1.\"Copyright (c) 1999 Jeroen Ruigrok van der Werven 2.\"All rights reserved. 3.\" 4.\"Redistribution and use in source and binary forms, with or without 5.\"modification, are permitted provided that the following conditions 6.\"are met: 7.\"1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\"2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 14.\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\"ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\"SUCH DAMAGE. 24.\" 25.\" $FreeBSD: src/share/man/man5/elf.5,v 1.6.2.8 2001/12/17 11:30:13 ru Exp $ 26.\" $DragonFly: src/share/man/man5/elf.5,v 1.11 2008/05/02 02:05:06 swildner Exp $ 27.\" 28.Dd February 9, 2008 29.Dt ELF 5 30.Os 31.Sh NAME 32.Nm elf 33.Nd format of ELF executable binary files 34.Sh SYNOPSIS 35.In elf.h 36.Sh DESCRIPTION 37The header file 38.In elf.h 39defines the format of ELF executable binary files. 40Amongst these files are 41normal executable files, relocatable object files, core files and shared 42libraries. 43.Pp 44An executable file using the ELF file format consists of an ELF header, 45followed by a program header table or a section header table, or both. 46The ELF header is always at offset zero of the file. 47The program header 48table and the section header table's offset in the file are defined in the 49ELF header. 50The two tables describe the rest of the particularities of 51the file. 52.Pp 53Applications which wish to process ELF binary files for their native 54architecture only should include 55.In elf.h 56in their source code. 57These applications should need to refer to 58all the types and structures by their generic names 59.Dq Elf_xxx 60and to the macros by 61.Dq ELF_xxx . 62Applications written this way can be compiled on any architecture, 63regardless whether the host is 32-bit or 64-bit. 64.Pp 65Should an application need to process ELF files of an unknown 66architecture then the application needs to include both 67.In sys/elf32.h 68and 69.In sys/elf64.h 70instead of 71.In elf.h . 72Furthermore, all types and structures need to be identified by either 73.Dq Elf32_xxx 74or 75.Dq Elf64_xxx . 76The macros need to be identified by 77.Dq ELF32_xxx 78or 79.Dq ELF64_xxx . 80.Pp 81Whatever the system's architecture is, it will always include 82.In sys/elf_common.h 83as well as 84.In sys/elf_generic.h . 85.Pp 86These header files describe the above mentioned headers as C structures 87and also include structures for dynamic sections, relocation sections and 88symbol tables. 89.Pp 90The following types are being used for 32-bit architectures: 91.Bd -literal -offset indent 92Elf32_Addr Unsigned 32-bit program address 93Elf32_Half Unsigned 16-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 97Elf32_Size Unsigned object size 98.Ed 99.Pp 100For 64-bit architectures we have the following types: 101.Bd -literal -offset indent 102Elf64_Addr Unsigned 64-bit program address 103Elf64_Half Unsigned 16-bit field 104Elf64_Lword Unsigned 64-bit field 105Elf64_Off Unsigned 64-bit file offset 106Elf64_Sword Signed 32-bit field 107Elf64_Sxword Signed 64-bit field or integer 108Elf64_Word Unsigned 32-bit field 109Elf64_Xword Unsigned 64-bit field or integer 110Elf64_Size Unsigned object size 111Elf64_Quarter Unsigned quarterword field 112.Ed 113.Pp 114All data structures that the file format defines follow the 115.Dq natural 116size and alignment guidelines for the relevant class. 117If necessary, 118data structures contain explicit padding to ensure 4-byte alignment 119for 4-byte objects, to force structure sizes to a multiple of 4, etc. 120.Pp 121The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: 122.Bd -literal -offset indent 123typedef struct { 124 unsigned char e_ident[EI_NIDENT]; 125 Elf32_Half e_type; 126 Elf32_Half e_machine; 127 Elf32_Word e_version; 128 Elf32_Addr e_entry; 129 Elf32_Off e_phoff; 130 Elf32_Off e_shoff; 131 Elf32_Word e_flags; 132 Elf32_Half e_ehsize; 133 Elf32_Half e_phentsize; 134 Elf32_Half e_phnum; 135 Elf32_Half e_shentsize; 136 Elf32_Half e_shnum; 137 Elf32_Half e_shstrndx; 138} Elf32_Ehdr; 139.Ed 140.Bd -literal -offset indent 141typedef struct { 142 unsigned char e_ident[EI_NIDENT]; 143 Elf64_Quarter e_type; 144 Elf64_Quarter e_machine; 145 Elf64_Half e_version; 146 Elf64_Addr e_entry; 147 Elf64_Off e_phoff; 148 Elf64_Off e_shoff; 149 Elf64_Half e_flags; 150 Elf64_Quarter e_ehsize; 151 Elf64_Quarter e_phentsize; 152 Elf64_Quarter e_phnum; 153 Elf64_Quarter e_shentsize; 154 Elf64_Quarter e_shnum; 155 Elf64_Quarter e_shstrndx; 156} Elf64_Ehdr; 157.Ed 158.Pp 159The fields have the following meanings: 160.Pp 161.Bl -tag -width ".Fa e_phentsize" -compact -offset indent 162.It Fa e_ident 163This array of bytes specifies to interpret the file, 164independent of the processor or the file's remaining contents. 165Within this array everything is named by macros, which start with 166the prefix 167.Sy EI_ 168and may contain values which start with the prefix 169.Sy ELF . 