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.\" $FreeBSD$ 26.\" 27.Dd December 18, 2005 28.Dt ELF 5 29.Os 30.Sh NAME 31.Nm elf 32.Nd format of ELF executable binary files 33.Sh SYNOPSIS 34.In elf.h 35.Sh DESCRIPTION 36The header file 37.In elf.h 38defines the format of ELF executable binary files. 39Amongst these files are 40normal executable files, relocatable object files, core files and shared 41libraries. 42.Pp 43An executable file using the ELF file format consists of an ELF header, 44followed by a program header table or a section header table, or both. 45The ELF header is always at offset zero of the file. 46The program header 47table and the section header table's offset in the file are defined in the 48ELF header. 49The two tables describe the rest of the particularities of 50the file. 51.Pp 52Applications which wish to process ELF binary files for their native 53architecture only should include 54.In elf.h 55in their source code. 56These applications should need to refer to 57all the types and structures by their generic names 58.Dq Elf_xxx 59and to the macros by 60.Dq ELF_xxx . 61Applications written this way can be compiled on any architecture, 62regardless whether the host is 32-bit or 64-bit. 63.Pp 64Should an application need to process ELF files of an unknown 65architecture then the application needs to include both 66.In sys/elf32.h 67and 68.In sys/elf64.h 69instead of 70.In elf.h . 71Furthermore, all types and structures need to be identified by either 72.Dq Elf32_xxx 73or 74.Dq Elf64_xxx . 75The macros need to be identified by 76.Dq ELF32_xxx 77or 78.Dq ELF64_xxx . 79.Pp 80Whatever the system's architecture is, it will always include 81.In sys/elf_common.h 82as well as 83.In sys/elf_generic.h . 84.Pp 85These header files describe the above mentioned headers as C structures 86and also include structures for dynamic sections, relocation sections and 87symbol tables. 88.Pp 89The following types are being used for 32-bit architectures: 90.Bd -literal -offset indent 91Elf32_Addr Unsigned 32-bit program address 92Elf32_Half Unsigned 16-bit field 93Elf32_Off Unsigned 32-bit file offset 94Elf32_Sword Signed 32-bit field or integer 95Elf32_Word Unsigned 32-bit field or integer 96.Ed 97.Pp 98For 64-bit architectures we have the following types: 99.Bd -literal -offset indent 100Elf64_Addr Unsigned 64-bit program address 101Elf64_Half Unsigned 16-bit field 102Elf64_Off Unsigned 64-bit file offset 103Elf64_Sword Signed 32-bit field 104Elf64_Word Unsigned 32-bit field 105Elf64_Sxword Signed 64-bit field or integer 106Elf64_Xword Unsigned 64-bit field or integer 107.Ed 108.Pp 109All data structures that the file format defines follow the 110.Dq natural 111size and alignment guidelines for the relevant class. 112If necessary, 113data structures contain explicit padding to ensure 4-byte alignment 114for 4-byte objects, to force structure sizes to a multiple of 4, etc. 115.Pp 116The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 117.Bd -literal -offset indent 118typedef struct { 119 unsigned char e_ident[EI_NIDENT]; 120 Elf32_Half e_type; 121 Elf32_Half e_machine; 122 Elf32_Word e_version; 123 Elf32_Addr e_entry; 124 Elf32_Off e_phoff; 125 Elf32_Off e_shoff; 126 Elf32_Word e_flags; 127 Elf32_Half e_ehsize; 128 Elf32_Half e_phentsize; 129 Elf32_Half e_phnum; 130 Elf32_Half e_shentsize; 131 Elf32_Half e_shnum; 132 Elf32_Half e_shstrndx; 133} Elf32_Ehdr; 134.Ed 135.Pp 136.Bd -literal -offset indent 137typedef struct { 138 unsigned char e_ident[EI_NIDENT]; 139 Elf64_Half e_type; 140 Elf64_Half e_machine; 141 Elf64_Word e_version; 142 Elf64_Addr e_entry; 143 Elf64_Off e_phoff; 144 Elf64_Off e_shoff; 145 Elf64_Word e_flags; 146 Elf64_Half e_ehsize; 147 Elf64_Half e_phentsize; 148 Elf64_Half e_phnum; 149 Elf64_Half e_shentsize; 150 Elf64_Half e_shnum; 151 Elf64_Half e_shstrndx; 152} Elf64_Ehdr; 153.Ed 154.Pp 155The fields have the following meanings: 156.Pp 157.Bl -tag -width "e_phentsize" -compact -offset indent 158.It Dv e_ident 159This array of bytes specifies to interpret the file, 160independent of the processor or the file's remaining contents. 161Within this array everything is named by macros, which start with 162the prefix 163.Sy EI_ 164and may contain values which start with the prefix 165.Sy ELF . 166The following macros are defined: 167.Pp 168.Bl -tag -width "EI_ABIVERSION" -compact 169.It Dv EI_MAG0 170The first byte of the magic number. 171It must be filled with 172.Sy ELFMAG0 . 173.It Dv EI_MAG1 174The second byte of the magic number. 175It must be filled with 176.Sy ELFMAG1 . 177.It Dv EI_MAG2 178The third byte of the magic number. 179It must be filled with 180.Sy ELFMAG2 . 181.It Dv EI_MAG3 182The fourth byte of the magic number. 183It must be filled with 184.Sy ELFMAG3 . 