This file documents the internals of the BFD library.

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BFD Internals
*************

This document describes some BFD internal information which may be helpful when working on BFD.  It is very incomplete.

This document is not updated regularly, and may be out of date.

The initial version of this document was written by Ian Lance Taylor ian@cygnus.com.

BFD overview
============

BFD is a library which provides a single interface to read and write object files, executables, archive files, and core files in any format.

BFD library interfaces
----------------------

One way to look at the BFD library is to divide it into four parts by type of interface.

The first interface is the set of generic functions which programs using the BFD library will call.  These generic function normally translate directly or indirectly into calls to routines which are specific to a particular object file format.  Many of these generic functions are actually defined as macros in bfd.h.  These functions comprise the official BFD interface.

The second interface is the set of functions which appear in the target vectors.  This is the bulk of the code in BFD.  A target vector is a set of function pointers specific to a particular object file format.  The target vector is used to implement the generic BFD functions.  These functions are always called through the target vector, and are never called directly.  The target vector is described in detail in BFD target vector.  The set of functions which appear in a particular target vector is often referred to as a BFD backend.

The third interface is a set of oddball functions which are typically specific to a particular object file format, are not generic functions, and are called from outside of the BFD library.  These are used as hooks by the linker and the assembler when a particular object file format requires some action which the BFD generic interface does not provide. These functions are typically declared in bfd.h, but in many cases they are only provided when BFD is configured with support for a particular object file format.  These functions live in a grey area, and are not really part of the official BFD interface.

The fourth interface is the set of BFD support functions which are called by the other BFD functions.  These manage issues like memory allocation, error handling, file access, hash tables, swapping, and the like.  These functions are never called from outside of the BFD library.

BFD library users
-----------------

Another way to look at the BFD library is to divide it into three parts by the manner in which it is used.

The first use is to read an object file.  The object file readers are programs like gdb, nm, objdump, and objcopy. These programs use BFD to view an object file in a generic form.  The official BFD interface is normally fully adequate for these programs.

The second use is to write an object file.  The object file writers are programs like gas and objcopy.  These programs use BFD to create an object file.  The official BFD interface is normally adequate for these programs, but for some object file formats the assembler needs some additional hooks in order to set particular flags or other information.  The official BFD interface includes functions to copy private information from one object file to another, and these functions are used by objcopy to avoid information loss.

The third use is to link object files.  There is only one object file linker, ld.  Originally, ld was an object file reader and an object file writer, and it did the link operation using the generic BFD structures.  However, this turned out to be too slow and too memory intensive.

The official BFD linker functions were written to permit specific BFD backends to perform the link without translating through the generic structures, in the normal case where all the input files and output file have the same object file format.  Not all of the backends currently implement the new interface, and there are default linking functions within BFD which use the generic structures and which work with all backends.

For several object file formats the linker needs additional hooks which are not provided by the official BFD interface, particularly for dynamic linking support.  These functions are typically called from the linker emulation template.

The BFD view of a file
----------------------

BFD uses generic structures to manage information.  It translates data into the generic form when reading files, and out of the generic form when writing files.

BFD describes a file as a pointer to the bfd type.  A bfd is composed of the following elements.  The BFD information can be displayed using the objdump program with various options.

general information
     The object file format, a few general flags, the start address.

architecture
     The architecture, including both a general processor type (m68k, MIPS etc.) and a specific machine number (m68000, R4000, etc.).

sections
     A list of sections.

symbols
     A symbol table.

BFD represents a section as a pointer to the asection type.  Each section has a name and a size.  Most sections also have an associated block of data, known as the section contents.  Sections also have associated flags, a virtual memory address, a load memory address, a required alignment, a list of relocations, and other miscellaneous information.

BFD represents a relocation as a pointer to the arelent type.  A relocation describes an action which the linker must take to modify the section contents.  Relocations have a symbol, an address, an addend, and a pointer to a howto structure which describes how to perform the relocation.  For more information, see BFD relocation handling.

BFD represents a symbol as a pointer to the asymbol type.  A symbol has a name, a pointer to a section, an offset within that section, and some flags.

Archive files do not have any sections or symbols.  Instead, BFD represents an archive file as a file which contains a list of bfds.  BFD also provides access to the archive symbol map, as a list of symbol names.  BFD provides a function to return the bfd within the archive which corresponds to a particular entry in the archive symbol map.

BFD loses information
---------------------

Most object file formats have information which BFD can not represent in its generic form, at least as currently defined.

There is often explicit information which BFD can not represent.  For example, the COFF version stamp, or the ELF program segments.  BFD provides special hooks to handle this information when copying, printing, or linking an object file.  The BFD support for a particular object file format will normally store this information in private data and handle it using the special hooks.

In some cases there is also implicit information which BFD can not represent.  For example, the MIPS processor distinguishes small and large symbols, and requires that all small symbols be within 32K of the GP register.  This means that the MIPS assembler must be able to mark variables as either small or large, and the MIPS linker must know to put small symbols within range of the GP register.  Since BFD can not represent this information, this means that the assembler and linker must have information that is specific to a particular object file format which is outside of the BFD library.

This loss of information indicates areas where the BFD paradigm breaks down.  It is not actually possible to represent the myriad differences among object file formats using a single generic interface, at least not in the manner which BFD does it today.

Nevertheless, the BFD library does greatly simplify the task of dealing with object files, and particular problems caused by information loss can normally be solved using some sort of relatively constrained hook into the library.

BFD programming guidelines
==========================

There is a lot of poorly written and confusing code in BFD.  New BFD code should be written to a higher standard.  Merely because some BFD code is written in a particular manner does not mean that you should emulate it.

Here are some general BFD programming guidelines:

  * Follow the GNU coding standards.

  * Avoid global variables.  We ideally want BFD to be fully reentrant, so that it can be used in multiple threads.  All uses of global or static variables interfere with that.  Initialized constant variables are OK, and they should be explicitly marked with const.  Instead of global variables, use data attached to a BFD or to a linker hash table.

  * All externally visible functions should have names which start with bfd_.  All such functions should be declared in some header file, typically bfd.h.  See, for example, the various declarations near the end of bfd-in.h, which mostly declare functions required by specific linker emulations.

  * All functions which need to be visible from one file to another within BFD, but should not be visible outside of BFD, should start with _bfd_.  Although external names beginning with _ are prohibited by the ANSI standard, in practice this usage will always work, and it is required by the GNU coding standards.

  * Always remember that people can compile using --enable-targets to build several, or all, targets at once.  It must be possible to link together the files for all targets.

  * BFD code should compile with few or no warnings using gcc -Wall. Some warnings are OK, like the absence of certain function declarations which may or may not be declared in system header files.  Warnings about ambiguous expressions and the like should always be fixed.

BFD target vector
=================

BFD supports multiple object file formats by using the target vector.  This is simply a set of function pointers which implement behaviour that is specific to a particular object file format.

In this section I list all of the entries in the target vector and describe what they do.

