2 * @brief Lightning memory-mapped database library
4 * @mainpage Lightning Memory-Mapped Database Manager (LMDB)
6 * @section intro_sec Introduction
7 * LMDB is a Btree-based database management library modeled loosely on the
8 * BerkeleyDB API, but much simplified. The entire database is exposed
9 * in a memory map, and all data fetches return data directly
10 * from the mapped memory, so no malloc's or memcpy's occur during
11 * data fetches. As such, the library is extremely simple because it
12 * requires no page caching layer of its own, and it is extremely high
13 * performance and memory-efficient. It is also fully transactional with
14 * full ACID semantics, and when the memory map is read-only, the
15 * database integrity cannot be corrupted by stray pointer writes from
18 * The library is fully thread-aware and supports concurrent read/write
19 * access from multiple processes and threads. Data pages use a copy-on-
20 * write strategy so no active data pages are ever overwritten, which
21 * also provides resistance to corruption and eliminates the need of any
22 * special recovery procedures after a system crash. Writes are fully
23 * serialized; only one write transaction may be active at a time, which
24 * guarantees that writers can never deadlock. The database structure is
25 * multi-versioned so readers run with no locks; writers cannot block
26 * readers, and readers don't block writers.
28 * Unlike other well-known database mechanisms which use either write-ahead
29 * transaction logs or append-only data writes, LMDB requires no maintenance
30 * during operation. Both write-ahead loggers and append-only databases
31 * require periodic checkpointing and/or compaction of their log or database
32 * files otherwise they grow without bound. LMDB tracks free pages within
33 * the database and re-uses them for new write operations, so the database
34 * size does not grow without bound in normal use.
36 * The memory map can be used as a read-only or read-write map. It is
37 * read-only by default as this provides total immunity to corruption.
38 * Using read-write mode offers much higher write performance, but adds
39 * the possibility for stray application writes thru pointers to silently
40 * corrupt the database. Of course if your application code is known to
41 * be bug-free (...) then this is not an issue.
43 * @section caveats_sec Caveats
44 * Troubleshooting the lock file, plus semaphores on BSD systems:
46 * - A broken lockfile can cause sync issues.
47 * Stale reader transactions left behind by an aborted program
48 * cause further writes to grow the database quickly, and
49 * stale locks can block further operation.
51 * Fix: Check for stale readers periodically, using the
52 * #mdb_reader_check function or the \ref mdb_stat_1 "mdb_stat" tool.
54 * locks with option MDB_ROBUST if supported (non-BSD). Or just
55 * make all programs using the database close it; the lockfile
56 * is always reset on first open of the environment.
58 * - On BSD systems or others configured with MDB_USE_POSIX_SEM,
59 * startup can fail due to semaphores owned by another userid.
61 * Fix: Open and close the database as the user which owns the
62 * semaphores (likely last user) or as root, while no other
63 * process is using the database.
65 * Restrictions/caveats (in addition to those listed for some functions):
67 * - Only the database owner should normally use the database on
68 * BSD systems or when otherwise configured with MDB_USE_POSIX_SEM.
69 * Multiple users can cause startup to fail later, as noted above.
71 * - There is normally no pure read-only mode, since readers need write
72 * access to locks and lock file. Exceptions: On read-only filesystems
73 * or with the #MDB_NOLOCK flag described under #mdb_env_open().
75 * - By default, in versions before 0.9.10, unused portions of the data
76 * file might receive garbage data from memory freed by other code.
77 * (This does not happen when using the #MDB_WRITEMAP flag.) As of
78 * 0.9.10 the default behavior is to initialize such memory before
79 * writing to the data file. Since there may be a slight performance
80 * cost due to this initialization, applications may disable it using
81 * the #MDB_NOMEMINIT flag. Applications handling sensitive data
82 * which must not be written should not use this flag. This flag is
83 * irrelevant when using #MDB_WRITEMAP.
85 * - A thread can only use one transaction at a time, plus any child
86 * transactions. Each transaction belongs to one thread. See below.
87 * The #MDB_NOTLS flag changes this for read-only transactions.
89 * - Use an MDB_env* in the process which opened it, without fork()ing.
91 * - Do not have open an LMDB database twice in the same process at
92 * the same time. Not even from a plain open() call - close()ing it
93 * breaks flock() advisory locking.
95 * - Avoid long-lived transactions. Read transactions prevent
96 * reuse of pages freed by newer write transactions, thus the
97 * database can grow quickly. Write transactions prevent
98 * other write transactions, since writes are serialized.
100 * - Avoid suspending a process with active transactions. These
101 * would then be "long-lived" as above. Also read transactions
102 * suspended when writers commit could sometimes see wrong data.
104 * ...when several processes can use a database concurrently:
106 * - Avoid aborting a process with an active transaction.
107 * The transaction becomes "long-lived" as above until a check
108 * for stale readers is performed or the lockfile is reset,
109 * since the process may not remove it from the lockfile.
110 * Except write-transactions on Unix with MDB_ROBUST or on Windows.
112 * - If you do that anyway, do a periodic check for stale readers. Or
113 * close the environment once in a while, so the lockfile can get reset.
115 * - Do not use LMDB databases on remote filesystems, even between
116 * processes on the same host. This breaks flock() on some OSes,
117 * possibly memory map sync, and certainly sync between programs
118 * on different hosts.
120 * - Opening a database can fail if another process is opening or
121 * closing it at exactly the same time.
123 * @author Howard Chu, Symas Corporation.
125 * @copyright Copyright 2011-2014 Howard Chu, Symas Corp. All rights reserved.
127 * Redistribution and use in source and binary forms, with or without
128 * modification, are permitted only as authorized by the OpenLDAP
131 * A copy of this license is available in the file LICENSE in the
132 * top-level directory of the distribution or, alternatively, at
133 * <http://www.OpenLDAP.org/license.html>.
136 * This code is derived from btree.c written by Martin Hedenfalk.
138 * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
140 * Permission to use, copy, modify, and distribute this software for any
141 * purpose with or without fee is hereby granted, provided that the above
142 * copyright notice and this permission notice appear in all copies.
144 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
145 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
146 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
147 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
148 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
149 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
150 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
155 #include <sys/types.h>
161 /** Unix permissions for creating files, or dummy definition for Windows */
163 typedef int mdb_mode_t;
165 typedef mode_t mdb_mode_t;
168 /** An abstraction for a file handle.
169 * On POSIX systems file handles are small integers. On Windows
170 * they're opaque pointers.
173 typedef void *mdb_filehandle_t;
175 typedef int mdb_filehandle_t;
178 /** @defgroup mdb LMDB API
180 * @brief OpenLDAP Lightning Memory-Mapped Database Manager
182 /** @defgroup Version Version Macros
185 /** Library major version */
186 #define MDB_VERSION_MAJOR 0
187 /** Library minor version */
188 #define MDB_VERSION_MINOR 9
189 /** Library patch version */
190 #define MDB_VERSION_PATCH 14
192 /** Combine args a,b,c into a single integer for easy version comparisons */
193 #define MDB_VERINT(a,b,c) (((a) << 24) | ((b) << 16) | (c))
195 /** The full library version as a single integer */
196 #define MDB_VERSION_FULL \
197 MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
199 /** The release date of this library version */
200 #define MDB_VERSION_DATE "September 20, 2014"
202 /** A stringifier for the version info */
203 #define MDB_VERSTR(a,b,c,d) "LMDB " #a "." #b "." #c ": (" d ")"
205 /** A helper for the stringifier macro */
206 #define MDB_VERFOO(a,b,c,d) MDB_VERSTR(a,b,c,d)
208 /** The full library version as a C string */
209 #define MDB_VERSION_STRING \
210 MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
213 /** @brief Opaque structure for a database environment.
