1NDBM(3) Library Functions Manual NDBM(3)
2
6 dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete, dbm_firstkey,
7 dbm_nextkey, dbm_error, dbm_clearerr - data base subroutines
8
10 #include <ndbm.h>
11 typedef struct {
13 char *dptr;
14 int dsize;
15 } datum;
16 DBM *dbm_open(file, flags, mode)
18 char *file;
19 int flags, mode;
20 void dbm_close(db)
22 DBM *db;
23 datum dbm_fetch(db, key)
25 DBM *db;
26 datum key;
27 int dbm_store(db, key, content, flags)
29 DBM *db;
30 datum key, content;
31 int flags;
32 int dbm_delete(db, key)
34 DBM *db;
35 datum key;
36 datum dbm_firstkey(db)
38 DBM *db;
39 datum dbm_nextkey(db)
41 DBM *db;
42 int dbm_error(db)
44 DBM *db;
45 int dbm_clearerr(db)
47 DBM *db;
48
50 These functions maintain key/content pairs in a data base. The func‐
51 tions will handle very large (a billion blocks) databases and will
52 access a keyed item in one or two file system accesses. This package
53 replaces the earlier dbm(3x) library, which managed only a single data‐
54 base.
55 Keys and contents are described by the datum typedef. A datum speci‐
57 fies a string of dsize bytes pointed to by dptr. Arbitrary binary
58 data, as well as normal ASCII strings, are allowed. The data base is
59 stored in two files. One file is a directory containing a bit map and
60 has `.dir' as its suffix. The second file contains all data and has
61 `.pag' as its suffix.
62 Before a database can be accessed, it must be opened by dbm_open. This
64 will open and/or create the files file.dir and file.pag depending on
65 the flags parameter (see open(2)).
66 Once open, the data stored under a key is accessed by dbm_fetch and
68 data is placed under a key by dbm_store. The flags field can be either
69 DBM_INSERT or DBM_REPLACE. DBM_INSERT will only insert new entries
70 into the database and will not change an existing entry with the same
71 key. DBM_REPLACE will replace an existing entry if it has the same
72 key. A key (and its associated contents) is deleted by dbm_delete. A
73 linear pass through all keys in a database may be made, in an (appar‐
74 ently) random order, by use of dbm_firstkey and dbm_nextkey.
75 Dbm_firstkey will return the first key in the database. Dbm_nextkey
76 will return the next key in the database. This code will traverse the
77 data base:
78 for (key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nex‐
80 tkey(db))
81 Dbm_error returns non-zero when an error has occurred reading or writ‐
83 ing the database. Dbm_clearerr resets the error condition on the named
84 database.
85
87 All functions that return an int indicate errors with negative values.
88 A zero return indicates ok. Routines that return a datum indicate
89 errors with a null (0) dptr. If dbm_store called with a flags value of
90 DBM_INSERT finds an existing entry with the same key it returns 1.
91
93 The `.pag' file will contain holes so that its apparent size is about
94 four times its actual content. Older UNIX systems may create real file
95 blocks for these holes when touched. These files cannot be copied by
96 normal means (cp, cat, tp, tar, ar) without filling in the holes.
97 Dptr pointers returned by these subroutines point into static storage
99 that is changed by subsequent calls.
100 The sum of the sizes of a key/content pair must not exceed the internal
102 block size (currently 4096 bytes). Moreover all key/content pairs that
103 hash together must fit on a single block. Dbm_store will return an
104 error in the event that a disk block fills with inseparable data.
105 Dbm_delete does not physically reclaim file space, although it does
107 make it available for reuse.
108 The order of keys presented by dbm_firstkey and dbm_nextkey depends on
110 a hashing function, not on anything interesting.
111
113 dbm(3X)
1144.3 Berkeley Distribution May 20, 1986 NDBM(3)