Provided by: lmdb-doc_0.9.23-0ubuntu1_all bug

NAME

       mdb - OpenLDAP Lightning Memory-Mapped Database Manager.

SYNOPSIS

   Modules
       Version Macros
       Environment Flags
       Database Flags
       Write Flags
       Copy Flags
       Return Codes

   Data Structures
       struct MDB_val
           Generic structure used for passing keys and data in and out of the database.
       struct MDB_stat
           Statistics for a database in the environment.
       struct MDB_envinfo
           Information about the environment.

   Macros
       #define mdb_open(txn,  name,  flags,  dbi)   mdb_dbi_open(txn,name,flags,dbi)
       #define mdb_close(env,  dbi)   mdb_dbi_close(env,dbi)

   Typedefs
       typedef unsigned int MDB_dbi
           A handle for an individual database in the DB environment.
       typedef int() MDB_cmp_func(const MDB_val *a, const MDB_val *b)
           A callback function used to compare two keys in a database.
       typedef void() MDB_rel_func(MDB_val *item, void *oldptr, void *newptr, void *relctx)
           A callback function used to relocate a position-dependent data item in a fixed-address
           database.
       typedef void MDB_assert_func(MDB_env *env, const char *msg)
           A callback function for most LMDB assert() failures, called before printing the
           message and aborting.
       typedef int() MDB_msg_func(const char *msg, void *ctx)
           A callback function used to print a message from the library.

   Enumerations
       enum MDB_cursor_op { MDB_FIRST, MDB_FIRST_DUP, MDB_GET_BOTH, MDB_GET_BOTH_RANGE,
           MDB_GET_CURRENT, MDB_GET_MULTIPLE, MDB_LAST, MDB_LAST_DUP, MDB_NEXT, MDB_NEXT_DUP,
           MDB_NEXT_MULTIPLE, MDB_NEXT_NODUP, MDB_PREV, MDB_PREV_DUP, MDB_PREV_NODUP, MDB_SET,
           MDB_SET_KEY, MDB_SET_RANGE, MDB_PREV_MULTIPLE } Cursor Get operations. "

   Functions
       char * mdb_version (int *major, int *minor, int *patch)
           Return the LMDB library version information.
       char * mdb_strerror (int err)
           Return a string describing a given error code.
       int mdb_env_create (MDB_env **env)
           Create an LMDB environment handle.
       int mdb_env_open (MDB_env *env, const char *path, unsigned int flags, mdb_mode_t mode)
           Open an environment handle.
       int mdb_env_copy (MDB_env *env, const char *path)
           Copy an LMDB environment to the specified path.
       int mdb_env_copyfd (MDB_env *env, mdb_filehandle_t fd)
           Copy an LMDB environment to the specified file descriptor.
       int mdb_env_copy2 (MDB_env *env, const char *path, unsigned int flags)
           Copy an LMDB environment to the specified path, with options.
       int mdb_env_copyfd2 (MDB_env *env, mdb_filehandle_t fd, unsigned int flags)
           Copy an LMDB environment to the specified file descriptor, with options.
       int mdb_env_stat (MDB_env *env, MDB_stat *stat)
           Return statistics about the LMDB environment.
       int mdb_env_info (MDB_env *env, MDB_envinfo *stat)
           Return information about the LMDB environment.
       int mdb_env_sync (MDB_env *env, int force)
           Flush the data buffers to disk.
       void mdb_env_close (MDB_env *env)
           Close the environment and release the memory map.
       int mdb_env_set_flags (MDB_env *env, unsigned int flags, int onoff)
           Set environment flags.
       int mdb_env_get_flags (MDB_env *env, unsigned int *flags)
           Get environment flags.
       int mdb_env_get_path (MDB_env *env, const char **path)
           Return the path that was used in mdb_env_open().
       int mdb_env_get_fd (MDB_env *env, mdb_filehandle_t *fd)
           Return the filedescriptor for the given environment.
       int mdb_env_set_mapsize (MDB_env *env, size_t size)
           Set the size of the memory map to use for this environment.
       int mdb_env_set_maxreaders (MDB_env *env, unsigned int readers)
           Set the maximum number of threads/reader slots for the environment.
       int mdb_env_get_maxreaders (MDB_env *env, unsigned int *readers)
           Get the maximum number of threads/reader slots for the environment.
       int mdb_env_set_maxdbs (MDB_env *env, MDB_dbi dbs)
           Set the maximum number of named databases for the environment.
       int mdb_env_get_maxkeysize (MDB_env *env)
           Get the maximum size of keys and MDB_DUPSORT data we can write.
       int mdb_env_set_userctx (MDB_env *env, void *ctx)
           Set application information associated with the MDB_env.
       void * mdb_env_get_userctx (MDB_env *env)
           Get the application information associated with the MDB_env.
       int mdb_env_set_assert (MDB_env *env, MDB_assert_func *func)
       int mdb_txn_begin (MDB_env *env, MDB_txn *parent, unsigned int flags, MDB_txn **txn)
           Create a transaction for use with the environment.
       MDB_env * mdb_txn_env (MDB_txn *txn)
           Returns the transaction's MDB_env.
       size_t mdb_txn_id (MDB_txn *txn)
           Return the transaction's ID.
       int mdb_txn_commit (MDB_txn *txn)
           Commit all the operations of a transaction into the database.
       void mdb_txn_abort (MDB_txn *txn)
           Abandon all the operations of the transaction instead of saving them.
       void mdb_txn_reset (MDB_txn *txn)
           Reset a read-only transaction.
       int mdb_txn_renew (MDB_txn *txn)
           Renew a read-only transaction.
       int mdb_dbi_open (MDB_txn *txn, const char *name, unsigned int flags, MDB_dbi *dbi)
           Open a database in the environment.
       int mdb_stat (MDB_txn *txn, MDB_dbi dbi, MDB_stat *stat)
           Retrieve statistics for a database.
       int mdb_dbi_flags (MDB_txn *txn, MDB_dbi dbi, unsigned int *flags)
           Retrieve the DB flags for a database handle.
       void mdb_dbi_close (MDB_env *env, MDB_dbi dbi)
           Close a database handle. Normally unnecessary. Use with care:
       int mdb_drop (MDB_txn *txn, MDB_dbi dbi, int del)
           Empty or delete+close a database.
       int mdb_set_compare (MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp)
           Set a custom key comparison function for a database.
       int mdb_set_dupsort (MDB_txn *txn, MDB_dbi dbi, MDB_cmp_func *cmp)
           Set a custom data comparison function for a MDB_DUPSORT database.
       int mdb_set_relfunc (MDB_txn *txn, MDB_dbi dbi, MDB_rel_func *rel)
           Set a relocation function for a MDB_FIXEDMAP database.
       int mdb_set_relctx (MDB_txn *txn, MDB_dbi dbi, void *ctx)
           Set a context pointer for a MDB_FIXEDMAP database's relocation function.
       int mdb_get (MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data)
           Get items from a database.
       int mdb_put (MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data, unsigned int flags)
           Store items into a database.
       int mdb_del (MDB_txn *txn, MDB_dbi dbi, MDB_val *key, MDB_val *data)
           Delete items from a database.
       int mdb_cursor_open (MDB_txn *txn, MDB_dbi dbi, MDB_cursor **cursor)
           Create a cursor handle.
       void mdb_cursor_close (MDB_cursor *cursor)
           Close a cursor handle.
       int mdb_cursor_renew (MDB_txn *txn, MDB_cursor *cursor)
           Renew a cursor handle.
       MDB_txn * mdb_cursor_txn (MDB_cursor *cursor)
           Return the cursor's transaction handle.
       MDB_dbi mdb_cursor_dbi (MDB_cursor *cursor)
           Return the cursor's database handle.
       int mdb_cursor_get (MDB_cursor *cursor, MDB_val *key, MDB_val *data, MDB_cursor_op op)
           Retrieve by cursor.
       int mdb_cursor_put (MDB_cursor *cursor, MDB_val *key, MDB_val *data, unsigned int flags)
           Store by cursor.
       int mdb_cursor_del (MDB_cursor *cursor, unsigned int flags)
           Delete current key/data pair.
       int mdb_cursor_count (MDB_cursor *cursor, size_t *countp)
           Return count of duplicates for current key.
       int mdb_cmp (MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b)
           Compare two data items according to a particular database.
       int mdb_dcmp (MDB_txn *txn, MDB_dbi dbi, const MDB_val *a, const MDB_val *b)
           Compare two data items according to a particular database.
       int mdb_reader_list (MDB_env *env, MDB_msg_func *func, void *ctx)
           Dump the entries in the reader lock table.
       int mdb_reader_check (MDB_env *env, int *dead)
           Check for stale entries in the reader lock table.

