mdb.c 258 KB
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/** @file mdb.c
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 *	@brief Lightning memory-mapped database library
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 *
 *	A Btree-based database management library modeled loosely on the
 *	BerkeleyDB API, but much simplified.
 */
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/*
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 * Copyright 2011-2014 Howard Chu, Symas Corp.
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 * 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>.
 *
 * 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.
 */
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#ifndef _GNU_SOURCE
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#define _GNU_SOURCE 1
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#endif
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#ifdef _WIN32
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#include <malloc.h>
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#include <windows.h>
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/** getpid() returns int; MinGW defines pid_t but MinGW64 typedefs it
 *  as int64 which is wrong. MSVC doesn't define it at all, so just
 *  don't use it.
 */
#define MDB_PID_T	int
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#define MDB_THR_T	DWORD
#include <sys/types.h>
#include <sys/stat.h>
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#ifdef __GNUC__
# include <sys/param.h>
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#else
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# define LITTLE_ENDIAN	1234
# define BIG_ENDIAN	4321
# define BYTE_ORDER	LITTLE_ENDIAN
# ifndef SSIZE_MAX
#  define SSIZE_MAX	INT_MAX
# endif
#endif
#else
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#include <sys/types.h>
#include <sys/stat.h>
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#define MDB_PID_T	pid_t
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#define MDB_THR_T	pthread_t
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#include <sys/param.h>
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#include <sys/uio.h>
#include <sys/mman.h>
#ifdef HAVE_SYS_FILE_H
#include <sys/file.h>
#endif
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#include <fcntl.h>
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#endif
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#if defined(__mips) && defined(__linux)
/* MIPS has cache coherency issues, requires explicit cache control */
#include <asm/cachectl.h>
extern int cacheflush(char *addr, int nbytes, int cache);
#define CACHEFLUSH(addr, bytes, cache)	cacheflush(addr, bytes, cache)
#else
#define CACHEFLUSH(addr, bytes, cache)
#endif

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#include <errno.h>
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#include <limits.h>
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#include <stddef.h>
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#include <inttypes.h>
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
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#if defined(__sun) || defined(ANDROID)
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/* Most platforms have posix_memalign, older may only have memalign */
#define HAVE_MEMALIGN	1
#include <malloc.h>
#endif

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#if !(defined(BYTE_ORDER) || defined(__BYTE_ORDER))
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#include <netinet/in.h>
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#include <resolv.h>	/* defines BYTE_ORDER on HPUX and Solaris */
#endif

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#if defined(__APPLE__) || defined (BSD)
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# define MDB_USE_SYSV_SEM	1
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# define MDB_FDATASYNC		fsync
#elif defined(ANDROID)
# define MDB_FDATASYNC		fsync
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#endif

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#ifndef _WIN32
#include <pthread.h>
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#ifdef MDB_USE_SYSV_SEM
#include <sys/ipc.h>
#include <sys/sem.h>
#ifdef _SEM_SEMUN_UNDEFINED
union semun {
	int val;
	struct semid_ds *buf;
	unsigned short *array;
};
#endif /* _SEM_SEMUN_UNDEFINED */
#endif /* MDB_USE_SYSV_SEM */
#endif /* !_WIN32 */
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#ifdef USE_VALGRIND
#include <valgrind/memcheck.h>
#define VGMEMP_CREATE(h,r,z)    VALGRIND_CREATE_MEMPOOL(h,r,z)
#define VGMEMP_ALLOC(h,a,s) VALGRIND_MEMPOOL_ALLOC(h,a,s)
#define VGMEMP_FREE(h,a) VALGRIND_MEMPOOL_FREE(h,a)
#define VGMEMP_DESTROY(h)	VALGRIND_DESTROY_MEMPOOL(h)
#define VGMEMP_DEFINED(a,s)	VALGRIND_MAKE_MEM_DEFINED(a,s)
#else
#define VGMEMP_CREATE(h,r,z)
#define VGMEMP_ALLOC(h,a,s)
#define VGMEMP_FREE(h,a)
#define VGMEMP_DESTROY(h)
#define VGMEMP_DEFINED(a,s)
#endif

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#ifndef BYTE_ORDER
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# if (defined(_LITTLE_ENDIAN) || defined(_BIG_ENDIAN)) && !(defined(_LITTLE_ENDIAN) && defined(_BIG_ENDIAN))
/* Solaris just defines one or the other */
#  define LITTLE_ENDIAN	1234
#  define BIG_ENDIAN	4321
#  ifdef _LITTLE_ENDIAN
#   define BYTE_ORDER  LITTLE_ENDIAN
#  else
#   define BYTE_ORDER  BIG_ENDIAN
#  endif
# else
#  define BYTE_ORDER   __BYTE_ORDER
# endif
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#endif
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#ifndef LITTLE_ENDIAN
#define LITTLE_ENDIAN	__LITTLE_ENDIAN
#endif
#ifndef BIG_ENDIAN
#define BIG_ENDIAN	__BIG_ENDIAN
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#endif

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#if defined(__i386) || defined(__x86_64) || defined(_M_IX86)
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#define MISALIGNED_OK	1
#endif

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#include "lmdb.h"
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#include "midl.h"

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#if (BYTE_ORDER == LITTLE_ENDIAN) == (BYTE_ORDER == BIG_ENDIAN)
# error "Unknown or unsupported endianness (BYTE_ORDER)"
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#elif (-6 & 5) || CHAR_BIT != 8 || UINT_MAX < 0xffffffff || ULONG_MAX % 0xFFFF
# error "Two's complement, reasonably sized integer types, please"
#endif

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#ifdef __GNUC__
/** Put infrequently used env functions in separate section */
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# ifdef __APPLE__
#  define	ESECT	__attribute__ ((section("__TEXT,text_env")))
# else
#  define	ESECT	__attribute__ ((section("text_env")))
# endif
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#else
#define ESECT
#endif

