nostrdb

lmdb.h (74013B)

---

 /** @file lmdb.h
  *	@brief Lightning memory-mapped database library
  *
  *	@mainpage	Lightning Memory-Mapped Database Manager (LMDB)
  *
  *	@section intro_sec Introduction
  *	LMDB is a Btree-based database management library modeled loosely on the
  *	BerkeleyDB API, but much simplified. The entire database is exposed
  *	in a memory map, and all data fetches return data directly
  *	from the mapped memory, so no malloc's or memcpy's occur during
  *	data fetches. As such, the library is extremely simple because it
  *	requires no page caching layer of its own, and it is extremely high
  *	performance and memory-efficient. It is also fully transactional with
  *	full ACID semantics, and when the memory map is read-only, the
  *	database integrity cannot be corrupted by stray pointer writes from
  *	application code.
  *
  *	The library is fully thread-aware and supports concurrent read/write
  *	access from multiple processes and threads. Data pages use a copy-on-
  *	write strategy so no active data pages are ever overwritten, which
  *	also provides resistance to corruption and eliminates the need of any
  *	special recovery procedures after a system crash. Writes are fully
  *	serialized; only one write transaction may be active at a time, which
  *	guarantees that writers can never deadlock. The database structure is
  *	multi-versioned so readers run with no locks; writers cannot block
  *	readers, and readers don't block writers.
  *
  *	Unlike other well-known database mechanisms which use either write-ahead
  *	transaction logs or append-only data writes, LMDB requires no maintenance
  *	during operation. Both write-ahead loggers and append-only databases
  *	require periodic checkpointing and/or compaction of their log or database
  *	files otherwise they grow without bound. LMDB tracks free pages within
  *	the database and re-uses them for new write operations, so the database
  *	size does not grow without bound in normal use.
  *
  *	The memory map can be used as a read-only or read-write map. It is
  *	read-only by default as this provides total immunity to corruption.
  *	Using read-write mode offers much higher write performance, but adds
  *	the possibility for stray application writes thru pointers to silently
  *	corrupt the database. Of course if your application code is known to
  *	be bug-free (...) then this is not an issue.
  *
  *	If this is your first time using a transactional embedded key/value
  *	store, you may find the \ref starting page to be helpful.
  *
  *	@section caveats_sec Caveats
  *	Troubleshooting the lock file, plus semaphores on BSD systems:
  *
  *	- A broken lockfile can cause sync issues.
  *	  Stale reader transactions left behind by an aborted program
  *	  cause further writes to grow the database quickly, and
  *	  stale locks can block further operation.
  *
  *	  Fix: Check for stale readers periodically, using the
  *	  #mdb_reader_check function or the \ref mdb_stat_1 "mdb_stat" tool.
  *	  Stale writers will be cleared automatically on some systems:
  *	  - Windows - automatic
  *	  - Linux, systems using POSIX mutexes with Robust option - automatic
  *	  - not on BSD, systems using POSIX semaphores.
  *	  Otherwise just make all programs using the database close it;
  *	  the lockfile is always reset on first open of the environment.
  *
  *	- On BSD systems or others configured with MDB_USE_POSIX_SEM,
  *	  startup can fail due to semaphores owned by another userid.
  *
  *	  Fix: Open and close the database as the user which owns the
  *	  semaphores (likely last user) or as root, while no other
  *	  process is using the database.
  *
  *	Restrictions/caveats (in addition to those listed for some functions):
  *
  *	- Only the database owner should normally use the database on
  *	  BSD systems or when otherwise configured with MDB_USE_POSIX_SEM.
  *	  Multiple users can cause startup to fail later, as noted above.
  *
  *	- There is normally no pure read-only mode, since readers need write
  *	  access to locks and lock file. Exceptions: On read-only filesystems
  *	  or with the #MDB_NOLOCK flag described under #mdb_env_open().
  *
  *	- An LMDB configuration will often reserve considerable \b unused
  *	  memory address space and maybe file size for future growth.
  *	  This does not use actual memory or disk space, but users may need
  *	  to understand the difference so they won't be scared off.
  *
  *	- By default, in versions before 0.9.10, unused portions of the data
  *	  file might receive garbage data from memory freed by other code.
  *	  (This does not happen when using the #MDB_WRITEMAP flag.) As of
  *	  0.9.10 the default behavior is to initialize such memory before
  *	  writing to the data file. Since there may be a slight performance
  *	  cost due to this initialization, applications may disable it using
  *	  the #MDB_NOMEMINIT flag. Applications handling sensitive data
  *	  which must not be written should not use this flag. This flag is
  *	  irrelevant when using #MDB_WRITEMAP.
  *
  *	- A thread can only use one transaction at a time, plus any child
  *	  transactions.  Each transaction belongs to one thread.  See below.
  *	  The #MDB_NOTLS flag changes this for read-only transactions.
  *
  *	- Use an MDB_env* in the process which opened it, not after fork().
  *
  *	- Do not have open an LMDB database twice in the same process at
  *	  the same time.  Not even from a plain open() call - close()ing it
  *	  breaks fcntl() advisory locking.  (It is OK to reopen it after
  *	  fork() - exec*(), since the lockfile has FD_CLOEXEC set.)
  *
  *	- Avoid long-lived transactions.  Read transactions prevent
  *	  reuse of pages freed by newer write transactions, thus the
  *	  database can grow quickly.  Write transactions prevent
  *	  other write transactions, since writes are serialized.
  *
  *	- Avoid suspending a process with active transactions.  These
  *	  would then be "long-lived" as above.  Also read transactions
  *	  suspended when writers commit could sometimes see wrong data.
  *
  *	...when several processes can use a database concurrently:
  *
  *	- Avoid aborting a process with an active transaction.
  *	  The transaction becomes "long-lived" as above until a check
  *	  for stale readers is performed or the lockfile is reset,
  *	  since the process may not remove it from the lockfile.
  *
  *	  This does not apply to write transactions if the system clears
  *	  stale writers, see above.
  *
  *	- If you do that anyway, do a periodic check for stale readers. Or
  *	  close the environment once in a while, so the lockfile can get reset.
  *
  *	- Do not use LMDB databases on remote filesystems, even between
  *	  processes on the same host.  This breaks flock() on some OSes,
  *	  possibly memory map sync, and certainly sync between programs
  *	  on different hosts.
  *
  *	- Opening a database can fail if another process is opening or
  *	  closing it at exactly the same time.
  *
  *	@author	Howard Chu, Symas Corporation.
  *
  *	@copyright Copyright 2011-2021 Howard Chu, Symas Corp. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted only as authorized by the OpenLDAP
  * Public License.
  *
  * A copy of this license is available in the file LICENSE in the
  * top-level directory of the distribution or, alternatively, at
  * <http://www.OpenLDAP.org/license.html>.
  *
  *	@par Derived From:
  * This code is derived from btree.c written by Martin Hedenfalk.
  *
  * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
  *
  * Permission to use, copy, modify, and distribute this software for any
  * purpose with or without fee is hereby granted, provided that the above
  * copyright notice and this permission notice appear in all copies.
  *
  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 #ifndef _LMDB_H_
 #define _LMDB_H_
 
 #include <sys/types.h>
 
 #ifdef __cplusplus
 extern "C" {
 #endif
 
 /** Unix permissions for creating files, or dummy definition for Windows */
 #ifdef _MSC_VER
 typedef	int	mdb_mode_t;
 #else
 typedef	mode_t	mdb_mode_t;
 #endif
 
 /** An abstraction for a file handle.
  *	On POSIX systems file handles are small integers. On Windows
  *	they're opaque pointers.
  */
 #ifdef _WIN32
 typedef	void *mdb_filehandle_t;
 #else
 typedef int mdb_filehandle_t;
 #endif
 
