1.\" $OpenBSD: elf.5,v 1.40 2021/04/15 18:13:24 kettenis 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: April 15 2021 $ 29.Dt ELF 5 30.Os 31.Sh NAME 32.Nm elf 33.Nd format of ELF executable binary files 34.Sh SYNOPSIS 35.In elf.h 36.Sh DESCRIPTION 37The header file 38.In elf.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.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 32-bit program address 83Elf32_Half Unsigned 16-bit field 84Elf32_Lword Unsigned 64-bit field 85Elf32_Off Unsigned 32-bit file offset 86Elf32_Sword Signed 32-bit field or integer 87Elf32_Word Unsigned 32-bit field or integer 88.Ed 89.Pp 90And the following types are used for 64-bit architectures: 91.Bd -literal -offset indent 92Elf64_Addr Unsigned 64-bit program address 93Elf64_Half Unsigned 16-bit field 94Elf64_Lword Unsigned 64-bit field 95Elf64_Off Unsigned 64-bit file offset 96Elf64_Sword Signed 32-bit field 97Elf64_Sxword Signed 64-bit field or integer 98Elf64_Word Unsigned 32-bit field 99Elf64_Xword Unsigned 64-bit field or integer 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_Half e_type; 132 Elf64_Half e_machine; 133 Elf64_Word e_version; 134 Elf64_Addr e_entry; 135 Elf64_Off e_phoff; 136 Elf64_Off e_shoff; 137 Elf64_Word e_flags; 138 Elf64_Half e_ehsize; 139 Elf64_Half e_phentsize; 140 Elf64_Half e_phnum; 141 Elf64_Half e_shentsize; 142 Elf64_Half e_shnum; 143 Elf64_Half 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 how 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 211This byte identifies the OS- or ABI-specific ELF extensions used by 212this object. 213Some fields in other ELF structures have flags and values that have 214platform specific meanings; the interpretation of those fields is 215determined by the value of this byte. 216The following values are currently defined: 217.Pp 218.Bl -tag -width "ELFOSABI_STANDALONE" -compact 219.It Dv ELFOSABI_SYSV 220UNIX System V ABI. 221.It Dv ELFOSABI_HPUX 222HP-UX operating system ABI. 223.It Dv ELFOSABI_NETBSD 224.Nx 225operating system ABI. 226.It Dv ELFOSABI_LINUX 227GNU/Linux operating system ABI. 228.It Dv ELFOSABI_HURD 229GNU/Hurd operating system ABI. 230.It Dv ELFOSABI_86OPEN 23186Open Common IA32 ABI. 232.It Dv ELFOSABI_SOLARIS 233Solaris operating system ABI. 234.It Dv ELFOSABI_MONTEREY 235Monterey project ABI. 236.It Dv ELFOSABI_IRIX 237IRIX operating system ABI. 238.It Dv ELFOSABI_FREEBSD 239.Fx 240operating system ABI. 241.It Dv ELFOSABI_TRU64 242TRU64 UNIX operating system ABI. 243.It Dv ELFOSABI_MODESTO 244Novell Modesto operating system ABI. 245.It Dv ELFOSABI_OPENBSD 246.Ox 247operating system ABI. 248.It Dv ELFOSABI_ARM 249ARM architecture ABI. 250.It Dv ELFOSABI_STANDALONE 251Stand-alone (embedded) ABI. 252.El 253.It Dv EI_ABIVERSION 254This byte identifies the version of the ABI 255to which the object is targeted. 256This field is used to distinguish among incompatible versions of an ABI. 257The interpretation of this version number 258is dependent on the ABI identified by the EI_OSABI field. 259.It Dv EI_PAD 260Start of padding. 261These bytes are reserved and set to zero. 262Programs which read them should ignore them. 263The value for EI_PAD will change in 264the future if currently unused bytes are given meanings. 265.It Dv EI_NIDENT 266The size of the e_ident array. 267.El 268.It Dv e_type 269This member of the structure identifies the object file type: 270.Pp 271.Bl -tag -width "ET_NONE" -compact 272.It Dv ET_NONE 273An unknown type. 274.It Dv ET_REL 275A relocatable file. 276.It Dv ET_EXEC 277An executable file. 278.It Dv ET_DYN 279A shared object. 280.It Dv ET_CORE 281A core file. 282.El 283.It Dv e_machine 284This member specifies the required architecture for an individual file: 285.Pp 286.Bl -tag -width "EM_MIPS_RS4_BE" -compact 287.It Dv EM_NONE 288An unknown machine. 289.It Dv EM_M32 290AT&T WE 32100. 291.It Dv EM_SPARC 292Sun Microsystems SPARC. 293.It Dv EM_386 294Intel 80386. 295.It Dv EM_68K 296Motorola 68000. 297.It Dv EM_88K 298Motorola 88000. 299.It Dv EM_486 300Intel 80486. 301.It Dv EM_860 302Intel 80860. 303.It Dv EM_MIPS 304MIPS RS3000 (big-endian only). 305.It Dv EM_MIPS_RS4_BE 306MIPS RS4000 (big-endian only). 307.It Dv EM_SPARC64 308SPARC v9 64-bit (unofficial). 309.It Dv EM_PARISC 310HPPA. 311.It Dv EM_SPARC32PLUS 312SPARC with enhanced instruction set. 313.It Dv EM_PPC 314PowerPC. 315.It Dv EM_PPC64 316PowerPC 64-bit. 317.It Dv EM_ARM 318Advanced RISC Machines ARM. 319.It Dv EM_ALPHA 320Compaq [DEC] Alpha. 321.It Dv EM_SH 322Hitachi/Renesas Super-H. 323.It Dv EM_SPARCV9 324SPARC v9 64-bit. 325.It Dv EM_IA_64 326Intel IA-64. 327.It Dv EM_AMD64 328AMD64. 329.It Dv EM_VAX 330DEC Vax. 331.It Dv EM_AARCH64 332ARM 64-bit. 333.It Dv EM_ALPHA_EXP 334Compaq [DEC] Alpha with enhanced instruction set. 335.El 336.It Dv e_version 337This member identifies the file version: 338.Pp 339.Bl -tag -width "EV_CURRENT" -compact 340.It Dv EV_NONE 341Invalid version. 