1.\" $OpenBSD: elf.5,v 1.10 2003/07/09 20:23:33 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.Pp 151.Bl -tag -width "e_phentsize" -compact -offset indent 152.It Dv e_ident 153This array of bytes specifies to interpret the file, 154independent of the processor or the file's remaining contents. 155Within this array everything is named by macros, which start with 156the prefix 157.Sy EI_ 158and may contain values which start with the prefix 159.Sy ELF . 160The following macros are defined: 161.Pp 162.Bl -tag -width "EI_VERSION" -compact \" EI_ABIVERSION 163.It Dv EI_MAG0 164The first byte of the magic number. 165It must be filled with 166.Sy ELFMAG0 . 167.It Dv EI_MAG1 168The second byte of the magic number. 169It must be filled with 170.Sy ELFMAG1 . 171.It Dv EI_MAG2 172The third byte of the magic number. 173It must be filled with 174.Sy ELFMAG2 . 175.It Dv EI_MAG3 176The fourth byte of the magic number. 177It must be filled with 178.Sy ELFMAG3 . 179.It Dv EI_CLASS 180The fifth byte identifies the architecture for this binary: 181.Pp 182.Bl -tag -width "ELFCLASSNONE" -compact 183.It Dv ELFCLASSNONE 184This class is invalid. 185.It Dv ELFCLASS32 186This defines the 32-bit architecture. 187It supports machines with files 188and virtual address spaces up to 4 Gigabytes. 189.It Dv ELFCLASS64 190This defines the 64-bit architecture. 191.El 192.It Dv EI_DATA 193The sixth byte specifies the data encoding of the processor-specific 194data in the file. 195Currently these encodings are supported: 196.Pp 197.Bl -tag -width "ELFDATA2LSB" -compact 198.It Dv ELFDATANONE 199Unknown data format. 200.It Dv ELFDATA2LSB 201Two's complement, little-endian. 202.It Dv ELFDATA2MSB 203Two's complement, big-endian. 204.El 205.It Dv EI_VERSION 206The version number of the ELF specification: 207.Pp 208.Bl -tag -width "EV_CURRENT" -compact 209.It Dv EV_NONE 210Invalid version. 211.It Dv EV_CURRENT 212Current version. 213.El 214.\" .It Dv EI_OSABI 215.\" This byte identifies the operating system 216.\" and ABI to which the object is targeted. 217.\" Some fields in other ELF structures have flags 218.\" and values that have platform specific meanings; 219.\" the interpretation of those fields is determined by the value of this byte. 220.\" The following values are currently defined: 221.\" .Pp 222.\" .Bl -tag -width "ELFOSABI_STANDALONE" -compact 223.\" .It Dv ELFOSABI_SYSV 224.\" UNIX System V ABI. 225.\" .It Dv ELFOSABI_HPUX 226.\" HP-UX operating system ABI. 227.\" .It Dv ELFOSABI_NETBSD 228.\" .Nx 229.\" operating system ABI. 230.\" .It Dv ELFOSABI_LINUX 231.\" GNU/Linux operating system ABI. 232.\" .It Dv ELFOSABI_HURD 233.\" GNU/Hurd operating system ABI. 234.\" .It Dv ELFOSABI_86OPEN 235.\" 86Open Common IA32 ABI. 236.\" .It Dv ELFOSABI_SOLARIS 237.\" Solaris operating system ABI. 238.\" .It Dv ELFOSABI_MONTEREY 239.\" Monterey project ABI. 240.\" .It Dv ELFOSABI_IRIX 241.\" IRIX operating system ABI. 242.\" .It Dv ELFOSABI_FREEBSD 243.\" .Fx 244.\" operating system ABI. 245.\" .It Dv ELFOSABI_TRU64 246.\" TRU64 UNIX operating system ABI. 247.\" .It Dv ELFOSABI_ARM 248.\" ARM architecture ABI. 249.\" .It Dv ELFOSABI_STANDALONE 250.\" Stand-alone (embedded) ABI. 251.\" .El 252.\" .It Dv EI_ABIVERSION 253.\" This byte identifies the version of the ABI 254.\" to which the object is targeted. 255.\" This field is used to distinguish among incompatible versions of an ABI. 256.\" The interpretation of this version number 257.\" is dependent on the ABI identified by the EI_OSABI field. 258.\" Applications conforming to this specification use the value 0. 259.It Dv EI_PAD 260Start of padding. 261These bytes are reserved and set to zero. 262Programs 263which read them should ignore them. 264The value for EI_PAD will change in 265the future if currently unused bytes are given meanings. 266.It Dv EI_BRAND 267Start of architecture identification. 268.It Dv EI_NIDENT 269The size of the e_ident array. 270.El 271.Pp 272.It Dv e_type 273This member of the structure identifies the object file type: 274.Pp 275.Bl -tag -width "ET_NONE" -compact 276.It Dv ET_NONE 277An unknown type. 278.It Dv ET_REL 279A relocatable file. 280.It Dv ET_EXEC 281An executable file. 282.It Dv ET_DYN 283A shared object. 284.It Dv ET_CORE 285A core file. 286.El 287.Pp 288.It Dv e_machine 289This member specifies the required architecture for an individual file: 290.Pp 291.Bl -tag -width "EM_MIPS_RS4_BE" -compact 292.It Dv EM_NONE 293An unknown machine. 