linker.texi

这个是LINUX下的GDB调度工具的源码

The function passed to
@code{_bfd_generic_link_add_archive_symbols} must read the
symbols of the archive element and decide whether the archive
element should be included in the link.  If the element is to
be included, the @code{add_archive_element} linker callback
routine must be called with the element as an argument, and
the elements symbols must be added to the linker hash table
just as though the element had itself been passed to the
@code{_bfd_link_add_symbols} function.

When the a.out @code{_bfd_link_add_symbols} function receives an
archive, it calls @code{_bfd_generic_link_add_archive_symbols}
passing @code{aout_link_check_archive_element} as the function
argument. @code{aout_link_check_archive_element} calls
@code{aout_link_check_ar_symbols}.  If the latter decides to add
the element (an element is only added if it provides a real,
non-common, definition for a previously undefined or common
symbol) it calls the @code{add_archive_element} callback and then
@code{aout_link_check_archive_element} calls
@code{aout_link_add_symbols} to actually add the symbols to the
linker hash table.

The ECOFF back end is unusual in that it does not normally
call @code{_bfd_generic_link_add_archive_symbols}, because ECOFF
archives already contain a hash table of symbols.  The ECOFF
back end searches the archive itself to avoid the overhead of
creating a new hash table.

@node Performing the Final Link, , Adding Symbols to the Hash Table, Linker Functions
@subsection Performing the final link
@cindex _bfd_link_final_link in target vector
@cindex target vector (_bfd_final_link)
When all the input files have been processed, the linker calls
the @code{_bfd_final_link} entry point of the output BFD.  This
routine is responsible for producing the final output file,
which has several aspects.  It must relocate the contents of
the input sections and copy the data into the output sections.
It must build an output symbol table including any local
symbols from the input files and the global symbols from the
hash table.  When producing relocatable output, it must
modify the input relocs and write them into the output file.
There may also be object format dependent work to be done.

The linker will also call the @code{write_object_contents} entry
point when the BFD is closed.  The two entry points must work
together in order to produce the correct output file.

The details of how this works are inevitably dependent upon
the specific object file format.  The a.out
@code{_bfd_final_link} routine is @code{NAME(aout,final_link)}.

@menu
* Information provided by the linker::
* Relocating the section contents::
* Writing the symbol table::
@end menu

@node Information provided by the linker, Relocating the section contents, Performing the Final Link, Performing the Final Link
@subsubsection Information provided by the linker
Before the linker calls the @code{_bfd_final_link} entry point,
it sets up some data structures for the function to use.

The @code{input_bfds} field of the @code{bfd_link_info} structure
will point to a list of all the input files included in the
link.  These files are linked through the @code{link_next} field
of the @code{bfd} structure.

Each section in the output file will have a list of
@code{link_order} structures attached to the @code{link_order_head}
field (the @code{link_order} structure is defined in
@code{bfdlink.h}).  These structures describe how to create the
contents of the output section in terms of the contents of
various input sections, fill constants, and, eventually, other
types of information.  They also describe relocs that must be
created by the BFD backend, but do not correspond to any input
file; this is used to support -Ur, which builds constructors
while generating a relocatable object file.

@node Relocating the section contents, Writing the symbol table, Information provided by the linker, Performing the Final Link
@subsubsection Relocating the section contents
The @code{_bfd_final_link} function should look through the
@code{link_order} structures attached to each section of the
output file.  Each @code{link_order} structure should either be
handled specially, or it should be passed to the function
@code{_bfd_default_link_order} which will do the right thing
(@code{_bfd_default_link_order} is defined in @code{linker.c}).

For efficiency, a @code{link_order} of type
@code{bfd_indirect_link_order} whose associated section belongs
to a BFD of the same format as the output BFD must be handled
specially.  This type of @code{link_order} describes part of an
output section in terms of a section belonging to one of the
input files.  The @code{_bfd_final_link} function should read the
contents of the section and any associated relocs, apply the
relocs to the section contents, and write out the modified
section contents.  If performing a relocatable link, the
relocs themselves must also be modified and written out.

@findex _bfd_relocate_contents
@findex _bfd_final_link_relocate
The functions @code{_bfd_relocate_contents} and
@code{_bfd_final_link_relocate} provide some general support for
performing the actual relocations, notably overflow checking.
Their arguments include information about the symbol the
relocation is against and a @code{reloc_howto_type} argument
which describes the relocation to perform.  These functions
are defined in @code{reloc.c}.

The a.out function which handles reading, relocating, and
writing section contents is @code{aout_link_input_section}.  The
actual relocation is done in @code{aout_link_input_section_std}
and @code{aout_link_input_section_ext}.

@node Writing the symbol table, , Relocating the section contents, Performing the Final Link
@subsubsection Writing the symbol table
The @code{_bfd_final_link} function must gather all the symbols
in the input files and write them out.  It must also write out
all the symbols in the global hash table.  This must be
controlled by the @code{strip} and @code{discard} fields of the
@code{bfd_link_info} structure.

The local symbols of the input files will not have been
entered into the linker hash table.  The @code{_bfd_final_link}
routine must consider each input file and include the symbols
in the output file.  It may be convenient to do this when
looking through the @code{link_order} structures, or it may be
done by stepping through the @code{input_bfds} list.

The @code{_bfd_final_link} routine must also traverse the global
hash table to gather all the externally visible symbols.  It
is possible that most of the externally visible symbols may be
written out when considering the symbols of each input file,
but it is still necessary to traverse the hash table since the
linker script may have defined some symbols that are not in
any of the input files.

The @code{strip} field of the @code{bfd_link_info} structure
controls which symbols are written out.  The possible values
are listed in @code{bfdlink.h}.  If the value is @code{strip_some},
then the @code{keep_hash} field of the @code{bfd_link_info}
structure is a hash table of symbols to keep; each symbol
should be looked up in this hash table, and only symbols which
are present should be included in the output file.

If the @code{strip} field of the @code{bfd_link_info} structure
permits local symbols to be written out, the @code{discard} field
is used to further controls which local symbols are included
in the output file.  If the value is @code{discard_l}, then all
local symbols which begin with a certain prefix are discarded;
this is controlled by the @code{bfd_is_local_label_name} entry point.

The a.out backend handles symbols by calling
@code{aout_link_write_symbols} on each input BFD and then
traversing the global hash table with the function
@code{aout_link_write_other_symbol}.  It builds a string table
while writing out the symbols, which is written to the output
file at the end of @code{NAME(aout,final_link)}.

@findex bfd_link_split_section
@subsubsection @code{bfd_link_split_section}
@strong{Synopsis}
@example
bfd_boolean bfd_link_split_section (bfd *abfd, asection *sec);
@end example
@strong{Description}@*
Return nonzero if @var{sec} should be split during a
reloceatable or final link.
@example
#define bfd_link_split_section(abfd, sec) \
       BFD_SEND (abfd, _bfd_link_split_section, (abfd, sec))

@end example

@findex bfd_section_already_linked
@subsubsection @code{bfd_section_already_linked}
@strong{Synopsis}
@example
void bfd_section_already_linked (bfd *abfd, asection *sec);
@end example
@strong{Description}@*
Check if @var{sec} has been already linked during a reloceatable
or final link.
@example
#define bfd_section_already_linked(abfd, sec) \
       BFD_SEND (abfd, _section_already_linked, (abfd, sec))

@end example