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).