294.It Dv EM_M32 295AT&T WE 32100. 296.It Dv EM_SPARC 297Sun Microsystems SPARC. 298.It Dv EM_386 299Intel 80386. 300.It Dv EM_68K 301Motorola 68000. 302.It Dv EM_88K 303Motorola 88000. 304.It Dv EM_486 305Intel 80486. 306.It Dv EM_860 307Intel 80860. 308.It Dv EM_MIPS 309MIPS RS3000 (big-endian only). 310.It Dv EM_MIPS_RS4_BE 311MIPS RS4000 (big-endian only). 312.It Dv EM_SPARC64 313SPARC v9 64-bit (unofficial). 314.It Dv EM_PARISC 315HPPA. 316.It Dv EM_SPARC32PLUS 317SPARC with enhanced instruction set. 318.It Dv EM_PPC 319PowerPC. 320.It Dv EM_ALPHA 321Compaq [DEC] Alpha. 322.It Dv EM_SPARCV9 323SPARC v9 64-bit. 324.It Dv EM_ALPHA_EXP 325Compaq [DEC] Alpha with enhanced instruction set. 326.It Dv EM_VAX 327DEC Vax. 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 names take the form EF_`machine_flag'. 355Currently no flags have been defined. 356.It Dv e_ehsize 357This member holds the ELF header's size in bytes. 358.It Dv e_phentsize 359This member holds the size in bytes of one entry in the file's program header 360table; all entries are the same size. 361.It Dv e_phnum 362This member holds the number of entries in the program header 363table. 364Thus the product of 365.Sy e_phentsize 366and 367.Sy e_phnum 368gives the table's size 369in bytes. 370If a file has no program header, 371.Sy e_phnum 372holds the value zero. 373.It Dv e_shentsize 374This member holds a sections header's size in bytes. 375A section header is one 376entry in the section header table; all entries are the same size. 377.It Dv e_shnum 378This member holds the number of entries in the section header table. 379Thus 380the product of 381.Sy e_shentsize 382and 383.Sy e_shnum 384gives the section header table's size in bytes. 385If a file has no section 386header table, 387.Sy e_shnum 388holds the value of zero. 389.It Dv e_shstrndx 390This member holds the section header table index of the entry associated 391with the section name string table. 392If the file has no section name string 393table, this member holds the value 394.Sy SHN_UNDEF . 395.Pp 396.Bl -tag -width "SHN_LORESERVE" -compact 397.It Dv SHN_UNDEF 398This value marks an undefined, missing, irrelevant, or otherwise meaningless 399section reference. 400For example, a symbol 401.Dq defined 402relative to section number 403.Sy SHN_UNDEF 404is an undefined symbol. 405.It Dv SHN_LORESERVE 406This value specifies the lower bound of the range of reserved indexes. 407.It Dv SHN_LOPROC 408Values greater than or equal to 409.Sy SHN_HIPROC 410is reserved for processor-specific semantics. 411.It Dv SHN_HIPROC 412Values less than or equal to 413.Sy SHN_LOPROC 414is reserved for processor-specific semantics. 415.It Dv SHN_ABS 416This value specifies absolute values for the corresponding reference. 417For 418example, symbols defined relative to section number 419.Sy SHN_ABS 420have absolute values and are not affected by relocation. 421.It Dv SHN_COMMON 422Symbols defined relative to this section are common symbols, such as Fortran 423COMMON or unallocated C external variables. 424.It Dv SHN_HIRESERVE 425This value specifies the upper bound of the range of reserved 426indices between 427.Sy SHN_LORESERVE 428and 429.Sy SHN_HIRESERVE , 430inclusive; the values do 431not reference the section header table. 432That is, the section header table 433does 434.Em not 435contain entries for the reserved indices. 436.El 437.El 438.Pp 439An executable or shared object file's program header table is an array of 440structures, each describing a segment or other information the system needs 441to prepare the program for execution. 442An object file 443.Em segment 444contains one or more 445.Em sections . 446Program headers are meaningful only for executable and shared object files. 447A file specifies its own program header size with the ELF header's 448.Sy e_phentsize 449and 450.Sy e_phnum 451members. 452As with the ELF executable header, the program header 453also has different versions depending on the architecture: 454.Bd -literal -offset indent 455typedef struct { 456 Elf32_Word p_type; 457 Elf32_Off p_offset; 458 Elf32_Addr p_vaddr; 459 Elf32_Addr p_paddr; 460 Elf32_Word p_filesz; 461 Elf32_Word p_memsz; 462 Elf32_Word p_flags; 463 Elf32_Word p_align; 464} Elf32_Phdr; 465.Ed 466.Bd -literal -offset indent 467typedef struct { 468 Elf64_Half p_type; 469 Elf64_Half p_flags; 470 Elf64_Off p_offset; 471 Elf64_Addr p_vaddr; 472 Elf64_Addr p_paddr; 473 Elf64_Xword p_filesz; 474 Elf64_Xword p_memsz; 475 Elf64_Xword p_align; 476} Elf64_Phdr; 477.Ed 478.Pp 479The main difference between the 32-bit and the 64-bit program header lies 480only in the location of a 481.Sy p_flags 482member in the total struct. 