#|
The function call_identity is supposed to be very boring, but here it is:

int call_identity(int (*function)()) {
  return function();
}

On macOS, I compiled this using the command:
gcc -dynamiclib -o libidentity.dylib libidentity.c
|#

CL-USER> (cffi:load-foreign-library "/tmp/libidentity.dylib")
#<CFFI:FOREIGN-LIBRARY LIBIDENTITY.DYLIB-554 "libidentity.dylib">
CL-USER> (cffi:defcallback makes-42 :int () 42)
MAKES-42
CL-USER> (cffi:defcfun "call_identity" :int (function :pointer))
CALL-IDENTITY
CL-USER> (call-identity (cffi:callback makes-42))
42