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.El 395.Pp 396An executable or shared object file's program header table is an array of 397structures, each describing a segment or other information the system needs 398to prepare the program for execution. 399An object file 400.Em segment 401contains one or more 402.Em sections . 403Program headers are meaningful only for executable and shared object files. 404A file specifies its own program header size with the ELF header's 405.Sy e_phentsize 406and 407.Sy e_phnum 408members. 409As with the Elf executable header, the program header 410also has different versions depending on the architecture: 411.Pp 412.Bd -literal -offset indent 413typedef struct { 414 Elf32_Word p_type; 415 Elf32_Off p_offset; 416 Elf32_Addr p_vaddr; 417 Elf32_Addr p_paddr; 418 Elf32_Size p_filesz; 419 Elf32_Size p_memsz; 420 Elf32_Word p_flags; 421 Elf32_Size p_align; 422} Elf32_Phdr; 423.Ed 424.Pp 425.Bd -literal -offset indent 426typedef struct { 427 Elf64_Half p_type; 428 Elf64_Half p_flags; 429 Elf64_Off p_offset; 430 Elf64_Addr p_vaddr; 431 Elf64_Addr p_paddr; 432 Elf64_Size p_filesz; 433 Elf64_Size p_memsz; 434 Elf64_Size p_align; 435} Elf64_Phdr; 436.Ed 437.Pp 438The main difference between the 32-bit and the 64-bit program header lies 439only in the location of a 440.Sy p_flags 441member in the total struct. 442.Pp 443.Bl -tag -width "p_offset" -compact -offset indent 444.It Dv p_type 445This member of the Phdr struct tells what kind of segment this array 446element describes or how to interpret the array element's information. 447.Bl -tag -width "PT_DYNAMIC" -compact 448.Pp 449.It Dv PT_NULL 450The array element is unused and the other members' values are undefined. 451This lets the program header have ignored entries. 452.It Dv PT_LOAD 453The array element specifies a loadable segment, described by 454.Sy p_filesz 455and 456.Sy p_memsz . 457The bytes from the file are mapped to the beginning of the memory 458segment. 459If the segment's memory size 460.Pq Sy p_memsz 461is larger than the file size 462.Pq Sy p_filesz , 463the 464.Dq extra 465bytes are defined to hold the value 0 and to follow the segment's 466initialized area. 467The file size may not be larger than the memory size. 468Loadable segment entries in the program header table appear in ascending 469order, sorted on the 470.Sy p_vaddr 471member. 472.It Dv PT_DYNAMIC 473The array element specifies dynamic linking information. 474.It Dv PT_INTERP 475The array element specifies the location and size of a null-terminated 476path name to invoke as an interpreter. 477This segment type is meaningful 478only for executable files (though it may occur for shared objects). However 479it may not occur more than once in a file. 480If it is present it must precede 481any loadable segment entry. 482.It Dv PT_NOTE 483The array element specifies the location and size for auxiliary information. 484.It Dv PT_SHLIB 485This segment type is reserved but has unspecified semantics. 486Programs that 487contain an array element of this type do not conform to the ABI. 488.It Dv PT_PHDR 489The array element, if present, specifies the location and size of the program 490header table itself, both in the file and in the memory image of the program. 491This segment type may not occur more than once in a file. 492Moreover, it may 493only occur if the program header table is part of the memory image of the 494program. 495If it is present it must precede any loadable segment entry. 496.It Dv PT_LOPROC 497This value up to and including 498.Sy PT_HIPROC 499are reserved for processor-specific semantics. 500.It Dv PT_HIPROC 501This value down to and including 502.Sy PT_LOPROC 503are reserved for processor-specific semantics. 504.El 505.Pp 506.It Dv p_offset 507This member holds the offset from the beginning of the file at which 508the first byte of the segment resides. 