Conclusion and Next Steps: Next Step — Writing a Linker

1. Introduction

After completing the development of an x86-64 assembler, the logical progression is to explore the next component in the toolchain pipeline: the linker. The linker is the system that binds together multiple object files, resolves symbol references, assigns final memory addresses, and produces an executable or shared object. Unlike assemblers, linkers operate at a higher semantic level and manage a broader context involving the layout of complete programs.

This section provides a foundational roadmap for writing a basic but extensible linker, tailored for integration with your assembler or other language frontends.

2. Linker Responsibilities

A linker performs several essential tasks:

• Symbol Resolution: Matches symbol references (e.g., function calls, global variables) to their definitions across input object files.

• Relocation Handling: Adjusts addresses in code and data sections so they point to the correct runtime locations after linking.

• Section Merging: Combines multiple sections (e.g., .text, .data, .rodata) from various object files into unified segments.

• Address Assignment: Determines the memory layout of the final binary, assigning virtual addresses to each section or symbol.

• Entry Point Definition: Identifies the program’s starting execution address, typically based on _start or main.

• Output Format Generation: Emits a valid executable file (e.g., ELF, PE, Mach-O) including headers, tables, and alignment constraints.

3. Linker Architecture

Designing a linker involves structuring it into clear phases:

1. Object File Loader

Parses input object files (produced by your assembler or others) and extracts:

• Section headers

• Symbol tables

• Relocation entries

• Raw section data

Support for formats such as ELF64 or COFF is essential. Begin with ELF64 for Unix-like systems due to its clarity and widespread use.

2. Symbol Table Construction

Builds a unified global symbol table from all object files:

• Identifies duplicate definitions or multiple weak symbols

• Tracks unresolved external references

• Marks visibility and binding types (local, global, weak)

3. Relocation Processor

Resolves relocations using the global symbol table:

• Patches instruction or data bytes with correct address or offset

• Supports different relocation types (e.g., R_X86_64_PC32, R_X86_64_64)

• Applies fixups at correct offsets in sections

4. Memory Layout and Alignment

Lays out sections with appropriate alignments, page boundaries, and padding:

• Align .text and .data as per platform ABI

• Calculate virtual addresses and file offsets

• Optionally support user-defined linker scripts

5. Final Output Generation

Writes the output file:

• Generates headers (ELF, PE, etc.)

• Serializes merged section content

• Includes program headers (for loaders) and section headers (for debugging)

• Supports relocation stripping, symbol table trimming, and other optimizations

4. Minimal ELF64 Linker Walkthrough

To write a basic static ELF64 linker:

1. Accept multiple .o files (object files from your assembler)

2. Read .symtab and .rel[a] sections

3. Resolve symbols like _start, main, and external functions

4. Merge .text, .data, and .bss into respective segments

5. Calculate final addresses based on default or fixed layout

6. Patch relocations

7. Emit ELF header, program headers, and section data

8. Write out the executable

You can omit dynamic linking, stripping, or full DWARF support in your first iteration. Focus on statically linking position-independent code or simple absolute relocations.

5. Extending the Linker

Once the basic linker is functional, you may expand its capabilities to:

• Support shared libraries: Parse dynamic symbol tables and produce DT_* entries in ELF.

• Accept linker scripts: Allow developers to control memory layout via directives.

• Emit debugging information: Retain or adjust .debug_* sections for use with gdb/lldb.

• Add optimizations: Dead code elimination, symbol inlining, section deduplication.

• Generate map files: Help developers understand how symbols were placed and resolved.

6. Language and Toolchain Considerations

If your assembler is written in C, C++, or Rust, you can use corresponding libraries for ELF/COFF parsing:

• C/C++: Use libelf, libbfd, or write a custom parser

• Rust: Leverage crates like goblin, object, and addr2line

• Cross-platform: Define an internal IR that abstracts format details and makes adding Mach-O/PE support easier in the future

Keep the linker modular, so it can evolve independently of the assembler. For instance, a standalone tool xlink could be developed alongside xasm.

7. Summary

Writing a linker is a natural follow-up to building an assembler. It connects the backend of your assembler to real-world executable output. A minimal static ELF linker is within reach once object parsing and symbol resolution are understood. This step offers deeper knowledge of binary formats, memory layout, and low-level platform conventions — and prepares you to build a complete, standalone compiler toolchain.