OpenVMS Alpha Galaxy Guide
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  15.2  System Services

  15.2.1  Enhanced Services
  The following system services have been enhanced to rec-
  ognize the new   shared memory global section          flag
  SEC$M_SHMGS:

  *   SYS$CRMPSC_GDZRO_64

  *   SYS$CREATE_GDZRO

  *   SYS$MGBLSC_64

  *   SYS$DGBLSC

  The following system services were enhanced to work with
  shared memory but no interfaces were changed:

  *   SYS$DELTVA

  *   SYS$DELTVA_64

  *   SYS$CREATE_BUFOBJ

  *   SYS$CREATE_BUFOBJ_64

  *   SYS$DELETE_BUFOBJ

  15.2.2  New Section Flag SEC$M_READ_ONLY_SHPT

  The new section flag SEC$M_READ_ONLY_SHPT is rec-
  ognized by the SYS$CREATE_GDZRO and SYS$CRMPSC_
  GDZRO_64 services.  When this bit is set, it directs the system
  to create shared page tables for the section that allow only
  read access.  This feature is particularly useful in an envi-
  ronment where a memory resident or Galaxy shared section
  is used by many readers but only a single writer.

  When you map a Galaxy shared or memory resident section
  that has an associated shared page table section, you have the
  following options:

                                                            Desired
                                                            ac-
  Shared page tables                                        cess

                               READ ONLY:                   READ
                                                            AND

                                                            /OR
                                                            WRITE:
  none created                 Do not set the SEC$M_WRT     Set the
                               flag in the map request.     SEC$M_
                               Private page tables will al- WRT
                                                            flag
                               ways be used, even if you arein the
                               specifying a shared page ta- map
                               ble region into which to map re-
                               the section.                 quest.

                                                            Private
                                                            page
                                                            tables
                                                            will
                                                            always
                                                            be
                                                            used,
                                                            even
                                                            if you
                                                            are
                                                            speci-
                                                            fying a
                                                            shared
                                                            page
                                                            table
                                                            region
                                                            into
                                                            which
                                                            to map
                                                            the
                                                            sec-
                                                            tion.

  write access                 Do not set the SEC$M_WRT     Set the
                               flag in the map request.     SEC$M_
                                                            WRT
                               Ensure that private page     flag
                               tables will be used.  Do not in the
                               specify a shared page table
                               region into which to map the map
                               section.  If you do, the errore-
                                                            quest.
                               status SS$_IVSECFLG is
                               returned.                    The
                                                            shared
                                                            page
                                                            table
                                                            section
                                                            will be
                                                            used
                                                            for
                                                            map-
                                                            ping
                                                            if you
                                                            spec-
                                                            ify a
                                                            shared
                                                            page
                                                            table
                                                            region
                                                            into
                                                            which
                                                            to map
                                                            the
                                                            sec-
                                                            tion.

  read access                  Do not set the SEC$M_WRT     Set the
                               flag in the map request.  TheSEC$M_
                               shared page table section wilWRT
                               be used for mapping if you   flag
                               specify a shared page table  in the
                               region into which to map the map
                               section.                     re-
                                                            quest.
                                                            Ensure
                                                            that
                                                            private
                                                            page
                                                            tables
                                                            will be
                                                            used.
                                                            Do not
                                                            spec-
                                                            ify a
                                                            shared
                                                            page
                                                            table
                                                            region
                                                            into
                                                            which
                                                            to map
                                                            the
                                                            sec-
                                                            tion.
                                                            If you
                                                            do the
                                                            error
                                                            status
                                                            SS$_
                                                            IVSECFLG
                                                            is re-
                                                            turned.

                                Notes

      Shared page tables for Galaxy shared sections are also
      implemented as Galaxy shared sections.  This im-
      plies that they allow either read access only on all

      OpenVMS instances connected to this section or they
      allow read/write access on all instances.  The setting of
      the SEC$M_READ_ONLY_SHPT flag as requested
      by the first instance to create the section is used on all
      instances.

      Using the SYS$CRMPSC_GDZRO_64 service always
      implies that the SEC$M_WRT flag is set and that
      you want to map the section for writing.  If you want
      to use this service to create a section with shared page
      tables for read only access, you must use private page
      tables and you cannot specify a shared page table
      region into which to map the section.
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