185.It Dv EI_CLASS 186The fifth byte identifies the architecture for this binary: 187.Pp 188.Bl -tag -width "ELFCLASSNONE" -compact 189.It Dv ELFCLASSNONE 190This class is invalid. 191.It Dv ELFCLASS32 192This defines the 32-bit architecture. 193It supports machines with files 194and virtual address spaces up to 4 Gigabytes. 195.It Dv ELFCLASS64 196This defines the 64-bit architecture. 197.El 198.It Dv EI_DATA 199The sixth byte specifies the data encoding of the processor-specific 200data in the file. 201Currently these encodings are supported: 202.Pp 203.Bl -tag -width "ELFDATA2LSB" -compact 204.It Dv ELFDATANONE 205Unknown data format. 206.It Dv ELFDATA2LSB 207Two's complement, little-endian. 208.It Dv ELFDATA2MSB 209Two's complement, big-endian. 210.El 211.It Dv EI_VERSION 212The version number of the ELF specification: 213.Pp 214.Bl -tag -width "EV_CURRENT" -compact 215.It Dv EV_NONE 216Invalid version. 217.It Dv EV_CURRENT 218Current version. 219.El 220.It Dv EI_OSABI 221This byte identifies the operating system 222and ABI to which the object is targeted. 223Some fields in other ELF structures have flags 224and values that have platform specific meanings; 225the interpretation of those fields is determined by the value of this byte. 226The following values are currently defined: 227.Pp 228.Bl -tag -width "ELFOSABI_STANDALONE" -compact 229.It Dv ELFOSABI_SYSV 230UNIX System V ABI. 231.It Dv ELFOSABI_HPUX 232HP-UX operating system ABI. 233.It Dv ELFOSABI_NETBSD 234.Nx 235operating system ABI. 236.It Dv ELFOSABI_LINUX 237GNU/Linux operating system ABI. 238.It Dv ELFOSABI_HURD 239GNU/Hurd operating system ABI. 240.It Dv ELFOSABI_86OPEN 24186Open Common IA32 ABI. 242.It Dv ELFOSABI_SOLARIS 243Solaris operating system ABI. 244.It Dv ELFOSABI_MONTEREY 245Monterey project ABI. 246.It Dv ELFOSABI_IRIX 247IRIX operating system ABI. 248.It Dv ELFOSABI_FREEBSD 249.Fx 250operating system ABI. 251.It Dv ELFOSABI_TRU64 252TRU64 UNIX operating system ABI. 253.It Dv ELFOSABI_ARM 254ARM architecture ABI. 255.It Dv ELFOSABI_STANDALONE 256Standalone (embedded) ABI. 257.El 258.It Dv EI_ABIVERSION 259This byte identifies the version of the ABI 260to which the object is targeted. 261This field is used to distinguish among incompatible versions of an ABI. 262The interpretation of this version number 263is dependent on the ABI identified by the EI_OSABI field. 264Applications conforming to this specification use the value 0. 265.It Dv EI_PAD 266Start of padding. 267These bytes are reserved and set to zero. 268Programs 269which read them should ignore them. 270The value for EI_PAD will change in 271the future if currently unused bytes are given meanings. 272.It Dv EI_BRAND 273Start of architecture identification. 274.It Dv EI_NIDENT 275The size of the e_ident array. 276.El 277.Pp 278.It Dv e_type 279This member of the structure identifies the object file type: 280.Pp 281.Bl -tag -width "ET_NONE" -compact 282.It Dv ET_NONE 283An unknown type. 284.It Dv ET_REL 285A relocatable file. 286.It Dv ET_EXEC 287An executable file. 288.It Dv ET_DYN 289A shared object. 290.It Dv ET_CORE 291A core file. 292.El 293.Pp 294.It Dv e_machine 295This member specifies the required architecture for an individual file: 296.Pp 297.Bl -tag -width "EM_MIPS_RS4_BE" -compact 298.It Dv EM_NONE 299An unknown machine. 300.It Dv EM_M32 301AT&T WE 32100. 302.It Dv EM_SPARC 303Sun Microsystems SPARC. 304.It Dv EM_386 305Intel 80386. 306.It Dv EM_68K 307Motorola 68000. 308.It Dv EM_88K 309Motorola 88000. 310.It Dv EM_486 311Intel 80486. 312.It Dv EM_860 313Intel 80860. 314.It Dv EM_MIPS 315MIPS RS3000 (big-endian only). 316.It Dv EM_MIPS_RS4_BE 317MIPS RS4000 (big-endian only). 318.It Dv EM_SPARC64 319SPARC v9 64-bit unofficial. 320.It Dv EM_PARISC 321HPPA. 322.It Dv EM_PPC 323PowerPC. 324.It Dv EM_ALPHA 325Compaq [DEC] Alpha. 326.El 327.Pp 328.It Dv e_version 329This member identifies the file version: 330.Pp 331.Bl -tag -width "EV_CURRENT" -compact 332.It Dv EV_NONE 333Invalid version 334.It Dv EV_CURRENT 335Current version 336.El 337.It Dv e_entry 338This member gives the virtual address to which the system first transfers 339control, thus starting the process. 340If the file has no associated entry 341point, this member holds zero. 342.It Dv e_phoff 343This member holds the program header table's file offset in bytes. 344If 345the file has no program header table, this member holds zero. 346.It Dv e_shoff 347This member holds the section header table's file offset in bytes. 348If the 349file has no section header table this member holds zero. 350.It Dv e_flags 351This member holds processor-specific flags associated with the file. 352Flag 353names take the form EF_`machine_flag'. 354Currently no flags have been defined. 