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 * @note The keys size must actually be equal to
156 * sizeof(int) or sizeof(long) otherwise there will be
157 * alignment issues. On some processors, accessing misaligned
158 * data will cause a SIGBUS.
160 #define MDB_INTEGERKEY 0x08
161 /** with #MDB_DUPSORT, sorted dup items have fixed size */
162 #define MDB_DUPFIXED 0x10
163 /** with #MDB_DUPSORT, dups are numeric in native byte order */
164 #define MDB_INTEGERDUP 0x20
165 /** with #MDB_DUPSORT, use reverse string dups */
166 #define MDB_REVERSEDUP 0x40
167 /** create DB if not already existing */
168 #define MDB_CREATE 0x40000
171 /** @defgroup mdb_put Write Flags
174 /** For put: Don't write if the key already exists. */
175 #define MDB_NOOVERWRITE 0x10
176 /** Only for #MDB_DUPSORT<br>
177 * For put: don't write if the key and data pair already exist.<br>
178 * For mdb_cursor_del: remove all duplicate data items.
180 #define MDB_NODUPDATA 0x20
181 /** For mdb_cursor_put: overwrite the current key/data pair */
182 #define MDB_CURRENT 0x40
185 /** Cursor operations */
186 typedef enum MDB_cursor_op {
187 MDB_FIRST, /**< Position at first key/data item */
188 MDB_FIRST_DUP, /**< Position at first data item of current key.
189 Only for #MDB_DUPSORT */
190 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
191 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
192 MDB_GET_MULTIPLE, /**< Return all the duplicate data items at the current
193 cursor position. Only for #MDB_DUPFIXED */
194 MDB_LAST, /**< Position at last key/data item */
195 MDB_LAST_DUP, /**< Position at last data item of current key.
196 Only for #MDB_DUPSORT */
197 MDB_NEXT, /**< Position at next data item */
198 MDB_NEXT_DUP, /**< Position at next data item of current key.
199 Only for #MDB_DUPSORT */
200 MDB_NEXT_MULTIPLE, /**< Return all duplicate data items at the next
201 cursor position. Only for #MDB_DUPFIXED */
202 MDB_NEXT_NODUP, /**< Position at first data item of next key.
203 Only for #MDB_DUPSORT */
204 MDB_PREV, /**< Position at previous data item */
205 MDB_PREV_DUP, /**< Position at previous data item of current key.
206 Only for #MDB_DUPSORT */
207 MDB_PREV_NODUP, /**< Position at last data item of previous key.
208 only for #MDB_DUPSORT */
209 MDB_SET, /**< Position at specified key */
210 MDB_SET_RANGE /**< Position at first key greater than or equal to specified key. */
213 /** @defgroup errors Return Codes
215 * BerkeleyDB uses -30800 to -30999, we'll go under them
218 /** Successful result */
219 #define MDB_SUCCESS 0
220 /** key/data pair already exists */
221 #define MDB_KEYEXIST (-30799)
222 /** key/data pair not found (EOF) */
223 #define MDB_NOTFOUND (-30798)
224 /** Requested page not found - this usually indicates corruption */
225 #define MDB_PAGE_NOTFOUND (-30797)
226 /** Located page was wrong type */
227 #define MDB_CORRUPTED (-30796)
228 /** Update of meta page failed, probably I/O error */
229 #define MDB_PANIC (-30795)
230 /** Environment version mismatch */
231 #define MDB_VERSION_MISMATCH (-30794)
234 /** Statistics for a database in the environment */
235 typedef struct MDB_stat {
236 unsigned int ms_psize; /**< Size of a database page.
237 This is currently the same for all databases. */
238 unsigned int ms_depth; /**< Depth (height) of the B-tree */
239 unsigned long ms_branch_pages; /**< Number of internal (non-leaf) pages */
240 unsigned long ms_leaf_pages; /**< Number of leaf pages */
241 unsigned long ms_overflow_pages; /**< Number of overflow pages */
242 unsigned long ms_entries; /**< Number of data items */
245 /** Return the mdb library version information.
246 * @param[out] major if non-NULL, the library major version number is copied here
247 * @param[out] minor if non-NULL, the library minor version number is copied here
248 * @param[out] patch if non-NULL, the library patch version number is copied here
249 * @retval "version string" The library version as a string
251 char *mdb_version(int *major, int *minor, int *patch);
253 /** Return a string describing a given error code.