342.It Dv EV_CURRENT 343Current version. 344.El 345.It Dv e_entry 346This member gives the virtual address to which the system first transfers 347control, thus starting the process. 348If the file has no associated entry 349point, this member holds zero. 350.It Dv e_phoff 351This member holds the program header table's file offset in bytes. 352If 353the file has no program header table, this member holds zero. 354.It Dv e_shoff 355This member holds the section header table's file offset in bytes. 356If the 357file has no section header table this member holds zero. 358.It Dv e_flags 359This member holds processor-specific flags associated with the file. 360Flag names take the form EF_`machine_flag'. 361Currently no flags have been defined. 362.It Dv e_ehsize 363This member holds the ELF header's size in bytes. 364.It Dv e_phentsize 365This member holds the size in bytes of one entry in the file's program header 366table; all entries are the same size. 367.It Dv e_phnum 368This member holds the number of entries in the program header 369table. 370Thus the product of 371.Sy e_phentsize 372and 373.Sy e_phnum 374gives the table's size 375in bytes. 376If a file has no program header, 377.Sy e_phnum 378holds the value zero. 379.It Dv e_shentsize 380This member holds a sections header's size in bytes. 381A section header is one 382entry in the section header table; all entries are the same size. 383.It Dv e_shnum 384This member holds the number of entries in the section header table. 385Thus 386the product of 387.Sy e_shentsize 388and 389.Sy e_shnum 390gives the section header table's size in bytes. 391If a file has no section 392header table, 393.Sy e_shnum 394holds the value of zero. 395.It Dv e_shstrndx 396This member holds the section header table index of the entry associated 397with the section name string table. 398If the file has no section name string 399table, this member holds the value 400.Dv SHN_UNDEF . 401.El 402.Pp 403An executable or shared object file's program header table is an array of 404structures, each describing a segment or other information the system needs 405to prepare the program for execution. 406An object file 407.Em segment 408contains one or more 409.Em sections . 410Program headers are meaningful only for executable and shared object files. 411A file specifies its own program header size with the ELF header's 412.Sy e_phentsize 413and 414.Sy e_phnum 415members. 416As with the ELF executable header, the program header 417also has different versions depending on the architecture: 418.Bd -literal -offset indent 419typedef struct { 420 Elf32_Word p_type; 421 Elf32_Off p_offset; 422 Elf32_Addr p_vaddr; 423 Elf32_Addr p_paddr; 424 Elf32_Word p_filesz; 425 Elf32_Word p_memsz; 426 Elf32_Word p_flags; 427 Elf32_Word p_align; 428} Elf32_Phdr; 429.Ed 430.Bd -literal -offset indent 431typedef struct { 432 Elf64_Word p_type; 433 Elf64_Word p_flags; 434 Elf64_Off p_offset; 435 Elf64_Addr p_vaddr; 436 Elf64_Addr p_paddr; 437 Elf64_Xword p_filesz; 438 Elf64_Xword p_memsz; 439 Elf64_Xword p_align; 440} Elf64_Phdr; 441.Ed 442.Pp 443The main difference between the 32-bit and the 64-bit program header lies 444only in the location of a 445.Sy p_flags 446member in the total struct. 447.Bl -tag -width "p_offset" -offset indent 448.It Dv p_type 449This member of the Phdr struct tells what kind of segment this array 450element describes or how to interpret the array element's information. 451.Bl -tag -width "PT_DYNAMIC" 452.It Dv PT_NULL 453The array element is unused and the other members' values are undefined. 454This lets the program header have ignored entries. 455.It Dv PT_LOAD 456The array element specifies a loadable segment, described by 457.Sy p_filesz 458and 459.Sy p_memsz . 460The bytes from the file are mapped to the beginning of the memory 461segment. 462If the segment's memory size 463.Pq Sy p_memsz 464is larger than the file size 465.Pq Sy p_filesz , 466the 467.Dq extra 468bytes are defined to hold the value 0 and to follow the segment's 469initialized area. 470The file size may not be larger than the memory size. 471Loadable segment entries in the program header table appear in ascending 472order, sorted on the 473.Sy p_vaddr 474member. 475.It Dv PT_DYNAMIC 476The array element specifies the location and size of the dynamic section, 477both in the file and in the memory image of the program. 478This segment type may not occur more than once in a file and may 479only occur if the dynamic section is part of the memory image of 480the program. 481.It Dv PT_INTERP 482The array element specifies the location and size of a null-terminated 483path name to invoke as an interpreter. 484This segment type is meaningful 485only for executable files (though it may occur for shared objects). 486However it may not occur more than once in a file. 487If it is present, it must precede any loadable segment entry. 