a.out is a
file format
A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free.
Some file formats ...
used in older versions of
Unix-like
A Unix-like (sometimes referred to as UN*X or *nix) 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-li ...
computer
operating systems
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems schedule tasks for efficient use of the system and may also inc ...
for
executable
In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, causes a computer "to perform indicated tasks according to encoded instruction (computer science), instructi ...
s,
object code
In computing, object code or object module is the product of a compiler.
In a general sense object code is a sequence of statements or instructions in a computer language, usually a machine code language (i.e., binary) or an intermediate langua ...
, and, in later systems,
shared libraries
In computer science, a library is a collection of non-volatile resources used by computer programs, often for software development. These may include configuration data, documentation, help data, message templates, pre-written code and subr ...
. This is an abbreviated form of "
assembler
Assembler may refer to:
Arts and media
* Nobukazu Takemura, avant-garde electronic musician, stage name Assembler
* Assemblers, a fictional race in the ''Star Wars'' universe
* Assemblers, an alternative name of the superhero group Champions of A ...
output", the filename of the output of
Ken Thompson
Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B programmi ...
's
PDP-7
The PDP-7 was a minicomputer produced by Digital Equipment Corporation as part of the PDP series. Introduced in 1964, shipped since 1965, it was the first to use their Flip-Chip technology. With a cost of , it was cheap but powerful by the st ...
assembler. The term was subsequently applied to the format of the resulting file to contrast with other formats for object code.
"a.out" remains the default output file name for executables created by certain
compiler
In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
s and
linker
Linker or linkers may refer to:
Computing
* Linker (computing), a computer program that takes one or more object files generated by a compiler or generated by an assembler and links them with libraries, generating an executable program or shar ...
s when no output name is specified, even though the created files actually are not in the a.out format.
Support for the ''a.out'' format was removed in
Linux kernel
The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...
version 5.19.
History
An ''a.out'' format for the
PDP-7
The PDP-7 was a minicomputer produced by Digital Equipment Corporation as part of the PDP series. Introduced in 1964, shipped since 1965, it was the first to use their Flip-Chip technology. With a cost of , it was cheap but powerful by the st ...
, similar to the ''a.out'' format used on the
PDP-11
The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were sold, ...
, appeared in the first edition of UNIX.
It was superseded by the
COFF
The Common Object File Format (COFF) is a format for executable, object code, and shared library computer files used on Unix systems. It was introduced in Unix System V, replaced the previously used a.out format, and formed the basis for exte ...
format in
AT&T Unix System V, which was in turn superseded by the
ELF
An elf () is a type of humanoid supernatural being in Germanic mythology and folklore. Elves appear especially in North Germanic mythology. They are subsequently mentioned in Snorri Sturluson's Icelandic Prose Edda. He distinguishes "ligh ...
format in
System V Release 4
Unix System V (pronounced: "System Five") 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, an ...
. ''a.out'' files are identified by the
magic numbers with
octal
The octal numeral system, or oct for short, is the radix, base-8 number system, and uses the Numerical digit, digits 0 to 7. This is to say that 10octal represents eight and 100octal represents sixty-four. However, English, like most languages, ...
codes 0407, 0410 or 0413.
Though
Berkeley Unix
The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...
continued using the ''a.out'' format for some time, modern BSD-systems have since switched to ELF.
NetBSD/i386 switched formally from ''a.out'' to ELF in its 1.5 release (Dec. 2000).
FreeBSD/i386 switched to ELF as a standard format during the 2.2 to 3.0 transition in 1998, however support for ''a.out'' remained in the system.
MINIX 3
Minix 3 is a small, Unix-like operating system. It is published under a BSD-3-Clause license and is a successor project to the earlier versions, Minix 1 and 2.
The project's main goal is for the system to be fault-tolerant by detecting and rep ...
switched to ELF in the 3.2.0 release.
Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which ...
also used ''a.out'' only until kernel 1.2 (Mar. 1995), when it was superseded by ELF for that platform as well. ELF support was added in the experimental 1.1.52 kernel. Linux's transition to ELF was more or less forced due to the complex nature of building ''a.out'' shared libraries on that platform, which included the need to register the virtual address space at which the library was located with a central authority, as the ''a.out'' ld.so in Linux was unable to relocate shared libraries. The various BSD flavors were able to continue using ''a.out'' binaries long after Linux was forced to switch to ELF, due to the somewhat more flexible nature of the BSD ''a.out'' format compared to that of Linux. The a.out file format on Linux was deprecated with the release of the 5.1 Linux kernel and the last parts of source code handling it were removed in 5.18.
Debugging
The ''a.out'' support for
debug
In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems.
Debugging tactics can involve int ...
information is done through the use of special entries in the symbol table called
stabs
stabs (sometimes written STABS) is a debugging data format for storing information about computer programs for use by symbolic and source-level debuggers. (The information is stored in symbol table strings; hence the name "stabs".) Cygnus Suppor ...
. The stabs format has also been used in many
COFF
The Common Object File Format (COFF) is a format for executable, object code, and shared library computer files used on Unix systems. It was introduced in Unix System V, replaced the previously used a.out format, and formed the basis for exte ...
and
ELF
An elf () is a type of humanoid supernatural being in Germanic mythology and folklore. Elves appear especially in North Germanic mythology. They are subsequently mentioned in Snorri Sturluson's Icelandic Prose Edda. He distinguishes "ligh ...
variations.
See also
*
Comparison of executable file formats
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 ma ...
*
Portable Executable
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. The PE format is a data structure that encapsulates the information necessary fo ...
References
*
External links
a.out manual pagefor
Version 6 Unix
Sixth Edition Unix, also called Version 6 Unix or just V6, was the first version of the Unix operating system to see wide release outside Bell Labs. It was released in May 1975 and, like its direct predecessor, targeted the DEC PDP-11 family of m ...
a.out manual pagefor 2.11
BSD
The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berk ...
a.out manual pagefor
FreeBSD
FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...
{{Executables
Executable file formats