215 * A DB environment supports multiple databases, all residing in the same
218 typedef struct MDB_env MDB_env;
220 /** @brief Opaque structure for a transaction handle.
222 * All database operations require a transaction handle. Transactions may be
223 * read-only or read-write.
225 typedef struct MDB_txn MDB_txn;
227 /** @brief A handle for an individual database in the DB environment. */
228 typedef unsigned int MDB_dbi;
230 /** @brief Opaque structure for navigating through a database */
231 typedef struct MDB_cursor MDB_cursor;
233 /** @brief Generic structure used for passing keys and data in and out
236 * Values returned from the database are valid only until a subsequent
237 * update operation, or the end of the transaction. Do not modify or
238 * free them, they commonly point into the database itself.
240 * Key sizes must be between 1 and #mdb_env_get_maxkeysize() inclusive.
241 * The same applies to data sizes in databases with the #MDB_DUPSORT flag.
242 * Other data items can in theory be from 0 to 0xffffffff bytes long.
244 typedef struct MDB_val {
245 size_t mv_size; /**< size of the data item */
246 void *mv_data; /**< address of the data item */
249 /** @brief A callback function used to compare two keys in a database */
250 typedef int (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
252 /** @brief A callback function used to relocate a position-dependent data item
253 * in a fixed-address database.
255 * The \b newptr gives the item's desired address in
256 * the memory map, and \b oldptr gives its previous address. The item's actual
257 * data resides at the address in \b item. This callback is expected to walk
258 * through the fields of the record in \b item and modify any
259 * values based at the \b oldptr address to be relative to the \b newptr address.
260 * @param[in,out] item The item that is to be relocated.
261 * @param[in] oldptr The previous address.
262 * @param[in] newptr The new address to relocate to.
263 * @param[in] relctx An application-provided context, set by #mdb_set_relctx().
264 * @todo This feature is currently unimplemented.
266 typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
268 /** @defgroup mdb_env Environment Flags
271 /** mmap at a fixed address (experimental) */
272 #define MDB_FIXEDMAP 0x01
273 /** no environment directory */
274 #define MDB_NOSUBDIR 0x4000
275 /** don't fsync after commit */
276 #define MDB_NOSYNC 0x10000
278 #define MDB_RDONLY 0x20000
279 /** don't fsync metapage after commit */
280 #define MDB_NOMETASYNC 0x40000
281 /** use writable mmap */
282 #define MDB_WRITEMAP 0x80000
283 /** use asynchronous msync when #MDB_WRITEMAP is used */
284 #define MDB_MAPASYNC 0x100000
285 /** tie reader locktable slots to #MDB_txn objects instead of to threads */
286 #define MDB_NOTLS 0x200000
287 /** don't do any locking, caller must manage their own locks */
288 #define MDB_NOLOCK 0x400000
289 /** don't do readahead (no effect on Windows) */
290 #define MDB_NORDAHEAD 0x800000
291 /** don't initialize malloc'd memory before writing to datafile */
292 #define MDB_NOMEMINIT 0x1000000
293 /** catch stale locks if supported (not on BSD, needs robust mutexes) */
294 #define MDB_ROBUST 0x2000000
297 /** @defgroup mdb_dbi_open Database Flags
300 /** use reverse string keys */
301 #define MDB_REVERSEKEY 0x02
302 /** use sorted duplicates */
303 #define MDB_DUPSORT 0x04
304 /** numeric keys in native byte order.
305 * The keys must all be of the same size. */
306 #define MDB_INTEGERKEY 0x08
307 /** with #MDB_DUPSORT, sorted dup items have fixed size */
308 #define MDB_DUPFIXED 0x10
309 /** with #MDB_DUPSORT, dups are numeric in native byte order */
310 #define MDB_INTEGERDUP 0x20
311 /** with #MDB_DUPSORT, use reverse string dups */
312 #define MDB_REVERSEDUP 0x40
313 /** create DB if not already existing */
314 #define MDB_CREATE 0x40000
317 /** @defgroup mdb_put Write Flags
320 /** For put: Don't write if the key already exists. */
321 #define MDB_NOOVERWRITE 0x10
322 /** Only for #MDB_DUPSORT<br>
323 * For put: don't write if the key and data pair already exist.<br>
324 * For mdb_cursor_del: remove all duplicate data items.
326 #define MDB_NODUPDATA 0x20
327 /** For mdb_cursor_put: overwrite the current key/data pair */
328 #define MDB_CURRENT 0x40
329 /** For put: Just reserve space for data, don't copy it. Return a
330 * pointer to the reserved space.
332 #define MDB_RESERVE 0x10000
333 /** Data is being appended, don't split full pages. */
334 #define MDB_APPEND 0x20000
335 /** Duplicate data is being appended, don't split full pages. */
336 #define MDB_APPENDDUP 0x40000
337 /** Store multiple data items in one call. Only for #MDB_DUPFIXED. */
338 #define MDB_MULTIPLE 0x80000
341 /** @defgroup mdb_copy Copy Flags
344 /** Compacting copy: Omit free space from copy, and renumber all
345 * pages sequentially.
347 #define MDB_CP_COMPACT 0x01
350 /** @brief Cursor Get operations.
