STREAMS/UX for the HP 9000 Reference Manual

Differences Between STREAMS/UX and System V Release 4 STREAMS

STREAMS/UX Data Structure Restrictions

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.