1GDBM(3)                       GDBM User Reference                      GDBM(3)
2
3
4

NAME

6       GDBM - The GNU database manager.  Includes dbm and ndbm compatibility.
7

SYNOPSIS

9       #include <gdbm.h>
10
11       extern gdbm_error gdbm_errno;
12       extern char *gdbm_version;
13       GDBM_FILE gdbm_open (const char *name, int block_size,
14                            int flags, int mode,
15                            void (*fatal_func)(const char *));
16       int gdbm_close (GDBM_FILE dbf);
17       int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
18       datum gdbm_fetch (GDBM_FILE dbf, datum key);
19       int gdbm_delete (GDBM_FILE dbf, datum key);
20       datum gdbm_firstkey (GDBM_FILE dbf);
21       datum gdbm_nextkey (GDBM_FILE dbf, datum key);
22       int gdbm_reorganize (GDBM_FILE dbf);
23       int gdbm_sync (GDBM_FILE dbf);
24       int gdbm_exists (GDBM_FILE dbf, datum key);
25       const char *gdbm_strerror (gdbm_error errno);
26       int gdbm_setopt (GDBM_FILE dbf, int option, int value, int size);
27       int gdbm_fdesc (GDBM_FILE dbf);
28
29   DBM Compatibility routines:
30       #include <dbm.h>
31
32       int dbminit (const char *name);
33       int store (datum key, datum content);
34       datum fetch (datum key);
35       int delete (datum key);
36       datum firstkey (void);
37       datum nextkey (datum key);
38       int dbmclose (void);
39
40   NDBM Compatibility routines:
41       #include <ndbm.h>
42
43       DBM *dbm_open (const char *name, int flags, int mode);
44       void dbm_close (DBM *file);
45       datum dbm_fetch (DBM *file, datum key);
46       int dbm_store (DBM *file, datum key, datum content, int flags);
47       int dbm_delete (DBM *file, datum key);
48       datum dbm_firstkey (DBM *file);
49       datum dbm_nextkey (DBM *file, datum key);
50       int dbm_error (DBM *file);
51       int dbm_clearerr (DBM *file);
52       int dbm_pagfno (DBM *file);
53       int dbm_dirfno (DBM *file);
54       int dbm_rdonly (DBM *file);
55

