Multiple streams are created above a driver/multiplexer by use of the open system call on either different minor device, or on a cloneable device file. Note that any driver that handles more than one minor device is considered an upper multiplexer.
To connect streams below a multiplexer requires additional software in the multiplexer. The main difference between STREAMS lower multiplexers and STREAMS device drivers is that multiplexers are pseudo-devices and multiplexers have two additional qinit structures, pointed to by fields in streamtab(9S): the lower half read-side qinit(9S) and the lower half write-side qinit(9S).
The multiplexer is divided into two parts: the lower half and the upper half. The multiplexer queue structures allocated when the multiplexer was opened use the usual qinit entries from the multiplexer's streamtab(9S). This is the same as any open of the STREAMS device. When a lower stream is linked beneath the multiplexer, the qinit structures at the stream head are substituted by the lower half qinit(9S) structures identified in the streamstab for the multiplexers. Once the linkage is made, the multiplexer switches messages between upper and lower streams. When messages reach the top of the lower stream, they are handled by put and service routines specified in the bottom half of the multiplexer.