term% cat index.txt A.OUT(5) File Formats Manual A.OUT(5)
NAME
a.out - assembler and link editor output
SYNOPSIS
#include <a.out.h>
DESCRIPTION
A.out is the output file of the assembler as(1) and the link editor
ld(1). Both programs make a.out executable if there were no errors and
no unresolved external references. The layout as given in the include
file is:
/*
* Header prepended to each a.out file.
*/
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 */
/*
* Macros which take exec structures as arguments and tell whether
* the file has a reasonable magic number or offsets to text|symbols|strings.
*/
#define N_BADMAG(x) \
(((x).a_magic)!=OMAGIC && ((x).a_magic)!=NMAGIC && ((x).a_magic)!=ZMAGIC)
#define N_TXTOFF(x) \
((x).a_magic==ZMAGIC ? 1024 : sizeof (struct exec))
#define N_SYMOFF(x) \
(N_TXTOFF(x) + (x).a_text+(x).a_data + (x).a_trsize+(x).a_drsize)
#define N_STROFF(x) \
(N_SYMOFF(x) + (x).a_syms)
The file has five sections: a header, the program text and data, relo‐
cation information, a symbol table and a string table (in that order).
The last three may be omitted if the program was loaded with the `-s'
option of ld or if the symbols and relocation have been removed by
strip(1).
In the header the sizes of each section are given in bytes. The size
of the header is not included in any of the other sizes.
When an a.out file is executed, three logical segments are set up: the
text segment, the data segment (with uninitialized data, which starts
off as all 0, following initialized), and a stack. The text segment
begins at 0 in the core image; the header is not loaded. If the magic
number in the header is OMAGIC (0407), it indicates that the text seg‐
ment is not to be write-protected and shared, so the data segment is
immediately contiguous with the text segment. This is the oldest kind
of executable program and is rarely used. If the magic number is
NMAGIC (0410) or ZMAGIC (0413), the data segment begins at the first 0
mod 1024 byte boundary following the text segment, and the text segment
is not writable by the program; if other processes are executing the
same file, they will share the text segment. For ZMAGIC format, the
text segment begins at a 0 mod 1024 byte boundary in the a.out file,
the remaining bytes after the header in the first block are reserved
and should be zero. In this case the text and data sizes must both be
multiples of 1024 bytes, and the pages of the file will be brought into
the running image as needed, and not pre-loaded as with the other for‐
mats. This is especially suitable for very large programs and is the
default format produced by ld(1).
The stack will occupy the highest possible locations in the core image:
growing downwards from 0x7ffff400. The stack is automatically extended
as required. The data segment is only extended as requested by
break(2).
After the header in the file follow the text, data, text relocation
data relocation, symbol table and string table in that order. The text
begins at the byte 1024 in the file for ZMAGIC format or just after the
header for the other formats. The N_TXTOFF macro returns this absolute
file position when given the name of an exec structure as argument.
The data segment is contiguous with the text and immediately followed
by the text relocation and then the data relocation information. The
symbol table follows all this; its position is computed by the N_SYMOFF
macro. Finally, the string table immediately follows the symbol table
at a position which can be gotten easily using N_STROFF. The first 4
bytes of the string table are not used for string storage, but rather
contain the size of the string table; this size INCLUDES the 4 bytes,
the minimum string table size is thus 4.
The layout of a symbol table entry and the principal flag values that
distinguish symbol types are given in the include file as follows:
/*
* Format of a symbol table entry.
*/
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, i.e. N_TEXT etc; see below */
char n_other;
short n_desc; /* see <stab.h> */
unsigned n_value; /* value of this symbol (or sdb offset) */
};
#define n_hash n_desc /* used internally by ld */
/*
* Simple values for n_type.
*/
#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 */
/*
* Other permanent symbol table entries have some of the N_STAB bits set.
* These are given in <stab.h>
*/
#define N_STAB 0xe0 /* if any of these bits set, don't discard */
/*
* Format for namelist values.
*/
#define N_FORMAT "%08x"
In the a.out file a symbol's n_un.n_strx field gives an index into the
string table. A n_strx value of 0 indicates that no name is associated
with a particular symbol table entry. The field n_un.n_name can be
used to refer to the symbol name only if the program sets this up using
n_strx and appropriate data from the string table.
If a symbol's type is undefined external, and the value field is non-
zero, the symbol is interpreted by the loader ld as the name of a com‐
mon region whose size is indicated by the value of the symbol.
The value of a byte in the text or data which is not a portion of a
reference to an undefined external symbol is exactly that value which
will appear in memory when the file is executed. If a byte in the text
or data involves a reference to an undefined external symbol, as indi‐
cated by the relocation information, then the value stored in the file
is an offset from the associated external symbol. When the file is
processed by the link editor and the external symbol becomes defined,
the value of the symbol will be added to the bytes in the file.
If relocation information is present, it amounts to eight bytes per re‐
locatable datum as in the following structure:
/*
* Format of a relocation datum.
*/
struct relocation_info {
int r_address; /* address which is relocated */
unsigned r_symbolnum:24, /* local symbol ordinal */
r_pcrel:1, /* was relocated pc relative already */
r_length:2, /* 0=byte, 1=word, 2=long */
r_extern:1, /* does not include value of sym referenced */
:4; /* nothing, yet */
};
There is no relocation information if a_trsize+a_drsize==0. If r_ex‐
tern is 0, then r_symbolnum is actually a n_type for the relocation
(i.e. N_TEXT meaning relative to segment text origin.)
SEE ALSO
adb(1), as(1), ld(1), nm(1), sdb(1), stab(5), strip(1)
BUGS
Not having the size of the string table in the header is a loss, but
expanding the header size would have meant stripped executable file in‐
compatibility, and we couldn't hack this just now.
A.OUT(5)