bfdint.texi revision 1.1.1.5
1\input texinfo 2@c Copyright (C) 1988-2018 Free Software Foundation, Inc. 3@setfilename bfdint.info 4 5@settitle BFD Internals 6@iftex 7@titlepage 8@title{BFD Internals} 9@author{Ian Lance Taylor} 10@author{Cygnus Solutions} 11@page 12@end iftex 13 14@copying 15This file documents the internals of the BFD library. 16 17Copyright @copyright{} 1988-2018 Free Software Foundation, Inc. 18Contributed by Cygnus Support. 19 20Permission is granted to copy, distribute and/or modify this document 21under the terms of the GNU Free Documentation License, Version 1.1 or 22any later version published by the Free Software Foundation; with the 23Invariant Sections being ``GNU General Public License'' and ``Funding 24Free Software'', the Front-Cover texts being (a) (see below), and with 25the Back-Cover Texts being (b) (see below). A copy of the license is 26included in the section entitled ``GNU Free Documentation License''. 27 28(a) The FSF's Front-Cover Text is: 29 30 A GNU Manual 31 32(b) The FSF's Back-Cover Text is: 33 34 You have freedom to copy and modify this GNU Manual, like GNU 35 software. Copies published by the Free Software Foundation raise 36 funds for GNU development. 37@end copying 38 39@node Top 40@top BFD Internals 41@raisesections 42@cindex bfd internals 43 44This document describes some BFD internal information which may be 45helpful when working on BFD. It is very incomplete. 46 47This document is not updated regularly, and may be out of date. 48 49The initial version of this document was written by Ian Lance Taylor 50@email{ian@@cygnus.com}. 51 52@menu 53* BFD overview:: BFD overview 54* BFD guidelines:: BFD programming guidelines 55* BFD target vector:: BFD target vector 56* BFD generated files:: BFD generated files 57* BFD multiple compilations:: Files compiled multiple times in BFD 58* BFD relocation handling:: BFD relocation handling 59* BFD ELF support:: BFD ELF support 60* BFD glossary:: Glossary 61* Index:: Index 62@end menu 63 64@node BFD overview 65@section BFD overview 66 67BFD is a library which provides a single interface to read and write 68object files, executables, archive files, and core files in any format. 69 70@menu 71* BFD library interfaces:: BFD library interfaces 72* BFD library users:: BFD library users 73* BFD view:: The BFD view of a file 74* BFD blindness:: BFD loses information 75@end menu 76 77@node BFD library interfaces 78@subsection BFD library interfaces 79 80One way to look at the BFD library is to divide it into four parts by 81type of interface. 82 83The first interface is the set of generic functions which programs using 84the BFD library will call. These generic function normally translate 85directly or indirectly into calls to routines which are specific to a 86particular object file format. Many of these generic functions are 87actually defined as macros in @file{bfd.h}. These functions comprise 88the official BFD interface. 89 90The second interface is the set of functions which appear in the target 91vectors. This is the bulk of the code in BFD. A target vector is a set 92of function pointers specific to a particular object file format. The 93target vector is used to implement the generic BFD functions. These 94functions are always called through the target vector, and are never 95called directly. The target vector is described in detail in @ref{BFD 96target vector}. The set of functions which appear in a particular 97target vector is often referred to as a BFD backend. 98 99The third interface is a set of oddball functions which are typically 100specific to a particular object file format, are not generic functions, 101and are called from outside of the BFD library. These are used as hooks 102by the linker and the assembler when a particular object file format 103requires some action which the BFD generic interface does not provide. 104These functions are typically declared in @file{bfd.h}, but in many 105cases they are only provided when BFD is configured with support for a 106particular object file format. These functions live in a grey area, and 107are not really part of the official BFD interface. 108 109The fourth interface is the set of BFD support functions which are 110called by the other BFD functions. These manage issues like memory 111allocation, error handling, file access, hash tables, swapping, and the 112like. These functions are never called from outside of the BFD library. 113 114@node BFD library users 115@subsection BFD library users 116 117Another way to look at the BFD library is to divide it into three parts 118by the manner in which it is used. 119 120The first use is to read an object file. The object file readers are 121programs like @samp{gdb}, @samp{nm}, @samp{objdump}, and @samp{objcopy}. 122These programs use BFD to view an object file in a generic form. The 123official BFD interface is normally fully adequate for these programs. 124 125The second use is to write an object file. The object file writers are 126programs like @samp{gas} and @samp{objcopy}. These programs use BFD to 127create an object file. The official BFD interface is normally adequate 128for these programs, but for some object file formats the assembler needs 129some additional hooks in order to set particular flags or other 130information. The official BFD interface includes functions to copy 131private information from one object file to another, and these functions 132are used by @samp{objcopy} to avoid information loss. 133 134The third use is to link object files. There is only one object file 135linker, @samp{ld}. Originally, @samp{ld} was an object file reader and 136an object file writer, and it did the link operation using the generic 137BFD structures. However, this turned out to be too slow and too memory 138intensive. 139 140The official BFD linker functions were written to permit specific BFD 141backends to perform the link without translating through the generic 142structures, in the normal case where all the input files and output file 143have the same object file format. Not all of the backends currently 144implement the new interface, and there are default linking functions 145within BFD which use the generic structures and which work with all 146backends. 147 148For several object file formats the linker needs additional hooks which 149are not provided by the official BFD interface, particularly for dynamic 150linking support. These functions are typically called from the linker 151emulation template. 152 153@node BFD view 154@subsection The BFD view of a file 155 156BFD uses generic structures to manage information. It translates data 157into the generic form when reading files, and out of the generic form 158when writing files. 159 160BFD describes a file as a pointer to the @samp{bfd} type. A @samp{bfd} 161is composed of the following elements. The BFD information can be 162displayed using the @samp{objdump} program with various options. 163 164@table @asis 165@item general information 166The object file format, a few general flags, the start address. 167@item architecture 168The architecture, including both a general processor type (m68k, MIPS 169etc.) and a specific machine number (m68000, R4000, etc.). 170@item sections 171A list of sections. 172@item symbols 173A symbol table. 174@end table 175 176BFD represents a section as a pointer to the @samp{asection} type. Each 177section has a name and a size. Most sections also have an associated 178block of data, known as the section contents. Sections also have 179associated flags, a virtual memory address, a load memory address, a 180required alignment, a list of relocations, and other miscellaneous 181information. 182 183BFD represents a relocation as a pointer to the @samp{arelent} type. A 184relocation describes an action which the linker must take to modify the 185section contents. Relocations have a symbol, an address, an addend, and 186a pointer to a howto structure which describes how to perform the 187relocation. For more information, see @ref{BFD relocation handling}. 188 189BFD represents a symbol as a pointer to the @samp{asymbol} type. A 190symbol has a name, a pointer to a section, an offset within that 191section, and some flags. 192 193Archive files do not have any sections or symbols. Instead, BFD 194represents an archive file as a file which contains a list of 195@samp{bfd}s. BFD also provides access to the archive symbol map, as a 196list of symbol names. BFD provides a function to return the @samp{bfd} 197within the archive which corresponds to a particular entry in the 198archive symbol map. 199 200@node BFD blindness 201@subsection BFD loses information 202 203Most object file formats have information which BFD can not represent in 204its generic form, at least as currently defined. 205 206There is often explicit information which BFD can not represent. For 207example, the COFF version stamp, or the ELF program segments. BFD 208provides special hooks to handle this information when copying, 209printing, or linking an object file. The BFD support for a particular 210object file format will normally store this information in private data 211and handle it using the special hooks. 212 213In some cases there is also implicit information which BFD can not 214represent. For example, the MIPS processor distinguishes small and 215large symbols, and requires that all small symbols be within 32K of the 216GP register. This means that the MIPS assembler must be able to mark 217variables as either small or large, and the MIPS linker must know to put 218small symbols within range of the GP register. Since BFD can not 219represent this information, this means that the assembler and linker 220must have information that is specific to a particular object file 221format which is outside of the BFD library. 222 223This loss of information indicates areas where the BFD paradigm breaks 224down. It is not actually possible to represent the myriad differences 225among object file formats using a single generic interface, at least not 226in the manner which BFD does it today. 227 228Nevertheless, the BFD library does greatly simplify the task of dealing 229with object files, and particular problems caused by information loss 230can normally be solved using some sort of relatively constrained hook 231into the library. 232 233 234 235@node BFD guidelines 236@section BFD programming guidelines 237@cindex bfd programming guidelines 238@cindex programming guidelines for bfd 239@cindex guidelines, bfd programming 240 241There is a lot of poorly written and confusing code in BFD. New BFD 242code should be written to a higher standard. Merely because some BFD 243code is written in a particular manner does not mean that you should 244emulate it. 245 246Here are some general BFD programming guidelines: 247 248@itemize @bullet 249@item 250Follow the GNU coding standards. 