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 some systems:
57 * - Windows - automatic
58 * - Linux, systems using POSIX mutexes with Robust option - automatic
59 * - not on BSD, systems using POSIX semaphores.
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_POSIX_SEM,
64 * startup can fail due to semaphores owned by another userid.
66 * Fix: Open and close the database as the user which owns the
67 * semaphores (likely last user) or as root, while no other
68 * process is using the database.
70 * Restrictions/caveats (in addition to those listed for some functions):
72 * - Only the database owner should normally use the database on
73 * BSD systems or when otherwise configured with MDB_USE_POSIX_SEM.
74 * Multiple users can cause startup to fail later, as noted above.
76 * - There is normally no pure read-only mode, since readers need write
77 * access to locks and lock file. Exceptions: On read-only filesystems
78 * or with the #MDB_NOLOCK flag described under #mdb_env_open().
80 * - By default, in versions before 0.9.10, unused portions of the data
81 * file might receive garbage data from memory freed by other code.
82 * (This does not happen when using the #MDB_WRITEMAP flag.) As of
83 * 0.9.10 the default behavior is to initialize such memory before
84 * writing to the data file. Since there may be a slight performance
85 * cost due to this initialization, applications may disable it using
86 * the #MDB_NOMEMINIT flag. Applications handling sensitive data
87 * which must not be written should not use this flag. This flag is
88 * irrelevant when using #MDB_WRITEMAP.
90 * - A thread can only use one transaction at a time, plus any child
91 * transactions. Each transaction belongs to one thread. See below.
92 * The #MDB_NOTLS flag changes this for read-only transactions.
94 * - Use an MDB_env* in the process which opened it, without fork()ing.
96 * - Do not have open an LMDB database twice in the same process at
97 * the same time. Not even from a plain open() call - close()ing it
98 * breaks flock() advisory locking.
100 * - Avoid long-lived transactions. Read transactions prevent
101 * reuse of pages freed by newer write transactions, thus the
102 * database can grow quickly. Write transactions prevent
103 * other write transactions, since writes are serialized.
105 * - Avoid suspending a process with active transactions. These
106 * would then be "long-lived" as above. Also read transactions
107 * suspended when writers commit could sometimes see wrong data.
109 * ...when several processes can use a database concurrently:
111 * - Avoid aborting a process with an active transaction.
112 * The transaction becomes "long-lived" as above until a check
113 * for stale readers is performed or the lockfile is reset,
114 * since the process may not remove it from the lockfile.
116 * This does not apply to write transactions if the system clears
117 * stale writers, see above.
119 * - If you do that anyway, do a periodic check for stale readers. Or
120 * close the environment once in a while, so the lockfile can get reset.
122 * - Do not use LMDB databases on remote filesystems, even between
123 * processes on the same host. This breaks flock() on some OSes,
124 * possibly memory map sync, and certainly sync between programs
125 * on different hosts.
127 * - Opening a database can fail if another process is opening or
128 * closing it at exactly the same time.
130 * @author Howard Chu, Symas Corporation.
132 * @copyright Copyright 2011-2015 Howard Chu, Symas Corp. All rights reserved.
134 * Redistribution and use in source and binary forms, with or without
135 * modification, are permitted only as authorized by the OpenLDAP
138 * A copy of this license is available in the file LICENSE in the
139 * top-level directory of the distribution or, alternatively, at
140 * <http://www.OpenLDAP.org/license.html>.
143 * This code is derived from btree.c written by Martin Hedenfalk.
145 * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
147 * Permission to use, copy, modify, and distribute this software for any
148 * purpose with or without fee is hereby granted, provided that the above
149 * copyright notice and this permission notice appear in all copies.
151 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
152 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
153 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
154 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
155 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
156 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
157 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
162 #include <sys/types.h>
168 /** Unix permissions for creating files, or dummy definition for Windows */
170 typedef int mdb_mode_t;
172 typedef mode_t mdb_mode_t;
175 /** An abstraction for a file handle.
176 * On POSIX systems file handles are small integers. On Windows
177 * they're opaque pointers.
180 typedef void *mdb_filehandle_t;
182 typedef int mdb_filehandle_t;
185 /** @defgroup mdb LMDB API
187 * @brief OpenLDAP Lightning Memory-Mapped Database Manager
189 /** @defgroup Version Version Macros
192 /** Library major version */
193 #define MDB_VERSION_MAJOR 0
194 /** Library minor version */
195 #define MDB_VERSION_MINOR 9
196 /** Library patch version */
197 #define MDB_VERSION_PATCH 18
199 /** Combine args a,b,c into a single integer for easy version comparisons */
200 #define MDB_VERINT(a,b,c) (((a) << 24) | ((b) << 16) | (c))
202 /** The full library version as a single integer */
203 #define MDB_VERSION_FULL \
204 MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
206 /** The release date of this library version */
207 #define MDB_VERSION_DATE "December 19, 2015"
209 /** A stringifier for the version info */
210 #define MDB_VERSTR(a,b,c,d) "LMDB " #a "." #b "." #c ": (" d ")"
212 /** A helper for the stringifier macro */
213 #define MDB_VERFOO(a,b,c,d) MDB_VERSTR(a,b,c,d)
215 /** The full library version as a C string */
216 #define MDB_VERSION_STRING \
217 MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
220 /** @brief Opaque structure for a database environment.
222 * A DB environment supports multiple databases, all residing in the same
225 typedef struct MDB_env MDB_env;
227 /** @brief Opaque structure for a transaction handle.
229 * All database operations require a transaction handle. Transactions may be
230 * read-only or read-write.
232 typedef struct MDB_txn MDB_txn;
234 /** @brief A handle for an individual database in the DB environment. */
235 typedef unsigned int MDB_dbi;
237 /** @brief Opaque structure for navigating through a database */
238 typedef struct MDB_cursor MDB_cursor;
240 /** @brief Generic structure used for passing keys and data in and out
243 * Values returned from the database are valid only until a subsequent
244 * update operation, or the end of the transaction. Do not modify or
245 * free them, they commonly point into the database itself.
247 * Key sizes must be between 1 and #mdb_env_get_maxkeysize() inclusive.
248 * The same applies to data sizes in databases with the #MDB_DUPSORT flag.
249 * Other data items can in theory be from 0 to 0xffffffff bytes long.
251 typedef struct MDB_val {
252 size_t mv_size; /**< size of the data item */
253 void *mv_data; /**< address of the data item */
256 /** @brief A callback function used to compare two keys in a database */
257 typedef int (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
259 /** @brief A callback function used to relocate a position-dependent data item
260 * in a fixed-address database.
262 * The \b newptr gives the item's desired address in
263 * the memory map, and \b oldptr gives its previous address. The item's actual
264 * data resides at the address in \b item. This callback is expected to walk
265 * through the fields of the record in \b item and modify any
266 * values based at the \b oldptr address to be relative to the \b newptr address.
267 * @param[in,out] item The item that is to be relocated.
268 * @param[in] oldptr The previous address.
269 * @param[in] newptr The new address to relocate to.
270 * @param[in] relctx An application-provided context, set by #mdb_set_relctx().
271 * @todo This feature is currently unimplemented.
