Hacker News Reader

Comment by bluGill | Hacker News Reader

Rust doesn't allos dynamic libraries in general, so it isn't going to work where (right or wrong) the code is based on plugins. You can work around this with C api interfaces, but that limits you if both sides are rust. (unsafe for what should be safe as I understand.)

View on HN

Instead of literally loading the plugin into the same address space as the host process, you could launch it as a separate process and use sockets or shared memory to communicate. With a slight shift in API design this might actually be better than traditional dynamically loaded plugins. It will tend to level the performance playing field across languages (e.g. no JNI penalty for plugins written in a JVM language). And it will prevent plugins crashing the host.

And the dispatch overhead will be a few orders of magnitude higher.

Passing a message via a shared memory queue (twice) is not 1000x slower than a non-inlineable function call.

That actually seems about the right order of magnitude.

With the associated context switch it's bigger, if anything. Busy looping to wait if some memory changes isn't really viable for many scenarios.

You're right, but that's also not the entire workload. You have to also wake up the remote process in some way and, depending on the expected duration, probably wake up for the response. On pretty much all major platforms that work is measured in 10s of microseconds, which puts you easily into the 1000x slower than a non-inlineable function call category.

Wdym doesn't allow dynamic library in general? You can have `dylib` as crate type.

We wrote some Rust *.so files, but had to use a C (not even C++) ABI to communicate between them. It's definitely possible, but quite clunky.

is it any better in C++, though? IIRC C++ doesn't have a stable ABI either.

If you either commit to dylib or C++'s DLLs, you still have to recompile everything on version change, unless you also use the C ABI in C++.

One common use case for DLLs in gamedev is for code hotloading. You can just recompile your game DLL and unload / reload the library, patch up some globals and vtable pointers and voila, your game logic has been updated without restarting the game. And all that you get just writing normal C++.

This is only for development. Shipping builds will usually statically link everything.

> And all that you get just writing normal C++.

But, as far as I understand, the boundary layer has to still be C (the side that loads DLLs and stuff), because of the natural limitations of templated languages and linkers.

And as soon as you change any interface you'd need to recompile more parts of the code. The same can be applied with Rust using dylib. At the end, the glue code always end up being C.

On any given compiler you can make C++ have a stable ABI. You can even do this commonly in practice across compiler versions, even, the standard library typically tries to achieve this.

It's easier to have a long term cross version cross compiler stable C ABI, but if you're talking a single toolchain that simplifies the problem tremendously and you can absolutely do that with C++ in practice at that point

It's pretty similar, yes.

If you don't care about the ABI being stable then you can use a Rust-based ABI, but you're essentially just static linking everything then. Not sure how that works out for the game developers.

C++ hes weird ABI rules, but if you follow them you can change implementations.

Reply on news.ycombinator.com