355.It Dv e_ehsize 356This member holds the ELF header's size in bytes. 357.It Dv e_phentsize 358This member holds the size in bytes of one entry in the file's program header 359table; all entries are the same size. 360.It Dv e_phnum 361This member holds the number of entries in the program header 362table. 363Thus the product of 364.Sy e_phentsize 365and 366.Sy e_phnum 367gives the table's size 368in bytes. 369If a file has no program header, 370.Sy e_phnum 371holds the value zero. 372.It Dv e_shentsize 373This member holds a sections header's size in bytes. 374A section header is one 375entry in the section header table; all entries are the same size. 376.It Dv e_shnum 377This member holds the number of entries in the section header table. 378Thus 379the product of 380.Sy e_shentsize 381and 382.Sy e_shnum 383gives the section header table's size in bytes. 384If a file has no section 385header table, 386.Sy e_shnum 387holds the value of zero. 388.It Dv e_shstrndx 389This member holds the section header table index of the entry associated 390with the section name string table. 391If the file has no section name string 392table, this member holds the value 393.Sy SHN_UNDEF . 394.El 395.Pp 396An executable or shared object file's program header table is an array of 397structures, each describing a segment or other information the system needs 398to prepare the program for execution. 399An object file 400.Em segment 401contains one or more 402.Em sections . 403Program headers are meaningful only for executable and shared object files. 404A file specifies its own program header size with the ELF header's 405.Sy e_phentsize 406and 407.Sy e_phnum 408members. 409As with the Elf executable header, the program header 410also has different versions depending on the architecture: 411.Pp 412.Bd -literal -offset indent 413typedef struct { 414 Elf32_Word p_type; 415 Elf32_Off p_offset; 416 Elf32_Addr p_vaddr; 417 Elf32_Addr p_paddr; 418 Elf32_Word p_filesz; 419 Elf32_Word p_memsz; 420 Elf32_Word p_flags; 421 Elf32_Word p_align; 422} Elf32_Phdr; 423.Ed 424.Pp 425.Bd -literal -offset indent 426typedef struct { 427 Elf64_Word p_type; 428 Elf64_Word p_flags; 429 Elf64_Off p_offset; 430 Elf64_Addr p_vaddr; 431 Elf64_Addr p_paddr; 432 Elf64_Xword p_filesz; 433 Elf64_Xword p_memsz; 434 Elf64_Xword p_align; 435} Elf64_Phdr; 436.Ed 437.Pp 438The main difference between the 32-bit and the 64-bit program header lies 439only in the location of a 440.Sy p_flags 441member in the total struct. 442.Pp 443.Bl -tag -width "p_offset" -compact -offset indent 444.It Dv p_type 445This member of the Phdr struct tells what kind of segment this array 446element describes or how to interpret the array element's information. 447.Bl -tag -width "PT_DYNAMIC" -compact 448.Pp 449.It Dv PT_NULL 450The array element is unused and the other members' values are undefined. 451This lets the program header have ignored entries. 452.It Dv PT_LOAD 453The array element specifies a loadable segment, described by 454.Sy p_filesz 455and 456.Sy p_memsz . 457The bytes from the file are mapped to the beginning of the memory 458segment. 459If the segment's memory size 460.Pq Sy p_memsz 461is larger than the file size 462.Pq Sy p_filesz , 463the 464.Dq extra 465bytes are defined to hold the value 0 and to follow the segment's 466initialized area. 467The file size may not be larger than the memory size. 468Loadable segment entries in the program header table appear in ascending 469order, sorted on the 470.Sy p_vaddr 471member. 472.It Dv PT_DYNAMIC 473The array element specifies dynamic linking information. 474.It Dv PT_INTERP 475The array element specifies the location and size of a null-terminated 476path name to invoke as an interpreter. 477This segment type is meaningful 478only for executable files (though it may occur for shared objects). 479However 480it may not occur more than once in a file. 481If it is present it must precede 482any loadable segment entry. 483.It Dv PT_NOTE 484The array element specifies the location and size for auxiliary information. 485.It Dv PT_SHLIB 486This segment type is reserved but has unspecified semantics. 487Programs that 488contain an array element of this type do not conform to the ABI. 489.It Dv PT_PHDR 490The array element, if present, specifies the location and size of the program 491header table itself, both in the file and in the memory image of the program. 492This segment type may not occur more than once in a file. 493Moreover, it may 494only occur if the program header table is part of the memory image of the 495program. 496If it is present it must precede any loadable segment entry. 497.It Dv PT_LOPROC 498This value up to and including 499.Sy PT_HIPROC 500are reserved for processor-specific semantics. 