forked from Imagelibrary/rtems
422 lines
13 KiB
Raku
422 lines
13 KiB
Raku
@c
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@c Copyright 2015 embedded brains GmbH
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@c All rights reserved.
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@chapter Linker Sets
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@cindex linkersets
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@section Introduction
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Linker sets are a flexible means to create arrays of items out of a set of
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object files at link-time. For example its possible to define an item @emph{I}
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of type @emph{T} in object file @emph{A} and an item @emph{J} of type @emph{T}
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in object file @emph{B} to be a member of a linker set @emph{S}. The linker
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will then collect these two items @emph{I} and @emph{J} and place them in
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consecutive memory locations, so that they can be accessed like a normal array
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defined in one object file. The size of a linker set is defined by its begin
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and end markers. A linker set may be empty. It should only contain items of
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the same type.
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The following macros are provided to create, populate and use linker sets.
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@itemize @bullet
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@item @code{RTEMS_LINKER_SET_BEGIN} - Designator of the linker set begin marker
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@item @code{RTEMS_LINKER_SET_END} - Designator of the linker set end marker
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@item @code{RTEMS_LINKER_SET_SIZE} - The linker set size in characters
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@item @code{RTEMS_LINKER_ROSET_DECLARE} - Declares a read-only linker set
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@item @code{RTEMS_LINKER_ROSET} - Defines a read-only linker set
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@item @code{RTEMS_LINKER_ROSET_ITEM_DECLARE} - Declares a read-only linker set item
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@item @code{RTEMS_LINKER_ROSET_ITEM_REFERENCE} - References a read-only linker set item
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@item @code{RTEMS_LINKER_ROSET_ITEM} - Defines a read-only linker set item
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@item @code{RTEMS_LINKER_ROSET_ITEM_ORDERED} - Defines an ordered read-only linker set item
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@item @code{RTEMS_LINKER_RWSET_DECLARE} - Declares a read-write linker set
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@item @code{RTEMS_LINKER_RWSET} - Defines a read-write linker set
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@item @code{RTEMS_LINKER_RWSET_ITEM_DECLARE} - Declares a read-write linker set item
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@item @code{RTEMS_LINKER_RWSET_ITEM_REFERENCE} - References a read-write linker set item
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@item @code{RTEMS_LINKER_RWSET_ITEM} - Defines a read-write linker set item
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@item @code{RTEMS_LINKER_RWSET_ITEM_ORDERED} - Defines an ordered read-write linker set item
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@end itemize
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@section Background
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Linker sets are used not only in RTEMS, but also for example in Linux, in
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FreeBSD, for the GNU C constructor extension and for global C++ constructors.
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They provide a space efficient and flexible means to initialize modules. A
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linker set consists of
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@itemize @bullet
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@item dedicated input sections for the linker (e.g. @code{.ctors} and
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@code{.ctors.*} in the case of global constructors),
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@item a begin marker (e.g. provided by @code{crtbegin.o}, and
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@item an end marker (e.g. provided by @code{ctrend.o}).
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@end itemize
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A module may place a certain data item into the dedicated input section. The
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linker will collect all such data items in this section and creates a begin and
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end marker. The initialization code can then use the begin and end markers to
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find all the collected data items (e.g. pointers to initialization functions).
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In the linker command file of the GNU linker we need the following output
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section descriptions.
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@example
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@group
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/* To be placed in a read-only memory region */
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.rtemsroset : @{
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KEEP (*(SORT(.rtemsroset.*)))
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@}
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/* To be placed in a read-write memory region */
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.rtemsrwset : @{
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KEEP (*(SORT(.rtemsrwset.*)))
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@}
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@end group
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@end example
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The @code{KEEP()} ensures that a garbage collection by the linker will not
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discard the content of this section. This would normally be the case since the
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linker set items are not referenced directly. The @code{SORT()} directive
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sorts the input sections lexicographically. Please note the lexicographical
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order of the @code{.begin}, @code{.content} and @code{.end} section name parts
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in the RTEMS linker sets macros which ensures that the position of the begin
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and end markers are right.
