the threads and procs that are created by libthread have ther stacks
malloc()ed, so one thread can allocate a structure or buffer on its
stack and pass a pointer to another thread or proc.  when you would do
a rfork(RFMEM) in a libthread program, the stacks would have been
shared by parent and child! libthread does some setjmp/longjmp magic to
switch the stack to the original plan9 stack segment to be able todo
a real rfork when spawning new procs.

also linux clone syscall takes a pointer to the routine running
in a new "thread" for a similar reason. they dont have the concept
of a always copy-on-write stack.

--
cinap