控制linux动态链接库（so, standard object）导出函数

参考：http://www.lampchina.net/article/htmls/201006/Mjg4MDMy.html

注意：参考文献中有一些函数是不能编译通过的。

编译环境：gcc 4.4.3

在windows中，我们可以指定__declspec(dllexport)定义来控制导出函数，在linux下，我们也有类似的控制参数。

在GCC帮助文档 -fvisibility=default|internal|hidden|protected参数下有这样一段描述：

a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLL's on Windows and with -fvisibility=hidden and "__attribute__ ((visibility("default")))"
instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects.

需要了解的是，在linux下，源文件中的所有函数都有一个默认的visibility属性为public，在编译命令中加入-fvisibility=hidden参数，会将所有默认的public的属性变为hidden。此时，如果对函数设置__attribute__ ((visibility("default")))参数，使特定的函数仍然按默认的public属性处理，则-fvisibility=hidden参数不会对该函数起作用。所以，设置了-fvisibility=hidden参数之后，只有设置了__attribute__
((visibility("default")))的函数才是对外可见的，如此则效果等同于Visual Studio下的__declspec(dllexport)定义。

下面看一个例子：

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#include<stdio.h>

#include<string.h>

#include<stdlib.h>

extern int test (int i);

__attribute ((visibility("default"))) int test2 (int i)

{

test(i);

printf("this is test2/n");

}

int test3 (int i)

{

printf("this is test 3/n");

}

此时调用以下命令编译：

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gcc -shared -o test.so -fvisibility=hidden test.c

可以获得test.so文件，用以下命令查看so文件属性：

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readelf -s test.so

得到以下结果：

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Symbol table '.dynsym' contains 12 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 00000000 0 NOTYPE LOCAL DEFAULT UND

1: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__

2: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses

3: 00000000 0 NOTYPE GLOBAL DEFAULT UND test

4: 00000000 0 FUNC GLOBAL DEFAULT UND puts@GLIBC_2.0 (2)

5: 00000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@GLIBC_2.1.3 (3)

6: 00002014 0 NOTYPE GLOBAL DEFAULT ABS _end

7: 0000200c 0 NOTYPE GLOBAL DEFAULT ABS _edata

8: 0000049c 31 FUNC GLOBAL DEFAULT 12 test2

9: 0000200c 0 NOTYPE GLOBAL DEFAULT ABS __bss_start

10: 00000380 0 FUNC GLOBAL DEFAULT 10 _init

11: 00000508 0 FUNC GLOBAL DEFAULT 13 _fini

Symbol table '.symtab' contains 56 entries:

Num: Value Size Type Bind Vis Ndx Name

0: 00000000 0 NOTYPE LOCAL DEFAULT UND

1: 000000f4 0 SECTION LOCAL DEFAULT 1

2: 00000118 0 SECTION LOCAL DEFAULT 2

3: 0000015c 0 SECTION LOCAL DEFAULT 3

4: 00000198 0 SECTION LOCAL DEFAULT 4

5: 00000258 0 SECTION LOCAL DEFAULT 5

6: 000002e0 0 SECTION LOCAL DEFAULT 6

7: 000002f8 0 SECTION LOCAL DEFAULT 7

8: 00000328 0 SECTION LOCAL DEFAULT 8

9: 00000370 0 SECTION LOCAL DEFAULT 9

10: 00000380 0 SECTION LOCAL DEFAULT 10

11: 000003b0 0 SECTION LOCAL DEFAULT 11

12: 000003e0 0 SECTION LOCAL DEFAULT 12

13: 00000508 0 SECTION LOCAL DEFAULT 13

14: 00000524 0 SECTION LOCAL DEFAULT 14

15: 00000544 0 SECTION LOCAL DEFAULT 15

16: 00001f04 0 SECTION LOCAL DEFAULT 16

17: 00001f0c 0 SECTION LOCAL DEFAULT 17

18: 00001f14 0 SECTION LOCAL DEFAULT 18

19: 00001f18 0 SECTION LOCAL DEFAULT 19

20: 00001fe8 0 SECTION LOCAL DEFAULT 20

21: 00001ff4 0 SECTION LOCAL DEFAULT 21

22: 00002008 0 SECTION LOCAL DEFAULT 22

23: 0000200c 0 SECTION LOCAL DEFAULT 23

24: 00000000 0 SECTION LOCAL DEFAULT 24

25: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c

26: 00001f04 0 OBJECT LOCAL DEFAULT 16 __CTOR_LIST__

27: 00001f0c 0 OBJECT LOCAL DEFAULT 17 __DTOR_LIST__

28: 00001f14 0 OBJECT LOCAL DEFAULT 18 __JCR_LIST__

29: 000003e0 0 FUNC LOCAL DEFAULT 12 __do_global_dtors_aux

30: 0000200c 1 OBJECT LOCAL DEFAULT 23 completed.7021

31: 00002010 4 OBJECT LOCAL DEFAULT 23 dtor_idx.7023

32: 00000460 0 FUNC LOCAL DEFAULT 12 frame_dummy

33: 00000000 0 FILE LOCAL DEFAULT ABS crtstuff.c

34: 00001f08 0 OBJECT LOCAL DEFAULT 16 __CTOR_END__

35: 00000544 0 OBJECT LOCAL DEFAULT 15 __FRAME_END__

36: 00001f14 0 OBJECT LOCAL DEFAULT 18 __JCR_END__

37: 000004d0 0 FUNC LOCAL DEFAULT 12 __do_global_ctors_aux

38: 00000000 0 FILE LOCAL DEFAULT ABS test.c

39: 00001ff4 0 OBJECT LOCAL HIDDEN ABS _GLOBAL_OFFSET_TABLE_

40: 00002008 0 OBJECT LOCAL HIDDEN 22 __dso_handle

41: 00001f10 0 OBJECT LOCAL HIDDEN 17 __DTOR_END__

42: 000004bb 20 FUNC LOCAL HIDDEN 12 test3

43: 00000497 0 FUNC LOCAL HIDDEN 12 __i686.get_pc_thunk.bx

44: 00001f18 0 OBJECT LOCAL HIDDEN ABS _DYNAMIC

45: 00000000 0 NOTYPE WEAK DEFAULT UND __gmon_start__

46: 00000000 0 NOTYPE WEAK DEFAULT UND _Jv_RegisterClasses

47: 00000508 0 FUNC GLOBAL DEFAULT 13 _fini

48: 0000049c 31 FUNC GLOBAL DEFAULT 12 test2

49: 0000200c 0 NOTYPE GLOBAL DEFAULT ABS __bss_start

50: 00000000 0 NOTYPE GLOBAL DEFAULT UND test

51: 00002014 0 NOTYPE GLOBAL DEFAULT ABS _end

52: 00000000 0 FUNC GLOBAL DEFAULT UND puts@@GLIBC_2.0

53: 0000200c 0 NOTYPE GLOBAL DEFAULT ABS _edata

54: 00000000 0 FUNC WEAK DEFAULT UND __cxa_finalize@@GLIBC_2.1

55: 00000380 0 FUNC GLOBAL DEFAULT 10 _init

可以看出，so文件中包含test2()和test3()这两个函数，并且test2()的属性为default（即可见），而test3()的属性则为hidden（不可见）。

转：/article/10323783.html