elf - Executable and Linking Format (ELF) files
The file name a.out is the default ELF-format output file name from the
link editor ld(1). 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
elf(3E). 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
Section 1 Segment 1
Section n Segment n
Section header table Section header table
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. A segment may contain one or
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 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. 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 table above 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
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 0's), 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)
as(1), cc(1), ld(1), brk(2), a.out(4)
IRIX System Programmer's Guide
ELF is the executable and object file format used on IRIX 5 and later
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