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POHow to properly call SQLite functions from background thread on iPhone?
 

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  1. POHow to properly call SQLite functions from background thread on iPhone?
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    <p>I'm using an SQLite database in my iPhone app. At startup, there are some database actions that I want to perform in a separate thread. (I'm doing this mainly to minimize startup time.)</p> <p>Occasionally/randomly, when these database calls are made from the background thread, the app will crash with these errors:</p> <pre><code>2009-04-13 17:36:09.932 Action Lists[1537:20b] *** Assertion failure in -[InboxRootViewController getInboxTasks], /Users/cperry/Dropbox/Projects/iPhone GTD/GTD/Classes/InboxRootViewController.m:74 2009-04-13 17:36:09.932 Action Lists[1537:3d0b] *** Assertion failure in +[Task deleteCompletedTasksInDatabase:completedMonthsAgo:], /Users/cperry/Dropbox/Projects/iPhone GTD/GTD/Classes/Data Classes/Task.m:957 2009-04-13 17:36:09.933 Action Lists[1537:20b] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Error: failed to prepare statement with message 'library routine called out of sequence'.' 2009-04-13 17:36:09.933 Action Lists[1537:3d0b] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Error: failed to prepare statement with message 'library routine called out of sequence'.' </code></pre> <p>Although I can't reliably reproduce the error, I've convinced myself that it is due to the fact that SQLite functions are being called in both active threads. How <em>should</em> I be calling SQLite functions from a separate thread? Is there a trick I'm missing? I'm pretty new to the iPhone, SQLite, and Objective-C, so it might be something that's obvious to you, but not so obviouse to me.</p> <p>Here are some code samples.</p> <p>MainApplication.m:</p> <pre><code>- (void)applicationDidFinishLaunching:(UIApplication *)application { // Take care of jobs that have to run at startup [NSThread detachNewThreadSelector:@selector(startUpJobs) toTarget:self withObject:nil]; } // Jobs that run in the background at startup - (void)startUpJobs { // Anticipating that this method will be called in its own NSThread, set up an autorelease pool. NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init]; // Get user preferences NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults]; // This Class Method calls SQLite functions and sometimes causes errors. [Task revertFutureTasksStatus:database]; [pool release]; } </code></pre> <p>Task.m:</p> <pre><code>static sqlite3_stmt *revert_future_statement = nil; + (void) revertFutureTasksStatus:(sqlite3 *)db { if (revert_future_statement == nil) { // Find all tasks that meet criteria static char *sql = "SELECT task_id FROM tasks where ((deleted IS NULL) OR (deleted=0)) AND (start_date &gt; ?) AND (status=0) AND (revert_status IS NOT NULL)"; if (sqlite3_prepare_v2(db, sql, -1, &amp;revert_future_statement, NULL) != SQLITE_OK) { NSAssert1(0, @"Error: failed to prepare update statement with message '%s'.", sqlite3_errmsg(db)); } } // Bind NOW to sql statement NSDate *now = [[NSDate alloc] init]; NSDateFormatter *formatter = [[NSDateFormatter alloc] init]; [formatter setDateFormat:@"yyyy-MM-dd"]; NSString *nowString = [formatter stringFromDate:now]; sqlite3_bind_text(revert_future_statement, 1, [nowString UTF8String], -1, SQLITE_TRANSIENT); [now release]; [formatter release]; // We "step" through the results - once for each row. while (sqlite3_step(revert_future_statement) == SQLITE_ROW) { // Do things to each returned row } // Reset the statement for future reuse. sqlite3_reset(revert_future_statement); } </code></pre>
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    1. COYou should really consider using queues - either through GCD dispatch functions or NSOperation. Creating a named queue creates simple synchronization of a class of work.
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