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So, what is the benefit of using linker sets to initialize modules? It can be
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used to initialize and include only those RTEMS managers and other components
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which are used by the application. For example, in case an application uses
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message queues, it must call @code{rtems_message_queue_create()}. In the
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module implementing this function, we can place a linker set item and register
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the message queue handler constructor. Otherwise, in case the application does
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not use message queues, there will be no reference to the
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@code{rtems_message_queue_create()} function and the constructor is not
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registered, thus nothing of the message queue handler will be in the final
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executable.
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For an example see test program @file{sptests/splinkersets01}.
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@section Directives
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@macro linkersetsset
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The @code{set} parameter itself must be a valid C designator on which no macro
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expansion is performed. It uniquely identifies the linker set.
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@end macro
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@macro linkersetstype
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The @code{type} parameter defines the type of the linker set items. The type
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must be the same for all macro invocations of a particular linker set.
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@end macro
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@macro linkersetsitem
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The @code{item} parameter itself must be a valid C designator on which no macro
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expansion is performed. It uniquely identifies an item in the linker set.
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@end macro
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@macro linkersetsorder
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The @code{order} parameter must be a valid linker input section name part on
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which macro expansion is performed. The items are lexicographically ordered
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according to the @code{order} parameter within a linker set. Ordered items are
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placed before unordered items in the linker set.
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@end macro
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@page
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@subsection RTEMS_LINKER_SET_BEGIN - Designator of the linker set begin marker
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_SET_BEGIN
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@example
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volatile type *begin = RTEMS_LINKER_SET_BEGIN( set );
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@end example
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@subheading DESCRIPTION:
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This macro generates the designator of the begin marker of the linker set
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identified by @code{set}. The item at the begin marker address is the first
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member of the linker set if it exists, e.g. the linker set is not empty. A
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linker set is empty, if and only if the begin and end markers have the same
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address.
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@linkersetsset
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@page
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@subsection RTEMS_LINKER_SET_END - Designator of the linker set end marker
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_SET_END
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@example
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volatile type *end = RTEMS_LINKER_SET_END( set );
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@end example
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@subheading DESCRIPTION:
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This macro generates the designator of the end marker of the linker set
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identified by @code{set}. The item at the end marker address is not a member
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of the linker set. @linkersetsset
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@page
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@subsection RTEMS_LINKER_SET_SIZE - The linker set size in characters
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_SET_SIZE
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@example
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size_t size = RTEMS_LINKER_SET_SIZE( set );
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@end example
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@subheading DESCRIPTION:
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This macro returns the size of the linker set identified by @code{set} in
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characters. @linkersetsset
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@page
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@subsection RTEMS_LINKER_ROSET_DECLARE - Declares a read-only linker set
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET_DECLARE
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@example
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RTEMS_LINKER_ROSET_DECLARE( set, type );
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@end example
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@subheading DESCRIPTION:
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This macro generates declarations for the begin and end markers of a read-only
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@page
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@subsection RTEMS_LINKER_ROSET - Defines a read-only linker set
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET
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@example
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RTEMS_LINKER_ROSET( set, type );
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@end example
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@subheading DESCRIPTION:
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This macro generates definitions for the begin and end markers of a read-only
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@page
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@subsection RTEMS_LINKER_ROSET_ITEM_DECLARE - Declares a read-only linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET_ITEM_DECLARE
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@example
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RTEMS_LINKER_ROSET_ITEM_DECLARE( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a declaration of an item contained in the read-only linker
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set identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_ROSET_ITEM_REFERENCE - References a read-only linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET_ITEM_REFERENCE
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@example
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RTEMS_LINKER_ROSET_ITEM_REFERENCE( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a reference to an item contained in the read-only linker set
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identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_ROSET_ITEM - Defines a read-only linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET_ITEM
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@example
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RTEMS_LINKER_ROSET_ITEM( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a definition of an item contained in the read-only linker set
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identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_ROSET_ITEM_ORDERED - Defines an ordered read-only linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_ROSET_ITEM_ORDERED
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@example
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RTEMS_LINKER_ROSET_ITEM_ORDERED( set, type, item, order );
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@end example
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@subheading DESCRIPTION:
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This macro generates a definition of an ordered item contained in the read-only
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@linkersetsitem @linkersetsorder
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@subheading NOTES:
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To be resilient to typos in the order parameter, it is recommended to use the
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following construct in macros defining items for a particular linker set (see
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enum in @code{XYZ_ITEM()}).
