term_t
: a reference to a
Prolog termThe principal data-type is term_t
. Type term_t
is what Quintus calls QP_term_ref
. This name indicates
better what the type represents: it is a handle for a term
rather than the term itself. Terms can only be represented and
manipulated using this type, as this is the only safe way to ensure the
Prolog kernel is aware of all terms referenced by foreign code and thus
allows the kernel to perform garbage-collection and/or stack-shifts
while foreign code is active, for example during a callback from C.
A term reference is a C unsigned long, representing the offset of a variable on the Prolog environment-stack. A foreign function is passed term references for the predicate-arguments, one for each argument. If references for intermediate results are needed, such references may be created using PL_new_term_ref() or PL_new_term_refs(). These references normally live till the foreign function returns control back to Prolog. Their scope can be explicitly limited using PL_open_foreign_frame() and PL_close_foreign_frame()/PL_discard_foreign_frame().
A term_t always refers to a valid Prolog term (variable, atom, integer, float or compound term). A term lives either until backtracking takes us back to a point before the term was created, the garbage collector has collected the term or the term was created after a PL_open_foreign_frame() and PL_discard_foreign_frame() has been called.
The foreign-interface functions can either read, unify or write to term-references. In the this document we use the following notation for arguments of type term_t:
term_t +t Accessed in read-mode. The `+' indicates the argument is `input'. term_t -t Accessed in write-mode. term_t ?t Accessed in unify-mode.
Term references are obtained in any of the following ways.
Term-references can safely be copied to other C-variables of type term_t, but all copies will always refer to the same term.