DESCRIPTION

57       GNU  dbm  is a library of routines that manages data files that contain
58       key/data pairs.  The access provided is that of storing, retrieval, and
59       deletion  by  key and a non-sorted traversal of all keys.  A process is
60       allowed to use multiple data files at the same time.
61
62       This manpage is a  short  description  of  the  GDBM  library.   For  a
63       detailed  discussion, including examples of the configuration and usage
64       recommendations, refer to the GDBM Manual available in Texinfo  format.
65       To access it, run:
66
67         info gdbm
68
69       Should  any  discrepancies occur between this manpage and the GDBM Man‐
70       ual, the later shall be considered the authoritative source.
71
72       A process that opens a gdbm file is  designated  as  a  "reader"  or  a
73       "writer".   Only  one  writer may open a gdbm file and many readers may
74       open the file.  Readers and writers can not open the gdbm file  at  the
75       same time. The procedure for opening a gdbm file is:
76
77       GDBM_FILE gdbm_open (const char *name, int block_size,
78                            int flags, int mode,
79                            void (*fatal_func)(const char *));
80
81       Name  is  the name of the file (the complete name, gdbm does not append
82       any characters to this name).  Block_size  is  the  size  of  a  single
83       transfer from disk to memory. This parameter is ignored unless the file
84       is a new file.  The minimum size is 512.  If it is less than  512,  dbm
85       will  use the stat block size for the file system.  Read_write can have
86       one of the following values:
87
88       GDBM_READER
89              reader
90
91       GDBM_WRITER
92              writer
93
94       GDBM_WRCREAT
95              writer - if database does not exist create new one
96
97       GDBM_NEWDB
98              writer - create new database regardless if one exists
99
100       The GDBM_NOMMAP added to read_write by bitwise or  instructs  gdbm_open
101       to disable the use of mmap(2).
102
103       For the last three (writers of the database) the following may be added
104       added to read_write by bitwise or:
105
106       GDBM_SYNC
107              Causes all database operations to be synchronized to the disk,
108
109       GDBM_NOLOCK
110              Prevents the library from performing any locking on the database
111              file.
112
113       The  option  GDBM_FAST  is now obsolete, since gdbm defaults to no-sync
114       mode.
115
116       Mode is the file mode (see chmod(2) and open(2)) if the  file  is  cre‐
117       ated.  (*Fatal_func)  ()  is a function for dbm to call if it detects a
118       fatal error. The only parameter of this function is a string.   If  the
119       value of 0 is provided, gdbm will use a default function.
120
121       The  return value is the pointer needed by all other routines to access
122       that gdbm file.  If the return is the NULL pointer, gdbm_open  was  not
123       successful.   The errors can be found in gdbm_errno for gdbm errors and
124       in errno for system errors.  (For error codes, see gdbmerrno.h.)
125
126       In all of the following calls, the parameter dbf refers to the  pointer
127       returned from gdbm_open.
128
129       It  is important that every file opened is also closed.  This is needed
130       to update the reader/writer count on the file.  This is done by:
131
132       int gdbm_close (GDBM_FILE dbf);
133
134       The database is used by 3 primary routines.  The first stores  data  in
135       the database.
136
137       int gdbm_store (GDBM_FILE dbf, datum key, datum content, int flag);
138
139       Dbf  is  the pointer returned by gdbm_open.  Key is the key data.  Con‐
140       tent is the data to be associated with the key.  Flag can have  one  of
141       the following values:
142
143       GDBM_INSERT
144              Insert only, generate an error if key exists;
145
146       GDBM_REPLACE
147              Replace contents if key exists.
148
149       If  a reader calls gdbm_store, the return value will be  -1.  If called
150       with GDBM_INSERT and key is in the database, the return value  will  be
151       1.  Otherwise, the return value is 0.
152
153       NOTICE:  If  you store data for a key that is already in the data base,
154       gdbm  replaces  the  old  data  with  the  new  data  if  called   with
155       GDBM_REPLACE.   You  do not get two data items for the same key and you
156       do not get an error from gdbm_store.
157
158       NOTICE: The size in gdbm is not restricted like in dbm or  ndbm.   Your
159       data can be as large as you want.
160
161       To search for some data, use:
162
163       datum gdbm_fetch (GDBM_FILE dbf, datum key);
164
165       Dbf is the pointer returned by gdbm_open.  Key is the key data.
166
167       If  the  dptr element of the return value is NULL, the gdbm_errno vari‐
168       able should be examined.  The value  of  GDBM_ITEM_NOT_FOUND  means  no
169       data was found for that key.  Other value means an error occurred.
170
171       Otherwise the return value is a pointer to the found data.  The storage
172       space for the dptr element is allocated using malloc(3).  Gdbm does not
173       automatically free this data.  It is the programmer's responsibility to
174       free this storage when it is no longer needed.
175
176       To search for some data, without retrieving it:
177
178       int gdbm_exists (GDBM_FILE dbf, datum key);
179
180       Dbf is the pointer returned by gdbm_open.   Key  is  the  key  data  to
181       search for.
182
183       If the key is found within the database, the return value will be true.
184       If nothing appropriate is found, false is returned.   This  routine  is
185       useful  for  checking for the existence of a record, without performing
186       the memory allocation done by gdbm_fetch.
187
188       To remove some data from the database:
189
190       int gdbm_delete (GDBM_FILE dbf, datum key);
191
192       Dbf is the pointer returned by gdbm_open.  Key is the key data.
193
194       The return value is -1 if the item is not present or the requester is a
195       reader.  The return value is 0 if there was a successful delete.
196
197       The  next  two  routines allow for accessing all items in the database.
198       This access is not key sequential, but it is guaranteed to visit  every
199       key in the database once.  (The order has to do with the hash values.)
200
201       datum gdbm_firstkey (GDBM_FILE dbf);
202       datum gdbm_nextkey (GDBM_FILE dbf, datum key);
203
204       Dbf is the pointer returned by gdbm_open. Key is the key data.
205
206       The  return  values are both of type datum.  If the dptr element of the
