Small. Fast. Reliable.
Choose any three.

About Sitemap Documentation Download License News Support |

## C/C++ Interface For SQLite Version 3

This page defined the C-language interface to SQLite.

This is not a tutorial. These pages are designed to be precise, not easy to read. For a tutorial introduction see SQLite In 3 Minutes Or Less and/or the Introduction To The SQLite C/C++ Interface.

This page contains all C-language interface information in a single HTML file. The same information is also available broken out into lots of small pages for easier viewing, if you prefer.

This document is created by a script which scans comments in the source code files.

## Experimental And Deprecated Interfaces

SQLite interfaces can be subdivided into three categories:

1. Stable
2. Experimental
3. Deprecated

Stable interfaces will be maintained indefinitely in a backwards compatible way. An application that uses only stable interfaces should always be able to relink against a newer version of SQLite without any changes.

Experiemental interfaces are subject to change. Applications that use experimental interfaces may need to be modified when upgrading to a newer SQLite release, though this is rare. When new interfaces are added to SQLite, they generally begin as experimental interfaces. After an interface has been in use for a while and the developers are confident that the design of the interface is sound and worthy of long-term support, the interface is marked as stable.

Deprecated interfaces have been superceded by better methods of accomplishing the same thing and should be avoided in new applications. Deprecated interfaces continue to be supported for the sake of backwards compatibility. At some point in the future, it is possible that deprecated interfaces may be removed.

Key points:

- Experimental interfaces are subject to change and/or removal at any time.
- Deprecated interfaces should not be used in new code and might be removed in some future release.

## Objects:

Note: Objects marked with "exp" are experimental and objects marked with "(obs)" are deprecated.

sqlite3 sqlite3_backup sqlite3_blob sqlite3_context sqlite3_file sqlite3_int64 sqlite3_uint64 sqlite_int64 | sqlite_uint64 sqlite3_io_methods sqlite3_mem_methods sqlite3_module sqlite3_mutex sqlite3_mutex_methods sqlite3_pcache sqlite3_pcache_methods | sqlite3_stmt sqlite3_temp_directory sqlite3_value sqlite3_vfs sqlite3_vtab sqlite3_vtab_cursor

## Constants:

Note: Constants marked with "(exp)" are experimental and constants marked with "(obs)" are deprecated

SQLITE_ABORT SQLITE_ACCESS_EXISTS SQLITE_ACCESS_READ SQLITE_ACCESS_READWRITE SQLITE_ALTER_TABLE SQLITE_ANALYZE SQLITE_ANY SQLITE_ATTACH SQLITE_AUTH SQLITE_BLOB SQLITE_BUSY SQLITE_BUSY_RECOVERY SQLITE_CANTOPEN SQLITE_CANTOPEN_NOTEMPDIR SQLITE_CONFIG_GETMALLOC SQLITE_CONFIG_GETMUTEX SQLITE_CONFIG_GETPCACHE SQLITE_CONFIG_HEAP SQLITE_CONFIG_LOG SQLITE_CONFIG_LOOKASIDE SQLITE_CONFIG_MALLOC SQLITE_CONFIG_MEMSTATUS SQLITE_CONFIG_MULTITHREAD SQLITE_CONFIG_MUTEX SQLITE_CONFIG_PAGECACHE SQLITE_CONFIG_PCACHE SQLITE_CONFIG_SCRATCH SQLITE_CONFIG_SERIALIZED SQLITE_CONFIG_SINGLETHREAD SQLITE_CONSTRAINT SQLITE_COPY SQLITE_CORRUPT SQLITE_CREATE_INDEX SQLITE_CREATE_TABLE SQLITE_CREATE_TEMP_INDEX SQLITE_CREATE_TEMP_TABLE SQLITE_CREATE_TEMP_TRIGGER SQLITE_CREATE_TEMP_VIEW SQLITE_CREATE_TRIGGER SQLITE_CREATE_VIEW SQLITE_CREATE_VTABLE SQLITE_DBCONFIG_LOOKASIDE SQLITE_DBSTATUS_CACHE_USED SQLITE_DBSTATUS_LOOKASIDE_USED SQLITE_DBSTATUS_MAX SQLITE_DELETE SQLITE_DENY SQLITE_DETACH SQLITE_DONE SQLITE_DROP_INDEX SQLITE_DROP_TABLE SQLITE_DROP_TEMP_INDEX SQLITE_DROP_TEMP_TABLE SQLITE_DROP_TEMP_TRIGGER SQLITE_DROP_TEMP_VIEW SQLITE_DROP_TRIGGER SQLITE_DROP_VIEW SQLITE_DROP_VTABLE SQLITE_EMPTY SQLITE_ERROR SQLITE_FCNTL_LOCKSTATE SQLITE_FCNTL_SIZE_HINT SQLITE_FLOAT SQLITE_FORMAT SQLITE_FULL SQLITE_FUNCTION SQLITE_GET_LOCKPROXYFILE SQLITE_IGNORE SQLITE_INDEX_CONSTRAINT_EQ SQLITE_INDEX_CONSTRAINT_GE SQLITE_INDEX_CONSTRAINT_GT SQLITE_INDEX_CONSTRAINT_LE SQLITE_INDEX_CONSTRAINT_LT SQLITE_INDEX_CONSTRAINT_MATCH SQLITE_INSERT SQLITE_INTEGER SQLITE_INTERNAL SQLITE_INTERRUPT SQLITE_IOCAP_ATOMIC SQLITE_IOCAP_ATOMIC16K SQLITE_IOCAP_ATOMIC1K SQLITE_IOCAP_ATOMIC2K SQLITE_IOCAP_ATOMIC32K SQLITE_IOCAP_ATOMIC4K SQLITE_IOCAP_ATOMIC512 SQLITE_IOCAP_ATOMIC64K SQLITE_IOCAP_ATOMIC8K SQLITE_IOCAP_SAFE_APPEND SQLITE_IOCAP_SEQUENTIAL SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN SQLITE_IOERR SQLITE_IOERR_ACCESS SQLITE_IOERR_BLOCKED SQLITE_IOERR_CHECKRESERVEDLOCK SQLITE_IOERR_CLOSE SQLITE_IOERR_DELETE SQLITE_IOERR_DIR_CLOSE SQLITE_IOERR_DIR_FSYNC SQLITE_IOERR_FSTAT SQLITE_IOERR_FSYNC SQLITE_IOERR_LOCK SQLITE_IOERR_NOMEM SQLITE_IOERR_RDLOCK SQLITE_IOERR_READ SQLITE_IOERR_SHMLOCK SQLITE_IOERR_SHMOPEN SQLITE_IOERR_SHMSIZE SQLITE_IOERR_SHORT_READ SQLITE_IOERR_TRUNCATE SQLITE_IOERR_UNLOCK SQLITE_IOERR_WRITE SQLITE_LAST_ERRNO SQLITE_LIMIT_ATTACHED | SQLITE_LIMIT_COLUMN SQLITE_LIMIT_COMPOUND_SELECT SQLITE_LIMIT_EXPR_DEPTH SQLITE_LIMIT_FUNCTION_ARG SQLITE_LIMIT_LENGTH SQLITE_LIMIT_LIKE_PATTERN_LENGTH SQLITE_LIMIT_SQL_LENGTH SQLITE_LIMIT_TRIGGER_DEPTH SQLITE_LIMIT_VARIABLE_NUMBER SQLITE_LIMIT_VDBE_OP SQLITE_LOCKED SQLITE_LOCKED_SHAREDCACHE SQLITE_LOCK_EXCLUSIVE SQLITE_LOCK_NONE SQLITE_LOCK_PENDING SQLITE_LOCK_RESERVED SQLITE_LOCK_SHARED SQLITE_MISMATCH SQLITE_MISUSE SQLITE_MUTEX_FAST SQLITE_MUTEX_RECURSIVE SQLITE_MUTEX_STATIC_LRU SQLITE_MUTEX_STATIC_LRU2 SQLITE_MUTEX_STATIC_MASTER SQLITE_MUTEX_STATIC_MEM SQLITE_MUTEX_STATIC_MEM2 SQLITE_MUTEX_STATIC_OPEN SQLITE_MUTEX_STATIC_PRNG SQLITE_NOLFS SQLITE_NOMEM SQLITE_NOTADB SQLITE_NOTFOUND SQLITE_NULL SQLITE_OK SQLITE_OPEN_AUTOPROXY SQLITE_OPEN_CREATE SQLITE_OPEN_DELETEONCLOSE SQLITE_OPEN_EXCLUSIVE SQLITE_OPEN_FULLMUTEX SQLITE_OPEN_MAIN_DB SQLITE_OPEN_MAIN_JOURNAL SQLITE_OPEN_MASTER_JOURNAL SQLITE_OPEN_NOMUTEX SQLITE_OPEN_PRIVATECACHE SQLITE_OPEN_READONLY SQLITE_OPEN_READWRITE SQLITE_OPEN_SHAREDCACHE SQLITE_OPEN_SUBJOURNAL SQLITE_OPEN_TEMP_DB SQLITE_OPEN_TEMP_JOURNAL SQLITE_OPEN_TRANSIENT_DB SQLITE_OPEN_WAL SQLITE_PERM SQLITE_PRAGMA SQLITE_PROTOCOL SQLITE_RANGE SQLITE_READ SQLITE_READONLY SQLITE_REINDEX SQLITE_ROW SQLITE_SAVEPOINT SQLITE_SCHEMA SQLITE_SELECT SQLITE_SET_LOCKPROXYFILE SQLITE_SHM_EXCLUSIVE SQLITE_SHM_LOCK SQLITE_SHM_NLOCK SQLITE_SHM_SHARED SQLITE_SHM_UNLOCK SQLITE_SOURCE_ID SQLITE_STATIC SQLITE_STATUS_MALLOC_SIZE SQLITE_STATUS_MEMORY_USED SQLITE_STATUS_PAGECACHE_OVERFLOW SQLITE_STATUS_PAGECACHE_SIZE SQLITE_STATUS_PAGECACHE_USED SQLITE_STATUS_PARSER_STACK SQLITE_STATUS_SCRATCH_OVERFLOW SQLITE_STATUS_SCRATCH_SIZE SQLITE_STATUS_SCRATCH_USED SQLITE_STMTSTATUS_AUTOINDEX SQLITE_STMTSTATUS_FULLSCAN_STEP SQLITE_STMTSTATUS_SORT SQLITE_SYNC_DATAONLY SQLITE_SYNC_FULL SQLITE_SYNC_NORMAL SQLITE_TESTCTRL_ALWAYS SQLITE_TESTCTRL_ASSERT SQLITE_TESTCTRL_BENIGN_MALLOC_HOOKS SQLITE_TESTCTRL_BITVEC_TEST SQLITE_TESTCTRL_FAULT_INSTALL SQLITE_TESTCTRL_FIRST SQLITE_TESTCTRL_ISKEYWORD SQLITE_TESTCTRL_LAST SQLITE_TESTCTRL_OPTIMIZATIONS SQLITE_TESTCTRL_PENDING_BYTE SQLITE_TESTCTRL_PGHDRSZ SQLITE_TESTCTRL_PRNG_RESET SQLITE_TESTCTRL_PRNG_RESTORE SQLITE_TESTCTRL_PRNG_SAVE SQLITE_TESTCTRL_RESERVE SQLITE_TEXT SQLITE_TOOBIG SQLITE_TRANSACTION SQLITE_TRANSIENT SQLITE_UPDATE SQLITE_UTF16 SQLITE_UTF16BE SQLITE_UTF16LE SQLITE_UTF16_ALIGNED SQLITE_UTF8 SQLITE_VERSION SQLITE_VERSION_NUMBER

## Functions:

Note: Functions marked with "(exp)" are experimental and functions marked with (obs) are deprecated.

sqlite3_aggregate_context sqlite3_aggregate_count (obs) sqlite3_auto_extension sqlite3_backup_finish sqlite3_backup_init sqlite3_backup_pagecount sqlite3_backup_remaining sqlite3_backup_step sqlite3_bind_blob sqlite3_bind_double sqlite3_bind_int sqlite3_bind_int64 sqlite3_bind_null sqlite3_bind_parameter_count sqlite3_bind_parameter_index sqlite3_bind_parameter_name sqlite3_bind_text sqlite3_bind_text16 sqlite3_bind_value sqlite3_bind_zeroblob sqlite3_blob_bytes sqlite3_blob_close sqlite3_blob_open sqlite3_blob_read sqlite3_blob_write sqlite3_busy_handler sqlite3_busy_timeout sqlite3_changes sqlite3_clear_bindings sqlite3_close sqlite3_collation_needed sqlite3_collation_needed16 sqlite3_column_blob sqlite3_column_bytes sqlite3_column_bytes16 sqlite3_column_count sqlite3_column_database_name sqlite3_column_database_name16 sqlite3_column_decltype sqlite3_column_decltype16 sqlite3_column_double sqlite3_column_int sqlite3_column_int64 sqlite3_column_name sqlite3_column_name16 sqlite3_column_origin_name sqlite3_column_origin_name16 sqlite3_column_table_name sqlite3_column_table_name16 sqlite3_column_text sqlite3_column_text16 sqlite3_column_type sqlite3_column_value sqlite3_commit_hook sqlite3_compileoption_get sqlite3_compileoption_used sqlite3_complete sqlite3_complete16 sqlite3_config sqlite3_context_db_handle sqlite3_create_collation sqlite3_create_collation16 sqlite3_create_collation_v2 | sqlite3_create_function sqlite3_create_function16 sqlite3_create_module sqlite3_create_module_v2 sqlite3_data_count sqlite3_db_config sqlite3_db_handle sqlite3_db_mutex sqlite3_db_status sqlite3_declare_vtab sqlite3_enable_load_extension sqlite3_enable_shared_cache sqlite3_errcode sqlite3_errmsg sqlite3_errmsg16 sqlite3_exec sqlite3_expired (obs) sqlite3_extended_errcode sqlite3_extended_result_codes sqlite3_file_control sqlite3_finalize sqlite3_free sqlite3_free_table sqlite3_get_autocommit sqlite3_get_auxdata sqlite3_get_table sqlite3_global_recover (obs) sqlite3_initialize sqlite3_interrupt sqlite3_last_insert_rowid sqlite3_libversion sqlite3_libversion_number sqlite3_limit sqlite3_load_extension sqlite3_log sqlite3_malloc sqlite3_memory_alarm (obs) sqlite3_memory_highwater sqlite3_memory_used sqlite3_mprintf sqlite3_mutex_alloc sqlite3_mutex_enter sqlite3_mutex_free sqlite3_mutex_held sqlite3_mutex_leave sqlite3_mutex_notheld sqlite3_mutex_try sqlite3_next_stmt sqlite3_open sqlite3_open16 sqlite3_open_v2 sqlite3_os_end sqlite3_os_init sqlite3_overload_function sqlite3_prepare sqlite3_prepare16 sqlite3_prepare16_v2 sqlite3_prepare_v2 sqlite3_profile sqlite3_progress_handler sqlite3_randomness sqlite3_realloc sqlite3_release_memory | sqlite3_reset sqlite3_reset_auto_extension sqlite3_result_blob sqlite3_result_double sqlite3_result_error sqlite3_result_error16 sqlite3_result_error_code sqlite3_result_error_nomem sqlite3_result_error_toobig sqlite3_result_int sqlite3_result_int64 sqlite3_result_null sqlite3_result_text sqlite3_result_text16 sqlite3_result_text16be sqlite3_result_text16le sqlite3_result_value sqlite3_result_zeroblob sqlite3_rollback_hook sqlite3_set_authorizer sqlite3_set_auxdata sqlite3_shutdown sqlite3_sleep sqlite3_snprintf sqlite3_soft_heap_limit sqlite3_sourceid sqlite3_sql sqlite3_status sqlite3_step sqlite3_stmt_status sqlite3_strnicmp sqlite3_table_column_metadata sqlite3_test_control sqlite3_thread_cleanup (obs) sqlite3_threadsafe sqlite3_total_changes sqlite3_trace sqlite3_transfer_bindings (obs) sqlite3_unlock_notify sqlite3_update_hook sqlite3_user_data sqlite3_value_blob sqlite3_value_bytes sqlite3_value_bytes16 sqlite3_value_double sqlite3_value_int sqlite3_value_int64 sqlite3_value_numeric_type sqlite3_value_text sqlite3_value_text16 sqlite3_value_text16be sqlite3_value_text16le sqlite3_value_type sqlite3_version, sqlite3_vfs_find sqlite3_vfs_register sqlite3_vfs_unregister sqlite3_vmprintf sqlite3_wal_autocheckpoint sqlite3_wal_checkpoint sqlite3_wal_hook

## Database Connection Configuration Options

>     #define SQLITE_DBCONFIG_LOOKASIDE    1001  /* void* int int */

These constants are the available integer configuration options that can be passed as the second argument to the sqlite3_db_config() interface.

New configuration options may be added in future releases of SQLite. Existing configuration options might be discontinued. Applications should check the return code from sqlite3_db_config() to make sure that the call worked. The sqlite3_db_config() interface will return a non-zero error code if a discontinued or unsupported configuration option is invoked.

SQLITE_DBCONFIG_LOOKASIDE
  This option takes three additional arguments that determine the lookaside memory allocator configuration for the database connection. The first argument (the third parameter to sqlite3_db_config() is a pointer to an memory buffer to use for lookaside memory. The first argument after the SQLITE_DBCONFIG_LOOKASIDE verb may be NULL in which case SQLite will allocate the lookaside buffer itself using sqlite3_malloc(). The second argument is the size of each lookaside buffer slot. The third argument is the number of slots. The size of the buffer in the first argument must be greater than or equal to the product of the second and third arguments. The buffer must be aligned to an 8-byte boundary. If the second argument to SQLITE_DBCONFIG_LOOKASIDE is not a multiple of 8, it is internally rounded down to the next smaller multiple of 8. See also: SQLITE_CONFIG_LOOKASIDE

## Maximum xShmLock index

>     #define SQLITE_SHM_NLOCK        8

The xShmLock method on sqlite3_io_methods may use values between 0 and this upper bound as its "offset" argument. The SQLite core will never attempt to acquire or release a lock outside of this range

## Online Backup Object

>     typedef struct sqlite3_backup sqlite3_backup;

The sqlite3_backup object records state information about an ongoing online backup operation. The sqlite3_backup object is created by a call to sqlite3_backup_init() and is destroyed by a call to sqlite3_backup_finish().

See Also: Using the SQLite Online Backup API

## SQL Function Context Object

>     typedef struct sqlite3_context sqlite3_context;

The context in which an SQL function executes is stored in an sqlite3_context object. A pointer to an sqlite3_context object is always first parameter to application-defined SQL functions. The application-defined SQL function implementation will pass this pointer through into calls to sqlite3_result(), sqlite3_aggregate_context(), sqlite3_user_data(), sqlite3_context_db_handle(), sqlite3_get_auxdata(), and/or sqlite3_set_auxdata().

## OS Interface Open File Handle

>     typedef struct sqlite3_file sqlite3_file;
>     struct sqlite3_file {
>       const struct sqlite3_io_methods *pMethods;  /* Methods for an open file */
>     };

An sqlite3_file object represents an open file in the OS interface layer. Individual OS interface implementations will want to subclass this object by appending additional fields for their own use. The pMethods entry is a pointer to an sqlite3_io_methods object that defines methods for performing I/O operations on the open file.

## OS Interface File Virtual Methods Object

>     typedef struct sqlite3_io_methods sqlite3_io_methods;
>     struct sqlite3_io_methods {
>       int iVersion;
>       int (*xClose)(sqlite3_file*);
>       int (*xRead)(sqlite3_file*, void*, int iAmt, sqlite3_int64 iOfst);
>       int (*xWrite)(sqlite3_file*, const void*, int iAmt, sqlite3_int64 iOfst);
>       int (*xTruncate)(sqlite3_file*, sqlite3_int64 size);
>       int (*xSync)(sqlite3_file*, int flags);
>       int (*xFileSize)(sqlite3_file*, sqlite3_int64 *pSize);
>       int (*xLock)(sqlite3_file*, int);
>       int (*xUnlock)(sqlite3_file*, int);
>       int (*xCheckReservedLock)(sqlite3_file*, int *pResOut);
>       int (*xFileControl)(sqlite3_file*, int op, void *pArg);
>       int (*xSectorSize)(sqlite3_file*);
>       int (*xDeviceCharacteristics)(sqlite3_file*);
>       /* Methods above are valid for version 1 */
>       int (*xShmMap)(sqlite3_file*, int iPg, int pgsz, int, void volatile**);
>       int (*xShmLock)(sqlite3_file*, int offset, int n, int flags);
>       void (*xShmBarrier)(sqlite3_file*);
>       int (*xShmUnmap)(sqlite3_file*, int deleteFlag);
>       /* Methods above are valid for version 2 */
>       /* Additional methods may be added in future releases */
>     };

Every file opened by the sqlite3_vfs xOpen method populates an sqlite3_file object (or, more commonly, a subclass of the sqlite3_file object) with a pointer to an instance of this object. This object defines the methods used to perform various operations against the open file represented by the sqlite3_file object.

If the xOpen method sets the sqlite3_file.pMethods element to a non-NULL pointer, then the sqlite3_io_methods.xClose method may be invoked even if the xOpen reported that it failed. The only way to prevent a call to xClose following a failed xOpen is for the xOpen to set the sqlite3_file.pMethods element to NULL.

The flags argument to xSync may be one of SQLITE_SYNC_NORMAL or SQLITE_SYNC_FULL. The first choice is the normal fsync(). The second choice is a Mac OS X style fullsync. The SQLITE_SYNC_DATAONLY flag may be ORed in to indicate that only the data of the file and not its inode needs to be synced.

The integer values to xLock() and xUnlock() are one of

- SQLITE_LOCK_NONE,
- SQLITE_LOCK_SHARED,
- SQLITE_LOCK_RESERVED,
- SQLITE_LOCK_PENDING, or
- SQLITE_LOCK_EXCLUSIVE.

xLock() increases the lock. xUnlock() decreases the lock. The xCheckReservedLock() method checks whether any database connection, either in this process or in some other process, is holding a RESERVED, PENDING, or EXCLUSIVE lock on the file. It returns true if such a lock exists and false otherwise.

The xFileControl() method is a generic interface that allows custom VFS implementations to directly control an open file using the sqlite3_file_control() interface. The second "op" argument is an integer opcode. The third argument is a generic pointer intended to point to a structure that may contain arguments or space in which to write return values. Potential uses for xFileControl() might be functions to enable blocking locks with timeouts, to change the locking strategy (for example to use dot-file locks), to inquire about the status of a lock, or to break stale locks. The SQLite core reserves all opcodes less than 100 for its own use. A list of opcodes less than 100 is available. Applications that define a custom xFileControl method should use opcodes greater than 100 to avoid conflicts.

The xSectorSize() method returns the sector size of the device that underlies the file. The sector size is the minimum write that can be performed without disturbing other bytes in the file. The xDeviceCharacteristics() method returns a bit vector describing behaviors of the underlying device:

- SQLITE_IOCAP_ATOMIC
- SQLITE_IOCAP_ATOMIC512
- SQLITE_IOCAP_ATOMIC1K
- SQLITE_IOCAP_ATOMIC2K
- SQLITE_IOCAP_ATOMIC4K
- SQLITE_IOCAP_ATOMIC8K
- SQLITE_IOCAP_ATOMIC16K
- SQLITE_IOCAP_ATOMIC32K
- SQLITE_IOCAP_ATOMIC64K
- SQLITE_IOCAP_SAFE_APPEND
- SQLITE_IOCAP_SEQUENTIAL

The SQLITE_IOCAP_ATOMIC property means that all writes of any size are atomic. The SQLITE_IOCAP_ATOMICnnn values mean that writes of blocks that are nnn bytes in size and are aligned to an address which is an integer multiple of nnn are atomic. The SQLITE_IOCAP_SAFE_APPEND value means that when data is appended to a file, the data is appended first then the size of the file is extended, never the other way around. The SQLITE_IOCAP_SEQUENTIAL property means that information is written to disk in the same order as calls to xWrite().

If xRead() returns SQLITE_IOERR_SHORT_READ it must also fill in the unread portions of the buffer with zeros. A VFS that fails to zero-fill short reads might seem to work. However, failure to zero-fill short reads will eventually lead to database corruption.

## Memory Allocation Routines

>     typedef struct sqlite3_mem_methods sqlite3_mem_methods;
>     struct sqlite3_mem_methods {
>       void *(*xMalloc)(int);         /* Memory allocation function */
>       void (*xFree)(void*);          /* Free a prior allocation */
>       void *(*xRealloc)(void*,int);  /* Resize an allocation */
>       int (*xSize)(void*);           /* Return the size of an allocation */
>       int (*xRoundup)(int);          /* Round up request size to allocation size */
>       int (*xInit)(void*);           /* Initialize the memory allocator */
>       void (*xShutdown)(void*);      /* Deinitialize the memory allocator */
>       void *pAppData;                /* Argument to xInit() and xShutdown() */
>     };

An instance of this object defines the interface between SQLite and low-level memory allocation routines.

This object is used in only one place in the SQLite interface. A pointer to an instance of this object is the argument to sqlite3_config() when the configuration option is SQLITE_CONFIG_MALLOC or SQLITE_CONFIG_GETMALLOC. By creating an instance of this object and passing it to sqlite3_config(SQLITE_CONFIG_MALLOC) during configuration, an application can specify an alternative memory allocation subsystem for SQLite to use for all of its dynamic memory needs.

Note that SQLite comes with several built-in memory allocators that are perfectly adequate for the overwhelming majority of applications and that this object is only useful to a tiny minority of applications with specialized memory allocation requirements. This object is also used during testing of SQLite in order to specify an alternative memory allocator that simulates memory out-of-memory conditions in order to verify that SQLite recovers gracefully from such conditions.

The xMalloc and xFree methods must work like the malloc() and free() functions from the standard C library. The xRealloc method must work like realloc() from the standard C library with the exception that if the second argument to xRealloc is zero, xRealloc must be a no-op - it must not perform any allocation or deallocation. SQLite guarantees that the second argument to xRealloc is always a value returned by a prior call to xRoundup. And so in cases where xRoundup always returns a positive number, xRealloc can perform exactly as the standard library realloc() and still be in compliance with this specification.

xSize should return the allocated size of a memory allocation previously obtained from xMalloc or xRealloc. The allocated size is always at least as big as the requested size but may be larger.

The xRoundup method returns what would be the allocated size of a memory allocation given a particular requested size. Most memory allocators round up memory allocations at least to the next multiple of 8. Some allocators round up to a larger multiple or to a power of 2. Every memory allocation request coming in through sqlite3_malloc() or sqlite3_realloc() first calls xRoundup. If xRoundup returns 0, that causes the corresponding memory allocation to fail.

The xInit method initializes the memory allocator. (For example, it might allocate any require mutexes or initialize internal data structures. The xShutdown method is invoked (indirectly) by sqlite3_shutdown() and should deallocate any resources acquired by xInit. The pAppData pointer is used as the only parameter to xInit and xShutdown.

SQLite holds the SQLITE_MUTEX_STATIC_MASTER mutex when it invokes the xInit method, so the xInit method need not be threadsafe. The xShutdown method is only called from sqlite3_shutdown() so it does not need to be threadsafe either. For all other methods, SQLite holds the SQLITE_MUTEX_STATIC_MEM mutex as long as the SQLITE_CONFIG_MEMSTATUS configuration option is turned on (which it is by default) and so the methods are automatically serialized. However, if SQLITE_CONFIG_MEMSTATUS is disabled, then the other methods must be threadsafe or else make their own arrangements for serialization.

SQLite will never invoke xInit() more than once without an intervening call to xShutdown().

## Mutex Handle

>     typedef struct sqlite3_mutex sqlite3_mutex;

The mutex module within SQLite defines sqlite3_mutex to be an abstract type for a mutex object. The SQLite core never looks at the internal representation of an sqlite3_mutex. It only deals with pointers to the sqlite3_mutex object.

Mutexes are created using sqlite3_mutex_alloc().

## Mutex Methods Object

>     typedef struct sqlite3_mutex_methods sqlite3_mutex_methods;
>     struct sqlite3_mutex_methods {
>       int (*xMutexInit)(void);
>       int (*xMutexEnd)(void);
>       sqlite3_mutex *(*xMutexAlloc)(int);
>       void (*xMutexFree)(sqlite3_mutex *);
>       void (*xMutexEnter)(sqlite3_mutex *);
>       int (*xMutexTry)(sqlite3_mutex *);
>       void (*xMutexLeave)(sqlite3_mutex *);
>       int (*xMutexHeld)(sqlite3_mutex *);
>       int (*xMutexNotheld)(sqlite3_mutex *);
>     };

An instance of this structure defines the low-level routines used to allocate and use mutexes.

Usually, the default mutex implementations provided by SQLite are sufficient, however the user has the option of substituting a custom implementation for specialized deployments or systems for which SQLite does not provide a suitable implementation. In this case, the user creates and populates an instance of this structure to pass to sqlite3_config() along with the SQLITE_CONFIG_MUTEX option. Additionally, an instance of this structure can be used as an output variable when querying the system for the current mutex implementation, using the SQLITE_CONFIG_GETMUTEX option.

The xMutexInit method defined by this structure is invoked as part of system initialization by the sqlite3_initialize() function. The xMutexInit routine is calle by SQLite exactly once for each effective call to sqlite3_initialize().

The xMutexEnd method defined by this structure is invoked as part of system shutdown by the sqlite3_shutdown() function. The implementation of this method is expected to release all outstanding resources obtained by the mutex methods implementation, especially those obtained by the xMutexInit method. The xMutexEnd() interface is invoked exactly once for each call to sqlite3_shutdown().

