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 * The keys must all be of the same size. */
156 #define MDB_INTEGERKEY 0x08
157 /** with #MDB_DUPSORT, sorted dup items have fixed size */
158 #define MDB_DUPFIXED 0x10
159 /** with #MDB_DUPSORT, dups are numeric in native byte order */
160 #define MDB_INTEGERDUP 0x20
161 /** with #MDB_DUPSORT, use reverse string dups */
162 #define MDB_REVERSEDUP 0x40
163 /** create DB if not already existing */
164 #define MDB_CREATE 0x40000
167 /** @defgroup mdb_put Write Flags
170 /** For put: Don't write if the key already exists. */
171 #define MDB_NOOVERWRITE 0x10
172 /** Only for #MDB_DUPSORT<br>
173 * For put: don't write if the key and data pair already exist.<br>
174 * For mdb_cursor_del: remove all duplicate data items.
176 #define MDB_NODUPDATA 0x20
177 /** For mdb_cursor_put: overwrite the current key/data pair */
178 #define MDB_CURRENT 0x40
181 /** Cursor operations */
182 typedef enum MDB_cursor_op {
183 MDB_FIRST, /**< Position at first key/data item */
184 MDB_FIRST_DUP, /**< Position at first data item of current key.
185 Only for #MDB_DUPSORT */
186 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
187 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
188 MDB_GET_MULTIPLE, /**< Return all the duplicate data items at the current
189 cursor position. Only for #MDB_DUPFIXED */
190 MDB_LAST, /**< Position at last key/data item */
191 MDB_LAST_DUP, /**< Position at last data item of current key.
192 Only for #MDB_DUPSORT */
193 MDB_NEXT, /**< Position at next data item */
194 MDB_NEXT_DUP, /**< Position at next data item of current key.
195 Only for #MDB_DUPSORT */
196 MDB_NEXT_MULTIPLE, /**< Return all duplicate data items at the next
197 cursor position. Only for #MDB_DUPFIXED */
198 MDB_NEXT_NODUP, /**< Position at first data item of next key.
199 Only for #MDB_DUPSORT */
200 MDB_PREV, /**< Position at previous data item */
201 MDB_PREV_DUP, /**< Position at previous data item of current key.
202 Only for #MDB_DUPSORT */
203 MDB_PREV_NODUP, /**< Position at last data item of previous key.
204 Only for #MDB_DUPSORT */
205 MDB_SET, /**< Position at specified key */
206 MDB_SET_RANGE /**< Position at first key greater than or equal to specified key. */
209 /** @defgroup errors Return Codes
211 * BerkeleyDB uses -30800 to -30999, we'll go under them
214 /** Successful result */
215 #define MDB_SUCCESS 0
216 /** key/data pair already exists */
217 #define MDB_KEYEXIST (-30799)
218 /** key/data pair not found (EOF) */
219 #define MDB_NOTFOUND (-30798)
220 /** Requested page not found - this usually indicates corruption */
221 #define MDB_PAGE_NOTFOUND (-30797)
222 /** Located page was wrong type */
223 #define MDB_CORRUPTED (-30796)
224 /** Update of meta page failed, probably I/O error */
225 #define MDB_PANIC (-30795)
226 /** Environment version mismatch */
227 #define MDB_VERSION_MISMATCH (-30794)
230 /** Statistics for a database in the environment */
231 typedef struct MDB_stat {
232 unsigned int ms_psize; /**< Size of a database page.
233 This is currently the same for all databases. */
234 unsigned int ms_depth; /**< Depth (height) of the B-tree */
235 unsigned long ms_branch_pages; /**< Number of internal (non-leaf) pages */
236 unsigned long ms_leaf_pages; /**< Number of leaf pages */
237 unsigned long ms_overflow_pages; /**< Number of overflow pages */
238 unsigned long ms_entries; /**< Number of data items */
241 /** Return the mdb library version information.
242 * @param[out] major if non-NULL, the library major version number is copied here
243 * @param[out] minor if non-NULL, the library minor version number is copied here
244 * @param[out] patch if non-NULL, the library patch version number is copied here
245 * @retval "version string" The library version as a string
247 char *mdb_version(int *major, int *minor, int *patch);
249 /** Return a string describing a given error code.
