2 * @brief memory-mapped database library
4 * @mainpage MDB Memory-Mapped Database Manager
5 * MDB is a Btree-based database management library modeled loosely on the
6 * BerkeleyDB API, but much simplified. The entire database is exposed
7 * in a read-only memory map, and all data fetches return data directly
8 * from the mapped memory, so no malloc's or memcpy's occur during
9 * data fetches. As such, the library is extremely simple because it
10 * requires no page caching layer of its own, and it is extremely high
11 * performance and memory-efficient. It is also fully transactional with
12 * full ACID semantics, and because the memory map is read-only, the
13 * database integrity cannot be corrupted by stray pointer writes from
16 * The library is fully thread-aware and supports concurrent read/write
17 * access from multiple processes and threads. Data pages use a copy-on-
18 * write strategy so no active data pages are ever overwritten, which
19 * also provides resistance to corruption and eliminates the need of any
20 * special recovery procedures after a system crash. Writes are fully
21 * serialized; only one write transaction may be active at a time, which
22 * guarantees that writers can never deadlock. The database structure is
23 * multi-versioned so readers run with no locks; writers cannot block
24 * readers, and readers don't block writers.
26 * Unlike other well-known database mechanisms which use either write-ahead
27 * transaction logs or append-only data writes, MDB requires no maintenance
28 * during operation. Both write-ahead loggers and append-only databases
29 * require periodic checkpointing and/or compaction of their log or database
30 * files otherwise they grow without bound. MDB tracks free pages within
31 * the database and re-uses them for new write operations, so the database
32 * size does not grow without bound in normal use.
34 * @author Howard Chu, Symas Corporation.
36 * @copyright Copyright 2011 Howard Chu, Symas Corp. All rights reserved.
38 * Redistribution and use in source and binary forms, with or without
39 * modification, are permitted only as authorized by the OpenLDAP
42 * A copy of this license is available in the file LICENSE in the
43 * top-level directory of the distribution or, alternatively, at
44 * <http://www.OpenLDAP.org/license.html>.
47 * This code is derived from btree.c written by Martin Hedenfalk.
49 * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
51 * Permission to use, copy, modify, and distribute this software for any
52 * purpose with or without fee is hereby granted, provided that the above
53 * copyright notice and this permission notice appear in all copies.
55 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
56 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
57 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
58 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
59 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
60 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
61 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
66 #include <sys/types.h>
68 /** @defgroup public Public API
71 /** @defgroup Version Version Macros
74 /** Library major version */
75 #define MDB_VERSION_MAJOR 0
76 /** Library minor version */
77 #define MDB_VERSION_MINOR 9
78 /** Library patch version */
79 #define MDB_VERSION_PATCH 0
81 /** Combine args a,b,c into a single integer for easy version comparisons */
82 #define MDB_VERINT(a,b,c) (((a) << 24) | ((b) << 16) | (c))
84 /** The full library version as a single integer */
85 #define MDB_VERSION_FULL \
86 MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
88 /** The release date of this library version */
89 #define MDB_VERSION_DATE "September 1, 2011"
91 /** A stringifier for the version info */
92 #define MDB_VERSTR(a,b,c,d) "MDB " #a "." #b "." #c ": (" #d ")"
94 /** A helper for the stringifier macro */
95 #define MDB_VERFOO(a,b,c,d) MDB_VERSTR(a,b,c,d)
97 /** The full library version as a C string */
98 #define MDB_VERSION_STRING \
99 MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
102 /** Opaque structure for a database environment. A DB environment supports multiple
103 * databases, all residing in the same shared-memory map.
105 typedef struct MDB_env MDB_env;
107 /** Opaque structure for a transaction handle. All database operations require
108 * a transaction handle. Transactions may be read-only or read-write.
110 typedef struct MDB_txn MDB_txn;
112 /** A handle for an individual database in the DB environment. */
113 typedef unsigned int MDB_dbi;
115 /** Opaque structure for navigating through a database */
116 typedef struct MDB_cursor MDB_cursor;
118 /** Generic structure used for passing keys and data in and out of the database. */
119 typedef struct MDB_val {
120 size_t mv_size; /**< size of the data item */
121 void *mv_data; /**< address of the data item */
124 /** A callback function used to compare two keys in a database */
125 typedef int (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
127 /** A callback function used to relocate a position-dependent data item
128 * in a fixed-address database. The \b newptr gives the item's current address in
129 * the memory map, and \b oldptr gives its previous address. This callback is
130 * expected to walk through the fields of the record in newptr and modify any
131 * values based at the \b oldptr address to be relative to the \b newptr address.
