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 July 31, 1999 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 program address 92Elf32_Half Unsigned halfword field 93Elf32_Off Unsigned file offset 94Elf32_Sword Signed large integer 95Elf32_Word Field or unsigned large 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 program address 102Elf64_Half Unsigned halfword field 103Elf64_Off Unsigned file offset 104Elf64_Sword Signed large integer 105Elf64_Word Field or unsigned large integer 106Elf64_Size Unsigned object size 107Elf64_Quarter Unsigned quarterword field 108.Ed 109.Pp 110All data structures that the file format defines follow the 111.Dq natural 112size and alignment guidelines for the relevant class. 113If necessary, 114data structures contain explicit padding to ensure 4-byte alignment 115for 4-byte objects, to force structure sizes to a multiple of 4, etc. 116.Pp 117The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 118.Bd -literal -offset indent 119typedef struct { 120 unsigned char e_ident[EI_NIDENT]; 121 Elf32_Half e_type; 122 Elf32_Half e_machine; 123 Elf32_Word e_version; 124 Elf32_Addr e_entry; 125 Elf32_Off e_phoff; 126 Elf32_Off e_shoff; 127 Elf32_Word e_flags; 128 Elf32_Half e_ehsize; 129 Elf32_Half e_phentsize; 130 Elf32_Half e_phnum; 131 Elf32_Half e_shentsize; 132 Elf32_Half e_shnum; 133 Elf32_Half e_shstrndx; 134} Elf32_Ehdr; 135.Ed 136.Pp 137.Bd -literal -offset indent 138typedef struct { 139 unsigned char e_ident[EI_NIDENT]; 140 Elf64_Quarter e_type; 141 Elf64_Quarter e_machine; 142 Elf64_Half e_version; 143 Elf64_Addr e_entry; 144 Elf64_Off e_phoff; 145 Elf64_Off e_shoff; 146 Elf64_Half e_flags; 147 Elf64_Quarter e_ehsize; 148 Elf64_Quarter e_phentsize; 149 Elf64_Quarter e_phnum; 150 Elf64_Quarter e_shentsize; 151 Elf64_Quarter e_shnum; 152 Elf64_Quarter e_shstrndx; 153} Elf64_Ehdr; 154.Ed 155.Pp 156The fields have the following meanings: 157.Pp 158.Bl -tag -width "e_phentsize" -compact -offset indent 159.It Dv e_ident 160This array of bytes specifies to interpret the file, 161independent of the processor or the file's remaining contents. 162Within this array everything is named by macros, which start with 163the prefix 164.Sy EI_ 165and may contain values which start with the prefix 166.Sy ELF . 167The following macros are defined: 168.Pp 169.Bl -tag -width "EI_ABIVERSION" -compact 170.It Dv EI_MAG0 171The first byte of the magic number. 172It must be filled with 173.Sy ELFMAG0 . 174.It Dv EI_MAG1 175The second byte of the magic number. 176It must be filled with 177.Sy ELFMAG1 . 178.It Dv EI_MAG2 179The third byte of the magic number. 180It must be filled with 181.Sy ELFMAG2 . 182.It Dv EI_MAG3 183The fourth byte of the magic number. 184It must be filled with 185.Sy ELFMAG3 . 186.It Dv EI_CLASS 187The fifth byte identifies the architecture for this binary: 188.Pp 189.Bl -tag -width "ELFCLASSNONE" -compact 190.It Dv ELFCLASSNONE 191This class is invalid. 192.It Dv ELFCLASS32 193This defines the 32-bit architecture. 194It supports machines with files 195and virtual address spaces up to 4 Gigabytes. 196.It Dv ELFCLASS64 197This defines the 64-bit architecture. 198.El 199.It Dv EI_DATA 200The sixth byte specifies the data encoding of the processor-specific 201data in the file. 202Currently these encodings are supported: 203.Pp 204.Bl -tag -width "ELFDATA2LSB" -compact 205.It Dv ELFDATANONE 206Unknown data format. 207.It Dv ELFDATA2LSB 208Two's complement, little-endian. 209.It Dv ELFDATA2MSB 210Two's complement, big-endian. 211.El 212.It Dv EI_VERSION 213The version number of the ELF specification: 214.Pp 215.Bl -tag -width "EV_CURRENT" -compact 216.It Dv EV_NONE 217Invalid version. 218.It Dv EV_CURRENT 219Current version. 220.El 221.It Dv EI_OSABI 222This byte identifies the operating system 223and ABI to which the object is targeted. 224Some fields in other ELF structures have flags 225and values that have platform specific meanings; 226the interpretation of those fields is determined by the value of this byte. 227The following values are currently defined: 228.Pp 229.Bl -tag -width "ELFOSABI_STANDALONE" -compact 230.It Dv ELFOSABI_SYSV 231UNIX System V ABI. 232.It Dv ELFOSABI_HPUX 233HP-UX operating system ABI. 234.It Dv ELFOSABI_NETBSD 235.Nx 236operating system ABI. 237.It Dv ELFOSABI_LINUX 238GNU/Linux operating system ABI. 239.It Dv ELFOSABI_HURD 240GNU/Hurd operating system ABI. 241.It Dv ELFOSABI_86OPEN 24286Open Common IA32 ABI. 243.It Dv ELFOSABI_SOLARIS 244Solaris operating system ABI. 245.It Dv ELFOSABI_MONTEREY 246Monterey project ABI. 247.It Dv ELFOSABI_IRIX 248IRIX operating system ABI. 249.It Dv ELFOSABI_FREEBSD 250.Fx 251operating system ABI. 252.It Dv ELFOSABI_TRU64 253TRU64 UNIX operating system ABI. 254.It Dv ELFOSABI_ARM 255ARM architecture ABI. 256.It Dv ELFOSABI_STANDALONE 257Standalone (embedded) ABI. 258.El 259.It Dv EI_ABIVERSION 260This byte identifies the version of the ABI 261to which the object is targeted. 