The remaining seven methods defined by this structure (xMutexAlloc, xMutexFree, xMutexEnter, xMutexTry, xMutexLeave, xMutexHeld and xMutexNotheld) implement the following interfaces (respectively):

- sqlite3_mutex_alloc()
- sqlite3_mutex_free()
- sqlite3_mutex_enter()
- sqlite3_mutex_try()
- sqlite3_mutex_leave()
- sqlite3_mutex_held()
- sqlite3_mutex_notheld()

The only difference is that the public sqlite3_XXX functions enumerated above silently ignore any invocations that pass a NULL pointer instead of a valid mutex handle. The implementations of the methods defined by this structure are not required to handle this case, the results of passing a NULL pointer instead of a valid mutex handle are undefined (i.e. it is acceptable to provide an implementation that segfaults if it is passed a NULL pointer).

The xMutexInit() method must be threadsafe. It must be harmless to invoke xMutexInit() mutiple times within the same process and without intervening calls to xMutexEnd(). Second and subsequent calls to xMutexInit() must be no-ops.

xMutexInit() must not use SQLite memory allocation (sqlite3_malloc() and its associates). Similarly, xMutexAlloc() must not use SQLite memory allocation for a static mutex. However xMutexAlloc() may use SQLite memory allocation for a fast or recursive mutex.

SQLite will invoke the xMutexEnd() method when sqlite3_shutdown() is called, but only if the prior call to xMutexInit returned SQLITE_OK. If xMutexInit fails in any way, it is expected to clean up after itself prior to returning.

## Custom Page Cache Object

>     typedef struct sqlite3_pcache sqlite3_pcache;

The sqlite3_pcache type is opaque. It is implemented by the pluggable module. The SQLite core has no knowledge of its size or internal structure and never deals with the sqlite3_pcache object except by holding and passing pointers to the object.

See sqlite3_pcache_methods for additional information.

## Name Of The Folder Holding Temporary Files

>     SQLITE_EXTERN char *sqlite3_temp_directory;

If this global variable is made to point to a string which is the name of a folder (a.k.a. directory), then all temporary files created by SQLite when using a built-in VFS will be placed in that directory. If this variable is a NULL pointer, then SQLite performs a search for an appropriate temporary file directory.

It is not safe to read or modify this variable in more than one thread at a time. It is not safe to read or modify this variable if a database connection is being used at the same time in a separate thread. It is intended that this variable be set once as part of process initialization and before any SQLite interface routines have been called and that this variable remain unchanged thereafter.

The temp_store_directory pragma may modify this variable and cause it to point to memory obtained from sqlite3_malloc. Furthermore, the temp_store_directory pragma always assumes that any string that this variable points to is held in memory obtained from sqlite3_malloc and the pragma may attempt to free that memory using sqlite3_free. Hence, if this variable is modified directly, either it should be made NULL or made to point to memory obtained from sqlite3_malloc or else the use of the temp_store_directory pragma should be avoided.

## OS Interface Object

>     typedef struct sqlite3_vfs sqlite3_vfs;
>     struct sqlite3_vfs {
>       int iVersion;            /* Structure version number (currently 2) */
>       int szOsFile;            /* Size of subclassed sqlite3_file */
>       int mxPathname;          /* Maximum file pathname length */
>       sqlite3_vfs *pNext;      /* Next registered VFS */
>       const char *zName;       /* Name of this virtual file system */
>       void *pAppData;          /* Pointer to application-specific data */
>       int (*xOpen)(sqlite3_vfs*, const char *zName, sqlite3_file*,
>                    int flags, int *pOutFlags);
>       int (*xDelete)(sqlite3_vfs*, const char *zName, int syncDir);
>       int (*xAccess)(sqlite3_vfs*, const char *zName, int flags, int *pResOut);
>       int (*xFullPathname)(sqlite3_vfs*, const char *zName, int nOut, char *zOut);
>       void *(*xDlOpen)(sqlite3_vfs*, const char *zFilename);
>       void (*xDlError)(sqlite3_vfs*, int nByte, char *zErrMsg);
>       void (*(*xDlSym)(sqlite3_vfs*,void*, const char *zSymbol))(void);
>       void (*xDlClose)(sqlite3_vfs*, void*);
>       int (*xRandomness)(sqlite3_vfs*, int nByte, char *zOut);
>       int (*xSleep)(sqlite3_vfs*, int microseconds);
>       int (*xCurrentTime)(sqlite3_vfs*, double*);
>       int (*xGetLastError)(sqlite3_vfs*, int, char *);
>       /*
>       ** The methods above are in version 1 of the sqlite_vfs object
>       ** definition.  Those that follow are added in version 2 or later
>       */
>       int (*xCurrentTimeInt64)(sqlite3_vfs*, sqlite3_int64*);
>       /*
>       ** The methods above are in versions 1 and 2 of the sqlite_vfs object.
>       ** New fields may be appended in figure versions.  The iVersion
>       ** value will increment whenever this happens.
>       */
>     };

An instance of the sqlite3_vfs object defines the interface between the SQLite core and the underlying operating system. The "vfs" in the name of the object stands for "virtual file system".

The value of the iVersion field is initially 1 but may be larger in future versions of SQLite. Additional fields may be appended to this object when the iVersion value is increased. Note that the structure of the sqlite3_vfs object changes in the transaction between SQLite version 3.5.9 and 3.6.0 and yet the iVersion field was not modified.

The szOsFile field is the size of the subclassed sqlite3_file structure used by this VFS. mxPathname is the maximum length of a pathname in this VFS.

Registered sqlite3_vfs objects are kept on a linked list formed by the pNext pointer. The sqlite3_vfs_register() and sqlite3_vfs_unregister() interfaces manage this list in a thread-safe way. The sqlite3_vfs_find() interface searches the list. Neither the application code nor the VFS implementation should use the pNext pointer.

The pNext field is the only field in the sqlite3_vfs structure that SQLite will ever modify. SQLite will only access or modify this field while holding a particular static mutex. The application should never modify anything within the sqlite3_vfs object once the object has been registered.

The zName field holds the name of the VFS module. The name must be unique across all VFS modules.

SQLite will guarantee that the zFilename parameter to xOpen is either a NULL pointer or string obtained from xFullPathname(). SQLite further guarantees that the string will be valid and unchanged until xClose() is called. Because of the previous sentence, the sqlite3_file can safely store a pointer to the filename if it needs to remember the filename for some reason. If the zFilename parameter is xOpen is a NULL pointer then xOpen must invent its own temporary name for the file. Whenever the xFilename parameter is NULL it will also be the case that the flags parameter will include SQLITE_OPEN_DELETEONCLOSE.

The flags argument to xOpen() includes all bits set in the flags argument to sqlite3_open_v2(). Or if sqlite3_open() or sqlite3_open16() is used, then flags includes at least SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE. If xOpen() opens a file read-only then it sets *pOutFlags to include SQLITE_OPEN_READONLY. Other bits in *pOutFlags may be set.

SQLite will also add one of the following flags to the xOpen() call, depending on the object being opened:

- SQLITE_OPEN_MAIN_DB
- SQLITE_OPEN_MAIN_JOURNAL
- SQLITE_OPEN_TEMP_DB
- SQLITE_OPEN_TEMP_JOURNAL
- SQLITE_OPEN_TRANSIENT_DB
- SQLITE_OPEN_SUBJOURNAL
- SQLITE_OPEN_MASTER_JOURNAL

The file I/O implementation can use the object type flags to change the way it deals with files. For example, an application that does not care about crash recovery or rollback might make the open of a journal file a no-op. Writes to this journal would also be no-ops, and any attempt to read the journal would return SQLITE_IOERR. Or the implementation might recognize that a database file will be doing page-aligned sector reads and writes in a random order and set up its I/O subsystem accordingly.

SQLite might also add one of the following flags to the xOpen method:

- SQLITE_OPEN_DELETEONCLOSE
- SQLITE_OPEN_EXCLUSIVE

The SQLITE_OPEN_DELETEONCLOSE flag means the file should be deleted when it is closed. The SQLITE_OPEN_DELETEONCLOSE will be set for TEMP databases, journals and for subjournals.

The SQLITE_OPEN_EXCLUSIVE flag is always used in conjunction with the SQLITE_OPEN_CREATE flag, which are both directly analogous to the O_EXCL and O_CREAT flags of the POSIX open() API. The SQLITE_OPEN_EXCLUSIVE flag, when paired with the SQLITE_OPEN_CREATE, is used to indicate that file should always be created, and that it is an error if it already exists. It is not used to indicate the file should be opened for exclusive access.

At least szOsFile bytes of memory are allocated by SQLite to hold the sqlite3_file structure passed as the third argument to xOpen. The xOpen method does not have to allocate the structure; it should just fill it in. Note that the xOpen method must set the sqlite3_file.pMethods to either a valid sqlite3_io_methods object or to NULL. xOpen must do this even if the open fails. SQLite expects that the sqlite3_file.pMethods element will be valid after xOpen returns regardless of the success or failure of the xOpen call.

The flags argument to xAccess() may be SQLITE_ACCESS_EXISTS to test for the existence of a file, or SQLITE_ACCESS_READWRITE to test whether a file is readable and writable, or SQLITE_ACCESS_READ to test whether a file is at least readable. The file can be a directory.

SQLite will always allocate at least mxPathname+1 bytes for the output buffer xFullPathname. The exact size of the output buffer is also passed as a parameter to both methods. If the output buffer is not large enough, SQLITE_CANTOPEN should be returned. Since this is handled as a fatal error by SQLite, vfs implementations should endeavor to prevent this by setting mxPathname to a sufficiently large value.

The xRandomness(), xSleep(), xCurrentTime(), and xCurrentTimeInt64() interfaces are not strictly a part of the filesystem, but they are included in the VFS structure for completeness. The xRandomness() function attempts to return nBytes bytes of good-quality randomness into zOut. The return value is the actual number of bytes of randomness obtained. The xSleep() method causes the calling thread to sleep for at least the number of microseconds given. The xCurrentTime() method returns a Julian Day Number for the current date and time as a floating point value. The xCurrentTimeInt64() method returns, as an integer, the Julian Day Number multipled by 86400000 (the number of milliseconds in a 24-hour day). SQLite will use the xCurrentTimeInt64() method to get the current date and time if that method is available (if iVersion is 2 or greater and the function pointer is not NULL) and will fall back to xCurrentTime() if xCurrentTimeInt64() is unavailable.

## Virtual Table Instance Object

>     struct sqlite3_vtab {
>       const sqlite3_module *pModule;  /* The module for this virtual table */
>       int nRef;                       /* NO LONGER USED */
>       char *zErrMsg;                  /* Error message from sqlite3_mprintf() */
>       /* Virtual table implementations will typically add additional fields */
>     };

Every virtual table module implementation uses a subclass of this object to describe a particular instance of the virtual table. Each subclass will be tailored to the specific needs of the module implementation. The purpose of this superclass is to define certain fields that are common to all module implementations.

Virtual tables methods can set an error message by assigning a string obtained from sqlite3_mprintf() to zErrMsg. The method should take care that any prior string is freed by a call to sqlite3_free() prior to assigning a new string to zErrMsg. After the error message is delivered up to the client application, the string will be automatically freed by sqlite3_free() and the zErrMsg field will be zeroed.

## Obtain Aggregate Function Context

>     void *sqlite3_aggregate_context(sqlite3_context*, int nBytes);

Implementions of aggregate SQL functions use this routine to allocate memory for storing their state.

The first time the sqlite3_aggregate_context(C,N) routine is called for a particular aggregate function, SQLite allocates N of memory, zeroes out that memory, and returns a pointer to the new memory. On second and subsequent calls to sqlite3_aggregate_context() for the same aggregate function instance, the same buffer is returned. Sqlite3_aggregate_context() is normally called once for each invocation of the xStep callback and then one last time when the xFinal callback is invoked. When no rows match an aggregate query, the xStep() callback of the aggregate function implementation is never called and xFinal() is called exactly once. In those cases, sqlite3_aggregate_context() might be called for the first time from within xFinal().

The sqlite3_aggregate_context(C,N) routine returns a NULL pointer if N is less than or equal to zero or if a memory allocate error occurs.

The amount of space allocated by sqlite3_aggregate_context(C,N) is determined by the N parameter on first successful call. Changing the value of N in subsequent call to sqlite3_aggregate_context() within the same aggregate function instance will not resize the memory allocation.

SQLite automatically frees the memory allocated by sqlite3_aggregate_context() when the aggregate query concludes.

The first parameter must be a copy of the SQL function context that is the first parameter to the xStep or xFinal callback routine that implements the aggregate function.

This routine must be called from the same thread in which the aggregate SQL function is running.

## Automatically Load An Extensions

>     int sqlite3_auto_extension(void (*xEntryPoint)(void));

This API can be invoked at program startup in order to register one or more statically linked extensions that will be available to all new database connections.

This routine stores a pointer to the extension entry point in an array that is obtained from sqlite3_malloc(). That memory is deallocated by sqlite3_reset_auto_extension().

This function registers an extension entry point that is automatically invoked whenever a new database connection is opened using sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2(). Duplicate extensions are detected so calling this routine multiple times with the same extension is harmless. Automatic extensions apply across all threads.

## Number Of SQL Parameters

>     int sqlite3_bind_parameter_count(sqlite3_stmt*);

This routine can be used to find the number of SQL parameters in a prepared statement. SQL parameters are tokens of the form "?", "?NNN", ":AAA", "$AAA", or "@AAA" that serve as placeholders for values that are bound to the parameters at a later time.

This routine actually returns the index of the largest (rightmost) parameter. For all forms except ?NNN, this will correspond to the number of unique parameters. If parameters of the ?NNN form are used, there may be gaps in the list.

See also: sqlite3_bind(), sqlite3_bind_parameter_name(), and sqlite3_bind_parameter_index().

## Index Of A Parameter With A Given Name

>     int sqlite3_bind_parameter_index(sqlite3_stmt*, const char *zName);

Return the index of an SQL parameter given its name. The index value returned is suitable for use as the second parameter to sqlite3_bind(). A zero is returned if no matching parameter is found. The parameter name must be given in UTF-8 even if the original statement was prepared from UTF-16 text using sqlite3_prepare16_v2().

See also: sqlite3_bind(), sqlite3_bind_parameter_count(), and sqlite3_bind_parameter_index().

## Name Of A Host Parameter

>     const char *sqlite3_bind_parameter_name(sqlite3_stmt*, int);

The sqlite3_bind_parameter_name(P,N) interface returns the name of the N-th SQL parameter in the prepared statement P. SQL parameters of the form "?NNN" or ":AAA" or "@AAA" or "$AAA" have a name which is the string "?NNN" or ":AAA" or "@AAA" or "$AAA" respectively. In other words, the initial ":" or "$" or "@" or "?" is included as part of the name. Parameters of the form "?" without a following integer have no name and are referred to as "nameless" or "anonymous parameters".

The first host parameter has an index of 1, not 0.

If the value N is out of range or if the N-th parameter is nameless, then NULL is returned. The returned string is always in UTF-8 encoding even if the named parameter was originally specified as UTF-16 in sqlite3_prepare16() or sqlite3_prepare16_v2().

See also: sqlite3_bind(), sqlite3_bind_parameter_count(), and sqlite3_bind_parameter_index().

## Return The Size Of An Open BLOB

>     int sqlite3_blob_bytes(sqlite3_blob *);

Returns the size in bytes of the BLOB accessible via the successfully opened BLOB handle in its only argument. The incremental blob I/O routines can only read or overwriting existing blob content; they cannot change the size of a blob.

This routine only works on a BLOB handle which has been created by a prior successful call to sqlite3_blob_open() and which has not been closed by sqlite3_blob_close(). Passing any other pointer in to this routine results in undefined and probably undesirable behavior.

## Close A BLOB Handle

>     int sqlite3_blob_close(sqlite3_blob *);

Closes an open BLOB handle.

Closing a BLOB shall cause the current transaction to commit if there are no other BLOBs, no pending prepared statements, and the database connection is in autocommit mode. If any writes were made to the BLOB, they might be held in cache until the close operation if they will fit.

Closing the BLOB often forces the changes out to disk and so if any I/O errors occur, they will likely occur at the time when the BLOB is closed. Any errors that occur during closing are reported as a non-zero return value.

The BLOB is closed unconditionally. Even if this routine returns an error code, the BLOB is still closed.

Calling this routine with a null pointer (such as would be returned by a failed call to sqlite3_blob_open()) is a harmless no-op.

## Open A BLOB For Incremental I/O

>     int sqlite3_blob_open(
>       sqlite3*,
>       const char *zDb,
>       const char *zTable,
>       const char *zColumn,
>       sqlite3_int64 iRow,
>       int flags,
>       sqlite3_blob **ppBlob
>     );

This interfaces opens a handle to the BLOB located in row iRow, column zColumn, table zTable in database zDb; in other words, the same BLOB that would be selected by:

    SELECT zColumn FROM zDb.zTable WHERE rowid = iRow;

If the flags parameter is non-zero, then the BLOB is opened for read and write access. If it is zero, the BLOB is opened for read access. It is not possible to open a column that is part of an index or primary key for writing. If foreign key constraints are enabled, it is not possible to open a column that is part of a child key for writing.

Note that the database name is not the filename that contains the database but rather the symbolic name of the database that appears after the AS keyword when the database is connected using ATTACH. For the main database file, the database name is "main". For TEMP tables, the database name is "temp".

On success, SQLITE_OK is returned and the new BLOB handle is written to *ppBlob. Otherwise an error code is returned and *ppBlob is set to be a null pointer. This function sets the database connection error code and message accessible via sqlite3_errcode() and sqlite3_errmsg() and related functions. Note that the *ppBlob variable is always initialized in a way that makes it safe to invoke sqlite3_blob_close() on *ppBlob regardless of the success or failure of this routine.

If the row that a BLOB handle points to is modified by an UPDATE, DELETE, or by ON CONFLICT side-effects then the BLOB handle is marked as "expired". This is true if any column of the row is changed, even a column other than the one the BLOB handle is open on. Calls to sqlite3_blob_read() and sqlite3_blob_write() for a expired BLOB handle fail with an return code of SQLITE_ABORT. Changes written into a BLOB prior to the BLOB expiring are not rolled back by the expiration of the BLOB. Such changes will eventually commit if the transaction continues to completion.

Use the sqlite3_blob_bytes() interface to determine the size of the opened blob. The size of a blob may not be changed by this interface. Use the UPDATE SQL command to change the size of a blob.

The sqlite3_bind_zeroblob() and sqlite3_result_zeroblob() interfaces and the built-in zeroblob SQL function can be used, if desired, to create an empty, zero-filled blob in which to read or write using this interface.

To avoid a resource leak, every open BLOB handle should eventually be released by a call to sqlite3_blob_close().

## Read Data From A BLOB Incrementally

>     int sqlite3_blob_read(sqlite3_blob *, void *Z, int N, int iOffset);

This function is used to read data from an open BLOB handle into a caller-supplied buffer. N bytes of data are copied into buffer Z from the open BLOB, starting at offset iOffset.

If offset iOffset is less than N bytes from the end of the BLOB, SQLITE_ERROR is returned and no data is read. If N or iOffset is less than zero, SQLITE_ERROR is returned and no data is read. The size of the blob (and hence the maximum value of N+iOffset) can be determined using the sqlite3_blob_bytes() interface.

An attempt to read from an expired BLOB handle fails with an error code of SQLITE_ABORT.

On success, sqlite3_blob_read() returns SQLITE_OK. Otherwise, an error code or an extended error code is returned.

This routine only works on a BLOB handle which has been created by a prior successful call to sqlite3_blob_open() and which has not been closed by sqlite3_blob_close(). Passing any other pointer in to this routine results in undefined and probably undesirable behavior.

See also: sqlite3_blob_write().

## Write Data Into A BLOB Incrementally

>     int sqlite3_blob_write(sqlite3_blob *, const void *z, int n, int iOffset);

This function is used to write data into an open BLOB handle from a caller-supplied buffer. N bytes of data are copied from the buffer Z into the open BLOB, starting at offset iOffset.

If the BLOB handle passed as the first argument was not opened for writing (the flags parameter to sqlite3_blob_open() was zero), this function returns SQLITE_READONLY.

This function may only modify the contents of the BLOB; it is not possible to increase the size of a BLOB using this API. If offset iOffset is less than N bytes from the end of the BLOB, SQLITE_ERROR is returned and no data is written. If N is less than zero SQLITE_ERROR is returned and no data is written. The size of the BLOB (and hence the maximum value of N+iOffset) can be determined using the sqlite3_blob_bytes() interface.

An attempt to write to an expired BLOB handle fails with an error code of SQLITE_ABORT. Writes to the BLOB that occurred before the BLOB handle expired are not rolled back by the expiration of the handle, though of course those changes might have been overwritten by the statement that expired the BLOB handle or by other independent statements.

On success, sqlite3_blob_write() returns SQLITE_OK. Otherwise, an error code or an extended error code is returned.

This routine only works on a BLOB handle which has been created by a prior successful call to sqlite3_blob_open() and which has not been closed by sqlite3_blob_close(). Passing any other pointer in to this routine results in undefined and probably undesirable behavior.

See also: sqlite3_blob_read().

## Register A Callback To Handle SQLITE_BUSY Errors

>     int sqlite3_busy_handler(sqlite3*, int(*)(void*,int), void*);

This routine sets a callback function that might be invoked whenever an attempt is made to open a database table that another thread or process has locked.

If the busy callback is NULL, then SQLITE_BUSY or SQLITE_IOERR_BLOCKED is returned immediately upon encountering the lock. If the busy callback is not NULL, then the callback might be invoked with two arguments.

The first argument to the busy handler is a copy of the void* pointer which is the third argument to sqlite3_busy_handler(). The second argument to the busy handler callback is the number of times that the busy handler has been invoked for this locking event. If the busy callback returns 0, then no additional attempts are made to access the database and SQLITE_BUSY or SQLITE_IOERR_BLOCKED is returned. If the callback returns non-zero, then another attempt is made to open the database for reading and the cycle repeats.

The presence of a busy handler does not guarantee that it will be invoked when there is lock contention. If SQLite determines that invoking the busy handler could result in a deadlock, it will go ahead and return SQLITE_BUSY or SQLITE_IOERR_BLOCKED instead of invoking the busy handler. Consider a scenario where one process is holding a read lock that it is trying to promote to a reserved lock and a second process is holding a reserved lock that it is trying to promote to an exclusive lock. The first process cannot proceed because it is blocked by the second and the second process cannot proceed because it is blocked by the first. If both processes invoke the busy handlers, neither will make any progress. Therefore, SQLite returns SQLITE_BUSY for the first process, hoping that this will induce the first process to release its read lock and allow the second process to proceed.

The default busy callback is NULL.

The SQLITE_BUSY error is converted to SQLITE_IOERR_BLOCKED when SQLite is in the middle of a large transaction where all the changes will not fit into the in-memory cache. SQLite will already hold a RESERVED lock on the database file, but it needs to promote this lock to EXCLUSIVE so that it can spill cache pages into the database file without harm to concurrent readers. If it is unable to promote the lock, then the in-memory cache will be left in an inconsistent state and so the error code is promoted from the relatively benign SQLITE_BUSY to the more severe SQLITE_IOERR_BLOCKED. This error code promotion forces an automatic rollback of the changes. See the CorruptionFollowingBusyError wiki page for a discussion of why this is important.

There can only be a single busy handler defined for each database connection. Setting a new busy handler clears any previously set handler. Note that calling sqlite3_busy_timeout() will also set or clear the busy handler.

The busy callback should not take any actions which modify the database connection that invoked the busy handler. Any such actions result in undefined behavior.

A busy handler must not close the database connection or prepared statement that invoked the busy handler.

## Set A Busy Timeout

>     int sqlite3_busy_timeout(sqlite3*, int ms);

This routine sets a busy handler that sleeps for a specified amount of time when a table is locked. The handler will sleep multiple times until at least "ms" milliseconds of sleeping have accumulated. After at least "ms" milliseconds of sleeping, the handler returns 0 which causes sqlite3_step() to return SQLITE_BUSY or SQLITE_IOERR_BLOCKED.

Calling this routine with an argument less than or equal to zero turns off all busy handlers.

There can only be a single busy handler for a particular database connection any any given moment. If another busy handler was defined (using sqlite3_busy_handler()) prior to calling this routine, that other busy handler is cleared.

## Count The Number Of Rows Modified

>     int sqlite3_changes(sqlite3*);

This function returns the number of database rows that were changed or inserted or deleted by the most recently completed SQL statement on the database connection specified by the first parameter. Only changes that are directly specified by the INSERT, UPDATE, or DELETE statement are counted. Auxiliary changes caused by triggers or foreign key actions are not counted. Use the sqlite3_total_changes() function to find the total number of changes including changes caused by triggers and foreign key actions.

Changes to a view that are simulated by an INSTEAD OF trigger are not counted. Only real table changes are counted.

A "row change" is a change to a single row of a single table caused by an INSERT, DELETE, or UPDATE statement. Rows that are changed as side effects of REPLACE constraint resolution, rollback, ABORT processing, DROP TABLE, or by any other mechanisms do not count as direct row changes.

A "trigger context" is a scope of execution that begins and ends with the script of a trigger. Most SQL statements are evaluated outside of any trigger. This is the "top level" trigger context. If a trigger fires from the top level, a new trigger context is entered for the duration of that one trigger. Subtriggers create subcontexts for their duration.

Calling sqlite3_exec() or sqlite3_step() recursively does not create a new trigger context.

This function returns the number of direct row changes in the most recent INSERT, UPDATE, or DELETE statement within the same trigger context.

Thus, when called from the top level, this function returns the number of changes in the most recent INSERT, UPDATE, or DELETE that also occurred at the top level. Within the body of a trigger, the sqlite3_changes() interface can be called to find the number of changes in the most recently completed INSERT, UPDATE, or DELETE statement within the body of the same trigger. However, the number returned does not include changes caused by subtriggers since those have their own context.

See also the sqlite3_total_changes() interface, the count_changes pragma, and the changes() SQL function.

If a separate thread makes changes on the same database connection while sqlite3_changes() is running then the value returned is unpredictable and not meaningful.

## Reset All Bindings On A Prepared Statement

>     int sqlite3_clear_bindings(sqlite3_stmt*);

Contrary to the intuition of many, sqlite3_reset() does not reset the bindings on a prepared statement. Use this routine to reset all host parameters to NULL.

## Closing A Database Connection

>     int sqlite3_close(sqlite3 *);

The sqlite3_close() routine is the destructor for the sqlite3 object. Calls to sqlite3_close() return SQLITE_OK if the sqlite3 object is successfullly destroyed and all associated resources are deallocated.

Applications must finalize all prepared statements and close all BLOB handles associated with the sqlite3 object prior to attempting to close the object. If sqlite3_close() is called on a database connection that still has outstanding prepared statements or BLOB handles, then it returns SQLITE_BUSY.

If sqlite3_close() is invoked while a transaction is open, the transaction is automatically rolled back.

The C parameter to sqlite3_close(C) must be either a NULL pointer or an sqlite3 object pointer obtained from sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2(), and not previously closed. Calling sqlite3_close() with a NULL pointer argument is a harmless no-op.

## Number Of Columns In A Result Set

>     int sqlite3_column_count(sqlite3_stmt *pStmt);

Return the number of columns in the result set returned by the prepared statement. This routine returns 0 if pStmt is an SQL statement that does not return data (for example an UPDATE).

## Configuring The SQLite Library

>     int sqlite3_config(int, ...);

The sqlite3_config() interface is used to make global configuration changes to SQLite in order to tune SQLite to the specific needs of the application. The default configuration is recommended for most applications and so this routine is usually not necessary. It is provided to support rare applications with unusual needs.

The sqlite3_config() interface is not threadsafe. The application must insure that no other SQLite interfaces are invoked by other threads while sqlite3_config() is running. Furthermore, sqlite3_config() may only be invoked prior to library initialization using sqlite3_initialize() or after shutdown by sqlite3_shutdown(). If sqlite3_config() is called after sqlite3_initialize() and before sqlite3_shutdown() then it will return SQLITE_MISUSE. Note, however, that sqlite3_config() can be called as part of the implementation of an application-defined sqlite3_os_init().

The first argument to sqlite3_config() is an integer configuration option that determines what property of SQLite is to be configured. Subsequent arguments vary depending on the configuration option in the first argument.

When a configuration option is set, sqlite3_config() returns SQLITE_OK. If the option is unknown or SQLite is unable to set the option then this routine returns a non-zero error code.

## Database Connection For Functions

>     sqlite3 *sqlite3_context_db_handle(sqlite3_context*);

The sqlite3_context_db_handle() interface returns a copy of the pointer to the database connection (the 1st parameter) of the sqlite3_create_function() and sqlite3_create_function16() routines that originally registered the application defined function.

## Number of columns in a result set

>     int sqlite3_data_count(sqlite3_stmt *pStmt);

The sqlite3_data_count(P) the number of columns in the of the result set of prepared statement P.

## Configure database connections

>     int sqlite3_db_config(sqlite3*, int op, ...);

The sqlite3_db_config() interface is used to make configuration changes to a database connection. The interface is similar to sqlite3_config() except that the changes apply to a single database connection (specified in the first argument). The sqlite3_db_config() interface should only be used immediately after the database connection is created using sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2().

The second argument to sqlite3_db_config(D,V,...) is the configuration verb - an integer code that indicates what aspect of the database connection is being configured. The only choice for this value is SQLITE_DBCONFIG_LOOKASIDE. New verbs are likely to be added in future releases of SQLite. Additional arguments depend on the verb.

Calls to sqlite3_db_config() return SQLITE_OK if and only if the call is considered successful.

