Multithreading in Lua

Question

I was having a discussion with my friend the other day. I was saying how that, in pure Lua, you couldn't build a preemptive multitasking system. He claims you can, because of the following reason:

Both C and Lua have no inbuilt threading libraries [OP's note: well, Lua technically does, but AFAIK it's not useful for our purposes]. Windows, which is written in mostly C(++) has pre-emptive multitasking, which they built from scratch. Therefore, you should be able to do the same in Lua. The big problem i see with that is that the main way preemptive multitasking works (to my knowledge) is that it generates regular interrupts which the manager uses to get control and determine what code it should be working on next. I also don't think Lua has any facility that can do that.

My question is: is it possible to write a pure-Lua library that allows people to have pre-emptive multitasking?

Answer 1

No. It's not possible to write a preemptive scheduler in pure Lua. At some point a preemptive scheduler needs some mechanism like an interrupt service routine to take control away from the current thread and give it to the scheduler which can then give it to another thread. Pure Lua doesn't have this mechanism.

You mention that Windows is written in mostly C/C++. The keyword is mostly. You can't write a preemptive scheduler in pure ANSI C/C++. Usually, part of the interrupt service routine is written in assembly language. Or, the C/C++ compiler implements a non-standard extension that allows interrupt service routines to be written in C/C++. Some compilers allow you to declare a functions with an __interrupt modifier that that causes the compiler to generate a prolong / epilog that allows the function to be used as an interrupt service routine.

Also, code that sets up the interrupt service routine fiddles with CPU registers with memory mapped IO, or a IO instructions. None of this code is portable ANSI C/C++. And, depends on the CPU architecture.