.\" $OpenBSD: elf.5,v 1.34 2017/10/27 08:36:42 mpi Exp $ .\"Copyright (c) 1999 Jeroen Ruigrok van der Werven .\"All rights reserved. .\" .\"Redistribution and use in source and binary forms, with or without .\"modification, are permitted provided that the following conditions .\"are met: .\"1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\"2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\"THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\"ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\"IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\"ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\"FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\"DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\"OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\"HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\"LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\"OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\"SUCH DAMAGE. .\" .\" $FreeBSD: src/share/man/man5/elf.5,v 1.21 2001/10/01 16:09:23 ru Exp $ .\" .Dd $Mdocdate: October 27 2017 $ .Dt ELF 5 .Os .Sh NAME .Nm elf .Nd format of ELF executable binary files .Sh SYNOPSIS .In elf.h .Sh DESCRIPTION The header file .In elf.h defines the format of ELF executable binary files. Amongst these files are normal executable files, relocatable object files, core files and shared libraries. .Pp An executable file using the ELF file format consists of an ELF header, followed by a program header table or a section header table, or both. The ELF header is always at offset zero of the file. The program header table and the section header table's offset in the file are defined in the ELF header. The two tables describe the rest of the particularities of the file. .Pp Applications which wish to process ELF binary files for their native architecture only should include .In elf.h in their source code. These applications should need to refer to all the types and structures by their generic names .Dq Elf_xxx and to the macros by .Dq ELF_xxx . Applications written this way can be compiled on any architecture, regardless of whether the host is 32-bit or 64-bit. .Pp Should an application need to process ELF files of an unknown architecture, then the application needs to explicitly use either .Dq Elf32_xxx or .Dq Elf64_xxx type and structure names. Likewise, the macros need to be identified by .Dq ELF32_xxx or .Dq ELF64_xxx . .Pp This header file describes the above mentioned headers as C structures and also includes structures for dynamic sections, relocation sections and symbol tables. .Pp The following types are used for 32-bit architectures: .Bd -literal -offset indent Elf32_Addr Unsigned program address Elf32_Off Unsigned file offset Elf32_Sword Signed large integer Elf32_Word Unsigned large integer Elf32_Half Unsigned medium integer .Ed .Pp And the following types are used for 64-bit architectures: .Bd -literal -offset indent Elf64_Addr Unsigned program address Elf64_Off Unsigned file offset Elf64_Shalf Signed halfword field Elf64_Sword Signed large integer Elf64_Word Field or unsigned large integer Elf64_Sxword Signed object size or alignment Elf64_Xword Unsigned object size or alignment Elf64_Half Unsigned halfword field Elf64_Quarter Unsigned quarterword field .Ed .Pp All data structures that the file format defines follow the .Dq natural size and alignment guidelines for the relevant class. If necessary, data structures contain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4, etc. .Pp The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr: .Bd -literal -offset indent typedef struct { unsigned char e_ident[EI_NIDENT]; Elf32_Half e_type; Elf32_Half e_machine; Elf32_Word e_version; Elf32_Addr e_entry; Elf32_Off e_phoff; Elf32_Off e_shoff; Elf32_Word e_flags; Elf32_Half e_ehsize; Elf32_Half e_phentsize; Elf32_Half e_phnum; Elf32_Half e_shentsize; Elf32_Half e_shnum; Elf32_Half e_shstrndx; } Elf32_Ehdr; .Ed .Bd -literal -offset indent typedef struct { unsigned char e_ident[EI_NIDENT]; Elf64_Quarter e_type; Elf64_Quarter e_machine; Elf64_Half e_version; Elf64_Addr e_entry; Elf64_Off e_phoff; Elf64_Off e_shoff; Elf64_Half e_flags; Elf64_Quarter e_ehsize; Elf64_Quarter e_phentsize; Elf64_Quarter e_phnum; Elf64_Quarter e_shentsize; Elf64_Quarter e_shnum; Elf64_Quarter e_shstrndx; } Elf64_Ehdr; .Ed .Pp The fields have the following meanings: .Bl -tag -width "e_phentsize" -offset indent .It Dv e_ident This array of bytes specifies how to interpret the file, independent of the processor or the file's remaining contents. Within this array everything is named by macros, which start with the prefix .Sy EI_ and may contain values which start with the prefix .Sy ELF . The following macros are defined: .Bl -tag -width "EI_VERSION" .It Dv EI_MAG0 The first byte of the magic number. It must be filled with .Dv ELFMAG0 . .It Dv EI_MAG1 The second byte of the magic number. It must be filled with .Dv ELFMAG1 . .It Dv EI_MAG2 The third byte of the