dbm(3N) dbm(3N)
NAME
dbm, dbminit, dbmclose, fetch, store, delete, firstkey,
nextkey - database subroutines
SYNOPSIS
cc [options] file -lnsl
#include <rpcsvc/dbm.h>
typedef struct {
char *dptr;
int dsize;
} datum;
int dbminit(const char *file);
int dbmclose(void);
datum fetch(datum key);
int store(datum key, datum content);
int delete(datum key);
datum firstkey(void);
datum nextkey(datum key);
DESCRIPTION
These functions maintain key/content pairs in a database. 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
-lnsl.
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 database 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.
A database may be closed by calling dbmclose. You must close
a database before opening a new one.
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
Copyright 1994 Novell, Inc. Page 1
dbm(3N) dbm(3N)
return the first key in the database. With any key nextkey
will return the next key in the database. This code will
traverse the database:
for (key = firstkey(); key.dptr != NULL; key = nextkey(key))
Return Values
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.
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 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.
There are no interlocks and no reliable cache flushing; thus
concurrent updating and reading is risky.
FILES
/usr/lib/libnsl.so
Copyright 1994 Novell, Inc. Page 2