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 1.2 2003/06/17 04:37:00 dillon Exp $ 27.\" 28.Dd July 31, 1999 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.Aq Pa 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.Pa 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.Pa sys/elf32.h 68and 69.Pa sys/elf64.h 70instead of 71.Pa 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.Pa sys/elf_common.h 83as well as 84.Pa 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 program address 93Elf32_Half Unsigned halfword field 94Elf32_Off Unsigned file offset 95Elf32_Sword Signed large integer 96Elf32_Word Field or unsigned large 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 program address 103Elf64_Half Unsigned halfword field 104Elf64_Off Unsigned file offset 105Elf64_Sword Signed large integer 106Elf64_Word Field or unsigned large integer 107Elf64_Size Unsigned object size 108Elf64_Quarter Unsigned quarterword field 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_Quarter e_type; 142 Elf64_Quarter e_machine; 143 Elf64_Half e_version; 144 Elf64_Addr e_entry; 145 Elf64_Off e_phoff; 146 Elf64_Off e_shoff; 147 Elf64_Half e_flags; 148 Elf64_Quarter e_ehsize; 149 Elf64_Quarter e_phentsize; 150 Elf64_Quarter e_phnum; 151 Elf64_Quarter e_shentsize; 152 Elf64_Quarter e_shnum; 153 Elf64_Quarter 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'. Currently no flags have been defined. 356.It Dv e_ehsize 357This member holds the ELF header's size in bytes. 358.It Dv e_phentsize 359This member holds the size in bytes of one entry in the file's program header 360table; all entries are the same size. 361.It Dv e_phnum 362This member holds the number of entries in the program header 363table. 364Thus the product of 365.Sy e_phentsize 366and 367.Sy e_phnum 368gives the table's size 369in bytes. 370If a file has no program header, 371.Sy e_phnum 372holds the value zero. 373.It Dv e_shentsize 374This member holds a sections header's size in bytes. 375A section header is one 376entry in the section header table; all entries are the same size. 377.It Dv e_shnum 378This member holds the number of entries in the section header table. 379Thus 380the product of 381.Sy e_shentsize 382and 383.Sy e_shnum 384gives the section header table's size in bytes. 385If a file has no section 386header table, 387.Sy e_shnum 388holds the value of zero. 389.It Dv e_shstrndx 390This member holds the section header table index of the entry associated 391with the section name string table. 392If the file has no section name string 393table, this member holds the value 394.Sy SHN_UNDEF . 395.Pp 396.Bl -tag -width "SHN_LORESERVE" -compact 397.It Dv SHN_UNDEF 398This value marks an undefined, missing, irrelevant, or otherwise meaningless 399section reference. 400For example, a symbol 401.Dq defined 402relative to section number 403.Sy SHN_UNDEF 404is an undefined symbol. 405.It Dv SHN_LORESERVE 406This value specifies the lower bound of the range of reserved indexes. 407.It Dv SHN_LOPROC 408This value up to and including 409.Sy SHN_HIPROC 410are reserved for processor-specific semantics. 411.It Dv SHN_HIPROC 412This value down to and including 413.Sy SHN_LOPROC 414are reserved for processor-specific semantics. 415.It Dv SHN_ABS 416This value specifies absolute values for the corresponding reference. 417For 418example, symbols defined relative to section number 419.Sy SHN_ABS 420have absolute values and are not affected by relocation. 421.It Dv SHN_COMMON 422Symbols defined relative to this section are common symbols, such as Fortran 423COMMON or unallocated C external variables. 424.It Dv SHN_HIRESERVE 425This value specifies the upper bound of the range of the range of reserved 426indices between 427.Sy SHN_LORESERVE 428and 429.Sy SHN_HIRESERVE , 430inclusive; the values do 431not reference the section header table. 