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/** @defgroup internal	LMDB Internals
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 *	@{
 */
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/** @defgroup compat	Compatibility Macros
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 *	A bunch of macros to minimize the amount of platform-specific ifdefs
 *	needed throughout the rest of the code. When the features this library
 *	needs are similar enough to POSIX to be hidden in a one-or-two line
 *	replacement, this macro approach is used.
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 *	@{
 */
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	/** Features under development */
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#ifndef MDB_DEVEL
#define MDB_DEVEL 0
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#endif

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#if defined(WIN32) || (defined(EOWNERDEAD) && !defined(MDB_USE_SYSV_SEM))
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#define MDB_ROBUST_SUPPORTED	1
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#endif

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	/** Wrapper around __func__, which is a C99 feature */
#if __STDC_VERSION__ >= 199901L
# define mdb_func_	__func__
#elif __GNUC__ >= 2 || _MSC_VER >= 1300
# define mdb_func_	__FUNCTION__
#else
/* If a debug message says <mdb_unknown>(), update the #if statements above */
# define mdb_func_	"<mdb_unknown>"
#endif

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#ifdef _WIN32
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#define MDB_USE_HASH	1
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#define MDB_PIDLOCK	0
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#define THREAD_RET	DWORD
#define pthread_t	HANDLE
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#define pthread_mutex_t	HANDLE
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#define pthread_cond_t	HANDLE
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typedef HANDLE mdb_mutex_t;
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#define pthread_key_t	DWORD
#define pthread_self()	GetCurrentThreadId()
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#define pthread_key_create(x,y)	\
	((*(x) = TlsAlloc()) == TLS_OUT_OF_INDEXES ? ErrCode() : 0)
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#define pthread_key_delete(x)	TlsFree(x)
#define pthread_getspecific(x)	TlsGetValue(x)
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#define pthread_setspecific(x,y)	(TlsSetValue(x,y) ? 0 : ErrCode())
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#define pthread_mutex_consistent(mutex) 0
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#define pthread_mutex_unlock(x)	ReleaseMutex(*x)
#define pthread_mutex_lock(x)	WaitForSingleObject(*x, INFINITE)
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#define pthread_cond_signal(x)	SetEvent(*x)
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#define pthread_cond_wait(cond,mutex)	do{SignalObjectAndWait(*mutex, *cond, INFINITE, FALSE); WaitForSingleObject(*mutex, INFINITE);}while(0)
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#define THREAD_CREATE(thr,start,arg)	thr=CreateThread(NULL,0,start,arg,0,NULL)
#define THREAD_FINISH(thr)	WaitForSingleObject(thr, INFINITE)
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#define MDB_MUTEX(env, rw)		((env)->me_##rw##mutex)
#define LOCK_MUTEX0(mutex)		WaitForSingleObject(mutex, INFINITE)
#define UNLOCK_MUTEX(mutex)		ReleaseMutex(mutex)
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#define getpid()	GetCurrentProcessId()
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#define	MDB_FDATASYNC(fd)	(!FlushFileBuffers(fd))
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#define	MDB_MSYNC(addr,len,flags)	(!FlushViewOfFile(addr,len))
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#define	ErrCode()	GetLastError()
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#define GET_PAGESIZE(x) {SYSTEM_INFO si; GetSystemInfo(&si); (x) = si.dwPageSize;}
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#define	close(fd)	(CloseHandle(fd) ? 0 : -1)
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#define	munmap(ptr,len)	UnmapViewOfFile(ptr)
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#ifdef PROCESS_QUERY_LIMITED_INFORMATION
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#define MDB_PROCESS_QUERY_LIMITED_INFORMATION PROCESS_QUERY_LIMITED_INFORMATION
#else
#define MDB_PROCESS_QUERY_LIMITED_INFORMATION 0x1000
#endif
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#define	Z	"I"
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#else
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#define THREAD_RET	void *
#define THREAD_CREATE(thr,start,arg)	pthread_create(&thr,NULL,start,arg)
#define THREAD_FINISH(thr)	pthread_join(thr,NULL)
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#define	Z	"z"			/**< printf format modifier for size_t */
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	/** For MDB_LOCK_FORMAT: True if readers take a pid lock in the lockfile */
#define MDB_PIDLOCK			1

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#ifdef MDB_USE_SYSV_SEM
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typedef struct mdb_mutex {
	int semid;
	int semnum;
} mdb_mutex_t;

#define MDB_MUTEX(env, rw)		(&(env)->me_##rw##mutex)
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#define LOCK_MUTEX0(mutex)		mdb_sem_wait(mutex)
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#define UNLOCK_MUTEX(mutex)		do { struct sembuf sb = { mutex->semnum, 1, SEM_UNDO }; semop(mutex->semid, &sb, 1); } while(0)
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static int
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mdb_sem_wait(mdb_mutex_t *sem)
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{
   int rc;
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   struct sembuf sb = { sem->semnum, -1, SEM_UNDO };
   while ((rc = semop(sem->semid, &sb, 1)) && (rc = errno) == EINTR) ;
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   return rc;
}

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#else
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	/** Pointer/HANDLE type of shared mutex/semaphore.
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	 */
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typedef pthread_mutex_t mdb_mutex_t;
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	/** Mutex for the reader table (rw = r) or write transaction (rw = w).
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	 */
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#define MDB_MUTEX(env, rw)	(&(env)->me_txns->mti_##rw##mutex)
	/** Lock the reader or writer mutex.
	 *	Returns 0 or a code to give #mdb_mutex_failed(), as in #LOCK_MUTEX().
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	 */
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#define LOCK_MUTEX0(mutex)	pthread_mutex_lock(mutex)
	/** Unlock the reader or writer mutex.
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	 */
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#define UNLOCK_MUTEX(mutex)	pthread_mutex_unlock(mutex)
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#endif	/* MDB_USE_SYSV_SEM */
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	/** Get the error code for the last failed system function.
	 */
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#define	ErrCode()	errno
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	/** An abstraction for a file handle.
	 *	On POSIX systems file handles are small integers. On Windows
	 *	they're opaque pointers.
	 */
#define	HANDLE	int

