Question

I’m getting an EXC_BAD_ACCESS (or a malloc error) on the following line of code:

NSLog(@"Points:");  

Which makes zero sense to me, as it should be accessing a string constant and nothing else. It’s usually an EXC_BAD_ACCESS, but it occasionally is the following error (on the same line):

Annotate23D(50572,0xac6bb2c0) malloc: *** error for object 0x7051004: incorrect checksum for freed object - object was probably modified after being freed.  

The stack trace for the malloc error (I’m having a hard time getting a full one out of the bad access error) is:

#0  0x960e7c97 in malloc_error_break ()
#1  0x960a94ce in szone_error ()
#2  0x960a954e in free_list_checksum_botch ()
#3  0x960afec0 in small_malloc_from_free_list ()
#4  0x960b124c in szone_malloc_should_clear ()
#5  0x960b166b in szone_malloc ()
#6  0x960e7962 in malloc_zone_malloc ()
#7  0x960e8882 in malloc ()
#8  0x020e5837 in uhash_hashUChars ()
#9  0x020e5c01 in uhash_setResizePolicy ()
#10 0x020e5ca5 in uhash_init ()
#11 0x02075896 in uhash_open ()
#12 0x021f5978 in icu::ZoneMeta::getCanonicalCLDRID ()
#13 0x021f6a4f in icu::ZoneMeta::getCanonicalCLDRID ()
#14 0x021ab551 in icu::TimeZone::getCanonicalID ()
#15 0x021bfaf2 in ucal_getCanonicalTimeZoneID ()
#16 0x024d8bf4 in __nameStringOK ()
#17 0x024d8ae4 in -[__NSPlaceholderTimeZone __initWithName:cache:] ()
#18 0x024d89d0 in -[__NSPlaceholderTimeZone initWithName:] ()
#19 0x024d884b in +[NSTimeZone timeZoneWithName:] ()
#20 0x024d8768 in +[NSTimeZone systemTimeZone] ()
#21 0x024d84d7 in +[NSTimeZone defaultTimeZone] ()
#22 0x024d846d in CFTimeZoneCopyDefault ()
#23 0x024e6726 in CFCalendarCreateWithIdentifier ()
#24 0x02509e67 in __CFLogCString ()
#25 0x02509db3 in _CFLogvEx ()
#26 0x00b99b63 in NSLogv ()
#27 0x00b99ad5 in NSLog ()
#28 0x00008323 in -[Cylinderoid generateMesh] (self=0x6a73fa0, _cmd=0x6e836) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/Cylinderoid.mm:154
#29 0x0006cd85 in -[MeshGenerator rendererForObjects:] (self=0x6a67b80, _cmd=0x6e246, workspace=0xa83f270) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/MeshGenerator.m:16
#30 0x00005367 in -[WorkspaceViewController renderButton:] (self=0xa8377f0, _cmd=0x6e4ac, sender=0x6a58260) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/WorkspaceViewController.m:304
#31 0x02562ec9 in -[NSObject performSelector:withObject:withObject:] ()
#32 0x002365c2 in -[UIApplication sendAction:to:from:forEvent:] ()
#33 0x0023655a in -[UIApplication sendAction:toTarget:fromSender:forEvent:] ()
#34 0x002dbb76 in -[UIControl sendAction:to:forEvent:] ()
#35 0x002dc03f in -[UIControl(Internal) _sendActionsForEvents:withEvent:] ()
#36 0x002db2fe in -[UIControl touchesEnded:withEvent:] ()
#37 0x004f4a2a in _UIGestureRecognizerUpdate ()
#38 0x025359ce in __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ ()
#39 0x024cc670 in __CFRunLoopDoObservers ()
#40 0x024984f6 in __CFRunLoopRun ()
#41 0x02497db4 in CFRunLoopRunSpecific ()
#42 0x02497ccb in CFRunLoopRunInMode ()
#43 0x0244a879 in GSEventRunModal ()
#44 0x0244a93e in GSEventRun ()
#45 0x00233a9b in UIApplicationMain ()
#46 0x00002b88 in main (argc=1, argv=0xbffff590) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/main.m:1  

It may be relevant that the file in question is an Objective-C++ file with ARC turned on. The method itself is huge, so I put it up here to keep the question shortish. The line in question is line 94. Any ideas on either how to fix it or how to debug are greatly appreciated. Thanks in advance!