501.It Dv PT_HIPROC 502This value down to and including 503.Sy PT_LOPROC 504are reserved for processor-specific semantics. 505.El 506.Pp 507.It Dv p_offset 508This member holds the offset from the beginning of the file at which 509the first byte of the segment resides. 510.It Dv p_vaddr 511This member holds the virtual address at which the first byte of the 512segment resides in memory. 513.It Dv p_paddr 514On systems for which physical addressing is relevant, this member is 515reserved for the segment's physical address. 516Under 517.Bx 518this member is 519not used and must be zero. 520.It Dv p_filesz 521This member holds the number of bytes in the file image of the segment. 522It may be zero. 523.It Dv p_memsz 524This member holds the number of bytes in the memory image of the segment. 525It may be zero. 526.It Dv p_flags 527This member holds flags relevant to the segment: 528.Pp 529.Bl -tag -width "PF_X" -compact 530.It Dv PF_X 531An executable segment. 532.It Dv PF_W 533A writable segment. 534.It Dv PF_R 535A readable segment. 536.El 537.Pp 538A text segment commonly has the flags 539.Sy PF_X 540and 541.Sy PF_R . 542A data segment commonly has 543.Sy PF_X , 544.Sy PF_W 545and 546.Sy PF_R . 547.It Dv p_align 548This member holds the value to which the segments are aligned in memory 549and in the file. 550Loadable process segments must have congruent values for 551.Sy p_vaddr 552and 553.Sy p_offset , 554modulo the page size. 555Values of zero and one mean no alignment is required. 556Otherwise, 557.Sy p_align 558should be a positive, integral power of two, and 559.Sy p_vaddr 560should equal 561.Sy p_offset , 562modulo 563.Sy p_align . 564.El 565.Pp 566An file's section header table lets one locate all the file's sections. 567The 568section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 569The 570ELF header's 571.Sy e_shoff 572member gives the byte offset from the beginning of the file to the section 573header table. 574.Sy e_shnum 575holds the number of entries the section header table contains. 576.Sy e_shentsize 577holds the size in bytes of each entry. 578.Pp 579A section header table index is a subscript into this array. 580Some section 581header table indices are reserved. 582An object file does not have sections for 583these special indices: 584.Pp 585.Bl -tag -width "SHN_LORESERVE" -compact 586.It Dv SHN_UNDEF 587This value marks an undefined, missing, irrelevant, or otherwise meaningless 588section reference. 589For example, a symbol 590.Dq defined 591relative to section number 592.Sy SHN_UNDEF 593is an undefined symbol. 594.It Dv SHN_LORESERVE 595This value specifies the lower bound of the range of reserved indices. 596.It Dv SHN_LOPROC 597This value up to and including 598.Sy SHN_HIPROC 599are reserved for processor-specific semantics. 600.It Dv SHN_HIPROC 601This value down to and including 602.Sy SHN_LOPROC 603are reserved for processor-specific semantics. 604.It Dv SHN_ABS 605This value specifies absolute values for the corresponding reference. 606For 607example, symbols defined relative to section number 608.Sy SHN_ABS 609have absolute values and are not affected by relocation. 610.It Dv SHN_COMMON 611Symbols defined relative to this section are common symbols, such as FORTRAN 612COMMON or unallocated C external variables. 613.It Dv SHN_HIRESERVE 614This value specifies the upper bound of the range of reserved indices. 615The 616system reserves indices between 617.Sy SHN_LORESERVE 618and 619.Sy SHN_HIRESERVE , 620inclusive. 621The section header table does not contain entries for the 622reserved indices. 623.El 624.Pp 625The section header has the following structure: 626.Bd -literal -offset indent 627typedef struct { 628 Elf32_Word sh_name; 629 Elf32_Word sh_type; 630 Elf32_Word sh_flags; 631 Elf32_Addr sh_addr; 632 Elf32_Off sh_offset; 633 Elf32_Word sh_size; 634 Elf32_Word sh_link; 635 Elf32_Word sh_info; 636 Elf32_Word sh_addralign; 637 Elf32_Word sh_entsize; 638} Elf32_Shdr; 639.Ed 640.Pp 641.Bd -literal -offset indent 642typedef struct { 643 Elf64_Word sh_name; 644 Elf64_Word sh_type; 645 Elf64_Xword sh_flags; 646 Elf64_Addr sh_addr; 647 Elf64_Off sh_offset; 648 Elf64_Xword sh_size; 649 Elf64_Word sh_link; 650 Elf64_Word sh_info; 651 Elf64_Xword sh_addralign; 652 Elf64_Xword sh_entsize; 653} Elf64_Shdr; 654.Ed 655.Pp 656.Bl -tag -width "sh_addralign" -compact 657.It Dv sh_name 658This member specifies the name of the section. 659Its value is an index 660into the section header string table section, giving the location of 661a null-terminated string. 662.It Dv sh_type 663This member categorizes the section's contents and semantics. 664.Pp 665.Bl -tag -width "SHT_PROGBITS" -compact 666.It Dv SHT_NULL 667This value marks the section header as inactive. 668It does not 669have an associated section. 