251 252@item 253Avoid global variables. We ideally want BFD to be fully reentrant, so 254that it can be used in multiple threads. All uses of global or static 255variables interfere with that. Initialized constant variables are OK, 256and they should be explicitly marked with @samp{const}. Instead of global 257variables, use data attached to a BFD or to a linker hash table. 258 259@item 260All externally visible functions should have names which start with 261@samp{bfd_}. All such functions should be declared in some header file, 262typically @file{bfd.h}. See, for example, the various declarations near 263the end of @file{bfd-in.h}, which mostly declare functions required by 264specific linker emulations. 265 266@item 267All functions which need to be visible from one file to another within 268BFD, but should not be visible outside of BFD, should start with 269@samp{_bfd_}. Although external names beginning with @samp{_} are 270prohibited by the ANSI standard, in practice this usage will always 271work, and it is required by the GNU coding standards. 272 273@item 274Always remember that people can compile using @samp{--enable-targets} to 275build several, or all, targets at once. It must be possible to link 276together the files for all targets. 277 278@item 279BFD code should compile with few or no warnings using @samp{gcc -Wall}. 280Some warnings are OK, like the absence of certain function declarations 281which may or may not be declared in system header files. Warnings about 282ambiguous expressions and the like should always be fixed. 283@end itemize 284 285@node BFD target vector 286@section BFD target vector 287@cindex bfd target vector 288@cindex target vector in bfd 289 290BFD supports multiple object file formats by using the @dfn{target 291vector}. This is simply a set of function pointers which implement 292behaviour that is specific to a particular object file format. 293 294In this section I list all of the entries in the target vector and 295describe what they do. 296 297@menu 298* BFD target vector miscellaneous:: Miscellaneous constants 299* BFD target vector swap:: Swapping functions 300* BFD target vector format:: Format type dependent functions 301* BFD_JUMP_TABLE macros:: BFD_JUMP_TABLE macros 302* BFD target vector generic:: Generic functions 303* BFD target vector copy:: Copy functions 304* BFD target vector core:: Core file support functions 305* BFD target vector archive:: Archive functions 306* BFD target vector symbols:: Symbol table functions 307* BFD target vector relocs:: Relocation support 308* BFD target vector write:: Output functions 309* BFD target vector link:: Linker functions 310* BFD target vector dynamic:: Dynamic linking information functions 311@end menu 312 313@node BFD target vector miscellaneous 314@subsection Miscellaneous constants 315 316The target vector starts with a set of constants. 317 318@table @samp 319@item name 320The name of the target vector. This is an arbitrary string. This is 321how the target vector is named in command line options for tools which 322use BFD, such as the @samp{--oformat} linker option. 323 324@item flavour 325A general description of the type of target. The following flavours are 326currently defined: 327 328@table @samp 329@item bfd_target_unknown_flavour 330Undefined or unknown. 331@item bfd_target_aout_flavour 332a.out. 333@item bfd_target_coff_flavour 334COFF. 335@item bfd_target_ecoff_flavour 336ECOFF. 337@item bfd_target_elf_flavour 338ELF. 339@item bfd_target_ieee_flavour 340IEEE-695. 341@item bfd_target_nlm_flavour 342NLM. 343@item bfd_target_oasys_flavour 344OASYS. 345@item bfd_target_tekhex_flavour 346Tektronix hex format. 347@item bfd_target_srec_flavour 348Motorola S-record format. 349@item bfd_target_ihex_flavour 350Intel hex format. 351@item bfd_target_som_flavour 352SOM (used on HP/UX). 353@item bfd_target_verilog_flavour 354Verilog memory hex dump format. 355@item bfd_target_os9k_flavour 356os9000. 357@item bfd_target_versados_flavour 358VERSAdos. 359@item bfd_target_msdos_flavour 360MS-DOS. 361@item bfd_target_evax_flavour 362openVMS. 363@item bfd_target_mmo_flavour 364Donald Knuth's MMIXware object format. 365@end table 366 367@item byteorder 368The byte order of data in the object file. One of 369@samp{BFD_ENDIAN_BIG}, @samp{BFD_ENDIAN_LITTLE}, or 370@samp{BFD_ENDIAN_UNKNOWN}. The latter would be used for a format such 371as S-records which do not record the architecture of the data. 372 373@item header_byteorder 374The byte order of header information in the object file. Normally the 375same as the @samp{byteorder} field, but there are certain cases where it 376may be different. 377 378@item object_flags 379Flags which may appear in the @samp{flags} field of a BFD with this 380format. 381 382@item section_flags 383Flags which may appear in the @samp{flags} field of a section within a 384BFD with this format. 385 386@item symbol_leading_char 387A character which the C compiler normally puts before a symbol. For 388example, an a.out compiler will typically generate the symbol 389@samp{_foo} for a function named @samp{foo} in the C source, in which 390case this field would be @samp{_}. If there is no such character, this 391field will be @samp{0}. 392 393@item ar_pad_char 394The padding character to use at the end of an archive name. Normally 395@samp{/}. 396 397@item ar_max_namelen 398The maximum length of a short name in an archive. Normally @samp{14}. 399 400@item backend_data 401A pointer to constant backend data. This is used by backends to store 402whatever additional information they need to distinguish similar target 403vectors which use the same sets of functions. 404@end table 405 406@node BFD target vector swap 407@subsection Swapping functions 408 409Every target vector has function pointers used for swapping information 410in and out of the target representation. There are two sets of 411functions: one for data information, and one for header information. 412Each set has three sizes: 64-bit, 32-bit, and 16-bit. Each size has 413three actual functions: put, get unsigned, and get signed. 414 415These 18 functions are used to convert data between the host and target 416representations. 417 418@node BFD target vector format 419@subsection Format type dependent functions 420 421Every target vector has three arrays of function pointers which are 422indexed by the BFD format type. The BFD format types are as follows: 423 424@table @samp 425@item bfd_unknown 426Unknown format. Not used for anything useful. 427@item bfd_object 428Object file. 429@item bfd_archive 430Archive file. 431@item bfd_core 432Core file. 433@end table 434 435The three arrays of function pointers are as follows: 436 437@table @samp 438@item bfd_check_format 439Check whether the BFD is of a particular format (object file, archive 440file, or core file) corresponding to this target vector. This is called 441by the @samp{bfd_check_format} function when examining an existing BFD. 442If the BFD matches the desired format, this function will initialize any 443format specific information such as the @samp{tdata} field of the BFD. 444This function must be called before any other BFD target vector function 445on a file opened for reading. 446 447@item bfd_set_format 448Set the format of a BFD which was created for output. This is called by 449the @samp{bfd_set_format} function after creating the BFD with a 450function such as @samp{bfd_openw}. This function will initialize format 451specific information required to write out an object file or whatever of 452the given format. This function must be called before any other BFD 453target vector function on a file opened for writing. 454 455@item bfd_write_contents 456Write out the contents of the BFD in the given format. This is called 457by @samp{bfd_close} function for a BFD opened for writing. This really 458should not be an array selected by format type, as the 459@samp{bfd_set_format} function provides all the required information. 460In fact, BFD will fail if a different format is used when calling 461through the @samp{bfd_set_format} and the @samp{bfd_write_contents} 462arrays; fortunately, since @samp{bfd_close} gets it right, this is a 463difficult error to make. 464@end table 465 466@node BFD_JUMP_TABLE macros 467@subsection @samp{BFD_JUMP_TABLE} macros 468@cindex @samp{BFD_JUMP_TABLE} 469 470Most target vectors are defined using @samp{BFD_JUMP_TABLE} macros. 471These macros take a single argument, which is a prefix applied to a set 472of functions. The macros are then used to initialize the fields in the 473target vector. 474 475For example, the @samp{BFD_JUMP_TABLE_RELOCS} macro defines three 476functions: @samp{_get_reloc_upper_bound}, @samp{_canonicalize_reloc}, 477and @samp{_bfd_reloc_type_lookup}. A reference like 478@samp{BFD_JUMP_TABLE_RELOCS (foo)} will expand into three functions 479prefixed with @samp{foo}: @samp{foo_get_reloc_upper_bound}, etc. The 480@samp{BFD_JUMP_TABLE_RELOCS} macro will be placed such that those three 481functions initialize the appropriate fields in the BFD target vector. 482 483This is done because it turns out that many different target vectors can 484share certain classes of functions. For example, archives are similar 485on most platforms, so most target vectors can use the same archive 486functions. Those target vectors all use @samp{BFD_JUMP_TABLE_ARCHIVE} 487with the same argument, calling a set of functions which is defined in 488@file{archive.c}. 489 490Each of the @samp{BFD_JUMP_TABLE} macros is mentioned below along with 491the description of the function pointers which it defines. The function 492pointers will be described using the name without the prefix which the 493@samp{BFD_JUMP_TABLE} macro defines. This name is normally the same as 494the name of the field in the target vector structure. Any differences 495will be noted. 496 497@node BFD target vector generic 498@subsection Generic functions 499@cindex @samp{BFD_JUMP_TABLE_GENERIC} 500 501The @samp{BFD_JUMP_TABLE_GENERIC} macro is used for some catch all 502functions which don't easily fit into other categories. 503 504@table @samp 505@item _close_and_cleanup 506Free any target specific information associated with the BFD. This is 507called when any BFD is closed (the @samp{bfd_write_contents} function 508mentioned earlier is only called for a BFD opened for writing). Most 509targets use @samp{bfd_alloc} to allocate all target specific 510information, and therefore don't have to do anything in this function. 