 /** @defgroup mdb LMDB API
  *	@{
  *	@brief OpenLDAP Lightning Memory-Mapped Database Manager
  */
 /** @defgroup Version Version Macros
  *	@{
  */
 /** Library major version */
 #define MDB_VERSION_MAJOR	0
 /** Library minor version */
 #define MDB_VERSION_MINOR	9
 /** Library patch version */
 #define MDB_VERSION_PATCH	31
 
 /** Combine args a,b,c into a single integer for easy version comparisons */
 #define MDB_VERINT(a,b,c)	(((a) << 24) | ((b) << 16) | (c))
 
 /** The full library version as a single integer */
 #define MDB_VERSION_FULL	\
 	MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
 
 /** The release date of this library version */
 #define MDB_VERSION_DATE	"July 10, 2023"
 
 /** A stringifier for the version info */
 #define MDB_VERSTR(a,b,c,d)	"LMDB " #a "." #b "." #c ": (" d ")"
 
 /** A helper for the stringifier macro */
 #define MDB_VERFOO(a,b,c,d)	MDB_VERSTR(a,b,c,d)
 
 /** The full library version as a C string */
 #define	MDB_VERSION_STRING	\
 	MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
 /**	@} */
 
 /** @brief Opaque structure for a database environment.
  *
  * A DB environment supports multiple databases, all residing in the same
  * shared-memory map.
  */
 typedef struct MDB_env MDB_env;
 
 /** @brief Opaque structure for a transaction handle.
  *
  * All database operations require a transaction handle. Transactions may be
  * read-only or read-write.
  */
 typedef struct MDB_txn MDB_txn;
 
 /** @brief A handle for an individual database in the DB environment. */
 typedef unsigned int	MDB_dbi;
 
 /** @brief Opaque structure for navigating through a database */
 typedef struct MDB_cursor MDB_cursor;
 
 /** @brief Generic structure used for passing keys and data in and out
  * of the database.
  *
  * Values returned from the database are valid only until a subsequent
  * update operation, or the end of the transaction. Do not modify or
  * free them, they commonly point into the database itself.
  *
  * Key sizes must be between 1 and #mdb_env_get_maxkeysize() inclusive.
  * The same applies to data sizes in databases with the #MDB_DUPSORT flag.
  * Other data items can in theory be from 0 to 0xffffffff bytes long.
  */
 typedef struct MDB_val {
 	size_t		 mv_size;	/**< size of the data item */
 	void		*mv_data;	/**< address of the data item */
 } MDB_val;
 
 /** @brief A callback function used to compare two keys in a database */
 typedef int  (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
 
 /** @brief A callback function used to relocate a position-dependent data item
  * in a fixed-address database.
  *
  * The \b newptr gives the item's desired address in
  * the memory map, and \b oldptr gives its previous address. The item's actual
  * data resides at the address in \b item.  This callback is expected to walk
  * through the fields of the record in \b item and modify any
  * values based at the \b oldptr address to be relative to the \b newptr address.
  * @param[in,out] item The item that is to be relocated.
  * @param[in] oldptr The previous address.
  * @param[in] newptr The new address to relocate to.
  * @param[in] relctx An application-provided context, set by #mdb_set_relctx().
  * @todo This feature is currently unimplemented.
  */
 typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
 
 /** @defgroup	mdb_env	Environment Flags
  *	@{
  */
 	/** mmap at a fixed address (experimental) */
 #define MDB_FIXEDMAP	0x01
 	/** no environment directory */
 #define MDB_NOSUBDIR	0x4000
 	/** don't fsync after commit */
 #define MDB_NOSYNC		0x10000
 	/** read only */
 #define MDB_RDONLY		0x20000
 	/** don't fsync metapage after commit */
 #define MDB_NOMETASYNC		0x40000
 	/** use writable mmap */
 #define MDB_WRITEMAP		0x80000
 	/** use asynchronous msync when #MDB_WRITEMAP is used */
 #define MDB_MAPASYNC		0x100000
 	/** tie reader locktable slots to #MDB_txn objects instead of to threads */
 #define MDB_NOTLS		0x200000
 	/** don't do any locking, caller must manage their own locks */
 #define MDB_NOLOCK		0x400000
 	/** don't do readahead (no effect on Windows) */
 #define MDB_NORDAHEAD	0x800000
 	/** don't initialize malloc'd memory before writing to datafile */
 #define MDB_NOMEMINIT	0x1000000
 /** @} */
 
 /**	@defgroup	mdb_dbi_open	Database Flags
  *	@{
  */
 	/** use reverse string keys */
 #define MDB_REVERSEKEY	0x02
 	/** use sorted duplicates */
 #define MDB_DUPSORT		0x04
 	/** numeric keys in native byte order: either unsigned int or size_t.
 	 *  The keys must all be of the same size. */
 #define MDB_INTEGERKEY	0x08
 	/** with #MDB_DUPSORT, sorted dup items have fixed size */
 #define MDB_DUPFIXED	0x10
 	/** with #MDB_DUPSORT, dups are #MDB_INTEGERKEY-style integers */
 #define MDB_INTEGERDUP	0x20
 	/** with #MDB_DUPSORT, use reverse string dups */
 #define MDB_REVERSEDUP	0x40
 	/** create DB if not already existing */
 #define MDB_CREATE		0x40000
 /** @} */
 
 /**	@defgroup mdb_put	Write Flags
  *	@{
  */
 /** For put: Don't write if the key already exists. */
 #define MDB_NOOVERWRITE	0x10
 /** Only for #MDB_DUPSORT<br>
  * For put: don't write if the key and data pair already exist.<br>
  * For mdb_cursor_del: remove all duplicate data items.
  */
 #define MDB_NODUPDATA	0x20
 /** For mdb_cursor_put: overwrite the current key/data pair */
 #define MDB_CURRENT	0x40
 /** For put: Just reserve space for data, don't copy it. Return a
  * pointer to the reserved space.
  */
 #define MDB_RESERVE	0x10000
 /** Data is being appended, don't split full pages. */
 #define MDB_APPEND	0x20000
 /** Duplicate data is being appended, don't split full pages. */
 #define MDB_APPENDDUP	0x40000
 /** Store multiple data items in one call. Only for #MDB_DUPFIXED. */
 #define MDB_MULTIPLE	0x80000
 /*	@} */
 
 /**	@defgroup mdb_copy	Copy Flags
  *	@{
  */
 /** Compacting copy: Omit free space from copy, and renumber all
  * pages sequentially.
  */
 #define MDB_CP_COMPACT	0x01
 /*	@} */
 
 /** @brief Cursor Get operations.
  *
  *	This is the set of all operations for retrieving data
  *	using a cursor.
  */
 typedef enum MDB_cursor_op {
 	MDB_FIRST,				/**< Position at first key/data item */
 	MDB_FIRST_DUP,			/**< Position at first data item of current key.
 								Only for #MDB_DUPSORT */
 	MDB_GET_BOTH,			/**< Position at key/data pair. Only for #MDB_DUPSORT */
 	MDB_GET_BOTH_RANGE,		/**< position at key, nearest data. Only for #MDB_DUPSORT */
 	MDB_GET_CURRENT,		/**< Return key/data at current cursor position */
 	MDB_GET_MULTIPLE,		/**< Return up to a page of duplicate data items
 								from current cursor position. Move cursor to prepare
 								for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
 	MDB_LAST,				/**< Position at last key/data item */
 	MDB_LAST_DUP,			/**< Position at last data item of current key.
 								Only for #MDB_DUPSORT */
 	MDB_NEXT,				/**< Position at next data item */
 	MDB_NEXT_DUP,			/**< Position at next data item of current key.
 								Only for #MDB_DUPSORT */
 	MDB_NEXT_MULTIPLE,		/**< Return up to a page of duplicate data items
 								from next cursor position. Move cursor to prepare
 								for #MDB_NEXT_MULTIPLE. Only for #MDB_DUPFIXED */
 	MDB_NEXT_NODUP,			/**< Position at first data item of next key */
 	MDB_PREV,				/**< Position at previous data item */
 	MDB_PREV_DUP,			/**< Position at previous data item of current key.
 								Only for #MDB_DUPSORT */
 	MDB_PREV_NODUP,			/**< Position at last data item of previous key */
 	MDB_SET,				/**< Position at specified key */
 	MDB_SET_KEY,			/**< Position at specified key, return key + data */
 	MDB_SET_RANGE,			/**< Position at first key greater than or equal to specified key. */
 	MDB_PREV_MULTIPLE		/**< Position at previous page and return up to
 								a page of duplicate data items. Only for #MDB_DUPFIXED */
 } MDB_cursor_op;
 
