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-\input texinfo
-@setfilename bfdint.info
-
-@settitle BFD Internals
-@iftex
-@titlepage
-@title{BFD Internals}
-@author{Ian Lance Taylor}
-@author{Cygnus Solutions}
-@page
-@end iftex
-
-@node Top
-@top BFD Internals
-@raisesections
-@cindex 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-This document is not updated regularly, and may be out of date. It was
-last modified on $Date$.
-
-The initial version of this document was written by Ian Lance Taylor
-@email{ian@@cygnus.com}.
-
-@menu
-* BFD overview:: BFD overview
-* BFD guidelines:: BFD programming guidelines
-* BFD target vector:: BFD target vector
-* BFD generated files:: BFD generated files
-* BFD multiple compilations:: Files compiled multiple times in BFD
-* BFD relocation handling:: BFD relocation handling
-* BFD ELF support:: BFD ELF support
-* BFD glossary:: Glossary
-* Index:: Index
-@end menu
-
-@node BFD overview
-@section 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.
-
-@menu
-* BFD library interfaces:: BFD library interfaces
-* BFD library users:: BFD library users
-* BFD view:: The BFD view of a file
-* BFD blindness:: BFD loses information
-@end menu
-
-@node BFD library interfaces
-@subsection 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 @file{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 @ref{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 @file{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.
-
-@node BFD library users
-@subsection 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 @samp{gdb}, @samp{nm}, @samp{objdump}, and @samp{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 @samp{gas} and @samp{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 @samp{objcopy} to avoid information loss.
-
-The third use is to link object files. There is only one object file
-linker, @samp{ld}. Originally, @samp{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.
-
-@node BFD view
-@subsection 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 @samp{bfd} type. A @samp{bfd}
-is composed of the following elements. The BFD information can be
-displayed using the @samp{objdump} program with various options.
-
-@table @asis
-@item general information
-The object file format, a few general flags, the start address.
-@item architecture
-The architecture, including both a general processor type (m68k, MIPS
-etc.) and a specific machine number (m68000, R4000, etc.).
-@item sections
-A list of sections.
-@item symbols
-A symbol table.
-@end table
-
-BFD represents a section as a pointer to the @samp{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 @samp{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 @ref{BFD relocation handling}.
-
-BFD represents a symbol as a pointer to the @samp{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
-@samp{bfd}s. BFD also provides access to the archive symbol map, as a
-list of symbol names. BFD provides a function to return the @samp{bfd}
-within the archive which corresponds to a particular entry in the
-archive symbol map.
-
-@node BFD blindness
-@subsection 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 symbls 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.
-
-
-
-@node BFD guidelines
-@section BFD programming guidelines
-@cindex bfd programming guidelines
-@cindex programming guidelines for bfd
-@cindex guidelines, bfd programming
-
-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:
-
-@itemize @bullet
-@item
-Follow the GNU coding standards.
-
-@item
-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.
-
-@item
-All externally visible functions should have names which start with
-@samp{bfd_}. All such functions should be declared in some header file,
-typically @file{bfd.h}. See, for example, the various declarations near
-the end of @file{bfd-in.h}, which mostly declare functions required by
-specific linker emulations.
-
-@item
-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
-@samp{_bfd_}. Although external names beginning with @samp{_} are
-prohibited by the ANSI standard, in practice this usage will always
-work, and it is required by the GNU coding standards.
-
-@item
-Always remember that people can compile using @samp{--enable-targets} to
-build several, or all, targets at once. It must be possible to link
-together the files for all targets.
-
-@item
-BFD code should compile with few or no warnings using @samp{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.
-@end itemize
-
-@node BFD target vector
-@section BFD target vector
-@cindex bfd target vector
-@cindex target vector in bfd
-
-BFD supports multiple object file formats by using the @dfn{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.
-
-@menu
-* BFD target vector miscellaneous:: Miscellaneous constants
-* BFD target vector swap:: Swapping functions
-* BFD target vector format:: Format type dependent functions
-* BFD_JUMP_TABLE macros:: BFD_JUMP_TABLE macros
-* BFD target vector generic:: Generic functions
-* BFD target vector copy:: Copy functions
-* BFD target vector core:: Core file support functions
-* BFD target vector archive:: Archive functions
-* BFD target vector symbols:: Symbol table functions
-* BFD target vector relocs:: Relocation support
-* BFD target vector write:: Output functions
-* BFD target vector link:: Linker functions
-* BFD target vector dynamic:: Dynamic linking information functions
-@end menu
-
-@node BFD target vector miscellaneous
-@subsection Miscellaneous constants
-
-The target vector starts with a set of constants.
-
-@table @samp
-@item 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 @samp{-oformat} linker option.
-
-@item flavour
-A general description of the type of target. The following flavours are
-currently defined:
-
-@table @samp
-@item bfd_target_unknown_flavour
-Undefined or unknown.
-@item bfd_target_aout_flavour
-a.out.
-@item bfd_target_coff_flavour
-COFF.
-@item bfd_target_ecoff_flavour
-ECOFF.
-@item bfd_target_elf_flavour
-ELF.
-@item bfd_target_ieee_flavour
-IEEE-695.
-@item bfd_target_nlm_flavour
-NLM.
-@item bfd_target_oasys_flavour
-OASYS.
-@item bfd_target_tekhex_flavour
-Tektronix hex format.
-@item bfd_target_srec_flavour
-Motorola S-record format.
-@item bfd_target_ihex_flavour
-Intel hex format.
-@item bfd_target_som_flavour
-SOM (used on HP/UX).
