1.\" $OpenBSD: elf.5,v 1.15 2007/05/31 19:19:58 jmc 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: May 31 2007 $ 29.Dt ELF 5 30.Os 31.Sh NAME 32.Nm elf 33.Nd format of ELF executable binary files 34.Sh SYNOPSIS 35.Fd #include <elf_abi.h> 36.Sh DESCRIPTION 37The header file 38.Aq Pa 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.Aq Pa 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_Half Unsigned halfword field 84Elf32_Off Unsigned file offset 85Elf32_Sword Signed large integer 86Elf32_Word Field or unsigned large integer 87.\" Elf32_Size Unsigned object size 88.Ed 89.Pp 90And the following types are used for 64-bit architectures: 91.Bd -literal -offset indent 92Elf64_Addr Unsigned program address 93Elf64_Shalf Signed halfword field 94Elf64_Half Unsigned halfword field 95Elf64_Off Unsigned file offset 96Elf64_Sword Signed large integer 97Elf64_Word Field or unsigned large integer 98.\" Elf64_Size Unsigned object size 99Elf64_Xword Unsigned object size or alignment 100Elf64_Sxword Signed object size or alignment 101Elf64_Quarter Unsigned quarterword field 102.Ed 103.Pp 104All data structures that the file format defines follow the 105.Dq natural 106size and alignment guidelines for the relevant class. 107If necessary, 108data structures contain explicit padding to ensure 4-byte alignment 109for 4-byte objects, to force structure sizes to a multiple of 4, etc. 110.Pp 111The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 112.Bd -literal -offset indent 113typedef struct { 114 unsigned char e_ident[EI_NIDENT]; 115 Elf32_Half e_type; 116 Elf32_Half e_machine; 117 Elf32_Word e_version; 118 Elf32_Addr e_entry; 119 Elf32_Off e_phoff; 120 Elf32_Off e_shoff; 121 Elf32_Word e_flags; 122 Elf32_Half e_ehsize; 123 Elf32_Half e_phentsize; 124 Elf32_Half e_phnum; 125 Elf32_Half e_shentsize; 126 Elf32_Half e_shnum; 127 Elf32_Half e_shstrndx; 128} Elf32_Ehdr; 129.Ed 130.Bd -literal -offset indent 131typedef struct { 132 unsigned char e_ident[EI_NIDENT]; 133 Elf64_Quarter e_type; 134 Elf64_Quarter e_machine; 135 Elf64_Half e_version; 136 Elf64_Addr e_entry; 137 Elf64_Off e_phoff; 138 Elf64_Off e_shoff; 139 Elf64_Half e_flags; 140 Elf64_Quarter e_ehsize; 141 Elf64_Quarter e_phentsize; 142 Elf64_Quarter e_phnum; 143 Elf64_Quarter e_shentsize; 144 Elf64_Quarter e_shnum; 145 Elf64_Quarter e_shstrndx; 146} Elf64_Ehdr; 147.Ed 148.Pp 149The fields have the following meanings: 150.Bl -tag -width "e_phentsize" -offset indent 151.It Dv e_ident 152This array of bytes specifies to interpret the file, 153independent of the processor or the file's remaining contents. 154Within this array everything is named by macros, which start with 155the prefix 156.Sy EI_ 157and may contain values which start with the prefix 158.Sy ELF . 159The following macros are defined: 160.Bl -tag -width "EI_VERSION" \" EI_ABIVERSION 161.It Dv EI_MAG0 162The first byte of the magic number. 163It must be filled with 164.Sy ELFMAG0 . 165.It Dv EI_MAG1 166The second byte of the magic number. 167It must be filled with 168.Sy ELFMAG1 . 169.It Dv EI_MAG2 170The third byte of the magic number. 171It must be filled with 172.Sy ELFMAG2 . 173.It Dv EI_MAG3 174The fourth byte of the magic number. 175It must be filled with 176.Sy ELFMAG3 . 177.It Dv EI_CLASS 178The fifth byte identifies the architecture for this binary: 179.Pp 180.Bl -tag -width "ELFCLASSNONE" -compact 181.It Dv ELFCLASSNONE 182This class is invalid. 183.It Dv ELFCLASS32 184This defines the 32-bit architecture. 185It supports machines with files 186and virtual address spaces up to 4 Gigabytes. 187.It Dv ELFCLASS64 188This defines the 64-bit architecture. 189.El 190.It Dv EI_DATA 191The sixth byte specifies the data encoding of the processor-specific 192data in the file. 193Currently these encodings are supported: 194.Pp 195.Bl -tag -width "ELFDATA2LSB" -compact 196.It Dv ELFDATANONE 197Unknown data format. 198.It Dv ELFDATA2LSB 199Two's complement, little-endian. 200.It Dv ELFDATA2MSB 201Two's complement, big-endian. 202.El 203.It Dv EI_VERSION 204The version number of the ELF specification: 205.Pp 206.Bl -tag -width "EV_CURRENT" -compact 207.It Dv EV_NONE 208Invalid version. 209.It Dv EV_CURRENT 210Current version. 211.El 212.\" .It Dv EI_OSABI 213.\" This byte identifies the operating system 214.\" and ABI to which the object is targeted. 215.\" Some fields in other ELF structures have flags 216.\" and values that have platform specific meanings; 217.\" the interpretation of those fields is determined by the value of this byte. 218.\" The following values are currently defined: 219.\" .Pp 220.\" .Bl -tag -width "ELFOSABI_STANDALONE" -compact 221.\" .It Dv ELFOSABI_SYSV 222.\" UNIX System V ABI. 223.\" .It Dv ELFOSABI_HPUX 224.\" HP-UX operating system ABI. 225.\" .It Dv ELFOSABI_NETBSD 226.\" .Nx 227.\" operating system ABI. 228.\" .It Dv ELFOSABI_LINUX 229.\" GNU/Linux operating system ABI. 230.\" .It Dv ELFOSABI_HURD 231.\" GNU/Hurd operating system ABI. 232.\" .It Dv ELFOSABI_86OPEN 233.\" 86Open Common IA32 ABI. 234.\" .It Dv ELFOSABI_SOLARIS 235.\" Solaris operating system ABI. 236.\" .It Dv ELFOSABI_MONTEREY 237.\" Monterey project ABI. 238.\" .It Dv ELFOSABI_IRIX 239.\" IRIX operating system ABI. 240.\" .It Dv ELFOSABI_FREEBSD 241.\" .Fx 242.\" operating system ABI. 243.\" .It Dv ELFOSABI_TRU64 244.\" TRU64 UNIX operating system ABI. 245.\" .It Dv ELFOSABI_ARM 246.\" ARM architecture ABI. 247.\" .It Dv ELFOSABI_STANDALONE 248.\" Stand-alone (embedded) ABI. 249.\" .El 250.\" .It Dv EI_ABIVERSION 251.\" This byte identifies the version of the ABI 252.\" to which the object is targeted. 253.\" This field is used to distinguish among incompatible versions of an ABI. 254.\" The interpretation of this version number 255.\" is dependent on the ABI identified by the EI_OSABI field. 256.\" Applications conforming to this specification use the value 0. 257.It Dv EI_PAD 258Start of padding. 259These bytes are reserved and set to zero. 260Programs 261which read them should ignore them. 262The value for EI_PAD will change in 263the future if currently unused bytes are given meanings. 264.It Dv EI_BRAND 265Start of architecture identification. 266.It Dv EI_NIDENT 267The size of the e_ident array. 268.El 269.Pp 270.It Dv e_type 271This member of the structure identifies the object file type: 272.Pp 273.Bl -tag -width "ET_NONE" -compact 274.It Dv ET_NONE 275An unknown type. 276.It Dv ET_REL 277A relocatable file. 278.It Dv ET_EXEC 279An executable file. 280.It Dv ET_DYN 281A shared object. 282.It Dv ET_CORE 283A core file. 284.El 285.Pp 286.It Dv e_machine 287This member specifies the required architecture for an individual file: 288.Pp 289.Bl -tag -width "EM_MIPS_RS4_BE" -compact 290.It Dv EM_NONE 291An unknown machine. 292.It Dv EM_M32 293AT&T WE 32100. 294.It Dv EM_SPARC 295Sun Microsystems SPARC. 296.It Dv EM_386 297Intel 80386. 298.It Dv EM_68K 299Motorola 68000. 300.It Dv EM_88K 301Motorola 88000. 302.It Dv EM_486 303Intel 80486. 304.It Dv EM_860 305Intel 80860. 306.It Dv EM_MIPS 307MIPS RS3000 (big-endian only). 308.It Dv EM_MIPS_RS4_BE 309MIPS RS4000 (big-endian only). 310.It Dv EM_SPARC64 311SPARC v9 64-bit (unofficial). 312.It Dv EM_PARISC 313HPPA. 314.It Dv EM_SPARC32PLUS 315SPARC with enhanced instruction set. 316.It Dv EM_PPC 317PowerPC. 318.It Dv EM_ALPHA 319Compaq [DEC] Alpha. 320.It Dv EM_SPARCV9 321SPARC v9 64-bit. 322.It Dv EM_ALPHA_EXP 323Compaq [DEC] Alpha with enhanced instruction set. 324.It Dv EM_VAX 325DEC Vax. 326.El 327.Pp 328.It Dv e_version 329This member identifies the file version: 330.Pp 331.Bl -tag -width "EV_CURRENT" -compact 332.It Dv EV_NONE 333Invalid version. 334.It Dv EV_CURRENT 335Current version. 336.El 337.It Dv e_entry 338This member gives the virtual address to which the system first transfers 339control, thus starting the process. 340If the file has no associated entry 341point, this member holds zero. 342.It Dv e_phoff 343This member holds the program header table's file offset in bytes. 344If 345the file has no program header table, this member holds zero. 346.It Dv e_shoff 347This member holds the section header table's file offset in bytes. 348If the 349file has no section header table this member holds zero. 350.It Dv e_flags 351This member holds processor-specific flags associated with the file. 352Flag names take the form EF_`machine_flag'. 353Currently no flags have been defined. 354.It Dv e_ehsize 355This member holds the ELF header's size in bytes. 356.It Dv e_phentsize 357This member holds the size in bytes of one entry in the file's program header 358table; all entries are the same size. 359.It Dv e_phnum 360This member holds the number of entries in the program header 361table. 