openldap/libraries/liblmdb/lmdb.h
2013-02-20 05:08:52 -08:00

1215 lines
53 KiB
C

/** @file lmdb.h
* @brief Lightning memory-mapped database library
*
* @mainpage Lightning Memory-Mapped Database Manager (MDB)
*
* @section intro_sec Introduction
* 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.
*
* @section caveats_sec Caveats
* Troubleshooting the lock file, plus semaphores on BSD systems:
*
* - A broken lockfile can cause sync issues.
* Stale reader transactions left behind by an aborted program
* cause further writes to grow the database quickly, and
* stale locks can block further operation.
*
* Fix: 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.
*
* - Avoid suspending a process with active transactions. These
* would then be "long-lived" as above. Also read transactions
* suspended when writers commit could sometimes see wrong data.
*
* ...when several processes can use a database concurrently:
*
* - Avoid aborting a process with an active transaction.
* The transaction becomes "long-lived" as above until 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 2011-2013 Howard Chu, Symas Corp. All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted only as authorized by the OpenLDAP
* Public License.
*
* A copy of this license is available in the file LICENSE in the
* top-level directory of the distribution or, alternatively, at
* <http://www.OpenLDAP.org/license.html>.
*
* @par Derived From:
* This code is derived from btree.c written by Martin Hedenfalk.
*
* Copyright (c) 2009, 2010 Martin Hedenfalk <martin@bzero.se>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
#ifndef _LMDB_H_
#define _LMDB_H_
#include <sys/types.h>
#ifdef __cplusplus
extern "C" {
#endif
/** @defgroup mdb MDB API
* @{
* @brief OpenLDAP Lightning Memory-Mapped Database Manager
*/
/** @defgroup Version Version Macros
* @{
*/
/** Library major version */
#define MDB_VERSION_MAJOR 0
/** Library minor version */
#define MDB_VERSION_MINOR 9
/** Library patch version */
#define MDB_VERSION_PATCH 6
/** 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 "January 10, 2013"
/** 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.
*
* Key sizes must be between 1 and the liblmdb build-time constant
* #MDB_MAXKEYSIZE inclusive. This currently defaults to 511. The
* same applies to data sizes in databases with the #MDB_DUPSORT flag.
* Other data items can in theory be from 0 to 0xffffffff bytes long.
*/
typedef struct MDB_val {
size_t mv_size; /**< size of the data item */
void *mv_data; /**< address of the data item */
} MDB_val;
/** @brief A callback function used to compare two keys in a database */
typedef int (MDB_cmp_func)(const MDB_val *a, const MDB_val *b);
/** @brief A callback function used to relocate a position-dependent data item
* in a fixed-address database.
*
* The \b newptr gives the item's desired address in
* the memory map, and \b oldptr gives its previous address. The item's actual
* data resides at the address in \b item. This callback is expected to walk
* through the fields of the record in \b item and modify any
* values based at the \b oldptr address to be relative to the \b newptr address.
* @param[in,out] item The item that is to be relocated.
* @param[in] oldptr The previous address.
* @param[in] newptr The new address to relocate to.
* @param[in] relctx An application-provided context, set by #mdb_set_relctx().
* @todo This feature is currently unimplemented.
*/
typedef void (MDB_rel_func)(MDB_val *item, void *oldptr, void *newptr, void *relctx);
/** @defgroup mdb_env Environment Flags
*
* Values do not overlap Database Flags.