207       return  value  is   NULL,   inspect   the   gdbm_errno.    If   it   is
208       GDBM_ITEM_NOT_FOUND,  there is no first key or next key.  Otherwise, an
209       error occurred.
210
211       Again, notice that dptr points to data allocated by malloc(3) and  gdbm
212       will not free it for you.
213
214       These  functions were intended to visit the database in read-only algo‐
215       rithms, for instance, to validate the database or similar operations.
216
217       File `visiting' is based on a `hash  table'.   gdbm_delete  re-arranges
218       the  hash  table  to  make sure that any collisions in the table do not
219       leave some item `un-findable'.  The original key order is  NOT  guaran‐
220       teed  to  remain  unchanged in ALL instances.  It is possible that some
221       key will not be visited if a loop like the following is executed:
222
223            key = gdbm_firstkey (dbf);
224            while (key.dptr)
225              {
226                nextkey = gdbm_nextkey (dbf, key);
227                if (some condition)
228                  gdbm_delete ( dbf, key );
229                free (key.dptr);
230                key = nextkey;
231              }
232
233       The following routine should be used very infrequently.
234
235       int gdbm_reorganize (GDBM_FILE dbf);
236
237       If you have had a lot of deletions and would like to shrink  the  space
238       used by the gdbm file, this routine will reorganize the database.  Gdbm
239       will not shorten the length of a gdbm file except by using this reorga‐
240       nization.  (Deleted file space will be reused.)
241
242       Unless  your database was opened with the GDBM_SYNC flag, gdbm does not
243       wait for writes to be flushed to the disk before continuing.  The  fol‐
244       lowing routine can be used to guarantee that the database is physically
245       written to the disk file.
246
247       int gdbm_sync (GDBM_FILE dbf);
248
249       It will not return until the disk file state is  syncronized  with  the
250       in-memory state of the database.
251
252       To convert a gdbm error code into English text, use this routine:
253
254       const char *gdbm_strerror (gdbm_error errno);
255
256       Gdbm now supports the ability to set certain options on an already open
257       database.
258
259       int gdbm_setopt (GDBM_FILE dbf, int option, int value, int size);
260
261       Where dbf is the return value from a previous call  to  gdbm_open,  and
262       option specifies which option to set.  The valid options are currently:
263
264       GDBM_CACHESIZE
265              Set  the size of the internal bucket cache. This option may only
266              be set once on each GDBM_FILE descriptor, and is  set  automati‐
267              cally to 100 upon the first access to the database.
268
269       GDBM_FASTMODE
270               Set fast mode to either on or off.  This allows fast mode to be
271              toggled on an already  open  and  active  database.  value  (see
272              below)  should  be  set to either TRUE or FALSE.  This option is
273              now obsolete.
274
275       GDBM_SYNCMODE
276              Turn on or off file  system  synchronization  operations.   This
277              setting  defaults  to  off;  value  (see below) should be set to
278              either TRUE or FALSE.
279
280       GDBM_CENTFREE
281              Set central free block pool to either on or off.  The default is
282              off, which is how previous versions of Gdbm handled free blocks.
283              If set, this option causes all  subsequent  free  blocks  to  be
284              placed  in the global pool, allowing (in thoery) more file space
285              to be reused more quickly. value (see below) should  be  set  to
286              either  TRUE  or  FALSE.   NOTICE:  This  feature is still under
287              study.
288
289       GDBM_COALESCEBLKS
290              Set free block merging to either on or off.  The default is off,
291              which  is  how previous versions of Gdbm handled free blocks. If
292              set, this option causes adjacent free blocks to be merged.  This
293              can become a CPU expensive process with time, though, especially
294              if used in conjunction with  GDBM_CENTFREE.  value  (see  below)
295              should  be set to either TRUE or FALSE.  NOTICE: This feature is
296              still under study.
297
298       value is the value to set option to, specified as an  integer  pointer.
299       size  is  the  size  of the data pointed to by value.  The return value
300       will be -1 upon failure,  or  0  upon  success.   The  global  variable
301       gdbm_errno will be set upon failure.
302
303       For  instance, to set a database to use a cache of 10, after opening it
304       with gdbm_open, but prior to accessing it in  any  way,  the  following
305       code could be used:
306
307            int value = 10;
308
309            ret = gdbm_setopt( dbf, GDBM_CACHESIZE, &value, sizeof(int));
310
311       If the database was opened with the GDBM_NOLOCK flag, the user may wish
312       to perform their own file locking on the database file in order to pre‐
313       vent multiple writers operating on the same file simultaneously.
314
315       In order to support this, the gdbm_fdesc routine is provided.
316
317       int gdbm_fdesc (GDBM_FILE dbf);
318
319       Where  dbf  is the return value from a previous call to gdbm_open.  The
320       return value will be the file descriptor of the database.
321
322       The following two external variables may be useful:
323
324       gdbm_errno is the variable that contains more  information  about  gdbm
325       errors.   (gdbm.h  has  the definitions of the error values and defines
326       gdbm_errno as an external variable.)
327
328       gdbm_version is the string containing the version information.
329
330       There are a few more things of interest.  First,  gdbm  files  are  not
331       "sparse".   You can copy them with the UNIX cp(1) command and they will
332       not expand in the copying process.  Also, there is a compatibility mode
333       for use with programs that already use UNIX dbm.  In this compatibility
334       mode, no gdbm file pointer is required by the programmer, and only  one
335       file  may  be  opened  at  a time.  All users in compatibility mode are
336       assumed to be writers.  If the gdbm file is a read only, it  will  fail
337       as  a  writer,  but will also try to open it as a reader.  All returned
338       pointers in datum structures point to data that gdbm WILL  free.   They
339       should be treated as static pointers (as standard UNIX dbm does).
340

