source: rtems-docs/c_user/linker_sets.rst @ 54514fe

.. comment SPDX-License-Identifier: CC-BY-SA-4.0

.. COMMENT: COPYRIGHT (c) 1989-2014.
.. COMMENT: On-Line Applications Research Corporation (OAR).
.. COMMENT: All rights reserved.

Linker Sets
###########

.. index:: linkersets

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

Linker sets are a flexible means to create arrays of items out of a set of
object files at link-time.  For example its possible to define an item *I* of
type *T* in object file *A* and an item *J* of type *T* in object file *B* to
be a member of a linker set *S*.  The linker will then collect these two items
*I* and *J* and place them in consecutive memory locations, so that they can be
accessed like a normal array defined in one object file.  The size of a linker
set is defined by its begin and end markers.  A linker set may be empty.  It
should only contain items of the same type.

The following macros are provided to create, populate and use linker sets.

- RTEMS_LINKER_SET_BEGIN_ - Designator of the linker set begin marker

- RTEMS_LINKER_SET_END_ - Designator of the linker set end marker

- RTEMS_LINKER_SET_SIZE_ - The linker set size in characters

- RTEMS_LINKER_ROSET_DECLARE_ - Declares a read-only linker set

- RTEMS_LINKER_ROSET_ - Defines a read-only linker set

- RTEMS_LINKER_ROSET_ITEM_DECLARE_ - Declares a read-only linker set item

- RTEMS_LINKER_ROSET_ITEM_REFERENCE_ - References a read-only linker set item

- RTEMS_LINKER_ROSET_ITEM_ - Defines a read-only linker set item

- RTEMS_LINKER_ROSET_ITEM_ORDERED_ - Defines an ordered read-only linker set item

- RTEMS_LINKER_RWSET_DECLARE_ - Declares a read-write linker set

- RTEMS_LINKER_RWSET_ - Defines a read-write linker set

- RTEMS_LINKER_RWSET_ITEM_DECLARE_ - Declares a read-write linker set item

- RTEMS_LINKER_RWSET_ITEM_REFERENCE_ - References a read-write linker set item

- RTEMS_LINKER_RWSET_ITEM_ - Defines a read-write linker set item

- RTEMS_LINKER_RWSET_ITEM_ORDERED_ - Defines an ordered read-write linker set item

Background
==========

Linker sets are used not only in RTEMS, but also for example in Linux, in
FreeBSD, for the GNU C constructor extension and for global C++ constructors.
They provide a space efficient and flexible means to initialize modules.  A
linker set consists of

- dedicated input sections for the linker (e.g. ``.ctors`` and ``.ctors.*`` in
  the case of global constructors),

- a begin marker (e.g. provided by ``crtbegin.o``, and

- an end marker (e.g. provided by ``ctrend.o``).

A module may place a certain data item into the dedicated input section.  The
linker will collect all such data items in this section and creates a begin and
end marker.  The initialization code can then use the begin and end markers to
find all the collected data items (e.g. pointers to initialization functions).

In the linker command file of the GNU linker we need the following output
section descriptions.

.. code-block:: c

    /* To be placed in a read-only memory region */
    .rtemsroset : {
        KEEP (*(SORT(.rtemsroset.*)))
    }
    /* To be placed in a read-write memory region */
    .rtemsrwset : {
        KEEP (*(SORT(.rtemsrwset.*)))
    }

The ``KEEP()`` ensures that a garbage collection by the linker will not discard
the content of this section.  This would normally be the case since the linker
set items are not referenced directly.  The ``SORT()`` directive sorts the
input sections lexicographically.  Please note the lexicographical order of the
``.begin``, ``.content`` and ``.end`` section name parts in the RTEMS linker
sets macros which ensures that the position of the begin and end markers are
right.

So, what is the benefit of using linker sets to initialize modules?  It can be
used to initialize and include only those RTEMS managers and other components
which are used by the application.  For example, in case an application uses
message queues, it must call ``rtems_message_queue_create()``.  In the module
implementing this function, we can place a linker set item and register the
message queue handler constructor.  Otherwise, in case the application does not
use message queues, there will be no reference to the
``rtems_message_queue_create()`` function and the constructor is not
registered, thus nothing of the message queue handler will be in the final
executable.

For an example see test program :file:`sptests/splinkersets01`.