352 * This is the set of all operations for retrieving data
355 typedef enum MDB_cursor_op {
356 MDB_FIRST, /**< Position at first key/data item */
357 MDB_FIRST_DUP, /**< Position at first data item of current key.
358 Only for #MDB_DUPSORT */
359 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
360 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
361 MDB_GET_CURRENT, /**< Return key/data at current cursor position */
362 MDB_GET_MULTIPLE, /**< Return key and up to a page of duplicate data items
363 from current cursor position. Move cursor to prepare
364 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
365 MDB_LAST, /**< Position at last key/data item */
366 MDB_LAST_DUP, /**< Position at last data item of current key.
367 Only for #MDB_DUPSORT */
368 MDB_NEXT, /**< Position at next data item */
369 MDB_NEXT_DUP, /**< Position at next data item of current key.
370 Only for #MDB_DUPSORT */
371 MDB_NEXT_MULTIPLE, /**< Return key and up to a page of duplicate data items
372 from next cursor position. Move cursor to prepare
373 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
374 MDB_NEXT_NODUP, /**< Position at first data item of next key */
375 MDB_PREV, /**< Position at previous data item */
376 MDB_PREV_DUP, /**< Position at previous data item of current key.
377 Only for #MDB_DUPSORT */
378 MDB_PREV_NODUP, /**< Position at last data item of previous key */
379 MDB_SET, /**< Position at specified key */
380 MDB_SET_KEY, /**< Position at specified key, return key + data */
381 MDB_SET_RANGE /**< Position at first key greater than or equal to specified key. */
384 /** @defgroup errors Return Codes
386 * BerkeleyDB uses -30800 to -30999, we'll go under them
389 /** Successful result */
390 #define MDB_SUCCESS 0
391 /** key/data pair already exists */
392 #define MDB_KEYEXIST (-30799)
393 /** key/data pair not found (EOF) */
394 #define MDB_NOTFOUND (-30798)
395 /** Requested page not found - this usually indicates corruption */
396 #define MDB_PAGE_NOTFOUND (-30797)
397 /** Located page was wrong type */
398 #define MDB_CORRUPTED (-30796)
399 /** Update of meta page failed or environment had fatal error */
400 #define MDB_PANIC (-30795)
401 /** Environment version mismatch */
402 #define MDB_VERSION_MISMATCH (-30794)
403 /** File is not a valid LMDB file */
404 #define MDB_INVALID (-30793)
405 /** Environment mapsize reached */
406 #define MDB_MAP_FULL (-30792)
407 /** Environment maxdbs reached */
408 #define MDB_DBS_FULL (-30791)
409 /** Environment maxreaders reached */
410 #define MDB_READERS_FULL (-30790)
411 /** Too many TLS keys in use - Windows only */
412 #define MDB_TLS_FULL (-30789)
413 /** Txn has too many dirty pages */
414 #define MDB_TXN_FULL (-30788)
415 /** Cursor stack too deep - internal error */
416 #define MDB_CURSOR_FULL (-30787)
417 /** Page has not enough space - internal error */
418 #define MDB_PAGE_FULL (-30786)
419 /** Database contents grew beyond environment mapsize */
420 #define MDB_MAP_RESIZED (-30785)
421 /** MDB_INCOMPATIBLE: Operation and DB incompatible, or DB flags changed */
422 #define MDB_INCOMPATIBLE (-30784)
423 /** Invalid reuse of reader locktable slot */
424 #define MDB_BAD_RSLOT (-30783)
425 /** Transaction cannot recover - it must be aborted */
426 #define MDB_BAD_TXN (-30782)
427 /** Unsupported size of key/DB name/data, or wrong DUPFIXED size */
428 #define MDB_BAD_VALSIZE (-30781)
429 /** The specified DBI was changed unexpectedly */
430 #define MDB_BAD_DBI (-30780)
431 /** The last defined error code */
432 #define MDB_LAST_ERRCODE MDB_BAD_DBI
435 /** @brief Statistics for a database in the environment */
436 typedef struct MDB_stat {
437 unsigned int ms_psize; /**< Size of a database page.
438 This is currently the same for all databases. */
439 unsigned int ms_depth; /**< Depth (height) of the B-tree */
440 size_t ms_branch_pages; /**< Number of internal (non-leaf) pages */
441 size_t ms_leaf_pages; /**< Number of leaf pages */
442 size_t ms_overflow_pages; /**< Number of overflow pages */
443 size_t ms_entries; /**< Number of data items */
446 /** @brief Information about the environment */
447 typedef struct MDB_envinfo {
448 void *me_mapaddr; /**< Address of map, if fixed */
449 size_t me_mapsize; /**< Size of the data memory map */
450 size_t me_last_pgno; /**< ID of the last used page */
451 size_t me_last_txnid; /**< ID of the last committed transaction */
452 unsigned int me_maxreaders; /**< max reader slots in the environment */
453 unsigned int me_numreaders; /**< max reader slots used in the environment */
456 /** @brief Return the LMDB library version information.
458 * @param[out] major if non-NULL, the library major version number is copied here
459 * @param[out] minor if non-NULL, the library minor version number is copied here
460 * @param[out] patch if non-NULL, the library patch version number is copied here
461 * @retval "version string" The library version as a string
463 char *mdb_version(int *major, int *minor, int *patch);
465 /** @brief Return a string describing a given error code.
467 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
468 * function. If the error code is greater than or equal to 0, then the string
469 * returned by the system function strerror(3) is returned. If the error code
470 * is less than 0, an error string corresponding to the LMDB library error is
471 * returned. See @ref errors for a list of LMDB-specific error codes.
472 * @param[in] err The error code
473 * @retval "error message" The description of the error
475 char *mdb_strerror(int err);
477 /** @brief Create an LMDB environment handle.
479 * This function allocates memory for a #MDB_env structure. To release
480 * the allocated memory and discard the handle, call #mdb_env_close().
481 * Before the handle may be used, it must be opened using #mdb_env_open().
482 * Various other options may also need to be set before opening the handle,
483 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
484 * depending on usage requirements.
485 * @param[out] env The address where the new handle will be stored
486 * @return A non-zero error value on failure and 0 on success.
488 int mdb_env_create(MDB_env **env);
490 /** @brief Open an environment handle.
492 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
493 * @param[in] env An environment handle returned by #mdb_env_create()
494 * @param[in] path The directory in which the database files reside. This
495 * directory must already exist and be writable.
496 * @param[in] flags Special options for this environment. This parameter
497 * must be set to 0 or by bitwise OR'ing together one or more of the
498 * values described here.
499 * Flags set by mdb_env_set_flags() are also used.
502 * use a fixed address for the mmap region. This flag must be specified
503 * when creating the environment, and is stored persistently in the environment.
504 * If successful, the memory map will always reside at the same virtual address
505 * and pointers used to reference data items in the database will be constant
506 * across multiple invocations. This option may not always work, depending on
507 * how the operating system has allocated memory to shared libraries and other uses.
508 * The feature is highly experimental.
510 * By default, LMDB creates its environment in a directory whose
511 * pathname is given in \b path, and creates its data and lock files
512 * under that directory. With this option, \b path is used as-is for
513 * the database main data file. The database lock file is the \b path
514 * with "-lock" appended.
516 * Open the environment in read-only mode. No write operations will be
517 * allowed. LMDB will still modify the lock file - except on read-only
518 * filesystems, where LMDB does not use locks.
520 * Initialize the lockfile to catch stale locks if robust mutexes
521 * are supported, so aborted processes will not block others.
522 * Ignored when another process has the environment open. Unsupported
523 * by liblmdb built with MDB_USE_POSIX_SEM (such as BSD systems).
524 * Enabled by default on Windows. Some locking slowdown on Unix.
526 * Use a writeable memory map unless MDB_RDONLY is set. This is faster
527 * and uses fewer mallocs, but loses protection from application bugs
528 * like wild pointer writes and other bad updates into the database.
529 * Incompatible with nested transactions.
530 * Processes with and without MDB_WRITEMAP on the same environment do
531 * not cooperate well.
532 * <li>#MDB_NOMETASYNC
533 * Flush system buffers to disk only once per transaction, omit the
534 * metadata flush. Defer that until the system flushes files to disk,
535 * or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization
536 * maintains database integrity, but a system crash may undo the last
537 * committed transaction. I.e. it preserves the ACI (atomicity,
538 * consistency, isolation) but not D (durability) database property.
539 * This flag may be changed at any time using #mdb_env_set_flags().
541 * Don't flush system buffers to disk when committing a transaction.
542 * This optimization means a system crash can corrupt the database or
543 * lose the last transactions if buffers are not yet flushed to disk.
544 * The risk is governed by how often the system flushes dirty buffers
545 * to disk and how often #mdb_env_sync() is called. However, if the
546 * filesystem preserves write order and the #MDB_WRITEMAP flag is not
547 * used, transactions exhibit ACI (atomicity, consistency, isolation)
548 * properties and only lose D (durability). I.e. database integrity
549 * is maintained, but a system crash may undo the final transactions.
550 * Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no
551 * hint for when to write transactions to disk, unless #mdb_env_sync()
552 * is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable.
553 * This flag may be changed at any time using #mdb_env_set_flags().
555 * When using #MDB_WRITEMAP, use asynchronous flushes to disk.
