| blob | c50285d001200890937ad87e81f891381edc03bd |
1 .\"Copyright (c) 1999 Jeroen Ruigrok van der Werven
2 .\"All rights reserved.
3 .\"
4 .\"Redistribution and use in source and binary forms, with or without
5 .\"modification, are permitted provided that the following conditions
6 .\"are met:
7 .\"1. Redistributions of source code must retain the above copyright
8 .\" notice, this list of conditions and the following disclaimer.
9 .\"2. Redistributions in binary form must reproduce the above copyright
10 .\" notice, this list of conditions and the following disclaimer in the
11 .\" documentation and/or other materials provided with the distribution.
12 .\"
13 .\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
14 .\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 .\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16 .\"ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
17 .\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18 .\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19 .\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20 .\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21 .\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22 .\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
23 .\"SUCH DAMAGE.
24 .\"
25 .\" $FreeBSD: src/share/man/man5/elf.5,v 1.6.2.8 2001/12/17 11:30:13 ru Exp $
26 .\" $DragonFly: src/share/man/man5/elf.5,v 1.8 2007/12/15 12:04:42 swildner Exp $
27 .\"
28 .Dd December 15, 2007
29 .Dt ELF 5
30 .Os
31 .Sh NAME
32 .Nm elf
33 .Nd format of ELF executable binary files
34 .Sh SYNOPSIS
35 .In elf.h
36 .Sh DESCRIPTION
37 The header file
38 .In elf.h
39 defines the format of ELF executable binary files.
40 Amongst these files are
41 normal executable files, relocatable object files, core files and shared
42 libraries.
43 .Pp
44 An executable file using the ELF file format consists of an ELF header,
45 followed by a program header table or a section header table, or both.
46 The ELF header is always at offset zero of the file.
47 The program header
48 table and the section header table's offset in the file are defined in the
49 ELF header.
50 The two tables describe the rest of the particularities of
51 the file.
52 .Pp
53 Applications which wish to process ELF binary files for their native
54 architecture only should include
55 .In elf.h
56 in their source code.
57 These applications should need to refer to
58 all the types and structures by their generic names
59 .Dq Elf_xxx
60 and to the macros by
61 .Dq ELF_xxx .
62 Applications written this way can be compiled on any architecture,
63 regardless whether the host is 32-bit or 64-bit.
64 .Pp
65 Should an application need to process ELF files of an unknown
66 architecture then the application needs to include both
67 .In sys/elf32.h
68 and
69 .In sys/elf64.h
70 instead of
71 .In elf.h .
72 Furthermore, all types and structures need to be identified by either
73 .Dq Elf32_xxx
74 or
75 .Dq Elf64_xxx .
76 The macros need to be identified by
77 .Dq ELF32_xxx
78 or
79 .Dq ELF64_xxx .
80 .Pp
81 Whatever the system's architecture is, it will always include
82 .In sys/elf_common.h
83 as well as
84 .In sys/elf_generic.h .
85 .Pp
86 These header files describe the above mentioned headers as C structures
87 and also include structures for dynamic sections, relocation sections and
88 symbol tables.
89 .Pp
90 The following types are being used for 32-bit architectures:
91 .Bd -literal -offset indent
92 Elf32_Addr Unsigned 32-bit program address
93 Elf32_Half Unsigned 16-bit field
94 Elf32_Off Unsigned 32-bit file offset
95 Elf32_Sword Signed 32-bit field or integer
96 Elf32_Word Unsigned 32-bit field or integer
97 Elf32_Size Unsigned object size
98 .Ed
99 .Pp
100 For 64-bit architectures we have the following types:
101 .Bd -literal -offset indent
102 Elf64_Addr Unsigned 64-bit program address
103 Elf64_Half Unsigned 16-bit field
104 Elf64_Lword Unsigned 64-bit field
105 Elf64_Off Unsigned 64-bit file offset
106 Elf64_Sword Signed 32-bit field
107 Elf64_Sxword Signed 64-bit field or integer
108 Elf64_Word Unsigned 32-bit field
109 Elf64_Xword Unsigned 64-bit field or integer
110 Elf64_Size Unsigned object size
111 Elf64_Quarter Unsigned quarterword field
112 .Ed
113 .Pp
114 All data structures that the file format defines follow the
115 .Dq natural
116 size and alignment guidelines for the relevant class.
117 If necessary,
118 data structures contain explicit padding to ensure 4-byte alignment
119 for 4-byte objects, to force structure sizes to a multiple of 4, etc.
120 .Pp
121 The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:
122 .Bd -literal -offset indent
123 typedef struct {
124 unsigned char e_ident[EI_NIDENT];
125 Elf32_Half e_type;
126 Elf32_Half e_machine;
127 Elf32_Word e_version;
128 Elf32_Addr e_entry;
129 Elf32_Off e_phoff;
130 Elf32_Off e_shoff;
131 Elf32_Word e_flags;
132 Elf32_Half e_ehsize;
133 Elf32_Half e_phentsize;
134 Elf32_Half e_phnum;
135 Elf32_Half e_shentsize;
136 Elf32_Half e_shnum;
137 Elf32_Half e_shstrndx;
138 } Elf32_Ehdr;
139 .Ed
140 .Pp
141 .Bd -literal -offset indent
142 typedef struct {
143 unsigned char e_ident[EI_NIDENT];
144 Elf64_Quarter e_type;
145 Elf64_Quarter e_machine;
146 Elf64_Half e_version;
147 Elf64_Addr e_entry;
148 Elf64_Off e_phoff;
149 Elf64_Off e_shoff;
150 Elf64_Half e_flags;
151 Elf64_Quarter e_ehsize;
152 Elf64_Quarter e_phentsize;
153 Elf64_Quarter e_phnum;
154 Elf64_Quarter e_shentsize;
155 Elf64_Quarter e_shnum;
156 Elf64_Quarter e_shstrndx;
157 } Elf64_Ehdr;
158 .Ed
159 .Pp
160 The fields have the following meanings:
161 .Pp
162 .Bl -tag -width ".Fa e_phentsize" -compact -offset indent
163 .It Fa e_ident
164 This array of bytes specifies to interpret the file,
165 independent of the processor or the file's remaining contents.
