ObjCopy Utility

The objcopy utility is a powerful tool used in software development for copying and transforming object files. It is part of the GNU Binutils package, which provides a suite of programs for handling binary and object files. objcopy is particularly useful for converting between different object file formats, stripping symbols, and modifying binary files in various ways.

Key Features and Capabilities

1 Format Conversion:

• objcopy can convert object files from one format to another. For example, it can convert ELF (Executable and Linkable Format) files to other formats like COFF (Common Object File Format) or binary.
• This feature is especially useful when working with different toolchains or platforms that require specific object file formats.

2 Stripping Symbols:

• The utility can remove symbols from object files, which is often done to reduce the size of the final executable or to prevent reverse engineering.
• By stripping debugging symbols, objcopy can create smaller, more efficient binaries for deployment.

3 Section Manipulation:

• objcopy allows for the inclusion or exclusion of specific sections within an object file.
• Users can rename, remove, or change the attributes of sections, tailoring the binary to specific requirements.

4 Binary Modification:

• Beyond format conversion and section manipulation, objcopy can be used to modify the contents of binary files directly.
• It supports operations like adding or removing headers, padding sections, and adjusting file alignment.

5 Symbol Operations:

• The tool can rename symbols, change their visibility, or even remove them entirely.
• This functionality is critical for linking and optimizing code, ensuring that symbols are handled appropriately for different stages of development and deployment.

Common Use Cases

1 Cross-Compilation:

• When developing software for different architectures, objcopy is often used to convert object files to the appropriate format required by the target architecture's toolchain.
• Example: Converting an ELF file to a flat binary format for embedded systems.

2 Firmware and Bootloader Development:

• objcopy can extract specific sections, such as the text (code) or data sections, from an object file to be used in firmware or bootloaders.
• Example: Extracting the .text section to create a raw binary image for a microcontroller.

3 Debugging and Analysis:

• Developers can use objcopy to strip out debugging symbols from a binary, creating a separate file with these symbols for debugging purposes.
• This allows the distribution of optimized binaries without exposing internal details, while still retaining the ability to debug using the separate symbol file.

4 Size Optimization:

• By stripping unused sections and symbols, objcopy helps in reducing the size of the final executable, which is crucial in resource-constrained environments like embedded systems.

Basic Usage and Examples

The objcopy utility provides a rich set of options to perform various operations. Here are some common command-line examples:

1 Converting Object File Formats:

objcopy -O binary input.elf output.bin

This command converts an ELF file (input.elf) to a raw binary format (output.bin).

2 Stripping Symbols:

objcopy --strip-all input.elf stripped.elf

This removes all symbols from input.elf, creating a smaller file stripped.elf.

3 Removing Specific Sections:

objcopy --remove-section=.bss input.elf output.elf

This command removes the .bss section from input.elf and writes the result to output.elf.

4 Extracting Sections:

objcopy -j .text -O binary input.elf text_section.bin

This extracts the .text section from input.elf and saves it as a binary file text_section.bin.

5 Renaming Sections:

objcopy --rename-section .data=.mydata input.elf output.elf

This renames the .data section to .mydata in input.elf and writes the result to output.elf.

6 Changing Symbol Visibility:

objcopy --localize-symbol=my_symbol input.elf output.elf

This changes the visibility of my_symbol to local in input.elf and writes the result to output.elf.

Advanced Options and Customization

objcopy supports a variety of advanced options for fine-grained control over the manipulation of object files:

1 Symbol Filtering:

objcopy --strip-unneeded input.elf output.elf

This removes all symbols that are not needed for relocation processing.

2 Prefixing Symbols:

objcopy --prefix-symbols=prefix_ input.elf output.elf

This adds a prefix to all symbols in the file, which can be useful for avoiding name collisions.

3 Interleaving Data:

objcopy --interleave=2 --interleave-width=4 input.elf output.elf

This interleaves the sections of the object file, which can be useful for certain memory layouts.