1.\" $OpenBSD: elf.5,v 1.12 2003/10/27 20:23:58 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 July 31, 1999 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 405Values greater than or equal to 406.Sy SHN_HIPROC 407are reserved for processor-specific semantics. 408.It Dv SHN_HIPROC 409Values less than or equal to 410.Sy SHN_LOPROC 411are reserved for processor-specific semantics. 412.It Dv SHN_ABS 413This value specifies absolute values for the corresponding reference. 414For 415example, symbols defined relative to section number 416.Sy SHN_ABS 417have absolute values and are not affected by relocation. 418.It Dv SHN_COMMON 419Symbols defined relative to this section are common symbols, such as Fortran 420COMMON or unallocated C external variables. 421.It Dv SHN_HIRESERVE 422This value specifies the upper bound of the range of 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_LOPROC 532Values greater than or equal to 533.Sy PT_HIPROC 534are reserved for processor-specific semantics. 535.It Dv PT_HIPROC 536Values less than or equal to 537.Sy PT_LOPROC 538are reserved for processor-specific semantics. 539.El 540.Pp 541.It Dv p_offset 542This member holds the offset from the beginning of the file at which 543the first byte of the segment resides. 544.It Dv p_vaddr 545This member holds the virtual address at which the first byte of the 546segment resides in memory. 547.It Dv p_paddr 548On systems for which physical addressing is relevant, this member is 549reserved for the segment's physical address. 550Under 551.Bx 552this member is 553not used and must be zero. 554.It Dv p_filesz 555This member holds the number of bytes in the file image of the segment. 556It may be zero. 557.It Dv p_memsz 558This member holds the number of bytes in the memory image of the segment. 559It may be zero. 560.It Dv p_flags 561This member holds flags relevant to the segment: 562.Pp 563.Bl -tag -width "PF_X" -compact 564.It Dv PF_X 565An executable segment. 566.It Dv PF_W 567A writable segment. 568.It Dv PF_R 569A readable segment. 570.El 571.Pp 572A text segment commonly has the flags 573.Sy PF_X 574and 575.Sy PF_R . 576A data segment commonly has 577.Sy PF_X , 578.Sy PF_W 579and 580.Sy PF_R . 581.It Dv p_align 582This member holds the value to which the segments are aligned in memory 583and in the file. 584Loadable process segments must have congruent values for 585.Sy p_vaddr 586and 587.Sy p_offset , 588modulo the page size. 589Values of zero and one mean no alignment is required. 590Otherwise, 591.Sy p_align 592should be a positive, integral power of two, and 593.Sy p_vaddr 594should equal 595.Sy p_offset , 596modulo 597.Sy p_align . 598.El 599.Pp 600A file's section header table lets one locate all the file's sections. 601The 602section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 603The 604ELF header's 605.Sy e_shoff 606member gives the byte offset from the beginning of the file to the section 607header table. 608.Sy e_shnum 609holds the number of entries the section header table contains. 610.Sy e_shentsize 611holds the size in bytes of each entry. 612.Pp 613A section header table index is a subscript into this array. 614Some section 615header table indices are reserved. 616An object file does not have sections for 617these special indices: 618.Bl -tag -width "SHN_LORESERVE" 619.It Dv SHN_UNDEF 620This value marks an undefined, missing, irrelevant or otherwise meaningless 621section reference. 622.It Dv SHN_LORESERVE 623This value specifies the lower bound of the range of reserved indices. 624.It Dv SHN_LOPROC 625Values greater than or equal to 626.Sy SHN_HIPROC 627are reserved for processor-specific semantics. 628.It Dv SHN_HIPROC 629Values less than or equal to 630.Sy SHN_LOPROC 631are reserved for processor-specific semantics. 632.It Dv SHN_ABS 633This value specifies the absolute value for the corresponding reference. 634For 635example, a symbol defined relative to section number 636.Sy SHN_ABS 637has an absolute value and is not affected by relocation. 638.It Dv SHN_COMMON 639Symbols defined relative to this section are common symbols, such as FORTRAN 640COMMON or unallocated C external variables. 