Opens a query and returns an identifier for it. ctx is the context module of the goal. When NULL, the context module of the calling context will be used, or user if there is no calling context (as may happen in embedded systems). Note that the context module only matters for meta-predicates. See meta_predicate/1, context_module/1 and module_transparent/1. The p argument specifies the predicate, and should be the result of a call to PL_pred() or PL_predicate(). Note that it is allowed to store this handle as global data and reuse it for future queries. The term reference t0 is the first of a vector of term references as returned by PL_new_term_refs(n).

The flags arguments provides some additional options concerning debugging and exception handling. It is a bitwise or of the following values:

PL_Q_NORMAL

Normal operation. The debugger inherits its settings from the environment. If an exception occurs that is not handled in Prolog, a message is printed and the tracer is started to debug the error.^{172Do not pass the integer 0 for normal operation, as this is interpreted as PL_Q_NODEBUG for backward compatibility reasons.}

PL_Q_NODEBUG

Switch off the debugger while executing the goal. This option is used by many calls to hook-predicates to avoid tracing the hooks. An example is print/1 calling portray/1 from foreign code.

PL_Q_CATCH_EXCEPTION

If an exception is raised while executing the goal, do not report it, but make it available for PL_exception().

PL_Q_PASS_EXCEPTION

As PL_Q_CATCH_EXCEPTION, but do not invalidate the exception-term while calling PL_close_query(). This option is experimental.

PL_Q_ALLOW_YIELD

Support the I_YIELD instruction for engine-based coroutining. See in boot/init.pl for details.

PL_Q_EXT_STATUS

Make PL_next_solution() return extended status. Instead of only TRUE or FALSE extended status as illustrated in the following table:

Extended	Normal
PL_S_EXCEPTION	FALSE	Exception available through PL_exception()
PL_S_FALSE	FALSE	Query failed
PL_S_TRUE	TRUE	Query succeeded with choicepoint
PL_S_LAST	TRUE	Query succeeded without choicepoint

PL_open_query() can return the query identifier `0' if there is not enough space on the environment stack. This function succeeds, even if the referenced predicate is not defined. In this case, running the query using PL_next_solution() will return an existence_error. See PL_exception().

The example below opens a query to the predicate is_a/2 to find the ancestor of `me'. The reference to the predicate is valid for the duration of the process and may be cached by the client.

char *
ancestor(const char *me)
{ term_t a0 = PL_new_term_refs(2);
  static predicate_t p;

  if ( !p )
    p = PL_predicate("is_a", 2, "database");

  PL_put_atom_chars(a0, me);
  PL_open_query(NULL, PL_Q_NORMAL, p, a0);
  ...
}