250 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
251 * function. If the error code is greater than or equal to 0, then the string
252 * returned by the system function strerror(3) is returned. If the error code
253 * is less than 0, an error string corresponding to the MDB library error is
254 * returned. See @ref errors for a list of MDB-specific error codes.
255 * @param[in] err The error code
256 * @retval "error message" The description of the error
258 char *mdb_strerror(int err);
260 /** Create an MDB environment handle.
261 * This function allocates memory for a #MDB_env structure. To release
262 * the allocated memory and discard the handle, call #mdb_env_close().
263 * Before the handle may be used, it must be opened using #mdb_env_open().
264 * Various other options may also need to be set before opening the handle,
265 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
266 * depending on usage requirements.
267 * @param[out] env The address where the new handle will be stored
268 * @return A non-zero error value on failure and 0 on success.
270 int mdb_env_create(MDB_env **env);
272 /** Open an environment handle.
273 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
274 * @param[in] env An environment handle returned by #mdb_env_create()
275 * @param[in] path The directory in which the database files reside. This
276 * directory must already exist and be writable.
277 * @param[in] flags Special options for this environment. This parameter
278 * must be set to 0 or by bitwise OR'ing together one or more of the
279 * values described here.
282 * use a fixed address for the mmap region. This flag must be specified
283 * when creating the environment, and is stored persistently in the environment.
284 * If successful, the memory map will always reside at the same virtual address
285 * and pointers used to reference data items in the database will be constant
286 * across multiple invocations. This option may not always work, depending on
287 * how the operating system has allocated memory to shared libraries and other uses.
288 * The feature is highly experimental.
290 * Don't perform a synchronous flush after committing a transaction. This means
291 * transactions will exhibit the ACI (atomicity, consistency, and isolation)
292 * properties, but not D (durability); that is database integrity will be
293 * maintained but it is possible some number of the most recently committed
294 * transactions may be undone after a system crash. The number of transactions
295 * at risk is governed by how often the system flushes dirty buffers to disk
296 * and how often #mdb_env_sync() is called. This flag may be changed
297 * at any time using #mdb_env_set_flags().
299 * Open the environment in read-only mode. No write operations will be allowed.
301 * @param[in] mode The UNIX permissions to set on created files. This parameter
302 * is ignored on Windows.
303 * @return A non-zero error value on failure and 0 on success. Some possible
306 * <li>#MDB_VERSION_MISMATCH - the version of the MDB library doesn't match the
307 * version that created the database environment.
308 * <li>EINVAL - the environment file headers are corrupted.
309 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
310 * <li>EACCES - the user didn't have permission to access the environment files.
311 * <li>EAGAIN - the environment was locked by another process.
314 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mode_t mode);
316 /** Return statistics about the MDB environment.
317 * @param[in] env An environment handle returned by #mdb_env_create()
318 * @param[out] stat The address of an #MDB_stat structure
319 * where the statistics will be copied
321 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
323 /** Flush the data buffers to disk.
324 * Data is always written to disk when #mdb_txn_commit() is called,
325 * but the operating system may keep it buffered. MDB always flushes
326 * the OS buffers upon commit as well, unless the environment was
327 * opened with #MDB_NOSYNC.
328 * @param[in] env An environment handle returned by #mdb_env_create()
329 * @param[in] force If non-zero, force the flush to occur. Otherwise
330 * if the environment has the #MDB_NOSYNC flag set the flushes
332 * @return A non-zero error value on failure and 0 on success. Some possible
335 * <li>EINVAL - an invalid parameter was specified.
336 * <li>EIO - an error occurred during synchronization.
339 int mdb_env_sync(MDB_env *env, int force);
341 /** Close the environment and release the memory map.
342 * Only a single thread may call this function. All transactions, databases,
343 * and cursors must already be closed before calling this function. Attempts to
344 * use any such handles after calling this function will cause a SIGSEGV.
345 * The environment handle will be freed and must not be used again after this call.
346 * @param[in] env An environment handle returned by #mdb_env_create()
348 void mdb_env_close(MDB_env *env);
350 /** Set environment flags.
351 * This may be used to set some flags that weren't already set during
352 * #mdb_env_open(), or to unset these flags. Currently only the
353 * #MDB_NOSYNC flag setting may be changed with this function.
