36. Linker Sets¶

36.1. 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

36.2. 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.

/* 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 sptests/splinkersets01.

36.3. Directives¶

36.3.1. RTEMS_LINKER_SET_BEGIN - Designator of the linker set begin marker¶

CALLING SEQUENCE:

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.

36.3.2. RTEMS_LINKER_SET_END - Designator of the linker set end marker¶

CALLING SEQUENCE:

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.

36.3.3. RTEMS_LINKER_SET_SIZE - The linker set size in characters¶

CALLING SEQUENCE:

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.

36.3.4. RTEMS_LINKER_ROSET_DECLARE - Declares a read-only linker set¶

CALLING SEQUENCE:

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.

36.3.5. RTEMS_LINKER_ROSET - Defines a read-only linker set¶

CALLING SEQUENCE:

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.

36.3.6. RTEMS_LINKER_ROSET_ITEM_DECLARE - Declares a read-only linker set item¶

CALLING SEQUENCE:

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.

36.3.7. RTEMS_LINKER_ROSET_ITEM_REFERENCE - References a read-only linker set item¶

CALLING SEQUENCE:

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.

36.3.8. RTEMS_LINKER_ROSET_ITEM - Defines a read-only linker set item¶

CALLING SEQUENCE:

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.

36.3.9. RTEMS_LINKER_ROSET_ITEM_ORDERED - Defines an ordered read-only linker set item¶

CALLING SEQUENCE:

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()).

#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 );

36.3.10. RTEMS_LINKER_RWSET_DECLARE - Declares a read-write linker set¶

CALLING SEQUENCE:

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.

36.3.11. RTEMS_LINKER_RWSET - Defines a read-write linker set¶

CALLING SEQUENCE:

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.

36.3.12. RTEMS_LINKER_RWSET_ITEM_DECLARE - Declares a read-write linker set item¶

CALLING SEQUENCE:

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.

36.3.13. RTEMS_LINKER_RWSET_ITEM_REFERENCE - References a read-write linker set item¶

CALLING SEQUENCE:

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.

36.3.14. RTEMS_LINKER_RWSET_ITEM - Defines a read-write linker set item¶

CALLING SEQUENCE:

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.

36.3.15. RTEMS_LINKER_RWSET_ITEM_ORDERED - Defines an ordered read-write linker set item¶

CALLING SEQUENCE:

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()).

#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 );