-Wl选项告诉编译器将后面的参数传递给链接器。
-soname则指定了动态库的soname(简单共享名，Short for shared object name)
soname的关键功能是它提供了兼容性的标准：
当要升级系统中的一个库时，并且新库的soname和老库的soname一样，用旧库链接生成的程序使用新库依然能正常运行。这个特性使得在Linux下，升级使得共享库的程序和定位错误变得十分容易。
在Linux中，应用程序通过使用soname，来指定所希望库的版本，库作者可以通过保留或改变soname来声明，哪些版本是兼容的，这使得程序员摆脱了共享库版本冲突问题的困扰。
可以通过readelf -d来查看每个动态库的SONAME
1. 声明libto.so.1，并生成libto.so.1.2
[root@localhost c]# gcc -fPIC -shared -Wl,-soname,libto.so.1 -o libto.so.1.2 to.c [root@localhost c]# ls -lh -rwxr-xr-x 1 root root 4268 Jan 10 17:22 libto.so.1.2 [root@localhost c]# ldconfig -n ./ lrwxrwxrwx 1 root root 12 Jan 10 17:23 libto.so.1 -> libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:22 libto.so.1.2
[root@localhost c]# readelf -d libto.so.1.2 Dynamic section at offset 0x504 contains 21 entries: Tag Type Name/Value 0x00000001 (NEEDED) Shared library: [libc.so.6] 0x0000000e (SONAME) Library soname: [libto.so.1] 0x0000000c (INIT) 0x2cc 0x0000000d (FINI) 0x4c4 0x6ffffef5 (GNU_HASH) 0xb4 0x00000005 (STRTAB) 0x1b4 0x00000006 (SYMTAB) 0xf4 0x0000000a (STRSZ) 150 (bytes) 0x0000000b (SYMENT) 16 (bytes) 0x00000003 (PLTGOT) 0x15d8 0x00000002 (PLTRELSZ) 24 (bytes) 0x00000014 (PLTREL) REL 0x00000017 (JMPREL) 0x2b4 0x00000011 (REL) 0x294 0x00000012 (RELSZ) 32 (bytes) 0x00000013 (RELENT) 8 (bytes) 0x6ffffffe (VERNEED) 0x264 0x6fffffff (VERNEEDNUM) 1 0x6ffffff0 (VERSYM) 0x24a 0x6ffffffa (RELCOUNT) 1 0x00000000 (NULL) 0x0
2. 声明libto.so.1，并生成libto.so.1.3
[root@localhost c]# gcc -fPIC -shared -Wl,-soname,libto.so.1 -o libto.so.1.3 to.c [root@localhost c]# ls -lh lrwxrwxrwx 1 root root 12 Jan 10 17:23 libto.so.1 -> libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:22 libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:23 libto.so.1.3 [root@localhost c]# ldconfig -n ./ lrwxrwxrwx 1 root root 12 Jan 10 17:24 libto.so.1 -> libto.so.1.3 #重新ldconfig，指向新的库文件 -rwxr-xr-x 1 root root 4.2K Jan 10 17:22 libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:23 libto.so.1.3
[root@localhost c]# readelf -d libto.so.1.3 Dynamic section at offset 0x504 contains 21 entries: Tag Type Name/Value 0x00000001 (NEEDED) Shared library: [libc.so.6] 0x0000000e (SONAME) Library soname: [libto.so.1] 0x0000000c (INIT) 0x2cc 0x0000000d (FINI) 0x4c4 0x6ffffef5 (GNU_HASH) 0xb4 0x00000005 (STRTAB) 0x1b4 0x00000006 (SYMTAB) 0xf4 0x0000000a (STRSZ) 150 (bytes) 0x0000000b (SYMENT) 16 (bytes) 0x00000003 (PLTGOT) 0x15d8 0x00000002 (PLTRELSZ) 24 (bytes) 0x00000014 (PLTREL) REL 0x00000017 (JMPREL) 0x2b4 0x00000011 (REL) 0x294 0x00000012 (RELSZ) 32 (bytes) 0x00000013 (RELENT) 8 (bytes) 0x6ffffffe (VERNEED) 0x264 0x6fffffff (VERNEEDNUM) 1 0x6ffffff0 (VERSYM) 0x24a 0x6ffffffa (RELCOUNT) 1 0x00000000 (NULL) 0x0
3. 声明libto.so.2，并生成libto.so.1.4
[root@localhost c]# gcc -fPIC -shared -Wl,-soname,libto.so.2 -o libto.so.1.4 to.c [root@localhost c]# ls -lh lrwxrwxrwx 1 root root 12 Jan 10 17:24 libto.so.1 -> libto.so.1.3 -rwxr-xr-x 1 root root 4.2K Jan 10 17:22 libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:23 libto.so.1.3 -rwxr-xr-x 1 root root 4.2K Jan 10 17:24 libto.so.1.4 [root@localhost c]# ldconfig -n ./ lrwxrwxrwx 1 root root 12 Jan 10 17:24 libto.so.1 -> libto.so.1.3 #重新ldconfig，不指向新的库文件，因为新库(1.4)的soname为libto.so.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:22 libto.so.1.2 -rwxr-xr-x 1 root root 4.2K Jan 10 17:23 libto.so.1.3 -rwxr-xr-x 1 root root 4.2K Jan 10 17:24 libto.so.1.4 lrwxrwxrwx 1 root root 12 Jan 10 17:24 libto.so.2 -> libto.so.1.4
[root@localhost c]# readelf -d libto.so.1.4 Dynamic section at offset 0x504 contains 21 entries: Tag Type Name/Value 0x00000001 (NEEDED) Shared library: [libc.so.6] 0x0000000e (SONAME) Library soname: [libto.so.2] 0x0000000c (INIT) 0x2cc 0x0000000d (FINI) 0x4c4 0x6ffffef5 (GNU_HASH) 0xb4 0x00000005 (STRTAB) 0x1b4 0x00000006 (SYMTAB) 0xf4 0x0000000a (STRSZ) 150 (bytes) 0x0000000b (SYMENT) 16 (bytes) 0x00000003 (PLTGOT) 0x15d8 0x00000002 (PLTRELSZ) 24 (bytes) 0x00000014 (PLTREL) REL 0x00000017 (JMPREL) 0x2b4 0x00000011 (REL) 0x294 0x00000012 (RELSZ) 32 (bytes) 0x00000013 (RELENT) 8 (bytes) 0x6ffffffe (VERNEED) 0x264 0x6fffffff (VERNEEDNUM) 1 0x6ffffff0 (VERSYM) 0x24a 0x6ffffffa (RELCOUNT) 1 0x00000000 (NULL) 0x0
总结：程式库主要的升级会破坏相容性；而次要的升级则可能不会；那麽以下面的方式来连结，所有的一切就都会相安无事了。
gcc -shared -Wl,-soname,libfoo.so.major -o libfoo.so.major.minor
gcc -shared -Wl,-soname,libfoo.so.major -o libfoo.so.major.minor
DT_NEEDED tag：
DT_NEEDED： 表示一个列表，列表里面以（NEEDED）为标志的项，就是当前库加载时要依赖的其它库。注意 DT_NEEDED 中的 DT 不是 DON'T 的意思。
1 gcc/g++链接时.o文件以及库的顺序问题
1.1 写在前面
最近换了xubuntu12.4，把原来的项目co出来编译的时候报“undefined reference to”。猜测是gcc的版本问题，用-v跟踪一下，发现gcc-4.6默认开 启了ld的–as-needed选项。关闭该选项（–no-as-needed）后编译正常。深 入挖掘发现还是一个比较有意思的问题。
1.3 技术铺垫
1.3.2 linux下查看一个可执行文件或动态库依赖哪些动态库的办法
你有一个library或者是可执行文件，你可以这样查看他的依赖关系：
- readelf -d
ocaml@ocaml:~$ readelf -d PyGalaxy.so Dynamic section at offset 0x7dd8 contains 26 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libGalaxyParser.so] ... 0x000000000000000c (INIT) 0x18e0 0x000000000000000d (FINI) 0x6398
- ldd工具,如下：
ocaml@ocaml:~$ ldd PyGalaxy.so
linux-vdso.so.1 => (0x00007fffc8ad3000)
libGalaxyParser.so => /home/ocaml/lib/libGalaxyParser.so (0x00007f1736e6d000)
...
