The foreign object type facility is Guile’s mechanism for
importing object and types from C or other languages into Guile’s
system. If you have a C struct foo type, for example, you can
define a corresponding Guile foreign object type that allows Scheme code
to handle struct foo * objects.
To define a new foreign object type, the programmer provides Guile with some essential information about the type — what its name is, how many fields it has, and its finalizer (if any) — and Guile allocates a fresh type for it. Foreign objects can be accessed from Scheme or from C.