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I am not exactly sure where this is coming from. I have a breakpoint on exceptions set but it does not show me what is really happening:

   libobjc.A.dylib`objc_exception_throw:
0x40308b9:  pushl  %ebp
0x40308ba:  movl   %esp, %ebp
0x40308bc:  pushl  %ebx
0x40308bd:  pushl  %edi
0x40308be:  pushl  %esi
0x40308bf:  subl   $0x7ec, %esp
0x40308c5:  calll  0x40308ca                 ; objc_exception_throw + 17
0x40308ca:  popl   %ebx
0x40308cb:  movl   $0x10, (%esp)
0x40308d2:  calll  0x4043698                 ; symbol stub for: __cxa_allocate_exception
0x40308d7:  movl   %eax, %esi
0x40308d9:  movl   0x8(%ebp), %eax
0x40308dc:  movl   %eax, (%esp)
0x40308df:  calll  *0x1a478a(%ebx)
0x40308e5:  movl   %eax, %edi
0x40308e7:  movl   0x1a4622(%ebx), %eax
0x40308ed:  movl   %eax, 0x4(%esp)
0x40308f1:  movl   %edi, (%esp)
0x40308f4:  calll  0x40400a4                 ; objc_msgSend
0x40308f9:  movl   %edi, (%esi)
0x40308fb:  leal   0x1a475e(%ebx), %eax
0x4030901:  movl   %eax, 0x4(%esi)
0x4030904:  movl   %edi, (%esp)
0x4030907:  calll  0x402ee20                 ; object_getClassName
0x403090c:  movl   %eax, 0x8(%esi)
0x403090f:  xorl   %eax, %eax
0x4030911:  testl  %edi, %edi
0x4030913:  je     0x4030917                 ; objc_exception_throw + 94
0x4030915:  movl   (%edi), %eax
0x4030917:  movl   %eax, 0xc(%esi)
0x403091a:  movl   0x1a37ae(%ebx), %eax
0x4030920:  movl   %eax, -0x7e0(%ebp)
0x4030926:  cmpb   $0x0, (%eax)
0x4030929:  je     0x403094d                 ; objc_exception_throw + 148
0x403092b:  movl   %edi, (%esp)
0x403092e:  calll  0x402ee20                 ; object_getClassName
0x4030933:  movl   %eax, 0xc(%esp)
0x4030937:  movl   %edi, 0x8(%esp)
0x403093b:  movl   %esi, 0x4(%esp)
0x403093f:  leal   0x141c2(%ebx), %eax
0x4030945:  movl   %eax, (%esp)
0x4030948:  calll  0x40304ad                 ; _objc_inform
0x403094d:  movl   0x1a37aa(%ebx), %eax
0x4030953:  cmpb   $0x0, (%eax)
0x4030956:  je     0x40309cd                 ; objc_exception_throw + 276
0x4030958:  movl   -0x7e0(%ebp), %eax
0x403095e:  cmpb   $0x0, (%eax)
0x4030961:  jne    0x4030985                 ; objc_exception_throw + 204
0x4030963:  movl   %edi, (%esp)
0x4030966:  calll  0x402ee20                 ; object_getClassName
0x403096b:  movl   %eax, 0xc(%esp)
0x403096f:  movl   %edi, 0x8(%esp)
0x4030973:  movl   %esi, 0x4(%esp)
0x4030977:  leal   0x141c2(%ebx), %eax
0x403097d:  movl   %eax, (%esp)
0x4030980:  calll  0x40304ad                 ; _objc_inform
0x4030985:  leal   -0x7dc(%ebp), %eax
0x403098b:  movl   %eax, (%esp)
0x403098e:  movl   $0x1f4, 0x4(%esp)
0x4030996:  calll  0x404375e                 ; symbol stub for: backtrace
0x403099b:  movl   %eax, -0x7e0(%ebp)
0x40309a1:  movl   0x1a37b6(%ebx), %eax
0x40309a7:  movl   (%eax), %eax
0x40309a9:  movl   %eax, (%esp)
0x40309ac:  calll  0x40437a0                 ; symbol stub for: fileno
0x40309b1:  movl   %eax, 0x8(%esp)
0x40309b5:  movl   -0x7e0(%ebp), %eax
0x40309bb:  movl   %eax, 0x4(%esp)
0x40309bf:  leal   -0x7dc(%ebp), %eax
0x40309c5:  movl   %eax, (%esp)
0x40309c8:  calll  0x404376a                 ; symbol stub for: backtrace_symbols_fd
0x40309cd:  movl   %ebx, -0x7e0(%ebp)
0x40309d3:  movl   %esi, %ebx
0x40309d5:  addl   $0x4, %esi
0x40309d8:  movl   %edi, (%esp)
0x40309db:  nop    
0x40309dc:  nopl   (%eax)
0x40309e0:  movl   -0x7e0(%ebp), %eax
0x40309e6:  leal   0x132(%eax), %eax
0x40309ec:  movl   %eax, 0x8(%esp)
0x40309f0:  movl   %esi, 0x4(%esp)
0x40309f4:  movl   %ebx, (%esp)
0x40309f7:  calll  0x40436bc                 ; symbol stub for: __cxa_throw

