Executable and Linkable Format

File:Elf-layout--en.svg

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 (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), and later in the Tool Interface Standard, 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:

• Program header table, describing zero or more memory segments
• Section header table, describing zero or more sections
• Data referred to by entries in the program header table or section header table

File:ELF Executable and Linkable Format diagram by Ange Albertini.png

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.

ELF 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

Offset		Size (bytes)		Field	Purpose
32-bit	64-bit	32-bit	64-bit
Template:Tt		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.
Template:Tt		1		e_ident[EI_CLASS]	This byte is set to either 1 or 2 to signify 32- or 64-bit format, respectively.
Template:Tt		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.
Template:Tt		1		e_ident[EI_VERSION]	Set to 1 for the original and current version of ELF.
Template:Tt		1		e_ident[EI_OSABI]	Identifies the target operating system ABI. Value ABI Template:Tt System V Template:Tt HP-UX Template:Tt NetBSD Template:Tt Linux Template:Tt GNU Hurd Template:Tt Solaris Template:Tt AIX (Monterey) Template:Tt IRIX Template:Tt FreeBSD Template:Tt Tru64 Template:Tt Novell Modesto Template:Tt OpenBSD Template:Tt OpenVMS Template:Tt NonStop Kernel Template:Tt AROS Template:Tt FenixOS Template:Tt Nuxi CloudABI Template:Tt Stratus Technologies OpenVOS
Template:Tt		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, 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: it defines a list of dynamic linker's features, 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.
Template:Tt		7		e_ident[EI_PAD]	Reserved padding bytes. Currently unused. Should be filled with zeros and ignored when read.
Template:Tt		2		e_type	Identifies object file type. Value Type Meaning Template:Tt ET_NONE Unknown. Template:Tt ET_REL Relocatable file. Template:Tt ET_EXEC Executable file. Template:Tt ET_DYN Shared object. Template:Tt ET_CORE Core file. Template:Tt ET_LOOS Reserved inclusive range. Operating system specific. Template:Tt ET_HIOS Template:Tt ET_LOPROC Reserved inclusive range. Processor specific. Template:Tt ET_HIPROC
Template:Tt		2		e_machine	Specifies target instruction set architecture. Some examples are: Value ISA Template:Tt No specific instruction set Template:Tt AT&T WE 32100 Template:Tt SPARC Template:Tt x86 Template:Tt Motorola 68000 (M68k) Template:Tt Motorola 88000 (M88k) Template:Tt Intel MCU Template:Tt Intel 80860 Template:Tt MIPS Template:Tt IBM System/370 Template:Tt MIPS RS3000 Little-endian Template:Tt Reserved for future use Template:Tt Hewlett-Packard PA-RISC Template:Tt Intel 80960 Template:Tt PowerPC Template:Tt PowerPC (64-bit) Template:Tt S390, including S390x Template:Tt IBM SPU/SPC Template:Tt Reserved for future use Template:Tt NEC V800 Template:Tt Fujitsu FR20 Template:Tt TRW RH-32 Template:Tt Motorola RCE Template:Tt Arm (up to Armv7/AArch32) Template:Tt Digital Alpha Template:Tt SuperH Template:Tt SPARC Version 9 Template:Tt Siemens TriCore embedded processor Template:Tt Argonaut RISC Core Template:Tt Hitachi H8/300 Template:Tt Hitachi H8/300H Template:Tt Hitachi H8S Template:Tt Hitachi H8/500 Template:Tt IA-64 Template:Tt Stanford MIPS-X Template:Tt Motorola ColdFire Template:Tt Motorola M68HC12 Template:Tt Fujitsu MMA Multimedia Accelerator Template:Tt Siemens PCP Template:Tt Sony nCPU embedded RISC processor Template:Tt Denso NDR1 microprocessor Template:Tt Motorola Star*Core processor Template:Tt Toyota ME16 processor Template:Tt STMicroelectronics ST100 processor Template:Tt Advanced Logic Corp. TinyJ embedded processor family Template:Tt AMD x86-64 Template:Tt Sony DSP Processor Template:Tt Digital Equipment Corp. PDP-10 Template:Tt Digital Equipment Corp. PDP-11 Template:Tt Siemens FX66 microcontroller Template:Tt STMicroelectronics ST9+ 8/16-bit microcontroller Template:Tt STMicroelectronics ST7 8-bit microcontroller Template:Tt Motorola MC68HC16 Microcontroller Template:Tt Motorola MC68HC11 Microcontroller Template:Tt Motorola MC68HC08 Microcontroller Template:Tt Motorola MC68HC05 Microcontroller Template:Tt Silicon Graphics SVx Template:Tt STMicroelectronics ST19 8-bit microcontroller Template:Tt Digital VAX Template:Tt Axis Communications 32-bit embedded processor Template:Tt Infineon Technologies 32-bit embedded processor Template:Tt Element 14 64-bit DSP Processor Template:Tt LSI Logic 16-bit DSP Processor Template:Tt TMS320C6000 Family Template:Tt MCST Elbrus e2k Template:Tt Arm 64-bits (Armv8/AArch64) Template:Tt Zilog Z80 Template:Tt RISC-V Template:Tt Berkeley Packet Filter Template:Tt WDC 65C816 Template:Tt LoongArch
Template:Tt		4		e_version	Set to 1 for the original version of ELF.
Template:Tt		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.
Template:Tt	Template:Tt	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.
Template:Tt	Template:Tt	4	8	e_shoff	Points to the start of the section header table.
Template:Tt	Template:Tt	4		e_flags	Interpretation of this field depends on the target architecture.
Template:Tt	Template:Tt	2		e_ehsize	Contains the size of this header, normally 64 Bytes for 64-bit and 52 Bytes for 32-bit format.
Template:Tt	Template:Tt	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).
Template:Tt	Template:Tt	2		e_phnum	Contains the number of entries in the program header table.
Template:Tt	Template:Tt	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).
Template:Tt	Template:Tt	2		e_shnum	Contains the number of entries in the section header table.
Template:Tt	Template:Tt	2		e_shstrndx	Contains index of the section header table entry that contains the section names.
Template:Tt	Template:Tt				End of ELF Header (size).