262This field is used to distinguish among incompatible versions of an ABI. 263The interpretation of this version number 264is dependent on the ABI identified by the EI_OSABI field. 265Applications conforming to this specification use the value 0. 266.It Dv EI_PAD 267Start of padding. 268These bytes are reserved and set to zero. 269Programs 270which read them should ignore them. 271The value for EI_PAD will change in 272the future if currently unused bytes are given meanings. 273.It Dv EI_BRAND 274Start of architecture identification. 275.It Dv EI_NIDENT 276The size of the e_ident array. 277.El 278.Pp 279.It Dv e_type 280This member of the structure identifies the object file type: 281.Pp 282.Bl -tag -width "ET_NONE" -compact 283.It Dv ET_NONE 284An unknown type. 285.It Dv ET_REL 286A relocatable file. 287.It Dv ET_EXEC 288An executable file. 289.It Dv ET_DYN 290A shared object. 291.It Dv ET_CORE 292A core file. 293.El 294.Pp 295.It Dv e_machine 296This member specifies the required architecture for an individual file: 297.Pp 298.Bl -tag -width "EM_MIPS_RS4_BE" -compact 299.It Dv EM_NONE 300An unknown machine. 301.It Dv EM_M32 302AT&T WE 32100. 303.It Dv EM_SPARC 304Sun Microsystems SPARC. 305.It Dv EM_386 306Intel 80386. 307.It Dv EM_68K 308Motorola 68000. 309.It Dv EM_88K 310Motorola 88000. 311.It Dv EM_486 312Intel 80486. 313.It Dv EM_860 314Intel 80860. 315.It Dv EM_MIPS 316MIPS RS3000 (big-endian only). 317.It Dv EM_MIPS_RS4_BE 318MIPS RS4000 (big-endian only). 319.It Dv EM_SPARC64 320SPARC v9 64-bit unofficial. 321.It Dv EM_PARISC 322HPPA. 323.It Dv EM_PPC 324PowerPC. 325.It Dv EM_ALPHA 326Compaq [DEC] Alpha. 327.El 328.Pp 329.It Dv e_version 330This member identifies the file version: 331.Pp 332.Bl -tag -width "EV_CURRENT" -compact 333.It Dv EV_NONE 334Invalid version 335.It Dv EV_CURRENT 336Current version 337.El 338.It Dv e_entry 339This member gives the virtual address to which the system first transfers 340control, thus starting the process. 341If the file has no associated entry 342point, this member holds zero. 343.It Dv e_phoff 344This member holds the program header table's file offset in bytes. 345If 346the file has no program header table, this member holds zero. 347.It Dv e_shoff 348This member holds the section header table's file offset in bytes. 349If the 350file has no section header table this member holds zero. 351.It Dv e_flags 352This member holds processor-specific flags associated with the file. 353Flag 354names take the form EF_`machine_flag'. Currently no flags have been defined. 355.It Dv e_ehsize 356This member holds the ELF header's size in bytes. 357.It Dv e_phentsize 358This member holds the size in bytes of one entry in the file's program header 359table; all entries are the same size. 360.It Dv e_phnum 361This member holds the number of entries in the program header 362table. 363Thus the product of 364.Sy e_phentsize 365and 366.Sy e_phnum 367gives the table's size 368in bytes. 369If a file has no program header, 370.Sy e_phnum 371holds the value zero. 372.It Dv e_shentsize 373This member holds a sections header's size in bytes. 374A section header is one 375entry in the section header table; all entries are the same size. 376.It Dv e_shnum 377This member holds the number of entries in the section header table. 378Thus 379the product of 380.Sy e_shentsize 381and 382.Sy e_shnum 383gives the section header table's size in bytes. 384If a file has no section 385header table, 386.Sy e_shnum 387holds the value of zero. 388.It Dv e_shstrndx 389This member holds the section header table index of the entry associated 390with the section name string table. 391If the file has no section name string 392table, this member holds the value 393.Sy SHN_UNDEF . 394.Pp 395.Bl -tag -width "SHN_LORESERVE" -compact 396.It Dv SHN_UNDEF 397This value marks an undefined, missing, irrelevant, or otherwise meaningless 398section reference. 