556 * As with #MDB_NOSYNC, a system crash can then corrupt the
557 * database or lose the last transactions. Calling #mdb_env_sync()
558 * ensures on-disk database integrity until next commit.
559 * This flag may be changed at any time using #mdb_env_set_flags().
561 * Don't use Thread-Local Storage. Tie reader locktable slots to
562 * #MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps
563 * the slot reseved for the #MDB_txn object. A thread may use parallel
564 * read-only transactions. A read-only transaction may span threads if
565 * the user synchronizes its use. Applications that multiplex many
566 * user threads over individual OS threads need this option. Such an
567 * application must also serialize the write transactions in an OS
568 * thread, since LMDB's write locking is unaware of the user threads.
570 * Don't do any locking. If concurrent access is anticipated, the
571 * caller must manage all concurrency itself. For proper operation
572 * the caller must enforce single-writer semantics, and must ensure
573 * that no readers are using old transactions while a writer is
574 * active. The simplest approach is to use an exclusive lock so that
575 * no readers may be active at all when a writer begins.
577 * Turn off readahead. Most operating systems perform readahead on
578 * read requests by default. This option turns it off if the OS
579 * supports it. Turning it off may help random read performance
580 * when the DB is larger than RAM and system RAM is full.
581 * The option is not implemented on Windows.
583 * Don't initialize malloc'd memory before writing to unused spaces
584 * in the data file. By default, memory for pages written to the data
585 * file is obtained using malloc. While these pages may be reused in
586 * subsequent transactions, freshly malloc'd pages will be initialized
587 * to zeroes before use. This avoids persisting leftover data from other
588 * code (that used the heap and subsequently freed the memory) into the
589 * data file. Note that many other system libraries may allocate
590 * and free memory from the heap for arbitrary uses. E.g., stdio may
591 * use the heap for file I/O buffers. This initialization step has a
592 * modest performance cost so some applications may want to disable
593 * it using this flag. This option can be a problem for applications
594 * which handle sensitive data like passwords, and it makes memory
595 * checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP,
596 * which writes directly to the mmap instead of using malloc for pages. The
597 * initialization is also skipped if #MDB_RESERVE is used; the
598 * caller is expected to overwrite all of the memory that was
599 * reserved in that case.
600 * This flag may be changed at any time using #mdb_env_set_flags().
602 * @param[in] mode The UNIX permissions to set on created files. This parameter
603 * is ignored on Windows.
604 * @return A non-zero error value on failure and 0 on success. Some possible
607 * <li>#MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the
608 * version that created the database environment.
609 * <li>#MDB_INVALID - the environment file headers are corrupted.
610 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
611 * <li>EACCES - the user didn't have permission to access the environment files.
612 * <li>EAGAIN - the environment was locked by another process.
615 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode);
617 /** @brief Copy an LMDB environment to the specified path.
619 * This function may be used to make a backup of an existing environment.
620 * No lockfile is created, since it gets recreated at need.
621 * @note This call can trigger significant file size growth if run in
622 * parallel with write transactions, because it employs a read-only
623 * transaction. See long-lived transactions under @ref caveats_sec.
624 * @param[in] env An environment handle returned by #mdb_env_create(). It
625 * must have already been opened successfully.
626 * @param[in] path The directory in which the copy will reside. This
627 * directory must already exist and be writable but must otherwise be
629 * @return A non-zero error value on failure and 0 on success.
631 int mdb_env_copy(MDB_env *env, const char *path);
633 /** @brief Copy an LMDB environment to the specified file descriptor.
635 * This function may be used to make a backup of an existing environment.
636 * No lockfile is created, since it gets recreated at need.
637 * @note This call can trigger significant file size growth if run in
638 * parallel with write transactions, because it employs a read-only
639 * transaction. See long-lived transactions under @ref caveats_sec.
640 * @param[in] env An environment handle returned by #mdb_env_create(). It
641 * must have already been opened successfully.
642 * @param[in] fd The filedescriptor to write the copy to. It must
643 * have already been opened for Write access.
644 * @return A non-zero error value on failure and 0 on success.
646 int mdb_env_copyfd(MDB_env *env, mdb_filehandle_t fd);
648 /** @brief Copy an LMDB environment to the specified path, with options.
650 * This function may be used to make a backup of an existing environment.
651 * No lockfile is created, since it gets recreated at need.
652 * @note This call can trigger significant file size growth if run in
653 * parallel with write transactions, because it employs a read-only
654 * transaction. See long-lived transactions under @ref caveats_sec.
655 * @param[in] env An environment handle returned by #mdb_env_create(). It
656 * must have already been opened successfully.
657 * @param[in] path The directory in which the copy will reside. This
658 * directory must already exist and be writable but must otherwise be
660 * @param[in] flags Special options for this operation. This parameter
661 * must be set to 0 or by bitwise OR'ing together one or more of the
662 * values described here.
664 * <li>#MDB_CP_COMPACT - Perform compaction while copying: omit free
665 * pages and sequentially renumber all pages in output. This option
666 * consumes more CPU and runs more slowly than the default.
668 * @return A non-zero error value on failure and 0 on success.
670 int mdb_env_copy2(MDB_env *env, const char *path, unsigned int flags);
672 /** @brief Copy an LMDB environment to the specified file descriptor,
675 * This function may be used to make a backup of an existing environment.
676 * No lockfile is created, since it gets recreated at need. See
677 * #mdb_env_copy2() for further details.
678 * @note This call can trigger significant file size growth if run in
679 * parallel with write transactions, because it employs a read-only
680 * transaction. See long-lived transactions under @ref caveats_sec.
681 * @param[in] env An environment handle returned by #mdb_env_create(). It
682 * must have already been opened successfully.
683 * @param[in] fd The filedescriptor to write the copy to. It must
684 * have already been opened for Write access.
685 * @param[in] flags Special options for this operation.
686 * See #mdb_env_copy2() for options.
687 * @return A non-zero error value on failure and 0 on success.
689 int mdb_env_copyfd2(MDB_env *env, mdb_filehandle_t fd, unsigned int flags);
691 /** @brief Return statistics about the LMDB environment.
693 * @param[in] env An environment handle returned by #mdb_env_create()
694 * @param[out] stat The address of an #MDB_stat structure
695 * where the statistics will be copied
697 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
699 /** @brief Return information about the LMDB environment.
701 * @param[in] env An environment handle returned by #mdb_env_create()
702 * @param[out] stat The address of an #MDB_envinfo structure
703 * where the information will be copied
705 int mdb_env_info(MDB_env *env, MDB_envinfo *stat);
707 /** @brief Flush the data buffers to disk.
709 * Data is always written to disk when #mdb_txn_commit() is called,
710 * but the operating system may keep it buffered. LMDB always flushes
711 * the OS buffers upon commit as well, unless the environment was
712 * opened with #MDB_NOSYNC or in part #MDB_NOMETASYNC.
713 * @param[in] env An environment handle returned by #mdb_env_create()
714 * @param[in] force If non-zero, force a synchronous flush. Otherwise
715 * if the environment has the #MDB_NOSYNC flag set the flushes
716 * will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
717 * @return A non-zero error value on failure and 0 on success. Some possible
720 * <li>EINVAL - an invalid parameter was specified.
721 * <li>EIO - an error occurred during synchronization.
724 int mdb_env_sync(MDB_env *env, int force);
726 /** @brief Close the environment and release the memory map.
