On the other hand, surprisingly enough, the flockfile/funlockfile
manpage tells me that:

       The stdio functions are thread-safe.  This is achieved by 
assigning  to  each  FILE
       object  a  lockcount  and  (if the lockcount is nonzero) an
owning thread.  For each
       library call, these functions wait until the FILE object is no
longer  locked  by  a
       different thread, then lock it, do the requested I/O, and unlock
the object again.

This leaves me wondering what is happening in the above-mentioned test.