#include <db.h> int DB_MPOOLFILE->set_fileid(DB_MPOOLFILE *mpf, u_int8_t *fileid);
         The DB_MPOOLFILE->set_fileid() method specifies a unique identifier
         for the file. (The shared memory buffer pool functions must be able to
         uniquely identify files in order that multiple processes wanting to
         share a file will correctly identify it in the cache.)
    
         On most UNIX/POSIX systems, the fileid field will not need
         to be set, and the memory pool functions will use the file's device and inode numbers for
         this purpose.  On Windows systems, the memory pool functions use the values returned by
         GetFileInformationByHandle() by default — these values are
         known to be constant between processes and over reboot in the case of NTFS (in which they
         are the NTFS MFT indices).
    
         On other filesystems (for example, FAT or NFS), these default values
         are not necessarily unique between processes or across system reboots.
         Applications wanting to maintain a shared
         cache between processes or across system reboots, in which the
         cache contains pages from files stored on such filesystems, must
         specify a unique file identifier using the DB_MPOOLFILE->set_fileid()
         method, and each process opening the file must provide the same unique
         identifier.
    
This call should not be necessary for most applications. Specifically, it is not necessary if the cache is not shared between processes and is reinstantiated after each system reboot, if the application is using the Berkeley DB access methods instead of calling the pool functions explicitly, or if the files in the cache are stored on filesystems in which the default values as described previously are invariant between process and across system reboots.
         The DB_MPOOLFILE->set_fileid() method configures a file in the 
         cache, not only operations performed using the specified 
         DB_MPOOLFILE  handle.
    
         The DB_MPOOLFILE->set_fileid() method may not be called after the
         DB_MPOOLFILE->open() 
         method is called. If the mpool file already exists when
         DB_MPOOLFILE->open() is called, the information
         specified to DB_MPOOLFILE->set_fileid() must be
         the same as that historically used to create the mpool file or
         corruption can occur.
    
         The DB_MPOOLFILE->set_fileid() 
            
                  method returns a non-zero error value on failure and 0 on success.