《coredump问题原理探究》Linux x86版6.4节虚函数
版权声明：本文为博主原创文章，未经博主允许不得转载。 https://cloud.tencent.com/developer/article/1344558
在上一节已经探究了类的成员变量的排列，现在看一下虚函数表和成员变量的排列及虚函数之间的排列.
先看一个例子:
1 #include <stdio.h>
2 class xuzhina_dump_c06_s3
3 {
4 private:
5 int m_a;
6 public:
7 xuzhina_dump_c06_s3() { m_a = 0; }
8 virtual void inc() { m_a++; }
9 virtual void dec() { m_a--; }
10 virtual void print()
11 {
12 printf( "%d\n", m_a );
13 }
14 };
15
16 int main()
17 {
18 xuzhina_dump_c06_s3* test = new xuzhina_dump_c06_s3;
19 if ( test != NULL )
20 {
21 test->inc();
22 test->inc();
23 test->print();
24 }
25 return 0;
26 }汇编代码：
(gdb) disassemble main
Dump of assembler code for function main:
0x08048560 <+0>: push %ebp
0x08048561 <+1>: mov %esp,%ebp
0x08048563 <+3>: push %ebx
0x08048564 <+4>: and $0xfffffff0,%esp
0x08048567 <+7>: sub $0x20,%esp
0x0804856a <+10>: movl $0x8,(%esp)
0x08048571 <+17>: call 0x8048450 <_Znwj@plt>
0x08048576 <+22>: mov %eax,%ebx
0x08048578 <+24>: mov %ebx,(%esp)
0x0804857b <+27>: call 0x80485cc <_ZN19xuzhina_dump_c06_s3C2Ev>
0x08048580 <+32>: mov %ebx,0x1c(%esp)
0x08048584 <+36>: cmpl $0x0,0x1c(%esp)
0x08048589 <+41>: je 0x80485c1 <main+97>
0x0804858b <+43>: mov 0x1c(%esp),%eax
0x0804858f <+47>: mov (%eax),%eax
0x08048591 <+49>: mov (%eax),%eax
0x08048593 <+51>: mov 0x1c(%esp),%edx
0x08048597 <+55>: mov %edx,(%esp)
0x0804859a <+58>: call *%eax
0x0804859c <+60>: mov 0x1c(%esp),%eax
0x080485a0 <+64>: mov (%eax),%eax
0x080485a2 <+66>: mov (%eax),%eax
0x080485a4 <+68>: mov 0x1c(%esp),%edx
0x080485a8 <+72>: mov %edx,(%esp)
0x080485ab <+75>: call *%eax
0x080485ad <+77>: mov 0x1c(%esp),%eax
0x080485b1 <+81>: mov (%eax),%eax
0x080485b3 <+83>: add $0x8,%eax
0x080485b6 <+86>: mov (%eax),%eax
0x080485b8 <+88>: mov 0x1c(%esp),%edx
0x080485bc <+92>: mov %edx,(%esp)
0x080485bf <+95>: call *%eax
0x080485c1 <+97>: mov $0x0,%eax
0x080485c6 <+102>: mov -0x4(%ebp),%ebx
0x080485c9 <+105>: leave
0x080485ca <+106>: ret
End of assembler dump.由上面代码可知，执行完了构造函数，test的m_a的值，会变成0。且由上面汇编可以看到this指针在调用构造函数前后，是放在ebx寄存器。
在0x08048578，0x08048580都打断点，看一下this指针所指向的地址是不是这样的结果。
(gdb) tbreak *0x08048578
Temporary breakpoint 1 at 0x8048578
(gdb) tbreak *0x08048580
Temporary breakpoint 2 at 0x8048580
(gdb) r
Starting program: /home/buckxu/work/6/3/xuzhina_dump_c6_s3
Temporary breakpoint 1, 0x08048578 in main ()
(gdb) x /4x $ebx
0x804a008: 0x00000000 0x00000000 0x00000000 0x00020ff1
(gdb) c
Continuing.
