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Creating C closures from Lua closures

Creating C closures from Lua closures

5 comments

·December 11, 2025
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psychoslave

What’s the thing emulating so much activity around closures lately?

https://news.ycombinator.com/item?id=46228597

https://news.ycombinator.com/item?id=46259334

Also talking about the Knuth boy/man test: https://news.ycombinator.com/item?id=46020151

Not a bad thing, but that really question if there is some active micro-

widdershins

This doesn't seem like something that should require generating assembly to solve. Couldn't `CALLBACK` just return a table, which contains both the userdata pointer to `REAL_CALLBACK` and a value for `findex`, eliminating the global variable? Then `Add` could extract that. You could even make the table returned by `CALLBACK` callable in Lua with a metatable.

Or, if you're worried about performance/memory, you could allocate a struct for `CALLBACK` to return as userdata, containing the function pointer and `findex`. If you made a little allocator for these it would be extremely fast.

I'm sure I'm missing things, but the solution you chose feels like the nuclear option.

CodesInChaos

Well designed C APIs have a context/userdata parameter on their callbacks, which is registered and stored alongside the function pointer. Unfortunately WNDPROC lacks this parameter.

GWLP_USERDATA should be the best option, though the API for setting it and setting the WNDPROC being separate looks error prone.

comex

The code is passing the function pointers into Win32 APIs, so the caller side isn’t controlled; the callbacks have to work as native C function pointers.

This was probably posted in response to this other link from two days ago, which is about about JIT-compiling wndproc callbacks in particular; the comments discuss the “proper” way to do it, which is to use GWLP_USERDATA:

https://news.ycombinator.com/item?id=46259334

At least, that’s the proper way to do it if you control the entire application. But for what’s apparently a thin wrapper that exposes Win32 APIs directly to Lua, I can understand picking an approach that makes life easier for the Lua code author, even if it’s hackier under the hood. It also avoids the need to write custom behavior for each API.

shakna

Why VirtualAlloc?

Lua has its own allocator, which will also collect for you. lua_newuserdata. At the expense of having to set executable yourself, but without all the inbuilt inefficiencies the article points out.