Example hexdump

00000000  7f 45 4c 46 02 01 01 00  00 00 00 00 00 00 00 00  |.ELF............|
00000010  02 00 3e 00 01 00 00 00  c5 48 40 00 00 00 00 00  |..>......H@.....|

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

Offset		Size (bytes)		Field	Purpose
32-bit	64-bit	32-bit	64-bit
Template:Tt		4		p_type	Identifies the type of the segment. Value Name Meaning Template:Tt PT_NULL Program header table entry unused. Template:Tt PT_LOAD Loadable segment. Template:Tt PT_DYNAMIC Dynamic linking information. Template:Tt PT_INTERP Interpreter information. Template:Tt PT_NOTE Auxiliary information. Template:Tt PT_SHLIB Reserved. Template:Tt PT_PHDR Segment containing program header table itself. Template:Tt PT_TLS Thread-Local Storage template. Template:Tt PT_LOOS Reserved inclusive range. Operating system specific. Template:Tt PT_HIOS Template:Tt PT_LOPROC Reserved inclusive range. Processor specific. Template:Tt PT_HIPROC
	Template:Tt		4	p_flags	Segment-dependent flags (position for 64-bit structure). Value Name Meaning Template:Tt PF_X Executable segment. Template:Tt PF_W Writeable segment. Template:Tt PF_R Readable segment.
Template:Tt	Template:Tt	4	8	p_offset	Offset of the segment in the file image.
Template:Tt	Template:Tt	4	8	p_vaddr	Virtual address of the segment in memory.
Template:Tt	Template:Tt	4	8	p_paddr	On systems where physical address is relevant, reserved for segment's physical address.
Template:Tt	Template:Tt	4	8	p_filesz	Size in bytes of the segment in the file image. May be 0.
Template:Tt	Template:Tt	4	8	p_memsz	Size in bytes of the segment in memory. May be 0.
Template:Tt		4		p_flags	Segment-dependent flags (position for 32-bit structure). See above p_flags field for flag definitions.
Template:Tt	Template:Tt	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.
Template:Tt	Template:Tt				End of Program Header (size).