432That is, the section header table 433does 434.Em not 435contain entries for the reserved indices. 436.El 437.El 438.Pp 439An executable or shared object file's program header table is an array of 440structures, each describing a segment or other information the system needs 441to prepare the program for execution. 442An object file 443.Em segment 444contains one or more 445.Em sections . 446Program headers are meaningful only for executable and shared object files. 447A file specifies its own program header size with the ELF header's 448.Sy e_phentsize 449and 450.Sy e_phnum 451members. 452As with the Elf executable header, the program header 453also has different versions depending on the architecture: 454.Pp 455.Bd -literal -offset indent 456typedef struct { 457 Elf32_Word p_type; 458 Elf32_Off p_offset; 459 Elf32_Addr p_vaddr; 460 Elf32_Addr p_paddr; 461 Elf32_Size p_filesz; 462 Elf32_Size p_memsz; 463 Elf32_Word p_flags; 464 Elf32_Size p_align; 465} Elf32_Phdr; 466.Ed 467.Pp 468.Bd -literal -offset indent 469typedef struct { 470 Elf64_Half p_type; 471 Elf64_Half p_flags; 472 Elf64_Off p_offset; 473 Elf64_Addr p_vaddr; 474 Elf64_Addr p_paddr; 475 Elf64_Size p_filesz; 476 Elf64_Size p_memsz; 477 Elf64_Size p_align; 478} Elf64_Phdr; 479.Ed 480.Pp 481The main difference between the 32-bit and the 64-bit program header lies 482only in the location of a 483.Sy p_flags 484member in the total struct. 485.Pp 486.Bl -tag -width "p_offset" -compact -offset indent 487.It Dv p_type 488This member of the Phdr struct tells what kind of segment this array 489element describes or how to interpret the array element's information. 490.Bl -tag -width "PT_DYNAMIC" -compact 491.Pp 492.It Dv PT_NULL 493The array element is unused and the other members' values are undefined. 494This lets the program header have ignored entries. 495.It Dv PT_LOAD 496The array element specifies a loadable segment, described by 497.Sy p_filesz 498and 499.Sy p_memsz . 500The bytes from the file are mapped to the beginning of the memory 501segment. 502If the segment's memory size 503.Pq Sy p_memsz 504is larger than the file size 505.Pq Sy p_filesz , 506the 507.Dq extra 508bytes are defined to hold the value 0 and to follow the segment's 509initialized area. 510The file size may not be larger than the memory size. 511Loadable segment entries in the program header table appear in ascending 512order, sorted on the 513.Sy p_vaddr 514member. 515.It Dv PT_DYNAMIC 516The array element specifies dynamic linking information. 517.It Dv PT_INTERP 518The array element specifies the location and size of a null-terminated 519path name to invoke as an interpreter. 520This segment type is meaningful 521only for executable files (though it may occur for shared objects). However 522it may not occur more than once in a file. 523If it is present it must precede 524any loadable segment entry. 525.It Dv PT_NOTE 526The array element specifies the location and size for auxiliary information. 527.It Dv PT_SHLIB 528This segment type is reserved but has unspecified semantics. 529Programs that 530contain an array element of this type do not conform to the ABI. 531.It Dv PT_PHDR 532The array element, if present, specifies the location and size of the program 533header table itself, both in the file and in the memory image of the program. 534This segment type may not occur more than once in a file. 535Moreover, it may 536only occur if the program header table is part of the memory image of the 537program. 538If it is present it must precede any loadable segment entry. 539.It Dv PT_LOPROC 540This value up to and including 541.Sy PT_HIPROC 542are reserved for processor-specific semantics. 543.It Dv PT_HIPROC 544This value down to and including 545.Sy PT_LOPROC 546are reserved for processor-specific semantics. 547.El 548.Pp 549.It Dv p_offset 550This member holds the offset from the beginning of the file at which 551the first byte of the of the segment resides. 