1.\" Copyright (c) 1999 Jeroen Ruigrok van der Werven 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd December 18, 2005 28.Dt ELF 5 29.Os 30.Sh NAME 31.Nm elf 32.Nd format of ELF executable binary files 33.Sh SYNOPSIS 34.In elf.h 35.Sh DESCRIPTION 36The header file 37.In elf.h 38defines the format of ELF executable binary files. 39Amongst these files are 40normal executable files, relocatable object files, core files and shared 41libraries. 42.Pp 43An executable file using the ELF file format consists of an ELF header, 44followed by a program header table or a section header table, or both. 45The ELF header is always at offset zero of the file. 46The program header 47table and the section header table's offset in the file are defined in the 48ELF header. 49The two tables describe the rest of the particularities of 50the file. 51.Pp 52Applications which wish to process ELF binary files for their native 53architecture only should include 54.In elf.h 55in their source code. 56These applications should need to refer to 57all the types and structures by their generic names 58.Dq Elf_xxx 59and to the macros by 60.Dq ELF_xxx . 61Applications written this way can be compiled on any architecture, 62regardless whether the host is 32-bit or 64-bit. 63.Pp 64Should an application need to process ELF files of an unknown 65architecture then the application needs to include both 66.In sys/elf32.h 67and 68.In sys/elf64.h 69instead of 70.In elf.h . 71Furthermore, all types and structures need to be identified by either 72.Dq Elf32_xxx 73or 74.Dq Elf64_xxx . 75The macros need to be identified by 76.Dq ELF32_xxx 77or 78.Dq ELF64_xxx . 79.Pp 80Whatever the system's architecture is, it will always include 81.In sys/elf_common.h 82as well as 83.In sys/elf_generic.h . 84.Pp 85These header files describe the above mentioned headers as C structures 86and also include structures for dynamic sections, relocation sections and 87symbol tables. 88.Pp 89The following types are being used for 32-bit architectures: 90.Bd -literal -offset indent 91Elf32_Addr Unsigned 32-bit program address 92Elf32_Half Unsigned 16-bit field 93Elf32_Lword Unsigned 64-bit field 94Elf32_Off Unsigned 32-bit file offset 95Elf32_Sword Signed 32-bit field or integer 96Elf32_Word Unsigned 32-bit field or integer 97.Ed 98.Pp 99For 64-bit architectures we have the following types: 100.Bd -literal -offset indent 101Elf64_Addr Unsigned 64-bit program address 102Elf64_Half Unsigned 16-bit field 103Elf64_Lword Unsigned 64-bit field 104Elf64_Off Unsigned 64-bit file offset 105Elf64_Sword Signed 32-bit field 106Elf64_Sxword Signed 64-bit field or integer 107Elf64_Word Unsigned 32-bit field 108Elf64_Xword Unsigned 64-bit field or integer 109.Ed 110.Pp 111All data structures that the file format defines follow the 112.Dq natural 113size and alignment guidelines for the relevant class. 114If necessary, 115data structures contain explicit padding to ensure 4-byte alignment 116for 4-byte objects, to force structure sizes to a multiple of 4, etc. 117.Pp 118The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 119.Bd -literal -offset indent 120typedef struct { 121 unsigned char e_ident[EI_NIDENT]; 122 Elf32_Half e_type; 123 Elf32_Half e_machine; 124 Elf32_Word e_version; 125 Elf32_Addr e_entry; 126 Elf32_Off e_phoff; 127 Elf32_Off e_shoff; 128 Elf32_Word e_flags; 129 Elf32_Half e_ehsize; 130 Elf32_Half e_phentsize; 131 Elf32_Half e_phnum; 132 Elf32_Half e_shentsize; 133 Elf32_Half e_shnum; 134 Elf32_Half e_shstrndx; 135} Elf32_Ehdr; 136.Ed 137.Pp 138.Bd -literal -offset indent 139typedef struct { 140 unsigned char e_ident[EI_NIDENT]; 141 Elf64_Half e_type; 142 Elf64_Half e_machine; 143 Elf64_Word e_version; 144 Elf64_Addr e_entry; 145 Elf64_Off e_phoff; 146 Elf64_Off e_shoff; 147 Elf64_Word e_flags; 148 Elf64_Half e_ehsize; 149 Elf64_Half e_phentsize; 150 Elf64_Half e_phnum; 151 Elf64_Half e_shentsize; 152 Elf64_Half e_shnum; 153 Elf64_Half e_shstrndx; 154} Elf64_Ehdr; 155.Ed 156.Pp 157The fields have the following meanings: 158.Pp 159.Bl -tag -width "e_phentsize" -compact -offset indent 160.It Dv e_ident 161This array of bytes specifies to interpret the file, 162independent of the processor or the file's remaining contents. 163Within this array everything is named by macros, which start with 164the prefix 165.Sy EI_ 166and may contain values which start with the prefix 167.Sy ELF . 168The following macros are defined: 169.Pp 170.Bl -tag -width "EI_ABIVERSION" -compact 171.It Dv EI_MAG0 172The first byte of the magic number. 173It must be filled with 174.Sy ELFMAG0 . 175.It Dv EI_MAG1 176The second byte of the magic number. 177It must be filled with 178.Sy ELFMAG1 . 179.It Dv EI_MAG2 180The third byte of the magic number. 181It must be filled with 182.Sy ELFMAG2 . 183.It Dv EI_MAG3 184The fourth byte of the magic number. 