399For example, a symbol 400.Dq defined 401relative to section number 402.Sy SHN_UNDEF 403is an undefined symbol. 404.It Dv SHN_LORESERVE 405This value specifies the lower bound of the range of reserved indexes. 406.It Dv SHN_LOPROC 407This value up to and including 408.Sy SHN_HIPROC 409are reserved for processor-specific semantics. 410.It Dv SHN_HIPROC 411This value down to and including 412.Sy SHN_LOPROC 413are reserved for processor-specific semantics. 414.It Dv SHN_ABS 415This value specifies absolute values for the corresponding reference. 416For 417example, symbols defined relative to section number 418.Sy SHN_ABS 419have absolute values and are not affected by relocation. 420.It Dv SHN_COMMON 421Symbols defined relative to this section are common symbols, such as Fortran 422COMMON or unallocated C external variables. 423.It Dv SHN_HIRESERVE 424This value specifies the upper bound of the range of the range of reserved 425indices between 426.Sy SHN_LORESERVE 427and 428.Sy SHN_HIRESERVE , 429inclusive; the values do 430not reference the section header table. 431That is, the section header table 432does 433.Em not 434contain entries for the reserved indices. 435.El 436.El 437.Pp 438An executable or shared object file's program header table is an array of 439structures, each describing a segment or other information the system needs 440to prepare the program for execution. 441An object file 442.Em segment 443contains one or more 444.Em sections . 445Program headers are meaningful only for executable and shared object files. 446A file specifies its own program header size with the ELF header's 447.Sy e_phentsize 448and 449.Sy e_phnum 450members. 451As with the Elf executable header, the program header 452also has different versions depending on the architecture: 453.Pp 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_Size p_filesz; 461 Elf32_Size p_memsz; 462 Elf32_Word p_flags; 463 Elf32_Size p_align; 464} Elf32_Phdr; 465.Ed 466.Pp 467.Bd -literal -offset indent 468typedef struct { 469 Elf64_Half p_type; 470 Elf64_Half p_flags; 471 Elf64_Off p_offset; 472 Elf64_Addr p_vaddr; 473 Elf64_Addr p_paddr; 474 Elf64_Size p_filesz; 475 Elf64_Size p_memsz; 476 Elf64_Size p_align; 477} Elf64_Phdr; 478.Ed 479.Pp 480The main difference between the 32-bit and the 64-bit program header lies 481only in the location of a 482.Sy p_flags 483member in the total struct. 484.Pp 485.Bl -tag -width "p_offset" -compact -offset indent 486.It Dv p_type 487This member of the Phdr struct tells what kind of segment this array 488element describes or how to interpret the array element's information. 489.Bl -tag -width "PT_DYNAMIC" -compact 490.Pp 491.It Dv PT_NULL 492The array element is unused and the other members' values are undefined. 493This lets the program header have ignored entries. 494.It Dv PT_LOAD 495The array element specifies a loadable segment, described by 496.Sy p_filesz 497and 498.Sy p_memsz . 499The bytes from the file are mapped to the beginning of the memory 500segment. 501If the segment's memory size 502.Pq Sy p_memsz 503is larger than the file size 504.Pq Sy p_filesz , 505the 506.Dq extra 507bytes are defined to hold the value 0 and to follow the segment's 508initialized area. 509The file size may not be larger than the memory size. 510Loadable segment entries in the program header table appear in ascending 511order, sorted on the 512.Sy p_vaddr 513member. 514.It Dv PT_DYNAMIC 515The array element specifies dynamic linking information. 516.It Dv PT_INTERP 517The array element specifies the location and size of a null-terminated 518path name to invoke as an interpreter. 519This segment type is meaningful 520only for executable files (though it may occur for shared objects). However 521it may not occur more than once in a file. 522If it is present it must precede 523any loadable segment entry. 524.It Dv PT_NOTE 525The array element specifies the location and size for auxiliary information. 526.It Dv PT_SHLIB 527This segment type is reserved but has unspecified semantics. 528Programs that 529contain an array element of this type do not conform to the ABI. 530.It Dv PT_PHDR 531The array element, if present, specifies the location and size of the program 532header table itself, both in the file and in the memory image of the program. 533This segment type may not occur more than once in a file. 534Moreover, it may 535only occur if the program header table is part of the memory image of the 536program. 