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HTML, HTTP, and Your CGI Program (cont'd)

The HTTP Headers

If HTML is responsible for gathering data to send to your CGI program, how does it get there? The data gathered by the browser gets to your CGI program through the magic of the HTTP request header. The HTML tags tell the browser what type of HTTP header to use to talk to the server-your CGI program. The basic HTTP headers for beginning communication with your CGI program are Get and Post.

If the HTML tag calling your program is a hypertext link, the default HTTP request method Get is used to communicate with your CGI program, as in this example:

<a href="www.domain.com/program.cgi">, call a CGI program </a>

If, instead of using a hypertext link to your program, you use the HTML Form tag, the Method attribute of the Form tag defines what type of HTTP request header is used to communicate with your CGI program. If the Method field is missing or is set to Get, the HTTP method request header type is Get. If the Method attribute is set to Post, a Post method request header is used to communicate with your CGI program. (The Get and Post methods are covered in Tutorials 4 and 5.)

After the method of sending the data is determined, the data is formatted and sent using one of two methods. If the Get method is used, the data is sent via the Uniform Resource Identifier (URI) field. (URI is covered in Tutorial 2.) If the Post method is used, the data is sent as a separate message, after all the other HTTP request headers have been sent.

After the browser determines how it is going to send the data, it creates an HTTP request header identifying where on the server your CGI program is located. The browser sends to the server this HTTP request header. The server receives the HTTP request header and calls your CGI program. Several other request headers can go along with the main request header to give the server and your CGI program useful information about the browser and this connection.

Your CGI program now performs some useful function and then tells the server what type of response it wants to send back to the server.

So where are we so far? The data has been gathered by the browser using the format defined by the HTML tags. The data/URI request has been sent to the server using HTTP request headers. The server used the HTTP request headers to find your CGI program and call it. Now your CGI program has done its thing and is ready to respond to the browser. What happens next? The server and your CGI program collaborate to send HTTP response headers back to the browser.

What about the data-the Web page-your CGI program generated? Well, that's why the HTTP response headers are used. They describe to the browser what type of data is being returned to the browser.

Your CGI program can generate all the HTTP response headers required for sending data back to the client/browser by calling itself a non-parsed header CGI program. If your CGI program is an NPH-CGI program, the server does not parse or look at the HTTP response headers generated by your CGI program; they are sent directly to the requesting browser, along with data/HTML generated by your CGI program.

The more common method of returning HTTP response headers is for your CGI program to generate the minimum required HTTP request headers; usually, just a Content-Type HTTP response header is required. The server then parses, or looks for, the response header your CGI program generated and determines what additional HTTP response headers should be returned to the browser.

The Content-Type HTTP response header identifies to the browser the type of data that will be returned to the browser. The browser uses the Content-Type response header to determine the types of viewers to activate so that the client can view things like inline images, movies, and HTML text.

The server adds the additional HTTP response headers it knows are required, bundles up the set of the headers and data in a nice TCP/IP package, and then sends it to the browser. The browser receives the HTTP response headers and displays the returned data as described by the HTTP response headers to your customer, the human.

So now you have the whole picture (which you will learn about in detail throughout the book), made up of the HTML used to format the data and the HTTP request and response headers used to communicate between the browser and server what type of data is being sent back and forth. Among all this is your very cool CGI program, aware of what is going on around it and driving the real applications in which your Web client really is interested.