273 typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
275 /** @defgroup mdb_env Environment Flags
278 /** mmap at a fixed address (experimental) */
279 #define MDB_FIXEDMAP 0x01
280 /** no environment directory */
281 #define MDB_NOSUBDIR 0x4000
282 /** don't fsync after commit */
283 #define MDB_NOSYNC 0x10000
285 #define MDB_RDONLY 0x20000
286 /** don't fsync metapage after commit */
287 #define MDB_NOMETASYNC 0x40000
288 /** use writable mmap */
289 #define MDB_WRITEMAP 0x80000
290 /** use asynchronous msync when #MDB_WRITEMAP is used */
291 #define MDB_MAPASYNC 0x100000
292 /** tie reader locktable slots to #MDB_txn objects instead of to threads */
293 #define MDB_NOTLS 0x200000
294 /** don't do any locking, caller must manage their own locks */
295 #define MDB_NOLOCK 0x400000
296 /** don't do readahead (no effect on Windows) */
297 #define MDB_NORDAHEAD 0x800000
298 /** don't initialize malloc'd memory before writing to datafile */
299 #define MDB_NOMEMINIT 0x1000000
302 /** @defgroup mdb_dbi_open Database Flags
305 /** use reverse string keys */
306 #define MDB_REVERSEKEY 0x02
307 /** use sorted duplicates */
308 #define MDB_DUPSORT 0x04
309 /** numeric keys in native byte order: either unsigned int or size_t.
310 * The keys must all be of the same size. */
311 #define MDB_INTEGERKEY 0x08
312 /** with #MDB_DUPSORT, sorted dup items have fixed size */
313 #define MDB_DUPFIXED 0x10
314 /** with #MDB_DUPSORT, dups are #MDB_INTEGERKEY-style integers */
315 #define MDB_INTEGERDUP 0x20
316 /** with #MDB_DUPSORT, use reverse string dups */
317 #define MDB_REVERSEDUP 0x40
318 /** create DB if not already existing */
319 #define MDB_CREATE 0x40000
322 /** @defgroup mdb_put Write Flags
325 /** For put: Don't write if the key already exists. */
326 #define MDB_NOOVERWRITE 0x10
327 /** Only for #MDB_DUPSORT<br>
328 * For put: don't write if the key and data pair already exist.<br>
329 * For mdb_cursor_del: remove all duplicate data items.
331 #define MDB_NODUPDATA 0x20
332 /** For mdb_cursor_put: overwrite the current key/data pair */
333 #define MDB_CURRENT 0x40
334 /** For put: Just reserve space for data, don't copy it. Return a
335 * pointer to the reserved space.
337 #define MDB_RESERVE 0x10000
338 /** Data is being appended, don't split full pages. */
339 #define MDB_APPEND 0x20000
340 /** Duplicate data is being appended, don't split full pages. */
341 #define MDB_APPENDDUP 0x40000
342 /** Store multiple data items in one call. Only for #MDB_DUPFIXED. */
343 #define MDB_MULTIPLE 0x80000
346 /** @defgroup mdb_copy Copy Flags
349 /** Compacting copy: Omit free space from copy, and renumber all
350 * pages sequentially.
352 #define MDB_CP_COMPACT 0x01
355 /** @brief Cursor Get operations.
357 * This is the set of all operations for retrieving data
360 typedef enum MDB_cursor_op {
361 MDB_FIRST, /**< Position at first key/data item */
362 MDB_FIRST_DUP, /**< Position at first data item of current key.
363 Only for #MDB_DUPSORT */
364 MDB_GET_BOTH, /**< Position at key/data pair. Only for #MDB_DUPSORT */
365 MDB_GET_BOTH_RANGE, /**< position at key, nearest data. Only for #MDB_DUPSORT */
366 MDB_GET_CURRENT, /**< Return key/data at current cursor position */
367 MDB_GET_MULTIPLE, /**< Return key and up to a page of duplicate data items
368 from current cursor position. Move cursor to prepare
369 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
370 MDB_LAST, /**< Position at last key/data item */
371 MDB_LAST_DUP, /**< Position at last data item of current key.
372 Only for #MDB_DUPSORT */
373 MDB_NEXT, /**< Position at next data item */
374 MDB_NEXT_DUP, /**< Position at next data item of current key.
375 Only for #MDB_DUPSORT */
376 MDB_NEXT_MULTIPLE, /**< Return key and up to a page of duplicate data items
377 from next cursor position. Move cursor to prepare
378 for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
379 MDB_NEXT_NODUP, /**< Position at first data item of next key */
380 MDB_PREV, /**< Position at previous data item */
381 MDB_PREV_DUP, /**< Position at previous data item of current key.
382 Only for #MDB_DUPSORT */
383 MDB_PREV_NODUP, /**< Position at last data item of previous key */
384 MDB_SET, /**< Position at specified key */
385 MDB_SET_KEY, /**< Position at specified key, return key + data */
386 MDB_SET_RANGE /**< Position at first key greater than or equal to specified key. */
389 /** @defgroup errors Return Codes
391 * BerkeleyDB uses -30800 to -30999, we'll go under them
394 /** Successful result */
395 #define MDB_SUCCESS 0
396 /** key/data pair already exists */
397 #define MDB_KEYEXIST (-30799)
398 /** key/data pair not found (EOF) */
399 #define MDB_NOTFOUND (-30798)
400 /** Requested page not found - this usually indicates corruption */
401 #define MDB_PAGE_NOTFOUND (-30797)
402 /** Located page was wrong type */
403 #define MDB_CORRUPTED (-30796)
404 /** Update of meta page failed or environment had fatal error */
405 #define MDB_PANIC (-30795)
406 /** Environment version mismatch */
407 #define MDB_VERSION_MISMATCH (-30794)
408 /** File is not a valid LMDB file */
409 #define MDB_INVALID (-30793)
410 /** Environment mapsize reached */
411 #define MDB_MAP_FULL (-30792)
412 /** Environment maxdbs reached */
413 #define MDB_DBS_FULL (-30791)
414 /** Environment maxreaders reached */
415 #define MDB_READERS_FULL (-30790)
416 /** Too many TLS keys in use - Windows only */
417 #define MDB_TLS_FULL (-30789)
418 /** Txn has too many dirty pages */
419 #define MDB_TXN_FULL (-30788)
420 /** Cursor stack too deep - internal error */
421 #define MDB_CURSOR_FULL (-30787)
422 /** Page has not enough space - internal error */
423 #define MDB_PAGE_FULL (-30786)
424 /** Database contents grew beyond environment mapsize */
425 #define MDB_MAP_RESIZED (-30785)
426 /** Operation and DB incompatible, or DB type changed. This can mean:
428 * <li>The operation expects an #MDB_DUPSORT / #MDB_DUPFIXED database.
429 * <li>Opening a named DB when the unnamed DB has #MDB_DUPSORT / #MDB_INTEGERKEY.
430 * <li>Accessing a data record as a database, or vice versa.
431 * <li>The database was dropped and recreated with different flags.
434 #define MDB_INCOMPATIBLE (-30784)
435 /** Invalid reuse of reader locktable slot */
436 #define MDB_BAD_RSLOT (-30783)
437 /** Transaction must abort, has a child, or is invalid */
438 #define MDB_BAD_TXN (-30782)
439 /** Unsupported size of key/DB name/data, or wrong DUPFIXED size */
440 #define MDB_BAD_VALSIZE (-30781)
441 /** The specified DBI was changed unexpectedly */
442 #define MDB_BAD_DBI (-30780)
443 /** The last defined error code */
444 #define MDB_LAST_ERRCODE MDB_BAD_DBI
447 /** @brief Statistics for a database in the environment */
448 typedef struct MDB_stat {
449 unsigned int ms_psize; /**< Size of a database page.
