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: src/share/man/man5/elf.5,v 1.36 2007/09/08 08:12:31 jkoshy Exp $ 26.\" 27.Dd August 30, 2008 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_Off Unsigned 32-bit file offset 94Elf32_Sword Signed 32-bit field or integer 95Elf32_Word Unsigned 32-bit field or integer 96Elf32_Size Unsigned object size 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 109Elf64_Size Unsigned object size 110.Ed 111.Pp 112All data structures that the file format defines follow the 113.Dq natural 114size and alignment guidelines for the relevant class. 115If necessary, 116data structures contain explicit padding to ensure 4-byte alignment 117for 4-byte objects, to force structure sizes to a multiple of 4, etc. 118.Pp 119The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 120.Bd -literal -offset indent 121typedef struct { 122 unsigned char e_ident[EI_NIDENT]; 123 Elf32_Half e_type; 124 Elf32_Half e_machine; 125 Elf32_Word e_version; 126 Elf32_Addr e_entry; 127 Elf32_Off e_phoff; 128 Elf32_Off e_shoff; 129 Elf32_Word e_flags; 130 Elf32_Half e_ehsize; 131 Elf32_Half e_phentsize; 132 Elf32_Half e_phnum; 133 Elf32_Half e_shentsize; 134 Elf32_Half e_shnum; 135 Elf32_Half e_shstrndx; 136} Elf32_Ehdr; 137.Ed 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 ".Fa e_phentsize" -compact -offset indent 160.It Fa 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 ".Dv 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 ".Dv 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 ".Dv 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 ".Dv 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 ".Dv ELFOSABI_STANDALONE" -compact 231.It Dv ELFOSABI_SYSV 232.At V 233ABI. 234.It Dv ELFOSABI_HPUX 235HP-UX operating system ABI. 236.It Dv ELFOSABI_NETBSD 237.Nx 238operating system ABI. 239.It Dv ELFOSABI_LINUX 240GNU/Linux operating system ABI. 241.It Dv ELFOSABI_HURD 242GNU/Hurd operating system ABI. 243.It Dv ELFOSABI_86OPEN 24486Open Common IA32 ABI. 245.It Dv ELFOSABI_SOLARIS 246Solaris operating system ABI. 247.It Dv ELFOSABI_MONTEREY 248Monterey project ABI. 249.It Dv ELFOSABI_IRIX 250IRIX operating system ABI. 251.It Dv ELFOSABI_FREEBSD 252.Fx 253operating system ABI. 254.It Dv ELFOSABI_TRU64 255TRU64 256.Ux 257operating system ABI. 258.It Dv ELFOSABI_ARM 259ARM architecture ABI. 260.It Dv ELFOSABI_STANDALONE 261Standalone (embedded) ABI. 262.El 263.It Dv EI_ABIVERSION 264This byte identifies the version of the ABI 265to which the object is targeted. 266This field is used to distinguish among incompatible versions of an ABI. 267The interpretation of this version number 268is dependent on the ABI identified by the 269.Dv EI_OSABI 270field. 271Applications conforming to this specification use the value 0. 272.It Dv EI_PAD 273Start of padding. 274These bytes are reserved and set to zero. 275Programs 276which read them should ignore them. 277The value for EI_PAD will change in 278the future if currently unused bytes are given meanings. 279.It Dv EI_NIDENT 280The size of the 281.Fa e_ident 282array. 283.El 284.Pp 285.It Fa e_type 286This member of the structure identifies the object file type: 287.Pp 288.Bl -tag -width ".Dv ET_NONE" -compact 289.It Dv ET_NONE 290An unknown type. 291.It Dv ET_REL 292A relocatable file. 293.It Dv ET_EXEC 294An executable file. 295.It Dv ET_DYN 296A shared object. 297.It Dv ET_CORE 298A core file. 299.El 300.Pp 301.It Fa e_machine 302This member specifies the required architecture for an individual file: 303.Pp 304.Bl -tag -width ".Dv EM_MIPS_RS4_BE" -compact 305.It Dv EM_NONE 306An unknown machine. 307.It Dv EM_M32 308AT&T WE 32100. 309.It Dv EM_SPARC 310Sun Microsystems SPARC. 311.It Dv EM_386 312Intel 80386. 313.It Dv EM_68K 314Motorola 68000. 315.It Dv EM_88K 316Motorola 88000. 317.It Dv EM_860 318Intel 80860. 319.It Dv EM_MIPS 320MIPS RS3000 (big-endian only). 321.It Dv EM_MIPS_RS4_BE 322MIPS RS4000 (big-endian only). 323.It Dv EM_SPARC64 324SPARC v9 64-bit unofficial. 325.It Dv EM_PARISC 326HPPA. 327.It Dv EM_PPC 328PowerPC. 329.It Dv EM_ALPHA 330Compaq [DEC] Alpha. 331.El 332.Pp 333.It Fa e_version 334This member identifies the file version: 335.Pp 336.Bl -tag -width ".Dv EV_CURRENT" -compact 337.It Dv EV_NONE 338Invalid version 339.It Dv EV_CURRENT 340Current version 341.El 342.It Fa e_entry 343This member gives the virtual address to which the system first transfers 344control, thus starting the process. 