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  • /** @file lmdb.h
     *	@brief Lightning memory-mapped database library
    
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     *	@mainpage	Lightning Memory-Mapped Database Manager (LMDB)
    
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     *
     *	@section intro_sec Introduction
    
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     *	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
    
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     *	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
    
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     *	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.
     *
    
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     *	@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
    
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     *	  #mdb_reader_check function or the \ref mdb_stat_1 "mdb_stat" tool. Or 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().
     *
    
     *	- By default, in versions before 0.9.10, unused portions of the data
     *	  file might receive garbage data from memory freed by other code.
    
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     *	  (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
    
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     *	  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.
    
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     *	  The #MDB_NOTLS flag changes this for read-only transactions.
    
     *
     *	- Use an MDB_env* in the process which opened it, without fork()ing.
     *
    
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     *	- 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 flock() advisory locking.
     *
     *	- 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.
    
     *	- 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.
    
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     *	- 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.
     *
    
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     *	@copyright Copyright 2011-2015 Howard Chu, Symas Corp. All rights reserved.
    
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     *
     * 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>.
     *
    
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     *	@par Derived From:
    
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     * 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_
    
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    #include <sys/types.h>
    
    
    #ifdef __cplusplus
    extern "C" {
    #endif
    
    
    /** Unix permissions for creating files, or dummy definition for Windows */
    
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    #ifdef _MSC_VER
    
    typedef	int	mdb_mode_t;
    #else
    typedef	mode_t	mdb_mode_t;
    
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    #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;
    
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    #endif
    
    
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    /** @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 */
    
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    #define MDB_VERSION_MINOR	9
    
    /** Library patch version */
    
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    #define MDB_VERSION_PATCH	16
    
    
    /** 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 */
    
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    #define MDB_VERSION_FULL	\
    	MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)
    
    
    /** The release date of this library version */
    
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    #define MDB_VERSION_DATE	"August 14, 2015"
    
    
    /** A stringifier for the version info */
    
    #define MDB_VERSTR(a,b,c,d)	"LMDB " #a "." #b "." #c ": (" d ")"
    
    
    /** A helper for the stringifier macro */
    
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    #define MDB_VERFOO(a,b,c,d)	MDB_VERSTR(a,b,c,d)
    
    
    /** The full library version as a C string */
    
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    #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.
    
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     */
    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.
    
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    typedef struct MDB_txn MDB_txn;
    
    /** @brief A handle for an individual database in the DB environment. */
    
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    typedef unsigned int	MDB_dbi;
    
    
    /** @brief Opaque structure for navigating through a database */
    
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    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.
    
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    typedef struct MDB_val {
    
    	size_t		 mv_size;	/**< size of the data item */
    	void		*mv_data;	/**< address of the data item */
    
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    } MDB_val;
    
    
    /** @brief A callback function used to compare two keys in a database */
    
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    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
    
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     * 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().
    
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     * @todo This feature is currently unimplemented.
    
    typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
    
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    /** @defgroup	mdb_env	Environment Flags
    
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     *	@{
     */
    
    	/** mmap at a fixed address (experimental) */
    
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    #define MDB_FIXEDMAP	0x01
    
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    	/** no environment directory */
    
    #define MDB_NOSUBDIR	0x4000
    
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    	/** 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
    
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    	/** use asynchronous msync when #MDB_WRITEMAP is used */
    
    #define MDB_MAPASYNC		0x100000
    
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    	/** tie reader locktable slots to #MDB_txn objects instead of to threads */
    #define MDB_NOTLS		0x200000
    
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    	/** 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
    
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    /** @} */
    
    
    /**	@defgroup	mdb_dbi_open	Database Flags
    
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     *	@{
     */
    	/** 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. */
    
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    #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 */
    
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    #define MDB_INTEGERDUP	0x20
    
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    	/** with #MDB_DUPSORT, use reverse string dups */
    #define MDB_REVERSEDUP	0x40
    
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    	/** create DB if not already existing */
    #define MDB_CREATE		0x40000
    /** @} */
    
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    /**	@defgroup mdb_put	Write Flags
    
    /** For put: Don't write if the key already exists. */
    
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    #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.
    
