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30 .\" @(#)dbopen.3 8.5 (Berkeley) 1/2/94
31 .\"
32 .Dd December 16, 2010
33 .Dt DBOPEN 3
34 .Os
35 .Sh NAME
36 .Nm dbopen ,
37 .Nm db
38 .Nd database access methods
39 .Sh SYNOPSIS
40 .In sys/types.h
41 .In limits.h
42 .In db.h
43 .In fcntl.h
44 .Ft DB *
45 .Fn dbopen "const char *file" "int flags" "mode_t mode" \
46 "DBTYPE type" "const void *openinfo"
47 .Sh DESCRIPTION
48 .Nm
49 is the library interface to database files.
50 The supported file formats are btree, hashed, and UNIX file oriented.
51 The btree format is a representation of a sorted, balanced tree
52 structure.
53 The hashed format is an extensible, dynamic hashing scheme.
54 The flat-file format is a byte stream file with fixed or variable
55 length records.
56 The formats and file format specific information are described in
57 detail in their respective manual pages
58 .Xr btree 3 ,
59 .Xr hash 3 ,
60 and
61 .Xr recno 3 .
62 .Pp
63 The
64 .Fn dbopen
65 function opens
66 .Fa file
67 for reading and/or writing.
68 Files never intended to be preserved on disk may be created by setting
69 the file parameter to
70 .Dv NULL .
71 .Pp
72 The
73 .Fa flags
74 and
75 .Fa mode
76 arguments are as specified to the
77 .Xr open 2
78 routine, however, only the
79 .Dv O_CREAT ,
80 .Dv O_EXCL ,
81 .Dv O_EXLOCK ,
82 .Dv O_NONBLOCK ,
83 .Dv O_RDONLY ,
84 .Dv O_RDWR ,
85 .Dv O_SHLOCK ,
86 and
87 .Dv O_TRUNC
88 flags are meaningful.
89 (Note, opening a database file
90 .Dv O_WRONLY
91 is not possible.)
92 .\"Three additional options may be specified by or'ing
93 .\"them into the
94 .\".Fa flags
95 .\"argument.
96 .\".Pp
97 .\".Dv DB_LOCK
98 .\"Do the necessary locking in the database to support concurrent access.
99 .\"If concurrent access isn't needed or the database is read-only this
100 .\"flag should not be set, as it tends to have an associated performance
101 .\"penalty.
102 .\".Pp
103 .\".Dv DB_SHMEM
104 .\"Place the underlying memory pool used by the database in shared
105 .\"memory.
106 .\"Necessary for concurrent access.
107 .\".Pp
108 .\".Dv DB_TXN
109 .\"Support transactions in the database.
110 .\"The
111 .\".Dv DB_LOCK
112 .\"and
113 .\".Dv DB_SHMEM
114 .\"flags must be set as well.
115 .Pp
116 The
117 .Fa type
118 argument is of type
119 .Vt DBTYPE
120 (as defined in the
121 .In db.h
122 include file) and may be set to
123 .Dv DB_BTREE ,
124 .Dv DB_HASH ,
125 or
126 .Dv DB_RECNO .
127 .Pp
128 The
129 .Fa openinfo
130 argument is a pointer to an access method specific structure described
131 in the access method's manual page.
132 If
133 .Fa openinfo
134 is
135 .Dv NULL ,
136 each access method will use defaults appropriate for the system and
137 the access method.
138 .Ss The DB Structure
139 The
140 .Fn dbopen
141 function returns a pointer to a DB structure on success and
142 .Dv NULL
143 on error.
144 The DB structure is defined in the
145 .In db.h
146 include file, and contains at least the following fields:
147 .Bd -literal -offset indent
148 typedef struct {
149 DBTYPE type;
150 int (*close)(const DB *db);
151 int (*del)(const DB *db, const DBT *key, u_int flags);
152 int (*fd)(const DB *db);
153 int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
154 int (*put)(const DB *db, DBT *key, const DBT *data,
155 u_int flags);
156 int (*sync)(const DB *db, u_int flags);
157 int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
158 } DB;
159 .Ed
160 .Pp
161 These elements describe a database type and a set of functions
162 performing various actions.
