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The Question is: Our database is currently self-serve user mode global sections accessed both directly using pointers and via interface functions. We've sometimes had corruption of the database because of a bug in a single task. We would like to protect against that. One alternative is to put the database in the private space of a database server. Probably at the expense of speed. Another alternative is to put the database in "system" space (not private, not public but protected). Requiring access via sys$cm*(). Any comme nts/suggestions/advise? New features that might be useful? In what other space should we put it if any? How come there's no sys$cmsuper() - at least not documented when there's sys$cmexec() and sys$cmkrnl()? Supervisor mode seems like the least severe in terms of invasion of system space. Thanks The Answer is : An appropriate approach would involve placing all routines with any knowledge of the shared memory into a common shareable image, and then debugging and maintaining and using this shareable image across all images accessing the information in the shared memory. When correctly architected, this approach also has the added advantage of allowing other storage and retrieval mechanisms to be explorered without requiring application modifications or even rebuilds. Mapping the memory privately or in system space is certainly possible, but no approach will avoid the possibility of corruption on access by errant software... Failures in code using inner-mode memory protection mechanisms can also result in process deletion or system crash -- if the existing code is unreliable, then moving it to executive or kernel mode will tend to exacerbate problems, not alleviate them. Another existing mechanism that permits the use of shared memory with a set of protections against structural corruptions, with correct handling of memory barriers, with support for operation across nodes within a cluster, and with full multi-process interlocking is RMS (with global buffers enabled). Also please see existing topics 1661, 2681, 2637, 3365, 3530, and 3635 for discussions relevent to shared memory. As for your question on supervisor mode, the OpenVMS FAQ section MISC15 might be of interest.
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