text: /*
  ** 2004 April 6
  **
  ** The author disclaims copyright to this source code.  In place of
  ** a legal notice, here is a blessing:
  **
  **    May you do good and not evil.
  **    May you find forgiveness for yourself and forgive others.
  **    May you share freely, never taking more than you give.
  **
  *************************************************************************
  ** This file implements a external (disk-based) database using BTrees.
  ** For a detailed discussion of BTrees, refer to
  **
  **     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
  **     "Sorting And Searching", pages 473-480. Addison-Wesley
  **     Publishing Company, Reading, Massachusetts.
  **
  ** The basic idea is that each page of the file contains N database
  ** entries and N+1 pointers to subpages.
  **
  **   ----------------------------------------------------------------
  **   |  Ptr(0) | Key(0) | Ptr(1) | Key(1) | ... | Key(N-1) | Ptr(N) |
  **   ----------------------------------------------------------------
  **
  ** All of the keys on the page that Ptr(0) points to have values less
  ** than Key(0).  All of the keys on page Ptr(1) and its subpages have
  ** values greater than Key(0) and less than Key(1).  All of the keys
  ** on Ptr(N) and its subpages have values greater than Key(N-1).  And
  ** so forth.
  **
  ** Finding a particular key requires reading O(log(M)) pages from the 
  ** disk where M is the number of entries in the tree.
  **
  ** In this implementation, a single file can hold one or more separate 
  ** BTrees.  Each BTree is identified by the index of its root page.  The
  ** key and data for any entry are combined to form the "payload".  A
  ** fixed amount of payload can be carried directly on the database
  ** page.  If the payload is larger than the preset amount then surplus
  ** bytes are stored on overflow pages.  The payload for an entry
  ** and the preceding pointer are combined to form a "Cell".  Each 
  ** page has a small header which contains the Ptr(N) pointer and other
  ** information such as the size of key and data.
  **
  ** FORMAT DETAILS
  **
  ** The file is divided into pages.  The first page is called page 1,
  ** the second is page 2, and so forth.  A page number of zero indicates
  ** "no such page".  The page size can be any power of 2 between 512 and 65536.
  ** Each page can be either a btree page, a freelist page, an overflow
  ** page, or a pointer-map page.
  **
  ** The first page is always a btree page.  The first 100 bytes of the first
  ** page contain a special header (the "file header") that describes the file.
  ** The format of the file header is as follows:
  **
  **   OFFSET   SIZE    DESCRIPTION
  **      0      16     Header string: "SQLite format 3\000"
  **     16       2     Page size in bytes.  (1 means 65536)
  **     18       1     File format write version
  **     19       1     File format read version
  **     20       1     Bytes of unused space at the end of each page
  **     21       1     Max embedded payload fraction (must be 64)
  **     22       1     Min embedded payload fraction (must be 32)
  **     23       1     Min leaf payload fraction (must be 32)
  **     24       4     File change counter
  **     28       4     Reserved for future use
  **     32       4     First freelist page
  **     36       4     Number of freelist pages in the file
  **     40      60     15 4-byte meta values passed to higher layers
  **
  **     40       4     Schema cookie
  **     44       4     File format of schema layer
  **     48       4     Size of page cache
  **     52       4     Largest root-page (auto/incr_vacuum)
  **     56       4     1=UTF-8 2=UTF16le 3=UTF16be
  **     60       4     User version
  **     64       4     Incremental vacuum mode
  **     68       4     Application-ID
  **     72      20     unused
  **     92       4     The version-valid-for number
  **     96       4     SQLITE_VERSION_NUMBER
  **
  ** All of the integer values are big-endian (most significant byte first).
  **
  ** The file change counter is incremented when the database is changed
  ** This counter allows other processes to know when the file has changed
  ** and thus when they need to flush their cache.
  **
  ** The max embedded payload fraction is the amount of the total usable
  ** space in a page that can be consumed by a single cell for standard
  ** B-tree (non-LEAFDATA) tables.  A value of 255 means 100%.  The default
  ** is to limit the maximum cell size so that at least 4 cells will fit
  ** on one page.  Thus the default max embedded payload fraction is 64.
  **
  ** If the payload for a cell is larger than the max payload, then extra
  ** payload is spilled to overflow pages.  Once an overflow page is allocated,
  ** as many bytes as possible are moved into the overflow pages without letting
  ** the cell size drop below the min embedded payload fraction.
  **
  ** The min leaf payload fraction is like the min embedded payload fraction
  ** except that it applies to leaf nodes in a LEAFDATA tree.  The maximum
  ** payload fraction for a LEAFDATA tree is always 100% (or 255) and it
  ** not specified in the header.
  **
  ** Each btree pages is divided into three sections:  The header, the
  ** cell pointer array, and the cell content area.  