552.It Dv p_vaddr 553This member holds the virtual address at which the first byte of the 554segment resides in memory. 555.It Dv p_paddr 556On systems for which physical addressing is relevant, this member is 557reserved for the segment's physical address. 558Under 559.Bx 560this member is 561not used and must be zero. 562.It Dv p_filesz 563This member holds the number of bytes in the file image of the segment. 564It may be zero. 565.It Dv p_memsz 566This member holds the number of bytes in the memory image of the segment. 567It may be zero. 568.It Dv p_flags 569This member holds flags relevant to the segment: 570.Pp 571.Bl -tag -width "PF_X" -compact 572.It Dv PF_X 573An executable segment. 574.It Dv PF_W 575A writable segment. 576.It Dv PF_R 577A readable segment. 578.El 579.Pp 580A text segment commonly has the flags 581.Sy PF_X 582and 583.Sy PF_R . 584A data segment commonly has 585.Sy PF_X , 586.Sy PF_W 587and 588.Sy PF_R . 589.It Dv p_align 590This member holds the value to which the segments are aligned in memory 591and in the file. 592Loadable process segments must have congruent values for 593.Sy p_vaddr 594and 595.Sy p_offset , 596modulo the page size. 597Values of zero and one mean no alignment is required. 598Otherwise, 599.Sy p_align 600should be a positive, integral power of two, and 601.Sy p_vaddr 602should equal 603.Sy p_offset , 604modulo 605.Sy p_align . 606.El 607.Pp 608An file's section header table lets one locate all the file's sections. 609The 610section header table is an array of Elf32_Shdr or Elf64_Shdr structures. 611The 612ELF header's 613.Sy e_shoff 614member gives the byte offset from the beginning of the file to the section 615header table. 616.Sy e_shnum 617holds the number of entries the section header table contains. 618.Sy e_shentsize 619holds the size in bytes of each entry. 620.Pp 621A section header table index is a subscript into this array. 622Some section 623header table indices are reserved. 624An object file does not have sections for 625these special indices: 626.Pp 627.Bl -tag -width "SHN_LORESERVE" -compact 628.It Dv SHN_UNDEF 629This value marks an undefined, missing, irrelevant or otherwise meaningless 630section reference. 631.It Dv SHN_LORESERVE 632This value specifies the lower bound of the range of reserved indices. 633.It Dv SHN_LOPROC 634This value up to and including 635.Sy SHN_HIPROC 636are reserved for processor-specific semantics. 637.It Dv SHN_HIPROC 638This value down to and including 639.Sy SHN_LOPROC 640are reserved for processor-specific semantics. 641.It Dv SHN_ABS 642This value specifies absolute values for the corresponding reference. 643For 644example, symbols defined relative to section number 645.Sy SHN_ABS 646have absolute values and are not affected by relocation. 647.It Dv SHN_COMMON 648Symbols defined relative to this section are common symbols, such as FORTRAN 649COMMON or unallocated C external variables. 650.It Dv SHN_HIRESERVE 651This value specifies the upper bound of the range of reserved indices. 652The 653system reserves indices between 654.Sy SHN_LORESERVE 655and 656.Sy SHN_HIRESERVE , 657inclusive. 658The section header table does not contain entries for the 659reserved indices. 660.El 661.Pp 662The section header has the following structure: 663.Bd -literal -offset indent 664typedef struct { 665 Elf32_Word sh_name; 666 Elf32_Word sh_type; 667 Elf32_Word sh_flags; 668 Elf32_Addr sh_addr; 669 Elf32_Off sh_offset; 670 Elf32_Size sh_size; 671 Elf32_Word sh_link; 672 Elf32_Word sh_info; 673 Elf32_Size sh_addralign; 674 Elf32_Size sh_entsize; 675} Elf32_Shdr; 676.Ed 677.Pp 678.Bd -literal -offset indent 679typedef struct { 680 Elf64_Half sh_name; 681 Elf64_Half sh_type; 682 Elf64_Size sh_flags; 683 Elf64_Addr sh_addr; 684 Elf64_Off sh_offset; 685 Elf64_Size sh_size; 686 Elf64_Half sh_link; 687 Elf64_Half sh_info; 688 Elf64_Size sh_addralign; 689 Elf64_Size sh_entsize; 690} Elf64_Shdr; 691.Ed 692.Pp 693.Bl -tag -width "sh_addralign" -compact 694.It Dv sh_name 695This member specifies the name of the section. 