362Thus the product of 363.Sy e_phentsize 364and 365.Sy e_phnum 366gives the table's size 367in bytes. 368If a file has no program header, 369.Sy e_phnum 370holds the value zero. 371.It Dv e_shentsize 372This member holds a sections header's size in bytes. 373A section header is one 374entry in the section header table; all entries are the same size. 375.It Dv e_shnum 376This member holds the number of entries in the section header table. 377Thus 378the product of 379.Sy e_shentsize 380and 381.Sy e_shnum 382gives the section header table's size in bytes. 383If a file has no section 384header table, 385.Sy e_shnum 386holds the value of zero. 387.It Dv e_shstrndx 388This member holds the section header table index of the entry associated 389with the section name string table. 390If the file has no section name string 391table, this member holds the value 392.Sy SHN_UNDEF . 393.Bl -tag -width "SHN_LORESERVE" 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 indices. 404.It Dv SHN_LOPROC 405This value up to and including 406.Sy SHN_HIPROC 407is reserved for processor-specific semantics. 408.It Dv SHN_HIPROC 409This value down to and including 410.Sy SHN_LOPROC 411is 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 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.Bd -literal -offset indent 452typedef struct { 453 Elf32_Word p_type; 454 Elf32_Off p_offset; 455 Elf32_Addr p_vaddr; 456 Elf32_Addr p_paddr; 457 Elf32_Word p_filesz; 458 Elf32_Word p_memsz; 459 Elf32_Word p_flags; 460 Elf32_Word p_align; 461} Elf32_Phdr; 462.Ed 463.Bd -literal -offset indent 464typedef struct { 465 Elf64_Half p_type; 466 Elf64_Half p_flags; 467 Elf64_Off p_offset; 468 Elf64_Addr p_vaddr; 469 Elf64_Addr p_paddr; 470 Elf64_Xword p_filesz; 471 Elf64_Xword p_memsz; 472 Elf64_Xword p_align; 473} Elf64_Phdr; 474.Ed 475.Pp 476The main difference between the 32-bit and the 64-bit program header lies 477only in the location of a 478.Sy p_flags 479member in the total struct. 480.Bl -tag -width "p_offset" -offset indent 481.It Dv p_type 482This member of the Phdr struct tells what kind of segment this array 483element describes or how to interpret the array element's information. 484.Bl -tag -width "PT_DYNAMIC" 485.It Dv PT_NULL 486The array element is unused and the other members' values are undefined. 487This lets the program header have ignored entries. 488.It Dv PT_LOAD 489The array element specifies a loadable segment, described by 490.Sy p_filesz 491and 492.Sy p_memsz . 493The bytes from the file are mapped to the beginning of the memory 494segment. 495If the segment's memory size 496.Pq Sy p_memsz 497is larger than the file size 498.Pq Sy p_filesz , 499the 500.Dq extra 501bytes are defined to hold the value 0 and to follow the segment's 502initialized area. 503The file size may not be larger than the memory size. 504Loadable segment entries in the program header table appear in ascending 505order, sorted on the 506.Sy p_vaddr 507member. 508.It Dv PT_DYNAMIC 509The array element specifies dynamic linking information. 510.It Dv PT_INTERP 511The array element specifies the location and size of a null-terminated 512path name to invoke as an interpreter. 513This segment type is meaningful 514only for executable files (though it may occur for shared objects). 515However it may not occur more than once in a file. 516If it is present, it must precede any loadable segment entry. 517.It Dv PT_NOTE 518The array element specifies the location and size for auxiliary information. 519.It Dv PT_SHLIB 520This segment type is reserved but has unspecified semantics. 521Programs that 522contain an array element of this type do not conform to the ABI. 523.It Dv PT_PHDR 524The array element, if present, specifies the location and size of the program 525header table itself, both in the file and in the memory image of the program. 526This segment type may not occur more than once in a file. 527Moreover, it may 528only occur if the program header table is part of the memory image of the 529program. 530If it is present, it must precede any loadable segment entry. 531.It Dv PT_LOOS 532This value up to and including 533.Sy PT_HIOS 534is reserverd for operating system-specific semantics. 535.It Dv PT_HIOS 536This value down to and including 537.Sy PT_LOOS 538is reserved for operating system-specific semantics. 