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'. 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.El 396.Pp 397An executable or shared object file's program header table is an array of 398structures, each describing a segment or other information the system needs 399to prepare the program for execution. 400An object file 401.Em segment 402contains one or more 403.Em sections . 404Program headers are meaningful only for executable and shared object files. 405A file specifies its own program header size with the ELF header's 406.Sy e_phentsize 407and 408.Sy e_phnum 409members. 410As with the Elf executable header, the program header 411also has different versions depending on the architecture: 412.Pp 413.Bd -literal -offset indent 414typedef struct { 415 Elf32_Word p_type; 416 Elf32_Off p_offset; 417 Elf32_Addr p_vaddr; 418 Elf32_Addr p_paddr; 419 Elf32_Size p_filesz; 420 Elf32_Size p_memsz; 421 Elf32_Word p_flags; 422 Elf32_Size p_align; 423} Elf32_Phdr; 424.Ed 425.Pp 426.Bd -literal -offset indent 427typedef struct { 428 Elf64_Half p_type; 429 Elf64_Half p_flags; 430 Elf64_Off p_offset; 431 Elf64_Addr p_vaddr; 432 Elf64_Addr p_paddr; 433 Elf64_Size p_filesz; 434 Elf64_Size p_memsz; 435 Elf64_Size p_align; 436} Elf64_Phdr; 437.Ed 438.Pp 439The main difference between the 32-bit and the 64-bit program header lies 440only in the location of a 441.Sy p_flags 442member in the total struct. 443.Pp 444.Bl -tag -width "p_offset" -compact -offset indent 445.It Dv p_type 446This member of the Phdr struct tells what kind of segment this array 447element describes or how to interpret the array element's information. 448.Bl -tag -width "PT_DYNAMIC" -compact 449.Pp 450.It Dv PT_NULL 451The array element is unused and the other members' values are undefined. 452This lets the program header have ignored entries. 453.It Dv PT_LOAD 454The array element specifies a loadable segment, described by 455.Sy p_filesz 456and 457.Sy p_memsz . 458The bytes from the file are mapped to the beginning of the memory 459segment. 460If the segment's memory size 461.Pq Sy p_memsz 462is larger than the file size 463.Pq Sy p_filesz , 464the 465.Dq extra 466bytes are defined to hold the value 0 and to follow the segment's 467initialized area. 468The file size may not be larger than the memory size. 469Loadable segment entries in the program header table appear in ascending 470order, sorted on the 471.Sy p_vaddr 472member. 473.It Dv PT_DYNAMIC 474The array element specifies dynamic linking information. 475.It Dv PT_INTERP 476The array element specifies the location and size of a null-terminated 477path name to invoke as an interpreter. 478This segment type is meaningful 479only for executable files (though it may occur for shared objects). 480However 481it may not occur more than once in a file. 482If it is present it must precede 483any loadable segment entry. 484.It Dv PT_NOTE 485The array element specifies the location and size for auxiliary information. 486.It Dv PT_SHLIB 487This segment type is reserved but has unspecified semantics. 488Programs that 489contain an array element of this type do not conform to the ABI. 490.It Dv PT_PHDR 491The array element, if present, specifies the location and size of the program 492header table itself, both in the file and in the memory image of the program. 493This segment type may not occur more than once in a file. 494Moreover, it may 495only occur if the program header table is part of the memory image of the 496program. 497If it is present it must precede any loadable segment entry. 498.It Dv PT_LOPROC 499This value up to and including 500.Sy PT_HIPROC 501are reserved for processor-specific semantics. 502.It Dv PT_HIPROC 503This value down to and including 504.Sy PT_LOPROC 505are reserved for processor-specific semantics. 506.El 507.Pp 508.It Dv p_offset 509This member holds the offset from the beginning of the file at which 510the first byte of the segment resides. 