-@item bfd_target_os9k_flavour
-os9000.
-@item bfd_target_versados_flavour
-VERSAdos.
-@item bfd_target_msdos_flavour
-MS-DOS.
-@item bfd_target_evax_flavour
-openVMS.
-@end table
-
-@item byteorder
-The byte order of data in the object file. One of
-@samp{BFD_ENDIAN_BIG}, @samp{BFD_ENDIAN_LITTLE}, or
-@samp{BFD_ENDIAN_UNKNOWN}. The latter would be used for a format such
-as S-records which do not record the architecture of the data.
-
-@item header_byteorder
-The byte order of header information in the object file. Normally the
-same as the @samp{byteorder} field, but there are certain cases where it
-may be different.
-
-@item object_flags
-Flags which may appear in the @samp{flags} field of a BFD with this
-format.
-
-@item section_flags
-Flags which may appear in the @samp{flags} field of a section within a
-BFD with this format.
-
-@item 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
-@samp{_foo} for a function named @samp{foo} in the C source, in which
-case this field would be @samp{_}. If there is no such character, this
-field will be @samp{0}.
-
-@item ar_pad_char
-The padding character to use at the end of an archive name. Normally
-@samp{/}.
-
-@item ar_max_namelen
-The maximum length of a short name in an archive. Normally @samp{14}.
-
-@item 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.
-@end table
-
-@node BFD target vector swap
-@subsection Swapping functions
-
-Every target vector has fuction 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.
-
-@node BFD target vector format
-@subsection 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:
-
-@table @samp
-@item bfd_unknown
-Unknown format. Not used for anything useful.
-@item bfd_object
-Object file.
-@item bfd_archive
-Archive file.
-@item bfd_core
-Core file.
-@end table
-
-The three arrays of function pointers are as follows:
-
-@table @samp
-@item 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 @samp{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 @samp{tdata} field of the BFD.
-This function must be called before any other BFD target vector function
-on a file opened for reading.
-
-@item bfd_set_format
-Set the format of a BFD which was created for output. This is called by
-the @samp{bfd_set_format} function after creating the BFD with a
-function such as @samp{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.
-
-@item bfd_write_contents
-Write out the contents of the BFD in the given format. This is called
-by @samp{bfd_close} function for a BFD opened for writing. This really
-should not be an array selected by format type, as the
-@samp{bfd_set_format} function provides all the required information.
-In fact, BFD will fail if a different format is used when calling
-through the @samp{bfd_set_format} and the @samp{bfd_write_contents}
-arrays; fortunately, since @samp{bfd_close} gets it right, this is a
-difficult error to make.
-@end table
-
-@node BFD_JUMP_TABLE macros
-@subsection @samp{BFD_JUMP_TABLE} macros
-@cindex @samp{BFD_JUMP_TABLE}
-
-Most target vectors are defined using @samp{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 @samp{BFD_JUMP_TABLE_RELOCS} macro defines three
-functions: @samp{_get_reloc_upper_bound}, @samp{_canonicalize_reloc},
-and @samp{_bfd_reloc_type_lookup}. A reference like
-@samp{BFD_JUMP_TABLE_RELOCS (foo)} will expand into three functions
-prefixed with @samp{foo}: @samp{foo_get_reloc_upper_found}, etc. The
-@samp{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 @samp{BFD_JUMP_TABLE_ARCHIVE}
-with the same argument, calling a set of functions which is defined in
-@file{archive.c}.
-
-Each of the @samp{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
-@samp{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.
-
-@node BFD target vector generic
-@subsection Generic functions
-@cindex @samp{BFD_JUMP_TABLE_GENERIC}
-
-The @samp{BFD_JUMP_TABLE_GENERIC} macro is used for some catch all
-functions which don't easily fit into other categories.
-
-@table @samp
-@item _close_and_cleanup
-Free any target specific information associated with the BFD. This is
-called when any BFD is closed (the @samp{bfd_write_contents} function
-mentioned earlier is only called for a BFD opened for writing). Most
-targets use @samp{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
-@samp{_bfd_generic_close_and_cleanup}, which simply returns true.
-
-@item _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 @samp{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
-@samp{_bfd_generic_free_cached_info}, which simply returns true.
-
-@item _new_section_hook
-This is called from @samp{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.
-
-@item _get_section_contents
-Get the contents of a section. This is called from
-@samp{bfd_get_section_contents}. Most targets set this to
-@samp{_bfd_generic_get_section_contents}, which does a @samp{bfd_seek}
-based on the section's @samp{filepos} field and a @samp{bfd_read}. The
-corresponding field in the target vector is named
-@samp{_bfd_get_section_contents}.
-
-@item _get_section_contents_in_window
-Set a @samp{bfd_window} to hold the contents of a section. This is
-called from @samp{bfd_get_section_contents_in_window}. The
-@samp{bfd_window} idea never really caught on, and I don't think this is
-ever called. Pretty much all targets implement this as
-@samp{bfd_generic_get_section_contents_in_window}, which uses
-@samp{bfd_get_section_contents} to do the right thing. The
-corresponding field in the target vector is named
-@samp{_bfd_get_section_contents_in_window}.
-@end table
-
-@node BFD target vector copy
-@subsection Copy functions
-@cindex @samp{BFD_JUMP_TABLE_COPY}
-
-The @samp{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.
-
-@table @samp
-@item _bfd_copy_private_bfd_data
-This is called when copying a BFD, via @samp{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.
-
-@item _bfd_merge_private_bfd_data
-This is called when linking, via @samp{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.
