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A.OUT(5)		  FreeBSD File Formats Manual		      A.OUT(5)

     a.out -- format of	executable binary files

     #include <a.out.h>

     The include file <a.out.h>	declares three structures and several macros.
     The structures describe the format	of executable machine code files
     (`binaries') on the system.

     A binary file consists of up to 7 sections.  In order, these sections

     exec header       Contains	parameters used	by the kernel to load a	binary
		       file into memory	and execute it,	and by the link	editor
		       ld(1) to	combine	a binary file with other binary	files.
		       This section is the only	mandatory one.

     text segment      Contains	machine	code and related data that are loaded
		       into memory when	a program executes.  May be loaded

     data segment      Contains	initialized data; always loaded	into writable

     text relocations  Contains	records	used by	the link editor	to update
		       pointers	in the text segment when combining binary

     data relocations  Like the	text relocation	section, but for data segment

     symbol table      Contains	records	used by	the link editor	to cross ref-
		       erence the addresses of named variables and functions
		       (`symbols') between binary files.

     string table      Contains	the character strings corresponding to the
		       symbol names.

     Every binary file begins with an exec structure:

	   struct exec {
		   unsigned long   a_midmag;
		   unsigned long   a_text;
		   unsigned long   a_data;
		   unsigned long   a_bss;
		   unsigned long   a_syms;
		   unsigned long   a_entry;
		   unsigned long   a_trsize;
		   unsigned long   a_drsize;

     The fields	have the following functions:

     a_midmag  This field is stored in host byte-order.	 It has	a number of
	       sub-components accessed by the macros N_GETFLAG(), N_GETMID(),
	       and N_GETMAGIC(), and set by the	macro N_SETMAGIC().

	       The macro N_GETFLAG() returns a few flags:

	       EX_DYNAMIC  indicates that the executable requires the services
			   of the run-time link	editor.

	       EX_PIC	   indicates that the object contains position inde-
			   pendent code.  This flag is set by as(1) when given
			   the `-k' flag and is	preserved by ld(1) if neces-

	       If both EX_DYNAMIC and EX_PIC are set, the object file is a po-
	       sition independent executable image (e.g. a shared library),
	       which is	to be loaded into the process address space by the
	       run-time	link editor.

	       The macro N_GETMID() returns the	machine-id.  This indicates
	       which machine(s)	the binary is intended to run on.

	       N_GETMAGIC() specifies the magic	number,	which uniquely identi-
	       fies binary files and distinguishes different loading conven-
	       tions.  The field must contain one of the following values:

	       OMAGIC  The text	and data segments immediately follow the
		       header and are contiguous.  The kernel loads both text
		       and data	segments into writable memory.

	       NMAGIC  As with OMAGIC, text and	data segments immediately fol-
		       low the header and are contiguous.  However, the	kernel
		       loads the text into read-only memory and	loads the data
		       into writable memory at the next	page boundary after
		       the text.

	       ZMAGIC  The kernel loads	individual pages on demand from	the
		       binary.	The header, text segment and data segment are
		       all padded by the link editor to	a multiple of the page
		       size.  Pages that the kernel loads from the text	seg-
		       ment are	read-only, while pages from the	data segment
		       are writable.

     a_text    Contains	the size of the	text segment in	bytes.

     a_data    Contains	the size of the	data segment in	bytes.

     a_bss     Contains	the number of bytes in the `bss	segment' and is	used
	       by the kernel to	set the	initial	break (brk(2)) after the data
	       segment.	 The kernel loads the program so that this amount of
	       writable	memory appears to follow the data segment and ini-
	       tially reads as zeroes.	(bss = block started by	symbol)

     a_syms    Contains	the size in bytes of the symbol	table section.

     a_entry   Contains	the address in memory of the entry point of the	pro-
	       gram after the kernel has loaded	it; the	kernel starts the exe-
	       cution of the program from the machine instruction at this ad-

     a_trsize  Contains	the size in bytes of the text relocation table.

     a_drsize  Contains	the size in bytes of the data relocation table.

