Any Prolog predicate can be made callable from foreign functions using
the declaration extern/1. This includes built-in predicates, as
well as predicates that are currently undefined and dynamic predicates
that currently have no clauses.
An extern/1 declaration not only makes the predicate known to
foreign languages, but also specifies how arguments will be passed to
and from it. When a predicate is declared callable using
extern/1 declaration, it becomes available to foreign functions
as soon as the declaration is loaded. This is equally true of
extern/1 declarations occurring in files that are loaded from
source form, pre-compiled QOF files whose source files contained
extern/1 declarations, and certain QOF files (e.g. those created
using save_program/1) that retain callability information (see
ref-sls-sst).
A Prolog predicate that has been made callable from foreign functions
is not otherwise changed in any way. It can still be abolished or
redefined just like any other predicate. There is no performance
penalty associated with making a predicate callable from foreign
functions. A predicate can be redeclared by loading a new or modified
extern/1 declaration.
When a predicate is made callable from foreign code, a new and closely
related Prolog predicate called an interface predicate is created
in the module in which extern/1 was declared. The interface
predicate has the same arity as the callable predicate, and its name
is the name of the declared predicate with an underscore prepended to
it. The interface predicate provides the link between foreign
languages and its Prolog predicate. It can be abolished or saved just
as any other predicate, but because they can only be created using an
extern/1 declaration, valid interface predicates cannot be made
dynamic or multifile.
The purpose of the interface predicate is to supply an entry point into Prolog for foreign functions, and a handle on the property of callability of Prolog procedures for manipulation in Prolog. It is possible to call an interface predicate from Prolog, but the call will simply fail.