 /** @defgroup  errors	Return Codes
  *
  *	BerkeleyDB uses -30800 to -30999, we'll go under them
  *	@{
  */
 	/**	Successful result */
 #define MDB_SUCCESS	 0
 	/** key/data pair already exists */
 #define MDB_KEYEXIST	(-30799)
 	/** key/data pair not found (EOF) */
 #define MDB_NOTFOUND	(-30798)
 	/** Requested page not found - this usually indicates corruption */
 #define MDB_PAGE_NOTFOUND	(-30797)
 	/** Located page was wrong type */
 #define MDB_CORRUPTED	(-30796)
 	/** Update of meta page failed or environment had fatal error */
 #define MDB_PANIC		(-30795)
 	/** Environment version mismatch */
 #define MDB_VERSION_MISMATCH	(-30794)
 	/** File is not a valid LMDB file */
 #define MDB_INVALID	(-30793)
 	/** Environment mapsize reached */
 #define MDB_MAP_FULL	(-30792)
 	/** Environment maxdbs reached */
 #define MDB_DBS_FULL	(-30791)
 	/** Environment maxreaders reached */
 #define MDB_READERS_FULL	(-30790)
 	/** Too many TLS keys in use - Windows only */
 #define MDB_TLS_FULL	(-30789)
 	/** Txn has too many dirty pages */
 #define MDB_TXN_FULL	(-30788)
 	/** Cursor stack too deep - internal error */
 #define MDB_CURSOR_FULL	(-30787)
 	/** Page has not enough space - internal error */
 #define MDB_PAGE_FULL	(-30786)
 	/** Database contents grew beyond environment mapsize */
 #define MDB_MAP_RESIZED	(-30785)
 	/** Operation and DB incompatible, or DB type changed. This can mean:
 	 *	<ul>
 	 *	<li>The operation expects an #MDB_DUPSORT / #MDB_DUPFIXED database.
 	 *	<li>Opening a named DB when the unnamed DB has #MDB_DUPSORT / #MDB_INTEGERKEY.
 	 *	<li>Accessing a data record as a database, or vice versa.
 	 *	<li>The database was dropped and recreated with different flags.
 	 *	</ul>
 	 */
 #define MDB_INCOMPATIBLE	(-30784)
 	/** Invalid reuse of reader locktable slot */
 #define MDB_BAD_RSLOT		(-30783)
 	/** Transaction must abort, has a child, or is invalid */
 #define MDB_BAD_TXN			(-30782)
 	/** Unsupported size of key/DB name/data, or wrong DUPFIXED size */
 #define MDB_BAD_VALSIZE		(-30781)
 	/** The specified DBI was changed unexpectedly */
 #define MDB_BAD_DBI		(-30780)
 	/** The last defined error code */
 #define MDB_LAST_ERRCODE	MDB_BAD_DBI
 /** @} */
 
 /** @brief Statistics for a database in the environment */
 typedef struct MDB_stat {
 	unsigned int	ms_psize;			/**< Size of a database page.
 											This is currently the same for all databases. */
 	unsigned int	ms_depth;			/**< Depth (height) of the B-tree */
 	size_t		ms_branch_pages;	/**< Number of internal (non-leaf) pages */
 	size_t		ms_leaf_pages;		/**< Number of leaf pages */
 	size_t		ms_overflow_pages;	/**< Number of overflow pages */
 	size_t		ms_entries;			/**< Number of data items */
 } MDB_stat;
 
 /** @brief Information about the environment */
 typedef struct MDB_envinfo {
 	void	*me_mapaddr;			/**< Address of map, if fixed */
 	size_t	me_mapsize;				/**< Size of the data memory map */
 	size_t	me_last_pgno;			/**< ID of the last used page */
 	size_t	me_last_txnid;			/**< ID of the last committed transaction */
 	unsigned int me_maxreaders;		/**< max reader slots in the environment */
 	unsigned int me_numreaders;		/**< max reader slots used in the environment */
 } MDB_envinfo;
 
 	/** @brief Return the LMDB library version information.
 	 *
 	 * @param[out] major if non-NULL, the library major version number is copied here
 	 * @param[out] minor if non-NULL, the library minor version number is copied here
 	 * @param[out] patch if non-NULL, the library patch version number is copied here
 	 * @retval "version string" The library version as a string
 	 */
 char *mdb_version(int *major, int *minor, int *patch);
 
 	/** @brief Return a string describing a given error code.
 	 *
 	 * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
 	 * function. If the error code is greater than or equal to 0, then the string
 	 * returned by the system function strerror(3) is returned. If the error code
 	 * is less than 0, an error string corresponding to the LMDB library error is
 	 * returned. See @ref errors for a list of LMDB-specific error codes.
 	 * @param[in] err The error code
 	 * @retval "error message" The description of the error
 	 */
 char *mdb_strerror(int err);
 
 	/** @brief Create an LMDB environment handle.
 	 *
 	 * This function allocates memory for a #MDB_env structure. To release
 	 * the allocated memory and discard the handle, call #mdb_env_close().
 	 * Before the handle may be used, it must be opened using #mdb_env_open().
 	 * Various other options may also need to be set before opening the handle,
 	 * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
 	 * depending on usage requirements.
 	 * @param[out] env The address where the new handle will be stored
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_create(MDB_env **env);
 
