10.6.2 Using Prolog engines from C

Prolog engines live as entities that are independent from threads. They are always supported in the multi-threaded version and may be enabled in the single threaded version by providing -DENGINES=ON during the cmake configuration. Multiple threads may use a pool of engines for handling calls to Prolog. A single thread may use multiple engines to achieve coroutining. Engines are suitable in the following identified cases:

• Many native threads with infrequent Prolog work
  Prolog threads are expensive in terms of memory and time to create and destroy them. For systems that use a large number of threads that only infrequently need to call Prolog, it is better to take an engine from a pool and return it there.

• Prolog status must be handed to another thread
  This situation has been identified by Uwe Lesta when creating a .NET interface for SWI-Prolog. .NET distributes work for an active internet connection over a pool of threads. If a Prolog engine contains the state for a connection, it must be possible to detach the engine from a thread and re-attach it to another thread handling the same connection.

• Achieving coroutines
  A single thread may use engines to implement coroutining. This is notably interesting when combined with yielding as described in section 12.4.1.2.

PL_engine_t PL_current_engine()

Returns the current engine of the calling thread or NULL if the thread has no Prolog engine.

PL_engine_t PL_create_engine(PL_thread_attr_t *attributes)

Create a new Prolog engine. attributes is described with PL_thread_attach_engine(). Any thread can make this call after PL_initialise() returns success. The returned engine is not attached to any thread and lives until PL_destroy_engine() is used on the returned handle.

In the single-threaded version this call always returns NULL, indicating failure.

bool PL_destroy_engine(PL_engine_t e)

Destroy the given engine. Destroying an engine is only allowed if the engine is not attached to any thread or attached to the calling thread. On success this function returns TRUE, on failure the return value is FALSE.

int PL_set_engine(PL_engine_t engine, PL_engine_t *old)

Make the calling thread ready to use engine. If old is non-NULL the current engine associated with the calling thread is stored at the given location. If engine equals PL_ENGINE_MAIN the initial engine is attached to the calling thread. If engine is PL_ENGINE_CURRENT the engine is not changed. This can be used to query the current engine. This call returns PL_ENGINE_SET if the engine was switched successfully, PL_ENGINE_INVAL if engine is not a valid engine handle and PL_ENGINE_INUSE if the engine is currently in use by another thread.

Engines can be changed at any time. For example, it is allowed to select an engine to initiate a Prolog goal, detach it and at a later moment execute the goal from another thread. Note, however, that the term_t, qid_t and fid_t types are interpreted relative to the engine for which they are created. Behaviour when passing one of these types from one engine to another is undefined. The engine to which a query belongs can be requested using PL_query_engine()

In versions that do not support engines this call only succeeds if engine refers to the main engine.

void PL_WITH_ENGINE(PL_engine_t e)

This macro implements a C for-loop where the body is executed with the engine e as current engine. The body is executed exactly once. After completion of the body the current engine of the calling thread is restored to old state (either the old current engine or no engine). The user may use break to terminate the body early. The user may not use return. Using return does not reset the old engine.