185It must be filled with 186.Sy ELFMAG3 . 187.It Dv EI_CLASS 188The fifth byte identifies the architecture for this binary: 189.Pp 190.Bl -tag -width "ELFCLASSNONE" -compact 191.It Dv ELFCLASSNONE 192This class is invalid. 193.It Dv ELFCLASS32 194This defines the 32-bit architecture. 195It supports machines with files 196and virtual address spaces up to 4 Gigabytes. 197.It Dv ELFCLASS64 198This defines the 64-bit architecture. 199.El 200.It Dv EI_DATA 201The sixth byte specifies the data encoding of the processor-specific 202data in the file. 203Currently these encodings are supported: 204.Pp 205.Bl -tag -width "ELFDATA2LSB" -compact 206.It Dv ELFDATANONE 207Unknown data format. 208.It Dv ELFDATA2LSB 209Two's complement, little-endian. 210.It Dv ELFDATA2MSB 211Two's complement, big-endian. 212.El 213.It Dv EI_VERSION 214The version number of the ELF specification: 215.Pp 216.Bl -tag -width "EV_CURRENT" -compact 217.It Dv EV_NONE 218Invalid version. 219.It Dv EV_CURRENT 220Current version. 221.El 222.It Dv EI_OSABI 223This byte identifies the operating system 224and ABI to which the object is targeted. 225Some fields in other ELF structures have flags 226and values that have platform specific meanings; 227the interpretation of those fields is determined by the value of this byte. 228The following values are currently defined: 229.Pp 230.Bl -tag -width "ELFOSABI_STANDALONE" -compact 231.It Dv ELFOSABI_SYSV 232UNIX System V ABI. 233.It Dv ELFOSABI_HPUX 234HP-UX operating system ABI. 235.It Dv ELFOSABI_NETBSD 236.Nx 237operating system ABI. 238.It Dv ELFOSABI_LINUX 239GNU/Linux operating system ABI. 240.It Dv ELFOSABI_HURD 241GNU/Hurd operating system ABI. 242.It Dv ELFOSABI_86OPEN 24386Open Common IA32 ABI. 244.It Dv ELFOSABI_SOLARIS 245Solaris operating system ABI. 246.It Dv ELFOSABI_MONTEREY 247Monterey project ABI. 248.It Dv ELFOSABI_IRIX 249IRIX operating system ABI. 250.It Dv ELFOSABI_FREEBSD 251.Fx 252operating system ABI. 253.It Dv ELFOSABI_TRU64 254TRU64 UNIX operating system ABI. 255.It Dv ELFOSABI_ARM 256ARM architecture ABI. 257.It Dv ELFOSABI_STANDALONE 258Standalone (embedded) ABI. 259.El 260.It Dv EI_ABIVERSION 261This byte identifies the version of the ABI 262to which the object is targeted. 263This field is used to distinguish among incompatible versions of an ABI. 264The interpretation of this version number 265is dependent on the ABI identified by the EI_OSABI field. 266Applications conforming to this specification use the value 0. 267.It Dv EI_PAD 268Start of padding. 269These bytes are reserved and set to zero. 270Programs 271which read them should ignore them. 272The value for EI_PAD will change in 273the future if currently unused bytes are given meanings. 274.It Dv EI_BRAND 275Start of architecture identification. 276.It Dv EI_NIDENT 277The size of the e_ident array. 278.El 279.Pp 280.It Dv e_type 281This member of the structure identifies the object file type: 282.Pp 283.Bl -tag -width "ET_NONE" -compact 284.It Dv ET_NONE 285An unknown type. 286.It Dv ET_REL 287A relocatable file. 288.It Dv ET_EXEC 289An executable file. 290.It Dv ET_DYN 291A shared object. 292.It Dv ET_CORE 293A core file. 294.El 295.Pp 296.It Dv e_machine 297This member specifies the required architecture for an individual file: 298.Pp 299.Bl -tag -width "EM_MIPS_RS4_BE" -compact 300.It Dv EM_NONE 301An unknown machine. 302.It Dv EM_M32 303AT&T WE 32100. 304.It Dv EM_SPARC 305Sun Microsystems SPARC. 306.It Dv EM_386 307Intel 80386. 308.It Dv EM_68K 309Motorola 68000. 310.It Dv EM_88K 311Motorola 88000. 312.It Dv EM_486 313Intel 80486. 314.It Dv EM_860 315Intel 80860. 316.It Dv EM_MIPS 317MIPS RS3000 (big-endian only). 318.It Dv EM_MIPS_RS4_BE 319MIPS RS4000 (big-endian only). 320.It Dv EM_SPARC64 321SPARC v9 64-bit unofficial. 322.It Dv EM_PARISC 323HPPA. 324.It Dv EM_PPC 325PowerPC. 326.It Dv EM_ALPHA 327Compaq [DEC] Alpha. 328.El 329.Pp 330.It Dv e_version 331This member identifies the file version: 332.Pp 333.Bl -tag -width "EV_CURRENT" -compact 334.It Dv EV_NONE 335Invalid version 336.It Dv EV_CURRENT 337Current version 338.El 339.It Dv e_entry 340This member gives the virtual address to which the system first transfers 341control, thus starting the process. 342If the file has no associated entry 343point, this member holds zero. 344.It Dv e_phoff 345This member holds the program header table's file offset in bytes. 346If 347the file has no program header table, this member holds zero. 348.It Dv e_shoff 349This member holds the section header table's file offset in bytes. 350If the 351file has no section header table this member holds zero. 352.It Dv e_flags 353This member holds processor-specific flags associated with the file. 354Flag 355names take the form EF_`machine_flag'. 356Currently no flags have been defined. 357.It Dv e_ehsize 358This member holds the ELF header's size in bytes. 