source: rtems-docs/c_user/partition_manager.rst @ 25d55d4

.. COMMENT: COPYRIGHT (c) 1988-2008.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

Partition Manager
#################

.. index:: partitions

Introduction
============

The partition manager provides facilities to dynamically allocate memory in
fixed-size units.  The directives provided by the partition manager are:

- rtems_partition_create_ - Create a partition

- rtems_partition_ident_ - Get ID of a partition

- rtems_partition_delete_ - Delete a partition

- rtems_partition_get_buffer_ - Get buffer from a partition

- rtems_partition_return_buffer_ - Return buffer to a partition

Background
==========

Partition Manager Definitions
-----------------------------
.. index:: partition, definition

A partition is a physically contiguous memory area divided into fixed-size
buffers that can be dynamically allocated and deallocated.

.. index:: buffers, definition

Partitions are managed and maintained as a list of buffers.  Buffers are
obtained from the front of the partition's free buffer chain and returned to
the rear of the same chain.  When a buffer is on the free buffer chain, RTEMS
uses two pointers of memory from each buffer as the free buffer chain.  When a
buffer is allocated, the entire buffer is available for application use.
Therefore, modifying memory that is outside of an allocated buffer could
destroy the free buffer chain or the contents of an adjacent allocated buffer.

Building a Partition Attribute Set
----------------------------------
.. index:: partition attribute set, building

In general, an attribute set is built by a bitwise OR of the desired attribute
components.  The set of valid partition attributes is provided in the following
table:

.. list-table::
 :class: rtems-table

 * - ``RTEMS_LOCAL``
   - local partition (default)
 * - ``RTEMS_GLOBAL``
   - global partition

Attribute values are specifically designed to be mutually exclusive, therefore
bitwise OR and addition operations are equivalent as long as each attribute
appears exactly once in the component list.  An attribute listed as a default
is not required to appear in the attribute list, although it is a good
programming practice to specify default attributes.  If all defaults are
desired, the attribute ``RTEMS_DEFAULT_ATTRIBUTES`` should be specified on this
call.  The attribute_set parameter should be ``RTEMS_GLOBAL`` to indicate that
the partition is to be known globally.

Operations
==========

Creating a Partition
--------------------

The ``rtems_partition_create`` directive creates a partition with a
user-specified name.  The partition's name, starting address, length and buffer
size are all specified to the ``rtems_partition_create`` directive.  RTEMS
allocates a Partition Control Block (PTCB) from the PTCB free list.  This data
structure is used by RTEMS to manage the newly created partition.  The number
of buffers in the partition is calculated based upon the specified partition
length and buffer size. If successful,the unique partition ID is returned to
the calling task.

Obtaining Partition IDs
-----------------------

When a partition is created, RTEMS generates a unique partition ID and assigned
it to the created partition until it is deleted.  The partition ID may be
obtained by either of two methods.  First, as the result of an invocation of
the ``rtems_partition_create`` directive, the partition ID is stored in a user
provided location.  Second, the partition ID may be obtained later using the
``rtems_partition_ident`` directive.  The partition ID is used by other
partition manager directives to access this partition.

Acquiring a Buffer
------------------

A buffer can be obtained by calling the ``rtems_partition_get_buffer``
directive.  If a buffer is available, then it is returned immediately with a
successful return code.  Otherwise, an unsuccessful return code is returned
immediately to the caller.  Tasks cannot block to wait for a buffer to become
available.

Releasing a Buffer
------------------

Buffers are returned to a partition's free buffer chain with the
``rtems_partition_return_buffer`` directive.  This directive returns an error
status code if the returned buffer was not previously allocated from this
partition.

Deleting a Partition
--------------------

The ``rtems_partition_delete`` directive allows a partition to be removed and
returned to RTEMS.  When a partition is deleted, the PTCB for that partition is
returned to the PTCB free list.  A partition with buffers still allocated
cannot be deleted.  Any task attempting to do so will be returned an error
status code.

Directives
==========

This section details the partition manager's directives.  A subsection is
dedicated to each of this manager's directives and describes the calling
sequence, related constants, usage, and status codes.

