The linkblk structure is used to connect a lower Stream to an upper STREAMS multiplexor driver. This structure is used in conjunction with the I_LINK, I_UNLINK, P_LINK,
and P_UNLINK ioctl commands. See streamio(7I). The M_DATA portion of the M_IOCTL message contains the linkblk structure. Note that the linkblk structure is allocated and initialized by the Stream head as a result of one of the above ioctl commands.