Hi Christian,
otherwise, using threads in PD is just like using threads anywhere else. use the pthread stuff (which also works on windoze mitlerweile), and just put some mutex's around your read/write operations.
threads in PD is more difficult if you need to use PD functions therein. For that you can use sys_lock/sys_unlock (flext::Lock(), flext::Unlock()) to be sure to be the only thread, but that's sub-optimal because it may cause priority inversion (locking to real-time thread for too long a time). In flext you don't need to lock if you just want to send messages to the outlets. These and some other functions are thread-safe. The devel branch has some other very nice feature, namely idle processing via callback... in many cases this is much better than using a thread.
all the best, Thomas