170The following macros are defined: 171.Pp 172.Bl -tag -width ".Dv EI_ABIVERSION" -compact 173.It Dv EI_MAG0 174The first byte of the magic number. 175It must be filled with 176.Sy ELFMAG0 . 177.It Dv EI_MAG1 178The second byte of the magic number. 179It must be filled with 180.Sy ELFMAG1 . 181.It Dv EI_MAG2 182The third byte of the magic number. 183It must be filled with 184.Sy ELFMAG2 . 185.It Dv EI_MAG3 186The fourth byte of the magic number. 187It must be filled with 188.Sy ELFMAG3 . 189.It Dv EI_CLASS 190The fifth byte identifies the architecture for this binary: 191.Pp 192.Bl -tag -width ".Dv ELFCLASSNONE" -compact 193.It Dv ELFCLASSNONE 194This class is invalid. 195.It Dv ELFCLASS32 196This defines the 32-bit architecture. 197It supports machines with files 198and virtual address spaces up to 4 Gigabytes. 199.It Dv ELFCLASS64 200This defines the 64-bit architecture. 201.El 202.It Dv EI_DATA 203The sixth byte specifies the data encoding of the processor-specific 204data in the file. 205Currently these encodings are supported: 206.Pp 207.Bl -tag -width ".Dv ELFDATA2LSB" -compact 208.It Dv ELFDATANONE 209Unknown data format. 210.It Dv ELFDATA2LSB 211Two's complement, little-endian. 212.It Dv ELFDATA2MSB 213Two's complement, big-endian. 214.El 215.It Dv EI_VERSION 216The version number of the ELF specification: 217.Pp 218.Bl -tag -width ".Dv EV_CURRENT" -compact 219.It Dv EV_NONE 220Invalid version. 221.It Dv EV_CURRENT 222Current version. 223.El 224.It Dv EI_OSABI 225This byte identifies the operating system 226and ABI to which the object is targeted. 227Some fields in other ELF structures have flags 228and values that have platform specific meanings; 229the interpretation of those fields is determined by the value of this byte. 230The following values are currently defined: 231.Pp 232.Bl -tag -width ".Dv ELFOSABI_STANDALONE" -compact 233.It Dv ELFOSABI_SYSV 234.At V 235ABI. 236.It Dv ELFOSABI_HPUX 237HP-UX operating system ABI. 238.It Dv ELFOSABI_NETBSD 239.Nx 240operating system ABI. 241.It Dv ELFOSABI_LINUX 242GNU/Linux operating system ABI. 243.It Dv ELFOSABI_HURD 244GNU/Hurd operating system ABI. 245.It Dv ELFOSABI_86OPEN 24686Open Common IA32 ABI. 247.It Dv ELFOSABI_SOLARIS 248Solaris operating system ABI. 249.It Dv ELFOSABI_MONTEREY 250Monterey project ABI. 251.It Dv ELFOSABI_IRIX 252IRIX operating system ABI. 253.It Dv ELFOSABI_FREEBSD 254.Fx 255operating system ABI. 256.It Dv ELFOSABI_TRU64 257TRU64 258.Ux 259operating system ABI. 260.It Dv ELFOSABI_ARM 261ARM architecture ABI. 262.It Dv ELFOSABI_STANDALONE 263Standalone (embedded) ABI. 264.El 265.It Dv EI_ABIVERSION 266This byte identifies the version of the ABI 267to which the object is targeted. 268This field is used to distinguish among incompatible versions of an ABI. 269The interpretation of this version number 270is dependent on the ABI identified by the 271.Dv EI_OSABI 272field. 273Applications conforming to this specification use the value 0. 274.It Dv EI_PAD 275Start of padding. 276These bytes are reserved and set to zero. 277Programs 278which read them should ignore them. 279The value for EI_PAD will change in 280the future if currently unused bytes are given meanings. 281.It Dv EI_NIDENT 282The size of the 283.Fa e_ident 284array. 285.El 286.Pp 287.It Fa e_type 288This member of the structure identifies the object file type: 289.Pp 290.Bl -tag -width ".Dv ET_NONE" -compact 291.It Dv ET_NONE 292An unknown type. 293.It Dv ET_REL 294A relocatable file. 295.It Dv ET_EXEC 296An executable file. 297.It Dv ET_DYN 298A shared object. 299.It Dv ET_CORE 300A core file. 301.El 302.Pp 303.It Fa e_machine 304This member specifies the required architecture for an individual file: 305.Pp 306.Bl -tag -width ".Dv EM_MIPS_RS4_BE" -compact 307.It Dv EM_NONE 308An unknown machine. 309.It Dv EM_M32 310AT&T WE 32100. 311.It Dv EM_SPARC 312Sun Microsystems SPARC. 313.It Dv EM_386 314Intel 80386. 315.It Dv EM_68K 316Motorola 68000. 317.It Dv EM_88K 318Motorola 88000. 319.It Dv EM_860 320Intel 80860. 321.It Dv EM_MIPS 322MIPS RS3000 (big-endian only). 323.It Dv EM_MIPS_RS4_BE 324MIPS RS4000 (big-endian only). 325.It Dv EM_SPARC64 326SPARC v9 64-bit unofficial. 327.It Dv EM_PARISC 328HPPA. 329.It Dv EM_PPC 330PowerPC. 331.It Dv EM_ALPHA 332Compaq [DEC] Alpha. 333.El 334.Pp 335.It Fa e_version 336This member identifies the file version: 337.Pp 338.Bl -tag -width ".Dv EV_CURRENT" -compact 339.It Dv EV_NONE 340Invalid version 341.It Dv EV_CURRENT 342Current version 343.El 344.It Fa e_entry 345This member gives the virtual address to which the system first transfers 346control, thus starting the process. 347If the file has no associated entry 348point, this member holds zero. 349.It Fa e_phoff 350This member holds the program header table's file offset in bytes. 351If 352the file has no program header table, this member holds zero. 353.It Fa e_shoff 354This member holds the section header table's file offset in bytes. 355If the 356file has no section header table this member holds zero. 357.It Fa e_flags 358This member holds processor-specific flags associated with the file. 359Flag names take the form EF_`machine_flag'. 