控制linux動態連結庫匯出函式
http://blog.csdn.net/wangjingfei/article/details/5693535
參考：http://www.lampchina.NET/article/htmls/201006/Mjg4MDMy.html
注意：參考文獻中有一些函式是不能編譯通過的。
編譯環境：gcc 4.4.3
在windows中，我們可以指定__declspec(dllexport)定義來控制匯出函式，在Linux下，我們也有類似的控制引數。
在GCC幫助文件 -fvisibility=default|internal|hidden|protected引數下有這樣一段描述：
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，原始檔中的所有函式都有一個預設的visibility屬性為public，在編譯命令中加入-fvisibility=hidden引數，會將所有預設的public的屬性變為hidden。此時，如果對函式設定__attribute__ ((visibility("default")))引數，使特定的函式仍然按預設的public屬性處理，則-fvisibility=hidden引數不會對該函式起作用。所以，設定了-fvisibility=hidden引數之後，只有設定了__attribute__ ((visibility("default")))的函式才是對外可見的，如此則效果等同於Visual Studio下的__declspec(dllexport)定義。
下面看一個例子：
[cpp] view plain copy
#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");
}
此時呼叫以下命令編譯：
[ruby] view plain copy
gcc -shared -o test.so -fvisibility=hidden test.c
可以獲得test.so檔案，用以下命令檢視so檔案屬性：
[ruby] view plain copy
readelf -s test.so
得到以下結果：
[c-sharp] view plain copy
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檔案中包含test2()和test3()這兩個函式，並且test2()的屬性為default（即可見），而test3()的屬性則為hid