Class LMDB


  • public class LMDB
    extends java.lang.Object
    Contains bindings to LMDB, the Symas Lightning Memory-Mapped Database.

    Getting Started

    LMDB is compact, fast, powerful, and robust and implements a simplified variant of the BerkeleyDB (BDB) API.

    Everything starts with an environment, created by env_create. Once created, this environment must also be opened with env_open. env_open gets passed a name which is interpreted as a directory path. Note that this directory must exist already, it is not created for you. Within that directory, a lock file and a storage file will be generated. If you don't want to use a directory, you can pass the NOSUBDIR option, in which case the path you provided is used directly as the data file, and another file with a "-lock" suffix added will be used for the lock file.

    Once the environment is open, a transaction can be created within it using txn_begin. Transactions may be read-write or read-only, and read-write transactions may be nested. A transaction must only be used by one thread at a time. Transactions are always required, even for read-only access. The transaction provides a consistent view of the data.

    Once a transaction has been created, a database can be opened within it using dbi_open. If only one database will ever be used in the environment, a NULL can be passed as the database name. For named databases, the CREATE flag must be used to create the database if it doesn't already exist. Also, env_set_maxdbs must be called after env_create and before env_open to set the maximum number of named databases you want to support.

    Note: a single transaction can open multiple databases. Generally databases should only be opened once, by the first transaction in the process. After the first transaction completes, the database handles can freely be used by all subsequent transactions.

    Within a transaction, get and put can store single key/value pairs if that is all you need to do (but see Cursors below if you want to do more).

    A key/value pair is expressed as two MDBVal structures. This struct has two fields, mv_size and mv_data. The data is a void pointer to an array of mv_size bytes.

    Because LMDB is very efficient (and usually zero-copy), the data returned in an MDBVal structure may be memory-mapped straight from disk. In other words look but do not touch (or free() for that matter). Once a transaction is closed, the values can no longer be used, so make a copy if you need to keep them after that.

    Cursors

    To do more powerful things, we must use a cursor.

    Within the transaction, a cursor can be created with cursor_open. With this cursor we can store/retrieve/delete (multiple) values using cursor_get, cursor_put, and cursor_del.

    cursor_get positions itself depending on the cursor operation requested, and for some operations, on the supplied key. For example, to list all key/value pairs in a database, use operation FIRST for the first call to cursor_get, and NEXT on subsequent calls, until the end is hit.

    To retrieve all keys starting from a specified key value, use SET.

    When using cursor_put, either the function will position the cursor for you based on the key, or you can use operation CURRENT to use the current position of the cursor. Note that key must then match the current position's key.

    Summarizing the Opening

    So we have a cursor in a transaction which opened a database in an environment which is opened from a filesystem after it was separately created.

    Or, we create an environment, open it from a filesystem, create a transaction within it, open a database within that transaction, and create a cursor within all of the above.

    Threads and Processes

    LMDB uses POSIX locks on files, and these locks have issues if one process opens a file multiple times. Because of this, do not env_open a file multiple times from a single process. Instead, share the LMDB environment that has opened the file across all threads. Otherwise, if a single process opens the same environment multiple times, closing it once will remove all the locks held on it, and the other instances will be vulnerable to corruption from other processes.

    Also note that a transaction is tied to one thread by default using Thread Local Storage. If you want to pass read-only transactions across threads, you can use the NOTLS option on the environment.

    Transactions, Rollbacks, etc.

    To actually get anything done, a transaction must be committed using txn_commit. Alternatively, all of a transaction's operations can be discarded using txn_abort. In a read-only transaction, any cursors will not automatically be freed. In a read-write transaction, all cursors will be freed and must not be used again.

    For read-only transactions, obviously there is nothing to commit to storage. The transaction still must eventually be aborted to close any database handle(s) opened in it, or committed to keep the database handles around for reuse in new transactions.

    In addition, as long as a transaction is open, a consistent view of the database is kept alive, which requires storage. A read-only transaction that no longer requires this consistent view should be terminated (committed or aborted) when the view is no longer needed (but see below for an optimization).

    There can be multiple simultaneously active read-only transactions but only one that can write. Once a single read-write transaction is opened, all further attempts to begin one will block until the first one is committed or aborted. This has no effect on read-only transactions, however, and they may continue to be opened at any time.