483.Pp 484.Bl -tag -width "p_offset" -compact -offset indent 485.It Dv p_type 486This member of the Phdr struct tells what kind of segment this array 487element describes or how to interpret the array element's information. 488.Bl -tag -width "PT_DYNAMIC" -compact 489.Pp 490.It Dv PT_NULL 491The array element is unused and the other members' values are undefined. 492This lets the program header have ignored entries. 493.It Dv PT_LOAD 494The array element specifies a loadable segment, described by 495.Sy p_filesz 496and 497.Sy p_memsz . 498The bytes from the file are mapped to the beginning of the memory 499segment. 500If the segment's memory size 501.Pq Sy p_memsz 502is larger than the file size 503.Pq Sy p_filesz , 504the 505.Dq extra 506bytes are defined to hold the value 0 and to follow the segment's 507initialized area. 508The file size may not be larger than the memory size. 509Loadable segment entries in the program header table appear in ascending 510order, sorted on the 511.Sy p_vaddr 512member. 513.It Dv PT_DYNAMIC 514The array element specifies dynamic linking information. 515.It Dv PT_INTERP 516The array element specifies the location and size of a null-terminated 517path name to invoke as an interpreter. 518This segment type is meaningful 519only for executable files (though it may occur for shared objects). 520However it may not occur more than once in a file. 521If it is present it must precede any loadable segment entry. 522.It Dv PT_NOTE 523The array element specifies the location and size for auxiliary information. 524.It Dv PT_SHLIB 525This segment type is reserved but has unspecified semantics. 526Programs that 527contain an array element of this type do not conform to the ABI. 528.It Dv PT_PHDR 529The array element, if present, specifies the location and size of the program 530header table itself, both in the file and in the memory image of the program. 531This segment type may not occur more than once in a file. 532Moreover, it may 533only occur if the program header table is part of the memory image of the 534program. 535If it is present it must precede any loadable segment entry. 536.It Dv PT_LOPROC 537Values greater than or equal to 538.Sy PT_HIPROC 539is reserved for processor-specific semantics. 540.It Dv PT_HIPROC 541Value less than or equal to 542.Sy PT_LOPROC 543is reserved for processor-specific semantics. 544.El 545.Pp 546.It Dv p_offset 547This member holds the offset from the beginning of the file at which 548the first byte of the segment resides. 549.It Dv p_vaddr 550This member holds the virtual address at which the first byte of the 551segment resides in memory. 552.It Dv p_paddr 553On systems for which physical addressing is relevant, this member is 554reserved for the segment's physical address. 555Under 556.Bx 557this member is 558not used and must be zero. 559.It Dv p_filesz 560This member holds the number of bytes in the file image of the segment. 561It may be zero. 562.It Dv p_memsz 563This member holds the number of bytes in the memory image of the segment. 564It may be zero. 565.It Dv p_flags 566This member holds flags relevant to the segment: 567.Pp 568.Bl -tag -width "PF_X" -compact 569.It Dv PF_X 570An executable segment. 571.It Dv PF_W 572A writable segment. 573.It Dv PF_R 574A readable segment. 575.El 576.Pp 577A text segment commonly has the flags 578.Sy PF_X 579and 580.Sy PF_R . 581A data segment commonly has 582.Sy PF_X , 583.Sy PF_W 584and 585.Sy PF_R . 586.It Dv p_align 587This member holds the value to which the segments are aligned in memory 588and in the file. 589Loadable process segments must have congruent values for 590.Sy p_vaddr 591and 592.Sy p_offset , 593modulo the page size. 594Values of zero and one mean no alignment is required. 595Otherwise, 596.Sy p_align 597should be a positive, integral power of two, and 598.Sy p_vaddr 599should equal 600.Sy p_offset , 601modulo 602.Sy p_align . 603.El 604.Pp 605A file's section header table lets one locate all the file's sections. 606The 607section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 608The 609ELF header's 610.Sy e_shoff 611member gives the byte offset from the beginning of the file to the section 612header table. 