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Linux 2.6 - man page for gdbm (linux section 3)

GDBM(3) 			     Library Functions Manual				  GDBM(3)

       GDBM - The GNU database manager.  Includes dbm and ndbm compatability. (Version 1.8.3.)

       #include <gdbm.h>

       extern gdbm_error

       extern char

       gdbm_open (name, block_size, read_write, mode, fatal_func)
       char * name;
       int block_size, read_write, mode;
       void (*fatal_func) ();

       gdbm_close (dbf)
       GDBM_FILE dbf;

       gdbm_store (dbf, key, content, flag)
       GDBM_FILE dbf;
       datum key, content;
       int flag;

       gdbm_fetch (dbf, key)
       GDBM_FILE dbf;
       datum key;

       gdbm_delete (dbf, key)
       GDBM_FILE dbf;
       datum key;

       gdbm_firstkey (dbf)
       GDBM_FILE dbf;

       gdbm_nextkey (dbf, key)
       GDBM_FILE dbf;
       datum key;

       gdbm_reorganize (dbf)
       GDBM_FILE dbf;

       gdbm_sync (dbf)
       GDBM_FILE dbf;

       gdbm_exists (dbf, key)
       GDBM_FILE dbf;
       datum key;

       char *
       gdbm_strerror (errno)
       gdbm_error errno;

       gdbm_setopt (dbf, option, value, size)
       GDBM_FILE dbf;
       int option;
       int *value;
       int size;

       gdbm_fdesc (dbf)
       GDBM_FILE dbf;

       DBM Compatability routines:

       #include <dbm.h>

       dbminit (name)
       char *name;

       store (key, content)
       datum key, content;

       fetch (key)
       datum key;

       delete (key)
       datum key;

       firstkey ()

       nextkey (key)
       datum key;

       dbmclose ()

       NDBM Compatability routines:

       #include <ndbm.h>

       *dbm_open (name, flags, mode)
       char *name;
       int flags, mode;

       dbm_close (file)
       DBM *file;

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

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

       dbm_delete (file, key)
       DBM *file;
       datum key;

       dbm_firstkey (file)
       DBM *file;

       dbm_nextkey (file)
       DBM *file;

       dbm_error (file)
       DBM *file;

       dbm_clearerr (file)
       DBM *file;

       dbm_pagfno (file)
       DBM *file;

       dbm_dirfno (file)
       DBM *file;

       dbm_rdonly (file)
       DBM *file;

       GNU dbm is a library of routines that manages data files that contain key/data pairs.  The
       access provided is that of storing, retrieval, and deletion by key and a  non-sorted  tra-
       versal of all keys.  A process is allowed to use multiple data files at the same time.

       A  process  that  opens	a  gdbm file is designated as a "reader" or a "writer".  Only one
       writer may open a gdbm file and many readers may open the file.	Readers and  writers  can
       not open the gdbm file at the same time. The procedure for opening a gdbm file is:

	 GDBM_FILE dbf;

	 dbf = gdbm_open ( name, block_size, read_write, mode, fatal_func )

       Name  is  the  name of the file (the complete name, gdbm does not append any characters to
       this name).  Block_size is the size of a single transfer from disk to memory. This parame-
       ter  is	ignored  unless  the file is a new file.  The minimum size is 512.  If it is less
       than 512, dbm will use the stat block size for the file system.	Read_write can	have  one
       of the following values:
       GDBM_READER reader
       GDBM_WRITER writer
       GDBM_WRCREAT writer - if database does not exist create new one
       GDBM_NEWDB writer - create new database regardless if one exists
       For  the  last  three  (writers	of  the  database)  the  following  may be added added to
       read_write by bitwise or: GDBM_SYNC, which causes all database operations to  be  synchro-
       nized to the disk, and GDBM_NOLOCK, which prevents the library from performing any locking
       on the database file.  The option GDBM_FAST is now obsolete, since gdbm	defaults  to  no-
       sync mode.
       Mode  is the file mode (see chmod(2) and open(2)) if the file is created. (*Fatal_func) ()
       is a function for dbm to call if it detects a fatal error.  The	only  parameter  of  this
       function is a string.  If the value of 0 is provided, gdbm will use a default function.

