NAME
a.out —
format of executable binary
files
SYNOPSIS
#include <sys/types.h>
#include <a.out.h>
DESCRIPTION
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 network byte-order so that binaries
for machines with alternative byte orders can be distinguished. 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.
-
-
- 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 {
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.
SEE ALSO
as(1),
gdb(1),
ld(1),
brk(2),
execve(2),
nlist(3),
core(5),
elf(5),
link(5),
stab(5)
HISTORY
The
a.out.h include file appeared in
Version 7 AT&T UNIX.
BUGS
Nobody seems to agree on what
bss stands for.
New binary file formats may be supported in the future, and they probably will
not be compatible at any level with this ancient format.