696Its value is an index 697into the section header string table section, giving the location of 698a null-terminated string. 699.It Dv sh_type 700This member categorizes the section's contents and semantics. 701.Pp 702.Bl -tag -width "SHT_PROGBITS" -compact 703.It Dv SHT_NULL 704This value marks the section header as inactive. 705It does not 706have an associated section. 707Other members of the section header 708have undefined values. 709.It Dv SHT_PROGBITS 710The section holds information defined by the program, whose 711format and meaning are determined solely by the program. 712.It Dv SHT_SYMTAB 713This section holds a symbol table. 714Typically, 715.Sy SHT_SYMTAB 716provides symbols for link editing, though it may also be used 717for dynamic linking. 718As a complete symbol table, it may contain 719many symbols unnecessary for dynamic linking. 720An object file can 721also contain a 722.Sy SHN_DYNSYM 723section. 724.It Dv SHT_STRTAB 725This section holds a string table. 726An object file may have multiple 727string table sections. 728.It Dv SHT_RELA 729This section holds relocation entries with explicit addends, such 730as type 731.Sy Elf32_Rela 732for the 32-bit class of object files. 733An object may have multiple 734relocation sections. 735.It Dv SHT_HASH 736This section holds a symbol hash table. 737All object participating in 738dynamic linking must contain a symbol hash table. 739An object file may 740have only one hash table. 741.It Dv SHT_DYNAMIC 742This section holds information for dynamic linking. 743An object file may 744have only one dynamic section. 745.It Dv SHT_NOTE 746This section holds information that marks the file in some way. 747.It Dv SHT_NOBITS 748A section of this type occupies no space in the file but otherwise 749resembles 750.Sy SHN_PROGBITS . 751Although this section contains no bytes, the 752.Sy sh_offset 753member contains the conceptual file offset. 754.It Dv SHT_REL 755This section holds relocation offsets without explicit addends, such 756as type 757.Sy Elf32_Rel 758for the 32-bit class of object files. 759An object file may have multiple 760relocation sections. 761.It Dv SHT_SHLIB 762This section is reserved but has unspecified semantics. 763.It Dv SHT_DYNSYM 764This section holds a minimal set of dynamic linking symbols. 765An 766object file can also contain a 767.Sy SHN_SYMTAB 768section. 769.It Dv SHT_LOPROC 770This value up to and including 771.Sy SHT_HIPROC 772are reserved for processor-specific semantics. 773.It Dv SHT_HIPROC 774This value down to and including 775.Sy SHT_LOPROC 776are reserved for processor-specific semantics. 777.It Dv SHT_LOUSER 778This value specifies the lower bound of the range of indices reserved for 779application programs. 780.It Dv SHT_HIUSER 781This value specifies the upper bound of the range of indices reserved for 782application programs. 783Section types between 784.Sy SHT_LOUSER 785and 786.Sy SHT_HIUSER 787may be used by the application, without conflicting with current or future 788system-defined section types. 789.El 790.Pp 791.It Dv sh_flags 792Sections support one-bit flags that describe miscellaneous attributes. 793If a flag bit is set in 794.Sy sh_flags , 795the attribute is 796.Dq on 797for the section. 798Otherwise, the attribute is 799.Dq off 800or does not apply. 801Undefined attributes are set to zero. 802.Pp 803.Bl -tag -width "SHF_EXECINSTR" -compact 804.It Dv SHF_WRITE 805This section contains data that should be writable during process 806execution. 807.It Dv SHF_ALLOC 808The section occupies memory during process execution. 809Some control 810sections do not reside in the memory image of an object file. 811This 812attribute is off for those sections. 