360Currently no flags have been defined. 361.It Fa e_ehsize 362This member holds the ELF header's size in bytes. 363.It Fa e_phentsize 364This member holds the size in bytes of one entry in the file's program header 365table; all entries are the same size. 366.It Fa e_phnum 367This member holds the number of entries in the program header 368table. 369Thus the product of 370.Fa e_phentsize 371and 372.Fa e_phnum 373gives the table's size 374in bytes. 375If a file has no program header, 376.Fa e_phnum 377holds the value zero. 378.It Fa e_shentsize 379This member holds a sections header's size in bytes. 380A section header is one 381entry in the section header table; all entries are the same size. 382.It Fa e_shnum 383This member holds the number of entries in the section header table. 384Thus 385the product of 386.Fa e_shentsize 387and 388.Fa e_shnum 389gives the section header table's size in bytes. 390If a file has no section 391header table, 392.Fa e_shnum 393holds the value of zero. 394.It Fa e_shstrndx 395This member holds the section header table index of the entry associated 396with the section name string table. 397If the file has no section name string 398table, this member holds the value 399.Dv SHN_UNDEF . 400.Pp 401.Bl -tag -width ".Dv SHN_LORESERVE" -compact 402.It Dv SHN_UNDEF 403This value marks an undefined, missing, irrelevant, or otherwise meaningless 404section reference. 405For example, a symbol 406.Dq defined 407relative to section number 408.Dv SHN_UNDEF 409is an undefined symbol. 410.It Dv SHN_LORESERVE 411This value specifies the lower bound of the range of reserved indexes. 412.It Dv SHN_LOPROC 413This value up to and including 414.Dv SHN_HIPROC 415are reserved for processor-specific semantics. 416.It Dv SHN_HIPROC 417This value down to and including 418.Dv SHN_LOPROC 419are reserved for processor-specific semantics. 420.It Dv SHN_ABS 421This value specifies absolute values for the corresponding reference. 422For 423example, symbols defined relative to section number 424.Dv SHN_ABS 425have absolute values and are not affected by relocation. 426.It Dv SHN_COMMON 427Symbols defined relative to this section are common symbols, such as Fortran 428COMMON or unallocated C external variables. 429.It Dv SHN_HIRESERVE 430This value specifies the upper bound of the range of the range of reserved 431indices between 432.Dv SHN_LORESERVE 433and 434.Dv SHN_HIRESERVE , 435inclusive; the values do 436not reference the section header table. 437That is, the section header table 438does 439.Em not 440contain entries for the reserved indices. 441.El 442.El 443.Pp 444An executable or shared object file's program header table is an array of 445structures, each describing a segment or other information the system needs 446to prepare the program for execution. 447An object file 448.Em segment 449contains one or more 450.Em sections . 451Program headers are meaningful only for executable and shared object files. 452A file specifies its own program header size with the ELF header's 453.Fa e_phentsize 454and 455.Fa e_phnum 456members. 457As with the Elf executable header, the program header 458also has different versions depending on the architecture: 459.Bd -literal -offset indent 460typedef struct { 461 Elf32_Word p_type; 462 Elf32_Off p_offset; 463 Elf32_Addr p_vaddr; 464 Elf32_Addr p_paddr; 465 Elf32_Size p_filesz; 466 Elf32_Size p_memsz; 467 Elf32_Word p_flags; 468 Elf32_Size p_align; 469} Elf32_Phdr; 470.Ed 471.Bd -literal -offset indent 472typedef struct { 473 Elf64_Half p_type; 474 Elf64_Half p_flags; 475 Elf64_Off p_offset; 476 Elf64_Addr p_vaddr; 477 Elf64_Addr p_paddr; 478 Elf64_Size p_filesz; 479 Elf64_Size p_memsz; 480 Elf64_Size p_align; 481} Elf64_Phdr; 482.Ed 483.Pp 484The main difference between the 32-bit and the 64-bit program header lies 485only in the location of a 486.Fa p_flags 487member in the total struct. 488.Pp 489.Bl -tag -width ".Fa p_offset" -compact -offset indent 490.It Fa p_type 491This member of the 492.Vt Phdr 493struct tells what kind of segment this array 494element describes or how to interpret the array element's information. 495.Pp 496.Bl -tag -width ".Dv PT_DYNAMIC" -compact 497.It Dv PT_NULL 498The array element is unused and the other members' values are undefined. 499This lets the program header have ignored entries. 500.It Dv PT_LOAD 501The array element specifies a loadable segment, described by 502.Fa p_filesz 503and 504.Fa p_memsz . 505The bytes from the file are mapped to the beginning of the memory 506segment. 507If the segment's memory size 508.Pq Fa p_memsz 509is larger than the file size 510.Pq Fa p_filesz , 511the 512.Dq extra 513bytes are defined to hold the value 0 and to follow the segment's 514initialized area. 515The file size may not be larger than the memory size. 516Loadable segment entries in the program header table appear in ascending 517order, sorted on the 518.Fa p_vaddr 519member. 