132 * @todo This feature is currently unimplemented.
134 typedef void (MDB_rel_func)(void *newptr, void *oldptr, size_t size);
136 /** @defgroup mdb_env environment flags
139 /** mmap at a fixed address */
140 #define MDB_FIXEDMAP 0x01
141 /** don't fsync after commit */
142 #define MDB_NOSYNC 0x10000
144 #define MDB_RDONLY 0x20000
147 /** @defgroup mdb_open database flags
150 /** use reverse string keys */
151 #define MDB_REVERSEKEY 0x02
152 /** use sorted duplicates */
153 #define MDB_DUPSORT 0x04
154 /** numeric keys in native byte order */
155 #define MDB_INTEGERKEY 0x08
156 /** with #MDB_DUPSORT, sorted dup items have fixed size */
157 #define MDB_DUPFIXED 0x10
158 /** with #MDB_DUPSORT, dups are numeric in native byte order */
159 #define MDB_INTEGERDUP 0x20
160 /** create DB if not already existing */
161 #define MDB_CREATE 0x40000
164 /** @defgroup mdb_put mdb_put flags
167 /** For mdb_put: don't write if the key already exists. */
168 #define MDB_NOOVERWRITE 0x10
169 /** For mdb_put: don't write if the key and data pair already exist.
170 * Only for #MDB_DUPSORT
172 #define MDB_NODUPDATA 0x20
175 /** Cursor operations */
176 typedef enum MDB_cursor_op {
177 MDB_FIRST, /**< Position at first key/data item */
178 MDB_FIRST_DUP, /**< Position at first data item of current key.
179 Only for #MDB_DUPSORT */
180 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
181 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
182 MDB_GET_MULTIPLE, /**< Return all the duplicate data items at the current
183 cursor position. Only for #MDB_DUPFIXED */
184 MDB_LAST, /**< Position at last key/data item */
185 MDB_LAST_DUP, /**< Position at last data item of current key.
186 Only for #MDB_DUPSORT */
187 MDB_NEXT, /**< Position at next data item */
188 MDB_NEXT_DUP, /**< Position at next data item of current key.
189 Only for #MDB_DUPSORT */
190 MDB_NEXT_MULTIPLE, /**< Return all duplicate data items at the next
191 cursor position. Only for #MDB_DUPFIXED */
192 MDB_NEXT_NODUP, /**< Position at first data item of next key.
193 Only for #MDB_DUPSORT */
194 MDB_PREV, /**< Position at previous data item */
195 MDB_PREV_DUP, /**< Position at previous data item of current key.
196 Only for #MDB_DUPSORT */
197 MDB_PREV_NODUP, /**< Position at last data item of previous key.
198 only for #MDB_DUPSORT */
199 MDB_SET, /**< Position at specified key */
200 MDB_SET_RANGE /**< Position at first key greater than or equal to specified key. */
203 /** @defgroup errors Return Codes
205 * BerkeleyDB uses -30800 to -30999, we'll go under them
208 /** Successful result */
209 #define MDB_SUCCESS 0
210 /** key/data pair already exists */
211 #define MDB_KEYEXIST (-30799)
212 /** key/data pair not found (EOF) */
213 #define MDB_NOTFOUND (-30798)
214 /** Requested page not found - this usually indicates corruption */
215 #define MDB_PAGE_NOTFOUND (-30797)
216 /** Located page was wrong type */
217 #define MDB_CORRUPTED (-30796)
218 /** Update of meta page failed, probably I/O error */
219 #define MDB_PANIC (-30795)
220 /** Environment version mismatch */
221 #define MDB_VERSION_MISMATCH (-30794)
224 /** Statistics for a database in the environment */
225 typedef struct MDB_stat {
226 unsigned int ms_psize; /**< Size of a database page.
227 This is currently the same for all databases. */
228 unsigned int ms_depth; /**< Depth (height) of the B-tree */
229 unsigned long ms_branch_pages; /**< Number of internal (non-leaf) pages */
230 unsigned long ms_leaf_pages; /**< Number of leaf pages */
231 unsigned long ms_overflow_pages; /**< Number of overflow pages */
232 unsigned long ms_entries; /**< Number of data items */
235 /** Return the mdb library version information.
236 * @param[out] major if non-NULL, the library major version number is copied here
237 * @param[out] minor if non-NULL, the library minor version number is copied here
238 * @param[out] patch if non-NULL, the library patch version number is copied here
239 * @retval "version string" The library version as a string
241 char *mdb_version(int *major, int *minor, int *patch);
243 /** Return a string describing a given error code.
244 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
245 * function. If the error code is greater than or equal to 0, then the string
246 * returned by the system function strerror(3) is returned. If the error code
247 * is less than 0, an error string corresponding to the MDB library error is
248 * returned. See @ref errors for a list of MDB-specific error codes.
249 * @param[in] err The error code
250 * @retval "error message" The description of the error
252 char *mdb_strerror(int err);
254 /** Create an MDB environment handle.
255 * This function allocates memory for a #MDB_env structure. To release
256 * the allocated memory and discard the handle, call #mdb_env_close().
257 * Before the handle may be used, it must be opened using #mdb_env_open().
258 * Various other options may also need to be set before opening the handle,
259 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
260 * depending on usage requirements.
261 * @param[out] env The address where the new handle will be stored
262 * @return A non-zero error value on failure and 0 on success.
264 int mdb_env_create(MDB_env **env);
266 /** Open an environment handle.
267 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
268 * @param[in] env An environment handle returned by #mdb_env_create()
269 * @param[in] path The directory in which the database files reside. This
270 * directory must already exist and be writable.
271 * @param[in] flags Special options for this environment. This parameter
272 * must be set to 0 or by bitwise OR'ing together one or more of the
273 * values described here.
276 * use a fixed address for the mmap region. This flag must be specified
277 * when creating the environment, and is stored persistently in the environment.
278 * If successful, the memory map will always reside at the same virtual address
279 * and pointers used to reference data items in the database will be constant
280 * across multiple invocations. This option may not always work, depending on
281 * how the operating system has allocated memory to shared libraries and other uses.
282 * The feature is highly experimental.
284 * Don't perform a synchronous flush after committing a transaction. This means
285 * transactions will exhibit the ACI (atomicity, consistency, and isolation)
286 * properties, but not D (durability); that is database integrity will be
287 * maintained but it is possible some number of the most recently committed
288 * transactions may be undone after a system crash. The number of transactions
289 * at risk is governed by how often the system flushes dirty buffers to disk
290 * and how often #mdb_env_sync() is called. This flag may be changed
291 * at any time using #mdb_env_set_flags().
293 * Open the environment in read-only mode. No write operations will be allowed.
295 * @param[in] mode The UNIX permissions to set on created files. This parameter
296 * is ignored on Windows.
297 * @return A non-zero error value on failure and 0 on success. Some possible
300 * <li>#MDB_VERSION_MISMATCH - the version of the MDB library doesn't match the
301 * version that created the database environment.
302 * <li>EINVAL - the environment file headers are corrupted.
303 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
304 * <li>EACCES - the user didn't have permission to access the environment files.
305 * <li>EAGAIN - the environment was locked by another process.
308 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mode_t mode);
310 /** Return statistics about the MDB environment.
311 * @param[in] env An environment handle returned by #mdb_env_create()
312 * @param[out] stat The address of an #MDB_stat structure
313 * where the statistics will be copied
315 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
317 /** Flush the data buffers to disk.
318 * Data is always written to disk when #mdb_txn_commit() is called,
319 * but the operating system may keep it buffered. MDB always flushes
320 * the OS buffers upon commit as well, unless the environment was
321 * opened with #MDB_NOSYNC.
322 * @param[in] env An environment handle returned by #mdb_env_create()
323 * @param[in] force If non-zero, force the flush to occur. Otherwise
324 * if the environment has the #MDB_NOSYNC flag set the flushes
326 * @return A non-zero error value on failure and 0 on success. Some possible
329 * <li>EINVAL - an invalid parameter was specified.
330 * <li>EIO - an error occurred during synchronization.
333 int mdb_env_sync(MDB_env *env, int force);
335 /** Close the environment and release the memory map.
336 * Only a single thread may call this function. All transactions, databases,
337 * and cursors must already be closed before calling this function. Attempts to
338 * use any such handles after calling this function will cause a SIGSEGV.
339 * The environment handle will be freed and must not be used again after this call.
340 * @param[in] env An environment handle returned by #mdb_env_create()
342 void mdb_env_close(MDB_env *env);
344 /** Set environment flags.
345 * This may be used to set some flags that weren't already set during
346 * #mdb_env_open(), or to unset these flags. Currently only the
347 * #MDB_NOSYNC flag setting may be changed with this function.
348 * @param[in] env An environment handle returned by #mdb_env_create()
349 * @param[in] flags The flags to change, bitwise OR'ed together
350 * @param[in] onoff A non-zero value sets the flags, zero clears them.
351 * @return A non-zero error value on failure and 0 on success. Some possible
354 * <li>EINVAL - an invalid parameter was specified.
357 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
359 /** Get environment flags.
360 * @param[in] env An environment handle returned by #mdb_env_create()
361 * @param[out] flags The address of an integer to store the flags
362 * @return A non-zero error value on failure and 0 on success. Some possible
365 * <li>EINVAL - an invalid parameter was specified.
368 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
370 /** Return the path that was used in #mdb_env_open().
371 * @param[in] env An environment handle returned by #mdb_env_create()
372 * @param[out] path Address of a string pointer to contain the path. This
373 * is the actual string in the environment, not a copy. It should not be
374 * altered in any way.
375 * @return A non-zero error value on failure and 0 on success. Some possible
378 * <li>EINVAL - an invalid parameter was specified.
381 int mdb_env_get_path(MDB_env *env, const char **path);
383 /** Set the size of the memory map to use for this environment.
384 * The size should be a multiple of the OS page size. The default is
385 * 10485760 bytes. The size of the memory map is also the maximum size
386 * of the database. The value should be chosen as large as possible,
387 * to accomodate future growth of the database.
388 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
389 * @param[in] env An environment handle returned by #mdb_env_create()
390 * @param[in] size The size in bytes
391 * @return A non-zero error value on failure and 0 on success. Some possible
394 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
397 int mdb_env_set_mapsize(MDB_env *env, size_t size);
399 /** Set the maximum number of threads for the environment.
400 * This defines the number of slots in the lock table that is used to track readers in the
401 * the environment. The default is 126.
402 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
403 * @param[in] env An environment handle returned by #mdb_env_create()
404 * @param[in] readers The maximum number of threads
405 * @return A non-zero error value on failure and 0 on success. Some possible
408 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
411 int mdb_env_set_maxreaders(MDB_env *env, int readers);
413 /** Get the maximum number of threads for the environment.
414 * @param[in] env An environment handle returned by #mdb_env_create()
415 * @param[out] readers Address of an integer to store the number of readers
416 * @return A non-zero error value on failure and 0 on success. Some possible
419 * <li>EINVAL - an invalid parameter was specified.
422 int mdb_env_get_maxreaders(MDB_env *env, int *readers);
424 /** Set the maximum number of databases for the environment.
425 * This function is only needed if multiple databases will be used in the
426 * environment. Simpler applications that only use a single database can ignore
428 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
429 * @param[in] env An environment handle returned by #mdb_env_create()
430 * @param[in] dbs The maximum number of databases
431 * @return A non-zero error value on failure and 0 on success. Some possible
434 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
437 int mdb_env_set_maxdbs(MDB_env *env, int dbs);
439 /** Create a transaction for use with the environment.
440 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
441 * @note Transactions may not span threads, a transaction must only be used by a
443 * @note Cursors may not span transactions; each cursor must be opened and closed
444 * within a single transaction.
445 * @param[in] env An environment handle returned by #mdb_env_create()
446 * @param[in] flags Special options for this transaction. This parameter
447 * must be set to 0 or by bitwise OR'ing together one or more of the
448 * values described here.
451 * This transaction will not perform any write operations.
453 * @param[out] txn Address where the new #MDB_txn handle will be stored
454 * @return A non-zero error value on failure and 0 on success. Some possible
457 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
459 * <li>ENOMEM - out of memory, or a read-only transaction was requested and
460 * the reader lock table is full. See #mdb_env_set_maxreaders().
463 int mdb_txn_begin(MDB_env *env, unsigned int flags, MDB_txn **txn);
465 /** Commit all the operations of a transaction into the database.
466 * All cursors opened within the transaction must be closed before the transaction
468 * The transaction handle will be freed and must not be used again after this call.
469 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
470 * @return A non-zero error value on failure and 0 on success. Some possible
473 * <li>EINVAL - an invalid parameter was specified.
474 * <li>ENOSPC - no more disk space.
475 * <li>EIO - a low-level I/O error occurred while writing.
478 int mdb_txn_commit(MDB_txn *txn);
480 /** Abandon all the operations of the transaction instead of saving them.
481 * All cursors opened within the transaction must be closed before the transaction
483 * The transaction handle will be freed and must not be used again after this call.
484 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
486 void mdb_txn_abort(MDB_txn *txn);
488 /** Reset a read-only transaction.
489 * This releases the current reader lock but doesn't free the
490 * transaction handle, allowing it to be used again later by #mdb_txn_renew().
491 * It otherwise has the same affect as #mdb_txn_abort() but saves some memory
492 * allocation/deallocation overhead if a thread is going to start a new
493 * read-only transaction again soon.
494 * All cursors opened within the transaction must be closed before the transaction
496 * Reader locks generally don't interfere with writers, but they keep old
497 * versions of database pages allocated. Thus they prevent the old pages
498 * from being reused when writers commit new data, and so under heavy load
499 * the database size may grow much more rapidly than otherwise.
500 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
502 void mdb_txn_reset(MDB_txn *txn);
504 /** Renew a read-only transaction.
505 * This acquires a new reader lock for a transaction handle that had been
506 * released by #mdb_txn_reset(). It must be called before a reset transaction
508 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
509 * @return A non-zero error value on failure and 0 on success. Some possible
512 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
514 * <li>EINVAL - an invalid parameter was specified.
517 int mdb_txn_renew(MDB_txn *txn);
519 /** Open a database in the environment.
520 * The database handle may be discarded by calling #mdb_close().
521 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
522 * @param[in] name The name of the database to open. If only a single
523 * database is needed in the enviroment, this value may be NULL.
524 * @param[in] flags Special options for this database. This parameter
525 * must be set to 0 or by bitwise OR'ing together one or more of the
526 * values described here.
528 * <li>#MDB_REVERSEKEY
529 * Keys are strings to be compared in reverse order, from the end
530 * of the strings to the beginning. By default, Keys are treated as strings and
531 * compared from beginning to end.
533 * Duplicate keys may be used in the database. (Or, from another perspective,
534 * keys may have multiple data items, stored in sorted order.) By default
535 * keys must be unique and may have only a single data item.
536 * <li>#MDB_INTEGERKEY
537 * Keys are binary integers in native byte order. On Big-Endian systems
538 * this flag has no effect. On Little-Endian systems this flag behaves
539 * the same as #MDB_REVERSEKEY. This flag is simply provided as a
540 * convenience so that applications don't need to detect Endianness themselves
541 * when using integers as keys.
543 * This flag may only be used in combination with #MDB_DUPSORT. This option
544 * tells the library that the data items for this database are all the same
545 * size, which allows further optimizations in storage and retrieval. When
546 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
547 * cursor operations may be used to retrieve multiple items at once.
548 * <li>#MDB_INTEGERDUP
549 * This option specifies that duplicate data items are also integers, and
550 * should be sorted as such.
552 * Create the named database if it doesn't exist. This option is not
553 * allowed in a read-only transaction or a read-only environment.
555 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
556 * @return A non-zero error value on failure and 0 on success. Some possible
559 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
560 * and #MDB_CREATE was not specified.
561 * <li>ENFILE - too many databases have been opened. See #mdb_env_set_maxdbs().
564 int mdb_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
566 /** Retrieve statistics for a database.
567 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
568 * @param[in] dbi A database handle returned by #mdb_open()
569 * @param[out] stat The address of an #MDB_stat structure
570 * where the statistics will be copied
571 * @return A non-zero error value on failure and 0 on success. Some possible
574 * <li>EINVAL - an invalid parameter was specified.
577 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
579 /** Close a database handle.
580 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
581 * @param[in] dbi A database handle returned by #mdb_open()
583 void mdb_close(MDB_txn *txn, MDB_dbi dbi);
585 /** Set a custom key comparison function for a database.
586 * The comparison function is called whenever it is necessary to compare a
587 * key specified by the application with a key currently stored in the database.
588 * If no comparison function is specified, and no special key flags were specified
589 * with #mdb_open(), the keys are compared lexically, with shorter keys collating
590 * before longer keys.