511.It Dv p_vaddr 512This member holds the virtual address at which the first byte of the 513segment resides in memory. 514.It Dv p_paddr 515On systems for which physical addressing is relevant, this member is 516reserved for the segment's physical address. 517Under 518.Bx 519this member is 520not used and must be zero. 521.It Dv p_filesz 522This member holds the number of bytes in the file image of the segment. 523It may be zero. 524.It Dv p_memsz 525This member holds the number of bytes in the memory image of the segment. 526It may be zero. 527.It Dv p_flags 528This member holds flags relevant to the segment: 529.Pp 530.Bl -tag -width "PF_X" -compact 531.It Dv PF_X 532An executable segment. 533.It Dv PF_W 534A writable segment. 535.It Dv PF_R 536A readable segment. 537.El 538.Pp 539A text segment commonly has the flags 540.Sy PF_X 541and 542.Sy PF_R . 543A data segment commonly has 544.Sy PF_X , 545.Sy PF_W 546and 547.Sy PF_R . 548.It Dv p_align 549This member holds the value to which the segments are aligned in memory 550and in the file. 551Loadable process segments must have congruent values for 552.Sy p_vaddr 553and 554.Sy p_offset , 555modulo the page size. 556Values of zero and one mean no alignment is required. 557Otherwise, 558.Sy p_align 559should be a positive, integral power of two, and 560.Sy p_vaddr 561should equal 562.Sy p_offset , 563modulo 564.Sy p_align . 565.El 566.Pp 567An file's section header table lets one locate all the file's sections. 568The 569section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 570The 571ELF header's 572.Sy e_shoff 573member gives the byte offset from the beginning of the file to the section 574header table. 575.Sy e_shnum 576holds the number of entries the section header table contains. 577.Sy e_shentsize 578holds the size in bytes of each entry. 579.Pp 580A section header table index is a subscript into this array. 581Some section 582header table indices are reserved. 583An object file does not have sections for 584these special indices: 585.Pp 586.Bl -tag -width "SHN_LORESERVE" -compact 587.It Dv SHN_UNDEF 588This value marks an undefined, missing, irrelevant, or otherwise meaningless 589section reference. 590For example, a symbol 591.Dq defined 592relative to section number 593.Sy SHN_UNDEF 594is an undefined symbol. 595.It Dv SHN_LORESERVE 596This value specifies the lower bound of the range of reserved indices. 597.It Dv SHN_LOPROC 598This value up to and including 599.Sy SHN_HIPROC 600are reserved for processor-specific semantics. 601.It Dv SHN_HIPROC 602This value down to and including 603.Sy SHN_LOPROC 604are reserved for processor-specific semantics. 605.It Dv SHN_ABS 606This value specifies absolute values for the corresponding reference. 607For 608example, symbols defined relative to section number 609.Sy SHN_ABS 610have absolute values and are not affected by relocation. 611.It Dv SHN_COMMON 612Symbols defined relative to this section are common symbols, such as FORTRAN 613COMMON or unallocated C external variables. 614.It Dv SHN_HIRESERVE 615This value specifies the upper bound of the range of reserved indices. 616The 617system reserves indices between 618.Sy SHN_LORESERVE 619and 620.Sy SHN_HIRESERVE , 621inclusive. 622The section header table does not contain entries for the 623reserved indices. 624.El 625.Pp 626The section header has the following structure: 627.Bd -literal -offset indent 628typedef struct { 629 Elf32_Word sh_name; 630 Elf32_Word sh_type; 631 Elf32_Word sh_flags; 632 Elf32_Addr sh_addr; 633 Elf32_Off sh_offset; 634 Elf32_Size sh_size; 635 Elf32_Word sh_link; 636 Elf32_Word sh_info; 637 Elf32_Size sh_addralign; 638 Elf32_Size sh_entsize; 639} Elf32_Shdr; 640.Ed 641.Pp 642.Bd -literal -offset indent 643typedef struct { 644 Elf64_Half sh_name; 645 Elf64_Half sh_type; 646 Elf64_Size sh_flags; 647 Elf64_Addr sh_addr; 648 Elf64_Off sh_offset; 649 Elf64_Size sh_size; 650 Elf64_Half sh_link; 651 Elf64_Half sh_info; 652 Elf64_Size sh_addralign; 653 Elf64_Size sh_entsize; 654} Elf64_Shdr; 655.Ed 656.Pp 657.Bl -tag -width "sh_addralign" -compact 658.It Dv sh_name 659This member specifies the name of the section. 