1.\" $OpenBSD: elf.5,v 1.26 2014/02/14 15:50:41 schwarze Exp $ 2.\"Copyright (c) 1999 Jeroen Ruigrok van der Werven 3.\"All rights reserved. 4.\" 5.\"Redistribution and use in source and binary forms, with or without 6.\"modification, are permitted provided that the following conditions 7.\"are met: 8.\"1. Redistributions of source code must retain the above copyright 9.\" notice, this list of conditions and the following disclaimer. 10.\"2. Redistributions in binary form must reproduce the above copyright 11.\" notice, this list of conditions and the following disclaimer in the 12.\" documentation and/or other materials provided with the distribution. 13.\" 14.\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 15.\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 16.\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 17.\"ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 18.\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 19.\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 20.\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 21.\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 22.\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 23.\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 24.\"SUCH DAMAGE. 25.\" 26.\" $FreeBSD: src/share/man/man5/elf.5,v 1.21 2001/10/01 16:09:23 ru Exp $ 27.\" 28.Dd $Mdocdate: February 14 2014 $ 29.Dt ELF 5 30.Os 31.Sh NAME 32.Nm elf 33.Nd format of ELF executable binary files 34.Sh SYNOPSIS 35.Fd #include <elf_abi.h> 36.Sh DESCRIPTION 37The header file 38.In elf_abi.h 39defines the format of ELF executable binary files. 40Amongst these files are 41normal executable files, relocatable object files, core files and shared 42libraries. 43.Pp 44An executable file using the ELF file format consists of an ELF header, 45followed by a program header table or a section header table, or both. 46The ELF header is always at offset zero of the file. 47The program header 48table and the section header table's offset in the file are defined in the 49ELF header. 50The two tables describe the rest of the particularities of 51the file. 52.Pp 53Applications which wish to process ELF binary files for their native 54architecture only should include 55.In elf_abi.h 56in their source code. 57These applications should need to refer to 58all the types and structures by their generic names 59.Dq Elf_xxx 60and to the macros by 61.Dq ELF_xxx . 62Applications written this way can be compiled on any architecture, 63regardless of whether the host is 32-bit or 64-bit. 64.Pp 65Should an application need to process ELF files of an unknown 66architecture, then the application needs to explicitly use either 67.Dq Elf32_xxx 68or 69.Dq Elf64_xxx 70type and structure names. 71Likewise, the macros need to be identified by 72.Dq ELF32_xxx 73or 74.Dq ELF64_xxx . 75.Pp 76This header file describes the above mentioned headers as C structures 77and also includes structures for dynamic sections, relocation sections and 78symbol tables. 79.Pp 80The following types are used for 32-bit architectures: 81.Bd -literal -offset indent 82Elf32_Addr Unsigned program address 83Elf32_Off Unsigned file offset 84Elf32_Sword Signed large integer 85Elf32_Word Unsigned large integer 86Elf32_Half Unsigned medium integer 87.Ed 88.Pp 89And the following types are used for 64-bit architectures: 90.Bd -literal -offset indent 91Elf64_Addr Unsigned program address 92Elf64_Off Unsigned file offset 93Elf64_Shalf Signed halfword field 94Elf64_Sword Signed large integer 95Elf64_Word Field or unsigned large integer 96Elf64_Sxword Signed object size or alignment 97Elf64_Xword Unsigned object size or alignment 98Elf64_Half Unsigned halfword field 99Elf64_Quarter Unsigned quarterword field 100.Ed 101.Pp 102All data structures that the file format defines follow the 103.Dq natural 104size and alignment guidelines for the relevant class. 105If necessary, 106data structures contain explicit padding to ensure 4-byte alignment 107for 4-byte objects, to force structure sizes to a multiple of 4, etc. 108.Pp 109The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 110.Bd -literal -offset indent 111typedef struct { 112 unsigned char e_ident[EI_NIDENT]; 113 Elf32_Half e_type; 114 Elf32_Half e_machine; 115 Elf32_Word e_version; 116 Elf32_Addr e_entry; 117 Elf32_Off e_phoff; 118 Elf32_Off e_shoff; 119 Elf32_Word e_flags; 120 Elf32_Half e_ehsize; 121 Elf32_Half e_phentsize; 122 Elf32_Half e_phnum; 123 Elf32_Half e_shentsize; 124 Elf32_Half e_shnum; 125 Elf32_Half e_shstrndx; 126} Elf32_Ehdr; 127.Ed 128.Bd -literal -offset indent 129typedef struct { 130 unsigned char e_ident[EI_NIDENT]; 131 Elf64_Quarter e_type; 132 Elf64_Quarter e_machine; 133 Elf64_Half e_version; 134 Elf64_Addr e_entry; 135 Elf64_Off e_phoff; 136 Elf64_Off e_shoff; 137 Elf64_Half e_flags; 138 Elf64_Quarter e_ehsize; 139 Elf64_Quarter e_phentsize; 140 Elf64_Quarter e_phnum; 141 Elf64_Quarter e_shentsize; 142 Elf64_Quarter e_shnum; 143 Elf64_Quarter e_shstrndx; 144} Elf64_Ehdr; 145.Ed 146.Pp 147The fields have the following meanings: 148.Bl -tag -width "e_phentsize" -offset indent 149.It Dv e_ident 150This array of bytes specifies to interpret the file, 151independent of the processor or the file's remaining contents. 