 at a given
time. The value initially associated with the returned key is <B>NULL</B>
in all currently executing threads. 
</P>
<P>The <I>destr_function</I> argument, if not <B>NULL</B>, specifies
a destructor function associated with the key. When a thread
terminates via <B>pthread_exit</B> or by cancellation, <I>destr_function</I>
is called with arguments the value associated with the key in that
thread. The <I>destr_function</I> is not called if that value is <B>NULL</B><SPAN STYLE="font-weight: medium">