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linux动态链接库导出函数控制

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windows 环境的vc的话,可以方便的指定__declspec(dllexport) 关键字来控制是否把dll中的函数导出。

我也来测试一下linux下面是如何做的:
先看gcc 和ld的相关选项

======================================
gcc 选项
       -shared
           Produce a shared object which can then be linked with other objects
           to form an executable. Not all systems support this option. For
           predictable results, you must also specify the same set of options
           that were used to generate code (-fpic, -fPIC, or model suboptions)
           when you specify this option.[1]


       -fpic
           Generate position-independent code (PIC) suitable for use in a
           shared library, if supported for the target machine. Such code
           accesses all constant addresses through a global offset table
           (GOT). The dynamic loader resolves the GOT entries when the
           program starts (the dynamic loader is not part of GCC; it is part
           of the operating system). If the GOT size for the linked
           executable exceeds a machine-specific maximum size, you get an
           error message from the linker indicating that -fpic does not work;
           in that case, recompile with -fPIC instead. (These maximums are 8k
           on the SPARC and 32k on the m68k and RS/6000. The 386 has no such
           limit.)

           Position-independent code requires special support, and therefore
           works only on certain machines. For the 386, GCC supports PIC for
           System V but not for the Sun 386i. Code generated for the IBM
           RS/6000 is always position-independent.

           When this flag is set, the macros "__pic__" and "__PIC__" are
           defined to 1.

       -fPIC
           If supported for the target machine, emit position-independent
           code, suitable for dynamic linking and avoiding any limit on the
           size of the global offset table. This option makes a difference on
           the m68k, PowerPC and SPARC.

           Position-independent code requires special support, and therefore
           works only on certain machines.

           When this flag is set, the macros "__pic__" and "__PIC__" are
           defined to 2.

       -rdynamic
           Pass the flag -export-dynamic to the ELF linker, on targets that
           support it. This instructs the linker to add all symbols, not only
           used ones, to the dynamic symbol table. This option is needed for
           some uses of "dlopen" or to allow obtaining backtraces from within
           a program.


      -fvisibility=default|internal|hidden|protected
           Set the default ELF image symbol visibility to the specified
           option---all symbols will be marked with this unless overridden
           within the code. Using this feature can very substantially improve
           linking and load times of shared object libraries, produce more
           optimized code, provide near-perfect API export and prevent symbol
           clashes. It is strongly recommended that you use this in any
           shared objects you distribute.

           Despite the nomenclature, "default" always means public ie;
           available to be linked against from outside the shared object.
           "protected" and "internal" are pretty useless in real-world usage
           so the only other commonly used option will be "hidden". The
           default if -fvisibility isn't specified is "default", i.e., make
           every symbol public---this causes the same behavior as previous
           versions of GCC.

           A good explanation of the benefits offered by ensuring ELF symbols
           have the correct visibility is given by "How To Write Shared
           Libraries" by Ulrich Drepper (which can be found at
           <http://people.redhat.com/~drepper/>)---however 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.


           For those adding visibility support to existing code, you may find
           #pragma GCC visibility of use. This works by you enclosing the
           declarations you wish to set visibility for with (for example)
           #pragma GCC visibility push(hidden) and #pragma GCC visibility pop.
           Bear in mind that symbol visibility should be viewed as part of the
           API interface contract and thus all new code should always specify
           visibility when it is not the default ie; declarations only for use
           within the local DSO should always be marked explicitly as hidden
           as so to avoid PLT indirection overheads---making this abundantly
           clear also aids readability and self-documentation of the code.
           Note that due to ISO C++ specification requirements, operator new
           and operator delete must always be of default visibility.

           Be aware that headers from outside your project, in particular
           system headers and headers from any other library you use, may not
           be expecting to be compiled with visibility other than the default.
           You may need to explicitly say #pragma GCC visibility push(default)
           before including any such headers.

           extern declarations are not affected by -fvisibility, so a lot of
           code can be recompiled with -fvisibility=hidden with no
           modifications. However, this means that calls to extern functions
           with no explicit visibility will use the PLT, so it is more
           effective to use __attribute ((visibility)) and/or #pragma GCC
           visibility to tell the compiler which extern declarations should be
           treated as hidden.

