Project 4: The Dynamic Loader in Action (dlopen)

A simple plugin-based program. The main application will load shared libraries (.so files) from a plugins/ directory at runtime, look for a specific function within them (e.g., run_plugin), and execute it.

Quick Reference

Attribute Value
Primary Language C
Alternative Languages N/A
Difficulty Level 2: Intermediate
Time Estimate Weekend
Knowledge Area Systems Programming / C
Tooling libdl library
Prerequisites Solid C skills, especially function pointers.

What You Will Build

A simple plugin-based program. The main application will load shared libraries (.so files) from a plugins/ directory at runtime, look for a specific function within them (e.g., run_plugin), and execute it.

Why It Matters

This project builds core skills that appear repeatedly in real-world systems and tooling.

Core Challenges

  • Compiling a shared library → maps to using the -fPIC and -shared flags correctly
  • Loading a library at runtime → maps to using dlopen and handling errors
  • Finding a symbol in the loaded library → maps to using dlsym and casting the returned void* to a function pointer
  • Managing library handles and memory → maps to understanding dlclose and its implications

Key Concepts

  • Dynamic Loading API: man 3 dlopen.
  • Function Pointers: “The C Programming Language” (K&R) Ch. 5.11.
  • PIC/Shared Libraries: GCC documentation on -fPIC.

Real-World Outcome

c #ifndef PLUGIN_H #define PLUGIN_H void run_plugin(); #endif

Implementation Guide

  1. Reproduce the simplest happy-path scenario.
  2. Build the smallest working version of the core feature.
  3. Add input validation and error handling.
  4. Add instrumentation/logging to confirm behavior.
  5. Refactor into clean modules with tests.

Milestones

  • Milestone 1: Minimal working program that runs end-to-end.
  • Milestone 2: Correct outputs for typical inputs.
  • Milestone 3: Robust handling of edge cases.
  • Milestone 4: Clean structure and documented usage.

Validation Checklist

  • Output matches the real-world outcome example
  • Handles invalid inputs safely
  • Provides clear errors and exit codes
  • Repeatable results across runs

References

  • Main guide: LEARN_C_LINKING_DEEP_DIVE.md
  • “Advanced Programming in the UNIX Environment” by Stevens & Rago