Developer's Topics
Chapter 8, ELF object files

File format

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

a view used during program linking
a view used during program execution

Figure 8-1, ``Object file format'' shows an object file's organization.

 ------------------------------------------------
|     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)     |
|---------------------|--|----------------------|

Figure 8-1 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.

NOTE: 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 specified order. Only the ELF header has a fixed position in the file.

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.

Table 8-1 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.