Using Lisp as a CGI Processor

Using Lisp as a CGI Processor on cubist.cs.washington.edu

This page explains how to use Gnu Common Lisp (gcl) to implement online conversational agents

There are several things you need to know about:

Accessing your account on our web server: Each student in the class has or will have an account on cubist.cs.washington.edu. The account names have been set up by the lab staff according to a class registration list. To log in on cubist, use your CSE kerberos password.
Creating your www directory. Each student needs to create a subdirectory of their home directory with the name www. Do this by typing in your home directory:

mkdir www

Installing an html page and testing it. Try creating an ordinary web page with a filename such as index.html and put it in your www directory. Use a web browser on some other machine and see if you can access the page. If your user name is joey, then the URL would be

http://cubist.cs.washington.edu/~joey/index.html

Installing a CGI script and testing it. The next thing to try is installing a simple Lisp program that begins with the line:

#! /usr/local/bin/gcl.exe -f

and that prints out the correct HTTP header info before anything else. It's the description of the type of content, followed by two newline characters. To keep life simple, just use the following line of Lisp:

(format t "Content-type: text/html~%~%")

Then put in some more FORMAT lines to create some text in the page. For example, to create a page with just a heading, you could use

(format t "<html><body><h1>This is my Lisp CGI script's")

(format t " output!</h1></body></html>~%")

Back in Linux's bash (i.e., on your regular command line) be sure to type something like
chmod +x mycgi.lsp
(or whatever your filename is, as long as it ends with .lsp) to make the program executable. Try accessing that script with your browser with a URL such as
http://cubist.cs.washington.edu/~joey/mycgi.lsp

How to process POST requests from the user. The key here is to read a string from standard input and parse it to obtain a collection of parameter-value pairs. These pairs are the info sent by the browser. The example program shows how to do this. You can run the sample program and you can examine the source code. It uses my package of Lisp CGI and HTML routines.
How to set a cookie on the user's browser. This will allow the next POST or GET to let the server know who the info is coming from. (See the same example program).
How to read a cookie on the user's browser. Once we have set the cookie, we need to read it in subsequent requests from the browser. (See the same example program).
How to create a session file. The way to maintain session continuity from one user request to the next is to keep a file on the server for each session in progress. The name of the file should be based on the user's unique information, so that it doesn't get mixed up with session files of other users. (See the same example program).
How to debug your script. "What do I do when I get `Internal Server Error'?". One thing you can do is look at the Web server's error messages. Here are today's error messages with your username.

Some documentation on GCL is available here.

Each student should be aware that CGI script writing involves system security risks. We all need to cooperate to see to it that we can all have a successful learning experience and avoid security lapses. Please read the WWW Security FAQ to get familiar with basic security issues.