613.Sy e_shnum 614holds the number of entries the section header table contains. 615.Sy e_shentsize 616holds the size in bytes of each entry. 617.Pp 618A section header table index is a subscript into this array. 619Some section 620header table indices are reserved. 621An object file does not have sections for 622these special indices: 623.Pp 624.Bl -tag -width "SHN_LORESERVE" -compact 625.It Dv SHN_UNDEF 626This value marks an undefined, missing, irrelevant or otherwise meaningless 627section reference. 628.It Dv SHN_LORESERVE 629This value specifies the lower bound of the range of reserved indices. 630.It Dv SHN_LOPROC 631Values greater than or equal to 632.Sy SHN_HIPROC 633is reserved for processor-specific semantics. 634.It Dv SHN_HIPROC 635Values less than or equal to 636.Sy SHN_LOPROC 637is reserved for processor-specific semantics. 638.It Dv SHN_ABS 639This value specifies the absolute value for the corresponding reference. 640For 641example, a symbol defined relative to section number 642.Sy SHN_ABS 643has an absolute value and is not affected by relocation. 644.It Dv SHN_COMMON 645Symbols defined relative to this section are common symbols, such as FORTRAN 646COMMON or unallocated C external variables. 647.It Dv SHN_HIRESERVE 648This value specifies the upper bound of the range of reserved indices. 649The 650system reserves indices between 651.Sy SHN_LORESERVE 652and 653.Sy SHN_HIRESERVE , 654inclusive. 655The section header table does not contain entries for the 656reserved indices. 657.El 658.Pp 659The section header has the following structure: 660.Bd -literal -offset indent 661typedef struct { 662 Elf32_Word sh_name; 663 Elf32_Word sh_type; 664 Elf32_Word sh_flags; 665 Elf32_Addr sh_addr; 666 Elf32_Off sh_offset; 667 Elf32_Word sh_size; 668 Elf32_Word sh_link; 669 Elf32_Word sh_info; 670 Elf32_Word sh_addralign; 671 Elf32_Word sh_entsize; 672} Elf32_Shdr; 673.Ed 674.Bd -literal -offset indent 675typedef struct { 676 Elf64_Half sh_name; 677 Elf64_Half sh_type; 678 Elf64_Xword sh_flags; 679 Elf64_Addr sh_addr; 680 Elf64_Off sh_offset; 681 Elf64_Xword sh_size; 682 Elf64_Half sh_link; 683 Elf64_Half sh_info; 684 Elf64_Xword sh_addralign; 685 Elf64_Xword sh_entsize; 686} Elf64_Shdr; 687.Ed 688.Pp 689.Bl -tag -width "sh_addralign" -compact 690.It Dv sh_name 691This member specifies the name of the section. 692Its value is an index 693into the section header string table section, giving the location of 694a null-terminated string. 695.It Dv sh_type 696This member categorizes the section's contents and semantics. 697.Pp 698.Bl -tag -width "SHT_PROGBITS" -compact 699.It Dv SHT_NULL 700This value marks the section header as inactive. 701It does not 702have an associated section. 703Other members of the section header 704have undefined values. 705.It Dv SHT_PROGBITS 706The section holds information defined by the program, whose 707format and meaning are determined solely by the program. 708.It Dv SHT_SYMTAB 709This section holds a symbol table. 710Typically, 711.Sy SHT_SYMTAB 712provides symbols for link editing, though it may also be used 713for dynamic linking. 714As a complete symbol table, it may contain 715many symbols unnecessary for dynamic linking. 716An object file can 717also contain a 718.Sy SHN_DYNSYM 719section. 720.It Dv SHT_STRTAB 721This section holds a string table. 722An object file may have multiple 723string table sections. 724.It Dv SHT_RELA 725This section holds relocation entries with explicit addends, such 726as type 727.Sy Elf32_Rela 728for the 32-bit class of object files. 729An object may have multiple 730relocation sections. 731.It Dv SHT_HASH 732This section holds a symbol hash table. 733An object participating in 734dynamic linking must contain a symbol hash table. 735An object file may 736have only one hash table. 737.It Dv SHT_DYNAMIC 738This section holds information for dynamic linking. 739An object file may 740have only one dynamic section. 741.It Dv SHT_NOTE 742This section holds information that marks the file in some way. 743.It Dv SHT_NOBITS 744A section of this type occupies no space in the file but otherwise 745resembles 746.Sy SHN_PROGBITS . 747Although this section contains no bytes, the 748.Sy sh_offset 749member contains the conceptual file offset. 750.It Dv SHT_REL 751This section holds relocation offsets without explicit addends, such 752as type 753.Sy Elf32_Rel 754for the 32-bit class of object files. 755An object file may have multiple 756relocation sections. 