345If the file has no associated entry 346point, this member holds zero. 347.It Fa e_phoff 348This member holds the program header table's file offset in bytes. 349If 350the file has no program header table, this member holds zero. 351.It Fa e_shoff 352This member holds the section header table's file offset in bytes. 353If the 354file has no section header table this member holds zero. 355.It Fa e_flags 356This member holds processor-specific flags associated with the file. 357Flag names take the form EF_`machine_flag'. 358Currently no flags have been defined. 359.It Fa e_ehsize 360This member holds the ELF header's size in bytes. 361.It Fa e_phentsize 362This member holds the size in bytes of one entry in the file's program header 363table; all entries are the same size. 364.It Fa e_phnum 365This member holds the number of entries in the program header 366table. 367Thus the product of 368.Fa e_phentsize 369and 370.Fa e_phnum 371gives the table's size 372in bytes. 373If a file has no program header, 374.Fa e_phnum 375holds the value zero. 376.It Fa e_shentsize 377This member holds a sections header's size in bytes. 378A section header is one 379entry in the section header table; all entries are the same size. 380.It Fa e_shnum 381This member holds the number of entries in the section header table. 382Thus 383the product of 384.Fa e_shentsize 385and 386.Fa e_shnum 387gives the section header table's size in bytes. 388If a file has no section 389header table, 390.Fa e_shnum 391holds the value of zero. 392.It Fa e_shstrndx 393This member holds the section header table index of the entry associated 394with the section name string table. 395If the file has no section name string 396table, this member holds the value 397.Dv SHN_UNDEF . 398.Pp 399.Bl -tag -width ".Dv SHN_LORESERVE" -compact 400.It Dv SHN_UNDEF 401This value marks an undefined, missing, irrelevant, or otherwise meaningless 402section reference. 403For example, a symbol 404.Dq defined 405relative to section number 406.Dv SHN_UNDEF 407is an undefined symbol. 408.It Dv SHN_LORESERVE 409This value specifies the lower bound of the range of reserved indexes. 410.It Dv SHN_LOPROC 411This value up to and including 412.Dv SHN_HIPROC 413are reserved for processor-specific semantics. 414.It Dv SHN_HIPROC 415This value down to and including 416.Dv SHN_LOPROC 417are reserved for processor-specific semantics. 418.It Dv SHN_ABS 419This value specifies absolute values for the corresponding reference. 420For 421example, symbols defined relative to section number 422.Dv SHN_ABS 423have absolute values and are not affected by relocation. 424.It Dv SHN_COMMON 425Symbols defined relative to this section are common symbols, such as Fortran 426COMMON or unallocated C external variables. 427.It Dv SHN_HIRESERVE 428This value specifies the upper bound of the range of the range of reserved 429indices between 430.Dv SHN_LORESERVE 431and 432.Dv SHN_HIRESERVE , 433inclusive; the values do 434not reference the section header table. 435That is, the section header table 436does 437.Em not 438contain entries for the reserved indices. 439.El 440.El 441.Pp 442An executable or shared object file's program header table is an array of 443structures, each describing a segment or other information the system needs 444to prepare the program for execution. 445An object file 446.Em segment 447contains one or more 448.Em sections . 449Program headers are meaningful only for executable and shared object files. 450A file specifies its own program header size with the ELF header's 451.Fa e_phentsize 452and 453.Fa e_phnum 454members. 455As with the Elf executable header, the program header 456also has different versions depending on the architecture: 457.Bd -literal -offset indent 458typedef struct { 459 Elf32_Word p_type; 460 Elf32_Off p_offset; 461 Elf32_Addr p_vaddr; 462 Elf32_Addr p_paddr; 463 Elf32_Size p_filesz; 464 Elf32_Size p_memsz; 465 Elf32_Word p_flags; 466 Elf32_Size p_align; 467} Elf32_Phdr; 468.Ed 469.Bd -literal -offset indent 470typedef struct { 471 Elf64_Word p_type; 472 Elf64_Word p_flags; 473 Elf64_Off p_offset; 474 Elf64_Addr p_vaddr; 475 Elf64_Addr p_paddr; 476 Elf64_Xword p_filesz; 477 Elf64_Xword p_memsz; 478 Elf64_Xword p_align; 479} Elf64_Phdr; 480.Ed 481.Pp 482The main difference between the 32-bit and the 64-bit program header lies 483only in the location of a 484.Fa p_flags 485member in the total struct. 