a.out(4) a.out(4)
NAME
a.out - ELF (Executable and Linking Format) files
SYNOPSIS
#include <elf.h>
DESCRIPTION
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(3E) 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.
Copyright 1994 Novell, Inc. Page 1
a.out(4) a.out(4)
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, and so
on. 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.
REFERENCES
as(1), brk(2), cc(1), elf(3E), ld(1)
Copyright 1994 Novell, Inc. Page 2