Renumber MDB_NOSUBDIR. Check mdb_env_open() flags.

[openldap] / libraries / libmdb / mdb.h

/** @file mdb.h
 *      @brief memory-mapped database library
 *
 *      @mainpage       MDB Memory-Mapped Database Manager
 *      MDB is a Btree-based database management library modeled loosely on the
 *      BerkeleyDB API, but much simplified. The entire database is exposed
 *      in a memory map, and all data fetches return data directly
 *      from the mapped memory, so no malloc's or memcpy's occur during
 *      data fetches. As such, the library is extremely simple because it
 *      requires no page caching layer of its own, and it is extremely high
 *      performance and memory-efficient. It is also fully transactional with
 *      full ACID semantics, and when the memory map is read-only, the
 *      database integrity cannot be corrupted by stray pointer writes from
 *      application code.
 *
 *      The library is fully thread-aware and supports concurrent read/write
 *      access from multiple processes and threads. Data pages use a copy-on-
 *      write strategy so no active data pages are ever overwritten, which
 *      also provides resistance to corruption and eliminates the need of any
 *      special recovery procedures after a system crash. Writes are fully
 *      serialized; only one write transaction may be active at a time, which
 *      guarantees that writers can never deadlock. The database structure is
 *      multi-versioned so readers run with no locks; writers cannot block
 *      readers, and readers don't block writers.
 *
 *      Unlike other well-known database mechanisms which use either write-ahead
 *      transaction logs or append-only data writes, MDB requires no maintenance
 *      during operation. Both write-ahead loggers and append-only databases
 *      require periodic checkpointing and/or compaction of their log or database
 *      files otherwise they grow without bound. MDB 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.
 *
 *      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: Terminate all programs using the database, or make
 *        them close it.  Next database user will reset the lockfile.
 *
 *      - 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.
 *
 *      - A thread can only use one transaction at a time, plus any child
 *        transactions.  Each transaction belongs to one thread.  See below.
 *
 *      - Use an MDB_env* in the process which opened it, without fork()ing.
 *
 *      - Do not have open an MDB 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.
 *
 *      ...when several processes can use a database concurrently:
 *
 *      - Avoid suspending a process with active transactions.  These
 *        would then be "long-lived" as above.
 *
 *      - Avoid aborting a process with an active transaction.
 *        The transaction becomes "long-lived" as above until the lockfile
 *        is reset, since the process may not remove it from the lockfile.
 *
 *      - If you do that anyway, close the environment once in a while,
 *        so the lockfile can get reset.
 *
 *      - Do not use MDB databases on remote filesystems, even between
 *        processes on the same host.  This breaks flock() on some OSes,
 *        possibly memory map sync, and certainly sync between programs
 *        on different hosts.
 *
 *      - Opening a database can fail if another process is opening or
 *        closing it at exactly the same time.
 *
 *      @author Howard Chu, Symas Corporation.
 *
 *      @copyright Copyright 2011-2012 Howard Chu, Symas Corp. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted only as authorized by the OpenLDAP
 * Public License.
 *
 * A copy of this license is available in the file LICENSE in the
 * top-level directory of the distribution or, alternatively, at
 * <http://www.OpenLDAP.org/license.html>.
 *
 *      @par Derived From:
 * This code is derived from btree.c written by Martin Hedenfalk.
 *
 * Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#ifndef _MDB_H_
#define _MDB_H_

#include <sys/types.h>

#ifdef __cplusplus
extern "C" {
#endif

/** @defgroup public Public API
 *      @{
 */
/** @defgroup Version Version Macros
 *      @{
 */
/** Library major version */
#define MDB_VERSION_MAJOR       0
/** Library minor version */
#define MDB_VERSION_MINOR       9
/** Library patch version */
#define MDB_VERSION_PATCH       4

/** Combine args a,b,c into a single integer for easy version comparisons */
#define MDB_VERINT(a,b,c)       (((a) << 24) | ((b) << 16) | (c))

/** The full library version as a single integer */
#define MDB_VERSION_FULL        \
        MDB_VERINT(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH)

/** The release date of this library version */
#define MDB_VERSION_DATE        "September 14, 2012"

/** A stringifier for the version info */
#define MDB_VERSTR(a,b,c,d)     "MDB " #a "." #b "." #c ": (" d ")"

/** A helper for the stringifier macro */
#define MDB_VERFOO(a,b,c,d)     MDB_VERSTR(a,b,c,d)

