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 * If this is your first time using a transactional embedded key/value
44 * store, you may find the \ref starting page to be helpful.
46 * @section caveats_sec Caveats
47 * Troubleshooting the lock file, plus semaphores on BSD systems:
49 * - A broken lockfile can cause sync issues.
50 * Stale reader transactions left behind by an aborted program
51 * cause further writes to grow the database quickly, and
52 * stale locks can block further operation.
54 * Fix: Check for stale readers periodically, using the
55 * #mdb_reader_check function or the \ref mdb_stat_1 "mdb_stat" tool.
56 * Stale writers will be cleared automatically on most systems:
57 * - Windows - automatic
58 * - BSD, systems using SysV semaphores - automatic
59 * - Linux, systems using POSIX mutexes with Robust option - automatic
60 * Otherwise just make all programs using the database close it;
61 * the lockfile is always reset on first open of the environment.
63 * - On BSD systems or others configured with MDB_USE_SYSV_SEM or
65 * startup can fail due to semaphores owned by another userid.
67 * Fix: Open and close the database as the user which owns the
68 * semaphores (likely last user) or as root, while no other
69 * process is using the database.
71 * Restrictions/caveats (in addition to those listed for some functions):
73 * - Only the database owner should normally use the database on
74 * BSD systems or when otherwise configured with MDB_USE_POSIX_SEM.
75 * Multiple users can cause startup to fail later, as noted above.
77 * - There is normally no pure read-only mode, since readers need write
78 * access to locks and lock file. Exceptions: On read-only filesystems
79 * or with the #MDB_NOLOCK flag described under #mdb_env_open().
81 * - An LMDB configuration will often reserve considerable \b unused
82 * memory address space and maybe file size for future growth.
83 * This does not use actual memory or disk space, but users may need
84 * to understand the difference so they won't be scared off.
86 * - By default, in versions before 0.9.10, unused portions of the data
87 * file might receive garbage data from memory freed by other code.
88 * (This does not happen when using the #MDB_WRITEMAP flag.) As of
89 * 0.9.10 the default behavior is to initialize such memory before
90 * writing to the data file. Since there may be a slight performance
91 * cost due to this initialization, applications may disable it using
92 * the #MDB_NOMEMINIT flag. Applications handling sensitive data
93 * which must not be written should not use this flag. This flag is
94 * irrelevant when using #MDB_WRITEMAP.
96 * - A thread can only use one transaction at a time, plus any child
97 * transactions. Each transaction belongs to one thread. See below.
98 * The #MDB_NOTLS flag changes this for read-only transactions.
100 * - Use an MDB_env* in the process which opened it, without fork()ing.
102 * - Do not have open an LMDB database twice in the same process at
103 * the same time. Not even from a plain open() call - close()ing it
104 * breaks flock() advisory locking.
106 * - Avoid long-lived transactions. Read transactions prevent
107 * reuse of pages freed by newer write transactions, thus the
108 * database can grow quickly. Write transactions prevent
109 * other write transactions, since writes are serialized.
111 * - Avoid suspending a process with active transactions. These
112 * would then be "long-lived" as above. Also read transactions
113 * suspended when writers commit could sometimes see wrong data.
115 * ...when several processes can use a database concurrently:
117 * - Avoid aborting a process with an active transaction.
118 * The transaction becomes "long-lived" as above until a check
119 * for stale readers is performed or the lockfile is reset,
120 * since the process may not remove it from the lockfile.
122 * This does not apply to write transactions if the system clears
123 * stale writers, see above.
125 * - If you do that anyway, do a periodic check for stale readers. Or
126 * close the environment once in a while, so the lockfile can get reset.
128 * - Do not use LMDB databases on remote filesystems, even between
129 * processes on the same host. This breaks flock() on some OSes,
130 * possibly memory map sync, and certainly sync between programs
131 * on different hosts.
133 * - Opening a database can fail if another process is opening or
134 * closing it at exactly the same time.
136 * @author Howard Chu, Symas Corporation.
138 * @copyright Copyright 2011-2015 Howard Chu, Symas Corp. All rights reserved.
140 * Redistribution and use in source and binary forms, with or without
141 * modification, are permitted only as authorized by the OpenLDAP
144 * A copy of this license is available in the file LICENSE in the
145 * top-level directory of the distribution or, alternatively, at
146 * <http://www.OpenLDAP.org/license.html>.
149 * This code is derived from btree.c written by Martin Hedenfalk.
151 * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
153 * Permission to use, copy, modify, and distribute this software for any
154 * purpose with or without fee is hereby granted, provided that the above
155 * copyright notice and this permission notice appear in all copies.
157 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
158 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
159 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
160 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
161 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
162 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
163 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
168 #include <sys/types.h>
169 #include <inttypes.h>
175 /** Unix permissions for creating files, or dummy definition for Windows */
177 typedef int mdb_mode_t;
179 typedef mode_t mdb_mode_t;
183 typedef uint64_t mdb_size_t;
184 #define mdb_env_create mdb_env_create_vl32 /**< Prevent mixing with non-VL32 builds */
186 typedef size_t mdb_size_t;
189 /** An abstraction for a file handle.
190 * On POSIX systems file handles are small integers. On Windows
191 * they're opaque pointers.
194 typedef void *mdb_filehandle_t;
196 typedef int mdb_filehandle_t;
199 /** @defgroup mdb LMDB API
201 * @brief OpenLDAP Lightning Memory-Mapped Database Manager
203 /** @defgroup Version Version Macros
206 /** Library major version */
207 #define MDB_VERSION_MAJOR 0
208 /** Library minor version */
209 #define MDB_VERSION_MINOR 9
210 /** Library patch version */
211 #define MDB_VERSION_PATCH 70
213 /** Combine args a,b,c into a single integer for easy version comparisons */
214 #define MDB_VERINT(a,b,c) (((a) << 24) | ((b) << 16) | (c))
216 /** The full library version as a single integer */
217 #define MDB_VERSION_FULL \
218 MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
220 /** The release date of this library version */
221 #define MDB_VERSION_DATE "December 19, 2015"
223 /** A stringifier for the version info */
224 #define MDB_VERSTR(a,b,c,d) "LMDB " #a "." #b "." #c ": (" d ")"
226 /** A helper for the stringifier macro */
227 #define MDB_VERFOO(a,b,c,d) MDB_VERSTR(a,b,c,d)
229 /** The full library version as a C string */
230 #define MDB_VERSION_STRING \
231 MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
234 /** @brief Opaque structure for a database environment.
236 * A DB environment supports multiple databases, all residing in the same
239 typedef struct MDB_env MDB_env;
241 /** @brief Opaque structure for a transaction handle.
243 * All database operations require a transaction handle. Transactions may be
244 * read-only or read-write.
246 typedef struct MDB_txn MDB_txn;
248 /** @brief A handle for an individual database in the DB environment. */
249 typedef unsigned int MDB_dbi;
251 /** @brief Opaque structure for navigating through a database */
252 typedef struct MDB_cursor MDB_cursor;
254 /** @brief Generic structure used for passing keys and data in and out
257 * Values returned from the database are valid only until a subsequent
258 * update operation, or the end of the transaction. Do not modify or
259 * free them, they commonly point into the database itself.
261 * Key sizes must be between 1 and #mdb_env_get_maxkeysize() inclusive.
262 * The same applies to data sizes in databases with the #MDB_DUPSORT flag.