Page 1 also has a 100-byte
  ** file header that occurs before the page header.
  **
  **      |----------------|
  **      | file header    |   100 bytes.  Page 1 only.
  **      |----------------|
  **      | page header    |   8 bytes for leaves.  12 bytes for interior nodes
  **      |----------------|
  **      | cell pointer   |   |  2 bytes per cell.  Sorted order.
  **      | array          |   |  Grows downward
  **      |                |   v
  **      |----------------|
  **      | unallocated    |
  **      | space          |
  **      |----------------|   ^  Grows upwards
  **      | cell content   |   |  Arbitrary order interspersed with freeblocks.
  **      | area           |   |  and free space fragments.
  **      |----------------|
  **
  ** The page headers looks like this:
  **
  **   OFFSET   SIZE     DESCRIPTION
  **      0       1      Flags. 1: intkey, 2: zerodata, 4: leafdata, 8: leaf
  **      1       2      byte offset to the first freeblock
  **      3       2      number of cells on this page
  **      5       2      first byte of the cell content area
  **      7       1      number of fragmented free bytes
  **      8       4      Right child (the Ptr(N) value).  Omitted on leaves.
  **
  ** The flags define the format of this btree page.  The leaf flag means that
  ** this page has no children.  The zerodata flag means that this page carries
  ** only keys and no data.  The intkey flag means that the key is a integer
  ** which is stored in the key size entry of the cell header rather than in
  ** the payload area.
  **
  ** The cell pointer array begins on the first byte after the page header.
  ** The cell pointer array contains zero or more 2-byte numbers which are
  ** offsets from the beginning of the page to the cell content in the cell
  ** content area.  The cell pointers occur in sorted order.  The system strives
  ** to keep free space after the last cell pointer so that new cells can
  ** be easily added without having to defragment the page.
  **
  ** Cell content is stored at the very end of the page and grows toward the
  ** beginning of the page.
  **
  ** Unused space within the cell content area is collected into a linked list of
  ** freeblocks.  Each freeblock is at least 4 bytes in size.  The byte offset
  ** to the first freeblock is given in the header.  Freeblocks occur in
  ** increasing order.  Because a freeblock must be at least 4 bytes in size,
  ** any group of 3 or fewer unused bytes in the cell content area cannot
  ** exist on the freeblock chain.  A group of 3 or fewer free bytes is called
  ** a fragment.  The total number of bytes in all fragments is recorded.
  ** in the page header at offset 7.
  **
  **    SIZE    DESCRIPTION
  **      2     Byte offset of the next freeblock
  **      2     Bytes in this freeblock
  **
  ** Cells are of variable length.  Cells are stored in the cell content area at
  ** the end of the page.  Pointers to the cells are in the cell pointer array
  ** that immediately follows the page header.  Cells is not necessarily
  ** contiguous or in order, but cell pointers are contiguous and in order.
  **
  ** Cell content makes use of variable length integers.  A variable
  ** length integer is 1 to 9 bytes where the lower 7 bits of each 
  ** byte are used.  The integer consists of all bytes that have bit 8 set and
  ** the first byte with bit 8 clear.  The most significant byte of the integer
  ** appears first.  A variable-length integer may not be more than 9 bytes long.
  ** As a special case, all 8 bytes of the 9th byte are used as data.  This
  ** allows a 64-bit integer to be encoded in 9 bytes.
  **
  **    0x00                      becomes  0x00000000
  **    0x7f                      becomes  0x0000007f
  **    0x81 0x00                 becomes  0x00000080
  **    0x82 0x00                 becomes  0x00000100
  **    0x80 0x7f                 becomes  0x0000007f
  **    0x8a 0x91 0xd1 0xac 0x78  becomes  0x12345678
  **    0x81 0x81 0x81 0x81 0x01  becomes  0x10204081
  **
  ** Variable length integers are used for rowids and to hold the number of
  ** bytes of key and data in a btree cell.
  **
  ** The content of a cell looks like this:
  **
  **    SIZE    DESCRIPTION
  **      4     Page number of the left child. Omitted if leaf flag is set.
  **     var    Number of bytes of data. Omitted if the zerodata flag is set.
  **     var    Number of bytes of key. Or the key itself if intkey flag is set.
  **      *     Payload
  **      4     First page of the overflow chain.  Omitted if no overflow
  **
  ** Overflow pages form a linked list.  Each page except the last is completely
  ** filled with data (pagesize - 4 bytes).  The last page can have as little
  ** as 1 byte of data.
  **
  **    SIZE    DESCRIPTION
  **      4     Page number of next overflow page
  **      *     Data
  **
  ** Freelist pages come in two subtypes: trunk pages and leaf pages.  The
  ** file header points to the first in a linked list of trunk page.  Each trunk
  ** page points to multiple leaf pages.  The content of a leaf page is
  ** unspecified.  A trunk page looks like this:
  **
  **    SIZE    DESCRIPTION
  **      4     Page number of next trunk page
  **      4     Number of leaf pointers on this page
  **      *     zero or more pages numbers of leaves
  */
  #include "sqliteInt.h"
  
