puzzzzzzle's blog

BLOG

0%

c++类链接机制与同名类链接方式

发表于 更新于 分类于

先看下示例代码

1
2
3
4
5
6
7
8
// class2_define.h

#pragma once
#include <cstdio>
class Class2 {
  public:
  void fun() { printf("Class2\n"); }
};
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
// test.cpp

#include <cstdio>

#include "class2_define.h"
class Class1 {
  public:
  void fun() { printf("test::Class2\n"); }
};
void test()
{
  printf("test start \n");
  Class1 c1{};
  c1.fun();
  Class2 c2{};
  c2.fun();
}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
// main.cpp

#include <cstdio>

#include "class2_define.h"
class Class1 {
  public:
  void fun() { printf("main::Class1\n"); }
};
void test();
/**
 * 输出:
 *
main start
main::Class1
Class2
test start
main::Class1
Class2
 * @return
 */
int main() {
  printf("main start \n");
  Class1 c1{};
  c1.fun();
  Class2 c2{};
  c2.fun();
  test();
}

问题

  • Class2 很正常, 类的实现inline到.h中了, 所以输出是一致的
  • Class1 重复定义了, 且func函数输出不一致, 但是能通过编译, 输出的都是main中的定义

分析

  • c++在编译时 每个编译单元(.o/.obj)都是独立编译的
    • 所有的.h会直接展开, 生成.i后, 再一起编译.s最终生成.o
  • 所以Class2在test.cpp.o和main.cpp.o中,都有一份汇编代码
    • 这么来说对翻译.s/.o的汇编器来说 Class1 和 Class2其实是一样的, 它都不知道其他.o中是否有重复定义
  • 但是这么做, 会导致二进制大小翻倍, 所以在ld过程中, ld对同一个符号, 只会保留一个定义
    • 这里main.cpp.o先链接, 采用首先找到就退出的方式, 所以全部链接main中的, test中的被丢弃了

编译/静态/动态 链接 相同符号的区别

  • 结论: 同符号函数, 只有编译直接链接非inline的函数时, 会报错, 否则使用第一个找到的
  • 编译链接:
    • 同符号的变量/函数会报错, 他们都是强符号
    • 同符号的函数, 如果被申明为inline的, 不会报错, 使用找到的第一个
    • 但是同符号的类方法, 不会报错, 类方法原地实现默认就是inline的
  • 静态链接:
    • 类似inline链接, 使用找到的第一个
      • 一些多版本依赖问题就是这个造成的
      • libA 依赖libX的v1版本, libB 依赖libX的v2版本, 最终链接后, 会定到一个版本中去, 导致数据结构和函数对不上
  • 动态链接
    • 类似静态链接, 相同符号找到第一个就退出
    • 利用这种机制, 我们可以覆盖一些libc函数

汇编分析

  • 反汇编在最后面
  • Class1::fun mangling 后的符号是: _ZN6Class23funEv
  • 可以看到_ZN6Class23funEv在main.cpp.o 和test.cpp.o中都有定义
  • 但是到了最终可执行文件中, 就只有一个了
  • 注意: data/rodata/bss段声明的数据不会被裁减, 即使完全没用到,
    • 可以看到test中声明的字符串, 在最终链接结果中还在
    • 可执行文件大小比较敏感时需要留意
1
2
3
4
5
6
7
8
9
10
11
12
# objdump -s -j .rodata ../../bin/src_cpp_language_class_link > rodata.txt


../../bin/src_cpp_language_class_link:     file format elf64-x86-64

Contents of section .rodata:
 2000 01000200 436c6173 7332006d 61696e3a  ....Class2.main:
 2010 3a436c61 73733100 6d61696e 20737461  :Class1.main sta
 2020 72742000 436c6173 73320074 6573743a  rt .Class2.test:
 2030 3a436c61 73733200 74657374 20737461  :Class2.test sta
 2040 72742000                             rt .            