Detailed Description

       OpenLDAP Lightning Memory-Mapped Database Manager.

Data Structure Documentation

struct MDB_val

       Generic structure used for passing keys and data in and out of the database.

       Values returned from the database are valid only until a subsequent update operation, or
       the end of the transaction. Do not modify or free them, they commonly point into the
       database itself.

       Key sizes must be between 1 and mdb_env_get_maxkeysize() inclusive. The same applies to
       data sizes in databases with the MDB_DUPSORT flag. Other data items can in theory be from
       0 to 0xffffffff bytes long.

   Data Fields

       size_t mv_size
       void * mv_data

Field Documentation

   size_t MDB_val::mv_size
       size of the data item

   void* MDB_val::mv_data
       address of the data item

struct MDB_stat

       Statistics for a database in the environment.

   Data Fields

       unsigned int ms_psize
       unsigned int ms_depth
       size_t ms_branch_pages
       size_t ms_leaf_pages
       size_t ms_overflow_pages
       size_t ms_entries

Field Documentation

   unsigned int MDB_stat::ms_psize
       Size of a database page. This is currently the same for all databases.

   unsigned int MDB_stat::ms_depth
       Depth (height) of the B-tree

   size_t MDB_stat::ms_branch_pages
       Number of internal (non-leaf) pages

   size_t MDB_stat::ms_leaf_pages
       Number of leaf pages

   size_t MDB_stat::ms_overflow_pages
       Number of overflow pages

   size_t MDB_stat::ms_entries
       Number of data items

struct MDB_envinfo

       Information about the environment.

   Data Fields

       void * me_mapaddr
       size_t me_mapsize
       size_t me_last_pgno
       size_t me_last_txnid
       unsigned int me_maxreaders
       unsigned int me_numreaders

Field Documentation

   void* MDB_envinfo::me_mapaddr
       Address of map, if fixed

   size_t MDB_envinfo::me_mapsize
       Size of the data memory map

   size_t MDB_envinfo::me_last_pgno
       ID of the last used page

   size_t MDB_envinfo::me_last_txnid
       ID of the last committed transaction

   unsigned int MDB_envinfo::me_maxreaders
       max reader slots in the environment

   unsigned int MDB_envinfo::me_numreaders
       max reader slots used in the environment

Macro Definition Documentation

   #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)
       Compat with version <= 0.9.4, avoid clash with libmdb from MDB Tools project

Typedef Documentation

   typedef void() MDB_rel_func(MDB_val *item, void *oldptr, void *newptr, void *relctx)
       A callback function used to relocate a position-dependent data item in a fixed-address
       database. The newptr gives the item's desired address in the memory map, and oldptr gives
       its previous address. The item's actual data resides at the address in item. This callback
       is expected to walk through the fields of the record in item and modify any values based
       at the oldptr address to be relative to the newptr address.