539.It Dv PT_LOPROC 540This value up to and including 541.Sy PT_HIPROC 542is reserved for processor-specific semantics. 543.It Dv PT_HIPROC 544This value down to and including 545.Sy PT_LOPROC 546is reserved for processor-specific semantics. 547.El 548.Pp 549.It Dv p_offset 550This member holds the offset from the beginning of the file at which 551the first byte of the segment resides. 552.It Dv p_vaddr 553This member holds the virtual address at which the first byte of the 554segment resides in memory. 555.It Dv p_paddr 556On systems for which physical addressing is relevant, this member is 557reserved for the segment's physical address. 558Under 559.Bx 560this member is 561not used and must be zero. 562.It Dv p_filesz 563This member holds the number of bytes in the file image of the segment. 564It may be zero. 565.It Dv p_memsz 566This member holds the number of bytes in the memory image of the segment. 567It may be zero. 568.It Dv p_flags 569This member holds flags relevant to the segment: 570.Pp 571.Bl -tag -width "PF_X" -compact 572.It Dv PF_X 573An executable segment. 574.It Dv PF_W 575A writable segment. 576.It Dv PF_R 577A readable segment. 578.El 579.Pp 580A text segment commonly has the flags 581.Sy PF_X 582and 583.Sy PF_R . 584A data segment commonly has 585.Sy PF_X , 586.Sy PF_W 587and 588.Sy PF_R . 589.It Dv p_align 590This member holds the value to which the segments are aligned in memory 591and in the file. 592Loadable process segments must have congruent values for 593.Sy p_vaddr 594and 595.Sy p_offset , 596modulo the page size. 597Values of zero and one mean no alignment is required. 598Otherwise, 599.Sy p_align 600should be a positive, integral power of two, and 601.Sy p_vaddr 602should equal 603.Sy p_offset , 604modulo 605.Sy p_align . 606.El 607.Pp 608A file's section header table lets one locate all the file's sections. 609The 610section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 611The 612ELF header's 613.Sy e_shoff 614member gives the byte offset from the beginning of the file to the section 615header table. 616.Sy e_shnum 617holds the number of entries the section header table contains. 618.Sy e_shentsize 619holds the size in bytes of each entry. 620.Pp 621A section header table index is a subscript into this array. 622Some section 623header table indices are reserved. 624An object file does not have sections for 625these special indices: 626.Bl -tag -width "SHN_LORESERVE" 627.It Dv SHN_UNDEF 628This value marks an undefined, missing, irrelevant or otherwise meaningless 629section reference. 630For example, a symbol 631.Dq defined 632relative to section number 633.Sy SHN_UNDEF 634is an undefined symbol. 635.It Dv SHN_LORESERVE 636This value specifies the lower bound of the range of reserved indices. 637.It Dv SHN_LOPROC 638This value up to and including 639.Sy SHN_HIPROC 640is reserved for processor-specific semantics. 641.It Dv SHN_HIPROC 642This value down to and including 643.Sy SHN_LOPROC 644is reserved for processor-specific semantics. 645.It Dv SHN_ABS 646This value specifies the absolute value for the corresponding reference. 647For 648example, a symbol defined relative to section number 649.Sy SHN_ABS 650has an absolute value and is not affected by relocation. 651.It Dv SHN_COMMON 652Symbols defined relative to this section are common symbols, such as FORTRAN 653COMMON or unallocated C external variables. 654.It Dv SHN_HIRESERVE 655This value specifies the upper bound of the range of reserved indices. 656The 657system reserves indices between 658.Sy SHN_LORESERVE 659and 660.Sy SHN_HIRESERVE , 661inclusive. 662The section header table does not contain entries for the 663reserved indices. 664.El 665.Pp 666The section header has the following structure: 667.Bd -literal -offset indent 668typedef struct { 669 Elf32_Word sh_name; 670 Elf32_Word sh_type; 671 Elf32_Word sh_flags; 672 Elf32_Addr sh_addr; 673 Elf32_Off sh_offset; 674 Elf32_Word sh_size; 675 Elf32_Word sh_link; 676 Elf32_Word sh_info; 677 Elf32_Word sh_addralign; 678 Elf32_Word sh_entsize; 679} Elf32_Shdr; 680.Ed 681.Bd -literal -offset indent 682typedef struct { 683 Elf64_Half sh_name; 684 Elf64_Half sh_type; 685 Elf64_Xword sh_flags; 686 Elf64_Addr sh_addr; 687 Elf64_Off sh_offset; 688 Elf64_Xword sh_size; 689 Elf64_Half sh_link; 690 Elf64_Half sh_info; 691 Elf64_Xword sh_addralign; 692 Elf64_Xword sh_entsize; 693} Elf64_Shdr; 694.Ed 695.Bl -tag -width "sh_addralign" 696.It Dv sh_name 697This member specifies the name of the section. 