 	/** @brief Open an environment handle.
 	 *
 	 * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] path The directory in which the database files reside. This
 	 * directory must already exist and be writable.
 	 * @param[in] flags Special options for this environment. This parameter
 	 * must be set to 0 or by bitwise OR'ing together one or more of the
 	 * values described here.
 	 * Flags set by mdb_env_set_flags() are also used.
 	 * <ul>
 	 *	<li>#MDB_FIXEDMAP
 	 *      use a fixed address for the mmap region. This flag must be specified
 	 *      when creating the environment, and is stored persistently in the environment.
 	 *		If successful, the memory map will always reside at the same virtual address
 	 *		and pointers used to reference data items in the database will be constant
 	 *		across multiple invocations. This option may not always work, depending on
 	 *		how the operating system has allocated memory to shared libraries and other uses.
 	 *		The feature is highly experimental.
 	 *	<li>#MDB_NOSUBDIR
 	 *		By default, LMDB creates its environment in a directory whose
 	 *		pathname is given in \b path, and creates its data and lock files
 	 *		under that directory. With this option, \b path is used as-is for
 	 *		the database main data file. The database lock file is the \b path
 	 *		with "-lock" appended.
 	 *	<li>#MDB_RDONLY
 	 *		Open the environment in read-only mode. No write operations will be
 	 *		allowed. LMDB will still modify the lock file - except on read-only
 	 *		filesystems, where LMDB does not use locks.
 	 *	<li>#MDB_WRITEMAP
 	 *		Use a writeable memory map unless MDB_RDONLY is set. This uses
 	 *		fewer mallocs but loses protection from application bugs
 	 *		like wild pointer writes and other bad updates into the database.
 	 *		This may be slightly faster for DBs that fit entirely in RAM, but
 	 *		is slower for DBs larger than RAM.
 	 *		Incompatible with nested transactions.
 	 *		Do not mix processes with and without MDB_WRITEMAP on the same
 	 *		environment.  This can defeat durability (#mdb_env_sync etc).
 	 *	<li>#MDB_NOMETASYNC
 	 *		Flush system buffers to disk only once per transaction, omit the
 	 *		metadata flush. Defer that until the system flushes files to disk,
 	 *		or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization
 	 *		maintains database integrity, but a system crash may undo the last
 	 *		committed transaction. I.e. it preserves the ACI (atomicity,
 	 *		consistency, isolation) but not D (durability) database property.
 	 *		This flag may be changed at any time using #mdb_env_set_flags().
 	 *	<li>#MDB_NOSYNC
 	 *		Don't flush system buffers to disk when committing a transaction.
 	 *		This optimization means a system crash can corrupt the database or
 	 *		lose the last transactions if buffers are not yet flushed to disk.
 	 *		The risk is governed by how often the system flushes dirty buffers
 	 *		to disk and how often #mdb_env_sync() is called.  However, if the
 	 *		filesystem preserves write order and the #MDB_WRITEMAP flag is not
 	 *		used, transactions exhibit ACI (atomicity, consistency, isolation)
 	 *		properties and only lose D (durability).  I.e. database integrity
 	 *		is maintained, but a system crash may undo the final transactions.
 	 *		Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no
 	 *		hint for when to write transactions to disk, unless #mdb_env_sync()
 	 *		is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable.
 	 *		This flag may be changed at any time using #mdb_env_set_flags().
 	 *	<li>#MDB_MAPASYNC
 	 *		When using #MDB_WRITEMAP, use asynchronous flushes to disk.
 	 *		As with #MDB_NOSYNC, a system crash can then corrupt the
 	 *		database or lose the last transactions. Calling #mdb_env_sync()
 	 *		ensures on-disk database integrity until next commit.
 	 *		This flag may be changed at any time using #mdb_env_set_flags().
 	 *	<li>#MDB_NOTLS
 	 *		Don't use Thread-Local Storage. Tie reader locktable slots to
 	 *		#MDB_txn objects instead of to threads. I.e. #mdb_txn_reset() keeps
 	 *		the slot reserved for the #MDB_txn object. A thread may use parallel
 	 *		read-only transactions. A read-only transaction may span threads if
 	 *		the user synchronizes its use. Applications that multiplex many
 	 *		user threads over individual OS threads need this option. Such an
 	 *		application must also serialize the write transactions in an OS
 	 *		thread, since LMDB's write locking is unaware of the user threads.
 	 *	<li>#MDB_NOLOCK
 	 *		Don't do any locking. If concurrent access is anticipated, the
 	 *		caller must manage all concurrency itself. For proper operation
 	 *		the caller must enforce single-writer semantics, and must ensure
 	 *		that no readers are using old transactions while a writer is
 	 *		active. The simplest approach is to use an exclusive lock so that
 	 *		no readers may be active at all when a writer begins.
 	 *	<li>#MDB_NORDAHEAD
 	 *		Turn off readahead. Most operating systems perform readahead on
 	 *		read requests by default. This option turns it off if the OS
 	 *		supports it. Turning it off may help random read performance
 	 *		when the DB is larger than RAM and system RAM is full.
 	 *		The option is not implemented on Windows.
 	 *	<li>#MDB_NOMEMINIT
 	 *		Don't initialize malloc'd memory before writing to unused spaces
 	 *		in the data file. By default, memory for pages written to the data
 	 *		file is obtained using malloc. While these pages may be reused in
 	 *		subsequent transactions, freshly malloc'd pages will be initialized
 	 *		to zeroes before use. This avoids persisting leftover data from other
 	 *		code (that used the heap and subsequently freed the memory) into the
 	 *		data file. Note that many other system libraries may allocate
 	 *		and free memory from the heap for arbitrary uses. E.g., stdio may
 	 *		use the heap for file I/O buffers. This initialization step has a
 	 *		modest performance cost so some applications may want to disable
 	 *		it using this flag. This option can be a problem for applications
 	 *		which handle sensitive data like passwords, and it makes memory
 	 *		checkers like Valgrind noisy. This flag is not needed with #MDB_WRITEMAP,
 	 *		which writes directly to the mmap instead of using malloc for pages. The
 	 *		initialization is also skipped if #MDB_RESERVE is used; the
 	 *		caller is expected to overwrite all of the memory that was
 	 *		reserved in that case.
 	 *		This flag may be changed at any time using #mdb_env_set_flags().
 	 * </ul>
 	 * @param[in] mode The UNIX permissions to set on created files and semaphores.
 	 * This parameter is ignored on Windows.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the
 	 *	version that created the database environment.
 	 *	<li>#MDB_INVALID - the environment file headers are corrupted.
 	 *	<li>ENOENT - the directory specified by the path parameter doesn't exist.
 	 *	<li>EACCES - the user didn't have permission to access the environment files.
 	 *	<li>EAGAIN - the environment was locked by another process.
 	 * </ul>
 	 */
 int  mdb_env_open(MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode);
 
 	/** @brief Copy an LMDB environment to the specified path.
 	 *
 	 * This function may be used to make a backup of an existing environment.
 	 * No lockfile is created, since it gets recreated at need.
 	 * @note This call can trigger significant file size growth if run in
 	 * parallel with write transactions, because it employs a read-only
 	 * transaction. See long-lived transactions under @ref caveats_sec.
 	 * @param[in] env An environment handle returned by #mdb_env_create(). It
 	 * must have already been opened successfully.
 	 * @param[in] path The directory in which the copy will reside. This
 	 * directory must already exist and be writable but must otherwise be
 	 * empty.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_copy(MDB_env *env, const char *path);
 
 	/** @brief Copy an LMDB environment to the specified file descriptor.
 	 *
 	 * This function may be used to make a backup of an existing environment.
 	 * No lockfile is created, since it gets recreated at need.
 	 * @note This call can trigger significant file size growth if run in
 	 * parallel with write transactions, because it employs a read-only
 	 * transaction. See long-lived transactions under @ref caveats_sec.
 	 * @param[in] env An environment handle returned by #mdb_env_create(). It
 	 * must have already been opened successfully.
 	 * @param[in] fd The filedescriptor to write the copy to. It must
 	 * have already been opened for Write access.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_copyfd(MDB_env *env, mdb_filehandle_t fd);
 
 	/** @brief Copy an LMDB environment to the specified path, with options.
 	 *
 	 * This function may be used to make a backup of an existing environment.
 	 * No lockfile is created, since it gets recreated at need.
 	 * @note This call can trigger significant file size growth if run in
 	 * parallel with write transactions, because it employs a read-only
 	 * transaction. See long-lived transactions under @ref caveats_sec.
 	 * @param[in] env An environment handle returned by #mdb_env_create(). It
 	 * must have already been opened successfully.
 	 * @param[in] path The directory in which the copy will reside. This
 	 * directory must already exist and be writable but must otherwise be
 	 * empty.
 	 * @param[in] flags Special options for this operation. This parameter
 	 * must be set to 0 or by bitwise OR'ing together one or more of the
 	 * values described here.
 	 * <ul>
 	 *	<li>#MDB_CP_COMPACT - Perform compaction while copying: omit free
 	 *		pages and sequentially renumber all pages in output. This option
 	 *		consumes more CPU and runs more slowly than the default.
 	 *		Currently it fails if the environment has suffered a page leak.
 	 * </ul>
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_copy2(MDB_env *env, const char *path, unsigned int flags);
 