263 * Other data items can in theory be from 0 to 0xffffffff bytes long.
265 typedef struct MDB_val {
266 size_t mv_size; /**< size of the data item */
267 void *mv_data; /**< address of the data item */
270 /** @brief A callback function used to compare two keys in a database */
271 typedef int (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
273 /** @brief A callback function used to relocate a position-dependent data item
274 * in a fixed-address database.
276 * The \b newptr gives the item's desired address in
277 * the memory map, and \b oldptr gives its previous address. The item's actual
278 * data resides at the address in \b item. This callback is expected to walk
279 * through the fields of the record in \b item and modify any
280 * values based at the \b oldptr address to be relative to the \b newptr address.
281 * @param[in,out] item The item that is to be relocated.
282 * @param[in] oldptr The previous address.
283 * @param[in] newptr The new address to relocate to.
284 * @param[in] relctx An application-provided context, set by #mdb_set_relctx().
285 * @todo This feature is currently unimplemented.
287 typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
289 /** @defgroup mdb_env Environment Flags
292 /** mmap at a fixed address (experimental) */
293 #define MDB_FIXEDMAP 0x01
294 /** no environment directory */
295 #define MDB_NOSUBDIR 0x4000
296 /** don't fsync after commit */
297 #define MDB_NOSYNC 0x10000
299 #define MDB_RDONLY 0x20000
300 /** don't fsync metapage after commit */
301 #define MDB_NOMETASYNC 0x40000
302 /** use writable mmap */
303 #define MDB_WRITEMAP 0x80000
304 /** use asynchronous msync when #MDB_WRITEMAP is used */
305 #define MDB_MAPASYNC 0x100000
306 /** tie reader locktable slots to #MDB_txn objects instead of to threads */
307 #define MDB_NOTLS 0x200000
308 /** don't do any locking, caller must manage their own locks */
309 #define MDB_NOLOCK 0x400000
310 /** don't do readahead (no effect on Windows) */
311 #define MDB_NORDAHEAD 0x800000
312 /** don't initialize malloc'd memory before writing to datafile */
313 #define MDB_NOMEMINIT 0x1000000
316 /** @defgroup mdb_dbi_open Database Flags
319 /** use reverse string keys */
320 #define MDB_REVERSEKEY 0x02
321 /** use sorted duplicates */
322 #define MDB_DUPSORT 0x04
323 /** numeric keys in native byte order: either unsigned int or size_t.
324 * The keys must all be of the same size. */
325 #define MDB_INTEGERKEY 0x08
326 /** with #MDB_DUPSORT, sorted dup items have fixed size */
327 #define MDB_DUPFIXED 0x10
328 /** with #MDB_DUPSORT, dups are #MDB_INTEGERKEY-style integers */
329 #define MDB_INTEGERDUP 0x20
330 /** with #MDB_DUPSORT, use reverse string dups */
331 #define MDB_REVERSEDUP 0x40
332 /** create DB if not already existing */
333 #define MDB_CREATE 0x40000
336 /** @defgroup mdb_put Write Flags
339 /** For put: Don't write if the key already exists. */
340 #define MDB_NOOVERWRITE 0x10
341 /** Only for #MDB_DUPSORT<br>
342 * For put: don't write if the key and data pair already exist.<br>
343 * For mdb_cursor_del: remove all duplicate data items.
345 #define MDB_NODUPDATA 0x20
346 /** For mdb_cursor_put: overwrite the current key/data pair */
347 #define MDB_CURRENT 0x40
348 /** For put: Just reserve space for data, don't copy it. Return a
349 * pointer to the reserved space.
351 #define MDB_RESERVE 0x10000
352 /** Data is being appended, don't split full pages. */
353 #define MDB_APPEND 0x20000
354 /** Duplicate data is being appended, don't split full pages. */
355 #define MDB_APPENDDUP 0x40000
356 /** Store multiple data items in one call. Only for #MDB_DUPFIXED. */
357 #define MDB_MULTIPLE 0x80000
360 /** @defgroup mdb_copy Copy Flags
363 /** Compacting copy: Omit free space from copy, and renumber all
364 * pages sequentially.
366 #define MDB_CP_COMPACT 0x01
369 /** @brief Cursor Get operations.
371 * This is the set of all operations for retrieving data
374 typedef enum MDB_cursor_op {
375 MDB_FIRST, /**< Position at first key/data item */
376 MDB_FIRST_DUP, /**< Position at first data item of current key.
377 Only for #MDB_DUPSORT */
378 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
379 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
380 MDB_GET_CURRENT, /**< Return key/data at current cursor position */
381 MDB_GET_MULTIPLE, /**< Return key and up to a page of duplicate data items
382 from current cursor position. Move cursor to prepare
383 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
384 MDB_LAST, /**< Position at last key/data item */
385 MDB_LAST_DUP, /**< Position at last data item of current key.
386 Only for #MDB_DUPSORT */
387 MDB_NEXT, /**< Position at next data item */
388 MDB_NEXT_DUP, /**< Position at next data item of current key.
389 Only for #MDB_DUPSORT */
390 MDB_NEXT_MULTIPLE, /**< Return key and up to a page of duplicate data items
391 from next cursor position. Move cursor to prepare
392 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
393 MDB_NEXT_NODUP, /**< Position at first data item of next key */
394 MDB_PREV, /**< Position at previous data item */
395 MDB_PREV_DUP, /**< Position at previous data item of current key.
396 Only for #MDB_DUPSORT */
397 MDB_PREV_NODUP, /**< Position at last data item of previous key */
398 MDB_SET, /**< Position at specified key */
399 MDB_SET_KEY, /**< Position at specified key, return key + data */
400 MDB_SET_RANGE, /**< Position at first key greater than or equal to specified key. */
401 MDB_PREV_MULTIPLE /**< Position at previous page and return key and up to
402 a page of duplicate data items. Only for #MDB_DUPFIXED */
405 /** @defgroup errors Return Codes
407 * BerkeleyDB uses -30800 to -30999, we'll go under them
410 /** Successful result */
411 #define MDB_SUCCESS 0
412 /** key/data pair already exists */
413 #define MDB_KEYEXIST (-30799)
414 /** key/data pair not found (EOF) */
415 #define MDB_NOTFOUND (-30798)
416 /** Requested page not found - this usually indicates corruption */
417 #define MDB_PAGE_NOTFOUND (-30797)
418 /** Located page was wrong type */
419 #define MDB_CORRUPTED (-30796)
420 /** Update of meta page failed or environment had fatal error */
421 #define MDB_PANIC (-30795)
422 /** Environment version mismatch */
423 #define MDB_VERSION_MISMATCH (-30794)
424 /** File is not a valid LMDB file */
425 #define MDB_INVALID (-30793)
426 /** Environment mapsize reached */
427 #define MDB_MAP_FULL (-30792)
428 /** Environment maxdbs reached */
429 #define MDB_DBS_FULL (-30791)
430 /** Environment maxreaders reached */
431 #define MDB_READERS_FULL (-30790)
432 /** Too many TLS keys in use - Windows only */
433 #define MDB_TLS_FULL (-30789)
434 /** Txn has too many dirty pages */
435 #define MDB_TXN_FULL (-30788)
436 /** Cursor stack too deep - internal error */
437 #define MDB_CURSOR_FULL (-30787)
438 /** Page has not enough space - internal error */
439 #define MDB_PAGE_FULL (-30786)
440 /** Database contents grew beyond environment mapsize */
441 #define MDB_MAP_RESIZED (-30785)
442 /** Operation and DB incompatible, or DB type changed. This can mean:
444 * <li>The operation expects an #MDB_DUPSORT / #MDB_DUPFIXED database.
