DEV Community

Cover image for The Deep Dive: Why and How Projects Use Multiple Programming Languages
Aadarsh Nagrath
Aadarsh Nagrath

Posted on

The Deep Dive: Why and How Projects Use Multiple Programming Languages

Programming languages are like specialized tools in a craftsman’s workshop—each designed for specific tasks. Some excel at rapid development and high-level abstractions (Python, JavaScript), while others provide fine-grained control over hardware and memory (C, Rust, Assembly). But why do some projects combine multiple languages? How do they work together under the hood?

This blog will explore:

  1. The compilation pipeline – How source code becomes executable binaries.
  2. Linking (static vs. dynamic) – The glue that binds different languages.
  3. Real-world examples – How projects like Linux, FFmpeg, and OpenSSL mix languages.
  4. ABI (Application Binary Interface) – Why calling conventions matter.
  5. Practical use cases – When and why you should mix languages.

1. The Compilation Pipeline: More Than Just "Code → Binary"

When you compile a program, the process is far more complex than just converting source code into an executable. Modern compilers follow a multi-stage pipeline, and understanding this is key to mixing languages.

The Four Key Phases (Using GCC as an Example)

1. Preprocessing

  • Removes comments.
  • Expands macros (#define).
  • Handles #include directives (pasting header files into the source).
  • Output: Preprocessed C code (still human-readable).

2. Compilation (to Assembly)

  • Translates high-level code (C, C++, Rust) into assembly language.
  • Myth Busted: Compilers don’t always go straight to machine code—many use intermediate representations (IR) like assembly.
  • Output: Assembly file (.s).

3. Assembly (to Machine Code)

  • The assembler converts assembly into machine code (binary).
  • Output: Object file (.o or .obj).

4. Linking (Combining Object Files)

  • Merges multiple object files (your code + libraries).
  • Resolves function calls between them.
  • Output: Final executable (.exe, .out, etc.).

Key Insight: Since compilation is modular, we can compile different parts of a project in different languages and link them later.

2. Linking: The Glue That Binds Languages

Linking is where the magic happens. There are two main types:

A. Static Linking

  • Library code is copied directly into the executable.
  • Pros: Self-contained, no runtime dependencies.
  • Cons: Larger binary, harder to update libraries.

B. Dynamic Linking

  • Libraries (.so on Linux, .dll on Windows) are loaded at runtime.
  • Pros: Saves disk space, allows library updates without recompiling.
  • Cons: Requires the library to be present on the system.

Example: Calling C from Rust

  1. Write a function in C: 
   // lib.c
   int add(int a, int b) { return a + b; }
Enter fullscreen mode Exit fullscreen mode
  1. Compile it as a static library: 
   gcc -c lib.c -o lib.o  
   ar rcs libadd.a lib.o
Enter fullscreen mode Exit fullscreen mode
  1. Call it from Rust: 
   // main.rs
   extern "C" { fn add(a: i32, b: i32) -> i32; }  
   fn main() { unsafe { println!("{}", add(2, 3)); } }
Enter fullscreen mode Exit fullscreen mode
  1. Compile & link: 
   rustc main.rs -l add -L .
Enter fullscreen mode Exit fullscreen mode

This works because both C and Rust produce object files that the linker can merge.

3. Real-World Examples of Multi-Language Projects

Project Languages Used Why?
Linux Kernel C (core), Assembly (critical sections) Performance for low-level hardware control.
FFmpeg C (core), Assembly (SIMD optimizations) Speed for video encoding/decoding.
Python Interpreter C (core), Python (standard library) C for speed, Python for flexibility.
Node.js C++ (V8 engine), JavaScript (runtime) C++ for performance, JS for usability.

4. The Big Challenge: ABI (Application Binary Interface)

Even if two languages compile to machine code, they might not work together if they disagree on:

  • How function arguments are passed (registers vs. stack).
  • How return values are handled.
  • Memory alignment of structures.

Example: ABI Mismatch Between Two Languages

Suppose:

  • Language A passes arguments in registers 0 and 1.
  • Language B expects them in registers 1 and 2.

Result: The function reads garbage values → Undefined behavior!

Solutions:

  • extern "C" in C++/Rust – Forces C-style calling conventions.
  • #[repr(C)] in Rust – Ensures structs match C memory layout.
  • Compiler flags (-fPIC, -mabi) to enforce compatibility.

5. When Should You Mix Languages?

Scenario Good Language Choices
High-performance computing C/Rust + Python (for scripting)
Embedded systems C (firmware) + Assembly (critical loops)
Game development C++ (engine) + Lua (scripting)
Web backends Go (API) + Python (ML components)

Pros of Mixing Languages

✅ Performance – Optimize bottlenecks in a lower-level language.

✅ Reuse existing libraries (e.g., TensorFlow in Python calls C++ under the hood).

✅ Flexibility – Use the best tool for each part of the project.

Cons

⚠ Build complexity – Managing multiple compilers and linkers.

⚠ Debugging challenges – Mixed-language stack traces.

⚠ ABI risks – If calling conventions don’t match.

Final Thoughts

Mixing programming languages is not just possible—it’s common in performance-critical and large-scale systems. The key lies in:

  1. Understanding the compilation pipeline (preprocessing → compilation → assembly → linking).
  2. Using the linker effectively (static vs. dynamic libraries).
  3. Ensuring ABI compatibility (so functions can call each other correctly).

Whether you're optimizing a game engine with C++ and Lua, speeding up Python with C extensions, or writing kernel modules in Rust and Assembly—knowing how languages interoperate unlocks new levels of performance and flexibility.

In the next part, we’ll explore how compiled languages (C, Rust) interact with interpreted ones (Python, JavaScript)—stay tuned!

Follow for More Deep Dives

Got questions? Drop them below! 🚀

Top comments (0)