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1.INTRODUCTION:

The Common Gateway Interface (CGI) is a standard for

interfacing external applications with information servers, such as HTTP orWeb servers. A plain HTML document that the web daemon retrieves is

static, which means it exits in a constant state: a text file that doesnt change.

A CGI program, on the other hand, is executed in real-time, so that it can

output dynamic information.

For example, you have used a fill-out or an image map on Web,

you have probably used a gateway script without even knowing it. With CGI

Scripts, everything seems to happen automatically. You enter data and click

a mouse button, and a moment later, a result is displayed. What happens

between the click of the mouse and the display of the result is what CGI

does.

Steps

Client sends request to the server

Server does not have the capacity to process the client request

So, it inturn contacts some server side program like CGI

Now CGI process the client request and sends response back to client

Then Server sends the response back to client in the form of html page

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2.WHAT ARE CGI SCRIPTS?

CGI scripts are external programs that run on the Web server. CGI

scripts are used to create highly interactive Web publications. The standard

that defines how external programs are used on Web servers and how they

interact with other applications is the Common Gateway Interface. The three

keywords that comprise the name of the standard---common, gateway and

interface---describe how the standard works.

Common: By specifying a common way for scripts to be

accessed, CGI enables anyone, regardless of platform, to pass information to

a CGI script.

Gateway: By defining the link (gateway) between the script,

the server, and other applications, CGI makes it possible for external

program to accept generalized input and pass information to other

applications.

Interface: By describing the external programs can be accessed

by users (the interface), CGI reduces the complex process of interfacing with

external programs to a few basic procedures.

3.CHOOSING A PROGRAMMING LANGUAGE FOR

SCRIPTS:

CGI scripts are also called gateway scripts. One can writegateway scripts in almost any computer languages that produce an

executable file. The most common language for scripts is the following:

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Bourne Shell

C Shell

C/C++ Shell

PERL

Python

Tcl

Visual Basic

Scripts located on server called server-side scripts (CGI). In the

contrast, some scripts, called client-side scripts, are embedded in HTML

documents and executed by browser. JavaScript and VB Script are two up

and coming client-side scripting languages.

The most programming language to write script is one

that works with Web server and meet needs. Preferably,

the language should already be available on the Web

server and user should be proficient in it (or at least have

some knowledge). The most user input in the form of text

that most manipulated in some way, which makes

supports for text strings and their manipulations critical.

PERL, C/C++, and UNIX shell are the most popularlanguages for scripts. PERL is rapidly becoming the

widely used language for CGI scripts.

4.WHY CGI SCRIPTS?

The primary reason to use CGI scripts is to automate what

would otherwise be a manual and probably time consuming process. UsingCGI scripts benefits both developer and user. The user gets simplicity,

automated response to input, easy ways to make submissions, and fast ways

to conduct searches. Gateway scripts enable to automatically process orders,

queries, and much more. CGI programs are commonly used to do the

following tasks:

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*Process input, typically search strings, and output a document

containing the results of the search.

*Validate user identification and password information and

grant readers access to restricted areas of the Web site.

*Process input from image maps and direct the reader to

associated documents.

*Add the readers feedback or survey responses to a database or

index.

*Track visitors to Web pages and post continually updated

numbers to the Web pages as it is accessed.

*Generate documents based on the type of browser the reader is

using.

*Perform post-submission processing and output results for the

reader.

5.HOW CGI SCRIPTS WORK?

Gateway scripts are used to process input submitted by readers

of Web publications. The input usually consists of environment variables

that the Web server passes to the gateway scripts. Environment variables

describe the information being passed, such as the version of CGI used on

the server, the type data, the size of the data, and other importantinformation. Gateway scripts can also receive command-line arguments and

standard input. To execute a CGI script, the script must exist on the server

that referencing. One must also have a server that is both capable of

executing gateway scripts and configured to handle the type of script one to

use.

Readers pass information to a CGI script by activating a link

containing a reference to the script. The gateway script processes the input

and formats the results as output that the Web server can use. The Web

server takes the results and passes them back to the readers browser. The

browser displays the output for the reader.