166 Within this array everything is named by macros, which start with
167 the prefix
168 .Sy EI_
169 and may contain values which start with the prefix
170 .Sy ELF .
171 The following macros are defined:
172 .Pp
173 .Bl -tag -width ".Dv EI_ABIVERSION" -compact
174 .It Dv EI_MAG0
175 The first byte of the magic number.
176 It must be filled with
177 .Sy ELFMAG0 .
178 .It Dv EI_MAG1
179 The second byte of the magic number.
180 It must be filled with
181 .Sy ELFMAG1 .
182 .It Dv EI_MAG2
183 The third byte of the magic number.
184 It must be filled with
185 .Sy ELFMAG2 .
186 .It Dv EI_MAG3
187 The fourth byte of the magic number.
188 It must be filled with
189 .Sy ELFMAG3 .
190 .It Dv EI_CLASS
191 The fifth byte identifies the architecture for this binary:
192 .Pp
193 .Bl -tag -width ".Dv ELFCLASSNONE" -compact
194 .It Dv ELFCLASSNONE
195 This class is invalid.
196 .It Dv ELFCLASS32
197 This defines the 32-bit architecture.
198 It supports machines with files
199 and virtual address spaces up to 4 Gigabytes.
200 .It Dv ELFCLASS64
201 This defines the 64-bit architecture.
202 .El
203 .It Dv EI_DATA
204 The sixth byte specifies the data encoding of the processor-specific
205 data in the file.
206 Currently these encodings are supported:
207 .Pp
208 .Bl -tag -width ".Dv ELFDATA2LSB" -compact
209 .It Dv ELFDATANONE
210 Unknown data format.
211 .It Dv ELFDATA2LSB
212 Two's complement, little-endian.
213 .It Dv ELFDATA2MSB
214 Two's complement, big-endian.
215 .El
216 .It Dv EI_VERSION
217 The version number of the ELF specification:
218 .Pp
219 .Bl -tag -width ".Dv EV_CURRENT" -compact
220 .It Dv EV_NONE
221 Invalid version.
222 .It Dv EV_CURRENT
223 Current version.
224 .El
225 .It Dv EI_OSABI
226 This byte identifies the operating system
227 and ABI to which the object is targeted.
228 Some fields in other ELF structures have flags
229 and values that have platform specific meanings;
230 the interpretation of those fields is determined by the value of this byte.
231 The following values are currently defined:
232 .Pp
233 .Bl -tag -width ".Dv ELFOSABI_STANDALONE" -compact
234 .It Dv ELFOSABI_SYSV
235 .At V
236 ABI.
237 .It Dv ELFOSABI_HPUX
238 HP-UX operating system ABI.
239 .It Dv ELFOSABI_NETBSD
240 .Nx
241 operating system ABI.
242 .It Dv ELFOSABI_LINUX
243 GNU/Linux operating system ABI.
244 .It Dv ELFOSABI_HURD
245 GNU/Hurd operating system ABI.
246 .It Dv ELFOSABI_86OPEN
247 86Open Common IA32 ABI.
248 .It Dv ELFOSABI_SOLARIS
249 Solaris operating system ABI.
250 .It Dv ELFOSABI_MONTEREY
251 Monterey project ABI.
252 .It Dv ELFOSABI_IRIX
253 IRIX operating system ABI.
254 .It Dv ELFOSABI_FREEBSD
255 .Fx
256 operating system ABI.
257 .It Dv ELFOSABI_TRU64
258 TRU64
259 .Ux
260 operating system ABI.
261 .It Dv ELFOSABI_ARM
262 ARM architecture ABI.
263 .It Dv ELFOSABI_STANDALONE
264 Standalone (embedded) ABI.
265 .El
266 .It Dv EI_ABIVERSION
267 This byte identifies the version of the ABI
268 to which the object is targeted.
269 This field is used to distinguish among incompatible versions of an ABI.
270 The interpretation of this version number
271 is dependent on the ABI identified by the
272 .Dv EI_OSABI
273 field.
274 Applications conforming to this specification use the value 0.
275 .It Dv EI_PAD
276 Start of padding.
277 These bytes are reserved and set to zero.
278 Programs
279 which read them should ignore them.
280 The value for EI_PAD will change in
281 the future if currently unused bytes are given meanings.
282 .It Dv EI_NIDENT
283 The size of the
284 .Fa e_ident
285 array.
