dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete,
dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - data base

#include <ndbm.h>

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

DBM *dbm_open(file, flags, mode)
const 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 data base. The
functions will handle very large (a billion blocks) databases and
will access a keyed item in one or two file system accesses. This
package replaces the earlier dbm(3)* library, which managed only a
single database.

Keys and contents are described by the datum typedef. A datum
specifies a string of dsize bytes pointed to by dptr. Arbitrary
binary data, as well as normal ASCII strings, are allowed. The
data base 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(2)).

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_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 ok. 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 UNIX systems may create
real file blocks for these holes when touched. These files cannot
be copied by normal means (cp, cat, tp, tar, ar) without filling in
the holes.

Dptr pointers returned by these subroutines point into static
storage that is changed by subsequent calls. This storage is not
necessarily aligned; stored "longs", for example, should be
copied to a properly aligned block of memory before being accessed.

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, not on anything interesting.


* Not currently supported under MachTen