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 July 31, 1999 28.Dt ELF 5 29.Os FreeBSD 30.Sh NAME 31.Nm elf 32.Nd format of ELF executable binary files 33.Sh SYNOPSIS 34.Fd #include <elf.h> 35.Sh DESCRIPTION 36The header file 37.Aq Pa 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.Pa 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.Pa sys/elf32.h 67and 68.Pa sys/elf64.h 69instead of 70.Pa 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.Pa sys/elf_common.h 82as well as 83.Pa 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 program address 92Elf32_Half Unsigned halfword field 93Elf32_Off Unsigned file offset 94Elf32_Sword Signed large integer 95Elf32_Word Field or unsigned large integer 96Elf32_Size Unsigned object size 97.Ed 98.Pp 99For 64-bit architectures we have the following types: 100.Bd -literal -offset indent 101Elf64_Addr Unsigned program address 102Elf64_Half Unsigned halfword field 103Elf64_Off Unsigned file offset 104Elf64_Sword Signed large integer 105Elf64_Word Field or unsigned large integer 106Elf64_Size Unsigned object size 107Elf64_Quarter Unsigned quarterword field 108.Ed 109.Pp 110All data structures that the file format defines follow the 111.Dq natural 112size and alignment guidelines for the relevant class. 113If necessary, 114data structures contain explicit padding to ensure 4-byte alignment 115for 4-byte objects, to force structure sizes to a multiple of 4, etc. 116.Pp 117The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 118.Bd -literal -offset indent 119typedef struct { 120 unsigned char e_ident[EI_NIDENT]; 121 Elf32_Half e_type; 122 Elf32_Half e_machine; 123 Elf32_Word e_version; 124 Elf32_Addr e_entry; 125 Elf32_Off e_phoff; 126 Elf32_Off e_shoff; 127 Elf32_Word e_flags; 128 Elf32_Half e_ehsize; 129 Elf32_Half e_phentsize; 130 Elf32_Half e_phnum; 131 Elf32_Half e_shentsize; 132 Elf32_Half e_shnum; 133 Elf32_Half e_shstrndx; 134} Elf32_Ehdr; 135.Ed 136.Pp 137.Bd -literal -offset indent 138typedef struct { 139 unsigned char e_ident[EI_NIDENT]; 140 Elf64_Quarter e_type; 141 Elf64_Quarter e_machine; 142 Elf64_Half e_version; 143 Elf64_Addr e_entry; 144 Elf64_Off e_phoff; 145 Elf64_Off e_shoff; 146 Elf64_Half e_flags; 147 Elf64_Quarter e_ehsize; 148 Elf64_Quarter e_phentsize; 149 Elf64_Quarter e_phnum; 150 Elf64_Quarter e_shentsize; 151 Elf64_Quarter e_shnum; 152 Elf64_Quarter e_shstrndx; 153} Elf64_Ehdr; 154.Ed 155.Pp 156The fields have the following meanings: 157.Pp 158.Bl -tag -width "e_phentsize" -compact -offset indent 159.It Dv e_ident 160This array of bytes specifies to interpret the file, 161independent of the processor or the file's remaining contents. 162Within this array everything is named by macros, which start with 163the prefix 164.Sy EI_ 165and may contain values which start with the prefix 166.Sy ELF . 167The following macros are defined: 168.Pp 169.Bl -tag -width "EI_ABIVERSION" -compact 170.It Dv EI_MAG0 171The first byte of the magic number. 172It must be filled with 173.Sy ELFMAG0 . 174.It Dv EI_MAG1 175The second byte of the magic number. 176It must be filled with 177.Sy ELFMAG1 . 178.It Dv EI_MAG2 179The third byte of the magic number. 180It must be filled with 181.Sy ELFMAG2 . 182.It Dv EI_MAG3 183The fourth byte of the magic number. 184It must be filled with 185.Sy ELFMAG3 . 186.It Dv EI_CLASS 187The fifth byte identifies the architecture for this binary: 188.Pp 189.Bl -tag -width "ELFCLASSNONE" -compact 190.It Dv ELFCLASSNONE 191This class is invalid. 192.It Dv ELFCLASS32 193This defines the 32-bit architecture. 194It supports machines with files 195and virtual address spaces up to 4 Gigabytes. 196.It Dv ELFCLASS64 197This defines the 64-bit architecture. 198.El 199.It Dv EI_DATA 200The sixth byte specifies the data encoding of the processor-specific 201data in the file. 202Currently these encodings are supported: 203.Pp 204.Bl -tag -width "ELFDATA2LSB" -compact 205.It Dv ELFDATANONE 206Unknown data format. 207.It Dv ELFDATA2LSB 208Two's complement, little-endian. 209.It Dv ELFDATA2MSB 210Two's complement, big-endian. 211.El 212.It Dv EI_VERSION 213The version number of the ELF specification: 214.Pp 215.Bl -tag -width "EV_CURRENT" -compact 216.It Dv EV_NONE 217Invalid version. 218.It Dv EV_CURRENT 219Current version. 220.El 221.It Dv EI_OSABI 222This byte identifies the operating system 223and ABI to which the object is targeted. 