Standard File Control Opcodes

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## SQLite C Interface

## 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
>     #define SQLITE_FCNTL_CHUNK_SIZE              6
>     #define SQLITE_FCNTL_FILE_POINTER            7
>     #define SQLITE_FCNTL_SYNC_OMITTED            8
>     #define SQLITE_FCNTL_WIN32_AV_RETRY          9
>     #define SQLITE_FCNTL_PERSIST_WAL            10
>     #define SQLITE_FCNTL_OVERWRITE              11
>     #define SQLITE_FCNTL_VFSNAME                12
>     #define SQLITE_FCNTL_POWERSAFE_OVERWRITE    13
>     #define SQLITE_FCNTL_PRAGMA                 14

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.

- The SQLITE_FCNTL_CHUNK_SIZE opcode is used to request that the VFS extends and truncates the database file in chunks of a size specified by the user. The fourth argument to sqlite3_file_control() should point to an integer (type int) containing the new chunk-size to use for the nominated database. Allocating database file space in large chunks (say 1MB at a time), may reduce file-system fragmentation and improve performance on some systems.

- The SQLITE_FCNTL_FILE_POINTER opcode is used to obtain a pointer to the sqlite3_file object associated with a particular database connection. See the sqlite3_file_control() documentation for additional information.

- The SQLITE_FCNTL_SYNC_OMITTED opcode is generated internally by SQLite and sent to all VFSes in place of a call to the xSync method when the database connection has PRAGMA synchronous set to OFF. Some specialized VFSes need this signal in order to operate correctly when PRAGMA synchronous=OFF is set, but most VFSes do not need this signal and should silently ignore this opcode. Applications should not call sqlite3_file_control() with this opcode as doing so may disrupt the operation of the specialized VFSes that do require it.

- The SQLITE_FCNTL_WIN32_AV_RETRY opcode is used to configure automatic retry counts and intervals for certain disk I/O operations for the windows VFS in order to provide robustness in the presence of anti-virus programs. By default, the windows VFS will retry file read, file write, and file delete operations up to 10 times, with a delay of 25 milliseconds before the first retry and with the delay increasing by an additional 25 milliseconds with each subsequent retry. This opcode allows these two values (10 retries and 25 milliseconds of delay) to be adjusted. The values are changed for all database connections within the same process. The argument is a pointer to an array of two integers where the first integer i the new retry count and the second integer is the delay. If either integer is negative, then the setting is not changed but instead the prior value of that setting is written into the array entry, allowing the current retry settings to be interrogated. The zDbName parameter is ignored.

- The SQLITE_FCNTL_PERSIST_WAL opcode is used to set or query the persistent Write Ahead Log setting. By default, the auxiliary write ahead log and shared memory files used for transaction control are automatically deleted when the latest connection to the database closes. Setting persistent WAL mode causes those files to persist after close. Persisting the files is useful when other processes that do not have write permission on the directory containing the database file want to read the database file, as the WAL and shared memory files must exist in order for the database to be readable. The fourth parameter to sqlite3_file_control() for this opcode should be a pointer to an integer. That integer is 0 to disable persistent WAL mode or 1 to enable persistent WAL mode. If the integer is -1, then it is overwritten with the current WAL persistence setting.

- The SQLITE_FCNTL_POWERSAFE_OVERWRITE opcode is used to set or query the persistent "powersafe-overwrite" or "PSOW" setting. The PSOW setting determines the SQLITE_IOCAP_POWERSAFE_OVERWRITE bit of the xDeviceCharacteristics methods. The fourth parameter to sqlite3_file_control() for this opcode should be a pointer to an integer. That integer is 0 to disable zero-damage mode or 1 to enable zero-damage mode. If the integer is -1, then it is overwritten with the current zero-damage mode setting.

- The SQLITE_FCNTL_OVERWRITE opcode is invoked by SQLite after opening a write transaction to indicate that, unless it is rolled back for some reason, the entire database file will be overwritten by the current transaction. This is used by VACUUM operations.

- The SQLITE_FCNTL_VFSNAME opcode can be used to obtain the names of all VFSes in the VFS stack. The names are of all VFS shims and the final bottom-level VFS are written into memory obtained from sqlite3_malloc() and the result is stored in the char* variable that the fourth parameter of sqlite3_file_control() points to. The caller is responsible for freeing the memory when done. As with all file-control actions, there is no guarantee that this will actually do anything. Callers should initialize the char* variable to a NULL pointer in case this file-control is not implemented. This file-control is intended for diagnostic use only.

- Whenever a PRAGMA statement is parsed, an SQLITE_FCNTL_PRAGMA file control is sent to the open sqlite3_file object corresponding to the database file to which the pragma statement refers. The argument to the SQLITE_FCNTL_PRAGMA file control is an array of pointers to strings (char**) in which the second element of the array is the name of the pragma and the third element is the argument to the pragma or NULL if the pragma has no argument. The handler for an SQLITE_FCNTL_PRAGMA file control can optionally make the first element of the char** argument point to a string obtained from sqlite3_mprintf() or the equivalent and that string will become the result of the pragma or the error message if the pragma fails. If the SQLITE_FCNTL_PRAGMA file control returns SQLITE_NOTFOUND, then normal PRAGMA processing continues. If the SQLITE_FCNTL_PRAGMA file control returns SQLITE_OK, then the parser assumes that the VFS has handled the PRAGMA itself and the parser generates a no-op prepared statement. If the SQLITE_FCNTL_PRAGMA file control returns any result code other than SQLITE_OK or SQLITE_NOTFOUND, that means that the VFS encountered an error while handling the PRAGMA and the compilation of the PRAGMA fails with an error. The SQLITE_FCNTL_PRAGMA file control occurs at the beginning of pragma statement analysis and so it is able to override built-in PRAGMA statements.

See also lists of Objects, Constants, and Functions.