ndbm: dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr -- database subroutines


cc ... -lndbm


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

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

void dbm_close (db) DBM*db;

datum dbm_fetch(db, key) DBM *db; datum key;

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

int dbm_delete(db, key) DBM *db; datum key;

datum dbm_firstkey(db) DBM*db;

datum dbm_nextkey(db) DBM*db;

int dbm_error(db) DBM *db;

int dbm_clearerr(db) DBM *db;


These functions maintain key/content pairs in a database. The functions will handle very large (a billion blocks) databases and will access a keyed item in one or two filesystem accesses. This package replaces the earlier dbm(NS) library, which managed only a single database.

keys and contents are described by the datum typedef. A datum specifies a string of dsize bytes to which dptr points. Arbitrary binary data, as well as normal ASCII strings, are allowed. The database is stored in two files. One file is a directory containing a bit map and has .dir as its suffix. The second file contains all data and has .pag as its suffix.

Before a database can be accessed, it must be opened by dbm_open. This will open and/or create the files file.dir and file.pag depending on the flags parameter (see open(S)).

A database is closed by calling dbm_close.

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) is deleted by dbm_delete. A linear pass through all keys in a database may be made, in an (apparently) random order, by use of dbm_firstkey() and dbm_nextkey. dbm_firstkey() will return the first key in the database. dbm_nextkey() will return the next key in the database. This code will traverse the data base:

   for (key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
dbm_error() returns non-zero when an error has occurred reading or writing the database. dbm_clearerr() resets the error condition on the named database.


All functions that return an int indicate errors with negative values. A zero return indicates no error. Routines that return a datum indicate errors with a NULL (0) dptr. If dbm_store called with a flags value of DBM_INSERT finds an existing entry with the same key, it returns 1.


The .pag file will contain holes so that its apparent size is about four times its actual content. Older versions of the UNIX operating system may create real file blocks for these holes when touched. These files cannot be copied by normal means (i.e. cp(C), cat(C), tar(C), ar(CP)) without filling in the holes.

dptr pointers returned by these subroutines point into static storage that is changed by subsequent calls.

The sum of the sizes of a key/content pair must not exceed the internal block size (currently 4096 bytes). Moreover all key/content pairs that hash together must fit on a single block. dbm_store() will return an error in the event that a disk block fills with inseparable data.

dbm_delete() does not physically reclaim file space, although it does make it available for reuse.

The order of keys presented by dbm_firstkey() and dbm_nextkey() depends on a hashing function.

There are no interlocks and no reliable cache flushing; thus concurrent updating and reading is risky.

See also

ar(CP), cat(C), cp(C), dbm(NS), open(S), tar(C)
© 2003 Caldera International, Inc. All rights reserved.
SCO OpenServer Release 5.0.7 -- 11 February 2003