NDBM(3) 1987 NDBM(3)
NAME
ndbm, dbm_open, dbm_close, dbm_fetch, dbm_store, dbm_delete,
dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - data
base subroutines
SYNOPSIS
#include <ndbm.h>
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;
DESCRIPTION
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(3X)
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 suf-
fix.
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(2V)).
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.
SEE ALSO
ar(1V), cat(1V), cp(1), tar(1), open(2V), dbm(3X)
DIAGNOSTICS
All functions that return an int indicate errors with nega-
tive 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.
BUGS
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 (cp(1), cat(1V), tar(1), ar(1V)) 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 sin-
gle 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.
There are no interlocks and no reliable cache flushing; thus
concurrent updating and reading is risky.
November Last change: 24 3