Miscellaneous constants
-----------------------

The target vector starts with a set of constants.

name
     The name of the target vector.  This is an arbitrary string.  This is how the target vector is named in command line options for tools which use BFD, such as the --oformat linker option.

flavour
     A general description of the type of target.  The following flavours are currently defined:

     bfd_target_unknown_flavour
          Undefined or unknown.

     bfd_target_aout_flavour
          a.out.

     bfd_target_coff_flavour
          COFF.

     bfd_target_ecoff_flavour
          ECOFF.

     bfd_target_elf_flavour
          ELF.

     bfd_target_ieee_flavour
          IEEE-695.

     bfd_target_nlm_flavour
          NLM.

     bfd_target_oasys_flavour
          OASYS.

     bfd_target_tekhex_flavour
          Tektronix hex format.

     bfd_target_srec_flavour
          Motorola S-record format.

     bfd_target_ihex_flavour
          Intel hex format.

     bfd_target_som_flavour
          SOM (used on HP/UX).

     bfd_target_os9k_flavour
          os9000.

     bfd_target_versados_flavour
          VERSAdos.

     bfd_target_msdos_flavour
          MS-DOS.

     bfd_target_evax_flavour
          openVMS.

     bfd_target_mmo_flavour
          Donald Knuth's MMIXware object format.

byteorder
     The byte order of data in the object file.  One of BFD_ENDIAN_BIG, BFD_ENDIAN_LITTLE, or BFD_ENDIAN_UNKNOWN.  The latter would be used for a format such as S-records which do not record the architecture of the data.

header_byteorder
     The byte order of header information in the object file.  Normally the same as the byteorder field, but there are certain cases where it may be different.

object_flags
     Flags which may appear in the flags field of a BFD with this format.

section_flags
     Flags which may appear in the flags field of a section within a BFD with this format.

symbol_leading_char
     A character which the C compiler normally puts before a symbol.  For example, an a.out compiler will typically generate the symbol _foo for a function named foo in the C source, in which case this field would be _.  If there is no such character, this field will be 0.

ar_pad_char
     The padding character to use at the end of an archive name.  Normally /.

ar_max_namelen
     The maximum length of a short name in an archive.  Normally 14.

backend_data
     A pointer to constant backend data.  This is used by backends to store whatever additional information they need to distinguish similar target vectors which use the same sets of functions.

Swapping functions
------------------

Every target vector has function pointers used for swapping information in and out of the target representation.  There are two sets of functions: one for data information, and one for header information. Each set has three sizes: 64-bit, 32-bit, and 16-bit.  Each size has three actual functions: put, get unsigned, and get signed.

These 18 functions are used to convert data between the host and target representations.

Format type dependent functions
-------------------------------

Every target vector has three arrays of function pointers which are indexed by the BFD format type.  The BFD format types are as follows:

bfd_unknown
     Unknown format.  Not used for anything useful.

bfd_object
     Object file.

bfd_archive
     Archive file.

bfd_core
     Core file.

The three arrays of function pointers are as follows:

bfd_check_format
     Check whether the BFD is of a particular format (object file, archive file, or core file) corresponding to this target vector.  This is called by the bfd_check_format function when examining an existing BFD. If the BFD matches the desired format, this function will initialize any format specific information such as the tdata field of the BFD. This function must be called before any other BFD target vector function on a file opened for reading.

bfd_set_format
     Set the format of a BFD which was created for output.  This is called by the bfd_set_format function after creating the BFD with a function such as bfd_openw.  This function will initialize format specific information required to write out an object file or whatever of the given format.  This function must be called before any other BFD target vector function on a file opened for writing.

bfd_write_contents
     Write out the contents of the BFD in the given format.  This is called by bfd_close function for a BFD opened for writing.  This really should not be an array selected by format type, as the bfd_set_format function provides all the required information. In fact, BFD will fail if a different format is used when calling through the bfd_set_format and the bfd_write_contents arrays; fortunately, since bfd_close gets it right, this is a difficult error to make.

BFD_JUMP_TABLE macros
---------------------

Most target vectors are defined using BFD_JUMP_TABLE macros. These macros take a single argument, which is a prefix applied to a set of functions.  The macros are then used to initialize the fields in the target vector.

For example, the BFD_JUMP_TABLE_RELOCS macro defines three functions: _get_reloc_upper_bound, _canonicalize_reloc, and _bfd_reloc_type_lookup.  A reference like BFD_JUMP_TABLE_RELOCS (foo) will expand into three functions prefixed with foo: foo_get_reloc_upper_bound, etc.  The BFD_JUMP_TABLE_RELOCS macro will be placed such that those three functions initialize the appropriate fields in the BFD target vector.

This is done because it turns out that many different target vectors can share certain classes of functions.  For example, archives are similar on most platforms, so most target vectors can use the same archive functions.  Those target vectors all use BFD_JUMP_TABLE_ARCHIVE with the same argument, calling a set of functions which is defined in archive.c.

Each of the BFD_JUMP_TABLE macros is mentioned below along with the description of the function pointers which it defines.  The function pointers will be described using the name without the prefix which the BFD_JUMP_TABLE macro defines.  This name is normally the same as the name of the field in the target vector structure.  Any differences will be noted.

Generic functions
-----------------

The BFD_JUMP_TABLE_GENERIC macro is used for some catch all functions which don't easily fit into other categories.

_close_and_cleanup
     Free any target specific information associated with the BFD.  This is called when any BFD is closed (the bfd_write_contents function mentioned earlier is only called for a BFD opened for writing).  Most targets use bfd_alloc to allocate all target specific information, and therefore don't have to do anything in this function. This function pointer is typically set to _bfd_generic_close_and_cleanup, which simply returns true.

_bfd_free_cached_info
     Free any cached information associated with the BFD which can be recreated later if necessary.  This is used to reduce the memory consumption required by programs using BFD.  This is normally called via the bfd_free_cached_info macro.  It is used by the default archive routines when computing the archive map.  Most targets do not do anything special for this entry point, and just set it to _bfd_generic_free_cached_info, which simply returns true.

_new_section_hook
     This is called from bfd_make_section_anyway whenever a new section is created.  Most targets use it to initialize section specific information.  This function is called whether or not the section corresponds to an actual section in an actual BFD.

_get_section_contents
     Get the contents of a section.  This is called from bfd_get_section_contents.  Most targets set this to _bfd_generic_get_section_contents, which does a bfd_seek based on the section's filepos field and a bfd_bread.  The corresponding field in the target vector is named _bfd_get_section_contents.

_get_section_contents_in_window
     Set a bfd_window to hold the contents of a section.  This is called from bfd_get_section_contents_in_window.  The bfd_window idea never really caught on, and I don't think this is ever called.  Pretty much all targets implement this as bfd_generic_get_section_contents_in_window, which uses bfd_get_section_contents to do the right thing.  The corresponding field in the target vector is named _bfd_get_section_contents_in_window.

Copy functions
--------------

The BFD_JUMP_TABLE_COPY macro is used for functions which are called when copying BFDs, and for a couple of functions which deal with internal BFD information.

