 Pthreads implementations.
<P>
In particular, it is very difficult for the collector to stop all other
threads in the system and examine the register contents.  This is currently
accomplished with very different mechanisms for some Pthreads
implementations.  The Solaris implementation temporarily disables much
of the user-level threads implementation by stopping kernel-level threads
("lwp"s).  The Linux/HPUX/OSF1 and Irix implementations sends signals to
individual Pthreads and has them wait in the signal h