a.out - format of executable binary files


#include <a.out.h>


The include file #include <a.out.h>
declares three structures and several macros. The struc
tures describe
the format of executable machine code files (`binaries') on
the system.
A binary file consists of up to 7 sections. In order, these
exec header Contains parameters used by the kernel to
load a binary
file into memory and execute it, and by
the link editor
ld(1) 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
data segment Contains initialized data; always loaded
into writable
text relocations Contains records used by the link editor
to update
pointers in the text segment when combin
ing binary
data relocations Like the text relocation section, but for
data segment
symbol table Contains records used by the link editor
to cross ref
erence the addresses of named variables
and functions
(`symbols') between binary files.
string table Contains the character strings correspond
ing to the
symbol names.
Every binary file begins with an exec structure:

struct exec {
unsigned long a_midmag;
unsigned long a_text;
unsigned long a_data;
unsigned long a_bss;
unsigned long a_syms;
unsigned long a_entry;
unsigned long a_trsize;
unsigned long a_drsize;
The fields have the following functions:
a_midmag This field is stored in host byte-order. It has a
number of
sub-components accessed by the macros N_GETFLAG(),


and N_GETMAGIC(), and set by the macro


The macro N_GETFLAG() returns a few flags:

EX_DYNAMIC indicates that the executable requires
the services
of the run-time link editor.
EX_PIC indicates that the object contains po
sition inde
pendent code. This flag is set by
as(1) when given
the `-k' flag and is preserved by
ld(1) if necessary.
If both EX_DYNAMIC and EX_PIC are set, the object
file is a
position independent executable image (e.g. a
shared library),
which is to be loaded into the process address
space by the
run-time link editor.
The macro N_GETMID() returns the machine-id. This
which machine(s) the binary is intended to run on.
N_GETMAGIC() specifies the magic number, which
uniquely identifies binary files and distinguishes different
loading conventions. The field must contain one of the follow
ing values:
OMAGIC The text and data segments immediately
follow the
header and are contiguous. The kernel
loads both text
and data segments into writable memory.
NMAGIC As with OMAGIC, text and data segments im
mediately fol
low the header and are contiguous. Howev
er, the kernel
loads the text into read-only memory and
loads the data
into writable memory at the next page
boundary after
the text.
ZMAGIC The kernel loads individual pages on de
mand from the
binary. The header, text segment and data
segment are
all padded by the link editor to a multi
ple of the page
size. Pages that the kernel loads from
the text segment are read-only, while pages from the
data segment
are writable.
a_text Contains the size of the text segment in bytes.
a_data Contains the size of the data segment in bytes.
a_bss Contains the number of bytes in the `bss segment'
and is used
by the kernel to set the initial break (brk(2))
after the data
segment. The kernel loads the program so that
this amount of
writable memory appears to follow the data segment
and initially reads as zeroes. (bss = block started by
a_syms Contains the size in bytes of the symbol table
a_entry Contains the address in memory of the entry point
of the pro
gram after the kernel has loaded it; the kernel
starts the execution of the program from the machine instruction
at this
a_trsize Contains the size in bytes of the text relocation
a_drsize Contains the size in bytes of the data relocation
The #include <a.out.h>
include file defines several macros which use an exec struc
ture to test
consistency or to locate section offsets in the binary file.
N_BADMAG(exec) Nonzero if the a_magic field does not con
tain a recog
nized value.
N_TXTOFF(exec) The byte offset in the binary file of the
beginning of
the text segment.
N_SYMOFF(exec) The byte offset of the beginning of the sym
bol table.
N_STROFF(exec) The byte offset of the beginning of the
string table.
Relocation records have a standard format which is described
by the
relocation_info structure:

