a.out(4)

NAME

     a.out - Executable and Linking Format (ELF) files

SYNOPSIS

     #include <elf.h>

DESCRIPTION

     The file name a.out is the default output file name from the
     link  editor, ld(1). The link editor will make an a.out exe-
     cutable if there were no errors in linking. The output  file
     of  the  assembler,  as(1),  also  follows the format of the
     a.out file although its default file name is different.

     Programs that manipulate ELF files may use the library  that
     elf(3ELF) describes. An overview of the file format follows.
     For more complete  information,  see  the  references  given
     below.

     ____________________________________________________________
    |        Linking View        |         Execution View       |
    |____________________________|______________________________|
    |         ELF header         |           ELF header         |
    |____________________________|______________________________|
    |    Program header table    |      Program header table    |
    |____________________________|______________________________|
    |          optional          |                              |
    |____________________________|______________________________|
    |          Section 1         |           Segment 1          |
    |____________________________|______________________________|
    |                            |                              |
    |____________________________|______________________________|
    |            . . .           |                              |
    |____________________________|______________________________|
    |          Section n         |           Segment 2          |
    |____________________________|______________________________|
    |                            |                              |
    |____________________________|______________________________|
    |            . . .           |                              |
    |____________________________|______________________________|
    |            . . .           |             . . .            |
    |____________________________|______________________________|
    |    Section header table    |      Section header table    |
    |____________________________|______________________________|
    |                            |            optional          |
    |____________________________|______________________________|

     An ELF header resides at the beginning and  holds  a  ``road
     map''  describing the file's organization. Sections hold the
     bulk of  object  file  information  for  the  linking  view:
     instructions,  data,  symbol  table, relocation information,
     and so on. Segments hold the object file information for the
     program  execution view. As shown, a segment may contain one
     or more sections.

     A program header table, if present, tells the system how  to
     create  a process image. Files used to build a process image
     (execute a program) must have a program header table;  relo-
     catable  files  do not need one. A section header table con-
     tains information describing the file's sections. Every sec-
     tion has an entry in the table; each entry gives information
     such as the section name, the section size, etc.  Files used
     during  linking  must  have  a  section  header table; other
     object files may or may not have one.

     Although the figure shows the program header  table  immedi-
     ately  after  the  ELF  header, and the section header table
     following the sections, actual files may  differ.  Moreover,
     sections  and segments have no specified order. Only the ELF
     header has a fixed position in the file.

     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,
     the  latter  actually  being  initialized to all 0's), and a
     stack. The text segment is not writable by the  program;  if
     other  processes  are  executing  the  same  a.out file, the
     processes will share a single text segment.

     The data segment starts at the next  maximal  page  boundary
     past the last text address. If the system supports more than
     one page size, the ``maximal page'' is the largest supported
     size.  When  the  process  image is created, the part of the
     file holding the end of text and the beginning of  data  may
     appear  twice.  The duplicated chunk of text that appears at
     the beginning of data is never executed; it is duplicated so
     that the operating system may bring in pieces of the file in
     multiples of the actual page size without having to  realign
     the  beginning  of  the  data  section  to  a page boundary.
     Therefore, the first data address is the  sum  of  the  next
     maximal  page  boundary  past  the  end  of  text  plus  the
     remainder of the last text address divided  by  the  maximal
     page  size.  If  the  last text address is a multiple of the
     maximal page size, no duplication is necessary. The stack is
     automatically  extended  as  required.  The  data segment is
     extended as requested by the brk(2) system call.

SEE ALSO

     as(1), cc(1B), ld(1), brk(2), elf(3ELF)

     ANSI C Programmer's Guide