-
-@item _bfd_copy_private_section_data
-This is similar to @samp{_bfd_copy_private_bfd_data}, but it is called
-for each section, via @samp{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.
-
-@item _bfd_copy_private_symbol_data
-This is called via @samp{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 @samp{asymbol} structure and storing private information in the
-extra space. Since @samp{objcopy} and other programs copy symbol
-information by copying pointers to @samp{asymbol} structures, the
-private symbol information is automatically copied as well. Most
-targets do not do anything in this function.
-
-@item _bfd_set_private_flags
-This is called via @samp{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.
-
-@item _bfd_print_private_bfd_data
-This is called by @samp{objdump} when the @samp{-p} option is used. It
-is called via @samp{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 @samp{objdump}. Most targets do not do
-anything in this function.
-@end table
-
-@node BFD target vector core
-@subsection Core file support functions
-@cindex @samp{BFD_JUMP_TABLE_CORE}
-
-The @samp{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.
-
-@table @samp
-@item _core_file_failing_command
-Given a core file, this returns the command which was run to produce the
-core file.
-
-@item _core_file_failing_signal
-Given a core file, this returns the signal number which produced the
-core file.
-
-@item _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.
-@end table
-
-@node BFD target vector archive
-@subsection Archive functions
-@cindex @samp{BFD_JUMP_TABLE_ARCHIVE}
-
-The @samp{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 @samp{_bfd_archive_coff} as the
-argument to @samp{BFD_JUMP_TABLE_ARCHIVE}. Some targets use BSD/a.out
-style archives, and these use @samp{_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
-@samp{_bfd_noarchive}. Finally, a few targets have unusual archive
-handling.
-
-@table @samp
-@item _slurp_armap
-Read in the archive symbol table, storing it in private BFD data. This
-is normally called from the archive @samp{check_format} routine. The
-corresponding field in the target vector is named
-@samp{_bfd_slurp_armap}.
-
-@item _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 @samp{check_format} routine. The corresponding field in the
-target vector is named @samp{_bfd_slurp_extended_name_table}.
-
-@item 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 @samp{write_contents} routine. The corresponding field in the
-target vector is named @samp{_bfd_construct_extended_name_table}.
-
-@item _truncate_arname
-This copies a file name into an archive header, truncating it as
-required. It is normally called from the archive @samp{write_contents}
-routine. This function is more interesting in targets which do not
-support extended name tables, but I think the GNU @samp{ar} program
-always uses extended name tables anyhow. The corresponding field in the
-target vector is named @samp{_bfd_truncate_arname}.
-
-@item _write_armap
-Write out the archive symbol table using calls to @samp{bfd_write}.
-This is normally called from the archive @samp{write_contents} routine.
-The corresponding field in the target vector is named @samp{write_armap}
-(no leading underscore).
-
-@item _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
-@samp{_bfd_read_ar_hdr_fn}.
-
-@item _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 @samp{bfd_openr_next_archived_file}. The corresponding field in the
-target vector is named @samp{openr_next_archived_file} (no leading
-underscore).
-
-@item _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 @samp{bfd_get_elt_at_index}. The corresponding field in the target
-vector is named @samp{_bfd_get_elt_at_index}.
-
-@item _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 @samp{bfd_stat_arch_elt}. The corresponding field in the
-target vector is named @samp{_bfd_stat_arch_elt}.
-
-@item _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 @samp{write_contents} routine. The corresponding
-field in the target vector is named @samp{_bfd_update_armap_timestamp}.
-@end table
-
-@node BFD target vector symbols
-@subsection Symbol table functions
-@cindex @samp{BFD_JUMP_TABLE_SYMBOLS}
-
-The @samp{BFD_JUMP_TABLE_SYMBOLS} macro is used for functions which deal
-with symbols.
-
-@table @samp
-@item _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 @samp{asymbol} pointers for all the
-symbols plus a trailing @samp{NULL} entry, and store the actual symbol
-information in BFD private data. This is called via
-@samp{bfd_get_symtab_upper_bound}. The corresponding field in the
-target vector is named @samp{_bfd_get_symtab_upper_bound}.
-
-@item _get_symtab
-Read in the symbol table. This is called via
-@samp{bfd_canonicalize_symtab}. The corresponding field in the target
-vector is named @samp{_bfd_canonicalize_symtab}.
-
-@item _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 @samp{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 @samp{asymbol}. This is called
-via @samp{bfd_make_empty_symbol}. The corresponding field in the target
-vector is named @samp{_bfd_make_empty_symbol}.
-
-@item _print_symbol
-Print information about the symbol. This is called via
-@samp{bfd_print_symbol}. One of the arguments indicates what sort of
-information should be printed:
-
-@table @samp
-@item bfd_print_symbol_name
-Just print the symbol name.
-@item bfd_print_symbol_more
-Print the symbol name and some interesting flags. I don't think
-anything actually uses this.
-@item bfd_print_symbol_all
-Print all information about the symbol. This is used by @samp{objdump}
-when run with the @samp{-t} option.
-@end table
-The corresponding field in the target vector is named
-@samp{_bfd_print_symbol}.
-
-@item _get_symbol_info
-Return a standard set of information about the symbol. This is called
-via @samp{bfd_symbol_info}. The corresponding field in the target
-vector is named @samp{_bfd_get_symbol_info}.
-
-@item _bfd_is_local_label_name
-Return whether the given string would normally represent the name of a
-local label. This is called via @samp{bfd_is_local_label} and
-@samp{bfd_is_local_label_name}. Local labels are normally discarded by
-the assembler. In the linker, this defines the difference between the
-@samp{-x} and @samp{-X} options.