  
  /* The following value is the maximum cell size assuming a maximum page
  ** size give above.
  */
  #define MX_CELL_SIZE(pBt)  ((int)(pBt->pageSize-8))
  
  /* The maximum number of cells on a single page of the database.  This
  ** assumes a minimum cell size of 6 bytes  (4 bytes for the cell itself
  ** plus 2 bytes for the index to the cell in the page header).  Such
  ** small cells will be rare, but they are possible.
  */
  #define MX_CELL(pBt) ((pBt->pageSize-8)/6)
  
  /* Forward declarations */
  typedef struct MemPage MemPage;
  typedef struct BtLock BtLock;
  
  /*
  ** This is a magic string that appears at the beginning of every
  ** SQLite database in order to identify the file as a real database.
  **
  ** You can change this value at compile-time by specifying a
  ** -DSQLITE_FILE_HEADER="..." on the compiler command-line.  The
  ** header must be exactly 16 bytes including the zero-terminator so
  ** the string itself should be 15 characters long.  If you change
  ** the header, then your custom library will not be able to read 
  ** databases generated by the standard tools and the standard tools
  ** will not be able to read databases created by your custom library.
  */
  #ifndef SQLITE_FILE_HEADER /* 123456789 123456 */
  #  define SQLITE_FILE_HEADER "SQLite format 3"
  #endif
  
  /*
  ** Page type flags.  An ORed combination of these flags appear as the
  ** first byte of on-disk image of every BTree page.
  */
  #define PTF_INTKEY    0x01
  #define PTF_ZERODATA  0x02
  #define PTF_LEAFDATA  0x04
  #define PTF_LEAF      0x08
  
