sometimes crash EXC_BAD_ACCESS with no message (set NSZombieEnabled)

Question

I'm handling RemoteIO to get mic inputs and modify them little.

Sometimes it crashes with EXC_BAD_ACCESS and there is no more message.

The lines that make crashes are these;

  int currPower = [[powers objectAtIndex:i] intValue];
  int prevPower = [[powers objectAtIndex:i - 1] intValue];

explaining the code,

1. "powers" is NSMutableArray.
2. [powers count] was always bigger than variable "i"

Struggling for a while, I found a good way to fix it.

A environment variables.

So I set NSZombieEnabled and also NSDebugEnabled so that I could see the reason of the crashes.

But even though I set the variables, Xcode shows no message. (But it correctly shows messages when a crash occurs from other line.)

Also a weird thing is that it doesn't crash just after the start of run; it crashes in a minute in average. (But the times really varied.)

And this is a little guess. When I decreased the rate to half than before, it was more stable.

So, is it a problem with NSMutableArray, because NSMutableArray method couldn't catch up the speed of the rate?

or do you see some other possible reasons?

=========================================================================================

There are some more codes. I allocated powers in this way..

  powers = [[NSMutableArray alloc] initWithCapacity:POWER_ARRAY_SIZE];

where I release the powers array is..

  - (void)dealloc {
     [powers release];
     [super dealloc];
  }

and no where else.

more detailed code is this.

  - (void)drawRect:(CGRect)rect
  { 
     ...//Do Something
     ...//Check "endindex" and "startindex" not to exceed boundary

     for (int i = endindex; i > startindex; i-=1)
     {

        int currPower = [[powers objectAtIndex:i] intValue];
        int prevPower = [[powers objectAtIndex:i - 1] intValue];

        ...//Doing something

     }
  }

this drawRect: method is calling from Main Thread(By Timer) in every millisecond.

-- updating(more specifically adding) powers in this method

  - (void)setPower:(int)p
  {

     [powers addObject:[NSNumber numberWithInt:p]]; 

     while ([powers count] > POWER_ARRAY_SIZE){
        [powers removeObjectAtIndex:0];
     }
  }

and also this method is calling in every millisecond. and this is calling in background thread.

so without @autoreleasepool XCode Shows message of alert of leaking

for this reason I blocked the method(setPower) with @autoreleasepool{..}

Answer 1

I found the answer.

Crashes are occurred because NSMutableArray of objected-C sometimes ack wrong.

That's when I try to do something in every milliseconds with it.

So I changed the Objective-C array to C array, like

  int power[ARRAYSIZE];

and after I changed it, it works fine.

May be NSMutableArray isn't that light to do something really fast.