520.It Dv PT_DYNAMIC 521The array element specifies dynamic linking information. 522.It Dv PT_INTERP 523The array element specifies the location and size of a null-terminated 524path name to invoke as an interpreter. 525This segment type is meaningful 526only for executable files (though it may occur for shared objects). 527However, it may not occur more than once in a file. 528If it is present it must precede 529any loadable segment entry. 530.It Dv PT_NOTE 531The array element specifies the location and size for auxiliary information. 532.It Dv PT_SHLIB 533This segment type is reserved but has unspecified semantics. 534Programs that 535contain an array element of this type do not conform to the ABI. 536.It Dv PT_PHDR 537The array element, if present, specifies the location and size of the program 538header table itself, both in the file and in the memory image of the program. 539This segment type may not occur more than once in a file. 540Moreover, it may 541only occur if the program header table is part of the memory image of the 542program. 543If it is present it must precede any loadable segment entry. 544.It Dv PT_LOPROC 545This value up to and including 546.Dv PT_HIPROC 547are reserved for processor-specific semantics. 548.It Dv PT_HIPROC 549This value down to and including 550.Dv PT_LOPROC 551are reserved for processor-specific semantics. 552.El 553.Pp 554.It Fa p_offset 555This member holds the offset from the beginning of the file at which 556the first byte of the segment resides. 557.It Fa p_vaddr 558This member holds the virtual address at which the first byte of the 559segment resides in memory. 560.It Fa p_paddr 561On systems for which physical addressing is relevant, this member is 562reserved for the segment's physical address. 563Under 564.Bx 565this member is 566not used and must be zero. 567.It Fa p_filesz 568This member holds the number of bytes in the file image of the segment. 569It may be zero. 570.It Fa p_memsz 571This member holds the number of bytes in the memory image of the segment. 572It may be zero. 573.It Fa p_flags 574This member holds flags relevant to the segment: 575.Pp 576.Bl -tag -width ".Dv PF_X" -compact 577.It Dv PF_X 578An executable segment. 579.It Dv PF_W 580A writable segment. 581.It Dv PF_R 582A readable segment. 583.El 584.Pp 585A text segment commonly has the flags 586.Dv PF_X 587and 588.Dv PF_R . 589A data segment commonly has 590.Dv PF_X , 591.Dv PF_W 592and 593.Dv PF_R . 594.It Fa p_align 595This member holds the value to which the segments are aligned in memory 596and in the file. 597Loadable process segments must have congruent values for 598.Fa p_vaddr 599and 600.Fa p_offset , 601modulo the page size. 602Values of zero and one mean no alignment is required. 603Otherwise, 604.Fa p_align 605should be a positive, integral power of two, and 606.Fa p_vaddr 607should equal 608.Fa p_offset , 609modulo 610.Fa p_align . 611.El 612.Pp 613An file's section header table lets one locate all the file's sections. 614The 615section header table is an array of 616.Vt Elf32_Shdr 617or 618.Vt Elf64_Shdr 619structures. 620The ELF header's 621.Fa e_shoff 622member gives the byte offset from the beginning of the file to the section 623header table. 624.Fa e_shnum 625holds the number of entries the section header table contains. 626.Fa e_shentsize 627holds the size in bytes of each entry. 628.Pp 629A section header table index is a subscript into this array. 630Some section 631header table indices are reserved. 632An object file does not have sections for 633these special indices: 634.Pp 635.Bl -tag -width ".Dv SHN_LORESERVE" -compact 636.It Dv SHN_UNDEF 637This value marks an undefined, missing, irrelevant, or otherwise meaningless 638section reference. 639.It Dv SHN_LORESERVE 640This value specifies the lower bound of the range of reserved indices. 641.It Dv SHN_LOPROC 642This value up to and including 643.Dv SHN_HIPROC 644are reserved for processor-specific semantics. 645.It Dv SHN_HIPROC 646This value down to and including 647.Dv SHN_LOPROC 648are reserved for processor-specific semantics. 649.It Dv SHN_ABS 650This value specifies absolute values for the corresponding reference. 651For 652example, symbols defined relative to section number 653.Dv SHN_ABS 654have absolute values and are not affected by relocation. 655.It Dv SHN_COMMON 656Symbols defined relative to this section are common symbols, such as FORTRAN 657COMMON or unallocated C external variables. 658.It Dv SHN_HIRESERVE 659This value specifies the upper bound of the range of reserved indices. 660The 661system reserves indices between 662.Dv SHN_LORESERVE 663and 664.Dv SHN_HIRESERVE , 665inclusive. 666The section header table does not contain entries for the 667reserved indices. 