509.It Dv p_vaddr 510This member holds the virtual address at which the first byte of the 511segment resides in memory. 512.It Dv p_paddr 513On systems for which physical addressing is relevant, this member is 514reserved for the segment's physical address. 515Under 516.Bx 517this member is 518not used and must be zero. 519.It Dv p_filesz 520This member holds the number of bytes in the file image of the segment. 521It may be zero. 522.It Dv p_memsz 523This member holds the number of bytes in the memory image of the segment. 524It may be zero. 525.It Dv p_flags 526This member holds flags relevant to the segment: 527.Pp 528.Bl -tag -width "PF_X" -compact 529.It Dv PF_X 530An executable segment. 531.It Dv PF_W 532A writable segment. 533.It Dv PF_R 534A readable segment. 535.El 536.Pp 537A text segment commonly has the flags 538.Sy PF_X 539and 540.Sy PF_R . 541A data segment commonly has 542.Sy PF_X , 543.Sy PF_W 544and 545.Sy PF_R . 546.It Dv p_align 547This member holds the value to which the segments are aligned in memory 548and in the file. 549Loadable process segments must have congruent values for 550.Sy p_vaddr 551and 552.Sy p_offset , 553modulo the page size. 554Values of zero and one mean no alignment is required. 555Otherwise, 556.Sy p_align 557should be a positive, integral power of two, and 558.Sy p_vaddr 559should equal 560.Sy p_offset , 561modulo 562.Sy p_align . 563.El 564.Pp 565An file's section header table lets one locate all the file's sections. 566The 567section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 568The 569ELF header's 570.Sy e_shoff 571member gives the byte offset from the beginning of the file to the section 572header table. 573.Sy e_shnum 574holds the number of entries the section header table contains. 575.Sy e_shentsize 576holds the size in bytes of each entry. 577.Pp 578A section header table index is a subscript into this array. 579Some section 580header table indices are reserved. 581An object file does not have sections for 582these special indices: 583.Pp 584.Bl -tag -width "SHN_LORESERVE" -compact 585.It Dv SHN_UNDEF 586This value marks an undefined, missing, irrelevant, or otherwise meaningless 587section reference. 588For example, a symbol 589.Dq defined 590relative to section number 591.Sy SHN_UNDEF 592is an undefined symbol. 593.It Dv SHN_LORESERVE 594This value specifies the lower bound of the range of reserved indices. 595.It Dv SHN_LOPROC 596This value up to and including 597.Sy SHN_HIPROC 598are reserved for processor-specific semantics. 599.It Dv SHN_HIPROC 600This value down to and including 601.Sy SHN_LOPROC 602are reserved for processor-specific semantics. 603.It Dv SHN_ABS 604This value specifies absolute values for the corresponding reference. 605For 606example, symbols defined relative to section number 607.Sy SHN_ABS 608have absolute values and are not affected by relocation. 609.It Dv SHN_COMMON 610Symbols defined relative to this section are common symbols, such as FORTRAN 611COMMON or unallocated C external variables. 612.It Dv SHN_HIRESERVE 613This value specifies the upper bound of the range of reserved indices. 614The 615system reserves indices between 616.Sy SHN_LORESERVE 617and 618.Sy SHN_HIRESERVE , 619inclusive. 620The section header table does not contain entries for the 621reserved indices. 622.El 623.Pp 624The section header has the following structure: 625.Bd -literal -offset indent 626typedef struct { 627 Elf32_Word sh_name; 628 Elf32_Word sh_type; 629 Elf32_Word sh_flags; 630 Elf32_Addr sh_addr; 631 Elf32_Off sh_offset; 632 Elf32_Size sh_size; 633 Elf32_Word sh_link; 634 Elf32_Word sh_info; 635 Elf32_Size sh_addralign; 636 Elf32_Size sh_entsize; 637} Elf32_Shdr; 638.Ed 639.Pp 640.Bd -literal -offset indent 641typedef struct { 642 Elf64_Half sh_name; 643 Elf64_Half sh_type; 644 Elf64_Size sh_flags; 645 Elf64_Addr sh_addr; 646 Elf64_Off sh_offset; 647 Elf64_Size sh_size; 648 Elf64_Half sh_link; 649 Elf64_Half sh_info; 650 Elf64_Size sh_addralign; 651 Elf64_Size sh_entsize; 652} Elf64_Shdr; 653.Ed 654.Pp 655.Bl -tag -width "sh_addralign" -compact 656.It Dv sh_name 657This member specifies the name of the section. 