="command">perl</tt>, then try either of thefollowing commands. They return anything on your filesystem named<tt class="command">perl</tt>, so they could return multiple results, andthe <tt class="command">find</tt> command will search your entirefilesystem, so depending on the size of the filesystem, this couldtake a while:</p><blockquote><pre class="code">$ locate perl$ find / -name perl -type f -print 2>/dev/null</pre></blockquote><p>Another thing to keep in mind: if you have<a name="INDEX-2938" /><a name="INDEX-2939" />multiple interpreters(i.e., different versions) for the same language, make sure that yourscripts reference the one you intend, or else you may see somemysterious effects. For example, on some systems,<em class="emphasis">perl4</em> is still installed in addition to<em class="emphasis">perl5</em>. Test the path you use with the<a name="INDEX-2940" /> <a name="INDEX-2,941" /><em class="emphasis">-v</em> flag to get itsversion.</p></div><a name="ch15-4-fm2xml" /><div class="sect2"><h3 class="sect2">15.1.3. Line Endings</h3><p>If you are working with a CGI script that downloaded from anothersite or edited with a different platform, then it is possible thatthe <a name="INDEX-2942" /><a name="INDEX-2943" />line endings do not match those of thecurrent system. For example, <tt class="command">perl</tt> on Unix willcomplain with multiple syntax errors if you attempt to run a filethat is formatted for Windows. You can clean these files up with<tt class="command">perl</tt> from the command line:</p><blockquote><pre class="code">$ perl -pi -e 's/\r\n/\n/' calendar.cgi</pre></blockquote></div><a name="ch15-5-fm2xml" /><div class="sect2"><h3 class="sect2">15.1.4. "Malformed" Header</h3><p>As we first discussed in <a href="ch02_01.htm">Chapter 2, "The Hypertext Transport Protocol "</a>, and <a href="ch03_01.htm">Chapter 3, "The Common Gateway Interface"</a>, and have seen in all the examples since, allCGI applications must return a<a name="INDEX-2944" /> <a name="INDEX-2,945" /> <a name="INDEX-2,946" /><a name="INDEX-2947" />validHTTP content-type header, followed by a newline, before the actualcontent, like this:</p><blockquote><pre class="code">Content-type: text/html(other headers)(content)</pre></blockquote><p>If you fail to follow this format, then a typical <em class="emphasis">500Server Error</em><a name="INDEX-2948" /> will ensue. The partial solution isto return all necessary<a name="INDEX-2949" /> <a name="INDEX-2,950" />HTML headers, includingcontent type, as early on in the CGI application as possible. We willlook at a very useful technique in the next section that will help uswith this task.</p><p>However, there are other reasons why we may see such an error. Ifyour CGI application generates errors that are printed to STDERR,these error messages may be returned to the web server before all ofthe header information. Because Perl<a name="INDEX-2951" /><a name="INDEX-2952" />buffers output to STDOUT, errors thatoccur after you have printed the headers may even cause this problem.</p><p>What's the moral? Make sure you check your application from thecommand line before you try to execute it from the Web. Ifyou're using Perl to develop CGI applications, then you can usethe <em class="emphasis">-wcT</em><a name="INDEX-2953" /> <a name="INDEX-2,954" /> <a name="INDEX-2,955" />switch to check for syntax errors:</p><blockquote><pre class="code">$ perl -wcT clock.cgisyntax error in file clock.cgi at line 9, at EOFclock.cgi had compilation errors.</pre></blockquote><p>If there are <a name="INDEX-2956" /><a name="INDEX-2957" />warnings, but no errors, you may seethe following:</p><blockquote><pre class="code">$ perl -wcT clock.cgiName "main::opt_g" used only once: possible typo at clock.cgi line 5.Name "main::opt_u" used only once: possible typo at clock.cgi line 6.Name "main::opt_f" used only once: possible typo at clock.cgi line 7.clock.cgi syntax OK</pre></blockquote><p>Pay attention to the warnings, as well. Perl's syntax checkerhas really improved over the years, and will alert you of manypossible errors, such as using non existent variables, uninitializedvariables, or file handles.</p><p>And finally, if there are no warnings or errors, you will see:</p><blockquote><pre class="code">$ perl -wcT clock.cgiclock.cgi syntax OK</pre></blockquote><p>To reiterate, make sure your application works from the command linebefore you even attempt to debug its functionality from the<a name="INDEX-2958" /><a name="INDEX-2959" /><a name="INDEX-2960" />Web.</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="ch14_05.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="ch15_02.htm"><img src="../gifs/txtnexta.gif" alt="Next" border="0" /></a></td></tr><tr><td width="172" valign="top" align="left">14.5. CGI Gateway to XML Middleware</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">15.2. Perl Coding Techniques</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 © 2001</a> O'Reilly & Associates. 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