Question

My app crashes sometimes at this point:

NSMutableData* position = [NSMutableData dataWithLength: 3 * sizeof(CGPoint)];

when NSMutableData calls the autorelease.

Here’s the crash log:

Exception Type:  EXC_BREAKPOINT (SIGTRAP)
Exception Codes: 0x00000001, 0xe7ffdefe
Crashed Thread:  0

Thread 0 name:
Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   CoreFoundation            0x343dea60 _CFAutoreleasePoolAddObject + 136
1   CoreFoundation                0x343de9cc -[NSObject(NSObject) autorelease] + 8
2   Foundation                0x30c49166 +[NSMutableData(NSMutableData) dataWithLength:] + 34

Do you have any idea why?

UPDATE:
The variable “position” is returned and used this way:

    NSMutableData *positionData = [[SFinder sharedFinder] move:self.crId] ;
CGPoint *path = [positionData mutableBytes];
CGPoint location0 = path[0];

CGPoint location1 = path[1];

CGPoint location = path[2];

UPDATE 2:

I removed the code with NSMutableData, but the app keeps crashing randomly. On the debug console there is a message:

*** attempt to pop an unknown autorelease pool (0x5830000)

stack:

Thread 0 Crashed:
   Dispatch queue: com.apple.main-thread
0   CoreFoundation               0x01348b69 _CFAutoreleasePoolPop + 201
1   UIKit                        0x008a947c _wrapRunLoopWithAutoreleasePoolHandler + 68
2   CoreFoundation               0x013edfbb __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 27
3   CoreFoundation               0x013830e7 __CFRunLoopDoObservers + 295
4   CoreFoundation               0x0134bbd7 __CFRunLoopRun + 1575
5   CoreFoundation               0x0134b240 CFRunLoopRunSpecific + 208
6   CoreFoundation               0x0134b161 CFRunLoopRunInMode + 97
7   GraphicsServices             0x029b3268 GSEventRunModal + 217
8   GraphicsServices             0x029b332d GSEventRun + 115
9   UIKit         0x008a942e UIApplicationMain + 1160
10  MyApp                        0x000029f4 main + 100 (main.m:13)
11  MyApp                        0x00002985 start + 53

Answer 1

Found the problem. There is an issue with NSMutableData. It occures when you init it with initWithLength, or dataWithLength and try to insert data trough mutableBytes. When you insert the last byte it makes some allocation somewhere else (even if that part of the memory is owned by an other object) causing EXC_BAD_ACCESS and other memory related errors. So initiating it with length+1 bytes solved the problem.