152Within this array everything is named by macros, which start with 153the prefix 154.Sy EI_ 155and may contain values which start with the prefix 156.Sy ELF . 157The following macros are defined: 158.Bl -tag -width "EI_VERSION" 159.It Dv EI_MAG0 160The first byte of the magic number. 161It must be filled with 162.Dv ELFMAG0 . 163.It Dv EI_MAG1 164The second byte of the magic number. 165It must be filled with 166.Dv ELFMAG1 . 167.It Dv EI_MAG2 168The third byte of the magic number. 169It must be filled with 170.Dv ELFMAG2 . 171.It Dv EI_MAG3 172The fourth byte of the magic number. 173It must be filled with 174.Dv ELFMAG3 . 175.It Dv EI_CLASS 176The fifth byte identifies the architecture for this binary: 177.Pp 178.Bl -tag -width "ELFCLASSNONE" -compact 179.It Dv ELFCLASSNONE 180This class is invalid. 181.It Dv ELFCLASS32 182This defines the 32-bit architecture. 183It supports machines with files 184and virtual address spaces up to 4 Gigabytes. 185.It Dv ELFCLASS64 186This defines the 64-bit architecture. 187.El 188.It Dv EI_DATA 189The sixth byte specifies the data encoding of the processor-specific 190data in the file. 191Currently these encodings are supported: 192.Pp 193.Bl -tag -width "ELFDATA2LSB" -compact 194.It Dv ELFDATANONE 195Unknown data format. 196.It Dv ELFDATA2LSB 197Two's complement, little-endian. 198.It Dv ELFDATA2MSB 199Two's complement, big-endian. 200.El 201.It Dv EI_VERSION 202The version number of the ELF specification: 203.Pp 204.Bl -tag -width "EV_CURRENT" -compact 205.It Dv EV_NONE 206Invalid version. 207.It Dv EV_CURRENT 208Current version. 209.El 210.\" .It Dv EI_OSABI 211.\" This byte identifies the operating system 212.\" and ABI to which the object is targeted. 213.\" Some fields in other ELF structures have flags 214.\" and values that have platform specific meanings; 215.\" the interpretation of those fields is determined by the value of this byte. 216.\" The following values are currently defined: 217.\" .Pp 218.\" .Bl -tag -width "ELFOSABI_STANDALONE" -compact 219.\" .It Dv ELFOSABI_SYSV 220.\" UNIX System V ABI. 221.\" .It Dv ELFOSABI_HPUX 222.\" HP-UX operating system ABI. 223.\" .It Dv ELFOSABI_NETBSD 224.\" .Nx 225.\" operating system ABI. 226.\" .It Dv ELFOSABI_LINUX 227.\" GNU/Linux operating system ABI. 228.\" .It Dv ELFOSABI_HURD 229.\" GNU/Hurd operating system ABI. 230.\" .It Dv ELFOSABI_86OPEN 231.\" 86Open Common IA32 ABI. 232.\" .It Dv ELFOSABI_SOLARIS 233.\" Solaris operating system ABI. 234.\" .It Dv ELFOSABI_MONTEREY 235.\" Monterey project ABI. 236.\" .It Dv ELFOSABI_IRIX 237.\" IRIX operating system ABI. 238.\" .It Dv ELFOSABI_FREEBSD 239.\" .Fx 240.\" operating system ABI. 241.\" .It Dv ELFOSABI_TRU64 242.\" TRU64 UNIX operating system ABI. 243.\" .It Dv ELFOSABI_ARM 244.\" ARM architecture ABI. 245.\" .It Dv ELFOSABI_STANDALONE 246.\" Stand-alone (embedded) ABI. 247.\" .El 248.\" .It Dv EI_ABIVERSION 249.\" This byte identifies the version of the ABI 250.\" to which the object is targeted. 251.\" This field is used to distinguish among incompatible versions of an ABI. 252.\" The interpretation of this version number 253.\" is dependent on the ABI identified by the EI_OSABI field. 254.\" Applications conforming to this specification use the value 0. 255.It Dv EI_PAD 256Start of padding. 257These bytes are reserved and set to zero. 258Programs 259which read them should ignore them. 260The value for EI_PAD will change in 261the future if currently unused bytes are given meanings. 262.It Dv EI_BRAND 263Start of architecture identification. 264.It Dv EI_NIDENT 265The size of the e_ident array. 266.El 267.It Dv e_type 268This member of the structure identifies the object file type: 269.Pp 270.Bl -tag -width "ET_NONE" -compact 271.It Dv ET_NONE 272An unknown type. 273.It Dv ET_REL 274A relocatable file. 275.It Dv ET_EXEC 276An executable file. 277.It Dv ET_DYN 278A shared object. 279.It Dv ET_CORE 280A core file. 281.El 282.It Dv e_machine 283This member specifies the required architecture for an individual file: 284.Pp 285.Bl -tag -width "EM_MIPS_RS4_BE" -compact 286.It Dv EM_NONE 287An unknown machine. 288.It Dv EM_M32 289AT&T WE 32100. 290.It Dv EM_SPARC 291Sun Microsystems SPARC. 292.It Dv EM_386 293Intel 80386. 294.It Dv EM_68K 295Motorola 68000. 296.It Dv EM_88K 297Motorola 88000. 298.It Dv EM_486 299Intel 80486. 300.It Dv EM_860 301Intel 80860. 302.It Dv EM_MIPS 303MIPS RS3000 (big-endian only). 304.It Dv EM_MIPS_RS4_BE 305MIPS RS4000 (big-endian only). 306.It Dv EM_SPARC64 307SPARC v9 64-bit (unofficial). 308.It Dv EM_PARISC 309HPPA. 310.It Dv EM_SPARC32PLUS 311SPARC with enhanced instruction set. 312.It Dv EM_PPC 313PowerPC. 314.It Dv EM_ARM 315Advanced RISC Machines ARM. 316.It Dv EM_ALPHA 317Compaq [DEC] Alpha. 318.It Dv EM_SH 319Hitachi/Renesas Super-H. 320.It Dv EM_SPARCV9 321SPARC v9 64-bit. 322.It Dv EM_IA_64 323Intel IA-64. 324.It Dv EM_AMD64 325AMD64. 326.It Dv EM_VAX 327DEC Vax. 328.It Dv EM_ALPHA_EXP 329Compaq [DEC] Alpha with enhanced instruction set. 330.El 331.It Dv e_version 332This member identifies the file version: 333.Pp 334.Bl -tag -width "EV_CURRENT" -compact 335.It Dv EV_NONE 336Invalid version. 337.It Dv EV_CURRENT 338Current version. 339.El 340.It Dv e_entry 341This member gives the virtual address to which the system first transfers 342control, thus starting the process. 