698Its value is an index 699into the section header string table section, giving the location of 700a null-terminated string. 701.It Dv sh_type 702This member categorizes the section's contents and semantics. 703.Bl -tag -width "SHT_PROGBITS" 704.It Dv SHT_NULL 705This value marks the section header as inactive. 706It does not 707have an associated section. 708Other members of the section header 709have undefined values. 710.It Dv SHT_PROGBITS 711This section holds information defined by the program, whose 712format and meaning are determined solely by the program. 713.It Dv SHT_SYMTAB 714This section holds a symbol table. 715Typically, 716.Sy SHT_SYMTAB 717provides symbols for link editing, though it may also be used 718for dynamic linking. 719As a complete symbol table, it may contain 720many symbols unnecessary for dynamic linking. 721An object file can 722also contain a 723.Sy SHN_DYNSYM 724section. 725.It Dv SHT_STRTAB 726This section holds a string table. 727An object file may have multiple 728string table sections. 729.It Dv SHT_RELA 730This section holds relocation entries with explicit addends, such 731as type 732.Sy Elf32_Rela 733for the 32-bit class of object files. 734An object may have multiple 735relocation sections. 736.It Dv SHT_HASH 737This section holds a symbol hash table. 738An object participating in 739dynamic linking must contain a symbol hash table. 740An object file may 741have only one hash table. 742.It Dv SHT_DYNAMIC 743This section holds information for dynamic linking. 744An object file may 745have only one dynamic section. 746.It Dv SHT_NOTE 747This section holds information that marks the file in some way. 748.It Dv SHT_NOBITS 749A section of this type occupies no space in the file but otherwise 750resembles 751.Sy SHN_PROGBITS . 752Although this section contains no bytes, the 753.Sy sh_offset 754member contains the conceptual file offset. 755.It Dv SHT_REL 756This section holds relocation offsets without explicit addends, such 757as type 758.Sy Elf32_Rel 759for the 32-bit class of object files. 760An object file may have multiple 761relocation sections. 762.It Dv SHT_SHLIB 763This section is reserved but has unspecified semantics. 764.It Dv SHT_DYNSYM 765This section holds a minimal set of dynamic linking symbols. 766An 767object file can also contain a 768.Sy SHN_SYMTAB 769section. 770.It Dv SHT_LOPROC 771This value up to and including 772.Sy SHT_HIPROC 773is reserved for processor-specific semantics. 774.It Dv SHT_HIPROC 775This value down to and including 776.Sy SHT_LOPROC 777is reserved for processor-specific semantics. 778.It Dv SHT_LOUSER 779This value specifies the lower bound of the range of indices reserved for 780application programs. 781.It Dv SHT_HIUSER 782This value specifies the upper bound of the range of indices reserved for 783application programs. 784Section types between 785.Sy SHT_LOUSER 786and 787.Sy SHT_HIUSER 788may be used by the application, without conflicting with current or future 789system-defined section types. 790.El 791.Pp 792.It Dv sh_flags 793Sections support one-bit flags that describe miscellaneous attributes. 794If a flag bit is set in 795.Sy sh_flags , 796the attribute is 797.Dq on 798for the section. 799Otherwise, the attribute is 800.Dq off 801or does not apply. 802Undefined attributes are set to zero. 803.Pp 804.Bl -tag -width "SHF_EXECINSTR" -compact 805.It Dv SHF_WRITE 806This section contains data that should be writable during process 807execution. 808.It Dv SHF_ALLOC 809This section occupies memory during process execution. 810Some control 811sections do not reside in the memory image of an object file. 812This 813attribute is off for those sections. 814.It Dv SHF_EXECINSTR 815This section contains executable machine instructions. 816.It Dv SHF_MASKPROC 817All bits included in this mask are reserved for processor-specific 818semantics. 819.El 820.Pp 821.It Dv sh_addr 822If this section appears in the memory image of a process, this member 823holds the address at which the section's first byte should reside. 824Otherwise, the member contains zero. 825.It Dv sh_offset 826This member's value holds the byte offset from the beginning of the file 827to the first byte in the section. 