I have been writing a cooperative thread system for ocaml, it only
works on linux and macosx though ...

you can find it at www.pps.jussieu.fr/~emmanuel/Public/Dev/HirondML/

I am giving you this link because I have had the same type of problems
at a time, it gave me a lot of headackes and sleepless nights ... so I
hope this will help

two little notes :

Ignore the thread migration mechanism if you are only interested in
the thread system

Do not expect nice code, the C code is full of hacks and disgusting
things that I don't understand myself anymore

I hope this helps ...

J