  /*
  ** As each page of the file is loaded into memory, an instance of the following
  ** structure is appended and initialized to zero.  This structure stores
  ** information about the page that is decoded from the raw file page.
  **
  ** The pParent field points back to the parent page.  This allows us to
  ** walk up the BTree from any leaf to the root.  Care must be taken to
  ** unref() the parent page pointer when this page is no longer referenced.
  ** The pageDestructor() routine handles that chore.
  **
  ** Access to all fields of this structure is controlled by the mutex
  ** stored in MemPage.pBt->mutex.
  */
  struct MemPage {
    u8 isInit;           /* True if previously initialized. MUST BE FIRST! */
    u8 nOverflow;        /* Number of overflow cell bodies in aCell[] */
    u8 intKey;           /* True if intkey flag is set */
    u8 leaf;             /* True if leaf flag is set */
    u8 hasData;          /* True if this page stores data */
    u8 hdrOffset;        /* 100 for page 1.  0 otherwise */
    u8 childPtrSize;     /* 0 if leaf==1.  4 if leaf==0 */
    u8 max1bytePayload;  /* min(maxLocal,127) */
    u16 maxLocal;        /* Copy of BtShared.maxLocal or BtShared.maxLeaf */
    u16 minLocal;        /* Copy of BtShared.minLocal or BtShared.minLeaf */
    u16 cellOffset;      /* Index in aData of first cell pointer */
    u16 nFree;           /* Number of free bytes on the page */
    u16 nCell;           /* Number of cells on this page, local and ovfl */
    u16 maskPage;        /* Mask for page offset */
    u16 aiOvfl[5];       /* Insert the i-th overflow cell before the aiOvfl-th
                         ** non-overflow cell */
    u8 *apOvfl[5];       /* Pointers to the body of overflow cells */
    BtShared *pBt;       /* Pointer to BtShared that this page is part of */
    u8 *aData;           /* Pointer to disk image of the page data */
    u8 *aDataEnd;        /* One byte past the end of usable data */
    u8 *aCellIdx;        /* The cell index area */
    DbPage *pDbPage;     /* Pager page handle */
    Pgno pgno;           /* Page number for this page */
  };
  
  /*
  ** The in-memory image of a disk page has the auxiliary information appended
  ** to the end.  EXTRA_SIZE is the number of bytes of space needed to hold
  ** that extra information.
  */
  #define EXTRA_SIZE sizeof(MemPage)
  
  /*
  ** A linked list of the following structures is stored at BtShared.pLock.
  ** Locks are added (or upgraded from READ_LOCK to WRITE_LOCK) when a cursor 
  ** is opened on the table with root page BtShared.iTable. Locks are removed
  ** from this list when a transaction is committed or rolled back, or when
  ** a btree handle is closed.
  */
  struct BtLock {
    Btree *pBtree;        /* Btree handle holding this lock */
    Pgno iTable;          /* Root page of table */
    u8 eLock;             /* READ_LOCK or WRITE_LOCK */
    BtLock *pNext;        /* Next in BtShared.pLock list */
  };
  
  /* Candidate values for BtLock.eLock */
  #define READ_LOCK     1
  #define WRITE_LOCK    2
  
  /* A Btree handle
  **
  ** A database connection contains a pointer to an instance of
  ** this object for every database file that it has open.  This structure
  ** is opaque to the database connection.  The database connection cannot
  ** see the internals of this structure and only deals with pointers to
  ** this structure.
  **
  ** For some database files, the same underlying database cache might be 
  ** shared between multiple connections.  In that case, each connection
  ** has it own instance of this object.  But each instance of this object
  ** points to the same BtShared object.  The database cache and the
  ** schema associated with the database file are all contained within
  ** the BtShared object.
  **
  ** All fields in this structure are accessed under sqlite3.mutex.
  ** The pBt pointer itself may not be changed while there exists cursors 
  ** in the referenced BtShared that point back to this Btree since those
  ** cursors have to go through this Btree to find their BtShared and
  ** they often do so without holding sqlite3.mutex.
  */
  struct Btree {
    sqlite3 *db;       /* The database connection holding this btree */
    BtShared *pBt;     /* Sharable content of this btree */
    u8 inTrans;        /* TRANS_NONE, TRANS_READ or TRANS_WRITE */
    u8 sharable;       /* True if we can share pBt with another db */
    u8 locked;         /* True if db currently has pBt locked */
    int wantToLock;    /* Number of nested calls to sqlite3BtreeEnter() */
    int nBackup;       /* Number of backup operations reading this btree */
    Btree *pNext;      /* List of other sharable Btrees from the same db */
    Btree *pPrev;      /* Back pointer of the same list */
  #ifndef SQLITE_OMIT_SHARED_CACHE
    BtLock lock;       /* Object used to lock page 1 */
  #endif
  };
  
