问题
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.
回答1:
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)
来源：https://stackoverflow.com/questions/25672754/nsnull-length-unrecognized-selector-sent-to-instance-0x43fe068