450 This is currently the same for all databases. */
451 unsigned int ms_depth; /**< Depth (height) of the B-tree */
452 size_t ms_branch_pages; /**< Number of internal (non-leaf) pages */
453 size_t ms_leaf_pages; /**< Number of leaf pages */
454 size_t ms_overflow_pages; /**< Number of overflow pages */
455 size_t ms_entries; /**< Number of data items */
458 /** @brief Information about the environment */
459 typedef struct MDB_envinfo {
460 void *me_mapaddr; /**< Address of map, if fixed */
461 size_t me_mapsize; /**< Size of the data memory map */
462 size_t me_last_pgno; /**< ID of the last used page */
463 size_t me_last_txnid; /**< ID of the last committed transaction */
464 unsigned int me_maxreaders; /**< max reader slots in the environment */
465 unsigned int me_numreaders; /**< max reader slots used in the environment */
468 /** @brief Return the LMDB library version information.
470 * @param[out] major if non-NULL, the library major version number is copied here
471 * @param[out] minor if non-NULL, the library minor version number is copied here
472 * @param[out] patch if non-NULL, the library patch version number is copied here
473 * @retval "version string" The library version as a string
475 char *mdb_version(int *major, int *minor, int *patch);
477 /** @brief Return a string describing a given error code.
479 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
480 * function. If the error code is greater than or equal to 0, then the string
481 * returned by the system function strerror(3) is returned. If the error code
482 * is less than 0, an error string corresponding to the LMDB library error is
483 * returned. See @ref errors for a list of LMDB-specific error codes.
484 * @param[in] err The error code
485 * @retval "error message" The description of the error
487 char *mdb_strerror(int err);
489 /** @brief Create an LMDB environment handle.
491 * This function allocates memory for a #MDB_env structure. To release
492 * the allocated memory and discard the handle, call #mdb_env_close().
493 * Before the handle may be used, it must be opened using #mdb_env_open().
494 * Various other options may also need to be set before opening the handle,
495 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
496 * depending on usage requirements.
497 * @param[out] env The address where the new handle will be stored
498 * @return A non-zero error value on failure and 0 on success.
500 int mdb_env_create(MDB_env **env);
502 /** @brief Open an environment handle.
504 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
505 * @param[in] env An environment handle returned by #mdb_env_create()
506 * @param[in] path The directory in which the database files reside. This
507 * directory must already exist and be writable.
508 * @param[in] flags Special options for this environment. This parameter
509 * must be set to 0 or by bitwise OR'ing together one or more of the
510 * values described here.
511 * Flags set by mdb_env_set_flags() are also used.
514 * use a fixed address for the mmap region. This flag must be specified
515 * when creating the environment, and is stored persistently in the environment.
516 * If successful, the memory map will always reside at the same virtual address
517 * and pointers used to reference data items in the database will be constant
518 * across multiple invocations. This option may not always work, depending on
519 * how the operating system has allocated memory to shared libraries and other uses.
520 * The feature is highly experimental.
522 * By default, LMDB creates its environment in a directory whose
523 * pathname is given in \b path, and creates its data and lock files
524 * under that directory. With this option, \b path is used as-is for
525 * the database main data file. The database lock file is the \b path
526 * with "-lock" appended.
528 * Open the environment in read-only mode. No write operations will be
529 * allowed. LMDB will still modify the lock file - except on read-only
530 * filesystems, where LMDB does not use locks.
532 * Use a writeable memory map unless MDB_RDONLY is set. This is faster
533 * and uses fewer mallocs, but loses protection from application bugs
534 * like wild pointer writes and other bad updates into the database.
535 * Incompatible with nested transactions.
536 * Do not mix processes with and without MDB_WRITEMAP on the same
537 * environment. This can defeat durability (#mdb_env_sync etc).
538 * <li>#MDB_NOMETASYNC
539 * Flush system buffers to disk only once per transaction, omit the
540 * metadata flush. Defer that until the system flushes files to disk,
541 * or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization
542 * maintains database integrity, but a system crash may undo the last
543 * committed transaction. I.e. it preserves the ACI (atomicity,
544 * consistency, isolation) but not D (durability) database property.
545 * This flag may be changed at any time using #mdb_env_set_flags().
547 * Don't flush system buffers to disk when committing a transaction.
548 * This optimization means a system crash can corrupt the database or
549 * lose the last transactions if buffers are not yet flushed to disk.
550 * The risk is governed by how often the system flushes dirty buffers
551 * to disk and how often #mdb_env_sync() is called. However, if the
552 * filesystem preserves write order and the #MDB_WRITEMAP flag is not
553 * used, transactions exhibit ACI (atomicity, consistency, isolation)
554 * properties and only lose D (durability). I.e. database integrity
555 * is maintained, but a system crash may undo the final transactions.
556 * Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no
557 * hint for when to write transactions to disk, unless #mdb_env_sync()
558 * is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable.
559 * This flag may be changed at any time using #mdb_env_set_flags().
561 * When using #MDB_WRITEMAP, use asynchronous flushes to disk.
562 * As with #MDB_NOSYNC, a system crash can then corrupt the
563 * database or lose the last transactions. Calling #mdb_env_sync()
564 * ensures on-disk database integrity until next commit.
565 * This flag may be changed at any time using #mdb_env_set_flags().
567 * Don't use Thread-Local Storage. Tie reader locktable slots to
568 * #MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps
569 * the slot reseved for the #MDB_txn object. A thread may use parallel
570 * read-only transactions. A read-only transaction may span threads if
571 * the user synchronizes its use. Applications that multiplex many
572 * user threads over individual OS threads need this option. Such an
573 * application must also serialize the write transactions in an OS
574 * thread, since LMDB's write locking is unaware of the user threads.
576 * Don't do any locking. If concurrent access is anticipated, the
577 * caller must manage all concurrency itself. For proper operation
578 * the caller must enforce single-writer semantics, and must ensure
579 * that no readers are using old transactions while a writer is
580 * active. The simplest approach is to use an exclusive lock so that
581 * no readers may be active at all when a writer begins.
583 * Turn off readahead. Most operating systems perform readahead on
584 * read requests by default. This option turns it off if the OS
585 * supports it. Turning it off may help random read performance
586 * when the DB is larger than RAM and system RAM is full.
587 * The option is not implemented on Windows.
589 * Don't initialize malloc'd memory before writing to unused spaces
590 * in the data file. By default, memory for pages written to the data
591 * file is obtained using malloc. While these pages may be reused in
592 * subsequent transactions, freshly malloc'd pages will be initialized
593 * to zeroes before use. This avoids persisting leftover data from other
594 * code (that used the heap and subsequently freed the memory) into the
595 * data file. Note that many other system libraries may allocate
596 * and free memory from the heap for arbitrary uses. E.g., stdio may
597 * use the heap for file I/O buffers. This initialization step has a
598 * modest performance cost so some applications may want to disable
599 * it using this flag. This option can be a problem for applications
600 * which handle sensitive data like passwords, and it makes memory
601 * checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP,
602 * which writes directly to the mmap instead of using malloc for pages. The
603 * initialization is also skipped if #MDB_RESERVE is used; the
604 * caller is expected to overwrite all of the memory that was
605 * reserved in that case.