224Some fields in other ELF structures have flags 225and values that have platform specific meanings; 226the interpretation of those fields is determined by the value of this byte. 227The following values are currently defined: 228.Pp 229.Bl -tag -width "ELFOSABI_STANDALONE" -compact 230.It Dv ELFOSABI_SYSV 231UNIX System V ABI. 232.It Dv ELFOSABI_HPUX 233HP-UX operating system ABI. 234.It Dv ELFOSABI_NETBSD 235NetBSD operating 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 249FreeBSD operating system ABI. 250.It Dv ELFOSABI_TRU64 251TRU64 UNIX operating system ABI. 252.It Dv ELFOSABI_ARM 253ARM architecture ABI. 254.It Dv ELFOSABI_STANDALONE 255Standalone (embedded) ABI. 256.El 257.It Dv EI_ABIVERSION 258This byte identifies the version of the ABI 259to which the object is targeted. 260This field is used to distinguish among incompatible versions of an ABI. 261The interpretation of this version number 262is dependent on the ABI identified by the EI_OSABI field. 263Applications conforming to this specification use the value 0. 264.It Dv EI_PAD 265Start of padding. 266These bytes are reserved and set to zero. 267Programs 268which read them should ignore them. 269The value for EI_PAD will change in 270the future if currently unused bytes are given meanings. 271.It Dv EI_BRAND 272Start of architecture identification. 273.It Dv EI_NIDENT 274The size of the e_ident array. 275.El 276.Pp 277.It Dv e_type 278This member of the structure identifies the object file type: 279.Pp 280.Bl -tag -width "ET_NONE" -compact 281.It Dv ET_NONE 282An unknown type. 283.It Dv ET_REL 284A relocatable file. 285.It Dv ET_EXEC 286An executable file. 287.It Dv ET_DYN 288A shared object. 289.It Dv ET_CORE 290A core file. 291.El 292.Pp 293.It Dv e_machine 294This member specifies the required architecture for an individual file: 295.Pp 296.Bl -tag -width "EM_MIPS_RS4_BE" -compact 297.It Dv EM_NONE 298An unknown machine. 299.It Dv EM_M32 300AT&T WE 32100. 301.It Dv EM_SPARC 302Sun Microsystems SPARC. 303.It Dv EM_386 304Intel 80386. 305.It Dv EM_68K 306Motorola 68000. 307.It Dv EM_88K 308Motorola 88000. 309.It Dv EM_486 310Intel 80486. 311.It Dv EM_860 312Intel 80860. 313.It Dv EM_MIPS 314MIPS RS3000 (big-endian only). 315.It Dv EM_MIPS_RS4_BE 316MIPS RS4000 (big-endian only). 317.It Dv EM_SPARC64 318SPARC v9 64-bit unofficial. 319.It Dv EM_PARISC 320HPPA. 321.It Dv EM_PPC 322PowerPC. 323.It Dv EM_ALPHA 324Compaq [DEC] Alpha. 325.El 326.Pp 327.It Dv e_version 328This member identifies the file version: 329.Pp 330.Bl -tag -width "EV_CURRENT" -compact 331.It Dv EV_NONE 332Invalid version 333.It Dv EV_CURRENT 334Current version 335.El 336.It Dv e_entry 337This member gives the virtual address to which the system first transfers 338control, thus starting the process. 339If the file has no associated entry 340point, this member holds zero. 341.It Dv e_phoff 342This member holds the program header table's file offset in bytes. 343If 344the file has no program header table, this member holds zero. 345.It Dv e_shoff 346This member holds the section header table's file offset in bytes. 347If the 348file has no section header table this member holds zero. 349.It Dv e_flags 350This member holds processor-specific flags associated with the file. 351Flag 352names take the form EF_`machine_flag'. Currently no flags have been defined. 353.It Dv e_ehsize 354This member holds the ELF header's size in bytes. 355.It Dv e_phentsize 356This member holds the size in bytes of one entry in the file's program header 357table; all entries are the same size. 358.It Dv e_phnum 359This member holds the number of entries in the program header 360table. 361Thus the product of 362.Sy e_phentsize 363and 364.Sy e_phnum 365gives the table's size 366in bytes. 367If a file has no program header, 368.Sy e_phnum 369holds the value zero. 370.It Dv e_shentsize 371This member holds a sections header's size in bytes. 372A section header is one 373entry in the section header table; all entries are the same size. 374.It Dv e_shnum 375This member holds the number of entries in the section header table. 376Thus 377the product of 378.Sy e_shentsize 379and 380.Sy e_shnum 381gives the section header table's size in bytes. 382If a file has no section 383header table, 384.Sy e_shnum 385holds the value of zero. 386.It Dv e_shstrndx 387This member holds the section header table index of the entry associated 388with the section name string table. 389If the file has no section name string 390table, this member holds the value 391.Sy SHN_UNDEF . 392.Pp 393.Bl -tag -width "SHN_LORESERVE" -compact 394.It Dv SHN_UNDEF 395This value marks an undefined, missing, irrelevant, or otherwise meaningless 396section reference. 397For example, a symbol 398.Dq defined 399relative to section number 400.Sy SHN_UNDEF 401is an undefined symbol. 402.It Dv SHN_LORESERVE 403This value specifies the lower bound of the range of reserved indexes. 