757.It Dv SHT_SHLIB 758This section is reserved but has unspecified semantics. 759.It Dv SHT_DYNSYM 760This section holds a minimal set of dynamic linking symbols. 761An 762object file can also contain a 763.Sy SHN_SYMTAB 764section. 765.It Dv SHT_LOPROC 766This value up to and including 767.Sy SHT_HIPROC 768is reserved for processor-specific semantics. 769.It Dv SHT_HIPROC 770This value down to and including 771.Sy SHT_LOPROC 772is reserved for processor-specific semantics. 773.It Dv SHT_LOUSER 774This value specifies the lower bound of the range of indices reserved for 775application programs. 776.It Dv SHT_HIUSER 777This value specifies the upper bound of the range of indices reserved for 778application programs. 779Section types between 780.Sy SHT_LOUSER 781and 782.Sy SHT_HIUSER 783may be used by the application, without conflicting with current or future 784system-defined section types. 785.El 786.Pp 787.It Dv sh_flags 788Sections support one-bit flags that describe miscellaneous attributes. 789If a flag bit is set in 790.Sy sh_flags , 791the attribute is 792.Dq on 793for the section. 794Otherwise, the attribute is 795.Dq off 796or does not apply. 797Undefined attributes are set to zero. 798.Pp 799.Bl -tag -width "SHF_EXECINSTR" -compact 800.It Dv SHF_WRITE 801This section contains data that should be writable during process 802execution. 803.It Dv SHF_ALLOC 804This section occupies memory during process execution. 805Some control 806sections do not reside in the memory image of an object file. 807This 808attribute is off for those sections. 809.It Dv SHF_EXECINSTR 810This section contains executable machine instructions. 811.It Dv SHF_MASKPROC 812All bits included in this mask are reserved for processor-specific 813semantics. 814.El 815.Pp 816.It Dv sh_addr 817If this section appears in the memory image of a process, this member 818holds the address at which the section's first byte should reside. 819Otherwise, the member contains zero. 820.It Dv sh_offset 821This member's value holds the byte offset from the beginning of the file 822to the first byte in the section. 823One section type, 824.Sy SHT_NOBITS , 825occupies no space in the file, and its 826.Sy sh_offset 827member locates the conceptual placement in the file. 828.It Dv sh_size 829This member holds the section's size in bytes. 830Unless the section type 831is 832.Sy SHT_NOBITS , 833the section occupies 834.Sy sh_size 835bytes in the file. 836A section of type 837.Sy SHT_NOBITS 838may have a non-zero size, but it occupies no space in the file. 839.It Dv sh_link 840This member holds a section header table index link, whose interpretation 841depends on the section type. 842.It Dv sh_info 843This member holds extra information, whose interpretation depends on the 844section type. 845.It Dv sh_addralign 846Some sections have address alignment constraints. 847If a section holds a 848doubleword, the system must ensure doubleword alignment for the entire 849section. 850That is, the value of 851.Sy sh_addr 852must be congruent to zero, modulo the value of 853.Sy sh_addralign . 854Only zero and positive integral powers of two are allowed. 855Values of zero 856or one mean the section has no alignment constraints. 857.It Dv sh_entsize 858Some sections hold a table of fixed-sized entries, such as a symbol table. 859For such a section, this member gives the size in bytes for each entry. 860This member contains zero if the section does not hold a table of 861fixed-size entries. 862.El 863.Pp 864Various sections hold program and control information: 865.Bl -tag -width ".shstrtab" -compact 866.It .bss 867This section holds uninitialized data that contributes to the program's 868memory image. 869By definition, the system initializes the data with zeros 870when the program begins to run. 871This section is of type 872.Sy SHT_NOBITS . 873The attributes types are 874.Sy SHF_ALLOC 875and 876.Sy SHF_WRITE . 877.It .comment 878This section holds version control information. 879This section is of type 880.Sy SHT_PROGBITS . 881No attribute types are used. 882.It .ctors 883This section holds initialized pointers to the C++ constructor functions. 884This section is of type 885.Sy SHT_PROGBITS . 886The attribute types are 887.Sy SHF_ALLOC 888and 889.Sy SHF_WRITE . 890.It .data 891This section holds initialized data that contribute to the program's 892memory image. 893This section is of type 894.Sy SHT_PROGBITS . 895The attribute types are 896.Sy SHF_ALLOC 897and 898.Sy SHF_WRITE . 