511This function pointer is typically set to 512@samp{_bfd_generic_close_and_cleanup}, which simply returns true. 513 514@item _bfd_free_cached_info 515Free any cached information associated with the BFD which can be 516recreated later if necessary. This is used to reduce the memory 517consumption required by programs using BFD. This is normally called via 518the @samp{bfd_free_cached_info} macro. It is used by the default 519archive routines when computing the archive map. Most targets do not 520do anything special for this entry point, and just set it to 521@samp{_bfd_generic_free_cached_info}, which simply returns true. 522 523@item _new_section_hook 524This is called from @samp{bfd_make_section_anyway} whenever a new 525section is created. Most targets use it to initialize section specific 526information. This function is called whether or not the section 527corresponds to an actual section in an actual BFD. 528 529@item _get_section_contents 530Get the contents of a section. This is called from 531@samp{bfd_get_section_contents}. Most targets set this to 532@samp{_bfd_generic_get_section_contents}, which does a @samp{bfd_seek} 533based on the section's @samp{filepos} field and a @samp{bfd_bread}. The 534corresponding field in the target vector is named 535@samp{_bfd_get_section_contents}. 536 537@item _get_section_contents_in_window 538Set a @samp{bfd_window} to hold the contents of a section. This is 539called from @samp{bfd_get_section_contents_in_window}. The 540@samp{bfd_window} idea never really caught on, and I don't think this is 541ever called. Pretty much all targets implement this as 542@samp{bfd_generic_get_section_contents_in_window}, which uses 543@samp{bfd_get_section_contents} to do the right thing. The 544corresponding field in the target vector is named 545@samp{_bfd_get_section_contents_in_window}. 546@end table 547 548@node BFD target vector copy 549@subsection Copy functions 550@cindex @samp{BFD_JUMP_TABLE_COPY} 551 552The @samp{BFD_JUMP_TABLE_COPY} macro is used for functions which are 553called when copying BFDs, and for a couple of functions which deal with 554internal BFD information. 555 556@table @samp 557@item _bfd_copy_private_bfd_data 558This is called when copying a BFD, via @samp{bfd_copy_private_bfd_data}. 559If the input and output BFDs have the same format, this will copy any 560private information over. This is called after all the section contents 561have been written to the output file. Only a few targets do anything in 562this function. 563 564@item _bfd_merge_private_bfd_data 565This is called when linking, via @samp{bfd_merge_private_bfd_data}. It 566gives the backend linker code a chance to set any special flags in the 567output file based on the contents of the input file. Only a few targets 568do anything in this function. 569 570@item _bfd_copy_private_section_data 571This is similar to @samp{_bfd_copy_private_bfd_data}, but it is called 572for each section, via @samp{bfd_copy_private_section_data}. This 573function is called before any section contents have been written. Only 574a few targets do anything in this function. 575 576@item _bfd_copy_private_symbol_data 577This is called via @samp{bfd_copy_private_symbol_data}, but I don't 578think anything actually calls it. If it were defined, it could be used 579to copy private symbol data from one BFD to another. However, most BFDs 580store extra symbol information by allocating space which is larger than 581the @samp{asymbol} structure and storing private information in the 582extra space. Since @samp{objcopy} and other programs copy symbol 583information by copying pointers to @samp{asymbol} structures, the 584private symbol information is automatically copied as well. Most 585targets do not do anything in this function. 586 587@item _bfd_set_private_flags 588This is called via @samp{bfd_set_private_flags}. It is basically a hook 589for the assembler to set magic information. For example, the PowerPC 590ELF assembler uses it to set flags which appear in the e_flags field of 591the ELF header. Most targets do not do anything in this function. 592 593@item _bfd_print_private_bfd_data 594This is called by @samp{objdump} when the @samp{-p} option is used. It 595is called via @samp{bfd_print_private_data}. It prints any interesting 596information about the BFD which can not be otherwise represented by BFD 597and thus can not be printed by @samp{objdump}. Most targets do not do 598anything in this function. 599@end table 600 601@node BFD target vector core 602@subsection Core file support functions 603@cindex @samp{BFD_JUMP_TABLE_CORE} 604 605The @samp{BFD_JUMP_TABLE_CORE} macro is used for functions which deal 606with core files. Obviously, these functions only do something 607interesting for targets which have core file support. 608 609@table @samp 610@item _core_file_failing_command 611Given a core file, this returns the command which was run to produce the 612core file. 613 614@item _core_file_failing_signal 615Given a core file, this returns the signal number which produced the 616core file. 617 618@item _core_file_matches_executable_p 619Given a core file and a BFD for an executable, this returns whether the 620core file was generated by the executable. 621@end table 622 623@node BFD target vector archive 624@subsection Archive functions 625@cindex @samp{BFD_JUMP_TABLE_ARCHIVE} 626 627The @samp{BFD_JUMP_TABLE_ARCHIVE} macro is used for functions which deal 628with archive files. Most targets use COFF style archive files 629(including ELF targets), and these use @samp{_bfd_archive_coff} as the 630argument to @samp{BFD_JUMP_TABLE_ARCHIVE}. Some targets use BSD/a.out 631style archives, and these use @samp{_bfd_archive_bsd}. (The main 632difference between BSD and COFF archives is the format of the archive 633symbol table). Targets with no archive support use 634@samp{_bfd_noarchive}. Finally, a few targets have unusual archive 635handling. 636 637@table @samp 638@item _slurp_armap 639Read in the archive symbol table, storing it in private BFD data. This 640is normally called from the archive @samp{check_format} routine. The 641corresponding field in the target vector is named 642@samp{_bfd_slurp_armap}. 643 644@item _slurp_extended_name_table 645Read in the extended name table from the archive, if there is one, 646storing it in private BFD data. This is normally called from the 647archive @samp{check_format} routine. The corresponding field in the 648target vector is named @samp{_bfd_slurp_extended_name_table}. 649 650@item construct_extended_name_table 651Build and return an extended name table if one is needed to write out 652the archive. This also adjusts the archive headers to refer to the 653extended name table appropriately. This is normally called from the 654archive @samp{write_contents} routine. The corresponding field in the 655target vector is named @samp{_bfd_construct_extended_name_table}. 656 657@item _truncate_arname 658This copies a file name into an archive header, truncating it as 659required. It is normally called from the archive @samp{write_contents} 660routine. This function is more interesting in targets which do not 661support extended name tables, but I think the GNU @samp{ar} program 662always uses extended name tables anyhow. The corresponding field in the 663target vector is named @samp{_bfd_truncate_arname}. 664 665@item _write_armap 666Write out the archive symbol table using calls to @samp{bfd_bwrite}. 667This is normally called from the archive @samp{write_contents} routine. 668The corresponding field in the target vector is named @samp{write_armap} 669(no leading underscore). 670 671@item _read_ar_hdr 672Read and parse an archive header. This handles expanding the archive 673header name into the real file name using the extended name table. This 674is called by routines which read the archive symbol table or the archive 675itself. The corresponding field in the target vector is named 676@samp{_bfd_read_ar_hdr_fn}. 677 678@item _openr_next_archived_file 679Given an archive and a BFD representing a file stored within the 680archive, return a BFD for the next file in the archive. This is called 681via @samp{bfd_openr_next_archived_file}. The corresponding field in the 682target vector is named @samp{openr_next_archived_file} (no leading 683underscore). 684 685@item _get_elt_at_index 686Given an archive and an index, return a BFD for the file in the archive 687corresponding to that entry in the archive symbol table. This is called 688via @samp{bfd_get_elt_at_index}. The corresponding field in the target 689vector is named @samp{_bfd_get_elt_at_index}. 690 691@item _generic_stat_arch_elt 692Do a stat on an element of an archive, returning information read from 693the archive header (modification time, uid, gid, file mode, size). This 694is called via @samp{bfd_stat_arch_elt}. The corresponding field in the 695target vector is named @samp{_bfd_stat_arch_elt}. 696 697@item _update_armap_timestamp 698After the entire contents of an archive have been written out, update 699the timestamp of the archive symbol table to be newer than that of the 700file. This is required for a.out style archives. This is normally 701called by the archive @samp{write_contents} routine. The corresponding 702field in the target vector is named @samp{_bfd_update_armap_timestamp}. 703@end table 704 705@node BFD target vector symbols 706@subsection Symbol table functions 707@cindex @samp{BFD_JUMP_TABLE_SYMBOLS} 708 709The @samp{BFD_JUMP_TABLE_SYMBOLS} macro is used for functions which deal 710with symbols. 711 712@table @samp 713@item _get_symtab_upper_bound 714Return a sensible upper bound on the amount of memory which will be 715required to read the symbol table. In practice most targets return the 716amount of memory required to hold @samp{asymbol} pointers for all the 717symbols plus a trailing @samp{NULL} entry, and store the actual symbol 718information in BFD private data. This is called via 719@samp{bfd_get_symtab_upper_bound}. The corresponding field in the 720target vector is named @samp{_bfd_get_symtab_upper_bound}. 721 722@item _canonicalize_symtab 723Read in the symbol table. This is called via 724@samp{bfd_canonicalize_symtab}. The corresponding field in the target 725vector is named @samp{_bfd_canonicalize_symtab}. 726 727@item _make_empty_symbol 728Create an empty symbol for the BFD. This is needed because most targets 729store extra information with each symbol by allocating a structure 730larger than an @samp{asymbol} and storing the extra information at the 731end. This function will allocate the right amount of memory, and return 732what looks like a pointer to an empty @samp{asymbol}. This is called 733via @samp{bfd_make_empty_symbol}. The corresponding field in the target 734vector is named @samp{_bfd_make_empty_symbol}. 735 736@item _print_symbol 737Print information about the symbol. This is called via 738@samp{bfd_print_symbol}. One of the arguments indicates what sort of 739information should be printed: 740 741@table @samp 742@item bfd_print_symbol_name 743Just print the symbol name. 744@item bfd_print_symbol_more 745Print the symbol name and some interesting flags. I don't think 746anything actually uses this. 747@item bfd_print_symbol_all 748Print all information about the symbol. This is used by @samp{objdump} 749when run with the @samp{-t} option. 750@end table 751The corresponding field in the target vector is named 752@samp{_bfd_print_symbol}. 