-
-@item _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.
-
-@item _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 @samp{bfd_find_nearest_line}. The corresponding field in the
-target vector is named @samp{_bfd_find_nearest_line}.
-
-@item _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
-@samp{N_DEBUG} section when it is written out. This is called via
-@samp{bfd_make_debug_symbol}.
-
-@item _read_minisymbols
-Minisymbols are used to reduce the memory requirements of programs like
-@samp{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 @samp{bfd_read_minisymbols}. Most targets do not implement this,
-and just use generic support which is based on using standard
-@samp{asymbol} structures.
-
-@item _minisymbol_to_symbol
-Convert a minisymbol to a standard @samp{asymbol}. This is called via
-@samp{bfd_minisymbol_to_symbol}.
-@end table
-
-@node BFD target vector relocs
-@subsection Relocation support
-@cindex @samp{BFD_JUMP_TABLE_RELOCS}
-
-The @samp{BFD_JUMP_TABLE_RELOCS} macro is used for functions which deal
-with relocations.
-
-@table @samp
-@item _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 @samp{arelent}
-pointers for all the relocations plus a trailing @samp{NULL} entry, and
-store the actual relocation information in BFD private data. This is
-called via @samp{bfd_get_reloc_upper_bound}.
-
-@item _canonicalize_reloc
-Return the relocation information for a section. This is called via
-@samp{bfd_canonicalize_reloc}. The corresponding field in the target
-vector is named @samp{_bfd_canonicalize_reloc}.
-
-@item _bfd_reloc_type_lookup
-Given a relocation code, return the corresponding howto structure
-(@pxref{BFD relocation codes}). This is called via
-@samp{bfd_reloc_type_lookup}. The corresponding field in the target
-vector is named @samp{reloc_type_lookup}.
-@end table
-
-@node BFD target vector write
-@subsection Output functions
-@cindex @samp{BFD_JUMP_TABLE_WRITE}
-
-The @samp{BFD_JUMP_TABLE_WRITE} macro is used for functions which deal
-with writing out a BFD.
-
-@table @samp
-@item _set_arch_mach
-Set the architecture and machine number for a BFD. This is called via
-@samp{bfd_set_arch_mach}. Most targets implement this by calling
-@samp{bfd_default_set_arch_mach}. The corresponding field in the target
-vector is named @samp{_bfd_set_arch_mach}.
-
-@item _set_section_contents
-Write out the contents of a section. This is called via
-@samp{bfd_set_section_contents}. The corresponding field in the target
-vector is named @samp{_bfd_set_section_contents}.
-@end table
-
-@node BFD target vector link
-@subsection Linker functions
-@cindex @samp{BFD_JUMP_TABLE_LINK}
-
-The @samp{BFD_JUMP_TABLE_LINK} macro is used for functions called by the
-linker.
-
-@table @samp
-@item _sizeof_headers
-Return the size of the header information required for a BFD. This is
-used to implement the @samp{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 @samp{bfd_sizeof_headers}. The
-corresponding field in the target vector is named
-@samp{_bfd_sizeof_headers}.
-
-@item _bfd_get_relocated_section_contents
-Read the contents of a section and apply the relocation information.
-This handles both a final link and a relocateable link; in the latter
-case, it adjust the relocation information as well. This is called via
-@samp{bfd_get_relocated_section_contents}. Most targets implement it by
-calling @samp{bfd_generic_get_relocated_section_contents}.
-
-@item _bfd_relax_section
-Try to use relaxation to shrink the size of a section. This is called
-by the linker when the @samp{-relax} option is used. This is called via
-@samp{bfd_relax_section}. Most targets do not support any sort of
-relaxation.
-
-@item _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
-@samp{bfd_link_hash_table_create}.
-
-@item _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 @samp{bfd_link_add_symbols}.
-
-@item _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 @samp{bfd_final_link}.
-
-@item _bfd_link_split_section
-I don't know what this is for. Nothing seems to call it. The only
-non-trivial definition is in @file{som.c}.
-@end table
-
-@node BFD target vector dynamic
-@subsection Dynamic linking information functions
-@cindex @samp{BFD_JUMP_TABLE_DYNAMIC}
-
-The @samp{BFD_JUMP_TABLE_DYNAMIC} macro is used for functions which read
-dynamic linking information.
-
-@table @samp
-@item _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 @samp{asymbol} pointers for
-all the symbols plus a trailing @samp{NULL} entry, and store the actual
-symbol information in BFD private data. This is called via
-@samp{bfd_get_dynamic_symtab_upper_bound}. The corresponding field in
-the target vector is named @samp{_bfd_get_dynamic_symtab_upper_bound}.
-
-@item _canonicalize_dynamic_symtab
-Read the dynamic symbol table. This is called via
-@samp{bfd_canonicalize_dynamic_symtab}. The corresponding field in the
-target vector is named @samp{_bfd_canonicalize_dynamic_symtab}.
-
-@item _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 @samp{arelent} pointers for
-all the relocations plus a trailing @samp{NULL} entry, and store the
-actual relocation information in BFD private data. This is called via
-@samp{bfd_get_dynamic_reloc_upper_bound}. The corresponding field in
-the target vector is named @samp{_bfd_get_dynamic_reloc_upper_bound}.
-
-@item _canonicalize_dynamic_reloc
-Read the dynamic relocations. This is called via
-@samp{bfd_canonicalize_dynamic_reloc}. The corresponding field in the
-target vector is named @samp{_bfd_canonicalize_dynamic_reloc}.