359.It Dv e_phentsize 360This member holds the size in bytes of one entry in the file's program header 361table; all entries are the same size. 362.It Dv e_phnum 363This member holds the number of entries in the program header 364table. 365Thus the product of 366.Sy e_phentsize 367and 368.Sy e_phnum 369gives the table's size 370in bytes. 371If a file has no program header, 372.Sy e_phnum 373holds the value zero. 374.It Dv e_shentsize 375This member holds a sections header's size in bytes. 376A section header is one 377entry in the section header table; all entries are the same size. 378.It Dv e_shnum 379This member holds the number of entries in the section header table. 380If the file is using extended section numbering, then the 381.Sy e_shnum 382field will be zero and the actual section number will be stored in the 383.Sy sh_size 384member of the section header at index 385.Dv SHN_UNDEF . 386If a file has no section header table, both the 387.Sy e_shnum 388and the 389.Sy e_shoff 390fields of the ELF header will be zero. 391The product of 392.Sy e_shentsize 393and the number of sections in the file gives the section header 394table's size in bytes. 395.It Dv e_shstrndx 396This member holds the section header table index of the entry associated 397with the section name string table. 398If extended section numbering is being used, this field will hold the 399value 400.Sy SHN_XINDEX , 401and the actual section header table index will be present in the 402.Sy sh_link 403field of the section header entry at index 404.Dv SHN_UNDEF . 405If the file has no section name string 406table, this member holds the value 407.Sy SHN_UNDEF . 408.El 409.Pp 410An executable or shared object file's program header table is an array of 411structures, each describing a segment or other information the system needs 412to prepare the program for execution. 413An object file 414.Em segment 415contains one or more 416.Em sections . 417Program headers are meaningful only for executable and shared object files. 418A file specifies its own program header size with the ELF header's 419.Sy e_phentsize 420and 421.Sy e_phnum 422members. 423As with the Elf executable header, the program header 424also has different versions depending on the architecture: 425.Pp 426.Bd -literal -offset indent 427typedef struct { 428 Elf32_Word p_type; 429 Elf32_Off p_offset; 430 Elf32_Addr p_vaddr; 431 Elf32_Addr p_paddr; 432 Elf32_Word p_filesz; 433 Elf32_Word p_memsz; 434 Elf32_Word p_flags; 435 Elf32_Word p_align; 436} Elf32_Phdr; 437.Ed 438.Pp 439.Bd -literal -offset indent 440typedef struct { 441 Elf64_Word p_type; 442 Elf64_Word p_flags; 443 Elf64_Off p_offset; 444 Elf64_Addr p_vaddr; 445 Elf64_Addr p_paddr; 446 Elf64_Xword p_filesz; 447 Elf64_Xword p_memsz; 448 Elf64_Xword p_align; 449} Elf64_Phdr; 450.Ed 451.Pp 452The main difference between the 32-bit and the 64-bit program header lies 453only in the location of a 454.Sy p_flags 455member in the total struct. 456.Pp 457.Bl -tag -width "p_offset" -compact -offset indent 458.It Dv p_type 459This member of the Phdr struct tells what kind of segment this array 460element describes or how to interpret the array element's information. 461.Bl -tag -width "PT_DYNAMIC" -compact 462.Pp 463.It Dv PT_NULL 464The array element is unused and the other members' values are undefined. 465This lets the program header have ignored entries. 466.It Dv PT_LOAD 467The array element specifies a loadable segment, described by 468.Sy p_filesz 469and 470.Sy p_memsz . 471The bytes from the file are mapped to the beginning of the memory 472segment. 473If the segment's memory size 474.Pq Sy p_memsz 475is larger than the file size 476.Pq Sy p_filesz , 477the 478.Dq extra 479bytes are defined to hold the value 0 and to follow the segment's 480initialized area. 481The file size may not be larger than the memory size. 482Loadable segment entries in the program header table appear in ascending 483order, sorted on the 484.Sy p_vaddr 485member. 486.It Dv PT_DYNAMIC 487The array element specifies dynamic linking information. 488.It Dv PT_INTERP 489The array element specifies the location and size of a null-terminated 490path name to invoke as an interpreter. 491This segment type is meaningful 492only for executable files (though it may occur for shared objects). 493However 494it may not occur more than once in a file. 495If it is present it must precede 496any loadable segment entry. 497.It Dv PT_NOTE 498The array element specifies the location and size for auxiliary information. 499.It Dv PT_SHLIB 500This segment type is reserved but has unspecified semantics. 501Programs that 502contain an array element of this type do not conform to the ABI. 503.It Dv PT_PHDR 504The array element, if present, specifies the location and size of the program 505header table itself, both in the file and in the memory image of the program. 