/** The full library version as a C string */
#define MDB_VERSION_STRING      \
        MDB_VERFOO(MDB_VERSION_MAJOR,MDB_VERSION_MINOR,MDB_VERSION_PATCH,MDB_VERSION_DATE)
/**     @} */

/** @brief Opaque structure for a database environment.
 *
 * A DB environment supports multiple databases, all residing in the same
 * shared-memory map.
 */
typedef struct MDB_env MDB_env;

/** @brief Opaque structure for a transaction handle.
 *
 * All database operations require a transaction handle. Transactions may be
 * read-only or read-write.
 */
typedef struct MDB_txn MDB_txn;

/** @brief A handle for an individual database in the DB environment. */
typedef unsigned int    MDB_dbi;

/** @brief Opaque structure for navigating through a database */
typedef struct MDB_cursor MDB_cursor;

/** @brief Generic structure used for passing keys and data in and out of the database. */
typedef struct MDB_val {
        size_t           mv_size;       /**< size of the data item */
        void            *mv_data;       /**< address of the data item */
} MDB_val;

/** @brief A callback function used to compare two keys in a database */
typedef int  (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);

/** @brief A callback function used to relocate a position-dependent data item
 * in a fixed-address database.
 *
 * The \b newptr gives the item's desired address in
 * the memory map, and \b oldptr gives its previous address. The item's actual
 * data resides at the address in \b item.  This callback is expected to walk
 * through the fields of the record in \b item and modify any
 * values based at the \b oldptr address to be relative to the \b newptr address.
 * @param[in,out] item The item that is to be relocated.
 * @param[in] oldptr The previous address.
 * @param[in] newptr The new address to relocate to.
 * @param[in] relctx An application-provided context, set by #mdb_set_relctx().
 * @todo This feature is currently unimplemented.
 */
typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);

/** @defgroup   mdb_env Environment Flags
 *
 *      Values do not overlap Database Flags.
 *      @{
 */
        /** mmap at a fixed address */
#define MDB_FIXEDMAP    0x01
        /** no environment directory */
#define MDB_NOSUBDIR    0x4000
        /** don't fsync after commit */
#define MDB_NOSYNC              0x10000
        /** read only */
#define MDB_RDONLY              0x20000
        /** don't fsync metapage after commit */
#define MDB_NOMETASYNC          0x40000
        /** use writable mmap */
#define MDB_WRITEMAP            0x80000
        /** use asynchronous msync */
#define MDB_MAPASYNC            0x100000
/** @} */

/**     @defgroup       mdb_open        Database Flags
 *
 *      Values do not overlap Environment Flags.
 *      @{
 */
        /** use reverse string keys */
#define MDB_REVERSEKEY  0x02
        /** use sorted duplicates */
#define MDB_DUPSORT             0x04
        /** numeric keys in native byte order.
         *  The keys must all be of the same size. */
#define MDB_INTEGERKEY  0x08
        /** with #MDB_DUPSORT, sorted dup items have fixed size */
#define MDB_DUPFIXED    0x10
        /** with #MDB_DUPSORT, dups are numeric in native byte order */
#define MDB_INTEGERDUP  0x20
        /** with #MDB_DUPSORT, use reverse string dups */
#define MDB_REVERSEDUP  0x40
        /** create DB if not already existing */
#define MDB_CREATE              0x40000
/** @} */

/**     @defgroup mdb_put       Write Flags
 *      @{
 */
/** For put: Don't write if the key already exists. */
#define MDB_NOOVERWRITE 0x10
/** Only for #MDB_DUPSORT<br>
 * For put: don't write if the key and data pair already exist.<br>
 * For mdb_cursor_del: remove all duplicate data items.
 */
#define MDB_NODUPDATA   0x20
/** For mdb_cursor_put: overwrite the current key/data pair */
#define MDB_CURRENT     0x40
/** For put: Just reserve space for data, don't copy it. Return a
 * pointer to the reserved space.
 */
#define MDB_RESERVE     0x10000
/** Data is being appended, don't split full pages. */
#define MDB_APPEND      0x20000
/** Duplicate data is being appended, don't split full pages. */
#define MDB_APPENDDUP   0x40000
/** Store multiple data items in one call. */
#define MDB_MULTIPLE    0x80000
/*      @} */