668.El 669.Pp 670The section header has the following structure: 671.Bd -literal -offset indent 672typedef struct { 673 Elf32_Word sh_name; 674 Elf32_Word sh_type; 675 Elf32_Word sh_flags; 676 Elf32_Addr sh_addr; 677 Elf32_Off sh_offset; 678 Elf32_Size sh_size; 679 Elf32_Word sh_link; 680 Elf32_Word sh_info; 681 Elf32_Size sh_addralign; 682 Elf32_Size sh_entsize; 683} Elf32_Shdr; 684.Ed 685.Bd -literal -offset indent 686typedef struct { 687 Elf64_Half sh_name; 688 Elf64_Half sh_type; 689 Elf64_Size sh_flags; 690 Elf64_Addr sh_addr; 691 Elf64_Off sh_offset; 692 Elf64_Size sh_size; 693 Elf64_Half sh_link; 694 Elf64_Half sh_info; 695 Elf64_Size sh_addralign; 696 Elf64_Size sh_entsize; 697} Elf64_Shdr; 698.Ed 699.Pp 700.Bl -tag -width ".Fa sh_addralign" -compact 701.It Fa sh_name 702This member specifies the name of the section. 703Its value is an index 704into the section header string table section, giving the location of 705a null-terminated string. 706.It Fa sh_type 707This member categorizes the section's contents and semantics. 708.Pp 709.Bl -tag -width ".Dv SHT_PROGBITS" -compact 710.It Dv SHT_NULL 711This value marks the section header as inactive. 712It does not 713have an associated section. 714Other members of the section header 715have undefined values. 716.It Dv SHT_PROGBITS 717The section holds information defined by the program, whose 718format and meaning are determined solely by the program. 719.It Dv SHT_SYMTAB 720This section holds a symbol table. 721Typically, 722.Dv SHT_SYMTAB 723provides symbols for link editing, though it may also be used 724for dynamic linking. 725As a complete symbol table, it may contain 726many symbols unnecessary for dynamic linking. 727An object file can 728also contain a 729.Dv SHN_DYNSYM 730section. 731.It Dv SHT_STRTAB 732This section holds a string table. 733An object file may have multiple 734string table sections. 735.It Dv SHT_RELA 736This section holds relocation entries with explicit addends, such 737as type 738.Vt Elf32_Rela 739for the 32-bit class of object files. 740An object may have multiple 741relocation sections. 742.It Dv SHT_HASH 743This section holds a symbol hash table. 744All object participating in 745dynamic linking must contain a symbol hash table. 746An object file may 747have only one hash table. 748.It Dv SHT_DYNAMIC 749This section holds information for dynamic linking. 750An object file may 751have only one dynamic section. 752.It Dv SHT_NOTE 753This section holds information that marks the file in some way. 754.It Dv SHT_NOBITS 755A section of this type occupies no space in the file but otherwise 756resembles 757.Dv SHN_PROGBITS . 758Although this section contains no bytes, the 759.Fa sh_offset 760member contains the conceptual file offset. 761.It Dv SHT_REL 762This section holds relocation offsets without explicit addends, such 763as type 764.Vt Elf32_Rel 765for the 32-bit class of object files. 766An object file may have multiple 767relocation sections. 768.It Dv SHT_SHLIB 769This section is reserved but has unspecified semantics. 770.It Dv SHT_DYNSYM 771This section holds a minimal set of dynamic linking symbols. 772An 773object file can also contain a 774.Dv SHN_SYMTAB 775section. 776.It Dv SHT_LOPROC 777This value up to and including 778.Dv SHT_HIPROC 779are reserved for processor-specific semantics. 780.It Dv SHT_HIPROC 781This value down to and including 782.Dv SHT_LOPROC 783are reserved for processor-specific semantics. 784.It Dv SHT_LOUSER 785This value specifies the lower bound of the range of indices reserved for 786application programs. 787.It Dv SHT_HIUSER 788This value specifies the upper bound of the range of indices reserved for 789application programs. 790Section types between 791.Dv SHT_LOUSER 792and 793.Dv SHT_HIUSER 794may be used by the application, without conflicting with current or future 795system-defined section types. 796.El 797.Pp 798.It Fa sh_flags 799Sections support one-bit flags that describe miscellaneous attributes. 800If a flag bit is set in 801.Fa sh_flags , 802the attribute is 803.Dq on 804for the section. 805Otherwise, the attribute is 806.Dq off 807or does not apply. 808Undefined attributes are set to zero. 809.Pp 810.Bl -tag -width ".Dv SHF_EXECINSTR" -compact 811.It Dv SHF_WRITE 812This section contains data that should be writable during process 813execution. 814.It Dv SHF_ALLOC 815The section occupies memory during process execution. 816Some control 817sections do not reside in the memory image of an object file. 818This 819attribute is off for those sections. 820.It Dv SHF_EXECINSTR 821The section contains executable machine instructions. 822.It Dv SHF_MASKPROC 823All bits included in this mask are reserved for processor-specific 824semantics. 825.El 826.Pp 827.It Fa sh_addr 828If the section will appear in the memory image of a process, this member 829holds the address at which the section's first byte should reside. 830Otherwise, the member contains zero. 831.It Fa sh_offset 832This member's value holds the byte offset from the beginning of the file 833to the first byte in the section. 834One section type, 835.Dv SHT_NOBITS , 836occupies no space in the file, and its 837.Fa sh_offset 838member locates the conceptual placement in the file. 839.It Fa sh_size 840This member holds the section's size in bytes. 