Question

I am currently developing on Objective-C using sqlite3. The following code seems like a bad memory access.

-(sqlite3_stmt *)PrepareStmt:(NSString *)query
{
    //...
    const char *query_stmt = [query UTF8String];
    sqlite3_stmt *stmt = nil;
    int retval = 0;

    if ((retval = sqlite3_prepare_v2(db, query_stmt, -1, &stmt, nil)) == SQLITE_OK)
    {
        return stmt;
    }
    else
    {
        //Error handling...
    }
}

- (void)SomeFunc
{
    NSString *query = @""; //Assume valid SQL statement
    sqlite3_stmt *stmt = [self PrepareStmt:query];

    //Use stmt, like step, etc.

    sqlite3_finalize(stmt);
}

The sqlite3_stmt in PrepareStmt is set to nil and it will be an out parameter from sqlite3_prepare_v2(). The memory should be allocated in that function. Therefore, it should be released by calling sqlite3_finalize().

My question here is that if we return sqlite3_stmt from PrepareStmt(), it should be still valid right? The local pointer in PrepareStmt() is already popped off the stack, but the memory allocated by sqlite3_prepare_v2() should still be valid.

Is this thinking valid? Or do I need to pass in an address of an pointer to PrepareStmt()?

Thank you!

Answer 1

Yes it’s valid in this case. But note that sqlite3_finalize() isn’t just about releasing memory (i.e. dealloc). It also sends a message to the DB to tell it to drop it’s precompiled SQL statement, etc…