Running a finite-length background task - ios

Question

I am doing some processing when the app enters the background state, I have put it under the beginBackgroundTaskWithExpirationHandler() and thereby, should be allotted ten minutes for the same. However, I believe the app is being suspended before that. The task I am performing is memory and CPU-intensive, so is it possible that the OS is putting my app to suspend state ? If so, is there any way to bypass these restrictions. (I am open to using private API's).
Here is the code for starting the background task:
if([[UIDevice currentDevice] respondsToSelector:#selector(isMultitaskingSupported)]){
if([[UIDevice currentDevice] isMultitaskingSupported]){
__block UIBackgroundTaskIdentifier bgTask;
UIApplication *application = [UIApplication sharedApplication];
bgTask = [application beginBackgroundTaskWithExpirationHandler:^{
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
}];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self captureImage];
[application endBackgroundTask:bgTask];
bgTask = UIBackgroundTaskInvalid;
});
}
}

Answer 1

I am not sure that Daij-Dan is completely right.
This part I agree with him:
the os may kill you at any time when its pressured.
However word "pressured" is quite vague. And I believe there should be a lot of pressure (low memory as example) to suspend app which legitimately requested more time. On one hand, it's a real scenario case. On other hand, it's happening not that often (iOS devices have quite a lot of memory nowdays).
From documentation (http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIApplication_Class/Reference/Reference.html):
Each call to this method must be balanced by a matching call to the endBackgroundTask:
method. Applications running background tasks have a finite amount of time in which to run them. (You can find out how much time is available using the backgroundTimeRemaining property.) If you do not call endBackgroundTask: for each task before time expires, the system kills the application. If you provide a block object in the handler parameter, the system calls your handler before time expires to give you a chance to end the task.
I would recommend:
Add some logs in completion handler
Add logs to the logs with logic (including backgroundTimeremaining)
And if your app got suspended review system console logs.
P.S. I removed iphone-privateapi tag, since it's public API.

Answer 2

the os may kill you at any time when its pressured. there are no guarantees about the ten minutes.
(you can also run much longer if you behave :D)
there is no way to force ios to run your app AFAIK