12.4.12 Calling Prolog from C

The Prolog engine can be called from C. There are two interfaces for this. For the first, a term is created that could be used as an argument to call/1, and then PL_call() is used to call Prolog. This system is simple, but does not allow to inspect the different answers to a non-deterministic goal and is relatively slow as the runtime system needs to find the predicate. The other interface is based on PL_open_query(), PL_next_solution(), and PL_cut_query() or PL_close_query(). This mechanism is more powerful, but also more complicated to use.

12.4.12.1 Predicate references

This section discusses the functions used to communicate about predicates. Though a Prolog predicate may be defined or not, redefined, etc., a Prolog predicate has a handle that is neither destroyed nor moved. This handle is known by the type predicate_t.

predicate_t PL_pred(functor_t f, module_t m)

Return a handle to a predicate for the specified name/arity in the given module. If the module argument m is NULL, the current context module is used. If the target predicate does not exist a handle to a new undefined predicate is returned. The predicate may fail, returning (predicate_t)0 after setting a resource exception, if the target module has a limit on the program_space, see set_module/1. Currently aborts the process with a fatal error when out of memory. Future versions may raise a resource exception and return (predicate_t)0.

predicate_t PL_predicate(const char *name, int arity, const char* module)

Same as PL_pred(), but provides a more convenient interface to the C programmer. If the module argument module is NULL, the current context module is used. The predicate_t handle may be stored as global data and reused for future queries^{228PL_predicate() involves 5 hash lookups (two to get the atoms, one to get the module, one to get the functor and the final one to get the predicate associated with the functor in the module)} as illustrated below.

static predicate_t p = 0;

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

Note that PL_cleanup() invalidates the predicate handle. Foreign libraries that use the above mechanism must implement the module uninstall() function to clear the predicate_t global variable.

bool PL_predicate_info(predicate_t p, atom_t *n, size_t *a, module_t *m)

Return information on the predicate p. The name is stored over n, the arity over a, while m receives the definition module. Note that the latter need not be the same as specified with PL_predicate(). If the predicate is imported into the module given to PL_predicate(), this function will return the module where the predicate is defined. Any of the arguments n, a and m can be NULL. Currently always returns TRUE.

12.4.12.2 Initiating a query from C

This section discusses the functions for creating and manipulating queries from C. Note that a foreign context can have at most one active query. This implies that it is allowed to make strictly nested calls between C and Prolog (Prolog calls C, calls Prolog, calls C, etc.), but it is not allowed to open multiple queries and start generating solutions for each of them by calling PL_next_solution(). Be sure to call PL_cut_query() or PL_close_query() on any query you opened before opening the next or returning control back to Prolog. Failure to do so results in "undefined behavior" (typically, a crash).

qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t +t0)

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 term reference t0 is the first of a vector of term references as returned by PL_new_term_refs(n). Raise a resource exception and returns (qid_t)0 on failure.

Every use of PL_open_query() must have a corresponding call to PL_cut_query() or PL_close_query() before the foreign predicate returns either TRUE or FALSE.

The flags arguments provides some additional options concerning debugging and exception handling. It is a bitwise or of the following values below. Note that exception propagation is defined by the flags PL_Q_NORMAL, PL_Q_CATCH_EXCEPTION and PL_Q_PASS_EXCEPTION. Exactly one of these flags must be specified (if none of them is specified, the behavior is as if PL_Q_NODEBUG is specified)..

PL_Q_NORMAL

Normal operation. It is named "normal" because it makes a call to Prolog behave as it did before exceptions were implemented, i.e., an error (now uncaught exception) triggers the debugger. See also the Prolog flag debug_on_error. This mode is still useful when calling Prolog from C if the C code is not willing to handle exceptions.

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. This is the default if flags is 0.

PL_Q_CATCH_EXCEPTION

If an exception is raised while executing the goal, make it available by calling PL_exception(qid), where qid is the qid_t returned by PL_open_query(). The exception is implicitly cleared from the environment when the query is closed and the exception term returned from PL_exception(qid) becomes invalid. Use PL_Q_PASS_EXCEPTION if you wish to propagate the exception.

