db/man/dbopen.3

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  32 .\"     @(#)dbopen.3    8.5 (Berkeley) 1/2/94
  33 .\" $FreeBSD: src/lib/libc/db/man/dbopen.3,v 1.5 2001/10/01 16:08:50 ru Exp $
  34 .\"
  35 .Dd January 2, 1994
  36 .Dt DBOPEN 3
  37 .Os
  38 .Sh NAME
  39 .Nm dbopen
  40 .Nd "database access methods"
  41 .Sh SYNOPSIS
  42 .In sys/types.h
  43 .In limits.h
  44 .In db.h
  45 .Ft DB *
  46 .Fn dbopen "const char *file" "int flags" "int mode" "DBTYPE type" "const void *openinfo"
  47 .Sh DESCRIPTION
  48 .Fn Dbopen
  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 structure.
  52 The hashed format is an extensible, dynamic hashing scheme.
  53 The flat-file format is a byte stream file with fixed or variable length
  54 records.
  55 The formats and file format specific information are described in detail
  56 in their respective manual pages
  57 .Xr btree 3 ,
  58 .Xr hash 3
  59 and
  60 .Xr recno 3 .
  61 .Pp
  62 .Fn Dbopen
  63 opens
  64 .Fa file
  65 for reading and/or writing.
  66 Files never intended to be preserved on disk may be created by setting
  67 the file parameter to
  68 .Dv NULL .
  69 .Pp
  70 The
  71 .Fa flags
  72 and
  73 .Fa mode
  74 arguments
  75 are as specified to the
  76 .Xr open 2
  77 routine, however, only the
  78 .Dv O_CREAT , O_EXCL , O_EXLOCK , O_NONBLOCK ,
  79 .Dv O_RDONLY , O_RDWR , O_SHLOCK
  80 and
  81 .Dv O_TRUNC
  82 flags are meaningful.
  83 (Note, opening a database file
  84 .Dv O_WRONLY
  85 is not possible.)
  86 .\"Three additional options may be specified by
  87 .\".Em or Ns 'ing
  88 .\"them into the
  89 .\".Fa flags
  90 .\"argument.
  91 .\".Bl -tag -width indent
  92 .\".It Dv DB_LOCK
  93 .\"Do the necessary locking in the database to support concurrent access.
  94 .\"If concurrent access isn't needed or the database is read-only this
  95 .\"flag should not be set, as it tends to have an associated performance
  96 .\"penalty.
  97 .\".It Dv DB_SHMEM
  98 .\"Place the underlying memory pool used by the database in shared
  99 .\"memory.
 100 .\"Necessary for concurrent access.
 101 .\".It Dv DB_TXN
 102 .\"Support transactions in the database.
 103 .\"The
 104 .\".Dv DB_LOCK
 105 .\"and
 106 .\".Dv DB_SHMEM
 107 .\"flags must be set as well.
 108 .\".El
 109 .Pp
 110 The
 111 .Fa type
 112 argument is of type
 113 .Ft DBTYPE
 114 (as defined in the
 115 .Aq Pa db.h
 116 include file) and
 117 may be set to
 118 .Dv DB_BTREE , DB_HASH
 119 or
 120 .Dv DB_RECNO .
 121 .Pp
 122 The
 123 .Fa openinfo
 124 argument is a pointer to an access method specific structure described
 125 in the access method's manual page.
 126 If
 127 .Fa openinfo
 128 is
 129 .Dv NULL ,
 130 each access method will use defaults appropriate for the system
 131 and the access method.
 132 .Pp
 133 .Fn Dbopen
 134 returns a pointer to a
 135 .Ft DB
 136 structure on success and
 137 .Dv NULL
 138 on error.
 139 The
 140 .Ft DB
 141 structure is defined in the
 142 .Aq Pa db.h
 143 include file, and contains at
 144 least the following fields:
 145 .Bd -literal
 146 typedef struct {
 147         DBTYPE type;
 148         int (*close)(const DB *db);
 149         int (*del)(const DB *db, const DBT *key, u_int flags);
 150         int (*fd)(const DB *db);
 151         int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
 152         int (*put)(const DB *db, DBT *key, const DBT *data,
 153              u_int flags);
 154         int (*sync)(const DB *db, u_int flags);
 155         int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
 156 } DB;
 157 .Ed
 158 .Pp
 159 These elements describe a database type and a set of functions performing
 160 various actions.