Temporary breakpoint 2, 0x08048580 in main ()
(gdb) x /4x $ebx
0x804a008: 0x080486d0 0x00000000 0x00000000 0x00020ff1非常奇怪，按照上一节的内容，地址0x804a008应该存放m_a，会初始化为0.究竟类xuzhina_dump_c06_s3的构造函数做了什么事情？而0x080486d0是什么东西来的？
看一下类xuzhina_dump_c06_s3的构造函数：
(gdb) disassemble _ZN19xuzhina_dump_c06_s3C2Ev
Dump of assembler code for function _ZN19xuzhina_dump_c06_s3C2Ev:
0x080485cc <+0>: push %ebp
0x080485cd <+1>: mov %esp,%ebp
0x080485cf <+3>: mov 0x8(%ebp),%eax
0x080485d2 <+6>: movl $0x80486d0,(%eax)
0x080485d8 <+12>: mov 0x8(%ebp),%eax
0x080485db <+15>: movl $0x0,0x4(%eax)
0x080485e2 <+22>: pop %ebp
0x080485e3 <+23>: ret
End of assembler dump.由构造函数的汇编可知，0x80486d0这个值是在构造函数设置，但还不清楚是什么东西。而
0x080485d8 <+12>: mov 0x8(%ebp),%eax
0x080485db <+15>: movl $0x0,0x4(%eax)却刚好对应了
7 xuzhina_dump_c06_s3() { m_a = 0; }也就是说，类xuzhina_dump_c06_s3的第一个成员变量m_a放在偏移this指针的地方，那么0x80486d0是什么东西，占了m_a的位置呢？
重新看一下main函数的汇编：
(gdb) disassemble main
Dump of assembler code for function main:
0x08048560 <+0>: push %ebp
0x08048561 <+1>: mov %esp,%ebp
0x08048563 <+3>: push %ebx
0x08048564 <+4>: and $0xfffffff0,%esp
0x08048567 <+7>: sub $0x20,%esp
0x0804856a <+10>: movl $0x8,(%esp)
0x08048571 <+17>: call 0x8048450 <_Znwj@plt>
0x08048576 <+22>: mov %eax,%ebx
0x08048578 <+24>: mov %ebx,(%esp)
0x0804857b <+27>: call 0x80485cc <_ZN19xuzhina_dump_c06_s3C2Ev>
=> 0x08048580 <+32>: mov %ebx,0x1c(%esp)
0x08048584 <+36>: cmpl $0x0,0x1c(%esp)
0x08048589 <+41>: je 0x80485c1 <main+97>
0x0804858b <+43>: mov 0x1c(%esp),%eax
0x0804858f <+47>: mov (%eax),%eax
0x08048591 <+49>: mov (%eax),%eax
0x08048593 <+51>: mov 0x1c(%esp),%edx
0x08048597 <+55>: mov %edx,(%esp)
0x0804859a <+58>: call *%eax
0x0804859c <+60>: mov 0x1c(%esp),%eax
0x080485a0 <+64>: mov (%eax),%eax
0x080485a2 <+66>: mov (%eax),%eax
0x080485a4 <+68>: mov 0x1c(%esp),%edx
0x080485a8 <+72>: mov %edx,(%esp)
0x080485ab <+75>: call *%eax
0x080485ad <+77>: mov 0x1c(%esp),%eax
0x080485b1 <+81>: mov (%eax),%eax
0x080485b3 <+83>: add $0x8,%eax
0x080485b6 <+86>: mov (%eax),%eax
0x080485b8 <+88>: mov 0x1c(%esp),%edx
0x080485bc <+92>: mov %edx,(%esp)
0x080485bf <+95>: call *%eax
0x080485c1 <+97>: mov $0x0,%eax
0x080485c6 <+102>: mov -0x4(%ebp),%ebx
0x080485c9 <+105>: leave
0x080485ca <+106>: ret
End of assembler dump.由
0x0804857b <+27>: call 0x80485cc <_ZN19xuzhina_dump_c06_s3C2Ev>
0x08048580 <+32>: mov %ebx,0x1c(%esp)可知，esp+0x1c用来存放this指针。
再看一下这几段指令：
0x0804858b <+43>: mov 0x1c(%esp),%eax
0x0804858f <+47>: mov (%eax),%eax
0x08048591 <+49>: mov (%eax),%eax
0x08048593 <+51>: mov 0x1c(%esp),%edx
0x08048597 <+55>: mov %edx,(%esp)
0x0804859a <+58>: call *%eax
0x0804859c <+60>: mov 0x1c(%esp),%eax
0x080485a0 <+64>: mov (%eax),%eax
0x080485a2 <+66>: mov (%eax),%eax
0x080485a4 <+68>: mov 0x1c(%esp),%edx
0x080485a8 <+72>: mov %edx,(%esp)
0x080485ab <+75>: call *%eax
0x080485ad <+77>: mov 0x1c(%esp),%eax
0x080485b1 <+81>: mov (%eax),%eax
0x080485b3 <+83>: add $0x8,%eax
0x080485b6 <+86>: mov (%eax),%eax
0x080485b8 <+88>: mov 0x1c(%esp),%edx
0x080485bc <+92>: mov %edx,(%esp)
0x080485bf <+95>: call *%eax由于是顺序结构，可知，这三段指令刚好对应
21 test->inc();
22 test->inc();
23 test->print();分析一下第三段汇编：
0x080485ad <+77>: mov 0x1c(%esp),%eax
0x080485b1 <+81>: mov (%eax),%eax
0x080485b3 <+83>: add $0x8,%eax
0x080485b6 <+86>: mov (%eax),%eax
0x080485b8 <+88>: mov 0x1c(%esp),%edx
0x080485bc <+92>: mov %edx,(%esp)
0x080485bf <+95>: call *%eax可见eax正好是放着print这个虚函数的指针。而这个指针最终是由esp+0x1c来取得。由
0x080485ad <+77>: mov 0x1c(%esp),%eax
0x080485b1 <+81>: mov (%eax),%eax可知，正好是从this指针的第一个成员取出来的，也就是说，这个成员是虚函数表指针。根据上面的分析，可知这个虚函数表指针的值是0x80486d0。来验证一下，它是不是虚函数表指针。
(gdb) x /4x 0x80486d0
0x80486d0 <_ZTV19xuzhina_dump_c06_s3+8>: 0x080485e4 0x080485f8 0x0804860c 0x75783931
(gdb) shell c++filt _ZTV19xuzhina_dump_c06_s3
vtable for xuzhina_dump_c06_s3
(gdb) info symbol 0x080485e4
xuzhina_dump_c06_s3::inc() in section .text of /home/buckxu/work/6/3/xuzhina_dump_c6_s3
(gdb) info symbol 0x080485f8
xuzhina_dump_c06_s3::dec() in section .text of /home/buckxu/work/6/3/xuzhina_dump_c6_s3
(gdb) info symbol 0x0804860c
xuzhina_dump_c06_s3::print() in section .text of /home/buckxu/work/6/3/xuzhina_dump_c6_s3可见，0x80486d0所指向正是虚函数表，且里面的表项顺序正好和虚函数的声明顺序一样。
由上面分析，test所指向的对象的内存布局如下图：
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