Coroutines [was: snapshot of Lua 4.1 (work)]

[Jean-Claude Wippler <jcw@...>]

Edgar Toernig <froese@gmx.de> wrote:

[coroutines]
>I'll definitely take a look.  In Sol I already have a coroutine library
>based on longjmp.  But it requires knowledge about the structure of the
>jmp_buf.  I recently found the POSIX functions getcontext/makecontext/
>swapcontext.  I'll change Sol's coro-lib to depend on these functions
>and add a prototype ucontext implementation for systems that do not have
>these functions [1].  I'll then port this lib to Lua 4.1.

Ouch.  C-level co-routines?  My first reaction was: why not simply
implement them at the Lua level?  Oh, of course: Lua intermingles the C
call stack with its own... like Tcl and standard Python - and unlike
Stackless Python (which implements "calling" without nesting the C
stack).  SP's trick is to have the VM push and pop call frames, instead
of going through a C function which nests the VM.

For Lua, this would require luaD_call to be split into two, and all
places where it gets called as well.  Quite a bit of surgery, unfortunately.

That's a pity.  If Lua were to avoid the C stack for ordinary function-
call nesting, then coroutines (at the Lua-level) would be easy to
implement - no special C code, and portable.  Simply a closure which
remembers the new_thread pointer, and saving/restoring the current VM
instruction pointer on that stack for suspend/resume.

-jcw