16.15 制作so文件时如何指定导出哪些符号

https://scz.617.cn/unix/201507241022.txt

Q:

制作so文件只想导出几个函数，其余函数不导出，就像Windows下dll一样。

A: Ulrich Drepper <drepper@redhat.com> 2005-01-22

一般来说至少有四种办法:

1) static
2) __attribute__ ((visibility("hidden")))
3) Export Maps
4) libtool: -export-symbols (实测无效)

更详细的介绍参看下文:

How To Write Shared Libraries
http://people.redhat.com/drepper/dsohowto.pdf

A: scz 2015-07-24 10:22

虽然下面以可执行程序举例，但相关技术同样适用于共享库。

$ gcc --version
gcc (Debian 4.9.2-10) 4.9.2

1)

$ vi test_0.c

--------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

static unsigned int bar ( void )
{
    return( 100 );
}  /* end of bar */

int main ( int argc, char * argv[] )
{
    unsigned int    a, b;

    a   = bar();
    b   = ( unsigned int )rand();
    return( a + b );
}  /* end of main */
--------------------------------------------------------------------------

$ gcc -Wl,--export-dynamic -Wall -pipe -O0 -s -o test_0 test_0.c
$ gcc -rdynamic -Wall -pipe -O0 -s -o test_0 test_0.c

"-Wl,--export-dynamic"与"-rdynamic"等效。

$ readelf -Ws test_0

Symbol table '.dynsym' contains 20 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
     2: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.0 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT  UND rand@GLIBC_2.0 (2)
     5: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     6: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     7: 080498d8     0 NOTYPE  GLOBAL DEFAULT   24 _edata
     8: 080498d0     0 NOTYPE  GLOBAL DEFAULT   24 __data_start
     9: 080498dc     0 NOTYPE  GLOBAL DEFAULT   25 _end
    10: 080498d0     0 NOTYPE  WEAK   DEFAULT   24 data_start
    11: 080486ac     4 OBJECT  GLOBAL DEFAULT   15 _IO_stdin_used
    12: 08048620    97 FUNC    GLOBAL DEFAULT   13 __libc_csu_init
    13: 080484e0     0 FUNC    GLOBAL DEFAULT   13 _start
    14: 080486a8     4 OBJECT  GLOBAL DEFAULT   15 _fp_hw
    15: 080498d8     0 NOTYPE  GLOBAL DEFAULT   25 __bss_start
    16: 080485e5    50 FUNC    GLOBAL DEFAULT   13 main
    17: 08048474     0 FUNC    GLOBAL DEFAULT   11 _init
    18: 08048690     2 FUNC    GLOBAL DEFAULT   13 __libc_csu_fini
    19: 08048694     0 FUNC    GLOBAL DEFAULT   14 _fini
$ nm -D test_0
080486ac R _IO_stdin_used
         w _ITM_deregisterTMCloneTable
         w _ITM_registerTMCloneTable
         w _Jv_RegisterClasses
080498d8 B __bss_start
080498d0 D __data_start
         w __gmon_start__
08048690 T __libc_csu_fini
08048620 T __libc_csu_init
         U __libc_start_main
080498d8 D _edata
080498dc B _end
08048694 T _fini
080486a8 R _fp_hw
08048474 T _init
080484e0 T _start
080498d0 W data_start
080485e5 T main
         U rand

符号bar未被导出，因为用了static修饰符。

2)

$ vi test_1.c

--------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

#define EXPORT  __attribute__ ((visibility("default")))

EXPORT unsigned int bar ( void )
{
    return( 100 );
}  /* end of bar */

int main ( int argc, char * argv[] )
{
    unsigned int    a, b;

    a   = bar();
    b   = ( unsigned int )rand();
    return( a + b );
}  /* end of main */
--------------------------------------------------------------------------