658Its value is an index 659into the section header string table section, giving the location of 660a null-terminated string. 661.It Dv sh_type 662This member categorizes the section's contents and semantics. 663.Pp 664.Bl -tag -width "SHT_PROGBITS" -compact 665.It Dv SHT_NULL 666This value marks the section header as inactive. 667It does not 668have an associated section. 669Other members of the section header 670have undefined values. 671.It Dv SHT_PROGBITS 672The section holds information defined by the program, whose 673format and meaning are determined solely by the program. 674.It Dv SHT_SYMTAB 675This section holds a symbol table. 676Typically, 677.Sy SHT_SYMTAB 678provides symbols for link editing, though it may also be used 679for dynamic linking. 680As a complete symbol table, it may contain 681many symbols unnecessary for dynamic linking. 682An object file can 683also contain a 684.Sy SHN_DYNSYM 685section. 686.It Dv SHT_STRTAB 687This section holds a string table. 688An object file may have multiple 689string table sections. 690.It Dv SHT_RELA 691This section holds relocation entries with explicit addends, such 692as type 693.Sy Elf32_Rela 694for the 32-bit class of object files. 695An object may have multiple 696relocation sections. 697.It Dv SHT_HASH 698This section holds a symbol hash table. 699All object participating in 700dynamic linking must contain a symbol hash table. 701An object file may 702have only one hash table. 703.It Dv SHT_DYNAMIC 704This section holds information for dynamic linking. 705An object file may 706have only one dynamic section. 707.It Dv SHT_NOTE 708This section holds information that marks the file in some way. 709.It Dv SHT_NOBITS 710A section of this type occupies no space in the file but otherwise 711resembles 712.Sy SHN_PROGBITS . 713Although this section contains no bytes, the 714.Sy sh_offset 715member contains the conceptual file offset. 716.It Dv SHT_REL 717This section holds relocation offsets without explicit addends, such 718as type 719.Sy Elf32_Rel 720for the 32-bit class of object files. 721An object file may have multiple 722relocation sections. 723.It Dv SHT_SHLIB 724This section is reserved but has unspecified semantics. 725.It Dv SHT_DYNSYM 726This section holds a minimal set of dynamic linking symbols. 727An 728object file can also contain a 729.Sy SHN_SYMTAB 730section. 731.It Dv SHT_LOPROC 732This value up to and including 733.Sy SHT_HIPROC 734are reserved for processor-specific semantics. 735.It Dv SHT_HIPROC 736This value down to and including 737.Sy SHT_LOPROC 738are reserved for processor-specific semantics. 739.It Dv SHT_LOUSER 740This value specifies the lower bound of the range of indices reserved for 741application programs. 742.It Dv SHT_HIUSER 743This value specifies the upper bound of the range of indices reserved for 744application programs. 745Section types between 746.Sy SHT_LOUSER 747and 748.Sy SHT_HIUSER 749may be used by the application, without conflicting with current or future 750system-defined section types. 751.El 752.Pp 753.It Dv sh_flags 754Sections support one-bit flags that describe miscellaneous attributes. 755If a flag bit is set in 756.Sy sh_flags , 757the attribute is 758.Dq on 759for the section. 760Otherwise, the attribute is 761.Dq off 762or does not apply. 763Undefined attributes are set to zero. 764.Pp 765.Bl -tag -width "SHF_EXECINSTR" -compact 766.It Dv SHF_WRITE 767This section contains data that should be writable during process 768execution. 769.It Dv SHF_ALLOC 770The section occupies memory during process execution. 771Some control 772sections do not reside in the memory image of an object file. 773This 774attribute is off for those sections. 