506This segment type may not occur more than once in a file. 507Moreover, it may 508only occur if the program header table is part of the memory image of the 509program. 510If it is present it must precede any loadable segment entry. 511.It Dv PT_LOPROC 512This value up to and including 513.Sy PT_HIPROC 514are reserved for processor-specific semantics. 515.It Dv PT_HIPROC 516This value down to and including 517.Sy PT_LOPROC 518are reserved for processor-specific semantics. 519.El 520.Pp 521.It Dv p_offset 522This member holds the offset from the beginning of the file at which 523the first byte of the segment resides. 524.It Dv p_vaddr 525This member holds the virtual address at which the first byte of the 526segment resides in memory. 527.It Dv p_paddr 528On systems for which physical addressing is relevant, this member is 529reserved for the segment's physical address. 530Under 531.Bx 532this member is 533not used and must be zero. 534.It Dv p_filesz 535This member holds the number of bytes in the file image of the segment. 536It may be zero. 537.It Dv p_memsz 538This member holds the number of bytes in the memory image of the segment. 539It may be zero. 540.It Dv p_flags 541This member holds flags relevant to the segment: 542.Pp 543.Bl -tag -width "PF_X" -compact 544.It Dv PF_X 545An executable segment. 546.It Dv PF_W 547A writable segment. 548.It Dv PF_R 549A readable segment. 550.El 551.Pp 552A text segment commonly has the flags 553.Sy PF_X 554and 555.Sy PF_R . 556A data segment commonly has 557.Sy PF_X , 558.Sy PF_W 559and 560.Sy PF_R . 561.It Dv p_align 562This member holds the value to which the segments are aligned in memory 563and in the file. 564Loadable process segments must have congruent values for 565.Sy p_vaddr 566and 567.Sy p_offset , 568modulo the page size. 569Values of zero and one mean no alignment is required. 570Otherwise, 571.Sy p_align 572should be a positive, integral power of two, and 573.Sy p_vaddr 574should equal 575.Sy p_offset , 576modulo 577.Sy p_align . 578.El 579.Pp 580An file's section header table lets one locate all the file's sections. 581The 582section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 583The 584ELF header's 585.Sy e_shoff 586member gives the byte offset from the beginning of the file to the section 587header table. 588.Sy e_shnum 589holds the number of entries the section header table contains. 590.Sy e_shentsize 591holds the size in bytes of each entry. 592.Pp 593A section header table index is a subscript into this array. 594Some section 595header table indices are reserved. 596An object file does not have sections for 597these special indices: 598.Pp 599.Bl -tag -width "SHN_LORESERVE" -compact 600.It Dv SHN_UNDEF 601This value marks an undefined, missing, irrelevant, or otherwise meaningless 602section reference. 603For example, a symbol 604.Dq defined 605relative to section number 606.Sy SHN_UNDEF 607is an undefined symbol. 608.It Dv SHN_LORESERVE 609This value specifies the lower bound of the range of reserved indices. 610.It Dv SHN_LOPROC 611This value up to and including 612.Sy SHN_HIPROC 613are reserved for processor-specific semantics. 614.It Dv SHN_HIPROC 615This value down to and including 616.Sy SHN_LOPROC 617are reserved for processor-specific semantics. 618.It Dv SHN_ABS 619This value specifies absolute values for the corresponding reference. 620For 621example, symbols defined relative to section number 622.Sy SHN_ABS 623have absolute values and are not affected by relocation. 624.It Dv SHN_COMMON 625Symbols defined relative to this section are common symbols, such as FORTRAN 626COMMON or unallocated C external variables. 627.It Dv SHN_HIRESERVE 628This value specifies the upper bound of the range of reserved indices. 629The 630system reserves indices between 631.Sy SHN_LORESERVE 632and 633.Sy SHN_HIRESERVE , 634inclusive. 635The section header table does not contain entries for the 636reserved indices. 637.El 638.Pp 639The section header has the following structure: 640.Bd -literal -offset indent 641typedef struct { 642 Elf32_Word sh_name; 643 Elf32_Word sh_type; 644 Elf32_Word sh_flags; 645 Elf32_Addr sh_addr; 646 Elf32_Off sh_offset; 647 Elf32_Word sh_size; 648 Elf32_Word sh_link; 649 Elf32_Word sh_info; 650 Elf32_Word sh_addralign; 651 Elf32_Word sh_entsize; 652} Elf32_Shdr; 653.Ed 654.Pp 655.Bd -literal -offset indent 656typedef struct { 657 Elf64_Word sh_name; 658 Elf64_Word sh_type; 659 Elf64_Xword sh_flags; 660 Elf64_Addr sh_addr; 661 Elf64_Off sh_offset; 662 Elf64_Xword sh_size; 663 Elf64_Word sh_link; 664 Elf64_Word sh_info; 665 Elf64_Xword sh_addralign; 666 Elf64_Xword sh_entsize; 667} Elf64_Shdr; 668.Ed 669.Pp 670.Bl -tag -width "sh_addralign" -compact 671.It Dv sh_name 672This member specifies the name of the section. 673Its value is an index 674into the section header string table section, giving the location of 675a null-terminated string. 