537If it is present it must precede any loadable segment entry. 538.It Dv PT_LOPROC 539This value up to and including 540.Sy PT_HIPROC 541are reserved for processor-specific semantics. 542.It Dv PT_HIPROC 543This value down to and including 544.Sy PT_LOPROC 545are reserved for processor-specific semantics. 546.El 547.Pp 548.It Dv p_offset 549This member holds the offset from the beginning of the file at which 550the first byte of the segment resides. 551.It Dv p_vaddr 552This member holds the virtual address at which the first byte of the 553segment resides in memory. 554.It Dv p_paddr 555On systems for which physical addressing is relevant, this member is 556reserved for the segment's physical address. 557Under 558.Bx 559this member is 560not used and must be zero. 561.It Dv p_filesz 562This member holds the number of bytes in the file image of the segment. 563It may be zero. 564.It Dv p_memsz 565This member holds the number of bytes in the memory image of the segment. 566It may be zero. 567.It Dv p_flags 568This member holds flags relevant to the segment: 569.Pp 570.Bl -tag -width "PF_X" -compact 571.It Dv PF_X 572An executable segment. 573.It Dv PF_W 574A writable segment. 575.It Dv PF_R 576A readable segment. 577.El 578.Pp 579A text segment commonly has the flags 580.Sy PF_X 581and 582.Sy PF_R . 583A data segment commonly has 584.Sy PF_X , 585.Sy PF_W 586and 587.Sy PF_R . 588.It Dv p_align 589This member holds the value to which the segments are aligned in memory 590and in the file. 591Loadable process segments must have congruent values for 592.Sy p_vaddr 593and 594.Sy p_offset , 595modulo the page size. 596Values of zero and one mean no alignment is required. 597Otherwise, 598.Sy p_align 599should be a positive, integral power of two, and 600.Sy p_vaddr 601should equal 602.Sy p_offset , 603modulo 604.Sy p_align . 605.El 606.Pp 607An file's section header table lets one locate all the file's sections. 608The 609section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 610The 611ELF header's 612.Sy e_shoff 613member gives the byte offset from the beginning of the file to the section 614header table. 615.Sy e_shnum 616holds the number of entries the section header table contains. 617.Sy e_shentsize 618holds the size in bytes of each entry. 619.Pp 620A section header table index is a subscript into this array. 621Some section 622header table indices are reserved. 623An object file does not have sections for 624these special indices: 625.Pp 626.Bl -tag -width "SHN_LORESERVE" -compact 627.It Dv SHN_UNDEF 628This value marks an undefined, missing, irrelevant or otherwise meaningless 629section reference. 630.It Dv SHN_LORESERVE 631This value specifies the lower bound of the range of reserved indices. 632.It Dv SHN_LOPROC 633This value up to and including 634.Sy SHN_HIPROC 635are reserved for processor-specific semantics. 636.It Dv SHN_HIPROC 637This value down to and including 638.Sy SHN_LOPROC 639are reserved for processor-specific semantics. 640.It Dv SHN_ABS 641This value specifies absolute values for the corresponding reference. 642For 643example, symbols defined relative to section number 644.Sy SHN_ABS 645have absolute values and are not affected by relocation. 646.It Dv SHN_COMMON 647Symbols defined relative to this section are common symbols, such as FORTRAN 648COMMON or unallocated C external variables. 649.It Dv SHN_HIRESERVE 650This value specifies the upper bound of the range of reserved indices. 651The 652system reserves indices between 653.Sy SHN_LORESERVE 654and 655.Sy SHN_HIRESERVE , 656inclusive. 657The section header table does not contain entries for the 658reserved indices. 659.El 660.Pp 661The section header has the following structure: 662.Bd -literal -offset indent 663typedef struct { 664 Elf32_Word sh_name; 665 Elf32_Word sh_type; 666 Elf32_Word sh_flags; 667 Elf32_Addr sh_addr; 668 Elf32_Off sh_offset; 669 Elf32_Size sh_size; 670 Elf32_Word sh_link; 671 Elf32_Word sh_info; 672 Elf32_Size sh_addralign; 673 Elf32_Size sh_entsize; 674} Elf32_Shdr; 675.Ed 676.Pp 677.Bd -literal -offset indent 678typedef struct { 679 Elf64_Half sh_name; 680 Elf64_Half sh_type; 681 Elf64_Size sh_flags; 682 Elf64_Addr sh_addr; 683 Elf64_Off sh_offset; 684 Elf64_Size sh_size; 685 Elf64_Half sh_link; 686 Elf64_Half sh_info; 687 Elf64_Size sh_addralign; 688 Elf64_Size sh_entsize; 689} Elf64_Shdr; 690.Ed 691.Pp 692.Bl -tag -width "sh_addralign" -compact 693.It Dv sh_name 694This member specifies the name of the section. 