775.It Dv SHF_EXECINSTR 776The section contains executable machine instructions. 777.It Dv SHF_MASKPROC 778All bits included in this mask are reserved for processor-specific 779semantics. 780.El 781.Pp 782.It Dv sh_addr 783If the section will appear in the memory image of a process, this member 784holds the address at which the section's first byte should reside. 785Otherwise, the member contains zero. 786.It Dv sh_offset 787This member's value holds the byte offset from the beginning of the file 788to the first byte in the section. 789One section type, 790.Sy SHT_NOBITS , 791occupies no space in the file, and its 792.Sy sh_offset 793member locates the conceptual placement in the file. 794.It Dv sh_size 795This member holds the section's size in bytes. 796Unless the section type 797is 798.Sy SHT_NOBITS , 799the section occupies 800.Sy sh_size 801bytes in the file. 802A section of type 803.Sy SHT_NOBITS 804may have a non-zero size, but it occupies no space in the file. 805.It Dv sh_link 806This member holds a section header table index link, whose interpretation 807depends on the section type. 808.It Dv sh_info 809This member holds extra information, whose interpretation depends on the 810section type. 811.It Dv sh_addralign 812Some sections have address alignment constraints. 813If a section holds a 814doubleword, the system must ensure doubleword alignment for the entire 815section. 816That is, the value of 817.Sy sh_addr 818must be congruent to zero, modulo the value of 819.Sy sh_addralign . 820Only zero and positive integral powers of two are allowed. 821Values of zero 822or one mean the section has no alignment constraints. 823.It Dv sh_entsize 824Some sections hold a table of fixed-sized entries, such as a symbol table. 825For such a section, this member gives the size in bytes for each entry. 826This member contains zero if the section does not hold a table of 827fixed-size entries. 828.El 829.Pp 830Various sections hold program and control information: 831.Bl -tag -width ".shstrtab" -compact 832.It .bss 833This section holds uninitialized data that contributes to the program's 834memory image. 835By definition, the system initializes the data with zeros 836when the program begins to run. 837This section is of type 838.Sy SHT_NOBITS . 839The attributes types are 840.Sy SHF_ALLOC 841and 842.Sy SHF_WRITE . 843.It .comment 844This section holds version control information. 845This section is of type 846.Sy SHT_PROGBITS . 847No attribute types are used. 848.It .data 849This section holds initialized data that contribute to the program's 850memory image. 851This section is of type 852.Sy SHT_PROGBITS . 853The attribute types are 854.Sy SHF_ALLOC 855and 856.Sy SHF_WRITE . 857.It .data1 858This section holds initialized data that contribute to the program's 859memory image. 860This section is of type 861.Sy SHT_PROGBITS . 862The attribute types are 863.Sy SHF_ALLOC 864and 865.Sy SHF_WRITE . 866.It .debug 867This section holds information for symbolic debugging. 868The contents 869are unspecified. 870This section is of type 871.Sy SHT_PROGBITS . 872No attribute types are used. 873.It .dynamic 874This section holds dynamic linking information. 875The section's attributes 876will include the 877.Sy SHF_ALLOC 878bit. 879Whether the 880.Sy SHF_WRITE 881bit is set is processor-specific. 882This section is of type 883.Sy SHT_DYNAMIC . 884See the attributes above. 885.It .dynstr 886This section holds strings needed for dynamic linking, most commonly 887the strings that represent the names associated with symbol table entries. 888This section is of type 889.Sy SHT_STRTAB . 890The attribute type used is 891.Sy SHF_ALLOC . 892.It .dynsym 893This section holds the dynamic linking symbol table. 894This section is of type 895.Sy SHT_DYNSYM . 896The attribute used is 897.Sy SHF_ALLOC . 898.It .fini 899This section holds executable instructions that contribute to the process 900termination code. 901When a program exits normally the system arranges to 902execute the code in this section. 