Dismantling a lower multiplexer is accomplished by disconnecting (unlinking) the lower streams. Unlinking can be initiated in three ways:
An I_UNLINK ioctl(2) referencing a specific stream
An I_UNLINK indicating all lower streams
The last close of the control stream
As in the link, an unlink sends a linkblk(9S) structure to the driver in an M_IOCTL message. The I_UNLINK call, which unlinks a single stream, uses the l_index value returned in the I_LINK to specify the lower stream to be unlinked. The latter two calls must designate a file corresponding to a control stream, which causes all the lower streams that were previously linked by this control stream to be unlinked. However, the driver sees a series of individual unlinks.
If no open references exist for a lower stream, a subsequent unlink will automatically close the stream. Otherwise, the lower stream must be closed by close(2) following the unlink. STREAMS automatically dismantles all cascaded multiplexers (below other multiplexing streams) if their controlling stream is closed. An I_UNLINK leaves lower, cascaded multiplexing streams intact unless the stream file descriptor was previously closed.