a.out

Introduction

a.out, short for “assembler output”, is the executable and object file format of early Unix. It takes its name from the file the assembler and the C compiler write when no output name is given. The BSDs and early Linux used it for programs, libraries and object files until the late 1990s or early 2000s, when it gave way to ELF, which can describe shared libraries and many sections far more easily. Modern Linux no longer runs a.out programs.

Compilers still name their output a.out by default, but on today’s systems that file is an ELF or Mach-O program; only the name is left.

Structure

An a.out file starts with a short header: on 32-bit systems, eight 32-bit words. The first is the magic number, which also says how the program is loaded: octal 0407 (OMAGIC) for a program whose code may be written to, 0410 (NMAGIC) for read-only code that can be shared, and 0413 (ZMAGIC) for a program paged in from the file on demand. On the PDP-11, 0407 was also the instruction to jump over the header. The other words give the sizes of the text (code), the data, the zero-filled BSS area, the symbol table and the two relocation tables, and the entry point.

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