杨胜吉他:ld的-rpath与-rpath-link选项

ld的-rpath与-rpath-link选项  

2011-05-13 14:07:15|  分类： toolchain |  标签： |字号大中小 订阅

rtspplayer这个程序在手机上运行的时候，要使用fbsink来作为videosink组件。这是使用了framebuffer的一个videosink组件。所以在代码中，很自然只需要在gst_element_factory_make的时候将xvimagesink换成fbsink就可以了。但是程序在scratchbox中编译了之后，传到了手机上，运行却发生错误，报告说:

No such element ...... `fbsink'

奇怪的是在手机上使用alp-gst-inspect|grep"fbsink"却能查到这个element。后来才知道，fbsink这个element存在于/opt/alp/lib/libalp_media.so文件中。而这个.so文件不在rtspplayer运行期动态链接库的查找范围之内，所以，fbsink创建就失败了。

如何让libalp_media.so这个文件存在于rtspplayer运行期的动态链接库的查找范围内呢？很简单，在编译的时候，加上 -L/opt/alp/lib-lalp_media即可。也就是说，虽然代码没有显式的调用这个.so中的函数，但是我们在编译的时候手动将这个.so作为一种dependency链接上，这样就OK了。但是，加上这样的内容后，编译发生错误：

gcc -Wall -g -o rtspplayer`pkg-config --cflags --libs gtk+-2.0` -I/usr/include/gstreamer-0.10-L/opt/alp/lib -lGSTfusion -lalp_media main.c gstplay.cgstplay-marshal.c
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_audiomgr.so, needed by /opt/alp/lib/libalp_media.so,not found (try using -rpath or -rpath-link)
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_gstaudiomgrsrc.so, needed by/opt/alp/lib/libalp_media.so, not found (try using -rpath or-rpath-link)
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_gstaudiomgrsink.so, needed by/opt/alp/lib/libalp_media.so, not found (try using -rpath or-rpath-link)
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_core.so, needed by /opt/alp/lib/libalp_media.so, notfound (try using -rpath or -rpath-link)
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_sysutils.so, needed by /opt/alp/lib/libalp_media.so,not found (try using -rpath or -rpath-link)
/scratchbox/compilers/arm-linux-gnueabi/bin/../lib/gcc/arm-none-linux-gnueabi/4.1.2/http://www.cnblogs.com/http://www.cnblogs.com/arm-none-linux-gnueabi/bin/ld:warning: libalp_drm1.so, needed by /opt/alp/lib/libalp_media.so, notfound (try using -rpath or -rpath-link)
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_fclose'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_message_get_message_ID'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_settings_open'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_message_unpack_start'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_log'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_settings_close'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_is_drm_file'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_settings_init'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_fseek'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_fopen_with_permission'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_settings_get'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_connection_register_disconnect_callback'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_message_unpack_end'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_channel_connect'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_connection_register_receive_callback'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_check_rights_info'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_message_unpack_int32'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_ftell'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_settings_value_clean'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_ipc_message_unpack_string'
/opt/alp/lib/libalp_media.so: undefined reference to `alp_drm_fread'
collect2: ld returned 1 exit status
make: *** [rtspplayer] Error 1

这个错误报告说找不到libalp_gstaudiomgrsrc.so等一堆so文件，这些so文件都位于/opt/alp/lib目录下，他们是libalp_media.so所需要的。换句话说，在libalp_media.so中还存在需要的动态链接库。

在这种情况下，-rpath,-rpath-link这两个选项应运而生了。其实我们费这么大劲，无非就是要把libalp_media.so这个加入到rtspplayer的动态链接库查找列表中去，形象一点来说，就是使用命令ldd ./rtspplayer的时候，能看到libalp_media.so这一行。

-rpath/-rpath-link其实都是ld的option，不是gcc的。gcc只是一个wrapper，将preprocessor, assemble,link三者结合了起来。-rpath的作用是手动将一个目录强行指定成一个.so文件的搜索目录，他的优先级在LD_LIBRARY_PATH和/etc/ld.so.conf...这些地方定义的.so文件搜索目录之上。由于这个目录是由程序的开发人员在编译的时候指定的，所以将来这个程序到了其他机器上运行的时候，这个目录是不能修改的。

-rpath-link和-rpath类似，只不过-rpath-link指定的是该程序需要的某个动态链接库，如果还需要其他的动态链接库（就象我们这里libalp_media.so还需要其他的.so一样）的时候，到哪个/哪些目录下去查找需要的.so。而且，和-rpath不同，在-rpath-link中定义的目录，有可能在ldd的输出中看不见，因为这里定义的目录不是该执行程序本身所需要的。

清楚了这两个option的含义之后，来看用法，要在gcc的命令行中直接使用这两个option，必须遵循语法：-Wl,......。比 如：-Wl,--rpath-link/opt/alp/lib。-Wl就是告诉gcc，后面的内容是传递给linker的option。如果直接使用ld的话，就不需要-Wl,了。所以，上面我们的编译命令就变成这样，就OK了：

gcc -Wall -g -o rtspplayer-Wl,--rpath-link /opt/alp/lib `pkg-config --cflags --libs gtk+-2.0`-I/usr/include/gstreamer-0.10 -L/opt/alp/lib -lGSTfusion -lalp_mediamain.c gstplay.c gstplay-marshal.c

这样，再次将生成的rtspplayer拷贝到手机中就可以运行了，不会再说找不到fbsink element了。

这里是gcc命令行中给assembler, preprocessor, linker传递option的具体关键字列表：

Code: Select all

  -Wa,            Pass comma-separated on to the assembler
  -Wp,            Pass comma-separated on to the preprocessor
  -Wl,            Pass comma-separated on to the linker

此外，定义LD_LIBRARY_PATH也是可以的。这样就不用加-Wl,--rpath-link /opt/alp/lib这个内容了。具体的ld是如何查找动态链接库的逻辑，请看man ld中有关-rpath, -rpath-link这两部分的详细解释。

-rpath=dir

Adda directory to the runtime library search path. This is used whenlinking an ELF executable with shared objects. All -rpath arguments areconcatenated and passed to the runtime linker, which uses them tolocate shared objects at runtime. The -rpath option is also used whenlocating shared objects which are needed by shared objects explicitlyincluded in the link; see the description of the -rpath-link option. If-rpath is not used when linking an ELF executable, the contents of theenvironment variable LD_RUN_PATH will be used if it is defined.

-rpath-link=dir

Whenusing ELF or SunOS, one shared library may require another. Thishappens when an ld -shared link includes a shared library as one of theinput files.

Whenthe linker encounters such a dependency when doing a non-shared,non-relocatable link, it will automatically try to locate the requiredshared library and include it in the link, if it is not includedexplicitly. In such a case, the -rpath-link option specifies the firstset of directories to search. The -rpath-link option may specify asequence of directory names either by specifying a list of namesseparated by colons, or by appearing multiple times.

Thisoption should be used with caution as it overrides the search path thatmay have been hard compiled into a shared library. In such a case it ispossible to use unintentionally a different search path than theruntime linker would do.

Thedifference between -rpath and -rpath-link is that directories specifiedby -rpath options are included in the executable and used at runtime,whereas the -rpath-link option is only effective at link time.Searching -rpath in this way is only supported by native linkers andcross linkers which have been configured with the --with-sysroot option.