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61

main.cpp.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <main>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	48 83 ec 10          	sub    $0x10,%rsp
   c:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
  13:	00 00 
  15:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
  19:	31 c0                	xor    %eax,%eax
  1b:	48 8d 45 f6          	lea    -0xa(%rbp),%rax
  1f:	48 89 c7             	mov    %rax,%rdi
  22:	e8 00 00 00 00       	call   27 <main+0x27>
  27:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
  2b:	48 89 c7             	mov    %rax,%rdi
  2e:	e8 00 00 00 00       	call   33 <main+0x33>
  33:	e8 00 00 00 00       	call   38 <main+0x38>
  38:	b8 00 00 00 00       	mov    $0x0,%eax
  3d:	48 8b 55 f8          	mov    -0x8(%rbp),%rdx
  41:	64 48 2b 14 25 28 00 	sub    %fs:0x28,%rdx
  48:	00 00 
  4a:	74 05                	je     51 <main+0x51>
  4c:	e8 00 00 00 00       	call   51 <main+0x51>
  51:	c9                   	leave  
  52:	c3                   	ret    

Disassembly of section .text._ZN6Class23funEv:

0000000000000000 <_ZN6Class23funEv>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	48 83 ec 10          	sub    $0x10,%rsp
   c:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
  10:	48 8d 05 00 00 00 00 	lea    0x0(%rip),%rax        # 17 <_ZN6Class23funEv+0x17>
  17:	48 89 c7             	mov    %rax,%rdi
  1a:	e8 00 00 00 00       	call   1f <_ZN6Class23funEv+0x1f>
  1f:	90                   	nop
  20:	c9                   	leave  
  21:	c3                   	ret    

Disassembly of section .text._ZN6Class13funEv:

0000000000000000 <_ZN6Class13funEv>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	48 83 ec 10          	sub    $0x10,%rsp
   c:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
  10:	48 8d 05 00 00 00 00 	lea    0x0(%rip),%rax        # 17 <_ZN6Class13funEv+0x17>
  17:	48 89 c7             	mov    %rax,%rdi
  1a:	e8 00 00 00 00       	call   1f <_ZN6Class13funEv+0x1f>
  1f:	90                   	nop
  20:	c9                   	leave  
  21:	c3                   	ret    

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42

test.cpp.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <_Z4testv>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	48 83 ec 10          	sub    $0x10,%rsp
   c:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
  13:	00 00 
  15:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
  19:	31 c0                	xor    %eax,%eax
  1b:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
  1f:	48 89 c7             	mov    %rax,%rdi
  22:	e8 00 00 00 00       	call   27 <_Z4testv+0x27>
  27:	90                   	nop
  28:	48 8b 45 f8          	mov    -0x8(%rbp),%rax
  2c:	64 48 2b 04 25 28 00 	sub    %fs:0x28,%rax
  33:	00 00 
  35:	74 05                	je     3c <_Z4testv+0x3c>
  37:	e8 00 00 00 00       	call   3c <_Z4testv+0x3c>
  3c:	c9                   	leave  
  3d:	c3                   	ret    

Disassembly of section .text._ZN6Class13funEv:

0000000000000000 <_ZN6Class13funEv>:
   0:	f3 0f 1e fa          	endbr64 
   4:	55                   	push   %rbp
   5:	48 89 e5             	mov    %rsp,%rbp
   8:	48 83 ec 10          	sub    $0x10,%rsp
   c:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
  10:	48 8d 05 00 00 00 00 	lea    0x0(%rip),%rax        # 17 <_ZN6Class13funEv+0x17>
  17:	48 89 c7             	mov    %rax,%rdi
  1a:	e8 00 00 00 00       	call   1f <_ZN6Class13funEv+0x1f>
  1f:	90                   	nop
  20:	c9                   	leave  
  21:	c3                   	ret    

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213

../../bin/src_cpp_language_class_link:     file format elf64-x86-64


Disassembly of section .init:

0000000000001000 <_init>:
    1000:	f3 0f 1e fa          	endbr64 
    1004:	48 83 ec 08          	sub    $0x8,%rsp
    1008:	48 8b 05 d9 2f 00 00 	mov    0x2fd9(%rip),%rax        # 3fe8 <__gmon_start__@Base>
    100f:	48 85 c0             	test   %rax,%rax
    1012:	74 02                	je     1016 <_init+0x16>
    1014:	ff d0                	call   *%rax
    1016:	48 83 c4 08          	add    $0x8,%rsp
    101a:	c3                   	ret    