## Find The Database Handle Of A Prepared Statement

>     sqlite3 *sqlite3_db_handle(sqlite3_stmt*);

The sqlite3_db_handle interface returns the database connection handle to which a prepared statement belongs. The database connection returned by sqlite3_db_handle is the same database connection that was the first argument to the sqlite3_prepare_v2() call (or its variants) that was used to create the statement in the first place.

## Retrieve the mutex for a database connection

>     sqlite3_mutex *sqlite3_db_mutex(sqlite3*);

This interface returns a pointer the sqlite3_mutex object that serializes access to the database connection given in the argument when the threading mode is Serialized. If the threading mode is Single-thread or Multi-thread then this routine returns a NULL pointer.

## Database Connection Status

>     int sqlite3_db_status(sqlite3*, int op, int *pCur, int *pHiwtr, int resetFlg);

This interface is used to retrieve runtime status information about a single database connection. The first argument is the database connection object to be interrogated. The second argument is an integer constant, taken from the set of SQLITE_DBSTATUS_* macros, that determiness the parameter to interrogate. The set of SQLITE_DBSTATUS_* macros is likely to grow in future releases of SQLite.

The current value of the requested parameter is written into *pCur and the highest instantaneous value is written into *pHiwtr. If the resetFlg is true, then the highest instantaneous value is reset back down to the current value.

See also: sqlite3_status() and sqlite3_stmt_status().

## Declare The Schema Of A Virtual Table

>     int sqlite3_declare_vtab(sqlite3*, const char *zSQL);

The xCreate and xConnect methods of a virtual table module call this interface to declare the format (the names and datatypes of the columns) of the virtual tables they implement.

## Enable Or Disable Extension Loading

>     int sqlite3_enable_load_extension(sqlite3 *db, int onoff);

So as not to open security holes in older applications that are unprepared to deal with extension loading, and as a means of disabling extension loading while evaluating user-entered SQL, the following API is provided to turn the sqlite3_load_extension() mechanism on and off.

Extension loading is off by default. See ticket #1863. Call the sqlite3_enable_load_extension() routine with onoff==1 to turn extension loading on and call it with onoff==0 to turn it back off again.

## One-Step Query Execution Interface

>     int sqlite3_exec(
>       sqlite3*,                                  /* An open database */
>       const char *sql,                           /* SQL to be evaluated */
>       int (*callback)(void*,int,char**,char**),  /* Callback function */
>       void *,                                    /* 1st argument to callback */
>       char **errmsg                              /* Error msg written here */
>     );

The sqlite3_exec() interface is a convenience wrapper around sqlite3_prepare_v2(), sqlite3_step(), and sqlite3_finalize(), that allows an application to run multiple statements of SQL without having to use a lot of C code.

The sqlite3_exec() interface runs zero or more UTF-8 encoded, semicolon-separate SQL statements passed into its 2nd argument, in the context of the database connection passed in as its 1st argument. If the callback function of the 3rd argument to sqlite3_exec() is not NULL, then it is invoked for each result row coming out of the evaluated SQL statements. The 4th argument to to sqlite3_exec() is relayed through to the 1st argument of each callback invocation. If the callback pointer to sqlite3_exec() is NULL, then no callback is ever invoked and result rows are ignored.

If an error occurs while evaluating the SQL statements passed into sqlite3_exec(), then execution of the current statement stops and subsequent statements are skipped. If the 5th parameter to sqlite3_exec() is not NULL then any error message is written into memory obtained from sqlite3_malloc() and passed back through the 5th parameter. To avoid memory leaks, the application should invoke sqlite3_free() on error message strings returned through the 5th parameter of of sqlite3_exec() after the error message string is no longer needed. If the 5th parameter to sqlite3_exec() is not NULL and no errors occur, then sqlite3_exec() sets the pointer in its 5th parameter to NULL before returning.

If an sqlite3_exec() callback returns non-zero, the sqlite3_exec() routine returns SQLITE_ABORT without invoking the callback again and without running any subsequent SQL statements.

The 2nd argument to the sqlite3_exec() callback function is the number of columns in the result. The 3rd argument to the sqlite3_exec() callback is an array of pointers to strings obtained as if from sqlite3_column_text(), one for each column. If an element of a result row is NULL then the corresponding string pointer for the sqlite3_exec() callback is a NULL pointer. The 4th argument to the sqlite3_exec() callback is an array of pointers to strings where each entry represents the name of corresponding result column as obtained from sqlite3_column_name().

If the 2nd parameter to sqlite3_exec() is a NULL pointer, a pointer to an empty string, or a pointer that contains only whitespace and/or SQL comments, then no SQL statements are evaluated and the database is not changed.

Restrictions:

- The application must insure that the 1st parameter to sqlite3_exec() is a valid and open database connection.
- The application must not close database connection specified by the 1st parameter to sqlite3_exec() while sqlite3_exec() is running.
- The application must not modify the SQL statement text passed into the 2nd parameter of sqlite3_exec() while sqlite3_exec() is running.

## Enable Or Disable Extended Result Codes

>     int sqlite3_extended_result_codes(sqlite3*, int onoff);

The sqlite3_extended_result_codes() routine enables or disables the extended result codes feature of SQLite. The extended result codes are disabled by default for historical compatibility.

## Low-Level Control Of Database Files

>     int sqlite3_file_control(sqlite3*, const char *zDbName, int op, void*);

The sqlite3_file_control() interface makes a direct call to the xFileControl method for the sqlite3_io_methods object associated with a particular database identified by the second argument. The name of the database "main" for the main database or "temp" for the TEMP database, or the name that appears after the AS keyword for databases that are added using the ATTACH SQL command. A NULL pointer can be used in place of "main" to refer to the main database file. The third and fourth parameters to this routine are passed directly through to the second and third parameters of the xFileControl method. The return value of the xFileControl method becomes the return value of this routine.

If the second parameter (zDbName) does not match the name of any open database file, then SQLITE_ERROR is returned. This error code is not remembered and will not be recalled by sqlite3_errcode() or sqlite3_errmsg(). The underlying xFileControl method might also return SQLITE_ERROR. There is no way to distinguish between an incorrect zDbName and an SQLITE_ERROR return from the underlying xFileControl method.

See also: SQLITE_FCNTL_LOCKSTATE

## Destroy A Prepared Statement Object

>     int sqlite3_finalize(sqlite3_stmt *pStmt);

The sqlite3_finalize() function is called to delete a prepared statement. If the statement was executed successfully or not executed at all, then SQLITE_OK is returned. If execution of the statement failed then an error code or extended error code is returned.

This routine can be called at any point during the execution of the prepared statement. If the virtual machine has not completed execution when this routine is called, that is like encountering an error or an interrupt. Incomplete updates may be rolled back and transactions canceled, depending on the circumstances, and the error code returned will be SQLITE_ABORT.

## Interrupt A Long-Running Query

>     void sqlite3_interrupt(sqlite3*);

This function causes any pending database operation to abort and return at its earliest opportunity. This routine is typically called in response to a user action such as pressing "Cancel" or Ctrl-C where the user wants a long query operation to halt immediately.

It is safe to call this routine from a thread different from the thread that is currently running the database operation. But it is not safe to call this routine with a database connection that is closed or might close before sqlite3_interrupt() returns.

If an SQL operation is very nearly finished at the time when sqlite3_interrupt() is called, then it might not have an opportunity to be interrupted and might continue to completion.

An SQL operation that is interrupted will return SQLITE_INTERRUPT. If the interrupted SQL operation is an INSERT, UPDATE, or DELETE that is inside an explicit transaction, then the entire transaction will be rolled back automatically.

The sqlite3_interrupt(D) call is in effect until all currently running SQL statements on database connection D complete. Any new SQL statements that are started after the sqlite3_interrupt() call and before the running statements reaches zero are interrupted as if they had been running prior to the sqlite3_interrupt() call. New SQL statements that are started after the running statement count reaches zero are not effected by the sqlite3_interrupt(). A call to sqlite3_interrupt(D) that occurs when there are no running SQL statements is a no-op and has no effect on SQL statements that are started after the sqlite3_interrupt() call returns.

If the database connection closes while sqlite3_interrupt() is running then bad things will likely happen.

## Last Insert Rowid

>     sqlite3_int64 sqlite3_last_insert_rowid(sqlite3*);

Each entry in an SQLite table has a unique 64-bit signed integer key called the "rowid". The rowid is always available as an undeclared column named ROWID, OID, or _ROWID_ as long as those names are not also used by explicitly declared columns. If the table has a column of type INTEGER PRIMARY KEY then that column is another alias for the rowid.

This routine returns the rowid of the most recent successful INSERT into the database from the database connection in the first argument. If no successful INSERTs have ever occurred on that database connection, zero is returned.

If an INSERT occurs within a trigger, then the rowid of the inserted row is returned by this routine as long as the trigger is running. But once the trigger terminates, the value returned by this routine reverts to the last value inserted before the trigger fired.

An INSERT that fails due to a constraint violation is not a successful INSERT and does not change the value returned by this routine. Thus INSERT OR FAIL, INSERT OR IGNORE, INSERT OR ROLLBACK, and INSERT OR ABORT make no changes to the return value of this routine when their insertion fails. When INSERT OR REPLACE encounters a constraint violation, it does not fail. The INSERT continues to completion after deleting rows that caused the constraint problem so INSERT OR REPLACE will always change the return value of this interface.

For the purposes of this routine, an INSERT is considered to be successful even if it is subsequently rolled back.

This function is accessible to SQL statements via the last_insert_rowid() SQL function.

If a separate thread performs a new INSERT on the same database connection while the sqlite3_last_insert_rowid() function is running and thus changes the last insert rowid, then the value returned by sqlite3_last_insert_rowid() is unpredictable and might not equal either the old or the new last insert rowid.

## Run-time Limits

>     int sqlite3_limit(sqlite3*, int id, int newVal);

This interface allows the size of various constructs to be limited on a connection by connection basis. The first parameter is the database connection whose limit is to be set or queried. The second parameter is one of the limit categories that define a class of constructs to be size limited. The third parameter is the new limit for that construct. The function returns the old limit.

If the new limit is a negative number, the limit is unchanged. For the limit category of SQLITE_LIMIT_XYZ there is a hard upper bound set by a compile-time C preprocessor macro named SQLITE_MAX_XYZ. (The "_LIMIT_" in the name is changed to "_MAX_".) Attempts to increase a limit above its hard upper bound are silently truncated to the hard upper bound.

Run-time limits are intended for use in applications that manage both their own internal database and also databases that are controlled by untrusted external sources. An example application might be a web browser that has its own databases for storing history and separate databases controlled by JavaScript applications downloaded off the Internet. The internal databases can be given the large, default limits. Databases managed by external sources can be given much smaller limits designed to prevent a denial of service attack. Developers might also want to use the sqlite3_set_authorizer() interface to further control untrusted SQL. The size of the database created by an untrusted script can be contained using the max_page_count PRAGMA.

New run-time limit categories may be added in future releases.

## Load An Extension

>     int sqlite3_load_extension(
>       sqlite3 *db,          /* Load the extension into this database connection */
>       const char *zFile,    /* Name of the shared library containing extension */
>       const char *zProc,    /* Entry point.  Derived from zFile if 0 */
>       char **pzErrMsg       /* Put error message here if not 0 */
>     );

This interface loads an SQLite extension library from the named file.

The sqlite3_load_extension() interface attempts to load an SQLite extension library contained in the file zFile.

The entry point is zProc. zProc may be 0, in which case the name of the entry point defaults to "sqlite3_extension_init". The sqlite3_load_extension() interface returns SQLITE_OK on success and SQLITE_ERROR if something goes wrong. If an error occurs and pzErrMsg is not 0, then the sqlite3_load_extension() interface shall attempt to fill *pzErrMsg with error message text stored in memory obtained from sqlite3_malloc(). The calling function should free this memory by calling sqlite3_free().

Extension loading must be enabled using sqlite3_enable_load_extension() prior to calling this API, otherwise an error will be returned.

See also the load_extension() SQL function.

## Error Logging Interface

>     void sqlite3_log(int iErrCode, const char *zFormat, ...);

The sqlite3_log() interface writes a message into the error log established by the SQLITE_CONFIG_LOG option to sqlite3_config(). If logging is enabled, the zFormat string and subsequent arguments are used with sqlite3_snprintf() to generate the final output string.

The sqlite3_log() interface is intended for use by extensions such as virtual tables, collating functions, and SQL functions. While there is nothing to prevent an application from calling sqlite3_log(), doing so is considered bad form.

The zFormat string must not be NULL.

To avoid deadlocks and other threading problems, the sqlite3_log() routine will not use dynamically allocated memory. The log message is stored in a fixed-length buffer on the stack. If the log message is longer than a few hundred characters, it will be truncated to the length of the buffer.

## Find the next prepared statement

>     sqlite3_stmt *sqlite3_next_stmt(sqlite3 *pDb, sqlite3_stmt *pStmt);

This interface returns a pointer to the next prepared statement after pStmt associated with the database connection pDb. If pStmt is NULL then this interface returns a pointer to the first prepared statement associated with the database connection pDb. If no prepared statement satisfies the conditions of this routine, it returns NULL.

The database connection pointer D in a call to sqlite3_next_stmt(D,S) must refer to an open database connection and in particular must not be a NULL pointer.

## Overload A Function For A Virtual Table

>     int sqlite3_overload_function(sqlite3*, const char *zFuncName, int nArg);

Virtual tables can provide alternative implementations of functions using the xFindFunction method of the virtual table module. But global versions of those functions must exist in order to be overloaded.

This API makes sure a global version of a function with a particular name and number of parameters exists. If no such function exists before this API is called, a new function is created. The implementation of the new function always causes an exception to be thrown. So the new function is not good for anything by itself. Its only purpose is to be a placeholder function that can be overloaded by a virtual table.

## Query Progress Callbacks

>     void sqlite3_progress_handler(sqlite3*, int, int(*)(void*), void*);

This routine configures a callback function - the progress callback - that is invoked periodically during long running calls to sqlite3_exec(), sqlite3_step() and sqlite3_get_table(). An example use for this interface is to keep a GUI updated during a large query.

If the progress callback returns non-zero, the operation is interrupted. This feature can be used to implement a "Cancel" button on a GUI progress dialog box.

The progress handler must not do anything that will modify the database connection that invoked the progress handler. Note that sqlite3_prepare_v2() and sqlite3_step() both modify their database connections for the meaning of "modify" in this paragraph.

## Pseudo-Random Number Generator

>     void sqlite3_randomness(int N, void *P);

SQLite contains a high-quality pseudo-random number generator (PRNG) used to select random ROWIDs when inserting new records into a table that already uses the largest possible ROWID. The PRNG is also used for the build-in random() and randomblob() SQL functions. This interface allows applications to access the same PRNG for other purposes.

A call to this routine stores N bytes of randomness into buffer P.

The first time this routine is invoked (either internally or by the application) the PRNG is seeded using randomness obtained from the xRandomness method of the default sqlite3_vfs object. On all subsequent invocations, the pseudo-randomness is generated internally and without recourse to the sqlite3_vfs xRandomness method.

## Attempt To Free Heap Memory

>     int sqlite3_release_memory(int);

The sqlite3_release_memory() interface attempts to free N bytes of heap memory by deallocating non-essential memory allocations held by the database library. Memory used to cache database pages to improve performance is an example of non-essential memory. sqlite3_release_memory() returns the number of bytes actually freed, which might be more or less than the amount requested.

## Reset A Prepared Statement Object

>     int sqlite3_reset(sqlite3_stmt *pStmt);

The sqlite3_reset() function is called to reset a prepared statement object back to its initial state, ready to be re-executed. Any SQL statement variables that had values bound to them using the sqlite3_bind_*() API retain their values. Use sqlite3_clear_bindings() to reset the bindings.

The sqlite3_reset(S) interface resets the prepared statement S back to the beginning of its program.

If the most recent call to sqlite3_step(S) for the prepared statement S returned SQLITE_ROW or SQLITE_DONE, or if sqlite3_step(S) has never before been called on S, then sqlite3_reset(S) returns SQLITE_OK.

If the most recent call to sqlite3_step(S) for the prepared statement S indicated an error, then sqlite3_reset(S) returns an appropriate error code.

The sqlite3_reset(S) interface does not change the values of any bindings on the prepared statement S.

## Reset Automatic Extension Loading

>     void sqlite3_reset_auto_extension(void);

This function disables all previously registered automatic extensions. It undoes the effect of all prior sqlite3_auto_extension() calls.

This function disables automatic extensions in all threads.

## Compile-Time Authorization Callbacks

>     int sqlite3_set_authorizer(
>       sqlite3*,
>       int (*xAuth)(void*,int,const char*,const char*,const char*,const char*),
>       void *pUserData
>     );

This routine registers a authorizer callback with a particular database connection, supplied in the first argument. The authorizer callback is invoked as SQL statements are being compiled by sqlite3_prepare() or its variants sqlite3_prepare_v2(), sqlite3_prepare16() and sqlite3_prepare16_v2(). At various points during the compilation process, as logic is being created to perform various actions, the authorizer callback is invoked to see if those actions are allowed. The authorizer callback should return SQLITE_OK to allow the action, SQLITE_IGNORE to disallow the specific action but allow the SQL statement to continue to be compiled, or SQLITE_DENY to cause the entire SQL statement to be rejected with an error. If the authorizer callback returns any value other than SQLITE_IGNORE, SQLITE_OK, or SQLITE_DENY then the sqlite3_prepare_v2() or equivalent call that triggered the authorizer will fail with an error message.

When the callback returns SQLITE_OK, that means the operation requested is ok. When the callback returns SQLITE_DENY, the sqlite3_prepare_v2() or equivalent call that triggered the authorizer will fail with an error message explaining that access is denied.

The first parameter to the authorizer callback is a copy of the third parameter to the sqlite3_set_authorizer() interface. The second parameter to the callback is an integer action code that specifies the particular action to be authorized. The third through sixth parameters to the callback are zero-terminated strings that contain additional details about the action to be authorized.

If the action code is SQLITE_READ and the callback returns SQLITE_IGNORE then the prepared statement statement is constructed to substitute a NULL value in place of the table column that would have been read if SQLITE_OK had been returned. The SQLITE_IGNORE return can be used to deny an untrusted user access to individual columns of a table. If the action code is SQLITE_DELETE and the callback returns SQLITE_IGNORE then the DELETE operation proceeds but the truncate optimization is disabled and all rows are deleted individually.

An authorizer is used when preparing SQL statements from an untrusted source, to ensure that the SQL statements do not try to access data they are not allowed to see, or that they do not try to execute malicious statements that damage the database. For example, an application may allow a user to enter arbitrary SQL queries for evaluation by a database. But the application does not want the user to be able to make arbitrary changes to the database. An authorizer could then be put in place while the user-entered SQL is being prepared that disallows everything except SELECT statements.

Applications that need to process SQL from untrusted sources might also consider lowering resource limits using sqlite3_limit() and limiting database size using the max_page_count PRAGMA in addition to using an authorizer.

Only a single authorizer can be in place on a database connection at a time. Each call to sqlite3_set_authorizer overrides the previous call. Disable the authorizer by installing a NULL callback. The authorizer is disabled by default.

The authorizer callback must not do anything that will modify the database connection that invoked the authorizer callback. Note that sqlite3_prepare_v2() and sqlite3_step() both modify their database connections for the meaning of "modify" in this paragraph.

When sqlite3_prepare_v2() is used to prepare a statement, the statement might be re-prepared during sqlite3_step() due to a schema change. Hence, the application should ensure that the correct authorizer callback remains in place during the sqlite3_step().

Note that the authorizer callback is invoked only during sqlite3_prepare() or its variants. Authorization is not performed during statement evaluation in sqlite3_step(), unless as stated in the previous paragraph, sqlite3_step() invokes sqlite3_prepare_v2() to reprepare a statement after a schema change.

## Suspend Execution For A Short Time

>     int sqlite3_sleep(int);

The sqlite3_sleep() function causes the current thread to suspend execution for at least a number of milliseconds specified in its parameter.

If the operating system does not support sleep requests with millisecond time resolution, then the time will be rounded up to the nearest second. The number of milliseconds of sleep actually requested from the operating system is returned.

SQLite implements this interface by calling the xSleep() method of the default sqlite3_vfs object.

## Impose A Limit On Heap Size

>     void sqlite3_soft_heap_limit(int);

The sqlite3_soft_heap_limit() interface places a "soft" limit on the amount of heap memory that may be allocated by SQLite. If an internal allocation is requested that would exceed the soft heap limit, sqlite3_release_memory() is invoked one or more times to free up some space before the allocation is performed.

The limit is called "soft" because if sqlite3_release_memory() cannot free sufficient memory to prevent the limit from being exceeded, the memory is allocated anyway and the current operation proceeds.

A negative or zero value for N means that there is no soft heap limit and sqlite3_release_memory() will only be called when memory is exhausted. The default value for the soft heap limit is zero.

SQLite makes a best effort to honor the soft heap limit. But if the soft heap limit cannot be honored, execution will continue without error or notification. This is why the limit is called a "soft" limit. It is advisory only.

Prior to SQLite version 3.5.0, this routine only constrained the memory allocated by a single thread - the same thread in which this routine runs. Beginning with SQLite version 3.5.0, the soft heap limit is applied to all threads. The value specified for the soft heap limit is an upper bound on the total memory allocation for all threads. In version 3.5.0 there is no mechanism for limiting the heap usage for individual threads.

## Retrieving Statement SQL

>     const char *sqlite3_sql(sqlite3_stmt *pStmt);

This interface can be used to retrieve a saved copy of the original SQL text used to create a prepared statement if that statement was compiled using either sqlite3_prepare_v2() or sqlite3_prepare16_v2().

## SQLite Runtime Status

>     int sqlite3_status(int op, int *pCurrent, int *pHighwater, int resetFlag);

This interface is used to retrieve runtime status information about the preformance of SQLite, and optionally to reset various highwater marks. The first argument is an integer code for the specific parameter to measure. Recognized integer codes are of the form SQLITE_STATUS_.... The current value of the parameter is returned into *pCurrent. The highest recorded value is returned in *pHighwater. If the resetFlag is true, then the highest record value is reset after *pHighwater is written. Some parameters do not record the highest value. For those parameters nothing is written into *pHighwater and the resetFlag is ignored. Other parameters record only the highwater mark and not the current value. For these latter parameters nothing is written into *pCurrent.

The sqlite3_db_status() routine returns SQLITE_OK on success and a non-zero error code on failure.

This routine is threadsafe but is not atomic. This routine can be called while other threads are running the same or different SQLite interfaces. However the values returned in *pCurrent and *pHighwater reflect the status of SQLite at different points in time and it is possible that another thread might change the parameter in between the times when *pCurrent and *pHighwater are written.

See also: sqlite3_db_status()

## Evaluate An SQL Statement

>     int sqlite3_step(sqlite3_stmt*);

After a prepared statement has been prepared using either sqlite3_prepare_v2() or sqlite3_prepare16_v2() or one of the legacy interfaces sqlite3_prepare() or sqlite3_prepare16(), this function must be called one or more times to evaluate the statement.

The details of the behavior of the sqlite3_step() interface depend on whether the statement was prepared using the newer "v2" interface sqlite3_prepare_v2() and sqlite3_prepare16_v2() or the older legacy interface sqlite3_prepare() and sqlite3_prepare16(). The use of the new "v2" interface is recommended for new applications but the legacy interface will continue to be supported.

In the legacy interface, the return value will be either SQLITE_BUSY, SQLITE_DONE, SQLITE_ROW, SQLITE_ERROR, or SQLITE_MISUSE. With the "v2" interface, any of the other result codes or extended result codes might be returned as well.

SQLITE_BUSY means that the database engine was unable to acquire the database locks it needs to do its job. If the statement is a COMMIT or occurs outside of an explicit transaction, then you can retry the statement. If the statement is not a COMMIT and occurs within a explicit transaction then you should rollback the transaction before continuing.

SQLITE_DONE means that the statement has finished executing successfully. sqlite3_step() should not be called again on this virtual machine without first calling sqlite3_reset() to reset the virtual machine back to its initial state.

If the SQL statement being executed returns any data, then SQLITE_ROW is returned each time a new row of data is ready for processing by the caller. The values may be accessed using the column access functions. sqlite3_step() is called again to retrieve the next row of data.

SQLITE_ERROR means that a run-time error (such as a constraint violation) has occurred. sqlite3_step() should not be called again on the VM. More information may be found by calling sqlite3_errmsg(). With the legacy interface, a more specific error code (for example, SQLITE_INTERRUPT, SQLITE_SCHEMA, SQLITE_CORRUPT, and so forth) can be obtained by calling sqlite3_reset() on the prepared statement. In the "v2" interface, the more specific error code is returned directly by sqlite3_step().

SQLITE_MISUSE means that the this routine was called inappropriately. Perhaps it was called on a prepared statement that has already been finalized or on one that had previously returned SQLITE_ERROR or SQLITE_DONE. Or it could be the case that the same database connection is being used by two or more threads at the same moment in time.

For all versions of SQLite up to and including 3.6.23.1, it was required after sqlite3_step() returned anything other than SQLITE_ROW that sqlite3_reset() be called before any subsequent invocation of sqlite3_step(). Failure to invoke sqlite3_reset() in this way would result in an SQLITE_MISUSE return from sqlite3_step(). But after version 3.6.23.1, sqlite3_step() began calling sqlite3_reset() automatically in this circumstance rather than returning SQLITE_MISUSE.

Goofy Interface Alert: In the legacy interface, the sqlite3_step() API always returns a generic error code, SQLITE_ERROR, following any error other than SQLITE_BUSY and SQLITE_MISUSE. You must call sqlite3_reset() or sqlite3_finalize() in order to find one of the specific error codes that better describes the error. We admit that this is a goofy design. The problem has been fixed with the "v2" interface. If you prepare all of your SQL statements using either sqlite3_prepare_v2() or sqlite3_prepare16_v2() instead of the legacy sqlite3_prepare() and sqlite3_prepare16() interfaces, then the more specific error codes are returned directly by sqlite3_step(). The use of the "v2" interface is recommended.

## Prepared Statement Status

>     int sqlite3_stmt_status(sqlite3_stmt*, int op,int resetFlg);

Each prepared statement maintains various counters that measure the number of times it has performed specific operations. These counters can be used to monitor the performance characteristics of the prepared statements. For example, if the number of table steps greatly exceeds the number of table searches or result rows, that would tend to indicate that the prepared statement is using a full table scan rather than an index.

This interface is used to retrieve and reset counter values from a prepared statement. The first argument is the prepared statement object to be interrogated. The second argument is an integer code for a specific counter to be interrogated. The current value of the requested counter is returned. If the resetFlg is true, then the counter is reset to zero after this interface call returns.

See also: sqlite3_status() and sqlite3_db_status().

## String Comparison

>     int sqlite3_strnicmp(const char *, const char *, int);

The sqlite3_strnicmp() API allows applications and extensions to compare the contents of two buffers containing UTF-8 strings in a case-indendent fashion, using the same definition of case independence that SQLite uses internally when comparing identifiers.

## Extract Metadata About A Column Of A Table

>     int sqlite3_table_column_metadata(
>       sqlite3 *db,                /* Connection handle */
>       const char *zDbName,        /* Database name or NULL */
>       const char *zTableName,     /* Table name */
>       const char *zColumnName,    /* Column name */
>       char const **pzDataType,    /* OUTPUT: Declared data type */
>       char const **pzCollSeq,     /* OUTPUT: Collation sequence name */
>       int *pNotNull,              /* OUTPUT: True if NOT NULL constraint exists */
>       int *pPrimaryKey,           /* OUTPUT: True if column part of PK */
>       int *pAutoinc               /* OUTPUT: True if column is auto-increment */
>     );

This routine returns metadata about a specific column of a specific database table accessible using the database connection handle passed as the first function argument.

The column is identified by the second, third and fourth parameters to this function. The second parameter is either the name of the database (i.e. "main", "temp", or an attached database) containing the specified table or NULL. If it is NULL, then all attached databases are searched for the table using the same algorithm used by the database engine to resolve unqualified table references.

The third and fourth parameters to this function are the table and column name of the desired column, respectively. Neither of these parameters may be NULL.

Metadata is returned by writing to the memory locations passed as the 5th and subsequent parameters to this function. Any of these arguments may be NULL, in which case the corresponding element of metadata is omitted.

> Parameter | Output Type | Description
> --- | --- | ---
> 5th | const char* | Data type
> 6th | const char* | Name of default collation sequence
> 7th | int | True if column has a NOT NULL constraint
> 8th | int | True if column is part of the PRIMARY KEY
> 9th | int | True if column is AUTOINCREMENT

The memory pointed to by the character pointers returned for the declaration type and collation sequence is valid only until the next call to any SQLite API function.

If the specified table is actually a view, an error code is returned.

If the specified column is "rowid", "oid" or "_rowid_" and an INTEGER PRIMARY KEY column has been explicitly declared, then the output parameters are set for the explicitly declared column. If there is no explicitly declared INTEGER PRIMARY KEY column, then the output parameters are set as follows:

    data type: "INTEGER"
    collation sequence: "BINARY"
    not null: 0
    primary key: 1
    auto increment: 0

This function may load one or more schemas from database files. If an error occurs during this process, or if the requested table or column cannot be found, an error code is returned and an error message left in the database connection (to be retrieved using sqlite3_errmsg()).

This API is only available if the library was compiled with the SQLITE_ENABLE_COLUMN_METADATA C-preprocessor symbol defined.

## Testing Interface

>     int sqlite3_test_control(int op, ...);

The sqlite3_test_control() interface is used to read out internal state of SQLite and to inject faults into SQLite for testing purposes. The first parameter is an operation code that determines the number, meaning, and operation of all subsequent parameters.

This interface is not for use by applications. It exists solely for verifying the correct operation of the SQLite library. Depending on how the SQLite library is compiled, this interface might not exist.

