In implementations that provide a foreign function interface as described
above, there is also usually a "callback" mechanism.  The programmer may
associate a foreign language function name with a Lisp function.  When a
foreign object file or library is loaded into the Lisp address space, it is
linked with these callback functions.  As with foreign functions, the
programmer must supply the argument and result data types so that Lisp may
perform conversions at the interface. Note that in such foreign function
interfaces Lisp is often left "in control" of things like memory
allocation, I/O channels, and startup code (this is a major nuisance
for lots of people).