354 * @param[in] env An environment handle returned by #mdb_env_create()
355 * @param[in] flags The flags to change, bitwise OR'ed together
356 * @param[in] onoff A non-zero value sets the flags, zero clears them.
357 * @return A non-zero error value on failure and 0 on success. Some possible
360 * <li>EINVAL - an invalid parameter was specified.
363 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
365 /** Get environment flags.
366 * @param[in] env An environment handle returned by #mdb_env_create()
367 * @param[out] flags The address of an integer to store the flags
368 * @return A non-zero error value on failure and 0 on success. Some possible
371 * <li>EINVAL - an invalid parameter was specified.
374 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
376 /** Return the path that was used in #mdb_env_open().
377 * @param[in] env An environment handle returned by #mdb_env_create()
378 * @param[out] path Address of a string pointer to contain the path. This
379 * is the actual string in the environment, not a copy. It should not be
380 * altered in any way.
381 * @return A non-zero error value on failure and 0 on success. Some possible
384 * <li>EINVAL - an invalid parameter was specified.
387 int mdb_env_get_path(MDB_env *env, const char **path);
389 /** Set the size of the memory map to use for this environment.
390 * The size should be a multiple of the OS page size. The default is
391 * 10485760 bytes. The size of the memory map is also the maximum size
392 * of the database. The value should be chosen as large as possible,
393 * to accomodate future growth of the database.
394 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
395 * @param[in] env An environment handle returned by #mdb_env_create()
396 * @param[in] size The size in bytes
397 * @return A non-zero error value on failure and 0 on success. Some possible
400 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
403 int mdb_env_set_mapsize(MDB_env *env, size_t size);
405 /** Set the maximum number of threads for the environment.
406 * This defines the number of slots in the lock table that is used to track readers in the
407 * the environment. The default is 126.
408 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
409 * @param[in] env An environment handle returned by #mdb_env_create()
410 * @param[in] readers The maximum number of threads
411 * @return A non-zero error value on failure and 0 on success. Some possible
414 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
417 int mdb_env_set_maxreaders(MDB_env *env, int readers);
419 /** Get the maximum number of threads for the environment.
420 * @param[in] env An environment handle returned by #mdb_env_create()
421 * @param[out] readers Address of an integer to store the number of readers
422 * @return A non-zero error value on failure and 0 on success. Some possible
425 * <li>EINVAL - an invalid parameter was specified.
428 int mdb_env_get_maxreaders(MDB_env *env, int *readers);
430 /** Set the maximum number of databases for the environment.
431 * This function is only needed if multiple databases will be used in the
432 * environment. Simpler applications that only use a single database can ignore
434 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
435 * @param[in] env An environment handle returned by #mdb_env_create()
436 * @param[in] dbs The maximum number of databases
437 * @return A non-zero error value on failure and 0 on success. Some possible
440 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
443 int mdb_env_set_maxdbs(MDB_env *env, int dbs);
445 /** Create a transaction for use with the environment.
446 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
447 * @note Transactions may not span threads, a transaction must only be used by a
449 * @note Cursors may not span transactions; each cursor must be opened and closed
450 * within a single transaction.
451 * @param[in] env An environment handle returned by #mdb_env_create()
452 * @param[in] flags Special options for this transaction. This parameter
453 * must be set to 0 or by bitwise OR'ing together one or more of the
454 * values described here.
457 * This transaction will not perform any write operations.
459 * @param[out] txn Address where the new #MDB_txn handle will be stored
460 * @return A non-zero error value on failure and 0 on success. Some possible
463 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
465 * <li>ENOMEM - out of memory, or a read-only transaction was requested and
466 * the reader lock table is full. See #mdb_env_set_maxreaders().
469 int mdb_txn_begin(MDB_env *env, unsigned int flags, MDB_txn **txn);
471 /** Commit all the operations of a transaction into the database.
472 * All cursors opened within the transaction must be closed before the transaction
474 * The transaction handle will be freed and must not be used again after this call.
475 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
476 * @return A non-zero error value on failure and 0 on success. Some possible
479 * <li>EINVAL - an invalid parameter was specified.
480 * <li>ENOSPC - no more disk space.
481 * <li>EIO - a low-level I/O error occurred while writing.
484 int mdb_txn_commit(MDB_txn *txn);
486 /** Abandon all the operations of the transaction instead of saving them.