488.It Dv PT_NOTE 489The array element specifies the location and size for auxiliary information. 490.It Dv PT_SHLIB 491This segment type is reserved but has unspecified semantics. 492Programs that 493contain an array element of this type do not conform to the ABI. 494.It Dv PT_PHDR 495The array element specifies the location and size of the program 496header table itself, both in the file and in the memory image of the program. 497This segment type may not occur more than once in a file and may 498only occur if the program header table is part of the memory image 499of the program. 500If it is present, it must precede any loadable segment entry. 501.It Dv PT_TLS 502The array element specifies the location and size of the 503thread-local storage for this file. 504Each thread in a process loading this file will have the segment's 505memory size 506.Pq Sy p_memsz 507allocated for it, where the bytes up to the segment's file size 508.Pq Sy p_filesz 509will be initialized with the data in this segment and the remaining 510.Dq extra 511bytes will be set to zero. 512This segment type may not occur more than once in a file and may 513only occur if the thread-local storage is part of the memory image 514of the program. 515.It Dv PT_GNU_EH_FRAME 516The array element specifies the location and size of the GNU exception 517frame header, both in the file and in the memory image of the program. 518This segment type may not occur more than once in a file and may 519only occur if the GNU exception frame header is part of the memory 520image of the program. 521.It Dv PT_GNU_RELRO 522The array element specifies the location and size of a part of the 523memory image of the program that should be made read-only once all 524immediate relocation processing for the file has been performed. 525This segment type may not occur more than once in a file. 526.It Dv PT_OPENBSD_RANDOMIZE 527The array element specifies the location and size of a part of the 528memory image of the program that must be filled with random data 529before any code in the object is executed. 530The memory region specified by a segment of this type may overlap 531the region specified by a 532.Dv PT_GNU_RELRO 533segment, in which case the intersection will be filled with random 534data before being marked read-only. 535This segment type may occur more than once in a file, but a limit 536on the total number of bytes in the segments for an object of no 537less than 65536 bytes may be imposed. 538.It Dv PT_OPENBSD_WXNEEDED 539The array element specifies that a process executing this file may 540need to be able to map or protect memory regions as simultaneously 541executable and writable. 542If the system is unable or unwilling to permit that for this executable 543then it may fail immediately. 544This segment type is meaningful only for executable files and is 545ignored in other objects. 546.It Dv PT_LOOS 547This value up to and including 548.Dv PT_HIOS 549is reserved for operating system-specific semantics. 550.It Dv PT_HIOS 551This value down to and including 552.Dv PT_LOOS 553is reserved for operating system-specific semantics. 554.It Dv PT_LOPROC 555This value up to and including 556.Dv PT_HIPROC 557is reserved for processor-specific semantics. 558.It Dv PT_HIPROC 559This value down to and including 560.Dv PT_LOPROC 561is reserved for processor-specific semantics. 562.El 563.It Dv p_offset 564This member holds the offset from the beginning of the file at which 565the first byte of the segment resides. 566.It Dv p_vaddr 567This member holds the virtual address at which the first byte of the 568segment resides in memory. 569.It Dv p_paddr 570On systems for which physical addressing is relevant, this member is 571reserved for the segment's physical address. 572Under 573.Bx 574this member is 575not used and must be zero. 576.It Dv p_filesz 577This member holds the number of bytes in the file image of the segment. 578It may be zero. 579.It Dv p_memsz 580This member holds the number of bytes in the memory image of the segment. 581It may be zero. 582.It Dv p_flags 583This member holds flags relevant to the segment: 584.Pp 585.Bl -tag -width "PF_X" -compact 586.It Dv PF_X 587An executable segment. 588.It Dv PF_W 589A writable segment. 590.It Dv PF_R 591A readable segment. 592.El 593.Pp 594A text segment commonly has the flags 595.Dv PF_X 596and 597.Dv PF_R . 598A data segment commonly has 599.Dv PF_X , 600.Dv PF_W 601and 602.Dv PF_R . 603.It Dv p_align 604This member holds the value to which the segments are aligned in memory 605and in the file. 606Loadable process segments must have congruent values for 607.Sy p_vaddr 608and 609.Sy p_offset , 610modulo the page size. 611Values of zero and one mean no alignment is required. 612Otherwise, 613.Sy p_align 614should be a positive, integral power of two, and 615.Sy p_vaddr 616should equal 617.Sy p_offset , 618modulo 619.Sy p_align . 620.El 621.Pp 622A file's section header table lets one locate all the file's sections. 