       Parameters:
           item The item that is to be relocated.
           oldptr The previous address.
           newptr The new address to relocate to.
           relctx An application-provided context, set by mdb_set_relctx().

       Todo
           This feature is currently unimplemented.

   typedef void MDB_assert_func(MDB_env *env, const char *msg)
       A callback function for most LMDB assert() failures, called before printing the message
       and aborting.

       Parameters:
           env An environment handle returned by mdb_env_create().
           msg The assertion message, not including newline.

   typedef int() MDB_msg_func(const char *msg, void *ctx)
       A callback function used to print a message from the library.

       Parameters:
           msg The string to be printed.
           ctx An arbitrary context pointer for the callback.

       Returns:
           < 0 on failure, >= 0 on success.

Enumeration Type Documentation

   enum MDB_cursor_op
       Cursor Get operations. This is the set of all operations for retrieving data using a
       cursor.

       Enumerator

       MDB_FIRST
              Position at first key/data item

       MDB_FIRST_DUP
              Position at first data item of current key. Only for MDB_DUPSORT

       MDB_GET_BOTH
              Position at key/data pair. Only for MDB_DUPSORT

       MDB_GET_BOTH_RANGE
              position at key, nearest data. Only for MDB_DUPSORT

       MDB_GET_CURRENT
              Return key/data at current cursor position

       MDB_GET_MULTIPLE
              Return up to a page of duplicate data items from current cursor position. Move
              cursor to prepare for MDB_NEXT_MULTIPLE. Only for MDB_DUPFIXED

       MDB_LAST
              Position at last key/data item

       MDB_LAST_DUP
              Position at last data item of current key. Only for MDB_DUPSORT

       MDB_NEXT
              Position at next data item

       MDB_NEXT_DUP
              Position at next data item of current key. Only for MDB_DUPSORT

       MDB_NEXT_MULTIPLE
              Return up to a page of duplicate data items from next cursor position. Move cursor
              to prepare for MDB_NEXT_MULTIPLE. Only for MDB_DUPFIXED

       MDB_NEXT_NODUP
              Position at first data item of next key

       MDB_PREV
              Position at previous data item

       MDB_PREV_DUP
              Position at previous data item of current key. Only for MDB_DUPSORT

       MDB_PREV_NODUP
              Position at last data item of previous key

       MDB_SET
              Position at specified key

       MDB_SET_KEY
              Position at specified key, return key + data

       MDB_SET_RANGE
              Position at first key greater than or equal to specified key.

       MDB_PREV_MULTIPLE
              Position at previous page and return up to a page of duplicate data items. Only for
              MDB_DUPFIXED

Function Documentation

   char* mdb_version (int * major, int * minor, int * patch)
       Return the LMDB library version information.

       Parameters:
           major if non-NULL, the library major version number is copied here
           minor if non-NULL, the library minor version number is copied here
           patch if non-NULL, the library patch version number is copied here

       Return values:
           version string The library version as a string

       Return the library version info.

   char* mdb_strerror (int err)
       Return a string describing a given error code. This function is a superset of the ANSI C
       X3.159-1989 (ANSI C) strerror(3) function. If the error code is greater than or equal to
       0, then the string returned by the system function strerror(3) is returned. If the error
       code is less than 0, an error string corresponding to the LMDB library error is returned.
       See Return Codes for a list of LMDB-specific error codes.

       Parameters:
           err The error code

       Return values:
           error message The description of the error

   int mdb_env_create (MDB_env ** env)
       Create an LMDB environment handle. This function allocates memory for a MDB_env structure.
       To release the allocated memory and discard the handle, call mdb_env_close(). Before the
       handle may be used, it must be opened using mdb_env_open(). Various other options may also
       need to be set before opening the handle, e.g. mdb_env_set_mapsize(),
       mdb_env_set_maxreaders(), mdb_env_set_maxdbs(), depending on usage requirements.

       Parameters:
           env The address where the new handle will be stored

       Returns:
           A non-zero error value on failure and 0 on success.

   int mdb_env_open (MDB_env * env, const char * path, unsigned int flags, mdb_mode_t mode)
       Open an environment handle. If this function fails, mdb_env_close() must be called to
       discard the MDB_env handle.

       Parameters:
           env An environment handle returned by mdb_env_create()
           path The directory in which the database files reside. This directory must already
           exist and be writable.
           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.