The details of the operation codes, their meanings, the parameters they take, and what they do are all subject to change without notice. Unlike most of the SQLite API, this function is not guaranteed to operate consistently from one release to the next.

## Test To See If The Library Is Threadsafe

>     int sqlite3_threadsafe(void);

The sqlite3_threadsafe() function returns zero if and only if SQLite was compiled mutexing code omitted due to the SQLITE_THREADSAFE compile-time option being set to 0.

SQLite can be compiled with or without mutexes. When the SQLITE_THREADSAFE C preprocessor macro is 1 or 2, mutexes are enabled and SQLite is threadsafe. When the SQLITE_THREADSAFE macro is 0, the mutexes are omitted. Without the mutexes, it is not safe to use SQLite concurrently from more than one thread.

Enabling mutexes incurs a measurable performance penalty. So if speed is of utmost importance, it makes sense to disable the mutexes. But for maximum safety, mutexes should be enabled. The default behavior is for mutexes to be enabled.

This interface can be used by an application to make sure that the version of SQLite that it is linking against was compiled with the desired setting of the SQLITE_THREADSAFE macro.

This interface only reports on the compile-time mutex setting of the SQLITE_THREADSAFE flag. If SQLite is compiled with SQLITE_THREADSAFE=1 or =2 then mutexes are enabled by default but can be fully or partially disabled using a call to sqlite3_config() with the verbs SQLITE_CONFIG_SINGLETHREAD, SQLITE_CONFIG_MULTITHREAD, or SQLITE_CONFIG_MUTEX. The return value of the sqlite3_threadsafe() function shows only the compile-time setting of thread safety, not any run-time changes to that setting made by sqlite3_config(). In other words, the return value from sqlite3_threadsafe() is unchanged by calls to sqlite3_config().

See the threading mode documentation for additional information.

## Total Number Of Rows Modified

>     int sqlite3_total_changes(sqlite3*);

This function returns the number of row changes caused by INSERT, UPDATE or DELETE statements since the database connection was opened. The count returned by sqlite3_total_changes() includes all changes from all trigger contexts and changes made by foreign key actions. However, the count does not include changes used to implement REPLACE constraints, do rollbacks or ABORT processing, or DROP TABLE processing. The count does not include rows of views that fire an INSTEAD OF trigger, though if the INSTEAD OF trigger makes changes of its own, those changes are counted. The sqlite3_total_changes() function counts the changes as soon as the statement that makes them is completed (when the statement handle is passed to sqlite3_reset() or sqlite3_finalize()).

See also the sqlite3_changes() interface, the count_changes pragma, and the total_changes() SQL function.

If a separate thread makes changes on the same database connection while sqlite3_total_changes() is running then the value returned is unpredictable and not meaningful.

## Unlock Notification

>     int sqlite3_unlock_notify(
>       sqlite3 *pBlocked,                          /* Waiting connection */
>       void (*xNotify)(void **apArg, int nArg),    /* Callback function to invoke */
>       void *pNotifyArg                            /* Argument to pass to xNotify */
>     );

When running in shared-cache mode, a database operation may fail with an SQLITE_LOCKED error if the required locks on the shared-cache or individual tables within the shared-cache cannot be obtained. See SQLite Shared-Cache Mode for a description of shared-cache locking. This API may be used to register a callback that SQLite will invoke when the connection currently holding the required lock relinquishes it. This API is only available if the library was compiled with the SQLITE_ENABLE_UNLOCK_NOTIFY C-preprocessor symbol defined.

See Also: Using the SQLite Unlock Notification Feature.

Shared-cache locks are released when a database connection concludes its current transaction, either by committing it or rolling it back.

When a connection (known as the blocked connection) fails to obtain a shared-cache lock and SQLITE_LOCKED is returned to the caller, the identity of the database connection (the blocking connection) that has locked the required resource is stored internally. After an application receives an SQLITE_LOCKED error, it may call the sqlite3_unlock_notify() method with the blocked connection handle as the first argument to register for a callback that will be invoked when the blocking connections current transaction is concluded. The callback is invoked from within the sqlite3_step or sqlite3_close call that concludes the blocking connections transaction.

If sqlite3_unlock_notify() is called in a multi-threaded application, there is a chance that the blocking connection will have already concluded its transaction by the time sqlite3_unlock_notify() is invoked. If this happens, then the specified callback is invoked immediately, from within the call to sqlite3_unlock_notify().

If the blocked connection is attempting to obtain a write-lock on a shared-cache table, and more than one other connection currently holds a read-lock on the same table, then SQLite arbitrarily selects one of the other connections to use as the blocking connection.

There may be at most one unlock-notify callback registered by a blocked connection. If sqlite3_unlock_notify() is called when the blocked connection already has a registered unlock-notify callback, then the new callback replaces the old. If sqlite3_unlock_notify() is called with a NULL pointer as its second argument, then any existing unlock-notify callback is cancelled. The blocked connections unlock-notify callback may also be canceled by closing the blocked connection using sqlite3_close().

The unlock-notify callback is not reentrant. If an application invokes any sqlite3_xxx API functions from within an unlock-notify callback, a crash or deadlock may be the result.

Unless deadlock is detected (see below), sqlite3_unlock_notify() always returns SQLITE_OK.

Callback Invocation Details

When an unlock-notify callback is registered, the application provides a single void* pointer that is passed to the callback when it is invoked. However, the signature of the callback function allows SQLite to pass it an array of void* context pointers. The first argument passed to an unlock-notify callback is a pointer to an array of void* pointers, and the second is the number of entries in the array.

When a blocking connections transaction is concluded, there may be more than one blocked connection that has registered for an unlock-notify callback. If two or more such blocked connections have specified the same callback function, then instead of invoking the callback function multiple times, it is invoked once with the set of void* context pointers specified by the blocked connections bundled together into an array. This gives the application an opportunity to prioritize any actions related to the set of unblocked database connections.

Deadlock Detection

Assuming that after registering for an unlock-notify callback a database waits for the callback to be issued before taking any further action (a reasonable assumption), then using this API may cause the application to deadlock. For example, if connection X is waiting for connection Y's transaction to be concluded, and similarly connection Y is waiting on connection X's transaction, then neither connection will proceed and the system may remain deadlocked indefinitely.

To avoid this scenario, the sqlite3_unlock_notify() performs deadlock detection. If a given call to sqlite3_unlock_notify() would put the system in a deadlocked state, then SQLITE_LOCKED is returned and no unlock-notify callback is registered. The system is said to be in a deadlocked state if connection A has registered for an unlock-notify callback on the conclusion of connection B's transaction, and connection B has itself registered for an unlock-notify callback when connection A's transaction is concluded. Indirect deadlock is also detected, so the system is also considered to be deadlocked if connection B has registered for an unlock-notify callback on the conclusion of connection C's transaction, where connection C is waiting on connection A. Any number of levels of indirection are allowed.

The "DROP TABLE" Exception

When a call to sqlite3_step() returns SQLITE_LOCKED, it is almost always appropriate to call sqlite3_unlock_notify(). There is however, one exception. When executing a "DROP TABLE" or "DROP INDEX" statement, SQLite checks if there are any currently executing SELECT statements that belong to the same connection. If there are, SQLITE_LOCKED is returned. In this case there is no "blocking connection", so invoking sqlite3_unlock_notify() results in the unlock-notify callback being invoked immediately. If the application then re-attempts the "DROP TABLE" or "DROP INDEX" query, an infinite loop might be the result.

One way around this problem is to check the extended error code returned by an sqlite3_step() call. If there is a blocking connection, then the extended error code is set to SQLITE_LOCKED_SHAREDCACHE. Otherwise, in the special "DROP TABLE/INDEX" case, the extended error code is just SQLITE_LOCKED.

## Data Change Notification Callbacks

>     void *sqlite3_update_hook(
>       sqlite3*,
>       void(*)(void *,int ,char const *,char const *,sqlite3_int64),
>       void*
>     );

The sqlite3_update_hook() interface registers a callback function with the database connection identified by the first argument to be invoked whenever a row is updated, inserted or deleted. Any callback set by a previous call to this function for the same database connection is overridden.

The second argument is a pointer to the function to invoke when a row is updated, inserted or deleted. The first argument to the callback is a copy of the third argument to sqlite3_update_hook(). The second callback argument is one of SQLITE_INSERT, SQLITE_DELETE, or SQLITE_UPDATE, depending on the operation that caused the callback to be invoked. The third and fourth arguments to the callback contain pointers to the database and table name containing the affected row. The final callback parameter is the rowid of the row. In the case of an update, this is the rowid after the update takes place.

The update hook is not invoked when internal system tables are modified (i.e. sqlite_master and sqlite_sequence).

In the current implementation, the update hook is not invoked when duplication rows are deleted because of an ON CONFLICT REPLACE clause. Nor is the update hook invoked when rows are deleted using the truncate optimization. The exceptions defined in this paragraph might change in a future release of SQLite.

The update hook implementation must not do anything that will modify the database connection that invoked the update hook. Any actions to modify the database connection must be deferred until after the completion of the sqlite3_step() call that triggered the update hook. Note that sqlite3_prepare_v2() and sqlite3_step() both modify their database connections for the meaning of "modify" in this paragraph.

The sqlite3_update_hook(D,C,P) function returns the P argument from the previous call on the same database connection D, or NULL for the first call on D.

See also the sqlite3_commit_hook() and sqlite3_rollback_hook() interfaces.

## User Data For Functions

>     void *sqlite3_user_data(sqlite3_context*);

The sqlite3_user_data() interface returns a copy of the pointer that was the pUserData parameter (the 5th parameter) of the sqlite3_create_function() and sqlite3_create_function16() routines that originally registered the application defined function.

This routine must be called from the same thread in which the application-defined function is running.

## Configure an auto-checkpoint

>     int sqlite3_wal_autocheckpoint(sqlite3 *db, int N);

The sqlite3_wal_autocheckpoint(D,N) is a wrapper around sqlite3_wal_hook() that causes any database on database connection D to automatically checkpoint after committing a transaction if there are N or more frames in the write-ahead log file. Passing zero or a negative value as the nFrame parameter disables automatic checkpoints entirely.

The callback registered by this function replaces any existing callback registered using sqlite3_wal_hook(). Likewise, registering a callback using sqlite3_wal_hook() disables the automatic checkpoint mechanism configured by this function.

The wal_autocheckpoint pragma can be used to invoke this interface from SQL.

Every new database connection defaults to having the auto-checkpoint enabled with a threshold of 1000 pages. The use of this interface is only necessary if the default setting is found to be suboptimal for a particular application.

## Checkpoint a database

>     int sqlite3_wal_checkpoint(sqlite3 *db, const char *zDb);

The sqlite3_wal_checkpoint(D,X) interface causes database named X on database connection D to be checkpointed. If X is NULL or an empty string, then a checkpoint is run on all databases of connection D. If the database connection D is not in write-ahead log mode then this interface is a harmless no-op.

The wal_checkpoint pragma can be used to invoke this interface from SQL. The sqlite3_wal_autocheckpoint() interface and the wal_autocheckpoint pragma can be used to cause this interface to be run whenever the WAL reaches a certain size threshold.

## Write-Ahead Log Commit Hook

>     void *sqlite3_wal_hook(
>       sqlite3*,
>       int(*)(void *,sqlite3*,const char*,int),
>       void*
>     );

The sqlite3_wal_hook() function is used to register a callback that will be invoked each time a database connection commits data to a write-ahead log (i.e. whenever a transaction is committed in journal_mode=WAL mode).

The callback is invoked by SQLite after the commit has taken place and the associated write-lock on the database released, so the implementation may read, write or checkpoint the database as required.

The first parameter passed to the callback function when it is invoked is a copy of the third parameter passed to sqlite3_wal_hook() when registering the callback. The second is a copy of the database handle. The third parameter is the name of the database that was written to - either "main" or the name of an ATTACH-ed database. The fourth parameter is the number of pages currently in the write-ahead log file, including those that were just committed.

The callback function should normally return SQLITE_OK. If an error code is returned, that error will propagate back up through the SQLite code base to cause the statement that provoked the callback to report an error, though the commit will have still occurred. If the callback returns SQLITE_ROW or SQLITE_DONE, or if it returns a value that does not correspond to any valid SQLite error code, the results are undefined.

A single database handle may have at most a single write-ahead log callback registered at one time. Calling sqlite3_wal_hook() replaces any previously registered write-ahead log callback. Note that the sqlite3_wal_autocheckpoint() interface and the wal_autocheckpoint pragma both invoke sqlite3_wal_hook() and will those overwrite any prior sqlite3_wal_hook() settings.

## Result Codes

>     #define SQLITE_OK           0   /* Successful result */
>     /* beginning-of-error-codes */
>     #define SQLITE_ERROR        1   /* SQL error or missing database */
>     #define SQLITE_INTERNAL     2   /* Internal logic error in SQLite */
>     #define SQLITE_PERM         3   /* Access permission denied */
>     #define SQLITE_ABORT        4   /* Callback routine requested an abort */
>     #define SQLITE_BUSY         5   /* The database file is locked */
>     #define SQLITE_LOCKED       6   /* A table in the database is locked */
>     #define SQLITE_NOMEM        7   /* A malloc() failed */
>     #define SQLITE_READONLY     8   /* Attempt to write a readonly database */
>     #define SQLITE_INTERRUPT    9   /* Operation terminated by sqlite3_interrupt()*/
>     #define SQLITE_IOERR       10   /* Some kind of disk I/O error occurred */
>     #define SQLITE_CORRUPT     11   /* The database disk image is malformed */
>     #define SQLITE_NOTFOUND    12   /* NOT USED. Table or record not found */
>     #define SQLITE_FULL        13   /* Insertion failed because database is full */
>     #define SQLITE_CANTOPEN    14   /* Unable to open the database file */
>     #define SQLITE_PROTOCOL    15   /* Database lock protocol error */
>     #define SQLITE_EMPTY       16   /* Database is empty */
>     #define SQLITE_SCHEMA      17   /* The database schema changed */
>     #define SQLITE_TOOBIG      18   /* String or BLOB exceeds size limit */
>     #define SQLITE_CONSTRAINT  19   /* Abort due to constraint violation */
>     #define SQLITE_MISMATCH    20   /* Data type mismatch */
>     #define SQLITE_MISUSE      21   /* Library used incorrectly */
>     #define SQLITE_NOLFS       22   /* Uses OS features not supported on host */
>     #define SQLITE_AUTH        23   /* Authorization denied */
>     #define SQLITE_FORMAT      24   /* Auxiliary database format error */
>     #define SQLITE_RANGE       25   /* 2nd parameter to sqlite3_bind out of range */
>     #define SQLITE_NOTADB      26   /* File opened that is not a database file */
>     #define SQLITE_ROW         100  /* sqlite3_step() has another row ready */
>     #define SQLITE_DONE        101  /* sqlite3_step() has finished executing */
>     /* end-of-error-codes */

Many SQLite functions return an integer result code from the set shown here in order to indicates success or failure.

New error codes may be added in future versions of SQLite.

See also: extended result codes

## Flags for the xAccess VFS method

>     #define SQLITE_ACCESS_EXISTS    0
>     #define SQLITE_ACCESS_READWRITE 1   /* Used by PRAGMA temp_store_directory */
>     #define SQLITE_ACCESS_READ      2   /* Unused */

These integer constants can be used as the third parameter to the xAccess method of an sqlite3_vfs object. They determine what kind of permissions the xAccess method is looking for. With SQLITE_ACCESS_EXISTS, the xAccess method simply checks whether the file exists. With SQLITE_ACCESS_READWRITE, the xAccess method checks whether the named directory is both readable and writable (in other words, if files can be added, removed, and renamed within the directory). The SQLITE_ACCESS_READWRITE constant is currently used only by the temp_store_directory pragma, though this could change in a future release of SQLite. With SQLITE_ACCESS_READ, the xAccess method checks whether the file is readable. The SQLITE_ACCESS_READ constant is currently unused, though it might be used in a future release of SQLite.

## Authorizer Action Codes

>     /******************************************* 3rd ************ 4th ***********/
>     #define SQLITE_CREATE_INDEX          1   /* Index Name      Table Name      */
>     #define SQLITE_CREATE_TABLE          2   /* Table Name      NULL            */
>     #define SQLITE_CREATE_TEMP_INDEX     3   /* Index Name      Table Name      */
>     #define SQLITE_CREATE_TEMP_TABLE     4   /* Table Name      NULL            */
>     #define SQLITE_CREATE_TEMP_TRIGGER   5   /* Trigger Name    Table Name      */
>     #define SQLITE_CREATE_TEMP_VIEW      6   /* View Name       NULL            */
>     #define SQLITE_CREATE_TRIGGER        7   /* Trigger Name    Table Name      */
>     #define SQLITE_CREATE_VIEW           8   /* View Name       NULL            */
>     #define SQLITE_DELETE                9   /* Table Name      NULL            */
>     #define SQLITE_DROP_INDEX           10   /* Index Name      Table Name      */
>     #define SQLITE_DROP_TABLE           11   /* Table Name      NULL            */
>     #define SQLITE_DROP_TEMP_INDEX      12   /* Index Name      Table Name      */
>     #define SQLITE_DROP_TEMP_TABLE      13   /* Table Name      NULL            */
>     #define SQLITE_DROP_TEMP_TRIGGER    14   /* Trigger Name    Table Name      */
>     #define SQLITE_DROP_TEMP_VIEW       15   /* View Name       NULL            */
>     #define SQLITE_DROP_TRIGGER         16   /* Trigger Name    Table Name      */
>     #define SQLITE_DROP_VIEW            17   /* View Name       NULL            */
>     #define SQLITE_INSERT               18   /* Table Name      NULL            */
>     #define SQLITE_PRAGMA               19   /* Pragma Name     1st arg or NULL */
>     #define SQLITE_READ                 20   /* Table Name      Column Name     */
>     #define SQLITE_SELECT               21   /* NULL            NULL            */
>     #define SQLITE_TRANSACTION          22   /* Operation       NULL            */
>     #define SQLITE_UPDATE               23   /* Table Name      Column Name     */
>     #define SQLITE_ATTACH               24   /* Filename        NULL            */
>     #define SQLITE_DETACH               25   /* Database Name   NULL            */
>     #define SQLITE_ALTER_TABLE          26   /* Database Name   Table Name      */
>     #define SQLITE_REINDEX              27   /* Index Name      NULL            */
>     #define SQLITE_ANALYZE              28   /* Table Name      NULL            */
>     #define SQLITE_CREATE_VTABLE        29   /* Table Name      Module Name     */
>     #define SQLITE_DROP_VTABLE          30   /* Table Name      Module Name     */
>     #define SQLITE_FUNCTION             31   /* NULL            Function Name   */
>     #define SQLITE_SAVEPOINT            32   /* Operation       Savepoint Name  */
>     #define SQLITE_COPY                  0   /* No longer used */

The sqlite3_set_authorizer() interface registers a callback function that is invoked to authorize certain SQL statement actions. The second parameter to the callback is an integer code that specifies what action is being authorized. These are the integer action codes that the authorizer callback may be passed.

These action code values signify what kind of operation is to be authorized. The 3rd and 4th parameters to the authorization callback function will be parameters or NULL depending on which of these codes is used as the second parameter. The 5th parameter to the authorizer callback is the name of the database ("main", "temp", etc.) if applicable. The 6th parameter to the authorizer callback is the name of the inner-most trigger or view that is responsible for the access attempt or NULL if this access attempt is directly from top-level SQL code.

## Text Encodings

>     #define SQLITE_UTF8           1
>     #define SQLITE_UTF16LE        2
>     #define SQLITE_UTF16BE        3
>     #define SQLITE_UTF16          4    /* Use native byte order */
>     #define SQLITE_ANY            5    /* sqlite3_create_function only */
>     #define SQLITE_UTF16_ALIGNED  8    /* sqlite3_create_collation only */

These constant define integer codes that represent the various text encodings supported by SQLite.

## Fundamental Datatypes

>     #define SQLITE_INTEGER  1
>     #define SQLITE_FLOAT    2
>     #define SQLITE_BLOB     4
>     #define SQLITE_NULL     5
>     #ifdef SQLITE_TEXT
>     # undef SQLITE_TEXT
>     #else
>     # define SQLITE_TEXT     3
>     #endif
>     #define SQLITE3_TEXT     3

Every value in SQLite has one of five fundamental datatypes:

- 64-bit signed integer
- 64-bit IEEE floating point number
- string
- BLOB
- NULL

These constants are codes for each of those types.

Note that the SQLITE_TEXT constant was also used in SQLite version 2 for a completely different meaning. Software that links against both SQLite version 2 and SQLite version 3 should use SQLITE3_TEXT, not SQLITE_TEXT.

## Extended Result Codes

>     #define SQLITE_IOERR_READ              (SQLITE_IOERR | (1<<8))
>     #define SQLITE_IOERR_SHORT_READ        (SQLITE_IOERR | (2<<8))
>     #define SQLITE_IOERR_WRITE             (SQLITE_IOERR | (3<<8))
>     #define SQLITE_IOERR_FSYNC             (SQLITE_IOERR | (4<<8))
>     #define SQLITE_IOERR_DIR_FSYNC         (SQLITE_IOERR | (5<<8))
>     #define SQLITE_IOERR_TRUNCATE          (SQLITE_IOERR | (6<<8))
>     #define SQLITE_IOERR_FSTAT             (SQLITE_IOERR | (7<<8))
>     #define SQLITE_IOERR_UNLOCK            (SQLITE_IOERR | (8<<8))
>     #define SQLITE_IOERR_RDLOCK            (SQLITE_IOERR | (9<<8))
>     #define SQLITE_IOERR_DELETE            (SQLITE_IOERR | (10<<8))
>     #define SQLITE_IOERR_BLOCKED           (SQLITE_IOERR | (11<<8))
>     #define SQLITE_IOERR_NOMEM             (SQLITE_IOERR | (12<<8))
>     #define SQLITE_IOERR_ACCESS            (SQLITE_IOERR | (13<<8))
>     #define SQLITE_IOERR_CHECKRESERVEDLOCK (SQLITE_IOERR | (14<<8))
>     #define SQLITE_IOERR_LOCK              (SQLITE_IOERR | (15<<8))
>     #define SQLITE_IOERR_CLOSE             (SQLITE_IOERR | (16<<8))
>     #define SQLITE_IOERR_DIR_CLOSE         (SQLITE_IOERR | (17<<8))
>     #define SQLITE_IOERR_SHMOPEN           (SQLITE_IOERR | (18<<8))
>     #define SQLITE_IOERR_SHMSIZE           (SQLITE_IOERR | (19<<8))
>     #define SQLITE_IOERR_SHMLOCK           (SQLITE_IOERR | (20<<8))
>     #define SQLITE_LOCKED_SHAREDCACHE      (SQLITE_LOCKED |  (1<<8))
>     #define SQLITE_BUSY_RECOVERY           (SQLITE_BUSY   |  (1<<8))
>     #define SQLITE_CANTOPEN_NOTEMPDIR      (SQLITE_CANTOPEN | (1<<8))

In its default configuration, SQLite API routines return one of 26 integer result codes. However, experience has shown that many of these result codes are too coarse-grained. They do not provide as much information about problems as programmers might like. In an effort to address this, newer versions of SQLite (version 3.3.8 and later) include support for additional result codes that provide more detailed information about errors. The extended result codes are enabled or disabled on a per database connection basis using the sqlite3_extended_result_codes() API.

Some of the available extended result codes are listed here. One may expect the number of extended result codes will be expand over time. Software that uses extended result codes should expect to see new result codes in future releases of SQLite.

The SQLITE_OK result code will never be extended. It will always be exactly zero.

## Configuration Options

>     #define SQLITE_CONFIG_SINGLETHREAD  1  /* nil */
>     #define SQLITE_CONFIG_MULTITHREAD   2  /* nil */
>     #define SQLITE_CONFIG_SERIALIZED    3  /* nil */
>     #define SQLITE_CONFIG_MALLOC        4  /* sqlite3_mem_methods* */
>     #define SQLITE_CONFIG_GETMALLOC     5  /* sqlite3_mem_methods* */
>     #define SQLITE_CONFIG_SCRATCH       6  /* void*, int sz, int N */
>     #define SQLITE_CONFIG_PAGECACHE     7  /* void*, int sz, int N */
>     #define SQLITE_CONFIG_HEAP          8  /* void*, int nByte, int min */
>     #define SQLITE_CONFIG_MEMSTATUS     9  /* boolean */
>     #define SQLITE_CONFIG_MUTEX        10  /* sqlite3_mutex_methods* */
>     #define SQLITE_CONFIG_GETMUTEX     11  /* sqlite3_mutex_methods* */
>     /* previously SQLITE_CONFIG_CHUNKALLOC 12 which is now unused. */
>     #define SQLITE_CONFIG_LOOKASIDE    13  /* int int */
>     #define SQLITE_CONFIG_PCACHE       14  /* sqlite3_pcache_methods* */
>     #define SQLITE_CONFIG_GETPCACHE    15  /* sqlite3_pcache_methods* */
>     #define SQLITE_CONFIG_LOG          16  /* xFunc, void* */

These constants are the available integer configuration options that can be passed as the first argument to the sqlite3_config() interface.

New configuration options may be added in future releases of SQLite. Existing configuration options might be discontinued. Applications should check the return code from sqlite3_config() to make sure that the call worked. The sqlite3_config() interface will return a non-zero error code if a discontinued or unsupported configuration option is invoked.

SQLITE_CONFIG_SINGLETHREAD
  There are no arguments to this option. This option sets the threading mode to Single-thread. In other words, it disables all mutexing and puts SQLite into a mode where it can only be used by a single thread. If SQLite is compiled with the SQLITE_THREADSAFE=0 compile-time option then it is not possible to change the threading mode from its default value of Single-thread and so sqlite3_config() will return SQLITE_ERROR if called with the SQLITE_CONFIG_SINGLETHREAD configuration option.

SQLITE_CONFIG_MULTITHREAD
  There are no arguments to this option. This option sets the threading mode to Multi-thread. In other words, it disables mutexing on database connection and prepared statement objects. The application is responsible for serializing access to database connections and prepared statements. But other mutexes are enabled so that SQLite will be safe to use in a multi-threaded environment as long as no two threads attempt to use the same database connection at the same time. If SQLite is compiled with the SQLITE_THREADSAFE=0 compile-time option then it is not possible to set the Multi-thread threading mode and sqlite3_config() will return SQLITE_ERROR if called with the SQLITE_CONFIG_MULTITHREAD configuration option.

SQLITE_CONFIG_SERIALIZED
  There are no arguments to this option. This option sets the threading mode to Serialized. In other words, this option enables all mutexes including the recursive mutexes on database connection and prepared statement objects. In this mode (which is the default when SQLite is compiled with SQLITE_THREADSAFE=1) the SQLite library will itself serialize access to database connections and prepared statements so that the application is free to use the same database connection or the same prepared statement in different threads at the same time. If SQLite is compiled with the SQLITE_THREADSAFE=0 compile-time option then it is not possible to set the Serialized threading mode and sqlite3_config() will return SQLITE_ERROR if called with the SQLITE_CONFIG_SERIALIZED configuration option.

SQLITE_CONFIG_MALLOC
  This option takes a single argument which is a pointer to an instance of the sqlite3_mem_methods structure. The argument specifies alternative low-level memory allocation routines to be used in place of the memory allocation routines built into SQLite. SQLite makes its own private copy of the content of the sqlite3_mem_methods structure before the sqlite3_config() call returns.

SQLITE_CONFIG_GETMALLOC
  This option takes a single argument which is a pointer to an instance of the sqlite3_mem_methods structure. The sqlite3_mem_methods structure is filled with the currently defined memory allocation routines. This option can be used to overload the default memory allocation routines with a wrapper that simulations memory allocation failure or tracks memory usage, for example.

SQLITE_CONFIG_MEMSTATUS
  This option takes single argument of type int, interpreted as a boolean, which enables or disables the collection of memory allocation statistics. When memory allocation statistics are disabled, the following SQLite interfaces become non-operational:
  - sqlite3_memory_used()
  - sqlite3_memory_highwater()
  - sqlite3_soft_heap_limit()
  - sqlite3_status()
  Memory allocation statistics are enabled by default unless SQLite is compiled with SQLITE_DEFAULT_MEMSTATUS=0 in which case memory allocation statistics are disabled by default.

SQLITE_CONFIG_SCRATCH
  This option specifies a static memory buffer that SQLite can use for scratch memory. There are three arguments: A pointer an 8-byte aligned memory buffer from which the scrach allocations will be drawn, the size of each scratch allocation (sz), and the maximum number of scratch allocations (N). The sz argument must be a multiple of 16. The sz parameter should be a few bytes larger than the actual scratch space required due to internal overhead. The first argument must be a pointer to an 8-byte aligned buffer of at least sz*N bytes of memory. SQLite will use no more than one scratch buffer per thread. So N should be set to the expected maximum number of threads. SQLite will never require a scratch buffer that is more than 6 times the database page size. If SQLite needs needs additional scratch memory beyond what is provided by this configuration option, then sqlite3_malloc() will be used to obtain the memory needed.

SQLITE_CONFIG_PAGECACHE
  This option specifies a static memory buffer that SQLite can use for the database page cache with the default page cache implemenation. This configuration should not be used if an application-define page cache implementation is loaded using the SQLITE_CONFIG_PCACHE option. There are three arguments to this option: A pointer to 8-byte aligned memory, the size of each page buffer (sz), and the number of pages (N). The sz argument should be the size of the largest database page (a power of two between 512 and 32768) plus a little extra for each page header. The page header size is 20 to 40 bytes depending on the host architecture. It is harmless, apart from the wasted memory, to make sz a little too large. The first argument should point to an allocation of at least sz*N bytes of memory. SQLite will use the memory provided by the first argument to satisfy its memory needs for the first N pages that it adds to cache. If additional page cache memory is needed beyond what is provided by this option, then SQLite goes to sqlite3_malloc() for the additional storage space. The implementation might use one or more of the N buffers to hold memory accounting information. The pointer in the first argument must be aligned to an 8-byte boundary or subsequent behavior of SQLite will be undefined.

