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** 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.
** 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:
** 0 16 Header string: "SQLite format 3\000"
** 16 2 Page size in bytes.
** 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
** 22 1 Min embedded payload fraction
** 23 1 Min leaf payload fraction
** 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 unused
** 72 4 unused
** 76 4 unused
** 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:
** 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.
** 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:
** 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.
** 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:
** 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"
** 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 */
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 */
struct _OvflCell { /* Cells that will not fit on aData[] */
u8 *pCell; /* Pointers to the body of the overflow cell */
u16 idx; /* Insert this cell before idx-th non-overflow cell */
} aOvfl[5];
BtShared *pBt; /* Pointer to BtShared that this page is part of */
u8 *aData; /* Pointer to disk image of the page data */
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 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 */
BtLock lock; /* Object used to lock page 1 */
** 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 as 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 readOnly; /* True if the underlying file is readonly */
u8 pageSizeFixed; /* True if the page size can no longer be changed */
u8 secureDelete; /* True if secure_delete is enabled */
u8 initiallyEmpty; /* Database is empty at start of transaction */
u8 openFlags; /* Flags to sqlite3BtreeOpen() */
u8 autoVacuum; /* True if auto-vacuum is enabled */
u8 incrVacuum; /* True if incr-vacuum is enabled */
u8 inTransaction; /* Transaction state */
u8 doNotUseWAL; /* If true, do not open write-ahead-log file */
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 */
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 */
u8 isExclusive; /* True if pWriter has an EXCLUSIVE lock on the db */
u8 isPending; /* If waiting for read-locks to clear */
u8 *pTmpSpace; /* BtShared.pageSize bytes of space for tmp use */
** 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.
** 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 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 pgnoRoot; /* The root page of this tree */
sqlite3_int64 cachedRowid; /* Next rowid cache. 0 means not valid */
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's last known position */
int skipNext; /* Prev() is noop if negative. Next() is noop if positive */
u8 wrFlag; /* True if writable */
u8 atLast; /* Cursor pointing to the last entry */
u8 validNKey; /* True if info.nKey is valid */
u8 eState; /* One of the CURSOR_XXX constants (see below) */
Pgno *aOverflow; /* Cache of overflow page locations */
u8 isIncrblobHandle; /* True if this cursor is an incr. io handle */
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 */
** Potential values for BtCursor.eState.
** Cursor points to a valid entry. getPayload() etc. may be called.
** 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.
** 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.
** 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_VALID 1
#define CURSOR_FAULT 3
** The database page the PENDING_BYTE occupies. This page is never used.
** 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_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.
#define ISAUTOVACUUM (pBt->autoVacuum)
** This structure is passed around through all the sanity checking routines
** in order to keep track of some global state information.
typedef struct IntegrityCk IntegrityCk;
struct IntegrityCk {
BtShared *pBt; /* The tree being checked out */
Pager *pPager; /* The associated pager. Also accessible by pBt->pPager */
Pgno nPage; /* Number of pages in the database */
int *anRef; /* Number of times each page is referenced */
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 */
** 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