286 .El
287 .Pp
288 .It Fa e_type
289 This member of the structure identifies the object file type:
290 .Pp
291 .Bl -tag -width ".Dv ET_NONE" -compact
292 .It Dv ET_NONE
293 An unknown type.
294 .It Dv ET_REL
295 A relocatable file.
296 .It Dv ET_EXEC
297 An executable file.
298 .It Dv ET_DYN
299 A shared object.
300 .It Dv ET_CORE
301 A core file.
302 .El
303 .Pp
304 .It Fa e_machine
305 This member specifies the required architecture for an individual file:
306 .Pp
307 .Bl -tag -width ".Dv EM_MIPS_RS4_BE" -compact
308 .It Dv EM_NONE
309 An unknown machine.
310 .It Dv EM_M32
311 AT&T WE 32100.
312 .It Dv EM_SPARC
313 Sun Microsystems SPARC.
314 .It Dv EM_386
315 Intel 80386.
316 .It Dv EM_68K
317 Motorola 68000.
318 .It Dv EM_88K
319 Motorola 88000.
320 .It Dv EM_860
321 Intel 80860.
322 .It Dv EM_MIPS
323 MIPS RS3000 (big-endian only).
324 .It Dv EM_MIPS_RS4_BE
325 MIPS RS4000 (big-endian only).
326 .It Dv EM_SPARC64
327 SPARC v9 64-bit unofficial.
328 .It Dv EM_PARISC
329 HPPA.
330 .It Dv EM_PPC
331 PowerPC.
332 .It Dv EM_ALPHA
333 Compaq [DEC] Alpha.
334 .El
335 .Pp
336 .It Fa e_version
337 This member identifies the file version:
338 .Pp
339 .Bl -tag -width ".Dv EV_CURRENT" -compact
340 .It Dv EV_NONE
341 Invalid version
342 .It Dv EV_CURRENT
343 Current version
344 .El
345 .It Fa e_entry
346 This member gives the virtual address to which the system first transfers
347 control, thus starting the process.
348 If the file has no associated entry
349 point, this member holds zero.
350 .It Fa e_phoff
351 This member holds the program header table's file offset in bytes.
352 If
353 the file has no program header table, this member holds zero.
354 .It Fa e_shoff
355 This member holds the section header table's file offset in bytes.
356 If the
357 file has no section header table this member holds zero.
358 .It Fa e_flags
359 This member holds processor-specific flags associated with the file.
360 Flag names take the form EF_`machine_flag'.
361 Currently no flags have been defined.
362 .It Fa e_ehsize
363 This member holds the ELF header's size in bytes.
364 .It Fa e_phentsize
365 This member holds the size in bytes of one entry in the file's program header
366 table; all entries are the same size.
367 .It Fa e_phnum
368 This member holds the number of entries in the program header
369 table.
370 Thus the product of
371 .Fa e_phentsize
372 and
373 .Fa e_phnum
374 gives the table's size
375 in bytes.
376 If a file has no program header,
377 .Fa e_phnum
378 holds the value zero.
379 .It Fa e_shentsize
380 This member holds a sections header's size in bytes.
381 A section header is one
382 entry in the section header table; all entries are the same size.
383 .It Fa e_shnum
384 This member holds the number of entries in the section header table.
385 Thus
386 the product of
387 .Fa e_shentsize
388 and
389 .Fa e_shnum
390 gives the section header table's size in bytes.
391 If a file has no section
392 header table,
393 .Fa e_shnum
394 holds the value of zero.
395 .It Fa e_shstrndx
396 This member holds the section header table index of the entry associated
397 with the section name string table.
398 If the file has no section name string
399 table, this member holds the value
400 .Dv SHN_UNDEF .
401 .Pp
402 .Bl -tag -width ".Dv SHN_LORESERVE" -compact
403 .It Dv SHN_UNDEF
404 This value marks an undefined, missing, irrelevant, or otherwise meaningless
405 section reference.
406 For example, a symbol
407 .Dq defined
408 relative to section number
409 .Dv SHN_UNDEF
410 is an undefined symbol.
411 .It Dv SHN_LORESERVE
412 This value specifies the lower bound of the range of reserved indexes.
413 .It Dv SHN_LOPROC
414 This value up to and including
415 .Dv SHN_HIPROC
416 are reserved for processor-specific semantics.
417 .It Dv SHN_HIPROC
418 This value down to and including
419 .Dv SHN_LOPROC
420 are reserved for processor-specific semantics.
421 .It Dv SHN_ABS
422 This value specifies absolute values for the corresponding reference.
423 For
424 example, symbols defined relative to section number
425 .Dv SHN_ABS
426 have absolute values and are not affected by relocation.
427 .It Dv SHN_COMMON
428 Symbols defined relative to this section are common symbols, such as Fortran
429 COMMON or unallocated C external variables.
430 .It Dv SHN_HIRESERVE
431 This value specifies the upper bound of the range of the range of reserved
432 indices between
433 .Dv SHN_LORESERVE
434 and
435 .Dv SHN_HIRESERVE ,
436 inclusive; the values do
437 not reference the section header table.
438 That is, the section header table
439 does
440 .Em not
441 contain entries for the reserved indices.
442 .El
443 .El
444 .Pp
445 An executable or shared object file's program header table is an array of
446 structures, each describing a segment or other information the system needs
447 to prepare the program for execution.
448 An object file
449 .Em segment
450 contains one or more
451 .Em sections .
452 Program headers are meaningful only for executable and shared object files.
453 A file specifies its own program header size with the ELF header's
454 .Fa e_phentsize
455 and
456 .Fa e_phnum
457 members.
458 As with the Elf executable header, the program header
459 also has different versions depending on the architecture:
460 .Pp
461 .Bd -literal -offset indent
462 typedef struct {
463 Elf32_Word p_type;
464 Elf32_Off p_offset;
465 Elf32_Addr p_vaddr;
466 Elf32_Addr p_paddr;
467 Elf32_Size p_filesz;
468 Elf32_Size p_memsz;
469 Elf32_Word p_flags;
470 Elf32_Size p_align;
471 } Elf32_Phdr;
472 .Ed
473 .Pp
474 .Bd -literal -offset indent
475 typedef struct {
476 Elf64_Half p_type;
477 Elf64_Half p_flags;
478 Elf64_Off p_offset;
479 Elf64_Addr p_vaddr;
480 Elf64_Addr p_paddr;
481 Elf64_Size p_filesz;
482 Elf64_Size p_memsz;
483 Elf64_Size p_align;
484 } Elf64_Phdr;
485 .Ed
486 .Pp
487 The main difference between the 32-bit and the 64-bit program header lies
488 only in the location of a
489 .Fa p_flags
490 member in the total struct.