/** @brief Cursor Get operations.
 *
 *      This is the set of all operations for retrieving data
 *      using a cursor.
 */
typedef enum MDB_cursor_op {
        MDB_FIRST,                              /**< Position at first key/data item */
        MDB_FIRST_DUP,                  /**< Position at first data item of current key.
                                                                Only for #MDB_DUPSORT */
        MDB_GET_BOTH,                   /**< Position at key/data pair. Only for #MDB_DUPSORT */
        MDB_GET_BOTH_RANGE,             /**< position at key, nearest data. Only for #MDB_DUPSORT */
        MDB_GET_CURRENT,                /**< Return key/data at current cursor position */
        MDB_GET_MULTIPLE,               /**< Return all the duplicate data items at the current
                                                                 cursor position. 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 all duplicate data items at the next
                                                                cursor position. Only for #MDB_DUPFIXED */
        MDB_NEXT_NODUP,                 /**< Position at first data item of next key.
                                                                Only for #MDB_DUPSORT */
        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.
                                                                Only for #MDB_DUPSORT */
        MDB_SET,                                /**< Position at specified key */
        MDB_SET_KEY,                    /**< Position at specified key, return key + data */
        MDB_SET_RANGE                   /**< Position at first key greater than or equal to specified key. */
} MDB_cursor_op;

/** @defgroup  errors   Return Codes
 *
 *      BerkeleyDB uses -30800 to -30999, we'll go under them
 *      @{
 */
        /**     Successful result */
#define MDB_SUCCESS      0
        /** key/data pair already exists */
#define MDB_KEYEXIST    (-30799)
        /** key/data pair not found (EOF) */
#define MDB_NOTFOUND    (-30798)
        /** Requested page not found - this usually indicates corruption */
#define MDB_PAGE_NOTFOUND       (-30797)
        /** Located page was wrong type */
#define MDB_CORRUPTED   (-30796)
        /** Update of meta page failed, probably I/O error */
#define MDB_PANIC               (-30795)
        /** Environment version mismatch */
#define MDB_VERSION_MISMATCH    (-30794)
        /** File is not a valid MDB 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)
        /** Nested 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)
#define MDB_LAST_ERRCODE        MDB_PAGE_FULL
/** @} */

/** @brief Statistics for a database in the environment */
typedef struct MDB_stat {
        unsigned int    ms_psize;                       /**< Size of a database page.
                                                                                        This is currently the same for all databases. */
        unsigned int    ms_depth;                       /**< Depth (height) of the B-tree */
        size_t          ms_branch_pages;        /**< Number of internal (non-leaf) pages */
        size_t          ms_leaf_pages;          /**< Number of leaf pages */
        size_t          ms_overflow_pages;      /**< Number of overflow pages */
        size_t          ms_entries;                     /**< Number of data items */
} MDB_stat;

/** @brief Information about the environment */
typedef struct MDB_envinfo {
        void    *me_mapaddr;                    /**< Address of map, if fixed */
        size_t  me_mapsize;                             /**< Size of the data memory map */
        size_t  me_last_pgno;                   /**< ID of the last used page */
        size_t  me_last_txnid;                  /**< ID of the last committed transaction */
        unsigned int me_maxreaders;             /**< maximum number of threads for the environment */
        unsigned int me_numreaders;             /**< maximum number of threads used in the environment */
} MDB_envinfo;

        /** @brief Return the mdb library version information.
         *
         * @param[out] major if non-NULL, the library major version number is copied here
         * @param[out] minor if non-NULL, the library minor version number is copied here
         * @param[out] patch if non-NULL, the library patch version number is copied here
         * @retval "version string" The library version as a string
         */
char *mdb_version(int *major, int *minor, int *patch);

        /** @brief Return a string describing a given error code.
         *
         * This function is a superset of the ANSI C X3.159-1989 (ANSI C) strerror(3)
         * function. If the error code is greater than or equal to 0, then the string
         * returned by the system function strerror(3) is returned. If the error code
         * is less than 0, an error string corresponding to the MDB library error is
         * returned. See @ref errors for a list of MDB-specific error codes.
         * @param[in] err The error code
         * @retval "error message" The description of the error
         */
char *mdb_strerror(int err);

        /** @brief Create an MDB environment handle.
         *
         * This function allocates memory for a #MDB_env structure. To release
         * the allocated memory and discard the handle, call #mdb_env_close().
         * Before the handle may be used, it must be opened using #mdb_env_open().
         * Various other options may also need to be set before opening the handle,
         * e.g. #mdb_env_set_mapsize(), #mdb_env_set_maxreaders(), #mdb_env_set_maxdbs(),
         * depending on usage requirements.
         * @param[out] env The address where the new handle will be stored
         * @return A non-zero error value on failure and 0 on success.
         */
int  mdb_env_create(MDB_env **env);