487 * All cursors opened within the transaction must be closed before the transaction
489 * The transaction handle will be freed and must not be used again after this call.
490 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
492 void mdb_txn_abort(MDB_txn *txn);
494 /** Reset a read-only transaction.
495 * This releases the current reader lock but doesn't free the
496 * transaction handle, allowing it to be used again later by #mdb_txn_renew().
497 * It otherwise has the same effect as #mdb_txn_abort() but saves some memory
498 * allocation/deallocation overhead if a thread is going to start a new
499 * read-only transaction again soon.
500 * All cursors opened within the transaction must be closed before the transaction
502 * Reader locks generally don't interfere with writers, but they keep old
503 * versions of database pages allocated. Thus they prevent the old pages
504 * from being reused when writers commit new data, and so under heavy load
505 * the database size may grow much more rapidly than otherwise.
506 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
508 void mdb_txn_reset(MDB_txn *txn);
510 /** Renew a read-only transaction.
511 * This acquires a new reader lock for a transaction handle that had been
512 * released by #mdb_txn_reset(). It must be called before a reset transaction
514 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
515 * @return A non-zero error value on failure and 0 on success. Some possible
518 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
520 * <li>EINVAL - an invalid parameter was specified.
523 int mdb_txn_renew(MDB_txn *txn);
525 /** Open a database in the environment.
526 * The database handle may be discarded by calling #mdb_close().
527 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
528 * @param[in] name The name of the database to open. If only a single
529 * database is needed in the enviroment, this value may be NULL.
530 * @param[in] flags Special options for this database. This parameter
531 * must be set to 0 or by bitwise OR'ing together one or more of the
532 * values described here.
534 * <li>#MDB_REVERSEKEY
535 * Keys are strings to be compared in reverse order, from the end
536 * of the strings to the beginning. By default, Keys are treated as strings and
537 * compared from beginning to end.
539 * Duplicate keys may be used in the database. (Or, from another perspective,
540 * keys may have multiple data items, stored in sorted order.) By default
541 * keys must be unique and may have only a single data item.
542 * <li>#MDB_INTEGERKEY
543 * Keys are binary integers in native byte order. Setting this option
544 * requires all keys to be the same size, typically sizeof(int)
547 * This flag may only be used in combination with #MDB_DUPSORT. This option
548 * tells the library that the data items for this database are all the same
549 * size, which allows further optimizations in storage and retrieval. When
550 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
551 * cursor operations may be used to retrieve multiple items at once.
552 * <li>#MDB_INTEGERDUP
553 * This option specifies that duplicate data items are also integers, and
554 * should be sorted as such.
555 * <li>#MDB_REVERSEDUP
556 * This option specifies that duplicate data items should be compared as
557 * strings in reverse order.
559 * Create the named database if it doesn't exist. This option is not
560 * allowed in a read-only transaction or a read-only environment.
562 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
563 * @return A non-zero error value on failure and 0 on success. Some possible
566 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
567 * and #MDB_CREATE was not specified.
568 * <li>ENFILE - too many databases have been opened. See #mdb_env_set_maxdbs().
571 int mdb_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
573 /** Retrieve statistics for a database.
574 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
575 * @param[in] dbi A database handle returned by #mdb_open()
576 * @param[out] stat The address of an #MDB_stat structure
577 * where the statistics will be copied
578 * @return A non-zero error value on failure and 0 on success. Some possible
581 * <li>EINVAL - an invalid parameter was specified.
584 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
586 /** Close a database handle.
587 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
588 * @param[in] dbi A database handle returned by #mdb_open()
590 void mdb_close(MDB_txn *txn, MDB_dbi dbi);
592 /** Set a custom key comparison function for a database.
593 * The comparison function is called whenever it is necessary to compare a
594 * key specified by the application with a key currently stored in the database.
595 * If no comparison function is specified, and no special key flags were specified
596 * with #mdb_open(), the keys are compared lexically, with shorter keys collating
597 * before longer keys.
598 * @warning This function must be called before any data access functions are used,
599 * otherwise data corruption may occur. The same function must be used by every
600 * process accessing the database, every time the database is used.
601 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
602 * @param[in] dbi A database handle returned by #mdb_open()
603 * @param[in] cmp A #MDB_cmp_func function
604 * @return A non-zero error value on failure and 0 on success. Some possible
607 * <li>EINVAL - an invalid parameter was specified.