753 754@item _get_symbol_info 755Return a standard set of information about the symbol. This is called 756via @samp{bfd_symbol_info}. The corresponding field in the target 757vector is named @samp{_bfd_get_symbol_info}. 758 759@item _bfd_is_local_label_name 760Return whether the given string would normally represent the name of a 761local label. This is called via @samp{bfd_is_local_label} and 762@samp{bfd_is_local_label_name}. Local labels are normally discarded by 763the assembler. In the linker, this defines the difference between the 764@samp{-x} and @samp{-X} options. 765 766@item _get_lineno 767Return line number information for a symbol. This is only meaningful 768for a COFF target. This is called when writing out COFF line numbers. 769 770@item _find_nearest_line 771Given an address within a section, use the debugging information to find 772the matching file name, function name, and line number, if any. This is 773called via @samp{bfd_find_nearest_line}. The corresponding field in the 774target vector is named @samp{_bfd_find_nearest_line}. 775 776@item _bfd_make_debug_symbol 777Make a debugging symbol. This is only meaningful for a COFF target, 778where it simply returns a symbol which will be placed in the 779@samp{N_DEBUG} section when it is written out. This is called via 780@samp{bfd_make_debug_symbol}. 781 782@item _read_minisymbols 783Minisymbols are used to reduce the memory requirements of programs like 784@samp{nm}. A minisymbol is a cookie pointing to internal symbol 785information which the caller can use to extract complete symbol 786information. This permits BFD to not convert all the symbols into 787generic form, but to instead convert them one at a time. This is called 788via @samp{bfd_read_minisymbols}. Most targets do not implement this, 789and just use generic support which is based on using standard 790@samp{asymbol} structures. 791 792@item _minisymbol_to_symbol 793Convert a minisymbol to a standard @samp{asymbol}. This is called via 794@samp{bfd_minisymbol_to_symbol}. 795@end table 796 797@node BFD target vector relocs 798@subsection Relocation support 799@cindex @samp{BFD_JUMP_TABLE_RELOCS} 800 801The @samp{BFD_JUMP_TABLE_RELOCS} macro is used for functions which deal 802with relocations. 803 804@table @samp 805@item _get_reloc_upper_bound 806Return a sensible upper bound on the amount of memory which will be 807required to read the relocations for a section. In practice most 808targets return the amount of memory required to hold @samp{arelent} 809pointers for all the relocations plus a trailing @samp{NULL} entry, and 810store the actual relocation information in BFD private data. This is 811called via @samp{bfd_get_reloc_upper_bound}. 812 813@item _canonicalize_reloc 814Return the relocation information for a section. This is called via 815@samp{bfd_canonicalize_reloc}. The corresponding field in the target 816vector is named @samp{_bfd_canonicalize_reloc}. 817 818@item _bfd_reloc_type_lookup 819Given a relocation code, return the corresponding howto structure 820(@pxref{BFD relocation codes}). This is called via 821@samp{bfd_reloc_type_lookup}. The corresponding field in the target 822vector is named @samp{reloc_type_lookup}. 823@end table 824 825@node BFD target vector write 826@subsection Output functions 827@cindex @samp{BFD_JUMP_TABLE_WRITE} 828 829The @samp{BFD_JUMP_TABLE_WRITE} macro is used for functions which deal 830with writing out a BFD. 831 832@table @samp 833@item _set_arch_mach 834Set the architecture and machine number for a BFD. This is called via 835@samp{bfd_set_arch_mach}. Most targets implement this by calling 836@samp{bfd_default_set_arch_mach}. The corresponding field in the target 837vector is named @samp{_bfd_set_arch_mach}. 838 839@item _set_section_contents 840Write out the contents of a section. This is called via 841@samp{bfd_set_section_contents}. The corresponding field in the target 842vector is named @samp{_bfd_set_section_contents}. 843@end table 844 845@node BFD target vector link 846@subsection Linker functions 847@cindex @samp{BFD_JUMP_TABLE_LINK} 848 849The @samp{BFD_JUMP_TABLE_LINK} macro is used for functions called by the 850linker. 851 852@table @samp 853@item _sizeof_headers 854Return the size of the header information required for a BFD. This is 855used to implement the @samp{SIZEOF_HEADERS} linker script function. It 856is normally used to align the first section at an efficient position on 857the page. This is called via @samp{bfd_sizeof_headers}. The 858corresponding field in the target vector is named 859@samp{_bfd_sizeof_headers}. 860 861@item _bfd_get_relocated_section_contents 862Read the contents of a section and apply the relocation information. 863This handles both a final link and a relocatable link; in the latter 864case, it adjust the relocation information as well. This is called via 865@samp{bfd_get_relocated_section_contents}. Most targets implement it by 866calling @samp{bfd_generic_get_relocated_section_contents}. 867 868@item _bfd_relax_section 869Try to use relaxation to shrink the size of a section. This is called 870by the linker when the @samp{-relax} option is used. This is called via 871@samp{bfd_relax_section}. Most targets do not support any sort of 872relaxation. 873 874@item _bfd_link_hash_table_create 875Create the symbol hash table to use for the linker. This linker hook 876permits the backend to control the size and information of the elements 877in the linker symbol hash table. This is called via 878@samp{bfd_link_hash_table_create}. 879 880@item _bfd_link_add_symbols 881Given an object file or an archive, add all symbols into the linker 882symbol hash table. Use callbacks to the linker to include archive 883elements in the link. This is called via @samp{bfd_link_add_symbols}. 884 885@item _bfd_final_link 886Finish the linking process. The linker calls this hook after all of the 887input files have been read, when it is ready to finish the link and 888generate the output file. This is called via @samp{bfd_final_link}. 889 890@item _bfd_link_split_section 891I don't know what this is for. Nothing seems to call it. The only 892non-trivial definition is in @file{som.c}. 893@end table 894 895@node BFD target vector dynamic 896@subsection Dynamic linking information functions 897@cindex @samp{BFD_JUMP_TABLE_DYNAMIC} 898 899The @samp{BFD_JUMP_TABLE_DYNAMIC} macro is used for functions which read 900dynamic linking information. 901 902@table @samp 903@item _get_dynamic_symtab_upper_bound 904Return a sensible upper bound on the amount of memory which will be 905required to read the dynamic symbol table. In practice most targets 906return the amount of memory required to hold @samp{asymbol} pointers for 907all the symbols plus a trailing @samp{NULL} entry, and store the actual 908symbol information in BFD private data. This is called via 909@samp{bfd_get_dynamic_symtab_upper_bound}. The corresponding field in 910the target vector is named @samp{_bfd_get_dynamic_symtab_upper_bound}. 911 912@item _canonicalize_dynamic_symtab 913Read the dynamic symbol table. This is called via 914@samp{bfd_canonicalize_dynamic_symtab}. The corresponding field in the 915target vector is named @samp{_bfd_canonicalize_dynamic_symtab}. 916 917@item _get_dynamic_reloc_upper_bound 918Return a sensible upper bound on the amount of memory which will be 919required to read the dynamic relocations. In practice most targets 920return the amount of memory required to hold @samp{arelent} pointers for 921all the relocations plus a trailing @samp{NULL} entry, and store the 922actual relocation information in BFD private data. This is called via 923@samp{bfd_get_dynamic_reloc_upper_bound}. The corresponding field in 924the target vector is named @samp{_bfd_get_dynamic_reloc_upper_bound}. 925 926@item _canonicalize_dynamic_reloc 927Read the dynamic relocations. This is called via 928@samp{bfd_canonicalize_dynamic_reloc}. The corresponding field in the 929target vector is named @samp{_bfd_canonicalize_dynamic_reloc}. 930@end table 931 932@node BFD generated files 933@section BFD generated files 934@cindex generated files in bfd 935@cindex bfd generated files 936 937BFD contains several automatically generated files. This section 938describes them. Some files are created at configure time, when you 939configure BFD. Some files are created at make time, when you build 940BFD. Some files are automatically rebuilt at make time, but only if 941you configure with the @samp{--enable-maintainer-mode} option. Some 942files live in the object directory---the directory from which you run 943configure---and some live in the source directory. All files that live 944in the source directory are checked into the git repository. 945 946@table @file 947@item bfd.h 948@cindex @file{bfd.h} 949@cindex @file{bfd-in3.h} 950Lives in the object directory. Created at make time from 951@file{bfd-in2.h} via @file{bfd-in3.h}. @file{bfd-in3.h} is created at 952configure time from @file{bfd-in2.h}. There are automatic dependencies 953to rebuild @file{bfd-in3.h} and hence @file{bfd.h} if @file{bfd-in2.h} 954changes, so you can normally ignore @file{bfd-in3.h}, and just think 955about @file{bfd-in2.h} and @file{bfd.h}. 956 957@file{bfd.h} is built by replacing a few strings in @file{bfd-in2.h}. 958To see them, search for @samp{@@} in @file{bfd-in2.h}. They mainly 959control whether BFD is built for a 32 bit target or a 64 bit target. 960 961@item bfd-in2.h 962@cindex @file{bfd-in2.h} 963Lives in the source directory. Created from @file{bfd-in.h} and several 964other BFD source files. If you configure with the 965@samp{--enable-maintainer-mode} option, @file{bfd-in2.h} is rebuilt 966automatically when a source file changes. 967 968@item elf32-target.h 969@itemx elf64-target.h 970@cindex @file{elf32-target.h} 971@cindex @file{elf64-target.h} 972Live in the object directory. Created from @file{elfxx-target.h}. 973These files are versions of @file{elfxx-target.h} customized for either 974a 32 bit ELF target or a 64 bit ELF target. 975 976@item libbfd.h 977@cindex @file{libbfd.h} 978Lives in the source directory. Created from @file{libbfd-in.h} and 979several other BFD source files. If you configure with the 980@samp{--enable-maintainer-mode} option, @file{libbfd.h} is rebuilt 981automatically when a source file changes. 982 983@item libcoff.h 984@cindex @file{libcoff.h} 985Lives in the source directory. Created from @file{libcoff-in.h} and 986@file{coffcode.h}. If you configure with the 987@samp{--enable-maintainer-mode} option, @file{libcoff.h} is rebuilt 988automatically when a source file changes. 