CGI (Common Gateway Interface) is a protocol that connects a web client to a web server. CGI program is a filter that gets input from a client, manipulate data on the server, and feedback result to the client. CGI turns a static web site into an interactive and dynamic web site. One CGI application accepts the HTML form data from a client and returns a HTML page. Another CGI application is gateway where CGI program forms a SQL query and generates user readable information retrived from a database. Finally, a CGI program is suitable to generate dynamic documents.

CGI program runs on a web server. Usually it is located on a subdirectory called cgi-bin. When server receives a command such as "get /cgi-bin/default.cgi HTTP 1.0", the default.cgi program will start to run. HTTP 1.0 is a communication protocol. CGI program will receive its input from stdin or environment variables in an operating system. It will send outputs to stdout. By using redirection CGI may easily change the output target.

_cgi command specifies a list of CGI headers. The 'Content-type' header specifies the type of CGI's returning data.

# launch snaml program
#!/usr/local/bin/snaml
_cgi "Content-type: text/html"
_html
_head
  _title; _quote "CGI"; title_
head_
_body
  _quote "Simplest CGI Program."
body_
html_