903This section is of type 904.Sy SHT_PROGBITS . 905The attributes used are 906.Sy SHF_ALLOC 907and 908.Sy SHF_EXECINSTR . 909.It .got 910This section holds the global offset table. 911This section is of type 912.Sy SHT_PROGBITS . 913The attributes are processor-specific. 914.It .hash 915This section holds a symbol hash table. 916This section is of type 917.Sy SHT_HASH . 918The attribute used is 919.Sy SHF_ALLOC . 920.It .init 921This section holds executable instructions that contribute to the process 922initialization code. 923When a program starts to run the system arranges to 924execute the code in this section before calling the main program entry point. 925This section is of type 926.Sy SHT_PROGBITS . 927The attributes used are 928.Sy SHF_ALLOC 929and 930.Sy SHF_EXECINSTR . 931.It .interp 932This section holds the pathname of a program interpreter. 933If the file has 934a loadable segment that includes the section, the section's attributes will 935include the 936.Sy SHF_ALLOC 937bit. 938Otherwise, that bit will be off. 939This section is of type 940.Sy SHT_PROGBITS . 941.It .line 942This section holds line number information for symbolic debugging, which 943describes the correspondence between the program source and the machine code. 944The contents are unspecified. 945This section is of type 946.Sy SHT_PROGBITS . 947No attribute types are used. 948.It .note 949This section holds information in the 950.Dq Note Section 951format described below. 952This section is of type 953.Sy SHT_NOTE . 954No attribute types are used. 955.It .plt 956This section holds the procedure linkage table. 957This section is of type 958.Sy SHT_PROGBITS . 959The attributes are processor-specific. 960.It .relNAME 961This section holds relocation information as described below. 962If the file 963has a loadable segment that includes relocation, the section's attributes 964will include the 965.Sy SHF_ALLOC 966bit. 967Otherwise the bit will be off. 968By convention, 969.Dq NAME 970is supplied by the section to which the relocations apply. 971Thus a relocation 972section for 973.Sy .text 974normally would have the name 975.Sy .rel.text . 976This section is of type 977.Sy SHT_REL . 978.It .relaNAME 979This section holds relocation information as described below. 980If the file 981has a loadable segment that includes relocation, the section's attributes 982will include the 983.Sy SHF_ALLOC 984bit. 985Otherwise the bit will be off. 986By convention, 987.Dq NAME 988is supplied by the section to which the relocations apply. 989Thus a relocation 990section for 991.Sy .text 992normally would have the name 993.Sy .rela.text . 994This section is of type 995.Sy SHT_RELA . 996.It .rodata 997This section holds read-only data that typically contributes to a 998non-writable segment in the process image. 999This section is of type 1000.Sy SHT_PROGBITS . 1001The attribute used is 1002.Sy SHF_ALLOC . 1003.It .rodata1 1004This section hold read-only data that typically contributes to a 1005non-writable segment in the process image. 1006This section is of type 1007.Sy SHT_PROGBITS . 1008The attribute used is 1009.Sy SHF_ALLOC . 1010.It .shstrtab 1011This section holds section names. 1012This section is of type 1013.Sy SHT_STRTAB . 1014No attribute types are used. 1015.It .strtab 1016This section holds strings, most commonly the strings that represent the 1017names associated with symbol table entries. 1018If the file has a loadable 1019segment that includes the symbol string table, the section's attributes 1020will include the 1021.Sy SHF_ALLOC 1022bit. 1023Otherwise the bit will be off. 1024This section is of type 1025.Sy SHT_STRTAB . 1026.It .symtab 1027This section holds a symbol table. 1028If the file has a loadable segment 1029that includes the symbol table, the section's attributes will include 1030the 1031.Sy SHF_ALLOC 1032bit. 1033Otherwise the bit will be off. 1034This section is of type 1035.Sy SHT_SYMTAB . 