641.It Dv SHN_HIRESERVE 642This value specifies the upper bound of the range of reserved indices. 643The 644system reserves indices between 645.Sy SHN_LORESERVE 646and 647.Sy SHN_HIRESERVE , 648inclusive. 649The section header table does not contain entries for the 650reserved indices. 651.El 652.Pp 653The section header has the following structure: 654.Bd -literal -offset indent 655typedef struct { 656 Elf32_Word sh_name; 657 Elf32_Word sh_type; 658 Elf32_Word sh_flags; 659 Elf32_Addr sh_addr; 660 Elf32_Off sh_offset; 661 Elf32_Word sh_size; 662 Elf32_Word sh_link; 663 Elf32_Word sh_info; 664 Elf32_Word sh_addralign; 665 Elf32_Word sh_entsize; 666} Elf32_Shdr; 667.Ed 668.Bd -literal -offset indent 669typedef struct { 670 Elf64_Half sh_name; 671 Elf64_Half sh_type; 672 Elf64_Xword sh_flags; 673 Elf64_Addr sh_addr; 674 Elf64_Off sh_offset; 675 Elf64_Xword sh_size; 676 Elf64_Half sh_link; 677 Elf64_Half sh_info; 678 Elf64_Xword sh_addralign; 679 Elf64_Xword sh_entsize; 680} Elf64_Shdr; 681.Ed 682.Bl -tag -width "sh_addralign" 683.It Dv sh_name 684This member specifies the name of the section. 685Its value is an index 686into the section header string table section, giving the location of 687a null-terminated string. 688.It Dv sh_type 689This member categorizes the section's contents and semantics. 690.Bl -tag -width "SHT_PROGBITS" 691.It Dv SHT_NULL 692This value marks the section header as inactive. 693It does not 694have an associated section. 695Other members of the section header 696have undefined values. 697.It Dv SHT_PROGBITS 698This section holds information defined by the program, whose 699format and meaning are determined solely by the program. 700.It Dv SHT_SYMTAB 701This section holds a symbol table. 702Typically, 703.Sy SHT_SYMTAB 704provides symbols for link editing, though it may also be used 705for dynamic linking. 706As a complete symbol table, it may contain 707many symbols unnecessary for dynamic linking. 708An object file can 709also contain a 710.Sy SHN_DYNSYM 711section. 712.It Dv SHT_STRTAB 713This section holds a string table. 714An object file may have multiple 715string table sections. 716.It Dv SHT_RELA 717This section holds relocation entries with explicit addends, such 718as type 719.Sy Elf32_Rela 720for the 32-bit class of object files. 721An object may have multiple 722relocation sections. 723.It Dv SHT_HASH 724This section holds a symbol hash table. 725An object participating in 726dynamic linking must contain a symbol hash table. 727An object file may 728have only one hash table. 729.It Dv SHT_DYNAMIC 730This section holds information for dynamic linking. 731An object file may 732have only one dynamic section. 733.It Dv SHT_NOTE 734This section holds information that marks the file in some way. 735.It Dv SHT_NOBITS 736A section of this type occupies no space in the file but otherwise 737resembles 738.Sy SHN_PROGBITS . 739Although this section contains no bytes, the 740.Sy sh_offset 741member contains the conceptual file offset. 742.It Dv SHT_REL 743This section holds relocation offsets without explicit addends, such 744as type 745.Sy Elf32_Rel 746for the 32-bit class of object files. 747An object file may have multiple 748relocation sections. 749.It Dv SHT_SHLIB 750This section is reserved but has unspecified semantics. 751.It Dv SHT_DYNSYM 752This section holds a minimal set of dynamic linking symbols. 753An 754object file can also contain a 755.Sy SHN_SYMTAB 756section. 757.It Dv SHT_LOPROC 758This value up to and including 759.Sy SHT_HIPROC 760is reserved for processor-specific semantics. 761.It Dv SHT_HIPROC 762This value down to and including 763.Sy SHT_LOPROC 764is reserved for processor-specific semantics. 765.It Dv SHT_LOUSER 766This value specifies the lower bound of the range of indices reserved for 767application programs. 768.It Dv SHT_HIUSER 769This value specifies the upper bound of the range of indices reserved for 770application programs. 771Section types between 772.Sy SHT_LOUSER 773and 774.Sy SHT_HIUSER 775may be used by the application, without conflicting with current or future 776system-defined section types. 777.El 778.Pp 779.It Dv sh_flags 780Sections support one-bit flags that describe miscellaneous attributes. 