445 * <li>Opening a named DB when the unnamed DB has #MDB_DUPSORT / #MDB_INTEGERKEY.
446 * <li>Accessing a data record as a database, or vice versa.
447 * <li>The database was dropped and recreated with different flags.
450 #define MDB_INCOMPATIBLE (-30784)
451 /** Invalid reuse of reader locktable slot */
452 #define MDB_BAD_RSLOT (-30783)
453 /** Transaction must abort, has a child, or is invalid */
454 #define MDB_BAD_TXN (-30782)
455 /** Unsupported size of key/DB name/data, or wrong DUPFIXED size */
456 #define MDB_BAD_VALSIZE (-30781)
457 /** The specified DBI was changed unexpectedly */
458 #define MDB_BAD_DBI (-30780)
459 /** The last defined error code */
460 #define MDB_LAST_ERRCODE MDB_BAD_DBI
463 /** @brief Statistics for a database in the environment */
464 typedef struct MDB_stat {
465 unsigned int ms_psize; /**< Size of a database page.
466 This is currently the same for all databases. */
467 unsigned int ms_depth; /**< Depth (height) of the B-tree */
468 mdb_size_t ms_branch_pages; /**< Number of internal (non-leaf) pages */
469 mdb_size_t ms_leaf_pages; /**< Number of leaf pages */
470 mdb_size_t ms_overflow_pages; /**< Number of overflow pages */
471 mdb_size_t ms_entries; /**< Number of data items */
474 /** @brief Information about the environment */
475 typedef struct MDB_envinfo {
476 void *me_mapaddr; /**< Address of map, if fixed */
477 mdb_size_t me_mapsize; /**< Size of the data memory map */
478 mdb_size_t me_last_pgno; /**< ID of the last used page */
479 mdb_size_t me_last_txnid; /**< ID of the last committed transaction */
480 unsigned int me_maxreaders; /**< max reader slots in the environment */
481 unsigned int me_numreaders; /**< max reader slots used in the environment */
484 /** @brief Return the LMDB library version information.
486 * @param[out] major if non-NULL, the library major version number is copied here
487 * @param[out] minor if non-NULL, the library minor version number is copied here
488 * @param[out] patch if non-NULL, the library patch version number is copied here
489 * @retval "version string" The library version as a string
491 char *mdb_version(int *major, int *minor, int *patch);
493 /** @brief Return a string describing a given error code.
495 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
496 * function. If the error code is greater than or equal to 0, then the string
497 * returned by the system function strerror(3) is returned. If the error code
498 * is less than 0, an error string corresponding to the LMDB library error is
499 * returned. See @ref errors for a list of LMDB-specific error codes.
500 * @param[in] err The error code
501 * @retval "error message" The description of the error
503 char *mdb_strerror(int err);
505 /** @brief Create an LMDB environment handle.
507 * This function allocates memory for a #MDB_env structure. To release
508 * the allocated memory and discard the handle, call #mdb_env_close().
509 * Before the handle may be used, it must be opened using #mdb_env_open().
510 * Various other options may also need to be set before opening the handle,
511 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
512 * depending on usage requirements.
513 * @param[out] env The address where the new handle will be stored
514 * @return A non-zero error value on failure and 0 on success.
516 int mdb_env_create(MDB_env **env);
518 /** @brief Open an environment handle.
520 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
521 * @param[in] env An environment handle returned by #mdb_env_create()
522 * @param[in] path The directory in which the database files reside. This
523 * directory must already exist and be writable.
524 * @param[in] flags Special options for this environment. This parameter
525 * must be set to 0 or by bitwise OR'ing together one or more of the
526 * values described here.
527 * Flags set by mdb_env_set_flags() are also used.
530 * use a fixed address for the mmap region. This flag must be specified
531 * when creating the environment, and is stored persistently in the environment.
532 * If successful, the memory map will always reside at the same virtual address
533 * and pointers used to reference data items in the database will be constant
534 * across multiple invocations. This option may not always work, depending on
535 * how the operating system has allocated memory to shared libraries and other uses.
536 * The feature is highly experimental.
538 * By default, LMDB creates its environment in a directory whose
539 * pathname is given in \b path, and creates its data and lock files
540 * under that directory. With this option, \b path is used as-is for
541 * the database main data file. The database lock file is the \b path
542 * with "-lock" appended.
544 * Open the environment in read-only mode. No write operations will be
545 * allowed. LMDB will still modify the lock file - except on read-only
546 * filesystems, where LMDB does not use locks.
548 * Use a writeable memory map unless MDB_RDONLY is set. This uses
549 * fewer mallocs but loses protection from application bugs
550 * like wild pointer writes and other bad updates into the database.
551 * This may be slightly faster for DBs that fit entirely in RAM, but
552 * is slower for DBs larger than RAM.
553 * Incompatible with nested transactions.
554 * Do not mix processes with and without MDB_WRITEMAP on the same
555 * environment. This can defeat durability (#mdb_env_sync etc).
556 * <li>#MDB_NOMETASYNC
557 * Flush system buffers to disk only once per transaction, omit the
558 * metadata flush. Defer that until the system flushes files to disk,
559 * or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization
560 * maintains database integrity, but a system crash may undo the last
561 * committed transaction. I.e. it preserves the ACI (atomicity,
562 * consistency, isolation) but not D (durability) database property.
563 * This flag may be changed at any time using #mdb_env_set_flags().
565 * Don't flush system buffers to disk when committing a transaction.
566 * This optimization means a system crash can corrupt the database or
567 * lose the last transactions if buffers are not yet flushed to disk.
568 * The risk is governed by how often the system flushes dirty buffers
569 * to disk and how often #mdb_env_sync() is called. However, if the
570 * filesystem preserves write order and the #MDB_WRITEMAP flag is not
571 * used, transactions exhibit ACI (atomicity, consistency, isolation)
572 * properties and only lose D (durability). I.e. database integrity
573 * is maintained, but a system crash may undo the final transactions.
574 * Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no
575 * hint for when to write transactions to disk, unless #mdb_env_sync()
576 * is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable.
577 * This flag may be changed at any time using #mdb_env_set_flags().
579 * When using #MDB_WRITEMAP, use asynchronous flushes to disk.
580 * As with #MDB_NOSYNC, a system crash can then corrupt the
581 * database or lose the last transactions. Calling #mdb_env_sync()
582 * ensures on-disk database integrity until next commit.
583 * This flag may be changed at any time using #mdb_env_set_flags().
585 * Don't use Thread-Local Storage. Tie reader locktable slots to
586 * #MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps
587 * the slot reseved for the #MDB_txn object. A thread may use parallel
588 * read-only transactions. A read-only transaction may span threads if
589 * the user synchronizes its use. Applications that multiplex many
590 * user threads over individual OS threads need this option. Such an
591 * application must also serialize the write transactions in an OS
592 * thread, since LMDB's write locking is unaware of the user threads.
594 * Don't do any locking. If concurrent access is anticipated, the
595 * caller must manage all concurrency itself. For proper operation
596 * the caller must enforce single-writer semantics, and must ensure
597 * that no readers are using old transactions while a writer is
598 * active. The simplest approach is to use an exclusive lock so that
599 * no readers may be active at all when a writer begins.
