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GDBM(3)                                                                GDBM(3)



NAME
       GDBM - The GNU database manager.  Includes dbm and ndbm compatability. (Version 1.8.)

SYNOPSIS
       #include <gdbm.h>

       extern gdbm_error
       gdbm_errno

       extern char
       *gdbm_version

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

       void
       gdbm_close (dbf)
       GDBM_FILE dbf;

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

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

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

       datum
       gdbm_firstkey (dbf)
       GDBM_FILE dbf;

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

       int
       gdbm_reorganize (dbf)
       GDBM_FILE dbf;

       void
       gdbm_sync (dbf)
       GDBM_FILE dbf;

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

       char *
       gdbm_strerror (errno)
       gdbm_error errno;

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

       int
       gdbm_fdesc (dbf)
       GDBM_FILE dbf;


       DBM Compatability routines:

       #include <dbm.h>

       int
       dbminit (name)
       char *name;

       int
       store (key, content)
       datum key, content;

       datum
       fetch (key)
       datum key;

       int
       delete (key)
       datum key;

       datum
       firstkey ()

       datum
       nextkey (key)
       datum key;

       int
       dbmclose ()


       NDBM Compatability routines:

       #include <ndbm.h>

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

       void
       dbm_close (file)
       DBM *file;

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

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

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

       datum
       dbm_firstkey (file)
       DBM *file;

       datum
       dbm_nextkey (file)
       DBM *file;

       int
       dbm_error (file)
       DBM *file;

       int
       dbm_clearerr (file)
       DBM *file;

       int
       dbm_pagfno (file)
       DBM *file;

       int
       dbm_dirfno (file)
       DBM *file;

       int
       dbm_rdonly (file)
       DBM *file;



DESCRIPTION
       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 traversal 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 parameter 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 synchronized 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 gdbmerrno.h.)

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

       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 automati-
       cally 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 existance 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  col-
       lisions  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
         TRUE or FALSE.

         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 success.  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 writ-
       ers.  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).



LINKING
       This library is accessed by specifying -lgdbm as the last parameter to the compile line, e.g.:

            gcc -o prog prog.c -lgdbm



ENVIRONMENT VARIABLES
       NDBM_LOCK - When the NDBM interface is used, the database file is locked by default. Locking might degrade per-
       formance when used on a NFS share. This environment variable can be set to false to tell GDBM not to  lock  the
       database file.


SEE ALSO
       dbm, ndbm


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

       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 war-
       ranty 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:  philATcs.edu
         us-mail:  Philip A. Nelson
       Computer Science Department
       Western Washington University
       Bellingham, WA 98226

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




                                    5/19/99                            GDBM(3)