343If the file has no associated entry 344point, this member holds zero. 345.It Dv e_phoff 346This member holds the program header table's file offset in bytes. 347If 348the file has no program header table, this member holds zero. 349.It Dv e_shoff 350This member holds the section header table's file offset in bytes. 351If the 352file has no section header table this member holds zero. 353.It Dv e_flags 354This member holds processor-specific flags associated with the file. 355Flag names take the form EF_`machine_flag'. 356Currently no flags have been defined. 357.It Dv e_ehsize 358This member holds the ELF header's size in bytes. 359.It Dv e_phentsize 360This member holds the size in bytes of one entry in the file's program header 361table; all entries are the same size. 362.It Dv e_phnum 363This member holds the number of entries in the program header 364table. 365Thus the product of 366.Sy e_phentsize 367and 368.Sy e_phnum 369gives the table's size 370in bytes. 371If a file has no program header, 372.Sy e_phnum 373holds the value zero. 374.It Dv e_shentsize 375This member holds a sections header's size in bytes. 376A section header is one 377entry in the section header table; all entries are the same size. 378.It Dv e_shnum 379This member holds the number of entries in the section header table. 380Thus 381the product of 382.Sy e_shentsize 383and 384.Sy e_shnum 385gives the section header table's size in bytes. 386If a file has no section 387header table, 388.Sy e_shnum 389holds the value of zero. 390.It Dv e_shstrndx 391This member holds the section header table index of the entry associated 392with the section name string table. 393If the file has no section name string 394table, this member holds the value 395.Dv SHN_UNDEF . 396.El 397.Pp 398An executable or shared object file's program header table is an array of 399structures, each describing a segment or other information the system needs 400to prepare the program for execution. 401An object file 402.Em segment 403contains one or more 404.Em sections . 405Program headers are meaningful only for executable and shared object files. 406A file specifies its own program header size with the ELF header's 407.Sy e_phentsize 408and 409.Sy e_phnum 410members. 411As with the ELF executable header, the program header 412also has different versions depending on the architecture: 413.Bd -literal -offset indent 414typedef struct { 415 Elf32_Word p_type; 416 Elf32_Off p_offset; 417 Elf32_Addr p_vaddr; 418 Elf32_Addr p_paddr; 419 Elf32_Word p_filesz; 420 Elf32_Word p_memsz; 421 Elf32_Word p_flags; 422 Elf32_Word p_align; 423} Elf32_Phdr; 424.Ed 425.Bd -literal -offset indent 426typedef struct { 427 Elf64_Half p_type; 428 Elf64_Half 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.Bl -tag -width "p_offset" -offset indent 443.It Dv p_type 444This member of the Phdr struct tells what kind of segment this array 445element describes or how to interpret the array element's information. 446.Bl -tag -width "PT_DYNAMIC" 447.It Dv PT_NULL 448The array element is unused and the other members' values are undefined. 449This lets the program header have ignored entries. 450.It Dv PT_LOAD 451The array element specifies a loadable segment, described by 452.Sy p_filesz 453and 454.Sy p_memsz . 455The bytes from the file are mapped to the beginning of the memory 456segment. 457If the segment's memory size 458.Pq Sy p_memsz 459is larger than the file size 460.Pq Sy p_filesz , 461the 462.Dq extra 463bytes are defined to hold the value 0 and to follow the segment's 464initialized area. 465The file size may not be larger than the memory size. 466Loadable segment entries in the program header table appear in ascending 467order, sorted on the 468.Sy p_vaddr 469member. 470.It Dv PT_DYNAMIC 471The array element specifies dynamic linking information. 472.It Dv PT_INTERP 473The array element specifies the location and size of a null-terminated 474path name to invoke as an interpreter. 475This segment type is meaningful 476only for executable files (though it may occur for shared objects). 477However it may not occur more than once in a file. 478If it is present, it must precede any loadable segment entry. 479.It Dv PT_NOTE 480The array element specifies the location and size for auxiliary information. 481.It Dv PT_SHLIB 482This segment type is reserved but has unspecified semantics. 483Programs that 484contain an array element of this type do not conform to the ABI. 485.It Dv PT_PHDR 486The array element, if present, specifies the location and size of the program 487header table itself, both in the file and in the memory image of the program. 488This segment type may not occur more than once in a file. 489Moreover, it may 490only occur if the program header table is part of the memory image of the 491program. 492If it is present, it must precede any loadable segment entry. 493.It Dv PT_TLS 494The array element, if present, specifies the location and size of the 495thread-local storage for this file. 496Each thread in a process loading this file will have the segment's 497memory size 498.Pq Sy p_memsz 499allocated for it, where the bytes up to the segment's file size 500.Pq Sy p_filesz 501will be initialized with the data in this segment and the remaining 502.Dq extra 503bytes will be set to zero. 504.It Dv PT_LOOS 505This value up to and including 506.Dv PT_HIOS 507is reserved for operating system-specific semantics. 