989 990@item targmatch.h 991@cindex @file{targmatch.h} 992Lives in the object directory. Created at make time from 993@file{config.bfd}. This file is used to map configuration triplets into 994BFD target vector variable names at run time. 995@end table 996 997@node BFD multiple compilations 998@section Files compiled multiple times in BFD 999Several files in BFD are compiled multiple times. By this I mean that 1000there are header files which contain function definitions. These header 1001files are included by other files, and thus the functions are compiled 1002once per file which includes them. 1003 1004Preprocessor macros are used to control the compilation, so that each 1005time the files are compiled the resulting functions are slightly 1006different. Naturally, if they weren't different, there would be no 1007reason to compile them multiple times. 1008 1009This is a not a particularly good programming technique, and future BFD 1010work should avoid it. 1011 1012@itemize @bullet 1013@item 1014Since this technique is rarely used, even experienced C programmers find 1015it confusing. 1016 1017@item 1018It is difficult to debug programs which use BFD, since there is no way 1019to describe which version of a particular function you are looking at. 1020 1021@item 1022Programs which use BFD wind up incorporating two or more slightly 1023different versions of the same function, which wastes space in the 1024executable. 1025 1026@item 1027This technique is never required nor is it especially efficient. It is 1028always possible to use statically initialized structures holding 1029function pointers and magic constants instead. 1030@end itemize 1031 1032The following is a list of the files which are compiled multiple times. 1033 1034@table @file 1035@item aout-target.h 1036@cindex @file{aout-target.h} 1037Describes a few functions and the target vector for a.out targets. This 1038is used by individual a.out targets with different definitions of 1039@samp{N_TXTADDR} and similar a.out macros. 1040 1041@item aoutf1.h 1042@cindex @file{aoutf1.h} 1043Implements standard SunOS a.out files. In principle it supports 64 bit 1044a.out targets based on the preprocessor macro @samp{ARCH_SIZE}, but 1045since all known a.out targets are 32 bits, this code may or may not 1046work. This file is only included by a few other files, and it is 1047difficult to justify its existence. 1048 1049@item aoutx.h 1050@cindex @file{aoutx.h} 1051Implements basic a.out support routines. This file can be compiled for 1052either 32 or 64 bit support. Since all known a.out targets are 32 bits, 1053the 64 bit support may or may not work. I believe the original 1054intention was that this file would only be included by @samp{aout32.c} 1055and @samp{aout64.c}, and that other a.out targets would simply refer to 1056the functions it defined. Unfortunately, some other a.out targets 1057started including it directly, leading to a somewhat confused state of 1058affairs. 1059 1060@item coffcode.h 1061@cindex @file{coffcode.h} 1062Implements basic COFF support routines. This file is included by every 1063COFF target. It implements code which handles COFF magic numbers as 1064well as various hook functions called by the generic COFF functions in 1065@file{coffgen.c}. This file is controlled by a number of different 1066macros, and more are added regularly. 1067 1068@item coffswap.h 1069@cindex @file{coffswap.h} 1070Implements COFF swapping routines. This file is included by 1071@file{coffcode.h}, and thus by every COFF target. It implements the 1072routines which swap COFF structures between internal and external 1073format. The main control for this file is the external structure 1074definitions in the files in the @file{include/coff} directory. A COFF 1075target file will include one of those files before including 1076@file{coffcode.h} and thus @file{coffswap.h}. There are a few other 1077macros which affect @file{coffswap.h} as well, mostly describing whether 1078certain fields are present in the external structures. 1079 1080@item ecoffswap.h 1081@cindex @file{ecoffswap.h} 1082Implements ECOFF swapping routines. This is like @file{coffswap.h}, but 1083for ECOFF. It is included by the ECOFF target files (of which there are 1084only two). The control is the preprocessor macro @samp{ECOFF_32} or 1085@samp{ECOFF_64}. 1086 1087@item elfcode.h 1088@cindex @file{elfcode.h} 1089Implements ELF functions that use external structure definitions. This 1090file is included by two other files: @file{elf32.c} and @file{elf64.c}. 1091It is controlled by the @samp{ARCH_SIZE} macro which is defined to be 1092@samp{32} or @samp{64} before including it. The @samp{NAME} macro is 1093used internally to give the functions different names for the two target 1094sizes. 1095 1096@item elfcore.h 1097@cindex @file{elfcore.h} 1098Like @file{elfcode.h}, but for functions that are specific to ELF core 1099files. This is included only by @file{elfcode.h}. 1100 1101@item elfxx-target.h 1102@cindex @file{elfxx-target.h} 1103This file is the source for the generated files @file{elf32-target.h} 1104and @file{elf64-target.h}, one of which is included by every ELF target. 1105It defines the ELF target vector. 1106 1107@item freebsd.h 1108@cindex @file{freebsd.h} 1109Presumably intended to be included by all FreeBSD targets, but in fact 1110there is only one such target, @samp{i386-freebsd}. This defines a 1111function used to set the right magic number for FreeBSD, as well as 1112various macros, and includes @file{aout-target.h}. 1113 1114@item netbsd.h 1115@cindex @file{netbsd.h} 1116Like @file{freebsd.h}, except that there are several files which include 1117it. 1118 1119@item nlm-target.h 1120@cindex @file{nlm-target.h} 1121Defines the target vector for a standard NLM target. 1122 1123@item nlmcode.h 1124@cindex @file{nlmcode.h} 1125Like @file{elfcode.h}, but for NLM targets. This is only included by 1126@file{nlm32.c} and @file{nlm64.c}, both of which define the macro 1127@samp{ARCH_SIZE} to an appropriate value. There are no 64 bit NLM 1128targets anyhow, so this is sort of useless. 1129 1130@item nlmswap.h 1131@cindex @file{nlmswap.h} 1132Like @file{coffswap.h}, but for NLM targets. This is included by each 1133NLM target, but I think it winds up compiling to the exact same code for 1134every target, and as such is fairly useless. 1135 1136@item peicode.h 1137@cindex @file{peicode.h} 1138Provides swapping routines and other hooks for PE targets. 1139@file{coffcode.h} will include this rather than @file{coffswap.h} for a 1140PE target. This defines PE specific versions of the COFF swapping 1141routines, and also defines some macros which control @file{coffcode.h} 1142itself. 1143@end table 1144 1145@node BFD relocation handling 1146@section BFD relocation handling 1147@cindex bfd relocation handling 1148@cindex relocations in bfd 1149 1150The handling of relocations is one of the more confusing aspects of BFD. 1151Relocation handling has been implemented in various different ways, all 1152somewhat incompatible, none perfect. 1153 1154@menu 1155* BFD relocation concepts:: BFD relocation concepts 1156* BFD relocation functions:: BFD relocation functions 1157* BFD relocation codes:: BFD relocation codes 1158* BFD relocation future:: BFD relocation future 1159@end menu 1160 1161@node BFD relocation concepts 1162@subsection BFD relocation concepts 1163 1164A relocation is an action which the linker must take when linking. It 1165describes a change to the contents of a section. The change is normally 1166based on the final value of one or more symbols. Relocations are 1167created by the assembler when it creates an object file. 1168 1169Most relocations are simple. A typical simple relocation is to set 32 1170bits at a given offset in a section to the value of a symbol. This type 1171of relocation would be generated for code like @code{int *p = &i;} where 1172@samp{p} and @samp{i} are global variables. A relocation for the symbol 1173@samp{i} would be generated such that the linker would initialize the 1174area of memory which holds the value of @samp{p} to the value of the 1175symbol @samp{i}. 1176 1177Slightly more complex relocations may include an addend, which is a 1178constant to add to the symbol value before using it. In some cases a 1179relocation will require adding the symbol value to the existing contents 1180of the section in the object file. In others the relocation will simply 1181replace the contents of the section with the symbol value. Some 1182relocations are PC relative, so that the value to be stored in the 1183section is the difference between the value of a symbol and the final 1184address of the section contents. 1185 1186In general, relocations can be arbitrarily complex. For example, 1187relocations used in dynamic linking systems often require the linker to 1188allocate space in a different section and use the offset within that 1189section as the value to store. In the IEEE object file format, 1190relocations may involve arbitrary expressions. 1191 1192When doing a relocatable link, the linker may or may not have to do 1193anything with a relocation, depending upon the definition of the 1194relocation. Simple relocations generally do not require any special 1195action. 1196 1197@node BFD relocation functions 1198@subsection BFD relocation functions 1199 1200In BFD, each section has an array of @samp{arelent} structures. Each 1201structure has a pointer to a symbol, an address within the section, an 1202addend, and a pointer to a @samp{reloc_howto_struct} structure. The 1203howto structure has a bunch of fields describing the reloc, including a 1204type field. The type field is specific to the object file format 1205backend; none of the generic code in BFD examines it. 1206 1207Originally, the function @samp{bfd_perform_relocation} was supposed to 1208handle all relocations. In theory, many relocations would be simple 1209enough to be described by the fields in the howto structure. For those 1210that weren't, the howto structure included a @samp{special_function} 1211field to use as an escape. 1212 1213While this seems plausible, a look at @samp{bfd_perform_relocation} 1214shows that it failed. The function has odd special cases. Some of the 1215fields in the howto structure, such as @samp{pcrel_offset}, were not 1216adequately documented. 1217 1218The linker uses @samp{bfd_perform_relocation} to do all relocations when 1219the input and output file have different formats (e.g., when generating 1220S-records). The generic linker code, which is used by all targets which 1221do not define their own special purpose linker, uses 1222@samp{bfd_get_relocated_section_contents}, which for most targets turns 1223into a call to @samp{bfd_generic_get_relocated_section_contents}, which 1224calls @samp{bfd_perform_relocation}. So @samp{bfd_perform_relocation} 1225is still widely used, which makes it difficult to change, since it is 1226difficult to test all possible cases. 1227 1228The assembler used @samp{bfd_perform_relocation} for a while. This 1229turned out to be the wrong thing to do, since 1230@samp{bfd_perform_relocation} was written to handle relocations on an 1231existing object file, while the assembler needed to create relocations 1232in a new object file. The assembler was changed to use the new function 1233@samp{bfd_install_relocation} instead, and @samp{bfd_install_relocation} 1234was created as a copy of @samp{bfd_perform_relocation}. 