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.\"     @(#)a.out.5     8.1 (Berkeley) 6/5/93
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.Dd June 10, 2010
.Dt A.OUT 5
.Os
.Sh NAME
.Nm a.out
.Nd format of executable binary files
.Sh SYNOPSIS
.In a.out.h
.Sh DESCRIPTION
The include file
.In a.out.h
declares three structures and several macros.
The structures describe the format of
executable machine code files
.Pq Sq binaries
on the system.
.Pp
A binary file consists of up to 7 sections.
In order, these sections are:
.Bl -tag -width "text relocations"
.It exec header
Contains parameters used by the kernel
to load a binary file into memory and execute it,
and by the link editor
.Xr ld 1
to combine a binary file with other binary files.
This section is the only mandatory one.
.It text segment
Contains machine code and related data
that are loaded into memory when a program executes.
May be loaded read-only.
.It data segment
Contains initialized data; always loaded into writable memory.
.It text relocations
Contains records used by the link editor
to update pointers in the text segment when combining binary files.
.It data relocations
Like the text relocation section, but for data segment pointers.
.It symbol table
Contains records used by the link editor
to cross reference the addresses of named variables and functions
.Pq Sq symbols
between binary files.
.It string table
Contains the character strings corresponding to the symbol names.
.El
.Pp
Every binary file begins with an
.Fa exec
structure:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
The fields have the following functions:
.Bl -tag -width a_trsize
.It Fa a_midmag
This field is stored in host byte-order.
It has a number of sub-components accessed by the macros
.Fn N_GETFLAG ,
.Fn N_GETMID ,
and
.Fn N_GETMAGIC ,
and set by the macro
.Fn N_SETMAGIC .
.Pp
The macro
.Fn N_GETFLAG
returns a few flags:
.Bl -tag -width EX_DYNAMIC
.It Dv EX_DYNAMIC
indicates that the executable requires the services of the run-time link editor.
.It Dv EX_PIC
indicates that the object contains position independent code.
This flag is
set by
.Xr as 1
when given the
.Sq -k
flag and is preserved by
.Xr ld 1
if necessary.
.El
.Pp
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.
.Pp
The macro
.Fn N_GETMID
returns the machine-id.
This indicates which machine(s) the binary is intended to run on.
.Pp
.Fn 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:
.Bl -tag -width ZMAGIC
.It Dv OMAGIC
The text and data segments immediately follow the header
and are contiguous.
The kernel loads both text and data segments into writable memory.
.It Dv NMAGIC
As with
.Dv 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.
.It Dv 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.
.El
.It Fa a_text
Contains the size of the text segment in bytes.
.It Fa a_data
Contains the size of the data segment in bytes.
.It Fa a_bss
Contains the number of bytes in the
.Sq bss segment
and is used by the kernel to set the initial break
.Pq Xr 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.
.Em ( bss
= block started by symbol)
.It Fa a_syms
Contains the size in bytes of the symbol table section.
.It Fa 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.
.It Fa a_trsize
Contains the size in bytes of the text relocation table.
.It Fa a_drsize
Contains the size in bytes of the data relocation table.
.El
.Pp
The
.In a.out.h
include file defines several macros which use an
.Fa exec
structure to test consistency or to locate section offsets in the binary file.
.Bl -tag -width N_BADMAG(exec)
.It Fn N_BADMAG exec
Nonzero if the
.Fa a_magic
field does not contain a recognized value.
.It Fn N_TXTOFF exec
The byte offset in the binary file of the beginning of the text segment.
.It Fn N_SYMOFF exec
The byte offset of the beginning of the symbol table.
.It Fn N_STROFF exec
The byte offset of the beginning of the string table.
.El
.Pp
Relocation records have a standard format which
is described by the
.Fa relocation_info
structure:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
The
.Fa relocation_info
fields are used as follows:
.Bl -tag -width r_symbolnum
.It Fa 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.
.It Fa r_symbolnum
Contains the ordinal number of a symbol structure
in the symbol table (it is
.Em 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
.Fa r_extern
bit is clear, the situation is different; see below.)
.It Fa 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.
.It Fa 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.
.It Fa r_extern
Set if this relocation requires an external reference;
the link editor must use a symbol address to update the pointer.
When the
.Fa r_extern
bit is clear, the relocation is
.Sq 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
.Fa r_baserel
is also set (see below).
In this case, the content of the
.Fa r_symbolnum
field is an
.Fa n_type
value (see below);
this type field tells the link editor
what segment the relocated pointer points into.
.It Fa r_baserel
If set, the symbol, as identified by the
.Fa 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.
.It Fa r_jmptable
If set, the symbol, as identified by the
.Fa r_symbolnum
field, is to be relocated to an offset into the Procedure Linkage Table.
.It Fa 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.
.It Fa r_copy
If set, this relocation record identifies a symbol whose contents should
be copied to the location given in
.Fa r_address .
The copying is done by the run-time link-editor from a suitable data
item in a shared object.
.El
.Pp
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
.Fa nlist
structures:
.Bd -literal -offset indent
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;
};
.Ed
.Pp
The fields are used as follows:
.Bl -tag -width n_un.n_strx
.It Fa 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
.Xr nlist 3
function,
this field is replaced with the
.Fa n_un.n_name
field, which is a pointer to the string in memory.
.It Fa n_type
Used by the link editor to determine
how to update the symbol's value.
The
.Fa n_type
field is broken down into three sub-fields using bitmasks.
The link editor treats symbols with the
.Dv N_EXT
type bit set as
.Sq external
symbols and permits references to them from other binary files.
The
.Dv N_TYPE
mask selects bits of interest to the link editor:
.Bl -tag -width N_TEXT
.It Dv 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
.Fa 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
.Fa 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.
.It Dv N_ABS
An absolute symbol.
The link editor does not update an absolute symbol.
.It Dv 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.
.It Dv N_DATA
A data symbol; similar to
.Dv 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.
.It Dv N_BSS
A bss symbol; like text or data symbols but
has no corresponding offset in the binary file.
.It Dv 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.
.El
.Pp
The
.Dv N_STAB
mask selects bits of interest to symbolic debuggers
such as
.Xr gdb 1 ;
the values are described in
.Xr stab 5 .
.It Fa 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
.Fa n_type
field.
Currently, the lower 4 bits of the
.Fa n_other
field hold one of two values:
.Dv AUX_FUNC
and
.Dv AUX_OBJECT
(see
.In link.h
for their definitions).
.Dv AUX_FUNC
associates the symbol with a callable function, while
.Dv 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
.Xr ld 1
for the construction of dynamic executables.
.It Fa n_desc
Reserved for use by debuggers; passed untouched by the link editor.
Different debuggers use this field for different purposes.
.It Fa 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.
.El
.Pp
The string table consists of an
.Em 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.
.Sh SEE ALSO
.Xr as 1 ,
.Xr gdb 1 ,
.Xr ld 1 ,
.Xr brk 2 ,
.Xr execve 2 ,
.Xr nlist 3 ,
.Xr core 5 ,
.Xr elf 5 ,
.Xr link 5 ,
.Xr stab 5
.Sh HISTORY
The
.In a.out.h
include file appeared in
.At v7 .
.Sh BUGS
Since not all of the supported architectures use the
.Fa 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
.Fa exec
header is machine-dependent.