On Sat, 2012-09-22 at 21:27 +0200, Frank Goenninger wrote:

(also posted to comp.lang.lisp)

Hi all:

I have a lisp image that contains the following functions:

(defun frgo (a) (format *debug-io* "~%~A~&" a) (values))

(cffi:defcallback frgo_helper :void ((a-cstr :string)) (frgo (cffi:foreign-string-to-lisp a-cstr)))

In a C dynamic library I have:

• file frgo_c.h:

extern void frgo_helper( char * a );

• file frgo_c.c:

void frgo( char * a ) { frgo_helper( a ); }

Now, the library does not compile - undefined symbol _frgo_helper() !

How do I make functions "known" to C land that are purely defined as callbacks in Lisp land?

You can't do that directly. Lisp callbacks must be passed to the C code as function pointers. You could write a C wrapper that calls the callback through a global variable and have the Lisp-side store the address of the callback in that variable.