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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>, contains 4–byte integers in big–endian
order:
typedef struct Exec {
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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 */
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} 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 */
#define D_MAGIC _MAGIC(19) /* amd 29000 */
#define E_MAGIC _MAGIC(20) /* arm 7–something */
#define Q_MAGIC _MAGIC(21) /* powerpc */
#define N_MAGIC _MAGIC(22) /* mips 4000 LE */
#define L_MAGIC _MAGIC(23) /* dec alpha */
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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 segment, and the stack. The text segment begins at a
virtual address which is a multiple of the machine–
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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 symbol
table entries, each laid out as:
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uchar value[4];
char type;
char name[n]; /* NUL–terminated */
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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 with the high
bit set:
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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 MC680X0
architectures there is a variable offset between the stack
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pointer and the frame pointer. Following the symbol table, MC680X0
executable files contain a spsz–byte table encoding the offset
of the stack frame pointer as a function of program location;
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 subtracted 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 MC680X0), 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. 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 conversion
of the absolute number to source file and line number:
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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
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values (in big–endian order) that represent file name components
from f symbols. These components, 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 clustered, one set for each object file in the program,
before any text symbols from that object file. The set of z symbols
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 represents 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 history 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.
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