* @{
*/
/** mmap at a fixed address (experimental) */
#define MDB_FIXEDMAP 0x01
/** no environment directory */
#define MDB_NOSUBDIR 0x4000
/** don't fsync after commit */
#define MDB_NOSYNC 0x10000
/** read only */
#define MDB_RDONLY 0x20000
/** don't fsync metapage after commit */
#define MDB_NOMETASYNC 0x40000
/** use writable mmap */
#define MDB_WRITEMAP 0x80000
/** use asynchronous msync when MDB_WRITEMAP is used */
#define MDB_MAPASYNC 0x100000
/** @} */
/** @defgroup mdb_dbi_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)
/** Txn has too many dirty pages */
#define MDB_TXN_FULL (-30788)
/** Cursor stack too deep - internal error */
#define MDB_CURSOR_FULL (-30787)
/** Page has not enough space - internal error */
#define MDB_PAGE_FULL (-30786)
/** Database contents grew beyond environment mapsize */
#define MDB_MAP_RESIZED (-30785)
/** Database flags changed or would change */
#define MDB_INCOMPATIBLE (-30784)
#define MDB_LAST_ERRCODE MDB_INCOMPATIBLE
/** @} */
/** @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_RDONLY
* Open the environment in read-only mode. No write operations will be
* allowed. MDB will still modify the lock file - except on read-only
* filesystems, where MDB does not use locks.
* <li>#MDB_WRITEMAP
* Use a writeable memory map unless MDB_RDONLY is set. This is faster
* and uses fewer mallocs, but loses protection from application bugs
* like wild pointer writes and other bad updates into the database.
* Incompatible with nested transactions.
* <li>#MDB_NOMETASYNC
* Flush system buffers to disk only once per transaction, omit the
* metadata flush. Defer that until the system flushes files to disk,
* or next non-MDB_RDONLY commit or #mdb_env_sync(). This optimization
* maintains database integrity, but a system crash may undo the last
* committed transaction. I.e. it preserves the ACI (atomicity,
* consistency, isolation) but not D (durability) database property.
* This flag may be changed at any time using #mdb_env_set_flags().
* <li>#MDB_NOSYNC
* Don't flush system buffers to disk when committing a transaction.
* This optimization means a system crash can corrupt the database or
* lose the last transactions if buffers are not yet flushed to disk.
* The risk is governed by how often the system flushes dirty buffers
* to disk and how often #mdb_env_sync() is called. However, if the
* filesystem preserves write order and the #MDB_WRITEMAP flag is not
* used, transactions exhibit ACI (atomicity, consistency, isolation)
* properties and only lose D (durability). I.e. database integrity
* is maintained, but a system crash may undo the final transactions.
* Note that (#MDB_NOSYNC | #MDB_WRITEMAP) leaves the system with no
* hint for when to write transactions to disk, unless #mdb_env_sync()
* is called. (#MDB_MAPASYNC | #MDB_WRITEMAP) may be preferable.
* This flag may be changed at any time using #mdb_env_set_flags().
* <li>#MDB_MAPASYNC
* When using #MDB_WRITEMAP, use asynchronous flushes to disk.
* As with #MDB_NOSYNC, a system crash can then corrupt the
* database or lose the last transactions. Calling #mdb_env_sync()
* ensures on-disk database integrity until next commit.
* This flag may be changed at any time using #mdb_env_set_flags().
* </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>#MDB_INVALID - the environment file headers are corrupted.
* <li>ENOENT - the directory specified by the path parameter doesn't exist.
* <li>EACCES - the user didn't have permission to access the environment files.
* <li>EAGAIN - the environment was locked by another process.
* </ul>
*/
int mdb_env_open(MDB_env *env, const char *path, unsigned int flags, 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 or in part #MDB_NOMETASYNC.
* @param[in] env An environment handle returned by #mdb_env_create()
* @param[in] force If non-zero, force a synchronous flush. Otherwise
* if the environment has the #MDB_NOSYNC flag set the flushes
* will be omitted, and with #MDB_MAPASYNC they will be asynchronous.
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>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().
* The size may be changed by closing and reopening the environment.
* Any attempt to set a size smaller than the space already consumed
* by the environment will be silently changed to the current size of the used space.
* @param[in] env An environment handle returned by #mdb_env_create()
* @param[in] size The size in bytes
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified, or the environment 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 named databases for the environment.
*
* This function is only needed if multiple databases will be used in the
* environment. Simpler applications that use the environment as a single
* unnamed database can ignore this option.