    Duplicate Keys

    get and put respectively have no and only some support for multiple key/value pairs with identical keys. If there are multiple values for a key, get will only return the first value.

    When multiple values for one key are required, pass the DUPSORT flag to dbi_open. In an DUPSORT database, by default put will not replace the value for a key if the key existed already. Instead it will add the new value to the key. In addition, del will pay attention to the value field too, allowing for specific values of a key to be deleted.

    Finally, additional cursor operations become available for traversing through and retrieving duplicate values.

    Some Optimization

    If you frequently begin and abort read-only transactions, as an optimization, it is possible to only reset and renew a transaction.

    txn_reset releases any old copies of data kept around for a read-only transaction. To reuse this reset transaction, call txn_renew on it. Any cursors in this transaction must also be renewed using cursor_renew.

    Note that txn_reset is similar to txn_abort and will close any databases you opened within the transaction.

    To permanently free a transaction, reset or not, use txn_abort.

    Cleaning Up

    For read-only transactions, any cursors created within it must be closed using cursor_close.

    It is very rarely necessary to close a database handle, and in general they should just be left open.

    • Method Detail

      • nmdb_version

        public static long nmdb_version​(long major,
                                        long minor,
                                        long patch)
        Unsafe version of: version
      • mdb_version

        @Nullable
        public static java.lang.String mdb_version​(@Nullable
                                                   java.nio.IntBuffer major,
                                                   @Nullable
                                                   java.nio.IntBuffer minor,
                                                   @Nullable
                                                   java.nio.IntBuffer patch)
        Returns 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
        Returns:
        the library version as a string
      • nmdb_strerror

        public static long nmdb_strerror​(int err)
        Unsafe version of: strerror
      • mdb_strerror

        public static java.lang.String mdb_strerror​(int err)
        Returns 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.

        Parameters:
        err - the error code
        Returns:
        the description of the error
      • nmdb_env_create

        public static int nmdb_env_create​(long env)
        Unsafe version of: env_create
      • mdb_env_create

        public static int mdb_env_create​(PointerBuffer env)
        Creates an LMDB environment handle.

        This function allocates memory for a MDB_env structure. To release the allocated memory and discard the handle, call env_close. Before the handle may be used, it must be opened using env_open. Various other options may also need to be set before opening the handle, e.g. env_set_mapsize, env_set_maxreaders, 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
      • nmdb_env_open

        public static int nmdb_env_open​(long env,
                                        long path,
                                        int flags,
                                        int mode)
        Unsafe version of: env_open
      • mdb_env_open

        public static int mdb_env_open​(long env,
                                       java.nio.ByteBuffer path,
                                       int flags,
                                       int mode)
        
        public static int mdb_env_open​(long env,
                                       java.lang.CharSequence path,
                                       int flags,
                                       int mode)
        
        Opens an environment handle.

        If this function fails, env_close must be called to discard the MDB_env handle.

        Parameters:
        env - an environment handle returned by 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 env_set_flags are also used.
        • 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.

        • 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.

        • 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.

        • WRITEMAP

          Use a writeable memory map unless 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 WRITEMAP on the same environment. This can defeat durability (env_sync etc).

        • 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-RDONLY commit or 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 env_set_flags.

        • 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 env_sync is called. However, if the filesystem preserves write order and the 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 (NOSYNC | WRITEMAP) leaves the system with no hint for when to write transactions to disk, unless env_sync is called. (MAPASYNC | WRITEMAP) may be preferable.

          This flag may be changed at any time using env_set_flags.

        • MAPASYNC

          When using WRITEMAP, use asynchronous flushes to disk. As with NOSYNC, a system crash can then corrupt the database or lose the last transactions. Calling env_sync ensures on-disk database integrity until next commit.

          This flag may be changed at any time using env_set_flags.

        • NOTLS

          Don't use Thread-Local Storage. Tie reader locktable slots to MDB_txn objects instead of to threads. I.e. 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.

        • 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.

        • 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.

        • 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 WRITEMAP, which writes directly to the mmap instead of using malloc for pages. The initialization is also skipped if 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 env_set_flags.

        • PREVMETA

          Open the environment with the previous meta page rather than the latest one. This loses the latest transaction, but may help work around some types of corruption.