508.It Dv PT_HIOS 509This value down to and including 510.Dv PT_LOOS 511is reserved for operating system-specific semantics. 512.It Dv PT_LOPROC 513This value up to and including 514.Dv PT_HIPROC 515is reserved for processor-specific semantics. 516.It Dv PT_HIPROC 517This value down to and including 518.Dv PT_LOPROC 519is reserved for processor-specific semantics. 520.El 521.It Dv p_offset 522This member holds the offset from the beginning of the file at which 523the first byte of the segment resides. 524.It Dv p_vaddr 525This member holds the virtual address at which the first byte of the 526segment resides in memory. 527.It Dv p_paddr 528On systems for which physical addressing is relevant, this member is 529reserved for the segment's physical address. 530Under 531.Bx 532this member is 533not used and must be zero. 534.It Dv p_filesz 535This member holds the number of bytes in the file image of the segment. 536It may be zero. 537.It Dv p_memsz 538This member holds the number of bytes in the memory image of the segment. 539It may be zero. 540.It Dv p_flags 541This member holds flags relevant to the segment: 542.Pp 543.Bl -tag -width "PF_X" -compact 544.It Dv PF_X 545An executable segment. 546.It Dv PF_W 547A writable segment. 548.It Dv PF_R 549A readable segment. 550.El 551.Pp 552A text segment commonly has the flags 553.Dv PF_X 554and 555.Dv PF_R . 556A data segment commonly has 557.Dv PF_X , 558.Dv PF_W 559and 560.Dv PF_R . 561.It Dv p_align 562This member holds the value to which the segments are aligned in memory 563and in the file. 564Loadable process segments must have congruent values for 565.Sy p_vaddr 566and 567.Sy p_offset , 568modulo the page size. 569Values of zero and one mean no alignment is required. 570Otherwise, 571.Sy p_align 572should be a positive, integral power of two, and 573.Sy p_vaddr 574should equal 575.Sy p_offset , 576modulo 577.Sy p_align . 578.El 579.Pp 580A file's section header table lets one locate all the file's sections. 581The 582section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 583The 584ELF header's 585.Sy e_shoff 586member gives the byte offset from the beginning of the file to the section 587header table. 588.Sy e_shnum 589holds the number of entries the section header table contains. 590.Sy e_shentsize 591holds the size in bytes of each entry. 592.Pp 593A section header table index is a subscript into this array. 594Some section 595header table indices are reserved. 596An object file does not have sections for 597these special indices: 598.Bl -tag -width "SHN_LORESERVE" 599.It Dv SHN_UNDEF 600This value marks an undefined, missing, irrelevant or otherwise meaningless 601section reference. 602For example, a symbol 603.Dq defined 604relative to section number 605.Dv SHN_UNDEF 606is an undefined symbol. 607.It Dv SHN_LORESERVE 608This value specifies the lower bound of the range of reserved indices. 609.It Dv SHN_LOPROC 610This value up to and including 611.Dv SHN_HIPROC 612is reserved for processor-specific semantics. 613.It Dv SHN_HIPROC 614This value down to and including 615.Dv SHN_LOPROC 616is reserved for processor-specific semantics. 617.It Dv SHN_ABS 618This value specifies the absolute value for the corresponding reference. 619For 620example, a symbol defined relative to section number 621.Dv SHN_ABS 622has an absolute value and is not affected by relocation. 623.It Dv SHN_COMMON 624Symbols defined relative to this section are common symbols, such as FORTRAN 625COMMON or unallocated C external variables. 626.It Dv SHN_HIRESERVE 627This value specifies the upper bound of the range of reserved indices. 628The 629system reserves indices between 630.Dv SHN_LORESERVE 631and 632.Dv SHN_HIRESERVE , 633inclusive. 634The section header table does not contain entries for the 635reserved indices. 636.El 637.Pp 638The section header has the following structure: 639.Bd -literal -offset indent 640typedef struct { 641 Elf32_Word sh_name; 642 Elf32_Word sh_type; 643 Elf32_Word sh_flags; 644 Elf32_Addr sh_addr; 645 Elf32_Off sh_offset; 646 Elf32_Word sh_size; 647 Elf32_Word sh_link; 648 Elf32_Word sh_info; 649 Elf32_Word sh_addralign; 650 Elf32_Word sh_entsize; 651} Elf32_Shdr; 652.Ed 653.Bd -literal -offset indent 654typedef struct { 655 Elf64_Half sh_name; 656 Elf64_Half sh_type; 657 Elf64_Xword sh_flags; 658 Elf64_Addr sh_addr; 659 Elf64_Off sh_offset; 660 Elf64_Xword sh_size; 661 Elf64_Half sh_link; 662 Elf64_Half sh_info; 663 Elf64_Xword sh_addralign; 664 Elf64_Xword sh_entsize; 665} Elf64_Shdr; 666.Ed 667.Bl -tag -width "sh_addralign" 668.It Dv sh_name 669This member specifies the name of the section. 670Its value is an index 671into the section header string table section, giving the location of 672a null-terminated string. 673.It Dv sh_type 674This member categorizes the section's contents and semantics. 675.Bl -tag -width "SHT_PROGBITS" 676.It Dv SHT_NULL 677This value marks the section header as inactive. 678It does not 679have an associated section. 680Other members of the section header 681have undefined values. 682.It Dv SHT_PROGBITS 683This section holds information defined by the program, whose 684format and meaning are determined solely by the program. 685.It Dv SHT_SYMTAB 686This section holds a symbol table. 