libdbm - data base subroutines

cc [ flags ] files -ldbm [ libraries ]

Note: the dbm library has been superceded by ndbm(3), and is now
implemented using ndbm. The libdbm 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. The functions are obtained with the
loader option -ldbm.

/usr/lib/libdbm.a -ldbm

ndbm(3), libc(3), intro(3)

#include <dbm.h>

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

char *file;

datum fetch(key)
datum key;

store(key, content)
datum key, content;

datum key;

datum firstkey()

datum nextkey(key)
datum key;

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 dbminit. At
the time of this call, the files file.dir and file.pag must exist.
(An empty database is created by creating zero-length ‘.dir’ and
‘.pag’ files.)

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

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

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.

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.

The sum of the sizes of a key/content pair must not exceed the
internal block size (currently 1024 bytes). Moreover all
key/content pairs that hash together must fit on a single block.
Store will return an error in the event that a disk block fills
with inseparable data.

Delete does not physically reclaim file space, although it does
make it available for reuse.

The order of keys presented by firstkey and nextkey depends on a
hashing function, not on anything interesting.