1036.It .text 1037This section holds the 1038.Dq text , 1039or executable instructions, of a program. 1040This section is of type 1041.Sy SHT_PROGBITS . 1042The attributes used are 1043.Sy SHF_ALLOC 1044and 1045.Sy SHF_EXECINSTR . 1046.It .jcr 1047This section holds information about Java classes that must 1048be registered. 1049.It .eh_frame 1050This section holds information used for C++ exception-handling. 1051.El 1052.Pp 1053String table sections hold null-terminated character sequences, commonly 1054called strings. 1055The object file uses these strings to represent symbol 1056and section names. 1057One references a string as an index into the string 1058table section. 1059The first byte, which is index zero, is defined to hold 1060a null character. 1061Similarly, a string table's last byte is defined to 1062hold a null character, ensuring null termination for all strings. 1063.Pp 1064An object file's symbol table holds information needed to locate and 1065relocate a program's symbolic definitions and references. 1066A symbol table 1067index is a subscript into this array. 1068.Pp 1069.Bd -literal -offset indent 1070typedef struct { 1071 Elf32_Word st_name; 1072 Elf32_Addr st_value; 1073 Elf32_Size st_size; 1074 unsigned char st_info; 1075 unsigned char st_other; 1076 Elf32_Half st_shndx; 1077} Elf32_Sym; 1078.Ed 1079.Pp 1080.Bd -literal -offset indent 1081typedef struct { 1082 Elf64_Half st_name; 1083 unsigned char st_info; 1084 unsigned char st_other; 1085 Elf64_Quarter st_shndx; 1086 Elf64_Addr st_value; 1087 Elf64_Size st_size; 1088} Elf64_Sym; 1089.Ed 1090.Pp 1091.Bl -tag -width "st_value" -compact 1092.It Dv st_name 1093This member holds an index into the object file's symbol string table, 1094which holds character representations of the symbol names. 1095If the value 1096is non-zero, it represents a string table index that gives the symbol 1097name. 1098Otherwise, the symbol table has no name. 1099.It Dv st_value 1100This member gives the value of the associated symbol. 1101.It Dv st_size 1102Many symbols have associated sizes. 1103This member holds zero if the symbol 1104has no size or an unknown size. 1105.It Dv st_info 1106This member specifies the symbol's type and binding attributes: 1107.Pp 1108.Bl -tag -width "STT_SECTION" -compact 1109.It Dv STT_NOTYPE 1110The symbol's type is not defined. 1111.It Dv STT_OBJECT 1112The symbol is associated with a data object. 1113.It Dv STT_FUNC 1114The symbol is associated with a function or other executable code. 1115.It Dv STT_SECTION 1116The symbol is associated with a section. 1117Symbol table entries of 1118this type exist primarily for relocation and normally have 1119.Sy STB_LOCAL 1120bindings. 1121.It Dv STT_FILE 1122By convention the symbol's name gives the name of the source file 1123associated with the object file. 1124A file symbol has 1125.Sy STB_LOCAL 1126bindings, its section index is 1127.Sy SHN_ABS , 1128and it precedes the other 1129.Sy STB_LOCAL 1130symbols of the file, if it is present. 1131.It Dv STT_LOPROC 1132This value up to and including 1133.Sy STT_HIPROC 1134are reserved for processor-specific semantics. 1135.It Dv STT_HIPROC 1136This value down to and including 1137.Sy STT_LOPROC 1138are reserved for processor-specific semantics. 1139.El 1140.Pp 1141.Bl -tag -width "STB_GLOBAL" -compact 1142.It Dv STB_LOCAL 1143Local symbols are not visible outside the object file containing their 1144definition. 1145Local symbols of the same name may exist in multiple file 1146without interfering with each other. 1147.It Dv STB_GLOBAL 1148Global symbols are visible to all object files being combined. 1149One file's 1150definition of a global symbol will satisfy another file's undefined 1151reference to the same symbol. 1152.It Dv STB_WEAK 1153Weak symbols resemble global symbols, but their definitions have lower 1154precedence. 