If an error condition occurs, this routine returns an integer
    indicating the type of error. Possible return values are as
    follows:

    Return      Description

    0           Successful completion.
    [EAGAIN]    The system lacked the necessary resources to create
                another thread-specific data key, or the limit on the
                total number of keys per process (PTHREAD_KEYS_MAX)
                has been exceeded.
    [EINVAL]    The value specified by key is invalid.
    [ENOMEM]    Insufficient memory to create the key.