A.OUT(5)

NAME
a.out - assembler and link editor output

SYNOPSIS
#include <a.out.h>

DESCRIPTION
A.out is the output file of the assembler as(1) and the link editor ld(1). Both programs make a.out executable if there were no errors and no unresolved external references. Layout information as given in the include file for the PDP11 is:

/*
 * Header prepended to each a.out file.
 */
struct	exec {
	int	a_magic;	/* magic number */
	unsigned int	a_text;		/* size of text segment */
	unsigned int	a_data;		/* size of initialized data */
	unsigned int	a_bss;		/* size of uninitialized data */
	unsigned int	a_syms;		/* size of symbol table */
	unsigned int	a_entry; 	/* entry point */
	unsigned int	a_unused;	/* not used */
	unsigned int	a_flag; 	/* relocation info stripped */
};
#define	NOVL	15		/* number of overlays */
struct	ovlhdr {
	int	max_ovl;	/* maximum overlay size */
	unsigned int	ov_siz[NOVL];	/* size of i'th overlay */
};
struct	xexec {
	struct exec e;
	struct ovlhdr o;
};
#define	A_MAGIC1	0407	/* normal */
#define	A_MAGIC2	0410	/* read-only text */
#define	A_MAGIC3	0411	/* separated I&D */
#define	A_MAGIC4	0405	/* overlay */
#define	A_MAGIC5	0430	/* auto-overlay (nonseparate) */
#define	A_MAGIC6	0431	/* auto-overlay (separate) */
/*
 * Macros which take exec structures as arguments and tell whether
 * the file has a reasonable magic number or offset to text.
 */
#define	N_BADMAG(x) \
	(((x).a_magic)!=A_MAGIC1 && ((x).a_magic)!=A_MAGIC2 && \
	((x).a_magic)!=A_MAGIC3 && ((x).a_magic)!=A_MAGIC4 && \
	((x).a_magic)!=A_MAGIC5 && ((x).a_magic)!=A_MAGIC6)
#define	N_TXTOFF(x) \
	((x).a_magic==A_MAGIC5 || (x).a_magic==A_MAGIC6 ? \
	sizeof(struct ovlhdr) + sizeof(struct exec) : sizeof(struct exec))
/*
 * The following were added as part of the new object file format.  They
 * call functions because calculating the sums of overlay sizes was too
 * messy (and verbose) to do 'inline'.
 *
 * NOTE: if the magic number is that of an overlaid object the program
 * must pass an extended header ('xexec') as the argument.
*/
off_t	n_stroff(), n_symoff(), n_datoff(), n_dreloc(), n_treloc();
#define	N_STROFF(e) (n_stroff(&e))
#define	N_SYMOFF(e) (n_symoff(&e))
#define	N_DATOFF(e) (n_datoff(&e))
#define	N_DRELOC(e) (n_dreloc(&e))
#define	N_TRELOC(e) (n_treloc(&e))

The file has five sections: a header, the program text and data, relocation information, a symbol table and a strings table (in that order). The last three may be omitted if the program was loaded with the `-s' option of ld or if the symbols and relocation have been removed by strip(1).

In the header the sizes of each section are given in bytes, but are even. The size of the header is not included in any of the other sizes.

When an a.out file is executed, three or four logical segments are set up: the text segment, a possible text overlay 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; the header is not loaded.

Non-overlaid objects: If the magic number in the header is A_MAGIC1 (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 the default; it should not be used for production binaries. If the magic number is A_MAGIC2 (0410), the data segment begins at the first 0 mod 8K byte boundary following the text segment, and the text segment is not writable by the program; if other processes are executing the same file, they will share the text segment. If the magic number is A_MAGIC3 (0411), the text segment is again pure, write-protected, and shared, and moreover instruction and data space are separated; the text and data segment both begin at location 0. This format is only runnable on processors which support separate instruction and data space but can provide significantly more data space than an A_MAGIC2 format of the same object.

Text replacement objects : If the magic number is A_MAGIC4 (0405), the text segment is overlaid on an existing non-overlaid pure (A_MAGIC2 or A_MAGIC3) or text replacement (A_MAGIC4) text segment and the existing data segment is preserved. The text segment of the previous memory image must be the same size as that of the text replacement object being loaded. There is, unfortunately, no loader support to help achieve this requirement. The text replacement format is useful for objects which need a large amount of data space on non-separate I&D processors.

