1. USING PERL FOR CGI PROGRAMMING
MODULE 3
USING PERL FOR CGI PROGRAMMING

2. The Common Gateway Interface
Computation is required to support sophisticated web applications
Computation can be done by the server or the client (browser) or both
The Common Gateway Interface (CGI) is a protocol describing a standard way of providing server-side active web content
Under circumstances determined by the server, an HTTP request will cause a program to run
The output from the program will be the response returned to the client making the request
Data from forms will be encoded in a request sent do the server
This data can be used by a CGI program

3. CGI Linkage
There are several common ways a web server can use to determine if a web request should cause a CGI program to execute
Usually, the determination is based on the target of the request
Certain directories can be designated as containing CGI programs
Often cgi-bin is used
Certain file extensions can be designated as signifying a CGI program
.pl usually identifies a Perl script

4. CGI Linkage
A request for a CGI program to executed can be made with a simple link (<a> tag in HTML)
This method is limited
Any data sent must be hard-coded into the link
The usual way for invoking CGI programs is through the action attribute of a form
Data from the form is then encoded and sent with the request

5. CGI Program Action
The CGI program generally creates the response by sending output to the standard output stream
Using print in Perl
The CGI program will usually have to provide the content- type header
Content-type: text/html
This will be the last line of the response headers and must be followed by a blank line
The rest of the response is created by printing the HTML code desired to standard output

6. Query String Format
Both GET and POST requests under HTTP can be used to carry form data from the browser to the server
The data is formatted into a query string
Each form of request includes the information in a different way
In a GET request, the query string is appended to the URL of the request, with a question mark used to separate it from the first part of the URL
In a POST request, the query string is sent as the data part of the request
In both cases, the query string is formatted the same

7. Query String Format Each unit of data sent is encoded as name = value
The name is usually the value of a name attribute of a form widget
The value is the string representation of the widget value
Several units are combined by separating them with ampersands, &
Special characters in name and value are encoded
The code is a percent sign, %, followed by the hexadecimal code for the character
A space is encoded as %20
Some browsers will encode spaces as +

8. Query String Access
When the POST method is used, the query string can be read from standard input
The CONTENT_LENGTH environment variable tells how many characters can be read
When The GET method is used, the query string is given by the value of the environment variable QUERY_STRING

9. The CGI.pm Module Writing a CGI program from scratch is very tedious
Creating HTML requires numerous print statements
Retrieving data from the query strings is tricky
One of the reasons for Perl’s popularity for CGI programming is the powerful pattern matching facilities which greatly ease the task of parsing a query string
The Perl module CGI.pm provides numerous functions to help with both of these problems

10. Shortcut Functions in CGI.pm
Shortcut functions return string values containing HTML code
Note, the string must be printed out to actually become part of the response
Some functions take no arguments
print br;
puts the tag <br/> into the response
Some functions can be given a single argument which becomes the content of the tag
print h1(“A Header”)
puts
<h1>A Header</h1>
into the response

11. Tag Attributes in Shortcut Functions
Attributes for tags are provided as attribute/value pairs in the argument list of the shortcut function
The arguments are provided in the form of a literal hash
Attribute names are preceded by a hyphen, -
print textarea(-name => "Description",
-rows => "2",
-cols => "35");
produces this in the response
<textarea name="Description" rows="2" cols="35">
</textarea>

12. Attributes and Content
Attributes and content can both be provided to a shortcut by giving the attributes explicitly as a hash reference
print a({-href => "fruit.html"},
Press here for fruit descriptions");
produces this in the response
<a href="fruit.html"> Press here for fruit descriptions </a>
If an array reference is provided for the content, a tag is created for each item, giving the tag all the specified attributes

13. More Shortcuts The head shortcut function provides a standard header
The start_html function provides the beginning part of an HTML document, through the <body> start tag
The function takes one argument, the document title
The param function takes a name as an argument
The function returns the value associated to the name, if any, in the request

14. Table Shortcut Functions
The Tr function will create a sequence of table rows from an argument that is a reference to a list
Tr is capitalized to distinguish it from the transliterate function tr
Similarly, the th and td functions will produce a sequence of tags from a reference to a list

15. Table Example table({-border => "border"},
caption("Sales Figures"),
Tr(
[th(["Salesperson", "Mon", "Tues", “Wed",
"Thu", "Fri"]), ] ) );
Note the period between the th and td function calls, this concatenates the results in order to make a single row

16. Table Example Results

17. Cookies
HTTP is a stateless protocol, that is, the server treats each request as completely separate from any other
This, however, makes some applications difficult
A shopping cart is an object that must be maintained across numerous requests and responses
The mechanism of cookies can be used to help maintain state by storing some information on the browser system
A cookie is a key/value pair that is keyed to the domain of the server
This key/value pair is sent along with any request made by the browser of the same server
A cookie has a lifetime which specifies a time at which the cookie is deleted from the browser

18. Cookies and Security
Cookies are only returned to the server that created them
Cookies can be used to determine usage patterns that might not otherwise be ascertained by a server
Browsers generally allow users to limit how cookies are used
Browsers usually allow users to remove all cookies currently stored by the browser
Systems that depend on cookies will fail if the browser refuses to store them

19. Cookie Functions
The cookie function takes a hash with three keys for the name, value and expiration time of a cookie
The cookie value produced by this function must be passed to the header function using the –cookie key
header(-cookie => $a_cookie)
Calling the cookie function with no arguments produces a hash of all cookies from the current request