  /*
  ** Btree.inTrans may take one of the following values.
  **
  ** If the shared-data extension is enabled, there may be multiple users
  ** of the Btree structure. At most one of these may open a write transaction,
  ** but any number may have active read transactions.
  */
  #define TRANS_NONE  0
  #define TRANS_READ  1
  #define TRANS_WRITE 2
  
  /*
  ** An instance of this object represents a single database file.
  ** 
  ** A single database file can be in use at the same time by two
  ** or more database connections.  When two or more connections are
  ** sharing the same database file, each connection has it own
  ** private Btree object for the file and each of those Btrees points
  ** to this one BtShared object.  BtShared.nRef is the number of
  ** connections currently sharing this database file.
  **
  ** Fields in this structure are accessed under the BtShared.mutex
  ** mutex, except for nRef and pNext which are accessed under the
  ** global SQLITE_MUTEX_STATIC_MASTER mutex.  The pPager field
  ** may not be modified once it is initially set as long as nRef>0.
  ** The pSchema field may be set once under BtShared.mutex and
  ** thereafter is unchanged as long as nRef>0.
  **
  ** isPending:
  **
  **   If a BtShared client fails to obtain a write-lock on a database
  **   table (because there exists one or more read-locks on the table),
  **   the shared-cache enters 'pending-lock' state and isPending is
  **   set to true.
  **
  **   The shared-cache leaves the 'pending lock' state when either of
  **   the following occur:
  **
  **     1) The current writer (BtShared.pWriter) concludes its transaction, OR
  **     2) The number of locks held by other connections drops to zero.
  **
  **   while in the 'pending-lock' state, no connection may start a new
  **   transaction.
  **
  **   This feature is included to help prevent writer-starvation.
  */
  struct BtShared {
    Pager *pPager;        /* The page cache */
    sqlite3 *db;          /* Database connection currently using this Btree */
    BtCursor *pCursor;    /* A list of all open cursors */
    MemPage *pPage1;      /* First page of the database */
    u8 openFlags;         /* Flags to sqlite3BtreeOpen() */
  #ifndef SQLITE_OMIT_AUTOVACUUM
    u8 autoVacuum;        /* True if auto-vacuum is enabled */
    u8 incrVacuum;        /* True if incr-vacuum is enabled */
    u8 bDoTruncate;       /* True to truncate db on commit */
  #endif
    u8 inTransaction;     /* Transaction state */
    u8 max1bytePayload;   /* Maximum first byte of cell for a 1-byte payload */
    u16 btsFlags;         /* Boolean parameters.  See BTS_* macros below */
    u16 maxLocal;         /* Maximum local payload in non-LEAFDATA tables */
    u16 minLocal;         /* Minimum local payload in non-LEAFDATA tables */
    u16 maxLeaf;          /* Maximum local payload in a LEAFDATA table */
    u16 minLeaf;          /* Minimum local payload in a LEAFDATA table */
    u32 pageSize;         /* Total number of bytes on a page */
    u32 usableSize;       /* Number of usable bytes on each page */
    int nTransaction;     /* Number of open transactions (read + write) */
    u32 nPage;            /* Number of pages in the database */
    void *pSchema;        /* Pointer to space allocated by sqlite3BtreeSchema() */
    void (*xFreeSchema)(void*);  /* Destructor for BtShared.pSchema */
    sqlite3_mutex *mutex; /* Non-recursive mutex required to access this object */
    Bitvec *pHasContent;  /* Set of pages moved to free-list this transaction */
  #ifndef SQLITE_OMIT_SHARED_CACHE
    int nRef;             /* Number of references to this structure */
    BtShared *pNext;      /* Next on a list of sharable BtShared structs */
    BtLock *pLock;        /* List of locks held on this shared-btree struct */
    Btree *pWriter;       /* Btree with currently open write transaction */
  #endif
    u8 *pTmpSpace;        /* BtShared.pageSize bytes of space for tmp use */
  };
  
