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, content)
datum key, content;
delete(key)
datum key;
datum firstkey()
datum nextkey(key)
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 -lyp.  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  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/libyp.a