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author | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2015-09-16 07:13:58 +0200 |
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committer | Sebastian Huber <sebastian.huber@embedded-brains.de> | 2015-12-08 09:25:22 +0100 |
commit | b618d8cfc54f84d4ed03dc7b7fa510c872e6128a (patch) | |
tree | 324b769a7866c0ac2ce8f5208861e5207834d9a4 /doc/user/linkersets.t | |
parent | score: Statically initialize system state (diff) | |
download | rtems-b618d8cfc54f84d4ed03dc7b7fa510c872e6128a.tar.bz2 |
Add RTEMS linker sets
Update #2408.
Diffstat (limited to '')
-rw-r--r-- | doc/user/linkersets.t | 421 |
1 files changed, 421 insertions, 0 deletions
diff --git a/doc/user/linkersets.t b/doc/user/linkersets.t new file mode 100644 index 0000000000..9d1abda513 --- /dev/null +++ b/doc/user/linkersets.t @@ -0,0 +1,421 @@ +@c +@c Copyright 2015 embedded brains GmbH +@c All rights reserved. + +@chapter Linker Sets + +@cindex linkersets + +@section 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 @emph{I} +of type @emph{T} in object file @emph{A} and an item @emph{J} of type @emph{T} +in object file @emph{B} to be a member of a linker set @emph{S}. The linker +will then collect these two items @emph{I} and @emph{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. + +@itemize @bullet +@item @code{RTEMS_LINKER_SET_BEGIN} - Designator of the linker set begin marker +@item @code{RTEMS_LINKER_SET_END} - Designator of the linker set end marker +@item @code{RTEMS_LINKER_SET_SIZE} - The linker set size in characters +@item @code{RTEMS_LINKER_ROSET_DECLARE} - Declares a read-only linker set +@item @code{RTEMS_LINKER_ROSET} - Defines a read-only linker set +@item @code{RTEMS_LINKER_ROSET_ITEM_DECLARE} - Declares a read-only linker set item +@item @code{RTEMS_LINKER_ROSET_ITEM_REFERENCE} - References a read-only linker set item +@item @code{RTEMS_LINKER_ROSET_ITEM} - Defines a read-only linker set item +@item @code{RTEMS_LINKER_ROSET_ITEM_ORDERED} - Defines an ordered read-only linker set item +@item @code{RTEMS_LINKER_RWSET_DECLARE} - Declares a read-write linker set +@item @code{RTEMS_LINKER_RWSET} - Defines a read-write linker set +@item @code{RTEMS_LINKER_RWSET_ITEM_DECLARE} - Declares a read-write linker set item +@item @code{RTEMS_LINKER_RWSET_ITEM_REFERENCE} - References a read-write linker set item +@item @code{RTEMS_LINKER_RWSET_ITEM} - Defines a read-write linker set item +@item @code{RTEMS_LINKER_RWSET_ITEM_ORDERED} - Defines an ordered read-write linker set item +@end itemize + +@section 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 +@itemize @bullet +@item dedicated input sections for the linker (e.g. @code{.ctors} and +@code{.ctors.*} in the case of global constructors), +@item a begin marker (e.g. provided by @code{crtbegin.o}, and +@item an end marker (e.g. provided by @code{ctrend.o}). +@end itemize +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. +@example +@group +/* 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.*))) +@} +@end group +@end example +The @code{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 @code{SORT()} directive +sorts the input sections lexicographically. Please note the lexicographical +order of the @code{.begin}, @code{.content} and @code{.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 @code{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 +@code{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}. + +@section Directives + +@macro linkersetsset +The @code{set} parameter itself must be a valid C designator on which no macro +expansion is performed. It uniquely identifies the linker set. +@end macro + +@macro linkersetstype +The @code{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. +@end macro + +@macro linkersetsitem +The @code{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. +@end macro + +@macro linkersetsorder +The @code{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 @code{order} parameter within a linker set. Ordered items are +placed before unordered items in the linker set. +@end macro + +@page + +@subsection RTEMS_LINKER_SET_BEGIN - Designator of the linker set begin marker + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_SET_BEGIN +@example +volatile type *begin = RTEMS_LINKER_SET_BEGIN( set ); +@end example + +@subheading DESCRIPTION: + +This macro generates the designator of the begin marker of the linker set +identified by @code{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. +@linkersetsset + +@page + +@subsection RTEMS_LINKER_SET_END - Designator of the linker set end marker + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_SET_END +@example +volatile type *end = RTEMS_LINKER_SET_END( set ); +@end example + +@subheading DESCRIPTION: + +This macro generates the designator of the end marker of the linker set +identified by @code{set}. The item at the end marker address is not a member +of the linker set. @linkersetsset + +@page + +@subsection RTEMS_LINKER_SET_SIZE - The linker set size in characters + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_SET_SIZE +@example +size_t size = RTEMS_LINKER_SET_SIZE( set ); +@end example + +@subheading DESCRIPTION: + +This macro returns the size of the linker set identified by @code{set} in +characters. @linkersetsset + +@page + +@subsection RTEMS_LINKER_ROSET_DECLARE - Declares a read-only linker set + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET_DECLARE +@example +RTEMS_LINKER_ROSET_DECLARE( set, type ); +@end example + +@subheading DESCRIPTION: + +This macro generates declarations for the begin and end markers of a read-only +linker set identified by @code{set}. @linkersetsset @linkersetstype + +@page + +@subsection RTEMS_LINKER_ROSET - Defines a read-only linker set + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET +@example +RTEMS_LINKER_ROSET( set, type ); +@end example + +@subheading DESCRIPTION: + +This macro generates definitions for the begin and end markers of a read-only +linker set identified by @code{set}. @linkersetsset @linkersetstype + +@page + +@subsection RTEMS_LINKER_ROSET_ITEM_DECLARE - Declares a read-only linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET_ITEM_DECLARE +@example +RTEMS_LINKER_ROSET_ITEM_DECLARE( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a declaration of an item contained in the read-only linker +set identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_ROSET_ITEM_REFERENCE - References a read-only linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET_ITEM_REFERENCE +@example +RTEMS_LINKER_ROSET_ITEM_REFERENCE( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a reference to an item contained in the read-only linker set +identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_ROSET_ITEM - Defines a read-only linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET_ITEM +@example +RTEMS_LINKER_ROSET_ITEM( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a definition of an item contained in the read-only linker set +identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_ROSET_ITEM_ORDERED - Defines an ordered read-only linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_ROSET_ITEM_ORDERED +@example +RTEMS_LINKER_ROSET_ITEM_ORDERED( set, type, item, order ); +@end example + +@subheading DESCRIPTION: + +This macro generates a definition of an ordered item contained in the read-only +linker set identified by @code{set}. @linkersetsset @linkersetstype +@linkersetsitem @linkersetsorder + +@subheading 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 @code{XYZ_ITEM()}). + +@example +#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 ); +@end example + +@page + +@subsection RTEMS_LINKER_RWSET_DECLARE - Declares a read-write linker set + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET_DECLARE +@example +RTEMS_LINKER_RWSET_DECLARE( set, type ); +@end example + +@subheading DESCRIPTION: + +This macro generates declarations for the begin and end markers of a read-write +linker set identified by @code{set}. @linkersetsset @linkersetstype + +@page + +@subsection RTEMS_LINKER_RWSET - Defines a read-write linker set + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET +@example +RTEMS_LINKER_RWSET( set, type ); +@end example + +@subheading DESCRIPTION: + +This macro generates definitions for the begin and end markers of a read-write +linker set identified by @code{set}. @linkersetsset @linkersetstype + +@page + +@subsection RTEMS_LINKER_RWSET_ITEM_DECLARE - Declares a read-write linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET_ITEM_DECLARE +@example +RTEMS_LINKER_RWSET_ITEM_DECLARE( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a declaration of an item contained in the read-write linker +set identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_RWSET_ITEM_REFERENCE - References a read-write linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET_ITEM_REFERENCE +@example +RTEMS_LINKER_RWSET_ITEM_REFERENCE( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a reference to an item contained in the read-write linker set +identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_RWSET_ITEM - Defines a read-write linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET_ITEM +@example +RTEMS_LINKER_RWSET_ITEM( set, type, item ); +@end example + +@subheading DESCRIPTION: + +This macro generates a definition of an item contained in the read-write linker set +identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem + +@page + +@subsection RTEMS_LINKER_RWSET_ITEM_ORDERED - Defines an ordered read-write linker set item + +@subheading CALLING SEQUENCE: + +@findex RTEMS_LINKER_RWSET_ITEM_ORDERED +@example +RTEMS_LINKER_RWSET_ITEM_ORDERED( set, type, item, order ); +@end example + +@subheading DESCRIPTION: + +This macro generates a definition of an ordered item contained in the read-write +linker set identified by @code{set}. @linkersetsset @linkersetstype +@linkersetsitem @linkersetsorder + +@subheading 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 @code{XYZ_ITEM()}). + +@example +#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 ); +@end example |