Disassembly of section .plt:

0000000000001020 <.plt>:
    1020:	ff 35 92 2f 00 00    	push   0x2f92(%rip)        # 3fb8 <_GLOBAL_OFFSET_TABLE_+0x8>
    1026:	f2 ff 25 93 2f 00 00 	bnd jmp *0x2f93(%rip)        # 3fc0 <_GLOBAL_OFFSET_TABLE_+0x10>
    102d:	0f 1f 00             	nopl   (%rax)
    1030:	f3 0f 1e fa          	endbr64 
    1034:	68 00 00 00 00       	push   $0x0
    1039:	f2 e9 e1 ff ff ff    	bnd jmp 1020 <_init+0x20>
    103f:	90                   	nop
    1040:	f3 0f 1e fa          	endbr64 
    1044:	68 01 00 00 00       	push   $0x1
    1049:	f2 e9 d1 ff ff ff    	bnd jmp 1020 <_init+0x20>
    104f:	90                   	nop

Disassembly of section .plt.got:

0000000000001050 <__cxa_finalize@plt>:
    1050:	f3 0f 1e fa          	endbr64 
    1054:	f2 ff 25 9d 2f 00 00 	bnd jmp *0x2f9d(%rip)        # 3ff8 <__cxa_finalize@GLIBC_2.2.5>
    105b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)

Disassembly of section .plt.sec:

0000000000001060 <puts@plt>:
    1060:	f3 0f 1e fa          	endbr64 
    1064:	f2 ff 25 5d 2f 00 00 	bnd jmp *0x2f5d(%rip)        # 3fc8 <puts@GLIBC_2.2.5>
    106b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)

0000000000001070 <__stack_chk_fail@plt>:
    1070:	f3 0f 1e fa          	endbr64 
    1074:	f2 ff 25 55 2f 00 00 	bnd jmp *0x2f55(%rip)        # 3fd0 <__stack_chk_fail@GLIBC_2.4>
    107b:	0f 1f 44 00 00       	nopl   0x0(%rax,%rax,1)

Disassembly of section .text:

0000000000001080 <_start>:
    1080:	f3 0f 1e fa          	endbr64 
    1084:	31 ed                	xor    %ebp,%ebp
    1086:	49 89 d1             	mov    %rdx,%r9
    1089:	5e                   	pop    %rsi
    108a:	48 89 e2             	mov    %rsp,%rdx
    108d:	48 83 e4 f0          	and    $0xfffffffffffffff0,%rsp
    1091:	50                   	push   %rax
    1092:	54                   	push   %rsp
    1093:	45 31 c0             	xor    %r8d,%r8d
    1096:	31 c9                	xor    %ecx,%ecx
    1098:	48 8d 3d ca 00 00 00 	lea    0xca(%rip),%rdi        # 1169 <main>
    109f:	ff 15 33 2f 00 00    	call   *0x2f33(%rip)        # 3fd8 <__libc_start_main@GLIBC_2.34>
    10a5:	f4                   	hlt    
    10a6:	66 2e 0f 1f 84 00 00 	cs nopw 0x0(%rax,%rax,1)
    10ad:	00 00 00 

00000000000010b0 <deregister_tm_clones>:
    10b0:	48 8d 3d 59 2f 00 00 	lea    0x2f59(%rip),%rdi        # 4010 <__TMC_END__>
    10b7:	48 8d 05 52 2f 00 00 	lea    0x2f52(%rip),%rax        # 4010 <__TMC_END__>
    10be:	48 39 f8             	cmp    %rdi,%rax
    10c1:	74 15                	je     10d8 <deregister_tm_clones+0x28>
    10c3:	48 8b 05 16 2f 00 00 	mov    0x2f16(%rip),%rax        # 3fe0 <_ITM_deregisterTMCloneTable@Base>
    10ca:	48 85 c0             	test   %rax,%rax
    10cd:	74 09                	je     10d8 <deregister_tm_clones+0x28>
    10cf:	ff e0                	jmp    *%rax
    10d1:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)
    10d8:	c3                   	ret    
    10d9:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

