4.5.1 Strings and foreign callables

To interface to a C function which takes a pointer to a string form and puts a string in the memory pointed to by result , declared like this:

you would define in Lisp:

(fli:define-foreign-function evalx 

    ((form (:reference-pass :ef-mb-string))

     (:ignore (:reference-return

               (:ef-mb-string :limit 1000)))))

and call

(evalx "(+ 2 3)")

=>

"5"

Now suppose instead that you want your C program to call a similar routine in a LispWorks for Windows DLL named "evaluator", like this:

{

 typedef void (_stdcall *evalx_func_type)(const char *form, char *result);

 HINSTANCE dll = LoadLibrary("evaluator");

 evalx_func_type evalx = (evalx_func_type) GetProcAddress(dll, "evalx");

 char result[1000];

 evalx("(+ 2 3)", result);

 printf("%s\n", result);

}

You would put this foreign callable in your DLL built with LispWorks:

(fli:define-foreign-callable

    ("evalx" :calling-convention :stdcall)

    ((form (:reference :ef-mb-string

            :lisp-to-foreign-p nil

            :foreign-to-lisp-p t))

     (result (:reference (:ef-mb-string :limit 1000)

              :lisp-to-foreign-p t

              :foreign-to-lisp-p nil)))

  (multiple-value-bind (res err)

      (ignore-errors (read-from-string form))

    (setq result

          (if (not (fixnump err))

              (format nil "Error reading: ~a"

                      err)

            (multiple-value-bind (res err)

                (ignore-errors (eval res))

              (if (and (not res) err)

                  (format nil "Error evaluating: ~a"

                          err)

                (princ-to-string res)))))))

Note: you could use :reference-return and :reference-pass in the foreign callable definition, but we have shown :reference with explicit lisp-to-foreign-p and foreign-to-lisp-p arguments to emphasise the direction of each conversion.

For information on how to create a LispWorks DLL, see "Creating a dynamic library" in the LispWorks User Guide and Reference Manual .