254 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
255 * function. If the error code is greater than or equal to 0, then the string
256 * returned by the system function strerror(3) is returned. If the error code
257 * is less than 0, an error string corresponding to the MDB library error is
258 * returned. See @ref errors for a list of MDB-specific error codes.
259 * @param[in] err The error code
260 * @retval "error message" The description of the error
262 char *mdb_strerror(int err);
264 /** Create an MDB environment handle.
265 * This function allocates memory for a #MDB_env structure. To release
266 * the allocated memory and discard the handle, call #mdb_env_close().
267 * Before the handle may be used, it must be opened using #mdb_env_open().
268 * Various other options may also need to be set before opening the handle,
269 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
270 * depending on usage requirements.
271 * @param[out] env The address where the new handle will be stored
272 * @return A non-zero error value on failure and 0 on success.
274 int mdb_env_create(MDB_env **env);
276 /** Open an environment handle.
277 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
278 * @param[in] env An environment handle returned by #mdb_env_create()
279 * @param[in] path The directory in which the database files reside. This
280 * directory must already exist and be writable.
281 * @param[in] flags Special options for this environment. This parameter
282 * must be set to 0 or by bitwise OR'ing together one or more of the
283 * values described here.
286 * use a fixed address for the mmap region. This flag must be specified
287 * when creating the environment, and is stored persistently in the environment.
288 * If successful, the memory map will always reside at the same virtual address
289 * and pointers used to reference data items in the database will be constant
290 * across multiple invocations. This option may not always work, depending on
291 * how the operating system has allocated memory to shared libraries and other uses.
292 * The feature is highly experimental.
294 * Don't perform a synchronous flush after committing a transaction. This means
295 * transactions will exhibit the ACI (atomicity, consistency, and isolation)
296 * properties, but not D (durability); that is database integrity will be
297 * maintained but it is possible some number of the most recently committed
298 * transactions may be undone after a system crash. The number of transactions
299 * at risk is governed by how often the system flushes dirty buffers to disk
300 * and how often #mdb_env_sync() is called. This flag may be changed
301 * at any time using #mdb_env_set_flags().
303 * Open the environment in read-only mode. No write operations will be allowed.
305 * @param[in] mode The UNIX permissions to set on created files. This parameter
306 * is ignored on Windows.
307 * @return A non-zero error value on failure and 0 on success. Some possible
310 * <li>#MDB_VERSION_MISMATCH - the version of the MDB library doesn't match the
311 * version that created the database environment.
312 * <li>EINVAL - the environment file headers are corrupted.
313 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
314 * <li>EACCES - the user didn't have permission to access the environment files.
315 * <li>EAGAIN - the environment was locked by another process.
318 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mode_t mode);
320 /** Return statistics about the MDB environment.
321 * @param[in] env An environment handle returned by #mdb_env_create()
322 * @param[out] stat The address of an #MDB_stat structure
323 * where the statistics will be copied
325 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
327 /** Flush the data buffers to disk.
328 * Data is always written to disk when #mdb_txn_commit() is called,
329 * but the operating system may keep it buffered. MDB always flushes
330 * the OS buffers upon commit as well, unless the environment was
331 * opened with #MDB_NOSYNC.
332 * @param[in] env An environment handle returned by #mdb_env_create()
333 * @param[in] force If non-zero, force the flush to occur. Otherwise
334 * if the environment has the #MDB_NOSYNC flag set the flushes
336 * @return A non-zero error value on failure and 0 on success. Some possible
339 * <li>EINVAL - an invalid parameter was specified.
340 * <li>EIO - an error occurred during synchronization.
343 int mdb_env_sync(MDB_env *env, int force);
345 /** Close the environment and release the memory map.
346 * Only a single thread may call this function. All transactions, databases,
347 * and cursors must already be closed before calling this function. Attempts to
348 * use any such handles after calling this function will cause a SIGSEGV.
349 * The environment handle will be freed and must not be used again after this call.
350 * @param[in] env An environment handle returned by #mdb_env_create()
352 void mdb_env_close(MDB_env *env);
354 /** Set environment flags.
