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.Pp 138.Bd -literal -offset indent 139typedef struct { 140 unsigned char e_ident[EI_NIDENT]; 141 Elf64_Half e_type; 142 Elf64_Half e_machine; 143 Elf64_Word e_version; 144 Elf64_Addr e_entry; 145 Elf64_Off e_phoff; 146 Elf64_Off e_shoff; 147 Elf64_Word e_flags; 148 Elf64_Half e_ehsize; 149 Elf64_Half e_phentsize; 150 Elf64_Half e_phnum; 151 Elf64_Half e_shentsize; 152 Elf64_Half e_shnum; 153 Elf64_Half e_shstrndx; 154} Elf64_Ehdr; 155.Ed 156.Pp 157The fields have the following meanings: 158.Pp 159.Bl -tag -width "e_phentsize" -compact -offset indent 160.It Dv e_ident 161This array of bytes specifies to interpret the file, 162independent of the processor or the file's remaining contents. 163Within this array everything is named by macros, which start with 164the prefix 165.Sy EI_ 166and may contain values which start with the prefix 167.Sy ELF . 168The following macros are defined: 169.Pp 170.Bl -tag -width "EI_ABIVERSION" -compact 171.It Dv EI_MAG0 172The first byte of the magic number. 173It must be filled with 174.Sy ELFMAG0 . 175.It Dv EI_MAG1 176The second byte of the magic number. 177It must be filled with 178.Sy ELFMAG1 . 179.It Dv EI_MAG2 180The third byte of the magic number. 181It must be filled with 182.Sy ELFMAG2 . 183.It Dv EI_MAG3 184The fourth byte of the magic number. 185It must be filled with 186.Sy ELFMAG3 . 187.It Dv EI_CLASS 188The fifth byte identifies the architecture for this binary: 189.Pp 190.Bl -tag -width "ELFCLASSNONE" -compact 191.It Dv ELFCLASSNONE 192This class is invalid. 193.It Dv ELFCLASS32 194This defines the 32-bit architecture. 195It supports machines with files 196and virtual address spaces up to 4 Gigabytes. 197.It Dv ELFCLASS64 198This defines the 64-bit architecture. 199.El 200.It Dv EI_DATA 201The sixth byte specifies the data encoding of the processor-specific 202data in the file. 203Currently these encodings are supported: 204.Pp 205.Bl -tag -width "ELFDATA2LSB" -compact 206.It Dv ELFDATANONE 207Unknown data format. 208.It Dv ELFDATA2LSB 209Two's complement, little-endian. 210.It Dv ELFDATA2MSB 211Two's complement, big-endian. 212.El 213.It Dv EI_VERSION 214The version number of the ELF specification: 215.Pp 216.Bl -tag -width "EV_CURRENT" -compact 217.It Dv EV_NONE 218Invalid version. 219.It Dv EV_CURRENT 220Current version. 221.El 222.It Dv EI_OSABI 223This byte identifies the operating system 224and ABI to which the object is targeted. 225Some fields in other ELF structures have flags 226and values that have platform specific meanings; 227the interpretation of those fields is determined by the value of this byte. 228The following values are currently defined: 229.Pp 230.Bl -tag -width "ELFOSABI_STANDALONE" -compact 231.It Dv ELFOSABI_SYSV 232UNIX System V ABI. 233.It Dv ELFOSABI_HPUX 234HP-UX operating system ABI. 235.It Dv ELFOSABI_NETBSD 236.Nx 237operating system ABI. 238.It Dv ELFOSABI_LINUX 239GNU/Linux operating system ABI. 240.It Dv ELFOSABI_HURD 241GNU/Hurd operating system ABI. 242.It Dv ELFOSABI_86OPEN 24386Open Common IA32 ABI. 244.It Dv ELFOSABI_SOLARIS 245Solaris operating system ABI. 246.It Dv ELFOSABI_MONTEREY 247Monterey project ABI. 248.It Dv ELFOSABI_IRIX 249IRIX operating system ABI. 250.It Dv ELFOSABI_FREEBSD 251.Fx 252operating system ABI. 253.It Dv ELFOSABI_TRU64 254TRU64 UNIX operating system ABI. 255.It Dv ELFOSABI_ARM 256ARM architecture ABI. 257.It Dv ELFOSABI_STANDALONE 258Standalone (embedded) ABI. 259.El 260.It Dv EI_ABIVERSION 261This byte identifies the version of the ABI 262to which the object is targeted. 263This field is used to distinguish among incompatible versions of an ABI. 264The interpretation of this version number 265is dependent on the ABI identified by the EI_OSABI field. 266Applications conforming to this specification use the value 0. 267.It Dv EI_PAD 268Start of padding. 269These bytes are reserved and set to zero. 270Programs 271which read them should ignore them. 272The value for EI_PAD will change in 273the future if currently unused bytes are given meanings. 274.It Dv EI_BRAND 275Start of architecture identification. 276.It Dv EI_NIDENT 277The size of the e_ident array. 278.El 279.Pp 280.It Dv e_type 281This member of the structure identifies the object file type: 282.Pp 283.Bl -tag -width "ET_NONE" -compact 284.It Dv ET_NONE 285An unknown type. 286.It Dv ET_REL 287A relocatable file. 288.It Dv ET_EXEC 289An executable file. 290.It Dv ET_DYN 291A shared object. 