491 .Pp
492 .Bl -tag -width ".Fa p_offset" -compact -offset indent
493 .It Fa p_type
494 This member of the
495 .Vt Phdr
496 struct tells what kind of segment this array
497 element describes or how to interpret the array element's information.
498 .Bl -tag -width ".Dv PT_DYNAMIC" -compact
499 .Pp
500 .It Dv PT_NULL
501 The array element is unused and the other members' values are undefined.
502 This lets the program header have ignored entries.
503 .It Dv PT_LOAD
504 The array element specifies a loadable segment, described by
505 .Fa p_filesz
506 and
507 .Fa p_memsz .
508 The bytes from the file are mapped to the beginning of the memory
509 segment.
510 If the segment's memory size
511 .Pq Fa p_memsz
512 is larger than the file size
513 .Pq Fa p_filesz ,
514 the
515 .Dq extra
516 bytes are defined to hold the value 0 and to follow the segment's
517 initialized area.
518 The file size may not be larger than the memory size.
519 Loadable segment entries in the program header table appear in ascending
520 order, sorted on the
521 .Fa p_vaddr
522 member.
523 .It Dv PT_DYNAMIC
524 The array element specifies dynamic linking information.
525 .It Dv PT_INTERP
526 The array element specifies the location and size of a null-terminated
527 path name to invoke as an interpreter.
528 This segment type is meaningful
529 only for executable files (though it may occur for shared objects).
530 However, it may not occur more than once in a file.
531 If it is present it must precede
532 any loadable segment entry.
533 .It Dv PT_NOTE
534 The array element specifies the location and size for auxiliary information.
535 .It Dv PT_SHLIB
536 This segment type is reserved but has unspecified semantics.
537 Programs that
538 contain an array element of this type do not conform to the ABI.
539 .It Dv PT_PHDR
540 The array element, if present, specifies the location and size of the program
541 header table itself, both in the file and in the memory image of the program.
542 This segment type may not occur more than once in a file.
543 Moreover, it may
544 only occur if the program header table is part of the memory image of the
545 program.
546 If it is present it must precede any loadable segment entry.
547 .It Dv PT_LOPROC
548 This value up to and including
549 .Dv PT_HIPROC
550 are reserved for processor-specific semantics.
551 .It Dv PT_HIPROC
552 This value down to and including
553 .Dv PT_LOPROC
554 are reserved for processor-specific semantics.
555 .El
556 .Pp
557 .It Fa p_offset
558 This member holds the offset from the beginning of the file at which
559 the first byte of the segment resides.
560 .It Fa p_vaddr
561 This member holds the virtual address at which the first byte of the
562 segment resides in memory.
563 .It Fa p_paddr
564 On systems for which physical addressing is relevant, this member is
565 reserved for the segment's physical address.
566 Under
567 .Bx
568 this member is
569 not used and must be zero.
570 .It Fa p_filesz
571 This member holds the number of bytes in the file image of the segment.
572 It may be zero.
573 .It Fa p_memsz
574 This member holds the number of bytes in the memory image of the segment.
575 It may be zero.
576 .It Fa p_flags
577 This member holds flags relevant to the segment:
578 .Pp
579 .Bl -tag -width ".Dv PF_X" -compact
580 .It Dv PF_X
581 An executable segment.
582 .It Dv PF_W
583 A writable segment.
584 .It Dv PF_R
585 A readable segment.
586 .El
587 .Pp
588 A text segment commonly has the flags
589 .Dv PF_X
590 and
591 .Dv PF_R .
592 A data segment commonly has
593 .Dv PF_X ,
594 .Dv PF_W
595 and
596 .Dv PF_R .
597 .It Fa p_align
598 This member holds the value to which the segments are aligned in memory
599 and in the file.
600 Loadable process segments must have congruent values for
601 .Fa p_vaddr
602 and
603 .Fa p_offset ,
604 modulo the page size.
605 Values of zero and one mean no alignment is required.
606 Otherwise,
607 .Fa p_align
608 should be a positive, integral power of two, and
609 .Fa p_vaddr
610 should equal
611 .Fa p_offset ,
612 modulo
613 .Fa p_align .
614 .El
615 .Pp
616 An file's section header table lets one locate all the file's sections.
617 The
618 section header table is an array of
619 .Vt Elf32_Shdr
620 or
621 .Vt Elf64_Shdr
622 structures.
623 The ELF header's
624 .Fa e_shoff
625 member gives the byte offset from the beginning of the file to the section
626 header table.
627 .Fa e_shnum
628 holds the number of entries the section header table contains.
629 .Fa e_shentsize
630 holds the size in bytes of each entry.
631 .Pp
632 A section header table index is a subscript into this array.
633 Some section
634 header table indices are reserved.
635 An object file does not have sections for
636 these special indices:
637 .Pp
638 .Bl -tag -width ".Dv SHN_LORESERVE" -compact
639 .It Dv SHN_UNDEF
640 This value marks an undefined, missing, irrelevant, or otherwise meaningless
641 section reference.
642 .It Dv SHN_LORESERVE
643 This value specifies the lower bound of the range of reserved indices.
644 .It Dv SHN_LOPROC
645 This value up to and including
646 .Dv SHN_HIPROC
647 are reserved for processor-specific semantics.
648 .It Dv SHN_HIPROC
649 This value down to and including
650 .Dv SHN_LOPROC
651 are reserved for processor-specific semantics.
652 .It Dv SHN_ABS
653 This value specifies absolute values for the corresponding reference.
654 For
655 example, symbols defined relative to section number
656 .Dv SHN_ABS
657 have absolute values and are not affected by relocation.
658 .It Dv SHN_COMMON
659 Symbols defined relative to this section are common symbols, such as FORTRAN
660 COMMON or unallocated C external variables.
661 .It Dv SHN_HIRESERVE
662 This value specifies the upper bound of the range of reserved indices.
663 The
664 system reserves indices between
665 .Dv SHN_LORESERVE
666 and
667 .Dv SHN_HIRESERVE ,
668 inclusive.