           Note that -fvisibility does affect C++ vague linkage entities. This
           means that, for instance, an exception class that will be thrown
           between DSOs must be explicitly marked with default visibility so
           that the type_info nodes will be unified between the DSOs.

           An overview of these techniques, their benefits and how to use them
           is at <http://gcc.gnu.org/wiki/Visibility>.

===================================
ld命令选项


       -E
       --export-dynamic
       --no-export-dynamic
           When creating a dynamically linked executable, using the -E option
           or the --export-dynamic option causes the linker to add all symbols
           to the dynamic symbol table. The dynamic symbol table is the set
           of symbols which are visible from dynamic objects at run time.

           If you do not use either of these options (or use the
           --no-export-dynamic option to restore the default behavior), the
           dynamic symbol table will normally contain only those symbols which
           are referenced by some dynamic object mentioned in the link.

           If you use "dlopen" to load a dynamic object which needs to refer
           back to the symbols defined by the program, rather than some other
           dynamic object, then you will probably need to use this option when
           linking the program itself.

           You can also use the dynamic list to control what symbols should be
           added to the dynamic symbol table if the output format supports it.
           See the description of --dynamic-list.

           Note that this option is specific to ELF targeted ports. PE
           targets support a similar function to export all symbols from a DLL
           or EXE; see the description of --export-all-symbols below.


       --dynamic-list=dynamic-list-file
           Specify the name of a dynamic list file to the linker. This is
           typically used when creating shared libraries to specify a list of
           global symbols whose references shouldn't be bound to the
           definition within the shared library, or creating dynamically
           linked executables to specify a list of symbols which should be
           added to the symbol table in the executable. This option is only
           meaningful on ELF platforms which support shared libraries.

           The format of the dynamic list is the same as the version node
           without scope and node name. See VERSION for more information.

       --dynamic-list-data
           Include all global data symbols to the dynamic list.

 

       --version-script=version-scriptfile
           Specify the name of a version script to the linker. This is typically used when creating shared libraries to
           specify additional information about the version hierarchy for the library being created. This option is only
           fully supported on ELF platforms which support shared libraries; see VERSION. It is partially supported on PE
           platforms, which can use version scripts to filter symbol visibility in auto-export mode: any symbols marked
           local in the version script will not be exported.

     #VERSION 文件格式可以参考这里     http://sourceware.org/binutils/docs-2.20/ld/VERSION.html#VERSION
=========================================

 

测试看看:

 

-----------------------main.c -----------------------------------
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

#include<dlfcn.h>

int test (int i)
{

printf("i=%d\n" ,i);
}

int main() 
{
    int (*test2)(int);
    int (*test3)(int);
    int *handler;


    handler=(int*)dlopen("test.so", RTLD_LAZY);
    if (!handler) {
        printf( "加载模块错误 %s\n", dlerror() );
        return;
    }

    test3 = dlsym(handler, "test3");
        if (test3) test3(3);

     test2 = dlsym(handler, "test2");
        if (test2) test2(2);

        dlclose(handler);

exit(0);

}

------------------------------------------------------------------
$ gcc -rdynamic main.c -o main -ldl

