A.OUT(5)

NAME
a.out - assembler and link editor output format

SYNOPSIS
#include <a.out.h>
#include <stab.h>
#include <nlist.h>

AVAILABILITY

Sun-2, Sun-3, and Sun-4 systems only. For Sun386i systems refer to coff(5).

DESCRIPTION

a.out is the output format of the assembler as(1) and the link editor ld(1). The link editor makes a.out executable files.

A file in a.out format consists of: a header, the program text, program data, text and data relocation information, a symbol table, and a string table (in that order). In the header, the sizes of each section are given in bytes. The last three sections may be absent if the program was loaded with the -s option of ld or if the symbols and relocation have been removed by strip(1).

The machine type in the header indicates the type of hardware on which the object code can be executed. Sun-2 code runs on Sun-3 systems, but not vice versa. Program files predating release 3.0 are recognized by a machine type of `0'. Sun-4 code may not be run on Sun-2 or Sun-3, nor vice versa.

  Header

The header consists of a exec structure. The exec structure has the form:

    struct exec {
    	unsigned char	a_dynamic:1;	/* has a __DYNAMIC */
    	unsigned char	a_toolversion:7;	/* version of toolset used to create this file */
    	unsigned char	a_machtype;	/* machine type */
    	unsigned short	a_magic;	/* magic number */
    	unsigned long	a_text;		/* size of text segment */
    	unsigned long	a_data;		/* size of initialized data */
    	unsigned long	a_bss;		/* size of uninitialized data */
    	unsigned long	a_syms;		/* size of symbol table */
    	unsigned long	a_entry;	/* entry point */
    	unsigned long	a_trsize;	/* size of text relocation */
    	unsigned long	a_drsize;	/* size of data relocation */
    };

The members of the structure are:

a_dynamic
    1 if the a.out file is dynamically linked or is a shared object, 0 otherwise.

a_toolversion
    The version number of the toolset (as, ld, etc.) used to create the file.

a_machtype
    One of the following:

    0   pre-3.0 executable image

    M_68010
        executable image using only MC68010 instructions that can run on Sun-2 or Sun-3 systems.

    M_68020
        executable image using MC68020 instructions that can run only on Sun-3 systems.

    M_SPARC
        executable image using SPARC instructions that can run only on Sun-4 systems.

a_magic
    One of the following:

    OMAGIC
        An text executable image which is not to be write-protected, so the data segment is immediately contiguous with the text segment.

    NMAGIC
        A write-protected text executable image. The data segment begins at the first segment boundary following the text segment, and the text segment is not writable by the program. When the image is started with execve(2V), the entire text and data segments will be read into memory.

    ZMAGIC
        A page-aligned text executable image. the data segment begins at the first segment boundary following the text segment, and the text segment is not writable by the program. The text and data sizes are both multiples of the page size, and 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 the default format produced by ld(1).

    The macro N_BADMAG takes an exec structure as an argument; it evaluates to 1 if the a_magic field of that structure is invalid, and evaluates to 0 if it is valid.

a_text
    The size of the text segment, in bytes.

a_data
    The size of the initialized portion of the data segment, in bytes.

a_bss
    The size of the “uninitialized” portion of the data segment, in bytes. This portion is actually initialized to zero. The zeroes are not stored in the a.out file; the data in this portion of the data segment is zeroed out when it is loaded.

a_syms
    The size of the symbol table, in bytes.

a_entry
    The virtual address of the entry point of the program; when the image is started with execve, the first instruction executed in the image is at this address.

a_trsize
    The size of the relocation information for the text segment.

a_drsize
    The size of the relocation information for the data segment.

The macros N_TXTADDR, N_DATADDR, and N_BSSADDR give the memory addresses at which the text, data, and bss segments, respectively, will be loaded.

In the ZMAGIC format, the size of the header is included in the size of the text section; in other formats, it is not.

When an a.out 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 data), and a stack. For the ZMAGIC format, the header is loaded with the text segment; for other formats it is not.

Program execution begins at the address given by the value of the a_entry field.

The stack starts at the highest possible location in the memory image, and grows downwards. The stack is automatically extended as required. The data segment is extended as requested by brk(2) or sbrk.

  Text and Data Segments

The text segment begins at the start of 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 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 information. The N_DATOFF macro returns the absolute file position of the beginning of the data segment when given an exec structure as argument.

  Relocation

The relocation information appears after the text and data segments. The N_TRELOFF macro returns the absolute file position of the relocation information for the text segment, when given an exec structure as argument. The N_DRELOFF macro returns the absolute file position of the relocation information for the data segment, when given an exec structure as argument. There is no relocation information if a_trsize+a_drsize==0.