SQLITE_CONFIG_HEAP
  This option specifies a static memory buffer that SQLite will use for all of its dynamic memory allocation needs beyond those provided for by SQLITE_CONFIG_SCRATCH and SQLITE_CONFIG_PAGECACHE. There are three arguments: An 8-byte aligned pointer to the memory, the number of bytes in the memory buffer, and the minimum allocation size. If the first pointer (the memory pointer) is NULL, then SQLite reverts to using its default memory allocator (the system malloc() implementation), undoing any prior invocation of SQLITE_CONFIG_MALLOC. If the memory pointer is not NULL and either SQLITE_ENABLE_MEMSYS3 or SQLITE_ENABLE_MEMSYS5 are defined, then the alternative memory allocator is engaged to handle all of SQLites memory allocation needs. The first pointer (the memory pointer) must be aligned to an 8-byte boundary or subsequent behavior of SQLite will be undefined.

SQLITE_CONFIG_MUTEX
  This option takes a single argument which is a pointer to an instance of the sqlite3_mutex_methods structure. The argument specifies alternative low-level mutex routines to be used in place the mutex routines built into SQLite. SQLite makes a copy of the content of the sqlite3_mutex_methods structure before the call to sqlite3_config() returns. If SQLite is compiled with the SQLITE_THREADSAFE=0 compile-time option then the entire mutexing subsystem is omitted from the build and hence calls to sqlite3_config() with the SQLITE_CONFIG_MUTEX configuration option will return SQLITE_ERROR.

SQLITE_CONFIG_GETMUTEX
  This option takes a single argument which is a pointer to an instance of the sqlite3_mutex_methods structure. The sqlite3_mutex_methods structure is filled with the currently defined mutex routines. This option can be used to overload the default mutex allocation routines with a wrapper used to track mutex usage for performance profiling or testing, for example. If SQLite is compiled with the SQLITE_THREADSAFE=0 compile-time option then the entire mutexing subsystem is omitted from the build and hence calls to sqlite3_config() with the SQLITE_CONFIG_GETMUTEX configuration option will return SQLITE_ERROR.

SQLITE_CONFIG_LOOKASIDE
  This option takes two arguments that determine the default memory allocation for the lookaside memory allocator on each database connection. The first argument is the size of each lookaside buffer slot and the second is the number of slots allocated to each database connection. This option sets the default lookaside size. The SQLITE_DBCONFIG_LOOKASIDE verb to sqlite3_db_config() can be used to change the lookaside configuration on individual connections.

SQLITE_CONFIG_PCACHE
  This option takes a single argument which is a pointer to an sqlite3_pcache_methods object. This object specifies the interface to a custom page cache implementation. SQLite makes a copy of the object and uses it for page cache memory allocations.

SQLITE_CONFIG_GETPCACHE
  This option takes a single argument which is a pointer to an sqlite3_pcache_methods object. SQLite copies of the current page cache implementation into that object.

SQLITE_CONFIG_LOG
  The SQLITE_CONFIG_LOG option takes two arguments: a pointer to a function with a call signature of void(*)(void*,int,const char*), and a pointer to void. If the function pointer is not NULL, it is invoked by sqlite3_log() to process each logging event. If the function pointer is NULL, the sqlite3_log() interface becomes a no-op. The void pointer that is the second argument to SQLITE_CONFIG_LOG is passed through as the first parameter to the application-defined logger function whenever that function is invoked. The second parameter to the logger function is a copy of the first parameter to the corresponding sqlite3_log() call and is intended to be a result code or an extended result code. The third parameter passed to the logger is log message after formatting via sqlite3_snprintf(). The SQLite logging interface is not reentrant; the logger function supplied by the application must not invoke any SQLite interface. In a multi-threaded application, the application-defined logger function must be threadsafe.

## Status Parameters for database connections

>     #define SQLITE_DBSTATUS_LOOKASIDE_USED     0
>     #define SQLITE_DBSTATUS_CACHE_USED         1
>     #define SQLITE_DBSTATUS_MAX                1   /* Largest defined DBSTATUS */

These constants are the available integer "verbs" that can be passed as the second argument to the sqlite3_db_status() interface.

New verbs may be added in future releases of SQLite. Existing verbs might be discontinued. Applications should check the return code from sqlite3_db_status() to make sure that the call worked. The sqlite3_db_status() interface will return a non-zero error code if a discontinued or unsupported verb is invoked.

SQLITE_DBSTATUS_LOOKASIDE_USED
  This parameter returns the number of lookaside memory slots currently checked out.

SQLITE_DBSTATUS_CACHE_USED
  This parameter returns the approximate number of of bytes of heap memory used by all pager caches associated with the database connection. The highwater mark associated with SQLITE_DBSTATUS_CACHE_USED is always 0.

## Authorizer Return Codes

>     #define SQLITE_DENY   1   /* Abort the SQL statement with an error */
>     #define SQLITE_IGNORE 2   /* Don't allow access, but don't generate an error */

The authorizer callback function must return either SQLITE_OK or one of these two constants in order to signal SQLite whether or not the action is permitted. See the authorizer documentation for additional information.

## Standard File Control Opcodes

>     #define SQLITE_FCNTL_LOCKSTATE        1
>     #define SQLITE_GET_LOCKPROXYFILE      2
>     #define SQLITE_SET_LOCKPROXYFILE      3
>     #define SQLITE_LAST_ERRNO             4
>     #define SQLITE_FCNTL_SIZE_HINT        5

These integer constants are opcodes for the xFileControl method of the sqlite3_io_methods object and for the sqlite3_file_control() interface.

The SQLITE_FCNTL_LOCKSTATE opcode is used for debugging. This opcode causes the xFileControl method to write the current state of the lock (one of SQLITE_LOCK_NONE, SQLITE_LOCK_SHARED, SQLITE_LOCK_RESERVED, SQLITE_LOCK_PENDING, or SQLITE_LOCK_EXCLUSIVE) into an integer that the pArg argument points to. This capability is used during testing and only needs to be supported when SQLITE_TEST is defined.

The SQLITE_FCNTL_SIZE_HINT opcode is used by SQLite to give the VFS layer a hint of how large the database file will grow to be during the current transaction. This hint is not guaranteed to be accurate but it is often close. The underlying VFS might choose to preallocate database file space based on this hint in order to help writes to the database file run faster.

## Virtual Table Indexing Information

>     struct sqlite3_index_info {
>       /* Inputs */
>       int nConstraint;           /* Number of entries in aConstraint */
>       struct sqlite3_index_constraint {
>          int iColumn;              /* Column on left-hand side of constraint */
>          unsigned char op;         /* Constraint operator */
>          unsigned char usable;     /* True if this constraint is usable */
>          int iTermOffset;          /* Used internally - xBestIndex should ignore */
>       } *aConstraint;            /* Table of WHERE clause constraints */
>       int nOrderBy;              /* Number of terms in the ORDER BY clause */
>       struct sqlite3_index_orderby {
>          int iColumn;              /* Column number */
>          unsigned char desc;       /* True for DESC.  False for ASC. */
>       } *aOrderBy;               /* The ORDER BY clause */
>       /* Outputs */
>       struct sqlite3_index_constraint_usage {
>         int argvIndex;           /* if >0, constraint is part of argv to xFilter */
>         unsigned char omit;      /* Do not code a test for this constraint */
>       } *aConstraintUsage;
>       int idxNum;                /* Number used to identify the index */
>       char *idxStr;              /* String, possibly obtained from sqlite3_malloc */
>       int needToFreeIdxStr;      /* Free idxStr using sqlite3_free() if true */
>       int orderByConsumed;       /* True if output is already ordered */
>       double estimatedCost;      /* Estimated cost of using this index */
>     };
>     #define SQLITE_INDEX_CONSTRAINT_EQ    2
>     #define SQLITE_INDEX_CONSTRAINT_GT    4
>     #define SQLITE_INDEX_CONSTRAINT_LE    8
>     #define SQLITE_INDEX_CONSTRAINT_LT    16
>     #define SQLITE_INDEX_CONSTRAINT_GE    32
>     #define SQLITE_INDEX_CONSTRAINT_MATCH 64

The sqlite3_index_info structure and its substructures is used to pass information into and receive the reply from the xBestIndex method of a virtual table module. The fields under **Inputs** are the inputs to xBestIndex and are read-only. xBestIndex inserts its results into the **Outputs** fields.

The aConstraint[] array records WHERE clause constraints of the form:

    column OP expr

where OP is =, <, <=, >, or >=. The particular operator is stored in aConstraint[].op. The index of the column is stored in aConstraint[].iColumn. aConstraint[].usable is TRUE if the expr on the right-hand side can be evaluated (and thus the constraint is usable) and false if it cannot.

The optimizer automatically inverts terms of the form "expr OP column" and makes other simplifications to the WHERE clause in an attempt to get as many WHERE clause terms into the form shown above as possible. The aConstraint[] array only reports WHERE clause terms that are relevant to the particular virtual table being queried.

Information about the ORDER BY clause is stored in aOrderBy[]. Each term of aOrderBy records a column of the ORDER BY clause.

The xBestIndex method must fill aConstraintUsage[] with information about what parameters to pass to xFilter. If argvIndex>0 then the right-hand side of the corresponding aConstraint[] is evaluated and becomes the argvIndex-th entry in argv. If aConstraintUsage[].omit is true, then the constraint is assumed to be fully handled by the virtual table and is not checked again by SQLite.

The idxNum and idxPtr values are recorded and passed into the xFilter method. sqlite3_free() is used to free idxPtr if and only if needToFreeIdxPtr is true.

The orderByConsumed means that output from xFilter/xNext will occur in the correct order to satisfy the ORDER BY clause so that no separate sorting step is required.

The estimatedCost value is an estimate of the cost of doing the particular lookup. A full scan of a table with N entries should have a cost of N. A binary search of a table of N entries should have a cost of approximately log(N).

## Device Characteristics

>     #define SQLITE_IOCAP_ATOMIC                 0x00000001
>     #define SQLITE_IOCAP_ATOMIC512              0x00000002
>     #define SQLITE_IOCAP_ATOMIC1K               0x00000004
>     #define SQLITE_IOCAP_ATOMIC2K               0x00000008
>     #define SQLITE_IOCAP_ATOMIC4K               0x00000010
>     #define SQLITE_IOCAP_ATOMIC8K               0x00000020
>     #define SQLITE_IOCAP_ATOMIC16K              0x00000040
>     #define SQLITE_IOCAP_ATOMIC32K              0x00000080
>     #define SQLITE_IOCAP_ATOMIC64K              0x00000100
>     #define SQLITE_IOCAP_SAFE_APPEND            0x00000200
>     #define SQLITE_IOCAP_SEQUENTIAL             0x00000400
>     #define SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN  0x00000800

The xDeviceCharacteristics method of the sqlite3_io_methods object returns an integer which is a vector of the these bit values expressing I/O characteristics of the mass storage device that holds the file that the sqlite3_io_methods refers to.

The SQLITE_IOCAP_ATOMIC property means that all writes of any size are atomic. The SQLITE_IOCAP_ATOMICnnn values mean that writes of blocks that are nnn bytes in size and are aligned to an address which is an integer multiple of nnn are atomic. The SQLITE_IOCAP_SAFE_APPEND value means that when data is appended to a file, the data is appended first then the size of the file is extended, never the other way around. The SQLITE_IOCAP_SEQUENTIAL property means that information is written to disk in the same order as calls to xWrite().

## File Locking Levels

>     #define SQLITE_LOCK_NONE          0
>     #define SQLITE_LOCK_SHARED        1
>     #define SQLITE_LOCK_RESERVED      2
>     #define SQLITE_LOCK_PENDING       3
>     #define SQLITE_LOCK_EXCLUSIVE     4

SQLite uses one of these integer values as the second argument to calls it makes to the xLock() and xUnlock() methods of an sqlite3_io_methods object.

## Mutex Types

>     #define SQLITE_MUTEX_FAST             0
>     #define SQLITE_MUTEX_RECURSIVE        1
>     #define SQLITE_MUTEX_STATIC_MASTER    2
>     #define SQLITE_MUTEX_STATIC_MEM       3  /* sqlite3_malloc() */
>     #define SQLITE_MUTEX_STATIC_MEM2      4  /* NOT USED */
>     #define SQLITE_MUTEX_STATIC_OPEN      4  /* sqlite3BtreeOpen() */
>     #define SQLITE_MUTEX_STATIC_PRNG      5  /* sqlite3_random() */
>     #define SQLITE_MUTEX_STATIC_LRU       6  /* lru page list */
>     #define SQLITE_MUTEX_STATIC_LRU2      7  /* lru page list */

The sqlite3_mutex_alloc() interface takes a single argument which is one of these integer constants.

The set of static mutexes may change from one SQLite release to the next. Applications that override the built-in mutex logic must be prepared to accommodate additional static mutexes.

## Flags For File Open Operations

>     #define SQLITE_OPEN_READONLY         0x00000001  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_READWRITE        0x00000002  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_CREATE           0x00000004  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_DELETEONCLOSE    0x00000008  /* VFS only */
>     #define SQLITE_OPEN_EXCLUSIVE        0x00000010  /* VFS only */
>     #define SQLITE_OPEN_AUTOPROXY        0x00000020  /* VFS only */
>     #define SQLITE_OPEN_MAIN_DB          0x00000100  /* VFS only */
>     #define SQLITE_OPEN_TEMP_DB          0x00000200  /* VFS only */
>     #define SQLITE_OPEN_TRANSIENT_DB     0x00000400  /* VFS only */
>     #define SQLITE_OPEN_MAIN_JOURNAL     0x00000800  /* VFS only */
>     #define SQLITE_OPEN_TEMP_JOURNAL     0x00001000  /* VFS only */
>     #define SQLITE_OPEN_SUBJOURNAL       0x00002000  /* VFS only */
>     #define SQLITE_OPEN_MASTER_JOURNAL   0x00004000  /* VFS only */
>     #define SQLITE_OPEN_NOMUTEX          0x00008000  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_FULLMUTEX        0x00010000  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_SHAREDCACHE      0x00020000  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_PRIVATECACHE     0x00040000  /* Ok for sqlite3_open_v2() */
>     #define SQLITE_OPEN_WAL              0x00080000  /* VFS only */

These bit values are intended for use in the 3rd parameter to the sqlite3_open_v2() interface and in the 4th parameter to the xOpen method of the sqlite3_vfs object.

## Flags for the xShmLock VFS method

>     #define SQLITE_SHM_UNLOCK       1
>     #define SQLITE_SHM_LOCK         2
>     #define SQLITE_SHM_SHARED       4
>     #define SQLITE_SHM_EXCLUSIVE    8

These integer constants define the various locking operations allowed by the xShmLock method of sqlite3_io_methods. The following are the only legal combinations of flags to the xShmLock method:

- SQLITE_SHM_LOCK | SQLITE_SHM_SHARED
- SQLITE_SHM_LOCK | SQLITE_SHM_EXCLUSIVE
- SQLITE_SHM_UNLOCK | SQLITE_SHM_SHARED
- SQLITE_SHM_UNLOCK | SQLITE_SHM_EXCLUSIVE

When unlocking, the same SHARED or EXCLUSIVE flag must be supplied as was given no the corresponding lock.

The xShmLock method can transition between unlocked and SHARED or between unlocked and EXCLUSIVE. It cannot transition between SHARED and EXCLUSIVE.

## Compile-Time Library Version Numbers

>     #define SQLITE_VERSION        "3.7.0"
>     #define SQLITE_VERSION_NUMBER 3007000
>     #define SQLITE_SOURCE_ID      "2010-07-21 16:16:28 b36b105eab6fd3195f4bfba6cb5cda0f063b7460"

The SQLITE_VERSION C preprocessor macro in the sqlite3.h header evaluates to a string literal that is the SQLite version in the format "X.Y.Z" where X is the major version number (always 3 for SQLite3) and Y is the minor version number and Z is the release number. The SQLITE_VERSION_NUMBER C preprocessor macro resolves to an integer with the value (X*1000000 + Y*1000 + Z) where X, Y, and Z are the same numbers used in SQLITE_VERSION. The SQLITE_VERSION_NUMBER for any given release of SQLite will also be larger than the release from which it is derived. Either Y will be held constant and Z will be incremented or else Y will be incremented and Z will be reset to zero.

Since version 3.6.18, SQLite source code has been stored in the Fossil configuration management system. The SQLITE_SOURCE_ID macro evalutes to a string which identifies a particular check-in of SQLite within its configuration management system. The SQLITE_SOURCE_ID string contains the date and time of the check-in (UTC) and an SHA1 hash of the entire source tree.

See also: sqlite3_libversion(), sqlite3_libversion_number(), sqlite3_sourceid(), sqlite_version() and sqlite_source_id().

## Constants Defining Special Destructor Behavior

>     typedef void (*sqlite3_destructor_type)(void*);
>     #define SQLITE_STATIC      ((sqlite3_destructor_type)0)
>     #define SQLITE_TRANSIENT   ((sqlite3_destructor_type)-1)

These are special values for the destructor that is passed in as the final argument to routines like sqlite3_result_blob(). If the destructor argument is SQLITE_STATIC, it means that the content pointer is constant and will never change. It does not need to be destroyed. The SQLITE_TRANSIENT value means that the content will likely change in the near future and that SQLite should make its own private copy of the content before returning.

The typedef is necessary to work around problems in certain C++ compilers. See ticket #2191.

## Status Parameters

>     #define SQLITE_STATUS_MEMORY_USED          0
>     #define SQLITE_STATUS_PAGECACHE_USED       1
>     #define SQLITE_STATUS_PAGECACHE_OVERFLOW   2
>     #define SQLITE_STATUS_SCRATCH_USED         3
>     #define SQLITE_STATUS_SCRATCH_OVERFLOW     4
>     #define SQLITE_STATUS_MALLOC_SIZE          5
>     #define SQLITE_STATUS_PARSER_STACK         6
>     #define SQLITE_STATUS_PAGECACHE_SIZE       7
>     #define SQLITE_STATUS_SCRATCH_SIZE         8

These integer constants designate various run-time status parameters that can be returned by sqlite3_status().

SQLITE_STATUS_MEMORY_USED
  This parameter is the current amount of memory checked out using sqlite3_malloc(), either directly or indirectly. The figure includes calls made to sqlite3_malloc() by the application and internal memory usage by the SQLite library. Scratch memory controlled by SQLITE_CONFIG_SCRATCH and auxiliary page-cache memory controlled by SQLITE_CONFIG_PAGECACHE is not included in this parameter. The amount returned is the sum of the allocation sizes as reported by the xSize method in sqlite3_mem_methods.

SQLITE_STATUS_MALLOC_SIZE
  This parameter records the largest memory allocation request handed to sqlite3_malloc() or sqlite3_realloc() (or their internal equivalents). Only the value returned in the *pHighwater parameter to sqlite3_status() is of interest. The value written into the *pCurrent parameter is undefined.

SQLITE_STATUS_PAGECACHE_USED
  This parameter returns the number of pages used out of the pagecache memory allocator that was configured using SQLITE_CONFIG_PAGECACHE. The value returned is in pages, not in bytes.

SQLITE_STATUS_PAGECACHE_OVERFLOW
  This parameter returns the number of bytes of page cache allocation which could not be statisfied by the SQLITE_CONFIG_PAGECACHE buffer and where forced to overflow to sqlite3_malloc(). The returned value includes allocations that overflowed because they where too large (they were larger than the "sz" parameter to SQLITE_CONFIG_PAGECACHE) and allocations that overflowed because no space was left in the page cache.

SQLITE_STATUS_PAGECACHE_SIZE
  This parameter records the largest memory allocation request handed to pagecache memory allocator. Only the value returned in the *pHighwater parameter to sqlite3_status() is of interest. The value written into the *pCurrent parameter is undefined.

SQLITE_STATUS_SCRATCH_USED
  This parameter returns the number of allocations used out of the scratch memory allocator configured using SQLITE_CONFIG_SCRATCH. The value returned is in allocations, not in bytes. Since a single thread may only have one scratch allocation outstanding at time, this parameter also reports the number of threads using scratch memory at the same time.

SQLITE_STATUS_SCRATCH_OVERFLOW
  This parameter returns the number of bytes of scratch memory allocation which could not be statisfied by the SQLITE_CONFIG_SCRATCH buffer and where forced to overflow to sqlite3_malloc(). The values returned include overflows because the requested allocation was too larger (that is, because the requested allocation was larger than the "sz" parameter to SQLITE_CONFIG_SCRATCH) and because no scratch buffer slots were available.

SQLITE_STATUS_SCRATCH_SIZE
  This parameter records the largest memory allocation request handed to scratch memory allocator. Only the value returned in the *pHighwater parameter to sqlite3_status() is of interest. The value written into the *pCurrent parameter is undefined.

SQLITE_STATUS_PARSER_STACK
  This parameter records the deepest parser stack. It is only meaningful if SQLite is compiled with YYTRACKMAXSTACKDEPTH.

New status parameters may be added from time to time.

## Status Parameters for prepared statements

>     #define SQLITE_STMTSTATUS_FULLSCAN_STEP     1
>     #define SQLITE_STMTSTATUS_SORT              2
>     #define SQLITE_STMTSTATUS_AUTOINDEX         3

These preprocessor macros define integer codes that name counter values associated with the sqlite3_stmt_status() interface. The meanings of the various counters are as follows:

SQLITE_STMTSTATUS_FULLSCAN_STEP
  This is the number of times that SQLite has stepped forward in a table as part of a full table scan. Large numbers for this counter may indicate opportunities for performance improvement through careful use of indices.

SQLITE_STMTSTATUS_SORT
  This is the number of sort operations that have occurred. A non-zero value in this counter may indicate an opportunity to improvement performance through careful use of indices.

SQLITE_STMTSTATUS_AUTOINDEX
  This is the number of rows inserted into transient indices that were created automatically in order to help joins run faster. A non-zero value in this counter may indicate an opportunity to improvement performance by adding permanent indices that do not need to be reinitialized each time the statement is run.

## Synchronization Type Flags

>     #define SQLITE_SYNC_NORMAL        0x00002
>     #define SQLITE_SYNC_FULL          0x00003
>     #define SQLITE_SYNC_DATAONLY      0x00010

When SQLite invokes the xSync() method of an sqlite3_io_methods object it uses a combination of these integer values as the second argument.

When the SQLITE_SYNC_DATAONLY flag is used, it means that the sync operation only needs to flush data to mass storage. Inode information need not be flushed. If the lower four bits of the flag equal SQLITE_SYNC_NORMAL, that means to use normal fsync() semantics. If the lower four bits equal SQLITE_SYNC_FULL, that means to use Mac OS X style fullsync instead of fsync().

## Testing Interface Operation Codes

>     #define SQLITE_TESTCTRL_FIRST                    5
>     #define SQLITE_TESTCTRL_PRNG_SAVE                5
>     #define SQLITE_TESTCTRL_PRNG_RESTORE             6
>     #define SQLITE_TESTCTRL_PRNG_RESET               7
>     #define SQLITE_TESTCTRL_BITVEC_TEST              8
>     #define SQLITE_TESTCTRL_FAULT_INSTALL            9
>     #define SQLITE_TESTCTRL_BENIGN_MALLOC_HOOKS     10
>     #define SQLITE_TESTCTRL_PENDING_BYTE            11
>     #define SQLITE_TESTCTRL_ASSERT                  12
>     #define SQLITE_TESTCTRL_ALWAYS                  13
>     #define SQLITE_TESTCTRL_RESERVE                 14
>     #define SQLITE_TESTCTRL_OPTIMIZATIONS           15
>     #define SQLITE_TESTCTRL_ISKEYWORD               16
>     #define SQLITE_TESTCTRL_PGHDRSZ                 17
>     #define SQLITE_TESTCTRL_LAST                    17

These constants are the valid operation code parameters used as the first argument to sqlite3_test_control().

These parameters and their meanings are subject to change without notice. These values are for testing purposes only. Applications should not use any of these parameters or the sqlite3_test_control() interface.

## Run-Time Limit Categories

>     #define SQLITE_LIMIT_LENGTH                    0
>     #define SQLITE_LIMIT_SQL_LENGTH                1
>     #define SQLITE_LIMIT_COLUMN                    2
>     #define SQLITE_LIMIT_EXPR_DEPTH                3
>     #define SQLITE_LIMIT_COMPOUND_SELECT           4
>     #define SQLITE_LIMIT_VDBE_OP                   5
>     #define SQLITE_LIMIT_FUNCTION_ARG              6
>     #define SQLITE_LIMIT_ATTACHED                  7
>     #define SQLITE_LIMIT_LIKE_PATTERN_LENGTH       8
>     #define SQLITE_LIMIT_VARIABLE_NUMBER           9
>     #define SQLITE_LIMIT_TRIGGER_DEPTH            10

These constants define various performance limits that can be lowered at run-time using sqlite3_limit(). The synopsis of the meanings of the various limits is shown below. Additional information is available at Limits in SQLite.

SQLITE_LIMIT_LENGTH
  The maximum size of any string or BLOB or table row.

SQLITE_LIMIT_SQL_LENGTH
  The maximum length of an SQL statement, in bytes.

SQLITE_LIMIT_COLUMN
  The maximum number of columns in a table definition or in the result set of a SELECT or the maximum number of columns in an index or in an ORDER BY or GROUP BY clause.

SQLITE_LIMIT_EXPR_DEPTH
  The maximum depth of the parse tree on any expression.

SQLITE_LIMIT_COMPOUND_SELECT
  The maximum number of terms in a compound SELECT statement.

SQLITE_LIMIT_VDBE_OP
  The maximum number of instructions in a virtual machine program used to implement an SQL statement.

SQLITE_LIMIT_FUNCTION_ARG
  The maximum number of arguments on a function.

SQLITE_LIMIT_ATTACHED
  The maximum number of attached databases.

SQLITE_LIMIT_LIKE_PATTERN_LENGTH
  The maximum length of the pattern argument to the LIKE or GLOB operators.

SQLITE_LIMIT_VARIABLE_NUMBER
  The maximum number of variables in an SQL statement that can be bound.

SQLITE_LIMIT_TRIGGER_DEPTH
  The maximum depth of recursion for triggers.

## 64-Bit Integer Types

>     #ifdef SQLITE_INT64_TYPE
>       typedef SQLITE_INT64_TYPE sqlite_int64;
>       typedef unsigned SQLITE_INT64_TYPE sqlite_uint64;
>     #elif defined(_MSC_VER) || defined(__BORLANDC__)
>       typedef __int64 sqlite_int64;
>       typedef unsigned __int64 sqlite_uint64;
>     #else
>       typedef long long int sqlite_int64;
>       typedef unsigned long long int sqlite_uint64;
>     #endif
>     typedef sqlite_int64 sqlite3_int64;
>     typedef sqlite_uint64 sqlite3_uint64;

Because there is no cross-platform way to specify 64-bit integer types SQLite includes typedefs for 64-bit signed and unsigned integers.

The sqlite3_int64 and sqlite3_uint64 are the preferred type definitions. The sqlite_int64 and sqlite_uint64 types are supported for backwards compatibility only.

The sqlite3_int64 and sqlite_int64 types can store integer values between -9223372036854775808 and +9223372036854775807 inclusive. The sqlite3_uint64 and sqlite_uint64 types can store integer values between 0 and +18446744073709551615 inclusive.

## Virtual Table Object

>     struct sqlite3_module {
>       int iVersion;
>       int (*xCreate)(sqlite3*, void *pAux,
>                    int argc, const char *const*argv,
>                    sqlite3_vtab **ppVTab, char**);
>       int (*xConnect)(sqlite3*, void *pAux,
>                    int argc, const char *const*argv,
>                    sqlite3_vtab **ppVTab, char**);
>       int (*xBestIndex)(sqlite3_vtab *pVTab, sqlite3_index_info*);
>       int (*xDisconnect)(sqlite3_vtab *pVTab);
>       int (*xDestroy)(sqlite3_vtab *pVTab);
>       int (*xOpen)(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor);
>       int (*xClose)(sqlite3_vtab_cursor*);
>       int (*xFilter)(sqlite3_vtab_cursor*, int idxNum, const char *idxStr,
>                     int argc, sqlite3_value **argv);
>       int (*xNext)(sqlite3_vtab_cursor*);
>       int (*xEof)(sqlite3_vtab_cursor*);
>       int (*xColumn)(sqlite3_vtab_cursor*, sqlite3_context*, int);
>       int (*xRowid)(sqlite3_vtab_cursor*, sqlite3_int64 *pRowid);
>       int (*xUpdate)(sqlite3_vtab *, int, sqlite3_value **, sqlite3_int64 *);
>       int (*xBegin)(sqlite3_vtab *pVTab);
>       int (*xSync)(sqlite3_vtab *pVTab);
>       int (*xCommit)(sqlite3_vtab *pVTab);
>       int (*xRollback)(sqlite3_vtab *pVTab);
>       int (*xFindFunction)(sqlite3_vtab *pVtab, int nArg, const char *zName,
>                            void (**pxFunc)(sqlite3_context*,int,sqlite3_value**),
>                            void **ppArg);
>       int (*xRename)(sqlite3_vtab *pVtab, const char *zNew);
>     };

This structure, sometimes called a a "virtual table module", defines the implementation of a virtual tables. This structure consists mostly of methods for the module.