606 * This flag may be changed at any time using #mdb_env_set_flags().
608 * @param[in] mode The UNIX permissions to set on created files and semaphores.
609 * This parameter is ignored on Windows.
610 * @return A non-zero error value on failure and 0 on success. Some possible
613 * <li>#MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the
614 * version that created the database environment.
615 * <li>#MDB_INVALID - the environment file headers are corrupted.
616 * <li>ENOENT - the directory specified by the path parameter doesn't exist.
617 * <li>EACCES - the user didn't have permission to access the environment files.
618 * <li>EAGAIN - the environment was locked by another process.
621 int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode);
623 /** @brief Copy an LMDB environment to the specified path.
625 * This function may be used to make a backup of an existing environment.
626 * No lockfile is created, since it gets recreated at need.
627 * @note This call can trigger significant file size growth if run in
628 * parallel with write transactions, because it employs a read-only
629 * transaction. See long-lived transactions under @ref caveats_sec.
630 * @param[in] env An environment handle returned by #mdb_env_create(). It
631 * must have already been opened successfully.
632 * @param[in] path The directory in which the copy will reside. This
633 * directory must already exist and be writable but must otherwise be
635 * @return A non-zero error value on failure and 0 on success.
637 int mdb_env_copy(MDB_env *env, const char *path);
639 /** @brief Copy an LMDB environment to the specified file descriptor.
641 * This function may be used to make a backup of an existing environment.
642 * No lockfile is created, since it gets recreated at need.
643 * @note This call can trigger significant file size growth if run in
644 * parallel with write transactions, because it employs a read-only
645 * transaction. See long-lived transactions under @ref caveats_sec.
646 * @param[in] env An environment handle returned by #mdb_env_create(). It
647 * must have already been opened successfully.
648 * @param[in] fd The filedescriptor to write the copy to. It must
649 * have already been opened for Write access.
650 * @return A non-zero error value on failure and 0 on success.
652 int mdb_env_copyfd(MDB_env *env, mdb_filehandle_t fd);
654 /** @brief Copy an LMDB environment to the specified path, with options.
656 * This function may be used to make a backup of an existing environment.
657 * No lockfile is created, since it gets recreated at need.
658 * @note This call can trigger significant file size growth if run in
659 * parallel with write transactions, because it employs a read-only
660 * transaction. See long-lived transactions under @ref caveats_sec.
661 * @param[in] env An environment handle returned by #mdb_env_create(). It
662 * must have already been opened successfully.
663 * @param[in] path The directory in which the copy will reside. This
664 * directory must already exist and be writable but must otherwise be
666 * @param[in] flags Special options for this operation. This parameter
667 * must be set to 0 or by bitwise OR'ing together one or more of the
668 * values described here.
670 * <li>#MDB_CP_COMPACT - Perform compaction while copying: omit free
671 * pages and sequentially renumber all pages in output. This option
672 * consumes more CPU and runs more slowly than the default.
674 * @return A non-zero error value on failure and 0 on success.
676 int mdb_env_copy2(MDB_env *env, const char *path, unsigned int flags);
678 /** @brief Copy an LMDB environment to the specified file descriptor,
681 * This function may be used to make a backup of an existing environment.
682 * No lockfile is created, since it gets recreated at need. See
683 * #mdb_env_copy2() for further details.
684 * @note This call can trigger significant file size growth if run in
685 * parallel with write transactions, because it employs a read-only
686 * transaction. See long-lived transactions under @ref caveats_sec.
687 * @param[in] env An environment handle returned by #mdb_env_create(). It
688 * must have already been opened successfully.
689 * @param[in] fd The filedescriptor to write the copy to. It must
690 * have already been opened for Write access.
691 * @param[in] flags Special options for this operation.
692 * See #mdb_env_copy2() for options.
693 * @return A non-zero error value on failure and 0 on success.
695 int mdb_env_copyfd2(MDB_env *env, mdb_filehandle_t fd, unsigned int flags);
697 /** @brief Return statistics about the LMDB environment.
699 * @param[in] env An environment handle returned by #mdb_env_create()
700 * @param[out] stat The address of an #MDB_stat structure
701 * where the statistics will be copied
703 int mdb_env_stat(MDB_env *env, MDB_stat *stat);
705 /** @brief Return information about the LMDB environment.
707 * @param[in] env An environment handle returned by #mdb_env_create()
708 * @param[out] stat The address of an #MDB_envinfo structure
709 * where the information will be copied
711 int mdb_env_info(MDB_env *env, MDB_envinfo *stat);
713 /** @brief Flush the data buffers to disk.
715 * Data is always written to disk when #mdb_txn_commit() is called,
716 * but the operating system may keep it buffered. LMDB always flushes
717 * the OS buffers upon commit as well, unless the environment was
718 * opened with #MDB_NOSYNC or in part #MDB_NOMETASYNC. This call is
719 * not valid if the environment was opened with #MDB_RDONLY.
720 * @param[in] env An environment handle returned by #mdb_env_create()
721 * @param[in] force If non-zero, force a synchronous flush. Otherwise
722 * if the environment has the #MDB_NOSYNC flag set the flushes
723 * will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
724 * @return A non-zero error value on failure and 0 on success. Some possible
727 * <li>EACCES - the environment is read-only.
728 * <li>EINVAL - an invalid parameter was specified.
729 * <li>EIO - an error occurred during synchronization.
732 int mdb_env_sync(MDB_env *env, int force);
734 /** @brief Close the environment and release the memory map.
736 * Only a single thread may call this function. All transactions, databases,
737 * and cursors must already be closed before calling this function. Attempts to
738 * use any such handles after calling this function will cause a SIGSEGV.
739 * The environment handle will be freed and must not be used again after this call.
740 * @param[in] env An environment handle returned by #mdb_env_create()
742 void mdb_env_close(MDB_env *env);
744 /** @brief Set environment flags.
746 * This may be used to set some flags in addition to those from
747 * #mdb_env_open(), or to unset these flags. If several threads
748 * change the flags at the same time, the result is undefined.
749 * @param[in] env An environment handle returned by #mdb_env_create()
750 * @param[in] flags The flags to change, bitwise OR'ed together
751 * @param[in] onoff A non-zero value sets the flags, zero clears them.
752 * @return A non-zero error value on failure and 0 on success. Some possible
755 * <li>EINVAL - an invalid parameter was specified.
758 int mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
760 /** @brief Get environment flags.
762 * @param[in] env An environment handle returned by #mdb_env_create()
763 * @param[out] flags The address of an integer to store the flags
764 * @return A non-zero error value on failure and 0 on success. Some possible
767 * <li>EINVAL - an invalid parameter was specified.
770 int mdb_env_get_flags(MDB_env *env, unsigned int *flags);
772 /** @brief Return the path that was used in #mdb_env_open().
774 * @param[in] env An environment handle returned by #mdb_env_create()
775 * @param[out] path Address of a string pointer to contain the path. This
776 * is the actual string in the environment, not a copy. It should not be
777 * altered in any way.
778 * @return A non-zero error value on failure and 0 on success. Some possible
781 * <li>EINVAL - an invalid parameter was specified.