        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:
        • VERSION_MISMATCH - the version of the LMDB library doesn't match the version that created the database environment.
        • 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.
      • nmdb_env_copy

        public static int nmdb_env_copy​(long env,
                                        long path)
        Unsafe version of: env_copy
      • mdb_env_copy

        public static int mdb_env_copy​(long env,
                                       java.nio.ByteBuffer path)
        
        public static int mdb_env_copy​(long env,
                                       java.lang.CharSequence path)
        
        Copies 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.

        This call can trigger significant file size growth if run in parallel with write transactions, because it employs a read-only transaction.

        Parameters:
        env - an environment handle returned by 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
      • nmdb_env_copy2

        public static int nmdb_env_copy2​(long env,
                                         long path,
                                         int flags)
        Unsafe version of: env_copy2
      • mdb_env_copy2

        public static int mdb_env_copy2​(long env,
                                        java.nio.ByteBuffer path,
                                        int flags)
        
        public static int mdb_env_copy2​(long env,
                                        java.lang.CharSequence path,
                                        int flags)
        
        Copies 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.

        This call can trigger significant file size growth if run in parallel with write transactions, because it employs a read-only transaction.

        Parameters:
        env - an environment handle returned by 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.
        • 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.
      • nmdb_env_stat

        public static int nmdb_env_stat​(long env,
                                        long stat)
        Unsafe version of: env_stat
      • mdb_env_stat

        public static int mdb_env_stat​(long env,
                                       MDBStat stat)
        Returns statistics about the LMDB environment.
        Parameters:
        env - an environment handle returned by env_create
        stat - the address of an MDBStat structure where the statistics will be copied
        Returns:
        a non-zero error value on failure and 0 on success
      • nmdb_env_info

        public static int nmdb_env_info​(long env,
                                        long stat)
        Unsafe version of: env_info
      • mdb_env_info

        public static int mdb_env_info​(long env,
                                       MDBEnvInfo stat)
        Returns information about the LMDB environment.
        Parameters:
        env - an environment handle returned by env_create
        stat - the address of an MDBEnvInfo structure where the information will be copied
        Returns:
        a non-zero error value on failure and 0 on success
      • nmdb_env_sync

        public static int nmdb_env_sync​(long env,
                                        int force)
        Unsafe version of: env_sync
      • mdb_env_sync

        public static int mdb_env_sync​(long env,
                                       boolean force)
        Flushes the data buffers to disk.

        Data is always written to disk when 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 NOSYNC or in part NOMETASYNC. This call is not valid if the environment was opened with RDONLY.

        Parameters:
        env - an environment handle returned by env_create
        force - if non-zero, force a synchronous flush. Otherwise if the environment has the NOSYNC flag set the flushes will be omitted, and with 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.
      • nmdb_env_close

        public static void nmdb_env_close​(long env)
        Unsafe version of: env_close
      • mdb_env_close

        public static void mdb_env_close​(long env)
        Closes the environment and releases 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 env_create
      • nmdb_env_set_flags

        public static int nmdb_env_set_flags​(long env,
                                             int flags,
                                             int onoff)
        Unsafe version of: env_set_flags
      • mdb_env_set_flags

        public static int mdb_env_set_flags​(long env,
                                            int flags,
                                            boolean onoff)
        Sets environment flags.

        This may be used to set some flags in addition to those from 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 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.
      • nmdb_env_get_flags

        public static int nmdb_env_get_flags​(long env,
                                             long flags)
        Unsafe version of: env_get_flags
      • mdb_env_get_flags

        public static int mdb_env_get_flags​(long env,
                                            java.nio.IntBuffer flags)
        Gets environment flags.
        Parameters:
        env - an environment handle returned by env_create
        flags - the address of an integer to store the flags
        Returns:
        a non-zero error value on failure and 0 on success
      • nmdb_env_get_path

        public static int nmdb_env_get_path​(long env,
                                            long path)
        Unsafe version of: env_get_path
      • mdb_env_get_path

        public static int mdb_env_get_path​(long env,
                                           PointerBuffer path)
        Returns the path that was used in env_open.
        Parameters:
        env - an environment handle returned by 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
      • nmdb_env_set_mapsize