           · 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.
           · MDB_NOSUBDIR By default, LMDB creates its environment in a directory whose pathname
             is given in path, and creates its data and lock files under that directory. With
             this option, path is used as-is for the database main data file. The database lock
             file is the path with '-lock' appended.
           · MDB_RDONLY Open the environment in read-only mode. No write operations will be
             allowed. LMDB will still modify the lock file - except on read-only filesystems,
             where LMDB does not use locks.
           · MDB_WRITEMAP Use a writeable memory map unless MDB_RDONLY is set. This uses fewer
             mallocs but loses protection from application bugs like wild pointer writes and
             other bad updates into the database. This may be slightly faster for DBs that fit
             entirely in RAM, but is slower for DBs larger than RAM. Incompatible with nested
             transactions. Do not mix processes with and without MDB_WRITEMAP on the same
             environment. This can defeat durability (mdb_env_sync etc).
           · 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().
           · 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().
           · 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().
           · MDB_NOTLS Don't use Thread-Local Storage. Tie reader locktable slots to MDB_txn
             objects instead of to threads. I.e. mdb_txn_reset() keeps the slot reseved for the
             MDB_txn object. A thread may use parallel read-only transactions. A read-only
             transaction may span threads if the user synchronizes its use. Applications that
             multiplex many user threads over individual OS threads need this option. Such an
             application must also serialize the write transactions in an OS thread, since LMDB's
             write locking is unaware of the user threads.
           · MDB_NOLOCK Don't do any locking. If concurrent access is anticipated, the caller
             must manage all concurrency itself. For proper operation the caller must enforce
             single-writer semantics, and must ensure that no readers are using old transactions
             while a writer is active. The simplest approach is to use an exclusive lock so that
             no readers may be active at all when a writer begins.
           · MDB_NORDAHEAD Turn off readahead. Most operating systems perform readahead on read
             requests by default. This option turns it off if the OS supports it. Turning it off
             may help random read performance when the DB is larger than RAM and system RAM is
             full. The option is not implemented on Windows.
           · MDB_NOMEMINIT Don't initialize malloc'd memory before writing to unused spaces in
             the data file. By default, memory for pages written to the data file is obtained
             using malloc. While these pages may be reused in subsequent transactions, freshly
             malloc'd pages will be initialized to zeroes before use. This avoids persisting
             leftover data from other code (that used the heap and subsequently freed the memory)
             into the data file. Note that many other system libraries may allocate and free
             memory from the heap for arbitrary uses. E.g., stdio may use the heap for file I/O
             buffers. This initialization step has a modest performance cost so some applications
             may want to disable it using this flag. This option can be a problem for
             applications which handle sensitive data like passwords, and it makes memory
             checkers like Valgrind noisy. This flag is not needed with MDB_WRITEMAP, which
             writes directly to the mmap instead of using malloc for pages. The initialization is
             also skipped if MDB_RESERVE is used; the caller is expected to overwrite all of the
             memory that was reserved in that case. This flag may be changed at any time using
             mdb_env_set_flags().
           mode The UNIX permissions to set on created files and semaphores. This parameter is
           ignored on Windows.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_VERSION_MISMATCH - the version of the LMDB library doesn't match the version
             that created the database environment.
           · MDB_INVALID - the environment file headers are corrupted.
           · ENOENT - the directory specified by the path parameter doesn't exist.
           · EACCES - the user didn't have permission to access the environment files.
           · EAGAIN - the environment was locked by another process.
   int mdb_env_copy (MDB_env * env, const char * path)
       Copy an LMDB environment to the specified path. This function may be used to make a backup
       of an existing environment. No lockfile is created, since it gets recreated at need.
       Note:
           This call can trigger significant file size growth if run in parallel with write
           transactions, because it employs a read-only transaction. See long-lived transactions
           under Caveats.
       Parameters:
           env An environment handle returned by mdb_env_create(). It must have already been
           opened successfully.
           path The directory in which the copy will reside. This directory must already exist
           and be writable but must otherwise be empty.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_env_copyfd (MDB_env * env, mdb_filehandle_t fd)
       Copy an LMDB environment to the specified file descriptor. This function may be used to
       make a backup of an existing environment. No lockfile is created, since it gets recreated
       at need.
       Note:
           This call can trigger significant file size growth if run in parallel with write
           transactions, because it employs a read-only transaction. See long-lived transactions
           under Caveats.
       Parameters:
           env An environment handle returned by mdb_env_create(). It must have already been
           opened successfully.
           fd The filedescriptor to write the copy to. It must have already been opened for Write
           access.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_env_copy2 (MDB_env * env, const char * path, unsigned int flags)
       Copy an LMDB environment to the specified path, with options. This function may be used to
       make a backup of an existing environment. No lockfile is created, since it gets recreated
       at need.
       Note:
           This call can trigger significant file size growth if run in parallel with write
           transactions, because it employs a read-only transaction. See long-lived transactions
           under Caveats.
       Parameters:
           env An environment handle returned by mdb_env_create(). It must have already been
           opened successfully.
           path The directory in which the copy will reside. This directory must already exist
           and be writable but must otherwise be empty.
           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.

