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.. index::
   single: pool; creating

.. _topic-pool:

Pools
=====

Within an :term:`arena` a client program creates one or more pools. A
pool is responsible for requesting memory from the :term:`arena` and
making it available for allocation.

.. c:type:: mps_pool_t

    The type of :term:`pools`.

    A pool is responsible for requesting memory from the :term:`arena`
    and making it available to the :term:`client program` via
    :c:func:`mps_alloc` or via an :term:`allocation point`.


.. c:function:: mps_res_t mps_pool_create_k(mps_pool_t *pool_o, mps_arena_t arena, mps_pool_class_t pool_class, mps_arg_s args[])

    Create a :term:`pool` in an :term:`arena`.

    ``pool_o`` points to a location that will hold a pointer to the new
    pool.

    ``arena`` is the arena in which to create the pool.

    ``pool_class`` is the :term:`pool class` of the new pool.

    ``args`` are :term:`keyword arguments` specific to the pool class.
    See the documentation for the pool class.

    Returns :c:macro:`MPS_RES_OK` if the pool is created successfully,
    or another :term:`result code` otherwise.

    The pool persists until it is destroyed by calling
    :c:func:`mps_pool_destroy`.


.. c:function:: void mps_pool_destroy(mps_pool_t pool)

    Destroy a :term:`pool`.

    ``pool`` is the pool to destroy.

    This function checks the consistency of the pool, destroys the
    pool's internal control structures and causes the pool's memory to
    be returned to the :term:`arena` for reuse by other pools, or to
    be returned to the operating system.  Blocks allocated from the
    pool may no longer be used.

    It is an error to destroy a pool without first destroying all
    :term:`allocation points` and :term:`segregated allocation caches`
    created in the pool.

    .. warning::

        It is not safe to carry on running the :term:`garbage
        collector` after destroying an :term:`automatically managed
        <automatic memory management>` pool that contains any objects
        that are :term:`reachable` from your roots, or any objects
        that have been registered for :term:`finalization` but not yet
        finalized.

        Our recommended approach is to destroy automatically managed
        pools just before destroying the arena, and then only while
        the arena is in the :term:`parked state`. Thus a safe
        tear-down sequence looks like this::

            mps_arena_park(arena);
            /* destroy threads and roots belonging to the arena */
            /* destroy allocation points and caches belonging to the pool */
            mps_pool_destroy(pool);
            /* destroy chains and formats belonging to the arena */
            mps_arena_destroy(arena);


.. index::
   single: pool class

Pool classes
------------

Pools belong to :term:`pool classes` that specify policies for how
their memory is managed. Some pools are :term:`manually managed
<manual memory management>` (you must call :c:func:`mps_free` to
return a block of memory to the pool) and others are
:term:`automatically managed <automatic memory management>` (the
:term:`garbage collector` reclaims :term:`unreachable` blocks).

See the :ref:`pool` for a list of pool classes.


.. c:type:: mps_pool_class_t

    The type of :term:`pool classes`.


.. index::
   pair: pool; introspection

Pool introspection
------------------

.. c:function:: size_t mps_pool_total_size(mps_pool_t pool)

    Return the total memory allocated from the arena and managed by
    the pool.

    ``pool`` is the pool.

    The result includes memory in use by the client program, memory
    that's available for use by the client program, and memory
    that's lost to fragmentation. It does not include memory used by
    the pool's internal control structures.


.. c:function:: size_t mps_pool_free_size(mps_pool_t pool)

    Return the free memory: memory managed by the pool but not in use
    by the client program.

    ``pool`` is the pool.

    The result includes memory that's available for use by the client
    program, and memory that's lost to fragmentation. It does not
    include memory used by the pool's internal control structures.


.. c:function:: mps_bool_t mps_addr_pool(mps_pool_t *pool_o, mps_arena_t arena, mps_addr_t addr)

    Determine the :term:`pool` to which an address belongs.

    ``pool_o`` points to a location that will hold the address of the
    pool, if one is found.

    ``arena`` is the arena whose pools will be considered.

    ``addr`` is the address.

    If ``addr`` is the address of a location inside a block allocated
    from a pool in ``arena``, then update the location pointed to by
    ``pool_o`` with the address of the pool, and return true.

    If ``addr`` points to a location that is not managed by ``arena``,
    return false.

    If neither of the above conditions is satisfied,
    :c:func:`mps_addr_pool` may return either true or false.

    .. note::

        This function might return a false positive by returning true
        if you ask about an address that happens to be inside memory
        managed by a pool, but which is not inside a block allocated
        by that pool. It never returns a false negative.

        The result from this function is valid only at the instant at
        which the function returned. In some circumstances the result
        may immediately become invalidated. For reliable results call
        this function and interpret the result while the arena is in
        the :term:`parked state`.

    .. seealso::

        To find out which :term:`object format` describes the object
        at the address, use :c:func:`mps_addr_fmt`. If you only care
        whether the address belongs to a particular :term:`arena`, use
        :c:func:`mps_arena_has_addr`.


.. c:function:: mps_res_t mps_pool_walk(mps_pool_t pool, mps_area_scan_t scan_area, void *closure)

    Visit all :term:`formatted objects` in a :term:`pool`. The pool
    must be :term:`automatically managed <automatic memory
    management>`. The pool's :term:`arena` must be in the
    :term:`parked state`.

    :c:data:`pool` is the pool whose formatted objects are visited.

    :c:data:`scan_area` is an area scanning function. See
    :ref:`topic-scanning-area`.

    :c:data:`closure` is an arbitrary pointer that will be passed to
    :c:data:`scan_area`.

    The scanning function is called multiple times with disjoint areas
    of memory that cover all formatted objects in the pool. The areas
    may also include :term:`padding objects` if the pool's format has
    a :term:`padding method`, but never includes :term:`forwarding
    objects` since the arena is in the parked state.

    The scanning function must follow the
    :ref:`topic-scanning-protocol`. In particular, it must :term:`fix`
    every :term:`reference` in the area. The scanning function may
    return :c:macro:`MPS_RES_OK` to continue visiting areas of
    formatted objects, or return other :ref:`topic-result-codes` to
    stop visiting and return to the caller.

    .. note::

        If the scanning function modifies a reference, it must scan
        the modified reference. It is safe to scan the original
        reference as well, but this may lead to unwanted
        :term:`retention`.