A determinate query can be made in a single C function call using
QP_query. The first argument passed to QP_query() is a
QP_pred_ref for the predicate to be called. Any arguments
after the first represent parameters to be passed to and from the
Prolog predicate.
The foreign language interface will interpret arguments passed to the Prolog
predicate according to the call specification given when the predicate
was made callable. Hence, it is important that the arguments to be passed
to and from the Prolog predicate should correspond to that call
specification. For certain parameter types (passing Prolog atoms in canonical
form) it is possible to detect inconsistencies between data supplied to
QP_query() and the call specification, but for the most part this is
impossible. Calls that are inconsistent with their call specifications
will produce undefined results.
QP_query() returns one of three values: QP_SUCCESS,
indicating that the query was made and a solution to the query was
computed; QP_FAILURE, meaning that the query was made but no
solution could be found; and QP_ERROR, which says that either the
query could not be made, or that an exception was signaled from Prolog
but not caught. In this case, see the reference page for
QP_exception_term(). Only when the return value is QP_SUCCESS
should the values in variables passed as outputs from Prolog be
considered valid. Otherwise, their contents are undefined.
It is important that a valid QP_pred_ref is passed to
QP_query(); in particular, it is advisable to check for an error
return from QP_predicate() or QP_pred() before calling
QP_query().