601 * Turn off readahead. Most operating systems perform readahead on
602 * read requests by default. This option turns it off if the OS
603 * supports it. Turning it off may help random read performance
604 * when the DB is larger than RAM and system RAM is full.
605 * The option is not implemented on Windows.
607 * Don't initialize malloc'd memory before writing to unused spaces
608 * in the data file. By default, memory for pages written to the data
609 * file is obtained using malloc. While these pages may be reused in
610 * subsequent transactions, freshly malloc'd pages will be initialized
611 * to zeroes before use. This avoids persisting leftover data from other
612 * code (that used the heap and subsequently freed the memory) into the
613 * data file. Note that many other system libraries may allocate
614 * and free memory from the heap for arbitrary uses. E.g., stdio may
615 * use the heap for file I/O buffers. This initialization step has a
616 * modest performance cost so some applications may want to disable
617 * it using this flag. This option can be a problem for applications
618 * which handle sensitive data like passwords, and it makes memory
619 * checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP,
620 * which writes directly to the mmap instead of using malloc for pages. The
621 * initialization is also skipped if #MDB_RESERVE is used; the
622 * caller is expected to overwrite all of the memory that was
623 * reserved in that case.
624 * This flag may be changed at any time using #mdb_env_set_flags().
626 * @param[in] mode The UNIX permissions to set on created files and semaphores.
627 * This parameter is ignored on Windows.
628 * @return A non-zero error value on failure and 0 on success. Some possible
631 * <li>#MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the
632 * version that created the database environment.
633 * <li>#MDB_INVALID - the environment file headers are corrupted.
634 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
635 * <li>EACCES - the user didn't have permission to access the environment files.
636 * <li>EAGAIN - the environment was locked by another process.
639 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode);
641 /** @brief Copy an LMDB environment to the specified path.
643 * This function may be used to make a backup of an existing environment.
644 * No lockfile is created, since it gets recreated at need.
645 * @note This call can trigger significant file size growth if run in
646 * parallel with write transactions, because it employs a read-only
647 * transaction. See long-lived transactions under @ref caveats_sec.
648 * @param[in] env An environment handle returned by #mdb_env_create(). It
649 * must have already been opened successfully.
650 * @param[in] path The directory in which the copy will reside. This
651 * directory must already exist and be writable but must otherwise be
653 * @return A non-zero error value on failure and 0 on success.
655 int mdb_env_copy(MDB_env *env, const char *path);
657 /** @brief Copy an LMDB environment to the specified file descriptor.
659 * This function may be used to make a backup of an existing environment.
660 * No lockfile is created, since it gets recreated at need.
661 * @note This call can trigger significant file size growth if run in
662 * parallel with write transactions, because it employs a read-only
663 * transaction. See long-lived transactions under @ref caveats_sec.
664 * @param[in] env An environment handle returned by #mdb_env_create(). It
665 * must have already been opened successfully.
666 * @param[in] fd The filedescriptor to write the copy to. It must
667 * have already been opened for Write access.
668 * @return A non-zero error value on failure and 0 on success.
670 int mdb_env_copyfd(MDB_env *env, mdb_filehandle_t fd);
672 /** @brief Copy an LMDB environment to the specified path, with options.
674 * This function may be used to make a backup of an existing environment.
675 * No lockfile is created, since it gets recreated at need.
676 * @note This call can trigger significant file size growth if run in
677 * parallel with write transactions, because it employs a read-only
678 * transaction. See long-lived transactions under @ref caveats_sec.
679 * @param[in] env An environment handle returned by #mdb_env_create(). It
680 * must have already been opened successfully.
681 * @param[in] path The directory in which the copy will reside. This
682 * directory must already exist and be writable but must otherwise be
684 * @param[in] flags Special options for this operation. This parameter
685 * must be set to 0 or by bitwise OR'ing together one or more of the
686 * values described here.
688 * <li>#MDB_CP_COMPACT - Perform compaction while copying: omit free
689 * pages and sequentially renumber all pages in output. This option
690 * consumes more CPU and runs more slowly than the default.
692 * @return A non-zero error value on failure and 0 on success.
694 int mdb_env_copy2(MDB_env *env, const char *path, unsigned int flags);
696 /** @brief Copy an LMDB environment to the specified file descriptor,
699 * This function may be used to make a backup of an existing environment.
700 * No lockfile is created, since it gets recreated at need. See
701 * #mdb_env_copy2() for further details.
702 * @note This call can trigger significant file size growth if run in
703 * parallel with write transactions, because it employs a read-only
704 * transaction. See long-lived transactions under @ref caveats_sec.
705 * @param[in] env An environment handle returned by #mdb_env_create(). It
706 * must have already been opened successfully.
707 * @param[in] fd The filedescriptor to write the copy to. It must
708 * have already been opened for Write access.
709 * @param[in] flags Special options for this operation.
710 * See #mdb_env_copy2() for options.
711 * @return A non-zero error value on failure and 0 on success.
713 int mdb_env_copyfd2(MDB_env *env, mdb_filehandle_t fd, unsigned int flags);
715 /** @brief Return statistics about the LMDB environment.
717 * @param[in] env An environment handle returned by #mdb_env_create()
718 * @param[out] stat The address of an #MDB_stat structure
719 * where the statistics will be copied
721 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
723 /** @brief Return information about the LMDB environment.
725 * @param[in] env An environment handle returned by #mdb_env_create()
726 * @param[out] stat The address of an #MDB_envinfo structure
727 * where the information will be copied
729 int mdb_env_info(MDB_env *env, MDB_envinfo *stat);
731 /** @brief Flush the data buffers to disk.
733 * Data is always written to disk when #mdb_txn_commit() is called,
734 * but the operating system may keep it buffered. LMDB always flushes
735 * the OS buffers upon commit as well, unless the environment was
736 * opened with #MDB_NOSYNC or in part #MDB_NOMETASYNC. This call is
737 * not valid if the environment was opened with #MDB_RDONLY.
738 * @param[in] env An environment handle returned by #mdb_env_create()
739 * @param[in] force If non-zero, force a synchronous flush. Otherwise
740 * if the environment has the #MDB_NOSYNC flag set the flushes
741 * will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
742 * @return A non-zero error value on failure and 0 on success. Some possible
745 * <li>EACCES - the environment is read-only.
746 * <li>EINVAL - an invalid parameter was specified.
747 * <li>EIO - an error occurred during synchronization.
750 int mdb_env_sync(MDB_env *env, int force);
752 /** @brief Close the environment and release the memory map.
754 * Only a single thread may call this function. All transactions, databases,
755 * and cursors must already be closed before calling this function. Attempts to
756 * use any such handles after calling this function will cause a SIGSEGV.
757 * The environment handle will be freed and must not be used again after this call.
758 * @param[in] env An environment handle returned by #mdb_env_create()
760 void mdb_env_close(MDB_env *env);
762 /** @brief Set environment flags.
764 * This may be used to set some flags in addition to those from
765 * #mdb_env_open(), or to unset these flags. If several threads
766 * change the flags at the same time, the result is undefined.
767 * @param[in] env An environment handle returned by #mdb_env_create()
768 * @param[in] flags The flags to change, bitwise OR'ed together
769 * @param[in] onoff A non-zero value sets the flags, zero clears them.
770 * @return A non-zero error value on failure and 0 on success. Some possible
773 * <li>EINVAL - an invalid parameter was specified.
