DBM(3X-B)        MISCELLANEOUS LIBRARY FUNCTIONS        DBM(3X-B)


NAME
     dbm:  dbminit,  fetch,  store,  delete, firstkey, nextkey --
     database subroutines


SYNOPSIS
     #include <dbm.h>

     typedef struct {
           char  dptr;
           int dsize;
     } datum;

     dbminit(char *file);

     datum fetch(datum key);

     store(datum key, datum content);

     delete(datum key);

     datum firstkey(void);

     datum nextkey(datum key);

     dbmclose(void);


DESCRIPTION
     These functions maintain key/content pairs  in  a  database.
     The functions handle very large (a billion blocks) databases
     and 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 database is stored in two files. One file is  a
     directory containing a bit map and has .dir as a suffix. The
     second file contains all data and has .pag as a 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 creat-
     ing zero-length .dir and .pag files.)

     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 can be made, in an apparently
     random  order,  by  using  firstkey  and  nextkey.  firstkey
     returns the first key in the database. With any key, nextkey
     returns  the  next  key  in the database. The following code
     traverses the database.



SUPER-UX            Last change: Oct  24, 1995                  1





DBM(3X-B)        MISCELLANEOUS LIBRARY FUNCTIONS        DBM(3X-B)


        for (key = firstkey();  key.dptr  !=  NULL;  key  =  nex-
        tkey(key))

     A  database  can  be  closed by calling dbmclose. Users must
     close a database before opening a new one.


RETURN VALUES
     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.


BUGS
     The .pag file contains holes so that its  apparent  size  is
     about  four times its actual content. Older UNIX systems can
     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 sin-
     gle block.  store returns 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.

     There  are  no  interlocks  and  no reliable cache flushing;
     thus, concurrent updating and reading is risky.




















SUPER-UX            Last change: Oct  24, 1995                  2