828One section type, 829.Sy SHT_NOBITS , 830occupies no space in the file, and its 831.Sy sh_offset 832member locates the conceptual placement in the file. 833.It Dv sh_size 834This member holds the section's size in bytes. 835Unless the section type 836is 837.Sy SHT_NOBITS , 838the section occupies 839.Sy sh_size 840bytes in the file. 841A section of type 842.Sy SHT_NOBITS 843may have a non-zero size, but it occupies no space in the file. 844.It Dv sh_link 845This member holds a section header table index link, whose interpretation 846depends on the section type. 847.It Dv sh_info 848This member holds extra information, whose interpretation depends on the 849section type. 850.It Dv sh_addralign 851Some sections have address alignment constraints. 852If a section holds a 853doubleword, the system must ensure doubleword alignment for the entire 854section. 855That is, the value of 856.Sy sh_addr 857must be congruent to zero, modulo the value of 858.Sy sh_addralign . 859Only zero and positive integral powers of two are allowed. 860Values of zero 861or one mean the section has no alignment constraints. 862.It Dv sh_entsize 863Some sections hold a table of fixed-sized entries, such as a symbol table. 864For such a section, this member gives the size in bytes for each entry. 865This member contains zero if the section does not hold a table of 866fixed-size entries. 867.El 868.Pp 869Various sections hold program and control information: 870.Bl -tag -width ".shstrtab" 871.It .bss 872This section holds uninitialized data that contributes to the program's 873memory image. 874By definition, the system initializes the data with zeros 875when the program begins to run. 876This section is of type 877.Sy SHT_NOBITS . 878The attribute types are 879.Sy SHF_ALLOC 880and 881.Sy SHF_WRITE . 882.It .comment 883This section holds version control information. 884This section is of type 885.Sy SHT_PROGBITS . 886No attribute types are used. 887.It .ctors 888This section holds initialized pointers to the C++ constructor functions. 889This section is of type 890.Sy SHT_PROGBITS . 891The attribute types are 892.Sy SHF_ALLOC 893and 894.Sy SHF_WRITE . 895.It .data 896This section holds initialized data that contribute to the program's 897memory image. 898This section is of type 899.Sy SHT_PROGBITS . 900The attribute types are 901.Sy SHF_ALLOC 902and 903.Sy SHF_WRITE . 904.It .data1 905This section holds initialized data that contribute to the program's 906memory image. 907This section is of type 908.Sy SHT_PROGBITS . 909The attribute types are 910.Sy SHF_ALLOC 911and 912.Sy SHF_WRITE . 913.It .debug 914This section holds information for symbolic debugging. 915The contents 916are unspecified. 917This section is of type 918.Sy SHT_PROGBITS . 919No attribute types are used. 920.It .dtors 921This section holds initialized pointers to the C++ destructor functions. 922This section is of type 923.Sy SHT_PROGBITS . 924The attribute types are 925.Sy SHF_ALLOC 926and 927.Sy SHF_WRITE . 928.It .dynamic 929This section holds dynamic linking information. 930The section's attributes 931will include the 932.Sy SHF_ALLOC 933bit. 934Whether the 935.Sy SHF_WRITE 936bit is set is processor-specific. 937This section is of type 938.Sy SHT_DYNAMIC . 939See the attributes above. 940.It .dynstr 941This section holds strings needed for dynamic linking, most commonly 942the strings that represent the names associated with symbol table entries. 943This section is of type 944.Sy SHT_STRTAB . 945The attribute type used is 946.Sy SHF_ALLOC . 947.It .dynsym 948This section holds the dynamic linking symbol table. 949This section is of type 950.Sy SHT_DYNSYM . 951The attribute used is 952.Sy SHF_ALLOC . 953.It .fini 954This section holds executable instructions that contribute to the process 955termination code. 956When a program exits normally the system arranges to 957execute the code in this section. 958This section is of type 959.Sy SHT_PROGBITS . 960The attributes used are 961.Sy SHF_ALLOC 962and 963.Sy SHF_EXECINSTR . 964.It .got 965This section holds the global offset table. 966This section is of type 967.Sy SHT_PROGBITS . 968The attributes are processor-specific. 969.It .hash 970This section holds a symbol hash table. 971This section is of type 972.Sy SHT_HASH . 973The attribute used is 974.Sy SHF_ALLOC . 