Re: [問題] c請益

看板C_and_CPP (C/C++)作者lungswu (宅爸爸)時間13年前 (2012/09/06 12:31)推噓3(3推 0噓 3→)

留言6則, 6人參與討論串1/1

※ 引述《slind (你別對著螢幕傻笑,很蠢耶)》之銘言： : void main(){ : int b[3]; : memset(b, 0, sizeof(b)); : char *a=(char*)b; : int i; : for(i=0; i<9; i++){ : a[i] = 0x5f; : } : printf("%x %x %x\n", b[0], b[1], b[2]); : } : 請問這段是怎麼運作的? 越詳細越好感謝我想它最詳細的運作方式是這樣的，再講下去要查Micro Instruction了 void main(){ 0: 55 push %rbp 1: 48 89 e5 mov %rsp,%rbp 4: 48 83 ec 20 sub $0x20,%rsp int b[3]; memset(b, 0, sizeof(b)); 8: 48 8d 45 e0 lea -0x20(%rbp),%rax c: ba 0c 00 00 00 mov $0xc,%edx 11: be 00 00 00 00 mov $0x0,%esi 16: 48 89 c7 mov %rax,%rdi 19: e8 00 00 00 00 callq 1e <main+0x1e> char *a=(char*)b; 1e: 48 8d 45 e0 lea -0x20(%rbp),%rax 22: 48 89 45 f0 mov %rax,-0x10(%rbp) 19: e8 00 00 00 00 callq 1e <main+0x1e> char *a=(char*)b; 1e: 48 8d 45 e0 lea -0x20(%rbp),%rax 22: 48 89 45 f0 mov %rax,-0x10(%rbp) int i; for(i=0; i<9; i++){ 26: c7 45 fc 00 00 00 00 movl $0x0,-0x4(%rbp) 2d: eb 10 jmp 3f <main+0x3f> a[i] = 0x5f; 2f: 8b 45 fc mov -0x4(%rbp),%eax 32: 48 98 cltq 34: 48 03 45 f0 add -0x10(%rbp),%rax 38: c6 00 5f movb $0x5f,(%rax) int b[3]; memset(b, 0, sizeof(b)); char *a=(char*)b; int i; for(i=0; i<9; i++){ 3b: 83 45 fc 01 addl $0x1,-0x4(%rbp) 3f: 83 7d fc 08 cmpl $0x8,-0x4(%rbp) 43: 7e ea jle 2f <main+0x2f> a[i] = 0x5f; } printf("%x %x %x\n", b[0], b[1], b[2]); 45: 8b 4d e8 mov -0x18(%rbp),%ecx 48: 8b 55 e4 mov -0x1c(%rbp),%edx 4b: 8b 45 e0 mov -0x20(%rbp),%eax 4e: 89 c6 mov %eax,%esi 50: bf 00 00 00 00 mov $0x0,%edi 55: b8 00 00 00 00 mov $0x0,%eax 5a: e8 00 00 00 00 callq 5f <main+0x5f> } 5f: c9 leaveq 60: c3 retq -- ※ 發信站: 批踢踢實業坊(ptt.cc) ◆ From: 134.122.249.1