dbopen() -- library interface to database files

SYNOPSIS

#include <sys/types.h>

#include <limits.h>

#include <db.h>

DB *dbopen(const char *file, int flags, int mode, DBTYPE type, const void *openinfo);

The dbopen() function is the library interface to database files. The supported file formats are btree, hashed and UNIX file oriented. The btree format is a representation of a sorted, balanced tree structure. The hashed format is an extensible, dynamic hashing scheme. The flat-file format is a byte stream file with fixed or variable length records. The formats and file format specific information are described in detail in their respective reference pages: db_btree, db_hash, and db_recno.

The dbopen() function opens the specified file for reading and/or writing. An in-memory database can be created by passing NULL for the file name.

The dbopen() function returns a pointer to a DB structure on success and NULL on error. The DB structure is defined in the <db.h> include file, and contains at least the following fields:

typedef struct {
	DBTYPE type;
	int (*close)(const DB *db);
	int (*del)(const DB *db, const DBT *key, u_int flag);
	int (*fd)(const DB *db);
	int (*get)(const DB *db, DBT *key, DBT *data, u_int flag);
	int (*put)(const DB *db, DBT *key, const DBT *data, u_int flags);
	int (*seq)(const DB *db, DBT *key, DBT *data, u_int flag);
	int (*sync)(const DB *db, u_int flags); 
} DB;

These elements describe a database type and a set of functions performing various actions. These functions take a pointer to a structure as returned by dbopen(), and sometimes one or more pointers to key/data structures and a flag value.

type

Is the type of the underlying access method (and file format).

close

Points to a routine to flush any cached information to disk, free any allocated resources, and close the underlying file(s). Since key/data pairs may be cached in memory, failing to sync the file with a close or sync function may result in inconsistent or lost information. Close routines return -1 on error (setting errno) and 0 on success.

del

Points to a routine to remove key/data pairs from the database. The parameter flag may be set to the following value:

R_CURSOR: Deletes the record referenced by the cursor. The cursor must have previously been initialized.

Del routines return -1 on error (setting errno), 0 on success, and 1 if the specified key was not in the file.

fd

Points to a routine which returns a file descriptor representative of the underlying database. A file descriptor referencing the same file is returned to all processes which call dbopen() with the same file name. This file descriptor may be safely used as an argument to the fcntl() and lockf() locking functions. The file descriptor is not necessarily associated with any of the underlying files used by the access method. No file descriptor is available for in-memory databases. Fd routines return -1 on error (setting errno), and the file descriptor on success.

get

Points to a routine which is the interface for keyed retrieval from the database. The address and length of the data associated with the specified key are returned in the structure referenced by data. Get routines return -1 on error (setting errno), 0 on success, and 1 if the key was not in the file.

put

Points to a routine to store key/data pairs in the database.

The parameter flag may be set to one of the following values:

R_CURSOR: Replaces the key/data pair referenced by the cursor. The cursor must have previously been initialized.
R_IAFTER: Appends 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. (Applicable only to the DB_RECNO access method.)
R_IBEFORE: Inserts 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. (Applicable only to the DB_RECNO access method.)
R_NOOVERWRITE: Enters the new key/data pair only if the key does not previously exist.
R_SETCURSOR: Stores the key/data pair, setting or initializing the position of the cursor to reference it. (Applicable only to the DB_BTREE and DB_RECNO access methods.)

R_SETCURSOR is available only for the DB_BTREE and DB_RECNO access methods because it implies that the keys have an inherent order which does not change.

R_IAFTER and R_IBEFORE are available only for the DB_RECNO access method because they each imply that the access method is able to create new keys. This is only true if the keys are ordered and independent, record numbers for example.

The default behavior of the put routines is to enter the new key/data pair, replacing any previously existing key. Put routines return -1 on error (setting errno), 0 on success, and 1 if the R_NOOVERWRITE flag was set and the key already exists in the file.

seq

Points to a routine which is the interface for sequential retrieval from the database. The address and length of the key are returned in the structure referenced by key, and the address and length of the data are returned in the structure referenced by data.