781If a flag bit is set in 782.Sy sh_flags , 783the attribute is 784.Dq on 785for the section. 786Otherwise, the attribute is 787.Dq off 788or does not apply. 789Undefined attributes are set to zero. 790.Pp 791.Bl -tag -width "SHF_EXECINSTR" -compact 792.It Dv SHF_WRITE 793This section contains data that should be writable during process 794execution. 795.It Dv SHF_ALLOC 796This section occupies memory during process execution. 797Some control 798sections do not reside in the memory image of an object file. 799This 800attribute is off for those sections. 801.It Dv SHF_EXECINSTR 802This section contains executable machine instructions. 803.It Dv SHF_MASKPROC 804All bits included in this mask are reserved for processor-specific 805semantics. 806.El 807.Pp 808.It Dv sh_addr 809If this section appears in the memory image of a process, this member 810holds the address at which the section's first byte should reside. 811Otherwise, the member contains zero. 812.It Dv sh_offset 813This member's value holds the byte offset from the beginning of the file 814to the first byte in the section. 815One section type, 816.Sy SHT_NOBITS , 817occupies no space in the file, and its 818.Sy sh_offset 819member locates the conceptual placement in the file. 820.It Dv sh_size 821This member holds the section's size in bytes. 822Unless the section type 823is 824.Sy SHT_NOBITS , 825the section occupies 826.Sy sh_size 827bytes in the file. 828A section of type 829.Sy SHT_NOBITS 830may have a non-zero size, but it occupies no space in the file. 831.It Dv sh_link 832This member holds a section header table index link, whose interpretation 833depends on the section type. 834.It Dv sh_info 835This member holds extra information, whose interpretation depends on the 836section type. 837.It Dv sh_addralign 838Some sections have address alignment constraints. 839If a section holds a 840doubleword, the system must ensure doubleword alignment for the entire 841section. 842That is, the value of 843.Sy sh_addr 844must be congruent to zero, modulo the value of 845.Sy sh_addralign . 846Only zero and positive integral powers of two are allowed. 847Values of zero 848or one mean the section has no alignment constraints. 849.It Dv sh_entsize 850Some sections hold a table of fixed-sized entries, such as a symbol table. 851For such a section, this member gives the size in bytes for each entry. 852This member contains zero if the section does not hold a table of 853fixed-size entries. 854.El 855.Pp 856Various sections hold program and control information: 857.Bl -tag -width ".shstrtab" 858.It .bss 859This section holds uninitialized data that contributes to the program's 860memory image. 861By definition, the system initializes the data with zeros 862when the program begins to run. 863This section is of type 864.Sy SHT_NOBITS . 865The attribute types are 866.Sy SHF_ALLOC 867and 868.Sy SHF_WRITE . 869.It .comment 870This section holds version control information. 871This section is of type 872.Sy SHT_PROGBITS . 873No attribute types are used. 874.It .ctors 875This section holds initialized pointers to the C++ constructor functions. 876This section is of type 877.Sy SHT_PROGBITS . 878The attribute types are 879.Sy SHF_ALLOC 880and 881.Sy SHF_WRITE . 882.It .data 883This section holds initialized data that contribute to the program's 884memory image. 885This section is of type 886.Sy SHT_PROGBITS . 887The attribute types are 888.Sy SHF_ALLOC 889and 890.Sy SHF_WRITE . 891.It .data1 892This section holds initialized data that contribute to the program's 893memory image. 894This section is of type 895.Sy SHT_PROGBITS . 896The attribute types are 897.Sy SHF_ALLOC 898and 899.Sy SHF_WRITE . 900.It .debug 901This section holds information for symbolic debugging. 902The contents 903are unspecified. 904This section is of type 905.Sy SHT_PROGBITS . 906No attribute types are used. 907.It .dtors 908This section holds initialized pointers to the C++ destructor functions. 909This section is of type 910.Sy SHT_PROGBITS . 911The attribute types are 912.Sy SHF_ALLOC 913and 914.Sy SHF_WRITE . 915.It .dynamic 916This section holds dynamic linking information. 