_bfd_copy_private_bfd_data
     This is called when copying a BFD, via bfd_copy_private_bfd_data. If the input and output BFDs have the same format, this will copy any private information over.  This is called after all the section contents have been written to the output file.  Only a few targets do anything in this function.

_bfd_merge_private_bfd_data
     This is called when linking, via bfd_merge_private_bfd_data.  It gives the backend linker code a chance to set any special flags in the output file based on the contents of the input file.  Only a few targets do anything in this function.

_bfd_copy_private_section_data
     This is similar to _bfd_copy_private_bfd_data, but it is called for each section, via bfd_copy_private_section_data.  This function is called before any section contents have been written.  Only a few targets do anything in this function.

_bfd_copy_private_symbol_data
     This is called via bfd_copy_private_symbol_data, but I don't think anything actually calls it.  If it were defined, it could be used to copy private symbol data from one BFD to another.  However, most BFDs store extra symbol information by allocating space which is larger than the asymbol structure and storing private information in the extra space.  Since objcopy and other programs copy symbol information by copying pointers to asymbol structures, the private symbol information is automatically copied as well.  Most targets do not do anything in this function.

_bfd_set_private_flags
     This is called via bfd_set_private_flags.  It is basically a hook for the assembler to set magic information.  For example, the PowerPC ELF assembler uses it to set flags which appear in the e_flags field of the ELF header.  Most targets do not do anything in this function.

_bfd_print_private_bfd_data
     This is called by objdump when the -p option is used.  It is called via bfd_print_private_data.  It prints any interesting information about the BFD which can not be otherwise represented by BFD and thus can not be printed by objdump.  Most targets do not do anything in this function.

Core file support functions
---------------------------

The BFD_JUMP_TABLE_CORE macro is used for functions which deal with core files.  Obviously, these functions only do something interesting for targets which have core file support.

_core_file_failing_command
     Given a core file, this returns the command which was run to produce the core file.

_core_file_failing_signal
     Given a core file, this returns the signal number which produced the core file.

_core_file_matches_executable_p
     Given a core file and a BFD for an executable, this returns whether the core file was generated by the executable.

Archive functions
-----------------

The BFD_JUMP_TABLE_ARCHIVE macro is used for functions which deal with archive files.  Most targets use COFF style archive files (including ELF targets), and these use _bfd_archive_coff as the argument to BFD_JUMP_TABLE_ARCHIVE.  Some targets use BSD/a.out style archives, and these use _bfd_archive_bsd.  (The main difference between BSD and COFF archives is the format of the archive symbol table).  Targets with no archive support use _bfd_noarchive.  Finally, a few targets have unusual archive handling.

_slurp_armap
     Read in the archive symbol table, storing it in private BFD data.  This is normally called from the archive check_format routine.  The corresponding field in the target vector is named _bfd_slurp_armap.

_slurp_extended_name_table
     Read in the extended name table from the archive, if there is one, storing it in private BFD data.  This is normally called from the archive check_format routine.  The corresponding field in the target vector is named _bfd_slurp_extended_name_table.

construct_extended_name_table
     Build and return an extended name table if one is needed to write out the archive.  This also adjusts the archive headers to refer to the extended name table appropriately.  This is normally called from the archive write_contents routine.  The corresponding field in the target vector is named _bfd_construct_extended_name_table.

_truncate_arname
     This copies a file name into an archive header, truncating it as required.  It is normally called from the archive write_contents routine.  This function is more interesting in targets which do not support extended name tables, but I think the GNU ar program always uses extended name tables anyhow.  The corresponding field in the target vector is named _bfd_truncate_arname.

_write_armap
     Write out the archive symbol table using calls to bfd_bwrite. This is normally called from the archive write_contents routine. The corresponding field in the target vector is named write_armap (no leading underscore).

_read_ar_hdr
     Read and parse an archive header.  This handles expanding the archive header name into the real file name using the extended name table.  This is called by routines which read the archive symbol table or the archive itself.  The corresponding field in the target vector is named _bfd_read_ar_hdr_fn.

_openr_next_archived_file
     Given an archive and a BFD representing a file stored within the archive, return a BFD for the next file in the archive.  This is called via bfd_openr_next_archived_file.  The corresponding field in the target vector is named openr_next_archived_file (no leading underscore).

_get_elt_at_index
     Given an archive and an index, return a BFD for the file in the archive corresponding to that entry in the archive symbol table.  This is called via bfd_get_elt_at_index.  The corresponding field in the target vector is named _bfd_get_elt_at_index.

_generic_stat_arch_elt
     Do a stat on an element of an archive, returning information read from the archive header (modification time, uid, gid, file mode, size).  This is called via bfd_stat_arch_elt.  The corresponding field in the target vector is named _bfd_stat_arch_elt.

_update_armap_timestamp
     After the entire contents of an archive have been written out, update the timestamp of the archive symbol table to be newer than that of the file.  This is required for a.out style archives.  This is normally called by the archive write_contents routine.  The corresponding field in the target vector is named _bfd_update_armap_timestamp.

Symbol table functions
----------------------

The BFD_JUMP_TABLE_SYMBOLS macro is used for functions which deal with symbols.

_get_symtab_upper_bound
     Return a sensible upper bound on the amount of memory which will be required to read the symbol table.  In practice most targets return the amount of memory required to hold asymbol pointers for all the symbols plus a trailing NULL entry, and store the actual symbol information in BFD private data.  This is called via bfd_get_symtab_upper_bound.  The corresponding field in the target vector is named _bfd_get_symtab_upper_bound.

_canonicalize_symtab
     Read in the symbol table.  This is called via bfd_canonicalize_symtab.  The corresponding field in the target vector is named _bfd_canonicalize_symtab.

_make_empty_symbol
     Create an empty symbol for the BFD.  This is needed because most targets store extra information with each symbol by allocating a structure larger than an asymbol and storing the extra information at the end.  This function will allocate the right amount of memory, and return what looks like a pointer to an empty asymbol.  This is called via bfd_make_empty_symbol.  The corresponding field in the target vector is named _bfd_make_empty_symbol.

_print_symbol
     Print information about the symbol.  This is called via bfd_print_symbol.  One of the arguments indicates what sort of information should be printed:

     bfd_print_symbol_name
          Just print the symbol name.

     bfd_print_symbol_more
          Print the symbol name and some interesting flags.  I don't think anything actually uses this.

     bfd_print_symbol_all
          Print all information about the symbol.  This is used by objdump when run with the -t option.

     The corresponding field in the target vector is named _bfd_print_symbol.

_get_symbol_info
     Return a standard set of information about the symbol.  This is called via bfd_symbol_info.  The corresponding field in the target vector is named _bfd_get_symbol_info.

_bfd_is_local_label_name
     Return whether the given string would normally represent the name of a local label.  This is called via bfd_is_local_label and bfd_is_local_label_name.  Local labels are normally discarded by the assembler.  In the linker, this defines the difference between the -x and -X options.

_get_lineno
     Return line number information for a symbol.  This is only meaningful for a COFF target.  This is called when writing out COFF line numbers.