  Relocation (Sun-2 and Sun-3 Systems)
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 is added to the bytes in the file. If a byte involves a reference to a relative location, or relocatable segment, then the value stored in the file is an offset from the associated segment.

If relocation information is present, it amounts to eight bytes per relocatable datum as in the following structure:

    struct reloc_info_68k {
    	long 	r_address;	/* address which is relocated */
    unsigned int	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 */
    	r_baserel:1,	/* linkage table relative */
    	r_jmptable:1,	/* pc-relative to jump table */
    	r_relative:1,	/* relative relocation */
    	:1;
    };

If r_extern is 0, then r_symbolnum is actually an n_type for the relocation (for instance, N_TEXT meaning relative to segment text origin.)

  Relocation (Sun-4 System)

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 ignored. Unlike the Sun-2 and Sun-3 system, the offset from the associated symbol is kept with the relocation record. When the file is processed by the link editor and the external symbol becomes defined, the value of the symbol is added to this offset, and the sum is inserted into the bytes in the text or data segment.

If relocation information is present, it amounts to twelve bytes per relocatable datum as in the following structure:

enum reloc_type
{
	RELOC_8,	RELOC_16,	RELOC_32,	/* simplest relocs */
	RELOC_DISP8,	RELOC_DISP16,	RELOC_DISP32,	/* Disp's (pc-rel) */
	RELOC_WDISP30,	RELOC_WDISP22,	/* SR word disp's */
	RELOC_HI22,	RELOC_22,	/* SR 22-bit relocs */
	RELOC_13,	RELOC_LO10,	/* SR 13&10-bit relocs */
	RELOC_SFA_BASE,	RELOC_SFA_OFF13,	/* SR S.F.A. relocs */
	RELOC_BASE10,	RELOC_BASE13,	RELOC_BASE22,	/* base_relative pic */
	RELOC_PC10,	RELOC_PC22,	/* special pc-rel pic*/
	RELOC_JMP_TBL,	/* jmp_tbl_rel in pic */
	RELOC_SEGOFF16,	/* ShLib offset-in-seg */
	RELOC_GLOB_DAT,	RELOC_JMP_SLOT,	RELOC_RELATIVE,	/* rtld relocs */
};

struct reloc_info_sparc	/* used when header.a_machtype == M_SPARC */
{
	unsigned long int	r_address;		/* relocation addr (offset in segment) */
	unsigned int	r_index	:24;	/* segment index or symbol index */
	unsigned int	r_extern	: 1;	/* if F, r_index==SEG#; if T, SYM idx */
	int	: 2;		/* <unused> */
	enum reloc_type	r_type	: 5;	/* type of relocation to perform */
	long int	r_addend;		/* addend for relocation value */
};

If r_extern is 0, then r_index is actually a n_type for the relocation (for instance, N_TEXT meaning relative to segment text origin.)

  Symbol Table

The N_SYMOFF macro returns the absolute file position of the symbol table when given an exec structure as argument. Within this symbol table, distinct symbols point to disjoint areas in the string table (even when two symbols have the same name). The string table immediately follows the symbol table; the N_STROFF macro returns the absolute file position of the string table when given an exec structure as argument. The first 4 bytes of the string table are not used for string storage, but rather contain the size of the string table. This size includes the 4 bytes; thus, the minimum string table size is 4. Layout information as given in the include file for the Sun system is shown below.

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 {
	union {
		char	*n_name;	/* for use when in-memory */
		long	n_strx;	/* index into file string table */
	} n_un;
	unsigned char	n_type;		/* type flag, that is, N_TEXT etc; see below */
	char	n_other;
	short	n_desc;		/* see <stab.h> */
	unsigned	n_value;		/* value of this symbol (or adb 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 of the N_STAB bits set.
* These are given in <stab.h>
*/
#define	N_STAB	0xe0		/* if any of these bits set, don't discard */

In the a.out file a symbol's n_un.n_strx field gives an index into the string table. A n_strx value of 0 indicates that no name is associated 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. Because of the union in the nlist declaration, it is impossible in C to statically initialize such a structure. If this must be done (as when using nlist(3V)) include the file <nlist.h>, rather than <a.out.h>. This contains the declaration without the union.

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.

SEE ALSO
adb(1), as(1), cc(1V), dbx(1), ld(1), nm(1), strip(1), brk(2), nlist(3V), coff(5)