917The section's attributes 918will include the 919.Sy SHF_ALLOC 920bit. 921Whether the 922.Sy SHF_WRITE 923bit is set is processor-specific. 924This section is of type 925.Sy SHT_DYNAMIC . 926See the attributes above. 927.It .dynstr 928This section holds strings needed for dynamic linking, most commonly 929the strings that represent the names associated with symbol table entries. 930This section is of type 931.Sy SHT_STRTAB . 932The attribute type used is 933.Sy SHF_ALLOC . 934.It .dynsym 935This section holds the dynamic linking symbol table. 936This section is of type 937.Sy SHT_DYNSYM . 938The attribute used is 939.Sy SHF_ALLOC . 940.It .fini 941This section holds executable instructions that contribute to the process 942termination code. 943When a program exits normally the system arranges to 944execute the code in this section. 945This section is of type 946.Sy SHT_PROGBITS . 947The attributes used are 948.Sy SHF_ALLOC 949and 950.Sy SHF_EXECINSTR . 951.It .got 952This section holds the global offset table. 953This section is of type 954.Sy SHT_PROGBITS . 955The attributes are processor-specific. 956.It .hash 957This section holds a symbol hash table. 958This section is of type 959.Sy SHT_HASH . 960The attribute used is 961.Sy SHF_ALLOC . 962.It .init 963This section holds executable instructions that contribute to the process 964initialization code. 965When a program starts to run the system arranges to 966execute the code in this section before calling the main program entry point. 967This section is of type 968.Sy SHT_PROGBITS . 969The attributes used are 970.Sy SHF_ALLOC 971and 972.Sy SHF_EXECINSTR . 973.It .interp 974This section holds the pathname of a program interpreter. 975If the file has 976a loadable segment that includes the section, the section's attributes will 977include the 978.Sy SHF_ALLOC 979bit. 980Otherwise, that bit will be off. 981This section is of type 982.Sy SHT_PROGBITS . 983.It .line 984This section holds line number information for symbolic debugging, which 985describes the correspondence between the program source and the machine code. 986The contents are unspecified. 987This section is of type 988.Sy SHT_PROGBITS . 989No attribute types are used. 990.It .note 991This section holds information in the 992.Dq Note Section 993format described below. 994This section is of type 995.Sy SHT_NOTE . 996No attribute types are used. 997.Ox 998native executables usually contain a 999.Sy .note.openbsd.ident 1000section to identify themselves, for the kernel to bypass any compatibility 1001ELF binary emulation tests when loading the file. 1002.It .plt 1003This section holds the procedure linkage table. 1004This section is of type 1005.Sy SHT_PROGBITS . 1006The attributes are processor-specific. 1007.It .relNAME 1008This section holds relocation information as described below. 1009If the file 1010has a loadable segment that includes relocation, the section's attributes 1011will include the 1012.Sy SHF_ALLOC 1013bit. 1014Otherwise the bit will be off. 1015By convention, 1016.Dq NAME 1017is supplied by the section to which the relocations apply. 1018Thus a relocation 1019section for 1020.Sy .text 1021normally would have the name 1022.Sy .rel.text . 1023This section is of type 1024.Sy SHT_REL . 1025.It .relaNAME 1026This section holds relocation information as described below. 1027If the file 1028has a loadable segment that includes relocation, the section's attributes 1029will include the 1030.Sy SHF_ALLOC 1031bit. 1032Otherwise the bit will be off. 1033By convention, 1034.Dq NAME 1035is supplied by the section to which the relocations apply. 1036Thus a relocation 1037section for 1038.Sy .text 1039normally would have the name 1040.Sy .rela.text . 1041This section is of type 1042.Sy SHT_RELA . 1043.It .rodata 1044This section holds read-only data that typically contributes to a 1045non-writable segment in the process image. 1046This section is of type 1047.Sy SHT_PROGBITS . 1048The attribute used is 1049.Sy SHF_ALLOC . 1050.It .rodata1 1051This section holds read-only data that typically contributes to a 1052non-writable segment in the process image. 1053This section is of type 1054.Sy SHT_PROGBITS . 1055The attribute used is 1056.Sy SHF_ALLOC . 