.. _rtems_partition_create:

PARTITION_CREATE - Create a partition
-------------------------------------
.. index:: create a partition

**CALLING SEQUENCE:**

.. index:: rtems_partition_create

.. code-block:: c

    rtems_status_code rtems_partition_create(
        rtems_name       name,
        void            *starting_address,
        uint32_t         length,
        uint32_t         buffer_size,
        rtems_attribute  attribute_set,
        rtems_id        *id
    );

**DIRECTIVE STATUS CODES:**

.. list-table::
 :class: rtems-table

 * - ``RTEMS_SUCCESSFUL``
   - partition created successfully
 * - ``RTEMS_INVALID_NAME``
   - invalid partition name
 * - ``RTEMS_TOO_MANY``
   - too many partitions created
 * - ``RTEMS_INVALID_ADDRESS``
   - address not on four byte boundary
 * - ``RTEMS_INVALID_ADDRESS``
   - ``starting_address`` is NULL
 * - ``RTEMS_INVALID_ADDRESS``
   - ``id`` is NULL
 * - ``RTEMS_INVALID_SIZE``
   - length or buffer size is 0
 * - ``RTEMS_INVALID_SIZE``
   - length is less than the buffer size
 * - ``RTEMS_INVALID_SIZE``
   - buffer size not a multiple of 4
 * - ``RTEMS_MP_NOT_CONFIGURED``
   - multiprocessing not configured
 * - ``RTEMS_TOO_MANY``
   - too many global objects

**DESCRIPTION:**

This directive creates a partition of fixed size buffers from a physically
contiguous memory space which starts at starting_address and is length bytes in
size.  Each allocated buffer is to be of ``buffer_size`` in bytes.  The
assigned partition id is returned in ``id``.  This partition id is used to
access the partition with other partition related directives.  For control and
maintenance of the partition, RTEMS allocates a PTCB from the local PTCB free
pool and initializes it.

**NOTES:**

This directive will not cause the calling task to be preempted.

The ``starting_address`` must be properly aligned for the target architecture.

The ``buffer_size`` parameter must be a multiple of the CPU alignment factor.
Additionally, ``buffer_size`` must be large enough to hold two pointers on the
target architecture.  This is required for RTEMS to manage the buffers when
they are free.

Memory from the partition is not used by RTEMS to store the Partition Control
Block.

The following partition attribute constants are defined by RTEMS:

.. list-table::
 :class: rtems-table

 * - ``RTEMS_LOCAL``
   - local partition (default)
 * - ``RTEMS_GLOBAL``
   - global partition

The PTCB for a global partition is allocated on the local node.  The memory
space used for the partition must reside in shared memory. Partitions should
not be made global unless remote tasks must interact with the partition.  This
is to avoid the overhead incurred by the creation of a global partition.  When
a global partition is created, the partition's name and id must be transmitted
to every node in the system for insertion in the local copy of the global
object table.

The total number of global objects, including partitions, is limited by the
maximum_global_objects field in the Configuration Table.

.. _rtems_partition_ident:

PARTITION_IDENT - Get ID of a partition
---------------------------------------
.. index:: get ID of a partition
.. index:: obtain ID of a partition

**CALLING SEQUENCE:**

.. index:: rtems_partition_ident

.. code-block:: c

    rtems_status_code rtems_partition_ident(
        rtems_name  name,
        uint32_t    node,
        rtems_id   *id
    );

**DIRECTIVE STATUS CODES:**

.. list-table::
 :class: rtems-table

 * - ``RTEMS_SUCCESSFUL``
   - partition identified successfully
 * - ``RTEMS_INVALID_ADDRESS``
   - ``id`` is NULL
 * - ``RTEMS_INVALID_NAME``
   - partition name not found
 * - ``RTEMS_INVALID_NODE``
   - invalid node id

**DESCRIPTION:**

This directive obtains the partition id associated with the partition name.  If
the partition name is not unique, then the partition id will match one of the
partitions with that name.  However, this partition id is not guaranteed to
correspond to the desired partition.  The partition id is used with other
partition related directives to access the partition.

**NOTES:**

This directive will not cause the running task to be preempted.