_find_nearest_line
     Given an address within a section, use the debugging information to find the matching file name, function name, and line number, if any.  This is called via bfd_find_nearest_line.  The corresponding field in the target vector is named _bfd_find_nearest_line.

_bfd_make_debug_symbol
     Make a debugging symbol.  This is only meaningful for a COFF target, where it simply returns a symbol which will be placed in the N_DEBUG section when it is written out.  This is called via bfd_make_debug_symbol.

_read_minisymbols
     Minisymbols are used to reduce the memory requirements of programs like nm.  A minisymbol is a cookie pointing to internal symbol information which the caller can use to extract complete symbol information.  This permits BFD to not convert all the symbols into generic form, but to instead convert them one at a time.  This is called via bfd_read_minisymbols.  Most targets do not implement this, and just use generic support which is based on using standard asymbol structures.

_minisymbol_to_symbol
     Convert a minisymbol to a standard asymbol.  This is called via bfd_minisymbol_to_symbol.

Relocation support
------------------

The BFD_JUMP_TABLE_RELOCS macro is used for functions which deal with relocations.

_get_reloc_upper_bound
     Return a sensible upper bound on the amount of memory which will be required to read the relocations for a section.  In practice most targets return the amount of memory required to hold arelent pointers for all the relocations plus a trailing NULL entry, and store the actual relocation information in BFD private data.  This is called via bfd_get_reloc_upper_bound.

_canonicalize_reloc
     Return the relocation information for a section.  This is called via bfd_canonicalize_reloc.  The corresponding field in the target vector is named _bfd_canonicalize_reloc.

_bfd_reloc_type_lookup
     Given a relocation code, return the corresponding howto structure (see BFD relocation codes).  This is called via bfd_reloc_type_lookup.  The corresponding field in the target vector is named reloc_type_lookup.

Output functions
----------------

The BFD_JUMP_TABLE_WRITE macro is used for functions which deal with writing out a BFD.

_set_arch_mach
     Set the architecture and machine number for a BFD.  This is called via bfd_set_arch_mach.  Most targets implement this by calling bfd_default_set_arch_mach.  The corresponding field in the target vector is named _bfd_set_arch_mach.

_set_section_contents
     Write out the contents of a section.  This is called via bfd_set_section_contents.  The corresponding field in the target vector is named _bfd_set_section_contents.

Linker functions
----------------

The BFD_JUMP_TABLE_LINK macro is used for functions called by the linker.

_sizeof_headers
     Return the size of the header information required for a BFD.  This is used to implement the SIZEOF_HEADERS linker script function.  It is normally used to align the first section at an efficient position on the page.  This is called via bfd_sizeof_headers.  The corresponding field in the target vector is named _bfd_sizeof_headers.

_bfd_get_relocated_section_contents
     Read the contents of a section and apply the relocation information. This handles both a final link and a relocatable link; in the latter case, it adjust the relocation information as well.  This is called via bfd_get_relocated_section_contents.  Most targets implement it by calling bfd_generic_get_relocated_section_contents.

_bfd_relax_section
     Try to use relaxation to shrink the size of a section.  This is called by the linker when the -relax option is used.  This is called via bfd_relax_section.  Most targets do not support any sort of relaxation.

_bfd_link_hash_table_create
     Create the symbol hash table to use for the linker.  This linker hook permits the backend to control the size and information of the elements in the linker symbol hash table.  This is called via bfd_link_hash_table_create.

_bfd_link_add_symbols
     Given an object file or an archive, add all symbols into the linker symbol hash table.  Use callbacks to the linker to include archive elements in the link.  This is called via bfd_link_add_symbols.

_bfd_final_link
     Finish the linking process.  The linker calls this hook after all of the input files have been read, when it is ready to finish the link and generate the output file.  This is called via bfd_final_link.

_bfd_link_split_section
     I don't know what this is for.  Nothing seems to call it.  The only non-trivial definition is in som.c.

Dynamic linking information functions
-------------------------------------

The BFD_JUMP_TABLE_DYNAMIC macro is used for functions which read dynamic linking information.

_get_dynamic_symtab_upper_bound
     Return a sensible upper bound on the amount of memory which will be required to read the dynamic symbol table.  In practice most targets return the amount of memory required to hold asymbol pointers for all the symbols plus a trailing NULL entry, and store the actual symbol information in BFD private data.  This is called via bfd_get_dynamic_symtab_upper_bound.  The corresponding field in the target vector is named _bfd_get_dynamic_symtab_upper_bound.

_canonicalize_dynamic_symtab
     Read the dynamic symbol table.  This is called via bfd_canonicalize_dynamic_symtab.  The corresponding field in the target vector is named _bfd_canonicalize_dynamic_symtab.

_get_dynamic_reloc_upper_bound
     Return a sensible upper bound on the amount of memory which will be required to read the dynamic relocations.  In practice most targets return the amount of memory required to hold arelent pointers for all the relocations plus a trailing NULL entry, and store the actual relocation information in BFD private data.  This is called via bfd_get_dynamic_reloc_upper_bound.  The corresponding field in the target vector is named _bfd_get_dynamic_reloc_upper_bound.

_canonicalize_dynamic_reloc
     Read the dynamic relocations.  This is called via bfd_canonicalize_dynamic_reloc.  The corresponding field in the target vector is named _bfd_canonicalize_dynamic_reloc.

BFD generated files
===================

BFD contains several automatically generated files.  This section describes them.  Some files are created at configure time, when you configure BFD.  Some files are created at make time, when you build BFD.  Some files are automatically rebuilt at make time, but only if you configure with the --enable-maintainer-mode option.  Some files live in the object directory---the directory from which you run configure---and some live in the source directory.  All files that live in the source directory are checked into the CVS repository.

bfd.h
     Lives in the object directory.  Created at make time from bfd-in2.h via bfd-in3.h.  bfd-in3.h is created at configure time from bfd-in2.h.  There are automatic dependencies to rebuild bfd-in3.h and hence bfd.h if bfd-in2.h changes, so you can normally ignore bfd-in3.h, and just think about bfd-in2.h and bfd.h.

     bfd.h is built by replacing a few strings in bfd-in2.h. To see them, search for @ in bfd-in2.h.  They mainly control whether BFD is built for a 32 bit target or a 64 bit target.

bfd-in2.h
     Lives in the source directory.  Created from bfd-in.h and several other BFD source files.  If you configure with the --enable-maintainer-mode option, bfd-in2.h is rebuilt automatically when a source file changes.

elf32-target.h
elf64-target.h
     Live in the object directory.  Created from elfxx-target.h. These files are versions of elfxx-target.h customized for either a 32 bit ELF target or a 64 bit ELF target.

libbfd.h
     Lives in the source directory.  Created from libbfd-in.h and several other BFD source files.  If you configure with the --enable-maintainer-mode option, libbfd.h is rebuilt automatically when a source file changes.

libcoff.h
     Lives in the source directory.  Created from libcoff-in.h and coffcode.h.  If you configure with the --enable-maintainer-mode option, libcoff.h is rebuilt automatically when a source file changes.

targmatch.h
     Lives in the object directory.  Created at make time from config.bfd.  This file is used to map configuration triplets into BFD target vector variable names at run time.

