5.5.2 Other foreign interface types

atom_t

An atom in Prologs internal representation. Atoms are pointers to an opaque structure. They are a unique representation for represented text, which implies that atom A represents the same text as atom B if-and-only-if A and B are the same pointer.

Atoms are the central representation for textual constants in Prolog The transformation of C a character string to an atom implies a hash-table lookup. If the same atom is needed often, it is advised to store its reference in a global variable to avoid repeated lookup.

functor_t

A functor is the internal representation of a name/arity pair. They are used to find the name and arity of a compound term as well as to construct new compound terms. Like atoms they live for the whole Prolog session and are unique.

predicate_t

Handle to a Prolog predicate. Predicate handles live forever (although they can loose their definition).

qid_t

Query Identifier. Used by PL_open_query()/PL_next_solution()/PL_close_query() to handle backtracking from C.

fid_t

Frame Identifier. Used by PL_open_foreign_frame()/PL_close_foreign_frame().

module_t

A module is a unique handle to a Prolog module. Modules are used only to call predicates in a specific module.

foreign_t

Return type for a C-function implementing a Prolog predicate.

control_t

Passed as additional argument to non-deterministic foreign functions. See PL_retry*() and PL_foreign_context*().

install_t

Type for the install() and uninstall() functions of shared or dynamic link libraries. See secrefshlib.