784 int mdb_env_get_path(MDB_env *env, const char **path);
786 /** @brief Return the filedescriptor for the given environment.
788 * @param[in] env An environment handle returned by #mdb_env_create()
789 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.
790 * @return A non-zero error value on failure and 0 on success. Some possible
793 * <li>EINVAL - an invalid parameter was specified.
796 int mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *fd);
798 /** @brief Set the size of the memory map to use for this environment.
800 * The size should be a multiple of the OS page size. The default is
801 * 10485760 bytes. The size of the memory map is also the maximum size
802 * of the database. The value should be chosen as large as possible,
803 * to accommodate future growth of the database.
804 * This function should be called after #mdb_env_create() and before #mdb_env_open().
805 * It may be called at later times if no transactions are active in
806 * this process. Note that the library does not check for this condition,
807 * the caller must ensure it explicitly.
809 * The new size takes effect immediately for the current process but
810 * will not be persisted to any others until a write transaction has been
811 * committed by the current process. Also, only mapsize increases are
812 * persisted into the environment.
814 * If the mapsize is increased by another process, and data has grown
815 * beyond the range of the current mapsize, #mdb_txn_begin() will
816 * return #MDB_MAP_RESIZED. This function may be called with a size
817 * of zero to adopt the new size.
819 * Any attempt to set a size smaller than the space already consumed
820 * by the environment will be silently changed to the current size of the used space.
821 * @param[in] env An environment handle returned by #mdb_env_create()
822 * @param[in] size The size in bytes
823 * @return A non-zero error value on failure and 0 on success. Some possible
826 * <li>EINVAL - an invalid parameter was specified, or the environment has
827 * an active write transaction.
830 int mdb_env_set_mapsize(MDB_env *env, size_t size);
832 /** @brief Set the maximum number of threads/reader slots for the environment.
834 * This defines the number of slots in the lock table that is used to track readers in the
835 * the environment. The default is 126.
836 * Starting a read-only transaction normally ties a lock table slot to the
837 * current thread until the environment closes or the thread exits. If
838 * MDB_NOTLS is in use, #mdb_txn_begin() instead ties the slot to the
839 * MDB_txn object until it or the #MDB_env object is destroyed.
840 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
841 * @param[in] env An environment handle returned by #mdb_env_create()
842 * @param[in] readers The maximum number of reader lock table slots
843 * @return A non-zero error value on failure and 0 on success. Some possible
846 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
849 int mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);
851 /** @brief Get the maximum number of threads/reader slots for the environment.
853 * @param[in] env An environment handle returned by #mdb_env_create()
854 * @param[out] readers Address of an integer to store the number of readers
855 * @return A non-zero error value on failure and 0 on success. Some possible
858 * <li>EINVAL - an invalid parameter was specified.
861 int mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers);
863 /** @brief Set the maximum number of named databases for the environment.
865 * This function is only needed if multiple databases will be used in the
866 * environment. Simpler applications that use the environment as a single
867 * unnamed database can ignore this option.
868 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
870 * Currently a moderate number of slots are cheap but a huge number gets
871 * expensive: 7-120 words per transaction, and every #mdb_dbi_open()
872 * does a linear search of the opened slots.
873 * @param[in] env An environment handle returned by #mdb_env_create()
874 * @param[in] dbs The maximum number of databases
875 * @return A non-zero error value on failure and 0 on success. Some possible
878 * <li>EINVAL - an invalid parameter was specified, or the environment is already open.
881 int mdb_env_set_maxdbs(MDB_env *env, MDB_dbi dbs);
883 /** @brief Get the maximum size of keys and #MDB_DUPSORT data we can write.
885 * Depends on the compile-time constant #MDB_MAXKEYSIZE. Default 511.
887 * @param[in] env An environment handle returned by #mdb_env_create()
888 * @return The maximum size of a key we can write
890 int mdb_env_get_maxkeysize(MDB_env *env);
892 /** @brief Set application information associated with the #MDB_env.
894 * @param[in] env An environment handle returned by #mdb_env_create()
895 * @param[in] ctx An arbitrary pointer for whatever the application needs.
896 * @return A non-zero error value on failure and 0 on success.
898 int mdb_env_set_userctx(MDB_env *env, void *ctx);
900 /** @brief Get the application information associated with the #MDB_env.
902 * @param[in] env An environment handle returned by #mdb_env_create()
903 * @return The pointer set by #mdb_env_set_userctx().
905 void *mdb_env_get_userctx(MDB_env *env);
907 /** @brief A callback function for most LMDB assert() failures,
908 * called before printing the message and aborting.
910 * @param[in] env An environment handle returned by #mdb_env_create().
911 * @param[in] msg The assertion message, not including newline.
913 typedef void MDB_assert_func(MDB_env *env, const char *msg);
915 /** Set or reset the assert() callback of the environment.
916 * Disabled if liblmdb is buillt with NDEBUG.
917 * @note This hack should become obsolete as lmdb's error handling matures.
918 * @param[in] env An environment handle returned by #mdb_env_create().
919 * @param[in] func An #MDB_assert_func function, or 0.
920 * @return A non-zero error value on failure and 0 on success.
922 int mdb_env_set_assert(MDB_env *env, MDB_assert_func *func);
924 /** @brief Create a transaction for use with the environment.
926 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
927 * @note A transaction and its cursors must only be used by a single
928 * thread, and a thread may only have a single transaction at a time.
929 * If #MDB_NOTLS is in use, this does not apply to read-only transactions.
930 * @note Cursors may not span transactions.
931 * @param[in] env An environment handle returned by #mdb_env_create()
932 * @param[in] parent If this parameter is non-NULL, the new transaction
933 * will be a nested transaction, with the transaction indicated by \b parent
934 * as its parent. Transactions may be nested to any level. A parent
935 * transaction and its cursors may not issue any other operations than
936 * mdb_txn_commit and mdb_txn_abort while it has active child transactions.
937 * @param[in] flags Special options for this transaction. This parameter
938 * must be set to 0 or by bitwise OR'ing together one or more of the
939 * values described here.
942 * This transaction will not perform any write operations.
944 * @param[out] txn Address where the new #MDB_txn handle will be stored
945 * @return A non-zero error value on failure and 0 on success. Some possible
948 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
950 * <li>#MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's
951 * mapsize and this environment's map must be resized as well.
952 * See #mdb_env_set_mapsize().
953 * <li>#MDB_READERS_FULL - a read-only transaction was requested and
954 * the reader lock table is full. See #mdb_env_set_maxreaders().
955 * <li>ENOMEM - out of memory.
958 int mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);
960 /** @brief Returns the transaction's #MDB_env
962 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
964 MDB_env *mdb_txn_env(MDB_txn *txn);
966 /** @brief Return the transaction's ID.
968 * This returns the identifier associated with this transaction. For a
969 * read-only transaction, this corresponds to the snapshot being read;
970 * concurrent readers will frequently have the same transaction ID.
972 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
973 * @return A transaction ID, valid if input is an active transaction.
975 size_t mdb_txn_id(MDB_txn *txn);
977 /** @brief Commit all the operations of a transaction into the database.
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()
984 * @return A non-zero error value on failure and 0 on success. Some possible
987 * <li>EINVAL - an invalid parameter was specified.
988 * <li>ENOSPC - no more disk space.