776 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
778 /** @brief Get environment flags.
780 * @param[in] env An environment handle returned by #mdb_env_create()
781 * @param[out] flags The address of an integer to store the flags
782 * @return A non-zero error value on failure and 0 on success. Some possible
785 * <li>EINVAL - an invalid parameter was specified.
788 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
790 /** @brief Return the path that was used in #mdb_env_open().
792 * @param[in] env An environment handle returned by #mdb_env_create()
793 * @param[out] path Address of a string pointer to contain the path. This
794 * is the actual string in the environment, not a copy. It should not be
795 * altered in any way.
796 * @return A non-zero error value on failure and 0 on success. Some possible
799 * <li>EINVAL - an invalid parameter was specified.
802 int mdb_env_get_path(MDB_env *env, const char **path);
804 /** @brief Return the filedescriptor for the given environment.
806 * @param[in] env An environment handle returned by #mdb_env_create()
807 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.
808 * @return A non-zero error value on failure and 0 on success. Some possible
811 * <li>EINVAL - an invalid parameter was specified.
814 int mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *fd);
816 /** @brief Set the size of the memory map to use for this environment.
818 * The size should be a multiple of the OS page size. The default is
819 * 10485760 bytes. The size of the memory map is also the maximum size
820 * of the database. The value should be chosen as large as possible,
821 * to accommodate future growth of the database.
822 * This function should be called after #mdb_env_create() and before #mdb_env_open().
823 * It may be called at later times if no transactions are active in
824 * this process. Note that the library does not check for this condition,
825 * the caller must ensure it explicitly.
827 * The new size takes effect immediately for the current process but
828 * will not be persisted to any others until a write transaction has been
829 * committed by the current process. Also, only mapsize increases are
830 * persisted into the environment.
832 * If the mapsize is increased by another process, and data has grown
833 * beyond the range of the current mapsize, #mdb_txn_begin() will
834 * return #MDB_MAP_RESIZED. This function may be called with a size
835 * of zero to adopt the new size.
837 * Any attempt to set a size smaller than the space already consumed
838 * by the environment will be silently changed to the current size of the used space.
839 * @param[in] env An environment handle returned by #mdb_env_create()
840 * @param[in] size The size in bytes
841 * @return A non-zero error value on failure and 0 on success. Some possible
844 * <li>EINVAL - an invalid parameter was specified, or the environment has
845 * an active write transaction.
848 int mdb_env_set_mapsize(MDB_env *env, mdb_size_t size);
850 /** @brief Set the maximum number of threads/reader slots for the environment.
852 * This defines the number of slots in the lock table that is used to track readers in the
853 * the environment. The default is 126.
854 * Starting a read-only transaction normally ties a lock table slot to the
855 * current thread until the environment closes or the thread exits. If
856 * MDB_NOTLS is in use, #mdb_txn_begin() instead ties the slot to the
857 * MDB_txn object until it or the #MDB_env object is destroyed.
858 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
859 * @param[in] env An environment handle returned by #mdb_env_create()
860 * @param[in] readers The maximum number of reader lock table slots
861 * @return A non-zero error value on failure and 0 on success. Some possible
864 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
867 int mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);
869 /** @brief Get the maximum number of threads/reader slots for the environment.
871 * @param[in] env An environment handle returned by #mdb_env_create()
872 * @param[out] readers Address of an integer to store the number of readers
873 * @return A non-zero error value on failure and 0 on success. Some possible
876 * <li>EINVAL - an invalid parameter was specified.
879 int mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers);
881 /** @brief Set the maximum number of named databases for the environment.
883 * This function is only needed if multiple databases will be used in the
884 * environment. Simpler applications that use the environment as a single
885 * unnamed database can ignore this option.
886 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
888 * Currently a moderate number of slots are cheap but a huge number gets
889 * expensive: 7-120 words per transaction, and every #mdb_dbi_open()
890 * does a linear search of the opened slots.
891 * @param[in] env An environment handle returned by #mdb_env_create()
892 * @param[in] dbs The maximum number of databases
893 * @return A non-zero error value on failure and 0 on success. Some possible
896 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
899 int mdb_env_set_maxdbs(MDB_env *env, MDB_dbi dbs);
901 /** @brief Get the maximum size of keys and #MDB_DUPSORT data we can write.
903 * Depends on the compile-time constant #MDB_MAXKEYSIZE. Default 511.
905 * @param[in] env An environment handle returned by #mdb_env_create()
906 * @return The maximum size of a key we can write
908 int mdb_env_get_maxkeysize(MDB_env *env);
910 /** @brief Set application information associated with the #MDB_env.
912 * @param[in] env An environment handle returned by #mdb_env_create()
913 * @param[in] ctx An arbitrary pointer for whatever the application needs.
914 * @return A non-zero error value on failure and 0 on success.
916 int mdb_env_set_userctx(MDB_env *env, void *ctx);
918 /** @brief Get the application information associated with the #MDB_env.
920 * @param[in] env An environment handle returned by #mdb_env_create()
921 * @return The pointer set by #mdb_env_set_userctx().
923 void *mdb_env_get_userctx(MDB_env *env);
925 /** @brief A callback function for most LMDB assert() failures,
926 * called before printing the message and aborting.
928 * @param[in] env An environment handle returned by #mdb_env_create().
929 * @param[in] msg The assertion message, not including newline.
931 typedef void MDB_assert_func(MDB_env *env, const char *msg);
933 /** Set or reset the assert() callback of the environment.
934 * Disabled if liblmdb is buillt with NDEBUG.
935 * @note This hack should become obsolete as lmdb's error handling matures.
936 * @param[in] env An environment handle returned by #mdb_env_create().
937 * @param[in] func An #MDB_assert_func function, or 0.
938 * @return A non-zero error value on failure and 0 on success.
940 int mdb_env_set_assert(MDB_env *env, MDB_assert_func *func);
942 /** @brief Create a transaction for use with the environment.
944 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
945 * @note A transaction and its cursors must only be used by a single
946 * thread, and a thread may only have a single transaction at a time.
947 * If #MDB_NOTLS is in use, this does not apply to read-only transactions.
948 * @note Cursors may not span transactions.
949 * @param[in] env An environment handle returned by #mdb_env_create()
950 * @param[in] parent If this parameter is non-NULL, the new transaction
951 * will be a nested transaction, with the transaction indicated by \b parent
952 * as its parent. Transactions may be nested to any level. A parent
953 * transaction and its cursors may not issue any other operations than
954 * mdb_txn_commit and mdb_txn_abort while it has active child transactions.
955 * @param[in] flags Special options for this transaction. This parameter
956 * must be set to 0 or by bitwise OR'ing together one or more of the
957 * values described here.
960 * This transaction will not perform any write operations.
962 * Don't flush system buffers to disk when committing this transaction.
963 * <li>#MDB_NOMETASYNC
964 * Flush system buffers but omit metadata flush when committing this transaction.
966 * @param[out] txn Address where the new #MDB_txn handle will be stored
967 * @return A non-zero error value on failure and 0 on success. Some possible
970 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
972 * <li>#MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's
973 * mapsize and this environment's map must be resized as well.
974 * See #mdb_env_set_mapsize().
975 * <li>#MDB_READERS_FULL - a read-only transaction was requested and
976 * the reader lock table is full. See #mdb_env_set_maxreaders().
