Executable and Linkable Format

In computing, the Executable and Linkable Format (ELF, formerly called Extensible Linking Format) is a common standard file format for executables, object code, shared libraries, and core dumps. First published in the System V Application Binary Interface specification,^[1] and later in the Tool Interface Standard,^[2] 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 by the 86open project.

ELF is flexible and extensible by design, and it is not bound to any particular processor or architecture. This has allowed it to be adopted by many different operating systems on many different platforms.

ELF file layout

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 of the binary.

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

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

The segments contain information that is necessary for runtime execution of the file, while sections contain important data for linking and relocation. Any byte in the entire file can be owned by at most one section, and there can be orphan bytes which are not owned by any section.

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.
• 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

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
• IRIX
• FreeBSD
• NetBSD
• OpenBSD
• DragonFly BSD
• Syllable
• HP-UX (except for 32-bit PA-RISC programs which continue to use SOM)
• QNX Neutrino
• MINIX^[3]

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

• OpenVMS, in its Itanium version
• 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, the open source reimplementation of BeOS.
• RISC OS^[4]

Some game consoles also use ELF:

• PlayStation Portable,^[5] PlayStation 2, PlayStation 3
• GP2X.
• Dreamcast
• GameCube, Wii

Other operating systems running on PowerPC using ELF:

• AmigaOS 4, the ELF executable has replaced the previous EHF (Extended Hunk Format) which was used on Amigas equipped with PPC processor expansion cards.
• MorphOS
• AROS

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

• Symbian OS v9 uses E32Image^[6] 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.

Some phones can run ELF files through the use of a patch that adds assembly code to the main firmware (known as the ELFPack, in the underground modding culture).

The ELF file format is also used with the Atmel AVR (8-bit), AVR32^[7] and with Texas Instruments MSP430 microcontroller architectures.

Specifications

• Generic:
  • System V Application Binary Interface Edition 4.1 (1997-03-18)
  • System V ABI Update (October 2009)
  • ELF-64 Object File Format Version 1.5 Draft 2 (May 1998)
• AMD64:
  • System V ABI, AMD64 Supplement
• ARM:
  • ELF for the ARM Architecture
• IA-32:
  • System V ABI, Intel386 Architecture Processor Supplement
• IA-64:
  • Itanium Software Conventions and Runtime Guide (September 2000)
• M32R:
  • M32R ELF ABI Supplement Version 1.2 (2004-08-26)
• MIPS:
  • System V ABI, MIPS RISC Processor Supplement
  • MIPS EABI documentation (2003-06-11)
• Motorola 6800:
  • Motorola 8 and 16 bit Embedded ABI
• PA-RISC:
  • ELF Supplement for PA-RISC Version 1.43 (October 6, 1997)
• PowerPC:
  • System V ABI, PPC Supplement
  • PowerPC Embedded Application Binary Interface 32-Bit Implementation (1995-10-01)
  • 64-bit PowerPC ELF Application Binary Interface Supplement Version 1.9 (2004)
• SPARC:
  • System V ABI, SPARC Supplement
• S/390:
  • S/390 32bit ELF ABI Supplement
• zSeries:
  • zSeries 64bit ELF ABI Supplement
• Symbian OS 9:
  • E32Image file format on Symbian OS 9

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, so as to encourage software developers to port to the architecture.^[8] 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 referred to as "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 started email discussions in 1997 and first met in person at the Santa Cruz Operation offices on 1997-08-22.

The steering committee was Marc Ewing, Dion Johnson, Evan Leibovitch, Bruce Perens, Andrew Roach, Bryan Sparks and Linus Torvalds. Other people on the project were Tim Bird, 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, Inc..

The project progressed and in mid-1998, SCO began developing lxrun, an open-source compatibility layer capable of running 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,^[9] and has since 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.^[10]

FatELF: Universal Binaries for Linux

FatELF is an ELF binary-format extension which adds Fat binary capabilities.^[11] 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 ABIs versions. A proof-of-concept Ubuntu 9.04 image (VM image of Ubuntu 9.04 with Fat Binary support) and development tools are available. As of 2011, support for FatELF is not integrated in the Linux kernel mainline.^[12][13][14]

See also

• Perbandingan -- executable file formats
• Portable Executable
• DWARF - Debug With Attributed Record Format
• Application binary interface
• VDSO - Virtual DSO

References

Further reading

• John R. Levine (October 1999). Linkers and Loaders. Morgan-Kauffman. ISBN 1-55860-496-0. 
• Ulrich Drepper (2006-08-20). How To Write Shared Libraries. 4.0. Retrieved 2007-06-20. 
• An unsung hero: The hardworking ELF by Peter Seebach (2005-12-20)
• LibElf and GElf — A Library to Manipulate ELF Files by Neelakanth Nadgir (August 2001)
• 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
• FreeBSD elf(5) manual page
• NetBSD ELF FAQ
• Solaris's Linker and Libraries Guide ©2001 Sun Microsystems
• The ERESI project : reverse engineering on ELF-based operating systems
• 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 on Groklaw, August 13, 2006