DB(3X) DB(3X)
NAME
DBopen, DBclose, DBget, DBput, DBdel, DBkey0, DBkeyn,
DBlock, DBunlock, DBsync, DBapp, DBins, DBcopy - database
subroutines
SYNOPSIS
#include <DB.h> datum DBkeyn(dp, key)
DBFILE * dp;
typedef struct { datum key;
char * dptr;
int dsize; void DBclose(dp)
} datum; DBFILE * dp;
int DBdebug; int DBlock(dp)
DBFILE * dp;
DBFILE * DBopen(file, flags)
char * file; int DBunlock(dp)
int flags; DBFILE * dp;
datum DBget(dp, key) void DBsync(dp)
DBFILE * dp; DBFILE * dp;
datum key;
int DBapp(dp, key, content)
int DBput(dp, key, content) DBFILE * dp;
DBFILE * dp; datum key, content;
datum key, content;
int DBins(dp, key, content)
int DBdel(dp, key) DBFILE * dp;
DBFILE * dp; datum key, content;
datum key;
int DBcopy(sdp, ddp)
datum DBkey0(dp) DBFILE * sdp, ddp;
DBFILE * dp;
DESCRIPTION
These functions maintain key/content pairs in a data base.
The functions will handle databases as large as the filesys-
tem can handle and will access a keyed item in one or two
file system accesses. The functions are obtained with the
loader option -lDB.
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 as either keys or records. 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
DB(3X) DB(3X)
DBopen. At the time of this call, the files file.dir and
file.pag must exist unless the DB_CREATE flag is set in the
flags argument. If this flag is specified and the files
exist, they are truncated. The only other meaningful flag
that may be specified is DB_RONLY, which indicates that the
DB library may not write on the files. If the files are
both not writable, this flag need not be specified and the
d_flags member of the returned DBFILE pointer will have this
bit set. Note that these flags are mutually exclusive, and
if one must be ignored, it will be DB_RONLY.
Once open, the data stored under a key is accessed by DBget
and data is placed under a key or overwritten by DBput. The
data denoted by a key may be appended to, or inserted
before, by DBapp and DBins, respectively. A key (and its
associated contents) is deleted by DBdel. A linear pass
through all keys in a database may be made, in an (appar-
ently) random order, by use of DBkey0 and DBkeyn. DBkey0
will return the first key in the database. With any key
argument, DBkeyn will return the next key in the database.
This code will traverse the data base:
for (key = DBkey0(dp); key.dptr != NULL; key =
DBkeyn(dp, key))
DBcopy copies the data indicated by keys in the database
referred to by its first argument to the database indicated
by its second argument using the same traversal.
DBlock and DBunlock set and clear advisory locks (see
ioctl(4)) on the database files. DBlock returns when the
lock is set by the invoking process.
DBsync flushes any modified buffers, and is called by
DBunlock and DBclose.
DIAGNOSTICS
All functions that return an int indicate errors with nega-
tive values. A zero return indicates success. Routines
that return a datum indicate errors with a null (0) dptr.
DBput will return an error in the event that a disk block
fills with inseparable data.
Debugging information will be printed out on stderr if
DBdebug is set to one of DBDBCORE, DBDBERR, DBDBWARN, or
DBDBINFO. These values provide increasing amounts of infor-
mation, ranging from conditions that will cause abnormal
termination of the program (DBDBCORE) to call traces of the
user-accessible routines (DBDBINFO).
BUGS
DB(3X) DB(3X)
The `.pag' file will contain holes so that its apparent size
is about four times its actual content. Older UNIX systems
may create real file blocks for these holes when touched.
These files cannot be copied by normal means (cp(1), cat(1),
tp(1), tar(1), ar(1)) without filling in the holes. A com-
mand to permit copying of these files (with holes intact) is
provided (DBcp(1)).
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 65536 (64K) bytes).
Moreover, all key/content pairs that hash together must fit
on a single block.
DBdel does not physically reclaim file space, although it
does make it available for reuse. Use DBcp(1) or DBcopy if
file space must be reclaimed.
The order of keys presented by DBkey0 and DBkeyn depends on
a hashing function, not on anything interesting.
Since these routines use binary data for internal housekeep-
ing, databases created on a VAX cannot be directly manipu-
lated on a 3B, and vice versa. DBcvt(1) converts a database
generated on either of these machines to a form palatable on
the other.
SEE ALSO
DB(1), DBcp(1), DBcvt(1).