        public static int nmdb_env_set_mapsize​(long env,
                                               long size)
        Unsafe version of: env_set_mapsize
      • mdb_env_set_mapsize

        public static int mdb_env_set_mapsize​(long env,
                                              long size)
        Sets 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 env_create and before 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, txn_begin will return 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 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.
      • nmdb_env_set_maxreaders

        public static int nmdb_env_set_maxreaders​(long env,
                                                  int readers)
        Unsafe version of: env_set_maxreaders
      • mdb_env_set_maxreaders

        public static int mdb_env_set_maxreaders​(long env,
                                                 int readers)
        Sets 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 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 NOTLS is in use, 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 env_create and before env_open.

        Parameters:
        env - an environment handle returned by 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.
      • nmdb_env_get_maxreaders

        public static int nmdb_env_get_maxreaders​(long env,
                                                  long readers)
        Unsafe version of: env_get_maxreaders
      • mdb_env_get_maxreaders

        public static int mdb_env_get_maxreaders​(long env,
                                                 java.nio.IntBuffer readers)
        Gets the maximum number of threads/reader slots for the environment.
        Parameters:
        env - an environment handle returned by 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
      • nmdb_env_set_maxdbs

        public static int nmdb_env_set_maxdbs​(long env,
                                              int dbs)
        Unsafe version of: env_set_maxdbs
      • mdb_env_set_maxdbs

        public static int mdb_env_set_maxdbs​(long env,
                                             int dbs)
        Sets 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 env_create and before env_open.

        Currently a moderate number of slots are cheap but a huge number gets expensive: 7-120 words per transaction, and every dbi_open does a linear search of the opened slots.

        Parameters:
        env - an environment handle returned by 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.
      • nmdb_env_get_maxkeysize

        public static int nmdb_env_get_maxkeysize​(long env)
        Unsafe version of: env_get_maxkeysize
      • mdb_env_get_maxkeysize

        public static int mdb_env_get_maxkeysize​(long env)
        Gets the maximum size of keys and DUPSORT data we can write.

        Depends on the compile-time constant MAXKEYSIZE. Default 511.

        Parameters:
        env - an environment handle returned by env_create
      • nmdb_env_set_userctx

        public static int nmdb_env_set_userctx​(long env,
                                               long ctx)
        Unsafe version of: env_set_userctx
      • mdb_env_set_userctx

        public static int mdb_env_set_userctx​(long env,
                                              long ctx)
        Set application information associated with the MDB_env.
        Parameters:
        env - an environment handle returned by env_create
        ctx - an arbitrary pointer for whatever the application needs
      • nmdb_env_get_userctx

        public static long nmdb_env_get_userctx​(long env)
        Unsafe version of: env_get_userctx
      • mdb_env_get_userctx

        public static long mdb_env_get_userctx​(long env)
        Gets the application information associated with the MDB_env.
        Parameters:
        env - an environment handle returned by env_create
      • nmdb_txn_begin

        public static int nmdb_txn_begin​(long env,
                                         long parent,
                                         int flags,
                                         long txn)
        Unsafe version of: txn_begin
      • mdb_txn_begin

        public static int mdb_txn_begin​(long env,
                                        long parent,
                                        int flags,
                                        PointerBuffer txn)
        Creates a transaction for use with the environment.

        The transaction handle may be discarded using txn_abort or txn_commit.

        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 NOTLS is in use, this does not apply to read-only transactions.

        Cursors may not span transactions.

        Parameters:
        env - an environment handle returned by 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 txn_commit and 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.
        • RDONLY - This transaction will not perform any write operations.
        • NOSYNC - Don't flush system buffers to disk when committing this transaction.
        • NOMETASYNC - Flush system buffers but omit metadata flush when committing this transaction.
        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:
        • PANIC - a fatal error occurred earlier and the environment must be shut down.
        • MAP_RESIZED - another process wrote data beyond this MDB_env's mapsize and this environment's map must be resized as well. See env_set_mapsize.
        • READERS_FULL - a read-only transaction was requested and the reader lock table is full. See env_set_maxreaders.
        • ENOMEM - out of memory.
      • nmdb_txn_env

        public static long nmdb_txn_env​(long txn)
        Unsafe version of: txn_env
      • mdb_txn_env