1235 1236Unfortunately, the work did not progress any farther, so 1237@samp{bfd_install_relocation} remains a simple copy of 1238@samp{bfd_perform_relocation}, with all the odd special cases and 1239confusing code. This again is difficult to change, because again any 1240change can affect any assembler target, and so is difficult to test. 1241 1242The new linker, when using the same object file format for all input 1243files and the output file, does not convert relocations into 1244@samp{arelent} structures, so it can not use 1245@samp{bfd_perform_relocation} at all. Instead, users of the new linker 1246are expected to write a @samp{relocate_section} function which will 1247handle relocations in a target specific fashion. 1248 1249There are two helper functions for target specific relocation: 1250@samp{_bfd_final_link_relocate} and @samp{_bfd_relocate_contents}. 1251These functions use a howto structure, but they @emph{do not} use the 1252@samp{special_function} field. Since the functions are normally called 1253from target specific code, the @samp{special_function} field adds 1254little; any relocations which require special handling can be handled 1255without calling those functions. 1256 1257So, if you want to add a new target, or add a new relocation to an 1258existing target, you need to do the following: 1259 1260@itemize @bullet 1261@item 1262Make sure you clearly understand what the contents of the section should 1263look like after assembly, after a relocatable link, and after a final 1264link. Make sure you clearly understand the operations the linker must 1265perform during a relocatable link and during a final link. 1266 1267@item 1268Write a howto structure for the relocation. The howto structure is 1269flexible enough to represent any relocation which should be handled by 1270setting a contiguous bitfield in the destination to the value of a 1271symbol, possibly with an addend, possibly adding the symbol value to the 1272value already present in the destination. 1273 1274@item 1275Change the assembler to generate your relocation. The assembler will 1276call @samp{bfd_install_relocation}, so your howto structure has to be 1277able to handle that. You may need to set the @samp{special_function} 1278field to handle assembly correctly. Be careful to ensure that any code 1279you write to handle the assembler will also work correctly when doing a 1280relocatable link. For example, see @samp{bfd_elf_generic_reloc}. 1281 1282@item 1283Test the assembler. Consider the cases of relocation against an 1284undefined symbol, a common symbol, a symbol defined in the object file 1285in the same section, and a symbol defined in the object file in a 1286different section. These cases may not all be applicable for your 1287reloc. 1288 1289@item 1290If your target uses the new linker, which is recommended, add any 1291required handling to the target specific relocation function. In simple 1292cases this will just involve a call to @samp{_bfd_final_link_relocate} 1293or @samp{_bfd_relocate_contents}, depending upon the definition of the 1294relocation and whether the link is relocatable or not. 1295 1296@item 1297Test the linker. Test the case of a final link. If the relocation can 1298overflow, use a linker script to force an overflow and make sure the 1299error is reported correctly. Test a relocatable link, whether the 1300symbol is defined or undefined in the relocatable output. For both the 1301final and relocatable link, test the case when the symbol is a common 1302symbol, when the symbol looked like a common symbol but became a defined 1303symbol, when the symbol is defined in a different object file, and when 1304the symbol is defined in the same object file. 1305 1306@item 1307In order for linking to another object file format, such as S-records, 1308to work correctly, @samp{bfd_perform_relocation} has to do the right 1309thing for the relocation. You may need to set the 1310@samp{special_function} field to handle this correctly. Test this by 1311doing a link in which the output object file format is S-records. 1312 1313@item 1314Using the linker to generate relocatable output in a different object 1315file format is impossible in the general case, so you generally don't 1316have to worry about that. The GNU linker makes sure to stop that from 1317happening when an input file in a different format has relocations. 1318 1319Linking input files of different object file formats together is quite 1320unusual, but if you're really dedicated you may want to consider testing 1321this case, both when the output object file format is the same as your 1322format, and when it is different. 1323@end itemize 1324 1325@node BFD relocation codes 1326@subsection BFD relocation codes 1327 1328BFD has another way of describing relocations besides the howto 1329structures described above: the enum @samp{bfd_reloc_code_real_type}. 1330 1331Every known relocation type can be described as a value in this 1332enumeration. The enumeration contains many target specific relocations, 1333but where two or more targets have the same relocation, a single code is 1334used. For example, the single value @samp{BFD_RELOC_32} is used for all 1335simple 32 bit relocation types. 1336 1337The main purpose of this relocation code is to give the assembler some 1338mechanism to create @samp{arelent} structures. In order for the 1339assembler to create an @samp{arelent} structure, it has to be able to 1340obtain a howto structure. The function @samp{bfd_reloc_type_lookup}, 1341which simply calls the target vector entry point 1342@samp{reloc_type_lookup}, takes a relocation code and returns a howto 1343structure. 1344 1345The function @samp{bfd_get_reloc_code_name} returns the name of a 1346relocation code. This is mainly used in error messages. 1347 1348Using both howto structures and relocation codes can be somewhat 1349confusing. There are many processor specific relocation codes. 1350However, the relocation is only fully defined by the howto structure. 1351The same relocation code will map to different howto structures in 1352different object file formats. For example, the addend handling may be 1353different. 1354 1355Most of the relocation codes are not really general. The assembler can 1356not use them without already understanding what sorts of relocations can 1357be used for a particular target. It might be possible to replace the 1358relocation codes with something simpler. 1359 1360@node BFD relocation future 1361@subsection BFD relocation future 1362 1363Clearly the current BFD relocation support is in bad shape. A 1364wholescale rewrite would be very difficult, because it would require 1365thorough testing of every BFD target. So some sort of incremental 1366change is required. 1367 1368My vague thoughts on this would involve defining a new, clearly defined, 1369howto structure. Some mechanism would be used to determine which type 1370of howto structure was being used by a particular format. 1371 1372The new howto structure would clearly define the relocation behaviour in 1373the case of an assembly, a relocatable link, and a final link. At 1374least one special function would be defined as an escape, and it might 1375make sense to define more. 1376 1377One or more generic functions similar to @samp{bfd_perform_relocation} 1378would be written to handle the new howto structure. 1379 1380This should make it possible to write a generic version of the relocate 1381section functions used by the new linker. The target specific code 1382would provide some mechanism (a function pointer or an initial 1383conversion) to convert target specific relocations into howto 1384structures. 1385 1386Ideally it would be possible to use this generic relocate section 1387function for the generic linker as well. That is, it would replace the 1388@samp{bfd_generic_get_relocated_section_contents} function which is 1389currently normally used. 1390 1391For the special case of ELF dynamic linking, more consideration needs to 1392be given to writing ELF specific but ELF target generic code to handle 1393special relocation types such as GOT and PLT. 1394 1395@node BFD ELF support 1396@section BFD ELF support 1397@cindex elf support in bfd 1398@cindex bfd elf support 1399 1400The ELF object file format is defined in two parts: a generic ABI and a 1401processor specific supplement. The ELF support in BFD is split in a 1402similar fashion. The processor specific support is largely kept within 1403a single file. The generic support is provided by several other files. 1404The processor specific support provides a set of function pointers and 1405constants used by the generic support. 1406 1407@menu 1408* BFD ELF sections and segments:: ELF sections and segments 1409* BFD ELF generic support:: BFD ELF generic support 1410* BFD ELF processor specific support:: BFD ELF processor specific support 1411* BFD ELF core files:: BFD ELF core files 1412* BFD ELF future:: BFD ELF future 1413@end menu 1414 1415@node BFD ELF sections and segments 1416@subsection ELF sections and segments 1417 1418The ELF ABI permits a file to have either sections or segments or both. 1419Relocatable object files conventionally have only sections. 1420Executables conventionally have both. Core files conventionally have 1421only program segments. 1422 1423ELF sections are similar to sections in other object file formats: they 1424have a name, a VMA, file contents, flags, and other miscellaneous 1425information. ELF relocations are stored in sections of a particular 1426type; BFD automatically converts these sections into internal relocation 1427information. 1428 1429ELF program segments are intended for fast interpretation by a system 1430loader. They have a type, a VMA, an LMA, file contents, and a couple of 1431other fields. When an ELF executable is run on a Unix system, the 1432system loader will examine the program segments to decide how to load 1433it. The loader will ignore the section information. Loadable program 1434segments (type @samp{PT_LOAD}) are directly loaded into memory. Other 1435program segments are interpreted by the loader, and generally provide 1436dynamic linking information. 