623The 624section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 625The 626ELF header's 627.Sy e_shoff 628member gives the byte offset from the beginning of the file to the section 629header table. 630.Sy e_shnum 631holds the number of entries the section header table contains. 632.Sy e_shentsize 633holds the size in bytes of each entry. 634.Pp 635A section header table index is a subscript into this array. 636Some section 637header table indices are reserved. 638An object file does not have sections for 639these special indices: 640.Bl -tag -width "SHN_LORESERVE" 641.It Dv SHN_UNDEF 642This value marks an undefined, missing, irrelevant or otherwise meaningless 643section reference. 644For example, a symbol 645.Dq defined 646relative to section number 647.Dv SHN_UNDEF 648is an undefined symbol. 649.It Dv SHN_LORESERVE 650This value specifies the lower bound of the range of reserved indices. 651.It Dv SHN_LOPROC 652This value up to and including 653.Dv SHN_HIPROC 654is reserved for processor-specific semantics. 655.It Dv SHN_HIPROC 656This value down to and including 657.Dv SHN_LOPROC 658is reserved for processor-specific semantics. 659.It Dv SHN_ABS 660This value specifies the absolute value for the corresponding reference. 661For 662example, a symbol defined relative to section number 663.Dv SHN_ABS 664has an absolute value and is not affected by relocation. 665.It Dv SHN_COMMON 666Symbols defined relative to this section are common symbols, such as FORTRAN 667COMMON or unallocated C external variables. 668.It Dv SHN_HIRESERVE 669This value specifies the upper bound of the range of reserved indices. 670The 671system reserves indices between 672.Dv SHN_LORESERVE 673and 674.Dv SHN_HIRESERVE , 675inclusive. 676The section header table does not contain entries for the 677reserved indices. 678.El 679.Pp 680The section header has the following structure: 681.Bd -literal -offset indent 682typedef struct { 683 Elf32_Word sh_name; 684 Elf32_Word sh_type; 685 Elf32_Word sh_flags; 686 Elf32_Addr sh_addr; 687 Elf32_Off sh_offset; 688 Elf32_Word sh_size; 689 Elf32_Word sh_link; 690 Elf32_Word sh_info; 691 Elf32_Word sh_addralign; 692 Elf32_Word sh_entsize; 693} Elf32_Shdr; 694.Ed 695.Bd -literal -offset indent 696typedef struct { 697 Elf64_Word sh_name; 698 Elf64_Word sh_type; 699 Elf64_Xword sh_flags; 700 Elf64_Addr sh_addr; 701 Elf64_Off sh_offset; 702 Elf64_Xword sh_size; 703 Elf64_Word sh_link; 704 Elf64_Word sh_info; 705 Elf64_Xword sh_addralign; 706 Elf64_Xword sh_entsize; 707} Elf64_Shdr; 708.Ed 709.Bl -tag -width "sh_addralign" 710.It Dv sh_name 711This member specifies the name of the section. 712Its value is an index 713into the section header string table section, giving the location of 714a null-terminated string. 715.It Dv sh_type 716This member categorizes the section's contents and semantics. 717.Bl -tag -width "SHT_PROGBITS" 718.It Dv SHT_NULL 719This value marks the section header as inactive. 720It does not 721have an associated section. 722Other members of the section header 723have undefined values. 724.It Dv SHT_PROGBITS 725This section holds information defined by the program, whose 726format and meaning are determined solely by the program. 727.It Dv SHT_SYMTAB 728This section holds a symbol table. 729Typically, 730.Dv SHT_SYMTAB 731provides symbols for link editing, though it may also be used 732for dynamic linking. 733As a complete symbol table, it may contain 734many symbols unnecessary for dynamic linking. 735An object file can 736also contain a 737.Dv SHT_DYNSYM 738section. 739.It Dv SHT_STRTAB 740This section holds a string table. 741An object file may have multiple 742string table sections. 743.It Dv SHT_RELA 744This section holds relocation entries with explicit addends, such 745as type 746.Sy Elf32_Rela 747for the 32-bit class of object files. 748An object may have multiple 749relocation sections. 750.It Dv SHT_HASH 751This section holds a symbol hash table. 752An object participating in 753dynamic linking must contain a symbol hash table. 754An object file may 755have only one hash table. 756.It Dv SHT_DYNAMIC 757This section holds information for dynamic linking. 758An object file may 759have only one dynamic section. 760.It Dv SHT_NOTE 761This section holds information that marks the file in some way. 762.It Dv SHT_NOBITS 763A section of this type occupies no space in the file but otherwise 764resembles 765.Dv SHT_PROGBITS . 766Although this section contains no bytes, the 767.Sy sh_offset 768member contains the conceptual file offset. 769.It Dv SHT_REL 770This section holds relocation offsets without explicit addends, such 771as type 772.Sy Elf32_Rel 773for the 32-bit class of object files. 774An object file may have multiple 775relocation sections. 776.It Dv SHT_SHLIB 777This section is reserved but has unspecified semantics. 778.It Dv SHT_DYNSYM 779This section holds a minimal set of dynamic linking symbols. 