404.It Dv SHN_LOPROC 405This value up to and including 406.Sy SHN_HIPROC 407are reserved for processor-specific semantics. 408.It Dv SHN_HIPROC 409This value down to and including 410.Sy SHN_LOPROC 411are reserved for processor-specific semantics. 412.It Dv SHN_ABS 413This value specifies absolute values for the corresponding reference. 414For 415example, symbols defined relative to section number 416.Sy SHN_ABS 417have absolute values and are not affected by relocation. 418.It Dv SHN_COMMON 419Symbols defined relative to this section are common symbols, such as Fortran 420COMMON or unallocated C external variables. 421.It Dv SHN_HIRESERVE 422This value specifies the upper bound of the range of the range of reserved 423indices between 424.Sy SHN_LORESERVE 425and 426.Sy SHN_HIRESERVE , 427inclusive; the values do 428not reference the section header table. 429That is, the section header table 430does 431.Em not 432contain entries for the reserved indices. 433.El 434.El 435.Pp 436An executable or shared object file's program header table is an array of 437structures, each describing a segment or other information the system needs 438to prepare the program for execution. 439An object file 440.Em segment 441contains one or more 442.Em sections . 443Program headers are meaningful only for executable and shared object files. 444A file specifies its own program header size with the ELF header's 445.Sy e_phentsize 446and 447.Sy e_phnum 448members. 449As with the Elf executable header, the program header 450also has different versions depending on the architecture: 451.Pp 452.Bd -literal -offset indent 453typedef struct { 454 Elf32_Word p_type; 455 Elf32_Off p_offset; 456 Elf32_Addr p_vaddr; 457 Elf32_Addr p_paddr; 458 Elf32_Size p_filesz; 459 Elf32_Size p_memsz; 460 Elf32_Word p_flags; 461 Elf32_Size p_align; 462} Elf32_Phdr; 463.Ed 464.Pp 465.Bd -literal -offset indent 466typedef struct { 467 Elf64_Half p_type; 468 Elf64_Half p_flags; 469 Elf64_Off p_offset; 470 Elf64_Addr p_vaddr; 471 Elf64_Addr p_paddr; 472 Elf64_Size p_filesz; 473 Elf64_Size p_memsz; 474 Elf64_Size p_align; 475} Elf64_Phdr; 476.Ed 477.Pp 478The main difference between the 32-bit and the 64-bit program header lies 479only in the location of a 480.Sy p_flags 481member in the total struct. 482.Pp 483.Bl -tag -width "p_offset" -compact -offset indent 484.It Dv p_type 485This member of the Phdr struct tells what kind of segment this array 486element describes or how to interpret the array element's information. 487.Bl -tag -width "PT_DYNAMIC" -compact 488.Pp 489.It Dv PT_NULL 490The array element is unused and the other members' values are undefined. 491This lets the program header have ignored entries. 492.It Dv PT_LOAD 493The array element specifies a loadable segment, described by 494.Sy p_filesz 495and 496.Sy p_memsz . 497The bytes from the file are mapped to the beginning of the memory 498segment. 499If the segment's memory size ( 500.Sy p_memsz 501) is larger than the file 502size ( 503.Sy p_filesz 504), the 505.Dq extra 506bytes are defined to hold the value 0 and to follow the segment's 507initialized area. 508The file size may not be larger than the memory size. 509Loadable segment entries in the program header table appear in ascending 510order, sorted on the 511.Sy p_vaddr 512member. 513.It Dv PT_DYNAMIC 514The array element specifies dynamic linking information. 515.It Dv PT_INTERP 516The array element specifies the location and size of a null-terminated 517path name to invoke as an interpreter. 518This segment type is meaningful 519only for executable files (though it may occur for shared objects). However 520it may not occur more than once in a file. 521If it is present it must precede 522any loadable segment entry. 523.It Dv PT_NOTE 524The array element specifies the location and size for auxiliary information. 525.It Dv PT_SHLIB 526This segment type is reserved but has unspecified semantics. 527Programs that 528contain an array element of this type do not conform to the ABI. 529.It Dv PT_PHDR 530The array element, if present, specifies the location and size of the program 531header table itself, both in the file and in the memory image of the program. 532This segment type may not occur more than once in a file. 533Moreover, it may 534only occur if the program header table is part of the memory image of the 535program. 536If it is present it must precede any loadable segment entry. 537.It Dv PT_LOPROC 538This value up to and including 539.Sy PT_HIPROC 540are reserved for processor-specific semantics. 541.It Dv PT_HIPROC 542This value down to and including 543.Sy PT_LOPROC 544are reserved for processor-specific semantics. 