1437 1438When an ELF file has both program segments and sections, an ELF program 1439segment may encompass one or more ELF sections, in the sense that the 1440portion of the file which corresponds to the program segment may include 1441the portions of the file corresponding to one or more sections. When 1442there is more than one section in a loadable program segment, the 1443relative positions of the section contents in the file must correspond 1444to the relative positions they should hold when the program segment is 1445loaded. This requirement should be obvious if you consider that the 1446system loader will load an entire program segment at a time. 1447 1448On a system which supports dynamic paging, such as any native Unix 1449system, the contents of a loadable program segment must be at the same 1450offset in the file as in memory, modulo the memory page size used on the 1451system. This is because the system loader will map the file into memory 1452starting at the start of a page. The system loader can easily remap 1453entire pages to the correct load address. However, if the contents of 1454the file were not correctly aligned within the page, the system loader 1455would have to shift the contents around within the page, which is too 1456expensive. For example, if the LMA of a loadable program segment is 1457@samp{0x40080} and the page size is @samp{0x1000}, then the position of 1458the segment contents within the file must equal @samp{0x80} modulo 1459@samp{0x1000}. 1460 1461BFD has only a single set of sections. It does not provide any generic 1462way to examine both sections and segments. When BFD is used to open an 1463object file or executable, the BFD sections will represent ELF sections. 1464When BFD is used to open a core file, the BFD sections will represent 1465ELF program segments. 1466 1467When BFD is used to examine an object file or executable, any program 1468segments will be read to set the LMA of the sections. This is because 1469ELF sections only have a VMA, while ELF program segments have both a VMA 1470and an LMA. Any program segments will be copied by the 1471@samp{copy_private} entry points. They will be printed by the 1472@samp{print_private} entry point. Otherwise, the program segments are 1473ignored. In particular, programs which use BFD currently have no direct 1474access to the program segments. 1475 1476When BFD is used to create an executable, the program segments will be 1477created automatically based on the section information. This is done in 1478the function @samp{assign_file_positions_for_segments} in @file{elf.c}. 1479This function has been tweaked many times, and probably still has 1480problems that arise in particular cases. 1481 1482There is a hook which may be used to explicitly define the program 1483segments when creating an executable: the @samp{bfd_record_phdr} 1484function in @file{bfd.c}. If this function is called, BFD will not 1485create program segments itself, but will only create the program 1486segments specified by the caller. The linker uses this function to 1487implement the @samp{PHDRS} linker script command. 1488 1489@node BFD ELF generic support 1490@subsection BFD ELF generic support 1491 1492In general, functions which do not read external data from the ELF file 1493are found in @file{elf.c}. They operate on the internal forms of the 1494ELF structures, which are defined in @file{include/elf/internal.h}. The 1495internal structures are defined in terms of @samp{bfd_vma}, and so may 1496be used for both 32 bit and 64 bit ELF targets. 1497 1498The file @file{elfcode.h} contains functions which operate on the 1499external data. @file{elfcode.h} is compiled twice, once via 1500@file{elf32.c} with @samp{ARCH_SIZE} defined as @samp{32}, and once via 1501@file{elf64.c} with @samp{ARCH_SIZE} defined as @samp{64}. 1502@file{elfcode.h} includes functions to swap the ELF structures in and 1503out of external form, as well as a few more complex functions. 1504 1505Linker support is found in @file{elflink.c}. The 1506linker support is only used if the processor specific file defines 1507@samp{elf_backend_relocate_section}, which is required to relocate the 1508section contents. If that macro is not defined, the generic linker code 1509is used, and relocations are handled via @samp{bfd_perform_relocation}. 1510 1511The core file support is in @file{elfcore.h}, which is compiled twice, 1512for both 32 and 64 bit support. The more interesting cases of core file 1513support only work on a native system which has the @file{sys/procfs.h} 1514header file. Without that file, the core file support does little more 1515than read the ELF program segments as BFD sections. 1516 1517The BFD internal header file @file{elf-bfd.h} is used for communication 1518among these files and the processor specific files. 1519 1520The default entries for the BFD ELF target vector are found mainly in 1521@file{elf.c}. Some functions are found in @file{elfcode.h}. 1522 1523The processor specific files may override particular entries in the 1524target vector, but most do not, with one exception: the 1525@samp{bfd_reloc_type_lookup} entry point is always processor specific. 1526 1527@node BFD ELF processor specific support 1528@subsection BFD ELF processor specific support 1529 1530By convention, the processor specific support for a particular processor 1531will be found in @file{elf@var{nn}-@var{cpu}.c}, where @var{nn} is 1532either 32 or 64, and @var{cpu} is the name of the processor. 1533 1534@menu 1535* BFD ELF processor required:: Required processor specific support 1536* BFD ELF processor linker:: Processor specific linker support 1537* BFD ELF processor other:: Other processor specific support options 1538@end menu 1539 1540@node BFD ELF processor required 1541@subsubsection Required processor specific support 1542 1543When writing a @file{elf@var{nn}-@var{cpu}.c} file, you must do the 1544following: 1545 1546@itemize @bullet 1547@item 1548Define either @samp{TARGET_BIG_SYM} or @samp{TARGET_LITTLE_SYM}, or 1549both, to a unique C name to use for the target vector. This name should 1550appear in the list of target vectors in @file{targets.c}, and will also 1551have to appear in @file{config.bfd} and @file{configure.ac}. Define 1552@samp{TARGET_BIG_SYM} for a big-endian processor, 1553@samp{TARGET_LITTLE_SYM} for a little-endian processor, and define both 1554for a bi-endian processor. 1555@item 1556Define either @samp{TARGET_BIG_NAME} or @samp{TARGET_LITTLE_NAME}, or 1557both, to a string used as the name of the target vector. This is the 1558name which a user of the BFD tool would use to specify the object file 1559format. It would normally appear in a linker emulation parameters 1560file. 1561@item 1562Define @samp{ELF_ARCH} to the BFD architecture (an element of the 1563@samp{bfd_architecture} enum, typically @samp{bfd_arch_@var{cpu}}). 1564@item 1565Define @samp{ELF_MACHINE_CODE} to the magic number which should appear 1566in the @samp{e_machine} field of the ELF header. As of this writing, 1567these magic numbers are assigned by Caldera; if you want to get a magic 1568number for a particular processor, try sending a note to 1569@email{registry@@caldera.com}. In the BFD sources, the magic numbers are 1570found in @file{include/elf/common.h}; they have names beginning with 1571@samp{EM_}. 1572@item 1573Define @samp{ELF_MAXPAGESIZE} to the maximum size of a virtual page in 1574memory. This can normally be found at the start of chapter 5 in the 1575processor specific supplement. For a processor which will only be used 1576in an embedded system, or which has no memory management hardware, this 1577can simply be @samp{1}. 1578@item 1579If the format should use @samp{Rel} rather than @samp{Rela} relocations, 1580define @samp{USE_REL}. This is normally defined in chapter 4 of the 1581processor specific supplement. 1582 1583In the absence of a supplement, it's easier to work with @samp{Rela} 1584relocations. @samp{Rela} relocations will require more space in object 1585files (but not in executables, except when using dynamic linking). 1586However, this is outweighed by the simplicity of addend handling when 1587using @samp{Rela} relocations. With @samp{Rel} relocations, the addend 1588must be stored in the section contents, which makes relocatable links 1589more complex. 1590 1591For example, consider C code like @code{i = a[1000];} where @samp{a} is 1592a global array. The instructions which load the value of @samp{a[1000]} 1593will most likely use a relocation which refers to the symbol 1594representing @samp{a}, with an addend that gives the offset from the 1595start of @samp{a} to element @samp{1000}. When using @samp{Rel} 1596relocations, that addend must be stored in the instructions themselves. 1597If you are adding support for a RISC chip which uses two or more 1598instructions to load an address, then the addend may not fit in a single 1599instruction, and will have to be somehow split among the instructions. 1600This makes linking awkward, particularly when doing a relocatable link 1601in which the addend may have to be updated. It can be done---the MIPS 1602ELF support does it---but it should be avoided when possible. 1603 1604It is possible, though somewhat awkward, to support both @samp{Rel} and 1605@samp{Rela} relocations for a single target; @file{elf64-mips.c} does it 1606by overriding the relocation reading and writing routines. 1607@item 1608Define howto structures for all the relocation types. 1609@item 1610Define a @samp{bfd_reloc_type_lookup} routine. This must be named 1611@samp{bfd_elf@var{nn}_bfd_reloc_type_lookup}, and may be either a 1612function or a macro. It must translate a BFD relocation code into a 1613howto structure. This is normally a table lookup or a simple switch. 1614@item 1615If using @samp{Rel} relocations, define @samp{elf_info_to_howto_rel}. 1616If using @samp{Rela} relocations, define @samp{elf_info_to_howto}. 1617Either way, this is a macro defined as the name of a function which 1618takes an @samp{arelent} and a @samp{Rel} or @samp{Rela} structure, and 1619sets the @samp{howto} field of the @samp{arelent} based on the 1620@samp{Rel} or @samp{Rela} structure. This is normally uses 1621@samp{ELF@var{nn}_R_TYPE} to get the ELF relocation type and uses it as 1622an index into a table of howto structures. 1623@end itemize 1624 1625You must also add the magic number for this processor to the 1626@samp{prep_headers} function in @file{elf.c}. 1627 1628You must also create a header file in the @file{include/elf} directory 1629called @file{@var{cpu}.h}. This file should define any target specific 1630information which may be needed outside of the BFD code. In particular 1631it should use the @samp{START_RELOC_NUMBERS}, @samp{RELOC_NUMBER}, 1632@samp{FAKE_RELOC}, @samp{EMPTY_RELOC} and @samp{END_RELOC_NUMBERS} 1633macros to create a table mapping the number used to identify a 1634relocation to a name describing that relocation. 1635 1636While not a BFD component, you probably also want to make the binutils 1637program @samp{readelf} parse your ELF objects. For this, you need to add 1638code for @code{EM_@var{cpu}} as appropriate in @file{binutils/readelf.c}. 1639 1640@node BFD ELF processor linker 1641@subsubsection Processor specific linker support 1642 1643The linker will be much more efficient if you define a relocate section 1644function. This will permit BFD to use the ELF specific linker support. 