 	/** @brief Copy an LMDB environment to the specified file descriptor,
 	 *	with options.
 	 *
 	 * This function may be used to make a backup of an existing environment.
 	 * No lockfile is created, since it gets recreated at need. See
 	 * #mdb_env_copy2() for further details.
 	 * @note This call can trigger significant file size growth if run in
 	 * parallel with write transactions, because it employs a read-only
 	 * transaction. See long-lived transactions under @ref caveats_sec.
 	 * @param[in] env An environment handle returned by #mdb_env_create(). It
 	 * must have already been opened successfully.
 	 * @param[in] fd The filedescriptor to write the copy to. It must
 	 * have already been opened for Write access.
 	 * @param[in] flags Special options for this operation.
 	 * See #mdb_env_copy2() for options.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_copyfd2(MDB_env *env, mdb_filehandle_t fd, unsigned int flags);
 
 	/** @brief Return statistics about the LMDB environment.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] stat The address of an #MDB_stat structure
 	 * 	where the statistics will be copied
 	 */
 int  mdb_env_stat(MDB_env *env, MDB_stat *stat);
 
 	/** @brief Return information about the LMDB environment.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] stat The address of an #MDB_envinfo structure
 	 * 	where the information will be copied
 	 */
 int  mdb_env_info(MDB_env *env, MDB_envinfo *stat);
 
 	/** @brief Flush the data buffers to disk.
 	 *
 	 * Data is always written to disk when #mdb_txn_commit() is called,
 	 * but the operating system may keep it buffered. LMDB always flushes
 	 * the OS buffers upon commit as well, unless the environment was
 	 * opened with #MDB_NOSYNC or in part #MDB_NOMETASYNC. This call is
 	 * not valid if the environment was opened with #MDB_RDONLY.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] force If non-zero, force a synchronous flush.  Otherwise
 	 *  if the environment has the #MDB_NOSYNC flag set the flushes
 	 *	will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EACCES - the environment is read-only.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 *	<li>EIO - an error occurred during synchronization.
 	 * </ul>
 	 */
 int  mdb_env_sync(MDB_env *env, int force);
 
 	/** @brief Close the environment and release the memory map.
 	 *
 	 * Only a single thread may call this function. All transactions, databases,
 	 * and cursors must already be closed before calling this function. Attempts to
 	 * use any such handles after calling this function will cause a SIGSEGV.
 	 * The environment handle will be freed and must not be used again after this call.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 */
 void mdb_env_close(MDB_env *env);
 
 	/** @brief Set environment flags.
 	 *
 	 * This may be used to set some flags in addition to those from
 	 * #mdb_env_open(), or to unset these flags.  If several threads
 	 * change the flags at the same time, the result is undefined.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] flags The flags to change, bitwise OR'ed together
 	 * @param[in] onoff A non-zero value sets the flags, zero clears them.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);
 
 	/** @brief Get environment flags.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] flags The address of an integer to store the flags
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_env_get_flags(MDB_env *env, unsigned int *flags);
 
 	/** @brief Return the path that was used in #mdb_env_open().
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] path Address of a string pointer to contain the path. This
 	 * is the actual string in the environment, not a copy. It should not be
 	 * altered in any way.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_env_get_path(MDB_env *env, const char **path);
 
 	/** @brief Return the filedescriptor for the given environment.
 	 *
 	 * This function may be called after fork(), so the descriptor can be
 	 * closed before exec*().  Other LMDB file descriptors have FD_CLOEXEC.
 	 * (Until LMDB 0.9.18, only the lockfile had that.)
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] fd Address of a mdb_filehandle_t to contain the descriptor.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_env_get_fd(MDB_env *env, mdb_filehandle_t *fd);
 
 	/** @brief Set the size of the memory map to use for this environment.
 	 *
 	 * The size should be a multiple of the OS page size. The default is
 	 * 10485760 bytes. The size of the memory map is also the maximum size
 	 * of the database. The value should be chosen as large as possible,
 	 * to accommodate future growth of the database.
 	 * This function should be called after #mdb_env_create() and before #mdb_env_open().
 	 * It may be called at later times if no transactions are active in
 	 * this process. Note that the library does not check for this condition,
 	 * the caller must ensure it explicitly.
 	 *
 	 * The new size takes effect immediately for the current process but
 	 * will not be persisted to any others until a write transaction has been
 	 * committed by the current process. Also, only mapsize increases are
 	 * persisted into the environment.
 	 *
 	 * If the mapsize is increased by another process, and data has grown
 	 * beyond the range of the current mapsize, #mdb_txn_begin() will
 	 * return #MDB_MAP_RESIZED. This function may be called with a size
 	 * of zero to adopt the new size.
 	 *
 	 * Any attempt to set a size smaller than the space already consumed
 	 * by the environment will be silently changed to the current size of the used space.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] size The size in bytes
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified, or the environment has
 	 *   	an active write transaction.
 	 * </ul>
 	 */
 int  mdb_env_set_mapsize(MDB_env *env, size_t size);
 
 	/** @brief Set the maximum number of threads/reader slots for the environment.
 	 *
 	 * This defines the number of slots in the lock table that is used to track readers in the
 	 * the environment. The default is 126.
 	 * Starting a read-only transaction normally ties a lock table slot to the
 	 * current thread until the environment closes or the thread exits. If
 	 * MDB_NOTLS is in use, #mdb_txn_begin() instead ties the slot to the
 	 * MDB_txn object until it or the #MDB_env object is destroyed.
 	 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] readers The maximum number of reader lock table slots
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified, or the environment is already open.
 	 * </ul>
 	 */
 int  mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);
 
 	/** @brief Get the maximum number of threads/reader slots for the environment.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] readers Address of an integer to store the number of readers
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_env_get_maxreaders(MDB_env *env, unsigned int *readers);
 
 	/** @brief Set the maximum number of named databases for the environment.
 	 *
 	 * This function is only needed if multiple databases will be used in the
 	 * environment. Simpler applications that use the environment as a single
 	 * unnamed database can ignore this option.
 	 * This function may only be called after #mdb_env_create() and before #mdb_env_open().
 	 *
 	 * Currently a moderate number of slots are cheap but a huge number gets
 	 * expensive: 7-120 words per transaction, and every #mdb_dbi_open()
 	 * does a linear search of the opened slots.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] dbs The maximum number of databases
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified, or the environment is already open.
 	 * </ul>
 	 */
 int  mdb_env_set_maxdbs(MDB_env *env, MDB_dbi dbs);
 
 	/** @brief Get the maximum size of keys and #MDB_DUPSORT data we can write.
 	 *
 	 * Depends on the compile-time constant #MDB_MAXKEYSIZE. Default 511.
 	 * See @ref MDB_val.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @return The maximum size of a key we can write
 	 */
 int  mdb_env_get_maxkeysize(MDB_env *env);
 
 	/** @brief Set application information associated with the #MDB_env.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] ctx An arbitrary pointer for whatever the application needs.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_set_userctx(MDB_env *env, void *ctx);
 
 	/** @brief Get the application information associated with the #MDB_env.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @return The pointer set by #mdb_env_set_userctx().
 	 */
 void *mdb_env_get_userctx(MDB_env *env);
 
 	/** @brief A callback function for most LMDB assert() failures,
 	 * called before printing the message and aborting.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create().
 	 * @param[in] msg The assertion message, not including newline.
 	 */
 typedef void MDB_assert_func(MDB_env *env, const char *msg);
 
 	/** Set or reset the assert() callback of the environment.
 	 * Disabled if liblmdb is built with NDEBUG.
 	 * @note This hack should become obsolete as lmdb's error handling matures.
 	 * @param[in] env An environment handle returned by #mdb_env_create().
 	 * @param[in] func An #MDB_assert_func function, or 0.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_env_set_assert(MDB_env *env, MDB_assert_func *func);
 
