freebsd/share/man/man5/a.out.5

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.\"	@(#)a.out.5	8.1 (Berkeley) 6/5/93
.\"
.Dd June 5, 1993
.Dt A.OUT 5
.Os
.Sh NAME
.Nm a.out
.Nd format of executable binary files
.Sh SYNOPSIS
.Fd #include <a.out.h>
.Sh DESCRIPTION
The include file
.Aq Pa 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 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;
};
.Ed
.Pp
The fields have the following functions:
.Bl -tag -width a_trsize
.It Fa a_mid
Contains a bit pattern that
identifies binaries that were built for
certain sub-classes of an architecture
.Pq Sq 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
.Fa a_mid
field is not present on some architectures;
in this case, the
.Fa a_magic
field has type
.Em unsigned long .
.It Fa a_magic
Contains a bit pattern
.Pq Sq magic number
that 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.
.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
.Pa 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,
			: 4;
};
.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.
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.
.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 {
		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 is currently unused.
.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 ld 1 ,
.Xr execve 2 ,
.Xr nlist 3 ,
.Xr core 5 ,
.Xr dbx 5 ,
.Xr stab 5
.Sh HISTORY
The
.Pa a.out.h
include file appeared in
.At v7 .
.Sh BUGS
Since not all of the supported architectures use the
.Fa 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 identifier,
the byte order of the
.Fa exec
header is machine-dependent.
.Pp
Nobody seems to agree on what
.Em bss
stands for.
.Pp
New binary file formats may be supported in the future,
and they probably will not be compatible at any level
with this ancient format.