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 that 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.
The pthread_key_delete function is callable from within destructor functions. Destructor functions are not invoked by pthread_key_delete. Any destructor function that may have been associated with key will no longer be called upon thread exit.
If successful, the pthread_key_delete function will return zero. Otherwise an error number will be returned to indicate the error.
The pthread_key_delete function will fail if: