Lib依赖库

1. 查看依赖库

# ldd xxx

当出现如下错误：error while loading shared libraries: libaio.so.1: cannot open shared object file: No such file or directory
解决办法：

如果是ubuntu，ld默认的目录是/lib和/usr/lib，不会查找/usr/lib64，需要手动添加。redhat会自动查找
先用查找命令在相应的lib目录下查找，是否此库文件存在
修改 /etc/ld.so.conf 文件，将路径添加到最后即可
或者在 /etc/ld.so.conf.d/ 下新建一文件，如 XXX.conf，其内容还是库路径
# sudo ldconfig

注：每次添加新的库文件时，都需要重新生效

2. 如果找不到

可能是库的权限问题，普通用户用不了

3. 如果还找不到

说明64位系统和32的支持

# sudo apt-get install ia32-libs

4. rpath

man ld，查找链接选项rpath的含义:

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
-rpath=dir
Add a directory to the runtime library search path. This is used when
linking an ELF executable with shared objects. All -rpath arguments
are concatenated and passed to the runtime linker,which uses them to
locate shared objects at runtime. The -rpath option is also used when
locating shared objects which are needed by shared objects explicitly
included in the link; see the description of the -rpath-link option.
If -rpath is not used when linking an ELF executable,the contents of
the environment variable “LD_RUN_PATH” will be used if it is defined.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
简单翻译下,rpath链接选项主要有两个功能：

程序运行时，优先到rpath指定的目录去寻找依赖库
程序链接时，在指定的目录中，隐式的链接那些动态库所需要的链接库

5. 动态链接的优先级

The linker uses @H_404_62@the following search paths to locate required shared libraries:
           1.  Any directories specified by -rpath-link options.
           2.  Any directories specified by -rpath options.  The difference
               between -rpath @H_404_62@and -rpath-link is that directories specified by
               -rpath options are included @H_404_62@in @H_404_62@the executable @H_404_62@and used at runtime,whereas @H_404_62@the -rpath-link option is only effective at link time.
               Searching -rpath @H_404_62@in this way is only supported by native linkers
               @H_404_62@and cross linkers which have been configured @H_404_62@with @H_404_62@the
               --with-sysroot option.
           3.  On @H_404_62@an ELF system,for native linkers,if @H_404_62@the -rpath @H_404_62@and -rpath-link
               options were @H_404_62@not used,search @H_404_62@the contents @H_404_62@of @H_404_62@the environment
               variable "LD_RUN_PATH".
           4.  On SunOS,if @H_404_62@the -rpath option was @H_404_62@not used,search any directories
               specified using -L options.
           5.  For @H_404_62@a native linker,@H_404_62@the search @H_404_62@the contents @H_404_62@of @H_404_62@the environment
               variable "LD_LIBRARY_PATH".
           6.  For @H_404_62@a native ELF linker,@H_404_62@the directories @H_404_62@in "DT_RUNPATH" @H_404_62@or
               "DT_RPATH" @H_404_62@of @H_404_62@a shared library are searched for shared libraries
               needed by it. The "DT_RPATH" entries are ignored if "DT_RUNPATH"
               entries exist.
           7.  The default directories,normally /lib @H_404_62@and /usr/lib.
           8.  For @H_404_62@a native linker on an ELF system,if the file /etc/ld.so.conf
               exists,@H_404_62@the list @H_404_62@of directories found @H_404_62@in that file.

           If @H_404_62@the required shared library is @H_404_62@not found,@H_404_62@the linker will issue @H_404_62@a
           warning @H_404_62@and continue @H_404_62@with @H_404_62@the link.

linux的默认搜索路径是/lib和/usr/lib，然后按照/etc/ld.so.conf里面的配置搜索绝对路径
LD_LIBRARY_PATH路径优先于系统默认路径之前查找
gcc的-R或-rpath选项来在编译时就指定库的查找路径，并且该库的路径信息保存在可执行文件中，运行时它会直接到该路径查找库

6. 动态库的链接

-L: “链接”的时候，去找的目录。都会先从 -L 指定的目录去找，然后是默认的地方。-L只是指定了程序编译连接时库的路径，并不影响程序执行时库的路径，系统还是会到默认路径下查找该程序所需要的库
-rpath-link：“链接”的时候，去找的目录。
-rpath: “运行”的时候，去找的目录。对于交叉编译，交叉编译链接器需已经配置 –with-sysroot 选项才能起作用。

7. ld链接选项

在gcc中使用ld链接选项时，需要在选项前面加上前缀-Wl(是字母l，不是1），以区别不是编译器的选项

# gcc -Wl,--start-group foo.o bar.o -Wl,--end-group