660Its value is an index 661into the section header string table section, giving the location of 662a null-terminated string. 663.It Dv sh_type 664This member categorizes the section's contents and semantics. 665.Pp 666.Bl -tag -width "SHT_PROGBITS" -compact 667.It Dv SHT_NULL 668This value marks the section header as inactive. 669It does not 670have an associated section. 671Other members of the section header 672have undefined values. 673.It Dv SHT_PROGBITS 674The section holds information defined by the program, whose 675format and meaning are determined solely by the program. 676.It Dv SHT_SYMTAB 677This section holds a symbol table. 678Typically, 679.Sy SHT_SYMTAB 680provides symbols for link editing, though it may also be used 681for dynamic linking. 682As a complete symbol table, it may contain 683many symbols unnecessary for dynamic linking. 684An object file can 685also contain a 686.Sy SHN_DYNSYM 687section. 688.It Dv SHT_STRTAB 689This section holds a string table. 690An object file may have multiple 691string table sections. 692.It Dv SHT_RELA 693This section holds relocation entries with explicit addends, such 694as type 695.Sy Elf32_Rela 696for the 32-bit class of object files. 697An object may have multiple 698relocation sections. 699.It Dv SHT_HASH 700This section holds a symbol hash table. 701All object participating in 702dynamic linking must contain a symbol hash table. 703An object file may 704have only one hash table. 705.It Dv SHT_DYNAMIC 706This section holds information for dynamic linking. 707An object file may 708have only one dynamic section. 709.It Dv SHT_NOTE 710This section holds information that marks the file in some way. 711.It Dv SHT_NOBITS 712A section of this type occupies no space in the file but otherwise 713resembles 714.Sy SHN_PROGBITS . 715Although this section contains no bytes, the 716.Sy sh_offset 717member contains the conceptual file offset. 718.It Dv SHT_REL 719This section holds relocation offsets without explicit addends, such 720as type 721.Sy Elf32_Rel 722for the 32-bit class of object files. 723An object file may have multiple 724relocation sections. 725.It Dv SHT_SHLIB 726This section is reserved but has unspecified semantics. 727.It Dv SHT_DYNSYM 728This section holds a minimal set of dynamic linking symbols. 729An 730object file can also contain a 731.Sy SHN_SYMTAB 732section. 733.It Dv SHT_LOPROC 734This value up to and including 735.Sy SHT_HIPROC 736are reserved for processor-specific semantics. 737.It Dv SHT_HIPROC 738This value down to and including 739.Sy SHT_LOPROC 740are reserved for processor-specific semantics. 741.It Dv SHT_LOUSER 742This value specifies the lower bound of the range of indices reserved for 743application programs. 744.It Dv SHT_HIUSER 745This value specifies the upper bound of the range of indices reserved for 746application programs. 747Section types between 748.Sy SHT_LOUSER 749and 750.Sy SHT_HIUSER 751may be used by the application, without conflicting with current or future 752system-defined section types. 753.El 754.Pp 755.It Dv sh_flags 756Sections support one-bit flags that describe miscellaneous attributes. 757If a flag bit is set in 758.Sy sh_flags , 759the attribute is 760.Dq on 761for the section. 762Otherwise, the attribute is 763.Dq off 764or does not apply. 765Undefined attributes are set to zero. 766.Pp 767.Bl -tag -width "SHF_EXECINSTR" -compact 768.It Dv SHF_WRITE 769This section contains data that should be writable during process 770execution. 771.It Dv SHF_ALLOC 772The section occupies memory during process execution. 773Some control 774sections do not reside in the memory image of an object file. 775This 776attribute is off for those sections. 