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     dbm(3X)                                                   dbm(3X)



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

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

          dbminit(file)
          char *file;

          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 data base.
          The functions will handle very large (a billion blocks) da-
          tabases and will access a keyed item in one or two file sys-
          tem accesses.  The functions are obtained with the loader
          option -ldbm.

          keys and contents are described by the datum typedef.  A da-
          tum 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 suf-
          fix.

          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.)

          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 as-
          sociated contents) is deleted by delete.  A linear pass
          through all keys in a database may be made, in an (apparent-
          ly) random order, by use of firstkey and nextkey.  firstkey



     April, 1990                                                     1

     dbm(3X)                                                   dbm(3X)



          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))


     RETURN VALUE
          All functions that return an int indicate errors with nega-
          tive values.  A zero return indicates okay.  Routines that
          return a datum indicate 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 UNIX 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 stat-
          ic 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 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.






















     2                                                     April, 1990