Availability:C-language interface function
term_t PL_exception(qid_t
qid)Return the pending exception. Exceptions may be raised by most
of the API calls described in this chapter, a common possibility being
resource_error exceptions. Some return type_error
or domain_error exceptions. A call to Prolog using
PL_next_solution()
may return any exception, including those thrown by explicit calls to throw/1.
If no exception is pending this function returns (term_t)0.
Normally qid should be 0. An explicit qid
must be used after a call to PL_next_solution()
that returns FALSE when the query was created using the PL_Q_PASS_EXCEPTION
flag (see PL_open_query()).
Note that an API may only raise an exception when it fails; if the
API call succeeds, the result of PL_exception(0)
will be 0.231Provided no exception
was pending before calling the API function. As clients must deal with
exceptions immediately after an API call raises one, this can not happen
in a well behaved client. The implementation of a foreign
predicate should normally cleanup and return
FALSE after an exception is raised (and typically also
after an API call failed for logical reasons; see PL_unify()
for an elaboration on this topic). If the call to Prolog is not the
implementation of a foreign predicate, e.g., when the overall process
control is in some other language, exceptions may be printed by calling print_message/2
and should be discarded by calling PL_clear_exception().
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