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a.out(5)                         File Formats                         a.out(5)

       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.

       tab() box; cw(2.69i) |cw(2.81i) cw(2.69i) |cw(2.81i) Linking ViewExecu‐
       tion View _ ELF headerELF header _ Program header  tableProgram  header
       table optional _ Section 1Segment 1 _ . . .  _ Section nSegment 2 _ . .
       .  _ . . .. . .  _ Section header tableSection header table optional

       An ELF header resides at the beginning and holds a "road map"  describ‐
       ing  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 infor‐
       mation 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 "max‐
       imal 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 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)

Oracle Solaris 11.4               30 Jul 2018                         a.out(5)
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