term% cat index.txt DB(3X) DB(3X)
NAME
DBopen, DBclose, DBget, DBput, DBdel, DBkey0, DBkeyn, DBlock, DBunlock,
DBsync, DBapp, DBins, DBcopy - database subroutines
SYNOPSIS
#include <DB.h>
typedef struct {
char * dptr;
int dsize;
} datum;
int DBdebug;
DBFILE * DBopen(file, flags)
char * file;
int flags;
datum DBget(dp, key)
DBFILE * dp;
datum key;
int DBput(dp, key, content)
DBFILE * dp;
datum key, content;
int DBdel(dp, key)
DBFILE * dp;
datum key;
datum DBkey0(dp)
DBFILE * dp;
datum DBkeyn(dp, key)
DBFILE * dp;
datum key;
void DBclose(dp)
DBFILE * dp;
int DBlock(dp)
DBFILE * dp;
int DBunlock(dp)
DBFILE * dp;
void DBsync(dp)
DBFILE * dp;
int DBapp(dp, key, content)
DBFILE * dp;
datum key, content;
int DBins(dp, key, content)
DBFILE * dp;
datum key, content;
int DBcopy(sdp, ddp)
DBFILE * sdp, ddp;
DESCRIPTION
These functions maintain key/content pairs in a data base. The func‐
tions will handle databases as large as the filesystem can handle and
will access a keyed item in one or two file system accesses. The func‐
tions are obtained with the loader option -lDB.
Keys and contents are described by the datum typedef. A datum speci‐
fies 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 direc‐
tory 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 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 spec‐
ified and the files exist, they are truncated. The only other meaning‐
ful 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, respec‐
tively. 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 re‐
turn 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 us‐
ing 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 DB‐
close.
DIAGNOSTICS
All functions that return an int indicate errors with negative 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 pro‐
vide increasing amounts of information, ranging from conditions that
will cause abnormal termination of the program (DBDBCORE) to call
traces of the user-accessible routines (DBDBINFO).
BUGS
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 command to permit copying of these files (with holes in‐
tact) 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 re‐
claimed.
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 housekeeping, data‐
bases created on a VAX cannot be directly manipulated on a 3B, and vice
versa. DBcvt(1) converts a database generated on either of these ma‐
chines to a form palatable on the other.
SEE ALSO
DB(1), DBcp(1), DBcvt(1).
DB(3X)