a.out is a

file format A file format is a Computer standard, 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 format, pr ...

used in older versions of

Unix-like A Unix-like (sometimes referred to as UN*X, *nix 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 Uni ...

computer

operating systems An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

for

executable In computer science, 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), in ...

object code In computing, object code or object module is the product of an assembler or compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' ...

, and, in later systems,

shared libraries In computing, a library is a collection of resources that can be leveraged during software development to implement a computer program. Commonly, a library consists of executable code such as compiled functions and classes, or a library can ...

. This is an abbreviated form of " assembler 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 is an 18-bit computing, 18-bit minicomputer produced by Digital Equipment Corporation as part of the Programmed Data Processor, PDP series. Introduced in 1964, shipped since 1965, it was the first to use their Flip-Chip module, Flip- ...

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 Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...

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, but often in

ELF An elf (: elves) is a type of humanoid supernatural being in Germanic peoples, Germanic folklore. Elves appear especially in Norse mythology, North Germanic mythology, being mentioned in the Icelandic ''Poetic Edda'' and the ''Prose Edda'' ...

. Support for the ''a.out'' format was deprecated in

Linux kernel The Linux kernel is a Free and open-source software, free and open source Unix-like kernel (operating system), kernel that is used in many computer systems worldwide. The kernel was created by Linus Torvalds in 1991 and was soon adopted as the k ...

version 5.1, and support for the format was removed across the 5.18 and 5.19 kernel releases.

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 originally sold by Digital Equipment Corporation (DEC) from 1970 into the late 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of a ...

, 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 ext ...

format in AT&T Unix System V, which was in turn superseded by the

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 Octal (base 8) is a numeral system with eight as the base. In the decimal system, each place is a power of ten. For example: : \mathbf_ = \mathbf \times 10^1 + \mathbf \times 10^0 In the octal system, each place is a power of eight. For ex ...

codes 0407, 0410 or 0413. MINIX 3 switched to ELF in the 3.2.0 release of February 2012.

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

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. Because modern compiler toolchains did not produce ''a.out'' files, and facilities for the format were suffering from bit rot, the ''a.out'' file format on Linux was deprecated with the release of the kernel version 5.1. Support for the format was removed from the

DEC Alpha Alpha (original name Alpha AXP) is a 64-bit reduced instruction set computer (RISC) instruction set architecture (ISA) developed by Digital Equipment Corporation (DEC). Alpha was designed to replace 32-bit VAX complex instruction set computers ( ...

and

Motorola 68000 The Motorola 68000 (sometimes shortened to Motorola 68k or m68k and usually pronounced "sixty-eight-thousand") is a 16/32-bit complex instruction set computer (CISC) microprocessor, introduced in 1979 by Motorola Semiconductor Products Sector ...

architectures, which were the last architectures that used it, in version 5.18. Support for ''a.out'' in the x86 architecture was removed completely in version 5.19. 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 in December 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. 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.

Debugging

The ''a.out'' support for

debug In engineering, debugging is the process of finding the root cause, workarounds, and possible fixes for bugs. For software, debugging tactics can involve interactive debugging, control flow analysis, log file analysis, monitoring at the ap ...

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 ("stabs"). History Cygnus Support attr ...

. 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 is a version of the Unix operating system first released in May 1975 and the first version of the Unix operating system to see wide release outside Bell Labs. Like its direct predecessor, t ...

a.out manual page
for 2.11

BSD The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...

a.out manual page
for

FreeBSD FreeBSD is a free-software Unix-like operating system descended from the Berkeley Software Distribution (BSD). The first version was released in 1993 developed from 386BSD, one of the first fully functional and free Unix clones on affordable ...

a.out manual page
for

NetBSD NetBSD is a free and open-source Unix-like operating system based on the Berkeley Software Distribution (BSD). It was the first open-source BSD descendant officially released after 386BSD was fork (software development), forked. It continues to ...

{{Executables Executable file formats

History

Debugging

See also

References

External links