676.It Dv sh_type 677This member categorizes the section's contents and semantics. 678.Pp 679.Bl -tag -width "SHT_PROGBITS" -compact 680.It Dv SHT_NULL 681This value marks the section header as inactive. 682It does not 683have an associated section. 684Other members of the section header 685have undefined values. 686.It Dv SHT_PROGBITS 687The section holds information defined by the program, whose 688format and meaning are determined solely by the program. 689.It Dv SHT_SYMTAB 690This section holds a symbol table. 691Typically, 692.Sy SHT_SYMTAB 693provides symbols for link editing, though it may also be used 694for dynamic linking. 695As a complete symbol table, it may contain 696many symbols unnecessary for dynamic linking. 697An object file can 698also contain a 699.Sy SHN_DYNSYM 700section. 701.It Dv SHT_STRTAB 702This section holds a string table. 703An object file may have multiple 704string table sections. 705.It Dv SHT_RELA 706This section holds relocation entries with explicit addends, such 707as type 708.Sy Elf32_Rela 709for the 32-bit class of object files. 710An object may have multiple 711relocation sections. 712.It Dv SHT_HASH 713This section holds a symbol hash table. 714All object participating in 715dynamic linking must contain a symbol hash table. 716An object file may 717have only one hash table. 718.It Dv SHT_DYNAMIC 719This section holds information for dynamic linking. 720An object file may 721have only one dynamic section. 722.It Dv SHT_NOTE 723This section holds information that marks the file in some way. 724.It Dv SHT_NOBITS 725A section of this type occupies no space in the file but otherwise 726resembles 727.Sy SHN_PROGBITS . 728Although this section contains no bytes, the 729.Sy sh_offset 730member contains the conceptual file offset. 731.It Dv SHT_REL 732This section holds relocation offsets without explicit addends, such 733as type 734.Sy Elf32_Rel 735for the 32-bit class of object files. 736An object file may have multiple 737relocation sections. 738.It Dv SHT_SHLIB 739This section is reserved but has unspecified semantics. 740.It Dv SHT_DYNSYM 741This section holds a minimal set of dynamic linking symbols. 742An 743object file can also contain a 744.Sy SHN_SYMTAB 745section. 746.It Dv SHT_LOPROC 747This value up to and including 748.Sy SHT_HIPROC 749are reserved for processor-specific semantics. 750.It Dv SHT_HIPROC 751This value down to and including 752.Sy SHT_LOPROC 753are reserved for processor-specific semantics. 754.It Dv SHT_LOUSER 755This value specifies the lower bound of the range of indices reserved for 756application programs. 757.It Dv SHT_HIUSER 758This value specifies the upper bound of the range of indices reserved for 759application programs. 760Section types between 761.Sy SHT_LOUSER 762and 763.Sy SHT_HIUSER 764may be used by the application, without conflicting with current or future 765system-defined section types. 766.El 767.Pp 768.It Dv sh_flags 769Sections support one-bit flags that describe miscellaneous attributes. 770If a flag bit is set in 771.Sy sh_flags , 772the attribute is 773.Dq on 774for the section. 775Otherwise, the attribute is 776.Dq off 777or does not apply. 778Undefined attributes are set to zero. 779.Pp 780.Bl -tag -width "SHF_EXECINSTR" -compact 781.It Dv SHF_WRITE 782This section contains data that should be writable during process 783execution. 784.It Dv SHF_ALLOC 785The section occupies memory during process execution. 786Some control 787sections do not reside in the memory image of an object file. 788This 789attribute is off for those sections. 790.It Dv SHF_EXECINSTR 791The section contains executable machine instructions. 792.It Dv SHF_MASKPROC 793All bits included in this mask are reserved for processor-specific 794semantics. 795.El 796.Pp 797.It Dv sh_addr 798If the section will appear in the memory image of a process, this member 799holds the address at which the section's first byte should reside. 800Otherwise, the member contains zero. 801.It Dv sh_offset 802This member's value holds the byte offset from the beginning of the file 803to the first byte in the section. 804One section type, 805.Sy SHT_NOBITS , 806occupies no space in the file, and its 807.Sy sh_offset 808member locates the conceptual placement in the file. 809.It Dv sh_size 810This member holds the section's size in bytes. 811Unless the section type 812is 813.Sy SHT_NOBITS , 814the section occupies 815.Sy sh_size 816bytes in the file. 817A section of type 818.Sy SHT_NOBITS 819may have a non-zero size, but it occupies no space in the file. 820.It Dv sh_link 821This member holds a section header table index link, whose interpretation 822depends on the section type. 823.It Dv sh_info 824This member holds extra information, whose interpretation depends on the 825section type. 826.It Dv sh_addralign 827Some sections have address alignment constraints. 