The
pthread_key_create() function creates a thread-specific data key visible to all threads in the process. Key values are opaque objects used to locate thread-specific data. The same key value may be used by different threads, but the values bound to the key by
pthread_setspecific() are maintained on a per-thread basis and persist for the life of the calling thread.
Upon key creation, the value
NULL is associated with the new key in all active threads. 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. At 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. The 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.
The
pthread_key_delete() function deletes a thread-specific data key previously returned by
pthread_key_create(). The thread-specific data values associated with
key need not be
NULL at the time of the call. It is the responsibility of the application to free any application storage or perform any cleanup actions for data structures related to the deleted key or associated thread-specific data in any threads; this cleanup can be done either before or after
pthread_key_delete() is called. Any attempt to use
key following the call to
pthread_key_delete() results in undefined behavior.
The
pthread_key_delete() function itself is callable from within destructor functions, but destructor functions are not invoked by the function. Any destructor function that may have been associated with
key will no longer be called upon thread exit.