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- Subject: Re: LuaSocket - COPAS - CoSocket
- From: "Thomas Harning Jr." <harningt@...>
- Date: Thu, 10 May 2007 08:50:39 -0400
On 5/10/07, email@example.com <firstname.lastname@example.org> wrote:
I have been trying to use COPAS to do asynchronous HTTP requests. I keep
getting the "attempt to yield across metamethod/C-call boundary" error
even after I converted my pcalls to calls etc.
There could be any number of things causing the metamethod/C-call
yield issue (looks like its somewhere inside your http.request
For a language as beautifully designed as Lua, the limitations on
coroutines seem very vague and constricting.
The limitations are not for the language, but for the fact that C does
not offer a way to do coroutines itself.
Quick explanation as to why you may run into limitations:
Lua sets up coroutine which does the following:
- calls a C function
- C function calls into Lua
- Lua yields -- ** boom **
Basic lua 'stack' at this point:
* Lua: Function that called the C function
* C Function
* Lua: Function that yielded
The 'yield' needs to switch the current stack to the one that started
the co-routine, but there is no way to switch a stack when C is
The better 'fix' is to rework the logic.
The quick fix is to use Coco , an extension that allows for
yielding through C functions. The downside is that you have to (IIRR)
predict how much stack will be used by all C functions inside that
co-routine. In any case you end up w/ another C stack instead of the
lightweight Lua stack.
Thomas Harning Jr.