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NAME    [Toc]    [Back]

     a.out - format of executable binary files

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <a.out.h>

DESCRIPTION    [Toc]    [Back]

     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

     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 reference
  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 {
                   u_int32_t       a_midmag;
                   u_int32_t       a_text;
                   u_int32_t       a_data;
                   u_int32_t       a_bss;
                   u_int32_t       a_syms;
                   u_int32_t       a_entry;
                   u_int32_t       a_trsize;
                   u_int32_t       a_drsize;

     The fields have the following functions:

     a_midmag  This field is stored in network byte-order so that
binaries for
               machines with alternate byte orders can be distinguished.  It
               has  a  number  of  sub-components accessed by the
               N_GETFLAG(), N_GETMID(), and N_GETMAGIC(), and set
by the macro

               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 independent
  code.   This  flag  is set by
as(1) when given
                           the -k flag and is preserved by  ld(1)
if necessary.

               If  both EX_DYNAMIC and EX_PIC are set, the object
file is a position
  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
               which machine(s) the binary is intended to run on.

               N_GETMAGIC() specifies  the  magic  number,  which
uniquely identifies
  binary  files  and  distinguishes  different
loading conventions.
  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 follow
 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 segment
  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 initially
 reads as zeroes.

     a_syms    Contains the size in bytes  of  the  symbol  table

     a_entry    Contains the address in memory of the entry point
of the program
 after the kernel has loaded  it;  the  kernel
starts the execution
 of the program from the machine instruction
at this address.

     a_trsize  Contains the size in bytes of the text  relocation

     a_drsize   Contains the size in bytes of the data relocation

     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 file.

     N_BADMAG(exec)   Non-zero if the a_magic field does not contain a recognized

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

     N_DATOFF(exec)   The byte offset of the beginning of the data segment.

     N_DRELOFF(exec)  The byte offset of the beginning of the data relocation

     N_TRELOFF(exec)  The byte offset of  the  beginning  of  the
text relocation

     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 linkedited.
   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 editor
 resolves the absolute address for this symbol, it adds
                  that  address to the pointer that is undergoing
                  (If the r_extern bit is clear, the situation is
                  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
 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
                  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
                  Table.  At run-time, the entry  in  the  Global
Offset Table at
                  this  offset  is  set  to be the address of the

     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
                  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
 name in the string table.  The symbol table is an array of nlist

           struct nlist {
                   union {
                           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 memory.

     n_type       Used by the link editor to determine how to update the symbol'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 permits
  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 non-zero
                          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 undefined
  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 address
 and the link editor  will  update
it when it
                          merges binary files.

                  N_DATA   A  data  symbol; similar to N_TEXT but
for data addresses.
  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 address
  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

                  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

                  The N_STAB mask selects  bits  of  interest  to
symbolic debuggers
  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_OBJECT  (see  <link.h>  for  their  definitions).  AUX_FUNC
                  associates the symbol with a callable function,
                  AUX_OBJECT associates the symbol with data, irrespective of
                  their  locations in either the text or the data
                  This field is intended to be used by ld(1)  for
the construction
 of dynamic executables.

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

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

     The string table consists of an u_int32_t length followed by
 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.

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     The a.out.h include file appeared in Version 3 AT&T UNIX.

BUGS    [Toc]    [Back]

     Nobody seems to agree on what BSS stands for.

     New  binary file formats may be supported in the future, and
they probably
     will not be compatible at any level with this  ancient  format.

OpenBSD      3.6                           June      5,      1993
[ Back ]
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