        public static long mdb_txn_env​(long txn)
        Returns the transaction's MDB_env.
        Parameters:
        txn - a transaction handle returned by txn_begin.
      • nmdb_txn_id

        public static long nmdb_txn_id​(long txn)
        Unsafe version of: txn_id
      • mdb_txn_id

        public static long mdb_txn_id​(long txn)
        Returns 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 txn_begin.
        Returns:
        a transaction ID, valid if input is an active transaction
      • nmdb_txn_commit

        public static int nmdb_txn_commit​(long txn)
        Unsafe version of: txn_commit
      • mdb_txn_commit

        public static int mdb_txn_commit​(long txn)
        Commits 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 cursor_renew.

        Earlier documentation incorrectly said all cursors would be freed. Only write-transactions free cursors.

        Parameters:
        txn - a transaction handle returned by 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.
      • nmdb_txn_abort

        public static void nmdb_txn_abort​(long txn)
        Unsafe version of: txn_abort
      • mdb_txn_abort

        public static void mdb_txn_abort​(long txn)
        Abandons 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 cursor_renew.

        Earlier documentation incorrectly said all cursors would be freed. Only write-transactions free cursors. "

        Parameters:
        txn - a transaction handle returned by txn_begin.
      • nmdb_txn_reset

        public static void nmdb_txn_reset​(long txn)
        Unsafe version of: txn_reset
      • mdb_txn_reset

        public static void mdb_txn_reset​(long txn)
        Resets a read-only transaction.

        Aborts the transaction like txn_abort, but keeps the transaction handle. 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 NOTLS is in use. The reader table lock is released, but the table slot stays tied to its thread or MDB_txn. Use txn_abort to discard a reset handle, and to free its lock table slot if NOTLS is in use.

        Cursors opened within the transaction must not be used again after this call, except with 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 txn_begin.
      • nmdb_txn_renew

        public static int nmdb_txn_renew​(long txn)
        Unsafe version of: txn_renew
      • mdb_txn_renew

        public static int mdb_txn_renew​(long txn)
        Renews a read-only transaction.

        This acquires a new reader lock for a transaction handle that had been released by txn_reset. It must be called before a reset transaction may be used again.

        Parameters:
        txn - a transaction handle returned by txn_begin.
      • nmdb_dbi_open

        public static int nmdb_dbi_open​(long txn,
                                        long name,
                                        int flags,
                                        long dbi)
        Unsafe version of: dbi_open
      • mdb_dbi_open

        public static int mdb_dbi_open​(long txn,
                                       @Nullable
                                       java.nio.ByteBuffer name,
                                       int flags,
                                       java.nio.IntBuffer dbi)
        
        public static int mdb_dbi_open​(long txn,
                                       @Nullable
                                       java.lang.CharSequence name,
                                       int flags,
                                       java.nio.IntBuffer dbi)
        
        Opens 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 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), 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 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.
        • 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.

        • 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.

        • INTEGERKEY

          Keys are binary integers in native byte order, either unsigned int or mdb_size_t, and will be sorted as such. The keys must all be of the same size.

        • DUPFIXED

          This flag may only be used in combination with 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 GET_MULTIPLE, NEXT_MULTIPLE and PREV_MULTIPLE cursor operations may be used to retrieve multiple items at once.

        • INTEGERDUP

          This option specifies that duplicate data items are binary integers, similar to INTEGERKEY keys.

        • REVERSEDUP

          This option specifies that duplicate data items should be compared as strings in reverse order.

        • 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:
      • nmdb_stat

        public static int nmdb_stat​(long txn,
                                    int dbi,
                                    long stat)
        Unsafe version of: stat
      • mdb_stat

        public static int mdb_stat​(long txn,
                                   int dbi,
                                   MDBStat stat)
        Retrieves statistics for a database.
        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        stat - the address of an MDBStat structure where the statistics will be copied
      • nmdb_dbi_flags

        public static int nmdb_dbi_flags​(long txn,
                                         int dbi,
                                         long flags)
        Unsafe version of: dbi_flags
      • mdb_dbi_flags

        public static int mdb_dbi_flags​(long txn,
                                        int dbi,
                                        java.nio.IntBuffer flags)
        Retrieve the DB flags for a database handle.
        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        flags - address where the flags will be returned
      • nmdb_dbi_close

        public static void nmdb_dbi_close​(long env,
                                          int dbi)
        Unsafe version of: dbi_close
      • mdb_dbi_close

        public static void mdb_dbi_close​(long env,
                                         int dbi)
        Closes 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 BAD_VALSIZE (since the DB name is gone).