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    #define MDB_NODUPDATA	0x20
    
    /** For mdb_cursor_put: overwrite the current key/data pair */
    #define MDB_CURRENT	0x40
    
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    /** 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
    
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    /** 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 */
    
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    	MDB_GET_CURRENT,		/**< Return key/data at current cursor position */
    
    	MDB_GET_MULTIPLE,		/**< Return key and 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 key and 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 */
    
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    	MDB_SET_KEY,			/**< Position at specified key, return key + data */
    
    	MDB_SET_RANGE			/**< Position at first key greater than or equal to specified key. */
    
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    } MDB_cursor_op;
    
    
    /** @defgroup  errors	Return Codes
     *
     *	BerkeleyDB uses -30800 to -30999, we'll go under them
     *	@{
     */
    	/**	Successful result */
    
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    #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, probably I/O error */
    
    #define MDB_PANIC		(-30795)
    	/** Environment version mismatch */
    #define MDB_VERSION_MISMATCH	(-30794)
    
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    	/** 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 cannot recover - it must be aborted */
    #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 */
    
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    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 */
    
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    } 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 */
    
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    	unsigned int me_maxreaders;		/**< max reader slots in the environment */
    	unsigned int me_numreaders;		/**< max reader slots used in the environment */
    
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    	/** @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
    	 */
    
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    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
    
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    	 * 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);
    
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    	/** @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.
    	 */
    
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    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()
    
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    	 * @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
    
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    	 *		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
    
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    	 *		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 is faster
    	 *		and uses fewer mallocs, but loses protection from application bugs
    	 *		like wild pointer writes and other bad updates into the database.
    	 *		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().
    
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    	 *	<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 reseved 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
    
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    	 *		thread, since LMDB's write locking is unaware of the user threads.
    
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    	 *	<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
    
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    	 *		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().
    
    	 * @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>
    
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    	 *	<li>#MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the
    
    	 *	version that created the database environment.
    
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    	 *	<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);
    
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    	/** @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);
    
    
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    	/** @brief Copy an LMDB environment to the specified file descriptor.
    
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    	 *
    	 * 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.
    
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    	 * @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);
    
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    	/** @brief Copy an LMDB environment to the specified path, with options.
    
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    	 *
    	 * This function may be used to make a backup of an existing environment.
    
    	 * No lockfile is created, since it gets recreated at need.
    
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    	 * @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.
    	 * </ul>
    
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    	 * @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);
    
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    	/** @brief Copy an LMDB environment to the specified file descriptor,
    
    	 *	with options.
    
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    	 *
    	 * 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.
    
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    	 * @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);
    
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    	/** @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
    	 */
    
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    int  mdb_env_stat(MDB_env *env, MDB_stat *stat);
    
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    	/** @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,
    
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    	 * 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>
    	 */
    
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    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()
    	 */
    
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    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>
    	 */
    
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    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>
    	 */
    
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    int  mdb_env_get_path(MDB_env *env, const char **path);
    
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    	/** @brief Return the filedescriptor for the given environment.
    	 *
    	 * @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
    
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    	 * 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,
    
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    	 * 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.
    
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    	 * 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.
    
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    int  mdb_env_set_mapsize(MDB_env *env, size_t size);
    
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    	/** @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.
    
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    	 * 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()
    
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    	 * @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);
    
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    	/** @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);
    
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    	/** @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.
    
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    	 * @param[in] env An environment handle returned by #mdb_env_create()
    
    	 * @return The maximum size of a key we can write
    
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    	 */
    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);
    
    
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    	/** @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 buillt 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().
    
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    	 * @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().
    
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    	 * @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
    
    	 *	<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().
    
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    	 *	<li>#MDB_READERS_FULL - a read-only transaction was requested and
    
    	 *		the reader lock table is full. See #mdb_env_set_maxreaders().
    
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    	 *	<li>ENOMEM - out of memory.
    
    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 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.
    
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    	 *	<li>ENOMEM - out of memory.
    
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    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()
    	 */
    
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    void mdb_txn_abort(MDB_txn *txn);
    
    
    	/** @brief Reset a read-only transaction.
    	 *
    
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    	 * 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()
    	 */