687Typically, 688.Dv SHT_SYMTAB 689provides symbols for link editing, though it may also be used 690for dynamic linking. 691As a complete symbol table, it may contain 692many symbols unnecessary for dynamic linking. 693An object file can 694also contain a 695.Dv SHT_DYNSYM 696section. 697.It Dv SHT_STRTAB 698This section holds a string table. 699An object file may have multiple 700string table sections. 701.It Dv SHT_RELA 702This section holds relocation entries with explicit addends, such 703as type 704.Sy Elf32_Rela 705for the 32-bit class of object files. 706An object may have multiple 707relocation sections. 708.It Dv SHT_HASH 709This section holds a symbol hash table. 710An object participating in 711dynamic linking must contain a symbol hash table. 712An object file may 713have only one hash table. 714.It Dv SHT_DYNAMIC 715This section holds information for dynamic linking. 716An object file may 717have only one dynamic section. 718.It Dv SHT_NOTE 719This section holds information that marks the file in some way. 720.It Dv SHT_NOBITS 721A section of this type occupies no space in the file but otherwise 722resembles 723.Dv SHT_PROGBITS . 724Although this section contains no bytes, the 725.Sy sh_offset 726member contains the conceptual file offset. 727.It Dv SHT_REL 728This section holds relocation offsets without explicit addends, such 729as type 730.Sy Elf32_Rel 731for the 32-bit class of object files. 732An object file may have multiple 733relocation sections. 734.It Dv SHT_SHLIB 735This section is reserved but has unspecified semantics. 736.It Dv SHT_DYNSYM 737This section holds a minimal set of dynamic linking symbols. 738An 739object file can also contain a 740.Dv SHT_SYMTAB 741section. 742.It Dv SHT_LOPROC 743This value up to and including 744.Dv SHT_HIPROC 745is reserved for processor-specific semantics. 746.It Dv SHT_HIPROC 747This value down to and including 748.Dv SHT_LOPROC 749is reserved for processor-specific semantics. 750.It Dv SHT_LOUSER 751This value specifies the lower bound of the range of indices reserved for 752application programs. 753.It Dv SHT_HIUSER 754This value specifies the upper bound of the range of indices reserved for 755application programs. 756Section types between 757.Dv SHT_LOUSER 758and 759.Dv SHT_HIUSER 760may be used by the application, without conflicting with current or future 761system-defined section types. 762.El 763.It Dv sh_flags 764Sections support one-bit flags that describe miscellaneous attributes. 765If a flag bit is set in 766.Sy sh_flags , 767the attribute is 768.Dq on 769for the section. 770Otherwise, the attribute is 771.Dq off 772or does not apply. 773Undefined attributes are set to zero. 774.Pp 775.Bl -tag -width "SHF_EXECINSTR" -compact 776.It Dv SHF_WRITE 777This section contains data that should be writable during process 778execution. 779.It Dv SHF_ALLOC 780This section occupies memory during process execution. 781Some control 782sections do not reside in the memory image of an object file. 783This 784attribute is off for those sections. 785.It Dv SHF_EXECINSTR 786This section contains executable machine instructions. 787.It Dv SHF_TLS 788This section is for thread-local storage. 789.It Dv SHF_MASKPROC 790All bits included in this mask are reserved for processor-specific 791semantics. 792.El 793.It Dv sh_addr 794If this section appears in the memory image of a process, this member 795holds the address at which the section's first byte should reside. 796Otherwise, the member contains zero. 797.It Dv sh_offset 798This member's value holds the byte offset from the beginning of the file 799to the first byte in the section. 800One section type, 801.Dv SHT_NOBITS , 802occupies no space in the file, and its 803.Sy sh_offset 804member locates the conceptual placement in the file. 805.It Dv sh_size 806This member holds the section's size in bytes. 807Unless the section type 808is 809.Dv SHT_NOBITS , 810the section occupies 811.Sy sh_size 812bytes in the file. 813A section of type 814.Dv SHT_NOBITS 815may have a non-zero size, but it occupies no space in the file. 816.It Dv sh_link 817This member holds a section header table index link, whose interpretation 818depends on the section type. 819.It Dv sh_info 820This member holds extra information, whose interpretation depends on the 821section type. 822.It Dv sh_addralign 823Some sections have address alignment constraints. 824If a section holds a 825doubleword, the system must ensure doubleword alignment for the entire 826section. 827That is, the value of 828.Sy sh_addr 829must be congruent to zero, modulo the value of 830.Sy sh_addralign . 831Only zero and positive integral powers of two are allowed. 832Values of zero 833or one mean the section has no alignment constraints. 834.It Dv sh_entsize 835Some sections hold a table of fixed-sized entries, such as a symbol table. 836For such a section, this member gives the size in bytes for each entry. 837This member contains zero if the section does not hold a table of 838fixed-size entries. 839.El 840.Pp 841Various sections hold program and control information: 842.Bl -tag -width ".shstrtab" 843.It .bss 844This section holds uninitialized data that contribute to the program's 845memory image. 