I have also tried: 

po 0x43fe068 - which shows me <null>

And

 po [(id)(0x43fe068) class] -which shows me NSNull

This makes sense but does not tell me where this really happens.

rmaddy
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Slee
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  • just go to the breakpoint panel and click the `+` to add an exception break point. this will show you exactly where the invalid `length` call is. – Brad Allred Sep 04 '14 at 18:43
  • First sentence in my question states I have done this already - appreciate the down votes everyone - thanks. – Slee Sep 04 '14 at 18:45
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    it would show you exactly where it is happening. you need to look at the *call stack* (the debug navigator pane) and go back to the last point in *your* code. dont bother looking at the ASM for `objc_exception_throw` that is completely useless. – Brad Allred Sep 04 '14 at 18:47
  • Have you tried my suggestion? it should be helpful anyway :) – Julian Sep 04 '14 at 18:49
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    Let me guess: You're dealing with JSON. If you dump the actual JSON you will see that there are values of `null` in some locations. This is translated to an NSNull object in the Objective-C representation. If you have the possibility of `null` JSON elements then you must test for their presence. In Objective-C you can do `suspectPtr == [NSNull null]` to test if a pointer addresses an NSNull object. – Hot Licks Sep 04 '14 at 18:53
  • And see http://stackoverflow.com/q/8100054/581994 – Hot Licks Sep 04 '14 at 18:55
  • @BradAllred there is nothing in my call stack in the debug navigator pane, that is why I am so stumped - normally I know that would lead me to the exact line of code f my error - that is not the case here. – Slee Sep 04 '14 at 19:08
  • I see, that isnt clear in your post. I assume typing `bt` in the debugger is equally useless? hard to imagine why the debugger doesnt have a call stack. this doesnt sound like a case of stack corruption. – Brad Allred Sep 04 '14 at 19:13
  • bt in debugger is totally useless as well. – Slee Sep 04 '14 at 19:22
  • When you're stopped at the exception breakpoint find the thread list, find the thread with the problem, and examine its stack. – Hot Licks Sep 04 '14 at 21:08
  • @HotLicks already suggested; he indicated "there is no stack" so either he doesnt know what we are talking about or something super weird is happening with his debugger (pretty sure he is just confused about what we mean). – Brad Allred Sep 05 '14 at 14:33

1 Answers1

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It is hard, according to what you posted, say what exactly is wrong except the obvious facts. The good start point might be adding a category on NSNull implementing length method and set there a break point and see when it gets called (probably you will need to import that category in a .pch file)

Julian
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  • why downvote? maybe some explanation to make me better person? :P – Julian Sep 04 '14 at 18:56
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    I think your answer is the only one that is going to get me anywhere, I will let you know. This fixes my issue: https://github.com/nicklockwood/NullSafe but I need to find the real cause not cover it up. – Slee Sep 04 '14 at 19:03