USING PERL FOR CGI PROGRAMMING

MODULE 3

The Common Gateway InterfaceComputation is required to support sophisticated web

applicationsComputation can be done by the server or the client

(browser) or bothThe Common Gateway Interface (CGI) is a protocol

describing a standard way of providing server-side active web contentUnder circumstances determined by the server, an HTTP

request will cause a program to runThe output from the program will be the response

returned to the client making the requestData from forms will be encoded in a request sent do

the serverThis data can be used by a CGI program

CGI LinkageThere 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 requestCertain directories can be designated as

containing CGI programsOften cgi-bin is used

Certain file extensions can be designated as signifying a CGI program.pl usually identifies a Perl script

CGI LinkageA request for a CGI program to executed

can be made with a simple link (<a> tag in HTML)This method is limitedAny data sent must be hard-coded into the

linkThe usual way for invoking CGI programs is

through the action attribute of a formData from the form is then encoded and sent

with the request

CGI Program ActionThe CGI program generally creates the

response by sending output to the standard output streamUsing print in Perl

The CGI program will usually have to provide the content-type headerContent-type: text/htmlThis will be the last line of the response headers

and must be followed by a blank lineThe rest of the response is created by printing

the HTML code desired to standard output

Query String FormatBoth 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 stringEach form of request includes the information

in a different wayIn 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

Query String FormatEach unit of data sent is encoded as

name = valueThe name is usually the value of a name attribute of a

form widgetThe value is the string representation of the widget

valueSeveral 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 %20Some browsers will encode spaces as +

Query String AccessWhen the POST method is used, the query

string can be read from standard inputThe CONTENT_LENGTH environment variable

tells how many characters can be readWhen The GET method is used, the query

string is given by the value of the environment variable QUERY_STRING

The CGI.pm ModuleWriting a CGI program from scratch is very

tediousCreating HTML requires numerous print

statementsRetrieving 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

Shortcut Functions in CGI.pmShortcut functions return string values

containing HTML codeNote, the string must be printed out to actually

become part of the responseSome 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

Tag Attributes in Shortcut FunctionsAttributes for tags are provided as attribute/value

pairs in the argument list of the shortcut functionThe arguments are provided in the form of a literal

hashAttribute 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>

Attributes and ContentAttributes 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

More ShortcutsThe head shortcut function provides a

standard headerThe start_html function provides the

beginning part of an HTML document, through the <body> start tagThe function takes one argument, the

document titleThe param function takes a name as an

argumentThe function returns the value associated

to the name, if any, in the request

Table Shortcut FunctionsThe Tr function will create a sequence of

table rows from an argument that is a reference to a listTr is capitalized to distinguish it from the

transliterate function trSimilarly, the th and td functions will

produce a sequence of tags from a reference to a list

Table Example table({-border => "border"},

caption("Sales Figures"),Tr([th(["Salesperson", "Mon", "Tues", “Wed",

"Thu", "Fri"]), th("Mary").td(\@marysales),

th("Freddie").td(\@freddiesales), th("Spot").td(\@spotsales),]

));

Note the period between the th and td function calls, this concatenates the results in order to make a single row

Table Example Results

CookiesHTTP is a stateless protocol, that is, the server

treats each request as completely separate from any other

This, however, makes some applications difficultA shopping cart is an object that must be maintained

across numerous requests and responsesThe 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 serverThis key/value pair is sent along with any request

made by the browser of the same serverA cookie has a lifetime which specifies a time at

which the cookie is deleted from the browser

Cookies and SecurityCookies are only returned to the server

that created themCookies 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 usedBrowsers usually allow users to remove all

cookies currently stored by the browserSystems that depend on cookies will fail if

the browser refuses to store them

Cookie FunctionsThe 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