Scala Native interop with C, C++, and assembly

Live coded for the London Scala User Group on July 23rd, 2025.

Watch the video, it's fun!

The app

This project implements a simple HTTP service with 2 endpoints, mimicking a curtain store where you can place orders (inspired by my traumatic renovation experience).

Exploring the code

1. We use a C++ library called httplib for our HTTP layer. The library in its entirety is a single header file which we place in the special folder which Scala Native treats as the location for C/C++/assembly files you want compiled alongside your application.

2. Scala Native cannot invoke C++ functions directly, only C ones. So we create a glue layer in C, exposing a minimal interface for our needs.

3. The C glue layer is then processed by sn-bindgen into a set of bindings for easier use from Scala

4. We implement our HTTP server in C++, relying on the C glue layer placeholders, implemented on the Scala side

5. As a fun exercise, we implement a tiny part of the logic in Assembly (the variant here is ARM64 assembly, for MacOS clang). if you struggle to build this application it might be down to this assembly part - assembly is not portable! If that's the case, just modify the main file to use the Scala variant of the function (or be brave and modify the assembly for your platform, that should be fun)

6. The app itself along with its logic is wired in our main file

Building and running

1. Start SBT shell by running sbt
2. Run show nativeLink - this will print the full path to the binary.
3. Run the binary
4. Test the server by running curl http://localhost:8899/list and curl -v http://localhost:8899/create -d "width=99&height=100"

Playing with debugger

To see assembly in action, you can run the app using LLDB:

lldb <path-to-binary>

This will start LLDB shell. You can set a breakpoint on our assembly function:

(lldb) br set --name rectangle_fits_asm
(lldb) run

Then, in a separate terminal, run curl -v http://localhost:8899/create -d "width=99&height=100" - this will trigger the breakpoint, and in lldb you should be able to see something like this:

Starting server on http://localhost:8899
Creating order with width 99 and height 100
Process 76887 stopped
* thread #2, stop reason = breakpoint 1.1
    frame #0: 0x000000010018f82c root-83`rectangle_fits_asm
root-83`rectangle_fits_asm:
->  0x10018f82c <+0>:  cmp    w2, w0
    0x10018f830 <+4>:  b.hi   0x10018f844    ; return_zero
    0x10018f834 <+8>:  cmp    w3, w1
    0x10018f838 <+12>: b.hi   0x10018f844    ; return_zero
Target 0: (root-83) stopped.