Question

In my code i am working with audio buffers, when i have a callback function that is being called many times per second. this callback is in a class that handle audio, and not in the main class of the app .

At the start i was getting this warning that is being log many times during callbacks:

Object 0x93cd5e0 of class __NSCFNumber autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() 

Then i was told to put this line in the callback func :

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

then this error disappeared .
But i cant understand how is that possible that i alloc the pool so many times in 1 second- and maybe i have a memory issues.
I saw than that i have to put this at the end :

[pool drain];

so i have this :

OSStatus status;
    status = AudioUnitRender(audioUnit, 
                             ioActionFlags, 
                             inTimeStamp, 
                             inBusNumber, 
                             inNumberFrames, 
                             &bufferList); 


    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];  // that line added

    //run on evert sample
    int16_t *q = (int16_t *)(&bufferList)->mBuffers[0].mData;
    for(int i=0; i < inNumberFrames; i++)
    {
    static int stateMachineSelector=1;
    static int sampelsCounter=0;

   // CODE TO HANDLE THE SAMPLES ...
    }
   [pool drain];  // issue here    

what exactly i did here?
why is that ?
is that ok from memory aspect ?

thanks a lot .

Answer 1

When starting a Autorelease pool with [[NSAutoreleasePool alloc] init] all further objects receiving a autorelease or where created with the convenience allocator (like NSArray *ary = [NSArray array];, or UIView *view = [[[UIView alloc] init] autorelease]; are in that pool.

So:

NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
NSArray *ary = [NSArray array];
[pool release]; // this will also release and dealoc the *ary from memory

If you have a callback running NOT no the main thread, you SHOULD do a new pool. If not, your objects may leek to nirvana. 🙂

If you process a lot of data with autorelease objects and you’d like to free the memory, then create a pool, process, release the pool.