670Other members of the section header 671have undefined values. 672.It Dv SHT_PROGBITS 673The section holds information defined by the program, whose 674format and meaning are determined solely by the program. 675.It Dv SHT_SYMTAB 676This section holds a symbol table. 677Typically, 678.Sy SHT_SYMTAB 679provides symbols for link editing, though it may also be used 680for dynamic linking. 681As a complete symbol table, it may contain 682many symbols unnecessary for dynamic linking. 683An object file can 684also contain a 685.Sy SHN_DYNSYM 686section. 687.It Dv SHT_STRTAB 688This section holds a string table. 689An object file may have multiple 690string table sections. 691.It Dv SHT_RELA 692This section holds relocation entries with explicit addends, such 693as type 694.Sy Elf32_Rela 695for the 32-bit class of object files. 696An object may have multiple 697relocation sections. 698.It Dv SHT_HASH 699This section holds a symbol hash table. 700All object participating in 701dynamic linking must contain a symbol hash table. 702An object file may 703have only one hash table. 704.It Dv SHT_DYNAMIC 705This section holds information for dynamic linking. 706An object file may 707have only one dynamic section. 708.It Dv SHT_NOTE 709This section holds information that marks the file in some way. 710.It Dv SHT_NOBITS 711A section of this type occupies no space in the file but otherwise 712resembles 713.Sy SHN_PROGBITS . 714Although this section contains no bytes, the 715.Sy sh_offset 716member contains the conceptual file offset. 717.It Dv SHT_REL 718This section holds relocation offsets without explicit addends, such 719as type 720.Sy Elf32_Rel 721for the 32-bit class of object files. 722An object file may have multiple 723relocation sections. 724.It Dv SHT_SHLIB 725This section is reserved but has unspecified semantics. 726.It Dv SHT_DYNSYM 727This section holds a minimal set of dynamic linking symbols. 728An 729object file can also contain a 730.Sy SHN_SYMTAB 731section. 732.It Dv SHT_LOPROC 733This value up to and including 734.Sy SHT_HIPROC 735are reserved for processor-specific semantics. 736.It Dv SHT_HIPROC 737This value down to and including 738.Sy SHT_LOPROC 739are reserved for processor-specific semantics. 740.It Dv SHT_LOUSER 741This value specifies the lower bound of the range of indices reserved for 742application programs. 743.It Dv SHT_HIUSER 744This value specifies the upper bound of the range of indices reserved for 745application programs. 746Section types between 747.Sy SHT_LOUSER 748and 749.Sy SHT_HIUSER 750may be used by the application, without conflicting with current or future 751system-defined section types. 752.El 753.Pp 754.It Dv sh_flags 755Sections support one-bit flags that describe miscellaneous attributes. 756If a flag bit is set in 757.Sy sh_flags , 758the attribute is 759.Dq on 760for the section. 761Otherwise, the attribute is 762.Dq off 763or does not apply. 764Undefined attributes are set to zero. 765.Pp 766.Bl -tag -width "SHF_EXECINSTR" -compact 767.It Dv SHF_WRITE 768This section contains data that should be writable during process 769execution. 770.It Dv SHF_ALLOC 771The section occupies memory during process execution. 772Some control 773sections do not reside in the memory image of an object file. 774This 775attribute is off for those sections. 776.It Dv SHF_EXECINSTR 777The section contains executable machine instructions. 778.It Dv SHF_MASKPROC 779All bits included in this mask are reserved for processor-specific 780semantics. 781.El 782.Pp 783.It Dv sh_addr 784If the section will appear in the memory image of a process, this member 785holds the address at which the section's first byte should reside. 786Otherwise, the member contains zero. 787.It Dv sh_offset 788This member's value holds the byte offset from the beginning of the file 789to the first byte in the section. 790One section type, 791.Sy SHT_NOBITS , 792occupies no space in the file, and its 793.Sy sh_offset 794member locates the conceptual placement in the file. 795.It Dv sh_size 796This member holds the section's size in bytes. 797Unless the section type 798is 799.Sy SHT_NOBITS , 800the section occupies 801.Sy sh_size 802bytes in the file. 803A section of type 804.Sy SHT_NOBITS 805may have a non-zero size, but it occupies no space in the file. 806.It Dv sh_link 807This member holds a section header table index link, whose interpretation 808depends on the section type. 809.It Dv sh_info 810This member holds extra information, whose interpretation depends on the 811section type. 812.It Dv sh_addralign 813Some sections have address alignment constraints. 814If a section holds a 815doubleword, the system must ensure doubleword alignment for the entire 816section. 