Files compiled multiple times in BFD
====================================

Several files in BFD are compiled multiple times.  By this I mean that there are header files which contain function definitions.  These header files are included by other files, and thus the functions are compiled once per file which includes them.

Preprocessor macros are used to control the compilation, so that each time the files are compiled the resulting functions are slightly different.  Naturally, if they weren't different, there would be no reason to compile them multiple times.

This is a not a particularly good programming technique, and future BFD work should avoid it.

  * Since this technique is rarely used, even experienced C programmers find it confusing.

  * It is difficult to debug programs which use BFD, since there is no way to describe which version of a particular function you are looking at.

  * Programs which use BFD wind up incorporating two or more slightly different versions of the same function, which wastes space in the executable.

  * This technique is never required nor is it especially efficient.  It is always possible to use statically initialized structures holding function pointers and magic constants instead.

The following is a list of the files which are compiled multiple times.

aout-target.h
     Describes a few functions and the target vector for a.out targets.  This is used by individual a.out targets with different definitions of N_TXTADDR and similar a.out macros.

aoutf1.h
     Implements standard SunOS a.out files.  In principle it supports 64 bit a.out targets based on the preprocessor macro ARCH_SIZE, but since all known a.out targets are 32 bits, this code may or may not work.  This file is only included by a few other files, and it is difficult to justify its existence.

aoutx.h
     Implements basic a.out support routines.  This file can be compiled for either 32 or 64 bit support.  Since all known a.out targets are 32 bits, the 64 bit support may or may not work.  I believe the original intention was that this file would only be included by aout32.c and aout64.c, and that other a.out targets would simply refer to the functions it defined.  Unfortunately, some other a.out targets started including it directly, leading to a somewhat confused state of affairs.

coffcode.h
     Implements basic COFF support routines.  This file is included by every COFF target.  It implements code which handles COFF magic numbers as well as various hook functions called by the generic COFF functions in coffgen.c.  This file is controlled by a number of different macros, and more are added regularly.

coffswap.h
     Implements COFF swapping routines.  This file is included by coffcode.h, and thus by every COFF target.  It implements the routines which swap COFF structures between internal and external format.  The main control for this file is the external structure definitions in the files in the include/coff directory.  A COFF target file will include one of those files before including coffcode.h and thus coffswap.h.  There are a few other macros which affect coffswap.h as well, mostly describing whether certain fields are present in the external structures.

ecoffswap.h
     Implements ECOFF swapping routines.  This is like coffswap.h, but for ECOFF.  It is included by the ECOFF target files (of which there are only two).  The control is the preprocessor macro ECOFF_32 or ECOFF_64.

elfcode.h
     Implements ELF functions that use external structure definitions.  This file is included by two other files: elf32.c and elf64.c. It is controlled by the ARCH_SIZE macro which is defined to be 32 or 64 before including it.  The NAME macro is used internally to give the functions different names for the two target sizes.

elfcore.h
     Like elfcode.h, but for functions that are specific to ELF core files.  This is included only by elfcode.h.

elfxx-target.h
     This file is the source for the generated files elf32-target.h and elf64-target.h, one of which is included by every ELF target. It defines the ELF target vector.

freebsd.h
     Presumably intended to be included by all FreeBSD targets, but in fact there is only one such target, i386-freebsd.  This defines a function used to set the right magic number for FreeBSD, as well as various macros, and includes aout-target.h.

netbsd.h
     Like freebsd.h, except that there are several files which include it.

nlm-target.h
     Defines the target vector for a standard NLM target.

nlmcode.h
     Like elfcode.h, but for NLM targets.  This is only included by nlm32.c and nlm64.c, both of which define the macro ARCH_SIZE to an appropriate value.  There are no 64 bit NLM targets anyhow, so this is sort of useless.

nlmswap.h
     Like coffswap.h, but for NLM targets.  This is included by each NLM target, but I think it winds up compiling to the exact same code for every target, and as such is fairly useless.

peicode.h
     Provides swapping routines and other hooks for PE targets. coffcode.h will include this rather than coffswap.h for a PE target.  This defines PE specific versions of the COFF swapping routines, and also defines some macros which control coffcode.h itself.

BFD relocation handling
=======================

The handling of relocations is one of the more confusing aspects of BFD. Relocation handling has been implemented in various different ways, all somewhat incompatible, none perfect.

BFD relocation concepts
-----------------------

A relocation is an action which the linker must take when linking.  It describes a change to the contents of a section.  The change is normally based on the final value of one or more symbols.  Relocations are created by the assembler when it creates an object file.

Most relocations are simple.  A typical simple relocation is to set 32 bits at a given offset in a section to the value of a symbol.  This type of relocation would be generated for code like int *p = &i; where p and i are global variables.  A relocation for the symbol i would be generated such that the linker would initialize the area of memory which holds the value of p to the value of the symbol i.

Slightly more complex relocations may include an addend, which is a constant to add to the symbol value before using it.  In some cases a relocation will require adding the symbol value to the existing contents of the section in the object file.  In others the relocation will simply replace the contents of the section with the symbol value.  Some relocations are PC relative, so that the value to be stored in the section is the difference between the value of a symbol and the final address of the section contents.

In general, relocations can be arbitrarily complex.  For example, relocations used in dynamic linking systems often require the linker to allocate space in a different section and use the offset within that section as the value to store.  In the IEEE object file format, relocations may involve arbitrary expressions.

When doing a relocatable link, the linker may or may not have to do anything with a relocation, depending upon the definition of the relocation.  Simple relocations generally do not require any special action.

BFD relocation functions
------------------------

In BFD, each section has an array of arelent structures.  Each structure has a pointer to a symbol, an address within the section, an addend, and a pointer to a reloc_howto_struct structure.  The howto structure has a bunch of fields describing the reloc, including a type field.  The type field is specific to the object file format backend; none of the generic code in BFD examines it.

Originally, the function bfd_perform_relocation was supposed to handle all relocations.  In theory, many relocations would be simple enough to be described by the fields in the howto structure.  For those that weren't, the howto structure included a special_function field to use as an escape.

While this seems plausible, a look at bfd_perform_relocation shows that it failed.  The function has odd special cases.  Some of the fields in the howto structure, such as pcrel_offset, were not adequately documented.

The linker uses bfd_perform_relocation to do all relocations when the input and output file have different formats (e.g., when generating S-records).  The generic linker code, which is used by all targets which do not define their own special purpose linker, uses bfd_get_relocated_section_contents, which for most targets turns into a call to bfd_generic_get_relocated_section_contents, which calls bfd_perform_relocation.  So bfd_perform_relocation is still widely used, which makes it difficult to change, since it is difficult to test all possible cases.

The assembler used bfd_perform_relocation for a while.  This turned out to be the wrong thing to do, since bfd_perform_relocation was written to handle relocations on an existing object file, while the assembler needed to create relocations in a new object file.  The assembler was changed to use the new function bfd_install_relocation instead, and bfd_install_relocation was created as a copy of bfd_perform_relocation.