        Closing a database handle is not necessary, but lets dbi_open reuse the handle value. Usually it's better to set a bigger env_set_maxdbs, unless that value would be large.

        Parameters:
        env - an environment handle returned by env_create
        dbi - a database handle returned by dbi_open
      • nmdb_drop

        public static int nmdb_drop​(long txn,
                                    int dbi,
                                    int del)
        Unsafe version of: drop
      • mdb_drop

        public static int mdb_drop​(long txn,
                                   int dbi,
                                   boolean del)
        Empties or deletes+closes a database.

        See dbi_close for restrictions about closing the DB handle.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        del - 0 to empty the DB, 1 to delete it from the environment and close the DB handle
      • nmdb_set_compare

        public static int nmdb_set_compare​(long txn,
                                           int dbi,
                                           long cmp)
        Unsafe version of: set_compare
      • mdb_set_compare

        public static int mdb_set_compare​(long txn,
                                          int dbi,
                                          MDBCmpFuncI cmp)
        Sets 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 dbi_open, the keys are compared lexically, with shorter keys collating before longer keys.

        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 txn_begin.
        dbi - a database handle returned by dbi_open
        cmp - an MDBCmpFunc function
      • nmdb_set_dupsort

        public static int nmdb_set_dupsort​(long txn,
                                           int dbi,
                                           long cmp)
        Unsafe version of: set_dupsort
      • mdb_set_dupsort

        public static int mdb_set_dupsort​(long txn,
                                          int dbi,
                                          MDBCmpFuncI cmp)
        Sets a custom data comparison function for a 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 DUPSORT flag.

        If no comparison function is specified, and no special key flags were specified with dbi_open, the data items are compared lexically, with shorter items collating before longer items.

        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 txn_begin.
        dbi - a database handle returned by dbi_open
        cmp - an MDBCmpFunc function
      • nmdb_set_relfunc

        public static int nmdb_set_relfunc​(long txn,
                                           int dbi,
                                           long rel)
        Unsafe version of: set_relfunc
      • mdb_set_relfunc

        public static int mdb_set_relfunc​(long txn,
                                          int dbi,
                                          MDBRelFuncI rel)
        Sets a relocation function for a FIXEDMAP database.

        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 FIXEDMAP option.

        Currently the relocation feature is unimplemented and setting this function has no effect.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        rel - an MDBRelFunc function
      • nmdb_set_relctx

        public static int nmdb_set_relctx​(long txn,
                                          int dbi,
                                          long ctx)
        Unsafe version of: set_relctx
      • mdb_set_relctx

        public static int mdb_set_relctx​(long txn,
                                         int dbi,
                                         long ctx)
        Sets a context pointer for a FIXEDMAP database's relocation function.

        See set_relfunc and MDBRelFunc for more details.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        ctx - an arbitrary pointer for whatever the application needs. It will be passed to the callback function set by MDBRelFunc as its relctx parameter whenever the callback is invoked.
      • nmdb_get

        public static int nmdb_get​(long txn,
                                   int dbi,
                                   long key,
                                   long data)
        Unsafe version of: get
      • mdb_get

        public static int mdb_get​(long txn,
                                  int dbi,
                                  MDBVal key,
                                  MDBVal data)
        Gets 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 (DUPSORT) then the first data item for the key will be returned. Retrieval of other items requires the use of cursor_get.

        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 txn_begin.
        dbi - a database handle returned by dbi_open
        key - the key to search for in the database
        data - the data corresponding to the key
      • nmdb_put

        public static int nmdb_put​(long txn,
                                   int dbi,
                                   long key,
                                   long data,
                                   int flags)
        Unsafe version of: put
      • mdb_put

        public static int mdb_put​(long txn,
                                  int dbi,
                                  MDBVal key,
                                  MDBVal data,
                                  int flags)
        Stores 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 (DUPSORT).

