Node:Conversion Declarations, Next:Conversions, Previous:Foreign Resources, Up:Calling C
Conversion declaration predicates:
foreign_resource(+ResourceName,+Functions) [Hook]
Specifies that a set of foreign functions,
to be called from Prolog, are to be found in the resource named by
ResourceName. Functions is a list of functions exported by
the resource. Only functions that are to be called from Prolog and
optionally one init function and one deinit function should
be listed. The init and deinit functions are specified as
init(Function) and deinit(Function)
respectively (see Init and Deinit Functions). This predicate should
be defined entirely in terms of facts (unit clauses) and will be called
in the relevant module, i.e. not necessarily in the user module.
For example:
foreign_resource('terminal', [scroll,pos_cursor,ask]).
specifies that functions scroll(), pos_cursor() and
ask() are to be found in the resource terminal.
foreign(+CFunctionName, +Predicate) [Hook]
foreign(+CFunctionName, +Language, +Predicate) [Hook]
Specify the Prolog interface to a C function. Language is at
present constrained to the atom c, so there is no advantage in
using foreign/3 over foreign/2.
CFunctionName is the name of a C function. Predicate
specifies the name of the Prolog predicate that will be used to call
CFunction(). Predicate also specifies how the predicate
arguments are to be translated to and from the corresponding C arguments. These
predicates should be defined entirely in terms of facts (unit clauses)
and will be called in the relevant module, i.e. not necessarily in the
user module. For example:
foreign(pos_cursor, c, move_cursor(+integer, +integer)).
The above example says that the C function pos_cursor() has two
integer value arguments and that we will use the predicate
move_cursor/2 to call this function. A goal move_cursor(5,
23) would translate into the C call pos_cursor(5,23);.
The third argument of the predicate foreign/3 specifies how to
translate between Prolog arguments and C arguments. A call to a foreign
predicate will raise an exception if an input arguments is
uninstantiated (instantiation_error/2) or has the wrong type
(type_error/4) or domain (domain_error/4). The call will
fail upon return from the function if the output arguments do not unify
with the actual arguments.
The available conversions are listed in the next subsection.