Coroutines & C boundaries

[David Given <dg@...>]

I'm trying to write some code in Lua using coroutines, and am having trouble.

My Lua code is being used as a plugin for an external program. The program is 
calling into Lua to tell it something's happening. As my algorithm is very 
stateful, doing everything from call-ins is a nightmare, which makes it a 
classic example for control inversion using coroutines.

Example: when input arrives on a socket, I'm called to tell me that it's 
happening.

The core of my Lua code is a scheduler. Whenever input arrives, my C code kick 
the scheduler and it runs the first task on the list for one iteration. It 
does this by calling the following function with lua_pcall():

function schedule_now()
	if (table.getn(taskqueue) == 0) then
		idle()
		return
	end
	
	local task = taskqueue[1]
	if (type(task) == "function") then
		task = coroutine.create(task)
		taskqueue[1] = task
	end
	
	coroutine.resume(task)
	
	if (coroutine.status(task) == "dead") then
		table.remove(taskqueue, 1)
	end
end

This ought to be straightforward enough. I schedule tasks by adding functions 
to the taskqueue table. They get run in stages until termination and then 
removed. Here's a sample task:

function a_task()
	print("Hello,")
	coroutine.yield()
	print("world!")
end

However, whenever I try to actually use this, the call to coroutine.yield() 
fails with a 'attempt to yield across metamethod/C-call boundary' error.

Why?

-- 
+- David Given --McQ-+ "I must have spent at least ten minutes out of my
|  dg@cowlark.com    | life talking to this joker like he was a sane
| (dg@tao-group.com) | person. I want a refund." --- Louann Miller, on
+- www.cowlark.com --+ rasfw

Attachment: pgp0Ns3MoAPPh.pgp
Description: PGP signature