ndbm(3BSD) (BSD System Compatibility) ndbm(3BSD)
NAME
ndbm: dbm_clearerr, dbm_close, dbm_delete, dbm_error,
dbm_fetch, dbm_firstkey, dbm_nextkey, dbm_open, dbm_store -
(BSD) data base subroutines
SYNOPSIS
/usr/ucb/cc [flag . . . ] file . . .
#include <ndbm.h>
typedef struct {
char *dptr;
int dsize;
} datum;
int dbm_clearerr(DBM *db);
void dbm_close(DBM *db);
int dbm_delete(DBM *db, datum key);
int dbm_error(DBM *db);
datum dbm_fetch(DBM *db, datum key);
datum dbm_firstkey(DBM *db);
datum dbm_nextkey(DBM *db);
DBM *dbm_open(char *file, int flags, int mode);
int dbm_store(DBM *db, datum key, datum content, int flags);
DESCRIPTION
These functions maintain key/content pairs in a data base.
The functions will handle very large (a billion blocks) data
base and will access a keyed item in one or two file system
accesses. This package replaces the earlier dbm(3BSD)
library, which managed only a single data base.
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 data base 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)].
A data base 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 data base and
Copyright 1994 Novell, Inc. Page 1
ndbm(3BSD) (BSD System Compatibility) ndbm(3BSD)
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 data base 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 data base. dbm_nextkey will return the next
key in the data base. 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 data base. dbm_clearerr resets the error
condition on the named data base.
REFERENCES
open(2), dbm(3BSD)
RETURN VALUE
All functions that return an int indicate errors with negative
values. A zero return indicates no error. Routines that
return a datum indicate errors with a NULL (0) dptr. If
dbm_store is called with a flags value of DBM_INSERT and finds
an existing entry with the same key, it returns 1.
NOTICES
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 [that is, cp(1), cat(1), tar(1), ar(1)] 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 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.
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ndbm(3BSD) (BSD System Compatibility) ndbm(3BSD)
The order of keys presented by dbm_firstkey and dbm_nextkey
depends on a hashing function.
There are no interlocks and no reliable cache flushing; thus
concurrent updating and reading is risky.
Copyright 1994 Novell, Inc. Page 3