This is a personal learning project. There are other good packages for integrating Rust and Lua. However, I did not find one well suited for the micro controller space. And I thought the exercise of integrating the two would be fun. 😃 I documenting the exercise in case I have to do something similar in the future and need a reference.
- A Rust development environment. Howto.
- You have Embassy running the blinky example on a RP2040 (RP2350 should work too). Howto.
- In step 2, make sure you got the debug probe working.
- Install xpack-arm-none-eabi-gcc-14.2.1-1.1-win32-x64.zip. I installed xPack outside of the cargo project tree and hardcoded the path in the build.rs file. You will need to do the same. This is icky; I know.
- The first issue is that Lua is written in C. How does one compile C code in a Rust project. Turned out to be easy using the cc crate. Refer. cc calls the C compiler supplied by xPack. See the build.rs file for the code that compiles the Lua source.
- To slim down the LUA runtime, I changed the loadedlibs array in linint.c to load only a few basic Lua libs. I have not removed the code from the compiled image.
- The second issue is that Lua expects to use the C standard library too. Research led me to Newlib-nano, a C standard library suited for embedded work. xPack provides a Newlib-nano that can be used on the RP2040. Newlib-nano binds to your "OS" via 17 syscalls. I have not implemented all 17, just the ones being used by this project. See syscalls.rs
- In alloc.rs I created wrappers around C malloc, realloc, and free (see alloc.rs). emballoc is used for dynamic memory management. Emballoc will provide a Rust global allocator if I need one later. Newlib-nano mostly uses these 3 calls for dynamic memory. But it will sometimes call _sbrk too. I provide a super simple_sbrk to cover these cases. I need to figure out where these _sbrk calls are coming from, and why they don't use my malloc/realloc/free.
- Need to start working on the _read syscall so that Lua can receive input from the rp2040 UART.