DBM(3B) DBM(3B)
NAME
dbm: dbminit, dbmclose, fetch, store, delete, firstkey, nextkey - data
base subroutines
SYNOPSIS
#include
typedef struct {
char *dptr;
int dsize;
} datum;
int dbminit(const char *file);
void dbmclose(void);
datum fetch(datum key);
int store(datum key, datum content);
int delete(datum key);
datum firstkey(void);
datum nextkey(datum key);
DESCRIPTION
Note: the dbm library has been superceded by ndbm(3B), and is now
implemented using ndbm. 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.
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. The following code will
traverse the data base:
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DBM(3B) DBM(3B)
for (key = firstkey(); key.dptr != NULL; key = nextkey(key))
DIAGNOSTICS
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.
SEE ALSO
ndbm(3B)
BUGS
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.
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