610 int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
612 /** Set a custom data comparison function for a #MDB_DUPSORT database.
613 * This comparison function is called whenever it is necessary to compare a data
614 * item specified by the application with a data item currently stored in the database.
615 * This function only takes effect if the database was opened with the #MDB_DUPSORT
617 * If no comparison function is specified, and no special key flags were specified
618 * with #mdb_open(), the data items are compared lexically, with shorter items collating
619 * before longer items.
620 * @warning This function must be called before any data access functions are used,
621 * otherwise data corruption may occur. The same function must be used by every
622 * process accessing the database, every time the database is used.
623 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
624 * @param[in] dbi A database handle returned by #mdb_open()
625 * @param[in] cmp A #MDB_cmp_func function
626 * @return A non-zero error value on failure and 0 on success. Some possible
629 * <li>EINVAL - an invalid parameter was specified.
632 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
634 /** Set a relocation function for a #MDB_FIXEDMAP database.
635 * @todo The relocation function is called whenever it is necessary to move the data
636 * of an item to a different position in the database (e.g. through tree
637 * balancing operations, shifts as a result of adds or deletes, etc.). It is
638 * intended to allow address/position-dependent data items to be stored in
639 * a database in an environment opened with the #MDB_FIXEDMAP option.
640 * Currently the relocation feature is unimplemented and setting
641 * this function has no effect.
642 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
643 * @param[in] dbi A database handle returned by #mdb_open()
644 * @param[in] rel A #MDB_rel_func function
645 * @return A non-zero error value on failure and 0 on success. Some possible
648 * <li>EINVAL - an invalid parameter was specified.
651 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
653 /** Get items from a database.
654 * This function retrieves key/data pairs from the database. The address
655 * and length of the data associated with the specified \b key are returned
656 * in the structure to which \b data refers.
657 * If the database supports duplicate keys (#MDB_DUPSORT) then the
658 * first data item for the key will be returned. Retrieval of other
659 * items requires the use of #mdb_cursor_get().
661 * @note The memory pointed to by the returned values is owned by the
662 * database. The caller need not dispose of the memory, and may not
663 * modify it in any way. For values returned in a read-only transaction
664 * any modification attempts will cause a SIGSEGV.
665 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
666 * @param[in] dbi A database handle returned by #mdb_open()
667 * @param[in] key The key to search for in the database
668 * @param[out] data The data corresponding to the key
669 * @return A non-zero error value on failure and 0 on success. Some possible
672 * <li>#MDB_NOTFOUND - the key was not in the database.
673 * <li>EINVAL - an invalid parameter was specified.
676 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
678 /** Store items into a database.
679 * This function stores key/data pairs in the database. The default behavior
680 * is to enter the new key/data pair, replacing any previously existing key
681 * if duplicates are disallowed, or adding a duplicate data item if
682 * duplicates are allowed (#MDB_DUPSORT).
683 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
684 * @param[in] dbi A database handle returned by #mdb_open()
685 * @param[in] key The key to store in the database
686 * @param[in,out] data The data to store
687 * @param[in] flags Special options for this operation. This parameter
688 * must be set to 0 or by bitwise OR'ing together one or more of the
689 * values described here.
691 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
692 * already appear in the database. This flag may only be specified
693 * if the database was opened with #MDB_DUPSORT. The function will
694 * return #MDB_KEYEXIST if the key/data pair already appears in the
696 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
697 * does not already appear in the database. The function will return
698 * #MDB_KEYEXIST if the key already appears in the database, even if
699 * the database supports duplicates (#MDB_DUPSORT). The \b data
700 * parameter will be set to point to the existing item.
702 * @return A non-zero error value on failure and 0 on success. Some possible
705 * <li>EACCESS - an attempt was made to write in a read-only transaction.
706 * <li>EINVAL - an invalid parameter was specified.
707 * <li>ENOMEM - the database is full, see #mdb_env_set_mapsize().
710 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
713 /** Delete items from a database.
714 * This function removes key/data pairs from the database.
715 * If the database does not support sorted duplicate data items
716 * (#MDB_DUPSORT) the data parameter is ignored.