669 The section header table does not contain entries for the
670 reserved indices.
671 .El
672 .Pp
673 The section header has the following structure:
674 .Bd -literal -offset indent
675 typedef struct {
676 Elf32_Word sh_name;
677 Elf32_Word sh_type;
678 Elf32_Word sh_flags;
679 Elf32_Addr sh_addr;
680 Elf32_Off sh_offset;
681 Elf32_Size sh_size;
682 Elf32_Word sh_link;
683 Elf32_Word sh_info;
684 Elf32_Size sh_addralign;
685 Elf32_Size sh_entsize;
686 } Elf32_Shdr;
687 .Ed
688 .Pp
689 .Bd -literal -offset indent
690 typedef struct {
691 Elf64_Half sh_name;
692 Elf64_Half sh_type;
693 Elf64_Size sh_flags;
694 Elf64_Addr sh_addr;
695 Elf64_Off sh_offset;
696 Elf64_Size sh_size;
697 Elf64_Half sh_link;
698 Elf64_Half sh_info;
699 Elf64_Size sh_addralign;
700 Elf64_Size sh_entsize;
701 } Elf64_Shdr;
702 .Ed
703 .Pp
704 .Bl -tag -width ".Fa sh_addralign" -compact
705 .It Fa sh_name
706 This member specifies the name of the section.
707 Its value is an index
708 into the section header string table section, giving the location of
709 a null-terminated string.
710 .It Fa sh_type
711 This member categorizes the section's contents and semantics.
712 .Pp
713 .Bl -tag -width ".Dv SHT_PROGBITS" -compact
714 .It Dv SHT_NULL
715 This value marks the section header as inactive.
716 It does not
717 have an associated section.
718 Other members of the section header
719 have undefined values.
720 .It Dv SHT_PROGBITS
721 The section holds information defined by the program, whose
722 format and meaning are determined solely by the program.
723 .It Dv SHT_SYMTAB
724 This section holds a symbol table.
725 Typically,
726 .Dv SHT_SYMTAB
727 provides symbols for link editing, though it may also be used
728 for dynamic linking.
729 As a complete symbol table, it may contain
730 many symbols unnecessary for dynamic linking.
731 An object file can
732 also contain a
733 .Dv SHN_DYNSYM
734 section.
735 .It Dv SHT_STRTAB
736 This section holds a string table.
737 An object file may have multiple
738 string table sections.
739 .It Dv SHT_RELA
740 This section holds relocation entries with explicit addends, such
741 as type
742 .Vt Elf32_Rela
743 for the 32-bit class of object files.
744 An object may have multiple
745 relocation sections.
746 .It Dv SHT_HASH
747 This section holds a symbol hash table.
748 All object participating in
749 dynamic linking must contain a symbol hash table.
750 An object file may
751 have only one hash table.
752 .It Dv SHT_DYNAMIC
753 This section holds information for dynamic linking.
754 An object file may
755 have only one dynamic section.
756 .It Dv SHT_NOTE
757 This section holds information that marks the file in some way.
758 .It Dv SHT_NOBITS
759 A section of this type occupies no space in the file but otherwise
760 resembles
761 .Dv SHN_PROGBITS .
762 Although this section contains no bytes, the
763 .Fa sh_offset
764 member contains the conceptual file offset.
765 .It Dv SHT_REL
766 This section holds relocation offsets without explicit addends, such
767 as type
768 .Vt Elf32_Rel
769 for the 32-bit class of object files.
770 An object file may have multiple
771 relocation sections.
772 .It Dv SHT_SHLIB
773 This section is reserved but has unspecified semantics.
774 .It Dv SHT_DYNSYM
775 This section holds a minimal set of dynamic linking symbols.
776 An
777 object file can also contain a
778 .Dv SHN_SYMTAB
779 section.
780 .It Dv SHT_LOPROC
781 This value up to and including
782 .Dv SHT_HIPROC
783 are reserved for processor-specific semantics.
784 .It Dv SHT_HIPROC
785 This value down to and including
786 .Dv SHT_LOPROC
787 are reserved for processor-specific semantics.
788 .It Dv SHT_LOUSER
789 This value specifies the lower bound of the range of indices reserved for
790 application programs.
791 .It Dv SHT_HIUSER
792 This value specifies the upper bound of the range of indices reserved for
793 application programs.
794 Section types between
795 .Dv SHT_LOUSER
796 and
797 .Dv SHT_HIUSER
798 may be used by the application, without conflicting with current or future
799 system-defined section types.
800 .El
801 .Pp
802 .It Fa sh_flags
803 Sections support one-bit flags that describe miscellaneous attributes.
804 If a flag bit is set in
805 .Fa sh_flags ,
806 the attribute is
807 .Dq on
808 for the section.
809 Otherwise, the attribute is
810 .Dq off
811 or does not apply.
812 Undefined attributes are set to zero.
813 .Pp
814 .Bl -tag -width ".Dv SHF_EXECINSTR" -compact
815 .It Dv SHF_WRITE
816 This section contains data that should be writable during process
817 execution.
818 .It Dv SHF_ALLOC
819 The section occupies memory during process execution.
820 Some control
821 sections do not reside in the memory image of an object file.
822 This
823 attribute is off for those sections.
824 .It Dv SHF_EXECINSTR
825 The section contains executable machine instructions.
826 .It Dv SHF_MASKPROC
827 All bits included in this mask are reserved for processor-specific
828 semantics.
829 .El
830 .Pp
831 .It Fa sh_addr
832 If the section will appear in the memory image of a process, this member
833 holds the address at which the section's first byte should reside.
834 Otherwise, the member contains zero.
835 .It Fa sh_offset
836 This member's value holds the byte offset from the beginning of the file
837 to the first byte in the section.