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	/**	A value for an invalid file handle.
	 *	Mainly used to initialize file variables and signify that they are
	 *	unused.
	 */
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#define INVALID_HANDLE_VALUE	(-1)
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	/** Get the size of a memory page for the system.
	 *	This is the basic size that the platform's memory manager uses, and is
	 *	fundamental to the use of memory-mapped files.
	 */
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#define	GET_PAGESIZE(x)	((x) = sysconf(_SC_PAGE_SIZE))
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#endif
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#if defined(_WIN32)
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#define MNAME_LEN	32
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#elif defined(MDB_USE_SYSV_SEM)
#define MNAME_LEN	0
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#else
#define MNAME_LEN	(sizeof(pthread_mutex_t))
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#endif

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/** @} */
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#ifdef MDB_ROBUST_SUPPORTED
	/** Lock mutex, handle any error, set rc = result.
	 *	Return 0 on success, nonzero (not rc) on error.
	 */
#define LOCK_MUTEX(rc, env, mutex) \
	(((rc) = LOCK_MUTEX0(mutex)) && \
	 ((rc) = mdb_mutex_failed(env, mutex, rc)))
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static int mdb_mutex_failed(MDB_env *env, mdb_mutex_t *mutex, int rc);
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#else
#define LOCK_MUTEX(rc, env, mutex) ((rc) = LOCK_MUTEX0(mutex))
#define mdb_mutex_failed(env, mutex, rc) (rc)
#endif

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#ifndef _WIN32
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/**	A flag for opening a file and requesting synchronous data writes.
 *	This is only used when writing a meta page. It's not strictly needed;
 *	we could just do a normal write and then immediately perform a flush.
 *	But if this flag is available it saves us an extra system call.
 *
 *	@note If O_DSYNC is undefined but exists in /usr/include,
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 * preferably set some compiler flag to get the definition.
 * Otherwise compile with the less efficient -DMDB_DSYNC=O_SYNC.
 */
#ifndef MDB_DSYNC
# define MDB_DSYNC	O_DSYNC
#endif
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#endif

/** Function for flushing the data of a file. Define this to fsync
 *	if fdatasync() is not supported.
 */
#ifndef MDB_FDATASYNC
# define MDB_FDATASYNC	fdatasync
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#endif

#ifndef MDB_MSYNC
# define MDB_MSYNC(addr,len,flags)	msync(addr,len,flags)
#endif

#ifndef MS_SYNC
#define	MS_SYNC	1
#endif

#ifndef MS_ASYNC
#define	MS_ASYNC	0
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#endif

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	/** A page number in the database.
	 *	Note that 64 bit page numbers are overkill, since pages themselves
	 *	already represent 12-13 bits of addressable memory, and the OS will
	 *	always limit applications to a maximum of 63 bits of address space.
	 *
	 *	@note In the #MDB_node structure, we only store 48 bits of this value,
	 *	which thus limits us to only 60 bits of addressable data.
	 */
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typedef MDB_ID	pgno_t;
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	/** A transaction ID.
	 *	See struct MDB_txn.mt_txnid for details.
	 */
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typedef MDB_ID	txnid_t;
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/** @defgroup debug	Debug Macros
 *	@{
 */
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#ifndef MDB_DEBUG
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	/**	Enable debug output.  Needs variable argument macros (a C99 feature).
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	 *	Set this to 1 for copious tracing. Set to 2 to add dumps of all IDLs
	 *	read from and written to the database (used for free space management).
	 */
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#define MDB_DEBUG 0
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#endif
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#if MDB_DEBUG
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static int mdb_debug;
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static txnid_t mdb_debug_start;
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	/**	Print a debug message with printf formatting.
	 *	Requires double parenthesis around 2 or more args.
	 */
# define DPRINTF(args) ((void) ((mdb_debug) && DPRINTF0 args))
# define DPRINTF0(fmt, ...) \
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	fprintf(stderr, "%s:%d " fmt "\n", mdb_func_, __LINE__, __VA_ARGS__)
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#else
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# define DPRINTF(args)	((void) 0)
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#endif
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	/**	Print a debug string.
	 *	The string is printed literally, with no format processing.
	 */
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#define DPUTS(arg)	DPRINTF(("%s", arg))
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	/** Debuging output value of a cursor DBI: Negative in a sub-cursor. */
#define DDBI(mc) \
	(((mc)->mc_flags & C_SUB) ? -(int)(mc)->mc_dbi : (int)(mc)->mc_dbi)
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/** @} */
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	/**	@brief The maximum size of a database page.
	 *
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	 *	It is 32k or 64k, since value-PAGEBASE must fit in
	 *	#MDB_page.%mp_upper.
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	 *
	 *	LMDB will use database pages < OS pages if needed.
	 *	That causes more I/O in write transactions: The OS must
	 *	know (read) the whole page before writing a partial page.
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	 *
	 *	Note that we don't currently support Huge pages. On Linux,
	 *	regular data files cannot use Huge pages, and in general
	 *	Huge pages aren't actually pageable. We rely on the OS
	 *	demand-pager to read our data and page it out when memory
	 *	pressure from other processes is high. So until OSs have
	 *	actual paging support for Huge pages, they're not viable.
	 */
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#define MAX_PAGESIZE	 (PAGEBASE ? 0x10000 : 0x8000)
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	/** The minimum number of keys required in a database page.
	 *	Setting this to a larger value will place a smaller bound on the
	 *	maximum size of a data item. Data items larger than this size will
	 *	be pushed into overflow pages instead of being stored directly in
	 *	the B-tree node. This value used to default to 4. With a page size
	 *	of 4096 bytes that meant that any item larger than 1024 bytes would
	 *	go into an overflow page. That also meant that on average 2-3KB of
	 *	each overflow page was wasted space. The value cannot be lower than
	 *	2 because then there would no longer be a tree structure. With this
	 *	value, items larger than 2KB will go into overflow pages, and on
	 *	average only 1KB will be wasted.
	 */
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#define MDB_MINKEYS	 2
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	/**	A stamp that identifies a file as an LMDB file.
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	 *	There's nothing special about this value other than that it is easily
	 *	recognizable, and it will reflect any byte order mismatches.
	 */
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#define MDB_MAGIC	 0xBEEFC0DE
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	/**	The version number for a database's datafile format. */
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#define MDB_DATA_VERSION	 ((MDB_DEVEL) ? 999 : 1)
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	/**	The version number for a database's lockfile format. */
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#define MDB_LOCK_VERSION	 ((MDB_DEVEL) ? 999 : 1)
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	/**	@brief The max size of a key we can write, or 0 for dynamic max.
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	 *
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	 *	Define this as 0 to compute the max from the page size.  511
	 *	is default for backwards compat: liblmdb <= 0.9.10 can break
	 *	when modifying a DB with keys/dupsort data bigger than its max.
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	 *	#MDB_DEVEL sets the default to 0.
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	 *
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	 *	Data items in an #MDB_DUPSORT database are also limited to
	 *	this size, since they're actually keys of a sub-DB.  Keys and
	 *	#MDB_DUPSORT data items must fit on a node in a regular page.
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	 */
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#ifndef MDB_MAXKEYSIZE
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#define MDB_MAXKEYSIZE	 ((MDB_DEVEL) ? 0 : 511)
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#endif

