STREAMS/UX has the same restrictions as those described in
the Kernel Data Structure chapter of the SVR4.2 Driver manual. Also, STREAMS/UX
limits which user written functions can access the queue structure
directly. A queue's open, close, put, or service routine can manipulate the queue structure as
specified by SVR4.2. On a uniprocessor system, a queue's entry points
can access the other queue in the queue pair in the same way that
they can access their own queue. On a multiprocessor system, a queue's
entry points can manipulate queues belonging to entities with which
they can share data. They can manipulate the queues in the same
way that they can manipulate their own queue. See "Writing
MP Scalable Modules and Drivers" in Chapter 4 for more
information about sharing data on multiprocessor systems.
It is difficult to program other functions (besides those
described above) to access the queue structure directly, especially
on multiprocessor systems. If a queue's entry points access queues
other than those described above, or if non-STREAMS/UX software
processes data in a STREAMS/UX queue, try to use the streams_put utility to manipulate the queues safely. streams_put is described in the "HP-UX Modifications to
STREAMS/UX Utilities" section of this chapter. If you cannot
use streams_put, the code that accesses a STREAMS/UX queue must,
at a minimum, follow these additional rules. The software must ensure
that it is accessing an allocated, opened queue. Also, it cannot
dereference the q_first, q_last, or q_next pointers. In other words, it cannot read or write
data pointed at by the pointers. For example, the function can check
if q_first is 0, but it cannot read the q_first-b_next field. Lastly, you must implement any additional
synchronization required for your modules and drivers to work correctly.
You may need to synchronize the function accessing the STREAMS/UX
queue with the queue's entry points. This is because the function
and the entry points may access the queue in parallel on a multiprocessor
system and may interrupt each other while accessing the queue on
a uniprocessor system.
