key

    Address of a variable that receives the key value. This value
    is used in calls to tis_getspecific() and tis_setspecific()  to
    obtain and set the value associated with this key.

 destructor

    Address of a routine that is called to destroy the context value
    when a thread terminates with a non-NULL value for the key. Note
    that this argument is used only when threads are present.