Executable and Linkable Format
Filename extension
none, .axf, .bin, .elf, .o, .out, .prx, .puff, .ko, .mod, and .so
Magic number0x7F 'E' 'L' 'F'
Developed byUnix System Laboratories[1]: 3 
Type of formatBinary, executable, object, shared library, core dump
Container forMany executable binary formats
An ELF file has two views: the program header shows the segments used at run time, whereas the section header lists the set of sections.

In computing, the Executable and Linkable Format[2] (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, object code, shared libraries, and core dumps. First published in the specification for the application binary interface (ABI) of the Unix operating system version named System V Release 4 (SVR4),[3] and later in the Tool Interface Standard,[1] it was quickly accepted among different vendors of Unix systems. In 1999, it was chosen as the standard binary file format for Unix and Unix-like systems on x86 processors by the 86open project.

By design, the ELF format is flexible, extensible, and cross-platform. For instance, it supports different endiannesses and address sizes so it does not exclude any particular CPU or instruction set architecture. This has allowed it to be adopted by many different operating systems on many different hardware platforms.

File layout

Each ELF file is made up of one ELF header, followed by file data. The data can include:

Structure of an ELF file with key entries highlighted

The segments contain information that is needed for run time execution of the file, while sections contain important data for linking and relocation. Any byte in the entire file can be owned by one section at most, and orphan bytes can occur which are unowned by any section.

File header

The ELF header defines whether to use 32-bit or 64-bit addresses. The header contains three fields that are affected by this setting and offset other fields that follow them. The ELF header is 52 or 64 bytes long for 32-bit and 64-bit binaries respectively.

ELF header[4]
Offset Size (bytes) Field Purpose
32-bit 64-bit 32-bit 64-bit
0x00 4 e_ident[EI_MAG0] through e_ident[EI_MAG3] 0x7F followed by ELF(45 4c 46) in ASCII; these four bytes constitute the magic number.
0x04 1 e_ident[EI_CLASS] This byte is set to either 1 or 2 to signify 32- or 64-bit format, respectively.
0x05 1 e_ident[EI_DATA] This byte is set to either 1 or 2 to signify little or big endianness, respectively. This affects interpretation of multi-byte fields starting with offset 0x10.
0x06 1 e_ident[EI_VERSION] Set to 1 for the original and current version of ELF.
0x07 1 e_ident[EI_OSABI] Identifies the target operating system ABI.
Value ABI
0x00 System V
0x01 HP-UX
0x02 NetBSD
0x03 Linux
0x04 GNU Hurd
0x06 Solaris
0x07 AIX (Monterey)
0x08 IRIX
0x09 FreeBSD
0x0A Tru64
0x0B Novell Modesto
0x0C OpenBSD
0x0D OpenVMS
0x0E NonStop Kernel
0x10 FenixOS
0x11 Nuxi CloudABI
0x12 Stratus Technologies OpenVOS
0x08 1 e_ident[EI_ABIVERSION] Further specifies the ABI version. Its interpretation depends on the target ABI. Linux kernel (after at least 2.6) has no definition of it,[5] so it is ignored for statically-linked executables. In that case, offset and size of EI_PAD are 8.

glibc 2.12+ in case e_ident[EI_OSABI] == 3 treats this field as ABI version of the dynamic linker:[6] it defines a list of dynamic linker's features,[7] treats e_ident[EI_ABIVERSION] as a feature level requested by the shared object (executable or dynamic library) and refuses to load it if an unknown feature is requested, i.e. e_ident[EI_ABIVERSION] is greater than the largest known feature.[8]