1.4 gcc/g++链接时对库的顺序要求
先看看gcc手册对-L和-l的描述
-Ldir
Add directory dir to the list of directories to be searched for -l.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for POSIX
compliance and is not recommended.)
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in
the order they are specified. Thus, `foo.o -lz bar.o' searches
library `z' after file foo.o but before bar.o. If bar.o refers to
functions in `z', those functions may not be loaded.
The linker searches a standard list of directories for the
library, which is actually a file named liblibrary.a. The linker
then uses this file as if it had been specified precisely by name.
The directories searched include several standard system
directories plus any that you specify with -L.
Normally the files found this way are library files—archive files
whose members are object files. The linker handles an archive file
by scanning through it for members which define symbols that have
so far been referenced but not defined. But if the file that is
found is an ordinary object file, it is linked in the usual
fashion. The only difference between using an -l option and
specifying a file name is that -l surrounds library with `lib' and
`.a' and searches several directories.
基本的意思就是从左向右查找，如果是链接成动态库会稍微不同一点。
1.5 –as-needed对链接动态库的影响
先看看与动态库链接相关的几个选项的man说明：
--as-needed
--no-as-needed
This option affects ELF DT_NEEDED tags for dynamic libraries mentioned on the command line after the --as-needed
option. Normally the linker will add a DT_NEEDED tag for each dynamic library mentioned on the command line,
regardless of whether the library is actually needed or not. --as-needed causes a DT_NEEDED tag to only be emitted for
a library that satisfies an undefined symbol reference from a regular object file or, if the library is not found in
the DT_NEEDED lists of other libraries linked up to that point, an undefined symbol reference from another dynamic
library. --no-as-needed restores the default behaviour.
--add-needed
--no-add-needed
These two options have been deprecated because of the similarity of their names to the --as-needed and --no-as-needed
options. They have been replaced by --copy-dt-needed-entries and --no-copy-dt-needed-entries.
--copy-dt-needed-entries
--no-copy-dt-needed-entries
This option affects the treatment of dynamic libraries referred to by DT_NEEDED tags inside ELF dynamic libraries
mentioned on the command line. Normally the linker won't add a DT_NEEDED tag to the output binary for each library
mentioned in a DT_NEEDED tag in an input dynamic library. With --copy-dt-needed-entries specified on the command line
however any dynamic libraries that follow it will have their DT_NEEDED entries added. The default behaviour can be
restored with --no-copy-dt-needed-entries.
This option also has an effect on the resolution of symbols in dynamic libraries. With --copy-dt-needed-entries
dynamic libraries mentioned on the command line will be recursively searched, following their DT_NEEDED tags to other
libraries, in order to resolve symbols required by the output binary. With the default setting however the searching
of dynamic libraries that follow it will stop with the dynamic library itself. No DT_NEEDED links will be traversed to
resolve symbols.
再看看ld的帮助
--add-needed Set DT_NEEDED tags for DT_NEEDED entries in following dynamic libs --no-add-needed Do not set DT_NEEDED tags for DT_NEEDED entries in following dynamic libs --as-needed Only set DT_NEEDED for following dynamic libs if used --no-as-needed Always set DT_NEEDED for following dynamic libs
关键的看–as-needed，意思是说：只给用到的动态库设置DT_NEEDED。比如：
g++ -shared PyGalaxy.o -lGalaxyParser -lxxx -lrt -o PyGalaxy.so
像这样链接一个PyGalaxy.so的时候，假设PyGalaxy.so里面用到了libGalaxyParser.so但是没 有用到libxxx.so。查看依赖关系如下：
ocaml@ocaml:~$ readelf -d PyGalaxy.so Dynamic section at offset 0x7dd8 contains 26 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libGalaxyParser.so] 0x0000000000000001 (NEEDED) Shared library: [libxxx.so] ...