A virtual table module is created by filling in a persistent instance of this structure and passing a pointer to that instance to sqlite3_create_module() or sqlite3_create_module_v2(). The registration remains valid until it is replaced by a different module or until the database connection closes. The content of this structure must not change while it is registered with any database connection.

## Virtual Table Cursor Object

>     struct sqlite3_vtab_cursor {
>       sqlite3_vtab *pVtab;      /* Virtual table of this cursor */
>       /* Virtual table implementations will typically add additional fields */
>     };

Every virtual table module implementation uses a subclass of the following structure to describe cursors that point into the virtual table and are used to loop through the virtual table. Cursors are created using the xOpen method of the module and are destroyed by the xClose method. Cursors are used by the xFilter, xNext, xEof, xColumn, and xRowid methods of the module. Each module implementation will define the content of a cursor structure to suit its own needs.

This superclass exists in order to define fields of the cursor that are common to all implementations.

## A Handle To An Open BLOB

>     typedef struct sqlite3_blob sqlite3_blob;

An instance of this object represents an open BLOB on which incremental BLOB I/O can be performed. Objects of this type are created by sqlite3_blob_open() and destroyed by sqlite3_blob_close(). The sqlite3_blob_read() and sqlite3_blob_write() interfaces can be used to read or write small subsections of the BLOB. The sqlite3_blob_bytes() interface returns the size of the BLOB in bytes.

## Database Connection Handle

>     typedef struct sqlite3 sqlite3;

Each open SQLite database is represented by a pointer to an instance of the opaque structure named "sqlite3". It is useful to think of an sqlite3 pointer as an object. The sqlite3_open(), sqlite3_open16(), and sqlite3_open_v2() interfaces are its constructors, and sqlite3_close() is its destructor. There are many other interfaces (such as sqlite3_prepare_v2(), sqlite3_create_function(), and sqlite3_busy_timeout() to name but three) that are methods on an sqlite3 object.

## Application Defined Page Cache.

>     typedef struct sqlite3_pcache_methods sqlite3_pcache_methods;
>     struct sqlite3_pcache_methods {
>       void *pArg;
>       int (*xInit)(void*);
>       void (*xShutdown)(void*);
>       sqlite3_pcache *(*xCreate)(int szPage, int bPurgeable);
>       void (*xCachesize)(sqlite3_pcache*, int nCachesize);
>       int (*xPagecount)(sqlite3_pcache*);
>       void *(*xFetch)(sqlite3_pcache*, unsigned key, int createFlag);
>       void (*xUnpin)(sqlite3_pcache*, void*, int discard);
>       void (*xRekey)(sqlite3_pcache*, void*, unsigned oldKey, unsigned newKey);
>       void (*xTruncate)(sqlite3_pcache*, unsigned iLimit);
>       void (*xDestroy)(sqlite3_pcache*);
>     };

The sqlite3_config(SQLITE_CONFIG_PCACHE, ...) interface can register an alternative page cache implementation by passing in an instance of the sqlite3_pcache_methods structure. The majority of the heap memory used by SQLite is used by the page cache to cache data read from, or ready to be written to, the database file. By implementing a custom page cache using this API, an application can control more precisely the amount of memory consumed by SQLite, the way in which that memory is allocated and released, and the policies used to determine exactly which parts of a database file are cached and for how long.

The contents of the sqlite3_pcache_methods structure are copied to an internal buffer by SQLite within the call to sqlite3_config. Hence the application may discard the parameter after the call to sqlite3_config() returns.

The xInit() method is called once for each call to sqlite3_initialize() (usually only once during the lifetime of the process). The xInit() method is passed a copy of the sqlite3_pcache_methods.pArg value. The xInit() method can set up up global structures and/or any mutexes required by the custom page cache implementation.

The xShutdown() method is called from within sqlite3_shutdown(), if the application invokes this API. It can be used to clean up any outstanding resources before process shutdown, if required.

SQLite holds a SQLITE_MUTEX_RECURSIVE mutex when it invokes the xInit method, so the xInit method need not be threadsafe. The xShutdown method is only called from sqlite3_shutdown() so it does not need to be threadsafe either. All other methods must be threadsafe in multithreaded applications.

SQLite will never invoke xInit() more than once without an intervening call to xShutdown().

The xCreate() method is used to construct a new cache instance. SQLite will typically create one cache instance for each open database file, though this is not guaranteed. The first parameter, szPage, is the size in bytes of the pages that must be allocated by the cache. szPage will not be a power of two. szPage will the page size of the database file that is to be cached plus an increment (here called "R") of about 100 or 200. SQLite will use the extra R bytes on each page to store metadata about the underlying database page on disk. The value of R depends on the SQLite version, the target platform, and how SQLite was compiled. R is constant for a particular build of SQLite. The second argument to xCreate(), bPurgeable, is true if the cache being created will be used to cache database pages of a file stored on disk, or false if it is used for an in-memory database. The cache implementation does not have to do anything special based with the value of bPurgeable; it is purely advisory. On a cache where bPurgeable is false, SQLite will never invoke xUnpin() except to deliberately delete a page. In other words, a cache created with bPurgeable set to false will never contain any unpinned pages.

The xCachesize() method may be called at any time by SQLite to set the suggested maximum cache-size (number of pages stored by) the cache instance passed as the first argument. This is the value configured using the SQLite "PRAGMA cache_size" command. As with the bPurgeable parameter, the implementation is not required to do anything with this value; it is advisory only.

The xPagecount() method should return the number of pages currently stored in the cache.

The xFetch() method is used to fetch a page and return a pointer to it. A 'page', in this context, is a buffer of szPage bytes aligned at an 8-byte boundary. The page to be fetched is determined by the key. The mimimum key value is 1. After it has been retrieved using xFetch, the page is considered to be "pinned".

If the requested page is already in the page cache, then the page cache implementation must return a pointer to the page buffer with its content intact. If the requested page is not already in the cache, then the behavior of the cache implementation is determined by the value of the createFlag parameter passed to xFetch, according to the following table:

createFlag | Behaviour when page is not already in cache
--- | ---
0 | Do not allocate a new page. Return NULL.
1 | Allocate a new page if it easy and convenient to do so. Otherwise return NULL.
2 | Make every effort to allocate a new page. Only return NULL if allocating a new page is effectively impossible.

SQLite will normally invoke xFetch() with a createFlag of 0 or 1. If a call to xFetch() with createFlag==1 returns NULL, then SQLite will attempt to unpin one or more cache pages by spilling the content of pinned pages to disk and synching the operating system disk cache. After attempting to unpin pages, the xFetch() method will be invoked again with a createFlag of 2.

xUnpin() is called by SQLite with a pointer to a currently pinned page as its second argument. If the third parameter, discard, is non-zero, then the page should be evicted from the cache. In this case SQLite assumes that the next time the page is retrieved from the cache using the xFetch() method, it will be zeroed. If the discard parameter is zero, then the page is considered to be unpinned. The cache implementation may choose to evict unpinned pages at any time.

The cache is not required to perform any reference counting. A single call to xUnpin() unpins the page regardless of the number of prior calls to xFetch().

The xRekey() method is used to change the key value associated with the page passed as the second argument from oldKey to newKey. If the cache previously contains an entry associated with newKey, it should be discarded. Any prior cache entry associated with newKey is guaranteed not to be pinned.

When SQLite calls the xTruncate() method, the cache must discard all existing cache entries with page numbers (keys) greater than or equal to the value of the iLimit parameter passed to xTruncate(). If any of these pages are pinned, they are implicitly unpinned, meaning that they can be safely discarded.

The xDestroy() method is used to delete a cache allocated by xCreate(). All resources associated with the specified cache should be freed. After calling the xDestroy() method, SQLite considers the sqlite3_pcache* handle invalid, and will not use it with any other sqlite3_pcache_methods functions.

## SQL Statement Object

>     typedef struct sqlite3_stmt sqlite3_stmt;

An instance of this object represents a single SQL statement. This object is variously known as a "prepared statement" or a "compiled SQL statement" or simply as a "statement".

The life of a statement object goes something like this:

1. Create the object using sqlite3_prepare_v2() or a related function.
2. Bind values to host parameters using the sqlite3_bind_*() interfaces.
3. Run the SQL by calling sqlite3_step() one or more times.
4. Reset the statement using sqlite3_reset() then go back to step 2. Do this zero or more times.
5. Destroy the object using sqlite3_finalize().

Refer to documentation on individual methods above for additional information.

## Dynamically Typed Value Object

>     typedef struct Mem sqlite3_value;

SQLite uses the sqlite3_value object to represent all values that can be stored in a database table. SQLite uses dynamic typing for the values it stores. Values stored in sqlite3_value objects can be integers, floating point values, strings, BLOBs, or NULL.

An sqlite3_value object may be either "protected" or "unprotected". Some interfaces require a protected sqlite3_value. Other interfaces will accept either a protected or an unprotected sqlite3_value. Every interface that accepts sqlite3_value arguments specifies whether or not it requires a protected sqlite3_value.

The terms "protected" and "unprotected" refer to whether or not a mutex is held. A internal mutex is held for a protected sqlite3_value object but no mutex is held for an unprotected sqlite3_value object. If SQLite is compiled to be single-threaded (with SQLITE_THREADSAFE=0 and with sqlite3_threadsafe() returning 0) or if SQLite is run in one of reduced mutex modes SQLITE_CONFIG_SINGLETHREAD or SQLITE_CONFIG_MULTITHREAD then there is no distinction between protected and unprotected sqlite3_value objects and they can be used interchangeably. However, for maximum code portability it is recommended that applications still make the distinction between between protected and unprotected sqlite3_value objects even when not strictly required.

The sqlite3_value objects that are passed as parameters into the implementation of application-defined SQL functions are protected. The sqlite3_value object returned by sqlite3_column_value() is unprotected. Unprotected sqlite3_value objects may only be used with sqlite3_result_value() and sqlite3_bind_value(). The sqlite3_value_type() family of interfaces require protected sqlite3_value objects.

## Deprecated Functions

>     #ifndef SQLITE_OMIT_DEPRECATED
>     int sqlite3_aggregate_count(sqlite3_context*);
>     int sqlite3_expired(sqlite3_stmt*);
>     int sqlite3_transfer_bindings(sqlite3_stmt*, sqlite3_stmt*);
>     int sqlite3_global_recover(void);
>     void sqlite3_thread_cleanup(void);
>     int sqlite3_memory_alarm(void(*)(void*,sqlite3_int64,int),void*,sqlite3_int64);
>     #endif

These functions are deprecated. In order to maintain backwards compatibility with older code, these functions continue to be supported. However, new applications should avoid the use of these functions. To help encourage people to avoid using these functions, we are not going to tell you what they do.

## Online Backup API.

>     sqlite3_backup *sqlite3_backup_init(
>       sqlite3 *pDest,                        /* Destination database handle */
>       const char *zDestName,                 /* Destination database name */
>       sqlite3 *pSource,                      /* Source database handle */
>       const char *zSourceName                /* Source database name */
>     );
>     int sqlite3_backup_step(sqlite3_backup *p, int nPage);
>     int sqlite3_backup_finish(sqlite3_backup *p);
>     int sqlite3_backup_remaining(sqlite3_backup *p);
>     int sqlite3_backup_pagecount(sqlite3_backup *p);

The backup API copies the content of one database into another. It is useful either for creating backups of databases or for copying in-memory databases to or from persistent files.

See Also: Using the SQLite Online Backup API

Exclusive access is required to the destination database for the duration of the operation. However the source database is only read-locked while it is actually being read; it is not locked continuously for the entire backup operation. Thus, the backup may be performed on a live source database without preventing other users from reading or writing to the source database while the backup is underway.

To perform a backup operation:

1. sqlite3_backup_init() is called once to initialize the backup,
2. sqlite3_backup_step() is called one or more times to transfer the data between the two databases, and finally
3. sqlite3_backup_finish() is called to release all resources associated with the backup operation.

There should be exactly one call to sqlite3_backup_finish() for each successful call to sqlite3_backup_init().

sqlite3_backup_init()

The D and N arguments to sqlite3_backup_init(D,N,S,M) are the database connection associated with the destination database and the database name, respectively. The database name is "main" for the main database, "temp" for the temporary database, or the name specified after the AS keyword in an ATTACH statement for an attached database. The S and M arguments passed to sqlite3_backup_init(D,N,S,M) identify the database connection and database name of the source database, respectively. The source and destination database connections (parameters S and D) must be different or else sqlite3_backup_init(D,N,S,M) will file with an error.

If an error occurs within sqlite3_backup_init(D,N,S,M), then NULL is returned and an error code and error message are store3d in the destination database connection D. The error code and message for the failed call to sqlite3_backup_init() can be retrieved using the sqlite3_errcode(), sqlite3_errmsg(), and/or sqlite3_errmsg16() functions. A successful call to sqlite3_backup_init() returns a pointer to an sqlite3_backup object. The sqlite3_backup object may be used with the sqlite3_backup_step() and sqlite3_backup_finish() functions to perform the specified backup operation.

sqlite3_backup_step()

Function sqlite3_backup_step(B,N) will copy up to N pages between the source and destination databases specified by sqlite3_backup object B. If N is negative, all remaining source pages are copied. If sqlite3_backup_step(B,N) successfully copies N pages and there are still more pages to be copied, then the function resturns SQLITE_OK. If sqlite3_backup_step(B,N) successfully finishes copying all pages from source to destination, then it returns SQLITE_DONE. If an error occurs while running sqlite3_backup_step(B,N), then an error code is returned. As well as SQLITE_OK and SQLITE_DONE, a call to sqlite3_backup_step() may return SQLITE_READONLY, SQLITE_NOMEM, SQLITE_BUSY, SQLITE_LOCKED, or an SQLITE_IOERR_XXX extended error code.

The sqlite3_backup_step() might return SQLITE_READONLY if

1. the destination database was opened read-only, or
2. the destination database is using write-ahead-log journaling and the destination and source page sizes differ, or
3. The destination database is an in-memory database and the destination and source page sizes differ.

If sqlite3_backup_step() cannot obtain a required file-system lock, then the busy-handler function is invoked (if one is specified). If the busy-handler returns non-zero before the lock is available, then SQLITE_BUSY is returned to the caller. In this case the call to sqlite3_backup_step() can be retried later. If the source database connection is being used to write to the source database when sqlite3_backup_step() is called, then SQLITE_LOCKED is returned immediately. Again, in this case the call to sqlite3_backup_step() can be retried later on. If SQLITE_IOERR_XXX, SQLITE_NOMEM, or SQLITE_READONLY is returned, then there is no point in retrying the call to sqlite3_backup_step(). These errors are considered fatal. The application must accept that the backup operation has failed and pass the backup operation handle to the sqlite3_backup_finish() to release associated resources.

The first call to sqlite3_backup_step() obtains an exclusive lock on the destination file. The exclusive lock is not released until either sqlite3_backup_finish() is called or the backup operation is complete and sqlite3_backup_step() returns SQLITE_DONE. Every call to sqlite3_backup_step() obtains a shared lock on the source database that lasts for the duration of the sqlite3_backup_step() call. Because the source database is not locked between calls to sqlite3_backup_step(), the source database may be modified mid-way through the backup process. If the source database is modified by an external process or via a database connection other than the one being used by the backup operation, then the backup will be automatically restarted by the next call to sqlite3_backup_step(). If the source database is modified by the using the same database connection as is used by the backup operation, then the backup database is automatically updated at the same time.

sqlite3_backup_finish()

When sqlite3_backup_step() has returned SQLITE_DONE, or when the application wishes to abandon the backup operation, the application should destroy the sqlite3_backup by passing it to sqlite3_backup_finish(). The sqlite3_backup_finish() interfaces releases all resources associated with the sqlite3_backup object. If sqlite3_backup_step() has not yet returned SQLITE_DONE, then any active write-transaction on the destination database is rolled back. The sqlite3_backup object is invalid and may not be used following a call to sqlite3_backup_finish().

The value returned by sqlite3_backup_finish is SQLITE_OK if no sqlite3_backup_step() errors occurred, regardless or whether or not sqlite3_backup_step() completed. If an out-of-memory condition or IO error occurred during any prior sqlite3_backup_step() call on the same sqlite3_backup object, then sqlite3_backup_finish() returns the corresponding error code.

A return of SQLITE_BUSY or SQLITE_LOCKED from sqlite3_backup_step() is not a permanent error and does not affect the return value of sqlite3_backup_finish().

sqlite3_backup_remaining(), sqlite3_backup_pagecount()

Each call to sqlite3_backup_step() sets two values inside the sqlite3_backup object: the number of pages still to be backed up and the total number of pages in the source databae file. The sqlite3_backup_remaining() and sqlite3_backup_pagecount() interfaces retrieve these two values, respectively.

The values returned by these functions are only updated by sqlite3_backup_step(). If the source database is modified during a backup operation, then the values are not updated to account for any extra pages that need to be updated or the size of the source database file changing.

Concurrent Usage of Database Handles

The source database connection may be used by the application for other purposes while a backup operation is underway or being initialized. If SQLite is compiled and configured to support threadsafe database connections, then the source database connection may be used concurrently from within other threads.

However, the application must guarantee that the destination database connection is not passed to any other API (by any thread) after sqlite3_backup_init() is called and before the corresponding call to sqlite3_backup_finish(). SQLite does not currently check to see if the application incorrectly accesses the destination database connection and so no error code is reported, but the operations may malfunction nevertheless. Use of the destination database connection while a backup is in progress might also also cause a mutex deadlock.

If running in shared cache mode, the application must guarantee that the shared cache used by the destination database is not accessed while the backup is running. In practice this means that the application must guarantee that the disk file being backed up to is not accessed by any connection within the process, not just the specific connection that was passed to sqlite3_backup_init().

The sqlite3_backup object itself is partially threadsafe. Multiple threads may safely make multiple concurrent calls to sqlite3_backup_step(). However, the sqlite3_backup_remaining() and sqlite3_backup_pagecount() APIs are not strictly speaking threadsafe. If they are invoked at the same time as another thread is invoking sqlite3_backup_step() it is possible that they return invalid values.

## Collation Needed Callbacks

>     int sqlite3_collation_needed(
>       sqlite3*,
>       void*,
>       void(*)(void*,sqlite3*,int eTextRep,const char*)
>     );
>     int sqlite3_collation_needed16(
>       sqlite3*,
>       void*,
>       void(*)(void*,sqlite3*,int eTextRep,const void*)
>     );

To avoid having to register all collation sequences before a database can be used, a single callback function may be registered with the database connection to be invoked whenever an undefined collation sequence is required.

If the function is registered using the sqlite3_collation_needed() API, then it is passed the names of undefined collation sequences as strings encoded in UTF-8. If sqlite3_collation_needed16() is used, the names are passed as UTF-16 in machine native byte order. A call to either function replaces the existing collation-needed callback.

When the callback is invoked, the first argument passed is a copy of the second argument to sqlite3_collation_needed() or sqlite3_collation_needed16(). The second argument is the database connection. The third argument is one of SQLITE_UTF8, SQLITE_UTF16BE, or SQLITE_UTF16LE, indicating the most desirable form of the collation sequence function required. The fourth parameter is the name of the required collation sequence.

The callback function should register the desired collation using sqlite3_create_collation(), sqlite3_create_collation16(), or sqlite3_create_collation_v2().

## Source Of Data In A Query Result

>     const char *sqlite3_column_database_name(sqlite3_stmt*,int);
>     const void *sqlite3_column_database_name16(sqlite3_stmt*,int);
>     const char *sqlite3_column_table_name(sqlite3_stmt*,int);
>     const void *sqlite3_column_table_name16(sqlite3_stmt*,int);
>     const char *sqlite3_column_origin_name(sqlite3_stmt*,int);
>     const void *sqlite3_column_origin_name16(sqlite3_stmt*,int);

These routines provide a means to determine the database, table, and table column that is the origin of a particular result column in SELECT statement. The name of the database or table or column can be returned as either a UTF-8 or UTF-16 string. The _database_ routines return the database name, the _table_ routines return the table name, and the origin_ routines return the column name. The returned string is valid until the prepared statement is destroyed using sqlite3_finalize() or until the same information is requested again in a different encoding.

The names returned are the original un-aliased names of the database, table, and column.

The first argument to these interfaces is a prepared statement. These functions return information about the Nth result column returned by the statement, where N is the second function argument. The left-most column is column 0 for these routines.

If the Nth column returned by the statement is an expression or subquery and is not a column value, then all of these functions return NULL. These routine might also return NULL if a memory allocation error occurs. Otherwise, they return the name of the attached database, table, or column that query result column was extracted from.

As with all other SQLite APIs, those whose names end with "16" return UTF-16 encoded strings and the other functions return UTF-8.

These APIs are only available if the library was compiled with the SQLITE_ENABLE_COLUMN_METADATA C-preprocessor symbol.

If two or more threads call one or more of these routines against the same prepared statement and column at the same time then the results are undefined.

If two or more threads call one or more column metadata interfaces for the same prepared statement and result column at the same time then the results are undefined.

## Declared Datatype Of A Query Result

>     const char *sqlite3_column_decltype(sqlite3_stmt*,int);
>     const void *sqlite3_column_decltype16(sqlite3_stmt*,int);

The first parameter is a prepared statement. If this statement is a SELECT statement and the Nth column of the returned result set of that SELECT is a table column (not an expression or subquery) then the declared type of the table column is returned. If the Nth column of the result set is an expression or subquery, then a NULL pointer is returned. The returned string is always UTF-8 encoded.

For example, given the database schema:

CREATE TABLE t1(c1 VARIANT);

and the following statement to be compiled:

SELECT c1 + 1, c1 FROM t1;

this routine would return the string "VARIANT" for the second result column (i==1), and a NULL pointer for the first result column (i==0).

SQLite uses dynamic run-time typing. So just because a column is declared to contain a particular type does not mean that the data stored in that column is of the declared type. SQLite is strongly typed, but the typing is dynamic not static. Type is associated with individual values, not with the containers used to hold those values.

## Column Names In A Result Set

>     const char *sqlite3_column_name(sqlite3_stmt*, int N);
>     const void *sqlite3_column_name16(sqlite3_stmt*, int N);

These routines return the name assigned to a particular column in the result set of a SELECT statement. The sqlite3_column_name() interface returns a pointer to a zero-terminated UTF-8 string and sqlite3_column_name16() returns a pointer to a zero-terminated UTF-16 string. The first parameter is the prepared statement that implements the SELECT statement. The second parameter is the column number. The leftmost column is number 0.

The returned string pointer is valid until either the prepared statement is destroyed by sqlite3_finalize() or until the next call to sqlite3_column_name() or sqlite3_column_name16() on the same column.

If sqlite3_malloc() fails during the processing of either routine (for example during a conversion from UTF-8 to UTF-16) then a NULL pointer is returned.

The name of a result column is the value of the "AS" clause for that column, if there is an AS clause. If there is no AS clause then the name of the column is unspecified and may change from one release of SQLite to the next.

## Commit And Rollback Notification Callbacks

>     void *sqlite3_commit_hook(sqlite3*, int(*)(void*), void*);
>     void *sqlite3_rollback_hook(sqlite3*, void(*)(void *), void*);

The sqlite3_commit_hook() interface registers a callback function to be invoked whenever a transaction is committed. Any callback set by a previous call to sqlite3_commit_hook() for the same database connection is overridden. The sqlite3_rollback_hook() interface registers a callback function to be invoked whenever a transaction is rolled back. Any callback set by a previous call to sqlite3_rollback_hook() for the same database connection is overridden. The pArg argument is passed through to the callback. If the callback on a commit hook function returns non-zero, then the commit is converted into a rollback.

The sqlite3_commit_hook(D,C,P) and sqlite3_rollback_hook(D,C,P) functions return the P argument from the previous call of the same function on the same database connection D, or NULL for the first call for each function on D.

The callback implementation must not do anything that will modify the database connection that invoked the callback. Any actions to modify the database connection must be deferred until after the completion of the sqlite3_step() call that triggered the commit or rollback hook in the first place. Note that sqlite3_prepare_v2() and sqlite3_step() both modify their database connections for the meaning of "modify" in this paragraph.

Registering a NULL function disables the callback.

When the commit hook callback routine returns zero, the COMMIT operation is allowed to continue normally. If the commit hook returns non-zero, then the COMMIT is converted into a ROLLBACK. The rollback hook is invoked on a rollback that results from a commit hook returning non-zero, just as it would be with any other rollback.

For the purposes of this API, a transaction is said to have been rolled back if an explicit "ROLLBACK" statement is executed, or an error or constraint causes an implicit rollback to occur. The rollback callback is not invoked if a transaction is automatically rolled back because the database connection is closed.

See also the sqlite3_update_hook() interface.

## Run-Time Library Compilation Options Diagnostics

>     #ifndef SQLITE_OMIT_COMPILEOPTION_DIAGS
>     int sqlite3_compileoption_used(const char *zOptName);
>     const char *sqlite3_compileoption_get(int N);
>     #endif

The sqlite3_compileoption_used() function returns 0 or 1 indicating whether the specified option was defined at compile time. The SQLITE_ prefix may be omitted from the option name passed to sqlite3_compileoption_used().

The sqlite3_compileoption_get() function allows interating over the list of options that were defined at compile time by returning the N-th compile time option string. If N is out of range, sqlite3_compileoption_get() returns a NULL pointer. The SQLITE_ prefix is omitted from any strings returned by sqlite3_compileoption_get().

Support for the diagnostic functions sqlite3_compileoption_used() and sqlite3_compileoption_get() may be omitted by specifing the SQLITE_OMIT_COMPILEOPTION_DIAGS option at compile time.

See also: SQL functions sqlite_compileoption_used() and sqlite_compileoption_get() and the compile_options pragma.

## Determine If An SQL Statement Is Complete

>     int sqlite3_complete(const char *sql);
>     int sqlite3_complete16(const void *sql);

These routines are useful during command-line input to determine if the currently entered text seems to form a complete SQL statement or if additional input is needed before sending the text into SQLite for parsing. These routines return 1 if the input string appears to be a complete SQL statement. A statement is judged to be complete if it ends with a semicolon token and is not a prefix of a well-formed CREATE TRIGGER statement. Semicolons that are embedded within string literals or quoted identifier names or comments are not independent tokens (they are part of the token in which they are embedded) and thus do not count as a statement terminator. Whitespace and comments that follow the final semicolon are ignored.

These routines return 0 if the statement is incomplete. If a memory allocation fails, then SQLITE_NOMEM is returned.

These routines do not parse the SQL statements thus will not detect syntactically incorrect SQL.

If SQLite has not been initialized using sqlite3_initialize() prior to invoking sqlite3_complete16() then sqlite3_initialize() is invoked automatically by sqlite3_complete16(). If that initialization fails, then the return value from sqlite3_complete16() will be non-zero regardless of whether or not the input SQL is complete.

The input to sqlite3_complete() must be a zero-terminated UTF-8 string.

The input to sqlite3_complete16() must be a zero-terminated UTF-16 string in native byte order.

## Define New Collating Sequences

>     int sqlite3_create_collation(
>       sqlite3*,
>       const char *zName,
>       int eTextRep,
>       void*,
>       int(*xCompare)(void*,int,const void*,int,const void*)
>     );
>     int sqlite3_create_collation_v2(
>       sqlite3*,
>       const char *zName,
>       int eTextRep,
>       void*,
>       int(*xCompare)(void*,int,const void*,int,const void*),
>       void(*xDestroy)(void*)
>     );
>     int sqlite3_create_collation16(
>       sqlite3*,
>       const void *zName,
>       int eTextRep,
>       void*,
>       int(*xCompare)(void*,int,const void*,int,const void*)
>     );

These functions are used to add new collation sequences to the database connection specified as the first argument.

The name of the new collation sequence is specified as a UTF-8 string for sqlite3_create_collation() and sqlite3_create_collation_v2() and a UTF-16 string for sqlite3_create_collation16(). In all cases the name is passed as the second function argument.

The third argument may be one of the constants SQLITE_UTF8, SQLITE_UTF16LE, or SQLITE_UTF16BE, indicating that the user-supplied routine expects to be passed pointers to strings encoded using UTF-8, UTF-16 little-endian, or UTF-16 big-endian, respectively. The third argument might also be SQLITE_UTF16 to indicate that the routine expects pointers to be UTF-16 strings in the native byte order, or the argument can be SQLITE_UTF16_ALIGNED if the the routine expects pointers to 16-bit word aligned strings of UTF-16 in the native byte order.

A pointer to the user supplied routine must be passed as the fifth argument. If it is NULL, this is the same as deleting the collation sequence (so that SQLite cannot call it anymore). Each time the application supplied function is invoked, it is passed as its first parameter a copy of the void* passed as the fourth argument to sqlite3_create_collation() or sqlite3_create_collation16().

The remaining arguments to the application-supplied routine are two strings, each represented by a (length, data) pair and encoded in the encoding that was passed as the third argument when the collation sequence was registered. The application defined collation routine should return negative, zero or positive if the first string is less than, equal to, or greater than the second string. i.e. (STRING1 - STRING2).

The sqlite3_create_collation_v2() works like sqlite3_create_collation() except that it takes an extra argument which is a destructor for the collation. The destructor is called when the collation is destroyed and is passed a copy of the fourth parameter void* pointer of the sqlite3_create_collation_v2(). Collations are destroyed when they are overridden by later calls to the collation creation functions or when the database connection is closed using sqlite3_close().

See also: sqlite3_collation_needed() and sqlite3_collation_needed16().

