12.3.11 SP_define_c_predicate()


     #include <sicstus/sicstus.h>
     typedef int
     SP_CPredFun(SP_term_ref goal,
                 void *stash);
     SP_define_c_predicate(char *name,
                           int arity,
                           char *module,
                           SP_CPredFun *proc,
                           void *stash);

Defines a Prolog predicate such that when the Prolog predicate is called it will call a C function with a term corresponding to the Prolog goal.


The predicate name.
The predicate arity.
The predicate module name.
The function.
See below.

Return Value

Nonzero on success, and 0 otherwise.


The Prolog predicate module:name/arity will be defined (the module module must already exist). The stash argument can be anything and is simply passed as the second argument to the C function proc.

The C function should return SP_SUCCESS for success and SP_FAILURE for failure. The C function may also call SP_fail() or SP_raise_exception() in which case the return value will be ignored.


Here is an end-to-end example of the above:

% square.pl
foreign_resource(square, [init(square_init)]). :- load_foreign_resource(square).
// square.c
#include <sicstus/sicstus.h> static int square_it(SP_term_ref goal, void *stash) { long arg1; SP_term_ref tmp = SP_new_term_ref(); SP_term_ref square_term = SP_new_term_ref(); long the_square; // goal will be a term like square(42,X) SP_get_arg(1,goal,tmp); // extract first arg if (!SP_get_integer(tmp,&arg1)) return SP_FAILURE; // type check first arg SP_put_integer(square_term, arg1*arg1); SP_get_arg(2,goal,tmp); // extract second arg // Unify output argument. SP_put_integer(tmp,...) would *not* work! return (SP_unify(tmp, square_term) ? SP_SUCCESS : SP_FAILURE); } void square_init(int when) { (void)when; // unused // Install square_it as user:square/2 SP_define_c_predicate("square", 2, "user", square_it, NULL); }
# terminal
% splfr square.pl square.c % sicstus -f -l square % compiling /home/matsc/tmp/square.pl... % loading foreign resource /home/matsc/tmp/square.so in module user % compiled /home/matsc/tmp/square.pl in module user, 0 msec 816 bytes SICStus 4.0.3 ... Licensed to SICS | ?- square(4711, X). X = 22193521 ? yes | ?- square(not_an_int, X). no

See Also

See Calling C from Prolog.

Send feedback on this subject.