163 These functions take a pointer to a structure as returned by
164 .Nm ,
165 and sometimes one or more pointers to key/data structures and a flag
166 value.
167 .Bl -tag -width closex -offset indent
168 .It Fa type
169 The type of the underlying access method (and file format).
170 .It Fa close
171 A pointer to a routine to flush any cached information to disk, free
172 any allocated resources, and close the underlying file(s).
173 Since key/data pairs may be cached in memory, failing to sync the file
174 with a
175 .Fa close
176 or
177 .Fa sync
178 function may result in inconsistent or lost information.
179 .Fa close
180 routines return \-1 on error (setting
181 .Va errno )
182 and 0 on success.
183 .It Fa del
184 A pointer to a routine to remove key/data pairs from the database.
185 .Pp
186 The parameter
187 .Fa flag
188 may be set to the following value:
189 .Bl -tag -width R_CURSORX
190 .It Dv R_CURSOR
191 Delete the record referenced by the cursor.
192 The cursor must have previously been initialized.
193 .El
194 .Pp
195 .Fa delete
196 routines return \-1 on error (setting
197 .Va errno ) ,
198 0 on success, and 1 if the specified
199 .Fa key
200 was not in the file.
201 .It Fa fd
202 A pointer to a routine which returns a file descriptor representative
203 of the underlying database.
204 A file descriptor referencing the same file will be returned to all
205 processes which call
206 .Nm
207 with the same
208 .Fa file
209 name.
210 This file descriptor may be safely used as an argument to the
211 .Xr fcntl 2
212 and
213 .Xr flock 2
214 locking functions.
215 The file descriptor is not necessarily associated with any of the
216 underlying files used by the access method.
217 No file descriptor is available for in memory databases.
218 .Fa fd
219 routines return \-1 on error (setting
220 .Va errno ) ,
221 and the file descriptor on success.
222 .It Fa get
223 A pointer to a routine which is the interface for keyed retrieval from
224 the database.
225 The address and length of the data associated with the specified
226 .Fa key
227 are returned in the structure referenced by
228 .Fa data .
229 .Fa get
230 routines return \-1 on error (setting
231 .Va errno ) ,
232 0 on success, and 1 if the
233 .Fa key
234 was not in the file.
235 .It Fa put
236 A pointer to a routine to store key/data pairs in the database.
237 .Pp
238 The parameter
239 .Fa flag
240 may be set to one of the following values:
241 .Bl -tag -width R_NOOVERWRITEX
242 .It Dv R_CURSOR
243 Replace the key/data pair referenced by the cursor.
244 The cursor must have previously been initialized.
245 .It Dv R_IAFTER
246 Append the data immediately after the data referenced by
247 .Fa key ,
248 creating a new key/data pair.
249 The record number of the appended key/data pair is returned in the
250 .Fa key
251 structure.
252 (Applicable only to the
253 .Dv DB_RECNO
254 access method.)
255 .It Dv R_IBEFORE
256 Insert the data immediately before the data referenced by
257 .Fa key ,
258 creating a new key/data pair.
259 The record number of the inserted key/data pair is returned in the
260 .Fa key
261 structure.
262 (Applicable only to the
263 .Dv DB_RECNO
264 access method.)
265 .It Dv R_NOOVERWRITE
266 Enter the new key/data pair only if the key does not previously
267 exist.
268 .It Dv R_SETCURSOR
269 Store the key/data pair, setting or initializing the position of the
270 cursor to reference it.
271 (Applicable only to the
272 .Dv DB_BTREE
273 and
274 .Dv DB_RECNO
275 access methods.)
