275 lines
7.7 KiB
Text
275 lines
7.7 KiB
Text
.TH A.OUT 6
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.SH NAME
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a.out \- object file format
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.SH SYNOPSIS
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.B #include <a.out.h>
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.SH DESCRIPTION
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An executable Plan 9 binary file has up to six sections:
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a header, the program text, the data,
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a symbol table, a PC/SP offset table (MC68020 only),
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and finally a PC/line number table.
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The header, given by a structure in
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.BR <a.out.h> ,
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contains 4-byte integers in big-endian order:
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.PP
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.EX
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.ta \w'#define 'u +\w'_MAGIC(b) 'u +\w'_MAGIC(10) 'u +4n +4n +4n +4n
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typedef struct Exec {
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long magic; /* magic number */
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long text; /* size of text segment */
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long data; /* size of initialized data */
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long bss; /* size of uninitialized data */
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long syms; /* size of symbol table */
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long entry; /* entry point */
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long spsz; /* size of pc/sp offset table */
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long pcsz; /* size of pc/line number table */
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} Exec;
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#define HDR_MAGIC 0x00008000 /* header expansion */
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#define _MAGIC(f, b) ((f)|((((4*(b))+0)*(b))+7))
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#define A_MAGIC _MAGIC(0, 8) /* 68020 */
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#define I_MAGIC _MAGIC(0, 11) /* intel 386 */
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#define J_MAGIC _MAGIC(0, 12) /* intel 960 (retired) */
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#define K_MAGIC _MAGIC(0, 13) /* sparc */
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#define V_MAGIC _MAGIC(0, 16) /* mips 3000 BE */
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#define X_MAGIC _MAGIC(0, 17) /* att dsp 3210 (retired) */
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#define M_MAGIC _MAGIC(0, 18) /* mips 4000 BE */
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#define D_MAGIC _MAGIC(0, 19) /* amd 29000 (retired) */
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#define E_MAGIC _MAGIC(0, 20) /* arm */
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#define Q_MAGIC _MAGIC(0, 21) /* powerpc */
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#define N_MAGIC _MAGIC(0, 22) /* mips 4000 LE */
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#define L_MAGIC _MAGIC(0, 23) /* dec alpha (retired) */
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#define P_MAGIC _MAGIC(0, 24) /* mips 3000 LE */
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#define U_MAGIC _MAGIC(0, 25) /* sparc64 */
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#define S_MAGIC _MAGIC(HDR_MAGIC, 26) /* amd64 */
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#define T_MAGIC _MAGIC(HDR_MAGIC, 27) /* powerpc64 */
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#define R_MAGIC _MAGIC(HDR_MAGIC, 28) /* arm64 */
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.EE
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.DT
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.PP
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Sizes are expressed in bytes.
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The size of the header is not included in any of the other sizes.
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.PP
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When a Plan 9 binary file is executed,
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a memory image of three segments is
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set up: the text segment, the data segment, and the stack.
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The text segment begins at a virtual address which is
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a multiple of the machine-dependent page size.
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The text segment consists of the header and the first
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.B text
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bytes of the binary file.
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The
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.B entry
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field gives the virtual address of the entry point of the program
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unless
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.B HDR_MAGIC
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flag is present in the
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.B magic
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field.
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In that case, the header is expanded
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by 8 bytes containing the 64-bit virtual address of the
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program entry point and the 32-bit
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.B entry
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field is reserved for physical kernel entry point.
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.PP
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The data segment starts at the first page-rounded virtual address
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after the text segment.
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It consists of the next
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.B data
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bytes of the binary file, followed by
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.B bss
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bytes initialized to zero.
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The stack occupies the highest possible locations
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in the core image, automatically growing downwards.
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The bss segment may be extended by
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.IR brk (2).
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.PP
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The next
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.B syms
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(possibly zero)
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bytes of the file contain symbol table
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entries, each laid out as:
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.IP
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.EX
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uchar value[4]; /* value[8] on 64 bit systems */
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char type;
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char name[\f2n\fP]; /* NUL-terminated */
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.EE
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.PP
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The
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.B value
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is in big-endian order and
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the size of the
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.B name
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field is not pre-defined: it is a zero-terminated array of
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variable length.