 161 These functions take a pointer to a structure as returned by
 162 .Fn dbopen ,
 163 and sometimes one or more pointers to key/data structures and a flag value.
 164 .Bl -tag -width indent
 165 .It Va type
 166 The type of the underlying access method (and file format).
 167 .It Va close
 168 A pointer to a routine to flush any cached information to disk, free any
 169 allocated resources, and close the underlying file(s).
 170 Since key/data pairs may be cached in memory, failing to sync the file
 171 with a
 172 .Va close
 173 or
 174 .Va sync
 175 function may result in inconsistent or lost information.
 176 .Va Close
 177 routines return -1 on error (setting
 178 .Va errno )
 179 and 0 on success.
 180 .It Va del
 181 A pointer to a routine to remove key/data pairs from the database.
 182 .Pp
 183 The parameter
 184 .Fa flags
 185 may be set to the following value:
 186 .Bl -tag -width indent
 187 .It Dv R_CURSOR
 188 Delete the record referenced by the cursor.
 189 The cursor must have previously been initialized.
 190 .El
 191 .Pp
 192 .Va Delete
 193 routines return -1 on error (setting
 194 .Va errno ) ,
 195 0 on success, and 1 if the specified
 196 .Fa key
 197 was not in the file.
 198 .It Va fd
 199 A pointer to a routine which returns a file descriptor representative
 200 of the underlying database.
 201 A file descriptor referencing the same file will be returned to all
 202 processes which call
 203 .Fn dbopen
 204 with the same
 205 .Fa file
 206 name.
 207 This file descriptor may be safely used as an argument to the
 208 .Xr fcntl 2
 209 and
 210 .Xr flock 2
 211 locking functions.
 212 The file descriptor is not necessarily associated with any of the
 213 underlying files used by the access method.
 214 No file descriptor is available for in memory databases.
 215 .Va \&Fd
 216 routines return -1 on error (setting
 217 .Va errno ) ,
 218 and the file descriptor on success.
 219 .It Va get
 220 A pointer to a routine which is the interface for keyed retrieval from
 221 the database.
 222 The address and length of the data associated with the specified
 223 .Fa key
 224 are returned in the structure referenced by
 225 .Fa data .
 226 .Va Get
 227 routines return -1 on error (setting
 228 .Va errno ) ,
 229 0 on success, and 1 if the
 230 .Fa key
 231 was not in the file.
 232 .It Va put
 233 A pointer to a routine to store key/data pairs in the database.
 234 .Pp
 235 The parameter
 236 .Fa flags
 237 may be set to one of the following values:
 238 .Bl -tag -width indent
 239 .It Dv R_CURSOR
 240 Replace the key/data pair referenced by the cursor.
 241 The cursor must have previously been initialized.
 242 .It Dv R_IAFTER
 243 Append the data immediately after the data referenced by
 244 .Fa key ,
 245 creating a new key/data pair.
 246 The record number of the appended key/data pair is returned in the
 247 .Fa key
 248 structure.
 249 (Applicable only to the
 250 .Dv DB_RECNO
 251 access method.)
 252 .It Dv R_IBEFORE
 253 Insert the data immediately before the data referenced by
 254 .Fa key ,
 255 creating a new key/data pair.
 256 The record number of the inserted key/data pair is returned in the
 257 .Fa key
 258 structure.
 259 (Applicable only to the
 260 .Dv DB_RECNO
 261 access method.)
 262 .It Dv R_NOOVERWRITE
 263 Enter the new key/data pair only if the key does not previously exist.
 264 .It Dv R_SETCURSOR
 265 Store the key/data pair, setting or initializing the position of the
 266 cursor to reference it.
 267 (Applicable only to the
 268 .Dv DB_BTREE
 269 and
 270 .Dv DB_RECNO
 271 access methods.)