The output from a gateway script begins with header containing

a directive to the server. Currently, the three valid server directives are

Content-type, Location, and Status. The header can consist of a directive in

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the format of an HTTP header followed by a blank line. The blank line

separates the header from the data passing back to the browser. Output

containing Location and Status directives usually consists of a single line

because the directive contained on the Location or Status line is all that

server, and when there is no subsequent date, no need to insert a blank line.

The server interprets the output, sets environment variables, and passes the

output to the client.

Any transaction between a client and server has many parts.

These parts can be broken down into following steps:

1.Browser shows the HTML document containing the form.

2.User enters data into the form as need then click the Submit button.

3.Optionally, a script in the browser performs client-side validation of

the forms contents.

4.The browser decodes the URL and contacts the server.

5.Browser requests the document file from the server.

6.The server translates the URL into a path and filename.

7.The server realizes that the URL points to a program instead of a

Static file.

8.The server prepares the environment and launches script.

9.The script executes and reads the environment variables and

STDIN.

10.The script sends the proper MIME headers to STDOUT for the

Forthcoming content.

11.The script sends the rest of its output to STDOUT and terminates.

12.The server notices that the script has finished and closes the

Connection to browser.

13.Browser displays the output from the script.

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6.INPUT TO CGI SCRIPTS:

When a user activates a link to gateway script, input is sent to

the server. The server formats this data into environment variables and

checks to see whether additional data was submitted via standard input

stream.

Environment variables:

Input to CGI scripts usually is in the form of environment

variables. The environment variables passed to gateway scripts are

associated with the browser requesting information from the server, theserver processing the request, and the data passed in the request.

Environment variables are case-sensitive. Although some environment

variables are system-specific, many environment variables are standard.

Environment variables are set automatically whenever reader input is passed

to a server.

Standardenvironmentvariables:

Variable Purpose

AUTH_TYPE Specifies the authentication method and is

Used to validate a users access.

CONTENT_LENGTH Used to provide a way of tracking the length

Of the data string as a numeric value.

CONTENT_TYPE Indicates the MIME type of data.

GATEWAY_INTERFACE Indicates which version of the CGI standard

The server is using.

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HTTP_ACCEPT Indicates the MIME type content types the

browser will accept, as passed to the

gateway script via the server.

HTTP_USER_AGENT Indicates the type of browser used to send

the request, as passed to the gateway script

Via the server.

PATH_INFO Identifies the extra information included in

the URL after the identification of the

CGI script.

PATH_TRANSLATED Set by server based on the PATH_INFOvariable. The server translates the

PATH_INFO variable into this variable.

QUERY_STRING Set to the query string (if the URL contains a

query string).

REMOTE_ADDR Identifies the Internet Protocol address of

the remote computer making the request.

REMOTE_HOST Identifies the name of the machine making

the request.

REMOTE_IDENT Identifies the machine-making request.

REMOTE_USER Identifies the user name as authenticated by

the user.

REQUEST_METHOD Identifies the method by which request was

made.

SCRIPT_NAME Identifies the virtual path to the script being

executed.

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SERVER_PORT Identifies the port number the server

received the request on.

SERVER_PROTOCOL Identifies the protocol of the request sent to

the server.

SERVER_SOFTWARE Identifies the Web server software.

CGI Standard Input:

Most users input sent to a Web server is used to sent environment

Variables, yet not all input fits neatly into an environment variable. When a

user submits data to be processed by a gateway script, this data is received

as an URL-encoded search string or through the standard input stream.

Sending data as standard input is the most direct way to send data.

The server opens the standard input stream and reads the specified amount

of data.

CGIINPUT:

Under HTTP 1.0, a two-submission method for forms exists. The

HTTP GET method uses URL-encoded search strings. When a server

receives an URL-encoded search string, the server assign the value of the

Search string to the QUERY_STRING environment variable.

The HTTP POST method uses the standard input streams. When

Receives data by the standard input stream, the server assigns the value with

the length of the input stream to the CONTENT_LENGTH variable.

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7.CGI SCRIPT STRUCTURE:

When the server invokes script, the server passes information to

the script via environment variables and, in the case of POST via STDIN.

The basic structure of CGI application is straightforward

initialization, processing, output and termination ideally a script follows

these steps in this order.

1.The program begins.

2.The program calls do-initialize.

3.The program calls do-process.

4.The program calls do-output.

