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
          testing.  A common object file consists of a file header, an
          optional 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
          exists 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



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     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 M68000 family of processors the stack begins at the
          end of memory and grows toward lower addresses.  The stack
          is automatically extended as required.  The data segment is
          extended 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
          processed 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), aouthdr(4), filehdr(4), ldfcn(4),
          linenum(4), reloc(4), scnhdr(4), syms(4).

























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