C99 变长数组 测试
#include <stdio.h>
#include <stdlib.h>
#define dprint(A) printf("%s:\t\t%p\n", #A, A);
void foo(int n, int m)
{
int xy = 8;
int a[n];
int x = 1;
float y = 1.0;
dprint(&xy);
dprint(&x);
dprint(&y);
printf("&a[%d]:\t%p\n",m, &a[m]);
dprint(a);
}
int main(int argc, char *argv[])
{
if(argc == 3)
foo(atoi(argv[1]), atoi(argv[2]));
return 0;
}
/* ---------------------
测试结果：
-->gcc -g -std=c99 Noname1.c -o a.exe
-->a 100 115
&xy: 0x22ff24
&x: 0x22ff20
&y: 0x22ff1c
&a[115]: 0x22ff1c
a: 0x22fd50
-->a 1000 1015
&xy: 0x22ff24
&x: 0x22ff20
&y: 0x22ff1c
&a[1015]: 0x22ff1c
a: 0x22ef40
------------------------ */
可以看出，变长数组被当做特殊的局部变量，相对于普通局部变量，它的位置总是在栈的低地址处。。 只是这个常数15该如何解释?
反汇编下，期待高人分析：
-->objdump -dS a.exe > tmp.txt
#define dprint(A) printf("%s:\t\t%p\n", #A, A);
void foo(int n, int m)
{
4013d9: 55 push %ebp
4013da: 89 e5 mov %esp,%ebp
4013dc: 57 push %edi
4013dd: 56 push %esi
4013de: 53 push %ebx
4013df: 83 ec 5c sub $0x5c,%esp
4013e2: 89 e0 mov %esp,%eax
4013e4: 89 c7 mov %eax,%edi
int xy = 8;
4013e6: c7 45 dc 08 00 00 00 movl $0x8,-0x24(%ebp)
int a[n];
4013ed: 8b 75 08 mov 0x8(%ebp),%esi
4013f0: 8d 46 ff lea -0x1(%esi),%eax
4013f3: 89 45 e0 mov %eax,-0x20(%ebp)
4013f6: 89 f0 mov %esi,%eax
4013f8: ba 00 00 00 00 mov $0x0,%edx
4013fd: 89 c1 mov %eax,%ecx
4013ff: 80 e5 ff and $0xff,%ch
401402: 89 d3 mov %edx,%ebx
401404: 83 e3 0f and $0xf,%ebx
401407: 89 c8 mov %ecx,%eax
401409: 89 da mov %ebx,%edx
40140b: 0f a4 c2 05 shld $0x5,%eax,%edx
40140f: c1 e0 05 shl $0x5,%eax
401412: 89 c1 mov %eax,%ecx
401414: 80 e5 ff and $0xff,%ch
401417: 89 4d c0 mov %ecx,-0x40(%ebp)
40141a: 89 d1 mov %edx,%ecx
40141c: 83 e1 0f and $0xf,%ecx
40141f: 89 4d c4 mov %ecx,-0x3c(%ebp)
401422: 8b 45 c0 mov -0x40(%ebp),%eax
401425: 8b 55 c4 mov -0x3c(%ebp),%edx
401428: 89 f0 mov %esi,%eax
40142a: ba 00 00 00 00 mov $0x0,%edx
40142f: 89 c1 mov %eax,%ecx
401431: 80 e5 ff and $0xff,%ch
401434: 89 4d b8 mov %ecx,-0x48(%ebp)
401437: 89 d1 mov %edx,%ecx
401439: 83 e1 0f and $0xf,%ecx
40143c: 89 4d bc mov %ecx,-0x44(%ebp)
40143f: 8b 45 b8 mov -0x48(%ebp),%eax
401442: 8b 55 bc mov -0x44(%ebp),%edx
401445: 0f a4 c2 05 shld $0x5,%eax,%edx
401449: c1 e0 05 shl $0x5,%eax
40144c: 89 c1 mov %eax,%ecx
40144e: 80 e5 ff and $0xff,%ch
