elf(4)

NAME
     elf - Executable and Linking Format (ELF) files

SYNOPSIS
     #include <elf.h>

DESCRIPTION
     The a.out file name is the default ELF-format output file name from
     the ld(1) link editor.  The link editor will make an a.out executable
     if there were no errors and no unresolved external references.

     Programs that manipulate ELF files may use the library described in
     the elf(3E) man page.  An overview of the file format follows.  For
     more complete information, see the references in the SEE ALSO section.

     Linking View                       Execution View

     ELF header                         ELF header

     Program header table (optional)    Program header table

     Section 1                          Segment 1

     ...                                ...

     Section n                          Segment n

     Section header table               Section header table (optional)

     An ELF header resides at the beginning and holds a ``road map'' that
     describes 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.  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 sections.  Every section has an entry in the table; each entry
     provides 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.  SGI executables and DSO files
     will be created with section header tables, so their absence in an
     object file, while not prohibited, may disable various tools.

     Although the overview of the file format indicates that the program
     header table immediately follows the ELF header, and the section
     header table follows the sections, actual files may differ.  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 kinds of
     logical segments are set up:  the text segment, the data segment
     (initialized data followed by uninitialized, the latter actually being
     initialized to all 0s), and a stack.  A program can have multiple text
     and data segments but only one stack segment.  The text segments are
     not writable by the program; if other processes are executing the same
     a.out file, the processes will share the same text segments.

     The first data segment generally 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 extended automatically as required.  The last
     data segment is extended as requested by the brk(2) system call.

NOTES
     ELF is the executable and object file format used on IRIX 5 and later
     systems.

SEE ALSO
     as(1), cc(1), ld(1)

     brk(2)

     a.out(4)