292.It Dv ET_CORE 293A core file. 294.El 295.Pp 296.It Dv e_machine 297This member specifies the required architecture for an individual file: 298.Pp 299.Bl -tag -width "EM_MIPS_RS4_BE" -compact 300.It Dv EM_NONE 301An unknown machine. 302.It Dv EM_M32 303AT&T WE 32100. 304.It Dv EM_SPARC 305Sun Microsystems SPARC. 306.It Dv EM_386 307Intel 80386. 308.It Dv EM_68K 309Motorola 68000. 310.It Dv EM_88K 311Motorola 88000. 312.It Dv EM_486 313Intel 80486. 314.It Dv EM_860 315Intel 80860. 316.It Dv EM_MIPS 317MIPS RS3000 (big-endian only). 318.It Dv EM_MIPS_RS4_BE 319MIPS RS4000 (big-endian only). 320.It Dv EM_SPARC64 321SPARC v9 64-bit unofficial. 322.It Dv EM_PARISC 323HPPA. 324.It Dv EM_PPC 325PowerPC. 326.It Dv EM_ALPHA 327Compaq [DEC] Alpha. 328.El 329.Pp 330.It Dv e_version 331This member identifies the file version: 332.Pp 333.Bl -tag -width "EV_CURRENT" -compact 334.It Dv EV_NONE 335Invalid version 336.It Dv EV_CURRENT 337Current version 338.El 339.It Dv e_entry 340This member gives the virtual address to which the system first transfers 341control, thus starting the process. 342If the file has no associated entry 343point, this member holds zero. 344.It Dv e_phoff 345This member holds the program header table's file offset in bytes. 346If 347the file has no program header table, this member holds zero. 348.It Dv e_shoff 349This member holds the section header table's file offset in bytes. 350If the 351file has no section header table this member holds zero. 352.It Dv e_flags 353This member holds processor-specific flags associated with the file. 354Flag 355names take the form EF_`machine_flag'. 356Currently no flags have been defined. 357.It Dv e_ehsize 358This member holds the ELF header's size in bytes. 359.It Dv e_phentsize 360This member holds the size in bytes of one entry in the file's program header 361table; all entries are the same size. 362.It Dv e_phnum 363This member holds the number of entries in the program header 364table. 365If the file is using extended program header numbering, then the 366.Sy e_phnum 367member will contain the value 368.Dv PN_XNUM 369and the actual number of program header table entries will be stored 370in the 371.Sy sh_info 372member of the section header at index 373.Dv SHN_UNDEF . 374The product of 375.Sy e_phentsize 376and the number of program header table entries gives the program 377header table's size in bytes. 378If a file has no program header, 379.Sy e_phnum 380holds the value zero. 381.It Dv e_shentsize 382This member holds a sections header's size in bytes. 383A section header is one 384entry in the section header table; all entries are the same size. 385.It Dv e_shnum 386This member holds the number of entries in the section header table. 387If the file is using extended section numbering, then the 388.Sy e_shnum 389member will be zero and the actual section number will be stored in the 390.Sy sh_size 391member of the section header at index 392.Dv SHN_UNDEF . 393If a file has no section header table, both the 394.Sy e_shnum 395and the 396.Sy e_shoff 397fields of the ELF header will be zero. 398The product of 399.Sy e_shentsize 400and the number of sections in the file gives the section header 401table's size in bytes. 402.It Dv e_shstrndx 403This member holds the section header table index of the entry associated 404with the section name string table. 405If extended section numbering is being used, this field will hold the 406value 407.Sy SHN_XINDEX , 408and the actual section header table index will be present in the 409.Sy sh_link 410field of the section header entry at index 411.Dv SHN_UNDEF . 412If the file has no section name string 413table, this member holds the value 414.Sy SHN_UNDEF . 415.El 416.Pp 417An executable or shared object file's program header table is an array of 418structures, each describing a segment or other information the system needs 419to prepare the program for execution. 420An object file 421.Em segment 422contains one or more 423.Em sections . 424Program headers are meaningful only for executable and shared object files. 425A file specifies its own program header size with the ELF header's 426.Sy e_phentsize 427and 428.Sy e_phnum 429members. 