0x09 7 e_ident[EI_PAD] Reserved padding bytes. Currently unused. Should be filled with zeros and ignored when read.
0x10 2 e_type Identifies object file type.
Value Type Meaning
0x00 ET_NONE Unknown.
0x01 ET_REL Relocatable file.
0x02 ET_EXEC Executable file.
0x03 ET_DYN Shared object.
0x04 ET_CORE Core file.
0xFE00 ET_LOOS Reserved inclusive range. Operating system specific.
0xFF00 ET_LOPROC Reserved inclusive range. Processor specific.
0x12 2 e_machine Specifies target instruction set architecture. Some examples are:
Value ISA
0x00 No specific instruction set
0x01 AT&T WE 32100
0x02 SPARC
0x03 x86
0x04 Motorola 68000 (M68k)
0x05 Motorola 88000 (M88k)
0x06 Intel MCU
0x07 Intel 80860
0x08 MIPS
0x09 IBM System/370
0x0A MIPS RS3000 Little-endian
0x0B - 0x0E Reserved for future use
0x0F Hewlett-Packard PA-RISC
0x13 Intel 80960
0x14 PowerPC
0x15 PowerPC (64-bit)
0x16 S390, including S390x
0x18 - 0x23 Reserved for future use
0x24 NEC V800
0x25 Fujitsu FR20
0x26 TRW RH-32
0x27 Motorola RCE
0x28 Arm (up to Armv7/AArch32)
0x29 Digital Alpha
0x2A SuperH
0x2B SPARC Version 9
0x2C Siemens TriCore embedded processor
0x2D Argonaut RISC Core
0x2E Hitachi H8/300
0x2F Hitachi H8/300H
0x30 Hitachi H8S
0x31 Hitachi H8/500
0x32 IA-64
0x33 Stanford MIPS-X
0x34 Motorola ColdFire
0x35 Motorola M68HC12
0x36 Fujitsu MMA Multimedia Accelerator
0x37 Siemens PCP
0x38 Sony nCPU embedded RISC processor
0x39 Denso NDR1 microprocessor
0x3A Motorola Star*Core processor
0x3B Toyota ME16 processor
0x3C STMicroelectronics ST100 processor
0x3D Advanced Logic Corp. TinyJ embedded processor family
0x3E AMD x86-64
0x3F Sony DSP Processor
0x40 Digital Equipment Corp. PDP-10
0x41 Digital Equipment Corp. PDP-11
0x42 Siemens FX66 microcontroller
0x43 STMicroelectronics ST9+ 8/16 bit microcontroller
0x44 STMicroelectronics ST7 8-bit microcontroller
0x45 Motorola MC68HC16 Microcontroller
0x46 Motorola MC68HC11 Microcontroller
0x47 Motorola MC68HC08 Microcontroller
0x48 Motorola MC68HC05 Microcontroller
0x49 Silicon Graphics SVx
0x4A STMicroelectronics ST19 8-bit microcontroller
0x4B Digital VAX
0x4C Axis Communications 32-bit embedded processor
0x4D Infineon Technologies 32-bit embedded processor
0x4E Element 14 64-bit DSP Processor
0x4F LSI Logic 16-bit DSP Processor
0x8C TMS320C6000 Family
0xAF MCST Elbrus e2k
0xB7 Arm 64-bits (Armv8/AArch64)
0xDC Zilog Z80
0xF7 Berkeley Packet Filter
0x101 WDC 65C816
0x102 LoongArch
0x14 4 e_version Set to 1 for the original version of ELF.
0x18 4 8 e_entry This is the memory address of the entry point from where the process starts executing. This field is either 32 or 64 bits long, depending on the format defined earlier (byte 0x04). If the file doesn't have an associated entry point, then this holds zero.
0x1C 0x20 4 8 e_phoff Points to the start of the program header table. It usually follows the file header immediately following this one, making the offset 0x34 or 0x40 for 32- and 64-bit ELF executables, respectively.
0x20 0x28 4 8 e_shoff Points to the start of the section header table.
0x24 0x30 4 e_flags Interpretation of this field depends on the target architecture.
0x28 0x34 2 e_ehsize Contains the size of this header, normally 64 Bytes for 64-bit and 52 Bytes for 32-bit format.
0x2A 0x36 2 e_phentsize Contains the size of a program header table entry. As explained below, this will typically be 0x20 (32 bit) or 0x38 (64 bit).
0x2C 0x38 2 e_phnum Contains the number of entries in the program header table.
0x2E 0x3A 2 e_shentsize Contains the size of a section header table entry. As explained below, this will typically be 0x28 (32 bit) or 0x40 (64 bit).
0x30 0x3C 2 e_shnum Contains the number of entries in the section header table.
0x32 0x3E 2 e_shstrndx Contains index of the section header table entry that contains the section names.
0x34 0x40 End of ELF Header (size).

