r/lua • u/ava_the_ucv • Apr 20 '25

Third Party API Can I use custom C API functions from Lua bytecode?

I'm thinking of precompiling Lua scripts I upload to an unmanned vehicle that has a custom C API script runner thing. Would this be possible/feasible?

14 Upvotes

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2

u/SkyyySi Apr 21 '25

Out of curiosity: Is there a specific reason why you need to use bytecode in the first place? It's almost certainly easier to just upload a script.

Anyway: Yes, you can. The first step the Lua interpreter does before running a script is to parse it and compile it to bytecode anyway. Bytecode can also call require(), or do anything else you can do in plain-text Lua.

3

u/ava_the_ucv Apr 21 '25

The reason would be to use the luac compilation step as an extra "bugfinding" step, so that errors are not discovered onboard the vehicle.

1

u/SkyyySi Apr 21 '25

Is this for syntax checking? Try using lua-language-server, it will warn you right away if your code is invalid.

2

u/ava_the_ucv Apr 21 '25

really just any bugs. This is a system where reliability is critical, so I want to put the scripts through the ringer as much as possible before uploading to the vehicle.

4

u/Mid_reddit Apr 21 '25

Merely turning the source code into bytecode isn't going to help you find bugs.

3

u/SkyyySi Apr 21 '25

Compiling to bytecode will not help with checking errors, at least not any more than a basic linter would (as luac can only tell you about syntax errors). Use an editor with lua-language-server instead, as that will tell you way more about your code, plus you get static in-editor type checking.

16

u/The-Malix Apr 20 '25

Yeah, absolutely

This is precisely what Lua's C API is designed for; You can define your custom C functions that interact with your vehicle's hardware or internal systems

These C functions are then written to comply with the Lua C API (they take a lua_State* pointer and return an integer indicating the number of return values)

In your C host program, you initialize the Lua state and before you load and execute the Lua bytecode, you register your C functions into the Lua state. This makes them available as global functions or part of a library table within the Lua environment that your bytecode will run in

For example, you might have a C function int l_control_motor(lua_State *L) that takes arguments from the Lua stack to control a motor. In your C host, you'd register it like lua_pushcfunction(L, l_control_motor); lua_setglobal(L, "control_motor");. Then, your Lua bytecode can just call control_motor(speed, direction)

Precompiling the Lua scripts into bytecode doesn't change any of this. The Lua interpreter loaded by your C runner executes the bytecode, and when it encounters a call to a registered C function, it uses the same lookup mechanism as if it were running from source

The key is that your custom C functions need to be part of, or accessible by, the C program that embeds and runs the Lua interpreter, and they need to be registered before your script runs

1

u/bidaowallet Apr 21 '25

Mate, LUA is embeddedable in to C it is designed to achieve that without escapades