977 * <li>ENOMEM - out of memory.
980 int mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);
982 /** @brief Returns the transaction's #MDB_env
984 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
986 MDB_env *mdb_txn_env(MDB_txn *txn);
988 /** @brief Return the transaction's ID.
990 * This returns the identifier associated with this transaction. For a
991 * read-only transaction, this corresponds to the snapshot being read;
992 * concurrent readers will frequently have the same transaction ID.
994 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
995 * @return A transaction ID, valid if input is an active transaction.
997 mdb_size_t mdb_txn_id(MDB_txn *txn);
999 /** @brief Commit all the operations of a transaction into the database.
1001 * The transaction handle is freed. It and its cursors must not be used
1002 * again after this call, except with #mdb_cursor_renew().
1003 * @note Earlier documentation incorrectly said all cursors would be freed.
1004 * Only write-transactions free cursors.
1005 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1006 * @return A non-zero error value on failure and 0 on success. Some possible
1009 * <li>EINVAL - an invalid parameter was specified.
1010 * <li>ENOSPC - no more disk space.
1011 * <li>EIO - a low-level I/O error occurred while writing.
1012 * <li>ENOMEM - out of memory.
1015 int mdb_txn_commit(MDB_txn *txn);
1017 /** @brief Abandon all the operations of the transaction instead of saving them.
1019 * The transaction handle is freed. It and its cursors must not be used
1020 * again after this call, except with #mdb_cursor_renew().
1021 * @note Earlier documentation incorrectly said all cursors would be freed.
1022 * Only write-transactions free cursors.
1023 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1025 void mdb_txn_abort(MDB_txn *txn);
1027 /** @brief Reset a read-only transaction.
1029 * Abort the transaction like #mdb_txn_abort(), but keep the transaction
1030 * handle. #mdb_txn_renew() may reuse the handle. This saves allocation
1031 * overhead if the process will start a new read-only transaction soon,
1032 * and also locking overhead if #MDB_NOTLS is in use. The reader table
1033 * lock is released, but the table slot stays tied to its thread or
1034 * #MDB_txn. Use mdb_txn_abort() to discard a reset handle, and to free
1035 * its lock table slot if MDB_NOTLS is in use.
1036 * Cursors opened within the transaction must not be used
1037 * again after this call, except with #mdb_cursor_renew().
1038 * Reader locks generally don't interfere with writers, but they keep old
1039 * versions of database pages allocated. Thus they prevent the old pages
1040 * from being reused when writers commit new data, and so under heavy load
1041 * the database size may grow much more rapidly than otherwise.
1042 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1044 void mdb_txn_reset(MDB_txn *txn);
1046 /** @brief Renew a read-only transaction.
1048 * This acquires a new reader lock for a transaction handle that had been
1049 * released by #mdb_txn_reset(). It must be called before a reset transaction
1050 * may be used again.
1051 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1052 * @return A non-zero error value on failure and 0 on success. Some possible
1055 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
1056 * must be shut down.
1057 * <li>EINVAL - an invalid parameter was specified.
1060 int mdb_txn_renew(MDB_txn *txn);
1062 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1063 #define mdb_open(txn,name,flags,dbi) mdb_dbi_open(txn,name,flags,dbi)
1064 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1065 #define mdb_close(env,dbi) mdb_dbi_close(env,dbi)
1067 /** @brief Open a database in the environment.
1069 * A database handle denotes the name and parameters of a database,
1070 * independently of whether such a database exists.
1071 * The database handle may be discarded by calling #mdb_dbi_close().
1072 * The old database handle is returned if the database was already open.
1073 * The handle may only be closed once.
1075 * The database handle will be private to the current transaction until
1076 * the transaction is successfully committed. If the transaction is
1077 * aborted the handle will be closed automatically.
1078 * After a successful commit the handle will reside in the shared
1079 * environment, and may be used by other transactions.
1081 * This function must not be called from multiple concurrent
1082 * transactions in the same process. A transaction that uses
1083 * this function must finish (either commit or abort) before
1084 * any other transaction in the process may use this function.
1086 * To use named databases (with name != NULL), #mdb_env_set_maxdbs()
1087 * must be called before opening the environment. Database names are
1088 * keys in the unnamed database, and may be read but not written.
1090 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1091 * @param[in] name The name of the database to open. If only a single
1092 * database is needed in the environment, this value may be NULL.
1093 * @param[in] flags Special options for this database. This parameter
1094 * must be set to 0 or by bitwise OR'ing together one or more of the
1095 * values described here.
1097 * <li>#MDB_REVERSEKEY
1098 * Keys are strings to be compared in reverse order, from the end
1099 * of the strings to the beginning. By default, Keys are treated as strings and
1100 * compared from beginning to end.
1102 * Duplicate keys may be used in the database. (Or, from another perspective,
1103 * keys may have multiple data items, stored in sorted order.) By default
1104 * keys must be unique and may have only a single data item.
1105 * <li>#MDB_INTEGERKEY
1106 * Keys are binary integers in native byte order, either unsigned int
1107 * or size_t, and will be sorted as such.
1108 * The keys must all be of the same size.
1110 * This flag may only be used in combination with #MDB_DUPSORT. This option
1111 * tells the library that the data items for this database are all the same
1112 * size, which allows further optimizations in storage and retrieval. When
1113 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
1114 * cursor operations may be used to retrieve multiple items at once.
1115 * <li>#MDB_INTEGERDUP
1116 * This option specifies that duplicate data items are binary integers,
1117 * similar to #MDB_INTEGERKEY keys.
1118 * <li>#MDB_REVERSEDUP
1119 * This option specifies that duplicate data items should be compared as
1120 * strings in reverse order.
1122 * Create the named database if it doesn't exist. This option is not
1123 * allowed in a read-only transaction or a read-only environment.
1125 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
1126 * @return A non-zero error value on failure and 0 on success. Some possible
1129 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
1130 * and #MDB_CREATE was not specified.
1131 * <li>#MDB_DBS_FULL - too many databases have been opened. See #mdb_env_set_maxdbs().
1134 int mdb_dbi_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
1136 /** @brief Retrieve statistics for a database.
1138 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1139 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1140 * @param[out] stat The address of an #MDB_stat structure
1141 * where the statistics will be copied
1142 * @return A non-zero error value on failure and 0 on success. Some possible
1145 * <li>EINVAL - an invalid parameter was specified.
1148 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
1150 /** @brief Retrieve the DB flags for a database handle.
1152 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1153 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1154 * @param[out] flags Address where the flags will be returned.
1155 * @return A non-zero error value on failure and 0 on success.
1157 int mdb_dbi_flags(MDB_txn *txn, MDB_dbi dbi, unsigned int *flags);
1159 /** @brief Close a database handle. Normally unnecessary. Use with care:
1161 * This call is not mutex protected. Handles should only be closed by
1162 * a single thread, and only if no other threads are going to reference
1163 * the database handle or one of its cursors any further. Do not close
1164 * a handle if an existing transaction has modified its database.
1165 * Doing so can cause misbehavior from database corruption to errors
1166 * like MDB_BAD_VALSIZE (since the DB name is gone).
1168 * Closing a database handle is not necessary, but lets #mdb_dbi_open()
1169 * reuse the handle value. Usually it's better to set a bigger
1170 * #mdb_env_set_maxdbs(), unless that value would be large.
