37. Linker Sets

37.1. 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.

37.2. 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.

37.3. Directives

37.3.1. RTEMS_LINKER_SET_BEGIN - Designator of the linker set begin marker

CALLING SEQUENCE:
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.

NOTE:

The compiler may try to be smart. In general it will not work to assign linker set begin and end addresses to pointer variables and treat them like ordinary pointers. The compiler may exploit the fact that actually two distinct objects are involved and use this to optimize. To avoid trouble use RTEMS_LINKER_SET_SIZE - The linker set size in characters, RTEMS_LINKER_SET_ITEM_COUNT - The linker set item count, RTEMS_LINKER_SET_IS_EMPTY - Is the linker set empty? and RTEMS_LINKER_SET_FOREACH - Iterate through the linker set items.

37.3.2. RTEMS_LINKER_SET_END - Designator of the linker set end marker

CALLING SEQUENCE:
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.

37.3.3. 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.

37.3.4. RTEMS_LINKER_SET_ITEM_COUNT - The linker set item count

CALLING SEQUENCE:
size_t item_count = RTEMS_LINKER_SET_ITEM_COUNT( set );
DESCRIPTION:

This macro returns the item count of the 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.

37.3.5. RTEMS_LINKER_SET_IS_EMPTY - Is the linker set empty?

CALLING SEQUENCE:
bool is_empty = RTEMS_LINKER_SET_IS_EMPTY( set );
DESCRIPTION:

This macro returns true if the linker set identified by set is empty, otherwise returns false. The set parameter itself must be a valid C designator on which no macro expansion is performed. It uniquely identifies the linker set.

37.3.6. RTEMS_LINKER_SET_FOREACH - Iterate through the linker set items

CALLING SEQUENCE:
RTEMS_LINKER_RWSET( myset, int );

int count( void )
{
  int *item;
  int n;

  n = 0;
  RTEMS_LINKER_SET_FOREACH( myset, item ) {
    n += *item;
  }

  return n;
}
DESCRIPTION:

This macro generates a for loop statement which iterates through each item of a 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 item parameter must be a pointer to an item of the linker set. It iterates through all items of the linker set from begin to end.

37.3.7. 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.

37.3.8. 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.

37.3.9. 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. For a description of the set, type, and item parameters see RTEMS_LINKER_ROSET_ITEM - Defines a read-only linker set item.

37.3.10. RTEMS_LINKER_ROSET_ITEM_ORDERED_DECLARE - Declares an ordered read-only linker set item

CALLING SEQUENCE:
RTEMS_LINKER_ROSET_ITEM_ORDERED_DECLARE( set, type, item, order );
DESCRIPTION:

This macro generates a declaration of an ordered item contained in the read-only linker set identified by set. For a description of the set, type, item, and order parameters see RTEMS_LINKER_ROSET_ITEM_ORDERED - Defines an ordered read-only linker set item.

37.3.11. 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.

37.3.12. 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.

37.3.13. 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 }; \
  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 );

37.3.14. RTEMS_LINKER_ROSET_CONTENT - Marks a declaration as a read-only linker set content

CALLING SEQUENCE:
RTEMS_LINKER_ROSET_CONTENT( set, decl );
DESCRIPTION:

This macro marks a declaration as a read-only linker set content. The linker set is 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 decl parameter must be an arbitrary variable declaration.

37.3.15. 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.

37.3.16. 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.

37.3.17. 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. For a description of the set, type, and item parameters see RTEMS_LINKER_RWSET_ITEM - Defines a read-write linker set item.

37.3.18. RTEMS_LINKER_RWSET_ITEM_ORDERED_DECLARE - Declares an ordered read-write linker set item

CALLING SEQUENCE:
RTEMS_LINKER_RWSET_ITEM_ORDERED_DECLARE( set, type, item, order );
DESCRIPTION:

This macro generates a declaration of an ordered item contained in the read-write linker set identified by set. For a description of the set, type, item, and order parameters see RTEMS_LINKER_RWSET_ITEM_ORDERED - Defines an ordered read-write linker set item.

37.3.19. 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.

37.3.20. 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.

37.3.21. 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 }; \
  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 );

37.3.22. RTEMS_LINKER_RWSET_CONTENT - Marks a declaration as a read-write linker set content

CALLING SEQUENCE:
RTEMS_LINKER_RWSET_CONTENT( set, decl );
DESCRIPTION:

This macro marks a declaration as a read-write linker set content. The linker set is 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 decl parameter must be an arbitrary variable declaration.