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diff --git a/bfd/doc/bfdint.texi b/bfd/doc/bfdint.texi deleted file mode 100644 index eb09b34a9d2..00000000000 --- a/bfd/doc/bfdint.texi +++ /dev/null @@ -1,1885 +0,0 @@ -\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 - -This document is not updated regularly, and may be out of date.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 |