486.Pp 487.Bl -tag -width ".Fa p_offset" -compact -offset indent 488.It Fa p_type 489This member of the 490.Vt Phdr 491struct tells what kind of segment this array 492element describes or how to interpret the array element's information. 493.Pp 494.Bl -tag -width ".Dv PT_DYNAMIC" -compact 495.It Dv PT_NULL 496The array element is unused and the other members' values are undefined. 497This lets the program header have ignored entries. 498.It Dv PT_LOAD 499The array element specifies a loadable segment, described by 500.Fa p_filesz 501and 502.Fa p_memsz . 503The bytes from the file are mapped to the beginning of the memory 504segment. 505If the segment's memory size 506.Pq Fa p_memsz 507is larger than the file size 508.Pq Fa p_filesz , 509the 510.Dq extra 511bytes are defined to hold the value 0 and to follow the segment's 512initialized area. 513The file size may not be larger than the memory size. 514Loadable segment entries in the program header table appear in ascending 515order, sorted on the 516.Fa p_vaddr 517member. 518.It Dv PT_DYNAMIC 519The array element specifies dynamic linking information. 520.It Dv PT_INTERP 521The array element specifies the location and size of a null-terminated 522path name to invoke as an interpreter. 523This segment type is meaningful 524only for executable files (though it may occur for shared objects). 525However, it may not occur more than once in a file. 526If it is present it must precede 527any loadable segment entry. 528.It Dv PT_NOTE 529The array element specifies the location and size for auxiliary information. 530.It Dv PT_SHLIB 531This segment type is reserved but has unspecified semantics. 532Programs that 533contain an array element of this type do not conform to the ABI. 534.It Dv PT_PHDR 535The array element, if present, specifies the location and size of the program 536header table itself, both in the file and in the memory image of the program. 537This segment type may not occur more than once in a file. 538Moreover, it may 539only occur if the program header table is part of the memory image of the 540program. 541If it is present it must precede any loadable segment entry. 542.It Dv PT_LOPROC 543This value up to and including 544.Dv PT_HIPROC 545are reserved for processor-specific semantics. 546.It Dv PT_HIPROC 547This value down to and including 548.Dv PT_LOPROC 549are reserved for processor-specific semantics. 550.El 551.Pp 552.It Fa p_offset 553This member holds the offset from the beginning of the file at which 554the first byte of the segment resides. 555.It Fa p_vaddr 556This member holds the virtual address at which the first byte of the 557segment resides in memory. 558.It Fa p_paddr 559On systems for which physical addressing is relevant, this member is 560reserved for the segment's physical address. 561Under 562.Bx 563this member is 564not used and must be zero. 565.It Fa p_filesz 566This member holds the number of bytes in the file image of the segment. 567It may be zero. 568.It Fa p_memsz 569This member holds the number of bytes in the memory image of the segment. 570It may be zero. 571.It Fa p_flags 572This member holds flags relevant to the segment: 573.Pp 574.Bl -tag -width ".Dv PF_X" -compact 575.It Dv PF_X 576An executable segment. 577.It Dv PF_W 578A writable segment. 579.It Dv PF_R 580A readable segment. 581.El 582.Pp 583A text segment commonly has the flags 584.Dv PF_X 585and 586.Dv PF_R . 587A data segment commonly has 588.Dv PF_X , 589.Dv PF_W 590and 591.Dv PF_R . 592.It Fa p_align 593This member holds the value to which the segments are aligned in memory 594and in the file. 595Loadable process segments must have congruent values for 596.Fa p_vaddr 597and 598.Fa p_offset , 599modulo the page size. 600Values of zero and one mean no alignment is required. 601Otherwise, 602.Fa p_align 603should be a positive, integral power of two, and 604.Fa p_vaddr 605should equal 606.Fa p_offset , 607modulo 608.Fa p_align . 609.El 610.Pp 611An file's section header table lets one locate all the file's sections. 