591 * @warning This function must be called before any data access functions are used,
592 * otherwise data corruption may occur. The same function must be used by every
593 * process accessing the database, every time the database is used.
594 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
595 * @param[in] dbi A database handle returned by #mdb_open()
596 * @param[in] cmp A #MDB_cmp_func function
597 * @return A non-zero error value on failure and 0 on success. Some possible
600 * <li>EINVAL - an invalid parameter was specified.
603 int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
605 /** Set a custom data comparison function for a #MDB_DUPSORT database.
606 * This comparison function is called whenever it is necessary to compare a data
607 * item specified by the application with a data item currently stored in the database.
608 * This function only takes effect if the database was opened with the #MDB_DUPSORT
610 * If no comparison function is specified, and no special key flags were specified
611 * with #mdb_open(), the data items are compared lexically, with shorter items collating
612 * before longer items.
613 * @warning This function must be called before any data access functions are used,
614 * otherwise data corruption may occur. The same function must be used by every
615 * process accessing the database, every time the database is used.
616 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
617 * @param[in] dbi A database handle returned by #mdb_open()
618 * @param[in] cmp A #MDB_cmp_func function
619 * @return A non-zero error value on failure and 0 on success. Some possible
622 * <li>EINVAL - an invalid parameter was specified.
625 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
627 /** Set a relocation function for a #MDB_FIXEDMAP database.
628 * @todo The relocation function is called whenever it is necessary to move the data
629 * of an item to a different position in the database (e.g. through tree
630 * balancing operations, shifts as a result of adds or deletes, etc.). It is
631 * intended to allow address/position-dependent data items to be stored in
632 * a database in an environment opened with the #MDB_FIXEDMAP option.
633 * Currently the relocation feature is unimplemented and setting
634 * this function has no effect.
635 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
636 * @param[in] dbi A database handle returned by #mdb_open()
637 * @param[in] rel A #MDB_rel_func function
638 * @return A non-zero error value on failure and 0 on success. Some possible
641 * <li>EINVAL - an invalid parameter was specified.
644 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
646 /** Get items from a database.
647 * This function retrieves key/data pairs from the database. The address
648 * and length of the data associated with the specified \b key are returned
649 * in the structure to which \b data refers.
650 * If the database supports duplicate keys (#MDB_DUPSORT) then the
651 * first data item for the key will be returned. Retrieval of other
652 * items requires the use of #mdb_cursor_get().
653 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
654 * @param[in] dbi A database handle returned by #mdb_open()
655 * @param[in] key The key to search for in the database
656 * @param[out] data The data corresponding to the key
657 * @return A non-zero error value on failure and 0 on success. Some possible
660 * <li>#MDB_NOTFOUND - the key was not in the database.
661 * <li>EINVAL - an invalid parameter was specified.
664 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
666 /** Store items into a database.
667 * This function stores key/data pairs in the database. The default behavior
668 * is to enter the new key/data pair, replacing any previously existing key
669 * if duplicates are disallowed, or adding a duplicate data item if
670 * duplicates are allowed (#MDB_DUPSORT).
671 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
672 * @param[in] dbi A database handle returned by #mdb_open()
673 * @param[in] key The key to store in the database
674 * @param[in] data The data to store
675 * @param[in] flags Special options for this operation. This parameter
676 * must be set to 0 or by bitwise OR'ing together one or more of the
677 * values described here.
679 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
680 * already appear in the database. This flag may only be specified
681 * if the database was opened with #MDB_DUPSORT. The function will
682 * return #MDB_KEYEXIST if the key/data pair already appears in the
684 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
685 * does not already appear in the database. The function will return
686 * #MDB_KEYEXIST if the key already appears in the database, even if
687 * the database supports duplicates (#MDB_DUPSORT).
689 * @return A non-zero error value on failure and 0 on success. Some possible
692 * <li>EACCESS - an attempt was made to write in a read-only transaction.
693 * <li>EINVAL - an invalid parameter was specified.
694 * <li>ENOMEM - the database is full, see #mdb_env_set_mapsize().
697 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
700 /** Delete items from a database.
701 * This function removes key/data pairs from the database.
702 * If the database does not support sorted duplicate data items
703 * (#MDB_DUPSORT) the data parameter is ignored.
704 * If the database supports sorted duplicates and the data parameter
705 * is NULL, all of the duplicate data items for the key will be
706 * deleted. Otherwise, if the data parameter is non-NULL
707 * only the matching data item will be deleted.