magic number. It must be filled with .Dv ELFMAG2 . .It Dv EI_MAG3 The fourth byte of the magic number. It must be filled with .Dv ELFMAG3 . .It Dv EI_CLASS The fifth byte identifies the architecture for this binary: .Pp .Bl -tag -width "ELFCLASSNONE" -compact .It Dv ELFCLASSNONE This class is invalid. .It Dv ELFCLASS32 This defines the 32-bit architecture. It supports machines with files and virtual address spaces up to 4 Gigabytes. .It Dv ELFCLASS64 This defines the 64-bit architecture. .El .It Dv EI_DATA The sixth byte specifies the data encoding of the processor-specific data in the file. Currently these encodings are supported: .Pp .Bl -tag -width "ELFDATA2LSB" -compact .It Dv ELFDATANONE Unknown data format. .It Dv ELFDATA2LSB Two's complement, little-endian. .It Dv ELFDATA2MSB Two's complement, big-endian. .El .It Dv EI_VERSION The version number of the ELF specification: .Pp .Bl -tag -width "EV_CURRENT" -compact .It Dv EV_NONE Invalid version. .It Dv EV_CURRENT Current version. .El .\" .It Dv EI_OSABI .\" This byte identifies the operating system .\" and ABI to which the object is targeted. .\" Some fields in other ELF structures have flags .\" and values that have platform specific meanings; .\" the interpretation of those fields is determined by the value of this byte. .\" The following values are currently defined: .\" .Pp .\" .Bl -tag -width "ELFOSABI_STANDALONE" -compact .\" .It Dv ELFOSABI_SYSV .\" UNIX System V ABI. .\" .It Dv ELFOSABI_HPUX .\" HP-UX operating system ABI. .\" .It Dv ELFOSABI_NETBSD .\" .Nx .\" operating system ABI. .\" .It Dv ELFOSABI_LINUX .\" GNU/Linux operating system ABI. .\" .It Dv ELFOSABI_HURD .\" GNU/Hurd operating system ABI. .\" .It Dv ELFOSABI_86OPEN .\" 86Open Common IA32 ABI. .\" .It Dv ELFOSABI_SOLARIS .\" Solaris operating system ABI. .\" .It Dv ELFOSABI_MONTEREY .\" Monterey project ABI. .\" .It Dv ELFOSABI_IRIX .\" IRIX operating system ABI. .\" .It Dv ELFOSABI_FREEBSD .\" .Fx .\" operating system ABI. .\" .It Dv ELFOSABI_TRU64 .\" TRU64 UNIX operating system ABI. .\" .It Dv ELFOSABI_ARM .\" ARM architecture ABI. .\" .It Dv ELFOSABI_STANDALONE .\" Stand-alone (embedded) ABI. .\" .El .\" .It Dv EI_ABIVERSION .\" This byte identifies the version of the ABI .\" to which the object is targeted. .\" This field is used to distinguish among incompatible versions of an ABI. .\" The interpretation of this version number .\" is dependent on the ABI identified by the EI_OSABI field. .\" Applications conforming to this specification use the value 0. .It Dv EI_PAD Start of padding. These bytes are reserved and set to zero. Programs which read them should ignore them. The value for EI_PAD will change in the future if currently unused bytes are given meanings. .It Dv EI_BRAND Start of architecture identification. .It Dv EI_NIDENT The size of the e_ident array. .El .It Dv e_type This member of the structure identifies the object file type: .Pp .Bl -tag -width "ET_NONE" -compact .It Dv ET_NONE An unknown type. .It Dv ET_REL A relocatable file. .It Dv ET_EXEC An executable file. .It Dv ET_DYN A shared object. .It Dv ET_CORE A core file. .El .It Dv e_machine This member specifies the required architecture for an individual file: .Pp .Bl -tag -width "EM_MIPS_RS4_BE" -compact .It Dv EM_NONE An unknown machine. .It Dv EM_M32 AT&T WE 32100. .It Dv EM_SPARC Sun Microsystems SPARC. .It Dv EM_386 Intel 80386. .It Dv EM_68K Motorola 68000. .It Dv EM_88K Motorola 88000. .It Dv EM_486 Intel 80486. .It Dv EM_860 Intel 80860. .It Dv EM_MIPS MIPS RS3000 (big-endian only). .It Dv EM_MIPS_RS4_BE MIPS RS4000 (big-endian only). .It Dv EM_SPARC64 SPARC v9 64-bit (unofficial). .It Dv EM_PARISC HPPA. .It Dv EM_SPARC32PLUS SPARC with enhanced instruction set. .It Dv EM_PPC PowerPC. .It Dv EM_ARM Advanced RISC Machines ARM. .It Dv EM_ALPHA Compaq [DEC] Alpha. .It Dv EM_SH Hitachi/Renesas Super-H. .It Dv EM_SPARCV9 SPARC v9 64-bit. .It Dv EM_IA_64 Intel IA-64. .It Dv EM_AMD64 AMD64. .It Dv EM_VAX DEC Vax. .It Dv EM_ALPHA_EXP Compaq [DEC] Alpha with enhanced instruction set. .El .It Dv e_version This member identifies the file version: .Pp .Bl -tag -width "EV_CURRENT" -compact .It Dv EV_NONE Invalid version. .It Dv EV_CURRENT Current version. .El .It Dv e_entry This member gives the virtual address to which the system first transfers control, thus starting the process. If the file has no associated entry point, this member holds zero. .It Dv e_phoff This member holds the program header table's file offset in bytes. If the file has no program header table, this member holds zero. .It Dv e_shoff This member holds the section header table's file offset in bytes. If the file has no section header table this member holds zero. .It Dv e_flags This member holds processor-specific flags associated with the file. Flag names take the form EF_`machine_flag'. Currently no flags have been defined. .It Dv e_ehsize This member holds the ELF header's size in bytes. .It Dv e_phentsize This member holds the size