612The 613section header table is an array of 614.Vt Elf32_Shdr 615or 616.Vt Elf64_Shdr 617structures. 618The ELF header's 619.Fa e_shoff 620member gives the byte offset from the beginning of the file to the section 621header table. 622.Fa e_shnum 623holds the number of entries the section header table contains. 624.Fa e_shentsize 625holds the size in bytes of each entry. 626.Pp 627A section header table index is a subscript into this array. 628Some section 629header table indices are reserved. 630An object file does not have sections for 631these special indices: 632.Pp 633.Bl -tag -width ".Dv SHN_LORESERVE" -compact 634.It Dv SHN_UNDEF 635This value marks an undefined, missing, irrelevant, or otherwise meaningless 636section reference. 637.It Dv SHN_LORESERVE 638This value specifies the lower bound of the range of reserved indices. 639.It Dv SHN_LOPROC 640This value up to and including 641.Dv SHN_HIPROC 642are reserved for processor-specific semantics. 643.It Dv SHN_HIPROC 644This value down to and including 645.Dv SHN_LOPROC 646are reserved for processor-specific semantics. 647.It Dv SHN_ABS 648This value specifies absolute values for the corresponding reference. 649For 650example, symbols defined relative to section number 651.Dv SHN_ABS 652have absolute values and are not affected by relocation. 653.It Dv SHN_COMMON 654Symbols defined relative to this section are common symbols, such as FORTRAN 655COMMON or unallocated C external variables. 656.It Dv SHN_HIRESERVE 657This value specifies the upper bound of the range of reserved indices. 658The 659system reserves indices between 660.Dv SHN_LORESERVE 661and 662.Dv SHN_HIRESERVE , 663inclusive. 664The section header table does not contain entries for the 665reserved indices. 666.El 667.Pp 668The section header has the following structure: 669.Bd -literal -offset indent 670typedef struct { 671 Elf32_Word sh_name; 672 Elf32_Word sh_type; 673 Elf32_Word sh_flags; 674 Elf32_Addr sh_addr; 675 Elf32_Off sh_offset; 676 Elf32_Size sh_size; 677 Elf32_Word sh_link; 678 Elf32_Word sh_info; 679 Elf32_Size sh_addralign; 680 Elf32_Size sh_entsize; 681} Elf32_Shdr; 682.Ed 683.Bd -literal -offset indent 684typedef struct { 685 Elf64_Word sh_name; 686 Elf64_Word sh_type; 687 Elf64_Xword sh_flags; 688 Elf64_Addr sh_addr; 689 Elf64_Off sh_offset; 690 Elf64_Xword sh_size; 691 Elf64_Word sh_link; 692 Elf64_Word sh_info; 693 Elf64_Xword sh_addralign; 694 Elf64_Xword sh_entsize; 695} Elf64_Shdr; 696.Ed 697.Pp 698.Bl -tag -width ".Fa sh_addralign" -compact 699.It Fa sh_name 700This member specifies the name of the section. 701Its value is an index 702into the section header string table section, giving the location of 703a null-terminated string. 704.It Fa sh_type 705This member categorizes the section's contents and semantics. 706.Pp 707.Bl -tag -width ".Dv SHT_PROGBITS" -compact 708.It Dv SHT_NULL 709This value marks the section header as inactive. 710It does not 711have an associated section. 712Other members of the section header 713have undefined values. 714.It Dv SHT_PROGBITS 715The section holds information defined by the program, whose 716format and meaning are determined solely by the program. 717.It Dv SHT_SYMTAB 718This section holds a symbol table. 719Typically, 720.Dv SHT_SYMTAB 721provides symbols for link editing, though it may also be used 722for dynamic linking. 723As a complete symbol table, it may contain 724many symbols unnecessary for dynamic linking. 725An object file can 726also contain a 727.Dv SHN_DYNSYM 728section. 729.It Dv SHT_STRTAB 730This section holds a string table. 731An object file may have multiple 732string table sections. 733.It Dv SHT_RELA 734This section holds relocation entries with explicit addends, such 735as type 736.Vt Elf32_Rela 737for the 32-bit class of object files. 738An object may have multiple 739relocation sections. 740.It Dv SHT_HASH 741This section holds a symbol hash table. 742All object participating in 743dynamic linking must contain a symbol hash table. 744An object file may 745have only one hash table. 746.It Dv SHT_DYNAMIC 747This section holds information for dynamic linking. 748An object file may 749have only one dynamic section. 