846By definition, the system initializes the data with zeros 847when the program begins to run. 848This section is of type 849.Dv SHT_NOBITS . 850The attribute types are 851.Dv SHF_ALLOC 852and 853.Dv SHF_WRITE . 854.It .comment 855This section holds version control information. 856This section is of type 857.Dv SHT_PROGBITS . 858No attribute types are used. 859.It .ctors 860This section holds initialized pointers to the C++ constructor functions. 861This section is of type 862.Dv SHT_PROGBITS . 863The attribute types are 864.Dv SHF_ALLOC 865and 866.Dv SHF_WRITE . 867.It .data 868This section holds initialized data that contribute to the program's 869memory image. 870This section is of type 871.Dv SHT_PROGBITS . 872The attribute types are 873.Dv SHF_ALLOC 874and 875.Dv SHF_WRITE . 876.It .data1 877This section holds initialized data that contribute to the program's 878memory image. 879This section is of type 880.Dv SHT_PROGBITS . 881The attribute types are 882.Dv SHF_ALLOC 883and 884.Dv SHF_WRITE . 885.It .debug 886This section holds information for symbolic debugging. 887The contents 888are unspecified. 889This section is of type 890.Dv SHT_PROGBITS . 891No attribute types are used. 892.It .dtors 893This section holds initialized pointers to the C++ destructor functions. 894This section is of type 895.Dv SHT_PROGBITS . 896The attribute types are 897.Dv SHF_ALLOC 898and 899.Dv SHF_WRITE . 900.It .dynamic 901This section holds dynamic linking information. 902The section's attributes 903will include the 904.Dv SHF_ALLOC 905bit. 906Whether the 907.Dv SHF_WRITE 908bit is set is processor-specific. 909This section is of type 910.Dv SHT_DYNAMIC . 911See the attributes above. 912.It .dynstr 913This section holds strings needed for dynamic linking, most commonly 914the strings that represent the names associated with symbol table entries. 915This section is of type 916.Dv SHT_STRTAB . 917The attribute type used is 918.Dv SHF_ALLOC . 919.It .dynsym 920This section holds the dynamic linking symbol table. 921This section is of type 922.Dv SHT_DYNSYM . 923The attribute used is 924.Dv SHF_ALLOC . 925.It .fini 926This section holds executable instructions that contribute to the process 927termination code. 928When a program exits normally the system arranges to 929execute the code in this section. 930This section is of type 931.Dv SHT_PROGBITS . 932The attributes used are 933.Dv SHF_ALLOC 934and 935.Dv SHF_EXECINSTR . 936.It .got 937This section holds the global offset table. 938This section is of type 939.Dv SHT_PROGBITS . 940The attributes are processor-specific. 941.It .hash 942This section holds a symbol hash table. 943This section is of type 944.Dv SHT_HASH . 945The attribute used is 946.Dv SHF_ALLOC . 947.It .init 948This section holds executable instructions that contribute to the process 949initialization code. 950When a program starts to run the system arranges to 951execute the code in this section before calling the main program entry point. 952This section is of type 953.Dv SHT_PROGBITS . 954The attributes used are 955.Dv SHF_ALLOC 956and 957.Dv SHF_EXECINSTR . 958.It .interp 959This section holds the pathname of a program interpreter. 960If the file has 961a loadable segment that includes the section, the section's attributes will 962include the 963.Dv SHF_ALLOC 964bit. 965Otherwise, that bit will be off. 966This section is of type 967.Dv SHT_PROGBITS . 968.It .line 969This section holds line number information for symbolic debugging, which 970describes the correspondence between the program source and the machine code. 971The contents are unspecified. 972This section is of type 973.Dv SHT_PROGBITS . 974No attribute types are used. 975.It .note 976This section holds information in the 977note section 978format described below. 979This section is of type 980.Dv SHT_NOTE . 981No attribute types are used. 982.Ox 983native executables usually contain a 984.Sy .note.openbsd.ident 985section to identify themselves, for the kernel to bypass any compatibility 986ELF binary emulation tests when loading the file. 987.It .plt 988This section holds the procedure linkage table. 989This section is of type 990.Dv SHT_PROGBITS . 991The attributes are processor-specific. 992.It .relNAME 993This section holds relocation information as described below. 994If the file 995has a loadable segment that includes relocation, the section's attributes 996will include the 997.Dv SHF_ALLOC 998bit. 999Otherwise the bit will be off. 1000By convention, 1001.Dq NAME 1002is supplied by the section to which the relocations apply. 1003Thus a relocation 1004section for 1005.Sy .text 1006normally would have the name 1007.Sy .rel.text . 1008This section is of type 1009.Dv SHT_REL . 1010.It .relaNAME 1011This section holds relocation information as described below. 1012If the file 1013has a loadable segment that includes relocation, the section's attributes 1014will include the 1015.Dv SHF_ALLOC 1016bit. 1017Otherwise the bit will be off. 1018By convention, 1019.Dq NAME 1020is supplied by the section to which the relocations apply. 1021Thus a relocation 1022section for 1023.Sy .text 1024normally would have the name 1025.Sy .rela.text . 1026This section is of type 1027.Dv SHT_RELA . 1028.It .rodata 1029This section holds read-only data that typically contribute to a 1030non-writable segment in the process image. 