1057.It .shstrtab 1058This section holds section names. 1059This section is of type 1060.Sy SHT_STRTAB . 1061No attribute types are used. 1062.It .strtab 1063This section holds strings, most commonly the strings that represent the 1064names associated with symbol table entries. 1065If the file has a loadable 1066segment that includes the symbol string table, the section's attributes 1067will include the 1068.Sy SHF_ALLOC 1069bit. 1070Otherwise the bit will be off. 1071This section is of type 1072.Sy SHT_STRTAB . 1073.It .symtab 1074This section holds a symbol table. 1075If the file has a loadable segment 1076that includes the symbol table, the section's attributes will include 1077the 1078.Sy SHF_ALLOC 1079bit. 1080Otherwise the bit will be off. 1081This section is of type 1082.Sy SHT_SYMTAB . 1083.It .text 1084This section holds the 1085.Dq text , 1086or executable instructions, of a program. 1087This section is of type 1088.Sy SHT_PROGBITS . 1089The attributes used are 1090.Sy SHF_ALLOC 1091and 1092.Sy SHF_EXECINSTR . 1093.El 1094.Pp 1095String table sections hold null-terminated character sequences, commonly 1096called strings. 1097The object file uses these strings to represent symbol 1098and section names. 1099One references a string as an index into the string 1100table section. 1101The first byte, which is index zero, is defined to hold 1102a null character. 1103Similarly, a string table's last byte is defined to 1104hold a null character, ensuring null termination for all strings. 1105.Pp 1106An object file's symbol table holds information needed to locate and 1107relocate a program's symbolic definitions and references. 1108A symbol table 1109index is a subscript into this array. 1110.Bd -literal -offset indent 1111typedef struct { 1112 Elf32_Word st_name; 1113 Elf32_Addr st_value; 1114 Elf32_Word st_size; 1115 unsigned char st_info; 1116 unsigned char st_other; 1117 Elf32_Half st_shndx; 1118} Elf32_Sym; 1119.Ed 1120.Bd -literal -offset indent 1121typedef struct { 1122 Elf64_Half st_name; 1123 Elf_Byte st_info; 1124 Elf_Byte st_other; 1125 Elf64_Quarter st_shndx; 1126 Elf64_Xword st_value; 1127 Elf64_Xword st_size; 1128} Elf64_Sym; 1129.Ed 1130.Bl -tag -width "st_value" 1131.It Dv st_name 1132This member holds an index into the object file's symbol string table, 1133which holds character representations of the symbol names. 1134If the value 1135is non-zero, it represents a string table index that gives the symbol 1136name. 1137Otherwise, the symbol table has no name. 1138.It Dv st_value 1139This member gives the value of the associated symbol. 1140.It Dv st_size 1141Many symbols have associated sizes. 1142This member holds zero if the symbol 1143has no size or an unknown size. 1144.It Dv st_info 1145This member specifies the symbol's type and binding attributes: 1146.Bl -tag -width "STT_SECTION" 1147.It Dv STT_NOTYPE 1148The symbol's type is not defined. 1149.It Dv STT_OBJECT 1150The symbol is associated with a data object. 1151.It Dv STT_FUNC 1152The symbol is associated with a function or other executable code. 1153.It Dv STT_SECTION 1154The symbol is associated with a section. 1155Symbol table entries of 1156this type exist primarily for relocation and normally have 1157.Sy STB_LOCAL 1158bindings. 1159.It Dv STT_FILE 1160By convention, the symbol's name gives the name of the source file 1161associated with the object file. 1162A file symbol has 1163.Sy STB_LOCAL 1164bindings, its section index is 1165.Sy SHN_ABS , 1166and it precedes the other 1167.Sy STB_LOCAL 1168symbols of the file, if it is present. 1169.It Dv STT_LOPROC 1170This value up to and including 1171.Sy STT_HIPROC 1172is reserved for processor-specific semantics. 1173.It Dv STT_HIPROC 1174This value down to and including 1175.Sy STT_LOPROC 1176is reserved for processor-specific semantics. 1177.El 1178.Bl -tag -width "STB_GLOBAL" 1179.It Dv STB_LOCAL 1180Local symbols are not visible outside the object file containing their 1181definition. 1182Local symbols of the same name may exist in multiple files 1183without interfering with each other. 1184.It Dv STB_GLOBAL 1185Global symbols are visible to all object files being combined. 