750.It Dv SHT_NOTE 751This section holds information that marks the file in some way. 752.It Dv SHT_NOBITS 753A section of this type occupies no space in the file but otherwise 754resembles 755.Dv SHN_PROGBITS . 756Although this section contains no bytes, the 757.Fa sh_offset 758member contains the conceptual file offset. 759.It Dv SHT_REL 760This section holds relocation offsets without explicit addends, such 761as type 762.Vt Elf32_Rel 763for the 32-bit class of object files. 764An object file may have multiple 765relocation sections. 766.It Dv SHT_SHLIB 767This section is reserved but has unspecified semantics. 768.It Dv SHT_DYNSYM 769This section holds a minimal set of dynamic linking symbols. 770An 771object file can also contain a 772.Dv SHN_SYMTAB 773section. 774.It Dv SHT_LOPROC 775This value up to and including 776.Dv SHT_HIPROC 777are reserved for processor-specific semantics. 778.It Dv SHT_HIPROC 779This value down to and including 780.Dv SHT_LOPROC 781are reserved for processor-specific semantics. 782.It Dv SHT_LOUSER 783This value specifies the lower bound of the range of indices reserved for 784application programs. 785.It Dv SHT_HIUSER 786This value specifies the upper bound of the range of indices reserved for 787application programs. 788Section types between 789.Dv SHT_LOUSER 790and 791.Dv SHT_HIUSER 792may be used by the application, without conflicting with current or future 793system-defined section types. 794.El 795.Pp 796.It Fa sh_flags 797Sections support one-bit flags that describe miscellaneous attributes. 798If a flag bit is set in 799.Fa sh_flags , 800the attribute is 801.Dq on 802for the section. 803Otherwise, the attribute is 804.Dq off 805or does not apply. 806Undefined attributes are set to zero. 807.Pp 808.Bl -tag -width ".Dv SHF_EXECINSTR" -compact 809.It Dv SHF_WRITE 810This section contains data that should be writable during process 811execution. 812.It Dv SHF_ALLOC 813The section occupies memory during process execution. 814Some control 815sections do not reside in the memory image of an object file. 816This 817attribute is off for those sections. 818.It Dv SHF_EXECINSTR 819The section contains executable machine instructions. 820.It Dv SHF_MASKPROC 821All bits included in this mask are reserved for processor-specific 822semantics. 823.El 824.Pp 825.It Fa sh_addr 826If the section will appear in the memory image of a process, this member 827holds the address at which the section's first byte should reside. 828Otherwise, the member contains zero. 829.It Fa sh_offset 830This member's value holds the byte offset from the beginning of the file 831to the first byte in the section. 832One section type, 833.Dv SHT_NOBITS , 834occupies no space in the file, and its 835.Fa sh_offset 836member locates the conceptual placement in the file. 837.It Fa sh_size 838This member holds the section's size in bytes. 839Unless the section type 840is 841.Dv SHT_NOBITS , 842the section occupies 843.Fa sh_size 844bytes in the file. 845A section of type 846.Dv SHT_NOBITS 847may have a non-zero size, but it occupies no space in the file. 848.It Fa sh_link 849This member holds a section header table index link, whose interpretation 850depends on the section type. 851.It Fa sh_info 852This member holds extra information, whose interpretation depends on the 853section type. 854.It Fa sh_addralign 855Some sections have address alignment constraints. 856If a section holds a 857doubleword, the system must ensure doubleword alignment for the entire 858section. 859That is, the value of 860.Fa sh_addr 861must be congruent to zero, modulo the value of 862.Fa sh_addralign . 863Only zero and positive integral powers of two are allowed. 864Values of zero 865or one mean the section has no alignment constraints. 866.It Fa sh_entsize 867Some sections hold a table of fixed-sized entries, such as a symbol table. 868For such a section, this member gives the size in bytes for each entry. 869This member contains zero if the section does not hold a table of 870fixed-size entries. 871.El 872.Pp 873Various sections hold program and control information: 874.Bl -tag -width ".Sy .shstrtab" -compact 875.It Sy .bss 876(Block Started by Symbol) 877This section holds uninitialized data that contributes to the program's 878memory image. 