841Unless the section type 842is 843.Dv SHT_NOBITS , 844the section occupies 845.Fa sh_size 846bytes in the file. 847A section of type 848.Dv SHT_NOBITS 849may have a non-zero size, but it occupies no space in the file. 850.It Fa sh_link 851This member holds a section header table index link, whose interpretation 852depends on the section type. 853.It Fa sh_info 854This member holds extra information, whose interpretation depends on the 855section type. 856.It Fa sh_addralign 857Some sections have address alignment constraints. 858If a section holds a 859doubleword, the system must ensure doubleword alignment for the entire 860section. 861That is, the value of 862.Fa sh_addr 863must be congruent to zero, modulo the value of 864.Fa sh_addralign . 865Only zero and positive integral powers of two are allowed. 866Values of zero 867or one mean the section has no alignment constraints. 868.It Fa sh_entsize 869Some sections hold a table of fixed-sized entries, such as a symbol table. 870For such a section, this member gives the size in bytes for each entry. 871This member contains zero if the section does not hold a table of 872fixed-size entries. 873.El 874.Pp 875Various sections hold program and control information: 876.Bl -tag -width ".Sy .shstrtab" -compact 877.It Sy .bss 878(Block Started by Symbol) 879This section holds uninitialized data that contributes to the program's 880memory image. 881By definition, the system initializes the data with zeros 882when the program begins to run. 883This section is of type 884.Dv SHT_NOBITS . 885The attributes types are 886.Dv SHF_ALLOC 887and 888.Dv SHF_WRITE . 889.It Sy .comment 890This section holds version control information. 891This section is of type 892.Dv SHT_PROGBITS . 893No attribute types are used. 894.It Sy .data 895This section holds initialized data that contribute to the program's 896memory image. 897This section is of type 898.Dv SHT_PROGBITS . 899The attribute types are 900.Dv SHF_ALLOC 901and 902.Dv SHF_WRITE . 903.It Sy .data1 904This section holds initialized data that contribute to the program's 905memory image. 906This section is of type 907.Dv SHT_PROGBITS . 908The attribute types are 909.Dv SHF_ALLOC 910and 911.Dv SHF_WRITE . 912.It Sy .debug 913This section holds information for symbolic debugging. 914The contents 915are unspecified. 916This section is of type 917.Dv SHT_PROGBITS . 918No attribute types are used. 919.It Sy .dynamic 920This section holds dynamic linking information. 921The section's attributes 922will include the 923.Dv SHF_ALLOC 924bit. 925Whether the 926.Dv SHF_WRITE 927bit is set is processor-specific. 928This section is of type 929.Dv SHT_DYNAMIC . 930See the attributes above. 931.It Sy .dynstr 932This section holds strings needed for dynamic linking, most commonly 933the strings that represent the names associated with symbol table entries. 934This section is of type 935.Dv SHT_STRTAB . 936The attribute type used is 937.Dv SHF_ALLOC . 938.It Sy .dynsym 939This section holds the dynamic linking symbol table. 940This section is of type 941.Dv SHT_DYNSYM . 942The attribute used is 943.Dv SHF_ALLOC . 944.It Sy .fini 945This section holds executable instructions that contribute to the process 946termination code. 947When a program exits normally the system arranges to 948execute the code in this section. 949This section is of type 950.Dv SHT_PROGBITS . 951The attributes used are 952.Dv SHF_ALLOC 953and 954.Dv SHF_EXECINSTR . 955.It Sy .got 956This section holds the global offset table. 957This section is of type 958.Dv SHT_PROGBITS . 959The attributes are processor-specific. 960.It Sy .hash 961This section holds a symbol hash table. 962This section is of type 963.Dv SHT_HASH . 964The attribute used is 965.Dv SHF_ALLOC . 966.It Sy .init 967This section holds executable instructions that contribute to the process 968initialization code. 969When a program starts to run the system arranges to 970execute the code in this section before calling the main program entry point. 971This section is of type 972.Dv SHT_PROGBITS . 973The attributes used are 974.Dv SHF_ALLOC 975and 976.Dv SHF_EXECINSTR . 977.It Sy .interp 978This section holds the pathname of a program interpreter. 979If the file has 980a loadable segment that includes the section, the section's attributes will 981include the 982.Dv SHF_ALLOC 983bit. 984Otherwise, that bit will be off. 985This section is of type 986.Dv SHT_PROGBITS . 987.It Sy .line 988This section holds line number information for symbolic debugging, which 989describes the correspondence between the program source and the machine code. 990The contents are unspecified. 991This section is of type 992.Dv SHT_PROGBITS . 993No attribute types are used. 994.It Sy .note 995This section holds information in the 996.Dq Note Section 997format described below. 998This section is of type 999.Dv SHT_NOTE . 1000No attribute types are used. 1001.It Sy .plt 1002This section holds the procedure linkage table. 