Filename extension | none, .axf, .bin, .elf, .o, .out, .prx, .puff, .ko, .mod, and .so |
---|---|
Magic number | 0x7F 'E' 'L' 'F' |
Developed by | Unix System Laboratories [1] : 3 |
Type of format | Binary, executable, object, shared library, core dump |
Container for | Many executable binary formats |
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.
Each ELF file is made up of one ELF header, followed by file data. The data can include:
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.
000000007f454c46020101000000000000000000|.ELF............|
0000001002003e0001000000c548400000000000|..>......H@.....|
Example hexdump of ELF file header [4]
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.
Offset | Size (bytes) | Field | Purpose | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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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.
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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, [6] 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: [7] it defines a list of dynamic linker's features, [8] 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. [9] | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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.
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0x12 | 2 | e_machine | Specifies target instruction set architecture. Some examples are:
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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). |
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:
Offset | Size (bytes) | Field | Purpose | |||||||||||||||||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
32-bit | 64-bit | 32-bit | 64-bit | |||||||||||||||||||||||||||||||||||||||
0x00 | 4 | p_type | Identifies the type of the segment.
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0x04 | 4 | p_flags | Segment-dependent flags (position for 64-bit structure).
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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). |
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.
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0x08 | 4 | 8 | sh_flags | Identifies the attributes of the section.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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). |
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. [11] 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. 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.The ELF format has replaced older executable formats in various environments. It has replaced a.out and COFF formats in Unix-like operating systems:
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. [17]
Some game consoles also use ELF:
Other (operating) systems running on PowerPC that use ELF:
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 [22] 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. [23] For example, that is the case for the System V ABI, AMD64 Supplement. [24] [25]
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. [26] 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, [27] 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. [28]
FatELF is an ELF binary-format extension that adds fat binary capabilities. [29] 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 [update] , FatELF has not been integrated into the mainline Linux kernel. [30] [31] [32]
MINIX is a Unix-like operating system based on a microkernel architecture. Since version 2.0, it has been Portable Operating System Interface (POSIX) compliant.
The Portable Operating System Interface is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system and user-level application programming interfaces (APIs), along with command line shells and utility interfaces, for software compatibility (portability) with variants of Unix and other operating systems. POSIX is also a trademark of the IEEE. POSIX is intended to be used by both application and system developers.
The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, and user commands. The core specifications of the SUS known as Base Specifications are developed and maintained by the Austin Group, which is a joint working group of IEEE, ISO/IEC JTC 1/SC 22/WG 15 and The Open Group. If an operating system is submitted to The Open Group for certification, and passes conformance tests, then it is deemed to be compliant with a UNIX standard such as UNIX 98 or UNIX 03.
The Netwide Assembler (NASM) is an assembler and disassembler for the Intel x86 architecture. It can be used to write 16-bit, 32-bit (IA-32) and 64-bit (x86-64) programs. It is considered one of the most popular assemblers for Linux and x86 chips.
The Portable Executable (PE) format is a file format for executables, object code, DLLs and others used in 32-bit and 64-bit versions of Windows operating systems, and in UEFI environments. The PE format is a data structure that encapsulates the information necessary for the Windows OS loader to manage the wrapped executable code. This includes dynamic library references for linking, API export and import tables, resource management data and thread-local storage (TLS) data. On NT operating systems, the PE format is used for EXE, DLL, SYS, MUI and other file types. The Unified Extensible Firmware Interface (UEFI) specification states that PE is the standard executable format in EFI environments.
In computing, a system call is the programmatic way in which a computer program requests a service from the operating system on which it is executed. This may include hardware-related services, creation and execution of new processes, and communication with integral kernel services such as process scheduling. System calls provide an essential interface between a process and the operating system.
In computing, tar is a computer software utility for collecting many files into one archive file, often referred to as a tarball, for distribution or backup purposes. The name is derived from "tape archive", as it was originally developed to write data to sequential I/O devices with no file system of their own, such as devices that use magnetic tape. The archive data sets created by tar contain various file system parameters, such as name, timestamps, ownership, file-access permissions, and directory organization. POSIX abandoned tar in favor of pax, yet tar sees continued widespread use.
compress is a Unix shell compression program based on the LZW compression algorithm. Compared to gzip's fastest setting, compress is slightly slower at compression, slightly faster at decompression, and has a significantly lower compression ratio. 1.8 MiB of memory is used to compress the Hutter Prize data, slightly more than gzip's slowest setting.
The archiver, also known simply as ar, is a Unix utility that maintains groups of files as a single archive file. Today, ar
is generally used only to create and update static library files that the link editor or linker uses and for generating .deb packages for the Debian family; it can be used to create archives for any purpose, but has been largely replaced by tar
for purposes other than static libraries. An implementation of ar
is included as one of the GNU Binutils.
A fat binary is a computer executable program or library which has been expanded with code native to multiple instruction sets which can consequently be run on multiple processor types. This results in a file larger than a normal one-architecture binary file, thus the name.
Unix System V is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, and 4. System V Release 4 (SVR4) was commercially the most successful version, being the result of an effort, marketed as Unix System Unification, which solicited the collaboration of the major Unix vendors. It was the source of several common commercial Unix features. System V is sometimes abbreviated to SysV.
Mach-O, short for Mach object file format, is a file format for executables, object code, shared libraries, dynamically loaded code, and core dumps. It was developed to replace the a.out format.
a.out is a file format used in older versions of Unix-like computer operating systems for executables, object code, and, in later systems, shared libraries. This is an abbreviated form of "assembler output", the filename of the output of Ken Thompson's PDP-7 assembler. The term was subsequently applied to the format of the resulting file to contrast with other formats for object code.
In computing, a dynamic linker is the part of an operating system that loads and links the shared libraries needed by an executable when it is executed, by copying the content of libraries from persistent storage to RAM, filling jump tables and relocating pointers. The specific operating system and executable format determine how the dynamic linker functions and how it is implemented.
Fiwix is an operating system kernel based on the UNIX architecture and fully focused on being POSIX compatible. It is designed and developed mainly as a hobbyist operating system, but it also serves for educational purposes. It runs on the i386 hardware platform and is compatible with a good base of existing GNU applications. It follows the UNIX System V application binary interface and is also mostly Linux 2.0 system call ABI compatible.
In computing, the System Object Model (SOM) is a proprietary executable file format developed by Hewlett-Packard for its HP-UX and MPE/ix operating systems. In particular, SOM is the native format used for 32-bit application executables, object code, and shared libraries running under the PA-RISC family of processors.
This is a comparison of binary executable file formats which, once loaded by a suitable executable loader, can be directly executed by the CPU rather than being interpreted by software. In addition to the binary application code, the executables may contain headers and tables with relocation and fixup information as well as various kinds of meta data. Among those formats listed, the ones in most common use are PE, ELF, Mach-O and MZ.
A Unix-like operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-like application is one that behaves like the corresponding Unix command or shell. Although there are general philosophies for Unix design, there is no technical standard defining the term, and opinions can differ about the degree to which a particular operating system or application is Unix-like.
Unix is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others.
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