12.4.22 Registering Foreign Predicates

bool PL_register_foreign_in_module(char *mod, char *name, int arity, foreign_t (*f)(), int flags, ...)

Register the C function f to implement a Prolog predicate. After this call returns successfully a predicate with name name (a char *) and arity arity (a C int) is created in module mod. If mod is NULL, the predicate is created in the module of the calling context, or if no context is present in the module user.

When called in Prolog, Prolog will call function. flags form a bitwise or’ed list of options for the installation. These are:

PL_FA_META	Provide meta-predicate info (see below)
PL_FA_TRANSPARENT	Predicate is module transparent (deprecated)
PL_FA_NONDETERMINISTIC	Predicate is non-deterministic. See also PL_retry().
PL_FA_NOTRACE	Predicate cannot be seen in the tracer
PL_FA_VARARGS	Use alternative calling convention.

If PL_FA_META is provided, PL_register_foreign_in_module() takes one extra argument. This argument is of type const char*. This string must be exactly as long as the number of arguments of the predicate and filled with characters from the set 0-9:^-+?. See meta_predicate/1 for details. PL_FA_TRANSPARENT is implied if at least one meta-argument is provided (0-9:^). Note that meta-arguments are not always passed as <module>:<term>. Always use PL_strip_module() to extract the module and plain term from a meta-argument.^{236It is encouraged to pass an additional NULL pointer for non-meta-predicates.}

Predicates may be registered either before or after PL_initialise(). When registered before initialisation the registration is recorded and executed after installing the system predicates and before loading the saved state.

Default calling (i.e. without PL_FA_VARARGS) function is passed the same number of term_t arguments as the arity of the predicate and, if the predicate is non-deterministic, an extra argument of type control_t (see section 12.4.1.1). If PL_FA_VARARGS is provided, function is called with three arguments. The first argument is a term_t handle to the first argument. Further arguments can be reached by adding the offset (see also PL_new_term_refs()). The second argument is the arity, which defines the number of valid term references in the argument vector. The last argument is used for non-deterministic calls. It is currently undocumented and should be defined of type void*. Here is an example:

static foreign_t
atom_checksum(term_t a0, int arity, void* context)
{ char *s;

  if ( PL_get_atom_chars(a0, &s) )
  { int sum;

    for(sum=0; *s; s++)
      sum += *s&0xff;

    return PL_unify_integer(a0+1, sum&0xff);
  }

  return FALSE;
}

install_t
install()
{ PL_register_foreign("atom_checksum", 2,
                      atom_checksum, PL_FA_VARARGS);
}

bool PL_register_foreign(const char *name, int arity, foreign_t (*function)(), int flags, ...)

Same as PL_register_foreign_in_module(), passing NULL for the module.

void PL_register_extensions_in_module(const char *module, PL_extension *e)

Register a series of predicates from an array of definitions of the type PL_extension in the given module. If module is NULL, the predicate is created in the module of the calling context, or if no context is present in the module user. The PL_extension type is defined as

typedef struct PL_extension
{ char          *predicate_name; /* Name of the predicate */
  short         arity;           /* Arity of the predicate */
  pl_function_t function;        /* Implementing functions */
  short         flags;           /* Or of PL_FA_... */
} PL_extension;

For details, see PL_register_foreign_in_module(). Here is an example of its usage:

static PL_extension predicates[] = {
{ "foo",        1,      pl_foo, 0 },
{ "bar",        2,      pl_bar, PL_FA_NONDETERMINISTIC },
{ NULL,         0,      NULL,   0 }
};

main(int argc, char **argv)
{ PL_register_extensions_in_module("user", predicates);

  if ( !PL_initialise(argc, argv) )
    PL_halt(1);

  ...
}

void PL_register_extensions( PL_extension *e)

Same as PL_register_extensions_in_module() using NULL for the module argument.