  /*
  ** Allowed values for BtShared.btsFlags
  */
  #define BTS_READ_ONLY        0x0001   /* Underlying file is readonly */
  #define BTS_PAGESIZE_FIXED   0x0002   /* Page size can no longer be changed */
  #define BTS_SECURE_DELETE    0x0004   /* PRAGMA secure_delete is enabled */
  #define BTS_INITIALLY_EMPTY  0x0008   /* Database was empty at trans start */
  #define BTS_NO_WAL           0x0010   /* Do not open write-ahead-log files */
  #define BTS_EXCLUSIVE        0x0020   /* pWriter has an exclusive lock */
  #define BTS_PENDING          0x0040   /* Waiting for read-locks to clear */
  
  /*
  ** An instance of the following structure is used to hold information
  ** about a cell.  The parseCellPtr() function fills in this structure
  ** based on information extract from the raw disk page.
  */
  typedef struct CellInfo CellInfo;
  struct CellInfo {
    i64 nKey;      /* The key for INTKEY tables, or number of bytes in key */
    u8 *pCell;     /* Pointer to the start of cell content */
    u32 nData;     /* Number of bytes of data */
    u32 nPayload;  /* Total amount of payload */
    u16 nHeader;   /* Size of the cell content header in bytes */
    u16 nLocal;    /* Amount of payload held locally */
    u16 iOverflow; /* Offset to overflow page number.  Zero if no overflow */
    u16 nSize;     /* Size of the cell content on the main b-tree page */
  };
  
  /*
  ** Maximum depth of an SQLite B-Tree structure. Any B-Tree deeper than
  ** this will be declared corrupt. This value is calculated based on a
  ** maximum database size of 2^31 pages a minimum fanout of 2 for a
  ** root-node and 3 for all other internal nodes.
  **
  ** If a tree that appears to be taller than this is encountered, it is
  ** assumed that the database is corrupt.
  */
  #define BTCURSOR_MAX_DEPTH 20
  
  /*
  ** A cursor is a pointer to a particular entry within a particular
  ** b-tree within a database file.
  **
  ** The entry is identified by its MemPage and the index in
  ** MemPage.aCell[] of the entry.
  **
  ** A single database file can be shared by two more database connections,
  ** but cursors cannot be shared.  Each cursor is associated with a
  ** particular database connection identified BtCursor.pBtree.db.
  **
  ** Fields in this structure are accessed under the BtShared.mutex
  ** found at self->pBt->mutex. 
  */
  struct BtCursor {
    Btree *pBtree;            /* The Btree to which this cursor belongs */
    BtShared *pBt;            /* The BtShared this cursor points to */
    BtCursor *pNext, *pPrev;  /* Forms a linked list of all cursors */
    struct KeyInfo *pKeyInfo; /* Argument passed to comparison function */
    Pgno *aOverflow;          /* Cache of overflow page locations */
    CellInfo info;            /* A parse of the cell we are pointing at */
    i64 nKey;                 /* Size of pKey, or last integer key */
    void *pKey;               /* Saved key that was cursor last known position */
    Pgno pgnoRoot;            /* The root page of this tree */
    int nOvflAlloc;           /* Allocated size of aOverflow[] array */
    int skipNext;    /* Prev() is noop if negative. Next() is noop if positive */
    u8 curFlags;              /* zero or more BTCF_* flags defined below */
    u8 eState;                /* One of the CURSOR_XXX constants (see below) */
    u8 hints;                             /* As configured by CursorSetHints() */
    i16 iPage;                            /* Index of current page in apPage */
    u16 aiIdx[BTCURSOR_MAX_DEPTH];        /* Current index in apPage[i] */
    MemPage *apPage[BTCURSOR_MAX_DEPTH];  /* Pages from root to current page */
  };
  
