Comment by themafia | Hacker News Reader

Comment by themafia | Hacker News Reader

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The C interface to ruby is just superb. It provides a very simple and first class access to the entire runtime and all it's features. It's absolutely my favorite language to write extensions in.

Lua is probably my second favorite. It's marred by it's initial creation as a stack based interpreter and requires more calls and contortions to achieve the same effects as you would in any other language; however, once you understand a handful of useful primitives it's quite easy to intuit the correct set of calls for any use case.

The blend of dynamic language with underlying compiled extensions is vastly underappreciated. I suspect it has a lot to do with the difficulty of packaging and distributing these extensions into current virtualized and cloud environments. Which is a pity given the extreme combination of flexibility and efficiency that it otherwise unlocks.

> The C interface to ruby is just superb.

How does it handle garbage collection? AFAIK GC is the main reason behind Lua's stack-based API: it's designed so that C code never needs to hold a pointer to a Lua object, which means an object will never be garbage-collected while C code is still trying to use it.

OTOH Python does allow C code to hold such pointers - so it requires that code to perform error-prone manual reference-counting.

How does Ruby solve this problem?

> How does Ruby solve this problem?

The interpreter keeps track of globals and all stack frames. So it knows what objects are in scope.

If you're creating an object that can contain native ruby values inside it's own opaque structure, then when you define the class that wraps this structure you reference the callback functions you want to be called during GC mark and when the GC frees your object. During the mark callback you simply call rb_gc_mark on any of those internal values so the GC knows they are in scope.

In practice it's quite easy to use and you can find many examples of this. There is no equivalent I'm aware of to mark in lua, but the free callback is effectively equivalent to the __gc metamethod.

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