	/**	The maximum size of a key we can write to the environment. */
#if MDB_MAXKEYSIZE
#define ENV_MAXKEY(env)	(MDB_MAXKEYSIZE)
#else
#define ENV_MAXKEY(env)	((env)->me_maxkey)
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#endif
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	/**	@brief The maximum size of a data item.
	 *
	 *	We only store a 32 bit value for node sizes.
	 */
#define MAXDATASIZE	0xffffffffUL

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#if MDB_DEBUG
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	/**	Key size which fits in a #DKBUF.
	 *	@ingroup debug
	 */
#define DKBUF_MAXKEYSIZE ((MDB_MAXKEYSIZE) > 0 ? (MDB_MAXKEYSIZE) : 511)
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	/**	A key buffer.
	 *	@ingroup debug
	 *	This is used for printing a hex dump of a key's contents.
	 */
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#define DKBUF	char kbuf[DKBUF_MAXKEYSIZE*2+1]
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	/**	Display a key in hex.
	 *	@ingroup debug
	 *	Invoke a function to display a key in hex.
	 */
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#define	DKEY(x)	mdb_dkey(x, kbuf)
#else
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#define	DKBUF
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#define DKEY(x)	0
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#endif
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	/** An invalid page number.
	 *	Mainly used to denote an empty tree.
	 */
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#define P_INVALID	 (~(pgno_t)0)
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	/** Test if the flags \b f are set in a flag word \b w. */
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#define F_ISSET(w, f)	 (((w) & (f)) == (f))

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	/** Round \b n up to an even number. */
#define EVEN(n)		(((n) + 1U) & -2) /* sign-extending -2 to match n+1U */

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	/**	Used for offsets within a single page.
	 *	Since memory pages are typically 4 or 8KB in size, 12-13 bits,
	 *	this is plenty.
	 */
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typedef uint16_t	 indx_t;

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	/**	Default size of memory map.
	 *	This is certainly too small for any actual applications. Apps should always set
	 *	the size explicitly using #mdb_env_set_mapsize().
	 */
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#define DEFAULT_MAPSIZE	1048576

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/**	@defgroup readers	Reader Lock Table
 *	Readers don't acquire any locks for their data access. Instead, they
 *	simply record their transaction ID in the reader table. The reader
 *	mutex is needed just to find an empty slot in the reader table. The
 *	slot's address is saved in thread-specific data so that subsequent read
 *	transactions started by the same thread need no further locking to proceed.
 *
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 *	If #MDB_NOTLS is set, the slot address is not saved in thread-specific data.
 *
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 *	No reader table is used if the database is on a read-only filesystem, or
 *	if #MDB_NOLOCK is set.
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 *
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 *	Since the database uses multi-version concurrency control, readers don't
 *	actually need any locking. This table is used to keep track of which
 *	readers are using data from which old transactions, so that we'll know
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 *	when a particular old transaction is no longer in use. Old transactions
 *	that have discarded any data pages can then have those pages reclaimed
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 *	for use by a later write transaction.
 *
 *	The lock table is constructed such that reader slots are aligned with the
 *	processor's cache line size. Any slot is only ever used by one thread.
 *	This alignment guarantees that there will be no contention or cache
 *	thrashing as threads update their own slot info, and also eliminates
 *	any need for locking when accessing a slot.
 *
 *	A writer thread will scan every slot in the table to determine the oldest
 *	outstanding reader transaction. Any freed pages older than this will be
 *	reclaimed by the writer. The writer doesn't use any locks when scanning
 *	this table. This means that there's no guarantee that the writer will
 *	see the most up-to-date reader info, but that's not required for correct
 *	operation - all we need is to know the upper bound on the oldest reader,
 *	we don't care at all about the newest reader. So the only consequence of
 *	reading stale information here is that old pages might hang around a
 *	while longer before being reclaimed. That's actually good anyway, because
 *	the longer we delay reclaiming old pages, the more likely it is that a
 *	string of contiguous pages can be found after coalescing old pages from
 *	many old transactions together.
 *	@{
 */
	/**	Number of slots in the reader table.
	 *	This value was chosen somewhat arbitrarily. 126 readers plus a
	 *	couple mutexes fit exactly into 8KB on my development machine.
	 *	Applications should set the table size using #mdb_env_set_maxreaders().
	 */
#define DEFAULT_READERS	126

