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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. The name stands for assembler output. Image File history File links This is a lossless scalable vector image. ... A filename extension is a suffix to the name of a computer file applied to show its format. ... AT&T Inc. ... A Hexdump of a JPEG image. ... In computer science, object file or object code is an intermediate representation of code generated by a compiler after it processes a source code file. ... In computer science, a library is a collection of subprograms used to develop software. ... A file format is a particular way to encode information for storage in a computer file. ... Diagram of the relationships between several Unix-like systems A Unix-like operating system is one that behaves in a manner similar to a Unix system, while not necessarily conforming to or being certified to any version of the Single UNIX Specification. ... In computing, an operating system (OS) is the system software responsible for the direct control and management of hardware and basic system operations. ... In computer science, object file or object code is an intermediate representation of code generated by a compiler after it processes a source code file. ... In computer science, a library is a collection of subprograms used to develop software. ... An assembly language is a low-level language used in the writing of computer programs. ...

a.out remained the default output file name for executables created by certain compilers / linkers when no output name was specified, even though these executables were no longer in the a.out format.^[1] This article is about the computing term. ... Figure of the linking process, where object files and static libraries are assembled into a new library or executable. ...

Use

An a.out format for the PDP-7, similar to the a.out format used on the PDP-11, appeared in the first edition of UNIX.^[2] It was superseded by the COFF format in AT&T Unix System V, which was in turn superseded by ELF in System V Release 4. A modified PDP-7 under restoration in Oslo, Norway The DEC PDP-7 is a minicomputer produced by Digital Equipment Corporation. ... The PDP-11 was a 16-bit minicomputer sold by Digital Equipment Corp. ... The Common Object File Format (COFF) is an object file format that was introduced in Unix System V Release 3, and was later adopted by Microsoft for Windows NT. It was superseded by the more powerful ELF in System V Release 4, but as of 2005, COFF is still used... It has been suggested that Traditional Unix be merged into this article or section. ... In computing, the Executable and Linkable Format (ELF, formerly called Extensible Linking Format) is a common standard file format for executables, object code, shared libraries, and core dumps. ...

Though Berkeley Unix kept using a.out 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. FreeBSD/i386 switched to ELF during the 2.2 to 3.0 transition. Berkeley Software Distribution (BSD, sometimes called Berkeley Unix) is the Unix derivative distributed by the University of California, Berkeley, starting in the 1970s. ... NetBSD is a freely redistributable, open source version of the Unix-like BSD computer operating system. ... A small forest elf (älva) rescuing an egg, from Solägget (1932), by Elsa Beskow An elf is a creature of Germanic paganism which still survives in northern European folklore. ... FreeBSD is a Unix-like free operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD) branch through the 386BSD and 4. ...

The a.out format has no direct support for debug information, but can be used with stabs, which uses special symbol table entries to store data. A debugger is a computer program that is used to test and debug other programs. ... The correct title of this article is . ...

Linux also used a.out until kernel 1.2 (ELF support was added in the experimental 1.1.52), when it was superseded by ELF for that platform as well.^[3] 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 which the library was located at with a central authority, as the a.out ld.so in Linux was unable to relocate shared libraries. The various BSD flavours 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.^[4] It has been suggested that Criticism of Linux be merged into this article or section. ... It has been suggested that Criticism of Linux be merged into this article or section. ... A small forest elf (älva) rescuing an egg, from Solägget (1932), by Elsa Beskow An elf is a creature of Germanic paganism which still survives in northern European folklore. ...

Format

a.out executables typically came in one of several variants, OMAGIC, NMAGIC, QMAGIC, or ZMAGIC. The OMAGIC format had contiguous segments after the header, with no separation of text and data. The NMAGIC format was similar to OMAGIC, however the data segment was loaded on the immediate next page after the end of the text segment, and the text segment was marked read-only. The ZMAGIC format added support for demand paging, and QMAGIC allowed the a.out header to be merged with the first page of the text segment, typically saving a page worth of memory. QMAGIC binaries were typically loaded one page above the bottom of the virtual address space, in order to permit trapping of null pointer dereferences via a segmentation fault.

An a.out file consists of up to 7 sections. In order, these sections are:

exec header

Contains parameters used by the kernel to load a binary file into memory and execute it, and by the link editor ld to combine a binary file with other binary files. This section is the only mandatory one.

text segment

Contains machine code and related data that are loaded into memory when a program executes. May be loaded read-only.

data segment

Contains initialized data; always loaded into writable memory.

text relocations

Contains records used by the link editor to update pointers in the text segment when combining binary files.

data relocations

Like the text relocation section, but for data segment pointers.

symbol table

Contains records used by the link editor to cross-reference the addresses of named variables and functions (symbols) between binary files.

string table

Contains the character strings corresponding to the symbol names.

ld is the name of the GNU linker, which creates an executable file on Unix and Unix-like systems from object files created during compilation. ...

References

External links

• a.out format
• Unix man page for a.out format