1172 * @param[in] env An environment handle returned by #mdb_env_create()
1173 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1175 void mdb_dbi_close(MDB_env *env, MDB_dbi dbi);
1177 /** @brief Empty or delete+close a database.
1179 * See #mdb_dbi_close() for restrictions about closing the DB handle.
1180 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1181 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1182 * @param[in] del 0 to empty the DB, 1 to delete it from the
1183 * environment and close the DB handle.
1184 * @return A non-zero error value on failure and 0 on success.
1186 int mdb_drop(MDB_txn *txn, MDB_dbi dbi, int del);
1188 /** @brief Set a custom key comparison function for a database.
1190 * The comparison function is called whenever it is necessary to compare a
1191 * key specified by the application with a key currently stored in the database.
1192 * If no comparison function is specified, and no special key flags were specified
1193 * with #mdb_dbi_open(), the keys are compared lexically, with shorter keys collating
1194 * before longer keys.
1195 * @warning This function must be called before any data access functions are used,
1196 * otherwise data corruption may occur. The same comparison function must be used by every
1197 * program accessing the database, every time the database is used.
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] cmp A #MDB_cmp_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_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1209 /** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
1211 * This comparison function is called whenever it is necessary to compare a data
1212 * item specified by the application with a data item currently stored in the database.
1213 * This function only takes effect if the database was opened with the #MDB_DUPSORT
1215 * If no comparison function is specified, and no special key flags were specified
1216 * with #mdb_dbi_open(), the data items are compared lexically, with shorter items collating
1217 * before longer items.
1218 * @warning This function must be called before any data access functions are used,
1219 * otherwise data corruption may occur. The same comparison function must be used by every
1220 * program accessing the database, every time the database is used.
1221 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1222 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1223 * @param[in] cmp A #MDB_cmp_func function
1224 * @return A non-zero error value on failure and 0 on success. Some possible
1227 * <li>EINVAL - an invalid parameter was specified.
1230 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1232 /** @brief Set a relocation function for a #MDB_FIXEDMAP database.
1234 * @todo The relocation function is called whenever it is necessary to move the data
1235 * of an item to a different position in the database (e.g. through tree
1236 * balancing operations, shifts as a result of adds or deletes, etc.). It is
1237 * intended to allow address/position-dependent data items to be stored in
1238 * a database in an environment opened with the #MDB_FIXEDMAP option.
1239 * Currently the relocation feature is unimplemented and setting
1240 * this function has no effect.
1241 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1242 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1243 * @param[in] rel A #MDB_rel_func function
1244 * @return A non-zero error value on failure and 0 on success. Some possible
1247 * <li>EINVAL - an invalid parameter was specified.
1250 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
1252 /** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
1254 * See #mdb_set_relfunc and #MDB_rel_func for more details.
1255 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1256 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1257 * @param[in] ctx An arbitrary pointer for whatever the application needs.
1258 * It will be passed to the callback function set by #mdb_set_relfunc
1259 * as its \b relctx parameter whenever the callback is invoked.
1260 * @return A non-zero error value on failure and 0 on success. Some possible
1263 * <li>EINVAL - an invalid parameter was specified.
1266 int mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);
1268 /** @brief Get items from a database.
1270 * This function retrieves key/data pairs from the database. The address
1271 * and length of the data associated with the specified \b key are returned
1272 * in the structure to which \b data refers.
1273 * If the database supports duplicate keys (#MDB_DUPSORT) then the
1274 * first data item for the key will be returned. Retrieval of other
1275 * items requires the use of #mdb_cursor_get().
1277 * @note The memory pointed to by the returned values is owned by the
1278 * database. The caller need not dispose of the memory, and may not
1279 * modify it in any way. For values returned in a read-only transaction
1280 * any modification attempts will cause a SIGSEGV.
1281 * @note Values returned from the database are valid only until a
1282 * subsequent update operation, or the end of the transaction.
1283 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1284 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1285 * @param[in] key The key to search for in the database
1286 * @param[out] data The data corresponding to the key
1287 * @return A non-zero error value on failure and 0 on success. Some possible
1290 * <li>#MDB_NOTFOUND - the key was not in the database.
1291 * <li>EINVAL - an invalid parameter was specified.
1294 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1296 /** @brief Store items into a database.
1298 * This function stores key/data pairs in the database. The default behavior
1299 * is to enter the new key/data pair, replacing any previously existing key
1300 * if duplicates are disallowed, or adding a duplicate data item if
1301 * duplicates are allowed (#MDB_DUPSORT).
1302 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1303 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1304 * @param[in] key The key to store in the database
1305 * @param[in,out] data The data to store
1306 * @param[in] flags Special options for this operation. This parameter
1307 * must be set to 0 or by bitwise OR'ing together one or more of the
1308 * values described here.
1310 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1311 * already appear in the database. This flag may only be specified
1312 * if the database was opened with #MDB_DUPSORT. The function will
1313 * return #MDB_KEYEXIST if the key/data pair already appears in the
1315 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1316 * does not already appear in the database. The function will return
1317 * #MDB_KEYEXIST if the key already appears in the database, even if
1318 * the database supports duplicates (#MDB_DUPSORT). The \b data
1319 * parameter will be set to point to the existing item.
1320 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1321 * don't copy the given data. Instead, return a pointer to the
1322 * reserved space, which the caller can fill in later - before
1323 * the next update operation or the transaction ends. This saves
1324 * an extra memcpy if the data is being generated later.
1325 * LMDB does nothing else with this memory, the caller is expected
1326 * to modify all of the space requested. This flag must not be
1327 * specified if the database was opened with #MDB_DUPSORT.
1328 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1329 * database. This option allows fast bulk loading when keys are
1330 * already known to be in the correct order. Loading unsorted keys
1331 * with this flag will cause a #MDB_KEYEXIST error.
1332 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1334 * @return A non-zero error value on failure and 0 on success. Some possible
1337 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1338 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1339 * <li>EACCES - an attempt was made to write in a read-only transaction.
1340 * <li>EINVAL - an invalid parameter was specified.
1343 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
1344 unsigned int flags);
1346 /** @brief Delete items from a database.
1348 * This function removes key/data pairs from the database.
1349 * If the database does not support sorted duplicate data items
1350 * (#MDB_DUPSORT) the data parameter is ignored.
1351 * If the database supports sorted duplicates and the data parameter
1352 * is NULL, all of the duplicate data items for the key will be
1353 * deleted. Otherwise, if the data parameter is non-NULL
1354 * only the matching data item will be deleted.
1355 * This function will return #MDB_NOTFOUND if the specified key/data
1356 * pair is not in the database.
1357 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1358 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1359 * @param[in] key The key to delete from the database
1360 * @param[in] data The data to delete
1361 * @return A non-zero error value on failure and 0 on success. Some possible
1364 * <li>EACCES - an attempt was made to write in a read-only transaction.
1365 * <li>EINVAL - an invalid parameter was specified.
1368 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1370 /** @brief Create a cursor handle.
1372 * A cursor is associated with a specific transaction and database.
1373 * A cursor cannot be used when its database handle is closed. Nor
1374 * when its transaction has ended, except with #mdb_cursor_renew().
1375 * It can be discarded with #mdb_cursor_close().
1376 * A cursor in a write-transaction can be closed before its transaction
1377 * ends, and will otherwise be closed when its transaction ends.
1378 * A cursor in a read-only transaction must be closed explicitly, before
1379 * or after its transaction ends. It can be reused with
1380 * #mdb_cursor_renew() before finally closing it.