879By definition, the system initializes the data with zeros 880when the program begins to run. 881This section is of type 882.Dv SHT_NOBITS . 883The attributes types are 884.Dv SHF_ALLOC 885and 886.Dv SHF_WRITE . 887.It Sy .comment 888This section holds version control information. 889This section is of type 890.Dv SHT_PROGBITS . 891No attribute types are used. 892.It Sy .data 893This section holds initialized data that contribute to the program's 894memory image. 895This section is of type 896.Dv SHT_PROGBITS . 897The attribute types are 898.Dv SHF_ALLOC 899and 900.Dv SHF_WRITE . 901.It Sy .data1 902This section holds initialized data that contribute to the program's 903memory image. 904This section is of type 905.Dv SHT_PROGBITS . 906The attribute types are 907.Dv SHF_ALLOC 908and 909.Dv SHF_WRITE . 910.It Sy .debug 911This section holds information for symbolic debugging. 912The contents 913are unspecified. 914This section is of type 915.Dv SHT_PROGBITS . 916No attribute types are used. 917.It Sy .dynamic 918This section holds dynamic linking information. 919The section's attributes 920will include the 921.Dv SHF_ALLOC 922bit. 923Whether the 924.Dv SHF_WRITE 925bit is set is processor-specific. 926This section is of type 927.Dv SHT_DYNAMIC . 928See the attributes above. 929.It Sy .dynstr 930This section holds strings needed for dynamic linking, most commonly 931the strings that represent the names associated with symbol table entries. 932This section is of type 933.Dv SHT_STRTAB . 934The attribute type used is 935.Dv SHF_ALLOC . 936.It Sy .dynsym 937This section holds the dynamic linking symbol table. 938This section is of type 939.Dv SHT_DYNSYM . 940The attribute used is 941.Dv SHF_ALLOC . 942.It Sy .fini 943This section holds executable instructions that contribute to the process 944termination code. 945When a program exits normally the system arranges to 946execute the code in this section. 947This section is of type 948.Dv SHT_PROGBITS . 949The attributes used are 950.Dv SHF_ALLOC 951and 952.Dv SHF_EXECINSTR . 953.It Sy .got 954This section holds the global offset table. 955This section is of type 956.Dv SHT_PROGBITS . 957The attributes are processor-specific. 958.It Sy .hash 959This section holds a symbol hash table. 960This section is of type 961.Dv SHT_HASH . 962The attribute used is 963.Dv SHF_ALLOC . 964.It Sy .init 965This section holds executable instructions that contribute to the process 966initialization code. 967When a program starts to run the system arranges to 968execute the code in this section before calling the main program entry point. 969This section is of type 970.Dv SHT_PROGBITS . 971The attributes used are 972.Dv SHF_ALLOC 973and 974.Dv SHF_EXECINSTR . 975.It Sy .interp 976This section holds the pathname of a program interpreter. 977If the file has 978a loadable segment that includes the section, the section's attributes will 979include the 980.Dv SHF_ALLOC 981bit. 982Otherwise, that bit will be off. 983This section is of type 984.Dv SHT_PROGBITS . 985.It Sy .line 986This section holds line number information for symbolic debugging, which 987describes the correspondence between the program source and the machine code. 988The contents are unspecified. 989This section is of type 990.Dv SHT_PROGBITS . 991No attribute types are used. 992.It Sy .note 993This section holds information in the 994.Dq Note Section 995format described below. 996This section is of type 997.Dv SHT_NOTE . 998No attribute types are used. 999.It Sy .plt 1000This section holds the procedure linkage table. 1001This section is of type 1002.Dv SHT_PROGBITS . 1003The attributes are processor-specific. 1004.It Sy .relNAME 1005This section holds relocation information as described below. 1006If the file 1007has a loadable segment that includes relocation, the section's attributes 1008will include the 1009.Dv SHF_ALLOC 1010bit. 1011Otherwise the bit will be off. 1012By convention, 1013.Dq NAME 1014is supplied by the section to which the relocations apply. 1015Thus a relocation 1016section for 1017.Sy .text 1018normally would have the name 1019.Sy .rel.text . 1020This section is of type 1021.Dv SHT_REL . 1022.It Sy .relaNAME 1023This section holds relocation information as described below. 