     The <a.out.h> include file	defines	several	macros which use an exec
     structure to test consistency or to locate	section	offsets	in the binary

     N_BADMAG(exec)  Nonzero if	the a_magic field does not contain a recog-
		     nized value.

     N_TXTOFF(exec)  The byte offset in	the binary file	of the beginning of
		     the text segment.

     N_SYMOFF(exec)  The byte offset of	the beginning of the symbol table.

     N_STROFF(exec)  The byte offset of	the beginning of the string table.

     Relocation	records	have a standard	format which is	described by the
     relocation_info structure:

	   struct relocation_info {
		   int		   r_address;
		   unsigned int	   r_symbolnum : 24,
				   r_pcrel : 1,
				   r_length : 2,
				   r_extern : 1,
				   r_baserel : 1,
				   r_jmptable :	1,
				   r_relative :	1,
				   r_copy : 1;

     The relocation_info fields	are used as follows:

     r_address	  Contains the byte offset of a	pointer	that needs to be link-
		  edited.  Text	relocation offsets are reckoned	from the start
		  of the text segment, and data	relocation offsets from	the
		  start	of the data segment.  The link editor adds the value
		  that is already stored at this offset	into the new value
		  that it computes using this relocation record.

     r_symbolnum  Contains the ordinal number of a symbol structure in the
		  symbol table (it is not a byte offset).  After the link edi-
		  tor resolves the absolute address for	this symbol, it	adds
		  that address to the pointer that is undergoing relocation.
		  (If the r_extern bit is clear, the situation is different;
		  see below.)

     r_pcrel	  If this is set, the link editor assumes that it is updating
		  a pointer that is part of a machine code instruction using
		  pc-relative addressing.  The address of the relocated
		  pointer is implicitly	added to its value when	the running
		  program uses it.

     r_length	  Contains the log base	2 of the length	of the pointer in
		  bytes; 0 for 1-byte displacements, 1 for 2-byte displace-
		  ments, 2 for 4-byte displacements.

     r_extern	  Set if this relocation requires an external reference; the
		  link editor must use a symbol	address	to update the pointer.
		  When the r_extern bit	is clear, the relocation is `local';
		  the link editor updates the pointer to reflect changes in
		  the load addresses of	the various segments, rather than
		  changes in the value of a symbol (except when	r_baserel is
		  also set (see	below).	 In this case, the content of the
		  r_symbolnum field is an n_type value (see below); this type
		  field	tells the link editor what segment the relocated
		  pointer points into.

     r_baserel	  If set, the symbol, as identified by the r_symbolnum field,
		  is to	be relocated to	an offset into the Global Offset Ta-
		  ble.	At run-time, the entry in the Global Offset Table at
		  this offset is set to	be the address of the symbol.

     r_jmptable	  If set, the symbol, as identified by the r_symbolnum field,
		  is to	be relocated to	an offset into the Procedure Linkage

     r_relative	  If set, this relocation is relative to the (run-time)	load
		  address of the image this object file	is going to be a part
		  of.  This type of relocation only occurs in shared objects.

     r_copy	  If set, this relocation record identifies a symbol whose
		  contents should be copied to the location given in
		  r_address.  The copying is done by the run-time link-editor
		  from a suitable data item in a shared	object.

     Symbols map names to addresses (or	more generally,	strings	to values).
     Since the link-editor adjusts addresses, a	symbol's name must be used to
     stand for its address until an absolute value has been assigned.  Symbols
     consist of	a fixed-length record in the symbol table and a	variable-
     length name in the	string table.  The symbol table	is an array of nlist

	   struct nlist	{
		   union {
			   const char	   *n_name;
			   long		   n_strx;
		   } n_un;
		   unsigned char	   n_type;
		   char			   n_other;
		   short		   n_desc;
		   unsigned long	   n_value;

