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DBM(3X)                                                                DBM(3X)



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

SYNOPSIS
       #include <&lt;dbm.h>&gt;

       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
       Note: the dbm() library has been superceded  by  ndbm(3),  and  is  now
       implemented using ndbm().

       These  functions  maintain key/content pairs in a data base.  The func-
       tions 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  speci-
       fies  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.)

       A  database  may be closed by calling dbmclose.  You must close a data-
       base 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 nex-
       tkey.  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 data base:

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

SEE ALSO
       ar(1V), cat(1V), cp(1), tar(1), ndbm(3)

DIAGNOSTICS
       All functions that return an int indicate errors with negative  values.
       A  zero  return indicates no error.  Routines that return a datum indi-
       cate 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 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(1V), tar(1), ar(1V))
       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.



                               24 November 1987                        DBM(3X)