355 * This may be used to set some flags that weren't already set during
356 * #mdb_env_open(), or to unset these flags. Currently only the
357 * #MDB_NOSYNC flag setting may be changed with this function.
358 * @param[in] env An environment handle returned by #mdb_env_create()
359 * @param[in] flags The flags to change, bitwise OR'ed together
360 * @param[in] onoff A non-zero value sets the flags, zero clears them.
361 * @return A non-zero error value on failure and 0 on success. Some possible
364 * <li>EINVAL - an invalid parameter was specified.
367 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
369 /** Get environment flags.
370 * @param[in] env An environment handle returned by #mdb_env_create()
371 * @param[out] flags The address of an integer to store the flags
372 * @return A non-zero error value on failure and 0 on success. Some possible
375 * <li>EINVAL - an invalid parameter was specified.
378 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
380 /** Return the path that was used in #mdb_env_open().
381 * @param[in] env An environment handle returned by #mdb_env_create()
382 * @param[out] path Address of a string pointer to contain the path. This
383 * is the actual string in the environment, not a copy. It should not be
384 * altered in any way.
385 * @return A non-zero error value on failure and 0 on success. Some possible
388 * <li>EINVAL - an invalid parameter was specified.
391 int mdb_env_get_path(MDB_env *env, const char **path);
393 /** Set the size of the memory map to use for this environment.
394 * The size should be a multiple of the OS page size. The default is
395 * 10485760 bytes. The size of the memory map is also the maximum size
396 * of the database. The value should be chosen as large as possible,
397 * to accomodate future growth of the database.
398 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
399 * @param[in] env An environment handle returned by #mdb_env_create()
400 * @param[in] size The size in bytes
401 * @return A non-zero error value on failure and 0 on success. Some possible
404 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
407 int mdb_env_set_mapsize(MDB_env *env, size_t size);
409 /** Set the maximum number of threads for the environment.
410 * This defines the number of slots in the lock table that is used to track readers in the
411 * the environment. The default is 126.
412 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
413 * @param[in] env An environment handle returned by #mdb_env_create()
414 * @param[in] readers The maximum number of threads
415 * @return A non-zero error value on failure and 0 on success. Some possible
418 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
421 int mdb_env_set_maxreaders(MDB_env *env, int readers);
423 /** Get the maximum number of threads for the environment.
424 * @param[in] env An environment handle returned by #mdb_env_create()
425 * @param[out] readers Address of an integer to store the number of readers
426 * @return A non-zero error value on failure and 0 on success. Some possible
429 * <li>EINVAL - an invalid parameter was specified.
432 int mdb_env_get_maxreaders(MDB_env *env, int *readers);
434 /** Set the maximum number of databases for the environment.
435 * This function is only needed if multiple databases will be used in the
436 * environment. Simpler applications that only use a single database can ignore
438 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
439 * @param[in] env An environment handle returned by #mdb_env_create()
440 * @param[in] dbs The maximum number of databases
441 * @return A non-zero error value on failure and 0 on success. Some possible
444 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
447 int mdb_env_set_maxdbs(MDB_env *env, int dbs);
449 /** Create a transaction for use with the environment.
450 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
451 * @note Transactions may not span threads, a transaction must only be used by a
453 * @note Cursors may not span transactions; each cursor must be opened and closed
454 * within a single transaction.
455 * @param[in] env An environment handle returned by #mdb_env_create()
456 * @param[in] flags Special options for this transaction. This parameter
457 * must be set to 0 or by bitwise OR'ing together one or more of the
458 * values described here.
461 * This transaction will not perform any write operations.
463 * @param[out] txn Address where the new #MDB_txn handle will be stored
464 * @return A non-zero error value on failure and 0 on success. Some possible
467 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
469 * <li>ENOMEM - out of memory, or a read-only transaction was requested and
470 * the reader lock table is full. See #mdb_env_set_maxreaders().
473 int mdb_txn_begin(MDB_env *env, unsigned int flags, MDB_txn **txn);
475 /** Commit all the operations of a transaction into the database.
476 * All cursors opened within the transaction must be closed before the transaction
478 * The transaction handle will be freed and must not be used again after this call.
479 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
480 * @return A non-zero error value on failure and 0 on success. Some possible
483 * <li>EINVAL - an invalid parameter was specified.
484 * <li>ENOSPC - no more disk space.