717 * If the database supports sorted duplicates and the data parameter
718 * is NULL, all of the duplicate data items for the key will be
719 * deleted. Otherwise, if the data parameter is non-NULL
720 * only the matching data item will be deleted.
721 * This function will return #MDB_NOTFOUND if the specified key/data
722 * pair is not in the database.
723 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
724 * @param[in] dbi A database handle returned by #mdb_open()
725 * @param[in] key The key to delete from the database
726 * @param[in] data The data to delete
727 * @return A non-zero error value on failure and 0 on success. Some possible
730 * <li>EACCESS - an attempt was made to write in a read-only transaction.
731 * <li>EINVAL - an invalid parameter was specified.
734 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
736 /** Create a cursor handle.
737 * Cursors are associated with a specific transaction and database and
738 * may not span threads.
739 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
740 * @param[in] dbi A database handle returned by #mdb_open()
741 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
742 * @return A non-zero error value on failure and 0 on success. Some possible
745 * <li>EINVAL - an invalid parameter was specified.
748 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
750 /** Close a cursor handle.
751 * The cursor handle will be freed and must not be used again after this call.
752 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
754 void mdb_cursor_close(MDB_cursor *cursor);
756 /** Retrieve by cursor.
757 * This function retrieves key/data pairs from the database. The address and length
758 * of the key are returned in the object to which \b key refers (except for the
759 * case of the #MDB_SET option, in which the \b key object is unchanged), and
760 * the address and length of the data are returned in the object to which \b data
762 * @bug Cursors are not coordinated with write operations. If a cursor in a
763 * write transaction is performing a sequential scan while records are being
764 * inserted or deleted in the same transaction, the cursor will be corrupted.
765 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
766 * @param[in,out] key The key for a retrieved item
767 * @param[in,out] data The data of a retrieved item
768 * @param[in] op A cursor operation #MDB_cursor_op
769 * @return A non-zero error value on failure and 0 on success. Some possible
772 * <li>#MDB_NOTFOUND - no matching key found.
773 * <li>EINVAL - an invalid parameter was specified.
776 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
780 * This function stores key/data pairs into the database.
781 * If the function fails for any reason, the state of the cursor will be
782 * unchanged. If the function succeeds and an item is inserted into the
783 * database, the cursor is always positioned to refer to the newly inserted item.
784 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
785 * @param[in] key The key operated on.
786 * @param[in] data The data operated on.
787 * @param[in] flags Options for this operation. This parameter
788 * must be set to 0 or one of the values described here.
790 * <li>#MDB_CURRENT - overwrite the data of the key/data pair to which
791 * the cursor refers with the specified data item. The \b key
792 * parameter is ignored.
793 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
794 * already appear in the database. This flag may only be specified
795 * if the database was opened with #MDB_DUPSORT. The function will
796 * return #MDB_KEYEXIST if the key/data pair already appears in the
798 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
799 * does not already appear in the database. The function will return
800 * #MDB_KEYEXIST if the key already appears in the database, even if
801 * the database supports duplicates (#MDB_DUPSORT).
803 * @return A non-zero error value on failure and 0 on success. Some possible
806 * <li>EACCES - an attempt was made to modify a read-only database.
807 * <li>EINVAL - an invalid parameter was specified.
810 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
813 /** Delete current key/data pair
814 * This function deletes the key/data pair to which the cursor refers.
815 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
816 * @param[in] flags Options for this operation. This parameter
817 * must be set to 0 or one of the values described here.
819 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
820 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
822 * @return A non-zero error value on failure and 0 on success. Some possible
825 * <li>EACCES - an attempt was made to modify a read-only database.
826 * <li>EINVAL - an invalid parameter was specified.
829 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
831 /** Return count of duplicates for current key.
832 * This call is only valid on databases that support sorted duplicate
833 * data items #MDB_DUPSORT.
834 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
835 * @param[out] countp Address where the count will be stored
836 * @return A non-zero error value on failure and 0 on success. Some possible
839 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
842 int mdb_cursor_count(MDB_cursor *cursor, unsigned long *countp);
844 /** Compare two data items according to a particular database.
845 * This returns a comparison as if the two data items were keys in the
846 * specified database.
847 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
848 * @param[in] dbi A database handle returned by #mdb_open()
849 * @param[in] a The first item to compare
850 * @param[in] b The second item to compare
851 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
853 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);