12.4.2 Atoms and functors

The following functions provide for communication using atoms and functors.

atom_t PL_new_atom(const char *)

Return an atom handle for the given C-string. This function always succeeds. The returned handle is valid as long as the atom is referenced (see section 12.4.2.1). The following atoms are provided as macros, giving access to the empty list symbol and the name of the list constructor. Prior to version 7, ATOM_nil is the same as PL_new_atom("[]") and ATOM_dot is the same as PL_new_atom("."). This is no long the case in SWI-Prolog version 7.

atom_t ATOM_nil(A)

tomic constant that represents the empty list. It is advised to use PL_get_nil(), PL_put_nil() or PL_unify_nil() where applicable.

atom_t ATOM_dot(A)

tomic constant that represents the name of the list constructor. The list constructor itself is created using PL_new_functor(ATOM_dot,2). It is advised to use PL_get_list(), PL_put_list() or PL_unify_list() where applicable.

atom_t PL_new_atom_mbchars(int rep, size_t len, const char *s)

This function generalizes PL_new_atom() and PL_new_atom_nchars() while allowing for multiple encodings. The rep argument is one of REP_ISO_LATIN_1, REP_UTF8 or REP_MB. If len is (size_t)-1, it is computed from s using strlen().

const char* PL_atom_chars(atom_t atom)

Return a C-string for the text represented by the given atom. The returned text will not be changed by Prolog. It is not allowed to modify the contents, not even‘temporary' as the string may reside in read-only memory. The returned string becomes invalid if the atom is garbage collected (see section 12.4.2.1). Foreign functions that require the text from an atom passed in a term_t normally use PL_get_atom_chars() or PL_get_atom_nchars().

functor_t PL_new_functor(atom_t name, int arity)

Returns a functor identifier, a handle for the name/arity pair. The returned handle is valid for the entire Prolog session.

atom_t PL_functor_name(functor_t f)

Return an atom representing the name of the given functor.

size_t PL_functor_arity(functor_t f)

Return the arity of the given functor.

12.4.2.1 Atoms and atom garbage collection

With the introduction of atom garbage collection in version 3.3.0, atoms no longer live as long as the process. Instead, their lifetime is guaranteed only as long as they are referenced. In the single-threaded version, atom garbage collections are only invoked at the call-port. In the multithreaded version (see chapter 10), they appear asynchronously, except for the invoking thread.

For dealing with atom garbage collection, two additional functions are provided:

void PL_register_atom(atom_t atom)

Increment the reference count of the atom by one. PL_new_atom() performs this automatically, returning an atom with a reference count of at least one.^{193Otherwise asynchronous atom garbage collection might destroy the atom before it is used.}

void PL_unregister_atom(atom_t atom)

Decrement the reference count of the atom. If the reference count drops below zero, an assertion error is raised.

Please note that the following two calls are different with respect to atom garbage collection:

PL_unify_atom_chars(t, "text");
PL_unify_atom(t, PL_new_atom("text"));

The latter increments the reference count of the atom text, which effectively ensures the atom will never be collected. It is advised to use the *_chars() or *_nchars() functions whenever applicable.