1381 * @note Earlier documentation said that cursors in every transaction
1382 * were closed when the transaction committed or aborted.
1383 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1384 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1385 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
1386 * @return A non-zero error value on failure and 0 on success. Some possible
1389 * <li>EINVAL - an invalid parameter was specified.
1392 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
1394 /** @brief Close a cursor handle.
1396 * The cursor handle will be freed and must not be used again after this call.
1397 * Its transaction must still be live if it is a write-transaction.
1398 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1400 void mdb_cursor_close(MDB_cursor *cursor);
1402 /** @brief Renew a cursor handle.
1404 * A cursor is associated with a specific transaction and database.
1405 * Cursors that are only used in read-only
1406 * transactions may be re-used, to avoid unnecessary malloc/free overhead.
1407 * The cursor may be associated with a new read-only transaction, and
1408 * referencing the same database handle as it was created with.
1409 * This may be done whether the previous transaction is live or dead.
1410 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1411 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1412 * @return A non-zero error value on failure and 0 on success. Some possible
1415 * <li>EINVAL - an invalid parameter was specified.
1418 int mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);
1420 /** @brief Return the cursor's transaction handle.
1422 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1424 MDB_txn *mdb_cursor_txn(MDB_cursor *cursor);
1426 /** @brief Return the cursor's database handle.
1428 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1430 MDB_dbi mdb_cursor_dbi(MDB_cursor *cursor);
1432 /** @brief Retrieve by cursor.
1434 * This function retrieves key/data pairs from the database. The address and length
1435 * of the key are returned in the object to which \b key refers (except for the
1436 * case of the #MDB_SET option, in which the \b key object is unchanged), and
1437 * the address and length of the data are returned in the object to which \b data
1439 * See #mdb_get() for restrictions on using the output values.
1440 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1441 * @param[in,out] key The key for a retrieved item
1442 * @param[in,out] data The data of a retrieved item
1443 * @param[in] op A cursor operation #MDB_cursor_op
1444 * @return A non-zero error value on failure and 0 on success. Some possible
1447 * <li>#MDB_NOTFOUND - no matching key found.
1448 * <li>EINVAL - an invalid parameter was specified.
1451 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1454 /** @brief Store by cursor.
1456 * This function stores key/data pairs into the database.
1457 * The cursor is positioned at the new item, or on failure usually near it.
1458 * @note Earlier documentation incorrectly said errors would leave the
1459 * state of the cursor unchanged.
1460 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1461 * @param[in] key The key operated on.
1462 * @param[in] data The data operated on.
1463 * @param[in] flags Options for this operation. This parameter
1464 * must be set to 0 or one of the values described here.
1466 * <li>#MDB_CURRENT - replace the item at the current cursor position.
1467 * The \b key parameter must still be provided, and must match it.
1468 * If using sorted duplicates (#MDB_DUPSORT) the data item must still
1469 * sort into the same place. This is intended to be used when the
1470 * new data is the same size as the old. Otherwise it will simply
1471 * perform a delete of the old record followed by an insert.
1472 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1473 * already appear in the database. This flag may only be specified
1474 * if the database was opened with #MDB_DUPSORT. The function will
1475 * return #MDB_KEYEXIST if the key/data pair already appears in the
1477 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1478 * does not already appear in the database. The function will return
1479 * #MDB_KEYEXIST if the key already appears in the database, even if
1480 * the database supports duplicates (#MDB_DUPSORT).
1481 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1482 * don't copy the given data. Instead, return a pointer to the
1483 * reserved space, which the caller can fill in later - before
1484 * the next update operation or the transaction ends. This saves
1485 * an extra memcpy if the data is being generated later. This flag
1486 * must not be specified if the database was opened with #MDB_DUPSORT.
1487 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1488 * database. No key comparisons are performed. This option allows
1489 * fast bulk loading when keys are already known to be in the
1490 * correct order. Loading unsorted keys with this flag will cause
1491 * a #MDB_KEYEXIST error.
1492 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1493 * <li>#MDB_MULTIPLE - store multiple contiguous data elements in a
1494 * single request. This flag may only be specified if the database
1495 * was opened with #MDB_DUPFIXED. The \b data argument must be an
1496 * array of two MDB_vals. The mv_size of the first MDB_val must be
1497 * the size of a single data element. The mv_data of the first MDB_val
1498 * must point to the beginning of the array of contiguous data elements.
1499 * The mv_size of the second MDB_val must be the count of the number
1500 * of data elements to store. On return this field will be set to
1501 * the count of the number of elements actually written. The mv_data
1502 * of the second MDB_val is unused.
1504 * @return A non-zero error value on failure and 0 on success. Some possible
1507 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1508 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1509 * <li>EACCES - an attempt was made to write in a read-only transaction.
1510 * <li>EINVAL - an invalid parameter was specified.
1513 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1514 unsigned int flags);
1516 /** @brief Delete current key/data pair
1518 * This function deletes the key/data pair to which the cursor refers.
1519 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1520 * @param[in] flags Options for this operation. This parameter
1521 * must be set to 0 or one of the values described here.
1523 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
1524 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
1526 * @return A non-zero error value on failure and 0 on success. Some possible
1529 * <li>EACCES - an attempt was made to write in a read-only transaction.
1530 * <li>EINVAL - an invalid parameter was specified.
1533 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
1535 /** @brief Return count of duplicates for current key.
1537 * This call is only valid on databases that support sorted duplicate
1538 * data items #MDB_DUPSORT.
1539 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1540 * @param[out] countp Address where the count will be stored
1541 * @return A non-zero error value on failure and 0 on success. Some possible
1544 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
1547 int mdb_cursor_count(MDB_cursor *cursor, size_t *countp);
1549 /** @brief Compare two data items according to a particular database.
1551 * This returns a comparison as if the two data items were keys in the
1552 * specified database.
1553 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1554 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1555 * @param[in] a The first item to compare
1556 * @param[in] b The second item to compare
1557 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1559 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1561 /** @brief Compare two data items according to a particular database.
1563 * This returns a comparison as if the two items were data items of
1564 * the specified database. The database must have the #MDB_DUPSORT flag.
1565 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1566 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1567 * @param[in] a The first item to compare
1568 * @param[in] b The second item to compare
1569 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1571 int mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1573 /** @brief A callback function used to print a message from the library.
1575 * @param[in] msg The string to be printed.
1576 * @param[in] ctx An arbitrary context pointer for the callback.
1577 * @return < 0 on failure, >= 0 on success.
1579 typedef int (MDB_msg_func)(const char *msg, void *ctx);
1581 /** @brief Dump the entries in the reader lock table.
1583 * @param[in] env An environment handle returned by #mdb_env_create()
1584 * @param[in] func A #MDB_msg_func function
1585 * @param[in] ctx Anything the message function needs
1586 * @return < 0 on failure, >= 0 on success.
1588 int mdb_reader_list(MDB_env *env, MDB_msg_func *func, void *ctx);
1590 /** @brief Check for stale entries in the reader lock table.
1592 * @param[in] env An environment handle returned by #mdb_env_create()
1593 * @param[out] dead Number of stale slots that were cleared
1594 * @return 0 on success, non-zero on failure.
1596 int mdb_reader_check(MDB_env *env, int *dead);
1602 /** @page tools LMDB Command Line Tools
1603 The following describes the command line tools that are available for LMDB.
1610 #endif /* _LMDB_H_ */