[iPhone] - Memory Warnings, available RAM and development

Question

Hello-
I keep receiving memory warnings, and occasional 101 exits from my iPhone app. According to Instruments, the RAM usage never goes above about 13MB. From what I have seen this should be well below the RAM available to apps, which I have read should be around 20-30 MB. We usually only see this after the phone has been on for a long time, with repeated builds and test sessions.
The question is, if we alloc memory during a testing phase, and then exit the app in a non-standard way (Xcode stop button), could we be leaving memory allocated, taking away the available RAM for the next build? Also, when does applicationDidReceiveMemoryWarning get called? When a certain percentage is left, a certain byte count?

Answer 1

Memory warnings are triggered when the OS is low on available memory - it might have nothing to do with your app. Something else happening on the device - an incoming text message, phone call, notification of some kind etc. all can cause memory warnings. With that said, you have to free up unused memory or you run the risk of your app being force quit by the OS.
There's some things you can do to help reduce your memory overhead. Only cache items you need cached, alloc and release objects as you use them rather than adding them to the autorelease pool. Auto released objects use memory until the pool is cleaned. For example creating an autoreleased NSString with
NSString* a = [NSString stringWithString:@"a"];
will get added to the autorelease pool and freed eventually. Instead, create it, use it and release it straight away.
NSString* a = [[NSString alloc] initWithString:@"a"];
// do something
[a release];
This way the memory is only allocated for the time it is in use. If you're going to use auto release pools, create them and drain them close together. I keep an autorelease pool around even though I am allocing and releasing most of my objects and then I drain the pool on a memory warning which frees up some memory.
I hope this helps, I seem to remember that the SQLite books sample code had some examples on inflating objects on demand and deflating them on receipt of a memory warning. Good luck.
Mike