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tdb - a trivial database system
tridge@linuxcare.com December 1999

This is a simple database API. It was inspired by the realisation that in Samba we have several ad-hoc bits of code that essentially implement small databases for sharing structures between parts of Samba. As I was about to add another I realised that a generic database module was called for to replace all the ad-hoc bits.

I based the interface on gdbm. I couldn't use gdbm as we need to be able to have multiple writers to the databases at one time.

Compilation

add HAVE_MMAP=1 to use mmap instead of read/write add TDB_DEBUG=1 for verbose debug info
add NOLOCK=1 to disable locking code

Testing

Compile tdbtest.c and link with gdbm for testing. tdbtest will perform identical operations via tdb and gdbm then make sure the result is the same

Also included is tdbtool, which allows simple database manipulation on the commandline.

tdbtest and tdbtool are not built as part of Samba, but are included for completeness.

File formats

append tdb.magic to your /etc/magic so that file will recognise tdb files.

Interface

The interface is very similar to gdbm except for the following:

different open interface. The tdb_open call is more similar to a traditional open()
no tdbm_reorganise() function
no tdbm_sync() function. No operations are cached in the library anyway
added a tdb_traverse() function for traversing the whole database

A general rule for using tdb is that the caller frees any returned TDB_DATA structures. Just call free(p.dptr) to free a TDB_DATA return value called p. This is the same as gdbm.

here is a full list of tdb functions with brief descriptions.

TDB_CONTEXT tdb_open(char name, int hash_size, int tdb_flags,

int open_flags, mode_t mode)

open the database, creating it if necessary

The open_flags and mode are passed straight to the open call on the database file. A flags value of O_WRONLY is invalid

The hash size is advisory, use zero for a default value.

return is NULL on error

possible tdb_flags are:
TDB_CLEAR_IF_FIRST - clear database if we are the only one with it open TDB_INTERNAL - don't use a file, instaed store the data in

memory. The filename is ignored in this case. TDB_NOLOCK - don't do any locking
TDB_NOMMAP - don't use mmap
TDB_CONVERT - Create a database in the reverse of native endian:

                  normally when the database is created (or cleared
                  with TDB_CLEAR_IF_FIRST), it is created in native
                  endian order. This flag is set (or unset) 
                  automat ically for existing databases.

enum TDB_ERROR tdb_error(TDB_CONTEXT *tdb);

tdb_error returns the current error state of the tdb database.

const char tdb_errorstr(TDB_CONTEXT tdb);

tdb_errorstr returns a printable string that describes the error state of the database.

int tdb_close(TDB_CONTEXT *tdb);

close a database

int tdb_update(TDB_CONTEXT *tdb, TDB_DATA key, TDB_DATA dbuf);

update an entry in place - this only works if the new data size is <= the old data size and the key exists. on failure return -1

TDB_DATA tdb_fetch(TDB_CONTEXT *tdb, TDB_DATA key);

fetch an entry in the database given a key if the return value has a null dptr then a error occurred

caller must free the resulting data

int tdb_exists(TDB_CONTEXT *tdb, TDB_DATA key);

check if an entry in the database exists

note that 1 is returned if the key is found and 0 is returned if not found this doesn't match the conventions in the rest of this module, but is compatible with gdbm

int tdb_traverse(TDB_CONTEXT tdb, int (fn)(TDB_CONTEXT *tdb,

TDB_DATA key, TDB_DATA dbuf, void state), void state);

traverse the entire database - calling fn(tdb, key, data, state) on each element.

return -1 on error or the record count traversed

if fn is NULL then it is not called

a non-zero return value from fn() indicates that the traversal should stop

TDB_DATA tdb_firstkey(TDB_CONTEXT *tdb);

find the first entry in the database and return its key

the caller must free the returned data

TDB_DATA tdb_nextkey(TDB_CONTEXT *tdb, TDB_DATA key);

find the next entry in the database, returning its key

the caller must free the returned data

int tdb_delete(TDB_CONTEXT *tdb, TDB_DATA key);

delete an entry in the database given a key

int tdb_store(TDB_CONTEXT *tdb, TDB_DATA key, TDB_DATA dbuf, int flag);

store an element in the database, replacing any existing element with the same key

If flag==TDB_INSERT then don't overwrite an existing entry If flag==TDB_MODIFY then don't create a new entry

return 0 on success, -1 on failure

int tdb_lockall(TDB_CONTEXT *tdb);

lock the database. If we already have it locked then don't do anything

int tdb_unlockall(TDB_CONTEXT *tdb);

unlock the database

int tdb_lockchain(TDB_CONTEXT *tdb, TDB_DATA key);

lock one hash chain. This is meant to be used to reduce locking contention - it cannot guarantee how many records will be locked

int tdb_unlockchain(TDB_CONTEXT *tdb, TDB_DATA key);

unlock one hash chain