6.1 Calling Prolog from a Python thread

Prolog may be called safely from any Python thread. The Prolog execution is embraced with Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS, which implies that Python is allowed to switch to another thread while Prolog is doing its work.

If the calling Python thread is not the one that initiated Janus, janus.query_once() and janus.query() attach and detach a temporary Prolog engine using PL_thread_attach_engine() and PL_thread_destroy_engine(). This is relatively costly. In addition we allow associating a Prolog engine persistently with the calling thread.

int janus.engine()

Return the identifier of the Prolog engine associated to the current thread, -1 if no engine is attached or -2 if this version of Prolog does not support engines.

int janus.attach_engine()

Attach a Prolog engine to the current thread using PL_thread_attach_engine(). On success, return the integer thread id of the created Prolog engine.^{8The current implementation passes NULL to PL_thread_attach_engine(). Future versions may provide access to the creation attributes.}

If the thread already has an engine the attach count is incremented and the current engine id is returned. The engine is detached after a matching number of calls to janus.detach_engine()

None janus.detach_engine()

Decrement the attach count of the attached Prolog engine. Destroy the engine if this count drops to zero. Raises an exception of the calling thread is not attached to a Prolog engine.