4.3BSD - /usr/man/cat3/ndbm.0

NDBM(3)                                                                NDBM(3)


NAME
       dbm_open,  dbm_close,  dbm_fetch,  dbm_store, dbm_delete, dbm_firstkey,
       dbm_nextkey, dbm_error, dbm_clearerr - data base subroutines

SYNOPSIS
       #include <ndbm.h>

       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
       database.

       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(2)).

       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_nextkey
       will return the next key in the database.  This code will traverse  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  writ‐
       ing the database.  Dbm_clearerr resets the error condition on the named
       database.

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.  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 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  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.

SEE ALSO
       dbm(3X)


4.3 Berkeley Distribution        May 20, 1986                          NDBM(3)