817That is, the value of 818.Sy sh_addr 819must be congruent to zero, modulo the value of 820.Sy sh_addralign . 821Only zero and positive integral powers of two are allowed. 822Values of zero 823or one mean the section has no alignment constraints. 824.It Dv sh_entsize 825Some sections hold a table of fixed-sized entries, such as a symbol table. 826For such a section, this member gives the size in bytes for each entry. 827This member contains zero if the section does not hold a table of 828fixed-size entries. 829.El 830.Pp 831Various sections hold program and control information: 832.Bl -tag -width ".shstrtab" -compact 833.It .bss 834(Block Started by Symbol) 835This section holds uninitialized data that contributes to the program's 836memory image. 837By definition, the system initializes the data with zeros 838when the program begins to run. 839This section is of type 840.Sy SHT_NOBITS . 841The attributes types are 842.Sy SHF_ALLOC 843and 844.Sy SHF_WRITE . 845.It .comment 846This section holds version control information. 847This section is of type 848.Sy SHT_PROGBITS . 849No attribute types are used. 850.It .data 851This section holds initialized data that contribute to the program's 852memory image. 853This section is of type 854.Sy SHT_PROGBITS . 855The attribute types are 856.Sy SHF_ALLOC 857and 858.Sy SHF_WRITE . 859.It .data1 860This section holds initialized data that contribute to the program's 861memory image. 862This section is of type 863.Sy SHT_PROGBITS . 864The attribute types are 865.Sy SHF_ALLOC 866and 867.Sy SHF_WRITE . 868.It .debug 869This section holds information for symbolic debugging. 870The contents 871are unspecified. 872This section is of type 873.Sy SHT_PROGBITS . 874No attribute types are used. 875.It .dynamic 876This section holds dynamic linking information. 877The section's attributes 878will include the 879.Sy SHF_ALLOC 880bit. 881Whether the 882.Sy SHF_WRITE 883bit is set is processor-specific. 884This section is of type 885.Sy SHT_DYNAMIC . 886See the attributes above. 887.It .dynstr 888This section holds strings needed for dynamic linking, most commonly 889the strings that represent the names associated with symbol table entries. 890This section is of type 891.Sy SHT_STRTAB . 892The attribute type used is 893.Sy SHF_ALLOC . 894.It .dynsym 895This section holds the dynamic linking symbol table. 896This section is of type 897.Sy SHT_DYNSYM . 898The attribute used is 899.Sy SHF_ALLOC . 900.It .fini 901This section holds executable instructions that contribute to the process 902termination code. 903When a program exits normally the system arranges to 904execute the code in this section. 905This section is of type 906.Sy SHT_PROGBITS . 907The attributes used are 908.Sy SHF_ALLOC 909and 910.Sy SHF_EXECINSTR . 911.It .got 912This section holds the global offset table. 913This section is of type 914.Sy SHT_PROGBITS . 915The attributes are processor-specific. 916.It .hash 917This section holds a symbol hash table. 918This section is of type 919.Sy SHT_HASH . 920The attribute used is 921.Sy SHF_ALLOC . 922.It .init 923This section holds executable instructions that contribute to the process 924initialization code. 925When a program starts to run the system arranges to 926execute the code in this section before calling the main program entry point. 927This section is of type 928.Sy SHT_PROGBITS . 929The attributes used are 930.Sy SHF_ALLOC 931and 932.Sy SHF_EXECINSTR . 933.It .interp 934This section holds the pathname of a program interpreter. 935If the file has 936a loadable segment that includes the section, the section's attributes will 937include the 938.Sy SHF_ALLOC 939bit. 940Otherwise, that bit will be off. 941This section is of type 942.Sy SHT_PROGBITS . 943.It .line 944This section holds line number information for symbolic debugging, which 945describes the correspondence between the program source and the machine code. 946The contents are unspecified. 947This section is of type 948.Sy SHT_PROGBITS . 949No attribute types are used. 950.It .note 951This section holds information in the 952.Dq Note Section 953format described below. 954This section is of type 955.Sy SHT_NOTE . 956No attribute types are used. 957.It .plt 958This section holds the procedure linkage table. 959This section is of type 960.Sy SHT_PROGBITS . 961The attributes are processor-specific. 962.It .relNAME 963This section holds relocation information as described below. 964If the file 965has a loadable segment that includes relocation, the section's attributes 966will include the 967.Sy SHF_ALLOC 968bit. 969Otherwise the bit will be off. 970By convention, 971.Dq NAME 972is supplied by the section to which the relocations apply. 973Thus a relocation 974section for 975.Sy .text 976normally would have the name 977.Sy .rel.text . 