989 * <li>EIO - a low-level I/O error occurred while writing.
990 * <li>ENOMEM - out of memory.
993 int mdb_txn_commit(MDB_txn *txn);
995 /** @brief Abandon all the operations of the transaction instead of saving them.
997 * The transaction handle is freed. It and its cursors must not be used
998 * again after this call, except with #mdb_cursor_renew().
999 * @note Earlier documentation incorrectly said all cursors would be freed.
1000 * Only write-transactions free cursors.
1001 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1003 void mdb_txn_abort(MDB_txn *txn);
1005 /** @brief Reset a read-only transaction.
1007 * Abort the transaction like #mdb_txn_abort(), but keep the transaction
1008 * handle. #mdb_txn_renew() may reuse the handle. This saves allocation
1009 * overhead if the process will start a new read-only transaction soon,
1010 * and also locking overhead if #MDB_NOTLS is in use. The reader table
1011 * lock is released, but the table slot stays tied to its thread or
1012 * #MDB_txn. Use mdb_txn_abort() to discard a reset handle, and to free
1013 * its lock table slot if MDB_NOTLS is in use.
1014 * Cursors opened within the transaction must not be used
1015 * again after this call, except with #mdb_cursor_renew().
1016 * Reader locks generally don't interfere with writers, but they keep old
1017 * versions of database pages allocated. Thus they prevent the old pages
1018 * from being reused when writers commit new data, and so under heavy load
1019 * the database size may grow much more rapidly than otherwise.
1020 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1022 void mdb_txn_reset(MDB_txn *txn);
1024 /** @brief Renew a read-only transaction.
1026 * This acquires a new reader lock for a transaction handle that had been
1027 * released by #mdb_txn_reset(). It must be called before a reset transaction
1028 * may be used again.
1029 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1030 * @return A non-zero error value on failure and 0 on success. Some possible
1033 * <li>#MDB_PANIC - a fatal error occurred earlier and the environment
1034 * must be shut down.
1035 * <li>EINVAL - an invalid parameter was specified.
1038 int mdb_txn_renew(MDB_txn *txn);
1040 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1041 #define mdb_open(txn,name,flags,dbi) mdb_dbi_open(txn,name,flags,dbi)
1042 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
1043 #define mdb_close(env,dbi) mdb_dbi_close(env,dbi)
1045 /** @brief Open a database in the environment.
1047 * A database handle denotes the name and parameters of a database,
1048 * independently of whether such a database exists.
1049 * The database handle may be discarded by calling #mdb_dbi_close().
1050 * The old database handle is returned if the database was already open.
1051 * The handle may only be closed once.
1053 * The database handle will be private to the current transaction until
1054 * the transaction is successfully committed. If the transaction is
1055 * aborted the handle will be closed automatically.
1056 * After a successful commit the handle will reside in the shared
1057 * environment, and may be used by other transactions.
1059 * This function must not be called from multiple concurrent
1060 * transactions in the same process. A transaction that uses
1061 * this function must finish (either commit or abort) before
1062 * any other transaction in the process may use this function.
1064 * To use named databases (with name != NULL), #mdb_env_set_maxdbs()
1065 * must be called before opening the environment. Database names are
1066 * keys in the unnamed database, and may be read but not written.
1068 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1069 * @param[in] name The name of the database to open. If only a single
1070 * database is needed in the environment, this value may be NULL.
1071 * @param[in] flags Special options for this database. This parameter
1072 * must be set to 0 or by bitwise OR'ing together one or more of the
1073 * values described here.
1075 * <li>#MDB_REVERSEKEY
1076 * Keys are strings to be compared in reverse order, from the end
1077 * of the strings to the beginning. By default, Keys are treated as strings and
1078 * compared from beginning to end.
1080 * Duplicate keys may be used in the database. (Or, from another perspective,
1081 * keys may have multiple data items, stored in sorted order.) By default
1082 * keys must be unique and may have only a single data item.
1083 * <li>#MDB_INTEGERKEY
1084 * Keys are binary integers in native byte order, either unsigned int
1085 * or size_t, and will be sorted as such.
1086 * The keys must all be of the same size.
1088 * This flag may only be used in combination with #MDB_DUPSORT. This option
1089 * tells the library that the data items for this database are all the same
1090 * size, which allows further optimizations in storage and retrieval. When
1091 * all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
1092 * cursor operations may be used to retrieve multiple items at once.
1093 * <li>#MDB_INTEGERDUP
1094 * This option specifies that duplicate data items are binary integers,
1095 * similar to #MDB_INTEGERKEY keys.
1096 * <li>#MDB_REVERSEDUP
1097 * This option specifies that duplicate data items should be compared as
1098 * strings in reverse order.
1100 * Create the named database if it doesn't exist. This option is not
1101 * allowed in a read-only transaction or a read-only environment.
1103 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
1104 * @return A non-zero error value on failure and 0 on success. Some possible
1107 * <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
1108 * and #MDB_CREATE was not specified.
1109 * <li>#MDB_DBS_FULL - too many databases have been opened. See #mdb_env_set_maxdbs().
1112 int mdb_dbi_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
1114 /** @brief Retrieve statistics for a database.
1116 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1117 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1118 * @param[out] stat The address of an #MDB_stat structure
1119 * where the statistics will be copied
1120 * @return A non-zero error value on failure and 0 on success. Some possible
1123 * <li>EINVAL - an invalid parameter was specified.
1126 int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
1128 /** @brief Retrieve the DB flags for a database handle.
1130 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1131 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1132 * @param[out] flags Address where the flags will be returned.
1133 * @return A non-zero error value on failure and 0 on success.
1135 int mdb_dbi_flags(MDB_txn *txn, MDB_dbi dbi, unsigned int *flags);
1137 /** @brief Close a database handle. Normally unnecessary. Use with care:
1139 * This call is not mutex protected. Handles should only be closed by
1140 * a single thread, and only if no other threads are going to reference
1141 * the database handle or one of its cursors any further. Do not close
1142 * a handle if an existing transaction has modified its database.
1143 * Doing so can cause misbehavior from database corruption to errors
1144 * like MDB_BAD_VALSIZE (since the DB name is gone).
1146 * Closing a database handle is not necessary, but lets #mdb_dbi_open()
1147 * reuse the handle value. Usually it's better to set a bigger
1148 * #mdb_env_set_maxdbs(), unless that value would be large.
1150 * @param[in] env An environment handle returned by #mdb_env_create()
1151 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1153 void mdb_dbi_close(MDB_env *env, MDB_dbi dbi);
1155 /** @brief Empty or delete+close a database.
1157 * See #mdb_dbi_close() for restrictions about closing the DB handle.
1158 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1159 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1160 * @param[in] del 0 to empty the DB, 1 to delete it from the
1161 * environment and close the DB handle.
1162 * @return A non-zero error value on failure and 0 on success.
1164 int mdb_drop(MDB_txn *txn, MDB_dbi dbi, int del);
1166 /** @brief Set a custom key comparison function for a database.
1168 * The comparison function is called whenever it is necessary to compare a
1169 * key specified by the application with a key currently stored in the database.
1170 * If no comparison function is specified, and no special key flags were specified
1171 * with #mdb_dbi_open(), the keys are compared lexically, with shorter keys collating
1172 * before longer keys.