695Its value is an index 696into the section header string table section, giving the location of 697a null-terminated string. 698.It Dv sh_type 699This member categorizes the section's contents and semantics. 700.Pp 701.Bl -tag -width "SHT_PROGBITS" -compact 702.It Dv SHT_NULL 703This value marks the section header as inactive. 704It does not 705have an associated section. 706Other members of the section header 707have undefined values. 708.It Dv SHT_PROGBITS 709The section holds information defined by the program, whose 710format and meaning are determined solely by the program. 711.It Dv SHT_SYMTAB 712This section holds a symbol table. 713Typically, 714.Sy SHT_SYMTAB 715provides symbols for link editing, though it may also be used 716for dynamic linking. 717As a complete symbol table, it may contain 718many symbols unnecessary for dynamic linking. 719An object file can 720also contain a 721.Sy SHN_DYNSYM 722section. 723.It Dv SHT_STRTAB 724This section holds a string table. 725An object file may have multiple 726string table sections. 727.It Dv SHT_RELA 728This section holds relocation entries with explicit addends, such 729as type 730.Sy Elf32_Rela 731for the 32-bit class of object files. 732An object may have multiple 733relocation sections. 734.It Dv SHT_HASH 735This section holds a symbol hash table. 736All object participating in 737dynamic linking must contain a symbol hash table. 738An object file may 739have only one hash table. 740.It Dv SHT_DYNAMIC 741This section holds information for dynamic linking. 742An object file may 743have only one dynamic section. 744.It Dv SHT_NOTE 745This section holds information that marks the file in some way. 746.It Dv SHT_NOBITS 747A section of this type occupies no space in the file but otherwise 748resembles 749.Sy SHN_PROGBITS . 750Although this section contains no bytes, the 751.Sy sh_offset 752member contains the conceptual file offset. 753.It Dv SHT_REL 754This section holds relocation offsets without explicit addends, such 755as type 756.Sy Elf32_Rel 757for the 32-bit class of object files. 758An object file may have multiple 759relocation sections. 760.It Dv SHT_SHLIB 761This section is reserved but has unspecified semantics. 762.It Dv SHT_DYNSYM 763This section holds a minimal set of dynamic linking symbols. 764An 765object file can also contain a 766.Sy SHN_SYMTAB 767section. 768.It Dv SHT_LOPROC 769This value up to and including 770.Sy SHT_HIPROC 771are reserved for processor-specific semantics. 772.It Dv SHT_HIPROC 773This value down to and including 774.Sy SHT_LOPROC 775are reserved for processor-specific semantics. 776.It Dv SHT_LOUSER 777This value specifies the lower bound of the range of indices reserved for 778application programs. 779.It Dv SHT_HIUSER 780This value specifies the upper bound of the range of indices reserved for 781application programs. 782Section types between 783.Sy SHT_LOUSER 784and 785.Sy SHT_HIUSER 786may be used by the application, without conflicting with current or future 787system-defined section types. 788.El 789.Pp 790.It Dv sh_flags 791Sections support one-bit flags that describe miscellaneous attributes. 792If a flag bit is set in 793.Sy sh_flags , 794the attribute is 795.Dq on 796for the section. 797Otherwise, the attribute is 798.Dq off 799or does not apply. 800Undefined attributes are set to zero. 801.Pp 802.Bl -tag -width "SHF_EXECINSTR" -compact 803.It Dv SHF_WRITE 804This section contains data that should be writable during process 805execution. 806.It Dv SHF_ALLOC 807The section occupies memory during process execution. 808Some control 809sections do not reside in the memory image of an object file. 810This 811attribute is off for those sections. 812.It Dv SHF_EXECINSTR 813The section contains executable machine instructions. 814.It Dv SHF_MASKPROC 815All bits included in this mask are reserved for processor-specific 816semantics. 817.El 818.Pp 819.It Dv sh_addr 820If the section will appear in the memory image of a process, this member 821holds the address at which the section's first byte should reside. 822Otherwise, the member contains zero. 823.It Dv sh_offset 824This member's value holds the byte offset from the beginning of the file 825to the first byte in the section. 826One section type, 827.Sy SHT_NOBITS , 828occupies no space in the file, and its 829.Sy sh_offset 830member locates the conceptual placement in the file. 831.It Dv sh_size 832This member holds the section's size in bytes. 