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 format ...

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 in ...

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 instructions", as opposed to a data fil ...

object code In computing, object code or object module is the product of a 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 ...

, 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 ...

. 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 ...

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 ...

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 s ...

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 tha ...

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 ...

version 5.19.

History

An ''a.out'' format for the

, 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 sol ...

, 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 e ...

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, ...

. ''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 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 ...

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, whi ...

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 i ...

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 Suppo ...

. The stabs format has also been used in many

and

variations.

References

External links

a.out manual page
for

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 ...

a.out manual page
for 2.11 BSD
a.out manual page
for

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

History

Debugging

See also

References

External links