POSIX threads and exiting from a thread
It is safe to return null from the thread functions; the code that waits for them should be OK.
POSIX says of pthread_exit():
An implicit call to pthread_exit() is made when a thread other than the thread in which main() was first invoked returns from the start routine that was used to create it.
You do need something to wait for the thread with pthread_join() unless the thread was created with the detached attribute or detached later with pthread_detach().
Calling pthread_exit(NULL) and returning NULL at the end of the thread's initial function should be equivalent. However, doing either of these alone will lead to a resource leak. To avoid that, you must either call pthread_join on the thread from another thread, or put the thread in the detached state by calling pthread_detach on it or setting it to start in the detached state before creating it.