728 * Only a single thread may call this function. All transactions, databases,
729 * and cursors must already be closed before calling this function. Attempts to
730 * use any such handles after calling this function will cause a SIGSEGV.
731 * The environment handle will be freed and must not be used again after this call.
732 * @param[in] env An environment handle returned by #mdb_env_create()
734 void mdb_env_close(MDB_env *env);
736 /** @brief Set environment flags.
738 * This may be used to set some flags in addition to those from
739 * #mdb_env_open(), or to unset these flags. If several threads
740 * change the flags at the same time, the result is undefined.
741 * Most flags cannot be changed after #mdb_env_open().
742 * @param[in] env An environment handle returned by #mdb_env_create()
743 * @param[in] flags The flags to change, bitwise OR'ed together
744 * @param[in] onoff A non-zero value sets the flags, zero clears them.
745 * @return A non-zero error value on failure and 0 on success. Some possible
748 * <li>EINVAL - an invalid parameter was specified.
751 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
753 /** @brief Get environment flags.
755 * @param[in] env An environment handle returned by #mdb_env_create()
756 * @param[out] flags The address of an integer to store the flags
757 * @return A non-zero error value on failure and 0 on success. Some possible
760 * <li>EINVAL - an invalid parameter was specified.
763 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
765 /** @brief Return the path that was used in #mdb_env_open().
767 * @param[in] env An environment handle returned by #mdb_env_create()
768 * @param[out] path Address of a string pointer to contain the path. This
769 * is the actual string in the environment, not a copy. It should not be
770 * altered in any way.
771 * @return A non-zero error value on failure and 0 on success. Some possible
774 * <li>EINVAL - an invalid parameter was specified.
777 int mdb_env_get_path(MDB_env *env, const char **path);
779 /** @brief Return the filedescriptor for the given environment.
781 * @param[in] env An environment handle returned by #mdb_env_create()
782 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.
783 * @return A non-zero error value on failure and 0 on success. Some possible
786 * <li>EINVAL - an invalid parameter was specified.
789 int mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *fd);
791 /** @brief Set the size of the memory map to use for this environment.
793 * The size should be a multiple of the OS page size. The default is
794 * 10485760 bytes. The size of the memory map is also the maximum size
795 * of the database. The value should be chosen as large as possible,
796 * to accommodate future growth of the database.
797 * This function should be called after #mdb_env_create() and before #mdb_env_open().
798 * It may be called at later times if no transactions are active in
799 * this process. Note that the library does not check for this condition,
800 * the caller must ensure it explicitly.
802 * The new size takes effect immediately for the current process but
803 * will not be persisted to any others until a write transaction has been
804 * committed by the current process. Also, only mapsize increases are
805 * persisted into the environment.
807 * If the mapsize is increased by another process, and data has grown
808 * beyond the range of the current mapsize, #mdb_txn_begin() will
809 * return #MDB_MAP_RESIZED. This function may be called with a size
810 * of zero to adopt the new size.
812 * Any attempt to set a size smaller than the space already consumed
813 * by the environment will be silently changed to the current size of the used space.
814 * @param[in] env An environment handle returned by #mdb_env_create()
815 * @param[in] size The size in bytes
816 * @return A non-zero error value on failure and 0 on success. Some possible
819 * <li>EINVAL - an invalid parameter was specified, or the environment has
820 * an active write transaction.
823 int mdb_env_set_mapsize(MDB_env *env, size_t size);
825 /** @brief Set the maximum number of threads/reader slots for the environment.
827 * This defines the number of slots in the lock table that is used to track readers in the
828 * the environment. The default is 126.
829 * Starting a read-only transaction normally ties a lock table slot to the
830 * current thread until the environment closes or the thread exits. If
831 * MDB_NOTLS is in use, #mdb_txn_begin() instead ties the slot to the
832 * MDB_txn object until it or the #MDB_env object is destroyed.
833 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
834 * @param[in] env An environment handle returned by #mdb_env_create()
835 * @param[in] readers The maximum number of reader lock table slots
836 * @return A non-zero error value on failure and 0 on success. Some possible
839 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
842 int mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);
844 /** @brief Get the maximum number of threads/reader slots for the environment.
846 * @param[in] env An environment handle returned by #mdb_env_create()
847 * @param[out] readers Address of an integer to store the number of readers
848 * @return A non-zero error value on failure and 0 on success. Some possible
851 * <li>EINVAL - an invalid parameter was specified.
854 int mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers);
856 /** @brief Set the maximum number of named databases for the environment.
858 * This function is only needed if multiple databases will be used in the
859 * environment. Simpler applications that use the environment as a single
860 * unnamed database can ignore this option.
861 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
863 * Currently a moderate number of slots are cheap but a huge number gets
864 * expensive: 7-120 words per transaction, and every #mdb_dbi_open()
865 * does a linear search of the opened slots.
866 * @param[in] env An environment handle returned by #mdb_env_create()
867 * @param[in] dbs The maximum number of databases
868 * @return A non-zero error value on failure and 0 on success. Some possible
871 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
874 int mdb_env_set_maxdbs(MDB_env *env, MDB_dbi dbs);
876 /** @brief Get the maximum size of keys and #MDB_DUPSORT data we can write.
878 * Depends on the compile-time constant #MDB_MAXKEYSIZE. Default 511.
880 * @param[in] env An environment handle returned by #mdb_env_create()
881 * @return The maximum size of a key we can write
883 int mdb_env_get_maxkeysize(MDB_env *env);
885 /** @brief Set application information associated with the #MDB_env.
887 * @param[in] env An environment handle returned by #mdb_env_create()
888 * @param[in] ctx An arbitrary pointer for whatever the application needs.
889 * @return A non-zero error value on failure and 0 on success.
891 int mdb_env_set_userctx(MDB_env *env, void *ctx);
893 /** @brief Get the application information associated with the #MDB_env.
895 * @param[in] env An environment handle returned by #mdb_env_create()
896 * @return The pointer set by #mdb_env_set_userctx().
898 void *mdb_env_get_userctx(MDB_env *env);
900 /** @brief A callback function for most LMDB assert() failures,
901 * called before printing the message and aborting.
903 * @param[in] env An environment handle returned by #mdb_env_create().
904 * @param[in] msg The assertion message, not including newline.
906 typedef void MDB_assert_func(MDB_env *env, const char *msg);
908 /** Set or reset the assert() callback of the environment.
909 * Disabled if liblmdb is buillt with NDEBUG.
910 * @note This hack should become obsolete as lmdb's error handling matures.
911 * @param[in] env An environment handle returned by #mdb_env_create().
912 * @param[in] func An #MDB_assert_func function, or 0.
913 * @return A non-zero error value on failure and 0 on success.
915 int mdb_env_set_assert(MDB_env *env, MDB_assert_func *func);
917 /** @brief Create a transaction for use with the environment.
919 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
920 * @note A transaction and its cursors must only be used by a single
921 * thread, and a thread may only have a single transaction at a time.
922 * If #MDB_NOTLS is in use, this does not apply to read-only transactions.
923 * @note Cursors may not span transactions.
924 * @param[in] env An environment handle returned by #mdb_env_create()
925 * @param[in] parent If this parameter is non-NULL, the new transaction
926 * will be a nested transaction, with the transaction indicated by \b parent
927 * as its parent. Transactions may be nested to any level. A parent
928 * transaction and its cursors may not issue any other operations than
929 * mdb_txn_commit and mdb_txn_abort while it has active child transactions.