430As with the Elf executable header, the program header 431also has different versions depending on the architecture: 432.Pp 433.Bd -literal -offset indent 434typedef struct { 435 Elf32_Word p_type; 436 Elf32_Off p_offset; 437 Elf32_Addr p_vaddr; 438 Elf32_Addr p_paddr; 439 Elf32_Word p_filesz; 440 Elf32_Word p_memsz; 441 Elf32_Word p_flags; 442 Elf32_Word p_align; 443} Elf32_Phdr; 444.Ed 445.Pp 446.Bd -literal -offset indent 447typedef struct { 448 Elf64_Word p_type; 449 Elf64_Word p_flags; 450 Elf64_Off p_offset; 451 Elf64_Addr p_vaddr; 452 Elf64_Addr p_paddr; 453 Elf64_Xword p_filesz; 454 Elf64_Xword p_memsz; 455 Elf64_Xword p_align; 456} Elf64_Phdr; 457.Ed 458.Pp 459The main difference between the 32-bit and the 64-bit program header lies 460only in the location of a 461.Sy p_flags 462member in the total struct. 463.Pp 464.Bl -tag -width "p_offset" -compact -offset indent 465.It Dv p_type 466This member of the Phdr struct tells what kind of segment this array 467element describes or how to interpret the array element's information. 468.Bl -tag -width "PT_DYNAMIC" -compact 469.Pp 470.It Dv PT_NULL 471The array element is unused and the other members' values are undefined. 472This lets the program header have ignored entries. 473.It Dv PT_LOAD 474The array element specifies a loadable segment, described by 475.Sy p_filesz 476and 477.Sy p_memsz . 478The bytes from the file are mapped to the beginning of the memory 479segment. 480If the segment's memory size 481.Pq Sy p_memsz 482is larger than the file size 483.Pq Sy p_filesz , 484the 485.Dq extra 486bytes are defined to hold the value 0 and to follow the segment's 487initialized area. 488The file size may not be larger than the memory size. 489Loadable segment entries in the program header table appear in ascending 490order, sorted on the 491.Sy p_vaddr 492member. 493.It Dv PT_DYNAMIC 494The array element specifies dynamic linking information. 495.It Dv PT_INTERP 496The array element specifies the location and size of a null-terminated 497path name to invoke as an interpreter. 498This segment type is meaningful 499only for executable files (though it may occur for shared objects). 500However 501it may not occur more than once in a file. 502If it is present it must precede 503any loadable segment entry. 504.It Dv PT_NOTE 505The array element specifies the location and size for auxiliary information. 506.It Dv PT_SHLIB 507This segment type is reserved but has unspecified semantics. 508Programs that 509contain an array element of this type do not conform to the ABI. 510.It Dv PT_PHDR 511The array element, if present, specifies the location and size of the program 512header table itself, both in the file and in the memory image of the program. 513This segment type may not occur more than once in a file. 514Moreover, it may 515only occur if the program header table is part of the memory image of the 516program. 517If it is present it must precede any loadable segment entry. 518.It Dv PT_LOPROC 519This value up to and including 520.Sy PT_HIPROC 521are reserved for processor-specific semantics. 522.It Dv PT_HIPROC 523This value down to and including 524.Sy PT_LOPROC 525are reserved for processor-specific semantics. 526.El 527.Pp 528.It Dv p_offset 529This member holds the offset from the beginning of the file at which 530the first byte of the segment resides. 531.It Dv p_vaddr 532This member holds the virtual address at which the first byte of the 533segment resides in memory. 534.It Dv p_paddr 535On systems for which physical addressing is relevant, this member is 536reserved for the segment's physical address. 537Under 538.Bx 539this member is 540not used and must be zero. 541.It Dv p_filesz 542This member holds the number of bytes in the file image of the segment. 543It may be zero. 544.It Dv p_memsz 545This member holds the number of bytes in the memory image of the segment. 546It may be zero. 547.It Dv p_flags 548This member holds flags relevant to the segment: 549.Pp 550.Bl -tag -width "PF_X" -compact 551.It Dv PF_X 552An executable segment. 553.It Dv PF_W 554A writable segment. 555.It Dv PF_R 556A readable segment. 557.El 558.Pp 559A text segment commonly has the flags 560.Sy PF_X 561and 562.Sy PF_R . 563A data segment commonly has 564.Sy PF_X , 565.Sy PF_W 566and 567.Sy PF_R . 568.It Dv p_align 569This member holds the value to which the segments are aligned in memory 570and in the file. 571Loadable process segments must have congruent values for 572.Sy p_vaddr 573and 574.Sy p_offset , 575modulo the page size. 576Values of zero and one mean no alignment is required. 577Otherwise, 578.Sy p_align 579should be a positive, integral power of two, and 580.Sy p_vaddr 581should equal 582.Sy p_offset , 583modulo 584.Sy p_align . 