Section header

Offset		Size (bytes)		Field	Purpose
32-bit	64-bit	32-bit	64-bit
0x00		4		Template:Tt	An offset to a string in the .shstrtab section that represents the name of this section.
0x04		4		Template:Tt	Identifies the type of this header. Value Name Meaning Template:Tt Template:Tt Section header table entry unused Template:Tt Template:Tt Program data Template:Tt Template:Tt Symbol table Template:Tt Template:Tt String table Template:Tt Template:Tt Relocation entries with addends Template:Tt Template:Tt Symbol hash table Template:Tt Template:Tt Dynamic linking information Template:Tt Template:Tt Notes Template:Tt Template:Tt Program space with no data (bss) Template:Tt Template:Tt Relocation entries, no addends Template:Tt Template:Tt Reserved Template:Tt Template:Tt Dynamic linker symbol table Template:Tt Template:Tt Array of constructors Template:Tt Template:Tt Array of destructors Template:Tt Template:Tt Array of pre-constructors Template:Tt Template:Tt Section group Template:Tt Template:Tt Extended section indices Template:Tt Template:Tt Number of defined types. Template:Tt Template:Tt Start OS-specific. ... ... ...
0x08		4	8	Template:Tt	Identifies the attributes of the section. Value Name Meaning Template:Tt Template:Tt Writable Template:Tt Template:Tt Occupies memory during execution Template:Tt Template:Tt Executable Template:Tt Template:Tt Might be merged Template:Tt Template:Tt Contains null-terminated strings Template:Tt Template:Tt 'sh_info' contains SHT index Template:Tt Template:Tt Preserve order after combining Template:Tt Template:Tt Non-standard OS specific handling required Template:Tt Template:Tt Section is member of a group Template:Tt Template:Tt Section hold thread-local data Template:Tt Template:Tt OS-specific Template:Tt Template:Tt Processor-specific Template:Tt Template:Tt Special ordering requirement (Solaris) Template:Tt Template:Tt Section is excluded unless referenced or allocated (Solaris)
Template:Tt	Template:Tt	4	8	Template:Tt	Virtual address of the section in memory, for sections that are loaded.
Template:Tt	Template:Tt	4	8	Template:Tt	Offset of the section in the file image.
Template:Tt	Template:Tt	4	8	Template:Tt	Size in bytes of the section. May be 0.
Template:Tt	Template:Tt	4		Template:Tt	Contains the section index of an associated section. This field is used for several purposes, depending on the type of section.
Template:Tt	Template:Tt	4		Template:Tt	Contains extra information about the section. This field is used for several purposes, depending on the type of section.
Template:Tt	Template:Tt	4	8	Template:Tt	Contains the required alignment of the section. This field must be a power of two.
Template:Tt	Template:Tt	4	8	Template:Tt	Contains the size, in bytes, of each entry, for sections that contain fixed-size entries. Otherwise, this field contains zero.
Template:Tt	Template:Tt				End of Section Header (size).

Tools

• readelf is a Unix binary utility that displays information about one or more ELF files. A free software implementation is provided by GNU Binutils.
• elfutils provides alternative tools to GNU Binutils purely for Linux.
• elfdump is a command for viewing ELF information in an ELF file, available under Solaris and FreeBSD.
• objdump provides a wide range of information about ELF files and other object formats. objdump uses the Binary File Descriptor library as a back-end to structure the ELF data.
• The Unix file utility can display some information about ELF files, including the instruction set architecture for which the code in a relocatable, executable, or shared object file is intended, or on which an ELF core dump was produced.