Program header

The program header table tells the system how to create a process image. It is found at file offset e_phoff, and consists of e_phnum entries, each with size e_phentsize. The layout is slightly different in 32-bit ELF vs 64-bit ELF, because the p_flags are in a different structure location for alignment reasons. Each entry is structured as:

Program header[9]
Offset Size (bytes) Field Purpose
32-bit 64-bit 32-bit 64-bit
0x00 4 p_type Identifies the type of the segment.
Value Name Meaning
0x00000000 PT_NULL Program header table entry unused.
0x00000001 PT_LOAD Loadable segment.
0x00000002 PT_DYNAMIC Dynamic linking information.
0x00000003 PT_INTERP Interpreter information.
0x00000004 PT_NOTE Auxiliary information.
0x00000005 PT_SHLIB Reserved.
0x00000006 PT_PHDR Segment containing program header table itself.
0x00000007 PT_TLS Thread-Local Storage template.
0x60000000 PT_LOOS Reserved inclusive range. Operating system specific.
0x70000000 PT_LOPROC Reserved inclusive range. Processor specific.
0x04 4 p_flags Segment-dependent flags (position for 64-bit structure).
Value Name Meaning
0x1 PF_X Executable segment.
0x2 PF_W Writeable segment.
0x4 PF_R Readable segment.
0x04 0x08 4 8 p_offset Offset of the segment in the file image.
0x08 0x10 4 8 p_vaddr Virtual address of the segment in memory.
0x0C 0x18 4 8 p_paddr On systems where physical address is relevant, reserved for segment's physical address.
0x10 0x20 4 8 p_filesz Size in bytes of the segment in the file image. May be 0.
0x14 0x28 4 8 p_memsz Size in bytes of the segment in memory. May be 0.
0x18 4 p_flags Segment-dependent flags (position for 32-bit structure). See above p_flags field for flag definitions.
0x1C 0x30 4 8 p_align 0 and 1 specify no alignment. Otherwise should be a positive, integral power of 2, with p_vaddr equating p_offset modulus p_align.
0x20 0x38 End of Program Header (size).

Section header

Offset Size (bytes) Field Purpose
32-bit 64-bit 32-bit 64-bit
0x00 4 sh_name An offset to a string in the .shstrtab section that represents the name of this section.
0x04 4 sh_type Identifies the type of this header.
Value Name Meaning
0x0 SHT_NULL Section header table entry unused
0x1 SHT_PROGBITS Program data
0x2 SHT_SYMTAB Symbol table
0x3 SHT_STRTAB String table
0x4 SHT_RELA Relocation entries with addends
0x5 SHT_HASH Symbol hash table
0x6 SHT_DYNAMIC Dynamic linking information
0x7 SHT_NOTE Notes
0x8 SHT_NOBITS Program space with no data (bss)
0x9 SHT_REL Relocation entries, no addends
0x0A SHT_SHLIB Reserved
0x0B SHT_DYNSYM Dynamic linker symbol table
0x0E SHT_INIT_ARRAY Array of constructors
0x0F SHT_FINI_ARRAY Array of destructors
0x10 SHT_PREINIT_ARRAY Array of pre-constructors
0x11 SHT_GROUP Section group
0x12 SHT_SYMTAB_SHNDX Extended section indices
0x13 SHT_NUM Number of defined types.
0x60000000 SHT_LOOS Start OS-specific.
... ... ...
0x08 4 8 sh_flags Identifies the attributes of the section.
Value Name Meaning
0x1 SHF_WRITE Writable
0x2 SHF_ALLOC Occupies memory during execution
0x4 SHF_EXECINSTR Executable
0x10 SHF_MERGE Might be merged
0x20 SHF_STRINGS Contains null-terminated strings
0x40 SHF_INFO_LINK 'sh_info' contains SHT index
0x80 SHF_LINK_ORDER Preserve order after combining
0x100 SHF_OS_NONCONFORMING Non-standard OS specific handling required
0x200 SHF_GROUP Section is member of a group
0x400 SHF_TLS Section hold thread-local data
0x0FF00000 SHF_MASKOS OS-specific
0xF0000000 SHF_MASKPROC Processor-specific
0x4000000 SHF_ORDERED Special ordering requirement (Solaris)
0x8000000 SHF_EXCLUDE Section is excluded unless referenced or allocated (Solaris)
0x0C 0x10 4 8 sh_addr Virtual address of the section in memory, for sections that are loaded.
0x10 0x18 4 8 sh_offset Offset of the section in the file image.
0x14 0x20 4 8 sh_size Size in bytes of the section in the file image. May be 0.
0x18 0x28 4 sh_link Contains the section index of an associated section. This field is used for several purposes, depending on the type of section.
0x1C 0x2C 4 sh_info Contains extra information about the section. This field is used for several purposes, depending on the type of section.
0x20 0x30 4 8 sh_addralign Contains the required alignment of the section. This field must be a power of two.
0x24 0x38 4 8 sh_entsize Contains the size, in bytes, of each entry, for sections that contain fixed-size entries. Otherwise, this field contains zero.
0x28 0x40 End of Section Header (size).