585.El 586.Pp 587An file's section header table lets one locate all the file's sections. 588The 589section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 590The 591ELF header's 592.Sy e_shoff 593member gives the byte offset from the beginning of the file to the section 594header table. 595.Sy e_shnum 596holds the number of entries the section header table contains. 597.Sy e_shentsize 598holds the size in bytes of each entry. 599.Pp 600A section header table index is a subscript into this array. 601Some section 602header table indices are reserved. 603An object file does not have sections for 604these special indices: 605.Pp 606.Bl -tag -width "SHN_LORESERVE" -compact 607.It Dv SHN_UNDEF 608This value marks an undefined, missing, irrelevant, or otherwise meaningless 609section reference. 610For example, a symbol 611.Dq defined 612relative to section number 613.Sy SHN_UNDEF 614is an undefined symbol. 615.It Dv SHN_LORESERVE 616This value specifies the lower bound of the range of reserved indices. 617.It Dv SHN_LOPROC 618This value up to and including 619.Sy SHN_HIPROC 620are reserved for processor-specific semantics. 621.It Dv SHN_HIPROC 622This value down to and including 623.Sy SHN_LOPROC 624are reserved for processor-specific semantics. 625.It Dv SHN_ABS 626This value specifies absolute values for the corresponding reference. 627For 628example, symbols defined relative to section number 629.Sy SHN_ABS 630have absolute values and are not affected by relocation. 631.It Dv SHN_COMMON 632Symbols defined relative to this section are common symbols, such as FORTRAN 633COMMON or unallocated C external variables. 634.It Dv SHN_HIRESERVE 635This value specifies the upper bound of the range of reserved indices. 636The 637system reserves indices between 638.Sy SHN_LORESERVE 639and 640.Sy SHN_HIRESERVE , 641inclusive. 642The section header table does not contain entries for the 643reserved indices. 644.El 645.Pp 646The section header has the following structure: 647.Bd -literal -offset indent 648typedef struct { 649 Elf32_Word sh_name; 650 Elf32_Word sh_type; 651 Elf32_Word sh_flags; 652 Elf32_Addr sh_addr; 653 Elf32_Off sh_offset; 654 Elf32_Word sh_size; 655 Elf32_Word sh_link; 656 Elf32_Word sh_info; 657 Elf32_Word sh_addralign; 658 Elf32_Word sh_entsize; 659} Elf32_Shdr; 660.Ed 661.Pp 662.Bd -literal -offset indent 663typedef struct { 664 Elf64_Word sh_name; 665 Elf64_Word sh_type; 666 Elf64_Xword sh_flags; 667 Elf64_Addr sh_addr; 668 Elf64_Off sh_offset; 669 Elf64_Xword sh_size; 670 Elf64_Word sh_link; 671 Elf64_Word sh_info; 672 Elf64_Xword sh_addralign; 673 Elf64_Xword sh_entsize; 674} Elf64_Shdr; 675.Ed 676.Pp 677.Bl -tag -width "sh_addralign" -compact 678.It Dv sh_name 679This member specifies the name of the section. 680Its value is an index 681into the section header string table section, giving the location of 682a null-terminated string. 683.It Dv sh_type 684This member categorizes the section's contents and semantics. 685.Pp 686.Bl -tag -width "SHT_PROGBITS" -compact 687.It Dv SHT_NULL 688This value marks the section header as inactive. 689It does not 690have an associated section. 691Other members of the section header 692have undefined values. 693.It Dv SHT_PROGBITS 694The section holds information defined by the program, whose 695format and meaning are determined solely by the program. 696.It Dv SHT_SYMTAB 697This section holds a symbol table. 698Typically, 699.Sy SHT_SYMTAB 700provides symbols for link editing, though it may also be used 701for dynamic linking. 702As a complete symbol table, it may contain 703many symbols unnecessary for dynamic linking. 704An object file can 705also contain a 706.Sy SHN_DYNSYM 707section. 708.It Dv SHT_STRTAB 709This section holds a string table. 710An object file may have multiple 711string table sections. 712.It Dv SHT_RELA 713This section holds relocation entries with explicit addends, such 714as type 715.Sy Elf32_Rela 716for the 32-bit class of object files. 717An object may have multiple 718relocation sections. 719.It Dv SHT_HASH 720This section holds a symbol hash table. 721All object participating in 722dynamic linking must contain a symbol hash table. 723An object file may 724have only one hash table. 725.It Dv SHT_DYNAMIC 726This section holds information for dynamic linking. 727An object file may 728have only one dynamic section. 