sqlite error database is locked

Question

I am have a sqlite database in the iPhone application I am writing. I get an error with following code that I am running in a background thread. In the background thread, I call this method:

 - (BOOL) songIsInDatabase:(NSString *)songTitle
{
NSString *docsDir;
NSArray *dirPaths;

dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
docsDir = [dirPaths objectAtIndex:0];


//Build the path to the database file
NSString *databasePath = [[NSString alloc] initWithString:[docsDir stringByAppendingPathComponent:@"Database.db"]];

const char *dbpath = [databasePath UTF8String];

sqlite3_stmt *statement;

if(sqlite3_open(dbpath, &DB) == SQLITE_OK){

    NSString *insertSQL = [NSString stringWithFormat:@"select * from Bpm_Table where song_title = '%@'", songTitle];

    const char *insert_stmt = [insertSQL UTF8String];
    if(sqlite3_prepare_v2(DB, insert_stmt, -1, &statement, NULL) == SQLITE_OK)
    {
        while (sqlite3_step(statement) == SQLITE_ROW)
        {
            return YES;
            break;
        }
    }else{
        NSLog(@"the error is %s", sqlite3_errmsg(DB));
    }
    sqlite3_finalize(statement); 
}

[databasePath release];
return NO;
}

Then I call this method:

- (void) addSongToDatabase: (NSString *) songTitle andBPM: (int)bpm andGenre: (NSString *) genre
{
NSString *docsDir;
NSArray *dirPaths;

dirPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
docsDir = [dirPaths objectAtIndex:0];


//Build the path to the database file
NSString *databasePath = [[NSString alloc] initWithString:[docsDir stringByAppendingPathComponent:@"Database.db"]];

const char *dbpath = [databasePath UTF8String];

sqlite3_stmt *statement;

if(sqlite3_open(dbpath, &DB) == SQLITE_OK){

    NSString *insertSQL = [NSString stringWithFormat:@"insert or replace into Bpm_Table (song_title, bpm, genre) values (\"%@\", \"%d\", \"%@\")", songTitle, bpm, genre];

    const char *insert_stmt = [insertSQL UTF8String];
    if(sqlite3_prepare(DB, insert_stmt, -1, &statement, NULL) == SQLITE_OK){
        if(sqlite3_step(statement) == SQLITE_DONE)
        {

        } else {
            NSLog(@"error: %s", sqlite3_errmsg(DB));
        }
    }sqlite3_finalize(statement);
}

[databasePath release];
}

If I run both these methods, one right after the other, I get an error that says database is locked. I added the sqlite3_finalize statements after poking around google in hopes that would fix the issue. If I comment out either of the methods, I don't get this error.

Does anyone know what's wrong?

Answer 1

If you have tried sqlite3_close(DB) it's probably because your methods are trying to access the same database at the same time. Try to place this line of code

sqlite3_busy_timeout(DB, 500);

somewhere between sqlite3_open and sqlite3_prepare_v2 in the method from where you get the "database is locked"-error message.

The database-connection in that method will then try to write/read in 500 milliseconds before it gives up, which is usually enough time to escape the locking.

Answer 2

I had the same problem a while ago. I tried copying the database file itself, renamed it to different filename, then checked it - It actually WORKED!

I am using SQLite3.

I hope this will also work for you!

Answer 3

I just spent an hour with the same problem. The problem for me arose when i unwittingly killed a process by closing the shell (when the script was running a while loop and timer), while a db journal temp file was still opened. Windows does not kill the process even though you have killed it in Python, regardless if you have used db.close() or finalize in the script; if you exit all Python apps there will still be one running in the Task manager.

Hence you will need to end all Python processes in Windows Task Manager, restart them, and it should be fine(if that is actually the cause of the problem).

Answer 4

you must always close the sqlite database after using it ... so add this line sqlite3_close(DB); just after sqlite3_finalize(statement);

Update -

You are returning YES in one of your while loop -

        while (sqlite3_step(statement) == SQLITE_ROW)
        {
            return YES;
            break;
        }

so here you are nither finalizing nor closing the database.. you need to close if everytime you open it

Answer 5

For one thing, you're not closing the database connection with sqlite3_close() before you open a new connection

Answer 6

But note that if you access the same SQLite DB from multiple threads, without some sort of separate synchronization (some way to know that you'll never have near-simultaneous accesses from multiple threads) then you're likely to get "database locked" errors even if you close everything correctly on each access.

SQLite does not provide any sort of "lock wait" mechanism.

Your basic options are:

1. Do all accesses from one thread.
2. Use a separate lock protocol around all accesses.
3. If you get a "database locked" error wait a brief period of time and retry.