838 One section type,
839 .Dv SHT_NOBITS ,
840 occupies no space in the file, and its
841 .Fa sh_offset
842 member locates the conceptual placement in the file.
843 .It Fa sh_size
844 This member holds the section's size in bytes.
845 Unless the section type
846 is
847 .Dv SHT_NOBITS ,
848 the section occupies
849 .Fa sh_size
850 bytes in the file.
851 A section of type
852 .Dv SHT_NOBITS
853 may have a non-zero size, but it occupies no space in the file.
854 .It Fa sh_link
855 This member holds a section header table index link, whose interpretation
856 depends on the section type.
857 .It Fa sh_info
858 This member holds extra information, whose interpretation depends on the
859 section type.
860 .It Fa sh_addralign
861 Some sections have address alignment constraints.
862 If a section holds a
863 doubleword, the system must ensure doubleword alignment for the entire
864 section.
865 That is, the value of
866 .Fa sh_addr
867 must be congruent to zero, modulo the value of
868 .Fa sh_addralign .
869 Only zero and positive integral powers of two are allowed.
870 Values of zero
871 or one mean the section has no alignment constraints.
872 .It Fa sh_entsize
873 Some sections hold a table of fixed-sized entries, such as a symbol table.
874 For such a section, this member gives the size in bytes for each entry.
875 This member contains zero if the section does not hold a table of
876 fixed-size entries.
877 .El
878 .Pp
879 Various sections hold program and control information:
880 .Bl -tag -width ".Sy .shstrtab" -compact
881 .It Sy .bss
882 (Block Started by Symbol)
883 This section holds uninitialized data that contributes to the program's
884 memory image.
885 By definition, the system initializes the data with zeros
886 when the program begins to run.
887 This section is of type
888 .Dv SHT_NOBITS .
889 The attributes types are
890 .Dv SHF_ALLOC
891 and
892 .Dv SHF_WRITE .
893 .It Sy .comment
894 This section holds version control information.
895 This section is of type
896 .Dv SHT_PROGBITS .
897 No attribute types are used.
898 .It Sy .data
899 This section holds initialized data that contribute to the program's
900 memory image.
901 This section is of type
902 .Dv SHT_PROGBITS .
903 The attribute types are
904 .Dv SHF_ALLOC
905 and
906 .Dv SHF_WRITE .
907 .It Sy .data1
908 This section holds initialized data that contribute to the program's
909 memory image.
910 This section is of type
911 .Dv SHT_PROGBITS .
912 The attribute types are
913 .Dv SHF_ALLOC
914 and
915 .Dv SHF_WRITE .
916 .It Sy .debug
917 This section holds information for symbolic debugging.
918 The contents
919 are unspecified.
920 This section is of type
921 .Dv SHT_PROGBITS .
922 No attribute types are used.
923 .It Sy .dynamic
924 This section holds dynamic linking information.
925 The section's attributes
926 will include the
927 .Dv SHF_ALLOC
928 bit.
929 Whether the
930 .Dv SHF_WRITE
931 bit is set is processor-specific.
932 This section is of type
933 .Dv SHT_DYNAMIC .
934 See the attributes above.
935 .It Sy .dynstr
936 This section holds strings needed for dynamic linking, most commonly
937 the strings that represent the names associated with symbol table entries.
938 This section is of type
939 .Dv SHT_STRTAB .
940 The attribute type used is
941 .Dv SHF_ALLOC .
942 .It Sy .dynsym
943 This section holds the dynamic linking symbol table.
944 This section is of type
945 .Dv SHT_DYNSYM .
946 The attribute used is
947 .Dv SHF_ALLOC .
948 .It Sy .fini
949 This section holds executable instructions that contribute to the process
950 termination code.
951 When a program exits normally the system arranges to
952 execute the code in this section.
953 This section is of type
954 .Dv SHT_PROGBITS .
955 The attributes used are
956 .Dv SHF_ALLOC
957 and
958 .Dv SHF_EXECINSTR .
959 .It Sy .got
960 This section holds the global offset table.
961 This section is of type
962 .Dv SHT_PROGBITS .
963 The attributes are processor-specific.
964 .It Sy .hash
965 This section holds a symbol hash table.
966 This section is of type
967 .Dv SHT_HASH .
968 The attribute used is
969 .Dv SHF_ALLOC .
970 .It Sy .init
971 This section holds executable instructions that contribute to the process
972 initialization code.
973 When a program starts to run the system arranges to
974 execute the code in this section before calling the main program entry point.
975 This section is of type
976 .Dv SHT_PROGBITS .
977 The attributes used are
978 .Dv SHF_ALLOC
979 and
980 .Dv SHF_EXECINSTR .
981 .It Sy .interp
982 This section holds the pathname of a program interpreter.
983 If the file has
984 a loadable segment that includes the section, the section's attributes will
985 include the
986 .Dv SHF_ALLOC
987 bit.
988 Otherwise, that bit will be off.
989 This section is of type
990 .Dv SHT_PROGBITS .
991 .It Sy .line
992 This section holds line number information for symbolic debugging, which
993 describes the correspondence between the program source and the machine code.
994 The contents are unspecified.
995 This section is of type
996 .Dv SHT_PROGBITS .
997 No attribute types are used.
998 .It Sy .note
999 This section holds information in the
1000 .Dq Note Section
1001 format described below.
1002 This section is of type
1003 .Dv SHT_NOTE .
1004 No attribute types are used.
1005 .It Sy .plt
1006 This section holds the procedure linkage table.
1007 This section is of type
1008 .Dv SHT_PROGBITS .
1009 The attributes are processor-specific.
1010 .It Sy .relNAME
1011 This section holds relocation information as described below.
1012 If the file
1013 has a loadable segment that includes relocation, the section's attributes
1014 will include the
1015 .Dv SHF_ALLOC
1016 bit.