PL_Q_PASS_EXCEPTION

As PL_Q_CATCH_EXCEPTION, making the exception on the inner environment available using PL_exception(0) in the parent environment. If PL_next_solution() returns FALSE, you must call PL_cut_query() or PL_close_query(). After that you may verify whether failure was due to logical failure of the called predicate or an exception by calling PL_exception(0). If the predicate failed due to an exception you should return with FALSE from the foreign predicate or call PL_clear_exception() to clear it. If you wish to process the exception in C, it is advised to use PL_Q_CATCH_EXCEPTION instead, but only if you have no need to raise an exception or re-raise the caught exception.

Note that PL_Q_PASS_EXCEPTION is used by the debugger to decide whether the exception is caught. If there is no matching catch/3 call in the current query and the query was started using PL_Q_PASS_EXCEPTION the debugger searches the parent queries until it either finds a matching catch/3, a query with PL_Q_CATCH_EXCEPTION (in which case it considers the exception handled by C) or the top of the query stack (in which case it considers the exception uncaught). Uncaught exceptions use the library(library(prolog_stack)) to add a backtrace to the exception and start the debugger as soon as possible if the Prolog flag debug_on_error is true.

PL_Q_ALLOW_YIELD

Support the I_YIELD instruction for engine-based coroutining. See $engine_yield/2 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 (and makes the exception available through PL_exception(0)). This function succeeds, even if the referenced predicate is not defined. In this case, running the query using PL_next_solution() may 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 or until PL_cleanup() is called (see PL_predicate() for details) 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_PASS_EXCEPTION, p, a0);
  ...
}

int PL_next_solution(qid_t qid)

Generate the first (next) solution for the given query. The return value is TRUE if a solution was found, or FALSE to indicate the query could not be proven. This function may be called repeatedly until it fails to generate all solutions to the query. The return value PL_S_NOT_INNER is returned if qid is not the innermost query.

If the PL_open_query() had the flag PL_Q_EXT_STATUS, there are additional return values (see section 12.4.1.2).

int PL_cut_query(qid_t qid)

Discards the query, but does not delete any of the data created by the query. It just invalidates qid, allowing for a new call to PL_open_query() in this context. PL_cut_query() may invoke cleanup handlers (see setup_call_cleanup/3) and therefore may experience exceptions. If an exception occurs the return value is FALSE and the exception is accessible through PL_exception(0).

An example of a handler that can trigger an exception in PL_cut_query() is:

test_setup_call_cleanup(X) :-
    setup_call_cleanup(
        true,
        between(1, 5, X),
        throw(error)).

where PL_next_solution() returns TRUE on the first result and the throw(error) will only run when PL_cut_query() or PL_close_query() is run. On the other hand, if the goal in setup_call_cleanup/3 has completed (failure, exception, determinitic success), the cleanup handler will have done its work before control gets back to Prolog and therefore PL_next_solution() will have generated the exception. The return value PL_S_NOT_INNER is returned if qid is not the innermost query.

int PL_close_query(qid_t qid)

As PL_cut_query(), but all data and bindings created by the query are destroyed as if the query is called as \+ \+ Goal. This reduces the need for garbage collection, but also rewinds side effects such as setting global variables using b_setval/2. The return value PL_S_NOT_INNER is returned if qid is not the innermost query.

qid_t PL_current_query(void)

Returns the query id of the current query or 0 if the current thread is not executing any queries.

PL_engine_t PL_query_engine(qid_t qid)

Return the engine to which qid belongs. Note that interacting with a query or the Prolog terms associated with a query requires the engine to be current. See PL_set_engine().

bool PL_call_predicate(module_t m, int flags, predicate_t pred, term_t +t0)

Shorthand for PL_open_query(), PL_next_solution(), PL_cut_query(), generating a single solution. The arguments are the same as for PL_open_query(), the return value is the same as PL_next_solution().

bool PL_call(term_t t, module_t m)

Call term t just like the Prolog predicate once/1. t is called in the module m, or in the context module if m == NULL. Returns TRUE if the call succeeds, FALSE otherwise. If the goal raises an exception the return value is FALSE and the exception term is available using PL_exception(0).^{229Up to version 9.1.11 the debugger was started and the exception was not propagated.} Figure 7 shows an example to obtain the number of defined atoms.