729.It Dv SHT_NOTE 730This section holds information that marks the file in some way. 731.It Dv SHT_NOBITS 732A section of this type occupies no space in the file but otherwise 733resembles 734.Sy SHN_PROGBITS . 735Although this section contains no bytes, the 736.Sy sh_offset 737member contains the conceptual file offset. 738.It Dv SHT_REL 739This section holds relocation offsets without explicit addends, such 740as type 741.Sy Elf32_Rel 742for the 32-bit class of object files. 743An object file may have multiple 744relocation sections. 745.It Dv SHT_SHLIB 746This section is reserved but has unspecified semantics. 747.It Dv SHT_DYNSYM 748This section holds a minimal set of dynamic linking symbols. 749An 750object file can also contain a 751.Sy SHN_SYMTAB 752section. 753.It Dv SHT_LOPROC 754This value up to and including 755.Sy SHT_HIPROC 756are reserved for processor-specific semantics. 757.It Dv SHT_HIPROC 758This value down to and including 759.Sy SHT_LOPROC 760are reserved for processor-specific semantics. 761.It Dv SHT_LOUSER 762This value specifies the lower bound of the range of indices reserved for 763application programs. 764.It Dv SHT_HIUSER 765This value specifies the upper bound of the range of indices reserved for 766application programs. 767Section types between 768.Sy SHT_LOUSER 769and 770.Sy SHT_HIUSER 771may be used by the application, without conflicting with current or future 772system-defined section types. 773.El 774.Pp 775.It Dv sh_flags 776Sections support one-bit flags that describe miscellaneous attributes. 777If a flag bit is set in 778.Sy sh_flags , 779the attribute is 780.Dq on 781for the section. 782Otherwise, the attribute is 783.Dq off 784or does not apply. 785Undefined attributes are set to zero. 786.Pp 787.Bl -tag -width "SHF_EXECINSTR" -compact 788.It Dv SHF_WRITE 789This section contains data that should be writable during process 790execution. 791.It Dv SHF_ALLOC 792The section occupies memory during process execution. 793Some control 794sections do not reside in the memory image of an object file. 795This 796attribute is off for those sections. 797.It Dv SHF_EXECINSTR 798The section contains executable machine instructions. 799.It Dv SHF_MASKPROC 800All bits included in this mask are reserved for processor-specific 801semantics. 802.El 803.Pp 804.It Dv sh_addr 805If the section will appear in the memory image of a process, this member 806holds the address at which the section's first byte should reside. 807Otherwise, the member contains zero. 808.It Dv sh_offset 809This member's value holds the byte offset from the beginning of the file 810to the first byte in the section. 811One section type, 812.Sy SHT_NOBITS , 813occupies no space in the file, and its 814.Sy sh_offset 815member locates the conceptual placement in the file. 816.It Dv sh_size 817This member holds the section's size in bytes. 818Unless the section type 819is 820.Sy SHT_NOBITS , 821the section occupies 822.Sy sh_size 823bytes in the file. 824A section of type 825.Sy SHT_NOBITS 826may have a non-zero size, but it occupies no space in the file. 827.It Dv sh_link 828This member holds a section header table index link, whose interpretation 829depends on the section type. 830.It Dv sh_info 831This member holds extra information, whose interpretation depends on the 832section type. 833.It Dv sh_addralign 834Some sections have address alignment constraints. 835If a section holds a 836doubleword, the system must ensure doubleword alignment for the entire 837section. 838That is, the value of 839.Sy sh_addr 840must be congruent to zero, modulo the value of 841.Sy sh_addralign . 842Only zero and positive integral powers of two are allowed. 843Values of zero 844or one mean the section has no alignment constraints. 845.It Dv sh_entsize 846Some sections hold a table of fixed-sized entries, such as a symbol table. 847For such a section, this member gives the size in bytes for each entry. 848This member contains zero if the section does not hold a table of 849fixed-size entries. 850.El 851.Pp 852Various sections hold program and control information: 853.Bl -tag -width ".shstrtab" -compact 854.It .bss 855(Block Started by Symbol) 856This section holds uninitialized data that contributes to the program's 857memory image. 858By definition, the system initializes the data with zeros 859when the program begins to run. 860This section is of type 861.Sy SHT_NOBITS . 862The attributes types are 863.Sy SHF_ALLOC 864and 865.Sy SHF_WRITE . 866.It .comment 867This section holds version control information. 