Unfortunately, the work did not progress any farther, so bfd_install_relocation remains a simple copy of bfd_perform_relocation, with all the odd special cases and confusing code.  This again is difficult to change, because again any change can affect any assembler target, and so is difficult to test.

The new linker, when using the same object file format for all input files and the output file, does not convert relocations into arelent structures, so it can not use bfd_perform_relocation at all.  Instead, users of the new linker are expected to write a relocate_section function which will handle relocations in a target specific fashion.

There are two helper functions for target specific relocation: _bfd_final_link_relocate and _bfd_relocate_contents. These functions use a howto structure, but they do not use the special_function field.  Since the functions are normally called from target specific code, the special_function field adds little; any relocations which require special handling can be handled without calling those functions.

So, if you want to add a new target, or add a new relocation to an existing target, you need to do the following:

  * Make sure you clearly understand what the contents of the section should look like after assembly, after a relocatable link, and after a final link.  Make sure you clearly understand the operations the linker must perform during a relocatable link and during a final link.

  * Write a howto structure for the relocation.  The howto structure is flexible enough to represent any relocation which should be handled by setting a contiguous bitfield in the destination to the value of a symbol, possibly with an addend, possibly adding the symbol value to the value already present in the destination.

  * Change the assembler to generate your relocation.  The assembler will call bfd_install_relocation, so your howto structure has to be able to handle that.  You may need to set the special_function field to handle assembly correctly.  Be careful to ensure that any code you write to handle the assembler will also work correctly when doing a relocatable link.  For example, see bfd_elf_generic_reloc.

  * Test the assembler.  Consider the cases of relocation against an undefined symbol, a common symbol, a symbol defined in the object file in the same section, and a symbol defined in the object file in a different section.  These cases may not all be applicable for your reloc.

  * If your target uses the new linker, which is recommended, add any required handling to the target specific relocation function.  In simple cases this will just involve a call to _bfd_final_link_relocate or _bfd_relocate_contents, depending upon the definition of the relocation and whether the link is relocatable or not.

  * Test the linker.  Test the case of a final link.  If the relocation can overflow, use a linker script to force an overflow and make sure the error is reported correctly.  Test a relocatable link, whether the symbol is defined or undefined in the relocatable output.  For both the final and relocatable link, test the case when the symbol is a common symbol, when the symbol looked like a common symbol but became a defined symbol, when the symbol is defined in a different object file, and when the symbol is defined in the same object file.

  * In order for linking to another object file format, such as S-records, to work correctly, bfd_perform_relocation has to do the right thing for the relocation.  You may need to set the special_function field to handle this correctly.  Test this by doing a link in which the output object file format is S-records.

  * Using the linker to generate relocatable output in a different object file format is impossible in the general case, so you generally don't have to worry about that.  The GNU linker makes sure to stop that from happening when an input file in a different format has relocations.

     Linking input files of different object file formats together is quite unusual, but if you're really dedicated you may want to consider testing this case, both when the output object file format is the same as your format, and when it is different.

BFD relocation codes
--------------------

BFD has another way of describing relocations besides the howto structures described above: the enum bfd_reloc_code_real_type.

Every known relocation type can be described as a value in this enumeration.  The enumeration contains many target specific relocations, but where two or more targets have the same relocation, a single code is used.  For example, the single value BFD_RELOC_32 is used for all simple 32 bit relocation types.

The main purpose of this relocation code is to give the assembler some mechanism to create arelent structures.  In order for the assembler to create an arelent structure, it has to be able to obtain a howto structure.  The function bfd_reloc_type_lookup, which simply calls the target vector entry point reloc_type_lookup, takes a relocation code and returns a howto structure.

The function bfd_get_reloc_code_name returns the name of a relocation code.  This is mainly used in error messages.

Using both howto structures and relocation codes can be somewhat confusing.  There are many processor specific relocation codes. However, the relocation is only fully defined by the howto structure. The same relocation code will map to different howto structures in different object file formats.  For example, the addend handling may be different.

Most of the relocation codes are not really general.  The assembler can not use them without already understanding what sorts of relocations can be used for a particular target.  It might be possible to replace the relocation codes with something simpler.

BFD relocation future
---------------------

Clearly the current BFD relocation support is in bad shape.  A wholescale rewrite would be very difficult, because it would require thorough testing of every BFD target.  So some sort of incremental change is required.

My vague thoughts on this would involve defining a new, clearly defined, howto structure.  Some mechanism would be used to determine which type of howto structure was being used by a particular format.

The new howto structure would clearly define the relocation behaviour in the case of an assembly, a relocatable link, and a final link.  At least one special function would be defined as an escape, and it might make sense to define more.

One or more generic functions similar to bfd_perform_relocation would be written to handle the new howto structure.

This should make it possible to write a generic version of the relocate section functions used by the new linker.  The target specific code would provide some mechanism (a function pointer or an initial conversion) to convert target specific relocations into howto structures.

Ideally it would be possible to use this generic relocate section function for the generic linker as well.  That is, it would replace the bfd_generic_get_relocated_section_contents function which is currently normally used.

For the special case of ELF dynamic linking, more consideration needs to be given to writing ELF specific but ELF target generic code to handle special relocation types such as GOT and PLT.

BFD ELF support
===============

The ELF object file format is defined in two parts: a generic ABI and a processor specific supplement.  The ELF support in BFD is split in a similar fashion.  The processor specific support is largely kept within a single file.  The generic support is provided by several other files. The processor specific support provides a set of function pointers and constants used by the generic support.

ELF sections and segments
-------------------------

The ELF ABI permits a file to have either sections or segments or both. Relocatable object files conventionally have only sections. Executables conventionally have both.  Core files conventionally have only program segments.

ELF sections are similar to sections in other object file formats: they have a name, a VMA, file contents, flags, and other miscellaneous information.  ELF relocations are stored in sections of a particular type; BFD automatically converts these sections into internal relocation information.

ELF program segments are intended for fast interpretation by a system loader.  They have a type, a VMA, an LMA, file contents, and a couple of other fields.  When an ELF executable is run on a Unix system, the system loader will examine the program segments to decide how to load it.  The loader will ignore the section information.  Loadable program segments (type PT_LOAD) are directly loaded into memory.  Other program segments are interpreted by the loader, and generally provide dynamic linking information.

When an ELF file has both program segments and sections, an ELF program segment may encompass one or more ELF sections, in the sense that the portion of the file which corresponds to the program segment may include the portions of the file corresponding to one or more sections.  When there is more than one section in a loadable program segment, the relative positions of the section contents in the file must correspond to the relative positions they should hold when the program segment is loaded.  This requirement should be obvious if you consider that the system loader will load an entire program segment at a time.

