ffi:callback

(ffi:callback function return-type argument-types)

Turns a Lisp function into a C function pointer of the given shape, so C code can call back into Lisp. The stub lives for the whole program. An error escaping the callback would unwind into the C frame above it and end the process, so one never does: the message is printed and the callback answers zero of its declared type. :string and struct types are refused in a callback shape -- take a :pointer.

Redefining a callback answers a NEW address: a stub cannot be re-targeted, so a C side holding the old one keeps calling the old function.