828If a section holds a 829doubleword, the system must ensure doubleword alignment for the entire 830section. 831That is, the value of 832.Sy sh_addr 833must be congruent to zero, modulo the value of 834.Sy sh_addralign . 835Only zero and positive integral powers of two are allowed. 836Values of zero 837or one mean the section has no alignment constraints. 838.It Dv sh_entsize 839Some sections hold a table of fixed-sized entries, such as a symbol table. 840For such a section, this member gives the size in bytes for each entry. 841This member contains zero if the section does not hold a table of 842fixed-size entries. 843.El 844.Pp 845Various sections hold program and control information: 846.Bl -tag -width ".shstrtab" -compact 847.It .bss 848(Block Started by Symbol) 849This section holds uninitialized data that contributes to the program's 850memory image. 851By definition, the system initializes the data with zeros 852when the program begins to run. 853This section is of type 854.Sy SHT_NOBITS . 855The attributes types are 856.Sy SHF_ALLOC 857and 858.Sy SHF_WRITE . 859.It .comment 860This section holds version control information. 861This section is of type 862.Sy SHT_PROGBITS . 863No attribute types are used. 864.It .data 865This section holds initialized data that contribute to the program's 866memory image. 867This section is of type 868.Sy SHT_PROGBITS . 869The attribute types are 870.Sy SHF_ALLOC 871and 872.Sy SHF_WRITE . 873.It .data1 874This section holds initialized data that contribute to the program's 875memory image. 876This section is of type 877.Sy SHT_PROGBITS . 878The attribute types are 879.Sy SHF_ALLOC 880and 881.Sy SHF_WRITE . 882.It .debug 883This section holds information for symbolic debugging. 884The contents 885are unspecified. 886This section is of type 887.Sy SHT_PROGBITS . 888No attribute types are used. 889.It .dynamic 890This section holds dynamic linking information. 891The section's attributes 892will include the 893.Sy SHF_ALLOC 894bit. 895Whether the 896.Sy SHF_WRITE 897bit is set is processor-specific. 898This section is of type 899.Sy SHT_DYNAMIC . 900See the attributes above. 901.It .dynstr 902This section holds strings needed for dynamic linking, most commonly 903the strings that represent the names associated with symbol table entries. 904This section is of type 905.Sy SHT_STRTAB . 906The attribute type used is 907.Sy SHF_ALLOC . 908.It .dynsym 909This section holds the dynamic linking symbol table. 910This section is of type 911.Sy SHT_DYNSYM . 912The attribute used is 913.Sy SHF_ALLOC . 914.It .fini 915This section holds executable instructions that contribute to the process 916termination code. 917When a program exits normally the system arranges to 918execute the code in this section. 919This section is of type 920.Sy SHT_PROGBITS . 921The attributes used are 922.Sy SHF_ALLOC 923and 924.Sy SHF_EXECINSTR . 925.It .got 926This section holds the global offset table. 927This section is of type 928.Sy SHT_PROGBITS . 929The attributes are processor-specific. 930.It .hash 931This section holds a symbol hash table. 932This section is of type 933.Sy SHT_HASH . 934The attribute used is 935.Sy SHF_ALLOC . 936.It .init 937This section holds executable instructions that contribute to the process 938initialization code. 939When a program starts to run the system arranges to 940execute the code in this section before calling the main program entry point. 941This section is of type 942.Sy SHT_PROGBITS . 943The attributes used are 944.Sy SHF_ALLOC 945and 946.Sy SHF_EXECINSTR . 947.It .interp 948This section holds the pathname of a program interpreter. 949If the file has 950a loadable segment that includes the section, the section's attributes will 951include the 952.Sy SHF_ALLOC 953bit. 954Otherwise, that bit will be off. 955This section is of type 956.Sy SHT_PROGBITS . 957.It .line 958This section holds line number information for symbolic debugging, which 959describes the correspondence between the program source and the machine code. 960The contents are unspecified. 961This section is of type 962.Sy SHT_PROGBITS . 963No attribute types are used. 964.It .note 965This section holds information in the 966.Dq Note Section 967format described below. 968This section is of type 969.Sy SHT_NOTE . 970No attribute types are used. 971.It .plt 972This section holds the procedure linkage table. 973This section is of type 974.Sy SHT_PROGBITS . 975The attributes are processor-specific. 976.It .relNAME 977This section holds relocation information as described below. 978If the file 979has a loadable segment that includes relocation, the section's attributes 980will include the 981.Sy SHF_ALLOC 982bit. 983Otherwise the bit will be off. 984By convention, 985.Dq NAME 986is supplied by the section to which the relocations apply. 