Quick edit:

I’m now also getting page corruption errors. From the debugger:

Annotate23D(50697) malloc: protecting edges
Annotate23D(50697) malloc: recording malloc stacks to disk using standard recorder
Annotate23D(50697) malloc: process 50678 no longer exists, stack logs deleted from /tmp/stack-logs.50678.Annotate23D.QDMh8a.index
Annotate23D(50697) malloc: stack logs being written into /tmp/stack-logs.50697.Annotate23D.XuPnEH.index
objc[50697]: autorelease pool page 0x7239000 corrupted
  magic 0 3f473a55 3f20c2f1 0
  pthread 0x43af3ce7

(gdb) bt
#0  0x026f28e5 in _objc_trap ()
#1  0x026f296e in _objc_fatal ()
#2  0x02706960 in objc_autoreleasePoolPush ()
#3  0x0248a95a in _CFAutoreleasePoolPush ()
#4  0x00b24e2b in NSPushAutoreleasePool ()
#5  0x00b99af4 in NSLogv ()
#6  0x00b99ad5 in NSLog ()
#7  0x00008323 in -[Cylinderoid generateMesh] (self=0x6ddd8a0, _cmd=0x6e836) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/Cylinderoid.mm:151
#8  0x0006cd85 in -[MeshGenerator rendererForObjects:] (self=0x6a9de30, _cmd=0x6e246, workspace=0x6dc1460) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/MeshGenerator.m:16
#9  0x00005367 in -[WorkspaceViewController renderButton:] (self=0x6db8a30, _cmd=0x6e4ac, sender=0x6dbd6a0) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/WorkspaceViewController.m:304
#10 0x02562ec9 in -[NSObject performSelector:withObject:withObject:] ()
#11 0x002365c2 in -[UIApplication sendAction:to:from:forEvent:] ()
#12 0x0023655a in -[UIApplication sendAction:toTarget:fromSender:forEvent:] ()
#13 0x002dbb76 in -[UIControl sendAction:to:forEvent:] ()
#14 0x002dc03f in -[UIControl(Internal) _sendActionsForEvents:withEvent:] ()
#15 0x002db2fe in -[UIControl touchesEnded:withEvent:] ()
#16 0x004f4a2a in _UIGestureRecognizerUpdate ()
#17 0x025359ce in __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ ()
#18 0x024cc670 in __CFRunLoopDoObservers ()
#19 0x024984f6 in __CFRunLoopRun ()
#20 0x02497db4 in CFRunLoopRunSpecific ()
#21 0x02497ccb in CFRunLoopRunInMode ()
#22 0x0244a879 in GSEventRunModal ()
#23 0x0244a93e in GSEventRun ()
#24 0x00233a9b in UIApplicationMain ()
#25 0x00002b88 in main (argc=1, argv=0xbffff590) at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/main.m:16
(gdb) info malloc-history 0x7239000
Alloc: Block address: 0x07239000 length: 4096
Stack - pthread: 0xac6bb2c0 number of frames: 4
    0: 0x960e76ee in malloc_zone_memalign
    1: 0x270697d in objc_autoreleasePoolPush
    2: 0x2b22 in main at /Users/haldean/Code/ipad/Annotate23D/Annotate23D/main.m:16
    3: 0x2ae5 in start

Answer 1

In the comments I suggested:

The problem is not with NSLog. The problem is probably that some memory is getting smashed and that is causing the crash in NSLog. It would probably help if you could break down that method you’re using to make it easier to read, perhaps even separating the functionality into a new object. Also, avoid the use of magic numbers (3 & 6).

It appears that @haldean went with this suggestion and tracked down the problem. I can’t claim credit for actually doing the hard work of tracking down the problem, but I’m happy the suggestion helped.