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2.7 Reading a Web Page

Retrieving a web page from a web server is as simple as retrieving email from an email server. We only have to use a similar, but not identical, protocol and a different port. The name of the protocol is HyperText Transfer Protocol (HTTP) and the port number is usually 80. As in the preceding section, ask your administrator about the name of your local web server or proxy web server and its port number for HTTP requests.

The following program employs a rather crude approach toward retrieving a web page. It uses the prehistoric syntax of HTTP 0.9, which almost all web servers still support. The most noticeable thing about it is that the program directs the request to the local proxy server whose name you insert in the special file name (which in turn calls ‘www.yahoo.com’):

BEGIN {
  RS = ORS = "\r\n"
  HttpService = "/inet/tcp/0/proxy/80"
  print "GET http://www.yahoo.com"     |& HttpService
  while ((HttpService |& getline) > 0)
     print $0
  close(HttpService)
}

Again, lines are separated by a redefined RS and ORS. The GET request that we send to the server is the only kind of HTTP request that existed when the web was created in the early 1990s. HTTP calls this GET request a “method,” which tells the service to transmit a web page (here the home page of the Yahoo! search engine). Version 1.0 added the request methods HEAD and POST. The current version of HTTP is 1.1,6 and knows the additional request methods OPTIONS, PUT, DELETE, and TRACE. You can fill in any valid web address, and the program prints the HTML code of that page to your screen.

Notice the similarity between the responses of the POP and HTTP services. First, you get a header that is terminated by an empty line, and then you get the body of the page in HTML. The lines of the headers also have the same form as in POP. There is the name of a parameter, then a colon, and finally the value of that parameter.

Images (.png or .gif files) can also be retrieved this way, but then you get binary data that should be redirected into a file. Another application is calling a CGI (Common Gateway Interface) script on some server. CGI scripts are used when the contents of a web page are not constant, but generated instantly at the moment you send a request for the page. For example, to get a detailed report about the current quotes of Motorola stock shares, call a CGI script at Yahoo! with the following:

get = "GET http://quote.yahoo.com/q?s=MOT&d=t"
print get |& HttpService

You can also request weather reports this way.


Footnotes

(6)

Version 1.0 of HTTP was defined in RFC 1945. HTTP 1.1 was initially specified in RFC 2068. In June 1999, RFC 2068 was made obsolete by RFC 2616, an update without any substantial changes.


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