Main article: GNU Binutils


Unix-like systems

This article is in list format but may read better as prose. You can help by converting this article, if appropriate. Editing help is available. (November 2016)

The ELF format has replaced older executable formats in various environments. It has replaced a.out and COFF formats in Unix-like operating systems:

Non-Unix adoption

ELF has also seen some adoption in non-Unix operating systems, such as:

Microsoft Windows also uses the ELF format, but only for its Windows Subsystem for Linux compatibility system.[16]

Game consoles

Some game consoles also use ELF:


Other (operating) systems running on PowerPC that use ELF:

Mobile phones

Some operating systems for mobile phones and mobile devices use ELF:

Some phones can run ELF files through the use of a patch that adds assembly code to the main firmware, which is a feature known as ELFPack in the underground modding culture. The ELF file format is also used with the Atmel AVR (8-bit), AVR32[21] and with Texas Instruments MSP430 microcontroller architectures. Some implementations of Open Firmware can also load ELF files, most notably Apple's implementation used in almost all PowerPC machines the company produced.


The Linux Standard Base (LSB) supplements some of the above specifications for architectures in which it is specified.[22] For example, that is the case for the System V ABI, AMD64 Supplement.[23][24]


86open was a project to form consensus on a common binary file format for Unix and Unix-like operating systems on the common PC compatible x86 architecture, to encourage software developers to port to the architecture.[25] The initial idea was to standardize on a small subset of Spec 1170, a predecessor of the Single UNIX Specification, and the GNU C Library (glibc) to enable unmodified binaries to run on the x86 Unix-like operating systems. The project was originally designated "Spec 150".

The format eventually chosen was ELF, specifically the Linux implementation of ELF, after it had turned out to be a de facto standard supported by all involved vendors and operating systems.

The group began email discussions in 1997 and first met together at the Santa Cruz Operation offices on August 22, 1997.

The steering committee was Marc Ewing, Dion Johnson, Evan Leibovitch, Bruce Perens, Andrew Roach, Bryan Wayne Sparks and Linus Torvalds. Other people on the project were Keith Bostic, Chuck Cranor, Michael Davidson, Chris G. Demetriou, Ulrich Drepper, Don Dugger, Steve Ginzburg, Jon "maddog" Hall, Ron Holt, Jordan Hubbard, Dave Jensen, Kean Johnston, Andrew Josey, Robert Lipe, Bela Lubkin, Tim Marsland, Greg Page, Ronald Joe Record, Tim Ruckle, Joel Silverstein, Chia-pi Tien, and Erik Troan. Operating systems and companies represented were BeOS, BSDI, FreeBSD, Intel, Linux, NetBSD, SCO and SunSoft.

The project progressed and in mid-1998, SCO began developing lxrun, an open-source compatibility layer able to run Linux binaries on OpenServer, UnixWare, and Solaris. SCO announced official support of lxrun at LinuxWorld in March 1999. Sun Microsystems began officially supporting lxrun for Solaris in early 1999,[26] and later moved to integrated support of the Linux binary format via Solaris Containers for Linux Applications.

