In computing, the Executable and Linking 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.

Unlike many proprietary executable file formats, ELF is very flexible and extensible, 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.

The ELF format has replaced older executable formats such as a.out and COFF in many Unix-like operating systems such as Linux, Solaris, IRIX, FreeBSD, NetBSD, OpenBSD, DragonFly BSD, Syllable, and HP-UX (except for 32-bit PA-RISC programs which continue to use SOM). ELF has also seen some adoption in non-Unix operating systems, such as the Itanium version of OpenVMS, and BeOS Revision 4 and later for x86 based computers (where it replaced the Portable Executable format; the PowerPC version stayed with Preferred Executable Format). The PlayStation Portable, PlayStation 2, PlayStation 3, Wii and GP2X consoles also use ELF. AmigaOS 4 and MorphOS also running on PowerPC machines, use ELF. On the Amiga platform the ELF executable has replaced the previous EHF (Extended Hunk Format) which was used on Amigas equipped with PPC processor expansion cards. The Symbian OS v9 uses E32Image^[3] format that is based on ELF file format.

Most Sony Ericsson (for example, the W800i, W610, K790, etc.), some Siemens (SGOLD and SGOLD2 platforms: from Siemens C65 to S75 and BenQ-Siemens E71/EL71) and Motorola (for example, the E398, SLVR L7, v360, v3i and all phone LTE2 which has the patch apply) 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 as a generic object and executable format for binary images used with embedded processors.

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.

This image describes the C process map and the categorization of different variables declared in a C code and their mapping in to the different ELF sections, it also shows the static and run time view of an executable.

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 the 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. Each byte in the entire file is taken by no more than one section at a time, but there can be orphan bytes, which are not covered by a section. In the normal case of a Unix executable one or more sections are enclosed in one segment.

Tools

• readelf is a Unix binary utility that displays information about one or more ELF files. A GPL implementation is provided by GNU Binutils.
• elfdump is a Solaris command for viewing ELF information in an elf file.
• objdump provides a wide range of information about ELF files and other object formats.
• Both readelf and objdump use the Binary File Descriptor library as a back-end to structure the ELF data.

Specifications

• Generic:
  • Generic ELF Specification (an update document, not the complete ELF spec)
  • System V Application Binary Interface Edition 4.1 (1997-03-18)
  • System V ABI Update (December 2003)
  • ELF-64 Object File Format Version 1.5 Draft 2 (May 1998)
• AMD64:
  • ELF Specification for the AMD64 Architecture
  • System V ABI, AMD64 Supplement
• ARM:
  • ELF for the ARM Architecture
• IA32:
  • ELF Specification for the IA32 Architecture
  • System V ABI, IA32 Supplement
• IA64:
  • ELF Specification for the IA64 Architecture
  • Itanium Software Conventions and Runtime Guide (September 2000)
• M32R:
  • ELF Specification for the M32R Architecture
  • 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:
  • Processor-Specific ELF Supplement for PA-RISC
  • 64-bit PA-RISC Run-time Architecture Version 3.3 (1997-10-06)
• PowerPC:
  • ELF Specification for the PPC32 Architecture
  • System V ABI, PPC Supplement
  • PowerPC Embedded Application Binary Interface 32-Bit Implementation (1995-10-01)
  • ELF Specification for the PPC64 Architecture
  • 64-bit PowerPC ELF Application Binary Interface Supplement Version 1.9 (2004)
• SPARC:
  • System V ABI, SPARC Supplement
• S/390:
  • ELF Specification for the S390 Architecture
  • S/390 ELF ABI Supplement
• zSeries:
  • ELF Specification for the S390X Architecture
  • zSeries 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.^[4]

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.

History

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, Linux, NetBSD, SCO and SunSoft, Inc..

The project progressed and in mid-1998, SCO began assisting in the development of 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,^[5] 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 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.^[6]

See also

• DWARF - Debug With Attributed Record Format
• Application binary interface

References

Further reading

• 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)

External links

• Elf library routines
• LGPL ELF object file access library
• ELFIO: a C++ library for reading and generating files in the ELF binary format.
• FreeBSD Handbook — Binary formats
• Description of the ELF binary format
• FreeBSD <tt>elf(5)</tt> manual page
• NetBSD ELF FAQ
• Sun's Linker and Libraries Guide
• The ERESI project : reverse engineering on ELF-based operating systems

x86open links

• 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

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