1017 Otherwise the bit will be off.
1018 By convention,
1019 .Dq NAME
1020 is supplied by the section to which the relocations apply.
1021 Thus a relocation
1022 section for
1023 .Sy .text
1024 normally would have the name
1025 .Sy .rel.text .
1026 This section is of type
1027 .Dv SHT_REL .
1028 .It Sy .relaNAME
1029 This section holds relocation information as described below.
1030 If the file
1031 has a loadable segment that includes relocation, the section's attributes
1032 will include the
1033 .Dv SHF_ALLOC
1034 bit.
1035 Otherwise the bit will be off.
1036 By convention,
1037 .Dq NAME
1038 is supplied by the section to which the relocations apply.
1039 Thus a relocation
1040 section for
1041 .Sy .text
1042 normally would have the name
1043 .Sy .rela.text .
1044 This section is of type
1045 .Dv SHT_RELA .
1046 .It Sy .rodata
1047 This section holds read-only data that typically contributes to a
1048 non-writable segment in the process image.
1049 This section is of type
1050 .Dv SHT_PROGBITS .
1051 The attribute used is
1052 .Dv SHF_ALLOC .
1053 .It Sy .rodata1
1054 This section hold read-only data that typically contributes to a
1055 non-writable segment in the process image.
1056 This section is of type
1057 .Dv SHT_PROGBITS .
1058 The attribute used is
1059 .Dv SHF_ALLOC .
1060 .It Sy .shstrtab
1061 This section holds section names.
1062 This section is of type
1063 .Dv SHT_STRTAB .
1064 No attribute types are used.
1065 .It Sy .strtab
1066 This section holds strings, most commonly the strings that represent the
1067 names associated with symbol table entries.
1068 If the file has a loadable
1069 segment that includes the symbol string table, the section's attributes
1070 will include the
1071 .Dv SHF_ALLOC
1072 bit.
1073 Otherwise the bit will be off.
1074 This section is of type
1075 .Dv SHT_STRTAB .
1076 .It Sy .symtab
1077 This section holds a symbol table.
1078 If the file has a loadable segment
1079 that includes the symbol table, the section's attributes will include
1080 the
1081 .Dv SHF_ALLOC
1082 bit.
1083 Otherwise the bit will be off.
1084 This section is of type
1085 .Dv SHT_SYMTAB .
1086 .It Sy .text
1087 This section holds the
1088 .Dq text ,
1089 or executable instructions, of a program.
1090 This section is of type
1091 .Dv SHT_PROGBITS .
1092 The attributes used are
1093 .Dv SHF_ALLOC
1094 and
1095 .Dv SHF_EXECINSTR .
1096 .El
1097 .Pp
1098 String table sections hold null-terminated character sequences, commonly
1099 called strings.
1100 The object file uses these strings to represent symbol
1101 and section names.
1102 One references a string as an index into the string
1103 table section.
1104 The first byte, which is index zero, is defined to hold
1105 a null character.
1106 Similarly, a string table's last byte is defined to
1107 hold a null character, ensuring null termination for all strings.
1108 .Pp
1109 An object file's symbol table holds information needed to locate and
1110 relocate a program's symbolic definitions and references.
1111 A symbol table
1112 index is a subscript into this array.
1113 .Pp
1114 .Bd -literal -offset indent
1115 typedef struct {
1116 Elf32_Word st_name;
1117 Elf32_Addr st_value;
1118 Elf32_Size st_size;
1119 unsigned char st_info;
1120 unsigned char st_other;
1121 Elf32_Half st_shndx;
1122 } Elf32_Sym;
1123 .Ed
1124 .Pp
1125 .Bd -literal -offset indent
1126 typedef struct {
1127 Elf64_Half st_name;
1128 unsigned char st_info;
1129 unsigned char st_other;
1130 Elf64_Quarter st_shndx;
1131 Elf64_Addr st_value;
1132 Elf64_Size st_size;
1133 } Elf64_Sym;
1134 .Ed
1135 .Pp
1136 .Bl -tag -width ".Fa st_value" -compact
1137 .It Fa st_name
1138 This member holds an index into the object file's symbol string table,
1139 which holds character representations of the symbol names.
1140 If the value
1141 is non-zero, it represents a string table index that gives the symbol
1142 name.
1143 Otherwise, the symbol table has no name.
1144 .It Fa st_value
1145 This member gives the value of the associated symbol.
1146 .It Fa st_size
1147 Many symbols have associated sizes.
1148 This member holds zero if the symbol
1149 has no size or an unknown size.
1150 .It Fa st_info
1151 This member specifies the symbol's type and binding attributes:
1152 .Pp
1153 .Bl -tag -width ".Dv STT_SECTION" -compact
1154 .It Dv STT_NOTYPE
1155 The symbol's type is not defined.
1156 .It Dv STT_OBJECT
1157 The symbol is associated with a data object.
1158 .It Dv STT_FUNC
1159 The symbol is associated with a function or other executable code.
1160 .It Dv STT_SECTION
1161 The symbol is associated with a section.
1162 Symbol table entries of
1163 this type exist primarily for relocation and normally have
1164 .Dv STB_LOCAL
1165 bindings.
1166 .It Dv STT_FILE
1167 By convention the symbol's name gives the name of the source file
1168 associated with the object file.
1169 A file symbol has
1170 .Dv STB_LOCAL
1171 bindings, its section index is
1172 .Dv SHN_ABS ,
1173 and it precedes the other
1174 .Dv STB_LOCAL
1175 symbols of the file, if it is present.
1176 .It Dv STT_LOPROC
1177 This value up to and including
1178 .Dv STT_HIPROC
1179 are reserved for processor-specific semantics.