With the BSDs having long supported Linux binaries (through a compatibility layer) and the main x86 Unix vendors having added support for the format, the project decided that Linux ELF was the format chosen by the industry and "declare[d] itself dissolved" on July 25, 1999.[27]

FatELF: universal binaries for Linux

FatELF is an ELF binary-format extension that adds fat binary capabilities.[28] It is aimed for Linux and other Unix-like operating systems. Additionally to the CPU architecture abstraction (byte order, word size, CPU instruction set etc.), there is the potential advantage of software-platform abstraction e.g., binaries which support multiple kernel ABI versions. As of 2021, FatELF has not been integrated into the mainline Linux kernel.[29][30][31]

See also


  1. ^ a b Tool Interface Standard (TIS) Executable and Linking Format (ELF) Specification Version 1.2 (May 1995)
  2. ^ Tool Interface Standard (TIS) Portable Formats Specification Version 1.1 (October 1993)
  3. ^ System V Application Binary Interface Edition 4.1 (1997-03-18)
  4. ^ "ELF Header". Sco.com. July 2000. Retrieved 2014-02-07.
  5. ^ "LXR linux/include/linux/elf.h". linux.no. Retrieved 27 April 2015.
  6. ^ "glibc 2.12 announce".
  7. ^ "sourceware.org Git - glibc.git/blob - libc-abis".
  8. ^ "sourceware.org Git - glibc.git/blob - sysdeps/gnu/ldsodefs.h". Archived from the original on 2021-03-07. Retrieved 2019-10-28.
  9. ^ "Program Header". Sco.com. July 2000. Retrieved 2017-04-05.
  10. ^ "elfutils". sourceware.org. Retrieved 30 April 2017.
  11. ^ "Binary Formats". Archived from the original on 2019-03-31. Retrieved 2019-03-31.
  12. ^ "MinixReleases – Minix Wiki". Wiki.minix3.org. Archived from the original on 2013-03-30. Retrieved 2014-01-19.
  13. ^ "Archived copy" (PDF). Archived from the original (PDF) on 2020-09-15. Retrieved 2016-10-19.((cite web)): CS1 maint: archived copy as title (link)
  14. ^ "GCCSDK – RISC OS". Riscos.info. 2012-04-22. Archived from the original on 2014-02-19. Retrieved 2014-01-19.
  15. ^ "Guardian Programmer's Guide" (PDF). Hewlett Packard Enterprise. Archived from the original (PDF) on 2018-05-30. Retrieved 2018-05-30. p. 44 archived from the original Archived 2018-05-30 at the Wayback Machine on 2018-5-30
  16. ^ Foley, Mary Jo. "Under the hood of Microsoft's Windows Subsystem for Linux | ZDNet". ZDNet. Retrieved 2016-08-19.
  17. ^ PlayStation Portable use encrypted & relocated ELF : PSP
  18. ^ Symbian OS executable file format
  19. ^ Rosen, Kenneth; Host, Douglas; Klee, Rachel; Rosinski, Richard (2007). UNIX: The Complete Reference (2 ed.). McGraw Hill Professional. p. 707. ISBN 9780071706988. Retrieved 2017-06-08. Dynamically linked libraries are also called shared objects (.so).
  20. ^ Thomas, Romain. "Android formats". Quarks Lab. Archived from the original on 16 Feb 2023. Retrieved 17 Jan 2023.
  21. ^ "Chapter 4: Object Files", System V Application Binary Interface, 2009-10-26, e_machine
  22. ^ "LSB Referenced Specifications". linuxfoundation.org. Retrieved 27 April 2015.
  23. ^ "Executable and Linking Format (ELF)". linuxfoundation.org. Retrieved 27 April 2015.
  24. ^ "Introduction". linuxfoundation.org. Retrieved 27 April 2015.
  25. ^ Leibovitch, Evan (1997-12-23). "86Open Frequently-Asked Questions". Archived from the original on 2007-03-11. Retrieved 2007-06-06.
  26. ^ Record, Ronald (1998-05-21). "Bulletin on status of 86open at SCO". Archived from the original on 2008-12-08. Retrieved 2008-05-06.
  27. ^ Leibovitch, Evan (1999-07-25). "The86open Project – Final Update". Archived from the original on 2007-02-27. Retrieved 2007-05-06.
  28. ^ Gordon, Ryan. "fatelf-specification v1". icculus.org. Retrieved 2010-07-25.
  29. ^ Gordon, Ryan. "FatELF: Turns out I liked the uncertainty better". icculus.org. Retrieved 2010-07-13.
  30. ^ Holwerda, Thom (2009-11-03). "Ryan Gordon Halts FatELF Project". osnews.com. Retrieved 2010-07-05.
  31. ^ Brockmeier, Joe (June 23, 2010). "SELF: Anatomy of an (alleged) failure". Linux Weekly News. Retrieved 2011-02-06.

Further reading