868This section is of type 869.Sy SHT_PROGBITS . 870No attribute types are used. 871.It .data 872This section holds initialized data that contribute to the program's 873memory image. 874This section is of type 875.Sy SHT_PROGBITS . 876The attribute types are 877.Sy SHF_ALLOC 878and 879.Sy SHF_WRITE . 880.It .data1 881This section holds initialized data that contribute to the program's 882memory image. 883This section is of type 884.Sy SHT_PROGBITS . 885The attribute types are 886.Sy SHF_ALLOC 887and 888.Sy SHF_WRITE . 889.It .debug 890This section holds information for symbolic debugging. 891The contents 892are unspecified. 893This section is of type 894.Sy SHT_PROGBITS . 895No attribute types are used. 896.It .dynamic 897This section holds dynamic linking information. 898The section's attributes 899will include the 900.Sy SHF_ALLOC 901bit. 902Whether the 903.Sy SHF_WRITE 904bit is set is processor-specific. 905This section is of type 906.Sy SHT_DYNAMIC . 907See the attributes above. 908.It .dynstr 909This section holds strings needed for dynamic linking, most commonly 910the strings that represent the names associated with symbol table entries. 911This section is of type 912.Sy SHT_STRTAB . 913The attribute type used is 914.Sy SHF_ALLOC . 915.It .dynsym 916This section holds the dynamic linking symbol table. 917This section is of type 918.Sy SHT_DYNSYM . 919The attribute used is 920.Sy SHF_ALLOC . 921.It .fini 922This section holds executable instructions that contribute to the process 923termination code. 924When a program exits normally the system arranges to 925execute the code in this section. 926This section is of type 927.Sy SHT_PROGBITS . 928The attributes used are 929.Sy SHF_ALLOC 930and 931.Sy SHF_EXECINSTR . 932.It .got 933This section holds the global offset table. 934This section is of type 935.Sy SHT_PROGBITS . 936The attributes are processor-specific. 937.It .hash 938This section holds a symbol hash table. 939This section is of type 940.Sy SHT_HASH . 941The attribute used is 942.Sy SHF_ALLOC . 943.It .init 944This section holds executable instructions that contribute to the process 945initialization code. 946When a program starts to run the system arranges to 947execute the code in this section before calling the main program entry point. 948This section is of type 949.Sy SHT_PROGBITS . 950The attributes used are 951.Sy SHF_ALLOC 952and 953.Sy SHF_EXECINSTR . 954.It .interp 955This section holds the pathname of a program interpreter. 956If the file has 957a loadable segment that includes the section, the section's attributes will 958include the 959.Sy SHF_ALLOC 960bit. 961Otherwise, that bit will be off. 962This section is of type 963.Sy SHT_PROGBITS . 964.It .line 965This section holds line number information for symbolic debugging, which 966describes the correspondence between the program source and the machine code. 967The contents are unspecified. 968This section is of type 969.Sy SHT_PROGBITS . 970No attribute types are used. 971.It .note 972This section holds information in the 973.Dq Note Section 974format described below. 975This section is of type 976.Sy SHT_NOTE . 977No attribute types are used. 978.It .plt 979This section holds the procedure linkage table. 980This section is of type 981.Sy SHT_PROGBITS . 982The attributes are processor-specific. 983.It .relNAME 984This section holds relocation information as described below. 985If the file 986has a loadable segment that includes relocation, the section's attributes 987will include the 988.Sy SHF_ALLOC 989bit. 990Otherwise the bit will be off. 991By convention, 992.Dq NAME 993is supplied by the section to which the relocations apply. 994Thus a relocation 995section for 996.Sy .text 997normally would have the name 998.Sy .rel.text . 999This section is of type 1000.Sy SHT_REL . 1001.It .relaNAME 1002This section holds relocation information as described below. 1003If the file 1004has a loadable segment that includes relocation, the section's attributes 1005will include the 1006.Sy SHF_ALLOC 1007bit. 1008Otherwise the bit will be off. 1009By convention, 1010.Dq NAME 1011is supplied by the section to which the relocations apply. 1012Thus a relocation 1013section for 1014.Sy .text 1015normally would have the name 1016.Sy .rela.text . 1017This section is of type 1018.Sy SHT_RELA . 1019.It .rodata 1020This section holds read-only data that typically contributes to a 1021non-writable segment in the process image. 