930 * @param[in] flags Special options for this transaction. This parameter
931 * must be set to 0 or by bitwise OR'ing together one or more of the
932 * values described here.
935 * This transaction will not perform any write operations.
937 * @param[out] txn Address where the new #MDB_txn handle will be stored
938 * @return A non-zero error value on failure and 0 on success. Some possible
941 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
943 * <li>#MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's
944 * mapsize and this environment's map must be resized as well.
945 * See #mdb_env_set_mapsize().
946 * <li>#MDB_READERS_FULL - a read-only transaction was requested and
947 * the reader lock table is full. See #mdb_env_set_maxreaders().
948 * <li>ENOMEM - out of memory.
951 int mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);
953 /** @brief Returns the transaction's #MDB_env
955 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
957 MDB_env *mdb_txn_env(MDB_txn *txn);
959 /** @brief Commit all the operations of a transaction into the database.
961 * The transaction handle is freed. It and its cursors must not be used
962 * again after this call, except with #mdb_cursor_renew().
963 * @note Earlier documentation incorrectly said all cursors would be freed.
964 * Only write-transactions free cursors.
965 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
966 * @return A non-zero error value on failure and 0 on success. Some possible
969 * <li>EINVAL - an invalid parameter was specified.
970 * <li>ENOSPC - no more disk space.
971 * <li>EIO - a low-level I/O error occurred while writing.
972 * <li>ENOMEM - out of memory.
975 int mdb_txn_commit(MDB_txn *txn);
977 /** @brief Abandon all the operations of the transaction instead of saving them.
979 * The transaction handle is freed. It and its cursors must not be used
980 * again after this call, except with #mdb_cursor_renew().
981 * @note Earlier documentation incorrectly said all cursors would be freed.
982 * Only write-transactions free cursors.
983 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
985 void mdb_txn_abort(MDB_txn *txn);
987 /** @brief Reset a read-only transaction.
989 * Abort the transaction like #mdb_txn_abort(), but keep the transaction
990 * handle. #mdb_txn_renew() may reuse the handle. This saves allocation
991 * overhead if the process will start a new read-only transaction soon,
992 * and also locking overhead if #MDB_NOTLS is in use. The reader table
993 * lock is released, but the table slot stays tied to its thread or
994 * #MDB_txn. Use mdb_txn_abort() to discard a reset handle, and to free
995 * its lock table slot if MDB_NOTLS is in use.
996 * Cursors opened within the transaction must not be used
997 * again after this call, except with #mdb_cursor_renew().
998 * Reader locks generally don't interfere with writers, but they keep old
999 * versions of database pages allocated. Thus they prevent the old pages
1000 * from being reused when writers commit new data, and so under heavy load
1001 * the database size may grow much more rapidly than otherwise.
1002 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1004 void mdb_txn_reset(MDB_txn *txn);
1006 /** @brief Renew a read-only transaction.
1008 * This acquires a new reader lock for a transaction handle that had been
1009 * released by #mdb_txn_reset(). It must be called before a reset transaction
1010 * may be used again.
1011 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1012 * @return A non-zero error value on failure and 0 on success. Some possible
1015 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
1016 * must be shut down.
1017 * <li>EINVAL - an invalid parameter was specified.
1020 int mdb_txn_renew(MDB_txn *txn);
1022 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1023 #define mdb_open(txn,name,flags,dbi) mdb_dbi_open(txn,name,flags,dbi)
1024 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1025 #define mdb_close(env,dbi) mdb_dbi_close(env,dbi)
1027 /** @brief Open a database in the environment.
1029 * A database handle denotes the name and parameters of a database,
1030 * independently of whether such a database exists.
1031 * The database handle may be discarded by calling #mdb_dbi_close().
1032 * The old database handle is returned if the database was already open.
1033 * The handle may only be closed once.
1034 * The database handle will be private to the current transaction until
1035 * the transaction is successfully committed. If the transaction is
1036 * aborted the handle will be closed automatically.
1037 * After a successful commit the
1038 * handle will reside in the shared environment, and may be used
1039 * by other transactions. This function must not be called from
1040 * multiple concurrent transactions. A transaction that uses this function
1041 * must finish (either commit or abort) before any other transaction may
1042 * use this function.
1044 * To use named databases (with name != NULL), #mdb_env_set_maxdbs()
1045 * must be called before opening the environment. Database names
1046 * are kept as keys in the unnamed database.
1047 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1048 * @param[in] name The name of the database to open. If only a single
1049 * database is needed in the environment, this value may be NULL.
1050 * @param[in] flags Special options for this database. This parameter
1051 * must be set to 0 or by bitwise OR'ing together one or more of the
1052 * values described here.
1054 * <li>#MDB_REVERSEKEY
1055 * Keys are strings to be compared in reverse order, from the end
1056 * of the strings to the beginning. By default, Keys are treated as strings and
1057 * compared from beginning to end.
1059 * Duplicate keys may be used in the database. (Or, from another perspective,
1060 * keys may have multiple data items, stored in sorted order.) By default
1061 * keys must be unique and may have only a single data item.
1062 * <li>#MDB_INTEGERKEY
1063 * Keys are binary integers in native byte order. Setting this option
1064 * requires all keys to be the same size, typically sizeof(int)
1065 * or sizeof(size_t).
1067 * This flag may only be used in combination with #MDB_DUPSORT. This option
1068 * tells the library that the data items for this database are all the same
1069 * size, which allows further optimizations in storage and retrieval. When
1070 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
1071 * cursor operations may be used to retrieve multiple items at once.
1072 * <li>#MDB_INTEGERDUP
1073 * This option specifies that duplicate data items are also integers, and
1074 * should be sorted as such.
1075 * <li>#MDB_REVERSEDUP
1076 * This option specifies that duplicate data items should be compared as
1077 * strings in reverse order.
1079 * Create the named database if it doesn't exist. This option is not
1080 * allowed in a read-only transaction or a read-only environment.
1082 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
1083 * @return A non-zero error value on failure and 0 on success. Some possible
1086 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
1087 * and #MDB_CREATE was not specified.
1088 * <li>#MDB_DBS_FULL - too many databases have been opened. See #mdb_env_set_maxdbs().
1091 int mdb_dbi_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
1093 /** @brief Retrieve statistics for a database.
1095 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1096 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1097 * @param[out] stat The address of an #MDB_stat structure
1098 * where the statistics will be copied
1099 * @return A non-zero error value on failure and 0 on success. Some possible
1102 * <li>EINVAL - an invalid parameter was specified.
1105 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
1107 /** @brief Retrieve the DB flags for a database handle.
1109 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1110 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1111 * @param[out] flags Address where the flags will be returned.
1112 * @return A non-zero error value on failure and 0 on success.
1114 int mdb_dbi_flags(MDB_txn *txn, MDB_dbi dbi, unsigned int *flags);
1116 /** @brief Close a database handle. Normally unnecessary. Use with care:
1118 * This call is not mutex protected. Handles should only be closed by
1119 * a single thread, and only if no other threads are going to reference
1120 * the database handle or one of its cursors any further. Do not close
1121 * a handle if an existing transaction has modified its database.
1122 * Doing so can cause misbehavior from database corruption to errors
1123 * like MDB_BAD_VALSIZE (since the DB name is gone).
1125 * Closing a database handle is not necessary, but lets #mdb_dbi_open()
1126 * reuse the handle value. Usually it's better to set a bigger
1127 * #mdb_env_set_maxdbs(), unless that value would be large.