975.It .init 976This section holds executable instructions that contribute to the process 977initialization code. 978When a program starts to run the system arranges to 979execute the code in this section before calling the main program entry point. 980This section is of type 981.Sy SHT_PROGBITS . 982The attributes used are 983.Sy SHF_ALLOC 984and 985.Sy SHF_EXECINSTR . 986.It .interp 987This section holds the pathname of a program interpreter. 988If the file has 989a loadable segment that includes the section, the section's attributes will 990include the 991.Sy SHF_ALLOC 992bit. 993Otherwise, that bit will be off. 994This section is of type 995.Sy SHT_PROGBITS . 996.It .line 997This section holds line number information for symbolic debugging, which 998describes the correspondence between the program source and the machine code. 999The contents are unspecified. 1000This section is of type 1001.Sy SHT_PROGBITS . 1002No attribute types are used. 1003.It .note 1004This section holds information in the 1005.Dq Note Section 1006format described below. 1007This section is of type 1008.Sy SHT_NOTE . 1009No attribute types are used. 1010.Ox 1011native executables usually contain a 1012.Sy .note.openbsd.ident 1013section to identify themselves, for the kernel to bypass any compatibility 1014ELF binary emulation tests when loading the file. 1015.It .plt 1016This section holds the procedure linkage table. 1017This section is of type 1018.Sy SHT_PROGBITS . 1019The attributes are processor-specific. 1020.It .relNAME 1021This section holds relocation information as described below. 1022If the file 1023has a loadable segment that includes relocation, the section's attributes 1024will include the 1025.Sy SHF_ALLOC 1026bit. 1027Otherwise the bit will be off. 1028By convention, 1029.Dq NAME 1030is supplied by the section to which the relocations apply. 1031Thus a relocation 1032section for 1033.Sy .text 1034normally would have the name 1035.Sy .rel.text . 1036This section is of type 1037.Sy SHT_REL . 1038.It .relaNAME 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.Sy 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 .rela.text . 1054This section is of type 1055.Sy SHT_RELA . 1056.It .rodata 1057This section holds read-only data that typically contributes to a 1058non-writable segment in the process image. 1059This section is of type 1060.Sy SHT_PROGBITS . 1061The attribute used is 1062.Sy SHF_ALLOC . 1063.It .rodata1 1064This section holds read-only data that typically contributes to a 1065non-writable segment in the process image. 1066This section is of type 1067.Sy SHT_PROGBITS . 1068The attribute used is 1069.Sy SHF_ALLOC . 1070.It .shstrtab 1071This section holds section names. 1072This section is of type 1073.Sy SHT_STRTAB . 1074No attribute types are used. 1075.It .strtab 1076This section holds strings, most commonly the strings that represent the 1077names associated with symbol table entries. 1078If the file has a loadable 1079segment that includes the symbol string table, the section's attributes 1080will include the 1081.Sy SHF_ALLOC 1082bit. 1083Otherwise the bit will be off. 1084This section is of type 1085.Sy SHT_STRTAB . 1086.It .symtab 1087This section holds a symbol table. 1088If the file has a loadable segment 1089that includes the symbol table, the section's attributes will include 1090the 1091.Sy SHF_ALLOC 1092bit. 1093Otherwise the bit will be off. 1094This section is of type 1095.Sy SHT_SYMTAB . 1096.It .text 1097This section holds the 1098.Dq text , 1099or executable instructions, of a program. 1100This section is of type 1101.Sy SHT_PROGBITS . 1102The attributes used are 1103.Sy SHF_ALLOC 1104and 1105.Sy 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.Sy 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.Sy STB_LOCAL 1177bindings, its section index is 1178.Sy SHN_ABS , 1179and it precedes the other 1180.Sy STB_LOCAL 1181symbols of the file, if it is present. 1182.It Dv STT_LOPROC 1183This value up to and including 1184.Sy STT_HIPROC 1185is reserved for processor-specific semantics. 1186.It Dv STT_HIPROC 1187This value down to and including 1188.Sy STT_LOPROC 1189is reserved for processor-specific semantics. 1190.El 1191.Bl -tag -width "STB_GLOBAL" 1192.It Dv STB_LOCAL 1193Local symbols are not visible outside the object file containing their 1194definition. 