780An 781object file can also contain a 782.Dv SHT_SYMTAB 783section. 784.It Dv SHT_LOPROC 785This value up to and including 786.Dv SHT_HIPROC 787is reserved for processor-specific semantics. 788.It Dv SHT_HIPROC 789This value down to and including 790.Dv SHT_LOPROC 791is reserved for processor-specific semantics. 792.It Dv SHT_LOUSER 793This value specifies the lower bound of the range of indices reserved for 794application programs. 795.It Dv SHT_HIUSER 796This value specifies the upper bound of the range of indices reserved for 797application programs. 798Section types between 799.Dv SHT_LOUSER 800and 801.Dv SHT_HIUSER 802may be used by the application, without conflicting with current or future 803system-defined section types. 804.El 805.It Dv sh_flags 806Sections support one-bit flags that describe miscellaneous attributes. 807If a flag bit is set in 808.Sy sh_flags , 809the attribute is 810.Dq on 811for the section. 812Otherwise, the attribute is 813.Dq off 814or does not apply. 815Undefined attributes are set to zero. 816.Pp 817.Bl -tag -width "SHF_EXECINSTR" -compact 818.It Dv SHF_WRITE 819This section contains data that should be writable during process 820execution. 821.It Dv SHF_ALLOC 822This section occupies memory during process execution. 823Some control 824sections do not reside in the memory image of an object file. 825This 826attribute is off for those sections. 827.It Dv SHF_EXECINSTR 828This section contains executable machine instructions. 829.It Dv SHF_TLS 830This section is for thread-local storage. 831.It Dv SHF_MASKPROC 832All bits included in this mask are reserved for processor-specific 833semantics. 834.El 835.It Dv sh_addr 836If this section appears in the memory image of a process, this member 837holds the address at which the section's first byte should reside. 838Otherwise, the member contains zero. 839.It Dv sh_offset 840This member's value holds the byte offset from the beginning of the file 841to the first byte in the section. 842One section type, 843.Dv SHT_NOBITS , 844occupies no space in the file, and its 845.Sy sh_offset 846member locates the conceptual placement in the file. 847.It Dv sh_size 848This member holds the section's size in bytes. 849Unless the section type 850is 851.Dv SHT_NOBITS , 852the section occupies 853.Sy sh_size 854bytes in the file. 855A section of type 856.Dv SHT_NOBITS 857may have a non-zero size, but it occupies no space in the file. 858.It Dv sh_link 859This member holds a section header table index link, whose interpretation 860depends on the section type. 861.It Dv sh_info 862This member holds extra information, whose interpretation depends on the 863section type. 864.It Dv sh_addralign 865Some sections have address alignment constraints. 866If a section holds a 867doubleword, the system must ensure doubleword alignment for the entire 868section. 869That is, the value of 870.Sy sh_addr 871must be congruent to zero, modulo the value of 872.Sy sh_addralign . 873Only zero and positive integral powers of two are allowed. 874Values of zero 875or one mean the section has no alignment constraints. 876.It Dv sh_entsize 877Some sections hold a table of fixed-sized entries, such as a symbol table. 878For such a section, this member gives the size in bytes for each entry. 879This member contains zero if the section does not hold a table of 880fixed-size entries. 881.El 882.Pp 883Various sections hold program and control information: 884.Bl -tag -width ".shstrtab" 885.It .SUNW_ctf 886This section contains the (un)compressed Compact C-Type Format data 887describing the object's types and symbols. 888This section is of type 889.Dv SHT_PROGBITS . 890.It .bss 891This section holds uninitialized data that contribute to the program's 892memory image. 893By definition, the system initializes the data with zeros 894when the program begins to run. 895This section is of type 896.Dv SHT_NOBITS . 897The attribute types are 898.Dv SHF_ALLOC 899and 900.Dv SHF_WRITE . 901.It .comment 902This section holds version control information. 903This section is of type 904.Dv SHT_PROGBITS . 905No attribute types are used. 906.It .ctors 907This section holds initialized pointers to the C++ constructor functions. 908This section is of type 909.Dv SHT_PROGBITS . 910The attribute types are 911.Dv SHF_ALLOC 912and 913.Dv SHF_WRITE . 914.It .data 915This section holds initialized data that contribute to the program's 916memory image. 917This section is of type 918.Dv SHT_PROGBITS . 919The attribute types are 920.Dv SHF_ALLOC 921and 922.Dv SHF_WRITE . 923.It .data1 924This section holds initialized data that contribute to the program's 925memory image. 926This section is of type 927.Dv SHT_PROGBITS . 928The attribute types are 929.Dv SHF_ALLOC 930and 931.Dv SHF_WRITE . 932.It .debug 933This section holds information for symbolic debugging. 934The contents 935are unspecified. 936This section is of type 937.Dv SHT_PROGBITS . 