# DbMpoolFile::set_fileid

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    #include <db_cxx.h>

    int
    DbMpoolFile::set(u_int8_t *fileid);

# Description

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 pool. The DbMpoolFile::set_fileid method specifies a unique identifier for the file. Unique file identifiers must be a DB_FILE_ID_LEN length array of bytes.

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 memory buffer pool between processes or across system reboots, in which the pool contains pages from files stored on such filesystems, must specify a unique file identifier using the DbMpoolFile::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 memory pool 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 memory pool are stored on filesystems in which the default values as described previously are invariant between process and across system reboots.

The DbMpoolFile::set_fileid method either returns a non-zero error value or throws an exception that encapsulates a non-zero error value on failure, and returns 0 on success.

# Errors

The DbMpoolFile::set_fileid method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods. If a catastrophic error has occurred, the DbMpoolFile::set_fileid method may fail and either return DB_RUNRECOVERY or throw an exception encapsulating DB_RUNRECOVERY, in which case all subsequent Berkeley DB calls will fail in the same way.

# Class

DbEnv, DbMpoolFile

# See Also

DbEnv::set_cachesize, DbEnv::set_mp_mmapsize, DbEnv::memp_fcreate, DbMpoolFile::close, DbMpoolFile::get, DbMpoolFile::open, DbMpoolFile::put, DbMpoolFile::set, DbMpoolFile::sync, DbEnv::memp_register, DbMpoolFile::set_clear_len, DbMpoolFile::set_fileid, DbMpoolFile::set_ftype, DbMpoolFile::set_lsn_offset, DbMpoolFile::set_pgcookie, DbEnv::memp_stat, DbEnv::memp_sync, and DbEnv::memp_trickle.`

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