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      script headers", possibly along with an error message
      generated by your CGI program. In this case, you will want to
      check each of the below sections to see what might be
      preventing your CGI program from emitting the proper HTTP
      headers.</dd>
    </dl>

    <h3><a name="permissions" id="permissions">File permissions</a></h3>
      

      <p>Remember that the server does not run as you. That is,
      when the server starts up, it is running with the permissions
      of an unprivileged user - usually <code>nobody</code>, or
      <code>www</code> - and so it will need extra permissions to
      execute files that are owned by you. Usually, the way to give
      a file sufficient permissions to be executed by <code>nobody</code>
      is to give everyone execute permission on the file:</p>

      <div class="example"><p><code>
        chmod a+x first.pl
      </code></p></div>

      <p>Also, if your program reads from, or writes to, any other
      files, those files will need to have the correct permissions
      to permit this.</p>

    

    <h3><a name="pathinformation" id="pathinformation">Path information and environment</a></h3>
      

      <p>When you run a program from your command line, you have
      certain information that is passed to the shell without you
      thinking about it. For example, you have a <code>PATH</code>,
      which tells the shell where it can look for files that you
      reference.</p>

      <p>When a program runs through the web server as a CGI program,
      it may not have the same <code>PATH</code>. Any programs that you
      invoke in your CGI program (like <code>sendmail</code>, for
      example) will need to be specified by a full path, so that the
      shell can find them when it attempts to execute your CGI
      program.</p>

      <p>A common manifestation of this is the path to the script
      interpreter (often <code>perl</code>) indicated in the first
      line of your CGI program, which will look something like:</p>

      <div class="example"><p><code>
        #!/usr/bin/perl
      </code></p></div>

      <p>Make sure that this is in fact the path to the
      interpreter.</p>

      <p>In addition, if your CGI program depends on other <a href="#env">environment variables</a>, you will need to
      assure that those variables are passed by Apache.</p>

    

    <h3><a name="syntaxerrors" id="syntaxerrors">Program errors</a></h3>
      

      <p>Most of the time when a CGI program fails, it's because of
      a problem with the program itself. This is particularly true
      once you get the hang of this CGI stuff, and no longer make
      the above two mistakes.  The first thing to do is to make
      sure that your program runs from the command line before
      testing it via the web server.  For example, try:</p>

      <div class="example"><p><code>
      cd /usr/local/apache2/cgi-bin<br />
      ./first.pl
      </code></p></div>

      <p>(Do not call the <code>perl</code> interpreter.  The shell
      and Apache should find the interpreter using the <a href="#pathinformation">path information</a> on the first line of
      the script.)</p>

      <p>The first thing you see written by your program should be
      a set of HTTP headers, including the <code>Content-Type</code>,
      followed by a blank line.  If you see anything else, Apache will
      return the <code>Premature end of script headers</code> error if
      you try to run it through the server. See <a href="#writing">Writing a CGI program</a> above for more
      details.</p>
    

    <h3><a name="errorlogs" id="errorlogs">Error logs</a></h3>
      

      <p>The error logs are your friend. Anything that goes wrong
      generates message in the error log. You should always look
      there first. If the place where you are hosting your web site
      does not permit you access to the error log, you should
      probably host your site somewhere else. Learn to read the
      error logs, and you'll find that almost all of your problems
      are quickly identified, and quickly solved.</p>
    

    <h3><a name="suexec" id="suexec">Suexec</a></h3>
      

      <p>The <a href="../suexec.html">suexec</a> support program
      allows CGI programs to be run under different user permissions,
      depending on which virtual host or user home directory they are
      located in. Suexec has very strict permission checking, and any
      failure in that checking will result in your CGI programs
      failing with <code>Premature end of script headers</code>.</p>

      <p>To check if you are using suexec, run <code>apachectl
      -V</code> and check for the location of <code>SUEXEC_BIN</code>.
      If Apache finds an <code class="program"><a href="../programs/suexec.html">suexec</a></code> binary there on startup,
      suexec will be activated.</p>

      <p>Unless you fully understand suexec, you should not be using it.
      To disable suexec, simply remove (or rename) the <code class="program"><a href="../programs/suexec.html">suexec</a></code>
      binary pointed to by <code>SUEXEC_BIN</code> and then restart the
      server.  If, after reading about <a href="../suexec.html">suexec</a>,
      you still wish to use it, then run <code>suexec -V</code> to find
      the location of the suexec log file, and use that log file to
      find what policy you are violating.</p>
    
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="behindscenes" id="behindscenes">What's going on behind the scenes?</a></h2>
    

    <p>As you become more advanced in CGI programming, it will
    become useful to understand more about what's happening behind
    the scenes. Specifically, how the browser and server
    communicate with one another. Because although it's all very
    well to write a program that prints "Hello, World.", it's not
    particularly useful.</p>

    <h3><a name="env" id="env">Environment variables</a></h3>
      

      <p>Environment variables are values that float around you as
      you use your computer. They are useful things like your path
      (where the computer searches for the actual file
      implementing a command when you type it), your username, your
      terminal type, and so on. For a full list of your normal,
      every day environment variables, type 
      <code>env</code> at a command prompt.</p>