938No attribute types are used. 939.It .dtors 940This section holds initialized pointers to the C++ destructor functions. 941This section is of type 942.Dv SHT_PROGBITS . 943The attribute types are 944.Dv SHF_ALLOC 945and 946.Dv SHF_WRITE . 947.It .dynamic 948This section holds dynamic linking information. 949The section's attributes 950will include the 951.Dv SHF_ALLOC 952bit. 953Whether the 954.Dv SHF_WRITE 955bit is set is processor-specific. 956This section is of type 957.Dv SHT_DYNAMIC . 958See the attributes above. 959.It .dynstr 960This section holds strings needed for dynamic linking, most commonly 961the strings that represent the names associated with symbol table entries. 962This section is of type 963.Dv SHT_STRTAB . 964The attribute type used is 965.Dv SHF_ALLOC . 966.It .dynsym 967This section holds the dynamic linking symbol table. 968This section is of type 969.Dv SHT_DYNSYM . 970The attribute used is 971.Dv SHF_ALLOC . 972.It .fini 973This section holds executable instructions that contribute to the process 974termination code. 975When a program exits normally the system arranges to 976execute the code in this section. 977This section is of type 978.Dv SHT_PROGBITS . 979The attributes used are 980.Dv SHF_ALLOC 981and 982.Dv SHF_EXECINSTR . 983.It .got 984This section holds the global offset table. 985This section is of type 986.Dv SHT_PROGBITS . 987The attributes are processor-specific. 988.It .hash 989This section holds a symbol hash table. 990This section is of type 991.Dv SHT_HASH . 992The attribute used is 993.Dv SHF_ALLOC . 994.It .init 995This section holds executable instructions that contribute to the process 996initialization code. 997When a program starts to run the system arranges to 998execute the code in this section before calling the main program entry point. 999This section is of type 1000.Dv SHT_PROGBITS . 1001The attributes used are 1002.Dv SHF_ALLOC 1003and 1004.Dv SHF_EXECINSTR . 1005.It .interp 1006This section holds the pathname of a program interpreter. 1007If the file has 1008a loadable segment that includes the section, the section's attributes will 1009include the 1010.Dv SHF_ALLOC 1011bit. 1012Otherwise, that bit will be off. 1013This section is of type 1014.Dv SHT_PROGBITS . 1015.It .line 1016This section holds line number information for symbolic debugging, which 1017describes the correspondence between the program source and the machine code. 1018The contents are unspecified. 1019This section is of type 1020.Dv SHT_PROGBITS . 1021No attribute types are used. 1022.It .note 1023This section holds information in the 1024note section 1025format described below. 1026This section is of type 1027.Dv SHT_NOTE . 1028No attribute types are used. 1029.Ox 1030native executables contain a 1031.Sy .note.openbsd.ident 1032section to identify themselves. 1033.It .plt 1034This section holds the procedure linkage table. 1035This section is of type 1036.Dv SHT_PROGBITS . 1037The attributes are processor-specific. 1038.It .relNAME 1039This section holds relocation information as described below. 1040If the file 1041has a loadable segment that includes relocation, the section's attributes 1042will include the 1043.Dv SHF_ALLOC 1044bit. 1045Otherwise the bit will be off. 1046By convention, 1047.Dq NAME 1048is supplied by the section to which the relocations apply. 1049Thus a relocation 1050section for 1051.Sy .text 1052normally would have the name 1053.Sy .rel.text . 1054This section is of type 1055.Dv SHT_REL . 1056.It .relaNAME 1057This section holds relocation information as described below. 1058If the file 1059has a loadable segment that includes relocation, the section's attributes 1060will include the 1061.Dv SHF_ALLOC 1062bit. 1063Otherwise the bit will be off. 1064By convention, 1065.Dq NAME 1066is supplied by the section to which the relocations apply. 1067Thus a relocation 1068section for 1069.Sy .text 1070normally would have the name 1071.Sy .rela.text . 1072This section is of type 1073.Dv SHT_RELA . 1074.It .rodata 1075This section holds read-only data that typically contribute to a 1076non-writable segment in the process image. 1077This section is of type 1078.Dv SHT_PROGBITS . 1079The attribute used is 1080.Dv SHF_ALLOC . 1081.It .rodata1 1082This section holds read-only data that typically contribute to a 1083non-writable segment in the process image. 1084This section is of type 1085.Dv SHT_PROGBITS . 1086The attribute used is 1087.Dv SHF_ALLOC . 1088.It .shstrtab 1089This section holds section names. 1090This section is of type 1091.Dv SHT_STRTAB . 1092No attribute types are used. 1093.It .strtab 1094This section holds strings, most commonly the strings that represent the 1095names associated with symbol table entries. 1096If the file has a loadable 1097segment that includes the symbol string table, the section's attributes 1098will include the 1099.Dv SHF_ALLOC 1100bit. 1101Otherwise the bit will be off. 1102This section is of type 1103.Dv SHT_STRTAB . 