833Unless the section type 834is 835.Sy SHT_NOBITS , 836the section occupies 837.Sy sh_size 838bytes in the file. 839A section of type 840.Sy SHT_NOBITS 841may have a non-zero size, but it occupies no space in the file. 842.It Dv sh_link 843This member holds a section header table index link, whose interpretation 844depends on the section type. 845.It Dv sh_info 846This member holds extra information, whose interpretation depends on the 847section type. 848.It Dv sh_addralign 849Some sections have address alignment constraints. 850If a section holds a 851doubleword, the system must ensure doubleword alignment for the entire 852section. 853That is, the value of 854.Sy sh_addr 855must be congruent to zero, modulo the value of 856.Sy sh_addralign . 857Only zero and positive integral powers of two are allowed. 858Values of zero 859or one mean the section has no alignment constraints. 860.It Dv sh_entsize 861Some sections hold a table of fixed-sized entries, such as a symbol table. 862For such a section, this member gives the size in bytes for each entry. 863This member contains zero if the section does not hold a table of 864fixed-size entries. 865.El 866.Pp 867Various sections hold program and control information: 868.Bl -tag -width ".shstrtab" -compact 869.It .bss 870This section holds uninitialized data that contributes to the program's 871memory image. 872By definition, the system initializes the data with zeros 873when the program begins to run. 874This section is of type 875.Sy SHT_NOBITS . 876The attributes types are 877.Sy SHF_ALLOC 878and 879.Sy SHF_WRITE . 880.It .comment 881This section holds version control information. 882This section is of type 883.Sy SHT_PROGBITS . 884No attribute types are used. 885.It .data 886This section holds initialized data that contribute to the program's 887memory image. 888This section is of type 889.Sy SHT_PROGBITS . 890The attribute types are 891.Sy SHF_ALLOC 892and 893.Sy SHF_WRITE . 894.It .data1 895This section holds initialized data that contribute to the program's 896memory image. 897This section is of type 898.Sy SHT_PROGBITS . 899The attribute types are 900.Sy SHF_ALLOC 901and 902.Sy SHF_WRITE . 903.It .debug 904This section holds information for symbolic debugging. 905The contents 906are unspecified. 907This section is of type 908.Sy SHT_PROGBITS . 909No attribute types are used. 910.It .dynamic 911This section holds dynamic linking information. 912The section's attributes 913will include the 914.Sy SHF_ALLOC 915bit. 916Whether the 917.Sy SHF_WRITE 918bit is set is processor-specific. 919This section is of type 920.Sy SHT_DYNAMIC . 921See the attributes above. 922.It .dynstr 923This section holds strings needed for dynamic linking, most commonly 924the strings that represent the names associated with symbol table entries. 925This section is of type 926.Sy SHT_STRTAB . 927The attribute type used is 928.Sy SHF_ALLOC . 929.It .dynsym 930This section holds the dynamic linking symbol table. 931This section is of type 932.Sy SHT_DYNSYM . 933The attribute used is 934.Sy SHF_ALLOC . 935.It .fini 936This section holds executable instructions that contribute to the process 937termination code. 938When a program exits normally the system arranges to 939execute the code in this section. 940This section is of type 941.Sy SHT_PROGBITS . 942The attributes used are 943.Sy SHF_ALLOC 944and 945.Sy SHF_EXECINSTR . 946.It .got 947This section holds the global offset table. 948This section is of type 949.Sy SHT_PROGBITS . 950The attributes are processor-specific. 951.It .hash 952This section holds a symbol hash table. 953This section is of type 954.Sy SHT_HASH . 955The attribute used is 956.Sy SHF_ALLOC . 957.It .init 958This section holds executable instructions that contribute to the process 959initialization code. 960When a program starts to run the system arranges to 961execute the code in this section before calling the main program entry point. 962This section is of type 963.Sy SHT_PROGBITS . 964The attributes used are 965.Sy SHF_ALLOC 966and 967.Sy SHF_EXECINSTR . 968.It .interp 969This section holds the pathname of a program interpreter. 970If the file has 971a loadable segment that includes the section, the section's attributes will 972include the 973.Sy SHF_ALLOC 974bit. 975Otherwise, that bit will be off. 976This section is of type 977.Sy SHT_PROGBITS . 