A.OUT(5) MachTen Programmer’s Manual A.OUT(5)

NAME
a.out - format of executable binary files

SYNOPSIS
#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 ref-
erence 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 short a_mid;
unsigned short a_magic;
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_mid Contains a bit pattern that identifies binaries that were built
for certain sub-classes of an architecture (‘machine IDs’) or
variants of the operating system on a given architecture. The
kernel may not support all machine IDs on a given architecture.
The a_mid field is not present on some architectures; in this

case, the a_magic field has type unsigned long.

a_magic Contains a bit pattern (‘magic number’) that uniquely identi-
fies binary files and distinguishes different loading conven-
tions. The field must contain one of the following values:

OMAGIC The text and data segments immediately follow the head-
er and are contiguous. The kernel loads both text and
data segments into writable memory.

NMAGIC As with OMAGIC, text and data segments immediately fol-
low 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 seg-
ment 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 initial-
ly 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 pro-
gram after the kernel has loaded it; the kernel starts the exe-
cution of the program from the machine instruction at this ad-
dress.

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 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 contain 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 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,
: 4;
};

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 edi-
tor 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 point-
er 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 displace-
ments, 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. 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 relo-
cated pointer points into.

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 memo-
ry.

n_type Used by the link editor to determine how to update 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’ symbols and per-
mits 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 unde-
fined 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 ad-
dress and the link editor will update it when it
merges binary files.

N_DATA A data symbol; similar to N_TEXT but for data ad-
dresses. 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 ad-
dress 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 debug-
gers such as gdb(1); the values are described in stab(5).

n_other This field is currently unused.

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 de-
bugger 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
ld(1), execve(2), nlist(3), core(5), dbx(5), stab(5)

HISTORY
The a.out.h include file appeared in Version 7 AT&T UNIX.

BUGS
Since not all of the supported architectures use the a_mid field, it can
be difficult to determine what architecture a binary will execute on
without examining its actual machine code. Even with a machine identifi-
er, the byte order of the exec header is machine-dependent.

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.

4.4BSD June 5, 1993 5