276 .El
277 .Pp
278 .Dv R_SETCURSOR
279 is available only for the
280 .Dv DB_BTREE
281 and
282 .Dv DB_RECNO
283 access methods because it implies that the keys have an inherent order
284 which does not change.
285 .Pp
286 .Dv R_IAFTER
287 and
288 .Dv R_IBEFORE
289 are available only for the
290 .Dv DB_RECNO
291 access method because they each imply that the access method is able
292 to create new keys.
293 This is only true if the keys are ordered and independent, record
294 numbers for example.
295 .Pp
296 The default behavior of the
297 .Fa put
298 routines is to enter the new key/data pair, replacing any previously
299 existing key.
300 .Pp
301 .Fa put
302 routines return \-1 on error (setting
303 .Va errno ) ,
304 0 on success, and 1 if the
305 .Dv R_NOOVERWRITE
306 .Fa flag
307 was set and the key already exists in the file.
308 .It Fa seq
309 A pointer to a routine which is the interface for sequential
310 retrieval from the database.
311 The address and length of the key are returned in the structure
312 referenced by
313 .Fa key ,
314 and the address and length of the data are returned in the
315 structure referenced by
316 .Fa data .
317 .Pp
318 Sequential key/data pair retrieval may begin at any time, and the
319 position of the
320 .Dq cursor
321 is not affected by calls to the
322 .Fa del ,
323 .Fa get ,
324 .Fa put ,
325 or
326 .Fa sync
327 routines.
328 Modifications to the database during a sequential scan will be
329 reflected in the scan, i.e., records inserted behind the cursor will
330 not be returned while records inserted in front of the cursor will be
331 returned.
332 .Pp
333 The flag value
334 .Em must
335 be set to one of the following values:
336 .Bl -tag -width R_CURSORX
337 .It Dv R_CURSOR
338 The data associated with the specified key is returned.
339 This differs from the
340 .Fa get
341 routines in that it sets or initializes the cursor to the location of
342 the key as well.
343 (Note, for the
344 .Dv DB_BTREE
345 access method, the returned key is not necessarily an exact match for
346 the specified key.
347 The returned key is the smallest key greater than or equal to the
348 specified key, permitting partial key matches and range searches.)
349 .It Dv R_FIRST
350 The first key/data pair of the database is returned, and the cursor
351 is set or initialized to reference it.
352 .It Dv R_LAST
353 The last key/data pair of the database is returned, and the cursor
354 is set or initialized to reference it.
355 (Applicable only to the
356 .Dv DB_BTREE
357 and
358 .Dv DB_RECNO
359 access methods.)
360 .It Dv R_NEXT
361 Retrieve the key/data pair immediately after the cursor.
362 If the cursor is not yet set, this is the same as the
363 .Dv R_FIRST
364 flag.
365 .It Dv R_PREV
366 Retrieve the key/data pair immediately before the cursor.
367 If the cursor is not yet set, this is the same as the
368 .Dv R_LAST
369 flag.
370 (Applicable only to the
371 .Dv DB_BTREE
372 and
373 .Dv DB_RECNO
374 access methods.)
375 .El
376 .Pp
377 .Dv R_LAST
378 and
379 .Dv R_PREV
380 are available only for the
381 .Dv DB_BTREE
382 and
383 .Dv DB_RECNO
384 access methods because they each imply that the keys have an inherent
385 order which does not change.
386 .Pp
387 .Fa seq
388 routines return \-1 on error (setting
389 .Va errno ) ,
390 0 on success and 1 if there are no key/data pairs less than or greater
391 than the specified or current key.
392 If the
393 .Dv DB_RECNO
394 access method is being used, and if the database file is a character
395 special file and no complete key/data pairs are currently available,
396 the
397 .Fa seq
398 routines return 2.
399 .It Fa sync
400 A pointer to a routine to flush any cached information to disk.
401 If the database is in memory only, the
402 .Fa sync
403 routine has no effect and will always succeed.
