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本帖最后由 kangels 于 2013-3-19 10:23 编辑 ! ]1 ], y, ]/ I7 P" G; Z
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丢失的类型信息
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# x" [! R0 q+ I, W指针变量应该存储两方面的信息：地址，类型信息/ v0 }: I5 |% Z! M) A9 d2 w
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实证
5 V# @/ @( L: l q用sizeof()，float *,int*,double *....都是4字节没有存储类型信息只有地址信息, A9 d/ c& ^+ R5 }+ c* m: j
反汇编一段代码来看看 @) V6 H4 H# v
实例1：' C5 ?7 X* A% ?6 n4 C% i; v, U
#include <stdio.h> int gi; int * pi; void main(int argc, char **argv) { pi = &gi; *pi = 12; } 1: #include <stdio.h> 2: int gi; 3: int * pi; 4: void main(int argc, char **argv) 5: { 00411370 push ebp 00411371 mov ebp,esp 00411373 sub esp,0C0h 00411379 push ebx 0041137A push esi 0041137B push edi 0041137C lea edi,[ebp+FFFFFF40h] 00411382 mov ecx,30h 00411387 mov eax,0CCCCCCCCh 0041138C rep stos dword ptr es:[edi] 6: pi = &gi; 0041138E mov dword ptr ds:[00417140h],417144h 7: *pi = 12; 00411398 mov eax,dword ptr ds:[00417140h] 0041139D mov dword ptr [eax],0Ch 8: } 004113A3 xor eax,eax 004113A5 pop edi 004113A6 pop esi 004113A7 pop ebx 004113A8 mov esp,ebp 004113AA pop ebp 004113AB ret 9 b) E7 w* {, \; m4 ^( B/ Z7 T
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实例2：
' a) f8 B q1 T! B6 T5 f#include <stdio.h> short gs; short * ps; void main(int argc, char **argv) { ps = &gs; *ps = 12; }
0 a" w M' Y9 L' X 1: #include <stdio.h> 2: short gs; 3: short * ps; 4: void main(int argc, char **argv) 5: { 00411370 push ebp 00411371 mov ebp,esp 00411373 sub esp,0C0h 00411379 push ebx 0041137A push esi 0041137B push edi 0041137C lea edi,[ebp+FFFFFF40h] 00411382 mov ecx,30h 00411387 mov eax,0CCCCCCCCh 0041138C rep stos dword ptr es:[edi] 6: ps = &gs; 0041138E mov dword ptr ds:[00417140h],417144h 7: *ps = 12; 00411398 mov eax,0Ch 0041139D mov ecx,dword ptr ds:[00417140h] 004113A3 mov word ptr [ecx],ax 8: } 004113A6 xor eax,eax 004113A8 pop edi 004113A9 pop esi 004113AA pop ebx 004113AB mov esp,ebp 004113AD pop ebp 004113AE ret 3 m* |4 |' h4 @
, |' X5 P4 ? I- R2 L; S8 Z7 m# Y: }实例3：% q1 W& b3 ?. a5 X0 u5 j! e: n; g& z
#include <stdio.h> char gc; char * pc; void main(int argc, char **argv) { pc = &gc; *pc = 12; } . B- x& w2 X! _+ v6 {7 \
1: #include <stdio.h> 2: char gc; 3: char * pc; 4: void main(int argc, char **argv) 5: { 00411370 push ebp 00411371 mov ebp,esp 00411373 sub esp,0C0h 00411379 push ebx 0041137A push esi 0041137B push edi 0041137C lea edi,[ebp+FFFFFF40h] 00411382 mov ecx,30h 00411387 mov eax,0CCCCCCCCh 0041138C rep stos dword ptr es:[edi] 6: pc = &gc; 0041138E mov dword ptr ds:[00417140h],417144h 7: *pc = 12; 00411398 mov eax,dword ptr ds:[00417140h] 0041139D mov byte ptr [eax],0Ch 8: } 004113A0 xor eax,eax 004113A2 pop edi 004113A3 pop esi 004113A4 pop ebx 004113A5 mov esp,ebp 004113A7 pop ebp 004113A8 ret
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; T. u& Y- [2 D3 }6 }可以看出指针的类型信息决定了赋值/读取时读/写多少个字节。
$ j y$ i2 C+ T/ f$ m u: [读/写多少字节的信息不是存放在指针变量中，而是放到了与该地址相关的赋值指令中。0 v% g3 f- ^) p, g ^1 D
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发表于 2013-3-12 20:59:39
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貌似很高深的反汇编
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