        /** @brief Open an environment handle.
         *
         * If this function fails, #mdb_env_close() must be called to discard the #MDB_env handle.
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] path The directory in which the database files reside. This
         * directory must already exist and be writable.
         * @param[in] flags Special options for this environment. This parameter
         * must be set to 0 or by bitwise OR'ing together one or more of the
         * values described here.
         * Flags set by mdb_env_set_flags() are also used.
         * <ul>
         *      <li>#MDB_FIXEDMAP
         *      use a fixed address for the mmap region. This flag must be specified
         *      when creating the environment, and is stored persistently in the environment.
         *              If successful, the memory map will always reside at the same virtual address
         *              and pointers used to reference data items in the database will be constant
         *              across multiple invocations. This option may not always work, depending on
         *              how the operating system has allocated memory to shared libraries and other uses.
         *              The feature is highly experimental.
         *      <li>#MDB_NOSUBDIR
         *              By default, MDB 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_NOSYNC
         *              Don't perform a synchronous flush after committing a transaction. This means
         *              transactions will exhibit the ACI (atomicity, consistency, and isolation)
         *              properties, but not D (durability); that is database integrity will be
         *              maintained but it is possible some number of the most recently committed
         *              transactions may be undone after a system crash. The number of transactions
         *              at risk is governed by how often the system flushes dirty buffers to disk
         *              and how often #mdb_env_sync() is called. This flag may be changed
         *              at any time using #mdb_env_set_flags().
         *      <li>#MDB_NOMETASYNC
         *              Don't perform a synchronous flush of the meta page after committing
         *              a transaction. This is similar to the #MDB_NOSYNC case, but safer
         *              because the transaction data is still flushed. The meta page for any
         *              transaction N will be flushed by the data flush of transaction N+1.
         *              In case of a system crash, the last committed transaction may be
         *              lost. This flag may be changed at any time using #mdb_env_set_flags().
         *      <li>#MDB_RDONLY
         *              Open the environment in read-only mode. No write operations will be allowed.
         * </ul>
         * @param[in] mode The UNIX permissions to set on created files. This parameter
         * is ignored on Windows.
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_VERSION_MISMATCH - the version of the MDB library doesn't match the
         *      version that created the database environment.
         *      <li>EINVAL - 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, mode_t mode);

        /** @brief Copy an MDB environment to the specified path.
         *
         * This function may be used to make a backup of an existing environment.
         * @param[in] env An environment handle returned by #mdb_env_create(). It
         * must have already been opened successfully.
         * @param[in] path The directory in which the copy will reside. This
         * directory must already exist and be writable but must otherwise be
         * empty.
         * @return A non-zero error value on failure and 0 on success.
         */
int  mdb_env_copy(MDB_env *env, const char *path);

        /** @brief Return statistics about the MDB environment.
         *
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[out] stat The address of an #MDB_stat structure
         *      where the statistics will be copied
         */
int  mdb_env_stat(MDB_env *env, MDB_stat *stat);

        /** @brief Return information about the MDB environment.
         *
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[out] stat The address of an #MDB_envinfo structure
         *      where the information will be copied
         */
int  mdb_env_info(MDB_env *env, MDB_envinfo *stat);

        /** @brief Flush the data buffers to disk.
         *
         * Data is always written to disk when #mdb_txn_commit() is called,
         * but the operating system may keep it buffered. MDB always flushes
         * the OS buffers upon commit as well, unless the environment was
         * opened with #MDB_NOSYNC.
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] force If non-zero, force the flush to occur. Otherwise
         *  if the environment has the #MDB_NOSYNC flag set the flushes
         *      will be omitted.
         * @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>EIO - an error occurred during synchronization.
         * </ul>
         */
int  mdb_env_sync(MDB_env *env, int force);

        /** @brief Close the environment and release the memory map.
         *
         * Only a single thread may call this function. All transactions, databases,
         * and cursors must already be closed before calling this function. Attempts to
         * use any such handles after calling this function will cause a SIGSEGV.
         * The environment handle will be freed and must not be used again after this call.
         * @param[in] env An environment handle returned by #mdb_env_create()
         */
void mdb_env_close(MDB_env *env);

        /** @brief Set environment flags.
         *
         * This may be used to set some flags in addition to those from
         * #mdb_env_open(), or to unset these flags.
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] flags The flags to change, bitwise OR'ed together
         * @param[in] onoff A non-zero value sets the flags, zero clears them.
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_env_set_flags(MDB_env *env, unsigned int flags, int onoff);

        /** @brief Get environment flags.
         *
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[out] flags The address of an integer to store the flags
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_env_get_flags(MDB_env *env, unsigned int *flags);