           · MDB_CP_COMPACT - Perform compaction while copying: omit free pages and sequentially
             renumber all pages in output. This option consumes more CPU and runs more slowly
             than the default. Currently it fails if the environment has suffered a page leak.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_env_copyfd2 (MDB_env * env, mdb_filehandle_t fd, unsigned int flags)
       Copy an LMDB environment to the specified file descriptor, with options. This function may
       be used to make a backup of an existing environment. No lockfile is created, since it gets
       recreated at need. See mdb_env_copy2() for further details.
       Note:
           This call can trigger significant file size growth if run in parallel with write
           transactions, because it employs a read-only transaction. See long-lived transactions
           under Caveats.
       Parameters:
           env An environment handle returned by mdb_env_create(). It must have already been
           opened successfully.
           fd The filedescriptor to write the copy to. It must have already been opened for Write
           access.
           flags Special options for this operation. See mdb_env_copy2() for options.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_env_stat (MDB_env * env, MDB_stat * stat)
       Return statistics about the LMDB environment.
       Parameters:
           env An environment handle returned by mdb_env_create()
           stat The address of an MDB_stat structure where the statistics will be copied
   int mdb_env_info (MDB_env * env, MDB_envinfo * stat)
       Return information about the LMDB environment.
       Parameters:
           env An environment handle returned by mdb_env_create()
           stat The address of an MDB_envinfo structure where the information will be copied
   int mdb_env_sync (MDB_env * env, int force)
       Flush the data buffers to disk. Data is always written to disk when mdb_txn_commit() is
       called, but the operating system may keep it buffered. LMDB always flushes the OS buffers
       upon commit as well, unless the environment was opened with MDB_NOSYNC or in part
       MDB_NOMETASYNC. This call is not valid if the environment was opened with MDB_RDONLY.
       Parameters:
           env An environment handle returned by mdb_env_create()
           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.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EACCES - the environment is read-only.
           · EINVAL - an invalid parameter was specified.
           · EIO - an error occurred during synchronization.
   void mdb_env_close (MDB_env * env)
       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.
       Parameters:
           env An environment handle returned by mdb_env_create()
   int mdb_env_set_flags (MDB_env * env, unsigned int flags, int onoff)
       Set environment flags. This may be used to set some flags in addition to those from
       mdb_env_open(), or to unset these flags. If several threads change the flags at the same
       time, the result is undefined.
       Parameters:
           env An environment handle returned by mdb_env_create()
           flags The flags to change, bitwise OR'ed together
           onoff A non-zero value sets the flags, zero clears them.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_env_get_flags (MDB_env * env, unsigned int * flags)
       Get environment flags.
       Parameters:
           env An environment handle returned by mdb_env_create()
           flags The address of an integer to store the flags
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_env_get_path (MDB_env * env, const char ** path)
       Return the path that was used in mdb_env_open().
       Parameters:
           env An environment handle returned by mdb_env_create()
           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.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_env_get_fd (MDB_env * env, mdb_filehandle_t * fd)
       Return the filedescriptor for the given environment. This function may be called after
       fork(), so the descriptor can be closed before exec*(). Other LMDB file descriptors have
       FD_CLOEXEC. (Until LMDB 0.9.18, only the lockfile had that.)
       Parameters:
           env An environment handle returned by mdb_env_create()
           fd Address of a mdb_filehandle_t to contain the descriptor.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_env_set_mapsize (MDB_env * env, size_t size)
       Set the size of the memory map to use for this environment. The size should be a multiple
       of the OS page size. The default is 10485760 bytes. The size of the memory map is also the
       maximum size of the database. The value should be chosen as large as possible, to
       accommodate future growth of the database. This function should be called after
       mdb_env_create() and before mdb_env_open(). It may be called at later times if no
       transactions are active in this process. Note that the library does not check for this
       condition, the caller must ensure it explicitly.
       The new size takes effect immediately for the current process but will not be persisted to
       any others until a write transaction has been committed by the current process. Also, only
       mapsize increases are persisted into the environment.
       If the mapsize is increased by another process, and data has grown beyond the range of the
       current mapsize, mdb_txn_begin() will return MDB_MAP_RESIZED. This function may be called
       with a size of zero to adopt the new size.
       Any attempt to set a size smaller than the space already consumed by the environment will
       be silently changed to the current size of the used space.
       Parameters:
           env An environment handle returned by mdb_env_create()
           size The size in bytes
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified, or the environment has an active write
             transaction.
   int mdb_env_set_maxreaders (MDB_env * env, unsigned int readers)
       Set the maximum number of threads/reader slots for the environment. This defines the
       number of slots in the lock table that is used to track readers in the the environment.
       The default is 126. Starting a read-only transaction normally ties a lock table slot to
       the current thread until the environment closes or the thread exits. If MDB_NOTLS is in
       use, mdb_txn_begin() instead ties the slot to the MDB_txn object until it or the MDB_env
       object is destroyed. This function may only be called after mdb_env_create() and before
       mdb_env_open().
       Parameters:
           env An environment handle returned by mdb_env_create()
           readers The maximum number of reader lock table slots
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified, or the environment is already open.
   int mdb_env_get_maxreaders (MDB_env * env, unsigned int * readers)
       Get the maximum number of threads/reader slots for the environment.
       Parameters:
           env An environment handle returned by mdb_env_create()
           readers Address of an integer to store the number of readers
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_env_set_maxdbs (MDB_env * env, MDB_dbi dbs)
       Set the maximum number of named databases for the environment. This function is only
       needed if multiple databases will be used in the environment. Simpler applications that
       use the environment as a single unnamed database can ignore this option. This function may
       only be called after mdb_env_create() and before mdb_env_open().
       Currently a moderate number of slots are cheap but a huge number gets expensive: 7-120
       words per transaction, and every mdb_dbi_open() does a linear search of the opened slots.
       Parameters:
           env An environment handle returned by mdb_env_create()
           dbs The maximum number of databases
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified, or the environment is already open.
   int mdb_env_get_maxkeysize (MDB_env * env)
       Get the maximum size of keys and MDB_DUPSORT data we can write. Depends on the compile-
       time constant MDB_MAXKEYSIZE. Default 511. See MDB_val.
       Parameters:
           env An environment handle returned by mdb_env_create()
       Returns:
           The maximum size of a key we can write
   int mdb_env_set_userctx (MDB_env * env, void * ctx)
       Set application information associated with the MDB_env.
       Parameters:
           env An environment handle returned by mdb_env_create()
           ctx An arbitrary pointer for whatever the application needs.
       Returns:
           A non-zero error value on failure and 0 on success.
   void* mdb_env_get_userctx (MDB_env * env)
       Get the application information associated with the MDB_env.
       Parameters:
           env An environment handle returned by mdb_env_create()
       Returns:
           The pointer set by mdb_env_set_userctx().
   int mdb_env_set_assert (MDB_env * env, MDB_assert_func * func)
       Set or reset the assert() callback of the environment. Disabled if liblmdb is buillt with
       NDEBUG.
       Note:
           This hack should become obsolete as lmdb's error handling matures.
       Parameters:
           env An environment handle returned by mdb_env_create().
           func An MDB_assert_func function, or 0.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_txn_begin (MDB_env * env, MDB_txn * parent, unsigned int flags, MDB_txn ** txn)
       Create a transaction for use with the environment. The transaction handle may be discarded
       using mdb_txn_abort() or mdb_txn_commit().
       Note:
           A transaction and its cursors must only be used by a single thread, and a thread may
           only have a single transaction at a time. If MDB_NOTLS is in use, this does not apply
           to read-only transactions.
           Cursors may not span transactions.
       Parameters:
           env An environment handle returned by mdb_env_create()
           parent If this parameter is non-NULL, the new transaction will be a nested
           transaction, with the transaction indicated by parent as its parent. Transactions may
           be nested to any level. A parent transaction and its cursors may not issue any other
           operations than mdb_txn_commit and mdb_txn_abort while it has active child
           transactions.
           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.