On a system which supports dynamic paging, such as any native Unix system, the contents of a loadable program segment must be at the same offset in the file as in memory, modulo the memory page size used on the system.  This is because the system loader will map the file into memory starting at the start of a page.  The system loader can easily remap entire pages to the correct load address.  However, if the contents of the file were not correctly aligned within the page, the system loader would have to shift the contents around within the page, which is too expensive.  For example, if the LMA of a loadable program segment is 0x40080 and the page size is 0x1000, then the position of the segment contents within the file must equal 0x80 modulo 0x1000.

BFD has only a single set of sections.  It does not provide any generic way to examine both sections and segments.  When BFD is used to open an object file or executable, the BFD sections will represent ELF sections. When BFD is used to open a core file, the BFD sections will represent ELF program segments.

When BFD is used to examine an object file or executable, any program segments will be read to set the LMA of the sections.  This is because ELF sections only have a VMA, while ELF program segments have both a VMA and an LMA.  Any program segments will be copied by the copy_private entry points.  They will be printed by the print_private entry point.  Otherwise, the program segments are ignored.  In particular, programs which use BFD currently have no direct access to the program segments.

When BFD is used to create an executable, the program segments will be created automatically based on the section information.  This is done in the function assign_file_positions_for_segments in elf.c. This function has been tweaked many times, and probably still has problems that arise in particular cases.

There is a hook which may be used to explicitly define the program segments when creating an executable: the bfd_record_phdr function in bfd.c.  If this function is called, BFD will not create program segments itself, but will only create the program segments specified by the caller.  The linker uses this function to implement the PHDRS linker script command.

BFD ELF generic support
-----------------------

In general, functions which do not read external data from the ELF file are found in elf.c.  They operate on the internal forms of the ELF structures, which are defined in include/elf/internal.h.  The internal structures are defined in terms of bfd_vma, and so may be used for both 32 bit and 64 bit ELF targets.

The file elfcode.h contains functions which operate on the external data.  elfcode.h is compiled twice, once via elf32.c with ARCH_SIZE defined as 32, and once via elf64.c with ARCH_SIZE defined as 64. elfcode.h includes functions to swap the ELF structures in and out of external form, as well as a few more complex functions.

Linker support is found in elflink.c.  The linker support is only used if the processor specific file defines elf_backend_relocate_section, which is required to relocate the section contents.  If that macro is not defined, the generic linker code is used, and relocations are handled via bfd_perform_relocation.

The core file support is in elfcore.h, which is compiled twice, for both 32 and 64 bit support.  The more interesting cases of core file support only work on a native system which has the sys/procfs.h header file.  Without that file, the core file support does little more than read the ELF program segments as BFD sections.

The BFD internal header file elf-bfd.h is used for communication among these files and the processor specific files.

The default entries for the BFD ELF target vector are found mainly in elf.c.  Some functions are found in elfcode.h.

The processor specific files may override particular entries in the target vector, but most do not, with one exception: the bfd_reloc_type_lookup entry point is always processor specific.

BFD ELF processor specific support
----------------------------------

By convention, the processor specific support for a particular processor will be found in elfNN-CPU.c, where NN is either 32 or 64, and CPU is the name of the processor.

Required processor specific support
...................................

When writing a elfNN-CPU.c file, you must do the following:

  * Define either TARGET_BIG_SYM or TARGET_LITTLE_SYM, or both, to a unique C name to use for the target vector.  This name should appear in the list of target vectors in targets.c, and will also have to appear in config.bfd and configure.in.  Define TARGET_BIG_SYM for a big-endian processor, TARGET_LITTLE_SYM for a little-endian processor, and define both for a bi-endian processor.

  * Define either TARGET_BIG_NAME or TARGET_LITTLE_NAME, or both, to a string used as the name of the target vector.  This is the name which a user of the BFD tool would use to specify the object file format.  It would normally appear in a linker emulation parameters file.

  * Define ELF_ARCH to the BFD architecture (an element of the bfd_architecture enum, typically bfd_arch_CPU).

  * Define ELF_MACHINE_CODE to the magic number which should appear in the e_machine field of the ELF header.  As of this writing, these magic numbers are assigned by Caldera; if you want to get a magic number for a particular processor, try sending a note to registry@caldera.com.  In the BFD sources, the magic numbers are found in include/elf/common.h; they have names beginning with EM_.

  * Define ELF_MAXPAGESIZE to the maximum size of a virtual page in memory.  This can normally be found at the start of chapter 5 in the processor specific supplement.  For a processor which will only be used in an embedded system, or which has no memory management hardware, this can simply be 1.

  * If the format should use Rel rather than Rela relocations, define USE_REL.  This is normally defined in chapter 4 of the processor specific supplement.

     In the absence of a supplement, it's easier to work with Rela relocations.  Rela relocations will require more space in object files (but not in executables, except when using dynamic linking). However, this is outweighed by the simplicity of addend handling when using Rela relocations.  With Rel relocations, the addend must be stored in the section contents, which makes relocatable links more complex.

     For example, consider C code like i = a[1000]; where a is a global array.  The instructions which load the value of a[1000] will most likely use a relocation which refers to the symbol representing a, with an addend that gives the offset from the start of a to element 1000.  When using Rel relocations, that addend must be stored in the instructions themselves. If you are adding support for a RISC chip which uses two or more instructions to load an address, then the addend may not fit in a single instruction, and will have to be somehow split among the instructions. This makes linking awkward, particularly when doing a relocatable link in which the addend may have to be updated.  It can be done---the MIPS ELF support does it---but it should be avoided when possible.

     It is possible, though somewhat awkward, to support both Rel and Rela relocations for a single target; elf64-mips.c does it by overriding the relocation reading and writing routines.

  * Define howto structures for all the relocation types.

  * Define a bfd_reloc_type_lookup routine.  This must be named bfd_elfNN_bfd_reloc_type_lookup, and may be either a function or a macro.  It must translate a BFD relocation code into a howto structure.  This is normally a table lookup or a simple switch.

  * If using Rel relocations, define elf_info_to_howto_rel. If using Rela relocations, define elf_info_to_howto. Either way, this is a macro defined as the name of a function which takes an arelent and a Rel or Rela structure, and sets the howto field of the arelent based on the Rel or Rela structure.  This is normally uses ELFNN_R_TYPE to get the ELF relocation type and uses it as an index into a table of howto structures.

You must also add the magic number for this processor to the prep_headers function in elf.c.

You must also create a header file in the include/elf directory called CPU.h.  This file should define any target specific information which may be needed outside of the BFD code.  In particular it should use the START_RELOC_NUMBERS, RELOC_NUMBER, FAKE_RELOC, EMPTY_RELOC and END_RELOC_NUMBERS macros to create a table mapping the number used to identify a relocation to a name describing that relocation.

While not a BFD component, you probably also want to make the binutils program readelf parse your ELF objects.  For this, you need to add code for EM_CPU as appropriate in binutils/readelf.c.

Processor specific linker support
.................................

The linker will be much more efficient if you define a relocate section function.  This will permit BFD to use the ELF specific linker support.

If you do not define a relocate section function, BFD must use the generic linker support, which requires converting all symbols and relocations into BFD asymbol and arelent structures.  In this case, relocations will be handled by calling bfd_perform_relocation, which will use the howto structures you have defined.  See BFD relocation handling.

