Question

My Project has a C++ dll in which a db is created in Sqllite with share deny = none. This dll will insert data into Db and I have a C# .exe which must access the db and display it. Display needs to be done asynchronously. Is there any way while launching connection to sqlitedb I can lock the table or db? If possible can anyone give me links or a sample on how it can be done? Thanks in advance.

Answer 1

Look into SQLite pragma-s. http://www.sqlite.org/pragma.html

PRAGMA locking_mode;
PRAGMA locking_mode = NORMAL | EXCLUSIVE

This pragma sets or queries the database connection locking-mode. The locking-mode is either NORMAL or EXCLUSIVE.

In NORMAL locking-mode (the default), a database connection unlocks the database file at the conclusion of each read or write transaction. When the locking-mode is set to EXCLUSIVE, the database connection never releases file-locks. The first time the database is read in EXCLUSIVE mode, a shared lock is obtained and held. The first time the database is written, an exclusive lock is obtained and held.

Database locks obtained by a connection in EXCLUSIVE mode may be released either by closing the database connection, or by setting the locking-mode back to NORMAL using this pragma and then accessing the database file (for read or write). Simply setting the locking-mode to NORMAL is not enough – locks are not be released until the next time the database file is accessed.

There are two reasons to set the locking-mode to EXCLUSIVE. One is if the application actually wants to prevent other processes from accessing the database file. The other is that a small number of filesystem operations are saved by optimizations enabled in this mode. This may be significant in embedded environments.

When the locking_mode pragma specifies a particular database, for example:

PRAGMA main.locking_mode=EXCLUSIVE;
Then the locking mode applies only to the named database. If no database name qualifier precedes the “locking_mode” keyword then the locking mode is applied to all databases, including any new databases added by subsequent ATTACH commands.

The “temp” database (in which TEMP tables and indices are stored) and in-memory databases always uses exclusive locking mode. The locking mode of temp and in-memory databases cannot be changed. All other databases use the normal locking mode by default and are affected by this pragma.

PRAGMA synchronous;
PRAGMA synchronous = 0 | OFF | 1 | NORMAL | 2 | FULL;

Query or change the setting of the “synchronous” flag. The first (query) form will return the synchronous setting as an integer. When synchronous is FULL (2), the SQLite database engine will pause at critical moments to make sure that data has actually been written to the disk surface before continuing. This ensures that if the operating system crashes or if there is a power failure, the database will be uncorrupted after rebooting. FULL synchronous is very safe, but it is also slower. When synchronous is NORMAL (1), the SQLite database engine will still pause at the most critical moments, but less often than in FULL mode. There is a very small (though non-zero) chance that a power failure at just the wrong time could corrupt the database in NORMAL mode. But in practice, you are more likely to suffer a catastrophic disk failure or some other unrecoverable hardware fault. With synchronous OFF (0), SQLite continues without pausing as soon as it has handed data off to the operating system. If the application running SQLite crashes, the data will be safe, but the database might become corrupted if the operating system crashes or the computer loses power before that data has been written to the disk surface. On the other hand, some operations are as much as 50 or more times faster with synchronous OFF.

The default setting is synchronous=FULL.