00000000000010e0 <register_tm_clones>:
    10e0:	48 8d 3d 29 2f 00 00 	lea    0x2f29(%rip),%rdi        # 4010 <__TMC_END__>
    10e7:	48 8d 35 22 2f 00 00 	lea    0x2f22(%rip),%rsi        # 4010 <__TMC_END__>
    10ee:	48 29 fe             	sub    %rdi,%rsi
    10f1:	48 89 f0             	mov    %rsi,%rax
    10f4:	48 c1 ee 3f          	shr    $0x3f,%rsi
    10f8:	48 c1 f8 03          	sar    $0x3,%rax
    10fc:	48 01 c6             	add    %rax,%rsi
    10ff:	48 d1 fe             	sar    %rsi
    1102:	74 14                	je     1118 <register_tm_clones+0x38>
    1104:	48 8b 05 e5 2e 00 00 	mov    0x2ee5(%rip),%rax        # 3ff0 <_ITM_registerTMCloneTable@Base>
    110b:	48 85 c0             	test   %rax,%rax
    110e:	74 08                	je     1118 <register_tm_clones+0x38>
    1110:	ff e0                	jmp    *%rax
    1112:	66 0f 1f 44 00 00    	nopw   0x0(%rax,%rax,1)
    1118:	c3                   	ret    
    1119:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

0000000000001120 <__do_global_dtors_aux>:
    1120:	f3 0f 1e fa          	endbr64 
    1124:	80 3d e5 2e 00 00 00 	cmpb   $0x0,0x2ee5(%rip)        # 4010 <__TMC_END__>
    112b:	75 2b                	jne    1158 <__do_global_dtors_aux+0x38>
    112d:	55                   	push   %rbp
    112e:	48 83 3d c2 2e 00 00 	cmpq   $0x0,0x2ec2(%rip)        # 3ff8 <__cxa_finalize@GLIBC_2.2.5>
    1135:	00 
    1136:	48 89 e5             	mov    %rsp,%rbp
    1139:	74 0c                	je     1147 <__do_global_dtors_aux+0x27>
    113b:	48 8b 3d c6 2e 00 00 	mov    0x2ec6(%rip),%rdi        # 4008 <__dso_handle>
    1142:	e8 09 ff ff ff       	call   1050 <__cxa_finalize@plt>
    1147:	e8 64 ff ff ff       	call   10b0 <deregister_tm_clones>
    114c:	c6 05 bd 2e 00 00 01 	movb   $0x1,0x2ebd(%rip)        # 4010 <__TMC_END__>
    1153:	5d                   	pop    %rbp
    1154:	c3                   	ret    
    1155:	0f 1f 00             	nopl   (%rax)
    1158:	c3                   	ret    
    1159:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)

0000000000001160 <frame_dummy>:
    1160:	f3 0f 1e fa          	endbr64 
    1164:	e9 77 ff ff ff       	jmp    10e0 <register_tm_clones>

0000000000001169 <main>:
    1169:	f3 0f 1e fa          	endbr64 
    116d:	55                   	push   %rbp
    116e:	48 89 e5             	mov    %rsp,%rbp
    1171:	48 83 ec 10          	sub    $0x10,%rsp
    1175:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    117c:	00 00 
    117e:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    1182:	31 c0                	xor    %eax,%eax
    1184:	48 8d 05 8d 0e 00 00 	lea    0xe8d(%rip),%rax        # 2018 <_IO_stdin_used+0x18>
    118b:	48 89 c7             	mov    %rax,%rdi
    118e:	e8 cd fe ff ff       	call   1060 <puts@plt>
    1193:	48 8d 45 f6          	lea    -0xa(%rbp),%rax
    1197:	48 89 c7             	mov    %rax,%rdi
    119a:	e8 4f 00 00 00       	call   11ee <_ZN6Class13funEv>
    119f:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    11a3:	48 89 c7             	mov    %rax,%rdi
    11a6:	e8 21 00 00 00       	call   11cc <_ZN6Class23funEv>
    11ab:	e8 60 00 00 00       	call   1210 <_Z4testv>
    11b0:	b8 00 00 00 00       	mov    $0x0,%eax
    11b5:	48 8b 55 f8          	mov    -0x8(%rbp),%rdx
    11b9:	64 48 2b 14 25 28 00 	sub    %fs:0x28,%rdx
    11c0:	00 00 
    11c2:	74 05                	je     11c9 <main+0x60>
    11c4:	e8 a7 fe ff ff       	call   1070 <__stack_chk_fail@plt>
    11c9:	c9                   	leave  
    11ca:	c3                   	ret    
    11cb:	90                   	nop