当开启–as-needed的时候，像
g++ -shared -Wl,--as-needed PyGalaxy.o -lGalaxyParser -lxxx -lrt -o PyGalaxy.so
这样链接PyGalaxy.so的时候，查看依赖关系如下：
ocaml@ocaml:~$ readelf -d PyGalaxy.so Dynamic section at offset 0x7dd8 contains 26 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libGalaxyParser.so] ...
–as-needed就是忽略链接时没有用到的动态库，只将用到的动态库set NEEDED。
开启–as-needed的一些常见的问题：
1.5.1 链接主程序模块或者是静态库的时的‘undefined reference to: xxx’
g++ -Wl,--as-needed -lGalaxyRT -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o mutex mutex.o
假设mutex依赖libGalaxyRT.so中的东西。想想，因为gcc对库的顺序要求 和–as-needed（因为libGalaxyRT.so在mutex.o的左边，所以gcc认为没有 用到它，–as-needed将其忽略），ld忽略libGalaxyRT.so，定位mutex.o的 符号的时候当然会找不到符号的定义！所以‘undefined reference to’这个 错误是正常地！
正确的链接方式是：
g++ -Wl,--as-needed mutex.o -lGalaxyRT -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o mutex
1.5.2 编译动态库（shared library）的时候会导致一个比较隐晦的错误
编译出来的动态库的时候没有问题，但是加载的时候有“undefined symbol: xxx”这样的错误。假如像这也链接PyGalaxy.so
g++ -shared -Wl,--as-needed -lGalaxyParser -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o PyGalaxy.so PyGalaxy.o
load PyGalaxy.so的时候会有上面的运行时错误!
简单分析原因：因为libGalaxyParser.so在mutex.o的左边，所以gcc认为没 有用到它，–as-needed将其忽略。但是前面说的动态库符号解析的特点导 致ld认为某些符号是加载的时候才去地址重定位的。但是 libGalaxyParser.so已经被忽略了。所以就算你写上了依赖的库，load的时 候也会找不到符号。但是为什么没有-Wl–as-needed的时候是正确的呢？没 有的话，ld会set NEEDED libGalaxyParser.so（用前面提到的查看动态库 依赖关系的办法可以验证）。load的时候还是可以找到符号的，所以正确。
正确的链接方式是：
g++ -shared -Wl,--as-needed PyGalaxy.o -lGalaxyParser -lc -lm -ldl -lpthread -L/home/ocaml/lib/ -lrt -o PyGalaxy.so
1.6 对链接顺序导致问题的解决方案
1.6.1 在项目开发过层中尽量让lib是垂直关系，避免循环依赖；越是底层的库，越是往后面写！
例如:
g++ ... obj($?) -l(上层逻辑lib) -l(中间封装lib) -l(基础lib) -l(系统lib) -o $@
这样写可以避免很多问题，这个是在搭建项目的构建环境的过程中需要考虑 清楚地，在编译和链接上浪费太多的生命不值得！
1.6.2 通过-(和-)强制repeat
-(和-),它能够强制"The specified archives are searched repeatedly", 这就是我们要找的啦。比如：
g++ -shared -Wl,--as-needed PyGalaxy.o Xlinker "-("-lGalaxyParser -lxxx -lrt"-)" -o PyGalaxy.so
简单解释一下，Xlinker是将后面的一个参数传给ld（这里就是 "-("-lGalaxyParser -lxxx -lrt"-)"），然后-(和-)强制repeat当然就 可以找到了。但是这样的repeat需要浪费一些时间。