What Is Common Gateway Interface (CGI)?

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∟What Is Common Gateway Interface (CGI)?

This section describes what is CGI (Common Gateway Interface) - a protocol defines how a Web server program interacts with application programs.

Common Gateway Interface (CGI) is a protocol that defines how a Web server program interacts with application programs. The interactions are illustrated in the following diagram:

Web                  Web      CGI        Appl.
Browser   Internet   Server   Protocol   Program
-------   --------   ------   --------   -------

          HTTP                Env. Var.
          request             stdin
Send --------------> Convert ----------> Process
                                            |
          HTTP                              |
          response            stdout        V
Receive <----------- Convert <---------- Process

There are several basic rules with CGI:

Web server provides most of the input information to application programs through environment variables.
Data send in the HTTP request with the GET method is converted to a special environment variable, QUERY_STRING.
Data send in the HTTP request with the POST method is converted to the standard input (stdin) channel.
Data printed to the standard output (stdout) channel is converted to the HTTP response.