	/**	The size of a CPU cache line in bytes. We want our lock structures
	 *	aligned to this size to avoid false cache line sharing in the
	 *	lock table.
	 *	This value works for most CPUs. For Itanium this should be 128.
	 */
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#ifndef CACHELINE
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#define CACHELINE	64
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#endif

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	/**	The information we store in a single slot of the reader table.
	 *	In addition to a transaction ID, we also record the process and
	 *	thread ID that owns a slot, so that we can detect stale information,
	 *	e.g. threads or processes that went away without cleaning up.
	 *	@note We currently don't check for stale records. We simply re-init
	 *	the table when we know that we're the only process opening the
	 *	lock file.
	 */
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typedef struct MDB_rxbody {
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	/**	Current Transaction ID when this transaction began, or (txnid_t)-1.
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	 *	Multiple readers that start at the same time will probably have the
	 *	same ID here. Again, it's not important to exclude them from
	 *	anything; all we need to know is which version of the DB they
	 *	started from so we can avoid overwriting any data used in that
	 *	particular version.
	 */
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	txnid_t		mrb_txnid;
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	/** The process ID of the process owning this reader txn. */
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	MDB_PID_T	mrb_pid;
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	/** The thread ID of the thread owning this txn. */
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	MDB_THR_T	mrb_tid;
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} MDB_rxbody;

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	/** The actual reader record, with cacheline padding. */
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typedef struct MDB_reader {
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	union {
		MDB_rxbody mrx;
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		/** shorthand for mrb_txnid */
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#define	mr_txnid	mru.mrx.mrb_txnid
#define	mr_pid	mru.mrx.mrb_pid
#define	mr_tid	mru.mrx.mrb_tid
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		/** cache line alignment */
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		char pad[(sizeof(MDB_rxbody)+CACHELINE-1) & ~(CACHELINE-1)];
	} mru;
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} MDB_reader;

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	/** The header for the reader table.
	 *	The table resides in a memory-mapped file. (This is a different file
	 *	than is used for the main database.)
	 *
	 *	For POSIX the actual mutexes reside in the shared memory of this
	 *	mapped file. On Windows, mutexes are named objects allocated by the
	 *	kernel; we store the mutex names in this mapped file so that other
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	 *	processes can grab them. This same approach is also used on
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	 *	MacOSX/Darwin (using named semaphores) since MacOSX doesn't support
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	 *	process-shared POSIX mutexes. For these cases where a named object
	 *	is used, the object name is derived from a 64 bit FNV hash of the
	 *	environment pathname. As such, naming collisions are extremely
	 *	unlikely. If a collision occurs, the results are unpredictable.
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	 */
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typedef struct MDB_txbody {
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		/** Stamp identifying this as an LMDB file. It must be set
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		 *	to #MDB_MAGIC. */
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	uint32_t	mtb_magic;
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		/** Format of this lock file. Must be set to #MDB_LOCK_FORMAT. */
	uint32_t	mtb_format;
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#if defined(_WIN32)
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	char	mtb_rmname[MNAME_LEN];
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#elif defined(MDB_USE_SYSV_SEM)
	int 	mtb_semid;
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#else
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		/** Mutex protecting access to this table.
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		 *	This is the #MDB_MUTEX(env,r) reader table lock.
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		 */
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	pthread_mutex_t	mtb_rmutex;
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#endif
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		/**	The ID of the last transaction committed to the database.
		 *	This is recorded here only for convenience; the value can always
		 *	be determined by reading the main database meta pages.
		 */
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	txnid_t		mtb_txnid;
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		/** The number of slots that have been used in the reader table.
		 *	This always records the maximum count, it is not decremented
		 *	when readers release their slots.
		 */
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	unsigned	mtb_numreaders;
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} MDB_txbody;

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	/** The actual reader table definition. */
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typedef struct MDB_txninfo {
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	union {
		MDB_txbody mtb;
#define mti_magic	mt1.mtb.mtb_magic
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#define mti_format	mt1.mtb.mtb_format
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#define mti_rmutex	mt1.mtb.mtb_rmutex
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#define mti_rmname	mt1.mtb.mtb_rmname
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#define mti_txnid	mt1.mtb.mtb_txnid
#define mti_numreaders	mt1.mtb.mtb_numreaders
		char pad[(sizeof(MDB_txbody)+CACHELINE-1) & ~(CACHELINE-1)];
	} mt1;
	union {
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#if defined(_WIN32)
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		char mt2_wmname[MNAME_LEN];
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#define	mti_wmname	mt2.mt2_wmname
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#elif defined(MDB_USE_SYSV_SEM)
#define	mti_semid	mt1.mtb.mtb_semid
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#else
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		pthread_mutex_t	mt2_wmutex;
#define mti_wmutex	mt2.mt2_wmutex
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#endif
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		char pad[(MNAME_LEN+CACHELINE-1) & ~(CACHELINE-1)];
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	} mt2;
	MDB_reader	mti_readers[1];
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} MDB_txninfo;
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	/** Lockfile format signature: version, features and field layout */
#define MDB_LOCK_FORMAT \
	((uint32_t) \
	 ((MDB_LOCK_VERSION) \
	  /* Flags which describe functionality */ \
	  + (((MDB_PIDLOCK) != 0) << 16)))
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/** @} */
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/** Common header for all page types.
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 * Overflow records occupy a number of contiguous pages with no
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 * headers on any page after the first.
 */
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typedef struct MDB_page {
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#define	mp_pgno	mp_p.p_pgno
#define	mp_next	mp_p.p_next
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	union {
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		pgno_t		p_pgno;	/**< page number */
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		struct MDB_page *p_next; /**< for in-memory list of freed pages */
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	} mp_p;
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	uint16_t	mp_pad;
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/**	@defgroup mdb_page	Page Flags
 *	@ingroup internal
 *	Flags for the page headers.
 *	@{
 */
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#define	P_BRANCH	 0x01		/**< branch page */
#define	P_LEAF		 0x02		/**< leaf page */
#define	P_OVERFLOW	 0x04		/**< overflow page */
#define	P_META		 0x08		/**< meta page */
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#define	P_DIRTY		 0x10		/**< dirty page, also set for #P_SUBP pages */
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#define	P_LEAF2		 0x20		/**< for #MDB_DUPFIXED records */
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#define	P_SUBP		 0x40		/**< for #MDB_DUPSORT sub-pages */
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#define	P_LOOSE		 0x4000		/**< page was dirtied then freed, can be reused */
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#define	P_KEEP		 0x8000		/**< leave this page alone during spill */
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/** @} */
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	uint16_t	mp_flags;		/**< @ref mdb_page */
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#define mp_lower	mp_pb.pb.pb_lower
#define mp_upper	mp_pb.pb.pb_upper
#define mp_pages	mp_pb.pb_pages
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	union {
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		struct {
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			indx_t		pb_lower;		/**< lower bound of free space */
			indx_t		pb_upper;		/**< upper bound of free space */
		} pb;
		uint32_t	pb_pages;	/**< number of overflow pages */
	} mp_pb;
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	indx_t		mp_ptrs[1];		/**< dynamic size */
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} MDB_page;