-@end table
-
-@node BFD generated files
-@section BFD generated files
-@cindex generated files in bfd
-@cindex 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
-time. Some files are automatically rebuilt at make time, but only if
-you configure with the @samp{--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.
-
-@table @file
-@item bfd.h
-@cindex @file{bfd.h}
-@cindex @file{bfd-in3.h}
-Lives in the object directory. Created at make time from
-@file{bfd-in2.h} via @file{bfd-in3.h}. @file{bfd-in3.h} is created at
-configure time from @file{bfd-in2.h}. There are automatic dependencies
-to rebuild @file{bfd-in3.h} and hence @file{bfd.h} if @file{bfd-in2.h}
-changes, so you can normally ignore @file{bfd-in3.h}, and just think
-about @file{bfd-in2.h} and @file{bfd.h}.
-
-@file{bfd.h} is built by replacing a few strings in @file{bfd-in2.h}.
-To see them, search for @samp{@@} in @file{bfd-in2.h}. They mainly
-control whether BFD is built for a 32 bit target or a 64 bit target.
-
-@item bfd-in2.h
-@cindex @file{bfd-in2.h}
-Lives in the source directory. Created from @file{bfd-in.h} and several
-other BFD source files. If you configure with the
-@samp{--enable-maintainer-mode} option, @file{bfd-in2.h} is rebuilt
-automatically when a source file changes.
-
-@item elf32-target.h
-@itemx elf64-target.h
-@cindex @file{elf32-target.h}
-@cindex @file{elf64-target.h}
-Live in the object directory. Created from @file{elfxx-target.h}.
-These files are versions of @file{elfxx-target.h} customized for either
-a 32 bit ELF target or a 64 bit ELF target.
-
-@item libbfd.h
-@cindex @file{libbfd.h}
-Lives in the source directory. Created from @file{libbfd-in.h} and
-several other BFD source files. If you configure with the
-@samp{--enable-maintainer-mode} option, @file{libbfd.h} is rebuilt
-automatically when a source file changes.
-
-@item libcoff.h
-@cindex @file{libcoff.h}
-Lives in the source directory. Created from @file{libcoff-in.h} and
-@file{coffcode.h}. If you configure with the
-@samp{--enable-maintainer-mode} option, @file{libcoff.h} is rebuilt
-automatically when a source file changes.
-
-@item targmatch.h
-@cindex @file{targmatch.h}
-Lives in the object directory. Created at make time from
-@file{config.bfd}. This file is used to map configuration triplets into
-BFD target vector variable names at run time.
-@end table
-
-@node BFD multiple compilations
-@section 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.
-
-@itemize @bullet
-@item
-Since this technique is rarely used, even experienced C programmers find
-it confusing.
-
-@item
-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.
-
-@item
-Programs which use BFD wind up incorporating two or more slightly
-different versions of the same function, which wastes space in the
-executable.
-
-@item
-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.
-@end itemize
-
-The following is a list of the files which are compiled multiple times.
-
-@table @file
-@item aout-target.h
-@cindex @file{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
-@samp{N_TXTADDR} and similar a.out macros.
-
-@item aoutf1.h
-@cindex @file{aoutf1.h}
-Implements standard SunOS a.out files. In principle it supports 64 bit
-a.out targets based on the preprocessor macro @samp{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.
-
-@item aoutx.h
-@cindex @file{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 @samp{aout32.c}
-and @samp{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.
-
-@item coffcode.h
-@cindex @file{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
-@file{coffgen.c}. This file is controlled by a number of different
-macros, and more are added regularly.
-
-@item coffswap.h
-@cindex @file{coffswap.h}
-Implements COFF swapping routines. This file is included by
-@file{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 @file{include/coff} directory. A COFF
-target file will include one of those files before including
-@file{coffcode.h} and thus @file{coffswap.h}. There are a few other
-macros which affect @file{coffswap.h} as well, mostly describing whether
-certain fields are present in the external structures.
-
-@item ecoffswap.h
-@cindex @file{ecoffswap.h}
-Implements ECOFF swapping routines. This is like @file{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 @samp{ECOFF_32} or
-@samp{ECOFF_64}.
-
-@item elfcode.h
-@cindex @file{elfcode.h}
-Implements ELF functions that use external structure definitions. This
-file is included by two other files: @file{elf32.c} and @file{elf64.c}.
-It is controlled by the @samp{ARCH_SIZE} macro which is defined to be
-@samp{32} or @samp{64} before including it. The @samp{NAME} macro is
-used internally to give the functions different names for the two target
-sizes.
-
-@item elfcore.h
-@cindex @file{elfcore.h}
-Like @file{elfcode.h}, but for functions that are specific to ELF core
-files. This is included only by @file{elfcode.h}.
-
-@item elflink.h
-@cindex @file{elflink.h}
-Like @file{elfcode.h}, but for functions used by the ELF linker. This
-is included only by @file{elfcode.h}.
-
-@item elfxx-target.h
-@cindex @file{elfxx-target.h}
-This file is the source for the generated files @file{elf32-target.h}
-and @file{elf64-target.h}, one of which is included by every ELF target.
-It defines the ELF target vector.
-
-@item freebsd.h
-@cindex @file{freebsd.h}
-Presumably intended to be included by all FreeBSD targets, but in fact
-there is only one such target, @samp{i386-freebsd}. This defines a
-function used to set the right magic number for FreeBSD, as well as
-various macros, and includes @file{aout-target.h}.
-
-@item netbsd.h
-@cindex @file{netbsd.h}
-Like @file{freebsd.h}, except that there are several files which include
-it.