708 * This function will return #MDB_NOTFOUND if the specified key/data
709 * pair is not in the database.
710 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
711 * @param[in] dbi A database handle returned by #mdb_open()
712 * @param[in] key The key to delete from the database
713 * @param[in] data The data to delete
714 * @return A non-zero error value on failure and 0 on success. Some possible
717 * <li>EACCESS - an attempt was made to write in a read-only transaction.
718 * <li>EINVAL - an invalid parameter was specified.
721 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
723 /** Create a cursor handle.
724 * Cursors are associated with a specific transaction and database and
725 * may not span threads.
726 * @todo Cursors only support read operations. Support for cursor_put() and
727 * cursor_del() needs to be added.
728 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
729 * @param[in] dbi A database handle returned by #mdb_open()
730 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
731 * @return A non-zero error value on failure and 0 on success. Some possible
734 * <li>EINVAL - an invalid parameter was specified.
737 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
739 /** Close a cursor handle.
740 * The cursor handle will be freed and must not be used again after this call.
741 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
743 void mdb_cursor_close(MDB_cursor *cursor);
745 /** Retrieve by cursor.
746 * This function retrieves key/data pairs from the database. The address and length
747 * of the key are returned in the object to which \b key refers (except for the
748 * case of the #MDB_SET option, in which the \b key object is unchanged), and
749 * the address and length of the data are returned in the object to which \b data
751 * @bug Cursors are not coordinated with write operations. If a cursor in a
752 * write transaction is performing a sequential scan while records are being
753 * inserted or deleted in the same transaction, the cursor will be corrupted.
754 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
755 * @param[in,out] key The key for a retrieved item
756 * @param[in,out] data The data of a retrieved item
757 * @param[in] op A cursor operation #MDB_cursor_op
758 * @return A non-zero error value on failure and 0 on success. Some possible
761 * <li>#MDB_NOTFOUND - no matching key found.
762 * <li>EINVAL - an invalid parameter was specified.
765 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
769 * This function stores key/data pairs into the database.
770 * If the function fails for any reason, the state of the cursor will be
771 * unchanged. If the function succeeds and an item is inserted into the
772 * database, the cursor is always positioned to refer to the newly inserted item.
773 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
774 * @param[in] key The key operated on.
775 * @param[in] data The data operated on.
776 * @param[in] flags Options for this operation. This parameter
777 * must be set to 0 or one of the values described here.
779 * <li>#MDB_CURRENT - overwrite the data of the key/data pair to which
780 * the cursor refers with the specified data item. The \b key
781 * parameter is ignored.
782 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
783 * already appear in the database. This flag may only be specified
784 * if the database was opened with #MDB_DUPSORT. The function will
785 * return #MDB_KEYEXIST if the key/data pair already appears in the
787 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
788 * does not already appear in the database. The function will return
789 * #MDB_KEYEXIST if the key already appears in the database, even if
790 * the database supports duplicates (#MDB_DUPSORT).
792 * @return A non-zero error value on failure and 0 on success. Some possible
795 * <li>EACCES - an attempt was made to modify a read-only database.
796 * <li>EINVAL - an invalid parameter was specified.
799 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
802 /** Delete current key/data pair
803 * This function deletes the key/data pair to which the cursor refers.
804 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
805 * @param[in] flags Options for this operation. This parameter
806 * must be set to 0 or one of the values described here.
808 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
809 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
811 * @return A non-zero error value on failure and 0 on success. Some possible
814 * <li>EACCES - an attempt was made to modify a read-only database.
815 * <li>EINVAL - an invalid parameter was specified.
818 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
820 /** Return count of duplicates for current key.
821 * This call is only valid on databases that support sorted duplicate
822 * data items #MDB_DUPSORT.
823 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
824 * @param[out] countp Address where the count will be stored
825 * @return A non-zero error value on failure and 0 on success. Some possible
828 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
831 int mdb_cursor_count(MDB_cursor *cursor, unsigned long *countp);
833 /** Compare two data items according to a particular database.
834 * This returns a comparison as if the two data items were keys in the
835 * specified database.
836 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
837 * @param[in] dbi A database handle returned by #mdb_open()
838 * @param[in] a The first item to compare
839 * @param[in] b The second item to compare
840 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
842 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);