10.6.1 A Prolog thread for each native thread (one-to-one)

In the one-to-one model, the thread that called PL_initialise() has a Prolog engine attached. If another C thread in the system wishes to call Prolog it must first attach an engine using PL_thread_attach_engine() and call PL_thread_destroy_engine() after all Prolog work is finished. This model is especially suitable with long running threads that need to do Prolog work regularly. See section 10.6.2 for the alternative many-to-many model.

int PL_thread_self()

Returns the integer Prolog identifier of the engine or -1 if the calling thread has no Prolog engine. This function is also provided in the single-threaded version of SWI-Prolog, where it returns -2.

int PL_unify_thread_id(term_t t, int i)

Unify t with the Prolog thread identifier for thread i. Thread identifiers are normally returned from PL_thread_self(). Returns -1 if the thread does not exist or the unification fails.

int PL_thread_attach_engine(const PL_thread_attr_t *attr)

Creates a new Prolog engine in the calling thread. If the calling thread already has an engine the reference count of the engine is incremented. The attr argument can be NULL to create a thread with default attributes. Otherwise it is a pointer to a structure with the definition below. For any field with value‘0’, the default is used. The cancel field may be filled with a pointer to a function that is called when PL_cleanup() terminates the running Prolog engines. If this function is not present or returns FALSE pthread_cancel() is used. The new thread inherits is properties from Prolog's main thread. The flags field defines the following flags:

PL_THREAD_NO_DEBUG

If this flag is present, the thread starts in normal no-debug status. By default, the debug status is inherited from the main thread.

PL_THREAD_NOT_DETACHED

By default the new thread is created in detached mode. With this flag it is created normally, allowing Prolog to join the thread.

PL_THREAD_CUR_STREAMS

By default the current_input and current_output are set to user_input and user_output of the main thread. Using this flag, these streams are copied from the main thread. See also the inherited_from option of thread_create/3.

typedef struct
{ size_t    stack_limit;                /* Total stack limit (bytes) */
  size_t    table_space;                /* Total tabling space limit (bytes) */
  char *    alias;                      /* alias name */
  int       (*cancel)(int thread);      /* cancel function */
  intptr_t  flags;                      /* PL_THREAD_* flags */
  size_t    max_queue_size;             /* Max size of associated queue */
} PL_thread_attr_t;

The structure may be destroyed after PL_thread_attach_engine() has returned. On success it returns the Prolog identifier for the thread (as returned by PL_thread_self()). If an error occurs, -1 is returned. If this Prolog is not compiled for multithreading, -2 is returned.

int PL_thread_destroy_engine()

Destroy the Prolog engine in the calling thread. Only takes effect if PL_thread_destroy_engine() is called as many times as PL_thread_attach_engine() in this thread. Returns TRUE on success and FALSE if the calling thread has no engine or this Prolog does not support threads.

Please note that construction and destruction of engines are relatively expensive operations. Only destroy an engine if performance is not critical and memory is a critical resource.

int PL_thread_at_exit(void (*function)(void *), void *closure, int global)

Register a handle to be called as the Prolog engine is destroyed. The handler function is called with one void * argument holding closure. If global is TRUE, the handler is installed for all threads. Globally installed handlers are executed after the thread-local handlers. If the handler is installed local for the current thread only (global == FALSE) it is stored in the same FIFO queue as used by thread_at_exit/1.