1024If the file 1025has a loadable segment that includes relocation, the section's attributes 1026will include the 1027.Dv SHF_ALLOC 1028bit. 1029Otherwise the bit will be off. 1030By convention, 1031.Dq NAME 1032is supplied by the section to which the relocations apply. 1033Thus a relocation 1034section for 1035.Sy .text 1036normally would have the name 1037.Sy .rela.text . 1038This section is of type 1039.Dv SHT_RELA . 1040.It Sy .rodata 1041This section holds read-only data that typically contributes to a 1042non-writable segment in the process image. 1043This section is of type 1044.Dv SHT_PROGBITS . 1045The attribute used is 1046.Dv SHF_ALLOC . 1047.It Sy .rodata1 1048This section hold read-only data that typically contributes to a 1049non-writable segment in the process image. 1050This section is of type 1051.Dv SHT_PROGBITS . 1052The attribute used is 1053.Dv SHF_ALLOC . 1054.It Sy .shstrtab 1055This section holds section names. 1056This section is of type 1057.Dv SHT_STRTAB . 1058No attribute types are used. 1059.It Sy .strtab 1060This section holds strings, most commonly the strings that represent the 1061names associated with symbol table entries. 1062If the file has a loadable 1063segment that includes the symbol string table, the section's attributes 1064will include the 1065.Dv SHF_ALLOC 1066bit. 1067Otherwise the bit will be off. 1068This section is of type 1069.Dv SHT_STRTAB . 1070.It Sy .symtab 1071This section holds a symbol table. 1072If the file has a loadable segment 1073that includes the symbol table, the section's attributes will include 1074the 1075.Dv SHF_ALLOC 1076bit. 1077Otherwise the bit will be off. 1078This section is of type 1079.Dv SHT_SYMTAB . 1080.It Sy .text 1081This section holds the 1082.Dq text , 1083or executable instructions, of a program. 1084This section is of type 1085.Dv SHT_PROGBITS . 1086The attributes used are 1087.Dv SHF_ALLOC 1088and 1089.Dv SHF_EXECINSTR . 1090.El 1091.Pp 1092String table sections hold null-terminated character sequences, commonly 1093called strings. 1094The object file uses these strings to represent symbol 1095and section names. 1096One references a string as an index into the string 1097table section. 1098The first byte, which is index zero, is defined to hold 1099a null character. 1100Similarly, a string table's last byte is defined to 1101hold a null character, ensuring null termination for all strings. 1102.Pp 1103An object file's symbol table holds information needed to locate and 1104relocate a program's symbolic definitions and references. 1105A symbol table 1106index is a subscript into this array. 1107.Bd -literal -offset indent 1108typedef struct { 1109 Elf32_Word st_name; 1110 Elf32_Addr st_value; 1111 Elf32_Size st_size; 1112 unsigned char st_info; 1113 unsigned char st_other; 1114 Elf32_Half st_shndx; 1115} Elf32_Sym; 1116.Ed 1117.Bd -literal -offset indent 1118typedef struct { 1119 Elf64_Word st_name; 1120 unsigned char st_info; 1121 unsigned char st_other; 1122 Elf64_Half st_shndx; 1123 Elf64_Addr st_value; 1124 Elf64_Xword st_size; 1125} Elf64_Sym; 1126.Ed 1127.Pp 1128.Bl -tag -width ".Fa st_value" -compact 1129.It Fa st_name 1130This member holds an index into the object file's symbol string table, 1131which holds character representations of the symbol names. 1132If the value 1133is non-zero, it represents a string table index that gives the symbol 1134name. 1135Otherwise, the symbol table has no name. 1136.It Fa st_value 1137This member gives the value of the associated symbol. 1138.It Fa st_size 1139Many symbols have associated sizes. 1140This member holds zero if the symbol 1141has no size or an unknown size. 1142.It Fa st_info 1143This member specifies the symbol's type and binding attributes: 1144.Pp 1145.Bl -tag -width ".Dv STT_SECTION" -compact 1146.It Dv STT_NOTYPE 1147The symbol's type is not defined. 1148.It Dv STT_OBJECT 1149The symbol is associated with a data object. 1150.It Dv STT_FUNC 1151The symbol is associated with a function or other executable code. 1152.It Dv STT_SECTION 1153The symbol is associated with a section. 1154Symbol table entries of 1155this type exist primarily for relocation and normally have 1156.Dv STB_LOCAL 1157bindings. 1158.It Dv STT_FILE 1159By convention the symbol's name gives the name of the source file 1160associated with the object file. 1161A file symbol has 1162.Dv STB_LOCAL 1163bindings, its section index is 1164.Dv SHN_ABS , 1165and it precedes the other 1166.Dv STB_LOCAL 1167symbols of the file, if it is present. 