## Register A Virtual Table Implementation

>     int sqlite3_create_module(
>       sqlite3 *db,               /* SQLite connection to register module with */
>       const char *zName,         /* Name of the module */
>       const sqlite3_module *p,   /* Methods for the module */
>       void *pClientData          /* Client data for xCreate/xConnect */
>     );
>     int sqlite3_create_module_v2(
>       sqlite3 *db,               /* SQLite connection to register module with */
>       const char *zName,         /* Name of the module */
>       const sqlite3_module *p,   /* Methods for the module */
>       void *pClientData,         /* Client data for xCreate/xConnect */
>       void(*xDestroy)(void*)     /* Module destructor function */
>     );

These routines are used to register a new virtual table module name. Module names must be registered before creating a new virtual table using the module and before using a preexisting virtual table for the module.

The module name is registered on the database connection specified by the first parameter. The name of the module is given by the second parameter. The third parameter is a pointer to the implementation of the virtual table module. The fourth parameter is an arbitrary client data pointer that is passed through into the xCreate and xConnect methods of the virtual table module when a new virtual table is be being created or reinitialized.

The sqlite3_create_module_v2() interface has a fifth parameter which is a pointer to a destructor for the pClientData. SQLite will invoke the destructor function (if it is not NULL) when SQLite no longer needs the pClientData pointer. The sqlite3_create_module() interface is equivalent to sqlite3_create_module_v2() with a NULL destructor.

## Error Codes And Messages

>     int sqlite3_errcode(sqlite3 *db);
>     int sqlite3_extended_errcode(sqlite3 *db);
>     const char *sqlite3_errmsg(sqlite3*);
>     const void *sqlite3_errmsg16(sqlite3*);

The sqlite3_errcode() interface returns the numeric result code or extended result code for the most recent failed sqlite3_* API call associated with a database connection. If a prior API call failed but the most recent API call succeeded, the return value from sqlite3_errcode() is undefined. The sqlite3_extended_errcode() interface is the same except that it always returns the extended result code even when extended result codes are disabled.

The sqlite3_errmsg() and sqlite3_errmsg16() return English-language text that describes the error, as either UTF-8 or UTF-16 respectively. Memory to hold the error message string is managed internally. The application does not need to worry about freeing the result. However, the error string might be overwritten or deallocated by subsequent calls to other SQLite interface functions.

When the serialized threading mode is in use, it might be the case that a second error occurs on a separate thread in between the time of the first error and the call to these interfaces. When that happens, the second error will be reported since these interfaces always report the most recent result. To avoid this, each thread can obtain exclusive use of the database connection D by invoking sqlite3_mutex_enter(sqlite3_db_mutex(D)) before beginning to use D and invoking sqlite3_mutex_leave(sqlite3_db_mutex(D)) after all calls to the interfaces listed here are completed.

If an interface fails with SQLITE_MISUSE, that means the interface was invoked incorrectly by the application. In that case, the error code and message may or may not be set.

## Memory Allocation Subsystem

>     void *sqlite3_malloc(int);
>     void *sqlite3_realloc(void*, int);
>     void sqlite3_free(void*);

The SQLite core uses these three routines for all of its own internal memory allocation needs. "Core" in the previous sentence does not include operating-system specific VFS implementation. The Windows VFS uses native malloc() and free() for some operations.

The sqlite3_malloc() routine returns a pointer to a block of memory at least N bytes in length, where N is the parameter. If sqlite3_malloc() is unable to obtain sufficient free memory, it returns a NULL pointer. If the parameter N to sqlite3_malloc() is zero or negative then sqlite3_malloc() returns a NULL pointer.

Calling sqlite3_free() with a pointer previously returned by sqlite3_malloc() or sqlite3_realloc() releases that memory so that it might be reused. The sqlite3_free() routine is a no-op if is called with a NULL pointer. Passing a NULL pointer to sqlite3_free() is harmless. After being freed, memory should neither be read nor written. Even reading previously freed memory might result in a segmentation fault or other severe error. Memory corruption, a segmentation fault, or other severe error might result if sqlite3_free() is called with a non-NULL pointer that was not obtained from sqlite3_malloc() or sqlite3_realloc().

The sqlite3_realloc() interface attempts to resize a prior memory allocation to be at least N bytes, where N is the second parameter. The memory allocation to be resized is the first parameter. If the first parameter to sqlite3_realloc() is a NULL pointer then its behavior is identical to calling sqlite3_malloc(N) where N is the second parameter to sqlite3_realloc(). If the second parameter to sqlite3_realloc() is zero or negative then the behavior is exactly the same as calling sqlite3_free(P) where P is the first parameter to sqlite3_realloc(). sqlite3_realloc() returns a pointer to a memory allocation of at least N bytes in size or NULL if sufficient memory is unavailable. If M is the size of the prior allocation, then min(N,M) bytes of the prior allocation are copied into the beginning of buffer returned by sqlite3_realloc() and the prior allocation is freed. If sqlite3_realloc() returns NULL, then the prior allocation is not freed.

The memory returned by sqlite3_malloc() and sqlite3_realloc() is always aligned to at least an 8 byte boundary.

In SQLite version 3.5.0 and 3.5.1, it was possible to define the SQLITE_OMIT_MEMORY_ALLOCATION which would cause the built-in implementation of these routines to be omitted. That capability is no longer provided. Only built-in memory allocators can be used.

The Windows OS interface layer calls the system malloc() and free() directly when converting filenames between the UTF-8 encoding used by SQLite and whatever filename encoding is used by the particular Windows installation. Memory allocation errors are detected, but they are reported back as SQLITE_CANTOPEN or SQLITE_IOERR rather than SQLITE_NOMEM.

The pointer arguments to sqlite3_free() and sqlite3_realloc() must be either NULL or else pointers obtained from a prior invocation of sqlite3_malloc() or sqlite3_realloc() that have not yet been released.

The application must not read or write any part of a block of memory after it has been released using sqlite3_free() or sqlite3_realloc().

## Convenience Routines For Running Queries

>     int sqlite3_get_table(
>       sqlite3 *db,          /* An open database */
>       const char *zSql,     /* SQL to be evaluated */
>       char ***pazResult,    /* Results of the query */
>       int *pnRow,           /* Number of result rows written here */
>       int *pnColumn,        /* Number of result columns written here */
>       char **pzErrmsg       /* Error msg written here */
>     );
>     void sqlite3_free_table(char **result);

Definition: A result table is memory data structure created by the sqlite3_get_table() interface. A result table records the complete query results from one or more queries.

The table conceptually has a number of rows and columns. But these numbers are not part of the result table itself. These numbers are obtained separately. Let N be the number of rows and M be the number of columns.

A result table is an array of pointers to zero-terminated UTF-8 strings. There are (N+1)*M elements in the array. The first M pointers point to zero-terminated strings that contain the names of the columns. The remaining entries all point to query results. NULL values result in NULL pointers. All other values are in their UTF-8 zero-terminated string representation as returned by sqlite3_column_text().

A result table might consist of one or more memory allocations. It is not safe to pass a result table directly to sqlite3_free(). A result table should be deallocated using sqlite3_free_table().

As an example of the result table format, suppose a query result is as follows:

>     Name        | Age
>     -----------------------
>     Alice       | 43
>     Bob         | 28
>     Cindy       | 21

There are two column (M==2) and three rows (N==3). Thus the result table has 8 entries. Suppose the result table is stored in an array names azResult. Then azResult holds this content:

>     azResult[0] = "Name";
>     azResult[1] = "Age";
>     azResult[2] = "Alice";
>     azResult[3] = "43";
>     azResult[4] = "Bob";
>     azResult[5] = "28";
>     azResult[6] = "Cindy";
>     azResult[7] = "21";

The sqlite3_get_table() function evaluates one or more semicolon-separated SQL statements in the zero-terminated UTF-8 string of its 2nd parameter and returns a result table to the pointer given in its 3rd parameter.

After the application has finished with the result from sqlite3_get_table(), it should pass the result table pointer to sqlite3_free_table() in order to release the memory that was malloced. Because of the way the sqlite3_malloc() happens within sqlite3_get_table(), the calling function must not try to call sqlite3_free() directly. Only sqlite3_free_table() is able to release the memory properly and safely.

The sqlite3_get_table() interface is implemented as a wrapper around sqlite3_exec(). The sqlite3_get_table() routine does not have access to any internal data structures of SQLite. It uses only the public interface defined here. As a consequence, errors that occur in the wrapper layer outside of the internal sqlite3_exec() call are not reflected in subsequent calls to sqlite3_errcode() or sqlite3_errmsg().

## Function Auxiliary Data

>     void *sqlite3_get_auxdata(sqlite3_context*, int N);
>     void sqlite3_set_auxdata(sqlite3_context*, int N, void*, void (*)(void*));

The following two functions may be used by scalar SQL functions to associate metadata with argument values. If the same value is passed to multiple invocations of the same SQL function during query execution, under some circumstances the associated metadata may be preserved. This may be used, for example, to add a regular-expression matching scalar function. The compiled version of the regular expression is stored as metadata associated with the SQL value passed as the regular expression pattern. The compiled regular expression can be reused on multiple invocations of the same function so that the original pattern string does not need to be recompiled on each invocation.

The sqlite3_get_auxdata() interface returns a pointer to the metadata associated by the sqlite3_set_auxdata() function with the Nth argument value to the application-defined function. If no metadata has been ever been set for the Nth argument of the function, or if the corresponding function parameter has changed since the meta-data was set, then sqlite3_get_auxdata() returns a NULL pointer.

The sqlite3_set_auxdata() interface saves the metadata pointed to by its 3rd parameter as the metadata for the N-th argument of the application-defined function. Subsequent calls to sqlite3_get_auxdata() might return this data, if it has not been destroyed. If it is not NULL, SQLite will invoke the destructor function given by the 4th parameter to sqlite3_set_auxdata() on the metadata when the corresponding function parameter changes or when the SQL statement completes, whichever comes first.

SQLite is free to call the destructor and drop metadata on any parameter of any function at any time. The only guarantee is that the destructor will be called before the metadata is dropped.

In practice, metadata is preserved between function calls for expressions that are constant at compile time. This includes literal values and parameters.

These routines must be called from the same thread in which the SQL function is running.

## Initialize The SQLite Library

>     int sqlite3_initialize(void);
>     int sqlite3_shutdown(void);
>     int sqlite3_os_init(void);
>     int sqlite3_os_end(void);

The sqlite3_initialize() routine initializes the SQLite library. The sqlite3_shutdown() routine deallocates any resources that were allocated by sqlite3_initialize(). These routines are designed to aid in process initialization and shutdown on embedded systems. Workstation applications using SQLite normally do not need to invoke either of these routines.

A call to sqlite3_initialize() is an "effective" call if it is the first time sqlite3_initialize() is invoked during the lifetime of the process, or if it is the first time sqlite3_initialize() is invoked following a call to sqlite3_shutdown(). Only an effective call of sqlite3_initialize() does any initialization. All other calls are harmless no-ops.

A call to sqlite3_shutdown() is an "effective" call if it is the first call to sqlite3_shutdown() since the last sqlite3_initialize(). Only an effective call to sqlite3_shutdown() does any deinitialization. All other valid calls to sqlite3_shutdown() are harmless no-ops.

The sqlite3_initialize() interface is threadsafe, but sqlite3_shutdown() is not. The sqlite3_shutdown() interface must only be called from a single thread. All open database connections must be closed and all other SQLite resources must be deallocated prior to invoking sqlite3_shutdown().

Among other things, sqlite3_initialize() will invoke sqlite3_os_init(). Similarly, sqlite3_shutdown() will invoke sqlite3_os_end().

The sqlite3_initialize() routine returns SQLITE_OK on success. If for some reason, sqlite3_initialize() is unable to initialize the library (perhaps it is unable to allocate a needed resource such as a mutex) it returns an error code other than SQLITE_OK.

The sqlite3_initialize() routine is called internally by many other SQLite interfaces so that an application usually does not need to invoke sqlite3_initialize() directly. For example, sqlite3_open() calls sqlite3_initialize() so the SQLite library will be automatically initialized when sqlite3_open() is called if it has not be initialized already. However, if SQLite is compiled with the SQLITE_OMIT_AUTOINIT compile-time option, then the automatic calls to sqlite3_initialize() are omitted and the application must call sqlite3_initialize() directly prior to using any other SQLite interface. For maximum portability, it is recommended that applications always invoke sqlite3_initialize() directly prior to using any other SQLite interface. Future releases of SQLite may require this. In other words, the behavior exhibited when SQLite is compiled with SQLITE_OMIT_AUTOINIT might become the default behavior in some future release of SQLite.

The sqlite3_os_init() routine does operating-system specific initialization of the SQLite library. The sqlite3_os_end() routine undoes the effect of sqlite3_os_init(). Typical tasks performed by these routines include allocation or deallocation of static resources, initialization of global variables, setting up a default sqlite3_vfs module, or setting up a default configuration using sqlite3_config().

The application should never invoke either sqlite3_os_init() or sqlite3_os_end() directly. The application should only invoke sqlite3_initialize() and sqlite3_shutdown(). The sqlite3_os_init() interface is called automatically by sqlite3_initialize() and sqlite3_os_end() is called by sqlite3_shutdown(). Appropriate implementations for sqlite3_os_init() and sqlite3_os_end() are built into SQLite when it is compiled for Unix, Windows, or OS/2. When built for other platforms (using the SQLITE_OS_OTHER=1 compile-time option) the application must supply a suitable implementation for sqlite3_os_init() and sqlite3_os_end(). An application-supplied implementation of sqlite3_os_init() or sqlite3_os_end() must return SQLITE_OK on success and some other error code upon failure.

## Run-Time Library Version Numbers

>     SQLITE_EXTERN const char sqlite3_version[];
>     const char *sqlite3_libversion(void);
>     const char *sqlite3_sourceid(void);
>     int sqlite3_libversion_number(void);

These interfaces provide the same information as the SQLITE_VERSION, SQLITE_VERSION_NUMBER, and SQLITE_SOURCE_ID C preprocessor macros but are associated with the library instead of the header file. Cautious programmers might include assert() statements in their application to verify that values returned by these interfaces match the macros in the header, and thus insure that the application is compiled with matching library and header files.

>     assert( sqlite3_libversion_number()==SQLITE_VERSION_NUMBER );
>     assert( strcmp(sqlite3_sourceid(),SQLITE_SOURCE_ID)==0 );
>     assert( strcmp(sqlite3_libversion(),SQLITE_VERSION)==0 );

The sqlite3_version[] string constant contains the text of SQLITE_VERSION macro. The sqlite3_libversion() function returns a pointer to the to the sqlite3_version[] string constant. The sqlite3_libversion() function is provided for use in DLLs since DLL users usually do not have direct access to string constants within the DLL. The sqlite3_libversion_number() function returns an integer equal to SQLITE_VERSION_NUMBER. The sqlite3_sourceid() function returns a pointer to a string constant whose value is the same as the SQLITE_SOURCE_ID C preprocessor macro.

See also: sqlite_version() and sqlite_source_id().

## Memory Allocator Statistics

>     sqlite3_int64 sqlite3_memory_used(void);
>     sqlite3_int64 sqlite3_memory_highwater(int resetFlag);

SQLite provides these two interfaces for reporting on the status of the sqlite3_malloc(), sqlite3_free(), and sqlite3_realloc() routines, which form the built-in memory allocation subsystem.

The sqlite3_memory_used() routine returns the number of bytes of memory currently outstanding (malloced but not freed). The sqlite3_memory_highwater() routine returns the maximum value of sqlite3_memory_used() since the high-water mark was last reset. The values returned by sqlite3_memory_used() and sqlite3_memory_highwater() include any overhead added by SQLite in its implementation of sqlite3_malloc(), but not overhead added by the any underlying system library routines that sqlite3_malloc() may call.

The memory high-water mark is reset to the current value of sqlite3_memory_used() if and only if the parameter to sqlite3_memory_highwater() is true. The value returned by sqlite3_memory_highwater(1) is the high-water mark prior to the reset.

## Formatted String Printing Functions

>     char *sqlite3_mprintf(const char*,...);
>     char *sqlite3_vmprintf(const char*, va_list);
>     char *sqlite3_snprintf(int,char*,const char*, ...);

These routines are work-alikes of the "printf()" family of functions from the standard C library.

The sqlite3_mprintf() and sqlite3_vmprintf() routines write their results into memory obtained from sqlite3_malloc(). The strings returned by these two routines should be released by sqlite3_free(). Both routines return a NULL pointer if sqlite3_malloc() is unable to allocate enough memory to hold the resulting string.

In sqlite3_snprintf() routine is similar to "snprintf()" from the standard C library. The result is written into the buffer supplied as the second parameter whose size is given by the first parameter. Note that the order of the first two parameters is reversed from snprintf(). This is an historical accident that cannot be fixed without breaking backwards compatibility. Note also that sqlite3_snprintf() returns a pointer to its buffer instead of the number of characters actually written into the buffer. We admit that the number of characters written would be a more useful return value but we cannot change the implementation of sqlite3_snprintf() now without breaking compatibility.

As long as the buffer size is greater than zero, sqlite3_snprintf() guarantees that the buffer is always zero-terminated. The first parameter "n" is the total size of the buffer, including space for the zero terminator. So the longest string that can be completely written will be n-1 characters.

These routines all implement some additional formatting options that are useful for constructing SQL statements. All of the usual printf() formatting options apply. In addition, there is are "%q", "%Q", and "%z" options.

The %q option works like %s in that it substitutes a null-terminated string from the argument list. But %q also doubles every '\'' character. %q is designed for use inside a string literal. By doubling each '\'' character it escapes that character and allows it to be inserted into the string.

For example, assume the string variable zText contains text as follows:

>     char *zText = "It's a happy day!";

One can use this text in an SQL statement as follows:

>     char *zSQL = sqlite3_mprintf("INSERT INTO table VALUES('%q')", zText);
>     sqlite3_exec(db, zSQL, 0, 0, 0);
>     sqlite3_free(zSQL);

Because the %q format string is used, the '\'' character in zText is escaped and the SQL generated is as follows:

>     INSERT INTO table1 VALUES('It''s a happy day!')

This is correct. Had we used %s instead of %q, the generated SQL would have looked like this:

>     INSERT INTO table1 VALUES('It's a happy day!');

This second example is an SQL syntax error. As a general rule you should always use %q instead of %s when inserting text into a string literal.

The %Q option works like %q except it also adds single quotes around the outside of the total string. Additionally, if the parameter in the argument list is a NULL pointer, %Q substitutes the text "NULL" (without single quotes). So, for example, one could say:

>     char *zSQL = sqlite3_mprintf("INSERT INTO table VALUES(%Q)", zText);
>     sqlite3_exec(db, zSQL, 0, 0, 0);
>     sqlite3_free(zSQL);

The code above will render a correct SQL statement in the zSQL variable even if the zText variable is a NULL pointer.

The "%z" formatting option works like "%s" but with the addition that after the string has been read and copied into the result, sqlite3_free() is called on the input string.

## Mutexes

>     sqlite3_mutex *sqlite3_mutex_alloc(int);
>     void sqlite3_mutex_free(sqlite3_mutex*);
>     void sqlite3_mutex_enter(sqlite3_mutex*);
>     int sqlite3_mutex_try(sqlite3_mutex*);
>     void sqlite3_mutex_leave(sqlite3_mutex*);

The SQLite core uses these routines for thread synchronization. Though they are intended for internal use by SQLite, code that links against SQLite is permitted to use any of these routines.

The SQLite source code contains multiple implementations of these mutex routines. An appropriate implementation is selected automatically at compile-time. The following implementations are available in the SQLite core:

- SQLITE_MUTEX_OS2
- SQLITE_MUTEX_PTHREAD
- SQLITE_MUTEX_W32
- SQLITE_MUTEX_NOOP

The SQLITE_MUTEX_NOOP implementation is a set of routines that does no real locking and is appropriate for use in a single-threaded application. The SQLITE_MUTEX_OS2, SQLITE_MUTEX_PTHREAD, and SQLITE_MUTEX_W32 implementations are appropriate for use on OS/2, Unix, and Windows.

If SQLite is compiled with the SQLITE_MUTEX_APPDEF preprocessor macro defined (with "-DSQLITE_MUTEX_APPDEF=1"), then no mutex implementation is included with the library. In this case the application must supply a custom mutex implementation using the SQLITE_CONFIG_MUTEX option of the sqlite3_config() function before calling sqlite3_initialize() or any other public sqlite3_ function that calls sqlite3_initialize().

The sqlite3_mutex_alloc() routine allocates a new mutex and returns a pointer to it. If it returns NULL that means that a mutex could not be allocated. SQLite will unwind its stack and return an error. The argument to sqlite3_mutex_alloc() is one of these integer constants:

- SQLITE_MUTEX_FAST
- SQLITE_MUTEX_RECURSIVE
- SQLITE_MUTEX_STATIC_MASTER
- SQLITE_MUTEX_STATIC_MEM
- SQLITE_MUTEX_STATIC_MEM2
- SQLITE_MUTEX_STATIC_PRNG
- SQLITE_MUTEX_STATIC_LRU
- SQLITE_MUTEX_STATIC_LRU2

The first two constants (SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) cause sqlite3_mutex_alloc() to create a new mutex. The new mutex is recursive when SQLITE_MUTEX_RECURSIVE is used but not necessarily so when SQLITE_MUTEX_FAST is used. The mutex implementation does not need to make a distinction between SQLITE_MUTEX_RECURSIVE and SQLITE_MUTEX_FAST if it does not want to. SQLite will only request a recursive mutex in cases where it really needs one. If a faster non-recursive mutex implementation is available on the host platform, the mutex subsystem might return such a mutex in response to SQLITE_MUTEX_FAST.

The other allowed parameters to sqlite3_mutex_alloc() (anything other than SQLITE_MUTEX_FAST and SQLITE_MUTEX_RECURSIVE) each return a pointer to a static preexisting mutex. Six static mutexes are used by the current version of SQLite. Future versions of SQLite may add additional static mutexes. Static mutexes are for internal use by SQLite only. Applications that use SQLite mutexes should use only the dynamic mutexes returned by SQLITE_MUTEX_FAST or SQLITE_MUTEX_RECURSIVE.

Note that if one of the dynamic mutex parameters (SQLITE_MUTEX_FAST or SQLITE_MUTEX_RECURSIVE) is used then sqlite3_mutex_alloc() returns a different mutex on every call. But for the static mutex types, the same mutex is returned on every call that has the same type number.

The sqlite3_mutex_free() routine deallocates a previously allocated dynamic mutex. SQLite is careful to deallocate every dynamic mutex that it allocates. The dynamic mutexes must not be in use when they are deallocated. Attempting to deallocate a static mutex results in undefined behavior. SQLite never deallocates a static mutex.

The sqlite3_mutex_enter() and sqlite3_mutex_try() routines attempt to enter a mutex. If another thread is already within the mutex, sqlite3_mutex_enter() will block and sqlite3_mutex_try() will return SQLITE_BUSY. The sqlite3_mutex_try() interface returns SQLITE_OK upon successful entry. Mutexes created using SQLITE_MUTEX_RECURSIVE can be entered multiple times by the same thread. In such cases the, mutex must be exited an equal number of times before another thread can enter. If the same thread tries to enter any other kind of mutex more than once, the behavior is undefined. SQLite will never exhibit such behavior in its own use of mutexes.

Some systems (for example, Windows 95) do not support the operation implemented by sqlite3_mutex_try(). On those systems, sqlite3_mutex_try() will always return SQLITE_BUSY. The SQLite core only ever uses sqlite3_mutex_try() as an optimization so this is acceptable behavior.

The sqlite3_mutex_leave() routine exits a mutex that was previously entered by the same thread. The behavior is undefined if the mutex is not currently entered by the calling thread or is not currently allocated. SQLite will never do either.

If the argument to sqlite3_mutex_enter(), sqlite3_mutex_try(), or sqlite3_mutex_leave() is a NULL pointer, then all three routines behave as no-ops.

See also: sqlite3_mutex_held() and sqlite3_mutex_notheld().

## Mutex Verification Routines

>     #ifndef NDEBUG
>     int sqlite3_mutex_held(sqlite3_mutex*);
>     int sqlite3_mutex_notheld(sqlite3_mutex*);
>     #endif

The sqlite3_mutex_held() and sqlite3_mutex_notheld() routines are intended for use inside assert() statements. The SQLite core never uses these routines except inside an assert() and applications are advised to follow the lead of the core. The SQLite core only provides implementations for these routines when it is compiled with the SQLITE_DEBUG flag. External mutex implementations are only required to provide these routines if SQLITE_DEBUG is defined and if NDEBUG is not defined.

These routines should return true if the mutex in their argument is held or not held, respectively, by the calling thread.

The implementation is not required to provided versions of these routines that actually work. If the implementation does not provide working versions of these routines, it should at least provide stubs that always return true so that one does not get spurious assertion failures.

If the argument to sqlite3_mutex_held() is a NULL pointer then the routine should return 1. This seems counter-intuitive since clearly the mutex cannot be held if it does not exist. But the the reason the mutex does not exist is because the build is not using mutexes. And we do not want the assert() containing the call to sqlite3_mutex_held() to fail, so a non-zero return is the appropriate thing to do. The sqlite3_mutex_notheld() interface should also return 1 when given a NULL pointer.

## Opening A New Database Connection

>     int sqlite3_open(
>       const char *filename,   /* Database filename (UTF-8) */
>       sqlite3 **ppDb          /* OUT: SQLite db handle */
>     );
>     int sqlite3_open16(
>       const void *filename,   /* Database filename (UTF-16) */
>       sqlite3 **ppDb          /* OUT: SQLite db handle */
>     );
>     int sqlite3_open_v2(
>       const char *filename,   /* Database filename (UTF-8) */
>       sqlite3 **ppDb,         /* OUT: SQLite db handle */
>       int flags,              /* Flags */
>       const char *zVfs        /* Name of VFS module to use */
>     );

These routines open an SQLite database file whose name is given by the filename argument. The filename argument is interpreted as UTF-8 for sqlite3_open() and sqlite3_open_v2() and as UTF-16 in the native byte order for sqlite3_open16(). A database connection handle is usually returned in *ppDb, even if an error occurs. The only exception is that if SQLite is unable to allocate memory to hold the sqlite3 object, a NULL will be written into *ppDb instead of a pointer to the sqlite3 object. If the database is opened (and/or created) successfully, then SQLITE_OK is returned. Otherwise an error code is returned. The sqlite3_errmsg() or sqlite3_errmsg16() routines can be used to obtain an English language description of the error following a failure of any of the sqlite3_open() routines.

The default encoding for the database will be UTF-8 if sqlite3_open() or sqlite3_open_v2() is called and UTF-16 in the native byte order if sqlite3_open16() is used.

Whether or not an error occurs when it is opened, resources associated with the database connection handle should be released by passing it to sqlite3_close() when it is no longer required.

The sqlite3_open_v2() interface works like sqlite3_open() except that it accepts two additional parameters for additional control over the new database connection. The flags parameter to sqlite3_open_v2() can take one of the following three values, optionally combined with the SQLITE_OPEN_NOMUTEX, SQLITE_OPEN_FULLMUTEX, SQLITE_OPEN_SHAREDCACHE, and/or SQLITE_OPEN_PRIVATECACHE flags:

SQLITE_OPEN_READONLY
  The database is opened in read-only mode. If the database does not already exist, an error is returned.

SQLITE_OPEN_READWRITE
  The database is opened for reading and writing if possible, or reading only if the file is write protected by the operating system. In either case the database must already exist, otherwise an error is returned.

SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE
  The database is opened for reading and writing, and is creates it if it does not already exist. This is the behavior that is always used for sqlite3_open() and sqlite3_open16().

If the 3rd parameter to sqlite3_open_v2() is not one of the combinations shown above or one of the combinations shown above combined with the SQLITE_OPEN_NOMUTEX, SQLITE_OPEN_FULLMUTEX, SQLITE_OPEN_SHAREDCACHE and/or SQLITE_OPEN_SHAREDCACHE flags, then the behavior is undefined.

If the SQLITE_OPEN_NOMUTEX flag is set, then the database connection opens in the multi-thread threading mode as long as the single-thread mode has not been set at compile-time or start-time. If the SQLITE_OPEN_FULLMUTEX flag is set then the database connection opens in the serialized threading mode unless single-thread was previously selected at compile-time or start-time. The SQLITE_OPEN_SHAREDCACHE flag causes the database connection to be eligible to use shared cache mode, regardless of whether or not shared cache is enabled using sqlite3_enable_shared_cache(). The SQLITE_OPEN_PRIVATECACHE flag causes the database connection to not participate in shared cache mode even if it is enabled.

If the filename is ":memory:", then a private, temporary in-memory database is created for the connection. This in-memory database will vanish when the database connection is closed. Future versions of SQLite might make use of additional special filenames that begin with the ":" character. It is recommended that when a database filename actually does begin with a ":" character you should prefix the filename with a pathname such as "./" to avoid ambiguity.

If the filename is an empty string, then a private, temporary on-disk database will be created. This private database will be automatically deleted as soon as the database connection is closed.

The fourth parameter to sqlite3_open_v2() is the name of the sqlite3_vfs object that defines the operating system interface that the new database connection should use. If the fourth parameter is a NULL pointer then the default sqlite3_vfs object is used.

Note to Windows users: The encoding used for the filename argument of sqlite3_open() and sqlite3_open_v2() must be UTF-8, not whatever codepage is currently defined. Filenames containing international characters must be converted to UTF-8 prior to passing them into sqlite3_open() or sqlite3_open_v2().

## Tracing And Profiling Functions

>     void *sqlite3_trace(sqlite3*, void(*xTrace)(void*,const char*), void*);
>     void *sqlite3_profile(sqlite3*,
>        void(*xProfile)(void*,const char*,sqlite3_uint64), void*);

These routines register callback functions that can be used for tracing and profiling the execution of SQL statements.