$ readelf -s main

Symbol table '.dynsym' contains 25 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 FUNC    GLOBAL DEFAULT UND dlclose@GLIBC_2.0 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.0 (3)
     5: 00000000     0 FUNC    GLOBAL DEFAULT UND dlsym@GLIBC_2.0 (2)
     6: 00000000     0 FUNC    GLOBAL DEFAULT UND dlopen@GLIBC_2.1 (4)
     7: 00000000     0 FUNC    GLOBAL DEFAULT UND printf@GLIBC_2.0 (3)
     8: 00000000     0 FUNC    GLOBAL DEFAULT UND dlerror@GLIBC_2.0 (2)
     9: 00000000     0 FUNC    GLOBAL DEFAULT UND exit@GLIBC_2.0 (3)
    10: 0804a020     0 NOTYPE GLOBAL DEFAULT   24 __data_start
    11: 0804a028    22 OBJECT GLOBAL DEFAULT   24 str
    12: 0804a048     0 NOTYPE GLOBAL DEFAULT ABS _end
    13: 0804a040     0 NOTYPE GLOBAL DEFAULT ABS _edata
    14: 0804a020     0 NOTYPE WEAK   DEFAULT   24 data_start
    15: 080486b0     0 FUNC    GLOBAL DEFAULT   14 _start
    16: 080488f8     4 OBJECT GLOBAL DEFAULT   16 _fp_hw
    17: 080488fc     4 OBJECT GLOBAL DEFAULT   16 _IO_stdin_used
    18: 08048850    90 FUNC    GLOBAL DEFAULT   14 __libc_csu_init
    19: 0804a040     0 NOTYPE GLOBAL DEFAULT ABS __bss_start
    20: 08048764    28 FUNC    GLOBAL DEFAULT   14 test   ///test 在这里导出了
    21: 08048780   178 FUNC    GLOBAL DEFAULT   14 main
    22: 080485ec     0 FUNC    GLOBAL DEFAULT   12 _init
    23: 08048840     5 FUNC    GLOBAL DEFAULT   14 __libc_csu_fini
    24: 080488dc     0 FUNC    GLOBAL DEFAULT   15 _fini


----------------------------------------------------------------------------------

$ gcc main.c -o main -ldl

$ readelf -s main

Symbol table '.dynsym' contains 11 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 FUNC    GLOBAL DEFAULT UND dlclose@GLIBC_2.0 (2)
     4: 00000000     0 FUNC    GLOBAL DEFAULT UND __libc_start_main@GLIBC_2.0 (3)
     5: 00000000     0 FUNC    GLOBAL DEFAULT UND dlsym@GLIBC_2.0 (2)
     6: 00000000     0 FUNC    GLOBAL DEFAULT UND dlopen@GLIBC_2.1 (4)
     7: 00000000     0 FUNC    GLOBAL DEFAULT UND printf@GLIBC_2.0 (3)
     8: 00000000     0 FUNC    GLOBAL DEFAULT UND dlerror@GLIBC_2.0 (2) ///没有把 test函数导出
     9: 00000000     0 FUNC    GLOBAL DEFAULT UND exit@GLIBC_2.0 (3)
    10: 080486fc     4 OBJECT GLOBAL DEFAULT   16 _IO_stdin_used

 

====================================================================
$ gcc -rdynamic -fPIC main.c -o main -ldl

$ readelf -s main

可以看到有 '.dynsym' 节有把函数导出   

=======================================================================

 

 

 

 

------test.c---------------------
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

extern int test (int i);

int test2 (int i)
{
     test(i);
     printf("this is test2\n");
}

int test3 (int i)
{
     printf("this is test 3\n");
}
-----------------------------------------------

$ gcc -shared -fPIC test.c -o test.so


$ readelf -s test.so

'.dynsym' 节里把所有函数导出

====================================================================

$ ./main
加载模块错误 test.so: cannot open shared object file: No such file or directory  

找不到 test.so文件,他不会自动在当前目录下搜索啊。
那就设置好这个LD_LIBRARY_PATH这个环境变量吧,我是不想复制到系统目录去了

$ echo $LD_LIBRARY_PATH

$ export LD_LIBRARY_PATH=`pwd`
$ echo $LD_LIBRARY_PATH
/home/widebright/桌面/测试用例

--------------------------
gcc -rdynamic -fPIC main.c -o main -ldl
gcc -shared -fPIC test.c -o test.so
使用上面编译选线,结果 
$ ./main
this is test 3
i=2
this is test2
--------------------------------
gcc -fPIC main.c -o main -ldl
gcc -shared -fPIC test.c -o test.so
使用上面编译选线,结果test.so就找不到 main中export出来的test函数了
$ ./main 
this is test 3
./main: symbol lookup error: /home/widebright/桌面/测试用例/test.so: undefined symbol: test
--------------------------------
gcc -rdynamic main.c -o main -ldl
gcc -shared -fPIC test.c -o test.so
使用上面编译选线,结果还是可以执行,说明 -fPIC对程序来说好像用处不大。
$ ./main
this is test 3
i=2
this is test2
------------------------------------------------
gcc -rdynamic main.c -o main -ldl
gcc -shared test.c -o test.so
使用上面编译选线,也还能正确运行 -fPIC 有什么作用啊,不明显
$ ./main
this is test 3
i=2
this is test2
---------------------------------------------