899.It .data1 900This section holds initialized data that contribute to the program's 901memory image. 902This section is of type 903.Sy SHT_PROGBITS . 904The attribute types are 905.Sy SHF_ALLOC 906and 907.Sy SHF_WRITE . 908.It .debug 909This section holds information for symbolic debugging. 910The contents 911are unspecified. 912This section is of type 913.Sy SHT_PROGBITS . 914No attribute types are used. 915.It .dtors 916This section holds initialized pointers to the C++ destructor functions. 917This section is of type 918.Sy SHT_PROGBITS . 919The attribute types are 920.Sy SHF_ALLOC 921and 922.Sy SHF_WRITE . 923.It .dynamic 924This section holds dynamic linking information. 925The section's attributes 926will include the 927.Sy SHF_ALLOC 928bit. 929Whether the 930.Sy SHF_WRITE 931bit is set is processor-specific. 932This section is of type 933.Sy SHT_DYNAMIC . 934See the attributes above. 935.It .dynstr 936This section holds strings needed for dynamic linking, most commonly 937the strings that represent the names associated with symbol table entries. 938This section is of type 939.Sy SHT_STRTAB . 940The attribute type used is 941.Sy SHF_ALLOC . 942.It .dynsym 943This section holds the dynamic linking symbol table. 944This section is of type 945.Sy SHT_DYNSYM . 946The attribute used is 947.Sy SHF_ALLOC . 948.It .fini 949This section holds executable instructions that contribute to the process 950termination code. 951When a program exits normally the system arranges to 952execute the code in this section. 953This section is of type 954.Sy SHT_PROGBITS . 955The attributes used are 956.Sy SHF_ALLOC 957and 958.Sy SHF_EXECINSTR . 959.It .got 960This section holds the global offset table. 961This section is of type 962.Sy SHT_PROGBITS . 963The attributes are processor-specific. 964.It .hash 965This section holds a symbol hash table. 966This section is of type 967.Sy SHT_HASH . 968The attribute used is 969.Sy SHF_ALLOC . 970.It .init 971This section holds executable instructions that contribute to the process 972initialization code. 973When a program starts to run the system arranges to 974execute the code in this section before calling the main program entry point. 975This section is of type 976.Sy SHT_PROGBITS . 977The attributes used are 978.Sy SHF_ALLOC 979and 980.Sy SHF_EXECINSTR . 981.It .interp 982This section holds the pathname of a program interpreter. 983If the file has 984a loadable segment that includes the section, the section's attributes will 985include the 986.Sy SHF_ALLOC 987bit. 988Otherwise, that bit will be off. 989This section is of type 990.Sy SHT_PROGBITS . 991.It .line 992This section holds line number information for symbolic debugging, which 993describes the correspondence between the program source and the machine code. 994The contents are unspecified. 995This section is of type 996.Sy SHT_PROGBITS . 997No attribute types are used. 998.It .note 999This section holds information in the 1000.Dq Note Section 1001format described below. 1002This section is of type 1003.Sy SHT_NOTE . 1004No attribute types are used. 1005.Ox 1006native executables usually contain a 1007.Sy .note.openbsd.ident 1008section to identify themselves, for the kernel to bypass any compatibility 1009ELF binary emulation tests when loading the file. 1010.It .plt 1011This section holds the procedure linkage table. 1012This section is of type 1013.Sy SHT_PROGBITS . 1014The attributes are processor-specific. 1015.It .relNAME 1016This section holds relocation information as described below. 1017If the file 1018has a loadable segment that includes relocation, the section's attributes 1019will include the 1020.Sy SHF_ALLOC 1021bit. 1022Otherwise the bit will be off. 1023By convention, 1024.Dq NAME 1025is supplied by the section to which the relocations apply. 1026Thus a relocation 1027section for 1028.Sy .text 1029normally would have the name 1030.Sy .rel.text . 1031This section is of type 1032.Sy SHT_REL . 1033.It .relaNAME 1034This section holds relocation information as described below. 1035If the file 1036has a loadable segment that includes relocation, the section's attributes 1037will include the 1038.Sy SHF_ALLOC 1039bit. 1040Otherwise the bit will be off. 1041By convention, 1042.Dq NAME 1043is supplied by the section to which the relocations apply. 1044Thus a relocation 1045section for 1046.Sy .text 1047normally would have the name 1048.Sy .rela.text . 