978This section is of type 979.Sy SHT_REL . 980.It .relaNAME 981This section holds relocation information as described below. 982If the file 983has a loadable segment that includes relocation, the section's attributes 984will include the 985.Sy SHF_ALLOC 986bit. 987Otherwise the bit will be off. 988By convention, 989.Dq NAME 990is supplied by the section to which the relocations apply. 991Thus a relocation 992section for 993.Sy .text 994normally would have the name 995.Sy .rela.text . 996This section is of type 997.Sy SHT_RELA . 998.It .rodata 999This section holds read-only data that typically contributes to a 1000non-writable segment in the process image. 1001This section is of type 1002.Sy SHT_PROGBITS . 1003The attribute used is 1004.Sy SHF_ALLOC . 1005.It .rodata1 1006This section hold read-only data that typically contributes to a 1007non-writable segment in the process image. 1008This section is of type 1009.Sy SHT_PROGBITS . 1010The attribute used is 1011.Sy SHF_ALLOC . 1012.It .shstrtab 1013This section holds section names. 1014This section is of type 1015.Sy SHT_STRTAB . 1016No attribute types are used. 1017.It .strtab 1018This section holds strings, most commonly the strings that represent the 1019names associated with symbol table entries. 1020If the file has a loadable 1021segment that includes the symbol string table, the section's attributes 1022will include the 1023.Sy SHF_ALLOC 1024bit. 1025Otherwise the bit will be off. 1026This section is of type 1027.Sy SHT_STRTAB . 1028.It .symtab 1029This section holds a symbol table. 1030If the file has a loadable segment 1031that includes the symbol table, the section's attributes will include 1032the 1033.Sy SHF_ALLOC 1034bit. 1035Otherwise the bit will be off. 1036This section is of type 1037.Sy SHT_SYMTAB . 1038.It .text 1039This section holds the 1040.Dq text , 1041or executable instructions, of a program. 1042This section is of type 1043.Sy SHT_PROGBITS . 1044The attributes used are 1045.Sy SHF_ALLOC 1046and 1047.Sy SHF_EXECINSTR . 1048.It .jcr 1049This section holds information about Java classes that must 1050be registered. 1051.It .eh_frame 1052This section holds information used for C++ exception-handling. 1053.El 1054.Pp 1055String table sections hold null-terminated character sequences, commonly 1056called strings. 1057The object file uses these strings to represent symbol 1058and section names. 1059One references a string as an index into the string 1060table section. 1061The first byte, which is index zero, is defined to hold 1062a null character. 1063Similarly, a string table's last byte is defined to 1064hold a null character, ensuring null termination for all strings. 1065.Pp 1066An object file's symbol table holds information needed to locate and 1067relocate a program's symbolic definitions and references. 1068A symbol table 1069index is a subscript into this array. 1070.Pp 1071.Bd -literal -offset indent 1072typedef struct { 1073 Elf32_Word st_name; 1074 Elf32_Addr st_value; 1075 Elf32_Word st_size; 1076 unsigned char st_info; 1077 unsigned char st_other; 1078 Elf32_Half st_shndx; 1079} Elf32_Sym; 1080.Ed 1081.Pp 1082.Bd -literal -offset indent 1083typedef struct { 1084 Elf64_Word st_name; 1085 unsigned char st_info; 1086 unsigned char st_other; 1087 Elf64_Half st_shndx; 1088 Elf64_Addr st_value; 1089 Elf64_Xword st_size; 1090} Elf64_Sym; 1091.Ed 1092.Pp 1093.Bl -tag -width "st_value" -compact 1094.It Dv st_name 1095This member holds an index into the object file's symbol string table, 1096which holds character representations of the symbol names. 1097If the value 1098is non-zero, it represents a string table index that gives the symbol 1099name. 1100Otherwise, the symbol table has no name. 1101.It Dv st_value 1102This member gives the value of the associated symbol. 1103.It Dv st_size 1104Many symbols have associated sizes. 1105This member holds zero if the symbol 1106has no size or an unknown size. 1107.It Dv st_info 1108This member specifies the symbol's type and binding attributes: 1109.Pp 1110.Bl -tag -width "STT_SECTION" -compact 1111.It Dv STT_NOTYPE 1112The symbol's type is not defined. 1113.It Dv STT_OBJECT 1114The symbol is associated with a data object. 1115.It Dv STT_FUNC 1116The symbol is associated with a function or other executable code. 1117.It Dv STT_SECTION 1118The symbol is associated with a section. 1119Symbol table entries of 1120this type exist primarily for relocation and normally have 1121.Sy STB_LOCAL 1122bindings. 1123.It Dv STT_FILE 1124By convention the symbol's name gives the name of the source file 1125associated with the object file. 1126A file symbol has 1127.Sy STB_LOCAL 1128bindings, its section index is 1129.Sy SHN_ABS , 1130and it precedes the other 1131.Sy STB_LOCAL 1132symbols of the file, if it is present. 