545.El 546.Pp 547.It Dv p_offset 548This member holds the offset from the beginning of the file at which 549the first byte of the of the segment resides. 550.It Dv p_vaddr 551This member holds the virtual address at which the first byte of the 552segment resides in memory. 553.It Dv p_paddr 554On systems for which physical addressing is relevant, this member is 555reserved for the segment's physical address. 556Under BSD this member is 557not used and must be zero. 558.It Dv p_filesz 559This member holds the number of bytes in the file image of the segment. 560It may be zero. 561.It Dv p_memsz 562This member holds the number of bytes in the memory image of the segment. 563It may be zero. 564.It Dv p_flags 565This member holds flags relevant to the segment: 566.Pp 567.Bl -tag -width "PF_X" -compact 568.It Dv PF_X 569An executable segment. 570.It Dv PF_W 571A writable segment. 572.It Dv PF_R 573A readable segment. 574.El 575.Pp 576A text segment commonly has the flags 577.Sy PF_X 578and 579.Sy PF_R . 580A data segment commonly has 581.Sy PF_X , 582.Sy PF_W 583and 584.Sy PF_R . 585.It Dv p_align 586This member holds the value to which the segments are aligned in memory 587and in the file. 588Loadable process segments must have congruent values for 589.Sy p_vaddr 590and 591.Sy p_offset , 592modulo the page size. 593Values of zero and one mean no alignment is required. 594Otherwise, 595.Sy p_align 596should be a positive, integral power of two, and 597.Sy p_vaddr 598should equal 599.Sy p_offset , 600modulo 601.Sy p_align . 602.El 603.Pp 604An file's section header table lets one locate all the file's sections. 605The 606section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 607The 608ELF header's 609.Sy e_shoff 610member gives the byte offset from the beginning of the file to the section 611header table. 612.Sy e_shnum 613holds the number of entries the section header table contains. 614.Sy e_shentsize 615holds the size in bytes of each entry. 616.Pp 617A section header table index is a subscript into this array. 618Some section 619header table indices are reserved. 620An object file does not have sections for 621these special indices: 622.Pp 623.Bl -tag -width "SHN_LORESERVE" -compact 624.It Dv SHN_UNDEF 625This value marks an undefined, missing, irrelevant or otherwise meaningless 626section reference. 627.It Dv SHN_LORESERVE 628This value specifies the lower bound of the range of reserved indices. 629.It Dv SHN_LOPROC 630This value up to and including 631.Sy SHN_HIPROC 632are reserved for processor-specific semantics. 633.It Dv SHN_HIPROC 634This value down to and including 635.Sy SHN_LOPROC 636are reserved for processor-specific semantics. 637.It Dv SHN_ABS 638This value specifies absolute values for the corresponding reference. 639For 640example, symbols defined relative to section number 641.Sy SHN_ABS 642have absolute values and are not affected by relocation. 643.It Dv SHN_COMMON 644Symbols defined relative to this section are common symbols, such as FORTRAN 645COMMON or unallocated C external variables. 646.It Dv SHN_HIRESERVE 647This value specifies the upper bound of the range of reserved indices. 648The 649system reserves indices between 650.Sy SHN_LORESERVE 651and 652.Sy SHN_HIRESERVE , 653inclusive. 654The section header table does not contain entries for the 655reserved indices. 656.El 657.Pp 658The section header has the following structure: 659.Bd -literal -offset indent 660typedef struct { 661 Elf32_Word sh_name; 662 Elf32_Word sh_type; 663 Elf32_Word sh_flags; 664 Elf32_Addr sh_addr; 665 Elf32_Off sh_offset; 666 Elf32_Size sh_size; 667 Elf32_Word sh_link; 668 Elf32_Word sh_info; 669 Elf32_Size sh_addralign; 670 Elf32_Size sh_entsize; 671} Elf32_Shdr; 672.Ed 673.Pp 674.Bd -literal -offset indent 675typedef struct { 676 Elf64_Half sh_name; 677 Elf64_Half sh_type; 678 Elf64_Size sh_flags; 679 Elf64_Addr sh_addr; 680 Elf64_Off sh_offset; 681 Elf64_Size sh_size; 682 Elf64_Half sh_link; 683 Elf64_Half sh_info; 684 Elf64_Size sh_addralign; 685 Elf64_Size sh_entsize; 686} Elf64_Shdr; 687.Ed 688.Pp 689.Bl -tag -width "sh_addralign" -compact 690.It Dv sh_name 691This member specifies the name of the section. 692Its value is an index 693into the section header string table section, giving the location of 694a null-terminated string. 695.It Dv sh_type 696This member categorizes the section's contents and semantics. 697.Pp 698.Bl -tag -width "SHT_PROGBITS" -compact 699.It Dv SHT_NULL 700This value marks the section header as inactive. 701It does not 702have an associated section. 703Other members of the section header 704have undefined values. 705.It Dv SHT_PROGBITS 706The section holds information defined by the program, whose 707format and meaning are determined solely by the program. 