The Executable and Linking Format (ELF)
The executable and linking format (ELF) was originally developed by Unix System Laboratories and is rapidly becoming the standard in file formats[8]. The ELF standard is growing in popularity because it has greater power and flexibility than the a.out and COFF binary formats[3]. ELF now appears as the default binary format on operating systems such as Linux, Solaris 2.x, and SVR4. Some of the capabilities of ELF are dynamic linking, dynamic loading, imposing runtime control on a program, and an improved method for creating shared libraries[3]. The ELF representation of control data in an object file is platform independent, an additional improvement over previous binary formats. The ELF representation permits object files to be identified, parsed, and interpreted similarly, making the ELF object files compatible across multiple platforms and architectures of different size.
The three main types of ELF files are executable, relocatable, and shared object files. These file types hold the code, data, and information about the program that the operating system and/or link editor need to perform the appropriate actions on these files. The three types of files are summarized as follows:
- An executable file supplies information necessary for the operating system to create a process image suitable for executing the code and accessing the data contained within the file.
- A relocatable file describes how it should be linked with other object files to create an executable file or shared library.
- A shared object file contains information needed in both static and dynamic linking.
In the next section we overview the ELF file format including a detailed description of each of the five section types that an ELF file might include. These five types are (1) the ELF header, (2) the program header table, (3) the section header table, (4) the ELF sections, and (5) the ELF segments. In Section 2.4.2, we describe the representation of data in an ELF file. The interested reader may consult reference [8] for additional information about the ELF format.