978.It .line 979This section holds line number information for symbolic debugging, which 980describes the correspondence between the program source and the machine code. 981The contents are unspecified. 982This section is of type 983.Sy SHT_PROGBITS . 984No attribute types are used. 985.It .note 986This section holds information in the 987.Dq Note Section 988format described below. 989This section is of type 990.Sy SHT_NOTE . 991No attribute types are used. 992.It .plt 993This section holds the procedure linkage table. 994This section is of type 995.Sy SHT_PROGBITS . 996The attributes are processor-specific. 997.It .relNAME 998This section holds relocation information as described below. 999If the file 1000has a loadable segment that includes relocation, the section's attributes 1001will include the 1002.Sy SHF_ALLOC 1003bit. 1004Otherwise the bit will be off. 1005By convention, 1006.Dq NAME 1007is supplied by the section to which the relocations apply. 1008Thus a relocation 1009section for 1010.Sy .text 1011normally would have the name 1012.Sy .rel.text . 1013This section is of type 1014.Sy SHT_REL . 1015.It .relaNAME 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 .rela.text . 1031This section is of type 1032.Sy SHT_RELA . 1033.It .rodata 1034This section holds read-only data that typically contributes to a 1035non-writable segment in the process image. 1036This section is of type 1037.Sy SHT_PROGBITS . 1038The attribute used is 1039.Sy SHF_ALLOC . 1040.It .rodata1 1041This section hold read-only data that typically contributes to a 1042non-writable segment in the process image. 1043This section is of type 1044.Sy SHT_PROGBITS . 1045The attribute used is 1046.Sy SHF_ALLOC . 1047.It .shstrtab 1048This section holds section names. 1049This section is of type 1050.Sy SHT_STRTAB . 1051No attribute types are used. 1052.It .strtab 1053This section holds strings, most commonly the strings that represent the 1054names associated with symbol table entries. 1055If the file has a loadable 1056segment that includes the symbol string table, the section's attributes 1057will include the 1058.Sy SHF_ALLOC 1059bit. 1060Otherwise the bit will be off. 1061This section is of type 1062.Sy SHT_STRTAB . 1063.It .symtab 1064This section holds a symbol table. 1065If the file has a loadable segment 1066that includes the symbol table, the section's attributes will include 1067the 1068.Sy SHF_ALLOC 1069bit. 1070Otherwise the bit will be off. 1071This section is of type 1072.Sy SHT_SYMTAB . 1073.It .text 1074This section holds the 1075.Dq text , 1076or executable instructions, of a program. 1077This section is of type 1078.Sy SHT_PROGBITS . 1079The attributes used are 1080.Sy SHF_ALLOC 1081and 1082.Sy SHF_EXECINSTR . 1083.It .jcr 1084This section holds information about Java classes that must 1085be registered. 1086.It .eh_frame 1087This section holds information used for C++ exception-handling. 1088.El 1089.Pp 1090String table sections hold null-terminated character sequences, commonly 1091called strings. 1092The object file uses these strings to represent symbol 1093and section names. 1094One references a string as an index into the string 1095table section. 1096The first byte, which is index zero, is defined to hold 1097a null character. 1098Similarly, a string table's last byte is defined to 1099hold a null character, ensuring null termination for all strings. 1100.Pp 1101An object file's symbol table holds information needed to locate and 1102relocate a program's symbolic definitions and references. 1103A symbol table 1104index is a subscript into this array. 1105.Pp 1106.Bd -literal -offset indent 1107typedef struct { 1108 Elf32_Word st_name; 1109 Elf32_Addr st_value; 1110 Elf32_Size st_size; 1111 unsigned char st_info; 1112 unsigned char st_other; 1113 Elf32_Half st_shndx; 1114} Elf32_Sym; 1115.Ed 1116.Pp 1117.Bd -literal -offset indent 1118typedef struct { 1119 Elf64_Half st_name; 1120 unsigned char st_info; 1121 unsigned char st_other; 1122 Elf64_Quarter st_shndx; 1123 Elf64_Addr st_value; 1124 Elf64_Size st_size; 1125} Elf64_Sym; 1126.Ed 1127.Pp 1128.Bl -tag -width "st_value" -compact 1129.It Dv 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 Dv st_value 1137This member gives the value of the associated symbol. 1138.It Dv st_size 1139Many symbols have associated sizes. 