        /** @brief Return the path that was used in #mdb_env_open().
         *
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[out] path Address of a string pointer to contain the path. This
         * is the actual string in the environment, not a copy. It should not be
         * altered in any way.
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_env_get_path(MDB_env *env, const char **path);

        /** @brief Set the size of the memory map to use for this environment.
         *
         * The size should be a multiple of the OS page size. The default is
         * 10485760 bytes. The size of the memory map is also the maximum size
         * of the database. The value should be chosen as large as possible,
         * to accommodate future growth of the database.
         * This function may only be called after #mdb_env_create() and before #mdb_env_open().
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] 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 is already open.
         * </ul>
         */
int  mdb_env_set_mapsize(MDB_env *env, size_t size);

        /** @brief Set the maximum number of threads 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.
         * This function may only be called after #mdb_env_create() and before #mdb_env_open().
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] readers The maximum number of threads
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified, or the environment is already open.
         * </ul>
         */
int  mdb_env_set_maxreaders(MDB_env *env, unsigned int readers);

        /** @brief Get the maximum number of threads 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 databases for the environment.
         *
         * This function is only needed if multiple databases will be used in the
         * environment. Simpler applications that only use a single database can ignore
         * this option.
         * This function may only be called after #mdb_env_create() and before #mdb_env_open().
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] dbs The maximum number of databases
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified, or the environment is already open.
         * </ul>
         */
int  mdb_env_set_maxdbs(MDB_env *env, MDB_dbi dbs);

        /** @brief Create a transaction for use with the environment.
         *
         * The transaction handle may be discarded using #mdb_txn_abort() or #mdb_txn_commit().
         * @note Transactions may not span threads; a transaction must only be used by a
         * single thread. Also, a thread may only have a single transaction.
         * @note Cursors may not span transactions; each cursor must be opened and closed
         * within a single transaction.
         * @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 may not issue any other operations besides mdb_txn_begin,
         * mdb_txn_abort, or mdb_txn_commit while it has active child transactions.
         * @param[in] flags Special options for this transaction. This parameter
         * must be set to 0 or by bitwise OR'ing together one or more of the
         * values described here.
         * <ul>
         *      <li>#MDB_RDONLY
         *              This transaction will not perform any write operations.
         * </ul>
         * @param[out] txn Address where the new #MDB_txn handle will be stored
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_PANIC - a fatal error occurred earlier and the environment
         *              must be shut down.
         *      <li>ENOMEM - out of memory, or a read-only transaction was requested and
         *              the reader lock table is full. See #mdb_env_set_maxreaders().
         * </ul>
         */
int  mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);

        /** @brief Commit all the operations of a transaction into the database.
         *
         * All cursors opened within the transaction will be closed by this call. The cursors
         * and transaction handle will be freed and must not be used again after this call.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         *      <li>ENOSPC - no more disk space.
         *      <li>EIO - a low-level I/O error occurred while writing.
         *      <li>ENOMEM - the transaction is nested and could not be merged into its parent.
         * </ul>
         */
int  mdb_txn_commit(MDB_txn *txn);

        /** @brief Abandon all the operations of the transaction instead of saving them.
         *
         * All cursors opened within the transaction will be closed by this call. The cursors
         * and transaction handle will be freed and must not be used again after this call.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         */
void mdb_txn_abort(MDB_txn *txn);

        /** @brief Reset a read-only transaction.
         *
         * This releases the current reader lock but doesn't free the
         * transaction handle, allowing it to be used again later by #mdb_txn_renew().
         * It otherwise has the same effect as #mdb_txn_abort() but saves some memory
         * allocation/deallocation overhead if a thread is going to start a new
         * read-only transaction again soon.
         * All cursors opened within the transaction must be closed before the transaction
         * is reset.
         * Reader locks generally don't interfere with writers, but they keep old
         * versions of database pages allocated. Thus they prevent the old pages
         * from being reused when writers commit new data, and so under heavy load
         * the database size may grow much more rapidly than otherwise.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         */
void mdb_txn_reset(MDB_txn *txn);

        /** @brief Renew a read-only transaction.
         *
         * This acquires a new reader lock for a transaction handle that had been
         * released by #mdb_txn_reset(). It must be called before a reset transaction
         * may be used again.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_PANIC - a fatal error occurred earlier and the environment
         *              must be shut down.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_txn_renew(MDB_txn *txn);