813.It Dv SHF_EXECINSTR 814The section contains executable machine instructions. 815.It Dv SHF_MASKPROC 816All bits included in this mask are reserved for processor-specific 817semantics. 818.El 819.Pp 820.It Dv sh_addr 821If the section will appear in the memory image of a process, this member 822holds the address at which the section's first byte should reside. 823Otherwise, the member contains zero. 824.It Dv sh_offset 825This member's value holds the byte offset from the beginning of the file 826to the first byte in the section. 827One section type, 828.Sy SHT_NOBITS , 829occupies no space in the file, and its 830.Sy sh_offset 831member locates the conceptual placement in the file. 832.It Dv sh_size 833This member holds the section's size in bytes. 834Unless the section type 835is 836.Sy SHT_NOBITS , 837the section occupies 838.Sy sh_size 839bytes in the file. 840A section of type 841.Sy SHT_NOBITS 842may have a non-zero size, but it occupies no space in the file. 843.It Dv sh_link 844This member holds a section header table index link, whose interpretation 845depends on the section type. 846.It Dv sh_info 847This member holds extra information, whose interpretation depends on the 848section type. 849.It Dv sh_addralign 850Some sections have address alignment constraints. 851If a section holds a 852doubleword, the system must ensure doubleword alignment for the entire 853section. 854That is, the value of 855.Sy sh_addr 856must be congruent to zero, modulo the value of 857.Sy sh_addralign . 858Only zero and positive integral powers of two are allowed. 859Values of zero 860or one mean the section has no alignment constraints. 861.It Dv sh_entsize 862Some sections hold a table of fixed-sized entries, such as a symbol table. 863For such a section, this member gives the size in bytes for each entry. 864This member contains zero if the section does not hold a table of 865fixed-size entries. 866.El 867.Pp 868Various sections hold program and control information: 869.Bl -tag -width ".shstrtab" -compact 870.It .bss 871This section holds uninitialized data that contributes to the program's 872memory image. 873By definition, the system initializes the data with zeros 874when the program begins to run. 875This section is of type 876.Sy SHT_NOBITS . 877The attributes types are 878.Sy SHF_ALLOC 879and 880.Sy SHF_WRITE . 881.It .comment 882This section holds version control information. 883This section is of type 884.Sy SHT_PROGBITS . 885No attribute types are used. 886.It .data 887This section holds initialized data that contribute to the program's 888memory image. 889This section is of type 890.Sy SHT_PROGBITS . 891The attribute types are 892.Sy SHF_ALLOC 893and 894.Sy SHF_WRITE . 895.It .data1 896This section holds initialized data that contribute to the program's 897memory image. 898This section is of type 899.Sy SHT_PROGBITS . 900The attribute types are 901.Sy SHF_ALLOC 902and 903.Sy SHF_WRITE . 904.It .debug 905This section holds information for symbolic debugging. 906The contents 907are unspecified. 908This section is of type 909.Sy SHT_PROGBITS . 910No attribute types are used. 911.It .dynamic 912This section holds dynamic linking information. 913The section's attributes 914will include the 915.Sy SHF_ALLOC 916bit. 917Whether the 918.Sy SHF_WRITE 919bit is set is processor-specific. 920This section is of type 921.Sy SHT_DYNAMIC . 922See the attributes above. 923.It .dynstr 924This section holds strings needed for dynamic linking, most commonly 925the strings that represent the names associated with symbol table entries. 926This section is of type 927.Sy SHT_STRTAB . 928The attribute type used is 929.Sy SHF_ALLOC . 930.It .dynsym 931This section holds the dynamic linking symbol table. 932This section is of type 933.Sy SHT_DYNSYM . 934The attribute used is 935.Sy SHF_ALLOC . 