If node is ``RTEMS_SEARCH_ALL_NODES``, all nodes are searched with the local
node being searched first.  All other nodes are searched with the lowest
numbered node searched first.

If node is a valid node number which does not represent the local node, then
only the partitions exported by the designated node are searched.

This directive does not generate activity on remote nodes.  It accesses only
the local copy of the global object table.

.. _rtems_partition_delete:

PARTITION_DELETE - Delete a partition
-------------------------------------
.. index:: delete a partition

**CALLING SEQUENCE:**

.. index:: rtems_partition_delete

.. code-block:: c

    rtems_status_code rtems_partition_delete(
        rtems_id id
    );

**DIRECTIVE STATUS CODES:**

.. list-table::
 :class: rtems-table

 * - ``RTEMS_SUCCESSFUL``
   - partition deleted successfully
 * - ``RTEMS_INVALID_ID``
   - invalid partition id
 * - ``RTEMS_RESOURCE_IN_USE``
   - buffers still in use
 * - ``RTEMS_ILLEGAL_ON_REMOTE_OBJECT``
   - cannot delete remote partition

**DESCRIPTION:**

This directive deletes the partition specified by id.  The partition cannot be
deleted if any of its buffers are still allocated.  The PTCB for the deleted
partition is reclaimed by RTEMS.

**NOTES:**

This directive will not cause the calling task to be preempted.

The calling task does not have to be the task that created the partition.  Any
local task that knows the partition id can delete the partition.

When a global partition is deleted, the partition id must be transmitted to
every node in the system for deletion from the local copy of the global object
table.

The partition must reside on the local node, even if the partition was created
with the ``RTEMS_GLOBAL`` option.

.. _rtems_partition_get_buffer:

PARTITION_GET_BUFFER - Get buffer from a partition
--------------------------------------------------
.. index:: get buffer from partition
.. index:: obtain buffer from partition

**CALLING SEQUENCE:**

.. index:: rtems_partition_get_buffer

.. code-block:: c

    rtems_status_code rtems_partition_get_buffer(
        rtems_id   id,
        void     **buffer
    );

**DIRECTIVE STATUS CODES:**

.. list-table::
 :class: rtems-table

 * - ``RTEMS_SUCCESSFUL``
   - buffer obtained successfully
 * - ``RTEMS_INVALID_ADDRESS``
   - ``buffer`` is NULL
 * - ``RTEMS_INVALID_ID``
   - invalid partition id
 * - ``RTEMS_UNSATISFIED``
   - all buffers are allocated

**DESCRIPTION:**

This directive allows a buffer to be obtained from the partition specified
in id.  The address of the allocated buffer is returned in buffer.

**NOTES:**

This directive will not cause the running task to be preempted.

All buffers begin on a four byte boundary.

A task cannot wait on a buffer to become available.

Getting a buffer from a global partition which does not reside on the local
node will generate a request telling the remote node to allocate a buffer from
the specified partition.

.. _rtems_partition_return_buffer:

PARTITION_RETURN_BUFFER - Return buffer to a partition
------------------------------------------------------
.. index:: return buffer to partitition

**CALLING SEQUENCE:**

.. index:: rtems_partition_return_buffer

.. code-block:: c

    rtems_status_code rtems_partition_return_buffer(
        rtems_id  id,
        void     *buffer
    );

**DIRECTIVE STATUS CODES:**

.. list-table::
 :class: rtems-table

 * - ``RTEMS_SUCCESSFUL``
   - buffer returned successfully
 * - ``RTEMS_INVALID_ADDRESS``
   - ``buffer`` is NULL
 * - ``RTEMS_INVALID_ID``
   - invalid partition id
 * - ``RTEMS_INVALID_ADDRESS``
   - buffer address not in partition

**DESCRIPTION:**

This directive returns the buffer specified by buffer to the partition
specified by id.

**NOTES:**

This directive will not cause the running task to be preempted.

Returning a buffer to a global partition which does not reside on the local
node will generate a request telling the remote node to return the buffer to
the specified partition.

Returning a buffer multiple times is an error.  It will corrupt the internal
state of the partition.