1022This section is of type 1023.Sy SHT_PROGBITS . 1024The attribute used is 1025.Sy SHF_ALLOC . 1026.It .rodata1 1027This section hold read-only data that typically contributes to a 1028non-writable segment in the process image. 1029This section is of type 1030.Sy SHT_PROGBITS . 1031The attribute used is 1032.Sy SHF_ALLOC . 1033.It .shstrtab 1034This section holds section names. 1035This section is of type 1036.Sy SHT_STRTAB . 1037No attribute types are used. 1038.It .strtab 1039This section holds strings, most commonly the strings that represent the 1040names associated with symbol table entries. 1041If the file has a loadable 1042segment that includes the symbol string table, the section's attributes 1043will include the 1044.Sy SHF_ALLOC 1045bit. 1046Otherwise the bit will be off. 1047This section is of type 1048.Sy SHT_STRTAB . 1049.It .symtab 1050This section holds a symbol table. 1051If the file has a loadable segment 1052that includes the symbol table, the section's attributes will include 1053the 1054.Sy SHF_ALLOC 1055bit. 1056Otherwise the bit will be off. 1057This section is of type 1058.Sy SHT_SYMTAB . 1059.It .text 1060This section holds the 1061.Dq text , 1062or executable instructions, of a program. 1063This section is of type 1064.Sy SHT_PROGBITS . 1065The attributes used are 1066.Sy SHF_ALLOC 1067and 1068.Sy SHF_EXECINSTR . 1069.It .jcr 1070This section holds information about Java classes that must 1071be registered. 1072.It .eh_frame 1073This section holds information used for C++ exception-handling. 1074.El 1075.Pp 1076String table sections hold null-terminated character sequences, commonly 1077called strings. 1078The object file uses these strings to represent symbol 1079and section names. 1080One references a string as an index into the string 1081table section. 1082The first byte, which is index zero, is defined to hold 1083a null character. 1084Similarly, a string table's last byte is defined to 1085hold a null character, ensuring null termination for all strings. 1086.Pp 1087An object file's symbol table holds information needed to locate and 1088relocate a program's symbolic definitions and references. 1089A symbol table 1090index is a subscript into this array. 1091.Pp 1092.Bd -literal -offset indent 1093typedef struct { 1094 Elf32_Word st_name; 1095 Elf32_Addr st_value; 1096 Elf32_Word st_size; 1097 unsigned char st_info; 1098 unsigned char st_other; 1099 Elf32_Half st_shndx; 1100} Elf32_Sym; 1101.Ed 1102.Pp 1103.Bd -literal -offset indent 1104typedef struct { 1105 Elf64_Word st_name; 1106 unsigned char st_info; 1107 unsigned char st_other; 1108 Elf64_Half st_shndx; 1109 Elf64_Addr st_value; 1110 Elf64_Xword st_size; 1111} Elf64_Sym; 1112.Ed 1113.Pp 1114.Bl -tag -width "st_value" -compact 1115.It Dv st_name 1116This member holds an index into the object file's symbol string table, 1117which holds character representations of the symbol names. 1118If the value 1119is non-zero, it represents a string table index that gives the symbol 1120name. 1121Otherwise, the symbol table has no name. 1122.It Dv st_value 1123This member gives the value of the associated symbol. 1124.It Dv st_size 1125Many symbols have associated sizes. 1126This member holds zero if the symbol 1127has no size or an unknown size. 1128.It Dv st_info 1129This member specifies the symbol's type and binding attributes: 1130.Pp 1131.Bl -tag -width "STT_SECTION" -compact 1132.It Dv STT_NOTYPE 1133The symbol's type is not defined. 1134.It Dv STT_OBJECT 1135The symbol is associated with a data object. 1136.It Dv STT_FUNC 1137The symbol is associated with a function or other executable code. 1138.It Dv STT_SECTION 1139The symbol is associated with a section. 1140Symbol table entries of 1141this type exist primarily for relocation and normally have 1142.Sy STB_LOCAL 1143bindings. 1144.It Dv STT_FILE 1145By convention the symbol's name gives the name of the source file 1146associated with the object file. 1147A file symbol has 1148.Sy STB_LOCAL 1149bindings, its section index is 1150.Sy SHN_ABS , 1151and it precedes the other 1152.Sy STB_LOCAL 1153symbols of the file, if it is present. 1154.It Dv STT_LOPROC 1155This value up to and including 1156.Sy STT_HIPROC 1157are reserved for processor-specific semantics. 1158.It Dv STT_HIPROC 1159This value down to and including 1160.Sy STT_LOPROC 1161are reserved for processor-specific semantics. 