3. The Common Gateway Interface

CGI (Common Gateway Interface) is the generic interface between the server and server-side 'gateway' programs. CGI specifies how data are sent to the gateway program, and how data can be returned by the gateway program, through the server and back to the browser that originally sent the data. The basic mechanisms are:

Sending Data to the Gateway Program

CGI (Common Gateway Interface) is the generic interface between the server and these server-side 'gateway' programs. The CGI specifies how data are sent to the gateway program (as environment variables or as data read, by the gateway program, from standard input) and what data are sent (in general, all the data sent by the client to the server, plus extra environment variables describing the status of the server).

Returning Data to the Client

To return data back to the client program (the user's web browsers) the gateway program just writes data to it's regular (standard) output. These data are sent back the the client, after some processing by the server to ensure they have the correct message headers describing the data and the state of the transaction. Of special note is the fact that the gateway program must write to standard output, before anything else, header messages explaining the MIME type of the data being returned by the program -- this is the only way that the server can know what type of data are being returned by the gateway program. Thus the gateway program must, at the very list, print out a a two- line header (the second of which must be a blank line):

  Content-type: text/html  The appropriate content-type header
                           A blank line separates the header from the data
  data  ....               The data being returned