[NSNull長度]：無法識別的選擇器發送到實例0x43fe068

Question

我不確定這是從哪裏來的。我有設置異常斷點，但它並沒有告訴我究竟發生什麼事：

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 

我也曾嘗試：

po 0x43fe068 - which shows me <null> 

而且

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

這是有道理的，但確實不告訴我這是真的發生了什麼。

Answer 1

根據你發佈的內容，很難說明除了顯而易見的事實之外究竟是什麼錯誤。好的起點可能是在NSNull實現長度方法上添加一個類別，並設置一個斷點並查看它何時被調用（可能需要在.pch文件中導入該類別）