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NDBM(3)                    Library Functions Manual                    NDBM(3)



NAME
       ndbm,    dbm_open,   dbm_close,   dbm_fetch,   dbm_store,   dbm_delete,
       dbm_firstkey, dbm_nextkey, dbm_error, dbm_clearerr - data base  subrou-
       tines

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

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

       DBM *dbm_open(file, flags, mode)
       char *file;
       int flags, mode;

       void dbm_close (db)
       DBM *db;

       datum dbm_fetch(db, key)
       DBM *db;
       datum key;

       int dbm_store(db, key, content, flags)
       DBM *db;
       datum key, content;
       int flags;

       int dbm_delete(db, key)
       DBM *db;
       datum key;

       datum dbm_firstkey(db)
       DBM *db;

       datum dbm_nextkey(db)
       DBM *db;

       int dbm_error(db)
       DBM *db;

       int dbm_clearerr(db)
       DBM *db;

DESCRIPTION
       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.  This package
       replaces the earlier dbm(3X) library, which managed only a single data-
       base.

       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 dbm_open.  This
       will  open  and/or  create the files file.dir and file.pag depending on
       the flags parameter (see open(2V)).

       A database is closed by calling dbm_close.

       Once open, the data stored under a key is accessed by  dbm_fetch()  and
       data is placed under a key by dbm_store.  The flags field can be either
       DBM_INSERT or DBM_REPLACE.  DBM_INSERT will  only  insert  new  entries
       into  the  database and will not change an existing entry with the same
       key.  DBM_REPLACE will replace an existing entry if  it  has  the  same
       key.   A key (and its associated contents) is deleted by dbm_delete.  A
       linear pass through all keys in a database may be made, in  an  (appar-
       ently)   random  order,  by  use  of  dbm_firstkey()  and  dbm_nextkey.
       dbm_firstkey() will return the first key  in  the  database.   dbm_nex-
       tkey()  will  return the next key in the database.  This code will tra-
       verse the data base:

              for (key = dbm_firstkey(db); key.dptr != NULL;  key  =  dbm_nex-
              tkey(db))

       dbm_error()  returns  non-zero  when  an  error has occurred reading or
       writing the database.  dbm_clearerr() resets the error condition on the
       named database.

SEE ALSO
       ar(1V), cat(1V), cp(1), tar(1), open(2V), dbm(3X)

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.  If dbm_store called with a flags
       value of DBM_INSERT finds an  existing  entry  with  the  same  key  it
       returns 1.

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 4096 bytes).  Moreover all key/content pairs that
       hash together must fit on a single block.  dbm_store() will  return  an
       error in the event that a disk block fills with inseparable data.

       dbm_delete()  does  not physically reclaim file space, although it does
       make it available for reuse.

       The order of keys presented by dbm_firstkey() and dbm_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                        NDBM(3)