1104.It .symtab 1105This section holds a symbol table. 1106If the file has a loadable segment 1107that includes the symbol table, the section's attributes will include 1108the 1109.Dv SHF_ALLOC 1110bit. 1111Otherwise the bit will be off. 1112This section is of type 1113.Dv SHT_SYMTAB . 1114.It .tbss 1115This section is the thread-local storage version of 1116.Sy .bss , 1117holding uninitialized data that contribute to the program's memory 1118image on a per-thread basis. 1119By definition, the system allocates and initializes the data with 1120zeros for each thread before it first accesses it. 1121This section is of type 1122.Dv SHT_NOBITS . 1123The attribute types are 1124.Dv SHF_ALLOC , 1125.Dv SHF_WRITE , 1126and 1127.Dv SHF_TLS . 1128.It .tdata 1129This section is the thread-local storage version of 1130.Sy .data , 1131holding initialized data that contribute to the program's memory 1132image on a per-thread basis. 1133The system allocates and initializes the data for each thread before 1134it first accesses it. 1135This section is of type 1136.Dv SHT_PROGBITS . 1137The attribute types are 1138.Dv SHF_ALLOC , 1139.Dv SHF_WRITE , 1140and 1141.Dv SHF_TLS . 1142.It .text 1143This section holds the 1144.Dq text , 1145or executable instructions, of a program. 1146This section is of type 1147.Dv SHT_PROGBITS . 1148The attributes used are 1149.Dv SHF_ALLOC 1150and 1151.Dv SHF_EXECINSTR . 1152.El 1153.Pp 1154String table sections hold null-terminated character sequences, commonly 1155called strings. 1156The object file uses these strings to represent symbol 1157and section names. 1158One references a string as an index into the string 1159table section. 1160The first byte, which is index zero, is defined to hold 1161a null character. 1162Similarly, a string table's last byte is defined to 1163hold a null character, ensuring null termination for all strings. 1164.Pp 1165An object file's symbol table holds information needed to locate and 1166relocate a program's symbolic definitions and references. 1167A symbol table 1168index is a subscript into this array. 1169.Bd -literal -offset indent 1170typedef struct { 1171 Elf32_Word st_name; 1172 Elf32_Addr st_value; 1173 Elf32_Word st_size; 1174 unsigned char st_info; 1175 unsigned char st_other; 1176 Elf32_Half st_shndx; 1177} Elf32_Sym; 1178.Ed 1179.Bd -literal -offset indent 1180typedef struct { 1181 Elf64_Word st_name; 1182 unsigned char st_info; 1183 unsigned char st_other; 1184 Elf64_Half st_shndx; 1185 Elf64_Addr st_value; 1186 Elf64_Xword st_size; 1187} Elf64_Sym; 1188.Ed 1189.Bl -tag -width "st_value" 1190.It Dv st_name 1191This member holds an index into the object file's symbol string table, 1192which holds character representations of the symbol names. 1193If the value 1194is non-zero, it represents a string table index that gives the symbol 1195name. 1196Otherwise, the symbol table has no name. 1197.It Dv st_value 1198This member gives the value of the associated symbol. 1199.It Dv st_size 1200Many symbols have associated sizes. 1201This member holds zero if the symbol 1202has no size or an unknown size. 1203.It Dv st_info 1204This member specifies the symbol's type and binding attributes: 1205.Bl -tag -width "STT_SECTION" 1206.It Dv STT_NOTYPE 1207The symbol's type is not defined. 1208.It Dv STT_OBJECT 1209The symbol is associated with a data object. 1210.It Dv STT_FUNC 1211The symbol is associated with a function or other executable code. 1212.It Dv STT_SECTION 1213The symbol is associated with a section. 1214Symbol table entries of 1215this type exist primarily for relocation and normally have 1216.Dv STB_LOCAL 1217bindings. 1218.It Dv STT_FILE 1219By convention, the symbol's name gives the name of the source file 1220associated with the object file. 1221A file symbol has 1222.Dv STB_LOCAL 1223bindings, its section index is 1224.Dv SHN_ABS , 1225and it precedes the other 1226.Dv STB_LOCAL 1227symbols of the file, if it is present. 1228.It Dv STT_TLS 1229The symbol is associated with an object in thread-local storage. 1230The symbol's value is its offset in the TLS storage for this file. 1231.It Dv STT_LOPROC 1232This value up to and including 1233.Dv STT_HIPROC 1234is reserved for processor-specific semantics. 1235.It Dv STT_HIPROC 1236This value down to and including 1237.Dv STT_LOPROC 1238is reserved for processor-specific semantics. 1239.El 1240.Bl -tag -width "STB_GLOBAL" 1241.It Dv STB_LOCAL 1242Local symbols are not visible outside the object file containing their 1243definition. 1244Local symbols of the same name may exist in multiple files 1245without interfering with each other. 1246.It Dv STB_GLOBAL 1247Global symbols are visible to all object files being combined. 1248One file's 1249definition of a global symbol will satisfy another file's undefined 1250reference to the same symbol. 1251.It Dv STB_WEAK 1252Weak symbols resemble global symbols, but their definitions have lower 1253precedence. 