Keeping Programs Trim with CGI_Lite
If we know that we want to put one or more of the form elements into scalar variables (and not keep them in the hash), we can do so by calling the method create_variables. For instance, in our example above, we first had to use parse_form_data in order to get the form elements into the hash %FORM. Then we had to assign $firstname in a separate step. If we had wanted to assign 10 variables based on the contents of the form, we would have needed to make 10 separate assignments, which is rather inefficient.
To get around this problem, we can use the create_variables method, which automatically creates local variables for us. If we want to turn each form element into its own variable, we can simply invoke:
When this method returns, we have a new variable defined for each element that was in the submitted form. Thus, if we have a form element named firstname, the value associated with that element is now available via the variable $firstname. The backslash in front of %FORM gives us a reference to the hash, a new feature in Perl 5 documented in great detail in the Perl manual pages (available by typing man perlref on most Linux systems).
There is one potential problem with create_variables, namely, your program might define variables with the same names as one or more form elements. For example, Listing 3 is a version of hello.pl in which we give the variable $firstname a value and call create_variables on the submitted form that included an element named firstname.
When $firstname is set to the value NOT CHANGED, as in Listing 3, the value of the HTML form element firstname is ignored when we call create_variables, and we get a greeting to NOT CHANGED, rather than the user's first name. If we comment out the line defining $firstname as NOT CHANGED, create_variables does its job just fine, creating a variable named $firstname and giving it the value that the user provided. This behavior is a good idea in terms of web security, but the silent failure of one or more variable assignments strikes me as a possible pitfall.
CGI.pm offers similar functionality with its import_names method. In this case, the authors encourage users to import names into a separate name space, ensuring that there are no name conflicts with existing variables.
Notice that in the Listing 3 version of hello.pl, I have removed the use strict line. This was to avoid possible conflicts when commenting out the line that defines a default value for $firstname. The strict module requires that you define variables before using them; however, if we are referencing variables that are created by create_variables, this is impossible.
CGI_Lite.pm is smart enough to grab form elements passed by either of the two methods: GET or POST. POST is generally considered to be the better method of the two, since it passes the contents of the form to a CGI program via standard input (stdin), rather than as part of the URL. However, if we were interested in passing a name to hello.pl as part of the URL, we could do so as follows:
Of course, if you are testing this program from a computer other than the web server, you need to replace localhost with the name of a server. For example, if your server runs on www.fictional.edu, you could use:
http://www.fictional.edu/cgi-bin/hello.pl?firstname=ReuvenNotice how we can set the variable's value after the question mark, known in CGI lingo as the “query string”. The query string is part of the URL, and URLs may not contain white space or other “dangerous” characters that might be misinterpreted by the browser and/or the server. For these reasons, certain characters must be sent in “percent-hex” format, in which the character's ASCII value in hexadecimal is prefaced by a percent sign. Obviously, the percent sign itself (ASCII value 0x25) must be encoded in this way. Thus, if my “first” name were actually two names, I could send the string as follows:
http://www.fictional.edu/cgi-bin/hello.pl?firstname=J%20EdgarSince the “space” character is ASCII 0x20 (32 in decimal), we can insert a space into the URL by sending a %20. CGI_Lite.pm automatically translates the percent-hex encoding into the ASCII codes we want.
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- Doing Astronomy with Python
- Problems with Ubuntu's Software Center and How Canonical Plans to Fix Them
- Firefox Security Exploit Targets Linux Users and Web Developers
- My Network Go-Bag
- Build a “Virtual SuperComputer” with Process Virtualization
- Three More Lessons
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