in bytes of one entry in the file's program header table; all entries are the same size. .It Dv e_phnum This member holds the number of entries in the program header table. Thus the product of .Sy e_phentsize and .Sy e_phnum gives the table's size in bytes. If a file has no program header, .Sy e_phnum holds the value zero. .It Dv e_shentsize This member holds a sections header's size in bytes. A section header is one entry in the section header table; all entries are the same size. .It Dv e_shnum This member holds the number of entries in the section header table. Thus the product of .Sy e_shentsize and .Sy e_shnum gives the section header table's size in bytes. If a file has no section header table, .Sy e_shnum holds the value of zero. .It Dv e_shstrndx This member holds the section header table index of the entry associated with the section name string table. If the file has no section name string table, this member holds the value .Dv SHN_UNDEF . .El .Pp An executable or shared object file's program header table is an array of structures, each describing a segment or other information the system needs to prepare the program for execution. An object file .Em segment contains one or more .Em sections . Program headers are meaningful only for executable and shared object files. A file specifies its own program header size with the ELF header's .Sy e_phentsize and .Sy e_phnum members. As with the ELF executable header, the program header also has different versions depending on the architecture: .Bd -literal -offset indent typedef struct { Elf32_Word p_type; Elf32_Off p_offset; Elf32_Addr p_vaddr; Elf32_Addr p_paddr; Elf32_Word p_filesz; Elf32_Word p_memsz; Elf32_Word p_flags; Elf32_Word p_align; } Elf32_Phdr; .Ed .Bd -literal -offset indent typedef struct { Elf64_Half p_type; Elf64_Half p_flags; Elf64_Off p_offset; Elf64_Addr p_vaddr; Elf64_Addr p_paddr; Elf64_Xword p_filesz; Elf64_Xword p_memsz; Elf64_Xword p_align; } Elf64_Phdr; .Ed .Pp The main difference between the 32-bit and the 64-bit program header lies only in the location of a .Sy p_flags member in the total struct. .Bl -tag -width "p_offset" -offset indent .It Dv p_type This member of the Phdr struct tells what kind of segment this array element describes or how to interpret the array element's information. .Bl -tag -width "PT_DYNAMIC" .It Dv PT_NULL The array element is unused and the other members' values are undefined. This lets the program header have ignored entries. .It Dv PT_LOAD The array element specifies a loadable segment, described by .Sy p_filesz and .Sy p_memsz . The bytes from the file are mapped to the beginning of the memory segment. If the segment's memory size .Pq Sy p_memsz is larger than the file size .Pq Sy p_filesz , the .Dq extra bytes are defined to hold the value 0 and to follow the segment's initialized area. The file size may not be larger than the memory size. Loadable segment entries in the program header table appear in ascending order, sorted on the .Sy p_vaddr member. .It Dv PT_DYNAMIC The array element specifies the location and size of the dynamic section, both in the file and in the memory image of the program. This segment type may not occur more than once in a file and may only occur if the dynamic section is part of the memory image of the program. .It Dv PT_INTERP The array element specifies the location and size of a null-terminated path name to invoke as an interpreter. This segment type is meaningful only for executable files (though it may occur for shared objects). However it may not occur more than once in a file. If it is present, it must precede any loadable segment entry. .It Dv PT_NOTE The array element specifies the location and size for auxiliary information. .It Dv PT_SHLIB This segment type is reserved but has unspecified semantics. Programs that contain an array element of this type do not conform to the ABI. .It Dv PT_PHDR The array element specifies the location and size of the program header table itself, both in the file and in the memory image of the program. This segment type may not occur more than once in a file and may only occur if the program header table is part of the memory image of the program. If it is present, it must precede any loadable segment entry. .It Dv PT_TLS The array element specifies the location and size of the thread-local storage for this file. Each thread in a process loading this file will have the segment's memory size .Pq Sy p_memsz allocated for it, where the bytes up to the segment's file size .Pq Sy p_filesz will be initialized with the data in this segment and the remaining .Dq extra bytes will be set to zero. This segment type may not occur more than once in a file and may only occur if the thread-local storage is part of the memory image of the program. .It Dv PT_GNU_EH_FRAME The array element specifies the location and size of the GNU exception frame header, both in the file and in the memory image of the program. This segment type may not occur