1140This member holds zero if the symbol 1141has no size or an unknown size. 1142.It Dv st_info 1143This member specifies the symbol's type and binding attributes: 1144.Pp 1145.Bl -tag -width "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.Sy 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.Sy STB_LOCAL 1163bindings, its section index is 1164.Sy SHN_ABS , 1165and it precedes the other 1166.Sy STB_LOCAL 1167symbols of the file, if it is present. 1168.It Dv STT_LOPROC 1169This value up to and including 1170.Sy STT_HIPROC 1171are reserved for processor-specific semantics. 1172.It Dv STT_HIPROC 1173This value down to and including 1174.Sy STT_LOPROC 1175are reserved for processor-specific semantics. 1176.El 1177.Pp 1178.Bl -tag -width "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.Sy STB_HIPROC 1195are reserved for processor-specific semantics. 1196.It Dv STB_HIPROC 1197This value down to and including 1198.Sy 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 "ELF32_ST_INFO(bind, type)" -compact 1204.It Xo 1205.Fn ELF32_ST_BIND info 1206.Xc 1207or 1208.Fn ELF64_ST_BIND info 1209extract a binding from an st_info value. 1210.It Xo 1211.Fn ELF64_ST_TYPE info 1212.Xc 1213or 1214.Fn ELF32_ST_TYPE info 1215extract a type from an st_info value. 1216.It Xo 1217.Fn ELF32_ST_INFO bind type 1218.Xc 1219or 1220.Fn ELF64_ST_INFO bind type 1221convert a binding and a type into an st_info value. 1222.El 1223.El 1224.Pp 1225.It Dv st_other 1226This member currently holds zero and has no defined meaning. 1227.It Dv st_shndx 1228Every symbol table entry is 1229.Dq defined 1230in relation to some action. 1231This member holds the relevant section 1232header table index. 1233.El 1234.Pp 1235Relocation is the process of connecting symbolic references with 1236symbolic definitions. 1237Relocatable files must have information that 1238describes how to modify their section contents, thus allowing executable 1239and shared object files to hold the right information for a process' 1240program image. 1241Relocation entries are these data. 1242.Pp 1243Relocation structures that do not need an addend: 1244.Pp 1245.Bd -literal -offset indent 1246typedef struct { 1247 Elf32_Addr r_offset; 1248 Elf32_Word r_info; 1249} Elf32_Rel; 1250.Ed 1251.Bd -literal -offset indent 1252typedef struct { 1253 Elf64_Addr r_offset; 1254 Elf64_Size r_info; 1255} Elf64_Rel; 1256.Ed 1257.Pp 1258Relocation structures that need an addend: 1259.Pp 1260.Bd -literal -offset indent 1261typedef struct { 1262 Elf32_Addr r_offset; 1263 Elf32_Word r_info; 1264 Elf32_Sword r_addend; 1265} Elf32_Rela; 1266.Ed 1267.Bd -literal -offset indent 1268typedef struct { 1269 Elf64_Addr r_offset; 1270 Elf64_Size r_info; 1271 Elf64_Off r_addend; 1272} Elf64_Rela; 1273.Ed 1274.Pp 1275.Bl -tag -width "r_offset" -compact 1276.It Dv r_offset 1277This member gives the location at which to apply the relocation action. 1278For a relocatable file, the value is the byte offset from the beginning 1279of the section to the storage unit affected by the relocation. 1280For an 1281executable file or shared object, the value is the virtual address of 1282the storage unit affected by the relocation. 1283.It Dv r_info 1284This member gives both the symbol table index with respect to which the 1285relocation must be made and the type of relocation to apply. 1286Relocation 1287types are processor-specific. 1288When the text refers to a relocation 1289entry's relocation type or symbol table index, it means the result of 1290applying 1291.Sy ELF_[32|64]_R_TYPE 1292or 1293.Sy ELF[32|64]_R_SYM , 1294respectively to the entry's 1295.Sy r_info 1296member. 1297.It Dv r_addend 1298This member specifies a constant addend used to compute the value to be 1299stored into the relocatable field. 1300.El 1301.Sh SEE ALSO 1302.Xr as 1 , 1303.Xr gdb 1 , 1304.Xr ld 1 , 1305.Xr objdump 1 , 1306.Xr execve 2 , 1307.Xr core 5 1308.Rs 1309.%A Hewlett Packard 1310.%B Elf-64 Object File Format 1311.Re 1312.Rs 1313.%A Santa Cruz Operation 1314.%B System V Application Binary Interface 1315.Re 1316.Rs 1317.%A Unix System Laboratories 1318.%T Object Files 1319.%B "Executable and Linking Format (ELF)" 1320.Re 1321.Sh HISTORY 1322The ELF header files made their appearance in 1323.Fx 2.2.6 . 1324ELF in itself first appeared in 1325.At V . 1326The ELF format is an adopted standard. 1327.Sh AUTHORS 1328This manual page was written by 1329.An Jeroen Ruigrok van der Werven 1330.Aq asmodai@FreeBSD.org 1331with inspiration from BSDi's 1332.Bsx 1333.Xr elf 5 1334manpage. 1335