       The return value dbf is the pointer needed by all other routines to access that gdbm file.
       If the return is the NULL pointer, gdbm_open was not successful.  The errors can be  found
       in  gdbm_errno for gdbm errors and in errno for system errors.  (For error codes, see gdb-

       In all of the following calls, the parameter dbf  refers  to  the  pointer  returned  from

       It  is  important  that	every  file  opened is also closed.  This is needed to update the
       reader/writer count on the file.  This is done by:

	 gdbm_close (dbf);

       The database is used by 3 primary routines.  The first stores data in the database.

	 ret = gdbm_store ( dbf, key, content, flag )

       Dbf is the pointer returned by gdbm_open.  Key is the key data.	Content is the data to be
       associated with the key.  Flag can have one of the following values:
       GDBM_INSERT insert only, generate an error if key exists
       GDBM_REPLACE replace contents if key exists.

       If a reader calls gdbm_store, the return value will be -1.  If called with GDBM_INSERT and
       key is in the database, the return value will be 1.  Otherwise, the return value is 0.

       NOTICE: If you store data for a key that is already in the data base,  gdbm  replaces  the
       old data with the new data if called with GDBM_REPLACE.	You do not get two data items for
       the same key and you do not get an error from gdbm_store.

       NOTICE: The size in gdbm is not restricted like dbm or ndbm.  Your data can be as large as
       you want.

       To search for some data:

	 content = gdbm_fetch ( dbf, key )

       Dbf is the pointer returned by gdbm_open.  Key is the key data.

       If  the dptr element of the return value is NULL, no data was found.  Otherwise the return
       value is a pointer to the found data.  The storage space for the dptr element is allocated
       using  malloc(3C).   Gdbm  does	not automatically free this data.  It is the programmer's
       responsibility to free this storage when it is no longer needed.

       To search for some data, without retrieving it:

	 ret = gdbm_exists ( dbf, key )

       Dbf is the pointer returned by gdbm_open.  Key is the key data to search for.

       If the key is found within the database, the return value ret will be  true.   If  nothing
       appropiate is found, ret will be false.	This routine is useful for checking for the exis-
       tance of a record, without performing the memory allocation done by gdbm_fetch.

       To remove some data from the database:

	 ret = gdbm_delete ( dbf, key )

       Dbf is the pointer returned by gdbm_open.  Key is the key data.

       The return value is -1 if the item is not present or  the  requester  is  a  reader.   The
       return value is 0 if there was a successful delete.

       The  next  two routines allow for accessing all items in the database.  This access is not
       key sequential, but it is guaranteed to visit every key in the database once.  (The  order
       has to do with the hash values.)

	 key = gdbm_firstkey ( dbf )

	 nextkey = gdbm_nextkey ( dbf, key )

       Dbf is the pointer returned by gdbm_open. Key is the key data.

       The  return  values  are  both  of type datum.  If the dptr element of the return value is
       NULL, there is no first key or next key.  Again notice that dptr points to data	allocated
       by malloc(3C) and gdbm will not free it for you.

       These functions were intended to visit the database in read-only algorithms, for instance,
       to validate the database or similar operations.

       File `visiting' is based on a `hash table'.  gdbm_delete re-arranges  the  hash	table  to
       make  sure  that  any  collisions  in the table do not leave some item `un-findable'.  The
       original key order is NOT guaranteed to remain unchanged in ALL instances.  It is possible
       that some key will not be visited if a loop like the following is executed:

	  key = gdbm_firstkey ( dbf );
	  while ( key.dptr ) {
	     nextkey = gdbm_nextkey ( dbf, key );
	     if ( some condition ) {
		gdbm_delete ( dbf, key );
		free ( key.dptr );
	     key = nextkey;

       The following routine should be used very infrequently.

	 ret = gdbm_reorganize ( dbf )

       If  you	have  had  a lot of deletions and would like to shrink the space used by the gdbm
       file, this routine will reorganize the database.  Gdbm will not shorten the  length  of	a
       gdbm file except by using this reorganization.  (Deleted file space will be reused.)

       Unless  your database was opened with the GDBM_SYNC flag, gdbm does not wait for writes to
       be flushed to the disk before continuing.  The following routine can be used to	guarantee
       that the database is physically written to the disk file.

	 gdbm_sync ( dbf )

       It  will  not  return until the disk file state is syncronized with the in-memory state of
       the database.

       To convert a gdbm error code into English text, use this routine:

	 ret = gdbm_strerror ( errno )

       Where errno is of type gdbm_error, usually the global variable gdbm_errno.  The appropiate
       phrase is returned.