708.It Dv SHT_SYMTAB 709This section holds a symbol table. 710Typically, 711.Sy SHT_SYMTAB 712provides symbols for link editing, though it may also be used 713for dynamic linking. 714As a complete symbol table, it may contain 715many symbols unnecessary for dynamic linking. 716An object file can 717also contain a 718.Sy SHN_DYNSYM 719section. 720.It Dv SHT_STRTAB 721This section holds a string table. 722An object file may have multiple 723string table sections. 724.It Dv SHT_RELA 725This section holds relocation entries with explicit addends, such 726as type 727.Sy Elf32_Rela 728for the 32-bit class of object files. 729An object may have multiple 730relocation sections. 731.It Dv SHT_HASH 732This section holds a symbol hash table. 733All object participating in 734dynamic linking must contain a symbol hash table. 735An object file may 736have only one hash table. 737.It Dv SHT_DYNAMIC 738This section holds information for dynamic linking. 739An object file may 740have only one dynamic section. 741.It Dv SHT_NOTE 742This section holds information that marks the file in some way. 743.It Dv SHT_NOBITS 744A section of this type occupies no space in the file but otherwise 745resembles 746.Sy SHN_PROGBITS . 747Although this section contains no bytes, the 748.Sy sh_offset 749member contains the conceptual file offset. 750.It Dv SHT_REL 751This section holds relocation offsets without explicit addends, such 752as type 753.Sy Elf32_Rel 754for the 32-bit class of object files. 755An object file may have multiple 756relocation sections. 757.It Dv SHT_SHLIB 758This section is reserved but has unspecified semantics. 759.It Dv SHT_DYNSYM 760This section holds a minimal set of dynamic linking symbols. 761An 762object file can also contain a 763.Sy SHN_SYMTAB 764section. 765.It Dv SHT_LOPROC 766This value up to and including 767.Sy SHT_HIPROC 768are reserved for processor-specific semantics. 769.It Dv SHT_HIPROC 770This value down to and including 771.Sy SHT_LOPROC 772are reserved for processor-specific semantics. 773.It Dv SHT_LOUSER 774This value specifies the lower bound of the range of indices reserved for 775application programs. 776.It Dv SHT_HIUSER 777This value specifies the upper bound of the range of indices reserved for 778application programs. 779Section types between 780.Sy SHT_LOUSER 781and 782.Sy SHT_HIUSER 783may be used by the application, without conflicting with current or future 784system-defined section types. 785.El 786.Pp 787.It Dv sh_flags 788Sections support one-bit flags that describe miscellaneous attributes. 789If a flag bit is set in 790.Sy sh_flags , 791the attribute is 792.Dq on 793for the section. 794Otherwise, the attribute is 795.Dq off 796or does not apply. 797Undefined attributes are set to zero. 798.Pp 799.Bl -tag -width "SHF_EXECINSTR" -compact 800.It Dv SHF_WRITE 801This section contains data that should be writable during process 802execution. 803.It Dv SHF_ALLOC 804The section occupies memory during process execution. 805Some control 806sections do not reside in the memory image of an object file. 807This 808attribute is off for those sections. 809.It Dv SHF_EXECINSTR 810The section contains executable machine instructions. 811.It Dv SHF_MASKPROC 812All bits included in this mask are reserved for processor-specific 813semantics. 814.El 815.Pp 816.It Dv sh_addr 817If the section will appear in the memory image of a process, this member 818holds the address at which the section's first byte should reside. 819Otherwise, the member contains zero. 820.It Dv sh_offset 821This member's value holds the byte offset from the beginning of the file 822to the first byte in the section. 823One section type, 824.Sy SHT_NOBITS , 825occupies no space in the file, and its 826.Sy sh_offset 827member locates the conceptual placement in the file. 828.It Dv sh_size 829This member holds the section's size in bytes. 830Unless the section type 831is 832.Sy SHT_NOBITS , 833the section occupies 834.Sy sh_size 835bytes in the file. 836A section of type 837.Sy SHT_NOBITS 838may have a non-zero size, but it occupies no space in the file. 839.It Dv sh_link 840This member holds a section header table index link, whose interpretation 841depends on the section type. 842.It Dv sh_info 843This member holds extra information, whose interpretation depends on the 844section type. 845.It Dv sh_addralign 846Some sections have address alignment constraints. 847If a section holds a 848doubleword, the system must ensure doubleword alignment for the entire 849section. 850That is, the value of 851.Sy sh_addr 852must be congruent to zero, modulo the value of 853.Sy sh_addralign . 