1003This section is of type 1004.Dv SHT_PROGBITS . 1005The attributes are processor-specific. 1006.It Sy .relNAME 1007This section holds relocation information as described below. 1008If the file 1009has a loadable segment that includes relocation, the section's attributes 1010will include the 1011.Dv SHF_ALLOC 1012bit. 1013Otherwise the bit will be off. 1014By convention, 1015.Dq NAME 1016is supplied by the section to which the relocations apply. 1017Thus a relocation 1018section for 1019.Sy .text 1020normally would have the name 1021.Sy .rel.text . 1022This section is of type 1023.Dv SHT_REL . 1024.It Sy .relaNAME 1025This section holds relocation information as described below. 1026If the file 1027has a loadable segment that includes relocation, the section's attributes 1028will include the 1029.Dv SHF_ALLOC 1030bit. 1031Otherwise the bit will be off. 1032By convention, 1033.Dq NAME 1034is supplied by the section to which the relocations apply. 1035Thus a relocation 1036section for 1037.Sy .text 1038normally would have the name 1039.Sy .rela.text . 1040This section is of type 1041.Dv SHT_RELA . 1042.It Sy .rodata 1043This section holds read-only data that typically contributes to a 1044non-writable segment in the process image. 1045This section is of type 1046.Dv SHT_PROGBITS . 1047The attribute used is 1048.Dv SHF_ALLOC . 1049.It Sy .rodata1 1050This section hold read-only data that typically contributes to a 1051non-writable segment in the process image. 1052This section is of type 1053.Dv SHT_PROGBITS . 1054The attribute used is 1055.Dv SHF_ALLOC . 1056.It Sy .shstrtab 1057This section holds section names. 1058This section is of type 1059.Dv SHT_STRTAB . 1060No attribute types are used. 1061.It Sy .strtab 1062This section holds strings, most commonly the strings that represent the 1063names associated with symbol table entries. 1064If the file has a loadable 1065segment that includes the symbol string table, the section's attributes 1066will include the 1067.Dv SHF_ALLOC 1068bit. 1069Otherwise the bit will be off. 1070This section is of type 1071.Dv SHT_STRTAB . 1072.It Sy .symtab 1073This section holds a symbol table. 1074If the file has a loadable segment 1075that includes the symbol table, the section's attributes will include 1076the 1077.Dv SHF_ALLOC 1078bit. 1079Otherwise the bit will be off. 1080This section is of type 1081.Dv SHT_SYMTAB . 1082.It Sy .text 1083This section holds the 1084.Dq text , 1085or executable instructions, of a program. 1086This section is of type 1087.Dv SHT_PROGBITS . 1088The attributes used are 1089.Dv SHF_ALLOC 1090and 1091.Dv SHF_EXECINSTR . 1092.El 1093.Pp 1094String table sections hold null-terminated character sequences, commonly 1095called strings. 1096The object file uses these strings to represent symbol 1097and section names. 1098One references a string as an index into the string 1099table section. 1100The first byte, which is index zero, is defined to hold 1101a null character. 1102Similarly, a string table's last byte is defined to 1103hold a null character, ensuring null termination for all strings. 1104.Pp 1105An object file's symbol table holds information needed to locate and 1106relocate a program's symbolic definitions and references. 1107A symbol table 1108index is a subscript into this array. 1109.Bd -literal -offset indent 1110typedef struct { 1111 Elf32_Word st_name; 1112 Elf32_Addr st_value; 1113 Elf32_Size st_size; 1114 unsigned char st_info; 1115 unsigned char st_other; 1116 Elf32_Half st_shndx; 1117} Elf32_Sym; 1118.Ed 1119.Bd -literal -offset indent 1120typedef struct { 1121 Elf64_Half st_name; 1122 unsigned char st_info; 1123 unsigned char st_other; 1124 Elf64_Quarter st_shndx; 1125 Elf64_Addr st_value; 1126 Elf64_Size st_size; 1127} Elf64_Sym; 1128.Ed 1129.Pp 1130.Bl -tag -width ".Fa st_value" -compact 1131.It Fa st_name 1132This member holds an index into the object file's symbol string table, 1133which holds character representations of the symbol names. 1134If the value 1135is non-zero, it represents a string table index that gives the symbol 1136name. 1137Otherwise, the symbol table has no name. 1138.It Fa st_value 1139This member gives the value of the associated symbol. 1140.It Fa st_size 1141Many symbols have associated sizes. 1142This member holds zero if the symbol 1143has no size or an unknown size. 1144.It Fa st_info 1145This member specifies the symbol's type and binding attributes: 1146.Pp 1147.Bl -tag -width ".Dv STT_SECTION" -compact 1148.It Dv STT_NOTYPE 1149The symbol's type is not defined. 1150.It Dv STT_OBJECT 1151The symbol is associated with a data object. 1152.It Dv STT_FUNC 1153The symbol is associated with a function or other executable code. 1154.It Dv STT_SECTION 1155The symbol is associated with a section. 1156Symbol table entries of 1157this type exist primarily for relocation and normally have 1158.Dv STB_LOCAL 1159bindings. 