The function int pthread_key_delete(pthread_key_t key); Shall delete a thread-specific data key previously returned by pthread_key_create. The thread-specific data values specified data values associated with 'key' need not be NULL at the time pthread_key_delete is called. 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. pthread_key_delete function shall be callable from within destructor functions. No destructor functions shall be invoked by pthread_key_delete. Any destructor function that may have been associated with 'key' shall no longer be called up thread exit. If successful, it returns zero. It may fail if: [EINVAL] - The 'key' value is invalid. it shall not return [EINTR]