854Only zero and positive integral powers of two are allowed. 855Values of zero 856or one mean the section has no alignment constraints. 857.It Dv sh_entsize 858Some sections hold a table of fixed-sized entries, such as a symbol table. 859For such a section, this member gives the size in bytes for each entry. 860This member contains zero if the section does not hold a table of 861fixed-size entries. 862.El 863.Pp 864Various sections hold program and control information: 865.Bl -tag -width ".shstrtab" -compact 866.It .bss 867This section holds uninitialized data that contributes to the program's 868memory image. 869By definition, the system initializes the data with zeros 870when the program begins to run. 871This section is of type 872.Sy SHT_NOBITS . 873The attributes types are 874.Sy SHF_ALLOC 875and 876.Sy SHF_WRITE . 877.It .comment 878This section holds version control information. 879This section is of type 880.Sy SHT_PROGBITS . 881No attribute types are used. 882.It .data 883This section holds initialized data that contribute to the program's 884memory image. 885This section is of type 886.Sy SHT_PROGBITS . 887The attribute types are 888.Sy SHF_ALLOC 889and 890.Sy SHF_WRITE . 891.It .data1 892This section holds initialized data that contribute to the program's 893memory image. 894This section is of type 895.Sy SHT_PROGBITS . 896The attribute types are 897.Sy SHF_ALLOC 898and 899.Sy SHF_WRITE . 900.It .debug 901This section holds information for symbolic debugging. 902The contents 903are unspecified. 904This section is of type 905.Sy SHT_PROGBITS . 906No attribute types are used. 907.It .dynamic 908This section holds dynamic linking information. 909The section's attributes 910will include the 911.Sy SHF_ALLOC 912bit. 913Whether the 914.Sy SHF_WRITE 915bit is set is processor-specific. 916This section is of type 917.Sy SHT_DYNAMIC . 918See the attributes above. 919.It .dynstr 920This section holds strings needed for dynamic linking, most commonly 921the strings that represent the names associated with symbol table entries. 922This section is of type 923.Sy SHT_STRTAB . 924The attribute type used is 925.Sy SHF_ALLOC . 926.It .dynsym 927This section holds the dynamic linking symbol table. 928This section is of type 929.Sy SHT_DYNSYM . 930The attribute used is 931.Sy SHF_ALLOC . 932.It .fini 933This section holds executable instructions that contribute to the process 934termination code. 935When a program exits normally the system arranges to 936execute the code in this section. 937This section is of type 938.Sy SHT_PROGBITS . 939The attributes used are 940.Sy SHF_ALLOC 941and 942.Sy SHF_EXECINSTR . 943.It .got 944This section holds the global offset table. 945This section is of type 946.Sy SHT_PROGBITS . 947The attributes are processor-specific. 948.It .hash 949This section holds a symbol hash table. 950This section is of type 951.Sy SHT_HASH . 952The attribute used is 953.Sy SHF_ALLOC . 954.It .init 955This section holds executable instructions that contribute to the process 956initialization code. 957When a program starts to run the system arranges to 958execute the code in this section before calling the main program entry point. 959This section is of type 960.Sy SHT_PROGBITS . 961The attributes used are 962.Sy SHF_ALLOC 963and 964.Sy SHF_EXECINSTR . 965.It .interp 966This section holds the pathname of a program interpreter. 967If the file has 968a loadable segment that includes the section, the section's attributes will 969include the 970.Sy SHF_ALLOC 971bit. 972Otherwise, that bit will be off. 973This section is of type 974.Sy SHT_PROGBITS . 975.It .line 976This section holds line number information for symbolic debugging, which 977describes the correspondence between the program source and the machine code. 978The contents are unspecified. 979This section is of type 980.Sy SHT_PROGBITS . 981No attribute types are used. 982.It .note 983This section holds information in the 984.Dq Note Section 985format described below. 986This section is of type 987.Sy SHT_NOTE . 988No attribute types are used. 989.It .plt 990This section holds the procedure linkage table. 991This section is of type 992.Sy SHT_PROGBITS . 993The attributes are processor-specific. 994.It .relNAME 995This section holds relocation information as described below. 996If the file 997has a loadable segment that includes relocation, the section's attributes 998will include the 999.Sy SHF_ALLOC 1000bit. 1001Otherwise the bit will be off. 1002By convention, 1003.Dq NAME 1004is supplied by the section to which the relocations apply. 1005Thus a relocation 1006section for 1007.Sy .text 1008normally would have the name 1009.Sy .rel.text . 1010This section is of type 1011.Sy SHT_REL . 