a.out(5)a.out(5)Namea.out - assembler and link editor output
Syntax
#include <a.out.h>
Description
The file is the output file of the assembler and the link editor Both
programs make executable if there were no errors and no unresolved
external references. Layout information as given in the include file
for the VAX is:
/*
* Header prepended to each a.out file.
*/
struct exec {
unsigned short a_magic; /* magic number */
unsigned short a_mode; /* mode parameter */
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 */
/*
* Compatibility modes
*/
#define A_BSD 0 /* All pre V2.4 a.outs and BSD */
#define A_SYSV 1 /* System V compliant process */
#define A_POSIX 2 /* IEEE P1003.1 compliant process */
/*
* 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 sections may be omitted if the program was loaded with
the -s option of or if the symbols and relocation have been removed by
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 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 and the header is not loaded. If the magic number
in the header is OMAGIC (0407), the number indicates that the text seg‐
ment will not 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 seg‐
ment, and the text segment is not writable by the program. If other
processes are executing the same file, they will share the text seg‐
ment. For ZMAGIC format, the text segment begins at a 0 mod 1024-byte
boundary in the 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. 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
large programs and is the default format produced by
The stack will occupy the highest possible locations in the core image,
growing downwards from 0x7ffff000. The stack is automatically extended
as required. The data segment is only extended as requested by
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 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 informa‐
tion. 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 that can be easily accessed using N_STROFF.
The first 4 bytes of the string table are not used for string storage;
instead they contain the size of the string table which 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; 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 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 file, a symbol's n_un.n_strx field gives an index into the
string table. An n_strx value of 0 indicates that no name is associ‐
ated 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
nonzero, the symbol is interpreted by the loader 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 that is not a portion of a ref‐
erence to an undefined external symbol is exactly the value that 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 8 bytes per relo‐
catable 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 an n_type for the reloca‐
tion (that is, N_TEXT meaning relative to segment text origin).
See Alsoadb(1), as(1), dbx(1), ld(1), nm(1), strip(1), stab(5)
VAX a.out(5)