1031This section is of type 1032.Dv SHT_PROGBITS . 1033The attribute used is 1034.Dv SHF_ALLOC . 1035.It .rodata1 1036This section holds read-only data that typically contribute to a 1037non-writable segment in the process image. 1038This section is of type 1039.Dv SHT_PROGBITS . 1040The attribute used is 1041.Dv SHF_ALLOC . 1042.It .shstrtab 1043This section holds section names. 1044This section is of type 1045.Dv SHT_STRTAB . 1046No attribute types are used. 1047.It .strtab 1048This section holds strings, most commonly the strings that represent the 1049names associated with symbol table entries. 1050If the file has a loadable 1051segment that includes the symbol string table, the section's attributes 1052will include the 1053.Dv SHF_ALLOC 1054bit. 1055Otherwise the bit will be off. 1056This section is of type 1057.Dv SHT_STRTAB . 1058.It .symtab 1059This section holds a symbol table. 1060If the file has a loadable segment 1061that includes the symbol table, the section's attributes will include 1062the 1063.Dv SHF_ALLOC 1064bit. 1065Otherwise the bit will be off. 1066This section is of type 1067.Dv SHT_SYMTAB . 1068.It .tbss 1069This section is the thread-local storage version of 1070.Sy .bss , 1071holding uninitialized data that contribute to the program's memory 1072image on a per-thread basis. 1073By definition, the system allocates and initializes the data with 1074zeros for each thread before it first accesses it. 1075This section is of type 1076.Dv SHT_NOBITS . 1077The attribute types are 1078.Dv SHF_ALLOC , 1079.Dv SHF_WRITE , 1080and 1081.Dv SHF_TLS . 1082.It .tdata 1083This section is the thread-local storage version of 1084.Sy .data , 1085holding initialized data that contribute to the program's memory 1086image on a per-thread basis. 1087The system allocates and initializes the data for each thread before 1088it first accesses it. 1089This section is of type 1090.Dv SHT_PROGBITS . 1091The attribute types are 1092.Dv SHF_ALLOC , 1093.Dv SHF_WRITE , 1094and 1095.Dv SHF_TLS . 1096.It .text 1097This section holds the 1098.Dq text , 1099or executable instructions, of a program. 1100This section is of type 1101.Dv SHT_PROGBITS . 1102The attributes used are 1103.Dv SHF_ALLOC 1104and 1105.Dv SHF_EXECINSTR . 1106.El 1107.Pp 1108String table sections hold null-terminated character sequences, commonly 1109called strings. 1110The object file uses these strings to represent symbol 1111and section names. 1112One references a string as an index into the string 1113table section. 1114The first byte, which is index zero, is defined to hold 1115a null character. 1116Similarly, a string table's last byte is defined to 1117hold a null character, ensuring null termination for all strings. 1118.Pp 1119An object file's symbol table holds information needed to locate and 1120relocate a program's symbolic definitions and references. 1121A symbol table 1122index is a subscript into this array. 1123.Bd -literal -offset indent 1124typedef struct { 1125 Elf32_Word st_name; 1126 Elf32_Addr st_value; 1127 Elf32_Word st_size; 1128 unsigned char st_info; 1129 unsigned char st_other; 1130 Elf32_Half st_shndx; 1131} Elf32_Sym; 1132.Ed 1133.Bd -literal -offset indent 1134typedef struct { 1135 Elf64_Half st_name; 1136 Elf_Byte st_info; 1137 Elf_Byte st_other; 1138 Elf64_Quarter st_shndx; 1139 Elf64_Xword st_value; 1140 Elf64_Xword st_size; 1141} Elf64_Sym; 1142.Ed 1143.Bl -tag -width "st_value" 1144.It Dv st_name 1145This member holds an index into the object file's symbol string table, 1146which holds character representations of the symbol names. 1147If the value 1148is non-zero, it represents a string table index that gives the symbol 1149name. 1150Otherwise, the symbol table has no name. 1151.It Dv st_value 1152This member gives the value of the associated symbol. 1153.It Dv st_size 1154Many symbols have associated sizes. 1155This member holds zero if the symbol 1156has no size or an unknown size. 1157.It Dv st_info 1158This member specifies the symbol's type and binding attributes: 1159.Bl -tag -width "STT_SECTION" 1160.It Dv STT_NOTYPE 1161The symbol's type is not defined. 1162.It Dv STT_OBJECT 1163The symbol is associated with a data object. 1164.It Dv STT_FUNC 1165The symbol is associated with a function or other executable code. 1166.It Dv STT_SECTION 1167The symbol is associated with a section. 1168Symbol table entries of 1169this type exist primarily for relocation and normally have 1170.Dv STB_LOCAL 1171bindings. 1172.It Dv STT_FILE 1173By convention, the symbol's name gives the name of the source file 1174associated with the object file. 1175A file symbol has 1176.Dv STB_LOCAL 1177bindings, its section index is 1178.Dv SHN_ABS , 1179and it precedes the other 1180.Dv STB_LOCAL 1181symbols of the file, if it is present. 1182.It Dv STT_TLS 1183The symbol is associated with an object in thread-local storage. 1184The symbol's value is its offset in the TLS storage for this file. 1185.It Dv STT_LOPROC 1186This value up to and including 1187.Dv STT_HIPROC 1188is reserved for processor-specific semantics. 1189.It Dv STT_HIPROC 1190This value down to and including 1191.Dv STT_LOPROC 1192is reserved for processor-specific semantics. 1193.El 1194.Bl -tag -width "STB_GLOBAL" 1195.It Dv STB_LOCAL 1196Local symbols are not visible outside the object file containing their 1197definition. 