more than once in a file and may only occur if the GNU exception frame header is part of the memory image of the program. .It Dv PT_GNU_RELRO The array element specifies the location and size of a part of the memory image of the program that should be made read-only once all immediate relocation processing for the file has been performed. This segment type may not occur more than once in a file. .It Dv PT_OPENBSD_RANDOMIZE The array element specifies the location and size of a part of the memory image of the program that must be filled with random data before any code in the object is executed. The memory region specified by a segment of this type may overlap the region specified by a .Dv PT_GNU_RELRO segment, in which case the intersection will be filled with random data before being marked read-only. This segment type may occur more than once in a file, but a limit on the total number of bytes in the segments for an object of no less than 65536 bytes may be imposed. .It Dv PT_OPENBSD_WXNEEDED The array element specifies that a process executing this file may need to be able to map or protect memory regions as simultaneously executable and writable. If the system is unable or unwilling to permit that for this executable then it may fail immediately. This segment type is meaningful only for executable files and is ignored in other objects. .It Dv PT_LOOS This value up to and including .Dv PT_HIOS is reserved for operating system-specific semantics. .It Dv PT_HIOS This value down to and including .Dv PT_LOOS is reserved for operating system-specific semantics. .It Dv PT_LOPROC This value up to and including .Dv PT_HIPROC is reserved for processor-specific semantics. .It Dv PT_HIPROC This value down to and including .Dv PT_LOPROC is reserved for processor-specific semantics. .El .It Dv p_offset This member holds the offset from the beginning of the file at which the first byte of the segment resides. .It Dv p_vaddr This member holds the virtual address at which the first byte of the segment resides in memory. .It Dv p_paddr On systems for which physical addressing is relevant, this member is reserved for the segment's physical address. Under .Bx this member is not used and must be zero. .It Dv p_filesz This member holds the number of bytes in the file image of the segment. It may be zero. .It Dv p_memsz This member holds the number of bytes in the memory image of the segment. It may be zero. .It Dv p_flags This member holds flags relevant to the segment: .Pp .Bl -tag -width "PF_X" -compact .It Dv PF_X An executable segment. .It Dv PF_W A writable segment. .It Dv PF_R A readable segment. .El .Pp A text segment commonly has the flags .Dv PF_X and .Dv PF_R . A data segment commonly has .Dv PF_X , .Dv PF_W and .Dv PF_R . .It Dv p_align This member holds the value to which the segments are aligned in memory and in the file. Loadable process segments must have congruent values for .Sy p_vaddr and .Sy p_offset , modulo the page size. Values of zero and one mean no alignment is required. Otherwise, .Sy p_align should be a positive, integral power of two, and .Sy p_vaddr should equal .Sy p_offset , modulo .Sy p_align . .El .Pp A file's section header table lets one locate all the file's sections. The section header table is an array of Elf32_Shdr or Elf64_Shdr structures. The ELF header's .Sy e_shoff member gives the byte offset from the beginning of the file to the section header table. .Sy e_shnum holds the number of entries the section header table contains. .Sy e_shentsize holds the size in bytes of each entry. .Pp A section header table index is a subscript into this array. Some section header table indices are reserved. An object file does not have sections for these special indices: .Bl -tag -width "SHN_LORESERVE" .It Dv SHN_UNDEF This value marks an undefined, missing, irrelevant or otherwise meaningless section reference. For example, a symbol .Dq defined relative to section number .Dv SHN_UNDEF is an undefined symbol. .It Dv SHN_LORESERVE This value specifies the lower bound of the range of reserved indices. .It Dv SHN_LOPROC This value up to and including .Dv SHN_HIPROC is reserved for processor-specific semantics. .It Dv SHN_HIPROC This value down to and including .Dv SHN_LOPROC is reserved for processor-specific semantics. .It Dv SHN_ABS This value specifies the absolute value for the corresponding reference. For example, a symbol defined relative to section number .Dv SHN_ABS has an absolute value and is not affected by relocation. .It Dv SHN_COMMON Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or unallocated C external variables. .It Dv SHN_HIRESERVE This value specifies the upper bound of the range of reserved indices. The system reserves indices between .Dv SHN_LORESERVE and .Dv SHN_HIRESERVE , inclusive. The section header table does not contain entries for the reserved indices. .El .Pp The section header has the following structure: .Bd -literal -offset indent typedef struct { Elf32_Word