404 .Pp
405 The flag value may be set to the following value:
406 .Bl -tag -width ".Dv R_RECNOSYNC"
407 .It Dv R_RECNOSYNC
408 If the
409 .Dv DB_RECNO
410 access method is being used, this flag causes the sync routine to
411 apply to the btree file which underlies the recno file, not the recno
412 file itself.
413 (See the
414 .Fa bfname
415 field of the
416 .Xr recno 3
417 manual page for more information.)
418 .El
419 .Pp
420 .Fa sync
421 routines return \-1 on error (setting
422 .Va errno )
423 and 0 on success.
424 .El
425 .Ss Key/data Pairs
426 Access to all file types is based on key/data pairs.
427 Both keys and data are represented by the following data structure:
428 .Bd -literal -offset indent
429 typedef struct {
430 void *data;
431 size_t size;
432 } DBT;
433 .Ed
434 .Pp
435 The elements of the DBT structure are defined as follows:
436 .Bl -tag -width datax -offset indent
437 .It Fa data
438 A pointer to a byte string.
439 .It Fa size
440 The length of the byte string.
441 .El
442 .Pp
443 Key and data byte strings may reference strings of essentially
444 unlimited length although any two of them must fit into available
445 memory at the same time.
446 It should be noted that the access methods provide no guarantees about
447 byte string alignment.
448 .Sh ERRORS
449 The
450 .Nm
451 routine may fail and set
452 .Va errno
453 for any of the errors specified for the library routines
454 .Xr open 2
455 and
456 .Xr malloc 3
457 or the following:
458 .Bl -tag -width Er
459 .It Er EFTYPE
460 A file is incorrectly formatted.
461 .It Er EINVAL
462 A parameter has been specified (hash function, pad byte, etc.) that is
463 incompatible with the current file specification or which is not
464 meaningful for the function (for example, use of the cursor without
465 prior initialization) or there is a mismatch between the version
466 number of file and the software.
467 .It Er EFBIG
468 The key could not be inserted due to limitations in the DB file format
469 (e.g., a hash database was out of overflow pages).
470 .El
471 .Pp
472 The
473 .Fa close
474 routines may fail and set
475 .Va errno
476 for any of the errors specified for the library routines
477 .Xr close 2 ,
478 .Xr read 2 ,
479 .Xr write 2 ,
480 .Xr free 3 ,
481 or
482 .Xr fsync 2 .
483 .Pp
484 The
485 .Fa del ,
486 .Fa get ,
487 .Fa put ,
488 and
489 .Fa seq
490 routines may fail and set
491 .Va errno
492 for any of the errors specified for the library routines
493 .Xr read 2 ,
494 .Xr write 2 ,
495 .Xr free 3 ,
496 or
497 .Xr malloc 3 .
498 .Pp
499 The
500 .Fa fd
501 routines will fail and set
502 .Va errno
503 to
504 .Er ENOENT
505 for in memory databases.
506 .Pp
507 The
508 .Fa sync
509 routines may fail and set
510 .Va errno
511 for any of the errors specified for the library routine
512 .Xr fsync 2 .
513 .Sh SEE ALSO
514 .Xr btree 3 ,
515 .Xr hash 3 ,
516 .Xr mpool 3 ,
517 .Xr recno 3
518 .Pp
519 .Rs
520 .%T LIBTP: Portable, Modular Transactions for UNIX
521 .%A Margo Seltzer
522 .%A Michael Olson
523 .%I USENIX Association
524 .%B Proceedings of the 1992 Winter USENIX Technical Conference
525 .%D 1992
526 .%P 9-25
527 .Re
528 .Sh BUGS
529 The typedef DBT is a mnemonic for
530 .Dq data base thang ,
531 and was used because no one could think of a reasonable name that
532 wasn't already used.
533 .Pp
534 The file descriptor interface is a kludge and will be deleted in a
535 future version of the interface.
536 .Pp
537 None of the access methods provide any form of concurrent access,
538 locking, or transactions.