00000000000011cc <_ZN6Class23funEv>:
    11cc:	f3 0f 1e fa          	endbr64 
    11d0:	55                   	push   %rbp
    11d1:	48 89 e5             	mov    %rsp,%rbp
    11d4:	48 83 ec 10          	sub    $0x10,%rsp
    11d8:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
    11dc:	48 8d 05 21 0e 00 00 	lea    0xe21(%rip),%rax        # 2004 <_IO_stdin_used+0x4>
    11e3:	48 89 c7             	mov    %rax,%rdi
    11e6:	e8 75 fe ff ff       	call   1060 <puts@plt>
    11eb:	90                   	nop
    11ec:	c9                   	leave  
    11ed:	c3                   	ret    

00000000000011ee <_ZN6Class13funEv>:
    11ee:	f3 0f 1e fa          	endbr64 
    11f2:	55                   	push   %rbp
    11f3:	48 89 e5             	mov    %rsp,%rbp
    11f6:	48 83 ec 10          	sub    $0x10,%rsp
    11fa:	48 89 7d f8          	mov    %rdi,-0x8(%rbp)
    11fe:	48 8d 05 06 0e 00 00 	lea    0xe06(%rip),%rax        # 200b <_IO_stdin_used+0xb>
    1205:	48 89 c7             	mov    %rax,%rdi
    1208:	e8 53 fe ff ff       	call   1060 <puts@plt>
    120d:	90                   	nop
    120e:	c9                   	leave  
    120f:	c3                   	ret    

0000000000001210 <_Z4testv>:
    1210:	f3 0f 1e fa          	endbr64 
    1214:	55                   	push   %rbp
    1215:	48 89 e5             	mov    %rsp,%rbp
    1218:	48 83 ec 10          	sub    $0x10,%rsp
    121c:	64 48 8b 04 25 28 00 	mov    %fs:0x28,%rax
    1223:	00 00 
    1225:	48 89 45 f8          	mov    %rax,-0x8(%rbp)
    1229:	31 c0                	xor    %eax,%eax
    122b:	48 8d 05 06 0e 00 00 	lea    0xe06(%rip),%rax        # 2038 <_IO_stdin_used+0x38>
    1232:	48 89 c7             	mov    %rax,%rdi
    1235:	e8 26 fe ff ff       	call   1060 <puts@plt>
    123a:	48 8d 45 f6          	lea    -0xa(%rbp),%rax
    123e:	48 89 c7             	mov    %rax,%rdi
    1241:	e8 a8 ff ff ff       	call   11ee <_ZN6Class13funEv>
    1246:	48 8d 45 f7          	lea    -0x9(%rbp),%rax
    124a:	48 89 c7             	mov    %rax,%rdi
    124d:	e8 7a ff ff ff       	call   11cc <_ZN6Class23funEv>
    1252:	90                   	nop
    1253:	48 8b 45 f8          	mov    -0x8(%rbp),%rax
    1257:	64 48 2b 04 25 28 00 	sub    %fs:0x28,%rax
    125e:	00 00 
    1260:	74 05                	je     1267 <_Z4testv+0x57>
    1262:	e8 09 fe ff ff       	call   1070 <__stack_chk_fail@plt>
    1267:	c9                   	leave  
    1268:	c3                   	ret    

Disassembly of section .fini:

000000000000126c <_fini>:
    126c:	f3 0f 1e fa          	endbr64 
    1270:	48 83 ec 08          	sub    $0x8,%rsp
    1274:	48 83 c4 08          	add    $0x8,%rsp
    1278:	c3                   	ret