Sequential key/data pair retrieval may begin at any time, and the position of the cursor is not affected by calls to the del, get, put, or sync routines. Modifications to the database during a sequential scan are reflected in the scan, that is, records inserted behind the cursor are not returned while records inserted in front of the cursor are returned.

The flag value must be set to one of the following values:

R_CURSOR: Returns the data associated with the specified key. This differs from the get routines in that it sets or initializes the cursor to the location of the key as well. (Note, for the DB_BTREE access method, the returned key is not necessarily an exact match for the specified key. The returned key is the smallest key greater than or equal to the specified key, permitting partial key matches and range searches.)
R_FIRST: Returns the first key/data pair of the database, and sets or initializes the cursor to reference it.
R_LAST: Returns the last key/data pair of the database, and sets or initializes the cursor to reference it. (Applicable only to the DB_BTREE and DB_RECNO access methods.)
R_NEXT: Retrieves the key/data pair immediately after the cursor. If the cursor is not yet set, this is the same as the R_FIRST flag.
R_PREV: Retrieves the key/data pair immediately before the cursor. If the cursor is not yet set, this is the same as the R_LAST flag. (Applicable only to the DB_BTREE and DB_RECNO access methods.)

R_LAST and R_PREV are available only for the DB_BTREE and DB_RECNO access methods because they each imply that the keys have an inherent order which does not change.

Seq routines return -1 on error (setting errno), 0 on success and 1 if there are no key/data pairs less than or greater than the specified or current key. If the DB_RECNO access method is being used, and if the database file is a character special file and no complete key/data pairs are currently available, the seq routines return 2.

sync

Points to a routine to flush any cached information to disk. If the database is in memory only, the sync routine has no effect and always succeeds.

The flag value may be set to the following value:

R_RECNOSYNC: If the DB_RECNO access method is being used, this flag causes the sync routine to apply to the btree file which underlies the recno file, not the recno file itself. (See the bfname field of the db_recno reference page for more information.)

Sync routines return -1 on error (setting errno) and 0 on success.

Key/Data Pairs

Access to all file types is based on key/data pairs. Both keys and data are represented by the following data structure:

typedef struct {
	void *data;
	size_t size;
} DBT;

The elements of the DBT structure are defined as follows:

data: Points to a byte string.
size: Is the length of the byte string.

Key and data byte strings may reference strings of essentially unlimited length although any two of them must fit into available memory at the same time. It should be noted that the access methods provide no guarantees about byte string alignment.

PARAMETERS

file: Specifies the database file to open, or NULL for an in-memory database.
flags: Is as specified to the open() function, however, only the O_BINARY, O_CREAT, O_EXCL, O_NONBLOCK, O_RDONLY, O_RDWR, and O_TRUNC flags are meaningful. (Note, opening a database file O_WRONLY is not possible.)
mode: Is as specified to the open() function.
type: Is of type DBTYPE (as defined in the <db.h> include file) and may be set to DB_BTREE, DB_HASH, or DB_RECNO.
openinfo: Points to an access method specific structure described in the access method's reference page. If openinfo is a null pointer, each access method uses defaults appropriate for the system and the access method.

RETURN VALUES

On success, dbopen() returns a pointer to the database structure. On error, it returns NULL and sets errno to any of the error code specified by open() or malloc(), or one of the following:

EFTYPE: A file is incorrectly formatted.
EINVAL: A parameter has been specified (hash function, pad byte etc.) that is incompatible with the current file specification or which is not meaningful for the function (for example, use of the cursor without prior initialization) or there is a mismatch between the version number of file and the software.

The close routines may fail and set errno for any of the errors specified for the functions close(), read(), write(), free(), or fsync().

The del, get, put and seq routines may fail and set errno for any of the errors specified for the functions read(), write(), free() or malloc().

The fd routines fail and set errno to ENOENT for in memory databases.

The sync routines may fail and set errno for any of the errors specified for fsync().

dbopen()
library interface to database files

Function

SYNOPSIS

DESCRIPTION

Key/Data Pairs

PARAMETERS

RETURN VALUES

CONFORMANCE

MULTITHREAD SAFETY LEVEL

PORTING ISSUES

AVAILABILITY

SEE ALSO

dbopen() library interface to database files

Function

dbopen()
library interface to database files