 	/** @brief Create a transaction for use with the environment.
 	 *
 	 * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
 	 * @note A transaction and its cursors must only be used by a single
 	 * thread, and a thread may only have a single transaction at a time.
 	 * If #MDB_NOTLS is in use, this does not apply to read-only transactions.
 	 * @note Cursors may not span transactions.
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] parent If this parameter is non-NULL, the new transaction
 	 * will be a nested transaction, with the transaction indicated by \b parent
 	 * as its parent. Transactions may be nested to any level. A parent
 	 * transaction and its cursors may not issue any other operations than
 	 * mdb_txn_commit and mdb_txn_abort while it has active child transactions.
 	 * @param[in] flags Special options for this transaction. This parameter
 	 * must be set to 0 or by bitwise OR'ing together one or more of the
 	 * values described here.
 	 * <ul>
 	 *	<li>#MDB_RDONLY
 	 *		This transaction will not perform any write operations.
 	 * </ul>
 	 * @param[out] txn Address where the new #MDB_txn handle will be stored
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_PANIC - a fatal error occurred earlier and the environment
 	 *		must be shut down.
 	 *	<li>#MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's
 	 *		mapsize and this environment's map must be resized as well.
 	 *		See #mdb_env_set_mapsize().
 	 *	<li>#MDB_READERS_FULL - a read-only transaction was requested and
 	 *		the reader lock table is full. See #mdb_env_set_maxreaders().
 	 *	<li>ENOMEM - out of memory.
 	 * </ul>
 	 */
 int  mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);
 
 	/** @brief Returns the transaction's #MDB_env
 	 *
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 */
 MDB_env *mdb_txn_env(MDB_txn *txn);
 
 	/** @brief Return the transaction's ID.
 	 *
 	 * This returns the identifier associated with this transaction. For a
 	 * read-only transaction, this corresponds to the snapshot being read;
 	 * concurrent readers will frequently have the same transaction ID.
 	 *
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @return A transaction ID, valid if input is an active transaction.
 	 */
 size_t mdb_txn_id(MDB_txn *txn);
 
 	/** @brief Commit all the operations of a transaction into the database.
 	 *
 	 * The transaction handle is freed. It and its cursors must not be used
 	 * again after this call, except with #mdb_cursor_renew().
 	 * @note Earlier documentation incorrectly said all cursors would be freed.
 	 * Only write-transactions free cursors.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 *	<li>ENOSPC - no more disk space.
 	 *	<li>EIO - a low-level I/O error occurred while writing.
 	 *	<li>ENOMEM - out of memory.
 	 * </ul>
 	 */
 int  mdb_txn_commit(MDB_txn *txn);
 
 	/** @brief Abandon all the operations of the transaction instead of saving them.
 	 *
 	 * The transaction handle is freed. It and its cursors must not be used
 	 * again after this call, except with #mdb_cursor_renew().
 	 * @note Earlier documentation incorrectly said all cursors would be freed.
 	 * Only write-transactions free cursors.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 */
 void mdb_txn_abort(MDB_txn *txn);
 
 	/** @brief Reset a read-only transaction.
 	 *
 	 * Abort the transaction like #mdb_txn_abort(), but keep the transaction
 	 * handle. #mdb_txn_renew() may reuse the handle. This saves allocation
 	 * overhead if the process will start a new read-only transaction soon,
 	 * and also locking overhead if #MDB_NOTLS is in use. The reader table
 	 * lock is released, but the table slot stays tied to its thread or
 	 * #MDB_txn. Use mdb_txn_abort() to discard a reset handle, and to free
 	 * its lock table slot if MDB_NOTLS is in use.
 	 * Cursors opened within the transaction must not be used
 	 * again after this call, except with #mdb_cursor_renew().
 	 * Reader locks generally don't interfere with writers, but they keep old
 	 * versions of database pages allocated. Thus they prevent the old pages
 	 * from being reused when writers commit new data, and so under heavy load
 	 * the database size may grow much more rapidly than otherwise.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 */
 void mdb_txn_reset(MDB_txn *txn);
 
 	/** @brief Renew a read-only transaction.
 	 *
 	 * This acquires a new reader lock for a transaction handle that had been
 	 * released by #mdb_txn_reset(). It must be called before a reset transaction
 	 * may be used again.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_PANIC - a fatal error occurred earlier and the environment
 	 *		must be shut down.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_txn_renew(MDB_txn *txn);
 
 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
 #define mdb_open(txn,name,flags,dbi)	mdb_dbi_open(txn,name,flags,dbi)
 /** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
 #define mdb_close(env,dbi)				mdb_dbi_close(env,dbi)
 
 	/** @brief Open a database in the environment.
 	 *
 	 * A database handle denotes the name and parameters of a database,
 	 * independently of whether such a database exists.
 	 * The database handle may be discarded by calling #mdb_dbi_close().
 	 * The old database handle is returned if the database was already open.
 	 * The handle may only be closed once.
 	 *
 	 * The database handle will be private to the current transaction until
 	 * the transaction is successfully committed. If the transaction is
 	 * aborted the handle will be closed automatically.
 	 * After a successful commit the handle will reside in the shared
 	 * environment, and may be used by other transactions.
 	 *
 	 * This function must not be called from multiple concurrent
 	 * transactions in the same process. A transaction that uses
 	 * this function must finish (either commit or abort) before
 	 * any other transaction in the process may use this function.
 	 *
 	 * To use named databases (with name != NULL), #mdb_env_set_maxdbs()
 	 * must be called before opening the environment.  Database names are
 	 * keys in the unnamed database, and may be read but not written.
 	 *
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] name The name of the database to open. If only a single
 	 * 	database is needed in the environment, this value may be NULL.
 	 * @param[in] flags Special options for this database. This parameter
 	 * must be set to 0 or by bitwise OR'ing together one or more of the
 	 * values described here.
 	 * <ul>
 	 *	<li>#MDB_REVERSEKEY
 	 *		Keys are strings to be compared in reverse order, from the end
 	 *		of the strings to the beginning. By default, Keys are treated as strings and
 	 *		compared from beginning to end.
 	 *	<li>#MDB_DUPSORT
 	 *		Duplicate keys may be used in the database. (Or, from another perspective,
 	 *		keys may have multiple data items, stored in sorted order.) By default
 	 *		keys must be unique and may have only a single data item.
 	 *	<li>#MDB_INTEGERKEY
 	 *		Keys are binary integers in native byte order, either unsigned int
 	 *		or size_t, and will be sorted as such.
 	 *		The keys must all be of the same size.
 	 *	<li>#MDB_DUPFIXED
 	 *		This flag may only be used in combination with #MDB_DUPSORT. This option
 	 *		tells the library that the data items for this database are all the same
 	 *		size, which allows further optimizations in storage and retrieval. When
 	 *		all data items are the same size, the #MDB_GET_MULTIPLE, #MDB_NEXT_MULTIPLE
 	 *		and #MDB_PREV_MULTIPLE cursor operations may be used to retrieve multiple
 	 *		items at once.
 	 *	<li>#MDB_INTEGERDUP
 	 *		This option specifies that duplicate data items are binary integers,
 	 *		similar to #MDB_INTEGERKEY keys.
 	 *	<li>#MDB_REVERSEDUP
 	 *		This option specifies that duplicate data items should be compared as
 	 *		strings in reverse order.
 	 *	<li>#MDB_CREATE
 	 *		Create the named database if it doesn't exist. This option is not
 	 *		allowed in a read-only transaction or a read-only environment.
 	 * </ul>
 	 * @param[out] dbi Address where the new #MDB_dbi handle will be stored
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
 	 *		and #MDB_CREATE was not specified.
 	 *	<li>#MDB_DBS_FULL - too many databases have been opened. See #mdb_env_set_maxdbs().
 	 * </ul>
 	 */
 int  mdb_dbi_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
 
 	/** @brief Retrieve statistics for a database.
 	 *
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[out] stat The address of an #MDB_stat structure
 	 * 	where the statistics will be copied
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
 
 	/** @brief Retrieve the DB flags for a database handle.
 	 *
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[out] flags Address where the flags will be returned.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int mdb_dbi_flags(MDB_txn *txn, MDB_dbi dbi, unsigned int *flags);
 
