recno - record number database access method

#include <sys/types.h>
#include <db.h>

Note well: This page documents interfaces provided up until glibc 2.1. Since glibc 2.2, glibc no longer provides these interfaces. Probably, you are looking for the APIs provided by the libdb library instead.

The routine dbopen(3) is the library interface to database files. One of the supported file formats is record number files. The general description of the database access methods is in dbopen(3), this manual page describes only the recno-specific information.

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 implies 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 recno access-method-specific data structure provided to dbopen(3) is defined in the <db.h> include file as follows:

typedef struct {
    unsigned long flags;
    unsigned int  cachesize;
    unsigned int  psize;
    int           lorder;
    size_t        reclen;
    unsigned char bval;
    char         *bfname;
} RECNOINFO;

The elements of this structure are defined as follows:

flags

The flag value is specified by ORing 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 dbopen(3), the sequential record retrieval fills in both the caller's key and data structures. If the R_NOKEY flag is specified, the cursor routines 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 dbopen(3) is called, instead of permitting any unmodified records to be read from the original file.

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 cache is used.

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 filesystem I/O block size. See btree(3) for more information.

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.

reclen

The length of a fixed-length record.

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.

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 filename for a dbopen(3) of a btree file.

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.

Because there can be no metadata 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.

In the interface specified by dbopen(3), 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.

btree(3), dbopen(3), hash(3), mpool(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.