-
-@item nlm-target.h
-@cindex @file{nlm-target.h}
-Defines the target vector for a standard NLM target.
-
-@item nlmcode.h
-@cindex @file{nlmcode.h}
-Like @file{elfcode.h}, but for NLM targets. This is only included by
-@file{nlm32.c} and @file{nlm64.c}, both of which define the macro
-@samp{ARCH_SIZE} to an appropriate value. There are no 64 bit NLM
-targets anyhow, so this is sort of useless.
-
-@item nlmswap.h
-@cindex @file{nlmswap.h}
-Like @file{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.
-
-@item peicode.h
-@cindex @file{peicode.h}
-Provides swapping routines and other hooks for PE targets.
-@file{coffcode.h} will include this rather than @file{coffswap.h} for a
-PE target. This defines PE specific versions of the COFF swapping
-routines, and also defines some macros which control @file{coffcode.h}
-itself.
-@end table
-
-@node BFD relocation handling
-@section BFD relocation handling
-@cindex bfd relocation handling
-@cindex relocations in bfd
-
-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.
-
-@menu
-* BFD relocation concepts:: BFD relocation concepts
-* BFD relocation functions:: BFD relocation functions
-* BFD relocation codes:: BFD relocation codes
-* BFD relocation future:: BFD relocation future
-@end menu
-
-@node BFD relocation concepts
-@subsection 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 @code{int *p = &i;} where
-@samp{p} and @samp{i} are global variables. A relocation for the symbol
-@samp{i} would be generated such that the linker would initialize the
-area of memory which holds the value of @samp{p} to the value of the
-symbol @samp{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 relocateable 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.
-
-@node BFD relocation functions
-@subsection BFD relocation functions
-
-In BFD, each section has an array of @samp{arelent} structures. Each
-structure has a pointer to a symbol, an address within the section, an
-addend, and a pointer to a @samp{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 @samp{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 @samp{special_function}
-field to use as an escape.
-
-While this seems plausible, a look at @samp{bfd_perform_relocation}
-shows that it failed. The function has odd special cases. Some of the
-fields in the howto structure, such as @samp{pcrel_offset}, were not
-adequately documented.
-
-The linker uses @samp{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
-@samp{bfd_get_relocated_section_contents}, which for most targets turns
-into a call to @samp{bfd_generic_get_relocated_section_contents}, which
-calls @samp{bfd_perform_relocation}. So @samp{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 @samp{bfd_perform_relocation} for a while. This
-turned out to be the wrong thing to do, since
-@samp{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
-@samp{bfd_install_relocation} instead, and @samp{bfd_install_relocation}
-was created as a copy of @samp{bfd_perform_relocation}.
-
-Unfortunately, the work did not progress any farther, so
-@samp{bfd_install_relocation} remains a simple copy of
-@samp{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
-@samp{arelent} structures, so it can not use
-@samp{bfd_perform_relocation} at all. Instead, users of the new linker
-are expected to write a @samp{relocate_section} function which will
-handle relocations in a target specific fashion.
-
-There are two helper functions for target specific relocation:
-@samp{_bfd_final_link_relocate} and @samp{_bfd_relocate_contents}.
-These functions use a howto structure, but they @emph{do not} use the
-@samp{special_function} field. Since the functions are normally called
-from target specific code, the @samp{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:
-
-@itemize @bullet
-@item
-Make sure you clearly understand what the contents of the section should
-look like after assembly, after a relocateable link, and after a final
-link. Make sure you clearly understand the operations the linker must
-perform during a relocateable link and during a final link.
-
-@item
-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.
-
-@item
-Change the assembler to generate your relocation. The assembler will
-call @samp{bfd_install_relocation}, so your howto structure has to be
-able to handle that. You may need to set the @samp{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
-relocateable link. For example, see @samp{bfd_elf_generic_reloc}.
-
-@item
-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.
-
-@item
-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 @samp{_bfd_final_link_relocate}
-or @samp{_bfd_relocate_contents}, depending upon the definition of the
-relocation and whether the link is relocateable or not.
-
-@item
-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 relocateable link, whether the
-symbol is defined or undefined in the relocateable output. For both the
-final and relocateable 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.
-
-@item
-In order for linking to another object file format, such as S-records,
-to work correctly, @samp{bfd_perform_relocation} has to do the right
-thing for the relocation. You may need to set the
-@samp{special_function} field to handle this correctly. Test this by
-doing a link in which the output object file format is S-records.
-
-@item
-Using the linker to generate relocateable output in a different object
-file format is impossible in the general case, so you generally don't
-have to worry about that. 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.
-@end itemize
-
-@node BFD relocation codes
-@subsection BFD relocation codes
-
-BFD has another way of describing relocations besides the howto
-structures described above: the enum @samp{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 @samp{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 @samp{arelent} structures. In order for the
-assembler to create an @samp{arelent} structure, it has to be able to
-obtain a howto structure. The function @samp{bfd_reloc_type_lookup},
-which simply calls the target vector entry point
-@samp{reloc_type_lookup}, takes a relocation code and returns a howto
-structure.
-
-The function @samp{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.
-
-@node BFD relocation future
-@subsection 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 relocateable 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 @samp{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
-@samp{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.
-
-@node BFD ELF support
-@section BFD ELF support
-@cindex elf support in bfd
-@cindex 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.
-
-@menu
-* BFD ELF sections and segments:: ELF sections and segments
-* BFD ELF generic support:: BFD ELF generic support
-* BFD ELF processor specific support:: BFD ELF processor specific support
-* BFD ELF core files:: BFD ELF core files
-* BFD ELF future:: BFD ELF future
-@end menu
-
-@node BFD ELF sections and segments
-@subsection ELF sections and segments
-
-The ELF ABI permits a file to have either sections or segments or both.