1049This section is of type 1050.Sy SHT_RELA . 1051.It .rodata 1052This section holds read-only data that typically contributes to a 1053non-writable segment in the process image. 1054This section is of type 1055.Sy SHT_PROGBITS . 1056The attribute used is 1057.Sy SHF_ALLOC . 1058.It .rodata1 1059This section holds read-only data that typically contributes to a 1060non-writable segment in the process image. 1061This section is of type 1062.Sy SHT_PROGBITS . 1063The attribute used is 1064.Sy SHF_ALLOC . 1065.It .shstrtab 1066This section holds section names. 1067This section is of type 1068.Sy SHT_STRTAB . 1069No attribute types are used. 1070.It .strtab 1071This section holds strings, most commonly the strings that represent the 1072names associated with symbol table entries. 1073If the file has a loadable 1074segment that includes the symbol string table, the section's attributes 1075will include the 1076.Sy SHF_ALLOC 1077bit. 1078Otherwise the bit will be off. 1079This section is of type 1080.Sy SHT_STRTAB . 1081.It .symtab 1082This section holds a symbol table. 1083If the file has a loadable segment 1084that includes the symbol table, the section's attributes will include 1085the 1086.Sy SHF_ALLOC 1087bit. 1088Otherwise the bit will be off. 1089This section is of type 1090.Sy SHT_SYMTAB . 1091.It .text 1092This section holds the 1093.Dq text , 1094or executable instructions, of a program. 1095This section is of type 1096.Sy SHT_PROGBITS . 1097The attributes used are 1098.Sy SHF_ALLOC 1099and 1100.Sy SHF_EXECINSTR . 1101.El 1102.Pp 1103String table sections hold null-terminated character sequences, commonly 1104called strings. 1105The object file uses these strings to represent symbol 1106and section names. 1107One references a string as an index into the string 1108table section. 1109The first byte, which is index zero, is defined to hold 1110a null character. 1111Similarly, a string table's last byte is defined to 1112hold a null character, ensuring null termination for all strings. 1113.Pp 1114An object file's symbol table holds information needed to locate and 1115relocate a program's symbolic definitions and references. 1116A symbol table 1117index is a subscript into this array. 1118.Bd -literal -offset indent 1119typedef struct { 1120 Elf32_Word st_name; 1121 Elf32_Addr st_value; 1122 Elf32_Word st_size; 1123 unsigned char st_info; 1124 unsigned char st_other; 1125 Elf32_Half st_shndx; 1126} Elf32_Sym; 1127.Ed 1128.Bd -literal -offset indent 1129typedef struct { 1130 Elf64_Half st_name; 1131 Elf_Byte st_info; 1132 Elf_Byte st_other; 1133 Elf64_Quarter st_shndx; 1134 Elf64_Xword st_value; 1135 Elf64_Xword st_size; 1136} Elf64_Sym; 1137.Ed 1138.Pp 1139.Bl -tag -width "st_value" -compact 1140.It Dv st_name 1141This member holds an index into the object file's symbol string table, 1142which holds character representations of the symbol names. 1143If the value 1144is non-zero, it represents a string table index that gives the symbol 1145name. 1146Otherwise, the symbol table has no name. 1147.It Dv st_value 1148This member gives the value of the associated symbol. 1149.It Dv st_size 1150Many symbols have associated sizes. 1151This member holds zero if the symbol 1152has no size or an unknown size. 1153.It Dv st_info 1154This member specifies the symbol's type and binding attributes: 1155.Pp 1156.Bl -tag -width "STT_SECTION" -compact 1157.It Dv STT_NOTYPE 1158The symbol's type is not defined. 1159.It Dv STT_OBJECT 1160The symbol is associated with a data object. 1161.It Dv STT_FUNC 1162The symbol is associated with a function or other executable code. 1163.It Dv STT_SECTION 1164The symbol is associated with a section. 1165Symbol table entries of 1166this type exist primarily for relocation and normally have 1167.Sy STB_LOCAL 1168bindings. 1169.It Dv STT_FILE 1170By convention the symbol's name gives the name of the source file 1171associated with the object file. 1172A file symbol has 1173.Sy STB_LOCAL 1174bindings, its section index is 1175.Sy SHN_ABS , 1176and it precedes the other 1177.Sy STB_LOCAL 1178symbols of the file, if it is present. 1179.It Dv STT_LOPROC 1180This value up to and including 1181.Sy STT_HIPROC 1182is reserved for processor-specific semantics. 1183.It Dv STT_HIPROC 1184This value down to and including 1185.Sy STT_LOPROC 1186is reserved for processor-specific semantics. 1187.El 1188.Pp 1189.Bl -tag -width "STB_GLOBAL" -compact 1190.It Dv STB_LOCAL 1191Local symbols are not visible outside the object file containing their 1192definition. 