Directives
==========

.. _RTEMS_LINKER_SET_BEGIN:

RTEMS_LINKER_SET_BEGIN - Designator of the linker set begin marker
------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_SET_BEGIN

.. code-block:: c

    volatile type *begin = RTEMS_LINKER_SET_BEGIN( set );

**DESCRIPTION:**

This macro generates the designator of the begin marker of the linker set
identified by ``set``.  The item at the begin marker address is the first
member of the linker set if it exists, e.g. the linker set is not empty.  A
linker set is empty, if and only if the begin and end markers have the same
address.

The ``set`` parameter itself must be a valid C designator on which no macro
expansion is performed.  It uniquely identifies the linker set.

.. _RTEMS_LINKER_SET_END:

RTEMS_LINKER_SET_END - Designator of the linker set end marker
--------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_SET_END

.. code-block:: c

    volatile type *end = RTEMS_LINKER_SET_END( set );

**DESCRIPTION:**

This macro generates the designator of the end marker of the linker set
identified by ``set``.  The item at the end marker address is not a member of
the linker set.  The ``set`` parameter itself must be a valid C designator on
which no macro expansion is performed.  It uniquely identifies the linker set.

.. _RTEMS_LINKER_SET_SIZE:

RTEMS_LINKER_SET_SIZE - The linker set size in characters
---------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_SET_SIZE

.. code-block:: c

    size_t size = RTEMS_LINKER_SET_SIZE( set );

**DESCRIPTION:**

This macro returns the size of the linker set identified by ``set`` in
characters.  The ``set`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies the linker set.

.. _RTEMS_LINKER_ROSET_DECLARE:

RTEMS_LINKER_ROSET_DECLARE - Declares a read-only linker set
------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET_DECLARE

.. code-block:: c

    RTEMS_LINKER_ROSET_DECLARE( set, type );

**DESCRIPTION:**

This macro generates declarations for the begin and end markers of a read-only
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set.

.. _RTEMS_LINKER_ROSET:

RTEMS_LINKER_ROSET - Defines a read-only linker set
---------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET

.. code-block:: c

    RTEMS_LINKER_ROSET( set, type );

**DESCRIPTION:**

This macro generates definitions for the begin and end markers of a read-only
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set.

.. _RTEMS_LINKER_ROSET_ITEM_DECLARE:

RTEMS_LINKER_ROSET_ITEM_DECLARE - Declares a read-only linker set item
----------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET_ITEM_DECLARE

.. code-block:: c

    RTEMS_LINKER_ROSET_ITEM_DECLARE( set, type, item );

**DESCRIPTION:**

This macro generates a declaration of an item contained in the read-only linker
set identified by ``set``.  The ``set`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_ROSET_ITEM_REFERENCE:

RTEMS_LINKER_ROSET_ITEM_REFERENCE - References a read-only linker set item
--------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET_ITEM_REFERENCE

.. code-block:: c

    RTEMS_LINKER_ROSET_ITEM_REFERENCE( set, type, item );

**DESCRIPTION:**

This macro generates a reference to an item contained in the read-only linker
set identified by ``set``.  The ``set`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_ROSET_ITEM:

RTEMS_LINKER_ROSET_ITEM - Defines a read-only linker set item
-------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET_ITEM

.. code-block:: c

    RTEMS_LINKER_ROSET_ITEM( set, type, item );

**DESCRIPTION:**

This macro generates a definition of an item contained in the read-only linker
set identified by ``set``.  The ``set`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_ROSET_ITEM_ORDERED:

RTEMS_LINKER_ROSET_ITEM_ORDERED - Defines an ordered read-only linker set item
------------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_ROSET_ITEM_ORDERED

.. code-block:: c

    RTEMS_LINKER_ROSET_ITEM_ORDERED( set, type, item, order );

**DESCRIPTION:**

This macro generates a definition of an ordered item contained in the read-only
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set.  The ``item`` parameter itself must be a valid C designator on
which no macro expansion is performed.  It uniquely identifies an item in the
linker set. The ``order`` parameter must be a valid linker input section name
part on which macro expansion is performed.  The items are lexicographically
ordered according to the ``order`` parameter within a linker set.  Ordered
items are placed before unordered items in the linker set.

**NOTES:**

To be resilient to typos in the order parameter, it is recommended to use the
following construct in macros defining items for a particular linker set (see
enum in ``XYZ_ITEM()``).