1195Local symbols of the same name may exist in multiple files 1196without interfering with each other. 1197.It Dv STB_GLOBAL 1198Global symbols are visible to all object files being combined. 1199One file's 1200definition of a global symbol will satisfy another file's undefined 1201reference to the same symbol. 1202.It Dv STB_WEAK 1203Weak symbols resemble global symbols, but their definitions have lower 1204precedence. 1205.It Dv STB_LOPROC 1206This value up to and including 1207.Sy STB_HIPROC 1208is reserved for processor-specific semantics. 1209.It Dv STB_HIPROC 1210This value down to and including 1211.Sy STB_LOPROC 1212is reserved for processor-specific semantics. 1213.Pp 1214There are macros for packing and unpacking the binding and type fields: 1215.Pp 1216.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1217.It Xo 1218.Fn ELF32_ST_BIND info 1219.Xc 1220or 1221.Fn ELF64_ST_BIND info 1222extract a binding from an st_info value. 1223.It Xo 1224.Fn ELF64_ST_TYPE info 1225.Xc 1226or 1227.Fn ELF32_ST_TYPE info 1228extract a type from an st_info value. 1229.It Xo 1230.Fn ELF32_ST_INFO bind type 1231.Xc 1232or 1233.Fn ELF64_ST_INFO bind type 1234convert a binding and a type into an st_info value. 1235.El 1236.El 1237.Pp 1238.It Dv st_other 1239This member currently holds zero and has no defined meaning. 1240.It Dv st_shndx 1241Every symbol table entry is 1242.Dq defined 1243in relation to some section. 1244This member holds the relevant section 1245header table index. 1246.El 1247.Pp 1248Relocation is the process of connecting symbolic references with 1249symbolic definitions. 1250Relocatable files must have information that 1251describes how to modify their section contents, thus allowing executable 1252and shared object files to hold the right information for a process' 1253program image. 1254Relocation entries are these data. 1255.Pp 1256Relocation structures that do not need an addend: 1257.Bd -literal -offset indent 1258typedef struct { 1259 Elf32_Addr r_offset; 1260 Elf32_Word r_info; 1261} Elf32_Rel; 1262.Ed 1263.Bd -literal -offset indent 1264typedef struct { 1265 Elf64_Xword r_offset; 1266 Elf64_Xword r_info; 1267} Elf64_Rel; 1268.Ed 1269.Pp 1270Relocation structures that need an addend: 1271.Bd -literal -offset indent 1272typedef struct { 1273 Elf32_Addr r_offset; 1274 Elf32_Word r_info; 1275 Elf32_Sword r_addend; 1276} Elf32_Rela; 1277.Ed 1278.Bd -literal -offset indent 1279typedef struct { 1280 Elf64_Xword r_offset; 1281 Elf64_Xword r_info; 1282 Elf64_Sxword r_addend; 1283} Elf64_Rela; 1284.Ed 1285.Bl -tag -width "r_offset" 1286.It Dv r_offset 1287This member gives the location at which to apply the relocation action. 1288For a relocatable file, the value is the byte offset from the beginning 1289of the section to the storage unit affected by the relocation. 1290For an 1291executable file or shared object, the value is the virtual address of 1292the storage unit affected by the relocation. 1293.It Dv r_info 1294This member gives both the symbol table index with respect to which the 1295relocation must be made and the type of relocation to apply. 1296Relocation 1297types are processor-specific. 1298When the text refers to a relocation 1299entry's relocation type or symbol table index, it means the result of 1300applying 1301.Sy ELF_[32|64]_R_TYPE 1302or 1303.Sy ELF[32|64]_R_SYM , 1304respectively, to the entry's 1305.Sy r_info 1306member. 1307.It Dv r_addend 1308This member specifies a constant addend used to compute the value to be 1309stored into the relocatable field. 1310.El 1311.Sh SEE ALSO 1312.Xr as 1 , 1313.Xr gdb 1 , 1314.Xr ld 1 , 1315.Xr objdump 1 , 1316.Xr execve 2 , 1317.Xr core 5 1318.Rs 1319.%A Hewlett-Packard 1320.%B Elf-64 Object File Format 1321.Re 1322.Rs 1323.%A Santa Cruz Operation 1324.%B System V Application Binary Interface 1325.Re 1326.Rs 1327.%A Unix System Laboratories 1328.%T Object Files 1329.%B "Executable and Linking Format (ELF)" 1330.Re 1331.Sh HISTORY 1332.Ox 1333ELF support first appeared in 1334.Ox 1.2 , 1335although not all supported platforms use it as the native 1336binary file format. 1337ELF in itself first appeared in 1338.At V . 1339The ELF format is an adopted standard. 1340.Sh AUTHORS 1341This manual page was written by 1342.An Jeroen Ruigrok van der Werven 1343.Aq asmodai@FreeBSD.org 1344with inspiration from BSDi's 1345.Bsx 1346.Nm elf 1347manpage. 1348