memory is growing in audio buffer code - objective-c

Question

I have a code that we use many times with our apps, its a class that take the buffer samples and process it ,then send back notification to the main class.
The code is c and objective-c.
It works just great, but there is a memory growing which i can see in instruments-allocations tool. the "overall bytes" is keep growing, in 100k a second. becuase of some parts of the code that i know who they are .
this is the callback function, with the line that makes problems.
it happens many times a second.
I also dont really understand where to put my *pool :
static OSStatus recordingCallback(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData)
{
AudioBuffer buffer;
buffer.mNumberChannels = 1;
buffer.mDataByteSize = inNumberFrames * 2;
//NSLog(#"%ld",inNumberFrames);
buffer.mData = malloc( inNumberFrames * 2 );
// Put buffer in a AudioBufferList
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0] = buffer;
// block A
OSStatus status;
status = AudioUnitRender(audioUnit,
ioActionFlags,
inTimeStamp,
inBusNumber,
inNumberFrames,
&bufferList);
//end block A
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
int16_t *q = (int16_t *)(&bufferList)->mBuffers[0].mData;
int16_t average ;
for(int i=0; i < inNumberFrames; i++)
{
average=q[i];
if(average>100) // lineB
reducer++;
//blockC
if(reducer==150 )
{
average= preSignal + alpha*(average-preSignal);
//NSLog(#"average:%d",average);
//call scene
[dict setObject:[NSNumber numberWithInt:average] forKey:#"amp" ] ;
[[NSNotificationCenter defaultCenter] postNotificationName:#"DigitalArrived" object:nil userInfo:dict];
reducer=0;
preSignal=average;
}
//end blockC
}
free(buffer.mData);
[pool release];
return noErr;
}
OK:
ignore blockC for a second.
removing blockA and lineB solve it all.
removing only one of them- leaks.
i just cant undetstand what is growing here .

Answer 1

Just a guess, but allocating a new NSAutoreleasePool inside of your recording callback function (which is a super time-critical function) is probably a bad idea.
Actually, why are you doing this here at all? Shouldn't you just have one pool for the entire app, in your main.m? This is probably causing some of your leaks.

Answer 2

You should not do anything the requires memory allocation inside an Audio Unit render callback. The real-time requirements are too tight for using generic Objective C.
Since you should not allocate a pool, or any other memory, inside an audio unit callback, you should not use any Objective C methods that potentially or actually create any objects, such as dictionary modifications or notification creation. You may have to drop back to using plain C inside the render callback (set a flag), and do your Objective C messaging outside the render callback in another thread (after polling the flag(s) in a timer callback, for instance).