5.The program ends.

INITIALIZATION:

The first thing script must do when it starts is its input,

environment and state. Basic operating system environment information canbe obtained the usual way (e.g. from the system registry in the windows)

State information will come from the input rather than the

operating environment or static variables. Each time CGI scripts are

invoked, its as if theyve never been invoked before. Every thing must be

initialized from scratch.

PROCESSING:

The processing, however, is the heart of your script. Processing

data means, generally, transforming it some how, in classical data processing

terminology, this is called the transform step because in batch-oriented

processing, the program reads a record, applies some rule to it (transforming

it), and then writes it back out.

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OUTPUT:

In simple the output is usually a header and some HTML. More

complex scripts might output graphics, graphic mixed with text, or all the

information necessary to call the script again with some additional

information.

TERMINATION:

Termination is nothing more than cleaning up after your self

and quitting. If youve locked any files, you must release them letting the

program end. If youve allocated memory, semaphores, or others, you must

free them.

8.OUTPUT FROM CGI SCRIPT

After the script finishes processing the input, the script should

return output to the server. The server will then return the output to the

client. Generally, a blank line and a body in the form of HTTP response that

includes a header follow this output. Although the CGI header output is

strictly formatted, the body of the output is formatted in the manner youspecify in the header. For example the body can contain an HTML document

for the client display.

CGI HEADERS:

CGI headers contain directives. Your CGI script outputs these

directives are valid

1.content-type

2.location

3.status

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A single header can contain one or all of the server directives,

your CGI script outputs these directives to the server. Although the header is

followed by blank line that separates the header from the body, the output

does not have to contain a body.

CONTENT TYPES USED IN CGI HEADERS:

The content-type field in a CGI header identifies the MIME

type of the data you are sending back to the client. Usually the data output

from a script is a fully formatted document, such as an HTML document.

Content-type: text/html.

LOCATIONS USED IN CGI HEADERS:

The output of your script doesnt have to be a document createdwithin the script. You can reference any document on the web using the

location field by its URL. Server process location references wither directly

or indirectly depending on the location of the file. If the server can find the

file locally, it passes the file to specify a location in a script as follows:

Location: http://www.typress.com/

STATUS USED IN THE CGI HEADERS:

The status field passes a status line to the server for forwarding

to the client. The status code shows the general status as follows:

1. Not yet allocated

2. Success

3. Redirection

4. Client error

5. Server error

Although servers use many status codes, the status codes you pass to a

client via your CGI script are usually client error codes.

Basic hello world CGI script in C, with basic HTML output added

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int main (int arc, char *arc[])

{

printf (Content-type: text/html\n\n);

printf ();

printf ();

printf (Hello, world!\n);

printf ();

printf (\n);

printf(

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maintenance, and the capability to perform the chosen task should also be

considered.

If one of your mandates is the capability to move code among

platforms with a minimum of functions from the ANSI C libraries and avoid

making other operating system calls. Unfortunately, following this will limit

scripts basic functionality if you wrap your platform-dependent code in self-

contained routines; however, you minimize the work needed to port from

one platform to the next. A properly designed program can have any module

replaced in its entirely without affecting the rest of the program. Using these

guidelines, you may have to replace a subroutine or two, and youll certainly

have to recompile step is used. Learning to write Perl is easy than C

Server Independence:

Server independence is fairly easy to achieve, but for some

reason to be stumbling block to beginning scriptwriters. To be server

independence, script must run without modification on any server using the

same operating system. Only server-independence programs can be useful as

shareware or freeware a doubt, server-independence is a requirement for

commercial software.

The following are some rules of server-independence:

1.Dont assume your environment.

2.Dont assume privileges.

3.Dont assume consistency of CGI variables.

4.Dont assume version-specific information.

5.Dont assume LAN or WAN configuration.

6.Dont assume the availability of system objects.

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10.CONCLUSION

The Common Gateway Interface opens the door for adding

advanced features to your Web publications. This workhorse running

quietly in the background less you use fill-out forms, database queries,and index searches and create documents on the fly. You use CGI

whenever you want to open a two-way communication channel with

the reader. CGI enhancement to Web means extra for Web publishes.

Still, the payoff associated with CGI makes the extra effort truly

worthwhile.

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