dbm(3N) dbm(3N)
NAME
dbm, dbminit, dbmclose, fetch, store, delete, firstkey, nextkey -
database subroutines
SYNOPSIS
#include <dbm.h>
typedef struct {
char *dptr;
int dsize;
} datum;
dbminit(file)
char *file;
dbmclose()
datum fetch(key)
datum key;
store(key, dat)
datum key, dat;
delete(key)
datum key;
datum firstkey()
datum nextkey(key)
datum key;
DESCRIPTION
These functions maintain key/dat 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 -lyp.
keys and dats 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.
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dbm(3N) dbm(3N)
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 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 VALUE
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.
NOTES
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 (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.a
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