 	/** @brief Close a database handle. Normally unnecessary. Use with care:
 	 *
 	 * This call is not mutex protected. Handles should only be closed by
 	 * a single thread, and only if no other threads are going to reference
 	 * the database handle or one of its cursors any further. Do not close
 	 * a handle if an existing transaction has modified its database.
 	 * Doing so can cause misbehavior from database corruption to errors
 	 * like MDB_BAD_VALSIZE (since the DB name is gone).
 	 *
 	 * Closing a database handle is not necessary, but lets #mdb_dbi_open()
 	 * reuse the handle value.  Usually it's better to set a bigger
 	 * #mdb_env_set_maxdbs(), unless that value would be large.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 */
 void mdb_dbi_close(MDB_env *env, MDB_dbi dbi);
 
 	/** @brief Empty or delete+close a database.
 	 *
 	 * See #mdb_dbi_close() for restrictions about closing the DB handle.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] del 0 to empty the DB, 1 to delete it from the
 	 * environment and close the DB handle.
 	 * @return A non-zero error value on failure and 0 on success.
 	 */
 int  mdb_drop(MDB_txn *txn, MDB_dbi dbi, int del);
 
 	/** @brief Set a custom key comparison function for a database.
 	 *
 	 * The comparison function is called whenever it is necessary to compare a
 	 * key specified by the application with a key currently stored in the database.
 	 * If no comparison function is specified, and no special key flags were specified
 	 * with #mdb_dbi_open(), the keys are compared lexically, with shorter keys collating
 	 * before longer keys.
 	 * @warning This function must be called before any data access functions are used,
 	 * otherwise data corruption may occur. The same comparison function must be used by every
 	 * program accessing the database, every time the database is used.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] cmp A #MDB_cmp_func function
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
 
 	/** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
 	 *
 	 * This comparison function is called whenever it is necessary to compare a data
 	 * item specified by the application with a data item currently stored in the database.
 	 * This function only takes effect if the database was opened with the #MDB_DUPSORT
 	 * flag.
 	 * If no comparison function is specified, and no special key flags were specified
 	 * with #mdb_dbi_open(), the data items are compared lexically, with shorter items collating
 	 * before longer items.
 	 * @warning This function must be called before any data access functions are used,
 	 * otherwise data corruption may occur. The same comparison function must be used by every
 	 * program accessing the database, every time the database is used.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] cmp A #MDB_cmp_func function
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
 
 	/** @brief Set a relocation function for a #MDB_FIXEDMAP database.
 	 *
 	 * @todo The relocation function is called whenever it is necessary to move the data
 	 * of an item to a different position in the database (e.g. through tree
 	 * balancing operations, shifts as a result of adds or deletes, etc.). It is
 	 * intended to allow address/position-dependent data items to be stored in
 	 * a database in an environment opened with the #MDB_FIXEDMAP option.
 	 * Currently the relocation feature is unimplemented and setting
 	 * this function has no effect.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] rel A #MDB_rel_func function
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
 
 	/** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
 	 *
 	 * See #mdb_set_relfunc and #MDB_rel_func for more details.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] ctx An arbitrary pointer for whatever the application needs.
 	 * It will be passed to the callback function set by #mdb_set_relfunc
 	 * as its \b relctx parameter whenever the callback is invoked.
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);
 
 	/** @brief Get items from a database.
 	 *
 	 * This function retrieves key/data pairs from the database. The address
 	 * and length of the data associated with the specified \b key are returned
 	 * in the structure to which \b data refers.
 	 * If the database supports duplicate keys (#MDB_DUPSORT) then the
 	 * first data item for the key will be returned. Retrieval of other
 	 * items requires the use of #mdb_cursor_get().
 	 *
 	 * @note The memory pointed to by the returned values is owned by the
 	 * database. The caller need not dispose of the memory, and may not
 	 * modify it in any way. For values returned in a read-only transaction
 	 * any modification attempts will cause a SIGSEGV.
 	 * @note Values returned from the database are valid only until a
 	 * subsequent update operation, or the end of the transaction.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] key The key to search for in the database
 	 * @param[out] data The data corresponding to the key
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_NOTFOUND - the key was not in the database.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
 
 	/** @brief Store items into a database.
 	 *
 	 * This function stores key/data pairs in the database. The default behavior
 	 * is to enter the new key/data pair, replacing any previously existing key
 	 * if duplicates are disallowed, or adding a duplicate data item if
 	 * duplicates are allowed (#MDB_DUPSORT).
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] key The key to store in the database
 	 * @param[in,out] data The data to store
 	 * @param[in] flags Special options for this operation. This parameter
 	 * must be set to 0 or by bitwise OR'ing together one or more of the
 	 * values described here.
 	 * <ul>
 	 *	<li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
 	 *		already appear in the database. This flag may only be specified
 	 *		if the database was opened with #MDB_DUPSORT. The function will
 	 *		return #MDB_KEYEXIST if the key/data pair already appears in the
 	 *		database.
 	 *	<li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
 	 *		does not already appear in the database. The function will return
 	 *		#MDB_KEYEXIST if the key already appears in the database, even if
 	 *		the database supports duplicates (#MDB_DUPSORT). The \b data
 	 *		parameter will be set to point to the existing item.
 	 *	<li>#MDB_RESERVE - reserve space for data of the given size, but
 	 *		don't copy the given data. Instead, return a pointer to the
 	 *		reserved space, which the caller can fill in later - before
 	 *		the next update operation or the transaction ends. This saves
 	 *		an extra memcpy if the data is being generated later.
 	 *		LMDB does nothing else with this memory, the caller is expected
 	 *		to modify all of the space requested. This flag must not be
 	 *		specified if the database was opened with #MDB_DUPSORT.
 	 *	<li>#MDB_APPEND - append the given key/data pair to the end of the
 	 *		database. This option allows fast bulk loading when keys are
 	 *		already known to be in the correct order. Loading unsorted keys
 	 *		with this flag will cause a #MDB_KEYEXIST error.
 	 *	<li>#MDB_APPENDDUP - as above, but for sorted dup data.
 	 * </ul>
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
 	 *	<li>#MDB_TXN_FULL - the transaction has too many dirty pages.
 	 *	<li>EACCES - an attempt was made to write in a read-only transaction.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
 			    unsigned int flags);
 
 	/** @brief Delete items from a database.
 	 *
 	 * This function removes key/data pairs from the database.
 	 * If the database does not support sorted duplicate data items
 	 * (#MDB_DUPSORT) the data parameter is ignored.
 	 * If the database supports sorted duplicates and the data parameter
 	 * is NULL, all of the duplicate data items for the key will be
 	 * deleted. Otherwise, if the data parameter is non-NULL
 	 * only the matching data item will be deleted.
 	 * This function will return #MDB_NOTFOUND if the specified key/data
 	 * pair is not in the database.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] key The key to delete from the database
 	 * @param[in] data The data to delete
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EACCES - an attempt was made to write in a read-only transaction.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
 
 	/** @brief Create a cursor handle.
 	 *
 	 * A cursor is associated with a specific transaction and database.
 	 * A cursor cannot be used when its database handle is closed.  Nor
 	 * when its transaction has ended, except with #mdb_cursor_renew().
 	 * It can be discarded with #mdb_cursor_close().
 	 * A cursor in a write-transaction can be closed before its transaction
 	 * ends, and will otherwise be closed when its transaction ends.
 	 * A cursor in a read-only transaction must be closed explicitly, before
 	 * or after its transaction ends. It can be reused with
 	 * #mdb_cursor_renew() before finally closing it.
 	 * @note Earlier documentation said that cursors in every transaction
 	 * were closed when the transaction committed or aborted.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[out] cursor Address where the new #MDB_cursor handle will be stored
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
 
 	/** @brief Close a cursor handle.
 	 *
 	 * The cursor handle will be freed and must not be used again after this call.
 	 * Its transaction must still be live if it is a write-transaction.
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 */
 void mdb_cursor_close(MDB_cursor *cursor);
 
