A.OUT(6) A.OUT(6)
NAME
a.out - object file format
SYNOPSIS
#include <a.out.h>
DESCRIPTION
An executable Plan 9 binary file has up to six sections: a
header, the program text, the data, a symbol table, a PC/SP
offset table (MC68020 only), and finally a PC/line number
table. The header, given by a structure in <a.out.h>, con-
tains 4-byte integers in big-endian order:
typedef struct Exec {
long magic; /* magic number */
long text; /* size of text segment */
long data; /* size of initialized data */
long bss; /* size of uninitialized data */
long syms; /* size of symbol table */
long entry; /* entry point */
long spsz; /* size of pc/sp offset table */
long pcsz; /* size of pc/line number table */
} Exec;
#define _MAGIC(b) ((((4*b)+0)*b)+7)
#define A_MAGIC _MAGIC(8)/* 68020 */
#define I_MAGIC _MAGIC(11)/* intel 386 */
#define J_MAGIC _MAGIC(12)/* intel 960 */
#define K_MAGIC _MAGIC(13)/* sparc */
#define V_MAGIC _MAGIC(16)/* mips 3000 */
#define X_MAGIC _MAGIC(17)/* att dsp 3210 */
#define M_MAGIC _MAGIC(18)/* mips 4000 */
Sizes are expressed in bytes. The size of the header is not
included in any of the other sizes.
When a Plan 9 binary file is executed, a memory image of
three segments is set up: the text segment, the data seg-
ment, and the stack. The text segment begins at a virtual
address which is a multiple of the machine-dependent page
size. The text segment consists of the header and the first
text bytes of the binary file. The entry field gives the
virtual address of the entry point of the program. The data
segment starts at the first page-rounded virtual address
after the text segment. It consists of the next data bytes
of the binary file, followed by bss bytes initialized to
zero. The stack occupies the highest possible locations in
the core image, automatically growing downwards. The bss
segment may be extended by brk(2).
The next syms (possibly zero) bytes of the file contain
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symbol table entries, each laid out as:
uchar value[4];
char type;
char name[n]; /* NUL-terminated */
The value is in big-endian order and the size of the name
field is not pre-defined: it is a zero-terminated array of
variable length.
The type field is one of the following characters:
T text segment symbol
t static text segment symbol
L leaf function text segment symbol
l static leaf function text segment symbol
D data segment symbol
d static data segment symbol
B bss segment symbol
b static bss segment symbol
a automatic (local) variable symbol
p function parameter symbol
A few others are described below. The symbols in the symbol
table appear in the same order as the program components
they describe.
The Plan 9 compilers implement a virtual stack frame pointer
rather than dedicating a register; moreover, on the MC68020
there is a variable offset between the stack pointer and the
frame pointer. Following the symbol table, MC68020 exe-
cutable files contain a spsz-byte table encoding the offset
of the stack frame pointer as a function of program loca-
tion; this section is not present for other architectures.
The PC/SP table is encoded as a byte stream. By setting the
PC to the base of the text segment and the offset to zero
and interpreting the stream, the offset can be computed for
any PC. A byte value of 0 is followed by four bytes that
hold, in big-endian order, a constant to be added to the
offset. A byte value of 1 to 64 is multiplied by four and
added, without sign extension, to the offset. A byte value
of 65 to 128 is reduced by 64, multiplied by four, and sub-
tracted from the offset. A byte value of 129 to 255 is
reduced by 129, multiplied by the quantum of instruction
size (e.g. two on the MC68020), and added to the current PC
without changing the offset. After any of these operations,
the instruction quantum is added to the PC.
A similar table, occupying pcsz-bytes, is the next section
in an executable; it is present for all architectures. The
same algorithm may be run using this table to recover the
absolute source line number from a given program location.
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A.OUT(6) A.OUT(6)
The absolute line number (starting from zero) counts the
newlines in the C-preprocessed source seen by the compiler.
Three symbol types in the main symbol table facilitate con-
version of the absolute number to source file and line num-
ber:
f source file name components
z source file name
Z source file line offset
The f symbol associates an integer (the value field of the
`symbol') with a unique file path name component (the name
of the `symbol'). These path components are used by the z
symbol to represent a file name: the first byte of the name
field is always 0; the remaining bytes hold a zero-
terminated array of 16-bit values (in big-endian order) that
represent file name components from f symbols. These compo-
nents, when separated by slashes, form a file name. The
initial slash of a file name is recorded in the symbol table
by an f symbol; when forming file names from z symbols an
initial slash is not to be assumed. The z symbols are clus-
tered, one set for each object file in the program, before
any text symbols from that object file. The set of z sym-
bols for an object file form a history stack of the included
source files from which the object file was compiled. The
value associated with each z symbol is the absolute line
number at which that file was included in the source; if the
name associated with the z symbol is null, the symbol repre-
sents the end of an included file, that is, a pop of the
history stack. If the value of the z symbol is 1 (one), it
represents the start of a new history stack. To recover the
source file and line number for a program location, find the
text symbol containing the location and then the first his-
tory stack preceding the text symbol in the symbol table.
Next, interpret the PC/line offset table to discover the
absolute line number for the program location. Using the
line number, scan the history stack to find the set of
source files open at that location. The line number within
the file can be found using the line numbers in the history
stack. The Z symbols correspond to #line directives in the
source; they specify an adjustment to the line number to be
printed by the above algorithm. The offset is associated
with the first previous z symbol in the symbol table.
SEE ALSO
db(1), acid(1), 2a(1), 2l(1), nm(1), strip(1), mach(2),
symbol(2)
BUGS
There is no type information in the symbol table.
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