           · MDB_RDONLY This transaction will not perform any write operations.
           txn Address where the new MDB_txn handle will be stored
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_PANIC - a fatal error occurred earlier and the environment must be shut down.
           · MDB_MAP_RESIZED - another process wrote data beyond this MDB_env's mapsize and this
             environment's map must be resized as well. See mdb_env_set_mapsize().
           · MDB_READERS_FULL - a read-only transaction was requested and the reader lock table
             is full. See mdb_env_set_maxreaders().
           · ENOMEM - out of memory.
   MDB_env* mdb_txn_env (MDB_txn * txn)
       Returns the transaction's MDB_env.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
   size_t mdb_txn_id (MDB_txn * txn)
       Return the transaction's ID. This returns the identifier associated with this transaction.
       For a read-only transaction, this corresponds to the snapshot being read; concurrent
       readers will frequently have the same transaction ID.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
       Returns:
           A transaction ID, valid if input is an active transaction.
   int mdb_txn_commit (MDB_txn * txn)
       Commit all the operations of a transaction into the database. The transaction handle is
       freed. It and its cursors must not be used again after this call, except with
       mdb_cursor_renew().
       Note:
           Earlier documentation incorrectly said all cursors would be freed. Only write-
           transactions free cursors.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
           · ENOSPC - no more disk space.
           · EIO - a low-level I/O error occurred while writing.
           · ENOMEM - out of memory.
   void mdb_txn_abort (MDB_txn * txn)
       Abandon all the operations of the transaction instead of saving them. The transaction
       handle is freed. It and its cursors must not be used again after this call, except with
       mdb_cursor_renew().
       Note:
           Earlier documentation incorrectly said all cursors would be freed. Only write-
           transactions free cursors.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
   void mdb_txn_reset (MDB_txn * txn)
       Reset a read-only transaction. Abort the transaction like mdb_txn_abort(), but keep the
       transaction handle. mdb_txn_renew() may reuse the handle. This saves allocation overhead
       if the process will start a new read-only transaction soon, and also locking overhead if
       MDB_NOTLS is in use. The reader table lock is released, but the table slot stays tied to
       its thread or MDB_txn. Use mdb_txn_abort() to discard a reset handle, and to free its lock
       table slot if MDB_NOTLS is in use. Cursors opened within the transaction must not be used
       again after this call, except with mdb_cursor_renew(). Reader locks generally don't
       interfere with writers, but they keep old versions of database pages allocated. Thus they
       prevent the old pages from being reused when writers commit new data, and so under heavy
       load the database size may grow much more rapidly than otherwise.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
   int mdb_txn_renew (MDB_txn * txn)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_PANIC - a fatal error occurred earlier and the environment must be shut down.
           · EINVAL - an invalid parameter was specified.
   int mdb_dbi_open (MDB_txn * txn, const char * name, unsigned int flags, MDB_dbi * dbi)
       Open a database in the environment. A database handle denotes the name and parameters of a
       database, independently of whether such a database exists. The database handle may be
       discarded by calling mdb_dbi_close(). The old database handle is returned if the database
       was already open. The handle may only be closed once.
       The database handle will be private to the current transaction until the transaction is
       successfully committed. If the transaction is aborted the handle will be closed
       automatically. After a successful commit the handle will reside in the shared environment,
       and may be used by other transactions.
       This function must not be called from multiple concurrent transactions in the same
       process. A transaction that uses this function must finish (either commit or abort) before
       any other transaction in the process may use this function.
       To use named databases (with name != NULL), mdb_env_set_maxdbs() must be called before
       opening the environment. Database names are keys in the unnamed database, and may be read
       but not written.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           name The name of the database to open. If only a single database is needed in the
           environment, this value may be NULL.
           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.

