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A.OUT(4)							      A.OUT(4)

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

       #include <elf.h>

       The  file name a.out is the default output file name from the link edi‐
       tor, ld(1). The link editor will make an a.out executable 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

       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 ta‐
       ble, 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; relocatable files do not need one.  A
       section	header	table  contains information describing the file's sec‐
       tions. Every section has an entry in the table; each entry gives infor‐
       mation such as the section name, the section size, etc. Files used dur‐
       ing linking must have a section header table; other object files may or
       may not have one.

       Although	 the  figure  shows the program header table immediately after
       the ELF header, and the section header table  following	the  sections,
       actual files may differ. Moreover, sections and segments have no speci‐
       fied 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 pro‐
       gram; if other processes are executing the same a.out  file,  the  pro‐
       cesses 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 begin‐
       ning  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 dupli‐
       cation  is  necessary. The stack is automatically extended as required.
       The data segment is extended as requested by the brk(2) system call.

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

       ANSI C Programmer's Guide

				 Aug 24, 2009			      A.OUT(4)

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