1162.El 1163.Pp 1164.Bl -tag -width "STB_GLOBAL" -compact 1165.It Dv STB_LOCAL 1166Local symbols are not visible outside the object file containing their 1167definition. 1168Local symbols of the same name may exist in multiple file 1169without interfering with each other. 1170.It Dv STB_GLOBAL 1171Global symbols are visible to all object files being combined. 1172One file's 1173definition of a global symbol will satisfy another file's undefined 1174reference to the same symbol. 1175.It Dv STB_WEAK 1176Weak symbols resemble global symbols, but their definitions have lower 1177precedence. 1178.It Dv STB_LOPROC 1179This value up to and including 1180.Sy STB_HIPROC 1181are reserved for processor-specific semantics. 1182.It Dv STB_HIPROC 1183This value down to and including 1184.Sy STB_LOPROC 1185are reserved for processor-specific semantics. 1186.Pp 1187There are macros for packing and unpacking the binding and type fields: 1188.Pp 1189.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1190.It Xo 1191.Fn ELF32_ST_BIND info 1192.Xc 1193or 1194.Fn ELF64_ST_BIND info 1195extract a binding from an st_info value. 1196.It Xo 1197.Fn ELF64_ST_TYPE info 1198.Xc 1199or 1200.Fn ELF32_ST_TYPE info 1201extract a type from an st_info value. 1202.It Xo 1203.Fn ELF32_ST_INFO bind type 1204.Xc 1205or 1206.Fn ELF64_ST_INFO bind type 1207convert a binding and a type into an st_info value. 1208.El 1209.El 1210.Pp 1211.It Dv st_other 1212This member currently holds zero and has no defined meaning. 1213.It Dv st_shndx 1214Every symbol table entry is 1215.Dq defined 1216in relation to some section. 1217This member holds the relevant section 1218header table index. 1219.El 1220.Pp 1221Relocation is the process of connecting symbolic references with 1222symbolic definitions. 1223Relocatable files must have information that 1224describes how to modify their section contents, thus allowing executable 1225and shared object files to hold the right information for a process' 1226program image. 1227Relocation entries are these data. 1228.Pp 1229Relocation structures that do not need an addend: 1230.Pp 1231.Bd -literal -offset indent 1232typedef struct { 1233 Elf32_Addr r_offset; 1234 Elf32_Word r_info; 1235} Elf32_Rel; 1236.Ed 1237.Bd -literal -offset indent 1238typedef struct { 1239 Elf64_Addr r_offset; 1240 Elf64_Xword r_info; 1241} Elf64_Rel; 1242.Ed 1243.Pp 1244Relocation structures that need an addend: 1245.Pp 1246.Bd -literal -offset indent 1247typedef struct { 1248 Elf32_Addr r_offset; 1249 Elf32_Word r_info; 1250 Elf32_Sword r_addend; 1251} Elf32_Rela; 1252.Ed 1253.Bd -literal -offset indent 1254typedef struct { 1255 Elf64_Addr r_offset; 1256 Elf64_Xword r_info; 1257 Elf64_Sxword r_addend; 1258} Elf64_Rela; 1259.Ed 1260.Pp 1261.Bl -tag -width "r_offset" -compact 1262.It Dv r_offset 1263This member gives the location at which to apply the relocation action. 1264For a relocatable file, the value is the byte offset from the beginning 1265of the section to the storage unit affected by the relocation. 1266For an 1267executable file or shared object, the value is the virtual address of 1268the storage unit affected by the relocation. 1269.It Dv r_info 1270This member gives both the symbol table index with respect to which the 1271relocation must be made and the type of relocation to apply. 1272Relocation 1273types are processor-specific. 1274When the text refers to a relocation 1275entry's relocation type or symbol table index, it means the result of 1276applying 1277.Sy ELF_[32|64]_R_TYPE 1278or 1279.Sy ELF[32|64]_R_SYM , 1280respectively to the entry's 1281.Sy r_info 1282member. 1283.It Dv r_addend 1284This member specifies a constant addend used to compute the value to be 1285stored into the relocatable field. 1286.El 1287.Sh SEE ALSO 1288.Xr as 1 , 1289.Xr gdb 1 , 1290.Xr ld 1 , 1291.Xr objdump 1 , 1292.Xr execve 2 , 1293.Xr core 5 1294.Rs 1295.%A Hewlett Packard 1296.%B Elf-64 Object File Format 1297.Re 1298.Rs 1299.%A Santa Cruz Operation 1300.%B System V Application Binary Interface 1301.Re 1302.Rs 1303.%A Unix System Laboratories 1304.%T Object Files 1305.%B "Executable and Linking Format (ELF)" 1306.Re 1307.Sh HISTORY 1308The ELF header files made their appearance in 1309.Fx 2.2.6 . 1310ELF in itself first appeared in 1311.At V . 1312The ELF format is an adopted standard. 1313.Sh AUTHORS 1314This manual page was written by 1315.An Jeroen Ruigrok van der Werven 1316.Aq asmodai@FreeBSD.org 1317with inspiration from BSDi's 1318.Bsx 1319.Xr elf 5 1320manpage. 1321