1129 * @param[in] env An environment handle returned by #mdb_env_create()
1130 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1132 void mdb_dbi_close(MDB_env *env, MDB_dbi dbi);
1134 /** @brief Empty or delete+close a database.
1136 * See #mdb_dbi_close() for restrictions about closing the DB handle.
1137 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1138 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1139 * @param[in] del 0 to empty the DB, 1 to delete it from the
1140 * environment and close the DB handle.
1141 * @return A non-zero error value on failure and 0 on success.
1143 int mdb_drop(MDB_txn *txn, MDB_dbi dbi, int del);
1145 /** @brief Set a custom key comparison function for a database.
1147 * The comparison function is called whenever it is necessary to compare a
1148 * key specified by the application with a key currently stored in the database.
1149 * If no comparison function is specified, and no special key flags were specified
1150 * with #mdb_dbi_open(), the keys are compared lexically, with shorter keys collating
1151 * before longer keys.
1152 * @warning This function must be called before any data access functions are used,
1153 * otherwise data corruption may occur. The same comparison function must be used by every
1154 * program accessing the database, every time the database is used.
1155 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1156 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1157 * @param[in] cmp A #MDB_cmp_func function
1158 * @return A non-zero error value on failure and 0 on success. Some possible
1161 * <li>EINVAL - an invalid parameter was specified.
1164 int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1166 /** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
1168 * This comparison function is called whenever it is necessary to compare a data
1169 * item specified by the application with a data item currently stored in the database.
1170 * This function only takes effect if the database was opened with the #MDB_DUPSORT
1172 * If no comparison function is specified, and no special key flags were specified
1173 * with #mdb_dbi_open(), the data items are compared lexically, with shorter items collating
1174 * before longer items.
1175 * @warning This function must be called before any data access functions are used,
1176 * otherwise data corruption may occur. The same comparison function must be used by every
1177 * program accessing the database, every time the database is used.
1178 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1179 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1180 * @param[in] cmp A #MDB_cmp_func function
1181 * @return A non-zero error value on failure and 0 on success. Some possible
1184 * <li>EINVAL - an invalid parameter was specified.
1187 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1189 /** @brief Set a relocation function for a #MDB_FIXEDMAP database.
1191 * @todo The relocation function is called whenever it is necessary to move the data
1192 * of an item to a different position in the database (e.g. through tree
1193 * balancing operations, shifts as a result of adds or deletes, etc.). It is
1194 * intended to allow address/position-dependent data items to be stored in
1195 * a database in an environment opened with the #MDB_FIXEDMAP option.
1196 * Currently the relocation feature is unimplemented and setting
1197 * this function has no effect.
1198 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1199 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1200 * @param[in] rel A #MDB_rel_func function
1201 * @return A non-zero error value on failure and 0 on success. Some possible
1204 * <li>EINVAL - an invalid parameter was specified.
1207 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
1209 /** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
1211 * See #mdb_set_relfunc and #MDB_rel_func for more details.
1212 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1213 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1214 * @param[in] ctx An arbitrary pointer for whatever the application needs.
1215 * It will be passed to the callback function set by #mdb_set_relfunc
1216 * as its \b relctx parameter whenever the callback is invoked.
1217 * @return A non-zero error value on failure and 0 on success. Some possible
1220 * <li>EINVAL - an invalid parameter was specified.
1223 int mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);
1225 /** @brief Get items from a database.
1227 * This function retrieves key/data pairs from the database. The address
1228 * and length of the data associated with the specified \b key are returned
1229 * in the structure to which \b data refers.
1230 * If the database supports duplicate keys (#MDB_DUPSORT) then the
1231 * first data item for the key will be returned. Retrieval of other
1232 * items requires the use of #mdb_cursor_get().
1234 * @note The memory pointed to by the returned values is owned by the
1235 * database. The caller need not dispose of the memory, and may not
1236 * modify it in any way. For values returned in a read-only transaction
1237 * any modification attempts will cause a SIGSEGV.
1238 * @note Values returned from the database are valid only until a
1239 * subsequent update operation, or the end of the transaction.
1240 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1241 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1242 * @param[in] key The key to search for in the database
1243 * @param[out] data The data corresponding to the key
1244 * @return A non-zero error value on failure and 0 on success. Some possible
1247 * <li>#MDB_NOTFOUND - the key was not in the database.
1248 * <li>EINVAL - an invalid parameter was specified.
1251 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1253 /** @brief Store items into a database.
1255 * This function stores key/data pairs in the database. The default behavior
1256 * is to enter the new key/data pair, replacing any previously existing key
1257 * if duplicates are disallowed, or adding a duplicate data item if
1258 * duplicates are allowed (#MDB_DUPSORT).
1259 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1260 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1261 * @param[in] key The key to store in the database
1262 * @param[in,out] data The data to store
1263 * @param[in] flags Special options for this operation. This parameter
1264 * must be set to 0 or by bitwise OR'ing together one or more of the
1265 * values described here.
1267 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1268 * already appear in the database. This flag may only be specified
1269 * if the database was opened with #MDB_DUPSORT. The function will
1270 * return #MDB_KEYEXIST if the key/data pair already appears in the
1272 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1273 * does not already appear in the database. The function will return
1274 * #MDB_KEYEXIST if the key already appears in the database, even if
1275 * the database supports duplicates (#MDB_DUPSORT). The \b data
1276 * parameter will be set to point to the existing item.
1277 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1278 * don't copy the given data. Instead, return a pointer to the
1279 * reserved space, which the caller can fill in later - before
1280 * the next update operation or the transaction ends. This saves
1281 * an extra memcpy if the data is being generated later.
1282 * LMDB does nothing else with this memory, the caller is expected
1283 * to modify all of the space requested.
1284 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1285 * database. No key comparisons are performed. This option allows
1286 * fast bulk loading when keys are already known to be in the
1287 * correct order. Loading unsorted keys with this flag will cause
1289 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1291 * @return A non-zero error value on failure and 0 on success. Some possible
1294 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1295 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1296 * <li>EACCES - an attempt was made to write in a read-only transaction.
1297 * <li>EINVAL - an invalid parameter was specified.
1300 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
1301 unsigned int flags);
1303 /** @brief Delete items from a database.
1305 * This function removes key/data pairs from the database.
1306 * If the database does not support sorted duplicate data items
1307 * (#MDB_DUPSORT) the data parameter is ignored.
1308 * If the database supports sorted duplicates and the data parameter
1309 * is NULL, all of the duplicate data items for the key will be
1310 * deleted. Otherwise, if the data parameter is non-NULL
1311 * only the matching data item will be deleted.
1312 * This function will return #MDB_NOTFOUND if the specified key/data
1313 * pair is not in the database.
1314 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1315 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1316 * @param[in] key The key to delete from the database
1317 * @param[in] data The data to delete
1318 * @return A non-zero error value on failure and 0 on success. Some possible
1321 * <li>EACCES - an attempt was made to write in a read-only transaction.
1322 * <li>EINVAL - an invalid parameter was specified.
1325 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1327 /** @brief Create a cursor handle.
1329 * A cursor is associated with a specific transaction and database.
1330 * A cursor cannot be used when its database handle is closed. Nor
1331 * when its transaction has ended, except with #mdb_cursor_renew().
1332 * It can be discarded with #mdb_cursor_close().
1333 * A cursor in a write-transaction can be closed before its transaction
1334 * ends, and will otherwise be closed when its transaction ends.