        /** @brief Open a database in the environment.
         *
         * The database handle may be discarded by calling #mdb_close().  The
         * database handle resides in the shared environment, it is not owned
         * by the given transaction. Only one thread should call this function;
         * it is not mutex-protected in a read-only transaction.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] name The name of the database to open. If only a single
         *      database is needed in the environment, this value may be NULL.
         * @param[in] flags Special options for this database. This parameter
         * must be set to 0 or by bitwise OR'ing together one or more of the
         * values described here.
         * <ul>
         *      <li>#MDB_REVERSEKEY
         *              Keys are strings to be compared in reverse order, from the end
         *              of the strings to the beginning. By default, Keys are treated as strings and
         *              compared from beginning to end.
         *      <li>#MDB_DUPSORT
         *              Duplicate keys may be used in the database. (Or, from another perspective,
         *              keys may have multiple data items, stored in sorted order.) By default
         *              keys must be unique and may have only a single data item.
         *      <li>#MDB_INTEGERKEY
         *              Keys are binary integers in native byte order. Setting this option
         *              requires all keys to be the same size, typically sizeof(int)
         *              or sizeof(size_t).
         *      <li>#MDB_DUPFIXED
         *              This flag may only be used in combination with #MDB_DUPSORT. This option
         *              tells the library that the data items for this database are all the same
         *              size, which allows further optimizations in storage and retrieval. When
         *              all data items are the same size, the #MDB_GET_MULTIPLE and #MDB_NEXT_MULTIPLE
         *              cursor operations may be used to retrieve multiple items at once.
         *      <li>#MDB_INTEGERDUP
         *              This option specifies that duplicate data items are also integers, and
         *              should be sorted as such.
         *      <li>#MDB_REVERSEDUP
         *              This option specifies that duplicate data items should be compared as
         *              strings in reverse order.
         *      <li>#MDB_CREATE
         *              Create the named database if it doesn't exist. This option is not
         *              allowed in a read-only transaction or a read-only environment.
         * </ul>
         * @param[out] dbi Address where the new #MDB_dbi handle will be stored
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_NOTFOUND - the specified database doesn't exist in the environment
         *              and #MDB_CREATE was not specified.
         *      <li>ENFILE - too many databases have been opened. See #mdb_env_set_maxdbs().
         * </ul>
         */
int  mdb_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);

        /** @brief Retrieve statistics for a database.
         *
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[out] stat The address of an #MDB_stat structure
         *      where the statistics will be copied
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);

        /** @brief Close a database handle.
         *
         * This call is not mutex protected. Handles should only be closed by
         * a single thread, and only if no other threads are going to reference
         * the database handle any further.
         * @param[in] env An environment handle returned by #mdb_env_create()
         * @param[in] dbi A database handle returned by #mdb_open()
         */
void mdb_close(MDB_env *env, MDB_dbi dbi);

        /** @brief Delete a database and/or free all its pages.
         *
         * If the \b del parameter is non-zero the DB handle will be closed
         * and the DB will be deleted.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] del non-zero to delete the DB from the environment,
         * otherwise just free its pages.
         * @return A non-zero error value on failure and 0 on success.
         */
int  mdb_drop(MDB_txn *txn, MDB_dbi dbi, int del);

        /** @brief Set a custom key comparison function for a database.
         *
         * The comparison function is called whenever it is necessary to compare a
         * key specified by the application with a key currently stored in the database.
         * If no comparison function is specified, and no special key flags were specified
         * with #mdb_open(), the keys are compared lexically, with shorter keys collating
         * before longer keys.
         * @warning This function must be called before any data access functions are used,
         * otherwise data corruption may occur. The same comparison function must be used by every
         * program accessing the database, every time the database is used.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] cmp A #MDB_cmp_func function
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);

        /** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
         *
         * This comparison function is called whenever it is necessary to compare a data
         * item specified by the application with a data item currently stored in the database.
         * This function only takes effect if the database was opened with the #MDB_DUPSORT
         * flag.
         * If no comparison function is specified, and no special key flags were specified
         * with #mdb_open(), the data items are compared lexically, with shorter items collating
         * before longer items.
         * @warning This function must be called before any data access functions are used,
         * otherwise data corruption may occur. The same comparison function must be used by every
         * program accessing the database, every time the database is used.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] cmp A #MDB_cmp_func function
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);

        /** @brief Set a relocation function for a #MDB_FIXEDMAP database.
         *
         * @todo The relocation function is called whenever it is necessary to move the data
         * of an item to a different position in the database (e.g. through tree
         * balancing operations, shifts as a result of adds or deletes, etc.). It is
         * intended to allow address/position-dependent data items to be stored in
         * a database in an environment opened with the #MDB_FIXEDMAP option.
         * Currently the relocation feature is unimplemented and setting
         * this function has no effect.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] rel A #MDB_rel_func function
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);

