Berkeley DB: dbm/ndbm
Description: Berkeley DB: An embedded database programmatic toolkit.
Keywords: embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++

### dbm/ndbm |

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    #define DB_DBM_HSEARCH    1
    #include <db.h>

    typedef struct {
    	char *dptr;
    	int dsize;
    } datum;

    Dbm Functions
    int
    dbminit(char *file);

    int
    dbmclose();

    datum
    fetch(datum key);

    int
    store(datum key, datum content);

    int
    delete(datum key);

    datum
    firstkey(void);

    datum
    nextkey(datum key);

    Ndbm Functions
    DBM *
    dbm_open(char *file, int flags, int mode);

    void
    dbm_close(DBM *db);

    datum
    dbm_fetch(DBM *db, datum key);

    int
    dbm_store(DBM *db, datum key, datum content, int flags);

    int
    dbm_delete(DBM *db, datum key);

    datum
    dbm_firstkey(DBM *db);

    datum
    dbm_nextkey(DBM *db);

    int
    dbm_error(DBM *db);

    int
    dbm_clearerr(DBM *db);

### Description: dbm/ndbm

The dbm functions are intended to provide high-performance implementations and source code compatibility for applications written to historic interfaces. They are not recommended for any other purpose. The historic dbm database format is not supported, and databases previously built using the real dbm libraries cannot be read by the Berkeley DB functions.

To compile dbm applications, replace the application's #include of the dbm or ndbm include file (for example, #include <dbm.h> or #include <ndbm.h>) with the following two lines:

>     #define DB_DBM_HSEARCH    1
>     #include <db.h>

and recompile. If the application attempts to load against a dbm library (for example, -ldbm), remove the library from the load line.

Key and content parameters are objects described by the datum typedef. A datum specifies a string of dsize bytes pointed to by dptr. Arbitrary binary data, as well as normal text strings, are allowed.

### Dbm Functions

Before a database can be accessed, it must be opened by dbminit. This will open and/or create the database file.db. If created, the database file is created read/write by owner only (as described in chmod(2)) and modified by the process' umask value at the time of creation (see umask(2)). The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB.

A database may be closed, and any held resources released, by calling dbmclose.

Once open, the data stored under a key is accessed by fetch, and data is placed under a key by store. A key (and its associated contents) are deleted by delete. A linear pass through all keys in a database may be made, in an (apparently) random order, by using firstkey and nextkey. The firstkey method will return the first key in the database. The nextkey method will return the next key in the database.

The following code will traverse the database:

>     for (key = firstkey();
>         key.dptr != NULL; key = nextkey(key)) {
>     	...
>     }

### Ndbm Functions

Before a database can be accessed, it must be opened by dbm_open. This will open and/or create the database file file.db, depending on the flags parameter (see open(2)). If created, the database file is created with mode mode (as described in chmod(2)) and modified by the process' umask value at the time of creation (see umask(2)). The group ownership of created files is based on the system and directory defaults, and is not further specified by Berkeley DB.

Once open, the data stored under a key is accessed by dbm_fetch, and data is placed under a key by dbm_store. The flags field can be either DBM_INSERT or DBM_REPLACE. DBM_INSERT will only insert new entries into the database, and will not change an existing entry with the same key. DBM_REPLACE will replace an existing entry if it has the same key. A key (and its associated contents) are deleted by dbm_delete. A linear pass through all keys in a database may be made, in an (apparently) random order, by using dbm_firstkey and dbm_nextkey. The dbm_firstkey method will return the first key in the database. The dbm_nextkey method will return the next key in the database.

The following code will traverse the database:

>     for (key = dbm_firstkey(db);
>         key.dptr != NULL; key = dbm_nextkey(db)) {
>     	...
>     }

### Compatibility Notes

The historic dbm library created two underlying database files, traditionally named file.dir and file.pag. The Berkeley DB library creates a single database file named file.db. Applications that are aware of the underlying database filenames may require additional source code modifications.

The historic dbminit function required that the underlying .dir and .pag files already exist (empty databases were created by first manually creating zero-length .dir and .pag files). Applications that expect to create databases using this method may require additional source code modifications.

The historic dbm_dirfno and dbm_pagfno macros are supported, but will return identical file descriptors because there is only a single underlying file used by the Berkeley DB hashing access method. Applications using both file descriptors for locking may require additional source code modifications.

If applications using the dbm function exits without first closing the database, it may lose updates because the Berkeley DB library buffers writes to underlying databases. Such applications will require additional source code modifications to work correctly with the Berkeley DB library.

### Dbm Diagnostics

The dbminit function returns -1 on failure, setting errno, and 0 on success.

The fetch function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

The store function returns -1 on failure, setting errno, and 0 on success.

The delete function returns -1 on failure, setting errno, and 0 on success.

The firstkey function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

The nextkey function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

### Errors

The dbminit, fetch, store, delete, firstkey, and nextkey functions may fail and return an error for errors specified for other Berkeley DB and C library or system functions.

### Ndbm Diagnostics

The dbm_close method returns non-zero when an error has occurred reading or writing the database.

The dbm_close method resets the error condition on the named database.

The dbm_open function returns NULL on failure, setting errno, and a DBM reference on success.

The dbm_fetch function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

The dbm_store function returns -1 on failure, setting errno, 0 on success, and 1 if DBM_INSERT was set and the specified key already existed in the database.

The dbm_delete function returns -1 on failure, setting errno, and 0 on success.

The dbm_firstkey function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

The dbm_nextkey function sets the dptr field of the returned datum to NULL on failure, setting errno, and returns a non-NULL dptr on success.

The dbm_close function returns -1 on failure, setting errno, and 0 on success.

The dbm_close function returns -1 on failure, setting errno, and 0 on success.

### Errors

The dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, and dbm_nextkey functions may fail and return an error for errors specified for other Berkeley DB and C library or system functions.

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