1335 * A cursor in a read-only transaction must be closed explicitly, before
1336 * or after its transaction ends. It can be reused with
1337 * #mdb_cursor_renew() before finally closing it.
1338 * @note Earlier documentation said that cursors in every transaction
1339 * were closed when the transaction committed or aborted.
1340 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1341 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1342 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
1343 * @return A non-zero error value on failure and 0 on success. Some possible
1346 * <li>EINVAL - an invalid parameter was specified.
1349 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
1351 /** @brief Close a cursor handle.
1353 * The cursor handle will be freed and must not be used again after this call.
1354 * Its transaction must still be live if it is a write-transaction.
1355 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1357 void mdb_cursor_close(MDB_cursor *cursor);
1359 /** @brief Renew a cursor handle.
1361 * A cursor is associated with a specific transaction and database.
1362 * Cursors that are only used in read-only
1363 * transactions may be re-used, to avoid unnecessary malloc/free overhead.
1364 * The cursor may be associated with a new read-only transaction, and
1365 * referencing the same database handle as it was created with.
1366 * This may be done whether the previous transaction is live or dead.
1367 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1368 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1369 * @return A non-zero error value on failure and 0 on success. Some possible
1372 * <li>EINVAL - an invalid parameter was specified.
1375 int mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);
1377 /** @brief Return the cursor's transaction handle.
1379 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1381 MDB_txn *mdb_cursor_txn(MDB_cursor *cursor);
1383 /** @brief Return the cursor's database handle.
1385 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1387 MDB_dbi mdb_cursor_dbi(MDB_cursor *cursor);
1389 /** @brief Retrieve by cursor.
1391 * This function retrieves key/data pairs from the database. The address and length
1392 * of the key are returned in the object to which \b key refers (except for the
1393 * case of the #MDB_SET option, in which the \b key object is unchanged), and
1394 * the address and length of the data are returned in the object to which \b data
1396 * See #mdb_get() for restrictions on using the output values.
1397 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1398 * @param[in,out] key The key for a retrieved item
1399 * @param[in,out] data The data of a retrieved item
1400 * @param[in] op A cursor operation #MDB_cursor_op
1401 * @return A non-zero error value on failure and 0 on success. Some possible
1404 * <li>#MDB_NOTFOUND - no matching key found.
1405 * <li>EINVAL - an invalid parameter was specified.
1408 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1411 /** @brief Store by cursor.
1413 * This function stores key/data pairs into the database.
1414 * The cursor is positioned at the new item, or on failure usually near it.
1415 * @note Earlier documentation incorrectly said errors would leave the
1416 * state of the cursor unchanged.
1417 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1418 * @param[in] key The key operated on.
1419 * @param[in] data The data operated on.
1420 * @param[in] flags Options for this operation. This parameter
1421 * must be set to 0 or one of the values described here.
1423 * <li>#MDB_CURRENT - replace the item at the current cursor position.
1424 * The \b key parameter must still be provided, and must match it.
1425 * If using sorted duplicates (#MDB_DUPSORT) the data item must still
1426 * sort into the same place. This is intended to be used when the
1427 * new data is the same size as the old. Otherwise it will simply
1428 * perform a delete of the old record followed by an insert.
1429 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1430 * already appear in the database. This flag may only be specified
1431 * if the database was opened with #MDB_DUPSORT. The function will
1432 * return #MDB_KEYEXIST if the key/data pair already appears in the
1434 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1435 * does not already appear in the database. The function will return
1436 * #MDB_KEYEXIST if the key already appears in the database, even if
1437 * the database supports duplicates (#MDB_DUPSORT).
1438 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1439 * don't copy the given data. Instead, return a pointer to the
1440 * reserved space, which the caller can fill in later. This saves
1441 * an extra memcpy if the data is being generated later.
1442 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1443 * database. No key comparisons are performed. This option allows
1444 * fast bulk loading when keys are already known to be in the
1445 * correct order. Loading unsorted keys with this flag will cause
1447 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1448 * <li>#MDB_MULTIPLE - store multiple contiguous data elements in a
1449 * single request. This flag may only be specified if the database
1450 * was opened with #MDB_DUPFIXED. The \b data argument must be an
1451 * array of two MDB_vals. The mv_size of the first MDB_val must be
1452 * the size of a single data element. The mv_data of the first MDB_val
1453 * must point to the beginning of the array of contiguous data elements.
1454 * The mv_size of the second MDB_val must be the count of the number
1455 * of data elements to store. On return this field will be set to
1456 * the count of the number of elements actually written. The mv_data
1457 * of the second MDB_val is unused.
1459 * @return A non-zero error value on failure and 0 on success. Some possible
1462 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1463 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1464 * <li>EACCES - an attempt was made to modify a read-only database.
1465 * <li>EINVAL - an invalid parameter was specified.
1468 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1469 unsigned int flags);
1471 /** @brief Delete current key/data pair
1473 * This function deletes the key/data pair to which the cursor refers.
1474 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1475 * @param[in] flags Options for this operation. This parameter
1476 * must be set to 0 or one of the values described here.
1478 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
1479 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
1481 * @return A non-zero error value on failure and 0 on success. Some possible
1484 * <li>EACCES - an attempt was made to modify a read-only database.
1485 * <li>EINVAL - an invalid parameter was specified.
1488 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
1490 /** @brief Return count of duplicates for current key.
1492 * This call is only valid on databases that support sorted duplicate
1493 * data items #MDB_DUPSORT.
1494 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1495 * @param[out] countp Address where the count will be stored
1496 * @return A non-zero error value on failure and 0 on success. Some possible
1499 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
1502 int mdb_cursor_count(MDB_cursor *cursor, size_t *countp);
1504 /** @brief Compare two data items according to a particular database.
1506 * This returns a comparison as if the two data items were keys in the
1507 * specified database.
1508 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1509 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1510 * @param[in] a The first item to compare
1511 * @param[in] b The second item to compare
1512 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1514 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1516 /** @brief Compare two data items according to a particular database.
1518 * This returns a comparison as if the two items were data items of
1519 * the specified database. The database must have the #MDB_DUPSORT flag.
1520 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1521 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1522 * @param[in] a The first item to compare
1523 * @param[in] b The second item to compare
1524 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1526 int mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1528 /** @brief A callback function used to print a message from the library.
1530 * @param[in] msg The string to be printed.
1531 * @param[in] ctx An arbitrary context pointer for the callback.
1532 * @return < 0 on failure, >= 0 on success.
1534 typedef int (MDB_msg_func)(const char *msg, void *ctx);
1536 /** @brief Dump the entries in the reader lock table.
1538 * @param[in] env An environment handle returned by #mdb_env_create()
1539 * @param[in] func A #MDB_msg_func function
1540 * @param[in] ctx Anything the message function needs
1541 * @return < 0 on failure, >= 0 on success.
1543 int mdb_reader_list(MDB_env *env, MDB_msg_func *func, void *ctx);
1545 /** @brief Check for stale entries in the reader lock table.
1547 * @param[in] env An environment handle returned by #mdb_env_create()
1548 * @param[out] dead Number of stale slots that were cleared
1549 * @return 0 on success, non-zero on failure.
1551 int mdb_reader_check(MDB_env *env, int *dead);
1557 /** @page tools LMDB Command Line Tools
1558 The following describes the command line tools that are available for LMDB.
1565 #endif /* _LMDB_H_ */