     The fields	are used as follows:

     n_un.n_strx  Contains a byte offset into the string table for the name of
		  this symbol.	When a program accesses	a symbol table with
		  the nlist(3) function, this field is replaced	with the
		  n_un.n_name field, which is a	pointer	to the string in mem-

     n_type	  Used by the link editor to determine how to update the sym-
		  bol's	value.	The n_type field is broken down	into three
		  sub-fields using bitmasks.  The link editor treats symbols
		  with the N_EXT type bit set as `external' symbols and	per-
		  mits references to them from other binary files.  The	N_TYPE
		  mask selects bits of interest	to the link editor:

		  N_UNDF  An undefined symbol.	The link editor	must locate an
			  external symbol with the same	name in	another	binary
			  file to determine the	absolute value of this symbol.
			  As a special case, if	the n_value field is nonzero
			  and no binary	file in	the link-edit defines this
			  symbol, the link-editor will resolve this symbol to
			  an address in	the bss	segment, reserving an amount
			  of bytes equal to n_value.  If this symbol is	unde-
			  fined	in more	than one binary	file and the binary
			  files	do not agree on	the size, the link editor
			  chooses the greatest size found across all binaries.

		  N_ABS	  An absolute symbol.  The link	editor does not	update
			  an absolute symbol.

		  N_TEXT  A text symbol.  This symbol's	value is a text	ad-
			  dress	and the	link editor will update	it when	it
			  merges binary	files.

		  N_DATA  A data symbol; similar to N_TEXT but for data	ad-
			  dresses.  The	values for text	and data symbols are
			  not file offsets but addresses; to recover the file
			  offsets, it is necessary to identify the loaded ad-
			  dress	of the beginning of the	corresponding section
			  and subtract it, then	add the	offset of the section.

		  N_BSS	  A bss	symbol;	like text or data symbols but has no
			  corresponding	offset in the binary file.

		  N_FN	  A filename symbol.  The link editor inserts this
			  symbol before	the other symbols from a binary	file
			  when merging binary files.  The name of the symbol
			  is the filename given	to the link editor, and	its
			  value	is the first text address from that binary
			  file.	 Filename symbols are not needed for link-
			  editing or loading, but are useful for debuggers.

		  The N_STAB mask selects bits of interest to symbolic debug-
		  gers such as gdb(1); the values are described	in stab(5).

     n_other	  This field provides information on the nature	of the symbol
		  independent of the symbol's location in terms	of segments as
		  determined by	the n_type field.  Currently, the lower	4 bits
		  of the n_other field hold one	of two values: AUX_FUNC	and
		  AUX_OBJECT (see <link.h> for their definitions).  AUX_FUNC
		  associates the symbol	with a callable	function, while
		  AUX_OBJECT associates	the symbol with	data, irrespective of
		  their	locations in either the	text or	the data segment.
		  This field is	intended to be used by ld(1) for the construc-
		  tion of dynamic executables.

     n_desc	  Reserved for use by debuggers; passed	untouched by the link
		  editor.  Different debuggers use this	field for different

     n_value	  Contains the value of	the symbol.  For text, data and	bss
		  symbols, this	is an address; for other symbols (such as de-
		  bugger symbols), the value may be arbitrary.

     The string	table consists of an unsigned long length followed by null-
     terminated	symbol strings.	 The length represents the size	of the entire
     table in bytes, so	its minimum value (or the offset of the	first string)
     is	always 4 on 32-bit machines.

     as(1), gdb(1), ld(1), brk(2), execve(2), nlist(3),	core(5), elf(5),
     link(5), stab(5)

     The <a.out.h> include file	appeared in Version 7 AT&T UNIX.

     Since not all of the supported architectures use the a_midmag field, it
     can be difficult to determine what	architecture a binary will execute on
     without examining its actual machine code.	 Even with a machine identi-
     fier, the byte order of the exec header is	machine-dependent.

FreeBSD	13.0			 June 10, 2010			  FreeBSD 13.0


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