1186One file's 1187definition of a global symbol will satisfy another file's undefined 1188reference to the same symbol. 1189.It Dv STB_WEAK 1190Weak symbols resemble global symbols, but their definitions have lower 1191precedence. 1192.It Dv STB_LOPROC 1193This value up to and including 1194.Sy STB_HIPROC 1195is reserved for processor-specific semantics. 1196.It Dv STB_HIPROC 1197This value down to and including 1198.Sy STB_LOPROC 1199is reserved for processor-specific semantics. 1200.Pp 1201There are macros for packing and unpacking the binding and type fields: 1202.Pp 1203.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1204.It Xo 1205.Fn ELF32_ST_BIND info 1206.Xc 1207or 1208.Fn ELF64_ST_BIND info 1209extract a binding from an st_info value. 1210.It Xo 1211.Fn ELF64_ST_TYPE info 1212.Xc 1213or 1214.Fn ELF32_ST_TYPE info 1215extract a type from an st_info value. 1216.It Xo 1217.Fn ELF32_ST_INFO bind type 1218.Xc 1219or 1220.Fn ELF64_ST_INFO bind type 1221convert a binding and a type into an st_info value. 1222.El 1223.El 1224.Pp 1225.It Dv st_other 1226This member currently holds zero and has no defined meaning. 1227.It Dv st_shndx 1228Every symbol table entry is 1229.Dq defined 1230in relation to some section. 1231This member holds the relevant section 1232header table index. 1233.El 1234.Pp 1235Relocation is the process of connecting symbolic references with 1236symbolic definitions. 1237Relocatable files must have information that 1238describes how to modify their section contents, thus allowing executable 1239and shared object files to hold the right information for a process' 1240program image. 1241Relocation entries are these data. 1242.Pp 1243Relocation structures that do not need an addend: 1244.Bd -literal -offset indent 1245typedef struct { 1246 Elf32_Addr r_offset; 1247 Elf32_Word r_info; 1248} Elf32_Rel; 1249.Ed 1250.Bd -literal -offset indent 1251typedef struct { 1252 Elf64_Xword r_offset; 1253 Elf64_Xword r_info; 1254} Elf64_Rel; 1255.Ed 1256.Pp 1257Relocation structures that need an addend: 1258.Bd -literal -offset indent 1259typedef struct { 1260 Elf32_Addr r_offset; 1261 Elf32_Word r_info; 1262 Elf32_Sword r_addend; 1263} Elf32_Rela; 1264.Ed 1265.Bd -literal -offset indent 1266typedef struct { 1267 Elf64_Xword r_offset; 1268 Elf64_Xword r_info; 1269 Elf64_Sxword r_addend; 1270} Elf64_Rela; 1271.Ed 1272.Bl -tag -width "r_offset" 1273.It Dv r_offset 1274This member gives the location at which to apply the relocation action. 1275For a relocatable file, the value is the byte offset from the beginning 1276of the section to the storage unit affected by the relocation. 1277For an 1278executable file or shared object, the value is the virtual address of 1279the storage unit affected by the relocation. 1280.It Dv r_info 1281This member gives both the symbol table index with respect to which the 1282relocation must be made and the type of relocation to apply. 1283Relocation 1284types are processor-specific. 1285When the text refers to a relocation 1286entry's relocation type or symbol table index, it means the result of 1287applying 1288.Sy ELF_[32|64]_R_TYPE 1289or 1290.Sy ELF[32|64]_R_SYM , 1291respectively, to the entry's 1292.Sy r_info 1293member. 1294.It Dv r_addend 1295This member specifies a constant addend used to compute the value to be 1296stored into the relocatable field. 1297.El 1298.Sh SEE ALSO 1299.Xr as 1 , 1300.Xr gdb 1 , 1301.Xr ld 1 , 1302.Xr objdump 1 , 1303.Xr execve 2 , 1304.Xr core 5 1305.Rs 1306.%A Hewlett-Packard 1307.%B Elf-64 Object File Format 1308.Re 1309.Rs 1310.%A Santa Cruz Operation 1311.%B System V Application Binary Interface 1312.Re 1313.Rs 1314.%A Unix System Laboratories 1315.%T Object Files 1316.%B "Executable and Linking Format (ELF)" 1317.Re 1318.Sh HISTORY 1319.Ox 1320ELF support first appeared in 1321.Ox 1.2 , 1322although not all supported platforms use it as the native 1323binary file format. 1324ELF in itself first appeared in 1325.At V . 1326The ELF format is an adopted standard. 1327.Sh AUTHORS 1328This manual page was written by 1329.An Jeroen Ruigrok van der Werven 1330.Aq asmodai@FreeBSD.org 1331with inspiration from BSDi's 1332.Bsx 1333.Nm elf 1334manpage. 1335