Question

I’m using SQLite3 in a Windows application. I have the source code (so-called SQLite amalgamation).

Sometimes I have to execute heavy queries. That is, I call sqlite3_step on a prepared statement, and it takes a lot of time to complete (due to the heavy I/O load).

I wonder if there’s a possibility to abort such a call. I would also be glad if there was an ability to do some background processing in the middle of the call within the same thread (since most of the time is spent in waiting for the I/O to complete).

I thought about modifying the SQLite code myself. In the simplest scenario I could check some condition (like an abort event handle for instance) before every invocation of either ReadFile/WriteFile, and return an error code appropriately. And in order to allow the background processing the file should be opened in the overlapped mode (this enables asynchronous ReadFile/WriteFile).

Is there a chance that interruption of WriteFile may in some circumstances leave the database in the inconsistent state, even with the journal enabled? I guess not, since the whole idea of the journal file is to be prepared for any error of any kind. But I’d like to hear more opinions about this.

Also, did someone tried something similar?

Thanks in advance.

EDIT:

Thanks to ereOn. I wasn’t aware of the existence of sqlite3_interrupt. This probably answers my question.

Now, for all of you who wonders how (and why) one expects to do some background processing during the I/O within the same thread.

Unfortunately not many people are familiar with so-called “Overlapped I/O”.

http://en.wikipedia.org/wiki/Overlapped_I/O

Using it one issues an I/O operation asynchronously, and the calling thread is not blocked. Then one receives the I/O completion status using one of the completion mechanisms: waitable event, new routine queued into the APC, or the completion port.

Using this technique one doesn’t have to create extra threads. Actually the only real legitimation for creating threads is when your bottleneck is the computation time (i.e. CPU load), and the machine has several CPUs (or cores).

And creating a thread just to let it be blocked by the OS most of the time – this doesn’t make sense. This leads to the unjustified waste of the OS resources, complicates the program (need for synchronization and etc.).

Unfortunately not all the libraries/APIs allow asynchronous mode of operation, thus making creating extra threads the necessarily evil.

EDIT2:

I’ve already found the solution, thansk to ereOn.

For all those who nevertheless insist that it’s not worth doing things “in background” while “waiting” for the I/O to complete using overlapped I/O. I disagree, and I think there’s no point to argue about this. At least this is not related to the subject.

I’m a Windows programmer (as you may noticed), and I have a very extensive experience in all kinds of multitasking. Plus I’m also a driver writer, so that I also know how things work “behind the scenes”.

I know that it’s a “common practice” to create several threads to do several things “in parallel”. But this doesn’t mean that this is a good practice. Please allow me not to follow the “common practice”.

Answer 1

I don’t understand why you want the interruption to come from the same thread and I even don’t understand how that would be possible: if the current thread is blocked, waiting for some IO, you can’t execute any other code. (Yeah, that’s what “blocked” means)

Perhaps if you give us more hints about why you want this, we might help further.

Usually, I use sqlite3_interrupt() to cancel calls. But this, obviously, involves that the call is made from another thread.