This routine obtains the detachstate attribute of a thread
    attributes object. This attribute specifies whether threads
    created using the specified thread attributes object are created
    in a detached state.

    On successful completion, this routine returns a zero and the
    detachstate attribute is set in detachstate. A value of PTHREAD_
    CREATE_JOINABLE indicates the thread is not detached, and a value
    of PTHREAD_CREATE_DETACHED indicates the thread is detached.

    See the pthread_attr_setdetachstate() description for information
    about the detachstate attribute.