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	/** Size of the page header, excluding dynamic data at the end */
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#define PAGEHDRSZ	 ((unsigned) offsetof(MDB_page, mp_ptrs))
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	/** Address of first usable data byte in a page, after the header */
#define METADATA(p)	 ((void *)((char *)(p) + PAGEHDRSZ))

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	/** ITS#7713, change PAGEBASE to handle 65536 byte pages */
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#define	PAGEBASE	((MDB_DEVEL) ? PAGEHDRSZ : 0)
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	/** Number of nodes on a page */
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#define NUMKEYS(p)	 (((p)->mp_lower - (PAGEHDRSZ-PAGEBASE)) >> 1)
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	/** The amount of space remaining in the page */
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#define SIZELEFT(p)	 (indx_t)((p)->mp_upper - (p)->mp_lower)
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	/** The percentage of space used in the page, in tenths of a percent. */
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#define PAGEFILL(env, p) (1000L * ((env)->me_psize - PAGEHDRSZ - SIZELEFT(p)) / \
				((env)->me_psize - PAGEHDRSZ))
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	/** The minimum page fill factor, in tenths of a percent.
	 *	Pages emptier than this are candidates for merging.
	 */
#define FILL_THRESHOLD	 250

	/** Test if a page is a leaf page */
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#define IS_LEAF(p)	 F_ISSET((p)->mp_flags, P_LEAF)
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	/** Test if a page is a LEAF2 page */
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#define IS_LEAF2(p)	 F_ISSET((p)->mp_flags, P_LEAF2)
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	/** Test if a page is a branch page */
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#define IS_BRANCH(p)	 F_ISSET((p)->mp_flags, P_BRANCH)
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	/** Test if a page is an overflow page */
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#define IS_OVERFLOW(p)	 F_ISSET((p)->mp_flags, P_OVERFLOW)
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	/** Test if a page is a sub page */
#define IS_SUBP(p)	 F_ISSET((p)->mp_flags, P_SUBP)
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	/** The number of overflow pages needed to store the given size. */
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#define OVPAGES(size, psize)	((PAGEHDRSZ-1 + (size)) / (psize) + 1)
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	/** Link in #MDB_txn.%mt_loose_pgs list */
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#define NEXT_LOOSE_PAGE(p)		(*(MDB_page **)((p) + 2))
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	/** Header for a single key/data pair within a page.
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	 * Used in pages of type #P_BRANCH and #P_LEAF without #P_LEAF2.
	 * We guarantee 2-byte alignment for 'MDB_node's.
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	 */
typedef struct MDB_node {
	/** lo and hi are used for data size on leaf nodes and for
	 * child pgno on branch nodes. On 64 bit platforms, flags
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	 * is also used for pgno. (Branch nodes have no flags).
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	 * They are in host byte order in case that lets some
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	 * accesses be optimized into a 32-bit word access.
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	 */
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#if BYTE_ORDER == LITTLE_ENDIAN
	unsigned short	mn_lo, mn_hi;	/**< part of data size or pgno */
#else
	unsigned short	mn_hi, mn_lo;
#endif
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/** @defgroup mdb_node Node Flags
 *	@ingroup internal
 *	Flags for node headers.
 *	@{
 */
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#define F_BIGDATA	 0x01			/**< data put on overflow page */
#define F_SUBDATA	 0x02			/**< data is a sub-database */
#define F_DUPDATA	 0x04			/**< data has duplicates */
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/** valid flags for #mdb_node_add() */
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#define	NODE_ADD_FLAGS	(F_DUPDATA|F_SUBDATA|MDB_RESERVE|MDB_APPEND)
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/** @} */
	unsigned short	mn_flags;		/**< @ref mdb_node */
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	unsigned short	mn_ksize;		/**< key size */
	char		mn_data[1];			/**< key and data are appended here */
} MDB_node;

	/** Size of the node header, excluding dynamic data at the end */
#define NODESIZE	 offsetof(MDB_node, mn_data)

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	/** Bit position of top word in page number, for shifting mn_flags */
#define PGNO_TOPWORD ((pgno_t)-1 > 0xffffffffu ? 32 : 0)

