DBM(3D)                 COMMAND REFERENCE                 DBM(3D)



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

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

     dbminit(filename)
     char *filename;

     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 -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 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 filename.dir
     and filename.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 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



Printed 5/12/88                                                 1

DBM(3D)                 COMMAND REFERENCE                 DBM(3D)



     any key nextkey will return the next key in the database.
     This code will traverse the data base:

          for (key = firstkey(); key.dptr != NULL; key =
          nextkey(key))

     Dbmclose may be called to close the current database files.

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.

CAVEATS
     The database is not locked so concurrent access by reading
     and writing processes is dangerous.

     Only one database may be opened at a time, though multiple
     databases may be handled by closing one and opening another.

     The .pag file will contain holes so that its apparent size
     is about four times its actual content.  Older 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 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.

SEE ALSO
     ar(1), cp(1), tar(1), and tp(1).








Printed 5/12/88                                                 2

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