sh_name; Elf32_Word sh_type; Elf32_Word sh_flags; Elf32_Addr sh_addr; Elf32_Off sh_offset; Elf32_Word sh_size; Elf32_Word sh_link; Elf32_Word sh_info; Elf32_Word sh_addralign; Elf32_Word sh_entsize; } Elf32_Shdr; .Ed .Bd -literal -offset indent typedef struct { Elf64_Half sh_name; Elf64_Half sh_type; Elf64_Xword sh_flags; Elf64_Addr sh_addr; Elf64_Off sh_offset; Elf64_Xword sh_size; Elf64_Half sh_link; Elf64_Half sh_info; Elf64_Xword sh_addralign; Elf64_Xword sh_entsize; } Elf64_Shdr; .Ed .Bl -tag -width "sh_addralign" .It Dv sh_name This member specifies the name of the section. Its value is an index into the section header string table section, giving the location of a null-terminated string. .It Dv sh_type This member categorizes the section's contents and semantics. .Bl -tag -width "SHT_PROGBITS" .It Dv SHT_NULL This value marks the section header as inactive. It does not have an associated section. Other members of the section header have undefined values. .It Dv SHT_PROGBITS This section holds information defined by the program, whose format and meaning are determined solely by the program. .It Dv SHT_SYMTAB This section holds a symbol table. Typically, .Dv SHT_SYMTAB provides symbols for link editing, though it may also be used for dynamic linking. As a complete symbol table, it may contain many symbols unnecessary for dynamic linking. An object file can also contain a .Dv SHT_DYNSYM section. .It Dv SHT_STRTAB This section holds a string table. An object file may have multiple string table sections. .It Dv SHT_RELA This section holds relocation entries with explicit addends, such as type .Sy Elf32_Rela for the 32-bit class of object files. An object may have multiple relocation sections. .It Dv SHT_HASH This section holds a symbol hash table. An object participating in dynamic linking must contain a symbol hash table. An object file may have only one hash table. .It Dv SHT_DYNAMIC This section holds information for dynamic linking. An object file may have only one dynamic section. .It Dv SHT_NOTE This section holds information that marks the file in some way. .It Dv SHT_NOBITS A section of this type occupies no space in the file but otherwise resembles .Dv SHT_PROGBITS . Although this section contains no bytes, the .Sy sh_offset member contains the conceptual file offset. .It Dv SHT_REL This section holds relocation offsets without explicit addends, such as type .Sy Elf32_Rel for the 32-bit class of object files. An object file may have multiple relocation sections. .It Dv SHT_SHLIB This section is reserved but has unspecified semantics. .It Dv SHT_DYNSYM This section holds a minimal set of dynamic linking symbols. An object file can also contain a .Dv SHT_SYMTAB section. .It Dv SHT_LOPROC This value up to and including .Dv SHT_HIPROC is reserved for processor-specific semantics. .It Dv SHT_HIPROC This value down to and including .Dv SHT_LOPROC is reserved for processor-specific semantics. .It Dv SHT_LOUSER This value specifies the lower bound of the range of indices reserved for application programs. .It Dv SHT_HIUSER This value specifies the upper bound of the range of indices reserved for application programs. Section types between .Dv SHT_LOUSER and .Dv SHT_HIUSER may be used by the application, without conflicting with current or future system-defined section types. .El .It Dv sh_flags Sections support one-bit flags that describe miscellaneous attributes. If a flag bit is set in .Sy sh_flags , the attribute is .Dq on for the section. Otherwise, the attribute is .Dq off or does not apply. Undefined attributes are set to zero. .Pp .Bl -tag -width "SHF_EXECINSTR" -compact .It Dv SHF_WRITE This section contains data that should be writable during process execution. .It Dv SHF_ALLOC This section occupies memory during process execution. Some control sections do not reside in the memory image of an object file. This attribute is off for those sections. .It Dv SHF_EXECINSTR This section contains executable machine instructions. .It Dv SHF_TLS This section is for thread-local storage. .It Dv SHF_MASKPROC All bits included in this mask are reserved for processor-specific semantics. .El .It Dv sh_addr If this section appears in the memory image of a process, this member holds the address at which the section's first byte should reside. Otherwise, the member contains zero. .It Dv sh_offset This member's value holds the byte offset from the beginning of the file to the first byte in the section. One section type, .Dv SHT_NOBITS , occupies no space in the file, and its .Sy sh_offset member locates the conceptual placement in the file. .It Dv sh_size This member holds the section's size in bytes. Unless the section type is .Dv SHT_NOBITS , the section occupies .Sy sh_size bytes in the file. A section of type .Dv SHT_NOBITS may have a non-zero size, but it occupies no space in the file. .It Dv sh_link This member holds a section header table index link, whose interpretation depends on the section type. .It