936.It .fini 937This section holds executable instructions that contribute to the process 938termination code. 939When a program exits normally the system arranges to 940execute the code in this section. 941This section is of type 942.Sy SHT_PROGBITS . 943The attributes used are 944.Sy SHF_ALLOC 945and 946.Sy SHF_EXECINSTR . 947.It .got 948This section holds the global offset table. 949This section is of type 950.Sy SHT_PROGBITS . 951The attributes are processor-specific. 952.It .hash 953This section holds a symbol hash table. 954This section is of type 955.Sy SHT_HASH . 956The attribute used is 957.Sy SHF_ALLOC . 958.It .init 959This section holds executable instructions that contribute to the process 960initialization code. 961When a program starts to run the system arranges to 962execute the code in this section before calling the main program entry point. 963This section is of type 964.Sy SHT_PROGBITS . 965The attributes used are 966.Sy SHF_ALLOC 967and 968.Sy SHF_EXECINSTR . 969.It .interp 970This section holds the pathname of a program interpreter. 971If the file has 972a loadable segment that includes the section, the section's attributes will 973include the 974.Sy SHF_ALLOC 975bit. 976Otherwise, that bit will be off. 977This section is of type 978.Sy SHT_PROGBITS . 979.It .line 980This section holds line number information for symbolic debugging, which 981describes the correspondence between the program source and the machine code. 982The contents are unspecified. 983This section is of type 984.Sy SHT_PROGBITS . 985No attribute types are used. 986.It .note 987This section holds information in the 988.Dq Note Section 989format described below. 990This section is of type 991.Sy SHT_NOTE . 992No attribute types are used. 993.It .plt 994This section holds the procedure linkage table. 995This section is of type 996.Sy SHT_PROGBITS . 997The attributes are processor-specific. 998.It .relNAME 999This section holds relocation information as described below. 1000If the file 1001has a loadable segment that includes relocation, the section's attributes 1002will include the 1003.Sy SHF_ALLOC 1004bit. 1005Otherwise the bit will be off. 1006By convention, 1007.Dq NAME 1008is supplied by the section to which the relocations apply. 1009Thus a relocation 1010section for 1011.Sy .text 1012normally would have the name 1013.Sy .rel.text . 1014This section is of type 1015.Sy SHT_REL . 1016.It .relaNAME 1017This section holds relocation information as described below. 1018If the file 1019has a loadable segment that includes relocation, the section's attributes 1020will include the 1021.Sy SHF_ALLOC 1022bit. 1023Otherwise the bit will be off. 1024By convention, 1025.Dq NAME 1026is supplied by the section to which the relocations apply. 1027Thus a relocation 1028section for 1029.Sy .text 1030normally would have the name 1031.Sy .rela.text . 1032This section is of type 1033.Sy SHT_RELA . 1034.It .rodata 1035This section holds read-only data that typically contributes to a 1036non-writable segment in the process image. 1037This section is of type 1038.Sy SHT_PROGBITS . 1039The attribute used is 1040.Sy SHF_ALLOC . 1041.It .rodata1 1042This section hold read-only data that typically contributes to a 1043non-writable segment in the process image. 1044This section is of type 1045.Sy SHT_PROGBITS . 1046The attribute used is 1047.Sy SHF_ALLOC . 1048.It .shstrtab 1049This section holds section names. 1050This section is of type 1051.Sy SHT_STRTAB . 1052No attribute types are used. 1053.It .strtab 1054This section holds strings, most commonly the strings that represent the 1055names associated with symbol table entries. 1056If the file has a loadable 1057segment that includes the symbol string table, the section's attributes 1058will include the 1059.Sy SHF_ALLOC 1060bit. 1061Otherwise the bit will be off. 1062This section is of type 1063.Sy SHT_STRTAB . 1064.It .symtab 1065This section holds a symbol table. 