The callback function registered by sqlite3_trace() is invoked at various times when an SQL statement is being run by sqlite3_step(). The sqlite3_trace() callback is invoked with a UTF-8 rendering of the SQL statement text as the statement first begins executing. Additional sqlite3_trace() callbacks might occur as each triggered subprogram is entered. The callbacks for triggers contain a UTF-8 SQL comment that identifies the trigger.

The callback function registered by sqlite3_profile() is invoked as each SQL statement finishes. The profile callback contains the original statement text and an estimate of wall-clock time of how long that statement took to run.

## Setting The Result Of An SQL Function

>     void sqlite3_result_blob(sqlite3_context*, const void*, int, void(*)(void*));
>     void sqlite3_result_double(sqlite3_context*, double);
>     void sqlite3_result_error(sqlite3_context*, const char*, int);
>     void sqlite3_result_error16(sqlite3_context*, const void*, int);
>     void sqlite3_result_error_toobig(sqlite3_context*);
>     void sqlite3_result_error_nomem(sqlite3_context*);
>     void sqlite3_result_error_code(sqlite3_context*, int);
>     void sqlite3_result_int(sqlite3_context*, int);
>     void sqlite3_result_int64(sqlite3_context*, sqlite3_int64);
>     void sqlite3_result_null(sqlite3_context*);
>     void sqlite3_result_text(sqlite3_context*, const char*, int, void(*)(void*));
>     void sqlite3_result_text16(sqlite3_context*, const void*, int, void(*)(void*));
>     void sqlite3_result_text16le(sqlite3_context*, const void*, int,void(*)(void*));
>     void sqlite3_result_text16be(sqlite3_context*, const void*, int,void(*)(void*));
>     void sqlite3_result_value(sqlite3_context*, sqlite3_value*);
>     void sqlite3_result_zeroblob(sqlite3_context*, int n);

These routines are used by the xFunc or xFinal callbacks that implement SQL functions and aggregates. See sqlite3_create_function() and sqlite3_create_function16() for additional information.

These functions work very much like the parameter binding family of functions used to bind values to host parameters in prepared statements. Refer to the SQL parameter documentation for additional information.

The sqlite3_result_blob() interface sets the result from an application-defined function to be the BLOB whose content is pointed to by the second parameter and which is N bytes long where N is the third parameter.

The sqlite3_result_zeroblob() interfaces set the result of the application-defined function to be a BLOB containing all zero bytes and N bytes in size, where N is the value of the 2nd parameter.

The sqlite3_result_double() interface sets the result from an application-defined function to be a floating point value specified by its 2nd argument.

The sqlite3_result_error() and sqlite3_result_error16() functions cause the implemented SQL function to throw an exception. SQLite uses the string pointed to by the 2nd parameter of sqlite3_result_error() or sqlite3_result_error16() as the text of an error message. SQLite interprets the error message string from sqlite3_result_error() as UTF-8. SQLite interprets the string from sqlite3_result_error16() as UTF-16 in native byte order. If the third parameter to sqlite3_result_error() or sqlite3_result_error16() is negative then SQLite takes as the error message all text up through the first zero character. If the third parameter to sqlite3_result_error() or sqlite3_result_error16() is non-negative then SQLite takes that many bytes (not characters) from the 2nd parameter as the error message. The sqlite3_result_error() and sqlite3_result_error16() routines make a private copy of the error message text before they return. Hence, the calling function can deallocate or modify the text after they return without harm. The sqlite3_result_error_code() function changes the error code returned by SQLite as a result of an error in a function. By default, the error code is SQLITE_ERROR. A subsequent call to sqlite3_result_error() or sqlite3_result_error16() resets the error code to SQLITE_ERROR.

The sqlite3_result_toobig() interface causes SQLite to throw an error indicating that a string or BLOB is too long to represent.

The sqlite3_result_nomem() interface causes SQLite to throw an error indicating that a memory allocation failed.

The sqlite3_result_int() interface sets the return value of the application-defined function to be the 32-bit signed integer value given in the 2nd argument. The sqlite3_result_int64() interface sets the return value of the application-defined function to be the 64-bit signed integer value given in the 2nd argument.

The sqlite3_result_null() interface sets the return value of the application-defined function to be NULL.

The sqlite3_result_text(), sqlite3_result_text16(), sqlite3_result_text16le(), and sqlite3_result_text16be() interfaces set the return value of the application-defined function to be a text string which is represented as UTF-8, UTF-16 native byte order, UTF-16 little endian, or UTF-16 big endian, respectively. SQLite takes the text result from the application from the 2nd parameter of the sqlite3_result_text* interfaces. If the 3rd parameter to the sqlite3_result_text* interfaces is negative, then SQLite takes result text from the 2nd parameter through the first zero character. If the 3rd parameter to the sqlite3_result_text* interfaces is non-negative, then as many bytes (not characters) of the text pointed to by the 2nd parameter are taken as the application-defined function result. If the 4th parameter to the sqlite3_result_text* interfaces or sqlite3_result_blob is a non-NULL pointer, then SQLite calls that function as the destructor on the text or BLOB result when it has finished using that result. If the 4th parameter to the sqlite3_result_text* interfaces or to sqlite3_result_blob is the special constant SQLITE_STATIC, then SQLite assumes that the text or BLOB result is in constant space and does not copy the content of the parameter nor call a destructor on the content when it has finished using that result. If the 4th parameter to the sqlite3_result_text* interfaces or sqlite3_result_blob is the special constant SQLITE_TRANSIENT then SQLite makes a copy of the result into space obtained from from sqlite3_malloc() before it returns.

The sqlite3_result_value() interface sets the result of the application-defined function to be a copy the unprotected sqlite3_value object specified by the 2nd parameter. The sqlite3_result_value() interface makes a copy of the sqlite3_value so that the sqlite3_value specified in the parameter may change or be deallocated after sqlite3_result_value() returns without harm. A protected sqlite3_value object may always be used where an unprotected sqlite3_value object is required, so either kind of sqlite3_value object can be used with this interface.

If these routines are called from within the different thread than the one containing the application-defined function that received the sqlite3_context pointer, the results are undefined.

## Obtaining SQL Function Parameter Values

>     const void *sqlite3_value_blob(sqlite3_value*);
>     int sqlite3_value_bytes(sqlite3_value*);
>     int sqlite3_value_bytes16(sqlite3_value*);
>     double sqlite3_value_double(sqlite3_value*);
>     int sqlite3_value_int(sqlite3_value*);
>     sqlite3_int64 sqlite3_value_int64(sqlite3_value*);
>     const unsigned char *sqlite3_value_text(sqlite3_value*);
>     const void *sqlite3_value_text16(sqlite3_value*);
>     const void *sqlite3_value_text16le(sqlite3_value*);
>     const void *sqlite3_value_text16be(sqlite3_value*);
>     int sqlite3_value_type(sqlite3_value*);
>     int sqlite3_value_numeric_type(sqlite3_value*);

The C-language implementation of SQL functions and aggregates uses this set of interface routines to access the parameter values on the function or aggregate.

The xFunc (for scalar functions) or xStep (for aggregates) parameters to sqlite3_create_function() and sqlite3_create_function16() define callbacks that implement the SQL functions and aggregates. The 4th parameter to these callbacks is an array of pointers to protected sqlite3_value objects. There is one sqlite3_value object for each parameter to the SQL function. These routines are used to extract values from the sqlite3_value objects.

These routines work only with protected sqlite3_value objects. Any attempt to use these routines on an unprotected sqlite3_value object results in undefined behavior.

These routines work just like the corresponding column access functions except that these routines take a single protected sqlite3_value object pointer instead of a sqlite3_stmt* pointer and an integer column number.

The sqlite3_value_text16() interface extracts a UTF-16 string in the native byte-order of the host machine. The sqlite3_value_text16be() and sqlite3_value_text16le() interfaces extract UTF-16 strings as big-endian and little-endian respectively.

The sqlite3_value_numeric_type() interface attempts to apply numeric affinity to the value. This means that an attempt is made to convert the value to an integer or floating point. If such a conversion is possible without loss of information (in other words, if the value is a string that looks like a number) then the conversion is performed. Otherwise no conversion occurs. The datatype after conversion is returned.

Please pay particular attention to the fact that the pointer returned from sqlite3_value_blob(), sqlite3_value_text(), or sqlite3_value_text16() can be invalidated by a subsequent call to sqlite3_value_bytes(), sqlite3_value_bytes16(), sqlite3_value_text(), or sqlite3_value_text16().

These routines must be called from the same thread as the SQL function that supplied the sqlite3_value* parameters.

## Virtual File System Objects

>     sqlite3_vfs *sqlite3_vfs_find(const char *zVfsName);
>     int sqlite3_vfs_register(sqlite3_vfs*, int makeDflt);
>     int sqlite3_vfs_unregister(sqlite3_vfs*);

A virtual filesystem (VFS) is an sqlite3_vfs object that SQLite uses to interact with the underlying operating system. Most SQLite builds come with a single default VFS that is appropriate for the host computer. New VFSes can be registered and existing VFSes can be unregistered. The following interfaces are provided.

The sqlite3_vfs_find() interface returns a pointer to a VFS given its name. Names are case sensitive. Names are zero-terminated UTF-8 strings. If there is no match, a NULL pointer is returned. If zVfsName is NULL then the default VFS is returned.

New VFSes are registered with sqlite3_vfs_register(). Each new VFS becomes the default VFS if the makeDflt flag is set. The same VFS can be registered multiple times without injury. To make an existing VFS into the default VFS, register it again with the makeDflt flag set. If two different VFSes with the same name are registered, the behavior is undefined. If a VFS is registered with a name that is NULL or an empty string, then the behavior is undefined.

Unregister a VFS with the sqlite3_vfs_unregister() interface. If the default VFS is unregistered, another VFS is chosen as the default. The choice for the new VFS is arbitrary.

## Binding Values To Prepared Statements

>     int sqlite3_bind_blob(sqlite3_stmt*, int, const void*, int n, void(*)(void*));
>     int sqlite3_bind_double(sqlite3_stmt*, int, double);
>     int sqlite3_bind_int(sqlite3_stmt*, int, int);
>     int sqlite3_bind_int64(sqlite3_stmt*, int, sqlite3_int64);
>     int sqlite3_bind_null(sqlite3_stmt*, int);
>     int sqlite3_bind_text(sqlite3_stmt*, int, const char*, int n, void(*)(void*));
>     int sqlite3_bind_text16(sqlite3_stmt*, int, const void*, int, void(*)(void*));
>     int sqlite3_bind_value(sqlite3_stmt*, int, const sqlite3_value*);
>     int sqlite3_bind_zeroblob(sqlite3_stmt*, int, int n);

In the SQL statement text input to sqlite3_prepare_v2() and its variants, literals may be replaced by a parameter that matches one of following templates:

- ?
- ?NNN
- :VVV
- @VVV
- $VVV

In the templates above, NNN represents an integer literal, and VVV represents an alphanumeric identifer. The values of these parameters (also called "host parameter names" or "SQL parameters") can be set using the sqlite3_bind_*() routines defined here.

The first argument to the sqlite3_bind_*() routines is always a pointer to the sqlite3_stmt object returned from sqlite3_prepare_v2() or its variants.

The second argument is the index of the SQL parameter to be set. The leftmost SQL parameter has an index of 1. When the same named SQL parameter is used more than once, second and subsequent occurrences have the same index as the first occurrence. The index for named parameters can be looked up using the sqlite3_bind_parameter_index() API if desired. The index for "?NNN" parameters is the value of NNN. The NNN value must be between 1 and the sqlite3_limit() parameter SQLITE_LIMIT_VARIABLE_NUMBER (default value: 999).

The third argument is the value to bind to the parameter.

In those routines that have a fourth argument, its value is the number of bytes in the parameter. To be clear: the value is the number of bytes in the value, not the number of characters. If the fourth parameter is negative, the length of the string is the number of bytes up to the first zero terminator.

The fifth argument to sqlite3_bind_blob(), sqlite3_bind_text(), and sqlite3_bind_text16() is a destructor used to dispose of the BLOB or string after SQLite has finished with it. If the fifth argument is the special value SQLITE_STATIC, then SQLite assumes that the information is in static, unmanaged space and does not need to be freed. If the fifth argument has the value SQLITE_TRANSIENT, then SQLite makes its own private copy of the data immediately, before the sqlite3_bind_*() routine returns.

The sqlite3_bind_zeroblob() routine binds a BLOB of length N that is filled with zeroes. A zeroblob uses a fixed amount of memory (just an integer to hold its size) while it is being processed. Zeroblobs are intended to serve as placeholders for BLOBs whose content is later written using incremental BLOB I/O routines. A negative value for the zeroblob results in a zero-length BLOB.

If any of the sqlite3_bind_*() routines are called with a NULL pointer for the prepared statement or with a prepared statement for which sqlite3_step() has been called more recently than sqlite3_reset(), then the call will return SQLITE_MISUSE. If any sqlite3_bind_() routine is passed a prepared statement that has been finalized, the result is undefined and probably harmful.

Bindings are not cleared by the sqlite3_reset() routine. Unbound parameters are interpreted as NULL.

The sqlite3_bind_* routines return SQLITE_OK on success or an error code if anything goes wrong. SQLITE_RANGE is returned if the parameter index is out of range. SQLITE_NOMEM is returned if malloc() fails.

See also: sqlite3_bind_parameter_count(), sqlite3_bind_parameter_name(), and sqlite3_bind_parameter_index().

## Compiling An SQL Statement

>     int sqlite3_prepare(
>       sqlite3 *db,            /* Database handle */
>       const char *zSql,       /* SQL statement, UTF-8 encoded */
>       int nByte,              /* Maximum length of zSql in bytes. */
>       sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
>       const char **pzTail     /* OUT: Pointer to unused portion of zSql */
>     );
>     int sqlite3_prepare_v2(
>       sqlite3 *db,            /* Database handle */
>       const char *zSql,       /* SQL statement, UTF-8 encoded */
>       int nByte,              /* Maximum length of zSql in bytes. */
>       sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
>       const char **pzTail     /* OUT: Pointer to unused portion of zSql */
>     );
>     int sqlite3_prepare16(
>       sqlite3 *db,            /* Database handle */
>       const void *zSql,       /* SQL statement, UTF-16 encoded */
>       int nByte,              /* Maximum length of zSql in bytes. */
>       sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
>       const void **pzTail     /* OUT: Pointer to unused portion of zSql */
>     );
>     int sqlite3_prepare16_v2(
>       sqlite3 *db,            /* Database handle */
>       const void *zSql,       /* SQL statement, UTF-16 encoded */
>       int nByte,              /* Maximum length of zSql in bytes. */
>       sqlite3_stmt **ppStmt,  /* OUT: Statement handle */
>       const void **pzTail     /* OUT: Pointer to unused portion of zSql */
>     );

To execute an SQL query, it must first be compiled into a byte-code program using one of these routines.

The first argument, "db", is a database connection obtained from a prior successful call to sqlite3_open(), sqlite3_open_v2() or sqlite3_open16(). The database connection must not have been closed.

The second argument, "zSql", is the statement to be compiled, encoded as either UTF-8 or UTF-16. The sqlite3_prepare() and sqlite3_prepare_v2() interfaces use UTF-8, and sqlite3_prepare16() and sqlite3_prepare16_v2() use UTF-16.

If the nByte argument is less than zero, then zSql is read up to the first zero terminator. If nByte is non-negative, then it is the maximum number of bytes read from zSql. When nByte is non-negative, the zSql string ends at either the first '\000' or '\u0000' character or the nByte-th byte, whichever comes first. If the caller knows that the supplied string is nul-terminated, then there is a small performance advantage to be gained by passing an nByte parameter that is equal to the number of bytes in the input string including the nul-terminator bytes.

If pzTail is not NULL then *pzTail is made to point to the first byte past the end of the first SQL statement in zSql. These routines only compile the first statement in zSql, so *pzTail is left pointing to what remains uncompiled.

*ppStmt is left pointing to a compiled prepared statement that can be executed using sqlite3_step(). If there is an error, *ppStmt is set to NULL. If the input text contains no SQL (if the input is an empty string or a comment) then *ppStmt is set to NULL. The calling procedure is responsible for deleting the compiled SQL statement using sqlite3_finalize() after it has finished with it. ppStmt may not be NULL.

On success, the sqlite3_prepare() family of routines return SQLITE_OK; otherwise an error code is returned.

The sqlite3_prepare_v2() and sqlite3_prepare16_v2() interfaces are recommended for all new programs. The two older interfaces are retained for backwards compatibility, but their use is discouraged. In the "v2" interfaces, the prepared statement that is returned (the sqlite3_stmt object) contains a copy of the original SQL text. This causes the sqlite3_step() interface to behave differently in three ways:

1. If the database schema changes, instead of returning SQLITE_SCHEMA as it always used to do, sqlite3_step() will automatically recompile the SQL statement and try to run it again. If the schema has changed in a way that makes the statement no longer valid, sqlite3_step() will still return SQLITE_SCHEMA. But unlike the legacy behavior, SQLITE_SCHEMA is now a fatal error. Calling sqlite3_prepare_v2() again will not make the error go away. Note: use sqlite3_errmsg() to find the text of the parsing error that results in an SQLITE_SCHEMA return.

2. When an error occurs, sqlite3_step() will return one of the detailed error codes or extended error codes. The legacy behavior was that sqlite3_step() would only return a generic SQLITE_ERROR result code and the application would have to make a second call to sqlite3_reset() in order to find the underlying cause of the problem. With the "v2" prepare interfaces, the underlying reason for the error is returned immediately.

3. If the value of a host parameter in the WHERE clause might change the query plan for a statement, then the statement may be automatically recompiled (as if there had been a schema change) on the first sqlite3_step() call following any change to the bindings of the parameter.

## Create Or Redefine SQL Functions

>     int sqlite3_create_function(
>       sqlite3 *db,
>       const char *zFunctionName,
>       int nArg,
>       int eTextRep,
>       void *pApp,
>       void (*xFunc)(sqlite3_context*,int,sqlite3_value**),
>       void (*xStep)(sqlite3_context*,int,sqlite3_value**),
>       void (*xFinal)(sqlite3_context*)
>     );
>     int sqlite3_create_function16(
>       sqlite3 *db,
>       const void *zFunctionName,
>       int nArg,
>       int eTextRep,
>       void *pApp,
>       void (*xFunc)(sqlite3_context*,int,sqlite3_value**),
>       void (*xStep)(sqlite3_context*,int,sqlite3_value**),
>       void (*xFinal)(sqlite3_context*)
>     );

These two functions (collectively known as "function creation routines") are used to add SQL functions or aggregates or to redefine the behavior of existing SQL functions or aggregates. The only difference between the two is that the second parameter, the name of the (scalar) function or aggregate, is encoded in UTF-8 for sqlite3_create_function() and UTF-16 for sqlite3_create_function16().

The first parameter is the database connection to which the SQL function is to be added. If an application uses more than one database connection then application-defined SQL functions must be added to each database connection separately.

The second parameter is the name of the SQL function to be created or redefined. The length of the name is limited to 255 bytes, exclusive of the zero-terminator. Note that the name length limit is in bytes, not characters. Any attempt to create a function with a longer name will result in SQLITE_ERROR being returned.

The third parameter (nArg) is the number of arguments that the SQL function or aggregate takes. If this parameter is -1, then the SQL function or aggregate may take any number of arguments between 0 and the limit set by sqlite3_limit(SQLITE_LIMIT_FUNCTION_ARG). If the third parameter is less than -1 or greater than 127 then the behavior is undefined.

The fourth parameter, eTextRep, specifies what text encoding this SQL function prefers for its parameters. Any SQL function implementation should be able to work work with UTF-8, UTF-16le, or UTF-16be. But some implementations may be more efficient with one encoding than another. An application may invoke sqlite3_create_function() or sqlite3_create_function16() multiple times with the same function but with different values of eTextRep. When multiple implementations of the same function are available, SQLite will pick the one that involves the least amount of data conversion. If there is only a single implementation which does not care what text encoding is used, then the fourth argument should be SQLITE_ANY.

The fifth parameter is an arbitrary pointer. The implementation of the function can gain access to this pointer using sqlite3_user_data().

The seventh, eighth and ninth parameters, xFunc, xStep and xFinal, are pointers to C-language functions that implement the SQL function or aggregate. A scalar SQL function requires an implementation of the xFunc callback only; NULL pointers should be passed as the xStep and xFinal parameters. An aggregate SQL function requires an implementation of xStep and xFinal and NULL should be passed for xFunc. To delete an existing SQL function or aggregate, pass NULL for all three function callbacks.

It is permitted to register multiple implementations of the same functions with the same name but with either differing numbers of arguments or differing preferred text encodings. SQLite will use the implementation that most closely matches the way in which the SQL function is used. A function implementation with a non-negative nArg parameter is a better match than a function implementation with a negative nArg. A function where the preferred text encoding matches the database encoding is a better match than a function where the encoding is different. A function where the encoding difference is between UTF16le and UTF16be is a closer match than a function where the encoding difference is between UTF8 and UTF16.

Built-in functions may be overloaded by new application-defined functions. The first application-defined function with a given name overrides all built-in functions in the same database connection with the same name. Subsequent application-defined functions of the same name only override prior application-defined functions that are an exact match for the number of parameters and preferred encoding.

An application-defined function is permitted to call other SQLite interfaces. However, such calls must not close the database connection nor finalize or reset the prepared statement in which the function is running.

## Test For Auto-Commit Mode

>     int sqlite3_get_autocommit(sqlite3*);

The sqlite3_get_autocommit() interface returns non-zero or zero if the given database connection is or is not in autocommit mode, respectively. Autocommit mode is on by default. Autocommit mode is disabled by a BEGIN statement. Autocommit mode is re-enabled by a COMMIT or ROLLBACK.

If certain kinds of errors occur on a statement within a multi-statement transaction (errors including SQLITE_FULL, SQLITE_IOERR, SQLITE_NOMEM, SQLITE_BUSY, and SQLITE_INTERRUPT) then the transaction might be rolled back automatically. The only way to find out whether SQLite automatically rolled back the transaction after an error is to use this function.

If another thread changes the autocommit status of the database connection while this routine is running, then the return value is undefined.

## Result Values From A Query

>     const void *sqlite3_column_blob(sqlite3_stmt*, int iCol);
>     int sqlite3_column_bytes(sqlite3_stmt*, int iCol);
>     int sqlite3_column_bytes16(sqlite3_stmt*, int iCol);
>     double sqlite3_column_double(sqlite3_stmt*, int iCol);
>     int sqlite3_column_int(sqlite3_stmt*, int iCol);
>     sqlite3_int64 sqlite3_column_int64(sqlite3_stmt*, int iCol);
>     const unsigned char *sqlite3_column_text(sqlite3_stmt*, int iCol);
>     const void *sqlite3_column_text16(sqlite3_stmt*, int iCol);
>     int sqlite3_column_type(sqlite3_stmt*, int iCol);
>     sqlite3_value *sqlite3_column_value(sqlite3_stmt*, int iCol);

These routines form the "result set" interface.

These routines return information about a single column of the current result row of a query. In every case the first argument is a pointer to the prepared statement that is being evaluated (the sqlite3_stmt* that was returned from sqlite3_prepare_v2() or one of its variants) and the second argument is the index of the column for which information should be returned. The leftmost column of the result set has the index 0. The number of columns in the result can be determined using sqlite3_column_count().

If the SQL statement does not currently point to a valid row, or if the column index is out of range, the result is undefined. These routines may only be called when the most recent call to sqlite3_step() has returned SQLITE_ROW and neither sqlite3_reset() nor sqlite3_finalize() have been called subsequently. If any of these routines are called after sqlite3_reset() or sqlite3_finalize() or after sqlite3_step() has returned something other than SQLITE_ROW, the results are undefined. If sqlite3_step() or sqlite3_reset() or sqlite3_finalize() are called from a different thread while any of these routines are pending, then the results are undefined.

The sqlite3_column_type() routine returns the datatype code for the initial data type of the result column. The returned value is one of SQLITE_INTEGER, SQLITE_FLOAT, SQLITE_TEXT, SQLITE_BLOB, or SQLITE_NULL. The value returned by sqlite3_column_type() is only meaningful if no type conversions have occurred as described below. After a type conversion, the value returned by sqlite3_column_type() is undefined. Future versions of SQLite may change the behavior of sqlite3_column_type() following a type conversion.

If the result is a BLOB or UTF-8 string then the sqlite3_column_bytes() routine returns the number of bytes in that BLOB or string. If the result is a UTF-16 string, then sqlite3_column_bytes() converts the string to UTF-8 and then returns the number of bytes. If the result is a numeric value then sqlite3_column_bytes() uses sqlite3_snprintf() to convert that value to a UTF-8 string and returns the number of bytes in that string. The value returned does not include the zero terminator at the end of the string. For clarity: the value returned is the number of bytes in the string, not the number of characters.

Strings returned by sqlite3_column_text() and sqlite3_column_text16(), even empty strings, are always zero terminated. The return value from sqlite3_column_blob() for a zero-length BLOB is an arbitrary pointer, possibly even a NULL pointer.

The sqlite3_column_bytes16() routine is similar to sqlite3_column_bytes() but leaves the result in UTF-16 in native byte order instead of UTF-8. The zero terminator is not included in this count.

The object returned by sqlite3_column_value() is an unprotected sqlite3_value object. An unprotected sqlite3_value object may only be used with sqlite3_bind_value() and sqlite3_result_value(). If the unprotected sqlite3_value object returned by sqlite3_column_value() is used in any other way, including calls to routines like sqlite3_value_int(), sqlite3_value_text(), or sqlite3_value_bytes(), then the behavior is undefined.

These routines attempt to convert the value where appropriate. For example, if the internal representation is FLOAT and a text result is requested, sqlite3_snprintf() is used internally to perform the conversion automatically. The following table details the conversions that are applied:

> Internal Type | Requested Type | Conversion
> --- | --- | ---
> NULL | INTEGER | Result is 0
> NULL | FLOAT | Result is 0.0
> NULL | TEXT | Result is NULL pointer
> NULL | BLOB | Result is NULL pointer
> INTEGER | FLOAT | Convert from integer to float
> INTEGER | TEXT | ASCII rendering of the integer
> INTEGER | BLOB | Same as INTEGER->TEXT
> FLOAT | INTEGER | Convert from float to integer
> FLOAT | TEXT | ASCII rendering of the float
> FLOAT | BLOB | Same as FLOAT->TEXT
> TEXT | INTEGER | Use atoi()
> TEXT | FLOAT | Use atof()
> TEXT | BLOB | No change
> BLOB | INTEGER | Convert to TEXT then use atoi()
> BLOB | FLOAT | Convert to TEXT then use atof()
> BLOB | TEXT | Add a zero terminator if needed

The table above makes reference to standard C library functions atoi() and atof(). SQLite does not really use these functions. It has its own equivalent internal routines. The atoi() and atof() names are used in the table for brevity and because they are familiar to most C programmers.

Note that when type conversions occur, pointers returned by prior calls to sqlite3_column_blob(), sqlite3_column_text(), and/or sqlite3_column_text16() may be invalidated. Type conversions and pointer invalidations might occur in the following cases:

- The initial content is a BLOB and sqlite3_column_text() or sqlite3_column_text16() is called. A zero-terminator might need to be added to the string.
- The initial content is UTF-8 text and sqlite3_column_bytes16() or sqlite3_column_text16() is called. The content must be converted to UTF-16.
- The initial content is UTF-16 text and sqlite3_column_bytes() or sqlite3_column_text() is called. The content must be converted to UTF-8.

Conversions between UTF-16be and UTF-16le are always done in place and do not invalidate a prior pointer, though of course the content of the buffer that the prior pointer points to will have been modified. Other kinds of conversion are done in place when it is possible, but sometimes they are not possible and in those cases prior pointers are invalidated.

The safest and easiest to remember policy is to invoke these routines in one of the following ways:

- sqlite3_column_text() followed by sqlite3_column_bytes()
- sqlite3_column_blob() followed by sqlite3_column_bytes()
- sqlite3_column_text16() followed by sqlite3_column_bytes16()

In other words, you should call sqlite3_column_text(), sqlite3_column_blob(), or sqlite3_column_text16() first to force the result into the desired format, then invoke sqlite3_column_bytes() or sqlite3_column_bytes16() to find the size of the result. Do not mix calls to sqlite3_column_text() or sqlite3_column_blob() with calls to sqlite3_column_bytes16(), and do not mix calls to sqlite3_column_text16() with calls to sqlite3_column_bytes().

The pointers returned are valid until a type conversion occurs as described above, or until sqlite3_step() or sqlite3_reset() or sqlite3_finalize() is called. The memory space used to hold strings and BLOBs is freed automatically. Do not pass the pointers returned sqlite3_column_blob(), sqlite3_column_text(), etc. into sqlite3_free().

If a memory allocation error occurs during the evaluation of any of these routines, a default value is returned. The default value is either the integer 0, the floating point number 0.0, or a NULL pointer. Subsequent calls to sqlite3_errcode() will return SQLITE_NOMEM.

## Enable Or Disable Shared Pager Cache

>     int sqlite3_enable_shared_cache(int);

This routine enables or disables the sharing of the database cache and schema data structures between connections to the same database. Sharing is enabled if the argument is true and disabled if the argument is false.

Cache sharing is enabled and disabled for an entire process. This is a change as of SQLite version 3.5.0. In prior versions of SQLite, sharing was enabled or disabled for each thread separately.

The cache sharing mode set by this interface effects all subsequent calls to sqlite3_open(), sqlite3_open_v2(), and sqlite3_open16(). Existing database connections continue use the sharing mode that was in effect at the time they were opened.

This routine returns SQLITE_OK if shared cache was enabled or disabled successfully. An error code is returned otherwise.

Shared cache is disabled by default. But this might change in future releases of SQLite. Applications that care about shared cache setting should set it explicitly.

See Also: SQLite Shared-Cache Mode