* This function may only be called after #mdb_env_create() and before #mdb_env_open().
* @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>#MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's
* mapsize and the environment must be shut down.
* <li>#MDB_READERS_FULL - a read-only transaction was requested and
* the reader lock table is full. See #mdb_env_set_maxreaders().
* <li>ENOMEM - out of memory.
* </ul>
*/
int mdb_txn_begin(MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn);
/** @brief 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 - out of memory.
* </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);
/** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
#define mdb_open(txn,name,flags,dbi) mdb_dbi_open(txn,name,flags,dbi)
/** Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project */
#define mdb_close(env,dbi) mdb_dbi_close(env,dbi)
/** @brief Open a database in the environment.
*
* The database handle may be discarded by calling #mdb_dbi_close().
* It denotes the name and parameters of a database, independently of
* whether such a database exists. It will not exist if the transaction
* which created it aborted, nor if another process deleted it. 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.
* Preexisting transactions, other than the current transaction and
* any parents, must not use the new handle. Nor must their children.
* To use named databases (with name != NULL), #mdb_env_set_maxdbs()
* must be called before opening the environment.
* @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>#MDB_DBS_FULL - too many databases have been opened. See #mdb_env_set_maxdbs().
* </ul>
*/
int mdb_dbi_open(MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi);
/** @brief Retrieve statistics for a database.
*
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[out] stat The address of an #MDB_stat structure
* where the statistics will be copied
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_stat(MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat);
/** @brief 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 or one of its cursors any further. Do not close
* a handle if an existing transaction has modified its database.
* @param[in] env An environment handle returned by #mdb_env_create()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
*/
void mdb_dbi_close(MDB_env *env, MDB_dbi dbi);
/** @brief Delete a database and/or free all its pages.
*
* If the \b del parameter is 1, 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_dbi_open()
* @param[in] del 1 to delete the DB from the environment,
* 0 to 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_dbi_open(), the keys are compared lexically, with shorter keys collating
* before longer keys.
* @warning This function must be called before any data access functions are used,
* otherwise data corruption may occur. The same comparison function must be used by every
* program accessing the database, every time the database is used.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] cmp A #MDB_cmp_func function
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_set_compare(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
/** @brief Set a custom data comparison function for a #MDB_DUPSORT database.
*
* This comparison function is called whenever it is necessary to compare a data
* item specified by the application with a data item currently stored in the database.
* This function only takes effect if the database was opened with the #MDB_DUPSORT
* flag.
* If no comparison function is specified, and no special key flags were specified
* with #mdb_dbi_open(), the data items are compared lexically, with shorter items collating
* before longer items.
* @warning This function must be called before any data access functions are used,
* otherwise data corruption may occur. The same comparison function must be used by every
* program accessing the database, every time the database is used.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] cmp A #MDB_cmp_func function
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_set_dupsort(MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp);
/** @brief Set a relocation function for a #MDB_FIXEDMAP database.
*
* @todo The relocation function is called whenever it is necessary to move the data
* of an item to a different position in the database (e.g. through tree
* balancing operations, shifts as a result of adds or deletes, etc.). It is
* intended to allow address/position-dependent data items to be stored in
* a database in an environment opened with the #MDB_FIXEDMAP option.
* Currently the relocation feature is unimplemented and setting
* this function has no effect.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] rel A #MDB_rel_func function
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_set_relfunc(MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel);
/** @brief Set a context pointer for a #MDB_FIXEDMAP database's relocation function.
*
* See #mdb_set_relfunc and #MDB_rel_func for more details.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] ctx An arbitrary pointer for whatever the application needs.
* It will be passed to the callback function set by #mdb_set_relfunc
* as its \b relctx parameter whenever the callback is invoked.
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_set_relctx(MDB_txn *txn, MDB_dbi dbi, void *ctx);
/** @brief Get items from a database.
*
* This function retrieves key/data pairs from the database. The address
* and length of the data associated with the specified \b key are returned
* in the structure to which \b data refers.
