dbm(3x)
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dbminit, fetch, store, delete, firstkey, nextkey Subroutine
database subroutines in DG/UX
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SYNTAX
typedef struct {
char *dptr;
int dsize;
} datum;
dbminit(file)
char *file;
datum fetch(key)
datum key;
store(key, content)
datum key, content;
delete(key)
datum key;
datum firstkey()
datum nextkey(key)
datum key;
DESCRIPTION
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. The
functions are obtained with the loader option -ldbm.
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
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dbm(3x)
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. Nextkey will return the next key in the
database after the specified key. This code will traverse the
data base:
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.
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, cat, tp, tar, ar)
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 if 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 only.
DG/UX 4.00 Page 2
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