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

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.^{222It 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);
}