1160.It Dv STT_FILE 1161By convention the symbol's name gives the name of the source file 1162associated with the object file. 1163A file symbol has 1164.Dv STB_LOCAL 1165bindings, its section index is 1166.Dv SHN_ABS , 1167and it precedes the other 1168.Dv STB_LOCAL 1169symbols of the file, if it is present. 1170.It Dv STT_LOPROC 1171This value up to and including 1172.Dv STT_HIPROC 1173are reserved for processor-specific semantics. 1174.It Dv STT_HIPROC 1175This value down to and including 1176.Dv STT_LOPROC 1177are reserved for processor-specific semantics. 1178.El 1179.Pp 1180.Bl -tag -width ".Dv STB_GLOBAL" -compact 1181.It Dv STB_LOCAL 1182Local symbols are not visible outside the object file containing their 1183definition. 1184Local symbols of the same name may exist in multiple file 1185without interfering with each other. 1186.It Dv STB_GLOBAL 1187Global symbols are visible to all object files being combined. 1188One file's 1189definition of a global symbol will satisfy another file's undefined 1190reference to the same symbol. 1191.It Dv STB_WEAK 1192Weak symbols resemble global symbols, but their definitions have lower 1193precedence. 1194.It Dv STB_LOPROC 1195This value up to and including 1196.Dv STB_HIPROC 1197are reserved for processor-specific semantics. 1198.It Dv STB_HIPROC 1199This value down to and including 1200.Dv STB_LOPROC 1201are reserved for processor-specific semantics. 1202.Pp 1203There are macros for packing and unpacking the binding and type fields: 1204.Pp 1205.Bl -tag -width ".Fn ELF32_ST_INFO bind type" -compact 1206.It Xo 1207.Fn ELF32_ST_BIND info 1208.Xc 1209or 1210.Fn ELF64_ST_BIND info 1211extract a binding from an 1212.Fa st_info 1213value. 1214.It Xo 1215.Fn ELF64_ST_TYPE info 1216.Xc 1217or 1218.Fn ELF32_ST_TYPE info 1219extract a type from an 1220.Fa st_info 1221value. 1222.It Xo 1223.Fn ELF32_ST_INFO bind type 1224.Xc 1225or 1226.Fn ELF64_ST_INFO bind type 1227convert a binding and a type into an 1228.Fa st_info 1229value. 1230.El 1231.El 1232.Pp 1233.It Fa st_other 1234This member currently holds zero and has no defined meaning. 1235.It Fa st_shndx 1236Every symbol table entry is 1237.Dq defined 1238in relation to some section. 1239This member holds the relevant section 1240header table index. 1241.El 1242.Pp 1243Relocation is the process of connecting symbolic references with 1244symbolic definitions. 1245Relocatable files must have information that 1246describes how to modify their section contents, thus allowing executable 1247and shared object files to hold the right information for a process' 1248program image. 1249Relocation entries are these data. 1250.Pp 1251Relocation structures that do not need an addend: 1252.Bd -literal -offset indent 1253typedef struct { 1254 Elf32_Addr r_offset; 1255 Elf32_Word r_info; 1256} Elf32_Rel; 1257.Ed 1258.Bd -literal -offset indent 1259typedef struct { 1260 Elf64_Addr r_offset; 1261 Elf64_Size r_info; 1262} Elf64_Rel; 1263.Ed 1264.Pp 1265Relocation structures that need an addend: 1266.Bd -literal -offset indent 1267typedef struct { 1268 Elf32_Addr r_offset; 1269 Elf32_Word r_info; 1270 Elf32_Sword r_addend; 1271} Elf32_Rela; 1272.Ed 1273.Bd -literal -offset indent 1274typedef struct { 1275 Elf64_Addr r_offset; 1276 Elf64_Size r_info; 1277 Elf64_Off r_addend; 1278} Elf64_Rela; 1279.Ed 1280.Pp 1281.Bl -tag -width ".Fa r_offset" -compact 1282.It Fa r_offset 1283This member gives the location at which to apply the relocation action. 1284For a relocatable file, the value is the byte offset from the beginning 1285of the section to the storage unit affected by the relocation. 1286For an 1287executable file or shared object, the value is the virtual address of 1288the storage unit affected by the relocation. 1289.It Fa r_info 1290This member gives both the symbol table index with respect to which the 1291relocation must be made and the type of relocation to apply. 1292Relocation 1293types are processor-specific. 1294When the text refers to a relocation 1295entry's relocation type or symbol table index, it means the result of 1296applying 1297.Fn ELF[32|64]_R_TYPE 1298or 1299.Fn ELF[32|64]_R_SYM , 1300respectively to the entry's 1301.Fa r_info 1302member. 1303.It Fa r_addend 1304This member specifies a constant addend used to compute the value to be 1305stored into the relocatable field. 1306.El 1307.Sh SEE ALSO 1308.Xr as 1 , 1309.Xr gdb 1 , 1310.Xr ld 1 , 1311.Xr objdump 1 , 1312.Xr readelf 1 , 1313.Xr execve 2 , 1314.Xr core 5 1315.Rs 1316.%A Hewlett Packard 1317.%B Elf-64 Object File Format 1318.Re 1319.Rs 1320.%A Santa Cruz Operation 1321.%B System V Application Binary Interface 1322.Re 1323.Rs 1324.%A Unix System Laboratories 1325.%T Object Files 1326.%B "Executable and Linking Format (ELF)" 1327.Re 1328.Sh HISTORY 1329The ELF header files made their appearance in 1330.Fx 2.2.6 . 1331ELF in itself first appeared in 1332.At V . 1333The ELF format is an adopted standard. 1334.Sh AUTHORS 1335This manual page was written by 1336.An Jeroen Ruigrok van der Werven 1337.Aq asmodai@FreeBSD.org 1338with inspiration from BSDi's 1339.Bsx 1340.Xr elf 5 1341manpage. 1342