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.PP
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The
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.B type
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field is one of the following characters with the high bit set:
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.RS
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.TP
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.B T
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text segment symbol
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.PD0
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.TP
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.B t
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static text segment symbol
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.TP
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.B L
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leaf function text segment symbol
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.TP
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.B l
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static leaf function text segment symbol
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.TP
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.B D
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data segment symbol
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.TP
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.B d
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static data segment symbol
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.TP
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.B B
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bss segment symbol
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.TP
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.B b
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static bss segment symbol
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.TP
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.B a
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automatic (local) variable symbol
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.TP
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.B p
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function parameter symbol
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.TP
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.B m
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frame symbol
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.RE
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.PD
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.PP
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A few others are described below.
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The symbols in the symbol table appear in the same order
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as the program components they describe.
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.PP
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The Plan 9 compilers implement a virtual stack frame pointer rather
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than dedicating a register;
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moreover, on the MC680X0 architectures
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there is a variable offset between the stack pointer and the
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frame pointer.
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Following the symbol table,
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MC680X0 executable files contain a
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.BR spsz -byte
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table encoding the offset
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of the stack frame pointer as a function of program location;
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this section is not present for other architectures.
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The PC/SP table is encoded as a byte stream.
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By setting the PC to the base of the text segment
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and the offset to zero and interpreting the stream,
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the offset can be computed for any PC.
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A byte value of 0 is followed by four bytes that hold, in big-endian order,
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a constant to be added to the offset.
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A byte value of 1 to 64 is multiplied by four and added, without sign
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extension, to the offset.
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A byte value of 65 to 128 is reduced by 64, multiplied by four, and
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subtracted from the offset.
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A byte value of 129 to 255 is reduced by 129, multiplied by the quantum
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of instruction size
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(e.g. two on the MC680X0),
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and added to the current PC without changing the offset.
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After any of these operations, the instruction quantum is added to the PC.
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.PP
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A similar table, occupying
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.BR pcsz -bytes,
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is the next section in an executable; it is present for all architectures.
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The same algorithm may be run using this table to
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recover the absolute source line number from a given program location.
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The absolute line number (starting from zero) counts the newlines
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in the C-preprocessed source seen by the compiler.
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Three symbol types in the main symbol table facilitate conversion of the absolute
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number to source file and line number:
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.RS
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.TP
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.B f
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source file name components
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.TP
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.B z
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source file name
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.TP
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.B Z
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source file line offset
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.RE
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.PP
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The
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.B f
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symbol associates an integer (the
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.B value
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field of the `symbol') with
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a unique file path name component (the
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.B name
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of the `symbol').
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These path components are used by the
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.B z
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symbol to represent a file name: the
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first byte of the name field is always 0; the remaining
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bytes hold a zero-terminated array of 16-bit values (in big-endian order)
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that represent file name components from
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.B f
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symbols.
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These components, when separated by slashes, form a file name.
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The initial slash of a file name is recorded in the symbol table by an
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.B f
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symbol; when forming file names from
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.B z
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symbols an initial slash is not to be assumed.
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The
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.B z
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symbols are clustered, one set for each object file in the program,
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before any text symbols from that object file.
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The set of
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.B z
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symbols for an object file form a
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.I history stack
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of the included source files from which the object file was compiled.
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The value associated with each
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.B z
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symbol is the absolute line number at which that file was included in the source;
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if the name associated with the
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.B z
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symbol is null, the symbol represents the end of an included file, that is,
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a pop of the history stack.
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If the value of the
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.B z
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symbol is 1 (one),
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it represents the start of a new history stack.
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To recover the source file and line number for a program location,
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find the text symbol containing the location
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and then the first history stack preceding the text symbol in the symbol table.
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Next, interpret the PC/line offset table to discover the absolute line number
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for the program location.
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Using the line number, scan the history stack to find the set of source
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files open at that location.
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The line number within the file can be found using the line numbers
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in the history stack.
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The
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.B Z
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symbols correspond to
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.B #line
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directives in the source; they specify an adjustment to the line number
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to be printed by the above algorithm. The offset is associated with the
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first previous
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.B z
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symbol in the symbol table.
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.SH "SEE ALSO"
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.IR db (1),
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.IR acid (1),
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.IR 2a (1),
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.IR 2l (1),
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.IR nm (1),
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.IR strip (1),
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.IR mach (2),
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.IR symbol (2)
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.SH BUGS
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There is no type information in the symbol table; however, the
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.B -a
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flags on the compilers will produce symbols for
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.IR acid (1).
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