1066If the file has a loadable segment 1067that includes the symbol table, the section's attributes will include 1068the 1069.Sy SHF_ALLOC 1070bit. 1071Otherwise the bit will be off. 1072This section is of type 1073.Sy SHT_SYMTAB . 1074.It .text 1075This section holds the 1076.Dq text , 1077or executable instructions, of a program. 1078This section is of type 1079.Sy SHT_PROGBITS . 1080The attributes used are 1081.Sy SHF_ALLOC 1082and 1083.Sy SHF_EXECINSTR . 1084.El 1085.Pp 1086String table sections hold null-terminated character sequences, commonly 1087called strings. 1088The object file uses these strings to represent symbol 1089and section names. 1090One references a string as an index into the string 1091table section. 1092The first byte, which is index zero, is defined to hold 1093a null character. 1094Similarly, a string table's last byte is defined to 1095hold a null character, ensuring null termination for all strings. 1096.Pp 1097An object file's symbol table holds information needed to locate and 1098relocate a program's symbolic definitions and references. 1099A symbol table 1100index is a subscript into this array. 1101.Pp 1102.Bd -literal -offset indent 1103typedef struct { 1104 Elf32_Word st_name; 1105 Elf32_Addr st_value; 1106 Elf32_Size st_size; 1107 unsigned char st_info; 1108 unsigned char st_other; 1109 Elf32_Half st_shndx; 1110} Elf32_Sym; 1111.Ed 1112.Pp 1113.Bd -literal -offset indent 1114typedef struct { 1115 Elf64_Half st_name; 1116 unsigned char st_info; 1117 unsigned char st_other; 1118 Elf64_Quarter st_shndx; 1119 Elf64_Addr st_value; 1120 Elf64_Size st_size; 1121} Elf64_Sym; 1122.Ed 1123.Pp 1124.Bl -tag -width "st_value" -compact 1125.It Dv st_name 1126This member holds an index into the object file's symbol string table, 1127which holds character representations of the symbol names. 1128If the value 1129is non-zero, it represents a string table index that gives the symbol 1130name. 1131Otherwise, the symbol table has no name. 1132.It Dv st_value 1133This member gives the value of the associated symbol. 1134.It Dv st_size 1135Many symbols have associated sizes. 1136This member holds zero if the symbol 1137has no size or an unknown size. 1138.It Dv st_info 1139This member specifies the symbol's type and binding attributes: 1140.Pp 1141.Bl -tag -width "STT_SECTION" -compact 1142.It Dv STT_NOTYPE 1143The symbol's type is not defined. 1144.It Dv STT_OBJECT 1145The symbol is associated with a data object. 1146.It Dv STT_FUNC 1147The symbol is associated with a function or other executable code. 1148.It Dv STT_SECTION 1149The symbol is associated with a section. 1150Symbol table entries of 1151this type exist primarily for relocation and normally have 1152.Sy STB_LOCAL 1153bindings. 1154.It Dv STT_FILE 1155By convention the symbol's name gives the name of the source file 1156associated with the object file. 1157A file symbol has 1158.Sy STB_LOCAL 1159bindings, its section index is 1160.Sy SHN_ABS , 1161and it precedes the other 1162.Sy STB_LOCAL 1163symbols of the file, if it is present. 1164.It Dv STT_LOPROC 1165This value up to and including 1166.Sy STT_HIPROC 1167are reserved for processor-specific semantics. 1168.It Dv STT_HIPROC 1169This value down to and including 1170.Sy STT_LOPROC 1171are reserved for processor-specific semantics. 1172.El 1173.Pp 1174.Bl -tag -width "STB_GLOBAL" -compact 1175.It Dv STB_LOCAL 1176Local symbols are not visible outside the object file containing their 1177definition. 1178Local symbols of the same name may exist in multiple file 1179without interfering with each other. 1180.It Dv STB_GLOBAL 1181Global symbols are visible to all object files being combined. 1182One file's 1183definition of a global symbol will satisfy another file's undefined 1184reference to the same symbol. 1185.It Dv STB_WEAK 1186Weak symbols resemble global symbols, but their definitions have lower 1187precedence. 