 272 .El
 273 .Pp
 274 .Dv R_SETCURSOR
 275 is available only for the
 276 .Dv DB_BTREE
 277 and
 278 .Dv DB_RECNO
 279 access
 280 methods because it implies that the keys have an inherent order
 281 which does not change.
 282 .Pp
 283 .Dv R_IAFTER
 284 and
 285 .Dv R_IBEFORE
 286 are available only for the
 287 .Dv DB_RECNO
 288 access method because they each imply that the access method is able to
 289 create new keys.
 290 This is only true if the keys are ordered and independent, record numbers
 291 for example.
 292 .Pp
 293 The default behavior of the
 294 .Va put
 295 routines is to enter the new key/data pair, replacing any previously
 296 existing key.
 297 .Pp
 298 .Va Put
 299 routines return -1 on error (setting
 300 .Va errno ) ,
 301 0 on success, and 1 if the
 302 .Dv R_NOOVERWRITE
 303 flag
 304 was set and the key already exists in the file.
 305 .It Va seq
 306 A pointer to a routine which is the interface for sequential
 307 retrieval from the database.
 308 The address and length of the key are returned in the structure
 309 referenced by
 310 .Fa key ,
 311 and the address and length of the data are returned in the
 312 structure referenced
 313 by
 314 .Fa data .
 315 .Pp
 316 Sequential key/data pair retrieval may begin at any time, and the
 317 position of the
 318 .Dq cursor
 319 is not affected by calls to the
 320 .Va del ,
 321 .Va get ,
 322 .Va put ,
 323 or
 324 .Va sync
 325 routines.
 326 Modifications to the database during a sequential scan will be reflected
 327 in the scan, i.e. records inserted behind the cursor will not be returned
 328 while records inserted in front of the cursor will be returned.
 329 .Pp
 330 The
 331 .Fa flags
 332 value
 333 .Em must
 334 be set to one of the following values:
 335 .Bl -tag -width indent
 336 .It Dv R_CURSOR
 337 The data associated with the specified key is returned.
 338 This differs from the
 339 .Va get
 340 routines in that it sets or initializes the cursor to the location of
 341 the key as well.
 342 (Note, for the
 343 .Dv DB_BTREE
 344 access method, the returned key is not necessarily an
 345 exact match for the specified key.
 346 The returned key is the smallest key greater than or equal to the specified
 347 key, permitting partial key matches and range searches.)
 348 .It Dv R_FIRST
 349 The first key/data pair of the database is returned, and the cursor
 350 is set or initialized to reference it.
 351 .It Dv R_LAST
 352 The last key/data pair of the database is returned, and the cursor
 353 is set or initialized to reference it.
 354 (Applicable only to the
 355 .Dv DB_BTREE
 356 and
 357 .Dv DB_RECNO
 358 access methods.)
 359 .It Dv R_NEXT
 360 Retrieve the key/data pair immediately after the cursor.
 361 If the cursor is not yet set, this is the same as the
 362 .Dv R_FIRST
 363 flag.
 364 .It Dv R_PREV
 365 Retrieve the key/data pair immediately before the cursor.
 366 If the cursor is not yet set, this is the same as the
 367 .Dv R_LAST
 368 flag.
 369 (Applicable only to the
 370 .Dv DB_BTREE
 371 and
 372 .Dv DB_RECNO
 373 access methods.)
 374 .El
 375 .Pp
 376 .Dv R_LAST
 377 and
 378 .Dv R_PREV
 379 are available only for the
 380 .Dv DB_BTREE
 381 and
 382 .Dv DB_RECNO
 383 access methods because they each imply that the keys have an inherent
 384 order which does not change.
 385 .Pp
 386 .Va Seq
 387 routines return -1 on error (setting
 388 .Va errno ) ,
 389 0 on success and 1 if there are no key/data pairs less than or greater
 390 than the specified or current key.
 391 If the
 392 .Dv DB_RECNO
 393 access method is being used, and if the database file
 394 is a character special file and no complete key/data pairs are currently
 395 available, the
 396 .Va seq
 397 routines return 2.
 398 .It Va sync
 399 A pointer to a routine to flush any cached information to disk.
 400 If the database is in memory only, the
 401 .Va sync
 402 routine has no effect and will always succeed.