LINKING

342       This  library is accessed by specifying -lgdbm as the last parameter to
343       the compile line, e.g.:
344
345            gcc -o prog prog.c -lgdbm
346
347       If you wish to use the dbm or ndbm  compatibility  routines,  you  must
348       link in the gdbm_compat library as well.  For example:
349
350            gcc -o prog proc.c -lgdbm -lgdbm_compat
351
352

BUG REPORTS

354       Send bug reports to <bug-gdbm@gnu.org>.
355

SEE ALSO

357       gdbm_dump(1), gdbm_load(1), gdbmtool(1).
358

AUTHORS

360       by Philip A. Nelson, Jason Downs and Sergey Poznyakoff.
361
363       Copyright © 1990 - 2016 Free Software Foundation, Inc.
364
365       GDBM  is  free software; you can redistribute it and/or modify it under
366       the terms of the GNU General Public License as published  by  the  Free
367       Software  Foundation;  either  version 1, or (at your option) any later
368       version.
369
370       GDBM is distributed in the hope that it will be useful, but WITHOUT ANY
371       WARRANTY;  without even the implied warranty of MERCHANTABILITY or FIT‐
372       NESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License  for
373       more details.
374
375       You should have received a copy of the GNU General Public License along
376       with GDBM.  If not, see <http://gnu.org/licenses/gpl.html>
377

CONTACTS

379       You may contact the original author by:
380          e-mail:  phil@cs.wwu.edu
381         us-mail:  Philip A. Nelson
382       Computer Science Department
383       Western Washington University
384       Bellingham, WA 98226
385
386       You may contact the current maintainers by:
387          e-mail:  downsj@downsj.com
388       and
389          e-mail:  gray@gnu.org
390
391
392
393
394GDBM                             July 1, 2018                          GDBM(3)
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