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：

#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：

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：

readelf -s test.so

得到以下结果：

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）。