Dv sh_info This member holds extra information, whose interpretation depends on the section type. .It Dv sh_addralign Some sections have address alignment constraints. If a section holds a doubleword, the system must ensure doubleword alignment for the entire section. That is, the value of .Sy sh_addr must be congruent to zero, modulo the value of .Sy sh_addralign . Only zero and positive integral powers of two are allowed. Values of zero or one mean the section has no alignment constraints. .It Dv sh_entsize Some sections hold a table of fixed-sized entries, such as a symbol table. For such a section, this member gives the size in bytes for each entry. This member contains zero if the section does not hold a table of fixed-size entries. .El .Pp Various sections hold program and control information: .Bl -tag -width ".shstrtab" .It .SUNW_ctf This section contains the (un)compressed Compact C-Type Format data describing the object's types and symbols. This section is of type .Dv SHT_PROGBITS . .It .bss This section holds uninitialized data that contribute to the program's memory image. By definition, the system initializes the data with zeros when the program begins to run. This section is of type .Dv SHT_NOBITS . The attribute types are .Dv SHF_ALLOC and .Dv SHF_WRITE . .It .comment This section holds version control information. This section is of type .Dv SHT_PROGBITS . No attribute types are used. .It .ctors This section holds initialized pointers to the C++ constructor functions. This section is of type .Dv SHT_PROGBITS . The attribute types are .Dv SHF_ALLOC and .Dv SHF_WRITE . .It .data This section holds initialized data that contribute to the program's memory image. This section is of type .Dv SHT_PROGBITS . The attribute types are .Dv SHF_ALLOC and .Dv SHF_WRITE . .It .data1 This section holds initialized data that contribute to the program's memory image. This section is of type .Dv SHT_PROGBITS . The attribute types are .Dv SHF_ALLOC and .Dv SHF_WRITE . .It .debug This section holds information for symbolic debugging. The contents are unspecified. This section is of type .Dv SHT_PROGBITS . No attribute types are used. .It .dtors This section holds initialized pointers to the C++ destructor functions. This section is of type .Dv SHT_PROGBITS . The attribute types are .Dv SHF_ALLOC and .Dv SHF_WRITE . .It .dynamic This section holds dynamic linking information. The section's attributes will include the .Dv SHF_ALLOC bit. Whether the .Dv SHF_WRITE bit is set is processor-specific. This section is of type .Dv SHT_DYNAMIC . See the attributes above. .It .dynstr This section holds strings needed for dynamic linking, most commonly the strings that represent the names associated with symbol table entries. This section is of type .Dv SHT_STRTAB . The attribute type used is .Dv SHF_ALLOC . .It .dynsym This section holds the dynamic linking symbol table. This section is of type .Dv SHT_DYNSYM . The attribute used is .Dv SHF_ALLOC . .It .fini This section holds executable instructions that contribute to the process termination code. When a program exits normally the system arranges to execute the code in this section. This section is of type .Dv SHT_PROGBITS . The attributes used are .Dv SHF_ALLOC and .Dv SHF_EXECINSTR . .It .got This section holds the global offset table. This section is of type .Dv SHT_PROGBITS . The attributes are processor-specific. .It .hash This section holds a symbol hash table. This section is of type .Dv SHT_HASH . The attribute used is .Dv SHF_ALLOC . .It .init This section holds executable instructions that contribute to the process initialization code. When a program starts to run the system arranges to execute the code in this section before calling the main program entry point. This section is of type .Dv SHT_PROGBITS . The attributes used are .Dv SHF_ALLOC and .Dv SHF_EXECINSTR . .It .interp This section holds the pathname of a program interpreter. If the file has a loadable segment that includes the section, the section's attributes will include the .Dv SHF_ALLOC bit. Otherwise, that bit will be off. This section is of type .Dv SHT_PROGBITS . .It .line This section holds line number information for symbolic debugging, which describes the correspondence between the program source and the machine code. The contents are unspecified. This section is of type .Dv SHT_PROGBITS . No attribute types are used. .It .note This section holds information in the note section format described below. This section is of type .Dv SHT_NOTE . No attribute types are used. .Ox native executables usually contain a .Sy .note.openbsd.ident section to identify themselves, for the kernel to bypass any compatibility ELF binary emulation tests when loading the file. .It .plt This section holds the procedure linkage table. This section is of type .Dv SHT_PROGBITS . The attributes are processor-specific. .It .relNAME This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the .Dv SHF_ALLOC bit. Otherwise