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