1173 * @warning This function must be called before any data access functions are used,
1174 * otherwise data corruption may occur. The same comparison function must be used by every
1175 * program accessing the database, every time the database is used.
1176 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1177 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1178 * @param[in] cmp A #MDB_cmp_func function
1179 * @return A non-zero error value on failure and 0 on success. Some possible
1182 * <li>EINVAL - an invalid parameter was specified.
1185 int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1187 /** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
1189 * This comparison function is called whenever it is necessary to compare a data
1190 * item specified by the application with a data item currently stored in the database.
1191 * This function only takes effect if the database was opened with the #MDB_DUPSORT
1193 * If no comparison function is specified, and no special key flags were specified
1194 * with #mdb_dbi_open(), the data items are compared lexically, with shorter items collating
1195 * before longer items.
1196 * @warning This function must be called before any data access functions are used,
1197 * otherwise data corruption may occur. The same comparison function must be used by every
1198 * program accessing the database, every time the database is used.
1199 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1200 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1201 * @param[in] cmp A #MDB_cmp_func function
1202 * @return A non-zero error value on failure and 0 on success. Some possible
1205 * <li>EINVAL - an invalid parameter was specified.
1208 int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
1210 /** @brief Set a relocation function for a #MDB_FIXEDMAP database.
1212 * @todo The relocation function is called whenever it is necessary to move the data
1213 * of an item to a different position in the database (e.g. through tree
1214 * balancing operations, shifts as a result of adds or deletes, etc.). It is
1215 * intended to allow address/position-dependent data items to be stored in
1216 * a database in an environment opened with the #MDB_FIXEDMAP option.
1217 * Currently the relocation feature is unimplemented and setting
1218 * this function has no effect.
1219 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1220 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1221 * @param[in] rel A #MDB_rel_func function
1222 * @return A non-zero error value on failure and 0 on success. Some possible
1225 * <li>EINVAL - an invalid parameter was specified.
1228 int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
1230 /** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
1232 * See #mdb_set_relfunc and #MDB_rel_func for more details.
1233 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1234 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1235 * @param[in] ctx An arbitrary pointer for whatever the application needs.
1236 * It will be passed to the callback function set by #mdb_set_relfunc
1237 * as its \b relctx parameter whenever the callback is invoked.
1238 * @return A non-zero error value on failure and 0 on success. Some possible
1241 * <li>EINVAL - an invalid parameter was specified.
1244 int mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);
1246 /** @brief Get items from a database.
1248 * This function retrieves key/data pairs from the database. The address
1249 * and length of the data associated with the specified \b key are returned
1250 * in the structure to which \b data refers.
1251 * If the database supports duplicate keys (#MDB_DUPSORT) then the
1252 * first data item for the key will be returned. Retrieval of other
1253 * items requires the use of #mdb_cursor_get().
1255 * @note The memory pointed to by the returned values is owned by the
1256 * database. The caller need not dispose of the memory, and may not
1257 * modify it in any way. For values returned in a read-only transaction
1258 * any modification attempts will cause a SIGSEGV.
1259 * @note Values returned from the database are valid only until a
1260 * subsequent update operation, or the end of the transaction.
1261 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1262 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1263 * @param[in] key The key to search for in the database
1264 * @param[out] data The data corresponding to the key
1265 * @return A non-zero error value on failure and 0 on success. Some possible
1268 * <li>#MDB_NOTFOUND - the key was not in the database.
1269 * <li>EINVAL - an invalid parameter was specified.
1272 int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1274 /** @brief Store items into a database.
1276 * This function stores key/data pairs in the database. The default behavior
1277 * is to enter the new key/data pair, replacing any previously existing key
1278 * if duplicates are disallowed, or adding a duplicate data item if
1279 * duplicates are allowed (#MDB_DUPSORT).
1280 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1281 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1282 * @param[in] key The key to store in the database
1283 * @param[in,out] data The data to store
1284 * @param[in] flags Special options for this operation. This parameter
1285 * must be set to 0 or by bitwise OR'ing together one or more of the
1286 * values described here.
1288 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1289 * already appear in the database. This flag may only be specified
1290 * if the database was opened with #MDB_DUPSORT. The function will
1291 * return #MDB_KEYEXIST if the key/data pair already appears in the
1293 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1294 * does not already appear in the database. The function will return
1295 * #MDB_KEYEXIST if the key already appears in the database, even if
1296 * the database supports duplicates (#MDB_DUPSORT). The \b data
1297 * parameter will be set to point to the existing item.
1298 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1299 * don't copy the given data. Instead, return a pointer to the
1300 * reserved space, which the caller can fill in later - before
1301 * the next update operation or the transaction ends. This saves
1302 * an extra memcpy if the data is being generated later.
1303 * LMDB does nothing else with this memory, the caller is expected
1304 * to modify all of the space requested. This flag must not be
1305 * specified if the database was opened with #MDB_DUPSORT.
1306 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1307 * database. This option allows fast bulk loading when keys are
1308 * already known to be in the correct order. Loading unsorted keys
1309 * with this flag will cause a #MDB_KEYEXIST error.
1310 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1312 * @return A non-zero error value on failure and 0 on success. Some possible
1315 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1316 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1317 * <li>EACCES - an attempt was made to write in a read-only transaction.
1318 * <li>EINVAL - an invalid parameter was specified.
1321 int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
1322 unsigned int flags);
1324 /** @brief Delete items from a database.
1326 * This function removes key/data pairs from the database.
1327 * If the database does not support sorted duplicate data items
1328 * (#MDB_DUPSORT) the data parameter is ignored.
1329 * If the database supports sorted duplicates and the data parameter
1330 * is NULL, all of the duplicate data items for the key will be
1331 * deleted. Otherwise, if the data parameter is non-NULL
1332 * only the matching data item will be deleted.
1333 * This function will return #MDB_NOTFOUND if the specified key/data
1334 * pair is not in the database.
1335 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1336 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1337 * @param[in] key The key to delete from the database
1338 * @param[in] data The data to delete
1339 * @return A non-zero error value on failure and 0 on success. Some possible
1342 * <li>EACCES - an attempt was made to write in a read-only transaction.
1343 * <li>EINVAL - an invalid parameter was specified.
1346 int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
1348 /** @brief Create a cursor handle.
1350 * A cursor is associated with a specific transaction and database.
1351 * A cursor cannot be used when its database handle is closed. Nor
1352 * when its transaction has ended, except with #mdb_cursor_renew().
1353 * It can be discarded with #mdb_cursor_close().
1354 * A cursor in a write-transaction can be closed before its transaction
1355 * ends, and will otherwise be closed when its transaction ends.
1356 * A cursor in a read-only transaction must be closed explicitly, before
1357 * or after its transaction ends. It can be reused with
1358 * #mdb_cursor_renew() before finally closing it.
1359 * @note Earlier documentation said that cursors in every transaction
1360 * were closed when the transaction committed or aborted.
1361 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1362 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1363 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
1364 * @return A non-zero error value on failure and 0 on success. Some possible
1367 * <li>EINVAL - an invalid parameter was specified.
1370 int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
1372 /** @brief Close a cursor handle.
1374 * The cursor handle will be freed and must not be used again after this call.
1375 * Its transaction must still be live if it is a write-transaction.
1376 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1378 void mdb_cursor_close(MDB_cursor *cursor);
1380 /** @brief Renew a cursor handle.
