DB_RECNO(3)

NAME
db_recno - record number database access method

DESCRIPTION
specific details of the recno access method.

ACCESS METHOD SPECIFIC INFORMATION
The recno access method specific data structure provided to db_open is typedef'd and named RECNOINFO. A RECNOINFO structure has at least the following fields, which may be initialized before calling db_open:

u_int8_t bval;
    The delimiting byte to be used to mark the end of a record for variable-length records, and the pad character for fixed-length records. If no value is specified, newlines (``\n'') are used to mark the end of variable-length records and fixed-length records are padded with spaces.

char *bfname;
    The recno access method stores the in-memory copies of its records in a btree. If bfname is non-NULL, it specifies the name of the btree file, as if specified as the file name for a db_open of a btree file.

u_int cachesize;
    A suggested maximum size, in bytes, of the memory cache. This value is only advisory, and the access method will allocate more memory rather than fail. If cachesize is 0 (no size is specified) a default size is used.

u_long flags;
    The flag value is specified by or'ing any of the following values:

    R_FIXEDLEN
        The records are fixed-length, not byte delimited. The structure element reclen specifies the length of the record, and the structure element bval is used as the pad character. Any records, inserted into the database, that are less than reclen bytes long are automatically padded.

    R_NOKEY
        In the interface specified by db_open, the sequential record retrieval fills in both the caller's key and data structures. If the R_NOKEY flag is specified, the cursor functions are not required to fill in the key structure. This permits applications to retrieve records at the end of files without reading all of the intervening records.

    R_SNAPSHOT
        This flag requires that a snapshot of the file be taken when db_open is called, instead of permitting any unmodified records to be read from the original file.

int lorder;
    The byte order for integers in the stored database metadata. The number should represent the order as an integer; for example, big endian order would be the number 4,321. If lorder is 0 (no order is specified) the current host order is used.

u_int psize;
    The recno access method stores the in-memory copies of its records in a btree. This value is the size (in bytes) of the pages used for nodes in that tree. If psize is 0 (no page size is specified) a page size is chosen based on the underlying file system I/O block size. See btree(3) for more information.

size_t reclen;
    The length of a fixed-length record.

DB OPERATIONS
The data part of the key/data pair used by the recno access method is the same as other access methods. The key is different. The data field of the key should be a pointer to a memory location of type recno_t, as defined in the <db.h> include file. This type is normally the largest unsigned integral type available to the implementation. The size field of the key should be the size of that type.

The record number data structure is either variable or fixed-length records stored in a flat-file format, accessed by the logical record number. The existence of record number five requires the existence of records one through four, and the deletion of record number one causes record number five to be renumbered to record number four, as well as the cursor, if positioned after record number one, to shift down one record. The creation of record number five when records one through four do not exist causes the logical creation of them with zero-length data.

Because there is no meta-data associated with the underlying recno access method files, any changes made to the default values (e.g. fixed record length or byte separator value) must be explicitly specified each time the file is opened.

The functions returned by db_open for the btree access method are as described in db_open(3), with the following exceptions and additions:

type
    The type is DB_RECNO.

put Using the put interface to create a new record will cause the creation of multiple, empty records if the record number is more than one greater than the largest record currently in the database.

    The put function takes the following additional flags:

    R_IAFTER
        Append the data immediately after the data referenced by key, creating a new key/data pair. The record number of the appended key/data pair is returned in the key structure.

    R_IBEFORE
        Insert the data immediately before the data referenced by key, creating a new key/data pair. The record number of the inserted key/data pair is returned in the key structure.

    R_SETCURSOR
        Store the key/data pair, setting or initializing the position of the cursor to reference it.

seq The seq function takes the following additional flags:

    R_LAST
        The last key/data pair of the database is returned, and the cursor is set or initialized to reference it.

    R_PREV
        Retrieve the key/data pair immediately before the cursor. If the cursor is not yet set, this is the same as the R_LAST flag.

    If the database file is a character special file and no complete key/data pairs are currently available, the seq function returns 2.

sync
    The sync function takes the following additional flag:

    R_RECNOSYNC
        This flag causes the sync function to apply to the btree file which underlies the recno file, not the recno file itself. (See the bfname field of RECNOINFO structure, above, for more information.)

ERRORS
The recno access method functions may fail and set errno for any of the errors specified for the library function db_open(3) or the following:

[EINVAL]
    An attempt was made to add a record to a fixed-length database that was too large to fit.

SEE ALSO
db_btree(3), db_hash(3), db_lock(3), db_log(3), db_mpool(3), db_open(3), db_txn(3)

Document Processing in a Relational Database System, Michael Stonebraker, Heidi Stettner, Joseph Kalash, Antonin Guttman, Nadene Lynn, Memorandum No. UCB/ERL M82/32, May 1982.

BUGS
The sync function's R_RECNOSYNC interface is a kluge, and will be deleted in a future version of the interface.