           · 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.
           · 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.
           · MDB_INTEGERKEY Keys are binary integers in native byte order, either unsigned int or
             size_t, and will be sorted as such. The keys must all be of the same size.
           · MDB_DUPFIXED This flag may only be used in combination with MDB_DUPSORT. This option
             tells the library that the data items for this database are all the same size, which
             allows further optimizations in storage and retrieval. When all data items are the
             same size, the MDB_GET_MULTIPLE, MDB_NEXT_MULTIPLE and MDB_PREV_MULTIPLE cursor
             operations may be used to retrieve multiple items at once.
           · MDB_INTEGERDUP This option specifies that duplicate data items are binary integers,
             similar to MDB_INTEGERKEY keys.
           · MDB_REVERSEDUP This option specifies that duplicate data items should be compared as
             strings in reverse order.
           · 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.
           dbi Address where the new MDB_dbi handle will be stored
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_NOTFOUND - the specified database doesn't exist in the environment and
             MDB_CREATE was not specified.
           · MDB_DBS_FULL - too many databases have been opened. See mdb_env_set_maxdbs().
   int mdb_stat (MDB_txn * txn, MDB_dbi dbi, MDB_stat * stat)
       Retrieve statistics for a database.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           stat The address of an MDB_stat structure where the statistics will be copied
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_dbi_flags (MDB_txn * txn, MDB_dbi dbi, unsigned int * flags)
       Retrieve the DB flags for a database handle.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           flags Address where the flags will be returned.
       Returns:
           A non-zero error value on failure and 0 on success.
   void mdb_dbi_close (MDB_env * env, MDB_dbi dbi)
       Close a database handle. Normally unnecessary. Use with care: This call is not mutex
       protected. Handles should only be closed by a single thread, and only if no other threads
       are going to reference the database handle or one of its cursors any further. Do not close
       a handle if an existing transaction has modified its database. Doing so can cause
       misbehavior from database corruption to errors like MDB_BAD_VALSIZE (since the DB name is
       gone).
       Closing a database handle is not necessary, but lets mdb_dbi_open() reuse the handle
       value. Usually it's better to set a bigger mdb_env_set_maxdbs(), unless that value would
       be large.
       Parameters:
           env An environment handle returned by mdb_env_create()
           dbi A database handle returned by mdb_dbi_open()
   int mdb_drop (MDB_txn * txn, MDB_dbi dbi, int del)
       Empty or delete+close a database. See mdb_dbi_close() for restrictions about closing the
       DB handle.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           del 0 to empty the DB, 1 to delete it from the environment and close the DB handle.
       Returns:
           A non-zero error value on failure and 0 on success.
   int mdb_set_compare (MDB_txn * txn, MDB_dbi dbi, MDB_cmp_func * cmp)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           cmp A MDB_cmp_func function
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_set_dupsort (MDB_txn * txn, MDB_dbi dbi, MDB_cmp_func * cmp)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           cmp A MDB_cmp_func function
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_set_relfunc (MDB_txn * txn, MDB_dbi dbi, MDB_rel_func * rel)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           rel A MDB_rel_func function
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_set_relctx (MDB_txn * txn, MDB_dbi dbi, void * ctx)
       Set a context pointer for a MDB_FIXEDMAP database's relocation function. See
       mdb_set_relfunc and MDB_rel_func for more details.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           ctx An arbitrary pointer for whatever the application needs. It will be passed to the
           callback function set by mdb_set_relfunc as its relctx parameter whenever the callback
           is invoked.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   int mdb_get (MDB_txn * txn, MDB_dbi dbi, MDB_val * key, MDB_val * data)
       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 key are returned in the
       structure to which 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.
           Values returned from the database are valid only until a subsequent update operation,
           or the end of the transaction.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           key The key to search for in the database
           data The data corresponding to the key
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_NOTFOUND - the key was not in the database.
           · EINVAL - an invalid parameter was specified.
   int mdb_put (MDB_txn * txn, MDB_dbi dbi, MDB_val * key, MDB_val * data, unsigned int flags)
       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).
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           key The key to store in the database
           data The data to store
           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.

           · 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.
           · 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
             data parameter will be set to point to the existing item.
           · MDB_RESERVE - reserve space for data of the given size, but don't copy the given
             data. Instead, return a pointer to the reserved space, which the caller can fill in
             later - before the next update operation or the transaction ends. This saves an
             extra memcpy if the data is being generated later. LMDB does nothing else with this
             memory, the caller is expected to modify all of the space requested. This flag must
             not be specified if the database was opened with MDB_DUPSORT.
           · MDB_APPEND - append the given key/data pair to the end of the database. This option
             allows fast bulk loading when keys are already known to be in the correct order.
             Loading unsorted keys with this flag will cause a MDB_KEYEXIST error.
           · MDB_APPENDDUP - as above, but for sorted dup data.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_MAP_FULL - the database is full, see mdb_env_set_mapsize().
           · MDB_TXN_FULL - the transaction has too many dirty pages.
           · EACCES - an attempt was made to write in a read-only transaction.
           · EINVAL - an invalid parameter was specified.
   int mdb_del (MDB_txn * txn, MDB_dbi dbi, MDB_val * key, MDB_val * data)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           key The key to delete from the database
           data The data to delete
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EACCES - an attempt was made to write in a read-only transaction.
           · EINVAL - an invalid parameter was specified.
   int mdb_cursor_open (MDB_txn * txn, MDB_dbi dbi, MDB_cursor ** cursor)
       Create a cursor handle. A cursor is associated with a specific transaction and database. A
       cursor cannot be used when its database handle is closed. Nor when its transaction has
       ended, except with mdb_cursor_renew(). It can be discarded with mdb_cursor_close(). A
       cursor in a write-transaction can be closed before its transaction ends, and will
       otherwise be closed when its transaction ends. A cursor in a read-only transaction must be
       closed explicitly, before or after its transaction ends. It can be reused with
       mdb_cursor_renew() before finally closing it.
       Note:
           Earlier documentation said that cursors in every transaction were closed when the
           transaction committed or aborted.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           cursor Address where the new MDB_cursor handle will be stored
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   void mdb_cursor_close (MDB_cursor * cursor)
       Close a cursor handle. The cursor handle will be freed and must not be used again after
       this call. Its transaction must still be live if it is a write-transaction.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
   int mdb_cursor_renew (MDB_txn * txn, MDB_cursor * cursor)
       Renew a cursor handle. A cursor is associated with a specific transaction and database.
       Cursors that are only used in read-only transactions may be re-used, to avoid unnecessary
       malloc/free overhead. The cursor may be associated with a new read-only transaction, and
       referencing the same database handle as it was created with. This may be done whether the
       previous transaction is live or dead.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           cursor A cursor handle returned by mdb_cursor_open()
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - an invalid parameter was specified.
   MDB_txn* mdb_cursor_txn (MDB_cursor * cursor)
       Return the cursor's transaction handle.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
   MDB_dbi mdb_cursor_dbi (MDB_cursor * cursor)
       Return the cursor's database handle.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
   int mdb_cursor_get (MDB_cursor * cursor, MDB_val * key, MDB_val * data, MDB_cursor_op op)
       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 key refers (except for the case
       of the MDB_SET option, in which the key object is unchanged), and the address and length
       of the data are returned in the object to which data refers. See mdb_get() for
       restrictions on using the output values.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
           key The key for a retrieved item
           data The data of a retrieved item
           op A cursor operation MDB_cursor_op
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_NOTFOUND - no matching key found.
           · EINVAL - an invalid parameter was specified.
   int mdb_cursor_put (MDB_cursor * cursor, MDB_val * key, MDB_val * data, unsigned int flags)
       Store by cursor. This function stores key/data pairs into the database. The cursor is
       positioned at the new item, or on failure usually near it.
       Note:
           Earlier documentation incorrectly said errors would leave the state of the cursor
           unchanged.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
           key The key operated on.
           data The data operated on.
           flags Options for this operation. This parameter must be set to 0 or one of the values
           described here.