987Thus a relocation 988section for 989.Sy .text 990normally would have the name 991.Sy .rel.text . 992This section is of type 993.Sy SHT_REL . 994.It .relaNAME 995This section holds relocation information as described below. 996If the file 997has a loadable segment that includes relocation, the section's attributes 998will include the 999.Sy SHF_ALLOC 1000bit. 1001Otherwise the bit will be off. 1002By convention, 1003.Dq NAME 1004is supplied by the section to which the relocations apply. 1005Thus a relocation 1006section for 1007.Sy .text 1008normally would have the name 1009.Sy .rela.text . 1010This section is of type 1011.Sy SHT_RELA . 1012.It .rodata 1013This section holds read-only data that typically contributes to a 1014non-writable segment in the process image. 1015This section is of type 1016.Sy SHT_PROGBITS . 1017The attribute used is 1018.Sy SHF_ALLOC . 1019.It .rodata1 1020This section hold read-only data that typically contributes to a 1021non-writable segment in the process image. 1022This section is of type 1023.Sy SHT_PROGBITS . 1024The attribute used is 1025.Sy SHF_ALLOC . 1026.It .shstrtab 1027This section holds section names. 1028This section is of type 1029.Sy SHT_STRTAB . 1030No attribute types are used. 1031.It .strtab 1032This section holds strings, most commonly the strings that represent the 1033names associated with symbol table entries. 1034If the file has a loadable 1035segment that includes the symbol string table, the section's attributes 1036will include the 1037.Sy SHF_ALLOC 1038bit. 1039Otherwise the bit will be off. 1040This section is of type 1041.Sy SHT_STRTAB . 1042.It .symtab 1043This section holds a symbol table. 1044If the file has a loadable segment 1045that includes the symbol table, the section's attributes will include 1046the 1047.Sy SHF_ALLOC 1048bit. 1049Otherwise the bit will be off. 1050This section is of type 1051.Sy SHT_SYMTAB . 1052.It .text 1053This section holds the 1054.Dq text , 1055or executable instructions, of a program. 1056This section is of type 1057.Sy SHT_PROGBITS . 1058The attributes used are 1059.Sy SHF_ALLOC 1060and 1061.Sy SHF_EXECINSTR . 1062.It .jcr 1063This section holds information about Java classes that must 1064be registered. 1065.It .eh_frame 1066This section holds information used for C++ exception-handling. 1067.El 1068.Pp 1069String table sections hold null-terminated character sequences, commonly 1070called strings. 1071The object file uses these strings to represent symbol 1072and section names. 1073One references a string as an index into the string 1074table section. 1075The first byte, which is index zero, is defined to hold 1076a null character. 1077Similarly, a string table's last byte is defined to 1078hold a null character, ensuring null termination for all strings. 1079.Pp 1080An object file's symbol table holds information needed to locate and 1081relocate a program's symbolic definitions and references. 1082A symbol table 1083index is a subscript into this array. 1084.Pp 1085.Bd -literal -offset indent 1086typedef struct { 1087 Elf32_Word st_name; 1088 Elf32_Addr st_value; 1089 Elf32_Word st_size; 1090 unsigned char st_info; 1091 unsigned char st_other; 1092 Elf32_Half st_shndx; 1093} Elf32_Sym; 1094.Ed 1095.Pp 1096.Bd -literal -offset indent 1097typedef struct { 1098 Elf64_Word st_name; 1099 unsigned char st_info; 1100 unsigned char st_other; 1101 Elf64_Half st_shndx; 1102 Elf64_Addr st_value; 1103 Elf64_Xword st_size; 1104} Elf64_Sym; 1105.Ed 1106.Pp 1107.Bl -tag -width "st_value" -compact 1108.It Dv st_name 1109This member holds an index into the object file's symbol string table, 1110which holds character representations of the symbol names. 1111If the value 1112is non-zero, it represents a string table index that gives the symbol 1113name. 1114Otherwise, the symbol table has no name. 1115.It Dv st_value 1116This member gives the value of the associated symbol. 1117.It Dv st_size 1118Many symbols have associated sizes. 1119This member holds zero if the symbol 1120has no size or an unknown size. 1121.It Dv st_info 1122This member specifies the symbol's type and binding attributes: 1123.Pp 1124.Bl -tag -width "STT_SECTION" -compact 1125.It Dv STT_NOTYPE 1126The symbol's type is not defined. 1127.It Dv STT_OBJECT 1128The symbol is associated with a data object. 1129.It Dv STT_FUNC 1130The symbol is associated with a function or other executable code. 1131.It Dv STT_SECTION 1132The symbol is associated with a section. 1133Symbol table entries of 1134this type exist primarily for relocation and normally have 1135.Sy STB_LOCAL 1136bindings. 1137.It Dv STT_FILE 1138By convention the symbol's name gives the name of the source file 1139associated with the object file. 1140A file symbol has 1141.Sy STB_LOCAL 1142bindings, its section index is 1143.Sy SHN_ABS , 1144and it precedes the other 1145.Sy STB_LOCAL 1146symbols of the file, if it is present. 