        /** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
         *
         * See #mdb_set_relfunc and #MDB_rel_func for more details.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] ctx An arbitrary pointer for whatever the application needs.
         * It will be passed to the callback function set by #mdb_set_relfunc
         * as its \b relctx parameter whenever the callback is invoked.
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);

        /** @brief Get items from a database.
         *
         * This function retrieves key/data pairs from the database. The address
         * and length of the data associated with the specified \b key are returned
         * in the structure to which \b data refers.
         * If the database supports duplicate keys (#MDB_DUPSORT) then the
         * first data item for the key will be returned. Retrieval of other
         * items requires the use of #mdb_cursor_get().
         *
         * @note The memory pointed to by the returned values is owned by the
         * database. The caller need not dispose of the memory, and may not
         * modify it in any way. For values returned in a read-only transaction
         * any modification attempts will cause a SIGSEGV.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] key The key to search for in the database
         * @param[out] data The data corresponding to the key
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_NOTFOUND - the key was not in the database.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);

        /** @brief Store items into a database.
         *
         * This function stores key/data pairs in the database. The default behavior
         * is to enter the new key/data pair, replacing any previously existing key
         * if duplicates are disallowed, or adding a duplicate data item if
         * duplicates are allowed (#MDB_DUPSORT).
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] key The key to store in the database
         * @param[in,out] data The data to store
         * @param[in] flags Special options for this operation. This parameter
         * must be set to 0 or by bitwise OR'ing together one or more of the
         * values described here.
         * <ul>
         *      <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
         *              already appear in the database. This flag may only be specified
         *              if the database was opened with #MDB_DUPSORT. The function will
         *              return #MDB_KEYEXIST if the key/data pair already appears in the
         *              database.
         *      <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
         *              does not already appear in the database. The function will return
         *              #MDB_KEYEXIST if the key already appears in the database, even if
         *              the database supports duplicates (#MDB_DUPSORT). The \b data
         *              parameter will be set to point to the existing item.
         *      <li>#MDB_RESERVE - reserve space for data of the given size, but
         *              don't copy the given data. Instead, return a pointer to the
         *              reserved space, which the caller can fill in later. This saves
         *              an extra memcpy if the data is being generated later.
         *      <li>#MDB_APPEND - append the given key/data pair to the end of the
         *              database. No key comparisons are performed. This option allows
         *              fast bulk loading when keys are already known to be in the
         *              correct order. Loading unsorted keys with this flag will cause
         *              data corruption.
         *      <li>#MDB_APPENDDUP - as above, but for sorted dup data.
         * </ul>
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EACCES - an attempt was made to write in a read-only transaction.
         *      <li>EINVAL - an invalid parameter was specified.
         *      <li>ENOMEM - the database is full, see #mdb_env_set_mapsize().
         * </ul>
         */
int  mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
                            unsigned int flags);

        /** @brief Delete items from a database.
         *
         * This function removes key/data pairs from the database.
         * If the database does not support sorted duplicate data items
         * (#MDB_DUPSORT) the data parameter is ignored.
         * If the database supports sorted duplicates and the data parameter
         * is NULL, all of the duplicate data items for the key will be
         * deleted. Otherwise, if the data parameter is non-NULL
         * only the matching data item will be deleted.
         * This function will return #MDB_NOTFOUND if the specified key/data
         * pair is not in the database.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] key The key to delete from the database
         * @param[in] data The data to delete
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EACCES - an attempt was made to write in a read-only transaction.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);

        /** @brief Create a cursor handle.
         *
         * Cursors are associated with a specific transaction and database and
         * may not span threads.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[out] cursor Address where the new #MDB_cursor handle will be stored
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);

        /** @brief Close a cursor handle.
         *
         * The cursor handle will be freed and must not be used again after this call.
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         */
void mdb_cursor_close(MDB_cursor *cursor);

        /** @brief Renew a cursor handle.
         *
         * Cursors are associated with a specific transaction and database and
         * may not span threads. Cursors that are only used in read-only
         * transactions may be re-used, to avoid unnecessary malloc/free overhead.
         * The cursor may be associated with a new read-only transaction, and
         * referencing the same database handle as it was created with.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_renew(MDB_txn *txn, MDB_cursor *cursor);

        /** @brief Return the cursor's transaction handle.
         *
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         */
MDB_txn *mdb_cursor_txn(MDB_cursor *cursor);

