《coredump问题原理探究》Linux x86版7.8节vector相关的iterator对象

在前面看过了一个vectorcoredump的样例,接触了vector的iterator,能够知道vector的iterator仅仅有一个成员_M_current指向vector某一个元素.

先看一个样例:

1 #include <vector>
  2
  3 void init( std::vector<int>& vec )
  4 {
  5     for ( int i = 0; i < 0x10; i++ )
  6     {
  7         vec.push_back( i );
  8     }
  9 }
 10
 11 int getSum( std::vector<int>& vec )
 12 {
 13     std::vector<int>::iterator iter;
 14     int result = 0;
 15
 16     for ( iter = vec.begin(); iter != vec.end(); iter++ )
 17     {
 18         result += *iter;
 19     }
 20
 21     return result;
 22 }
 23
 24 int main()
 25 {
 26     std::vector<int> vec;
 27     init( vec );
 28
 29     return getSum( vec );
 30 }

因为仅仅是考察iterator,仅仅看getSum的汇编:

(gdb) disassemble getSum
Dump of assembler code for function _Z6getSumRSt6vectorIiSaIiEE:
   0x080486cd <+0>:	push   %ebp
   0x080486ce <+1>:	mov    %esp,%ebp
   0x080486d0 <+3>:	sub    $0x38,%esp
   0x080486d3 <+6>:	lea    -0x18(%ebp),%eax
   0x080486d6 <+9>:	mov    %eax,(%esp)
   0x080486d9 <+12>:	call   0x8048840 <_ZN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEC2Ev>
   0x080486de <+17>:	movl   $0x0,-0xc(%ebp)
   0x080486e5 <+24>:	lea    -0x1c(%ebp),%eax
   0x080486e8 <+27>:	mov    0x8(%ebp),%edx
   0x080486eb <+30>:	mov    %edx,0x4(%esp)
   0x080486ef <+34>:	mov    %eax,(%esp)
   0x080486f2 <+37>:	call   0x804884e <_ZNSt6vectorIiSaIiEE5beginEv>
   0x080486f7 <+42>:	sub    $0x4,%esp
   0x080486fa <+45>:	mov    -0x1c(%ebp),%eax
   0x080486fd <+48>:	mov    %eax,-0x18(%ebp)
   0x08048700 <+51>:	jmp    0x804872f <_Z6getSumRSt6vectorIiSaIiEE+98>
   0x08048702 <+53>:	lea    -0x18(%ebp),%eax
   0x08048705 <+56>:	mov    %eax,(%esp)
   0x08048708 <+59>:	call   0x80488f8 <_ZNK9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEdeEv>
   0x0804870d <+64>:	mov    (%eax),%eax
   0x0804870f <+66>:	add    %eax,-0xc(%ebp)
   0x08048712 <+69>:	lea    -0x10(%ebp),%eax
   0x08048715 <+72>:	movl   $0x0,0x8(%esp)
   0x0804871d <+80>:	lea    -0x18(%ebp),%edx
   0x08048720 <+83>:	mov    %edx,0x4(%esp)
   0x08048724 <+87>:	mov    %eax,(%esp)
   0x08048727 <+90>:	call   0x80488c4 <_ZN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEppEi>
   0x0804872c <+95>:	sub    $0x4,%esp
   0x0804872f <+98>:	lea    -0x14(%ebp),%eax
   0x08048732 <+101>:	mov    0x8(%ebp),%edx
   0x08048735 <+104>:	mov    %edx,0x4(%esp)
   0x08048739 <+108>:	mov    %eax,(%esp)
   0x0804873c <+111>:	call   0x8048872 <_ZNSt6vectorIiSaIiEE3endEv>
   0x08048741 <+116>:	sub    $0x4,%esp
   0x08048744 <+119>:	lea    -0x14(%ebp),%eax
   0x08048747 <+122>:	mov    %eax,0x4(%esp)
   0x0804874b <+126>:	lea    -0x18(%ebp),%eax
   0x0804874e <+129>:	mov    %eax,(%esp)
   0x08048751 <+132>:	call   0x8048898 <_ZN9__gnu_cxxneIPiSt6vectorIiSaIiEEEEbRKNS_17__normal_iteratorIT_T0_EESA_>
   0x08048756 <+137>:	test   %al,%al
   0x08048758 <+139>:	jne    0x8048702 <_Z6getSumRSt6vectorIiSaIiEE+53>
   0x0804875a <+141>:	mov    -0xc(%ebp),%eax
   0x0804875d <+144>:	leave
   0x0804875e <+145>:	ret
End of assembler dump.

在0x0804874b打断点.由上面汇编可知,iter的this指针是ebp-0x18,而vec的this指针放在ebp+0x8.

运行到断点处,看一下vec的内容:

(gdb) x $ebp+8
0xbffff5a0:	0xbffff5b4
(gdb) x /4x 0xbffff5b4
0xbffff5b4:	0x0804b068	0x0804b0a8	0x0804b0a8	0x080491f0
(gdb) x /16x 0x0804b068
0x804b068:	0x00000000	0x00000001	0x00000002	0x00000003
0x804b078:	0x00000004	0x00000005	0x00000006	0x00000007
0x804b088:	0x00000008	0x00000009	0x0000000a	0x0000000b
0x804b098:	0x0000000c	0x0000000d	0x0000000e	0x0000000f

看一下iter的内容怎样变化,运行完几次c命令之后(注意,断点地址在运行iter++之后):

Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) ()
(gdb) x /4x $ebp-0x18
0xbffff580:	0x0804b068	0x0804b0a8	0x00000001	0x00000000
(gdb) c
Continuing.

Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) ()
(gdb) x /4x $ebp-0x18
0xbffff580:	0x0804b06c	0x0804b0a8	0x0804b068	0x00000000
(gdb) c
Continuing.

Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) ()
(gdb) x /4x $ebp-0x18
0xbffff580:	0x0804b070	0x0804b0a8	0x0804b06c	0x00000001
(gdb) c
Continuing.

Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) ()
(gdb) x /4x $ebp-0x18
0xbffff580:	0x0804b074	0x0804b0a8	0x0804b070	0x00000003
(gdb) c
Continuing.

Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) ()
(gdb) x /4x $ebp-0x18
0xbffff580:	0x0804b078	0x0804b0a8	0x0804b074	0x00000006

可见vector的iterator确实仅仅有一个成员_Ptr,它的取值范围是

vec. _M_start  <= _M_current < vec. _M_finish