Applications

Unix-like systems

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

• Linux
• Solaris / Illumos
• IRIX
• FreeBSD
• NetBSD
• OpenBSD
• Redox
• DragonFly BSD
• Syllable
• HP-UX (except for 32-bit PA-RISC programs which continue to use SOM)
• QNX Neutrino
• MINIX

Non-Unix adoption

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

• OpenVMS, in its Itanium and amd64 versions
• BeOS Revision 4 and later for x86 based computers (where it replaced the Portable Executable format; the PowerPC version stayed with Preferred Executable Format)
• Haiku, an open source reimplementation of BeOS
• RISC OS
• Stratus VOS, in PA-RISC and x86 versions
• SkyOS
• Fuchsia OS
• Z/TPF
• HPE NonStop OS
• Deos

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

Game consoles

Some game consoles also use ELF:

• PlayStation Portable, PlayStation Vita, PlayStation, PlayStation 2, PlayStation 3, PlayStation 4, PlayStation 5
• GP2X
• Dreamcast
• GameCube
• Nintendo 64
• Wii
• Wii U

PowerPC

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

• AmigaOS 4, the ELF executable has replaced the prior Extended Hunk Format (EHF) which was used on Amigas equipped with PPC processor expansion cards.
• MorphOS
• AROS
• Café OS (The operating system run by the Wii U)

Mobile phones

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

• Symbian OS v9 uses E32Image format that is based on the ELF file format;
• Sony Ericsson, for example, the W800i, W610, W300, etc.
• Siemens, the SGOLD and SGOLD2 platforms: from Siemens C65 to S75 and BenQ-Siemens E71/EL71;
• Motorola, for example, the E398, SLVR L7, v360, v3i (and all phone LTE2 which has the patch applied).
• Bada, for example, the Samsung Wave S8500.
• Nokia phones or tablets running the Maemo or the Meego OS, for example, the Nokia N900.
• Android uses ELF .so (shared object) libraries for the Java Native Interface. With Android Runtime (ART), the default since Android 5.0 "Lollipop", all applications are compiled into native ELF binaries on installation. It's also possible to use native Linux software from package managers like Termux, or compile them from sources via Clang or GCC, that are available in repositories.

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 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.

Blockchain platforms

• Solana uses ELF format for its on-chain programs (smart contracts). The platform processes ELF files compiled to BPF (Berkeley Packet Filter) byte-code, which are then deployed as shared objects and executed in Solana's runtime environment. The BPF loader validates and processes these ELF files during program deployment.

86open

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. 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, 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.

FatELF: universal binaries for Linux

FatELF is an ELF binary-format extension that adds fat binary capabilities. 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. since 2021Template:Dated maintenance category (articles), FatELF has not been integrated into the mainline Linux kernel.

See also

• Application binary interface
• Comparison of executable file formats
• DWARFTemplate:Snd a format for debugging data
• Intel Binary Compatibility Standard
• Portable ExecutableTemplate:Snd format used by Windows
• vDSOTemplate:Snd virtual DSO
• Position-independent code

Further reading

• Code:[1]Template:Cbignore Errata: [2]
• Ulrich Drepper, How To Write Shared Libraries, Version 4.1.2 (2011). Published on the author's web page, https://www.akkadia.org/drepper.
• An unsung hero: The hardworking ELF by Peter Seebach, December 20, 2005, archived from the original on February 24, 2007
• Template:Webarchive
• The ELF Object File Format: Introduction, The ELF Object File Format by Dissection by Eric Youngdale (1995-05-01)
• A Whirlwind Tutorial on Creating Really Teensy ELF Executables for Linux by Brian Raiter
• ELF relocation into non-relocatable objects by Julien Vanegue (2003-08-13)
• Embedded ELF debugging without ptrace by the ELFsh team (2005-08-01)
• Study of ELF loading and relocs by Pat Beirne (1999-08-03)

External links

• FreeBSD Handbook: Binary formats (archived version)
• FreeBSD elf(5) manual page
• NetBSD ELF FAQ
• Linux elf(5) manual page
• Oracle Solaris Linker and Libraries Guide
• The ERESI project: reverse engineering on ELF-based operating systems Template:Webarchive
• Linux Today article on 86open July 26, 1999
• Announcement of 86open on Debian Announce mailing list October 10, 1997, Bruce Perens
• Declaration of Ulrich Drepper (PDF) in The SCO Group vs IBM, September 19, 2006
• 86open and ELF discussion Template:Webarchive on Groklaw, August 13, 2006