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A STREAMS multiplexor is a driver with multiple Streams connected to it. The primary function of the multiplexing driver is to switch messages among the connected Streams. Multiplexor configurations are created at user level by system calls.
STREAMS-related system calls set up the ``plumbing,'' or Stream interconnections, for multiplexing drivers. The subset of these calls that allows a user to connect (and disconnect) Streams below a driver is referred to as the multiplexing facility. This type of connection is referred to as a 1-to-M, or lower, multiplexor configuration. This configuration must always contain a multiplexing driver, which is recognized by STREAMS as having special characteristics.
Multiple Streams can be connected above a driver by open calls. There is no difference between the connections to these drivers, only the functions performed by the driver are different. In the multiplexing case, the driver routes data between multiple Streams. In the device driver case, the driver routes data between user processes and associated physical ports. Multiplexing with Streams connected above is referred to as an N-to-1, or upper, multiplexor. STREAMS does not provide any facilities beyond open and close to connect or disconnect upper Streams for multiplexing purposes.
From the driver's perspective, upper and lower configurations differ only in how they are initially connected to the driver. The implementation requirements are the same: route the data and handle flow control. All multiplexor drivers require special developer-provided software to perform the multiplexing data routing and to handle flow control. STREAMS does not directly support flow control among multiplexed Streams.
M-to-N multiplexing configurations are implemented by using both of the above mechanisms in a driver.
The multiple Streams that represent minor devices are actually distinct Streams in which the driver keeps track of each Stream attached to it. The STREAMS subsystem does not recognize any relationship between the Streams. The same is true for STREAMS multiplexors of any configuration. The multiplexed Streams are distinct and the driver must be implemented to do most of the work.
In addition to upper and lower multiplexors, more complex configurations can be created by connecting Streams containing multiplexors to other multiplexor drivers. With such a diversity of needs for multiplexors, it is not possible to provide general-purpose multiplexor drivers. Rather, STREAMS provides a general purpose multiplexing facility that allows users to set up the intermodule/driver plumbing to create multiplexor configurations of generally unlimited interconnection.