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	/** Size of a node in a branch page with a given key.
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	 *	This is just the node header plus the key, there is no data.
	 */
#define INDXSIZE(k)	 (NODESIZE + ((k) == NULL ? 0 : (k)->mv_size))

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	/** Size of a node in a leaf page with a given key and data.
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	 *	This is node header plus key plus data size.
	 */
#define LEAFSIZE(k, d)	 (NODESIZE + (k)->mv_size + (d)->mv_size)

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	/** Address of node \b i in page \b p */
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#define NODEPTR(p, i)	 ((MDB_node *)((char *)(p) + (p)->mp_ptrs[i] + PAGEBASE))
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	/** Address of the key for the node */
#define NODEKEY(node)	 (void *)((node)->mn_data)

	/** Address of the data for a node */
#define NODEDATA(node)	 (void *)((char *)(node)->mn_data + (node)->mn_ksize)

	/** Get the page number pointed to by a branch node */
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#define NODEPGNO(node) \
	((node)->mn_lo | ((pgno_t) (node)->mn_hi << 16) | \
	 (PGNO_TOPWORD ? ((pgno_t) (node)->mn_flags << PGNO_TOPWORD) : 0))
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	/** Set the page number in a branch node */
#define SETPGNO(node,pgno)	do { \
	(node)->mn_lo = (pgno) & 0xffff; (node)->mn_hi = (pgno) >> 16; \
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	if (PGNO_TOPWORD) (node)->mn_flags = (pgno) >> PGNO_TOPWORD; } while(0)
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	/** Get the size of the data in a leaf node */
#define NODEDSZ(node)	 ((node)->mn_lo | ((unsigned)(node)->mn_hi << 16))
	/** Set the size of the data for a leaf node */
#define SETDSZ(node,size)	do { \
	(node)->mn_lo = (size) & 0xffff; (node)->mn_hi = (size) >> 16;} while(0)
	/** The size of a key in a node */
#define NODEKSZ(node)	 ((node)->mn_ksize)

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	/** Copy a page number from src to dst */
#ifdef MISALIGNED_OK
#define COPY_PGNO(dst,src)	dst = src
#else
#if SIZE_MAX > 4294967295UL
#define COPY_PGNO(dst,src)	do { \
	unsigned short *s, *d;	\
	s = (unsigned short *)&(src);	\
	d = (unsigned short *)&(dst);	\
	*d++ = *s++;	\
	*d++ = *s++;	\
	*d++ = *s++;	\
	*d = *s;	\
} while (0)
#else
#define COPY_PGNO(dst,src)	do { \
	unsigned short *s, *d;	\
	s = (unsigned short *)&(src);	\
	d = (unsigned short *)&(dst);	\
	*d++ = *s++;	\
	*d = *s;	\
} while (0)
#endif
#endif
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	/** The address of a key in a LEAF2 page.
	 *	LEAF2 pages are used for #MDB_DUPFIXED sorted-duplicate sub-DBs.
	 *	There are no node headers, keys are stored contiguously.
	 */
#define LEAF2KEY(p, i, ks)	((char *)(p) + PAGEHDRSZ + ((i)*(ks)))

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	/** Set the \b node's key into \b keyptr, if requested. */
#define MDB_GET_KEY(node, keyptr)	{ if ((keyptr) != NULL) { \
	(keyptr)->mv_size = NODEKSZ(node); (keyptr)->mv_data = NODEKEY(node); } }

	/** Set the \b node's key into \b key. */
#define MDB_GET_KEY2(node, key)	{ key.mv_size = NODEKSZ(node); key.mv_data = NODEKEY(node); }
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	/** Information about a single database in the environment. */
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typedef struct MDB_db {
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	uint32_t	md_pad;		/**< also ksize for LEAF2 pages */
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	uint16_t	md_flags;	/**< @ref mdb_dbi_open */
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	uint16_t	md_depth;	/**< depth of this tree */
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	pgno_t		md_branch_pages;	/**< number of internal pages */
	pgno_t		md_leaf_pages;		/**< number of leaf pages */
	pgno_t		md_overflow_pages;	/**< number of overflow pages */
	size_t		md_entries;		/**< number of data items */
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	pgno_t		md_root;		/**< the root page of this tree */
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} MDB_db;

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	/** mdb_dbi_open flags */
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#define MDB_VALID	0x8000		/**< DB handle is valid, for me_dbflags */
#define PERSISTENT_FLAGS	(0xffff & ~(MDB_VALID))
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#define VALID_FLAGS	(MDB_REVERSEKEY|MDB_DUPSORT|MDB_INTEGERKEY|MDB_DUPFIXED|\
	MDB_INTEGERDUP|MDB_REVERSEDUP|MDB_CREATE)

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	/** Handle for the DB used to track free pages. */
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#define	FREE_DBI	0
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	/** Handle for the default DB. */
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#define	MAIN_DBI	1

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	/** Meta page content.
	 *	A meta page is the start point for accessing a database snapshot.
	 *	Pages 0-1 are meta pages. Transaction N writes meta page #(N % 2).
	 */
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typedef struct MDB_meta {
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		/** Stamp identifying this as an LMDB file. It must be set
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		 *	to #MDB_MAGIC. */
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	uint32_t	mm_magic;
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		/** Version number of this lock file. Must be set to #MDB_DATA_VERSION. */
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	uint32_t	mm_version;
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	void		*mm_address;		/**< address for fixed mapping */
	size_t		mm_mapsize;			/**< size of mmap region */
	MDB_db		mm_dbs[2];			/**< first is free space, 2nd is main db */
	/** The size of pages used in this DB */
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#define	mm_psize	mm_dbs[0].md_pad
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	/** Any persistent environment flags. @ref mdb_env */
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#define	mm_flags	mm_dbs[0].md_flags
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	pgno_t		mm_last_pg;			/**< last used page in file */
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	txnid_t		mm_txnid;			/**< txnid that committed this page */
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} MDB_meta;

