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Berkeley DB Reference Guide: Building Berkeley DB for Windows systems

Windows notes

• Various Berkeley DB interfaces take a mode argument, intended to specify the underlying file permissions for created files. Berkeley DB currently ignores this argument on Windows systems.

  It would be possible to construct a set of security attributes to pass to CreateFile that accurately represents the mode. In the worst case, this would involve looking up user and all group names and creating an entry for each. Alternatively, we could call the _chmod (partial emulation) function after file creation, although this leaves us with an obvious race.

  Practically speaking, however, these efforts would be largely meaningless on FAT, the most common file system, which only has a "readable" and "writeable" flag, applying to all users.

• When using the DB_SYSTEM_MEM flag, Berkeley DB shared regions are created without ACLs, which means that the regions are only accessible to a single user. If wider sharing is appropriate (e.g., both user applications and Windows/NT service applications need to access the Berkeley DB regions), the Berkeley DB code will need to be modified to create the shared regions with the correct ACLs. Alternatively, by not specifying the DB_SYSTEM_MEM flag, file-system backed regions will be created instead, and the permissions on those files may be directly specified through the DBENV->open interface.
• On Windows/9X, files opened by multiple processes do not share data correctly. For this reason, the DB_SYSTEM_MEM flag is implied for any application that does not specify the DB_PRIVATE flag, causing the system paging file to be used for sharing data. However, paging file memory is freed on last close, implying that multiple processes sharing an environment must arrange for at least one process to always have the environment open, or, alternatively, that any process joining the environment be prepared to re-create it. If a shared environment is closed by all processes, a subsequent open without specifying the DB_CREATE flag will result in the return of a system EAGAIN error code.

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