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As part of constructing a Stream, a module can be added (pushed) with an ioctl I_PUSH (see streamio(M)) system call. The push inserts a module beneath the Stream head. Because of the similarity of STREAMS components, the push operation is similar to the driver open. First, the address of the qinit structure for the module is obtained.
Next, STREAMS allocates a pair of queue structures and initializes their contents as in the driver open.
Then, q_next values are set and modified so that the module is interposed between the Stream head and its neighbor immediately downstream. Finally, the module open procedure (located using qinit) is called.
Each push of a module is independent, even in the same Stream. If the same module is pushed more than once on a Stream, there will be multiple occurrences of that module in the Stream. The total number of pushable modules that may be contained on any one Stream is limited by the kernel parameter NSTRPUSH.
An ioctl I_POP (see streamio(M)) system call removes (pops) the module immediately below the Stream head. The pop calls the module close procedure. On return from the module close, any messages left on the module's message queues are freed (deallocated). Then, STREAMS connects the Stream head to the component previously below the popped module and deallocates the module's queue pair. I_PUSH and I_POP enable a user process to alter dynamically the configuration of a Stream by pushing and popping modules as required. For example, a module may be removed and a new one inserted below the Stream head. Then the original module can be pushed back after the new module has been pushed.