1198Local symbols of the same name may exist in multiple files 1199without interfering with each other. 1200.It Dv STB_GLOBAL 1201Global symbols are visible to all object files being combined. 1202One file's 1203definition of a global symbol will satisfy another file's undefined 1204reference to the same symbol. 1205.It Dv STB_WEAK 1206Weak symbols resemble global symbols, but their definitions have lower 1207precedence. 1208.It Dv STB_LOPROC 1209This value up to and including 1210.Dv STB_HIPROC 1211is reserved for processor-specific semantics. 1212.It Dv STB_HIPROC 1213This value down to and including 1214.Dv STB_LOPROC 1215is reserved for processor-specific semantics. 1216.Pp 1217There are macros for packing and unpacking the binding and type fields: 1218.Pp 1219.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1220.It Xo 1221.Fn ELF32_ST_BIND info 1222.Xc 1223or 1224.Fn ELF64_ST_BIND info 1225extract a binding from an st_info value. 1226.It Xo 1227.Fn ELF64_ST_TYPE info 1228.Xc 1229or 1230.Fn ELF32_ST_TYPE info 1231extract a type from an st_info value. 1232.It Xo 1233.Fn ELF32_ST_INFO bind type 1234.Xc 1235or 1236.Fn ELF64_ST_INFO bind type 1237convert a binding and a type into an st_info value. 1238.El 1239.El 1240.It Dv st_other 1241This member currently holds zero and has no defined meaning. 1242.It Dv st_shndx 1243Every symbol table entry is 1244.Dq defined 1245in relation to some section. 1246This member holds the relevant section 1247header table index. 1248.El 1249.Pp 1250Relocation is the process of connecting symbolic references with 1251symbolic definitions. 1252Relocatable files must have information that 1253describes how to modify their section contents, thus allowing executable 1254and shared object files to hold the right information for a process' 1255program image. 1256Relocation entries are these data. 1257.Pp 1258Relocation structures that do not need an addend: 1259.Bd -literal -offset indent 1260typedef struct { 1261 Elf32_Addr r_offset; 1262 Elf32_Word r_info; 1263} Elf32_Rel; 1264.Ed 1265.Bd -literal -offset indent 1266typedef struct { 1267 Elf64_Xword r_offset; 1268 Elf64_Xword r_info; 1269} Elf64_Rel; 1270.Ed 1271.Pp 1272Relocation structures that need an addend: 1273.Bd -literal -offset indent 1274typedef struct { 1275 Elf32_Addr r_offset; 1276 Elf32_Word r_info; 1277 Elf32_Sword r_addend; 1278} Elf32_Rela; 1279.Ed 1280.Bd -literal -offset indent 1281typedef struct { 1282 Elf64_Xword r_offset; 1283 Elf64_Xword r_info; 1284 Elf64_Sxword r_addend; 1285} Elf64_Rela; 1286.Ed 1287.Bl -tag -width "r_offset" 1288.It Dv r_offset 1289This member gives the location at which to apply the relocation action. 1290For a relocatable file, the value is the byte offset from the beginning 1291of the section to the storage unit affected by the relocation. 1292For an 1293executable file or shared object, the value is the virtual address of 1294the storage unit affected by the relocation. 1295.It Dv r_info 1296This member gives both the symbol table index with respect to which the 1297relocation must be made and the type of relocation to apply. 1298Relocation 1299types are processor-specific. 1300When the text refers to a relocation 1301entry's relocation type or symbol table index, it means the result of 1302applying 1303.Dv ELF[32|64]_R_TYPE 1304or 1305.Dv ELF[32|64]_R_SYM , 1306respectively, to the entry's 1307.Sy r_info 1308member. 1309.It Dv r_addend 1310This member specifies a constant addend used to compute the value to be 1311stored into the relocatable field. 1312.El 1313.Pp 1314The note section is used to hold vendor-specific information that 1315may be used to help identify a binary's ABI. 1316It should start with an Elf_Note struct, 1317followed by the section name and the section description. 1318The actual note contents follow thereafter. 1319.Bd -literal -offset indent 1320typedef struct { 1321Elf32_Word namesz; 1322Elf32_Word descsz; 1323Elf32_Word type; 1324} Elf32_Note; 1325 1326typedef struct { 1327Elf64_Half namesz; 1328Elf64_Half descsz; 1329Elf64_Half type; 1330} Elf64_Note; 1331.Ed 1332.Bl -tag -width "r_offset" 1333.It Dv namesz 1334Length of the note name, rounded up to a 4-byte boundary. 1335.It Dv descsz 1336Length of the note description, rounded up to a 4-byte boundary. 1337.It Dv type 1338A vendor-specific note type. 1339.El 1340.Pp 1341The name and description strings follow the note structure. 1342Each string is aligned on a 4-byte boundary. 1343.Sh SEE ALSO 1344.Xr as 1 , 1345.Xr gdb 1 , 1346.Xr ld 1 , 1347.Xr objdump 1 , 1348.Xr execve 2 , 1349.Xr core 5 1350.Rs 1351.%A Hewlett-Packard 1352.%B Elf-64 Object File Format 1353.Re 1354.Rs 1355.%A Santa Cruz Operation 1356.%B System V Application Binary Interface 1357.Re 1358.Rs 1359.%A Unix System Laboratories 1360.%T Object Files 1361.%B "Executable and Linking Format (ELF)" 1362.Re 1363.Sh HISTORY 1364.Ox 1365ELF support first appeared in 1366.Ox 1.2 . 1367Starting with 1368.Ox 5.4 , 1369all supported platforms use it as the native binary file format. 1370ELF in itself first appeared in 1371.At V . 1372The ELF format is an adopted standard. 1373.Sh AUTHORS 1374This manual page was written by 1375.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org 1376with inspiration from BSDi's 1377.Bsx 1378.Nm elf 1379manpage. 1380