 	/** @brief Renew a cursor handle.
 	 *
 	 * A cursor is associated with a specific transaction and database.
 	 * Cursors that are only used in read-only
 	 * transactions may be re-used, to avoid unnecessary malloc/free overhead.
 	 * The cursor may be associated with a new read-only transaction, and
 	 * referencing the same database handle as it was created with.
 	 * This may be done whether the previous transaction is live or dead.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);
 
 	/** @brief Return the cursor's transaction handle.
 	 *
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 */
 MDB_txn *mdb_cursor_txn(MDB_cursor *cursor);
 
 	/** @brief Return the cursor's database handle.
 	 *
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 */
 MDB_dbi mdb_cursor_dbi(MDB_cursor *cursor);
 
 	/** @brief Retrieve by cursor.
 	 *
 	 * This function retrieves key/data pairs from the database. The address and length
 	 * of the key are returned in the object to which \b key refers (except for the
 	 * case of the #MDB_SET option, in which the \b key object is unchanged), and
 	 * the address and length of the data are returned in the object to which \b data
 	 * refers.
 	 * See #mdb_get() for restrictions on using the output values.
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 * @param[in,out] key The key for a retrieved item
 	 * @param[in,out] data The data of a retrieved item
 	 * @param[in] op A cursor operation #MDB_cursor_op
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_NOTFOUND - no matching key found.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
 			    MDB_cursor_op op);
 
 	/** @brief Store by cursor.
 	 *
 	 * This function stores key/data pairs into the database.
 	 * The cursor is positioned at the new item, or on failure usually near it.
 	 * @note Earlier documentation incorrectly said errors would leave the
 	 * state of the cursor unchanged.
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 * @param[in] key The key operated on.
 	 * @param[in] data The data operated on.
 	 * @param[in] flags Options for this operation. This parameter
 	 * must be set to 0 or one of the values described here.
 	 * <ul>
 	 *	<li>#MDB_CURRENT - replace the item at the current cursor position.
 	 *		The \b key parameter must still be provided, and must match it.
 	 *		If using sorted duplicates (#MDB_DUPSORT) the data item must still
 	 *		sort into the same place. This is intended to be used when the
 	 *		new data is the same size as the old. Otherwise it will simply
 	 *		perform a delete of the old record followed by an insert.
 	 *	<li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
 	 *		already appear in the database. This flag may only be specified
 	 *		if the database was opened with #MDB_DUPSORT. The function will
 	 *		return #MDB_KEYEXIST if the key/data pair already appears in the
 	 *		database.
 	 *	<li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
 	 *		does not already appear in the database. The function will return
 	 *		#MDB_KEYEXIST if the key already appears in the database, even if
 	 *		the database supports duplicates (#MDB_DUPSORT).
 	 *	<li>#MDB_RESERVE - reserve space for data of the given size, but
 	 *		don't copy the given data. Instead, return a pointer to the
 	 *		reserved space, which the caller can fill in later - before
 	 *		the next update operation or the transaction ends. This saves
 	 *		an extra memcpy if the data is being generated later. This flag
 	 *		must not be specified if the database was opened with #MDB_DUPSORT.
 	 *	<li>#MDB_APPEND - append the given key/data pair to the end of the
 	 *		database. No key comparisons are performed. This option allows
 	 *		fast bulk loading when keys are already known to be in the
 	 *		correct order. Loading unsorted keys with this flag will cause
 	 *		a #MDB_KEYEXIST error.
 	 *	<li>#MDB_APPENDDUP - as above, but for sorted dup data.
 	 *	<li>#MDB_MULTIPLE - store multiple contiguous data elements in a
 	 *		single request. This flag may only be specified if the database
 	 *		was opened with #MDB_DUPFIXED. The \b data argument must be an
 	 *		array of two MDB_vals. The mv_size of the first MDB_val must be
 	 *		the size of a single data element. The mv_data of the first MDB_val
 	 *		must point to the beginning of the array of contiguous data elements.
 	 *		The mv_size of the second MDB_val must be the count of the number
 	 *		of data elements to store. On return this field will be set to
 	 *		the count of the number of elements actually written. The mv_data
 	 *		of the second MDB_val is unused.
 	 * </ul>
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
 	 *	<li>#MDB_TXN_FULL - the transaction has too many dirty pages.
 	 *	<li>EACCES - an attempt was made to write in a read-only transaction.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
 				unsigned int flags);
 
 	/** @brief Delete current key/data pair
 	 *
 	 * This function deletes the key/data pair to which the cursor refers.
 	 * This does not invalidate the cursor, so operations such as MDB_NEXT
 	 * can still be used on it.
 	 * Both MDB_NEXT and MDB_GET_CURRENT will return the same record after
 	 * this operation.
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 * @param[in] flags Options for this operation. This parameter
 	 * must be set to 0 or one of the values described here.
 	 * <ul>
 	 *	<li>#MDB_NODUPDATA - delete all of the data items for the current key.
 	 *		This flag may only be specified if the database was opened with #MDB_DUPSORT.
 	 * </ul>
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EACCES - an attempt was made to write in a read-only transaction.
 	 *	<li>EINVAL - an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);
 
 	/** @brief Return count of duplicates for current key.
 	 *
 	 * This call is only valid on databases that support sorted duplicate
 	 * data items #MDB_DUPSORT.
 	 * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
 	 * @param[out] countp Address where the count will be stored
 	 * @return A non-zero error value on failure and 0 on success. Some possible
 	 * errors are:
 	 * <ul>
 	 *	<li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
 	 * </ul>
 	 */
 int  mdb_cursor_count(MDB_cursor *cursor, size_t *countp);
 
 	/** @brief Compare two data items according to a particular database.
 	 *
 	 * This returns a comparison as if the two data items were keys in the
 	 * specified database.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] a The first item to compare
 	 * @param[in] b The second item to compare
 	 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
 	 */
 int  mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
 
 	/** @brief Compare two data items according to a particular database.
 	 *
 	 * This returns a comparison as if the two items were data items of
 	 * the specified database. The database must have the #MDB_DUPSORT flag.
 	 * @param[in] txn A transaction handle returned by #mdb_txn_begin()
 	 * @param[in] dbi A database handle returned by #mdb_dbi_open()
 	 * @param[in] a The first item to compare
 	 * @param[in] b The second item to compare
 	 * @return < 0 if a < b, 0 if a == b, > 0 if a > b
 	 */
 int  mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
 
 	/** @brief A callback function used to print a message from the library.
 	 *
 	 * @param[in] msg The string to be printed.
 	 * @param[in] ctx An arbitrary context pointer for the callback.
 	 * @return < 0 on failure, >= 0 on success.
 	 */
 typedef int (MDB_msg_func)(const char *msg, void *ctx);
 
 	/** @brief Dump the entries in the reader lock table.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[in] func A #MDB_msg_func function
 	 * @param[in] ctx Anything the message function needs
 	 * @return < 0 on failure, >= 0 on success.
 	 */
 int	mdb_reader_list(MDB_env *env, MDB_msg_func *func, void *ctx);
 
 	/** @brief Check for stale entries in the reader lock table.
 	 *
 	 * @param[in] env An environment handle returned by #mdb_env_create()
 	 * @param[out] dead Number of stale slots that were cleared
 	 * @return 0 on success, non-zero on failure.
 	 */
 int	mdb_reader_check(MDB_env *env, int *dead);
 /**	@} */
 
 #ifdef __cplusplus
 }
 #endif
 /** @page tools LMDB Command Line Tools
 	The following describes the command line tools that are available for LMDB.
 	\li \ref mdb_copy_1
 	\li \ref mdb_dump_1
 	\li \ref mdb_load_1
 	\li \ref mdb_stat_1
 */
 
 #endif /* _LMDB_H_ */