2.9.1 PlCheckFail(), PlCheckEx(), and PlCheck_PL() convenience functions

If one of the C "PL_" functions in SWI-Prolog.h returns failure, this can be either a Prolog-style failure (e.g. from PL_unify() or PL_next_solution()) or an error. If the failure is due to an error, it's usually best to immediately return to Prolog - and this can be done with the PlCheckEx() function, which turns a Prolog error into a C++ PlException. PlCheckFail() calls PlCheckEx() and additionally throws PlFail() if the failure is for Prolog failure.

The code for PlCheck() is just

void PlCheck(int rc)
{ if ( !PlCheckEx(rc) ) throw PlFail(); }

PlCheckEx() calls PL_exception() to see if there is a Prolog exception; if so, the Prolog exception is converted to a PlException object, which is then thrown. For more details on the C++ exceptions, see section 2.18.