 


----visibility.txt------------------

    global:
            test3;   
    local: *;
};
-------------------

gcc -rdynamic main.c -o main -ldl
gcc -shared test.c -o test.so -Wl,--version-script=visibility.txt

$ gcc -shared test.c -o test.so -Wl,--version-script=visibility.txt
$ ./main 
this is test 3

可以看到 使用--version-script=visibility.txt 参数控制之后,只有test3才被导出了

:~/桌面/测试readelf -s test.so

Symbol table '.dynsym' contains 7 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: 0000041b    20 FUNC    GLOBAL DEFAULT   12 test3    //visibility.txt文件里面指定了 test3才导出了, test2不再导出了

======================================

 

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

----当 -fvisibility=hidden 设置为hidden之后,所有的都函数不再导出了。
readelf -s test.so

Symbol table '.dynsym' contains 11 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    ///这个还是会在这里,知道是外部函数来的
     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: 0000202c     0 NOTYPE GLOBAL DEFAULT ABS _end
     7: 00002024     0 NOTYPE GLOBAL DEFAULT ABS _edata
     8: 00002024     0 NOTYPE GLOBAL DEFAULT ABS __bss_start
     9: 0000035c     0 FUNC    GLOBAL DEFAULT   10 _init
    10: 000004e8     0 FUNC    GLOBAL DEFAULT   13 _fini

==============================================

我们给要导出到函数加上 属性控制,改成这样,
----------test.c-----------------
#include<stdio.h>
#include<string.h>
#include<stdlib.h>

extern int test (int i);

__attribute ((visibility)) int test2 (int i)
{
     test(i);
     printf("this is test2\n");
}

__attribute ((visibility)) int test3 (int i)
{
     printf("this is test 3\n");
}

-------------------------------

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


$ readelf -s test.so

Symbol table '.dynsym' contains 13 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: 0000202c     0 NOTYPE GLOBAL DEFAULT ABS _end
     7: 00002024     0 NOTYPE GLOBAL DEFAULT ABS _edata
     8: 000004bc    31 FUNC    GLOBAL DEFAULT   12 test2    //这次就会导出了
     9: 00002024     0 NOTYPE GLOBAL DEFAULT ABS __bss_start
    10: 000003a0     0 FUNC    GLOBAL DEFAULT   10 _init
    11: 00000528     0 FUNC    GLOBAL DEFAULT   13 _fini
    12: 000004db    20 FUNC    GLOBAL DEFAULT   12 test3


$ ./main 
this is test 3
i=2
this is test2
-----------------------------------------------------

 

 


总结:

通过gcc命令的-fvisibility=hidden 选项和 "__attribute__ ((visibility("default")))" 语法扩展 可以得到 vc中
__declspec(dllexport)"的效果。

采用ld的 --version-script=version-scriptfile 参数 类似vc中到 *.DEF 文件,也可以用来统一链接库到输出与否。


测完才发现,gcc帮助文档提到的 “How To Write Shared Libraries” 这篇文章上面解释的更完整一些,
一个pdf文档,在google搜索一下就可以找到了。

其他的办法还包括:
1. static 定义函数

2、libtool的-export-symbols参数
$ libtool --mode=link gcc -o libfoo.la \
foo.lo -export-symbols=foo.sym

3.使用alias属性来隐藏具体的函数名
int next (void) {    
return ++last_int;         
}                           
extern __typeof (next) next_int
                                       
__attribute ((alias ("next"),
                                         
                visibility ("hidden")));

Tags:函数 链接 动态
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