Provided by: lmdb-doc_0.9.21-1ubuntu0.1_all 
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 key and up to a page of duplicate data items from current cursor position. Move cursor to
prepare for MDB_NEXT_MULTIPLE. Only for MDB_DUPFIXED
MDB_LAST
Position at last key/data item
MDB_LAST_DUP
Position at last data item of current key. Only for MDB_DUPSORT
MDB_NEXT
Position at next data item
MDB_NEXT_DUP
Position at next data item of current key. Only for MDB_DUPSORT
MDB_NEXT_MULTIPLE
Return key and up to a page of duplicate data items from next cursor position. Move cursor to
prepare for MDB_NEXT_MULTIPLE. Only for MDB_DUPFIXED
MDB_NEXT_NODUP
Position at first data item of next key
MDB_PREV
Position at previous data item
MDB_PREV_DUP
Position at previous data item of current key. Only for MDB_DUPSORT
MDB_PREV_NODUP
Position at last data item of previous key
MDB_SET
Position at specified key
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 key and 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.
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.
Author
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LMDB Thu Feb 21 2019 mdb(3)