1012.It .relaNAME 1013This section holds relocation information as described below. 1014If the file 1015has a loadable segment that includes relocation, the section's attributes 1016will include the 1017.Sy SHF_ALLOC 1018bit. 1019Otherwise the bit will be off. 1020By convention, 1021.Dq NAME 1022is supplied by the section to which the relocations apply. 1023Thus a relocation 1024section for 1025.Sy .text 1026normally would have the name 1027.Sy .rela.text . 1028This section is of type 1029.Sy SHT_RELA . 1030.It .rodata 1031This section holds read-only data that typically contributes to a 1032non-writable segment in the process image. 1033This section is of type 1034.Sy SHT_PROGBITS . 1035The attribute used is 1036.Sy SHF_ALLOC . 1037.It .rodata1 1038This section hold read-only data that typically contributes to a 1039non-writable segment in the process image. 1040This section is of type 1041.Sy SHT_PROGBITS . 1042The attribute used is 1043.Sy SHF_ALLOC . 1044.It .shstrtab 1045This section holds section names. 1046This section is of type 1047.Sy SHT_STRTAB . 1048No attribute types are used. 1049.It .strtab 1050This section holds strings, most commonly the strings that represent the 1051names associated with symbol table entries. 1052If the file has a loadable 1053segment that includes the symbol string table, the section's attributes 1054will include the 1055.Sy SHF_ALLOC 1056bit. 1057Otherwise the bit will be off. 1058This section is of type 1059.Sy SHT_STRTAB . 1060.It .symtab 1061This section holds a symbol table. 1062If the file has a loadable segment 1063that includes the symbol table, the section's attributes will include 1064the 1065.Sy SHF_ALLOC 1066bit. 1067Otherwise the bit will be off. 1068This section is of type 1069.Sy SHT_SYMTAB . 1070.It .text 1071This section holds the 1072.Dq text , 1073or executable instructions, of a program. 1074This section is of type 1075.Sy SHT_PROGBITS . 1076The attributes used are 1077.Sy SHF_ALLOC 1078and 1079.Sy SHF_EXECINSTR . 1080.El 1081.Pp 1082String table sections hold null-terminated character sequences, commonly 1083called strings. 1084The object file uses these strings to represent symbol 1085and section names. 1086One references a string as an index into the string 1087table section. 1088The first byte, which is index zero, is defined to hold 1089a null character. 1090Similarly, a string table's last byte is defined to 1091hold a null character, ensuring null termination for all strings. 1092.Pp 1093An object file's symbol table holds information needed to locate and 1094relocate a program's symbolic definitions and references. 1095A symbol table 1096index is a subscript into this array. 1097.Pp 1098.Bd -literal -offset indent 1099typedef struct { 1100 Elf32_Word st_name; 1101 Elf32_Addr st_value; 1102 Elf32_Size st_size; 1103 unsigned char st_info; 1104 unsigned char st_other; 1105 Elf32_Half st_shndx; 1106} Elf32_Sym; 1107.Ed 1108.Pp 1109.Bd -literal -offset indent 1110typedef struct { 1111 Elf64_Half st_name; 1112 unsigned char st_info; 1113 unsigned char st_other; 1114 Elf64_Quarter st_shndx; 1115 Elf64_Addr st_value; 1116 Elf64_Size st_size; 1117} Elf64_Sym; 1118.Ed 1119.Pp 1120.Bl -tag -width "st_value" -compact 1121.It Dv st_name 1122This member holds an index into the object file's symbol string table, 1123which holds character representations of the symbol names. 1124If the value 1125is non-zero, it represents a string table index that gives the symbol 1126name. 1127Otherwise, the symbol table has no name. 1128.It Dv st_value 1129This member gives the value of the associated symbol. 1130.It Dv st_size 1131Many symbols have associated sizes. 1132This member holds zero if the symbol 1133has no size or an unknown size. 1134.It Dv st_info 1135This member specifies the symbol's type and binding attributes: 1136.Pp 1137.Bl -tag -width "STT_SECTION" -compact 1138.It Dv STT_NOTYPE 1139The symbol's type is not defined. 1140.It Dv STT_OBJECT 1141The symbol is associated with a data object. 1142.It Dv STT_FUNC 1143The symbol is associated with a function or other executable code. 1144.It Dv STT_SECTION 1145The symbol is associated with a section. 1146Symbol table entries of 1147this type exist primarily for relocation and normally have 1148.Sy STB_LOCAL 1149bindings. 1150.It Dv STT_FILE 1151By convention the symbol's name gives the name of the source file 1152associated with the object file. 1153A file symbol has 1154.Sy STB_LOCAL 1155bindings, its section index is 1156.Sy SHN_ABS , 1157and it precedes the other 1158.Sy STB_LOCAL 1159symbols of the file, if it is present. 1160.It Dv STT_LOPROC 1161This value up to and including 1162.Sy STT_HIPROC 1163are reserved for processor-specific semantics. 