485 * <li>EIO - a low-level I/O error occurred while writing.
488 int mdb_txn_commit(MDB_txn *txn);
490 /** Abandon all the operations of the transaction instead of saving them.
491 * All cursors opened within the transaction must be closed before the transaction
493 * The transaction handle will be freed and must not be used again after this call.
494 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
496 void mdb_txn_abort(MDB_txn *txn);
498 /** Reset a read-only transaction.
499 * This releases the current reader lock but doesn't free the
500 * transaction handle, allowing it to be used again later by #mdb_txn_renew().
501 * It otherwise has the same affect as #mdb_txn_abort() but saves some memory
502 * allocation/deallocation overhead if a thread is going to start a new
503 * read-only transaction again soon.
504 * All cursors opened within the transaction must be closed before the transaction
506 * Reader locks generally don't interfere with writers, but they keep old
507 * versions of database pages allocated. Thus they prevent the old pages
508 * from being reused when writers commit new data, and so under heavy load
509 * the database size may grow much more rapidly than otherwise.
510 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
512 void mdb_txn_reset(MDB_txn *txn);
514 /** Renew a read-only transaction.
515 * This acquires a new reader lock for a transaction handle that had been
516 * released by #mdb_txn_reset(). It must be called before a reset transaction
518 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
519 * @return A non-zero error value on failure and 0 on success. Some possible
522 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
524 * <li>EINVAL - an invalid parameter was specified.
527 int mdb_txn_renew(MDB_txn *txn);
529 /** Open a database in the environment.
530 * The database handle may be discarded by calling #mdb_close().
531 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
532 * @param[in] name The name of the database to open. If only a single
533 * database is needed in the enviroment, this value may be NULL.
534 * @param[in] flags Special options for this database. This parameter
535 * must be set to 0 or by bitwise OR'ing together one or more of the
536 * values described here.
538 * <li>#MDB_REVERSEKEY
539 * Keys are strings to be compared in reverse order, from the end
540 * of the strings to the beginning. By default, Keys are treated as strings and
541 * compared from beginning to end.
543 * Duplicate keys may be used in the database. (Or, from another perspective,
544 * keys may have multiple data items, stored in sorted order.) By default
545 * keys must be unique and may have only a single data item.
546 * <li>#MDB_INTEGERKEY
547 * Keys are binary integers in native byte order. On Big-Endian systems
548 * this flag has no effect. On Little-Endian systems this flag behaves
549 * the same as #MDB_REVERSEKEY. This flag is simply provided as a
550 * convenience so that applications don't need to detect Endianness themselves
551 * when using integers as keys.
553 * This flag may only be used in combination with #MDB_DUPSORT. This option
554 * tells the library that the data items for this database are all the same
555 * size, which allows further optimizations in storage and retrieval. When
556 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
557 * cursor operations may be used to retrieve multiple items at once.
558 * <li>#MDB_INTEGERDUP
559 * This option specifies that duplicate data items are also integers, and
560 * should be sorted as such.
562 * Create the named database if it doesn't exist. This option is not
563 * allowed in a read-only transaction or a read-only environment.
565 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
566 * @return A non-zero error value on failure and 0 on success. Some possible
569 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
570 * and #MDB_CREATE was not specified.
571 * <li>ENFILE - too many databases have been opened. See #mdb_env_set_maxdbs().
574 int mdb_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
576 /** Retrieve statistics for a database.
577 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
578 * @param[in] dbi A database handle returned by #mdb_open()
579 * @param[out] stat The address of an #MDB_stat structure
580 * where the statistics will be copied
581 * @return A non-zero error value on failure and 0 on success. Some possible
584 * <li>EINVAL - an invalid parameter was specified.
587 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
589 /** Close a database handle.
590 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
591 * @param[in] dbi A database handle returned by #mdb_open()
593 void mdb_close(MDB_txn *txn, MDB_dbi dbi);
595 /** Set a custom key comparison function for a database.
596 * The comparison function is called whenever it is necessary to compare a
597 * key specified by the application with a key currently stored in the database.
598 * If no comparison function is specified, and no special key flags were specified
599 * with #mdb_open(), the keys are compared lexically, with shorter keys collating
600 * before longer keys.
