A.OUT(5) Standards, Environments, and Macros A.OUT(5)


a.out - Executable and Linking Format (ELF) files


#include <elf.h>


The file name a.out is the default output file name from the link editor,
ld(1). The link editor will make an a.out executable if there were no
errors in linking. The output file of the assembler, as(1), also follows
the format of the a.out file although its default file name is different.

Programs that manipulate ELF files may use the library that elf(3ELF)
describes. An overview of the file format follows. For more complete
information, see the references given below.

| Linking View | Execution View |
|ELF header | ELF header |
|Program header table | Program header table |
|optional | |
|Section 1 | Segment 1 |
|... | |
|Section n | Segment 2 |
|... | |
|... | ... |
|Section header table | Section header table |
| | optional |

An ELF header resides at the beginning and holds a ``road map''
describing the file's organization. Sections hold the bulk of object file
information for the linking view: instructions, data, symbol table,
relocation information, and so on. Segments hold the object file
information for the program execution view. As shown, a segment may
contain one or more sections.

A program header table, if present, tells the system how to create a
process image. Files used to build a process image (execute a program)
must have a program header table; relocatable files do not need one. A
section header table contains information describing the file's sections.
Every section has an entry in the table; each entry gives information
such as the section name, the section size, etc. Files used during
linking must have a section header table; other object files may or may
not have one.

Although the figure shows the program header table immediately after the
ELF header, and the section header table following the sections, actual
files may differ. Moreover, sections and segments have no specified
order. Only the ELF header has a fixed position in the file.

When an a.out file is loaded into memory for execution, three logical
segments are set up: the text segment, the data segment (initialized data
followed by uninitialized, the latter actually being initialized to all
0's), and a stack. The text segment is not writable by the program; if
other processes are executing the same a.out file, the processes will
share a single text segment.

The data segment starts at the next maximal page boundary past the last
text address. If the system supports more than one page size, the
``maximal page'' is the largest supported size. When the process image is
created, the part of the file holding the end of text and the beginning
of data may appear twice. The duplicated chunk of text that appears at
the beginning of data is never executed; it is duplicated so that the
operating system may bring in pieces of the file in multiples of the
actual page size without having to realign the beginning of the data
section to a page boundary. Therefore, the first data address is the sum
of the next maximal page boundary past the end of text plus the remainder
of the last text address divided by the maximal page size. If the last
text address is a multiple of the maximal page size, no duplication is
necessary. The stack is automatically extended as required. The data
segment is extended as requested by the brk(2) system call.


as(1), ld(1), brk(2), elf(3ELF)

ANSI C Programmer's Guide

August 24, 2009 A.OUT(5)