* If the database supports duplicate keys (#MDB_DUPSORT) then the
* first data item for the key will be returned. Retrieval of other
* items requires the use of #mdb_cursor_get().
*
* @note The memory pointed to by the returned values is owned by the
* database. The caller need not dispose of the memory, and may not
* modify it in any way. For values returned in a read-only transaction
* any modification attempts will cause a SIGSEGV.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] key The key to search for in the database
* @param[out] data The data corresponding to the key
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>#MDB_NOTFOUND - the key was not in the database.
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_get(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
/** @brief Store items into a database.
*
* This function stores key/data pairs in the database. The default behavior
* is to enter the new key/data pair, replacing any previously existing key
* if duplicates are disallowed, or adding a duplicate data item if
* duplicates are allowed (#MDB_DUPSORT).
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] key The key to store in the database
* @param[in,out] data The data to store
* @param[in] flags Special options for this operation. This parameter
* must be set to 0 or by bitwise OR'ing together one or more of the
* values described here.
* <ul>
* <li>#MDB_NODUPDATA - enter the new key/data pair only if it does not
* already appear in the database. This flag may only be specified
* if the database was opened with #MDB_DUPSORT. The function will
* return #MDB_KEYEXIST if the key/data pair already appears in the
* database.
* <li>#MDB_NOOVERWRITE - enter the new key/data pair only if the key
* does not already appear in the database. The function will return
* #MDB_KEYEXIST if the key already appears in the database, even if
* the database supports duplicates (#MDB_DUPSORT). The \b data
* parameter will be set to point to the existing item.
* <li>#MDB_RESERVE - reserve space for data of the given size, but
* don't copy the given data. Instead, return a pointer to the
* reserved space, which the caller can fill in later. 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>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
* <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
* <li>EACCES - an attempt was made to write in a read-only transaction.
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_put(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data,
unsigned int flags);
/** @brief Delete items from a database.
*
* This function removes key/data pairs from the database.
* If the database does not support sorted duplicate data items
* (#MDB_DUPSORT) the data parameter is ignored.
* If the database supports sorted duplicates and the data parameter
* is NULL, all of the duplicate data items for the key will be
* deleted. Otherwise, if the data parameter is non-NULL
* only the matching data item will be deleted.
* This function will return #MDB_NOTFOUND if the specified key/data
* pair is not in the database.
* @param[in] txn A transaction handle returned by #mdb_txn_begin()
* @param[in] dbi A database handle returned by #mdb_dbi_open()
* @param[in] key The key to delete from the database
* @param[in] data The data to delete
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EACCES - an attempt was made to write in a read-only transaction.
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_del(MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data);
/** @brief Create a cursor handle.
*
* 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_dbi_open()
* @param[out] cursor Address where the new #MDB_cursor handle will be stored
* @return A non-zero error value on failure and 0 on success. Some possible
* errors are:
* <ul>
* <li>EINVAL - an invalid parameter was specified.
* </ul>
*/
int mdb_cursor_open(MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor);
/** @brief Close a cursor handle.
*
* The cursor handle will be freed and must not be used again after this call.
* @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>#MDB_MAP_FULL - the database is full, see #mdb_env_set_mapsize().
* <li>#MDB_TXN_FULL - the transaction has too many dirty pages.
* <li>EACCES - an attempt was made to 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_dbi_open()
* @param[in] a The first item to compare
* @param[in] b The second item to compare
* @return < 0 if a < b, 0 if a == b, > 0 if a > b
*/
int mdb_cmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
/** @brief Compare two data items according to a particular database.
*
* This returns a comparison as if the two items were data items of
* 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_dbi_open()
* @param[in] a The first item to compare
* @param[in] b The second item to compare
* @return < 0 if a < b, 0 if a == b, > 0 if a > b
*/
int mdb_dcmp(MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b);
/** @} */
#ifdef __cplusplus
}
#endif
#endif /* _LMDB_H_ */