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@example
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#include <rtems/linkersets.h>
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typedef struct @{
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int foo;
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@} xyz_item;
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/* The XYZ-order defines */
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#define XYZ_ORDER_FIRST 0x00001000
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#define XYZ_ORDER_AND_SO_ON 0x00002000
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/* Defines an ordered XYZ-item */
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#define XYZ_ITEM( item, order ) \
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enum @{ xyz_##item = order - order @}; \
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RTEMS_LINKER_ROSET_ITEM_ORDERED( \
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xyz, const xyz_item *, item, order \
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) = @{ &item @}
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/* Example item */
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static const xyz_item some_item = @{ 123 @};
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XYZ_ITEM( some_item, XYZ_ORDER_FIRST );
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@end example
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@page
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@subsection RTEMS_LINKER_RWSET_DECLARE - Declares a read-write linker set
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET_DECLARE
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@example
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RTEMS_LINKER_RWSET_DECLARE( set, type );
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@end example
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@subheading DESCRIPTION:
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This macro generates declarations for the begin and end markers of a read-write
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@page
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@subsection RTEMS_LINKER_RWSET - Defines a read-write linker set
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET
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@example
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RTEMS_LINKER_RWSET( set, type );
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@end example
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@subheading DESCRIPTION:
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This macro generates definitions for the begin and end markers of a read-write
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@page
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@subsection RTEMS_LINKER_RWSET_ITEM_DECLARE - Declares a read-write linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET_ITEM_DECLARE
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@example
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RTEMS_LINKER_RWSET_ITEM_DECLARE( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a declaration of an item contained in the read-write linker
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set identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_RWSET_ITEM_REFERENCE - References a read-write linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET_ITEM_REFERENCE
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@example
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RTEMS_LINKER_RWSET_ITEM_REFERENCE( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a reference to an item contained in the read-write linker set
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identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_RWSET_ITEM - Defines a read-write linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET_ITEM
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@example
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RTEMS_LINKER_RWSET_ITEM( set, type, item );
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@end example
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@subheading DESCRIPTION:
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This macro generates a definition of an item contained in the read-write linker set
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identified by @code{set}. @linkersetsset @linkersetstype @linkersetsitem
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@page
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@subsection RTEMS_LINKER_RWSET_ITEM_ORDERED - Defines an ordered read-write linker set item
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@subheading CALLING SEQUENCE:
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@findex RTEMS_LINKER_RWSET_ITEM_ORDERED
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@example
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RTEMS_LINKER_RWSET_ITEM_ORDERED( set, type, item, order );
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@end example
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@subheading DESCRIPTION:
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This macro generates a definition of an ordered item contained in the read-write
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linker set identified by @code{set}. @linkersetsset @linkersetstype
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@linkersetsitem @linkersetsorder
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@subheading NOTES:
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To be resilient to typos in the order parameter, it is recommended to use the
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following construct in macros defining items for a particular linker set (see
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enum in @code{XYZ_ITEM()}).
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@example
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#include <rtems/linkersets.h>
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typedef struct @{
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int foo;
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@} xyz_item;
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/* The XYZ-order defines */
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#define XYZ_ORDER_FIRST 0x00001000
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#define XYZ_ORDER_AND_SO_ON 0x00002000
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/* Defines an ordered XYZ-item */
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#define XYZ_ITEM( item, order ) \
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enum @{ xyz_##item = order - order @}; \
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RTEMS_LINKER_RWSET_ITEM_ORDERED( \
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xyz, const xyz_item *, item, order \
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) = @{ &item @}
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/* Example item */
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static const xyz_item some_item = @{ 123 @};
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XYZ_ITEM( some_item, XYZ_ORDER_FIRST );
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@end example
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