CGI Programming on the World Wide WebBy Shishir Gundavaram
1st Edition March 1996
This book is out of print, but it has been made available online through the O'Reilly Open Books Project.
By now, you should be reasonably comfortable designing CGI programs that create simple virtual documents, like this one:
#!/usr/local/bin/perl print "Content-type: text/html", "\n\n"; print "<HTML>", "\n"; print "<HEAD><TITLE>Simple Virtual HTML Document</TITLE></HEAD>", "\n"; print "<BODY>", "\n"; print "<H1>", "Virtual HTML", "</H1>", "<HR>", "\n"; print "Hey look, I just created a virtual (yep, virtual) HTML document!", "\n"; print "</BODY></HTML>", "\n"; exit (0);
Up to this point, we have taken the line that outputs "Content-type" for granted. But this is only one type of header that CGI programs can use. "Content-type" is an HTTP header that contains a MIME content type describing the format of the data that follows. Other headers can describe:
- The size of the data
- Another document that the server should return (that is, instead of returning a virtual document created by the script itself)
- HTTP status codes
This chapter will discuss how HTTP headers can be used to fine-tune your CGI documents. First, however, Table 3.1 provides a quick listing of all the HTTP headers you might find useful.
Table 3.1: Valid HTTP Headers
The length (in bytes) of the output stream. Implies binary data.
The MIME content type of the output stream.
Date and time when the document is no longer valid and should be reloaded by the browser.
Server redirection (cannot be sent as part of a complete header).
Turns document caching on and off.
Status of the request (cannot be sent as part of a complete header).
The following headers are "understood" only by Netscape-compatible browsers (i.e., Netscape Navigator and Microsoft Internet Explorer).
Table 3.2: Netscape-Compatible Headers
Client reloads specified document.
Client stores specified data. Useful for keeping track of data between requests.
You can see a complete list of HTTP headers at http://www.w3.org/hypertext/WWW/Protocols/HTTP/Object_Headers.html
Also, there are a couple of things you should know about header syntax:
- Header lines don't have to be in any special order.
In general, the headers you generate from a CGI program can be output in any order you like.
- The header block has to end with a blank line.
HTTP is a very simple protocol. The way the server knows that you're done with your header information is that it looks for a blank line. Everything before the blank line is taken as header information; everything after the blank line is assumed to be data. In Perl, the blank line is generated by two newline characters (\n\n) that are output after the last line of the header. If you don't include the blank line after the header, the server will assume incorrectly that the entire information stream is an HTTP header, and will generate a server error.
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