.. code-block:: c

    #include <rtems/linkersets.h>

    typedef struct {
        int foo;
    } xyz_item;

    /* The XYZ-order defines */
    #define XYZ_ORDER_FIRST 0x00001000
    #define XYZ_ORDER_AND_SO_ON 0x00002000

    /* Defines an ordered XYZ-item */
    #define XYZ_ITEM( item, order ) \
                enum { xyz_##item = order - order }; \
                RTEMS_LINKER_ROSET_ITEM_ORDERED( \
                    xyz, const xyz_item *, item, order \
                ) = { &item }

    /* Example item */
    static const xyz_item some_item = { 123 };
    XYZ_ITEM( some_item, XYZ_ORDER_FIRST );

.. _RTEMS_LINKER_RWSET_DECLARE:

RTEMS_LINKER_RWSET_DECLARE - Declares a read-write linker set
-------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET_DECLARE

.. code-block:: c

    RTEMS_LINKER_RWSET_DECLARE( set, type );

**DESCRIPTION:**

This macro generates declarations for the begin and end markers of a read-write
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set.

.. _RTEMS_LINKER_RWSET:

RTEMS_LINKER_RWSET - Defines a read-write linker set
----------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET

.. code-block:: c

    RTEMS_LINKER_RWSET( set, type );

**DESCRIPTION:**

This macro generates definitions for the begin and end markers of a read-write
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set.

.. _RTEMS_LINKER_RWSET_ITEM_DECLARE:

RTEMS_LINKER_RWSET_ITEM_DECLARE - Declares a read-write linker set item
-----------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET_ITEM_DECLARE

.. code-block:: c

    RTEMS_LINKER_RWSET_ITEM_DECLARE( set, type, item );

**DESCRIPTION:**

This macro generates a declaration of an item contained in the read-write
linker set identified by ``set``.  The ``set`` parameter itself must be a valid
C designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_RWSET_ITEM_REFERENCE:

RTEMS_LINKER_RWSET_ITEM_REFERENCE - References a read-write linker set item
---------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET_ITEM_REFERENCE

.. code-block:: c

    RTEMS_LINKER_RWSET_ITEM_REFERENCE( set, type, item );

**DESCRIPTION:**

This macro generates a reference to an item contained in the read-write linker
set identified by ``set``.  The ``set`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_RWSET_ITEM:

RTEMS_LINKER_RWSET_ITEM - Defines a read-write linker set item
--------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET_ITEM

.. code-block:: c

    RTEMS_LINKER_RWSET_ITEM( set, type, item );

**DESCRIPTION:**

This macro generates a definition of an item contained in the read-write linker
set identified by ``set``.  The ``set`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies
the linker set. The ``type`` parameter defines the type of the linker set
items.  The type must be the same for all macro invocations of a particular
linker set. The ``item`` parameter itself must be a valid C designator on which
no macro expansion is performed.  It uniquely identifies an item in the linker
set.

.. _RTEMS_LINKER_RWSET_ITEM_ORDERED:

RTEMS_LINKER_RWSET_ITEM_ORDERED - Defines an ordered read-write linker set item
-------------------------------------------------------------------------------

**CALLING SEQUENCE:**

.. index:: RTEMS_LINKER_RWSET_ITEM_ORDERED

.. code-block:: c

    RTEMS_LINKER_RWSET_ITEM_ORDERED( set, type, item, order );

**DESCRIPTION:**

This macro generates a definition of an ordered item contained in the
read-write linker set identified by ``set``.  The ``set`` parameter itself must
be a valid C designator on which no macro expansion is performed.  It uniquely
identifies the linker set. The ``type`` parameter defines the type of the
linker set items.  The type must be the same for all macro invocations of a
particular linker set.  The ``item`` parameter itself must be a valid C
designator on which no macro expansion is performed.  It uniquely identifies an
item in the linker set. The ``order`` parameter must be a valid linker input
section name part on which macro expansion is performed.  The items are
lexicographically ordered according to the ``order`` parameter within a linker
set.  Ordered items are placed before unordered items in the linker set.

**NOTES:**

To be resilient to typos in the order parameter, it is recommended to use the
following construct in macros defining items for a particular linker set (see
enum in ``XYZ_ITEM()``).

.. code-block:: c

    #include <rtems/linkersets.h>

    typedef struct {
        int foo;
    } xyz_item;

    /* The XYZ-order defines */
    #define XYZ_ORDER_FIRST 0x00001000
    #define XYZ_ORDER_AND_SO_ON 0x00002000

    /* Defines an ordered XYZ-item */
    #define XYZ_ITEM( item, order ) \
                enum { xyz_##item = order - order }; \
                RTEMS_LINKER_RWSET_ITEM_ORDERED( \
                    xyz, const xyz_item *, item, order \
                ) = { &item }
    /* Example item */
    static const xyz_item some_item = { 123 };
    XYZ_ITEM( some_item, XYZ_ORDER_FIRST );