777.It Dv SHF_EXECINSTR 778The section contains executable machine instructions. 779.It Dv SHF_MASKPROC 780All bits included in this mask are reserved for processor-specific 781semantics. 782.El 783.Pp 784.It Dv sh_addr 785If the section will appear in the memory image of a process, this member 786holds the address at which the section's first byte should reside. 787Otherwise, the member contains zero. 788.It Dv sh_offset 789This member's value holds the byte offset from the beginning of the file 790to the first byte in the section. 791One section type, 792.Sy SHT_NOBITS , 793occupies no space in the file, and its 794.Sy sh_offset 795member locates the conceptual placement in the file. 796.It Dv sh_size 797This member holds the section's size in bytes. 798Unless the section type 799is 800.Sy SHT_NOBITS , 801the section occupies 802.Sy sh_size 803bytes in the file. 804A section of type 805.Sy SHT_NOBITS 806may have a non-zero size, but it occupies no space in the file. 807.It Dv sh_link 808This member holds a section header table index link, whose interpretation 809depends on the section type. 810.It Dv sh_info 811This member holds extra information, whose interpretation depends on the 812section type. 813.It Dv sh_addralign 814Some sections have address alignment constraints. 815If a section holds a 816doubleword, the system must ensure doubleword alignment for the entire 817section. 818That is, the value of 819.Sy sh_addr 820must be congruent to zero, modulo the value of 821.Sy sh_addralign . 822Only zero and positive integral powers of two are allowed. 823Values of zero 824or one mean the section has no alignment constraints. 825.It Dv sh_entsize 826Some sections hold a table of fixed-sized entries, such as a symbol table. 827For such a section, this member gives the size in bytes for each entry. 828This member contains zero if the section does not hold a table of 829fixed-size entries. 830.El 831.Pp 832Various sections hold program and control information: 833.Bl -tag -width ".shstrtab" -compact 834.It .bss 835Block Started by Symbol - 836This section holds uninitialized data that contributes to the program's 837memory image. 838By definition, the system initializes the data with zeros 839when the program begins to run. 840This section is of type 841.Sy SHT_NOBITS . 842The attributes types are 843.Sy SHF_ALLOC 844and 845.Sy SHF_WRITE . 846.It .comment 847This section holds version control information. 848This section is of type 849.Sy SHT_PROGBITS . 850No attribute types are used. 851.It .data 852This section holds initialized data that contribute to the program's 853memory image. 854This section is of type 855.Sy SHT_PROGBITS . 856The attribute types are 857.Sy SHF_ALLOC 858and 859.Sy SHF_WRITE . 860.It .data1 861This section holds initialized data that contribute to the program's 862memory image. 863This section is of type 864.Sy SHT_PROGBITS . 865The attribute types are 866.Sy SHF_ALLOC 867and 868.Sy SHF_WRITE . 869.It .debug 870This section holds information for symbolic debugging. 871The contents 872are unspecified. 873This section is of type 874.Sy SHT_PROGBITS . 875No attribute types are used. 876.It .dynamic 877This section holds dynamic linking information. 878The section's attributes 879will include the 880.Sy SHF_ALLOC 881bit. 882Whether the 883.Sy SHF_WRITE 884bit is set is processor-specific. 885This section is of type 886.Sy SHT_DYNAMIC . 887See the attributes above. 888.It .dynstr 889This section holds strings needed for dynamic linking, most commonly 890the strings that represent the names associated with symbol table entries. 891This section is of type 892.Sy SHT_STRTAB . 893The attribute type used is 894.Sy SHF_ALLOC . 895.It .dynsym 896This section holds the dynamic linking symbol table. 897This section is of type 898.Sy SHT_DYNSYM . 899The attribute used is 900.Sy SHF_ALLOC . 