function creates a thread-specific data key visible to all threads in the
Key values provided by
are opaque objects used to locate thread-specific data.
Although the same
key value may be used by different threads, the values bound to the key
are maintained on a per-thread basis and persist for the life of the calling
Upon key creation, the value NULL is associated with the new key in all
Upon thread creation, the value NULL is associated with all
defined keys in the new thread.
An optional destructor function may be associated with each key value.
thread exit, if a key value has a non-NULL destructor pointer, and the
thread has a non-NULL value associated with the key, the function pointed
to is called with the current associated value as its sole argument.
order of destructor calls is unspecified if more than one destructor exists
for a thread when it exits.
If, after all the destructors have been called for all non-NULL values
with associated destructors, there are still some non-NULL values with
associated destructors, then the process is repeated.
If, after at least
[PTHREAD_DESTRUCTOR_ITERATIONS] iterations of destructor calls for
outstanding non-NULL values, there are still some non-NULL values with
associated destructors, the implementation stops calling destructors.