  /*
  ** Legal values for BtCursor.curFlags
  */
  #define BTCF_WriteFlag    0x01   /* True if a write cursor */
  #define BTCF_ValidNKey    0x02   /* True if info.nKey is valid */
  #define BTCF_ValidOvfl    0x04   /* True if aOverflow is valid */
  #define BTCF_AtLast       0x08   /* Cursor is pointing ot the last entry */
  #define BTCF_Incrblob     0x10   /* True if an incremental I/O handle */
  
  /*
  ** Potential values for BtCursor.eState.
  **
  ** CURSOR_INVALID:
  **   Cursor does not point to a valid entry. This can happen (for example) 
  **   because the table is empty or because BtreeCursorFirst() has not been
  **   called.
  **
  ** CURSOR_VALID:
  **   Cursor points to a valid entry. getPayload() etc. may be called.
  **
  ** CURSOR_SKIPNEXT:
  **   Cursor is valid except that the Cursor.skipNext field is non-zero
  **   indicating that the next sqlite3BtreeNext() or sqlite3BtreePrevious()
  **   operation should be a no-op.
  **
  ** CURSOR_REQUIRESEEK:
  **   The table that this cursor was opened on still exists, but has been 
  **   modified since the cursor was last used. The cursor position is saved
  **   in variables BtCursor.pKey and BtCursor.nKey. When a cursor is in 
  **   this state, restoreCursorPosition() can be called to attempt to
  **   seek the cursor to the saved position.
  **
  ** CURSOR_FAULT:
  **   A unrecoverable error (an I/O error or a malloc failure) has occurred
  **   on a different connection that shares the BtShared cache with this
  **   cursor.  The error has left the cache in an inconsistent state.
  **   Do nothing else with this cursor.  Any attempt to use the cursor
  **   should return the error code stored in BtCursor.skip
  */
  #define CURSOR_INVALID           0
  #define CURSOR_VALID             1
  #define CURSOR_SKIPNEXT          2
  #define CURSOR_REQUIRESEEK       3
  #define CURSOR_FAULT             4
  
  /* 
  ** The database page the PENDING_BYTE occupies. This page is never used.
  */
  # define PENDING_BYTE_PAGE(pBt) PAGER_MJ_PGNO(pBt)
  
  /*
  ** These macros define the location of the pointer-map entry for a 
  ** database page. The first argument to each is the number of usable
  ** bytes on each page of the database (often 1024). The second is the
  ** page number to look up in the pointer map.
  **
  ** PTRMAP_PAGENO returns the database page number of the pointer-map
  ** page that stores the required pointer. PTRMAP_PTROFFSET returns
  ** the offset of the requested map entry.
  **
  ** If the pgno argument passed to PTRMAP_PAGENO is a pointer-map page,
  ** then pgno is returned. So (pgno==PTRMAP_PAGENO(pgsz, pgno)) can be
  ** used to test if pgno is a pointer-map page. PTRMAP_ISPAGE implements
  ** this test.
  */
  #define PTRMAP_PAGENO(pBt, pgno) ptrmapPageno(pBt, pgno)
  #define PTRMAP_PTROFFSET(pgptrmap, pgno) (5*(pgno-pgptrmap-1))
  #define PTRMAP_ISPAGE(pBt, pgno) (PTRMAP_PAGENO((pBt),(pgno))==(pgno))
  
