dbm(3X)            UNIX System V(BSD Compatibility Package)             dbm(3X)


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

SYNOPSIS
      cc [ flag. . . ] file . . . -ldbm

      #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
      Note: the dbm library has been superceded by ndbm(3), and is now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 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 -libdbm.

      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 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.


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dbm(3X)            UNIX System V(BSD Compatibility Package)             dbm(3X)


      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
      data base:

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

SEE ALSO
      ndbm(3)

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.










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