|
HP OpenVMS Systemsask the wizard |
|
The Question is: I use the LIB$INS... and LIB$REM... queue routines both for queues and stacks. Sometimes I have a need to insert, remove or just examine an entry at other locations than the head or tail. Is there a recommended way to do that? This must be an issue also with the OpenVMS timer and AST queues assuming that these queues are implemented using these routines (except for the wrapper). One way I can think of to for exmaple remove an entry in the middle is to remove head and insert as tail to a temporary work queue until the entry is encountered. And after removing the entry then reattach the temporary work queue using the tail of that queue and the head of the remaining queue as routine arguments. Another way is maybe to not use a temporary work queue but instead insert a marker entry as tail and then remove head and insert as tail in the same queue until the entry is encountered. And after removing the entry then continue removing head and inserting as tail until the marker is encountered and removed but not inserted (obviously). This approach is though more wasteful. Thank you very much. The Answer is : As you are undoubtedly aware, the interlocked queue operations use hardware- (VAX) or PALcode-assisted (Alpha) interlocks that are located in the queue header data structure. There are no general guidelines for using these interlocked queue operations away from the header, other than the obvious requirement that the access to the located entries within the queue -- entries that are not adjacent to the queue header -- and any necessary queue entry traversals involved in this access be fully interlocked against any conflicting access. To ensure that the operations are reentrant for threading or for AST activites, in other words. The most appropriate solution depends on the application. Specific details of the implementation of the interlocked operations and of reentrancy are in the OpenVMS programming concepts and in the Alpha Architecture Reference Manual, and topics (2681) and (5199) may be of interest. Some general programming tips for asynchronous programming are in topic (1661). Details of achieving thread-reentrancy and the associated locking are in the DECthreads documentation.
|