1188.It Dv STB_LOPROC 1189This value up to and including 1190.Sy STB_HIPROC 1191are reserved for processor-specific semantics. 1192.It Dv STB_HIPROC 1193This value down to and including 1194.Sy STB_LOPROC 1195are reserved for processor-specific semantics. 1196.Pp 1197There are macros for packing and unpacking the binding and type fields: 1198.Pp 1199.Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact 1200.It Xo 1201.Fn ELF32_ST_BIND info 1202.Xc 1203or 1204.Fn ELF64_ST_BIND info 1205extract a binding from an st_info value. 1206.It Xo 1207.Fn ELF64_ST_TYPE info 1208.Xc 1209or 1210.Fn ELF32_ST_TYPE info 1211extract a type from an st_info value. 1212.It Xo 1213.Fn ELF32_ST_INFO bind type 1214.Xc 1215or 1216.Fn ELF64_ST_INFO bind type 1217convert a binding and a type into an st_info value. 1218.El 1219.El 1220.Pp 1221.It Dv st_other 1222This member currently holds zero and has no defined meaning. 1223.It Dv st_shndx 1224Every symbol table entry is 1225.Dq defined 1226in relation to some action. 1227This member holds the relevant section 1228header table index. 1229.El 1230.Pp 1231Relocation is the process of connecting symbolic references with 1232symbolic definitions. 1233Relocatable files must have information that 1234describes how to modify their section contents, thus allowing executable 1235and shared object files to hold the right information for a process' 1236program image. 1237Relocation entries are these data. 1238.Pp 1239Relocation structures that do not need an addend: 1240.Pp 1241.Bd -literal -offset indent 1242typedef struct { 1243 Elf32_Addr r_offset; 1244 Elf32_Word r_info; 1245} Elf32_Rel; 1246.Ed 1247.Bd -literal -offset indent 1248typedef struct { 1249 Elf64_Addr r_offset; 1250 Elf64_Size r_info; 1251} Elf64_Rel; 1252.Ed 1253.Pp 1254Relocation structures that need an addend: 1255.Pp 1256.Bd -literal -offset indent 1257typedef struct { 1258 Elf32_Addr r_offset; 1259 Elf32_Word r_info; 1260 Elf32_Sword r_addend; 1261} Elf32_Rela; 1262.Ed 1263.Bd -literal -offset indent 1264typedef struct { 1265 Elf64_Addr r_offset; 1266 Elf64_Size r_info; 1267 Elf64_Off r_addend; 1268} Elf64_Rela; 1269.Ed 1270.Pp 1271.Bl -tag -width "r_offset" -compact 1272.It Dv r_offset 1273This member gives the location at which to apply the relocation action. 1274For a relocatable file, the value is the byte offset from the beginning 1275of the section to the storage unit affected by the relocation. 1276For an 1277executable file or shared object, the value is the virtual address of 1278the storage unit affected by the relocation. 1279.It Dv r_info 1280This member gives both the symbol table index with respect to which the 1281relocation must be made and the type of relocation to apply. 1282Relocation 1283types are processor-specific. 1284When the text refers to a relocation 1285entry's relocation type or symbol table index, it means the result of 1286applying 1287.Sy ELF_[32|64]_R_TYPE 1288or 1289.Sy ELF[32|64]_R_SYM , 1290respectively to the entry's 1291.Sy r_info 1292member. 1293.It Dv r_addend 1294This member specifies a constant addend used to compute the value to be 1295stored into the relocatable field. 1296.El 1297.Sh SEE ALSO 1298.Xr as 1 , 1299.Xr gdb 1 , 1300.Xr ld 1 , 1301.Xr objdump 1 , 1302.Xr execve 2 , 1303.Xr core 5 1304.Rs 1305.%A Hewlett Packard 1306.%B Elf-64 Object File Format 1307.Re 1308.Rs 1309.%A Santa Cruz Operation 1310.%B System V Application Binary Interface 1311.Re 1312.Rs 1313.%A Unix System Laboratories 1314.%T Object Files 1315.%B "Executable and Linking Format (ELF)" 1316.Re 1317.Sh HISTORY 1318The ELF header files made their appearance in 1319.Fx 2.2.6 . 1320ELF in itself first appeared in 1321.At V . 1322The ELF format is an adopted standard. 1323.Sh AUTHORS 1324This manual page was written by 1325.An Jeroen Ruigrok van der Werven 1326.Aq asmodai@FreeBSD.org 1327with inspiration from BSDi's 1328.Bsx 1329.Xr elf 5 1330manpage. 1331