[Chapter 3] 3.2 CGI and Response Headers

3.2 CGI and Response Headers

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
Header	Description
Content-length	The length (in bytes) of the output stream. Implies binary data.
Content-type	The MIME content type of the output stream.
Expires	Date and time when the document is no longer valid and should be reloaded by the browser.
Location	Server redirection (cannot be sent as part of a complete header).
Pragma	Turns document caching on and off.
Status	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
Header	Description
Refresh	Client reloads specified document.
Set-Cookie	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.