Plan 9's libthread [1] implements cooperative threading entirely in userspace library. This is apart of Plan 9's fork() [2] syscall which allows both forking pr

Plan 9's libthread [1] implements cooperative threading entirely in userspace library. This is apart of Plan 9's fork() [2] syscall which allows both forking processes and creating kernel-level threads. Thanks to libthread being orthogonal to processes, it turned out to be easily portable. [1] http://man.cat-v.org/plan_9/2/thread http://man.cat-v.org/plan_9/2/thread [2] http://man.cat-v.org/plan_9/2/fork http://man.cat-v.org/plan_9/2/fork -- this is what linux' clone(3) syscall was modeled after.