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	/** Buffer for a stack-allocated meta page.
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	 *	The members define size and alignment, and silence type
	 *	aliasing warnings.  They are not used directly; that could
	 *	mean incorrectly using several union members in parallel.
	 */
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typedef union MDB_metabuf {
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	MDB_page	mb_page;
	struct {
		char		mm_pad[PAGEHDRSZ];
		MDB_meta	mm_meta;
	} mb_metabuf;
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} MDB_metabuf;
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	/** Auxiliary DB info.
	 *	The information here is mostly static/read-only. There is
	 *	only a single copy of this record in the environment.
	 */
typedef struct MDB_dbx {
	MDB_val		md_name;		/**< name of the database */
	MDB_cmp_func	*md_cmp;	/**< function for comparing keys */
	MDB_cmp_func	*md_dcmp;	/**< function for comparing data items */
	MDB_rel_func	*md_rel;	/**< user relocate function */
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	void		*md_relctx;		/**< user-provided context for md_rel */
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} MDB_dbx;
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	/** A database transaction.
	 *	Every operation requires a transaction handle.
	 */
struct MDB_txn {
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	MDB_txn		*mt_parent;		/**< parent of a nested txn */
	MDB_txn		*mt_child;		/**< nested txn under this txn */
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	pgno_t		mt_next_pgno;	/**< next unallocated page */
	/** The ID of this transaction. IDs are integers incrementing from 1.
	 *	Only committed write transactions increment the ID. If a transaction
	 *	aborts, the ID may be re-used by the next writer.
	 */
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	txnid_t		mt_txnid;
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	MDB_env		*mt_env;		/**< the DB environment */
	/** The list of pages that became unused during this transaction.
	 */
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	MDB_IDL		mt_free_pgs;
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	/** The list of loose pages that became unused and may be reused
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	 *	in this transaction, linked through #NEXT_LOOSE_PAGE(page).
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	 */
	MDB_page	*mt_loose_pgs;
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	/* #Number of loose pages (#mt_loose_pgs) */
	int			mt_loose_count;
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	/** The sorted list of dirty pages we temporarily wrote to disk
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	 *	because the dirty list was full. page numbers in here are
	 *	shifted left by 1, deleted slots have the LSB set.
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	 */
	MDB_IDL		mt_spill_pgs;
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	union {
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		/** For write txns: Modified pages. Sorted when not MDB_WRITEMAP. */
		MDB_ID2L	dirty_list;
		/** For read txns: This thread/txn's reader table slot, or NULL. */
		MDB_reader	*reader;
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	} mt_u;
	/** Array of records for each DB known in the environment. */
	MDB_dbx		*mt_dbxs;
	/** Array of MDB_db records for each known DB */
	MDB_db		*mt_dbs;
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	/** Array of sequence numbers for each DB handle */
	unsigned int	*mt_dbiseqs;
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/** @defgroup mt_dbflag	Transaction DB Flags
 *	@ingroup internal
 * @{
 */
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#define DB_DIRTY	0x01		/**< DB was modified or is DUPSORT data */
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#define DB_STALE	0x02		/**< Named-DB record is older than txnID */
#define DB_NEW		0x04		/**< Named-DB handle opened in this txn */
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#define DB_VALID	0x08		/**< DB handle is valid, see also #MDB_VALID */
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/** @} */
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	/** In write txns, array of cursors for each DB */
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	MDB_cursor	**mt_cursors;
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	/** Array of flags for each DB */
	unsigned char	*mt_dbflags;
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	/**	Number of DB records in use. This number only ever increments;
	 *	we don't decrement it when individual DB handles are closed.
	 */
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	MDB_dbi		mt_numdbs;
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/** @defgroup mdb_txn	Transaction Flags
 *	@ingroup internal
 *	@{
 */
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#define MDB_TXN_RDONLY		0x01		/**< read-only transaction */
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#define MDB_TXN_ERROR		0x02		/**< txn is unusable after an error */
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#define MDB_TXN_DIRTY		0x04		/**< must write, even if dirty list is empty */
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#define MDB_TXN_SPILLS		0x08		/**< txn or a parent has spilled pages */
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/** @} */
	unsigned int	mt_flags;		/**< @ref mdb_txn */
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	/** #dirty_list room: Array size - \#dirty pages visible to this txn.
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	 *	Includes ancestor txns' dirty pages not hidden by other txns'
	 *	dirty/spilled pages. Thus commit(nested txn) has room to merge
	 *	dirty_list into mt_parent after freeing hidden mt_parent pages.
	 */
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	unsigned int	mt_dirty_room;
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};
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/** Enough space for 2^32 nodes with minimum of 2 keys per node. I.e., plenty.
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 * At 4 keys per node, enough for 2^64 nodes, so there's probably no need to
 * raise this on a 64 bit machine.
 */
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#define CURSOR_STACK		 32
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struct MDB_xcursor;
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	/** Cursors are used for all DB operations.
	 *	A cursor holds a path of (page pointer, key index) from the DB
	 *	root to a position in the DB, plus other state. #MDB_DUPSORT
	 *	cursors include an xcursor to the current data item. Write txns
	 *	track their cursors and keep them up to date when data moves.
	 *	Exception: An xcursor's pointer to a #P_SUBP page can be stale.
	 *	(A node with #F_DUPDATA but no #F_SUBDATA contains a subpage).
	 */
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struct MDB_cursor {
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	/** Next cursor on this DB in this txn */
	MDB_cursor	*mc_next;
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	/** Backup of the original cursor if this cursor is a shadow */
	MDB_cursor	*mc_backup;
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	/** Context used for databases with #MDB_DUPSORT, otherwise NULL */
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	struct MDB_xcursor	*mc_xcursor;
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	/** The transaction that owns this cursor */
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	MDB_txn		*mc_txn;