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by 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.
        • 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 DUPSORT. The function will return KEYEXIST if the key/data pair already appears in the database.
        • NOOVERWRITE - enter the new key/data pair only if the key does not already appear in the database. The function will return KEYEXIST if the key already appears in the database, even if the database supports duplicates (DUPSORT). The data parameter will be set to point to the existing item.
        • 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 DUPSORT.

        • 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 KEYEXIST error.
        • APPENDDUP - as above, but for sorted dup data.
      • nmdb_del

        public static int nmdb_del​(long txn,
                                   int dbi,
                                   long key,
                                   long data)
        Unsafe version of: del
      • mdb_del

        public static int mdb_del​(long txn,
                                  int dbi,
                                  MDBVal key,
                                  @Nullable
                                  MDBVal data)
        Deletes items from a database.

        This function removes key/data pairs from the database. If the database does not support sorted duplicate data items (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 NOTFOUND if the specified key/data pair is not in the database.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        key - the key to delete from the database
        data - the data to delete
      • nmdb_cursor_open

        public static int nmdb_cursor_open​(long txn,
                                           int dbi,
                                           long cursor)
        Unsafe version of: cursor_open
      • mdb_cursor_open

        public static int mdb_cursor_open​(long txn,
                                          int dbi,
                                          PointerBuffer cursor)
        Creates 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 cursor_renew.

        It can be discarded with 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 cursor_renew before finally closing it.

        Earlier documentation said that cursors in every transaction were closed when the transaction committed or aborted.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by dbi_open
        cursor - address where the new MDB_cursor handle will be stored
      • nmdb_cursor_close

        public static void nmdb_cursor_close​(long cursor)
        Unsafe version of: cursor_close
      • mdb_cursor_close

        public static void mdb_cursor_close​(long cursor)
        Closes 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 cursor_open
      • nmdb_cursor_renew

        public static int nmdb_cursor_renew​(long txn,
                                            long cursor)
        Unsafe version of: cursor_renew
      • mdb_cursor_renew

        public static int mdb_cursor_renew​(long txn,
                                           long cursor)
        Renews 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 txn_begin.
        cursor - a cursor handle returned by cursor_open
      • nmdb_cursor_txn

        public static long nmdb_cursor_txn​(long cursor)
        Unsafe version of: cursor_txn
      • mdb_cursor_txn

        public static long mdb_cursor_txn​(long cursor)
        Returns the cursor's transaction handle.
        Parameters:
        cursor - a cursor handle returned by cursor_open
      • nmdb_cursor_dbi

        public static int nmdb_cursor_dbi​(long cursor)
        Unsafe version of: cursor_dbi
      • mdb_cursor_dbi

        public static int mdb_cursor_dbi​(long cursor)
        Return the cursor's database handle.
        Parameters:
        cursor - a cursor handle returned by cursor_open
      • nmdb_cursor_get

        public static int nmdb_cursor_get​(long cursor,
                                          long key,
                                          long data,
                                          int op)
        Unsafe version of: cursor_get
      • nmdb_cursor_put

        public static int nmdb_cursor_put​(long cursor,
                                          long key,
                                          long data,
                                          int flags)
        Unsafe version of: cursor_put
      • mdb_cursor_put

        public static int mdb_cursor_put​(long cursor,
                                         MDBVal key,
                                         MDBVal data,
                                         int flags)
        Stores 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.

        Earlier documentation incorrectly said errors would leave the state of the cursor unchanged.

        Parameters:
        cursor - a cursor handle returned by 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.
        • CURRENT - replace the item at the current cursor position. The key parameter must still be provided, and must match it. If using sorted duplicates (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.
        • 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 DUPSORT. The function will return KEYEXIST if the key/data pair already appears in the database.
        • NOOVERWRITE - enter the new key/data pair only if the key does not already appear in the database. The function will return KEYEXIST if the key already appears in the database, even if the database supports duplicates (DUPSORT).
        • 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 DUPSORT.
        • 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 KEYEXIST error.
        • APPENDDUP - as above, but for sorted dup data.
        • MULTIPLE - store multiple contiguous data elements in a single request. This flag may only be specified if the database was opened with DUPFIXED. The data argument must be an array of two MDBVal. The mv_size of the first MDBVal must be the size of a single data element. The mv_data of the first MDBVal must point to the beginning of the array of contiguous data elements. The mv_size of the second MDBVal 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 MDBVal is unused.
      • nmdb_cursor_del

        public static int nmdb_cursor_del​(long cursor,
                                          int flags)
        Unsafe version of: cursor_del
      • mdb_cursor_del

        public static int mdb_cursor_del​(long cursor,
                                         int flags)
        Deletes 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 NEXT can still be used on it. Both NEXT and GET_CURRENT will return the same record after this operation.

