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researchv10 Dan Cross

.TH A.OUT 5
.CT 1 lib_obj
.SH NAME
a.out \- object file format
.SH SYNOPSIS
.B #include <a.out.h>
.SH DESCRIPTION
.I A.out
is the default name of the output file of the assembler
.IR as (1)
or the link editor
.IR ld (1).
Both programs make
.I a.out
executable if there were no
errors and no unresolved external references.
An object file has five sections:
a header, the program text and data,
relocation information, a symbol table and a string table (in that order).
The last three may be absent; see
.IR strip (1)
and option
.B -s
of
.IR ld (1).
The header format, given in 
.BR <a.out.h> ,
is
.PP
.EX
.ta \w'#define  'u +\w'unsigned  'u +\w'a_dirsize  'u +4n
struct exec {
	long	a_magic;	/* magic number */
	unsigned	a_text;	/* size of text segment */
	unsigned	a_data;	/* size of initialized data */
	unsigned	a_bss;	/* size of uninitialized data */
	unsigned	a_syms;	/* size of symbol table */
	unsigned	a_entry;	/* entry point */
	unsigned	a_trsize;	/* size of text relocation */
	unsigned	a_drsize;	/* size of data relocation */
};
#define	OMAGIC	0407	/* old impure format */
#define	NMAGIC	0410	/* read-only text */
#define	ZMAGIC	0413	/* demand load format */
.EE
.PP
Macros which take
.L exec
structures as arguments and tell whether
the file has a reasonable magic number or return offsets:
.EX
#define	N_BADMAG(x) (((x).a_magic)!=OMAGIC && \e
	((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
#define	N_TXTOFF(x) \e
	((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
#define N_SYMOFF(x) (N_TXTOFF(x) + (x).a_text+(x).a_data + \e
	(x).a_trsize+(x).a_drsize)
#define	N_STROFF(x) (N_SYMOFF(x) + (x).a_syms)
.EE
.DT
.PP
Sizes are expressed in bytes.
The size of the header is not included in any of the other sizes.
.PP
When an
.F a.out
file is executed, a memory image of three segments is
set up: the text segment, the data segment,
and a stack.
The text segment begins at virtual address 0.
Following the text segment is the data segment, in which
explicitly initialized data come first, then
other data, initialized to 0.
The stack occupies the highest possible locations
in the core image, automatically growing downwards from
.lg 0
.BR 0x7ffff400 
.lg 1
as needed.
The data segment may be extended by
.IR brk (2).
.PP
If the magic number in the header is
.BR OMAGIC ,
the text
segment is neither write-protected nor shared and
the data segment is immediately contiguous
with the text segment.
This kind of executable program is rarely used.
If the magic number is
.BR NMAGIC 
or
.BR ZMAGIC ,
the data segment is loaded at the first 0 mod 1024 address
following the text segment,
and the text segment is write-protected and shared among
all processes that are executing the same file.
.BR ZMAGIC
format, which
.IR ld (1)
produces by default, supports demand paging: the
text and data segments are multiples of
1024 bytes long and begin at byte offsets of 0 mod 1024
in the
.F a.out
file.
.PP
Macros are provided to compute the absolute offset of various
parts of the file:
.TF N_TXTOFF
.TP 
.B N_TXTOFF
Text segment.
.TP
.B N_SYMOFF
Symbol table.
.TP
.B N_STROFF
String table.
.PD
.PP
The offsets of the data segment, text relocation information,
and data relocation information are obtained by successively
adding to 
.B N_TXTOFF
the size fields
.BR a_text ,
.BR a_data ,
and
.BR a_trsize .
The first 4 bytes of the string table contain it size, including
the 4 bytes.
.PP
The layout of a symbol table entry, as given in
.BR <a.out.h> ,
is
.PP
.EX
.ta \w'#define  'u +\w'char'u-1u +\w'unsigned  'u+1u +\w'*n_name 'u
struct nlist {
	union {
		char	*n_name;	/* for use when in-core */
		long	n_strx;	/* index into file string table */
	} n_un;
	unsigned char	n_type;	/* type flag; see below */
	char	n_other;
	short	n_desc;	/* see \fIstab\fR(5)\f5 */
	unsigned	n_value;	/* value of this symbol (or struct offset) */
};
.EE
.PP
Basic values for
.BR n_type :
.PP
.EX
#define	N_UNDF	0x0	/* undefined */
#define	N_ABS	0x2	/* absolute */
#define	N_TEXT	0x4	/* text */
#define	N_DATA	0x6	/* data */
#define	N_BSS	0x8	/* bss */
#define	N_COMM	0x12	/* common (internal to ld) */
#define	N_FN	0x1f	/* file name symbol */
#define	N_EXT	01	/* external bit, or'ed in */
#define	N_TYPE	0x1e	/* mask for all the type bits */
.EE
.PP
Other permanent symbol table entries have some
.B N_STAB
bits set.
These are given in
.LR <stab.h> :
.EX
#define	N_STAB	0xe0	/* if any of these bits set, keep */
.EE
.DT
.PP
The field
.B n_un.n_strx
gives an index into the
string table; 0
indicates that no name is associated
with the entry.
The field
.B n_un.n_name
can be used
to refer to the symbol name only if the program sets this up using
.B n_strx
and appropriate data from the string table.
.PP
A symbol of type undefined external with nonzero value field
names a common region; the value specifies its size.
.PP
Relocation
information occupies eight bytes per
relocatable datum:
.PP
.EX
.ta \w'#define  'u +\w'unsigned  'u +\w'r_symbolnum:24,  'u +4n
struct relocation_info {
	int	r_address;	/* address of datum to be relocated */
	unsigned	r_symbolnum:24,	/* local symbol ordinal */
		r_pcrel:1, 	/* is referenced relative to pc */
		r_length:2,	/* 0=byte, 1=word, 2=long */
		r_extern:1,	/* symbol value unknown */
		:4;	/* nothing, yet */
};
.EE
.DT
.PP
If
.B r_extern 
is 1, the datum designated by
.B r_address 
and
.B r_length
will be relocated by adding to it the value of the associated
external symbol.
If
.B r_extern
is 0, 
.BR r_symbolnum
is encoded in the style of
.B n_type
and the value will be relocated by adding the relocated base of
the designated area (text, initialized data, or common data).
.SH "SEE ALSO"
.IR adb (1), 
.IR as (1), 
.IR ld (1), 
.IR nm (1), 
.IR stab (5), 
.IR strip (1)

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