2011/6/30 Richard Hipp <d...@sqlite.org> > > > I am writing a desktop application in which I want to have exclusive > > rights. > > > In this way I do not need to check if the data has changed when the > user > > of > > > my program wants to change records. Is this possible? > > > > You can open a transaction as > > > > BEGIN EXCLUSIVE > > > > <http://www.sqlite.org/lang_transaction.html> > > > > "After a BEGIN EXCLUSIVE, no other database connection except for > > read_uncommitted connections will be able to read the database and no > other > > connection without exception will be able to write the database until the > > transaction is complete." > > > > This is how you do that sort of thing. So what you do is … > > > > 1) BEGIN EXCLUSIVE > > 2) Check the result code from step 1 and make sure you got the lock. > > 3) Do SELECTs to check the conditions for your data change and decide > what > > to do. > > 4) If the results of step suggest changes, make them. > > 5) END > > > > It is extremely common to see programs where step 3 of the above is done > > before step 1. Obviously, from your question, you understand this. > > > > Note to SQLite experts: it's not clear to me at what level the exclusive > > lock is maintained on the file. If an app crashes in while an EXCLUSIVE > > lock is held, is the lock released ? Does this vary by platform ? > > > > The BEGIN EXCLUSIVE...COMMIT technique shown above only gives you exclusive > access in rollback mode. If you select WAL mode, then BEGIN EXCLUSIVE > makes > you the exclusive writer, but other processes can continue to read. To get > exclusive access in WAL mode, use PRAGMA locking_mode=EXCLUSIVE. >
Is good enough for me. My only problem is that between reading the data and writing the changes, I want to be sure that no one has changed the data. For me that is enough. But it is good to know for when I want really exclusive access. Thanks. I am going to try it. -- Cecil Westerhof _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