1147.It Dv STT_LOPROC 1148This value up to and including 1149.Sy STT_HIPROC 1150are reserved for processor-specific semantics. 1151.It Dv STT_HIPROC 1152This value down to and including 1153.Sy STT_LOPROC 1154are reserved for processor-specific semantics. 1155.El 1156.Pp 1157.Bl -tag -width "STB_GLOBAL" -compact 1158.It Dv STB_LOCAL 1159Local symbols are not visible outside the object file containing their 1160definition. 1161Local symbols of the same name may exist in multiple file 1162without interfering with each other. 1163.It Dv STB_GLOBAL 1164Global symbols are visible to all object files being combined. 1165One file's 1166definition of a global symbol will satisfy another file's undefined 1167reference to the same symbol. 1168.It Dv STB_WEAK 1169Weak symbols resemble global symbols, but their definitions have lower 1170precedence. 1171.It Dv STB_LOPROC 1172This value up to and including 1173.Sy STB_HIPROC 1174are reserved for processor-specific semantics. 1175.It Dv STB_HIPROC 1176This value down to and including 1177.Sy STB_LOPROC 1178are reserved for processor-specific semantics. 1179.Pp 1180There are macros for packing and unpacking the binding and type fields: 1181.Pp 1182.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1183.It Xo 1184.Fn ELF32_ST_BIND info 1185.Xc 1186or 1187.Fn ELF64_ST_BIND info 1188extract a binding from an st_info value. 1189.It Xo 1190.Fn ELF64_ST_TYPE info 1191.Xc 1192or 1193.Fn ELF32_ST_TYPE info 1194extract a type from an st_info value. 1195.It Xo 1196.Fn ELF32_ST_INFO bind type 1197.Xc 1198or 1199.Fn ELF64_ST_INFO bind type 1200convert a binding and a type into an st_info value. 1201.El 1202.El 1203.Pp 1204.It Dv st_other 1205This member currently holds zero and has no defined meaning. 1206.It Dv st_shndx 1207Every symbol table entry is 1208.Dq defined 1209in relation to some section. 1210This member holds the relevant section 1211header table index. 1212.El 1213.Pp 1214Relocation is the process of connecting symbolic references with 1215symbolic definitions. 1216Relocatable files must have information that 1217describes how to modify their section contents, thus allowing executable 1218and shared object files to hold the right information for a process' 1219program image. 1220Relocation entries are these data. 1221.Pp 1222Relocation structures that do not need an addend: 1223.Pp 1224.Bd -literal -offset indent 1225typedef struct { 1226 Elf32_Addr r_offset; 1227 Elf32_Word r_info; 1228} Elf32_Rel; 1229.Ed 1230.Bd -literal -offset indent 1231typedef struct { 1232 Elf64_Addr r_offset; 1233 Elf64_Xword r_info; 1234} Elf64_Rel; 1235.Ed 1236.Pp 1237Relocation structures that need an addend: 1238.Pp 1239.Bd -literal -offset indent 1240typedef struct { 1241 Elf32_Addr r_offset; 1242 Elf32_Word r_info; 1243 Elf32_Sword r_addend; 1244} Elf32_Rela; 1245.Ed 1246.Bd -literal -offset indent 1247typedef struct { 1248 Elf64_Addr r_offset; 1249 Elf64_Xword r_info; 1250 Elf64_Sxword r_addend; 1251} Elf64_Rela; 1252.Ed 1253.Pp 1254.Bl -tag -width "r_offset" -compact 1255.It Dv r_offset 1256This member gives the location at which to apply the relocation action. 1257For a relocatable file, the value is the byte offset from the beginning 1258of the section to the storage unit affected by the relocation. 1259For an 1260executable file or shared object, the value is the virtual address of 1261the storage unit affected by the relocation. 1262.It Dv r_info 1263This member gives both the symbol table index with respect to which the 1264relocation must be made and the type of relocation to apply. 1265Relocation 1266types are processor-specific. 1267When the text refers to a relocation 1268entry's relocation type or symbol table index, it means the result of 1269applying 1270.Sy ELF_[32|64]_R_TYPE 1271or 1272.Sy ELF[32|64]_R_SYM , 1273respectively to the entry's 1274.Sy r_info 1275member. 1276.It Dv r_addend 1277This member specifies a constant addend used to compute the value to be 1278stored into the relocatable field. 1279.El 1280.Sh SEE ALSO 1281.Xr as 1 , 1282.Xr gdb 1 , 1283.Xr ld 1 , 1284.Xr objdump 1 , 1285.Xr execve 2 , 1286.Xr core 5 1287.Rs 1288.%A Hewlett Packard 1289.%B Elf-64 Object File Format 1290.Re 1291.Rs 1292.%A Santa Cruz Operation 1293.%B System V Application Binary Interface 1294.Re 1295.Rs 1296.%A Unix System Laboratories 1297.%T Object Files 1298.%B "Executable and Linking Format (ELF)" 1299.Re 1300.Sh HISTORY 1301The ELF header files made their appearance in 1302.Fx 2.2.6 . 1303ELF in itself first appeared in 1304.At V . 1305The ELF format is an adopted standard. 1306.Sh AUTHORS 1307This manual page was written by 1308.An Jeroen Ruigrok van der Werven 1309.Aq asmodai@FreeBSD.org 1310with inspiration from BSDi's 1311.Bsx 1312.Xr elf 5 1313manpage. 1314