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BSD 2.11 - man page for ndbm (bsd section 3)

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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 functions 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 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 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  (apparently) 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_nextkey(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.

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 (cur-
       rently 4096 bytes).  Moreover all key/content pairs that hash together must fit on a  sin-
       gle  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)
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