Uniplexor (UNI multiplexor)

The uniplexor (UNI multiplexor) is a special SVR5 DDI module that is placed between a module or driver and the rest of the Stream when they have different levels of multiprocessor support (one is multithreaded and one is not). See ``Multithreaded drivers''. The uniplexor is a multithreaded 1-to-1 multiplexor where either the upper Stream or lower Stream is bound to the same CPU as non-MP safe modules and drivers. See ``Multiplexing, STREAMS''.

For example, for a non-MP module that is pushed on a STREAM that contains an MP driver:

• The Stream head, non-MP module, and the upper half of the uniplexor are bound to the same CPU as the non-MP module.

• The lower half of the uniplexor and the multithreaded driver are unbound and can run on any CPU.

If you follow the q_next pointers on such a Stream, they seem to end at the uniplexor, although the uniplexor itself stores information internally about which is the next Stream.

There is no portable and DDI compliant way to look at another queue's next pointer (q_next). Instead, you should do one of the following:

• Make both the driver and the module have the same level of MP-ness, either multithreaded or non-multithreaded, so that the uniplexor is not required. This solution provides the best performance but requires that you have the source for both the driver and the module.

• Create a protocol message that returns a pointer back to the lower Stream. It may take longer to receive the q_ptr information with this implementation, because the uniplexor places messages on a queue to be processed ``at a later time'' by service procedures, since one side of the uniplexor is bound to a CPU. Often, such a message is exchanged once during the driver and module initialization, so this delay is acceptable.