Introduction

This chapter describes the object file format, called ELF (Executable and Linking Format). There are three main types of object files.

A relocatable file holds code and data suitable for linking with other object files to create an executable or a shared object file.
An executable file holds a program suitable for execution; the file specifies how exec(BA_OS) creates a program's process image.
A shared object file holds code and data suitable for linking in two contexts. First, the link editor [see ld(BA_OS)] processes the shared object file with other relocatable and shared object files to create another object file. Second, the dynamic linker combines it with an executable file and other shared objects to create a process image.

Created by the assembler and link editor, object files are binary representations of programs intended to be executed directly on a processor. Programs that require other abstract machines, such as shell scripts, are excluded.

After the introductory material, this chapter focuses on the file format and how it pertains to building programs. Chapter 5 also describes parts of the object file, concentrating on the information necessary to execute a program.

File Format

Object files participate in program linking (building a program) and program execution (running a program). For convenience and efficiency, the object file format provides parallel views of a file's contents, reflecting the differing needs of those activities. Figure 4-1 shows an object file's organization.

Figure 4-1: Object File Format

**Linking View**
ELF Header
Program header table optional
Section 1
...
Section n
...
Section header table required

**Execution View**
ELF Header
Program header table required
Segment 1
Segment 2
Segment 3
...
Section header table optional

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. Descriptions of special sections appear later in the chapter. Chapter 5 discusses segments and the program execution view of the file.

A program header table 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, and so on. Files used during 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 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 either 32-bit or 64-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.

Figure 4-2: 32-Bit Data Types

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

64-Bit Data Types

Name	Size	Alignment	Purpose
`Elf64_Addr`	`8`	`8`	Unsigned program address
`Elf64_Off`	`8`	`8`	Unsigned file offset
`Elf64_Half`	`2`	`2`	Unsigned medium integer
`Elf64_Word`	`4`	`4`	Unsigned integer
`Elf64_Sword`	`4`	`4`	Signed integer
`Elf64_Xword`	`8`	`8`	Unsigned long integer
`Elf64_Sxword`	`8`	`8`	Signed long 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 8-byte alignment for 8-byte objects, 4-byte alignment for 4-byte objects, to force structure sizes to a multiple of 4 or 8, 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.