a.out
—
format of executable binary files
#include
<a.out.h>
The include file
<a.out.h>
declares three structures and several macros. The structures describe the
format of executable machine code files (‘binaries’) on the
system.
A binary 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(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 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.
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
(),
N_GETMID
(), and
N_GETMAGIC
(), and set by the macro
N_SETMAGIC
().
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 position independent 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 indicates 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 following 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 immediately follow the header and are contiguous. However,
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 demand from the binary. The
header, text segment and data segment are all padded by the link
editor to a multiple 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
symbol)
- a_syms
- Contains the size in bytes of the symbol table section.
- a_entry
- Contains the address in memory of the entry point of the program after the
kernel has loaded it; the kernel starts the execution of the program from
the machine instruction at this address.
- a_trsize
- Contains the size in bytes of the text relocation table.
- a_drsize
- Contains the size in bytes of the data relocation table.
The
<a.out.h>
include file defines several macros which use an
exec structure to test consistency or to
locate section offsets in the binary file.
N_BADMAG
(exec)
- Nonzero if the a_magic field does not
contain a recognized 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 symbol 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 offsets 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 structure in the symbol table (it
is not a byte offset). After the link editor
resolves the absolute address for this symbol, it adds that address to the
pointer that is undergoing relocation. (If the
r_extern bit is clear, the situation is
different; 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 instruction using pc-relative addressing. The
address of the relocated 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 content of the r_symbolnum
field is an n_type value (see below);
this type field tells the link editor what segment the relocated 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 Offset Table. At run-time, the entry in the
Global Offset Table at this offset is set to be the address of the
symbol.
- r_jmptable
- If set, the symbol, as identified by the
r_symbolnum field, is to be relocated to
an offset into the Procedure Linkage Table.
- 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 in
r_address. The copying is done by the
run-time link-editor 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 assigned. Symbols consist of a
fixed-length record in the symbol table and a variable-length name in the
string table. The symbol table is an array of
nlist structures:
struct nlist {
union {
const 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 update the symbol'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’ symbols and permits references to them from other
binary files. The N_TYPE
mask selects
bits of interest to the link editor:
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 defines this symbol, the link-editor will
resolve this symbol to an address in the bss segment, reserving 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 data symbols are not
file offsets but addresses; to recover the file offsets, it is
necessary to identify the loaded address of the beginning of the
corresponding section and subtract it, then add the offset of the
section.
N_BSS
- A bss symbol; like text or data symbols but has no corresponding
offset in the binary file.
N_FN
- A filename symbol. The link editor inserts 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 editor, and its value
is the first text address from that binary file. Filename symbols are
not needed for link-editing or loading, but are useful for
debuggers.
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_FUNC
and
AUX_OBJECT
(see
<link.h>
for their definitions). AUX_FUNC
associates the symbol with a callable function, while
AUX_OBJECT
associates the symbol with
data, irrespective of their locations in either the text or the data
segment. This field is intended to be used by
ld(1)
for the construction of dynamic executables.
- n_desc
- Reserved for use by debuggers; passed untouched by the link editor.
Different debuggers use this field for different purposes.
- 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 followed by null-terminated 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),
elf(5),
link(5),
stab(5)
The
<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 machine identifier, the byte order of the
exec header is machine-dependent.