4.3BSD-Reno/share/man/cat5/a.out.0
A.OUT(5) 1986 A.OUT(5)
NNAAMMEE
a.out - assembler and link editor output
SSYYNNOOPPSSIISS
##iinncclluuddee <<aa..oouutt..hh>>
DDEESSCCRRIIPPTTIIOONN
_A._o_u_t is the output file of the assembler _a_s(1) and the link
editor _l_d(1). Both programs make _a._o_u_t executable if there
were no errors and no unresolved external references. Lay-
out information as given in the include file for the VAX-11
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, relocation 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 _l_d or if the sym-
bols and relocation have been removed by _s_t_r_i_p(1).
Printed 7/27/90 May 1
A.OUT(5) 1986 A.OUT(5)
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._o_u_t file is executed, three logical segments are
set up: the text segment, the data segment (with uninitial-
ized data, which starts off as all 0, following initial-
ized), 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
segment 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 execut-
ing 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._o_u_t 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 formats. This is especially suitable for very
large programs and is the default format produced by _l_d(1).
The stack will occupy the highest possible locations in the
core image, growing downwards from USRSTACK (from
<_m_a_c_h_i_n_e/_v_m_p_a_r_a_m._h>). The stack is automatically extended
as required. The data segment is only extended as requested
by _b_r_k(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 argu-
ment. 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
Printed 7/27/90 May 2
A.OUT(5) 1986 A.OUT(5)
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 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._o_u_t 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
_l_d as the name of a common region whose size is indicated by
Printed 7/27/90 May 3
A.OUT(5) 1986 A.OUT(5)
the value of the symbol.
The value of a byte in the text or data which is not a por-
tion 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 indicated 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 relocatable 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_extern is 0, then r_symbolnum is actually a n_type for
the relocation (i.e. N_TEXT meaning relative to segment text
origin.)
SSEEEE AALLSSOO
adb(1), as(1), ld(1), nm(1), dbx(1), stab(5), strip(1)
BBUUGGSS
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 incompatibility, and we couldn't
hack this just now.
Printed 7/27/90 May 4