1133.It Dv STT_LOPROC 1134This value up to and including 1135.Sy STT_HIPROC 1136are reserved for processor-specific semantics. 1137.It Dv STT_HIPROC 1138This value down to and including 1139.Sy STT_LOPROC 1140are reserved for processor-specific semantics. 1141.El 1142.Pp 1143.Bl -tag -width "STB_GLOBAL" -compact 1144.It Dv STB_LOCAL 1145Local symbols are not visible outside the object file containing their 1146definition. 1147Local symbols of the same name may exist in multiple file 1148without interfering with each other. 1149.It Dv STB_GLOBAL 1150Global symbols are visible to all object files being combined. 1151One file's 1152definition of a global symbol will satisfy another file's undefined 1153reference to the same symbol. 1154.It Dv STB_WEAK 1155Weak symbols resemble global symbols, but their definitions have lower 1156precedence. 1157.It Dv STB_LOPROC 1158This value up to and including 1159.Sy STB_HIPROC 1160are reserved for processor-specific semantics. 1161.It Dv STB_HIPROC 1162This value down to and including 1163.Sy STB_LOPROC 1164are reserved for processor-specific semantics. 1165.Pp 1166There are macros for packing and unpacking the binding and type fields: 1167.Pp 1168.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1169.It Xo 1170.Fn ELF32_ST_BIND info 1171.Xc 1172or 1173.Fn ELF64_ST_BIND info 1174extract a binding from an st_info value. 1175.It Xo 1176.Fn ELF64_ST_TYPE info 1177.Xc 1178or 1179.Fn ELF32_ST_TYPE info 1180extract a type from an st_info value. 1181.It Xo 1182.Fn ELF32_ST_INFO bind type 1183.Xc 1184or 1185.Fn ELF64_ST_INFO bind type 1186convert a binding and a type into an st_info value. 1187.El 1188.El 1189.Pp 1190.It Dv st_other 1191This member currently holds zero and has no defined meaning. 1192.It Dv st_shndx 1193Every symbol table entry is 1194.Dq defined 1195in relation to some section. 1196This member holds the relevant section 1197header table index. 1198.El 1199.Pp 1200Relocation is the process of connecting symbolic references with 1201symbolic definitions. 1202Relocatable files must have information that 1203describes how to modify their section contents, thus allowing executable 1204and shared object files to hold the right information for a process' 1205program image. 1206Relocation entries are these data. 1207.Pp 1208Relocation structures that do not need an addend: 1209.Pp 1210.Bd -literal -offset indent 1211typedef struct { 1212 Elf32_Addr r_offset; 1213 Elf32_Word r_info; 1214} Elf32_Rel; 1215.Ed 1216.Bd -literal -offset indent 1217typedef struct { 1218 Elf64_Addr r_offset; 1219 Elf64_Xword r_info; 1220} Elf64_Rel; 1221.Ed 1222.Pp 1223Relocation structures that need an addend: 1224.Pp 1225.Bd -literal -offset indent 1226typedef struct { 1227 Elf32_Addr r_offset; 1228 Elf32_Word r_info; 1229 Elf32_Sword r_addend; 1230} Elf32_Rela; 1231.Ed 1232.Bd -literal -offset indent 1233typedef struct { 1234 Elf64_Addr r_offset; 1235 Elf64_Xword r_info; 1236 Elf64_Sxword r_addend; 1237} Elf64_Rela; 1238.Ed 1239.Pp 1240.Bl -tag -width "r_offset" -compact 1241.It Dv r_offset 1242This member gives the location at which to apply the relocation action. 1243For a relocatable file, the value is the byte offset from the beginning 1244of the section to the storage unit affected by the relocation. 1245For an 1246executable file or shared object, the value is the virtual address of 1247the storage unit affected by the relocation. 1248.It Dv r_info 1249This member gives both the symbol table index with respect to which the 1250relocation must be made and the type of relocation to apply. 1251Relocation 1252types are processor-specific. 1253When the text refers to a relocation 1254entry's relocation type or symbol table index, it means the result of 1255applying 1256.Sy ELF_[32|64]_R_TYPE 1257or 1258.Sy ELF[32|64]_R_SYM , 1259respectively to the entry's 1260.Sy r_info 1261member. 1262.It Dv r_addend 1263This member specifies a constant addend used to compute the value to be 1264stored into the relocatable field. 1265.El 1266.Sh SEE ALSO 1267.Xr as 1 , 1268.Xr gdb 1 , 1269.Xr ld 1 , 1270.Xr objdump 1 , 1271.Xr execve 2 , 1272.Xr core 5 1273.Rs 1274.%A Hewlett Packard 1275.%B Elf-64 Object File Format 1276.Re 1277.Rs 1278.%A Santa Cruz Operation 1279.%B System V Application Binary Interface 1280.Re 1281.Rs 1282.%A Unix System Laboratories 1283.%T Object Files 1284.%B "Executable and Linking Format (ELF)" 1285.Re 1286.Sh HISTORY 1287The ELF header files made their appearance in 1288.Fx 2.2.6 . 1289ELF in itself first appeared in 1290.At V . 1291The ELF format is an adopted standard. 1292.Sh AUTHORS 1293This manual page was written by 1294.An Jeroen Ruigrok van der Werven 1295.Aq asmodai@FreeBSD.org 1296with inspiration from BSDi's 1297.Bsx 1298.Xr elf 5 1299manpage. 1300