901.It .fini 902This section holds executable instructions that contribute to the process 903termination code. 904When a program exits normally the system arranges to 905execute the code in this section. 906This section is of type 907.Sy SHT_PROGBITS . 908The attributes used are 909.Sy SHF_ALLOC 910and 911.Sy SHF_EXECINSTR . 912.It .got 913This section holds the global offset table. 914This section is of type 915.Sy SHT_PROGBITS . 916The attributes are processor-specific. 917.It .hash 918This section holds a symbol hash table. 919This section is of type 920.Sy SHT_HASH . 921The attribute used is 922.Sy SHF_ALLOC . 923.It .init 924This section holds executable instructions that contribute to the process 925initialization code. 926When a program starts to run the system arranges to 927execute the code in this section before calling the main program entry point. 928This section is of type 929.Sy SHT_PROGBITS . 930The attributes used are 931.Sy SHF_ALLOC 932and 933.Sy SHF_EXECINSTR . 934.It .interp 935This section holds the pathname of a program interpreter. 936If the file has 937a loadable segment that includes the section, the section's attributes will 938include the 939.Sy SHF_ALLOC 940bit. 941Otherwise, that bit will be off. 942This section is of type 943.Sy SHT_PROGBITS . 944.It .line 945This section holds line number information for symbolic debugging, which 946describes the correspondence between the program source and the machine code. 947The contents are unspecified. 948This section is of type 949.Sy SHT_PROGBITS . 950No attribute types are used. 951.It .note 952This section holds information in the 953.Dq Note Section 954format described below. 955This section is of type 956.Sy SHT_NOTE . 957No attribute types are used. 958.It .plt 959This section holds the procedure linkage table. 960This section is of type 961.Sy SHT_PROGBITS . 962The attributes are processor-specific. 963.It .relNAME 964This section holds relocation information as described below. 965If the file 966has a loadable segment that includes relocation, the section's attributes 967will include the 968.Sy SHF_ALLOC 969bit. 970Otherwise the bit will be off. 971By convention, 972.Dq NAME 973is supplied by the section to which the relocations apply. 974Thus a relocation 975section for 976.Sy .text 977normally would have the name 978.Sy .rel.text . 979This section is of type 980.Sy SHT_REL . 981.It .relaNAME 982This section holds relocation information as described below. 983If the file 984has a loadable segment that includes relocation, the section's attributes 985will include the 986.Sy SHF_ALLOC 987bit. 988Otherwise the bit will be off. 989By convention, 990.Dq NAME 991is supplied by the section to which the relocations apply. 992Thus a relocation 993section for 994.Sy .text 995normally would have the name 996.Sy .rela.text . 997This section is of type 998.Sy SHT_RELA . 999.It .rodata 1000This section holds read-only data that typically contributes to a 1001non-writable segment in the process image. 1002This section is of type 1003.Sy SHT_PROGBITS . 1004The attribute used is 1005.Sy SHF_ALLOC . 1006.It .rodata1 1007This section hold read-only data that typically contributes to a 1008non-writable segment in the process image. 1009This section is of type 1010.Sy SHT_PROGBITS . 1011The attribute used is 1012.Sy SHF_ALLOC . 1013.It .shstrtab 1014This section holds section names. 1015This section is of type 1016.Sy SHT_STRTAB . 1017No attribute types are used. 1018.It .strtab 1019This section holds strings, most commonly the strings that represent the 1020names associated with symbol table entries. 1021If the file has a loadable 1022segment that includes the symbol string table, the section's attributes 1023will include the 1024.Sy SHF_ALLOC 1025bit. 1026Otherwise the bit will be off. 1027This section is of type 1028.Sy SHT_STRTAB . 1029.It .symtab 1030This section holds a symbol table. 1031If the file has a loadable segment 1032that includes the symbol table, the section's attributes will include 1033the 1034.Sy SHF_ALLOC 1035bit. 1036Otherwise the bit will be off. 1037This section is of type 1038.Sy SHT_SYMTAB . 