the bit will be off. By convention, .Dq NAME is supplied by the section to which the relocations apply. Thus a relocation section for .Sy .text normally would have the name .Sy .rel.text . This section is of type .Dv SHT_REL . .It .relaNAME This section holds relocation information as described below. If the file has a loadable segment that includes relocation, the section's attributes will include the .Dv SHF_ALLOC bit. Otherwise the bit will be off. By convention, .Dq NAME is supplied by the section to which the relocations apply. Thus a relocation section for .Sy .text normally would have the name .Sy .rela.text . This section is of type .Dv SHT_RELA . .It .rodata This section holds read-only data that typically contribute to a non-writable segment in the process image. This section is of type .Dv SHT_PROGBITS . The attribute used is .Dv SHF_ALLOC . .It .rodata1 This section holds read-only data that typically contribute to a non-writable segment in the process image. This section is of type .Dv SHT_PROGBITS . The attribute used is .Dv SHF_ALLOC . .It .shstrtab This section holds section names. This section is of type .Dv SHT_STRTAB . No attribute types are used. .It .strtab This section holds strings, most commonly the strings that represent the names associated with symbol table entries. If the file has a loadable segment that includes the symbol string table, the section's attributes will include the .Dv SHF_ALLOC bit. Otherwise the bit will be off. This section is of type .Dv SHT_STRTAB . .It .symtab This section holds a symbol table. If the file has a loadable segment that includes the symbol table, the section's attributes will include the .Dv SHF_ALLOC bit. Otherwise the bit will be off. This section is of type .Dv SHT_SYMTAB . .It .tbss This section is the thread-local storage version of .Sy .bss , holding uninitialized data that contribute to the program's memory image on a per-thread basis. By definition, the system allocates and initializes the data with zeros for each thread before it first accesses it. This section is of type .Dv SHT_NOBITS . The attribute types are .Dv SHF_ALLOC , .Dv SHF_WRITE , and .Dv SHF_TLS . .It .tdata This section is the thread-local storage version of .Sy .data , holding initialized data that contribute to the program's memory image on a per-thread basis. The system allocates and initializes the data for each thread before it first accesses it. This section is of type .Dv SHT_PROGBITS . The attribute types are .Dv SHF_ALLOC , .Dv SHF_WRITE , and .Dv SHF_TLS . .It .text This section holds the .Dq text , or executable instructions, of a program. This section is of type .Dv SHT_PROGBITS . The attributes used are .Dv SHF_ALLOC and .Dv SHF_EXECINSTR . .El .Pp String table sections hold null-terminated character sequences, commonly called strings. The object file uses these strings to represent symbol and section names. One references a string as an index into the string table section. The first byte, which is index zero, is defined to hold a null character. Similarly, a string table's last byte is defined to hold a null character, ensuring null termination for all strings. .Pp An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions and references. A symbol table index is a subscript into this array. .Bd -literal -offset indent typedef struct { Elf32_Word st_name; Elf32_Addr st_value; Elf32_Word st_size; unsigned char st_info; unsigned char st_other; Elf32_Half st_shndx; } Elf32_Sym; .Ed .Bd -literal -offset indent typedef struct { Elf64_Half st_name; Elf_Byte st_info; Elf_Byte st_other; Elf64_Quarter st_shndx; Elf64_Xword st_value; Elf64_Xword st_size; } Elf64_Sym; .Ed .Bl -tag -width "st_value" .It Dv st_name This member holds an index into the object file's symbol string table, which holds character representations of the symbol names. If the value is non-zero, it represents a string table index that gives the symbol name. Otherwise, the symbol table has no name. .It Dv st_value This member gives the value of the associated symbol. .It Dv st_size Many symbols have associated sizes. This member holds zero if the symbol has no size or an unknown size. .It Dv st_info This member specifies the symbol's type and binding attributes: .Bl -tag -width "STT_SECTION" .It Dv STT_NOTYPE The symbol's type is not defined. .It Dv STT_OBJECT The symbol is associated with a data object. .It Dv STT_FUNC The symbol is associated with a function or other executable code. .It Dv STT_SECTION The symbol is associated with a section. Symbol table entries of this type exist primarily for relocation and normally have .Dv STB_LOCAL bindings. .It Dv STT_FILE By convention, the symbol's name gives the name of the source file associated with the object file. A file symbol has .Dv STB_LOCAL bindings, its section index is .Dv SHN_ABS , and it precedes the other .Dv STB_LOCAL symbols of the file, if it is present. .It Dv STT_TLS The symbol is associated with an object in thread-local storage. The