401451: 89 4d b0 mov %ecx,-0x50(%ebp)
401454: 89 d1 mov %edx,%ecx
401456: 83 e1 0f and $0xf,%ecx
401459: 89 4d b4 mov %ecx,-0x4c(%ebp)
40145c: 8b 45 b0 mov -0x50(%ebp),%eax
40145f: 8b 55 b4 mov -0x4c(%ebp),%edx
401462: 89 f0 mov %esi,%eax
401464: c1 e0 02 shl $0x2,%eax
401467: 83 c0 0f add $0xf,%eax
40146a: 83 c0 0f add $0xf,%eax
40146d: c1 e8 04 shr $0x4,%eax
401470: c1 e0 04 shl $0x4,%eax
401473: e8 f8 02 00 00 call 401770 <___chkstk>
401478: 8d 44 24 0c lea 0xc(%esp),%eax
40147c: 83 c0 0f add $0xf,%eax
40147f: c1 e8 04 shr $0x4,%eax
401482: c1 e0 04 shl $0x4,%eax
401485: 89 45 e4 mov %eax,-0x1c(%ebp)
int x = 1;
401488: c7 45 d8 01 00 00 00 movl $0x1,-0x28(%ebp)
float y = 1.0;
40148f: b8 00 00 80 3f mov $0x3f800000,%eax
401494: 89 45 d4 mov %eax,-0x2c(%ebp)
dprint(&xy);
401497: 8d 45 dc lea -0x24(%ebp),%eax
40149a: 89 44 24 08 mov %eax,0x8(%esp)
40149e: c7 44 24 04 64 80 40 movl $0x408064,0x4(%esp)
4014a5: 00
4014a6: c7 04 24 68 80 40 00 movl $0x408068,(%esp)
4014ad: e8 fe fe ff ff call 4013b0 <_printf>
dprint(&x);
4014b2: 8d 45 d8 lea -0x28(%ebp),%eax
4014b5: 89 44 24 08 mov %eax,0x8(%esp)
4014b9: c7 44 24 04 71 80 40 movl $0x408071,0x4(%esp)
4014c0: 00
4014c1: c7 04 24 68 80 40 00 movl $0x408068,(%esp)
4014c8: e8 e3 fe ff ff call 4013b0 <_printf>
dprint(&y);
4014cd: 8d 45 d4 lea -0x2c(%ebp),%eax
4014d0: 89 44 24 08 mov %eax,0x8(%esp)
4014d4: c7 44 24 04 74 80 40 movl $0x408074,0x4(%esp)
4014db: 00
4014dc: c7 04 24 68 80 40 00 movl $0x408068,(%esp)
4014e3: e8 c8 fe ff ff call 4013b0 <_printf>
printf("&a[%d]:\t%p\n",m, &a[m]);
4014e8: 8b 45 e4 mov -0x1c(%ebp),%eax
4014eb: 8b 55 0c mov 0xc(%ebp),%edx
4014ee: c1 e2 02 shl $0x2,%edx
4014f1: 01 d0 add %edx,%eax
4014f3: 89 44 24 08 mov %eax,0x8(%esp)
4014f7: 8b 45 0c mov 0xc(%ebp),%eax
4014fa: 89 44 24 04 mov %eax,0x4(%esp)
4014fe: c7 04 24 77 80 40 00 movl $0x408077,(%esp)
401505: e8 a6 fe ff ff call 4013b0 <_printf>
dprint(a);
40150a: 8b 45 e4 mov -0x1c(%ebp),%eax
40150d: 89 44 24 08 mov %eax,0x8(%esp)
401511: c7 44 24 04 83 80 40 movl $0x408083,0x4(%esp)
401518: 00
401519: c7 04 24 68 80 40 00 movl $0x408068,(%esp)
401520: e8 8b fe ff ff call 4013b0 <_printf>
401525: 89 fc mov %edi,%esp
}
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