           · MDB_CURRENT - replace the item at the current cursor position. The key parameter
             must still be provided, and must match it. If using sorted duplicates (MDB_DUPSORT)
             the data item must still sort into the same place. This is intended to be used when
             the new data is the same size as the old. Otherwise it will simply perform a delete
             of the old record followed by an insert.
           · 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.
           · 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).
           · MDB_RESERVE - reserve space for data of the given size, but don't copy the given
             data. Instead, return a pointer to the reserved space, which the caller can fill in
             later - before the next update operation or the transaction ends. This saves an
             extra memcpy if the data is being generated later. This flag must not be specified
             if the database was opened with MDB_DUPSORT.
           · MDB_APPEND - append the given key/data pair to the end of the database. No key
             comparisons are performed. This option allows fast bulk loading when keys are
             already known to be in the correct order. Loading unsorted keys with this flag will
             cause a MDB_KEYEXIST error.
           · MDB_APPENDDUP - as above, but for sorted dup data.
           · MDB_MULTIPLE - store multiple contiguous data elements in a single request. This
             flag may only be specified if the database was opened with MDB_DUPFIXED. The data
             argument must be an array of two MDB_vals. The mv_size of the first MDB_val must be
             the size of a single data element. The mv_data of the first MDB_val must point to
             the beginning of the array of contiguous data elements. The mv_size of the second
             MDB_val must be the count of the number of data elements to store. On return this
             field will be set to the count of the number of elements actually written. The
             mv_data of the second MDB_val is unused.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · MDB_MAP_FULL - the database is full, see mdb_env_set_mapsize().
           · MDB_TXN_FULL - the transaction has too many dirty pages.
           · EACCES - an attempt was made to write in a read-only transaction.
           · EINVAL - an invalid parameter was specified.
   int mdb_cursor_del (MDB_cursor * cursor, unsigned int flags)
       Delete current key/data pair. This function deletes the key/data pair to which the cursor
       refers. This does not invalidate the cursor, so operations such as MDB_NEXT can still be
       used on it. Both MDB_NEXT and MDB_GET_CURRENT will return the same record after this
       operation.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
           flags Options for this operation. This parameter must be set to 0 or one of the values
           described here.

           · 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.
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EACCES - an attempt was made to write in a read-only transaction.
           · EINVAL - an invalid parameter was specified.
   int mdb_cursor_count (MDB_cursor * cursor, size_t * countp)
       Return count of duplicates for current key. This call is only valid on databases that
       support sorted duplicate data items MDB_DUPSORT.
       Parameters:
           cursor A cursor handle returned by mdb_cursor_open()
           countp Address where the count will be stored
       Returns:
           A non-zero error value on failure and 0 on success. Some possible errors are:

           · EINVAL - cursor is not initialized, or an invalid parameter was specified.
   int mdb_cmp (MDB_txn * txn, MDB_dbi dbi, const MDB_val * a, const MDB_val * b)
       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.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           a The first item to compare
           b The second item to compare
       Returns:
           < 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)
       Compare two data items according to a particular database. This returns a comparison as if
       the two items were data items of the specified database. The database must have the
       MDB_DUPSORT flag.
       Parameters:
           txn A transaction handle returned by mdb_txn_begin()
           dbi A database handle returned by mdb_dbi_open()
           a The first item to compare
           b The second item to compare
       Returns:
           < 0 if a < b, 0 if a == b, > 0 if a > b
   int mdb_reader_list (MDB_env * env, MDB_msg_func * func, void * ctx)
       Dump the entries in the reader lock table.
       Parameters:
           env An environment handle returned by mdb_env_create()
           func A MDB_msg_func function
           ctx Anything the message function needs
       Returns:
           < 0 on failure, >= 0 on success.
   int mdb_reader_check (MDB_env * env, int * dead)
       Check for stale entries in the reader lock table.
       Parameters:
           env An environment handle returned by mdb_env_create()
           dead Number of stale slots that were cleared
       Returns:
           0 on success, non-zero on failure.

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