Overlaid objects : If the magic number is A_MAGIC5 (0430), a base text segment is write-protected and shared and is followed by a text overlay segment. There are a maximum of NOVL overlays, all pure and shared. The base segment runs from 0 to txtsiz. The overlay region begins at the next 0 mod 8k byte boundary, which is as large as the largest overlay. When running, any one of the overlays can be mapped into this region. The data segment begins at the following 0 mod 8k byte boundary. If the magic number is A_MAGIC6 (0431), the situation is the same as for type A_MAGIC5 except that instruction and data spaces are separated and both begin at location 0. As with the A_MAGIC3 format, an a.out in A_MAGIC6 format can only be run on a processor which supports separate I&D, but again can provide significantly more data space than A_MAGIC5 format. Both A_MAGIC5 and A_MAGIC6 executable files have a second header between the normal a.out header and the start of the text image; it contains the maximum overlay size and the sizes of each of the overlays. The text images of the overlays follow the text in the object file.

The stack segment will occupy the highest possible locations in the core image: growing downwards from 0177776(8). The stack segment is automatically extended as required. The data segment is only extended as requested by brk(2).

The include file a.out.h defines _AOUT_INCLUDE_, the include file nlist.h does not. This permits compile time initialization of the n_name field for programs that are not looking at the executable header.

The layout of a symbol table entry and the principal flag values that distinguish symbol types are given in the include file as follows:

struct	nlist {
#ifdef	_AOUT_INCLUDE_
	union {
		char *n_name;	/* In memory address of symbol name */
		off_t n_strx;	/* String table offset (file) */
	} n_un;
#else
	char	*n_name;	/* symbol name (in memory) */
#endif
	u_char	n_type;		/* Type of symbol - see below */
	char	n_ovly;		/* Overlay number */
	u_int	n_value;	/* Symbol value */
};
/*
 * Simple values for n_type.
 */
#define	N_UNDF	0x0		/* undefined */
#define	N_ABS	0x1		/* absolute */
#define	N_TEXT	0x2		/* text symbol */
#define	N_DATA	0x3		/* data symbol */
#define	N_BSS	0x4		/* bss symbol */
#define	N_REG	0x14		/* register name */
#define	N_FN	0x1f		/* file name symbol */
#define	N_EXT	0x20		/* external bit, or'ed in */
#define	N_TYPE	0x1f		/* mask for all the type bits */
/*
 * Format for namelist values.
 */
#define	N_FORMAT	"%06o"

If a symbol's type is undefined external, and the value field is non-zero, the symbol is interpreted by the loader ld as the name of a common region whose size is indicated by the value of the symbol.

The value of a word in the text or data which is not a portion of a reference to an undefined external symbol is exactly that value which will appear in memory when the file is executed. If a word 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 into the word in the file.

If relocation information is present, it amounts to one word per word of program text or initialized data. There is no relocation information if the `relocation info stripped' flag in the header is on. Automatic-overlay (A_MAGIC5 and A_MAGIC6) files do not contain relocation information.

Bits 1-3 of a relocation word indicate the segment referred to by the text or data word associated with the relocation word:

000 absolute number

002 reference to text segment

004 reference to initialized data

006 reference to uninitialized data (bss)

010 reference to undefined external symbol

Bit 0 of the relocation word indicates, if 1, that the reference is relative to the pc (e.g. `clr x'); if 0, that the reference is to the actual symbol (e.g., `clr *$x').

The remainder of the relocation word (bits 15-4) contains a symbol number in the case of external references, and is unused otherwise.

The string table begins with a longword containing the length of the string table (including the longword itself). All strings are null terminated.

The first symbol is numbered 0, the second 1, etc.

SEE ALSO
as(1), ld(1), nm(1), strip(1), nlist(3)

BUGS
The current implementation places a maximum length of 32 characters for symbol names in a.out files. This is (relatively) easily raised with the caveat that the linker and other programs which look at symbol tables will slow down even more than they already have.

The 4BSD a.out format has been implemented. This involved modifying the first phase of the C compiler (/lib/c0), the assembler (/bin/as), the debugger adb(1), the linker ld(1), and then simply porting the 4.3BSD/Net-2 ar(1), nm(1), ranlib(1), strip(1) and nlist(3).

As part of this effort the include file short_names.h has gone away.