601 * @warning This function must be called before any data access functions are used,
602 * otherwise data corruption may occur. The same function must be used by every
603 * process accessing the database, every time the database is used.
604 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
605 * @param[in] dbi A database handle returned by #mdb_open()
606 * @param[in] cmp A #MDB_cmp_func function
607 * @return A non-zero error value on failure and 0 on success. Some possible
610 * <li>EINVAL - an invalid parameter was specified.
613 int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
615 /** Set a custom data comparison function for a #MDB_DUPSORT database.
616 * This comparison function is called whenever it is necessary to compare a data
617 * item specified by the application with a data item currently stored in the database.
618 * This function only takes effect if the database was opened with the #MDB_DUPSORT
620 * If no comparison function is specified, and no special key flags were specified
621 * with #mdb_open(), the data items are compared lexically, with shorter items collating
622 * before longer items.
623 * @warning This function must be called before any data access functions are used,
624 * otherwise data corruption may occur. The same function must be used by every
625 * process accessing the database, every time the database is used.
626 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
627 * @param[in] dbi A database handle returned by #mdb_open()
628 * @param[in] cmp A #MDB_cmp_func function
629 * @return A non-zero error value on failure and 0 on success. Some possible
632 * <li>EINVAL - an invalid parameter was specified.
635 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
637 /** Set a relocation function for a #MDB_FIXEDMAP database.
638 * @todo The relocation function is called whenever it is necessary to move the data
639 * of an item to a different position in the database (e.g. through tree
640 * balancing operations, shifts as a result of adds or deletes, etc.). It is
641 * intended to allow address/position-dependent data items to be stored in
642 * a database in an environment opened with the #MDB_FIXEDMAP option.
643 * Currently the relocation feature is unimplemented and setting
644 * this function has no effect.
645 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
646 * @param[in] dbi A database handle returned by #mdb_open()
647 * @param[in] rel A #MDB_rel_func function
648 * @return A non-zero error value on failure and 0 on success. Some possible
651 * <li>EINVAL - an invalid parameter was specified.
654 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
656 /** Get items from a database.
657 * This function retrieves key/data pairs from the database. The address
658 * and length of the data associated with the specified \b key are returned
659 * in the structure to which \b data refers.
660 * If the database supports duplicate keys (#MDB_DUPSORT) then the
661 * first data item for the key will be returned. Retrieval of other
662 * items requires the use of #mdb_cursor_get().
663 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
664 * @param[in] dbi A database handle returned by #mdb_open()
665 * @param[in] key The key to search for in the database
666 * @param[out] data The data corresponding to the key
667 * @return A non-zero error value on failure and 0 on success. Some possible
670 * <li>#MDB_NOTFOUND - the key was not in the database.
671 * <li>EINVAL - an invalid parameter was specified.
674 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
676 /** Store items into a database.
677 * This function stores key/data pairs in the database. The default behavior
678 * is to enter the new key/data pair, replacing any previously existing key
679 * if duplicates are disallowed, or adding a duplicate data item if
680 * duplicates are allowed (#MDB_DUPSORT).
681 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
682 * @param[in] dbi A database handle returned by #mdb_open()
683 * @param[in] key The key to store in the database
684 * @param[in] data The data to store
685 * @param[in] flags Special options for this operation. This parameter
686 * must be set to 0 or by bitwise OR'ing together one or more of the
687 * values described here.
689 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
690 * already appear in the database. This flag may only be specified
691 * if the database was opened with #MDB_DUPSORT. The function will
692 * return #MDB_KEYEXIST if the key/data pair already appears in the
694 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
695 * does not already appear in the database. The function will return
696 * #MDB_KEYEXIST if the key already appears in the database, even if
697 * the database supports duplicates (#MDB_DUPSORT).
699 * @return A non-zero error value on failure and 0 on success. Some possible
702 * <li>EACCESS - an attempt was made to write in a read-only transaction.
703 * <li>EINVAL - an invalid parameter was specified.
704 * <li>ENOMEM - the database is full, see #mdb_env_set_mapsize().
707 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
710 /** Delete items from a database.
711 * This function removes key/data pairs from the database.
712 * If the database does not support sorted duplicate data items
713 * (#MDB_DUPSORT) the data parameter is ignored.
