A few years ago, Web programming started out with the Common Gateway Interface, or CGI. Here's a quick review of the basics concepts of CGI.

1. When a user makes a CGI request of a Web server, something in the URL will tip off the server to process it as a CGI request. The hint in the URL could look like one of the following examples.
   • The URL requested by the user is in a /cgi-bin/ directory: http://www.someWhere.com/cgi-bin/randomCGIprogram
   • The Web server might be configured to automatically recognize certain file extensions as being CGI executables: http://www.erehwon.org/goSearch.py .py commonly denotes Python programs, another popular language for Web programming.
   • The file extension might be a "generic" CGI extension: http://www.xyz.net/doSomething.cgi
2. In these cases, the Web server "hands off" to a program specified by the URL, spawning it as a child process and providing it with the information it needs to be a "Web program": generally environment variables (ie, QUERY_STRING) and Standard Input (STDIN).
3. The program will run and produce information, which it sends to Standard Output (STDOUT). Usually, the program will then create a minimal amount of HTTP header information, at the very least, as part of its Output.
4. The Web server will "capture" the STDOUT stream and redirect it to the user via the Web. The user's browser will interpret the information according to the HTTP header. This will usually be HTML text, but CGI programs can just as easily create byte streams to be reconstructed as JPEG images or RealAudio feeds.

A simple C program:

     #include <stdio.h>

     int main () {
         print("Hello, world!\n");
     }

which we can turn into a simple CGI program by simply adding an HTTP header:

     #include <stdio.h>

     int main () {
         print("Content-type: text/plain\n\n");
         print("Hello, world!\n");
     }

Now all that's left to do is compile this code and put the resulting binary in my Web directory structure and then set permissions appropriately.

NOTE: print("Content-type: text/plain\n\n"); should look like the Perl examples we looked at during  Week 6 which also used print("Content-type: text/html\n\n");

CGI is still used quite a bit in the Web world, but serious complaints have been made about it.

• Spawning a child process is hard work and costs time and memory. This problem with speed has prompted many a complaint from producers of high-traffic Web sites.
• Web servers contain a lot more information than just environment variables and STDIN. It would be handy sometimes for Web programs to gain access to these extra tidbits.
• The whole classical programming paradigm has proven cumbersome for most Web-programming needs. What you're really trying to do is write a program that will intelligently compose HTML on your behalf. So, why does it look like computer code? Why can't it look more like HTML?

Web-Embedded Programming

More modern ways of programming Web applications have arisen over the past few years. Their roots lie in Server-parsed HTML - Server Side Includes (SSIs) (.shtml - UNIX or .stm - Windows), a programming option that's been on the scene since almost the beginning, but was never powerful enough for serious application programming.

The new languages and techniques revolve around embedding programming code into HTML files. Some popular examples are:

• Active Server Pages (.asp files) are used by the Microsoft IIS Webserver. ASP files can be activated with several different scripting engines, including VBScript, JavaScript, and PerlScript.
  (Code Example) & (Form Using Get & Form Using Post - Form.asp - v1 & Form.asp - v2)
   NOTE: ASP has been ported to UNIX and Linux using two different development tracks:
  • Commercial (ChiliSoft) - expensive
  • Perl Module - free
• Allaire Cold Fusion (.cfm files) provide a very handy commercial Web-development environment. Though it started off only being available on the Windows NT side of the house, lately it's cropped up on the UNIX and Linux as well.
  (Code Example)
• PHP (.php or .php3 files) like Active Server Pages except on steroids. PHP incorporates features from Shell programming, C programming, Perl, and Java. Works on NT, UNIX, and Linux platforms.
  (Code Example - Script in Action)