iPhone - NSLog does not print when app is in background

Question

One of my VoIP enabled app prints log for every 5 mins interval when app is in background.

But this log did not show at the time of log time.

All log shows at the moment of application coming to foreground by pressing on it.

But why?

Any help please.

Answer 1

Update for Swift print function:

The print() function does log to the Target Output on Xcode in background. I verified it on iOS 8.1 (on the phone, not simulator) and the logs are printed by background location updates.