a.out(4)                                                 a.out(4)



     NAME
          a.out - common assembler and link editor output

     DESCRIPTION
          a.out is the output file from the assembler  as(1)  and  the
          link  editor  ld(1).   a.out  can  be executed on the target
          machine if there were no errors in assembling or linking and
          no unresolved external references.

          The object file format supports  user-defined  sections  and
          contains  extensive  information for symbolic software test-
          ing.  A common object file consists of a file header, an op-
          tional  aout  header, a table of section headers, relocation
          information, (optional) line numbers, and  a  symbol  table.
          The order is given below.

                    File header.
                    Optional aout header.
                    Section 1 header.
                    ...
                    Section n header.
                    Section 1 data.
                    ...
                    Section n data.
                    Section 1 relocation.
                    ...
                    Section n relocation.
                    Section 1 line numbers.
                    ...
                    Section n line numbers.
                    Symbol table.
                    String table.

          The last four sections  (relocation,  line  numbers,  symbol
          table,  and  string table) may be missing if the program was
          linked with the -s option of ld(1) or if  the  symbol  table
          and  relocation  bits  were  removed by strip(1).  Also note
          that if the program was linked without the  -r  option,  the
          relocation information will be absent.  The string table ex-
          ists only if necessary.

          When an a.out file is  loaded  into  memory  for  execution,
          three  logical  segments  are  set up: the text segment, the
          data segment (initialized  data  followed  by  uninitialized
          data, the latter actually being initialized to all 0's), and
          a stack.  The text segment begins at location 0 in the  core
          image;  the  header is not loaded.  If the magic number (the
          first field in the optional aout header) is 407 (octal),  it
          indicates that the text segment is not to be write-protected
          or shared, so the data segment will be contiguous  with  the
          text  segment.  If the magic number is 410 (octal), the data
          segment begins at the next segment  boundary  following  the



     April, 1990                                                     1

     a.out(4)                                                 a.out(4)



          text  segment,  and  the text segment is not writable by the
          program.  If other processes are executing  the  same  a.out
          file, they will share a single text segment.

          On the Macintosh II with A/UX the stack begins at the end of
          memory  and  grows toward lower addresses.  The stack is au-
          tomatically extended as required.  The data segment  is  ex-
          tended  only  as  requested by the brk(2) and sbrk(2) system
          calls.

          The value of a word in the text or data portions that is not
          a  reference  to an undefined external symbol is exactly the
          value that will appear in memory when the file is  executed.
          If  a  word in the text involves a reference to an undefined
          external symbol, the storage class of the symbol-table entry
          for  that word will be marked as an ``external symbol'', and
          the section number will be set to 0.  When the file is  pro-
          cessed  by  the  link editor and the external symbol becomes
          defined, the value of the symbol will be added to  the  word
          in the file.

          See aouthdr(4), filehdr(4), linenum(4), scnhdr(4), reloc(4),
          and  syms(4)  for  descriptions  of  the  individuals parts.
          Every section created by as(1) contains  a  multiple-of-four
          number  of  bytes;  directives to ld(1) can create a section
          with an odd number of bytes.

     SEE ALSO
          as(1), cc(1), ld(1), ldfcn(3X), aouthdr(4), filehdr(4),
          linenum(4), reloc(4), scnhdr(4), syms(4).

























     2                                                     April, 1990