a is indexed by TSD keys.
The TSD area associates values of type <B>void *</B> to TSD keys. TSD
keys are common to all threads, but the value associated with a given
TSD key can be different in each thread. 
</P>
<P>For concreteness, the TSD areas can be viewed as arrays of <B>void
*</B> pointers, TSD keys as integer indices into these arrays, and
the value of a TSD key as the value of the corresponding array
element in the calling thread. 
</P>
<P>When a thread is created, its TSD area initially associates