1155.It Dv STB_LOPROC 1156This value up to and including 1157.Sy STB_HIPROC 1158are reserved for processor-specific semantics. 1159.It Dv STB_HIPROC 1160This value down to and including 1161.Sy STB_LOPROC 1162are reserved for processor-specific semantics. 1163.Pp 1164There are macros for packing and unpacking the binding and type fields: 1165.Pp 1166.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1167.It Xo 1168.Fn ELF32_ST_BIND info 1169.Xc 1170or 1171.Fn ELF64_ST_BIND info 1172extract a binding from an st_info value. 1173.It Xo 1174.Fn ELF64_ST_TYPE info 1175.Xc 1176or 1177.Fn ELF32_ST_TYPE info 1178extract a type from an st_info value. 1179.It Xo 1180.Fn ELF32_ST_INFO bind type 1181.Xc 1182or 1183.Fn ELF64_ST_INFO bind type 1184convert a binding and a type into an st_info value. 1185.El 1186.El 1187.Pp 1188.It Dv st_other 1189This member currently holds zero and has no defined meaning. 1190.It Dv st_shndx 1191Every symbol table entry is 1192.Dq defined 1193in relation to some section. 1194This member holds the relevant section 1195header table index. 1196.El 1197.Pp 1198Relocation is the process of connecting symbolic references with 1199symbolic definitions. 1200Relocatable files must have information that 1201describes how to modify their section contents, thus allowing executable 1202and shared object files to hold the right information for a process' 1203program image. 1204Relocation entries are these data. 1205.Pp 1206Relocation structures that do not need an addend: 1207.Pp 1208.Bd -literal -offset indent 1209typedef struct { 1210 Elf32_Addr r_offset; 1211 Elf32_Word r_info; 1212} Elf32_Rel; 1213.Ed 1214.Bd -literal -offset indent 1215typedef struct { 1216 Elf64_Addr r_offset; 1217 Elf64_Size r_info; 1218} Elf64_Rel; 1219.Ed 1220.Pp 1221Relocation structures that need an addend: 1222.Pp 1223.Bd -literal -offset indent 1224typedef struct { 1225 Elf32_Addr r_offset; 1226 Elf32_Word r_info; 1227 Elf32_Sword r_addend; 1228} Elf32_Rela; 1229.Ed 1230.Bd -literal -offset indent 1231typedef struct { 1232 Elf64_Addr r_offset; 1233 Elf64_Size r_info; 1234 Elf64_Off r_addend; 1235} Elf64_Rela; 1236.Ed 1237.Pp 1238.Bl -tag -width "r_offset" -compact 1239.It Dv r_offset 1240This member gives the location at which to apply the relocation action. 1241For a relocatable file, the value is the byte offset from the beginning 1242of the section to the storage unit affected by the relocation. 1243For an 1244executable file or shared object, the value is the virtual address of 1245the storage unit affected by the relocation. 1246.It Dv r_info 1247This member gives both the symbol table index with respect to which the 1248relocation must be made and the type of relocation to apply. 1249Relocation 1250types are processor-specific. 1251When the text refers to a relocation 1252entry's relocation type or symbol table index, it means the result of 1253applying 1254.Sy ELF_[32|64]_R_TYPE 1255or 1256.Sy ELF[32|64]_R_SYM , 1257respectively to the entry's 1258.Sy r_info 1259member. 1260.It Dv r_addend 1261This member specifies a constant addend used to compute the value to be 1262stored into the relocatable field. 1263.El 1264.Sh SEE ALSO 1265.Xr as 1 , 1266.Xr gdb 1 , 1267.Xr ld 1 , 1268.Xr objdump 1 , 1269.Xr execve 2 , 1270.Xr core 5 1271.Rs 1272.%A Hewlett Packard 1273.%B Elf-64 Object File Format 1274.Re 1275.Rs 1276.%A Santa Cruz Operation 1277.%B System V Application Binary Interface 1278.Re 1279.Rs 1280.%A Unix System Laboratories 1281.%T Object Files 1282.%B "Executable and Linking Format (ELF)" 1283.Re 1284.Sh HISTORY 1285The ELF header files made their appearance in 1286.Fx 2.2.6 . 1287ELF in itself first appeared in 1288.At V . 1289The ELF format is an adopted standard. 1290.Sh AUTHORS 1291This manual page was written by 1292.An Jeroen Ruigrok van der Werven 1293.Aq asmodai@FreeBSD.org 1294with inspiration from BSDi's 1295.Bsx 1296.Xr elf 5 1297manpage. 1298