CFFI User Manual - Common Lisp.net

Chapter 4: An Introduction to Foreign Interfaces and CFFI 10
cluttered than the equivalent disjoint-function set would be; in addition, setting a new option
in an old libcurl can generate a run-time error rather than breaking the compile. Lisp can
just as concisely generate functions as compare values, and the “undefined function” error
is just as useful as any explicit error we could define here might be.
4.7 Option functions in Lisp
We could use foreign-funcall directly every time we wanted to call curl_easy_setopt.
However, we can encapsulate some of the necessary information with the following.
;;; We will use this type later in a more creative way. For
;;; now, just consider it a marker that this isn’t just any
;;; pointer.
(defctype easy-handle :pointer)
(defmacro curl-easy-setopt (easy-handle enumerated-name
value-type new-value)
"Call ‘curl_easy_setopt’ on EASY-HANDLE, using ENUMERATED-NAME
as the OPTION. VALUE-TYPE is the CFFI foreign type of the third
argument, and NEW-VALUE is the Lisp data to be translated to the
third argument. VALUE-TYPE is not evaluated."
‘(foreign-funcall "curl_easy_setopt" easy-handle ,easy-handle
curl-option ,enumerated-name
,value-type ,new-value curl-code))
Now we define a function for each kind of argument that encodes the correct value-type
in the above. This can be done reasonably in the define-curl-options macroexpansion;
after all, that is where the different options are listed!
We could make cl:defun forms in the expansion that simply call curl-easy-setopt;
however, it is probably easier and clearer to use defcfun. define-curl-options was
becoming unwieldy, so I defined some helpers in this new definition.
(defun curry-curl-option-setter (function-name option-keyword)
"Wrap the function named by FUNCTION-NAME with a version that
curries the second argument as OPTION-KEYWORD.
This function is intended for use in DEFINE-CURL-OPTION-SETTER."
(setf (symbol-function function-name)
(let ((c-function (symbol-function function-name)))
(lambda (easy-handle new-value)
(funcall c-function easy-handle option-keyword
new-value)))))
(defmacro define-curl-option-setter (name option-type
option-value foreign-type)
"Define (with DEFCFUN) a function NAME that calls
curl_easy_setopt. OPTION-TYPE and OPTION-VALUE are the CFFI
foreign type and value to be passed as the second argument to
easy_setopt, and FOREIGN-TYPE is the CFFI foreign type to be used
for the resultant function’s third argument.
This macro is intended for use in DEFINE-CURL-OPTIONS."
‘(progn
(defcfun ("curl_easy_setopt" ,name) curl-code