        /** @brief Return the cursor's database handle.
         *
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         */
MDB_dbi mdb_cursor_dbi(MDB_cursor *cursor);

        /** @brief Retrieve by cursor.
         *
         * This function retrieves key/data pairs from the database. The address and length
         * of the key are returned in the object to which \b key refers (except for the
         * case of the #MDB_SET option, in which the \b key object is unchanged), and
         * the address and length of the data are returned in the object to which \b data
         * refers.
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         * @param[in,out] key The key for a retrieved item
         * @param[in,out] data The data of a retrieved item
         * @param[in] op A cursor operation #MDB_cursor_op
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>#MDB_NOTFOUND - no matching key found.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_get(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
                            MDB_cursor_op op);

        /** @brief Store by cursor.
         *
         * This function stores key/data pairs into the database.
         * If the function fails for any reason, the state of the cursor will be
         * unchanged. If the function succeeds and an item is inserted into the
         * database, the cursor is always positioned to refer to the newly inserted item.
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         * @param[in] key The key operated on.
         * @param[in] data The data operated on.
         * @param[in] flags Options for this operation. This parameter
         * must be set to 0 or one of the values described here.
         * <ul>
         *      <li>#MDB_CURRENT - overwrite the data of the key/data pair to which
         *              the cursor refers with the specified data item. The \b key
         *              parameter is ignored.
         *      <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
         *              already appear in the database. This flag may only be specified
         *              if the database was opened with #MDB_DUPSORT. The function will
         *              return #MDB_KEYEXIST if the key/data pair already appears in the
         *              database.
         *      <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
         *              does not already appear in the database. The function will return
         *              #MDB_KEYEXIST if the key already appears in the database, even if
         *              the database supports duplicates (#MDB_DUPSORT).
         *      <li>#MDB_RESERVE - reserve space for data of the given size, but
         *              don't copy the given data. Instead, return a pointer to the
         *              reserved space, which the caller can fill in later. This saves
         *              an extra memcpy if the data is being generated later.
         *      <li>#MDB_APPEND - append the given key/data pair to the end of the
         *              database. No key comparisons are performed. This option allows
         *              fast bulk loading when keys are already known to be in the
         *              correct order. Loading unsorted keys with this flag will cause
         *              data corruption.
         *      <li>#MDB_APPENDDUP - as above, but for sorted dup data.
         * </ul>
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EACCES - an attempt was made to modify a read-only database.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_put(MDB_cursor *cursor, MDB_val *key, MDB_val *data,
                                unsigned int flags);

        /** @brief Delete current key/data pair
         *
         * This function deletes the key/data pair to which the cursor refers.
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         * @param[in] flags Options for this operation. This parameter
         * must be set to 0 or one of the values described here.
         * <ul>
         *      <li>#MDB_NODUPDATA - delete all of the data items for the current key.
         *              This flag may only be specified if the database was opened with #MDB_DUPSORT.
         * </ul>
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EACCES - an attempt was made to modify a read-only database.
         *      <li>EINVAL - an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_del(MDB_cursor *cursor, unsigned int flags);

        /** @brief Return count of duplicates for current key.
         *
         * This call is only valid on databases that support sorted duplicate
         * data items #MDB_DUPSORT.
         * @param[in] cursor A cursor handle returned by #mdb_cursor_open()
         * @param[out] countp Address where the count will be stored
         * @return A non-zero error value on failure and 0 on success. Some possible
         * errors are:
         * <ul>
         *      <li>EINVAL - cursor is not initialized, or an invalid parameter was specified.
         * </ul>
         */
int  mdb_cursor_count(MDB_cursor *cursor, size_t *countp);

        /** @brief Compare two data items according to a particular database.
         *
         * This returns a comparison as if the two data items were keys in the
         * specified database.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] a The first item to compare
         * @param[in] b The second item to compare
         * @return < 0 if a < b, 0 if a == b, > 0 if a > b
         */
int  mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);

        /** @brief Compare two data items according to a particular database.
         *
         * This returns a comparison as if the two items were data items of
         * a sorted duplicates #MDB_DUPSORT database.
         * @param[in] txn A transaction handle returned by #mdb_txn_begin()
         * @param[in] dbi A database handle returned by #mdb_open()
         * @param[in] a The first item to compare
         * @param[in] b The second item to compare
         * @return < 0 if a < b, 0 if a == b, > 0 if a > b
         */
int  mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
/**     @} */

#ifdef __cplusplus
}
#endif
#endif /* _MDB_H_ */