1645 1646If you do not define a relocate section function, BFD must use the 1647generic linker support, which requires converting all symbols and 1648relocations into BFD @samp{asymbol} and @samp{arelent} structures. In 1649this case, relocations will be handled by calling 1650@samp{bfd_perform_relocation}, which will use the howto structures you 1651have defined. @xref{BFD relocation handling}. 1652 1653In order to support linking into a different object file format, such as 1654S-records, @samp{bfd_perform_relocation} must work correctly with your 1655howto structures, so you can't skip that step. However, if you define 1656the relocate section function, then in the normal case of linking into 1657an ELF file the linker will not need to convert symbols and relocations, 1658and will be much more efficient. 1659 1660To use a relocation section function, define the macro 1661@samp{elf_backend_relocate_section} as the name of a function which will 1662take the contents of a section, as well as relocation, symbol, and other 1663information, and modify the section contents according to the relocation 1664information. In simple cases, this is little more than a loop over the 1665relocations which computes the value of each relocation and calls 1666@samp{_bfd_final_link_relocate}. The function must check for a 1667relocatable link, and in that case normally needs to do nothing other 1668than adjust the addend for relocations against a section symbol. 1669 1670The complex cases generally have to do with dynamic linker support. GOT 1671and PLT relocations must be handled specially, and the linker normally 1672arranges to set up the GOT and PLT sections while handling relocations. 1673When generating a shared library, random relocations must normally be 1674copied into the shared library, or converted to RELATIVE relocations 1675when possible. 1676 1677@node BFD ELF processor other 1678@subsubsection Other processor specific support options 1679 1680There are many other macros which may be defined in 1681@file{elf@var{nn}-@var{cpu}.c}. These macros may be found in 1682@file{elfxx-target.h}. 1683 1684Macros may be used to override some of the generic ELF target vector 1685functions. 1686 1687Several processor specific hook functions which may be defined as 1688macros. These functions are found as function pointers in the 1689@samp{elf_backend_data} structure defined in @file{elf-bfd.h}. In 1690general, a hook function is set by defining a macro 1691@samp{elf_backend_@var{name}}. 1692 1693There are a few processor specific constants which may also be defined. 1694These are again found in the @samp{elf_backend_data} structure. 1695 1696I will not define the various functions and constants here; see the 1697comments in @file{elf-bfd.h}. 1698 1699Normally any odd characteristic of a particular ELF processor is handled 1700via a hook function. For example, the special @samp{SHN_MIPS_SCOMMON} 1701section number found in MIPS ELF is handled via the hooks 1702@samp{section_from_bfd_section}, @samp{symbol_processing}, 1703@samp{add_symbol_hook}, and @samp{output_symbol_hook}. 1704 1705Dynamic linking support, which involves processor specific relocations 1706requiring special handling, is also implemented via hook functions. 1707 1708@node BFD ELF core files 1709@subsection BFD ELF core files 1710@cindex elf core files 1711 1712On native ELF Unix systems, core files are generated without any 1713sections. Instead, they only have program segments. 1714 1715When BFD is used to read an ELF core file, the BFD sections will 1716actually represent program segments. Since ELF program segments do not 1717have names, BFD will invent names like @samp{segment@var{n}} where 1718@var{n} is a number. 1719 1720A single ELF program segment may include both an initialized part and an 1721uninitialized part. The size of the initialized part is given by the 1722@samp{p_filesz} field. The total size of the segment is given by the 1723@samp{p_memsz} field. If @samp{p_memsz} is larger than @samp{p_filesz}, 1724then the extra space is uninitialized, or, more precisely, initialized 1725to zero. 1726 1727BFD will represent such a program segment as two different sections. 1728The first, named @samp{segment@var{n}a}, will represent the initialized 1729part of the program segment. The second, named @samp{segment@var{n}b}, 1730will represent the uninitialized part. 1731 1732ELF core files store special information such as register values in 1733program segments with the type @samp{PT_NOTE}. BFD will attempt to 1734interpret the information in these segments, and will create additional 1735sections holding the information. Some of this interpretation requires 1736information found in the host header file @file{sys/procfs.h}, and so 1737will only work when BFD is built on a native system. 1738 1739BFD does not currently provide any way to create an ELF core file. In 1740general, BFD does not provide a way to create core files. The way to 1741implement this would be to write @samp{bfd_set_format} and 1742@samp{bfd_write_contents} routines for the @samp{bfd_core} type; see 1743@ref{BFD target vector format}. 1744 1745@node BFD ELF future 1746@subsection BFD ELF future 1747 1748The current dynamic linking support has too much code duplication. 1749While each processor has particular differences, much of the dynamic 1750linking support is quite similar for each processor. The GOT and PLT 1751are handled in fairly similar ways, the details of -Bsymbolic linking 1752are generally similar, etc. This code should be reworked to use more 1753generic functions, eliminating the duplication. 1754 1755Similarly, the relocation handling has too much duplication. Many of 1756the @samp{reloc_type_lookup} and @samp{info_to_howto} functions are 1757quite similar. The relocate section functions are also often quite 1758similar, both in the standard linker handling and the dynamic linker 1759handling. Many of the COFF processor specific backends share a single 1760relocate section function (@samp{_bfd_coff_generic_relocate_section}), 1761and it should be possible to do something like this for the ELF targets 1762as well. 1763 1764The appearance of the processor specific magic number in 1765@samp{prep_headers} in @file{elf.c} is somewhat bogus. It should be 1766possible to add support for a new processor without changing the generic 1767support. 1768 1769The processor function hooks and constants are ad hoc and need better 1770documentation. 1771 1772@node BFD glossary 1773@section BFD glossary 1774@cindex glossary for bfd 1775@cindex bfd glossary 1776 1777This is a short glossary of some BFD terms. 1778 1779@table @asis 1780@item a.out 1781The a.out object file format. The original Unix object file format. 1782Still used on SunOS, though not Solaris. Supports only three sections. 1783 1784@item archive 1785A collection of object files produced and manipulated by the @samp{ar} 1786program. 1787 1788@item backend 1789The implementation within BFD of a particular object file format. The 1790set of functions which appear in a particular target vector. 1791 1792@item BFD 1793The BFD library itself. Also, each object file, archive, or executable 1794opened by the BFD library has the type @samp{bfd *}, and is sometimes 1795referred to as a bfd. 1796 1797@item COFF 1798The Common Object File Format. Used on Unix SVR3. Used by some 1799embedded targets, although ELF is normally better. 1800 1801@item DLL 1802A shared library on Windows. 1803 1804@item dynamic linker 1805When a program linked against a shared library is run, the dynamic 1806linker will locate the appropriate shared library and arrange to somehow 1807include it in the running image. 1808 1809@item dynamic object 1810Another name for an ELF shared library. 1811 1812@item ECOFF 1813The Extended Common Object File Format. Used on Alpha Digital Unix 1814(formerly OSF/1), as well as Ultrix and Irix 4. A variant of COFF. 1815 1816@item ELF 1817The Executable and Linking Format. The object file format used on most 1818modern Unix systems, including GNU/Linux, Solaris, Irix, and SVR4. Also 1819used on many embedded systems. 1820 1821@item executable 1822A program, with instructions and symbols, and perhaps dynamic linking 1823information. Normally produced by a linker. 1824 1825@item LMA 1826Load Memory Address. This is the address at which a section will be 1827loaded. Compare with VMA, below. 1828 1829@item NLM 1830NetWare Loadable Module. Used to describe the format of an object which 1831be loaded into NetWare, which is some kind of PC based network server 1832program. 1833 1834@item object file 1835A binary file including machine instructions, symbols, and relocation 1836information. Normally produced by an assembler. 1837 1838@item object file format 1839The format of an object file. Typically object files and executables 1840for a particular system are in the same format, although executables 1841will not contain any relocation information. 1842 1843@item PE 1844The Portable Executable format. This is the object file format used for 1845Windows (specifically, Win32) object files. It is based closely on 1846COFF, but has a few significant differences. 1847 1848@item PEI 1849The Portable Executable Image format. This is the object file format 1850used for Windows (specifically, Win32) executables. It is very similar 1851to PE, but includes some additional header information. 1852 1853@item relocations 1854Information used by the linker to adjust section contents. Also called 1855relocs. 1856 1857@item section 1858Object files and executable are composed of sections. Sections have 1859optional data and optional relocation information. 1860 1861@item shared library 1862A library of functions which may be used by many executables without 1863actually being linked into each executable. There are several different 1864implementations of shared libraries, each having slightly different 1865features. 1866 1867@item symbol 1868Each object file and executable may have a list of symbols, often 1869referred to as the symbol table. A symbol is basically a name and an 1870address. There may also be some additional information like the type of 1871symbol, although the type of a symbol is normally something simple like 1872function or object, and should be confused with the more complex C 1873notion of type. Typically every global function and variable in a C 1874program will have an associated symbol. 1875 1876@item target vector 1877A set of functions which implement support for a particular object file 1878format. The @samp{bfd_target} structure. 1879 1880@item Win32 1881The current Windows API, implemented by Windows 95 and later and Windows 1882NT 3.51 and later, but not by Windows 3.1. 1883 1884@item XCOFF 1885The eXtended Common Object File Format. Used on AIX. A variant of 1886COFF, with a completely different symbol table implementation. 1887 1888@item VMA 1889Virtual Memory Address. This is the address a section will have when 1890an executable is run. Compare with LMA, above. 1891@end table 1892 1893@node Index 1894@unnumberedsec Index 1895@printindex cp 1896 1897@contents 1898@bye 1899