1382 * A cursor is associated with a specific transaction and database.
1383 * Cursors that are only used in read-only
1384 * transactions may be re-used, to avoid unnecessary malloc/free overhead.
1385 * The cursor may be associated with a new read-only transaction, and
1386 * referencing the same database handle as it was created with.
1387 * This may be done whether the previous transaction is live or dead.
1388 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1389 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1390 * @return A non-zero error value on failure and 0 on success. Some possible
1393 * <li>EINVAL - an invalid parameter was specified.
1396 int mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);
1398 /** @brief Return the cursor's transaction handle.
1400 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1402 MDB_txn *mdb_cursor_txn(MDB_cursor *cursor);
1404 /** @brief Return the cursor's database handle.
1406 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1408 MDB_dbi mdb_cursor_dbi(MDB_cursor *cursor);
1410 /** @brief Retrieve by cursor.
1412 * This function retrieves key/data pairs from the database. The address and length
1413 * of the key are returned in the object to which \b key refers (except for the
1414 * case of the #MDB_SET option, in which the \b key object is unchanged), and
1415 * the address and length of the data are returned in the object to which \b data
1417 * See #mdb_get() for restrictions on using the output values.
1418 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1419 * @param[in,out] key The key for a retrieved item
1420 * @param[in,out] data The data of a retrieved item
1421 * @param[in] op A cursor operation #MDB_cursor_op
1422 * @return A non-zero error value on failure and 0 on success. Some possible
1425 * <li>#MDB_NOTFOUND - no matching key found.
1426 * <li>EINVAL - an invalid parameter was specified.
1429 int mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1432 /** @brief Store by cursor.
1434 * This function stores key/data pairs into the database.
1435 * The cursor is positioned at the new item, or on failure usually near it.
1436 * @note Earlier documentation incorrectly said errors would leave the
1437 * state of the cursor unchanged.
1438 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1439 * @param[in] key The key operated on.
1440 * @param[in] data The data operated on.
1441 * @param[in] flags Options for this operation. This parameter
1442 * must be set to 0 or one of the values described here.
1444 * <li>#MDB_CURRENT - replace the item at the current cursor position.
1445 * The \b key parameter must still be provided, and must match it.
1446 * If using sorted duplicates (#MDB_DUPSORT) the data item must still
1447 * sort into the same place. This is intended to be used when the
1448 * new data is the same size as the old. Otherwise it will simply
1449 * perform a delete of the old record followed by an insert.
1450 * <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
1451 * already appear in the database. This flag may only be specified
1452 * if the database was opened with #MDB_DUPSORT. The function will
1453 * return #MDB_KEYEXIST if the key/data pair already appears in the
1455 * <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
1456 * does not already appear in the database. The function will return
1457 * #MDB_KEYEXIST if the key already appears in the database, even if
1458 * the database supports duplicates (#MDB_DUPSORT).
1459 * <li>#MDB_RESERVE - reserve space for data of the given size, but
1460 * don't copy the given data. Instead, return a pointer to the
1461 * reserved space, which the caller can fill in later - before
1462 * the next update operation or the transaction ends. This saves
1463 * an extra memcpy if the data is being generated later. This flag
1464 * must not be specified if the database was opened with #MDB_DUPSORT.
1465 * <li>#MDB_APPEND - append the given key/data pair to the end of the
1466 * database. No key comparisons are performed. This option allows
1467 * fast bulk loading when keys are already known to be in the
1468 * correct order. Loading unsorted keys with this flag will cause
1469 * a #MDB_KEYEXIST error.
1470 * <li>#MDB_APPENDDUP - as above, but for sorted dup data.
1471 * <li>#MDB_MULTIPLE - store multiple contiguous data elements in a
1472 * single request. This flag may only be specified if the database
1473 * was opened with #MDB_DUPFIXED. The \b data argument must be an
1474 * array of two MDB_vals. The mv_size of the first MDB_val must be
1475 * the size of a single data element. The mv_data of the first MDB_val
1476 * must point to the beginning of the array of contiguous data elements.
1477 * The mv_size of the second MDB_val must be the count of the number
1478 * of data elements to store. On return this field will be set to
1479 * the count of the number of elements actually written. The mv_data
1480 * of the second MDB_val is unused.
1482 * @return A non-zero error value on failure and 0 on success. Some possible
1485 * <li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
1486 * <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
1487 * <li>EACCES - an attempt was made to write in a read-only transaction.
1488 * <li>EINVAL - an invalid parameter was specified.
1491 int mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
1492 unsigned int flags);
1494 /** @brief Delete current key/data pair
1496 * This function deletes the key/data pair to which the cursor refers.
1497 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1498 * @param[in] flags Options for this operation. This parameter
1499 * must be set to 0 or one of the values described here.
1501 * <li>#MDB_NODUPDATA - delete all of the data items for the current key.
1502 * This flag may only be specified if the database was opened with #MDB_DUPSORT.
1504 * @return A non-zero error value on failure and 0 on success. Some possible
1507 * <li>EACCES - an attempt was made to write in a read-only transaction.
1508 * <li>EINVAL - an invalid parameter was specified.
1511 int mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
1513 /** @brief Return count of duplicates for current key.
1515 * This call is only valid on databases that support sorted duplicate
1516 * data items #MDB_DUPSORT.
1517 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
1518 * @param[out] countp Address where the count will be stored
1519 * @return A non-zero error value on failure and 0 on success. Some possible
1522 * <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
1525 int mdb_cursor_count(MDB_cursor *cursor, size_t *countp);
1527 /** @brief Compare two data items according to a particular database.
1529 * This returns a comparison as if the two data items were keys in the
1530 * specified database.
1531 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1532 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1533 * @param[in] a The first item to compare
1534 * @param[in] b The second item to compare
1535 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1537 int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1539 /** @brief Compare two data items according to a particular database.
1541 * This returns a comparison as if the two items were data items of
1542 * the specified database. The database must have the #MDB_DUPSORT flag.
1543 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
1544 * @param[in] dbi A database handle returned by #mdb_dbi_open()
1545 * @param[in] a The first item to compare
1546 * @param[in] b The second item to compare
1547 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
1549 int mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
1551 /** @brief A callback function used to print a message from the library.
1553 * @param[in] msg The string to be printed.
1554 * @param[in] ctx An arbitrary context pointer for the callback.
1555 * @return < 0 on failure, >= 0 on success.
1557 typedef int (MDB_msg_func)(const char *msg, void *ctx);
1559 /** @brief Dump the entries in the reader lock table.
1561 * @param[in] env An environment handle returned by #mdb_env_create()
1562 * @param[in] func A #MDB_msg_func function
1563 * @param[in] ctx Anything the message function needs
1564 * @return < 0 on failure, >= 0 on success.
1566 int mdb_reader_list(MDB_env *env, MDB_msg_func *func, void *ctx);
1568 /** @brief Check for stale entries in the reader lock table.
1570 * @param[in] env An environment handle returned by #mdb_env_create()
1571 * @param[out] dead Number of stale slots that were cleared
1572 * @return 0 on success, non-zero on failure.
1574 int mdb_reader_check(MDB_env *env, int *dead);
1580 /** @page tools LMDB Command Line Tools
1581 The following describes the command line tools that are available for LMDB.
1588 #endif /* _LMDB_H_ */