jeremy.marzhillstudios.com/_content/Perl-and-CGI-part-I-2005-5-4.yml

title: Perl and CGI part I
time: 2005-05-04 00:47:13
tags:
    - Uncategorized
content-type: html
content: |
Perl and CGI part I Cookies, Query Strings, and Post variables.... OH My!!!! If you're just starting out in perl and trying to figure out how to handle all that cgi stuff you have a number of options. You can use a CPAN module (CGI, CGI-Lite, FCGI...) or you can use a do it yourself solution. The CPAN Modules have the benefit of handling everything for you with easy(supposedly) to use methods. On the other hand if you don't need all the features of a CPAN Module a do it yourself solution may be less confusing and have a smaller codebase. In this article I will walk you through making a custom module for your CGI to handle the basics of a CGI application. This article assumes familiarity with basic Perl Syntax and Modules. All those Cookies, Query Strings and Post Variables are made possible in the HTTP world by something called HTTP headers. The HTTP header tells the application (browser, Server, rss reader.....) making the page request useful information for displaying the page. Some of the Header is built automatically by the HTTP server(apache, IIS....) or application making a request. However, if you want to include cookies, or retrieve query strings, post variables and cookies values on the server side, you have to be able to retrieve the values from those headers. Now in my case I needed to be able to set and read cookies and retrieve Query strings and Post variables. I didn't need to be able to write out html automatically or output anything else other than that header. Rather than try to figure out how to use the CPAN modules for just those tasks and nothing else I decided to stretch myself a little and write my own. I like learning new things and it turned out not to be any more difficult than wading through the documentation for the CPAN modules would have been. The first thing to remember when woking on a CGI Module is the last line of the HTTP Header. The last line you say? Why do we start on the last line? The reason is that any header regardless of whether it has anything else in it must have this last line. Additionally this line must be last because, when the browser sees it, it stops processing the header and assumes everything after it is part of the page itself. What is this magical line: "Content-type: text/html\n\n" This line tells the browser application what mime type the page is. In this case it is html text. I could just as easily have set it to text/xml, text/rss, or any other mime type I cared to, including my own custom mime types. For more information about mime types you can look here. For our custom module we are are going to use an object to represent the header. What we want, is to be able to add cookies to the header. We also want to leave it open to possibly adding other things to the header later. When our header is created we will retrieve the string by using a method. The first thing our Header object needs is a constructor method, which we will call new_header, in the interest of readable code. <blockquote><code>package pk_cgi; require Exporter; use strict; our @ISA = qw(Exporter); our @EXPORT = qw(cgi_request new_header get_cookie_list get_cookie); sub new_header { my $proto = shift; my $class = ref($proto) || $proto; my $header = "Content-type: text/html\n\n"; return bless(\$header, $class); }</code></blockquote> What we did here is create a string with that all important last line in the header. This header is actually completely viable now. we could output it and it would be perfectly acceptable to the requesting application. It does not however have any cookies defined in it. Those will be added later if we should want them. We will use an object method for those. As you can see the object creation is almost absurdly simple. Just a string blessed into the object. Should you wish, you could also add arguments to set the mime type to something else. Now, what about those cookies? How do we handle those? Cookies are handled by lines in the header like this one "Set-Cookie: cookiename=cookievalue\n". That is a basic cookie header. you could also add some optional parameters: "Set-