In this model Prolog engines live as entities that are independent
from threads. If a thread needs to call Prolog it takes one of the
engines from the pool and returns the engine when done. This model is
suitable in the following identified cases:
- Compatibility with the single-threaded version
In the single-threaded version, foreign threads must serialise access to
the one and only thread engine. Functions from this section allow
sharing one engine among multiple threads.
- 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.
- PL_engine_t PL_create_engine(PL_thread_attr_t
- Create a new Prolog engine. attributes is described with
Any thread can make this call after
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
- int PL_destroy_engine(PL_engine_t
- 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
- int PL_set_engine(PL_engine_t
engine, PL_engine_t *old)
- Make the calling thread ready to use engine. If old
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
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
PL_ENGINE_INUSE if the engine is currently in use by
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
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.
In the single-threaded version this call only succeeds if engine
refers to the main engine.