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10 (www server-utils cgi-prep)

Often the server cannot do everything by itself, and makes use of external programs invoked in a common gateway interface environment. These programs are also known as CGI scripts.

The (www server-utils cgi-prep) module provide a procedure to set up such an environment. Actually invoking the CGI script is not covered.

Procedure: cgi-environment-manager initial-bindings

Return a closure encapsulating initial-bindings, a list of pairs (name . value), where name is a symbol listed in the following table, and value is a string unless otherwise noted.

If name is not recognized, signal "unrecognized key" error. Encapsulation includes name=value formatting.

The closure accepts these commands:

name value

Encapsulate an additional binding. name and value are as above.


Drop the additional bindings. Note that initial bindings can never be dropped (you can always create a new closure).


Return a list of strings suitable for passing to environ or as the second argument to execle.

Any other command results in a "bad command" error.


Following is a simple example of how to use cgi-environment-manager. A more realistic example would include port and connection management, input validation, error handling, logging, etc. First, we set up the manager with more-or-less constant bindings.

(define M (cgi-environment-manager
            '((server-software . "FooServe/24")
              (server-protocol . "HTTP/1.0")
              (server-port . 80))))

Later, we add connection-specific bindings. We use receive-request from the parse-request module.

(define PORT ...)
(define REQUEST (receive-request PORT))
(define UPATH (request-upath REQUEST))
(define QMARK (string-index UPATH #\?))
(define CGI (substring UPATH 0 QMARK))

(M 'script-name CGI)
(M 'query-string (substring UPATH (1+ QMARK)))

Lastly, we spawn the child process, passing the constructed environment as the second arg to execle, and drop the connection-specific bindings afterwards.

(let ((pid (primitive-fork)))
  (if (zero? pid)
      (execle CGI (M #:environ-list) (list CGI)) ; child
      (waitpid pid)))                            ; parent

(M #:clear!)

Now we can re-use M for another connection.

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