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gdbm(3)                       GDBM User Reference                      gdbm(3)




NAME

       GDBM  - The GNU database manager.  Includes dbm and ndbm compatibility.


SYNOPSIS

       #include <gdbm.h>

       extern gdbm_error gdbm_errno;
       extern char *gdbm_version;
       GDBM_FILE gdbm_open (const char *name, int block_size,
                            int flags, int mode,
                            void (*fatal_func)(const char *));
       void gdbm_close (GDBM_FILE dbf);
       int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
       datum gdbm_fetch (GDBM_FILE dbf, datum key);
       int gdbm_delete (GDBM_FILE dbf, datum key);
       datum gdbm_firstkey (GDBM_FILE dbf);
       datum gdbm_nextkey (GDBM_FILE dbf, datum key);
       int gdbm_reorganize (GDBM_FILE dbf);
       void gdbm_sync (GDBM_FILE dbf);
       int gdbm_exists (GDBM_FILE dbf, datum key);
       const char *gdbm_strerror (gdbm_error errno);
       int gdbm_setopt (GDBM_FILE dbf, int option, int value, int size);
       int gdbm_fdesc (GDBM_FILE dbf);


   DBM Compatibility routines:
       #include <dbm.h>

       int dbminit (const char *name);
       int store (datum key, datum content);
       datum fetch (datum key);
       int delete (datum key);
       datum firstkey (void);
       datum nextkey (datum key);
       int dbmclose (void);


   NDBM Compatibility routines:
       #include <ndbm.h>

       DBM *dbm_open (const char *name, int flags, int mode);
       void dbm_close (DBM *file);
       datum dbm_fetch (DBM *file, datum key);
       int dbm_store (DBM *file, datum key, datum content, int flags);
       int dbm_delete (DBM *file, datum key);
       datum dbm_firstkey (DBM *file);
       datum dbm_nextkey (DBM *file, datum key);
       int dbm_error (DBM *file);
       int dbm_clearerr (DBM *file);
       int dbm_pagfno (DBM *file);
       int dbm_dirfno (DBM *file);
       int dbm_rdonly (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.

       This  manpage  is  a  short  description  of  the  GDBM library.  For a
       detailed discussion, including examples of the configuration and  usage
       recommendations,  refer to the GDBM Manual available in Texinfo format.
       To access it, run:

         info gdbm

       Should any discrepancies occur between this manpage and the  GDBM  Man-
       ual, the later shall be considered the authoritative source.

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

       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

       The  GDBM_NOMMAP  added to read_write by bitwise or instructs gdbm_open
       to disable the use of mmap(2).

       For the last three (writers of the database) the following may be added
       added to read_write by bitwise or:

       GDBM_SYNC
              Causes all database operations to be synchronized to the disk,

       GDBM_NOLOCK
              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 cre-
       ated. (*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 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:

       void gdbm_close (GDBM_FILE dbf);

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

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

       Dbf is the pointer returned by gdbm_open.  Key is the key  data.   Con-
       tent  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 in dbm or ndbm.  Your
       data can be as large as you want.

       To search for some data, use:

       datum gdbm_fetch (GDBM_FILE dbf, datum key);

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

       If the dptr element of the return value is NULL, the  gdbm_errno  vari-
       able  should  be  examined.   The value of GDBM_ITEM_NOT_FOUND means no
       data was found for that key.  Other value means an error occurred.

       Otherwise the return value is a pointer to the found data.  The storage
       space for the dptr element is allocated using malloc(3).  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:

       int gdbm_exists (GDBM_FILE dbf, datum 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 will be true.
       If  nothing  appropriate  is found, false is returned.  This routine is
       useful for checking for the existence of a record,  without  performing
       the memory allocation done by gdbm_fetch.

       To remove some data from the database:

       int gdbm_delete (GDBM_FILE dbf, datum 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.)

       datum gdbm_firstkey (GDBM_FILE dbf);
       datum gdbm_nextkey (GDBM_FILE dbf, datum 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,   inspect   the   gdbm_errno.    If   it   is
       GDBM_ITEM_NOT_FOUND,  there is no first key or next key.  Otherwise, an
       error occurred.

       Again, notice that dptr points to data allocated by malloc(3) and  gdbm
       will not free it for you.

       These  functions were intended to visit the database in read-only algo-
       rithms, 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  guaran-
       teed  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.

       int gdbm_reorganize (GDBM_FILE 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 reorga-
       nization.  (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  fol-
       lowing routine can be used to guarantee that the database is physically
       written to the disk file.

       void gdbm_sync (GDBM_FILE 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:

       const char *gdbm_strerror (gdbm_error errno);

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

       int gdbm_setopt (GDBM_FILE dbf, int option, int value, int 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 automati-
              cally 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 pre-
       vent multiple writers operating on the same file simultaneously.

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

       int gdbm_fdesc (GDBM_FILE 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).


LINKING

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

            gcc -o prog prog.c -lgdbm

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

            gcc -o prog proc.c -lgdbm -lgdbm_compat



BUG REPORTS

       Send bug reports to <bug-gdbm@gnu.org>.


SEE ALSO

       gdbm_dump(1), gdbm_load(1), gdbmtool(1).


AUTHORS

       by Philip A. Nelson, Jason Downs and Sergey Poznyakoff.


COPYRIGHT

       Copyright (C) 1990 - 2016 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 warranty of MERCHANTABILITY or  FIT-
       NESS  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.  If not, see <http://gnu.org/licenses/gpl.html>


CONTACTS

       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 maintainers by:
          e-mail:  downsj@downsj.com
       and
          e-mail:  gray@gnu.org




GDBM                             July 8, 2016                          gdbm(3)

gdbm 1.14.1 - Generated Sat Jan 6 09:09:48 CST 2018
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