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Article by Ayman Alheraki in January 24 2025 01:40 PM

Concise Description of Executable Headers for Multiple Operating Systems

Concise Description of Executable Headers for Multiple Operating Systems

Executable headers are a fundamental part of any executable file in various operating systems. These headers contain vital information that helps the operating system understand how to load and execute the program. These headers differ across operating systems, such as Windows, Linux, and macOS, and play a crucial role in defining the structure of the executable file and how it interacts with the system.

In this article, we will explore the types of executable headers, the differences between them, their mechanisms of operation, how they are integrated with compiled programs, and the tools required for their creation. We will also provide illustrative examples for each type of header.

Types of Executable Headers

  1. PE Header (Portable Executable)

    • Operating System: Windows

    • Description: The Windows operating system uses the PE format for executable files such as .exe and .dll. The PE header contains information such as entry points, program sections, imports, and exports.

    • Structure: The PE header consists of several parts, including the DOS Header, PE Header, Optional Header, and data sections.

  2. ELF Header (Executable and Linkable Format)

    • Operating System: Linux and Unix-like systems

    • Description: The Linux operating system uses the ELF format for executable files and shared libraries. The ELF header contains information such as the file type (executable, library, etc.), architecture, and entry points.

    • Structure: The ELF header consists of the ELF Header, Program Headers, and Section Headers.

  3. Mach-O Header (Mach Object)

    • Operating System: macOS and iOS

    • Description: The macOS operating system uses the Mach-O format for executable files and libraries. The Mach-O header contains information such as the file type, number of commands, and command sizes.

    • Structure: The Mach-O header consists of the Mach Header, Load Commands, and Segment Commands.

Differences Between Executable Headers

  1. Structure and Format

    • PE: Relies on a complex binary format with multiple sections.

    • ELF: More flexible and used in Unix-like systems.

    • Mach-O: Specifically designed for Apple systems and relies on the concept of commands.

  2. Entry Points

    • PE: Contains a specific entry point in the Optional Header.

    • ELF: Contains an entry point in the ELF Header.

    • Mach-O: Contains an entry point in the Load Commands.

  3. Memory Management

    • PE: Uses sections like .text and .data for memory management.

    • ELF: Uses Program Headers for memory management.

    • Mach-O: Uses Segment Commands for memory management.

Mechanism of Executable Headers

1. PE Header - Mechanism

  • DOS Header: Contains basic information such as the file signature and a small stub program that runs when the file is executed in DOS.

  • PE Header: Contains information such as the file type, number of sections, and timestamps.

  • Optional Header: Contains vital information such as the entry point, memory addresses, image size, and data directories.

  • Sections: The actual parts of the program, such as .text (executable code), .data (initialized data), and .rdata (read-only data).

Example of PE Header:

2. ELF Header - Mechanism

  • ELF Header: Contains general information such as the file signature, file type, architecture, and entry point.

  • Program Headers: Describe how segments are loaded into memory.

  • Section Headers: Describe sections such as .text (executable code), .data (initialized data), and .bss (uninitialized data).

Example of ELF Header:

3. Mach-O Header - Mechanism

  • Mach Header: Contains information such as the file signature, CPU type, file type, and number of commands.

  • Load Commands: Describe how the file is loaded into memory.

  • Segment Commands: Describe segments such as __TEXT (executable code) and __DATA (data).

Example of Mach-O Header:

How Executable Headers Are Integrated with Compiled Programs

  1. First Stage: Assembly

    • The source code is converted into machine code using an assembler. In this stage, program sections such as .text (for code) and .data (for data) are created.

  2. Second Stage: Linking

    • The compiled code is linked with libraries and imports using a linker. In this stage, the executable header is created, and necessary information such as entry points and sections is added.

  3. Third Stage: Loading

    • When the program is run, the operating system loads the executable file into memory using the information in the header. The entry point location is determined, and the program is executed.

Tools Required for Creating Executable Files

  1. Assembler

    • Windows: MASM (Microsoft Macro Assembler)

    • Linux: as (GNU Assembler)

    • macOS: as (GNU Assembler)

  2. Linker

    • Windows: link.exe (Microsoft Linker)

    • Linux: ld (GNU Linker)

    • macOS: ld (GNU Linker)

  3. Compiler

    • Windows: cl.exe (Microsoft C/C++ Compiler)

    • Linux: gcc (GNU Compiler Collection)

    • macOS: clang (LLVM Compiler)

  4. Analysis Tools

    • PE: PEview, CFF Explorer

    • ELF: readelf, objdump

    • Mach-O: otool, MachOView

Conclusion

Executable headers are a critical component of any operating system, providing the necessary information for loading and executing programs. These headers vary across operating systems, each with its own structure and format. By understanding these headers and the tools used to create them, developers can optimize program performance and ensure compatibility across different operating systems. The detailed knowledge of PE, ELF, and Mach-O headers, along with the tools for assembly, linking, and analysis, empowers developers to create robust and efficient executable files tailored to specific operating environments.

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