Re: Use of Lua corotines in games

[Graham Wakefield <lists@...>]

From the manual:

"coroutine.yield (···)

Suspends the execution of the calling coroutine. The coroutine cannot  
be running a C function, a metamethod, or an iterator. Any arguments  
to yield are passed as extra results to resume."

The bit I don't understand is how a coroutine in Lua can call yield()  
while it is running a C function?  A code example would help here.

The manual entry for lua_resume() doesn't mention anything about  
yielding across C calls.

"lua_yield
int lua_yield  (lua_State *L, int nresults);
Yields a coroutine.

This function should only be called as the return expression of a C  
function, as follows:

     return lua_yield (L, nresults);
When a C function calls lua_yield in that way, the running coroutine  
suspends its execution, and the call to lua_resume that started this  
coroutine returns. The parameter nresults is the number of values  
from the stack that are passed as results to lua_resume."

What's not clear here is whether the 'running coroutine' is one  
created in Lua, that called a C function ending in lua_yield, or  
whether the running coroutine was a thread created in C via  
lua_newthread, or whether either case is valid.

Perhaps 'not yielding across C calls' simply means that lua_yield  
should not be called inside a C function, but only at the end as a  
return value?


On Mar 19, 2007, at 5:36 PM, David Burgess wrote:

> lua_resume()->lua code->resume()->C function->pcall()->yield()
>
> I think I have this right. So when the coroutine starts running it  
> might
> call a C function which might pcall some Lua code that yields. THis
> means that the yield() needs to cross a C/Lua boundary.
> The same problem occurs when starting with lua_resume() and then
> executing Lua pcall() which in turn may call lua_yield().
>
> db
>
> On 3/20/07, Graham Wakefield <lists@grahamwakefield.net> wrote:
> > I keep hearing this phrase, but what exactly does it mean? Thanks!
> >
> > On Mar 19, 2007, at 6:46 AM, Eike Decker wrote:
> >
> > > Only problem is, that lua coroutines can't yield across C calls