1168.It Dv STT_LOPROC 1169This value up to and including 1170.Dv STT_HIPROC 1171are reserved for processor-specific semantics. 1172.It Dv STT_HIPROC 1173This value down to and including 1174.Dv STT_LOPROC 1175are reserved for processor-specific semantics. 1176.El 1177.Pp 1178.Bl -tag -width ".Dv STB_GLOBAL" -compact 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 file 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.Dv STB_HIPROC 1195are reserved for processor-specific semantics. 1196.It Dv STB_HIPROC 1197This value down to and including 1198.Dv STB_LOPROC 1199are 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 ".Fn 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 1210.Fa st_info 1211value. 1212.It Xo 1213.Fn ELF64_ST_TYPE info 1214.Xc 1215or 1216.Fn ELF32_ST_TYPE info 1217extract a type from an 1218.Fa st_info 1219value. 1220.It Xo 1221.Fn ELF32_ST_INFO bind type 1222.Xc 1223or 1224.Fn ELF64_ST_INFO bind type 1225convert a binding and a type into an 1226.Fa st_info 1227value. 1228.El 1229.El 1230.Pp 1231.It Fa st_other 1232This member currently holds zero and has no defined meaning. 1233.It Fa st_shndx 1234Every symbol table entry is 1235.Dq defined 1236in relation to some section. 1237This member holds the relevant section 1238header table index. 1239.El 1240.Pp 1241Relocation is the process of connecting symbolic references with 1242symbolic definitions. 1243Relocatable files must have information that 1244describes how to modify their section contents, thus allowing executable 1245and shared object files to hold the right information for a process' 1246program image. 1247Relocation entries are these data. 1248.Pp 1249Relocation structures that do not need an addend: 1250.Bd -literal -offset indent 1251typedef struct { 1252 Elf32_Addr r_offset; 1253 Elf32_Word r_info; 1254} Elf32_Rel; 1255.Ed 1256.Bd -literal -offset indent 1257typedef struct { 1258 Elf64_Addr r_offset; 1259 Elf64_Xword r_info; 1260} Elf64_Rel; 1261.Ed 1262.Pp 1263Relocation structures that need an addend: 1264.Bd -literal -offset indent 1265typedef struct { 1266 Elf32_Addr r_offset; 1267 Elf32_Word r_info; 1268 Elf32_Sword r_addend; 1269} Elf32_Rela; 1270.Ed 1271.Bd -literal -offset indent 1272typedef struct { 1273 Elf64_Addr r_offset; 1274 Elf64_Xword r_info; 1275 Elf64_Sxword r_addend; 1276} Elf64_Rela; 1277.Ed 1278.Pp 1279.Bl -tag -width ".Fa r_offset" -compact 1280.It Fa r_offset 1281This member gives the location at which to apply the relocation action. 1282For a relocatable file, the value is the byte offset from the beginning 1283of the section to the storage unit affected by the relocation. 1284For an 1285executable file or shared object, the value is the virtual address of 1286the storage unit affected by the relocation. 1287.It Fa r_info 1288This member gives both the symbol table index with respect to which the 1289relocation must be made and the type of relocation to apply. 1290Relocation 1291types are processor-specific. 1292When the text refers to a relocation 1293entry's relocation type or symbol table index, it means the result of 1294applying 1295.Fn ELF[32|64]_R_TYPE 1296or 1297.Fn ELF[32|64]_R_SYM , 1298respectively to the entry's 1299.Fa r_info 1300member. 1301.It Fa r_addend 1302This member specifies a constant addend used to compute the value to be 1303stored into the relocatable field. 1304.El 1305.Sh SEE ALSO 1306.Xr as 1 , 1307.Xr gdb 1 , 1308.Xr ld 1 , 1309.Xr objdump 1 , 1310.Xr readelf 1 , 1311.Xr execve 2 , 1312.Xr core 5 1313.Rs 1314.%A Hewlett Packard 1315.%B Elf-64 Object File Format 1316.Re 1317.Rs 1318.%A Santa Cruz Operation 1319.%B System V Application Binary Interface 1320.Re 1321.Rs 1322.%A Unix System Laboratories 1323.%T Object Files 1324.%B "Executable and Linking Format (ELF)" 1325.Re 1326.Sh HISTORY 1327The ELF header files made their appearance in 1328.Fx 2.2.6 . 1329ELF in itself first appeared in 1330.At V . 1331The ELF format is an adopted standard. 1332.Sh AUTHORS 1333This manual page was written by 1334.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org 1335with inspiration from BSDi's 1336.Bsx 1337.Xr elf 5 1338manpage. 1339