1193Local symbols of the same name may exist in multiple files 1194without interfering with each other. 1195.It Dv STB_GLOBAL 1196Global symbols are visible to all object files being combined. 1197One file's 1198definition of a global symbol will satisfy another file's undefined 1199reference to the same symbol. 1200.It Dv STB_WEAK 1201Weak symbols resemble global symbols, but their definitions have lower 1202precedence. 1203.It Dv STB_LOPROC 1204This value up to and including 1205.Sy STB_HIPROC 1206is reserved for processor-specific semantics. 1207.It Dv STB_HIPROC 1208This value down to and including 1209.Sy STB_LOPROC 1210is reserved for processor-specific semantics. 1211.Pp 1212There are macros for packing and unpacking the binding and type fields: 1213.Pp 1214.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1215.It Xo 1216.Fn ELF32_ST_BIND info 1217.Xc 1218or 1219.Fn ELF64_ST_BIND info 1220extract a binding from an st_info value. 1221.It Xo 1222.Fn ELF64_ST_TYPE info 1223.Xc 1224or 1225.Fn ELF32_ST_TYPE info 1226extract a type from an st_info value. 1227.It Xo 1228.Fn ELF32_ST_INFO bind type 1229.Xc 1230or 1231.Fn ELF64_ST_INFO bind type 1232convert a binding and a type into an st_info value. 1233.El 1234.El 1235.Pp 1236.It Dv st_other 1237This member currently holds zero and has no defined meaning. 1238.It Dv st_shndx 1239Every symbol table entry is 1240.Dq defined 1241in relation to some action. 1242This member holds the relevant section 1243header table index. 1244.El 1245.Pp 1246Relocation is the process of connecting symbolic references with 1247symbolic definitions. 1248Relocatable files must have information that 1249describes how to modify their section contents, thus allowing executable 1250and shared object files to hold the right information for a process' 1251program image. 1252Relocation entries are these data. 1253.Pp 1254Relocation structures that do not need an addend: 1255.Bd -literal -offset indent 1256typedef struct { 1257 Elf32_Addr r_offset; 1258 Elf32_Word r_info; 1259} Elf32_Rel; 1260.Ed 1261.Bd -literal -offset indent 1262typedef struct { 1263 Elf64_Xword r_offset; 1264 Elf64_Xword r_info; 1265} Elf64_Rel; 1266.Ed 1267.Pp 1268Relocation structures that need an addend: 1269.Bd -literal -offset indent 1270typedef struct { 1271 Elf32_Addr r_offset; 1272 Elf32_Word r_info; 1273 Elf32_Sword r_addend; 1274} Elf32_Rela; 1275.Ed 1276.Bd -literal -offset indent 1277typedef struct { 1278 Elf64_Xword r_offset; 1279 Elf64_Xword r_info; 1280 Elf64_Sxword r_addend; 1281} Elf64_Rela; 1282.Ed 1283.Pp 1284.Bl -tag -width "r_offset" -compact 1285.It Dv r_offset 1286This member gives the location at which to apply the relocation action. 1287For a relocatable file, the value is the byte offset from the beginning 1288of the section to the storage unit affected by the relocation. 1289For an 1290executable file or shared object, the value is the virtual address of 1291the storage unit affected by the relocation. 1292.It Dv r_info 1293This member gives both the symbol table index with respect to which the 1294relocation must be made and the type of relocation to apply. 1295Relocation 1296types are processor-specific. 1297When the text refers to a relocation 1298entry's relocation type or symbol table index, it means the result of 1299applying 1300.Sy ELF_[32|64]_R_TYPE 1301or 1302.Sy ELF[32|64]_R_SYM , 1303respectively, to the entry's 1304.Sy r_info 1305member. 1306.It Dv r_addend 1307This member specifies a constant addend used to compute the value to be 1308stored into the relocatable field. 1309.El 1310.Sh SEE ALSO 1311.Xr as 1 , 1312.Xr gdb 1 , 1313.Xr ld 1 , 1314.Xr objdump 1 , 1315.Xr execve 2 , 1316.Xr core 5 1317.Rs 1318.%A Hewlett-Packard 1319.%B Elf-64 Object File Format 1320.Re 1321.Rs 1322.%A Santa Cruz Operation 1323.%B System V Application Binary Interface 1324.Re 1325.Rs 1326.%A Unix System Laboratories 1327.%T Object Files 1328.%B "Executable and Linking Format (ELF)" 1329.Re 1330.Sh HISTORY 1331.Ox 1332ELF support first appeared in 1333.Ox 1.2 , 1334although not all supported platforms use it as the native 1335binary file format. 1336ELF in itself first appeared in 1337.At V . 1338The ELF format is an adopted standard. 1339.Sh AUTHORS 1340This manual page was written by 1341.An Jeroen Ruigrok van der Werven 1342.Aq asmodai@FreeBSD.org 1343with inspiration from BSDi's 1344.Bsx 1345.Nm elf 1346manpage. 1347