$ gcc -rdynamic -fvisibility=hidden -Wall -pipe -O0 -s -o test_1 test_1.c

"-fvisibility=hidden"将缺省visibility由default改为hidden，test_1.c中没有明
确EXPORT的符号便不会导出。

$ readelf -Ws test_1

Symbol table '.dynsym' contains 20 entries:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
     0: 00000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_deregisterTMCloneTable
     2: 00000000     0 NOTYPE  WEAK   DEFAULT  UND __gmon_start__
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.0 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT  UND rand@GLIBC_2.0 (2)
     5: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _Jv_RegisterClasses
     6: 00000000     0 NOTYPE  WEAK   DEFAULT  UND _ITM_registerTMCloneTable
     7: 080498d8     0 NOTYPE  GLOBAL DEFAULT   24 _edata
     8: 080485db    10 FUNC    GLOBAL DEFAULT   13 bar
     9: 080498d0     0 NOTYPE  GLOBAL DEFAULT   24 __data_start
    10: 080498dc     0 NOTYPE  GLOBAL DEFAULT   25 _end
    11: 080498d0     0 NOTYPE  WEAK   DEFAULT   24 data_start
    12: 080486ac     4 OBJECT  GLOBAL DEFAULT   15 _IO_stdin_used
    13: 08048620    97 FUNC    GLOBAL DEFAULT   13 __libc_csu_init
    14: 080484e0     0 FUNC    GLOBAL DEFAULT   13 _start
    15: 080486a8     4 OBJECT  GLOBAL DEFAULT   15 _fp_hw
    16: 080498d8     0 NOTYPE  GLOBAL DEFAULT   25 __bss_start
    17: 08048478     0 FUNC    GLOBAL DEFAULT   11 _init
    18: 08048690     2 FUNC    GLOBAL DEFAULT   13 __libc_csu_fini
    19: 08048694     0 FUNC    GLOBAL DEFAULT   14 _fini
$ nm -D test_1
080486ac R _IO_stdin_used
         w _ITM_deregisterTMCloneTable
         w _ITM_registerTMCloneTable
         w _Jv_RegisterClasses
080498d8 B __bss_start
080498d0 D __data_start
         w __gmon_start__
08048690 T __libc_csu_fini
08048620 T __libc_csu_init
         U __libc_start_main
080498d8 D _edata
080498dc B _end
08048694 T _fini
080486a8 R _fp_hw
08048478 T _init
080484e0 T _start
080485db T bar
080498d0 W data_start
         U rand

与Export Maps技术相比，visibility技术生成的代码更高效，hidden的函数不经过
PLT。

3)

$ vi test_2.c

--------------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>

unsigned int bar ( void )
{
    return( 100 );
}  /* end of bar */

int main ( int argc, char * argv[] )
{
    unsigned int    a, b;

    a   = bar();
    b   = ( unsigned int )rand();
    return( a + b );
}  /* end of main */
--------------------------------------------------------------------------

$ vi test_2.map

--------------------------------------------------------------------------
{
    global  :

        bar;
        _IO_stdin_used;

    local   :

        *;
};
--------------------------------------------------------------------------

$ gcc -Wl,--version-script=test_2.map -rdynamic -Wall -pipe -O0 -s -o test_2 test_2.c

Export Maps必须与-rdynamic一起使用，否则无意义。

test_2.map中额外指定了_IO_stdin_used，否则该符号不会被导出，而test_1导出了
该符号。这是个啥，有必要导出吗？

$ readelf -Ws test_2

Symbol table '.dynsym' contains 6 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 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.0 (2)
     3: 00000000     0 FUNC    GLOBAL DEFAULT  UND rand@GLIBC_2.0 (2)
     4: 0804840b    10 FUNC    GLOBAL DEFAULT   13 bar
     5: 080484dc     4 OBJECT  GLOBAL DEFAULT   15 _IO_stdin_used
$ nm -D test_2
080484dc R _IO_stdin_used
         w __gmon_start__
         U __libc_start_main
0804840b T bar
         U rand

4)

libtool的"-export-symbols"是传说中的第4种方案，据我实测无效。

$ vi test_2.sym

--------------------------------------------------------------------------
bar
_IO_stdin_used
--------------------------------------------------------------------------

$ libtool --mode=compile gcc -Wall -pipe -O0 -c -o test_3.o test_2.c
$ libtool --mode=link gcc -export-symbols test_2.sym -rdynamic -Wall -pipe -O0 -s -o test_3 test_3.o

或

$ libtool --mode=link gcc -export-symbols-regex "^bar" -rdynamic -Wall -pipe -O0 -s -o test_4 test_2.c

"-export-symbols"或"-export-symbols-regex"直接被忽略了。放狗搜了一下，好像
在Linux上就是无效。