1180 .It Dv STT_HIPROC
1181 This value down to and including
1182 .Dv STT_LOPROC
1183 are reserved for processor-specific semantics.
1184 .El
1185 .Pp
1186 .Bl -tag -width ".Dv STB_GLOBAL" -compact
1187 .It Dv STB_LOCAL
1188 Local symbols are not visible outside the object file containing their
1189 definition.
1190 Local symbols of the same name may exist in multiple file
1191 without interfering with each other.
1192 .It Dv STB_GLOBAL
1193 Global symbols are visible to all object files being combined.
1194 One file's
1195 definition of a global symbol will satisfy another file's undefined
1196 reference to the same symbol.
1197 .It Dv STB_WEAK
1198 Weak symbols resemble global symbols, but their definitions have lower
1199 precedence.
1200 .It Dv STB_LOPROC
1201 This value up to and including
1202 .Dv STB_HIPROC
1203 are reserved for processor-specific semantics.
1204 .It Dv STB_HIPROC
1205 This value down to and including
1206 .Dv STB_LOPROC
1207 are reserved for processor-specific semantics.
1208 .Pp
1209 There are macros for packing and unpacking the binding and type fields:
1210 .Pp
1211 .Bl -tag -width ".Fn ELF32_ST_INFO bind type" -compact
1212 .It Xo
1213 .Fn ELF32_ST_BIND info
1214 .Xc
1215 or
1216 .Fn ELF64_ST_BIND info
1217 extract a binding from an
1218 .Fa st_info
1219 value.
1220 .It Xo
1221 .Fn ELF64_ST_TYPE info
1222 .Xc
1223 or
1224 .Fn ELF32_ST_TYPE info
1225 extract a type from an
1226 .Fa st_info
1227 value.
1228 .It Xo
1229 .Fn ELF32_ST_INFO bind type
1230 .Xc
1231 or
1232 .Fn ELF64_ST_INFO bind type
1233 convert a binding and a type into an
1234 .Fa st_info
1235 value.
1236 .El
1237 .El
1238 .Pp
1239 .It Fa st_other
1240 This member currently holds zero and has no defined meaning.
1241 .It Fa st_shndx
1242 Every symbol table entry is
1243 .Dq defined
1244 in relation to some section.
1245 This member holds the relevant section
1246 header table index.
1247 .El
1248 .Pp
1249 Relocation is the process of connecting symbolic references with
1250 symbolic definitions.
1251 Relocatable files must have information that
1252 describes how to modify their section contents, thus allowing executable
1253 and shared object files to hold the right information for a process'
1254 program image.
1255 Relocation entries are these data.
1256 .Pp
1257 Relocation structures that do not need an addend:
1258 .Pp
1259 .Bd -literal -offset indent
1260 typedef struct {
1261 Elf32_Addr r_offset;
1262 Elf32_Word r_info;
1263 } Elf32_Rel;
1264 .Ed
1265 .Bd -literal -offset indent
1266 typedef struct {
1267 Elf64_Addr r_offset;
1268 Elf64_Size r_info;
1269 } Elf64_Rel;
1270 .Ed
1271 .Pp
1272 Relocation structures that need an addend:
1273 .Pp
1274 .Bd -literal -offset indent
1275 typedef struct {
1276 Elf32_Addr r_offset;
1277 Elf32_Word r_info;
1278 Elf32_Sword r_addend;
1279 } Elf32_Rela;
1280 .Ed
1281 .Bd -literal -offset indent
1282 typedef struct {
1283 Elf64_Addr r_offset;
1284 Elf64_Size r_info;
1285 Elf64_Off r_addend;
1286 } Elf64_Rela;
1287 .Ed
1288 .Pp
1289 .Bl -tag -width ".Fa r_offset" -compact
1290 .It Fa r_offset
1291 This member gives the location at which to apply the relocation action.
1292 For a relocatable file, the value is the byte offset from the beginning
1293 of the section to the storage unit affected by the relocation.
1294 For an
1295 executable file or shared object, the value is the virtual address of
1296 the storage unit affected by the relocation.
1297 .It Fa r_info
1298 This member gives both the symbol table index with respect to which the
1299 relocation must be made and the type of relocation to apply.
1300 Relocation
1301 types are processor-specific.
1302 When the text refers to a relocation
1303 entry's relocation type or symbol table index, it means the result of
1304 applying
1305 .Fn ELF[32|64]_R_TYPE
1306 or
1307 .Fn ELF[32|64]_R_SYM ,
1308 respectively to the entry's
1309 .Fa r_info
1310 member.
1311 .It Fa r_addend
1312 This member specifies a constant addend used to compute the value to be
1313 stored into the relocatable field.
1314 .El
1315 .Sh SEE ALSO
1316 .Xr as 1 ,
1317 .Xr gdb 1 ,
1318 .Xr ld 1 ,
1319 .Xr objdump 1 ,
1320 .Xr execve 2 ,
1321 .Xr core 5
1322 .Rs
1323 .%A Hewlett Packard
1324 .%B Elf-64 Object File Format
1325 .Re
1326 .Rs
1327 .%A Santa Cruz Operation
1328 .%B System V Application Binary Interface
1329 .Re
1330 .Rs
1331 .%A Unix System Laboratories
1332 .%T Object Files
1333 .%B "Executable and Linking Format (ELF)"
1334 .Re
1335 .Sh HISTORY
1336 The ELF header files made their appearance in
1337 .Fx 2.2.6 .
1338 ELF in itself first appeared in
1339 .At V .
1340 The ELF format is an adopted standard.
1341 .Sh AUTHORS
1342 This manual page was written by
1343 .An Jeroen Ruigrok van der Werven
1344 .Aq asmodai@FreeBSD.org
1345 with inspiration from BSDi's
1346 .Bsx
1347 .Xr elf 5
1348 manpage.