 403 .Pp
 404 The
 405 .Fa flags
 406 value may be set to the following value:
 407 .Bl -tag -width indent
 408 .It Dv R_RECNOSYNC
 409 If the
 410 .Dv DB_RECNO
 411 access method is being used, this flag causes
 412 the
 413 .Va sync
 414 routine to apply to the btree file which underlies the
 415 recno file, not the recno file itself.
 416 (See the
 417 .Va bfname
 418 field of the
 419 .Xr recno 3
 420 manual page for more information.)
 421 .El
 422 .Pp
 423 .Va Sync
 424 routines return -1 on error (setting
 425 .Va errno )
 426 and 0 on success.
 427 .El
 428 .Sh "KEY/DATA PAIRS"
 429 Access to all file types is based on key/data pairs.
 430 Both keys and data are represented by the following data structure:
 431 .Bd -literal
 432 typedef struct {
 433         void *data;
 434         size_t size;
 435 } DBT;
 436 .Ed
 437 .Pp
 438 The elements of the
 439 .Ft DBT
 440 structure are defined as follows:
 441 .Bl -tag -width "data"
 442 .It Va data
 443 A pointer to a byte string.
 444 .It Va size
 445 The length of the byte string.
 446 .El
 447 .Pp
 448 Key and data byte strings may reference strings of essentially unlimited
 449 length although any two of them must fit into available memory at the same
 450 time.
 451 It should be noted that the access methods provide no guarantees about
 452 byte string alignment.
 453 .Sh ERRORS
 454 The
 455 .Fn dbopen
 456 routine may fail and set
 457 .Va errno
 458 for any of the errors specified for the library routines
 459 .Xr open 2
 460 and
 461 .Xr malloc 3
 462 or the following:
 463 .Bl -tag -width Er
 464 .It Bq Er EFTYPE
 465 A file is incorrectly formatted.
 466 .It Bq Er EINVAL
 467 A parameter has been specified (hash function, pad byte etc.) that is
 468 incompatible with the current file specification or which is not
 469 meaningful for the function (for example, use of the cursor without
 470 prior initialization) or there is a mismatch between the version
 471 number of file and the software.
 472 .El
 473 .Pp
 474 The
 475 .Va close
 476 routines may fail and set
 477 .Va errno
 478 for any of the errors specified for the library routines
 479 .Xr close 2 ,
 480 .Xr read 2 ,
 481 .Xr write 2 ,
 482 .Xr free 3 ,
 483 or
 484 .Xr fsync 2 .
 485 .Pp
 486 The
 487 .Va del ,
 488 .Va get ,
 489 .Va put
 490 and
 491 .Va seq
 492 routines may fail and set
 493 .Va errno
 494 for any of the errors specified for the library routines
 495 .Xr read 2 ,
 496 .Xr write 2 ,
 497 .Xr free 3
 498 or
 499 .Xr malloc 3 .
 500 .Pp
 501 The
 502 .Va fd
 503 routines will fail and set
 504 .Va errno
 505 to
 506 .Er ENOENT
 507 for in memory databases.
 508 .Pp
 509 The
 510 .Va sync
 511 routines may fail and set
 512 .Va errno
 513 for any of the errors specified for the library routine
 514 .Xr fsync 2 .
 515 .Sh SEE ALSO
 516 .Xr btree 3 ,
 517 .Xr hash 3 ,
 518 .Xr mpool 3 ,
 519 .Xr recno 3
 520 .Rs
 521 .%T "LIBTP: Portable, Modular Transactions for UNIX"
 522 .%A Margo Seltzer
 523 .%A Michael Olson
 524 .%R "USENIX proceedings"
 525 .%D Winter 1992
 526 .Re
 527 .Sh BUGS
 528 The typedef
 529 .Ft DBT
 530 is a mnemonic for
 531 .Dq "data base thang" ,
 532 and was used
 533 because noone could think of a reasonable name that wasn't already used.
 534 .Pp
 535 The file descriptor interface is a kluge and will be deleted in a
 536 future version of the interface.
 537 .Pp
 538 None of the access methods provide any form of concurrent access,
 539 locking, or transactions.