In order to support linking into a different object file format, such as S-records, bfd_perform_relocation must work correctly with your howto structures, so you can't skip that step.  However, if you define the relocate section function, then in the normal case of linking into an ELF file the linker will not need to convert symbols and relocations, and will be much more efficient.

To use a relocation section function, define the macro elf_backend_relocate_section as the name of a function which will take the contents of a section, as well as relocation, symbol, and other information, and modify the section contents according to the relocation information.  In simple cases, this is little more than a loop over the relocations which computes the value of each relocation and calls _bfd_final_link_relocate.  The function must check for a relocatable link, and in that case normally needs to do nothing other than adjust the addend for relocations against a section symbol.

The complex cases generally have to do with dynamic linker support.  GOT and PLT relocations must be handled specially, and the linker normally arranges to set up the GOT and PLT sections while handling relocations. When generating a shared library, random relocations must normally be copied into the shared library, or converted to RELATIVE relocations when possible.

Other processor specific support options
........................................

There are many other macros which may be defined in elfNN-CPU.c.  These macros may be found in elfxx-target.h.

Macros may be used to override some of the generic ELF target vector functions.

Several processor specific hook functions which may be defined as macros.  These functions are found as function pointers in the elf_backend_data structure defined in elf-bfd.h.  In general, a hook function is set by defining a macro elf_backend_NAME.

There are a few processor specific constants which may also be defined. These are again found in the elf_backend_data structure.

I will not define the various functions and constants here; see the comments in elf-bfd.h.

Normally any odd characteristic of a particular ELF processor is handled via a hook function.  For example, the special SHN_MIPS_SCOMMON section number found in MIPS ELF is handled via the hooks section_from_bfd_section, symbol_processing, add_symbol_hook, and output_symbol_hook.

Dynamic linking support, which involves processor specific relocations requiring special handling, is also implemented via hook functions.

BFD ELF core files
------------------

On native ELF Unix systems, core files are generated without any sections.  Instead, they only have program segments.

When BFD is used to read an ELF core file, the BFD sections will actually represent program segments.  Since ELF program segments do not have names, BFD will invent names like segmentN where N is a number.

A single ELF program segment may include both an initialized part and an uninitialized part.  The size of the initialized part is given by the p_filesz field.  The total size of the segment is given by the p_memsz field.  If p_memsz is larger than p_filesz, then the extra space is uninitialized, or, more precisely, initialized to zero.

BFD will represent such a program segment as two different sections. The first, named segmentNa, will represent the initialized part of the program segment.  The second, named segmentNb, will represent the uninitialized part.

ELF core files store special information such as register values in program segments with the type PT_NOTE.  BFD will attempt to interpret the information in these segments, and will create additional sections holding the information.  Some of this interpretation requires information found in the host header file sys/procfs.h, and so will only work when BFD is built on a native system.

BFD does not currently provide any way to create an ELF core file.  In general, BFD does not provide a way to create core files.  The way to implement this would be to write bfd_set_format and bfd_write_contents routines for the bfd_core type; see BFD target vector format.

BFD ELF future
--------------

The current dynamic linking support has too much code duplication. While each processor has particular differences, much of the dynamic linking support is quite similar for each processor.  The GOT and PLT are handled in fairly similar ways, the details of -Bsymbolic linking are generally similar, etc.  This code should be reworked to use more generic functions, eliminating the duplication.

Similarly, the relocation handling has too much duplication.  Many of the reloc_type_lookup and info_to_howto functions are quite similar.  The relocate section functions are also often quite similar, both in the standard linker handling and the dynamic linker handling.  Many of the COFF processor specific backends share a single relocate section function (_bfd_coff_generic_relocate_section), and it should be possible to do something like this for the ELF targets as well.

The appearance of the processor specific magic number in prep_headers in elf.c is somewhat bogus.  It should be possible to add support for a new processor without changing the generic support.

The processor function hooks and constants are ad hoc and need better documentation.

BFD glossary
============

This is a short glossary of some BFD terms.

a.out
     The a.out object file format.  The original Unix object file format. Still used on SunOS, though not Solaris.  Supports only three sections.

archive
     A collection of object files produced and manipulated by the ar program.

backend
     The implementation within BFD of a particular object file format.  The set of functions which appear in a particular target vector.

BFD
     The BFD library itself.  Also, each object file, archive, or executable opened by the BFD library has the type bfd *, and is sometimes referred to as a bfd.

COFF
     The Common Object File Format.  Used on Unix SVR3.  Used by some embedded targets, although ELF is normally better.

DLL
     A shared library on Windows.

dynamic linker
     When a program linked against a shared library is run, the dynamic linker will locate the appropriate shared library and arrange to somehow include it in the running image.

dynamic object
     Another name for an ELF shared library.

ECOFF
     The Extended Common Object File Format.  Used on Alpha Digital Unix (formerly OSF/1), as well as Ultrix and Irix 4.  A variant of COFF.

ELF
     The Executable and Linking Format.  The object file format used on most modern Unix systems, including GNU/Linux, Solaris, Irix, and SVR4.  Also used on many embedded systems.

executable
     A program, with instructions and symbols, and perhaps dynamic linking information.  Normally produced by a linker.

LMA
     Load Memory Address.  This is the address at which a section will be loaded.  Compare with VMA, below.

NLM
     NetWare Loadable Module.  Used to describe the format of an object which be loaded into NetWare, which is some kind of PC based network server program.

object file
     A binary file including machine instructions, symbols, and relocation information.  Normally produced by an assembler.

object file format
     The format of an object file.  Typically object files and executables for a particular system are in the same format, although executables will not contain any relocation information.

PE
     The Portable Executable format.  This is the object file format used for Windows (specifically, Win32) object files.  It is based closely on COFF, but has a few significant differences.

PEI
     The Portable Executable Image format.  This is the object file format used for Windows (specifically, Win32) executables.  It is very similar to PE, but includes some additional header information.

relocations
     Information used by the linker to adjust section contents.  Also called relocs.

section
     Object files and executable are composed of sections.  Sections have optional data and optional relocation information.

shared library
     A library of functions which may be used by many executables without actually being linked into each executable.  There are several different implementations of shared libraries, each having slightly different features.

symbol
     Each object file and executable may have a list of symbols, often referred to as the symbol table.  A symbol is basically a name and an address.  There may also be some additional information like the type of symbol, although the type of a symbol is normally something simple like function or object, and should be confused with the more complex C notion of type.  Typically every global function and variable in a C program will have an associated symbol.

target vector
     A set of functions which implement support for a particular object file format.  The bfd_target structure.

Win32
     The current Windows API, implemented by Windows 95 and later and Windows NT 3.51 and later, but not by Windows 3.1.

XCOFF
     The eXtended Common Object File Format.  Used on AIX.  A variant of COFF, with a completely different symbol table implementation.

VMA
     Virtual Memory Address.  This is the address a section will have when an executable is run.  Compare with LMA, above.