struct relocation_info {
int r_address;
unsigned int r_symbolnum : 24,
r_pcrel : 1,
r_length : 2,
r_extern : 1,
r_baserel : 1,
r_jmptable : 1,
r_relative : 1,
r_copy : 1;
The relocation_info fields are used as follows:
r_address Contains the byte offset of a pointer that
needs to be link
edited. Text relocation offsets are reckoned
from the start
of the text segment, and data relocation off
sets from the
start of the data segment. The link editor
adds the value
that is already stored at this offset into the
new value
that it computes using this relocation record.
r_symbolnum Contains the ordinal number of a symbol struc
ture in the
symbol table (it is not a byte offset). After
the link editor resolves the absolute address for this sym
bol, it adds
that address to the pointer that is undergoing
(If the r_extern bit is clear, the situation is
see below.)
r_pcrel If this is set, the link editor assumes that it
is updating
a pointer that is part of a machine code in
struction using
pc-relative addressing. The address of the re
pointer is implicitly added to its value when
the running
program uses it.
r_length Contains the log base 2 of the length of the
pointer in
bytes; 0 for 1-byte displacements, 1 for 2-byte
displacements, 2 for 4-byte displacements.
r_extern Set if this relocation requires an external
reference; the
link editor must use a symbol address to update
the pointer.
When the r_extern bit is clear, the relocation
is `local';
the link editor updates the pointer to reflect
changes in
the load addresses of the various segments,
rather than
changes in the value of a symbol (except when
r_baserel is
also set (see below). In this case, the con
tent of the
r_symbolnum field is an n_type value (see be
low); this type
field tells the link editor what segment the
pointer points into.
r_baserel If set, the symbol, as identified by the
r_symbolnum field,
is to be relocated to an offset into the Global
Table. At run-time, the entry in the Global
Offset Table at
this offset is set to be the address of the
r_jmptable If set, the symbol, as identified by the
r_symbolnum field,
is to be relocated to an offset into the Proce
dure Linkage
r_relative If set, this relocation is relative to the
(run-time) load
address of the image this object file is going
to be a part
of. This type of relocation only occurs in
shared objects.
r_copy If set, this relocation record identifies a
symbol whose
contents should be copied to the location given
r_address. The copying is done by the run-time
from a suitable data item in a shared object.
Symbols map names to addresses (or more generally, strings
to values).
Since the link-editor adjusts addresses, a symbol's name
must be used to
stand for its address until an absolute value has been as
signed. Symbols
consist of a fixed-length record in the symbol table and a
variablelength name in the string table. The symbol table is an ar
ray of nlist

struct nlist {
union {
char *n_name;
long n_strx;
} n_un;
unsigned char n_type;
char n_other;
short n_desc;
unsigned long n_value;
The fields are used as follows:
n_un.n_strx Contains a byte offset into the string table
for the name of
this symbol. When a program accesses a symbol
table with
the nlist(3) function, this field is replaced
with the
n_un.n_name field, which is a pointer to the
string in memory.
n_type Used by the link editor to determine how to up
date the sym
bol's value. The n_type field is broken down
into three
sub-fields using bitmasks. The link editor
treats symbols
with the N_EXT type bit set as `external' sym
bols and permits references to them from other binary
files. The N_TYPE
mask selects bits of interest to the link edi
N_UNDF An undefined symbol. The link editor
must locate an
external symbol with the same name in
another binary
file to determine the absolute value of
this symbol.
As a special case, if the n_value field
is nonzero
and no binary file in the link-edit de
fines this
symbol, the link-editor will resolve
this symbol to
an address in the bss segment, reserv
ing an amount
of bytes equal to n_value. If this
symbol is undefined in more than one binary file and
the binary
files do not agree on the size, the
link editor
chooses the greatest size found across
all binaries.
N_ABS An absolute symbol. The link editor
does not update
an absolute symbol.
N_TEXT A text symbol. This symbol's value is
a text
address and the link editor will update
it when it
merges binary files.
N_DATA A data symbol; similar to N_TEXT but
for data
addresses. The values for text and da
ta symbols are
not file offsets but addresses; to re
cover the file
offsets, it is necessary to identify
the loaded
address of the beginning of the corre
sponding section and subtract it, then add the off
set of the
N_BSS A bss symbol; like text or data symbols
but has no
corresponding offset in the binary
N_FN A filename symbol. The link editor in
serts this
symbol before the other symbols from a
binary file
when merging binary files. The name of
the symbol
is the filename given to the link edi
tor, and its
value is the first text address from
that binary
file. Filename symbols are not needed
for linkediting or loading, but are useful for
The N_STAB mask selects bits of interest to
symbolic debuggers such as gdb(1); the values are described
in stab(5).
n_other This field provides information on the nature
of the symbol
independent of the symbol's location in terms
of segments as
determined by the n_type field. Currently, the
lower 4 bits
of the n_other field hold one of two values:
AUX_OBJECT (see #include <link.h> for their definitions). AUX_FUNC associates
the symbol with
a callable function, while AUX_OBJECT associ
ates the symbol
with data, irrespective of their locations in
either the
text or the data segment. This field is in
tended to be used
by ld(1) for the construction of dynamic exe
n_desc Reserved for use by debuggers; passed untouched
by the link
editor. Different debuggers use this field for
n_value Contains the value of the symbol. For text,
data and bss
symbols, this is an address; for other symbols
(such as
debugger symbols), the value may be arbitrary.
The string table consists of an unsigned long length fol
lowed by nullterminated symbol strings. The length represents the size
of the entire
table in bytes, so its minimum value (or the offset of the
first string)
is always 4 on 32-bit machines.


as(1), gdb(1), ld(1), brk(2), execve(2), nlist(3), core(5),
link(5), stab(5)


The #include <a.out.h>
include file appeared in Version 7 AT&T UNIX.


Since not all of the supported architectures use the
a_midmag field, it
can be difficult to determine what architecture a binary
will execute on
without examining its actual machine code. Even with a ma
chine identifier, the byte order of the exec header is machine-depen
BSD June 5, 1993
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