714 * If the database supports sorted duplicates and the data parameter
715 * is NULL, all of the duplicate data items for the key will be
716 * deleted. Otherwise, if the data parameter is non-NULL
717 * only the matching data item will be deleted.
718 * This function will return #MDB_NOTFOUND if the specified key/data
719 * pair is not in the database.
720 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
721 * @param[in] dbi A database handle returned by #mdb_open()
722 * @param[in] key The key to delete from the database
723 * @param[in] data The data to delete
724 * @return A non-zero error value on failure and 0 on success. Some possible
727 * <li>EACCESS - an attempt was made to write in a read-only transaction.
728 * <li>EINVAL - an invalid parameter was specified.
731 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
733 /** Create a cursor handle.
734 * Cursors are associated with a specific transaction and database and
735 * may not span threads.
736 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
737 * @param[in] dbi A database handle returned by #mdb_open()
738 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
739 * @return A non-zero error value on failure and 0 on success. Some possible
742 * <li>EINVAL - an invalid parameter was specified.
745 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
747 /** Close a cursor handle.
748 * The cursor handle will be freed and must not be used again after this call.
749 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
751 void mdb_cursor_close(MDB_cursor *cursor);
753 /** Retrieve by cursor.
754 * This function retrieves key/data pairs from the database. The address and length
755 * of the key are returned in the object to which \b key refers (except for the
756 * case of the #MDB_SET option, in which the \b key object is unchanged), and
757 * the address and length of the data are returned in the object to which \b data
759 * @bug Cursors are not coordinated with write operations. If a cursor in a
760 * write transaction is performing a sequential scan while records are being
761 * inserted or deleted in the same transaction, the cursor will be corrupted.
762 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
763 * @param[in,out] key The key for a retrieved item
764 * @param[in,out] data The data of a retrieved item
765 * @param[in] op A cursor operation #MDB_cursor_op
766 * @return A non-zero error value on failure and 0 on success. Some possible
769 * <li>#MDB_NOTFOUND - no matching key found.
770 * <li>EINVAL - an invalid parameter was specified.
773 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
777 * This function stores key/data pairs into the database.
778 * If the function fails for any reason, the state of the cursor will be
779 * unchanged. If the function succeeds and an item is inserted into the
780 * database, the cursor is always positioned to refer to the newly inserted item.
781 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
782 * @param[in] key The key operated on.
783 * @param[in] data The data operated on.
784 * @param[in] flags Options for this operation. This parameter
785 * must be set to 0 or one of the values described here.
787 * <li>#MDB_CURRENT - overwrite the data of the key/data pair to which
788 * the cursor refers with the specified data item. The \b key
789 * parameter is ignored.
790 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
791 * already appear in the database. This flag may only be specified
792 * if the database was opened with #MDB_DUPSORT. The function will
793 * return #MDB_KEYEXIST if the key/data pair already appears in the
795 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
796 * does not already appear in the database. The function will return
797 * #MDB_KEYEXIST if the key already appears in the database, even if
798 * the database supports duplicates (#MDB_DUPSORT).
800 * @return A non-zero error value on failure and 0 on success. Some possible
803 * <li>EACCES - an attempt was made to modify a read-only database.
804 * <li>EINVAL - an invalid parameter was specified.
807 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
810 /** Delete current key/data pair
811 * This function deletes the key/data pair to which the cursor refers.
812 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
813 * @param[in] flags Options for this operation. This parameter
814 * must be set to 0 or one of the values described here.
816 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
817 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
819 * @return A non-zero error value on failure and 0 on success. Some possible
822 * <li>EACCES - an attempt was made to modify a read-only database.
823 * <li>EINVAL - an invalid parameter was specified.
826 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
828 /** Return count of duplicates for current key.
829 * This call is only valid on databases that support sorted duplicate
830 * data items #MDB_DUPSORT.
831 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
832 * @param[out] countp Address where the count will be stored
833 * @return A non-zero error value on failure and 0 on success. Some possible
836 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
839 int mdb_cursor_count(MDB_cursor *cursor, unsigned long *countp);
841 /** Compare two data items according to a particular database.
842 * This returns a comparison as if the two data items were keys in the
843 * specified database.
844 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
845 * @param[in] dbi A database handle returned by #mdb_open()
846 * @param[in] a The first item to compare
847 * @param[in] b The second item to compare
848 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
850 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);