symbol's value is its offset in the TLS storage for this file. .It Dv STT_LOPROC This value up to and including .Dv STT_HIPROC is reserved for processor-specific semantics. .It Dv STT_HIPROC This value down to and including .Dv STT_LOPROC is reserved for processor-specific semantics. .El .Bl -tag -width "STB_GLOBAL" .It Dv STB_LOCAL Local symbols are not visible outside the object file containing their definition. Local symbols of the same name may exist in multiple files without interfering with each other. .It Dv STB_GLOBAL Global symbols are visible to all object files being combined. One file's definition of a global symbol will satisfy another file's undefined reference to the same symbol. .It Dv STB_WEAK Weak symbols resemble global symbols, but their definitions have lower precedence. .It Dv STB_LOPROC This value up to and including .Dv STB_HIPROC is reserved for processor-specific semantics. .It Dv STB_HIPROC This value down to and including .Dv STB_LOPROC is reserved for processor-specific semantics. .Pp There are macros for packing and unpacking the binding and type fields: .Pp .Bl -tag -width "ELF32_ST_INFO(bind, type)" -compact .It Xo .Fn ELF32_ST_BIND info .Xc or .Fn ELF64_ST_BIND info extract a binding from an st_info value. .It Xo .Fn ELF64_ST_TYPE info .Xc or .Fn ELF32_ST_TYPE info extract a type from an st_info value. .It Xo .Fn ELF32_ST_INFO bind type .Xc or .Fn ELF64_ST_INFO bind type convert a binding and a type into an st_info value. .El .El .It Dv st_other This member currently holds zero and has no defined meaning. .It Dv st_shndx Every symbol table entry is .Dq defined in relation to some section. This member holds the relevant section header table index. .El .Pp Relocation is the process of connecting symbolic references with symbolic definitions. Relocatable files must have information that describes how to modify their section contents, thus allowing executable and shared object files to hold the right information for a process' program image. Relocation entries are these data. .Pp Relocation structures that do not need an addend: .Bd -literal -offset indent typedef struct { Elf32_Addr r_offset; Elf32_Word r_info; } Elf32_Rel; .Ed .Bd -literal -offset indent typedef struct { Elf64_Xword r_offset; Elf64_Xword r_info; } Elf64_Rel; .Ed .Pp Relocation structures that need an addend: .Bd -literal -offset indent typedef struct { Elf32_Addr r_offset; Elf32_Word r_info; Elf32_Sword r_addend; } Elf32_Rela; .Ed .Bd -literal -offset indent typedef struct { Elf64_Xword r_offset; Elf64_Xword r_info; Elf64_Sxword r_addend; } Elf64_Rela; .Ed .Bl -tag -width "r_offset" .It Dv r_offset This member gives the location at which to apply the relocation action. For a relocatable file, the value is the byte offset from the beginning of the section to the storage unit affected by the relocation. For an executable file or shared object, the value is the virtual address of the storage unit affected by the relocation. .It Dv r_info This member gives both the symbol table index with respect to which the relocation must be made and the type of relocation to apply. Relocation types are processor-specific. When the text refers to a relocation entry's relocation type or symbol table index, it means the result of applying .Dv ELF[32|64]_R_TYPE or .Dv ELF[32|64]_R_SYM , respectively, to the entry's .Sy r_info member. .It Dv r_addend This member specifies a constant addend used to compute the value to be stored into the relocatable field. .El .Pp The note section is used to hold vendor-specific information that may be used to help identify a binary's ABI. It should start with an Elf_Note struct, followed by the section name and the section description. The actual note contents follow thereafter. .Bd -literal -offset indent typedef struct { Elf32_Word namesz; Elf32_Word descsz; Elf32_Word type; } Elf32_Note; typedef struct { Elf64_Half namesz; Elf64_Half descsz; Elf64_Half type; } Elf64_Note; .Ed .Bl -tag -width "r_offset" .It Dv namesz Length of the note name, rounded up to a 4-byte boundary. .It Dv descsz Length of the note description, rounded up to a 4-byte boundary. .It Dv type A vendor-specific note type. .El .Pp The name and description strings follow the note structure. Each string is aligned on a 4-byte boundary. .Sh SEE ALSO .Xr as 1 , .Xr gdb 1 , .Xr ld 1 , .Xr objdump 1 , .Xr execve 2 , .Xr core 5 .Rs .%A Hewlett-Packard .%B Elf-64 Object File Format .Re .Rs .%A Santa Cruz Operation .%B System V Application Binary Interface .Re .Rs .%A Unix System Laboratories .%T Object Files .%B "Executable and Linking Format (ELF)" .Re .Sh HISTORY .Ox ELF support first appeared in .Ox 1.2 . Starting with .Ox 5.4 , all supported platforms use it as the native binary file format. ELF in itself first appeared in .At V . The ELF format is an adopted standard. .Sh AUTHORS This manual page was written by .An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org with inspiration from BSDi's .Bsx .Nm elf manpage.