        Parameters:
        cursor - a cursor handle returned by cursor_open
        flags - options for this operation. This parameter must be set to 0 or one of the values described here.
        • NODUPDATA - delete all of the data items for the current key. This flag may only be specified if the database was opened with DUPSORT.
      • nmdb_cursor_count

        public static int nmdb_cursor_count​(long cursor,
                                            long countp)
        Unsafe version of: cursor_count
      • mdb_cursor_count

        public static int mdb_cursor_count​(long cursor,
                                           PointerBuffer countp)
        Returns count of duplicates for current key.

        This call is only valid on databases that support sorted duplicate data items DUPSORT.

        Parameters:
        cursor - a cursor handle returned by cursor_open
        countp - address where the count will be stored
      • nmdb_cmp

        public static int nmdb_cmp​(long txn,
                                   int dbi,
                                   long a,
                                   long b)
        Unsafe version of: cmp
      • mdb_cmp

        public static int mdb_cmp​(long txn,
                                  int dbi,
                                  MDBVal a,
                                  MDBVal b)
        Compares 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 txn_begin.
        dbi - a database handle returned by 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
      • nmdb_dcmp

        public static int nmdb_dcmp​(long txn,
                                    int dbi,
                                    long a,
                                    long b)
        Unsafe version of: dcmp
      • mdb_dcmp

        public static int mdb_dcmp​(long txn,
                                   int dbi,
                                   MDBVal a,
                                   MDBVal b)
        Compares 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 DUPSORT flag.

        Parameters:
        txn - a transaction handle returned by txn_begin.
        dbi - a database handle returned by 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
      • nmdb_reader_list

        public static int nmdb_reader_list​(long env,
                                           long func,
                                           long ctx)
        Unsafe version of: reader_list
      • mdb_reader_list

        public static int mdb_reader_list​(long env,
                                          MDBMsgFuncI func,
                                          long ctx)
        Dumps the entries in the reader lock table.
        Parameters:
        env - an environment handle returned by env_create
        func - an MDBMsgFunc function
        ctx - anything the message function needs
      • nmdb_reader_check

        public static int nmdb_reader_check​(long env,
                                            long dead)
        Unsafe version of: reader_check
      • mdb_reader_check

        public static int mdb_reader_check​(long env,
                                           java.nio.IntBuffer dead)
        Checks for stale entries in the reader lock table.
        Parameters:
        env - an environment handle returned by env_create
        dead - number of stale slots that were cleared
      • mdb_version

        @Nullable
        public static java.lang.String mdb_version​(@Nullable
                                                   int[] major,
                                                   @Nullable
                                                   int[] minor,
                                                   @Nullable
                                                   int[] patch)
        Array version of: version
      • mdb_env_get_flags

        public static int mdb_env_get_flags​(long env,
                                            int[] flags)
        Array version of: env_get_flags
      • mdb_env_get_maxreaders

        public static int mdb_env_get_maxreaders​(long env,
                                                 int[] readers)
        Array version of: env_get_maxreaders
      • mdb_dbi_open

        public static int mdb_dbi_open​(long txn,
                                       @Nullable
                                       java.nio.ByteBuffer name,
                                       int flags,
                                       int[] dbi)
        
        public static int mdb_dbi_open​(long txn,
                                       @Nullable
                                       java.lang.CharSequence name,
                                       int flags,
                                       int[] dbi)
        
        Array version of: dbi_open
      • mdb_dbi_flags

        public static int mdb_dbi_flags​(long txn,
                                        int dbi,
                                        int[] flags)
        Array version of: dbi_flags
      • mdb_reader_check

        public static int mdb_reader_check​(long env,
                                           int[] dead)
        Array version of: reader_check