       Gdbm now supports the ability to set certain options on an already open database.

	 ret = gdbm_setopt ( dbf, option, value, size )

       Where  dbf  is  the  return  value from a previous call to gdbm_open, and option specifies
       which option to set.  The valid options are currently:

	 GDBM_CACHESIZE - Set the size of the internal bucket
	 cache. This option may only be set once on each GDBM_FILE
	 descriptor, and is set automatically to 100 upon the first
	 access to the database.

	 GDBM_FASTMODE - Set fast mode to either on or off.  This
	 allows fast mode to be toggled on an already open and
	 active database. value (see below) should be set to either
	 TRUE or FALSE.  This option is now obsolete.

	 GDBM_SYNCMODE - Turn on or off file system synchronization operations.
	 This setting defaults to off; value (see below) should be set to either

	 GDBM_CENTFREE - Set central free block pool to either on or off.
	 The default is off, which is how previous versions of Gdbm
	 handled free blocks. If set, this option causes all subsequent free
	 blocks to be placed in the global pool, allowing (in thoery)
	 more file space to be reused more quickly. value (see below) should
	 be set to either TRUE or FALSE.
	 NOTICE: This feature is still under study.

	 GDBM_COALESCEBLKS - Set free block merging to either on or off.
	 The default is off, which is how previous versions of Gdbm
	 handled free blocks. If set, this option causes adjacent free blocks
	 to be merged. This can become a CPU expensive process with time, though,
	 especially if used in conjunction with GDBM_CENTFREE. value
	 (see below) should be set to either TRUE or FALSE.
	 NOTICE: This feature is still under study.

       value is the value to set option to, specified as an integer pointer.  size is the size of
       the  data  pointed  to by value.  The return value will be -1 upon failure, or 0 upon suc-
       cess.  The global variable gdbm_errno will be set upon failure.

       For instance, to set a database to use a cache of 10, after opening it with gdbm_open, but
       prior to accessing it in any way, the following code could be used:

	 int value = 10;

	 ret = gdbm_setopt( dbf, GDBM_CACHESIZE, &value, sizeof(int));

       If  the	database was opened with the GDBM_NOLOCK flag, the user may wish to perform their
       own file locking on the database file in order to prevent multiple  writers  operating  on
       the same file simultaneously.

       In order to support this, the gdbm_fdesc routine is provided.

	 ret = gdbm_fdesc ( dbf )

       Where dbf is the return value from a previous call to gdbm_open.  The return value will be
       the file descriptor of the database.

       The following two external variables may be useful:

       gdbm_errno is the variable that contains more information about gdbm errors.  (gdbm.h  has
       the definitions of the error values and defines gdbm_errno as an external variable.)
       gdbm_version is the string containing the version information.

       There  are  a  few  more things of interest.  First, gdbm files are not "sparse".  You can
       copy them with the UNIX cp(1) command and they will not expand  in  the	copying  process.
       Also,  there  is a compatibility mode for use with programs that already use UNIX dbm.  In
       this compatibility mode, no gdbm file pointer is required by the programmer, and only  one
       file  may be opened at a time.  All users in compatibility mode are assumed to be writers.
       If the gdbm file is a read only, it will fail as a writer, but will also try to open it as
       a  reader.   All  returned pointers in datum structures point to data that gdbm WILL free.
       They should be treated as static pointers (as standard UNIX dbm does).

       This library is accessed by specifying -lgdbm as the last parameter to the  compile  line,

	    gcc -o prog prog.c -lgdbm

       If  you wish to use the dbm or ndbm compatibility routines, you must link in the gdbm_com-
       pat library as well.  For example:

	    gcc -o prog proc.c -lgdbm -lgdbm_compat

       dbm, ndbm

       by Philip A. Nelson and Jason Downs.  Copyright (C) 1990 - 1999 Free Software  Foundation,

       GDBM is free software; you can redistribute it and/or modify it under the terms of the GNU
       General Public License as published by the Free Software Foundation; either version 1,  or
       (at your option) any later version.

       GDBM  is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
       even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
       GNU General Public License for more details.

       You should have received a copy of the GNU General Public License along with GDBM; see the
       file COPYING.  If not, write to the Free Software Foundation, 675 Mass Ave, Cambridge,  MA
       02139, USA.

       You may contact the original author by:
	  e-mail:  phil@cs.wwu.edu
	 us-mail:  Philip A. Nelson
       Computer Science Department
       Western Washington University
       Bellingham, WA 98226

       You may contact the current maintainer by:
	  e-mail:  downsj@downsj.com

					    10/15/2002					  GDBM(3)

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