File format

The ELF object file format provides two views of a file's contents:

1. a view used during program linking

2. a view used during program execution

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)

Object file format

Common to both views is an ELF header that resides at the beginning and holds a ``road map'' describing the file's organization.

In the linking view, sections hold the bulk of object file information which includes instructions, data, symbol table, and relocation information.

A program header table tells the system how to create a process image from the ELF object. In the program execution view, 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, and so forth. Files used during linking must have a section header table; other object files may or may not have one.

Data representation

As described here, the object file format supports various processors with 8-bit bytes and 32-bit architectures. Nevertheless, it is intended to be extensible to larger (or smaller) architectures. Object files therefore represent some control data with a machine-independent format, making it possible to identify object files and interpret their contents in a common way. Remaining data in an object file use the encoding of the target processor, regardless of the machine on which the file was created.

32-bit data types

Name	Size	Alignment	Purpose
Elf32_Addr	4	4	Unsigned program address
Elf32_Half	2	2	Unsigned medium integer
Elf32_Off	4	4	Unsigned file offset
Elf32_Sword	4	4	Signed large integer
Elf32_Word	4	4	Unsigned large integer
unsigned char	1	1	Unsigned small integer

All data structures that the object file format defines follow the ``natural'' size and alignment guidelines for the relevant class. If necessary, data structures contain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4, and so forth. Data also have suitable alignment from the beginning of the file. Thus, for example, a structure containing an Elf32_Addr member will be aligned on a 4-byte boundary within the file. For portability reasons, ELF uses no bit-fields.