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- Is LuaThreads a separate package or is it the coroutines facility within standard
Lua for doing non-pre-emptive threading in a Lua script? Does it sit on top
of Posix threads (saw references to LuaThreads on top of eCos threads
elsewhere)?

I'm not knowledgable on LuaThreads (I'm sure Diego will answer your question). However, from my own experience and what I read on this list, it is probably not a good idea to run several pre-emptive threads on one and the same Lua state. The locking overhead can be quite high. I'd stick with a Lua state per thread and handle marshalling of any data yourself.

- I can see a lua_newthread() API that can be called from C - is this an
alternative to creating my own Posix thread and then lua_open()'ing a new
Lua VM within that thread (which was what I was trying to do now)?

No. lua_newthread is for coroutine support. It creates a "Lua thread" (essentially a separate call stack) as a data structure in a Lua state. It does not offer or support pre-emptive threading automatically (i.e. you still need some extension). There are endless discussions about coroutines on this list and they are explained in the manual and in PIL.

- lua_set/gettable(L, LUA_GLOBALSINDEX) vs. lua_set/getglobal(L, "global_id")
as a way of setting/getting globals in a Lua VM. Are these equivalent or
have I confused two concepts? 

Equivalent.  The _global versions save you an additional push call.

- Also, lua_pcall vs. lua_cpcall. lua_cpcall seems to be the preferred method
for invoking a Lua script and not having the calling program terminate if an
error occurs since the C function passed to lua_cpcall will return to the
function calling lua_cpcall in the event of an error?

Depends on your situation. To safely call a function on the Lua stack lua_pcall is just what you'll need.

--
Wim