  /*
  ** The pointer map is a lookup table that identifies the parent page for
  ** each child page in the database file.  The parent page is the page that
  ** contains a pointer to the child.  Every page in the database contains
  ** 0 or 1 parent pages.  (In this context 'database page' refers
  ** to any page that is not part of the pointer map itself.)  Each pointer map
  ** entry consists of a single byte 'type' and a 4 byte parent page number.
  ** The PTRMAP_XXX identifiers below are the valid types.
  **
  ** The purpose of the pointer map is to facility moving pages from one
  ** position in the file to another as part of autovacuum.  When a page
  ** is moved, the pointer in its parent must be updated to point to the
  ** new location.  The pointer map is used to locate the parent page quickly.
  **
  ** PTRMAP_ROOTPAGE: The database page is a root-page. The page-number is not
  **                  used in this case.
  **
  ** PTRMAP_FREEPAGE: The database page is an unused (free) page. The page-number 
  **                  is not used in this case.
  **
  ** PTRMAP_OVERFLOW1: The database page is the first page in a list of 
  **                   overflow pages. The page number identifies the page that
  **                   contains the cell with a pointer to this overflow page.
  **
  ** PTRMAP_OVERFLOW2: The database page is the second or later page in a list of
  **                   overflow pages. The page-number identifies the previous
  **                   page in the overflow page list.
  **
  ** PTRMAP_BTREE: The database page is a non-root btree page. The page number
  **               identifies the parent page in the btree.
  */
  #define PTRMAP_ROOTPAGE 1
  #define PTRMAP_FREEPAGE 2
  #define PTRMAP_OVERFLOW1 3
  #define PTRMAP_OVERFLOW2 4
  #define PTRMAP_BTREE 5
  
  /* A bunch of assert() statements to check the transaction state variables
  ** of handle p (type Btree*) are internally consistent.
  */
  #define btreeIntegrity(p) \
    assert( p->pBt->inTransaction!=TRANS_NONE || p->pBt->nTransaction==0 ); \
    assert( p->pBt->inTransaction>=p->inTrans ); 
  
  
  /*
  ** The ISAUTOVACUUM macro is used within balance_nonroot() to determine
  ** if the database supports auto-vacuum or not. Because it is used
  ** within an expression that is an argument to another macro 
  ** (sqliteMallocRaw), it is not possible to use conditional compilation.
  ** So, this macro is defined instead.
  */
  #ifndef SQLITE_OMIT_AUTOVACUUM
  #define ISAUTOVACUUM (pBt->autoVacuum)
  #else
  #define ISAUTOVACUUM 0
  #endif
  
  
  /*
  ** This structure is passed around through all the sanity checking routines
  ** in order to keep track of some global state information.
  **
  ** The aRef[] array is allocated so that there is 1 bit for each page in
  ** the database. As the integrity-check proceeds, for each page used in
  ** the database the corresponding bit is set. This allows integrity-check to 
  ** detect pages that are used twice and orphaned pages (both of which 
  ** indicate corruption).
  */
  typedef struct IntegrityCk IntegrityCk;
  struct IntegrityCk {
    BtShared *pBt;    /* The tree being checked out */
    Pager *pPager;    /* The associated pager.  Also accessible by pBt->pPager */
    u8 *aPgRef;       /* 1 bit per page in the db (see above) */
    Pgno nPage;       /* Number of pages in the database */
    int mxErr;        /* Stop accumulating errors when this reaches zero */
    int nErr;         /* Number of messages written to zErrMsg so far */
    int mallocFailed; /* A memory allocation error has occurred */
    StrAccum errMsg;  /* Accumulate the error message text here */
  };
  
  /*
  ** Routines to read or write a two- and four-byte big-endian integer values.
  */
  #define get2byte(x)   ((x)[0]<<8 | (x)[1])
  #define put2byte(p,v) ((p)[0] = (u8)((v)>>8), (p)[1] = (u8)(v))
  #define get4byte sqlite3Get4byte
  #define put4byte sqlite3Put4byte
  