1164.It Dv STT_HIPROC 1165This value down to and including 1166.Sy STT_LOPROC 1167are reserved for processor-specific semantics. 1168.El 1169.Pp 1170.Bl -tag -width "STB_GLOBAL" -compact 1171.It Dv STB_LOCAL 1172Local symbols are not visible outside the object file containing their 1173definition. 1174Local symbols of the same name may exist in multiple file 1175without interfering with each other. 1176.It Dv STB_GLOBAL 1177Global symbols are visible to all object files being combined. 1178One file's 1179definition of a global symbol will satisfy another file's undefined 1180reference to the same symbol. 1181.It Dv STB_WEAK 1182Weak symbols resemble global symbols, but their definitions have lower 1183precedence. 1184.It Dv STB_LOPROC 1185This value up to and including 1186.Sy STB_HIPROC 1187are reserved for processor-specific semantics. 1188.It Dv STB_HIPROC 1189This value down to and including 1190.Sy STB_LOPROC 1191are reserved for processor-specific semantics. 1192.Pp 1193There are macros for packing and unpacking the binding and type fields: 1194.Pp 1195.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1196.It Xo 1197.Fn ELF32_ST_BIND info 1198.Xc 1199or 1200.Fn ELF64_ST_BIND info 1201extract a binding from an st_info value. 1202.It Xo 1203.Fn ELF64_ST_TYPE info 1204.Xc 1205or 1206.Fn ELF32_ST_TYPE info 1207extract a type from an st_info value. 1208.It Xo 1209.Fn ELF32_ST_INFO bind type 1210.Xc 1211or 1212.Fn ELF64_ST_INFO bind type 1213convert a binding and a type into an st_info value. 1214.El 1215.El 1216.Pp 1217.It Dv st_other 1218This member currently holds zero and has no defined meaning. 1219.It Dv st_shndx 1220Every symbol table entry is 1221.Dq defined 1222in relation to some action. 1223This member holds the relevant section 1224header table index. 1225.El 1226.Pp 1227Relocation is the process of connecting symbolic references with 1228symbolic definitions. 1229Relocatable files must have information that 1230describes how to modify their section contents, thus allowing executable 1231and shared object files to hold the right information for a process' 1232program image. 1233Relocation entries are these data. 1234.Pp 1235Relocation structures that do not need an addend: 1236.Pp 1237.Bd -literal -offset indent 1238typedef struct { 1239 Elf32_Addr r_offset; 1240 Elf32_Word r_info; 1241} Elf32_Rel; 1242.Ed 1243.Bd -literal -offset indent 1244typedef struct { 1245 Elf64_Addr r_offset; 1246 Elf64_Size r_info; 1247} Elf64_Rel; 1248.Ed 1249.Pp 1250Relocation structures that need an addend: 1251.Pp 1252.Bd -literal -offset indent 1253typedef struct { 1254 Elf32_Addr r_offset; 1255 Elf32_Word r_info; 1256 Elf32_Sword r_addend; 1257} Elf32_Rela; 1258.Ed 1259.Bd -literal -offset indent 1260typedef struct { 1261 Elf64_Addr r_offset; 1262 Elf64_Size r_info; 1263 Elf64_Off r_addend; 1264} Elf64_Rela; 1265.Ed 1266.Pp 1267.Bl -tag -width "r_offset" -compact 1268.It Dv r_offset 1269This member gives the location at which to apply the relocation action. 1270For a relocatable file, the value is the byte offset from the beginning 1271of the section to the storage unit affected by the relocation. 1272For an 1273executable file or shared object, the value is the virtual address of 1274the storage unit affected by the relocation. 1275.It Dv r_info 1276This member gives both the symbol table index with respect to which the 1277relocation must be made and the type of relocation to apply. 1278Relocation 1279types are processor-specific. 1280When the text refers to a relocation 1281entry's relocation type or symbol table index, it means the result of 1282applying 1283.Sy ELF_[32|64]_R_TYPE 1284or 1285.Sy ELF[32|64]_R_SYM , 1286respectively to the entry's 1287.Sy r_info 1288member. 1289.It Dv r_addend 1290This member specifies a constant addend used to compute the value to be 1291stored into the relocatable field. 1292.El 1293.Sh SEE ALSO 1294.Xr as 1 , 1295.Xr gdb 1 , 1296.Xr ld 1 , 1297.Xr objdump 1 , 1298.Xr execve 2 , 1299.Xr core 5 1300.Rs 1301.%A Hewlett Packard 1302.%B Elf-64 Object File Format 1303.Re 1304.Rs 1305.%A Santa Cruz Operation 1306.%B System V Application Binary Interface 1307.Re 1308.Rs 1309.%A Unix System Laboratories 1310.%T Object Files 1311.%B "Executable and Linking Format (ELF)" 1312.Re 1313.Sh HISTORY 1314The ELF header files made their appearance in 1315.Fx 2.2.6 . 1316ELF in itself first appeared in 1317.At V . 1318The ELF format is an adopted standard. 1319.Sh AUTHORS 1320This manual page was written by 1321.An Jeroen Ruigrok van der Werven 1322.Aq asmodai@FreeBSD.org 1323with inspiration from BSDi's BSD/OS 1324.Xr elf 5 1325manpage. 1326