-Relocateable 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 @samp{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
-@samp{0x40080} and the page size is @samp{0x1000}, then the position of
-the segment contents within the file must equal @samp{0x80} modulo
-@samp{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
-@samp{copy_private} entry points. They will be printed by the
-@samp{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 @samp{assign_file_positions_for_segments} in @file{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 @samp{bfd_record_phdr}
-function in @file{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 @samp{PHDRS} linker script command.
-
-@node BFD ELF generic support
-@subsection BFD ELF generic support
-
-In general, functions which do not read external data from the ELF file
-are found in @file{elf.c}. They operate on the internal forms of the
-ELF structures, which are defined in @file{include/elf/internal.h}. The
-internal structures are defined in terms of @samp{bfd_vma}, and so may
-be used for both 32 bit and 64 bit ELF targets.
-
-The file @file{elfcode.h} contains functions which operate on the
-external data. @file{elfcode.h} is compiled twice, once via
-@file{elf32.c} with @samp{ARCH_SIZE} defined as @samp{32}, and once via
-@file{elf64.c} with @samp{ARCH_SIZE} defined as @samp{64}.
-@file{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 @file{elflink.c} and @file{elflink.h}. The
-latter file is compiled twice, for both 32 and 64 bit support. The
-linker support is only used if the processor specific file defines
-@samp{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 @samp{bfd_perform_relocation}.
-
-The core file support is in @file{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 @file{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 @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
-@file{elf.c}. Some functions are found in @file{elfcode.h}.
-
-The processor specific files may override particular entries in the
-target vector, but most do not, with one exception: the
-@samp{bfd_reloc_type_lookup} entry point is always processor specific.
-
-@node BFD ELF processor specific support
-@subsection BFD ELF processor specific support
-
-By convention, the processor specific support for a particular processor
-will be found in @file{elf@var{nn}-@var{cpu}.c}, where @var{nn} is
-either 32 or 64, and @var{cpu} is the name of the processor.
-
-@menu
-* BFD ELF processor required:: Required processor specific support
-* BFD ELF processor linker:: Processor specific linker support
-* BFD ELF processor other:: Other processor specific support options
-@end menu
-
-@node BFD ELF processor required
-@subsubsection Required processor specific support
-
-When writing a @file{elf@var{nn}-@var{cpu}.c} file, you must do the
-following:
-
-@itemize @bullet
-@item
-Define either @samp{TARGET_BIG_SYM} or @samp{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 @file{targets.c}, and will also
-have to appear in @file{config.bfd} and @file{configure.in}. Define
-@samp{TARGET_BIG_SYM} for a big-endian processor,
-@samp{TARGET_LITTLE_SYM} for a little-endian processor, and define both
-for a bi-endian processor.
-@item
-Define either @samp{TARGET_BIG_NAME} or @samp{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.
-@item
-Define @samp{ELF_ARCH} to the BFD architecture (an element of the
-@samp{bfd_architecture} enum, typically @samp{bfd_arch_@var{cpu}}).
-@item
-Define @samp{ELF_MACHINE_CODE} to the magic number which should appear
-in the @samp{e_machine} field of the ELF header. As of this writing,
-these magic numbers are assigned by SCO; if you want to get a magic
-number for a particular processor, try sending a note to
-@email{registry@@sco.com}. In the BFD sources, the magic numbers are
-found in @file{include/elf/common.h}; they have names beginning with
-@samp{EM_}.
-@item
-Define @samp{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 @samp{1}.
-@item
-If the format should use @samp{Rel} rather than @samp{Rela} relocations,
-define @samp{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 @samp{Rela}
-relocations. @samp{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 @samp{Rela} relocations. With @samp{Rel} relocations, the addend
-must be stored in the section contents, which makes relocateable links
-more complex.
-
-For example, consider C code like @code{i = a[1000];} where @samp{a} is
-a global array. The instructions which load the value of @samp{a[1000]}
-will most likely use a relocation which refers to the symbol
-representing @samp{a}, with an addend that gives the offset from the
-start of @samp{a} to element @samp{1000}. When using @samp{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 relocateable 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 @samp{Rel} and
-@samp{Rela} relocations for a single target; @file{elf64-mips.c} does it
-by overriding the relocation reading and writing routines.
-@item
-Define howto structures for all the relocation types.
-@item
-Define a @samp{bfd_reloc_type_lookup} routine. This must be named
-@samp{bfd_elf@var{nn}_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.
-@item
-If using @samp{Rel} relocations, define @samp{elf_info_to_howto_rel}.
-If using @samp{Rela} relocations, define @samp{elf_info_to_howto}.
-Either way, this is a macro defined as the name of a function which
-takes an @samp{arelent} and a @samp{Rel} or @samp{Rela} structure, and
-sets the @samp{howto} field of the @samp{arelent} based on the
-@samp{Rel} or @samp{Rela} structure. This is normally uses
-@samp{ELF@var{nn}_R_TYPE} to get the ELF relocation type and uses it as
-an index into a table of howto structures.
-@end itemize
-
-You must also add the magic number for this processor to the
-@samp{prep_headers} function in @file{elf.c}.
-
-You must also create a header file in the @file{include/elf} directory
-called @file{@var{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 @samp{START_RELOC_NUMBERS}, @samp{RELOC_NUMBER},
-@samp{FAKE_RELOC}, @samp{EMPTY_RELOC} and @samp{END_RELOC_NUMBERS}
-macros to create a table mapping the number used to indentify a
-relocation to a name describing that relocation.