1039.It .text 1040This section holds the 1041.Dq text , 1042or executable instructions, of a program. 1043This section is of type 1044.Sy SHT_PROGBITS . 1045The attributes used are 1046.Sy SHF_ALLOC 1047and 1048.Sy SHF_EXECINSTR . 1049.It .jcr 1050This section holds information about Java classes that must 1051be registered. 1052.It .eh_frame 1053This section holds information used for C++ exception-handling. 1054.El 1055.Pp 1056String table sections hold null-terminated character sequences, commonly 1057called strings. 1058The object file uses these strings to represent symbol 1059and section names. 1060One references a string as an index into the string 1061table section. 1062The first byte, which is index zero, is defined to hold 1063a null character. 1064Similarly, a string table's last byte is defined to 1065hold a null character, ensuring null termination for all strings. 1066.Pp 1067An object file's symbol table holds information needed to locate and 1068relocate a program's symbolic definitions and references. 1069A symbol table 1070index is a subscript into this array. 1071.Pp 1072.Bd -literal -offset indent 1073typedef struct { 1074 Elf32_Word st_name; 1075 Elf32_Addr st_value; 1076 Elf32_Size st_size; 1077 unsigned char st_info; 1078 unsigned char st_other; 1079 Elf32_Half st_shndx; 1080} Elf32_Sym; 1081.Ed 1082.Pp 1083.Bd -literal -offset indent 1084typedef struct { 1085 Elf64_Half st_name; 1086 unsigned char st_info; 1087 unsigned char st_other; 1088 Elf64_Quarter st_shndx; 1089 Elf64_Addr st_value; 1090 Elf64_Size st_size; 1091} Elf64_Sym; 1092.Ed 1093.Pp 1094.Bl -tag -width "st_value" -compact 1095.It Dv st_name 1096This member holds an index into the object file's symbol string table, 1097which holds character representations of the symbol names. 1098If the value 1099is non-zero, it represents a string table index that gives the symbol 1100name. 1101Otherwise, the symbol table has no name. 1102.It Dv st_value 1103This member gives the value of the associated symbol. 1104.It Dv st_size 1105Many symbols have associated sizes. 1106This member holds zero if the symbol 1107has no size or an unknown size. 1108.It Dv st_info 1109This member specifies the symbol's type and binding attributes: 1110.Pp 1111.Bl -tag -width "STT_SECTION" -compact 1112.It Dv STT_NOTYPE 1113The symbol's type is not defined. 1114.It Dv STT_OBJECT 1115The symbol is associated with a data object. 1116.It Dv STT_FUNC 1117The symbol is associated with a function or other executable code. 1118.It Dv STT_SECTION 1119The symbol is associated with a section. 1120Symbol table entries of 1121this type exist primarily for relocation and normally have 1122.Sy STB_LOCAL 1123bindings. 1124.It Dv STT_FILE 1125By convention the symbol's name gives the name of the source file 1126associated with the object file. 1127A file symbol has 1128.Sy STB_LOCAL 1129bindings, its section index is 1130.Sy SHN_ABS , 1131and it precedes the other 1132.Sy STB_LOCAL 1133symbols of the file, if it is present. 1134.It Dv STT_LOPROC 1135This value up to and including 1136.Sy STT_HIPROC 1137are reserved for processor-specific semantics. 1138.It Dv STT_HIPROC 1139This value down to and including 1140.Sy STT_LOPROC 1141are reserved for processor-specific semantics. 1142.El 1143.Pp 1144.Bl -tag -width "STB_GLOBAL" -compact 1145.It Dv STB_LOCAL 1146Local symbols are not visible outside the object file containing their 1147definition. 1148Local symbols of the same name may exist in multiple file 1149without interfering with each other. 1150.It Dv STB_GLOBAL 1151Global symbols are visible to all object files being combined. 1152One file's 1153definition of a global symbol will satisfy another file's undefined 1154reference to the same symbol. 1155.It Dv STB_WEAK 1156Weak symbols resemble global symbols, but their definitions have lower 1157precedence. 