On successful completion, this routine returns the calling
    thread's previous cancelability state in the oldstate argument.

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

    Return      Description

    0           Successful completion.
    [EINVAL]    The specified state is not PTHREAD_CANCEL_ENABLE or
                PTHREAD_CANCEL_DISABLE.