      <p>During the CGI transaction, the server and the browser
      also set environment variables, so that they can communicate
      with one another. These are things like the browser type
      (Netscape, IE, Lynx), the server type (Apache, IIS, WebSite),
      the name of the CGI program that is being run, and so on.</p>

      <p>These variables are available to the CGI programmer, and
      are half of the story of the client-server communication. The
      complete list of required variables is at 
      <a href="http://hoohoo.ncsa.uiuc.edu/cgi/env.html">http://hoohoo.ncsa.uiuc.edu/cgi/env.html</a>.</p>

      <p>This simple Perl CGI program will display all of the
      environment variables that are being passed around. Two
      similar programs are included in the 
      <code>cgi-bin</code>

      directory of the Apache distribution. Note that some
      variables are required, while others are optional, so you may
      see some variables listed that were not in the official list.
      In addition, Apache provides many different ways for you to 
      <a href="../env.html">add your own environment variables</a>
      to the basic ones provided by default.</p>

      <div class="example"><p><code>
        #!/usr/bin/perl<br />
        print "Content-type: text/html\n\n";<br />
        foreach $key (keys %ENV) {<br />
        <span class="indent">
          print "$key --&gt; $ENV{$key}&lt;br&gt;";<br />
        </span>
        }
      </code></p></div>
    

    <h3><a name="stdin" id="stdin">STDIN and STDOUT</a></h3>
      

      <p>Other communication between the server and the client
      happens over standard input (<code>STDIN</code>) and standard
      output (<code>STDOUT</code>). In normal everyday context, 
      <code>STDIN</code> means the keyboard, or a file that a 
      program is given to act on, and <code>STDOUT</code>
      usually means the console or screen.</p> 

      <p>When you <code>POST</code> a web form to a CGI program,
      the data in that form is bundled up into a special format
      and gets delivered to your CGI program over <code>STDIN</code>.
      The program then can process that data as though it was
      coming in from the keyboard, or from a file</p>

      <p>The "special format" is very simple. A field name and
      its value are joined together with an equals (=) sign, and
      pairs of values are joined together with an ampersand
      (&amp;). Inconvenient characters like spaces, ampersands, and
      equals signs, are converted into their hex equivalent so that
      they don't gum up the works. The whole data string might look
      something like:</p>

      <div class="example"><p><code>
        name=Rich%20Bowen&amp;city=Lexington&amp;state=KY&amp;sidekick=Squirrel%20Monkey
      </code></p></div>

      <p>You'll sometimes also see this type of string appended to
      a URL. When that is done, the server puts that string
      into the environment variable called 
      <code>QUERY_STRING</code>. That's called a <code>GET</code>
      request. Your HTML form specifies whether a <code>GET</code>
      or a <code>POST</code> is used to deliver the data, by setting the 
      <code>METHOD</code> attribute in the <code>FORM</code> tag.</p>

      <p>Your program is then responsible for splitting that string
      up into useful information. Fortunately, there are libraries
      and modules available to help you process this data, as well
      as handle other of the aspects of your CGI program.</p>
    
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="libraries" id="libraries">CGI modules/libraries</a></h2>
    

    <p>When you write CGI programs, you should consider using a
    code library, or module, to do most of the grunt work for you.
    This leads to fewer errors, and faster development.</p>

    <p>If you're writing CGI programs in Perl, modules are
    available on <a href="http://www.cpan.org/">CPAN</a>. The most
    popular module for this purpose is <code>CGI.pm</code>. You might
    also consider <code>CGI::Lite</code>, which implements a minimal
    set of functionality, which is all you need in most programs.</p>

    <p>If you're writing CGI programs in C, there are a variety of
    options. One of these is the <code>CGIC</code> library, from 
    <a href="http://www.boutell.com/cgic/">http://www.boutell.com/cgic/</a>.</p>
  </div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="moreinfo" id="moreinfo">For more information</a></h2>
    

    <p>There are a large number of CGI resources on the web. You
    can discuss CGI problems with other users on the Usenet group
    <a href="news:comp.infosystems.www.authoring.cgi">comp.infosystems.www.authoring.cgi</a>. And the -servers mailing
    list from the HTML Writers Guild is a great source of answers
    to your questions. You can find out more at 
    <a href="http://www.hwg.org/lists/hwg-servers/">http://www.hwg.org/lists/hwg-servers/</a>.</p>

    <p>And, of course, you should probably read the CGI
    specification, which has all the details on the operation of
    CGI programs. You can find the original version at the 
    <a href="http://hoohoo.ncsa.uiuc.edu/cgi/interface.html">NCSA</a> and there is an updated draft at the 
    <a href="http://web.golux.com/coar/cgi/">Common Gateway
    Interface RFC project</a>.</p>

    <p>When you post a question about a CGI problem that you're
    having, whether to a mailing list, or to a newsgroup, make sure
    you provide enough information about what happened, what you
    expected to happen, and how what actually happened was
    different, what server you're running, what language your CGI
    program was in, and, if possible, the offending code. This will
    make finding your problem much simpler.</p>

    <p>Note that questions about CGI problems should <strong>never</strong>
    be posted to the Apache bug database unless you are sure you
    have found a problem in the Apache source code.</p>
  </div></div>
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