1158.It Dv STB_LOPROC 1159This value up to and including 1160.Sy STB_HIPROC 1161are reserved for processor-specific semantics. 1162.It Dv STB_HIPROC 1163This value down to and including 1164.Sy STB_LOPROC 1165are reserved for processor-specific semantics. 1166.Pp 1167There are macros for packing and unpacking the binding and type fields: 1168.Pp 1169.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1170.It Xo 1171.Fn ELF32_ST_BIND info 1172.Xc 1173or 1174.Fn ELF64_ST_BIND info 1175extract a binding from an st_info value. 1176.It Xo 1177.Fn ELF64_ST_TYPE info 1178.Xc 1179or 1180.Fn ELF32_ST_TYPE info 1181extract a type from an st_info value. 1182.It Xo 1183.Fn ELF32_ST_INFO bind type 1184.Xc 1185or 1186.Fn ELF64_ST_INFO bind type 1187convert a binding and a type into an st_info value. 1188.El 1189.El 1190.Pp 1191.It Dv st_other 1192This member currently holds zero and has no defined meaning. 1193.It Dv st_shndx 1194Every symbol table entry is 1195.Dq defined 1196in relation to some section. 1197This member holds the relevant section 1198header table index. 1199.El 1200.Pp 1201Relocation is the process of connecting symbolic references with 1202symbolic definitions. 1203Relocatable files must have information that 1204describes how to modify their section contents, thus allowing executable 1205and shared object files to hold the right information for a process' 1206program image. 1207Relocation entries are these data. 1208.Pp 1209Relocation structures that do not need an addend: 1210.Pp 1211.Bd -literal -offset indent 1212typedef struct { 1213 Elf32_Addr r_offset; 1214 Elf32_Word r_info; 1215} Elf32_Rel; 1216.Ed 1217.Bd -literal -offset indent 1218typedef struct { 1219 Elf64_Addr r_offset; 1220 Elf64_Size r_info; 1221} Elf64_Rel; 1222.Ed 1223.Pp 1224Relocation structures that need an addend: 1225.Pp 1226.Bd -literal -offset indent 1227typedef struct { 1228 Elf32_Addr r_offset; 1229 Elf32_Word r_info; 1230 Elf32_Sword r_addend; 1231} Elf32_Rela; 1232.Ed 1233.Bd -literal -offset indent 1234typedef struct { 1235 Elf64_Addr r_offset; 1236 Elf64_Size r_info; 1237 Elf64_Off r_addend; 1238} Elf64_Rela; 1239.Ed 1240.Pp 1241.Bl -tag -width "r_offset" -compact 1242.It Dv r_offset 1243This member gives the location at which to apply the relocation action. 1244For a relocatable file, the value is the byte offset from the beginning 1245of the section to the storage unit affected by the relocation. 1246For an 1247executable file or shared object, the value is the virtual address of 1248the storage unit affected by the relocation. 1249.It Dv r_info 1250This member gives both the symbol table index with respect to which the 1251relocation must be made and the type of relocation to apply. 1252Relocation 1253types are processor-specific. 1254When the text refers to a relocation 1255entry's relocation type or symbol table index, it means the result of 1256applying 1257.Sy ELF_[32|64]_R_TYPE 1258or 1259.Sy ELF[32|64]_R_SYM , 1260respectively to the entry's 1261.Sy r_info 1262member. 1263.It Dv r_addend 1264This member specifies a constant addend used to compute the value to be 1265stored into the relocatable field. 1266.El 1267.Sh SEE ALSO 1268.Xr as 1 , 1269.Xr gdb 1 , 1270.Xr ld 1 , 1271.Xr objdump 1 , 1272.Xr execve 2 , 1273.Xr core 5 1274.Rs 1275.%A Hewlett Packard 1276.%B Elf-64 Object File Format 1277.Re 1278.Rs 1279.%A Santa Cruz Operation 1280.%B System V Application Binary Interface 1281.Re 1282.Rs 1283.%A Unix System Laboratories 1284.%T Object Files 1285.%B "Executable and Linking Format (ELF)" 1286.Re 1287.Sh HISTORY 1288The ELF header files made their appearance in 1289.Fx 2.2.6 . 1290ELF in itself first appeared in 1291.At V . 1292The ELF format is an adopted standard. 1293.Sh AUTHORS 1294This manual page was written by 1295.An Jeroen Ruigrok van der Werven 1296.Aq asmodai@FreeBSD.org 1297with inspiration from BSDi's 1298.Bsx 1299.Xr elf 5 1300manpage. 1301