1254.It Dv STB_LOPROC 1255This value up to and including 1256.Dv STB_HIPROC 1257is reserved for processor-specific semantics. 1258.It Dv STB_HIPROC 1259This value down to and including 1260.Dv STB_LOPROC 1261is reserved for processor-specific semantics. 1262.Pp 1263There are macros for packing and unpacking the binding and type fields: 1264.Pp 1265.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1266.It Xo 1267.Fn ELF32_ST_BIND info 1268.Xc 1269or 1270.Fn ELF64_ST_BIND info 1271extract a binding from an st_info value. 1272.It Xo 1273.Fn ELF64_ST_TYPE info 1274.Xc 1275or 1276.Fn ELF32_ST_TYPE info 1277extract a type from an st_info value. 1278.It Xo 1279.Fn ELF32_ST_INFO bind type 1280.Xc 1281or 1282.Fn ELF64_ST_INFO bind type 1283convert a binding and a type into an st_info value. 1284.El 1285.El 1286.It Dv st_other 1287This member currently holds zero and has no defined meaning. 1288.It Dv st_shndx 1289Every symbol table entry is 1290.Dq defined 1291in relation to some section. 1292This member holds the relevant section 1293header table index. 1294.El 1295.Pp 1296Relocation is the process of connecting symbolic references with 1297symbolic definitions. 1298Relocatable files must have information that 1299describes how to modify their section contents, thus allowing executable 1300and shared object files to hold the right information for a process' 1301program image. 1302Relocation entries are these data. 1303.Pp 1304Relocation structures that do not need an addend: 1305.Bd -literal -offset indent 1306typedef struct { 1307 Elf32_Addr r_offset; 1308 Elf32_Word r_info; 1309} Elf32_Rel; 1310.Ed 1311.Bd -literal -offset indent 1312typedef struct { 1313 Elf64_Addr r_offset; 1314 Elf64_Xword r_info; 1315} Elf64_Rel; 1316.Ed 1317.Pp 1318Relocation structures that need an addend: 1319.Bd -literal -offset indent 1320typedef struct { 1321 Elf32_Addr r_offset; 1322 Elf32_Word r_info; 1323 Elf32_Sword r_addend; 1324} Elf32_Rela; 1325.Ed 1326.Bd -literal -offset indent 1327typedef struct { 1328 Elf64_Addr r_offset; 1329 Elf64_Xword r_info; 1330 Elf64_Sxword r_addend; 1331} Elf64_Rela; 1332.Ed 1333.Bl -tag -width "r_offset" 1334.It Dv r_offset 1335This member gives the location at which to apply the relocation action. 1336For a relocatable file, the value is the byte offset from the beginning 1337of the section to the storage unit affected by the relocation. 1338For an 1339executable file or shared object, the value is the virtual address of 1340the storage unit affected by the relocation. 1341.It Dv r_info 1342This member gives both the symbol table index with respect to which the 1343relocation must be made and the type of relocation to apply. 1344Relocation 1345types are processor-specific. 1346When the text refers to a relocation 1347entry's relocation type or symbol table index, it means the result of 1348applying 1349.Dv ELF[32|64]_R_TYPE 1350or 1351.Dv ELF[32|64]_R_SYM , 1352respectively, to the entry's 1353.Sy r_info 1354member. 1355.It Dv r_addend 1356This member specifies a constant addend used to compute the value to be 1357stored into the relocatable field. 1358.El 1359.Pp 1360The note section is used to hold vendor-specific information that 1361may be used to help identify a binary's ABI. 1362It should start with an Elf_Note struct, 1363followed by the section name and the section description. 1364The actual note contents follow thereafter. 1365.Bd -literal -offset indent 1366typedef struct { 1367 Elf32_Word namesz; 1368 Elf32_Word descsz; 1369 Elf32_Word type; 1370} Elf32_Note; 1371 1372typedef struct { 1373 Elf64_Word namesz; 1374 Elf64_Word descsz; 1375 Elf64_Word type; 1376} Elf64_Note; 1377.Ed 1378.Bl -tag -width "r_offset" 1379.It Dv namesz 1380Length of the note name, rounded up to a 4-byte boundary. 1381.It Dv descsz 1382Length of the note description, rounded up to a 4-byte boundary. 1383.It Dv type 1384A vendor-specific note type. 1385.El 1386.Pp 1387The name and description strings follow the note structure. 1388Each string is aligned on a 4-byte boundary. 1389.Sh SEE ALSO 1390.Xr as 1 , 1391.Xr gdb 1 , 1392.Xr ld 1 , 1393.Xr objdump 1 , 1394.Xr execve 2 , 1395.Xr core 5 1396.Rs 1397.%A Hewlett-Packard 1398.%B Elf-64 Object File Format 1399.Re 1400.Rs 1401.%A Santa Cruz Operation 1402.%B System V Application Binary Interface 1403.Re 1404.Rs 1405.%A Unix System Laboratories 1406.%T Object Files 1407.%B "Executable and Linking Format (ELF)" 1408.Re 1409.Sh HISTORY 1410.Ox 1411ELF support first appeared in 1412.Ox 1.2 . 1413Starting with 1414.Ox 5.4 , 1415all supported platforms use it as the native binary file format. 1416ELF in itself first appeared in 1417.At V . 1418The ELF format is an adopted standard. 1419.Sh AUTHORS 1420This manual page was written by 1421.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org 1422with inspiration from BSDi's 1423.Bsx 1424.Nm elf 1425manpage. 1426