-
-@node BFD ELF processor linker
-@subsubsection 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 @samp{asymbol} and @samp{arelent} structures. In
-this case, relocations will be handled by calling
-@samp{bfd_perform_relocation}, which will use the howto structures you
-have defined. @xref{BFD relocation handling}.
-
-In order to support linking into a different object file format, such as
-S-records, @samp{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
-@samp{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
-@samp{_bfd_final_link_relocate}. The function must check for a
-relocateable 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.
-
-@node BFD ELF processor other
-@subsubsection Other processor specific support options
-
-There are many other macros which may be defined in
-@file{elf@var{nn}-@var{cpu}.c}. These macros may be found in
-@file{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
-@samp{elf_backend_data} structure defined in @file{elf-bfd.h}. In
-general, a hook function is set by defining a macro
-@samp{elf_backend_@var{name}}.
-
-There are a few processor specific constants which may also be defined.
-These are again found in the @samp{elf_backend_data} structure.
-
-I will not define the various functions and constants here; see the
-comments in @file{elf-bfd.h}.
-
-Normally any odd characteristic of a particular ELF processor is handled
-via a hook function. For example, the special @samp{SHN_MIPS_SCOMMON}
-section number found in MIPS ELF is handled via the hooks
-@samp{section_from_bfd_section}, @samp{symbol_processing},
-@samp{add_symbol_hook}, and @samp{output_symbol_hook}.
-
-Dynamic linking support, which involves processor specific relocations
-requiring special handling, is also implemented via hook functions.
-
-@node BFD ELF core files
-@subsection BFD ELF core files
-@cindex 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 @samp{segment@var{n}} where
-@var{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
-@samp{p_filesz} field. The total size of the segment is given by the
-@samp{p_memsz} field. If @samp{p_memsz} is larger than @samp{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 @samp{segment@var{n}a}, will represent the initialized
-part of the program segment. The second, named @samp{segment@var{n}b},
-will represent the uninitialized part.
-
-ELF core files store special information such as register values in
-program segments with the type @samp{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 @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 @samp{bfd_set_format} and
-@samp{bfd_write_contents} routines for the @samp{bfd_core} type; see
-@ref{BFD target vector format}.
-
-@node BFD ELF future
-@subsection 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 @samp{reloc_type_lookup} and @samp{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 (@samp{_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
-@samp{prep_headers} in @file{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.
-
-When a linker script uses @samp{SIZEOF_HEADERS}, the ELF backend must
-guess at the number of program segments which will be required, in
-@samp{get_program_header_size}. This is because the linker calls
-@samp{bfd_sizeof_headers} before it knows all the section addresses and
-sizes. The ELF backend may later discover, when creating program
-segments, that more program segments are required. This is currently
-reported as an error in @samp{assign_file_positions_for_segments}.
-
-In practice this makes it difficult to use @samp{SIZEOF_HEADERS} except
-with a carefully defined linker script. Unfortunately,
-@samp{SIZEOF_HEADERS} is required for fast program loading on a native
-system, since it permits the initial code section to appear on the same
-page as the program segments, saving a page read when the program starts
-running. Fortunately, native systems permit careful definition of the
-linker script. Still, ideally it would be possible to use relaxation to
-compute the number of program segments.
-
-@node BFD glossary
-@section BFD glossary
-@cindex glossary for bfd
-@cindex bfd glossary
-
-This is a short glossary of some BFD terms.
-
-@table @asis
-@item 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.
-
-@item archive
-A collection of object files produced and manipulated by the @samp{ar}
-program.
-
-@item backend
-The implementation within BFD of a particular object file format. The
-set of functions which appear in a particular target vector.
-
-@item BFD
-The BFD library itself. Also, each object file, archive, or exectable
-opened by the BFD library has the type @samp{bfd *}, and is sometimes
-referred to as a bfd.
-
-@item COFF
-The Common Object File Format. Used on Unix SVR3. Used by some
-embedded targets, although ELF is normally better.
-
-@item DLL
-A shared library on Windows.
-
-@item 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.
-
-@item dynamic object
-Another name for an ELF shared library.
-
-@item 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.
-
-@item 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.
-
-@item executable
-A program, with instructions and symbols, and perhaps dynamic linking
-information. Normally produced by a linker.
-
-@item LMA
-Load Memory Address. This is the address at which a section will be
-loaded. Compare with VMA, below.
-
-@item 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.
-
-@item object file
-A binary file including machine instructions, symbols, and relocation
-information. Normally produced by an assembler.
-
-@item 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.
-
-@item 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.
-
-@item 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.
-
-@item relocations
-Information used by the linker to adjust section contents. Also called
-relocs.
-
-@item section
-Object files and executable are composed of sections. Sections have
-optional data and optional relocation information.
-
-@item 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.
-
-@item 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.
-
-@item target vector
-A set of functions which implement support for a particular object file
-format. The @samp{bfd_target} structure.
-
-@item Win32
-The current Windows API, implemented by Windows 95 and later and Windows
-NT 3.51 and later, but not by Windows 3.1.
-
-@item XCOFF
-The eXtended Common Object File Format. Used on AIX. A variant of
-COFF, with a completely different symbol table implementation.
-
-@item VMA
-Virtual Memory Address. This is the address a section will have when
-an executable is run. Compare with LMA, above.
-@end table
-
-@node Index
-@unnumberedsec Index
-@printindex cp
-
-@contents
-@bye