gcc 库顺序问题解决方法

使用gcc生成可执行文件时,大部分时候我们需要连接我们自己打包(AR)好的一些库文件,对于中大型(50万代码行以上)项目上，你将面对数个项目组, 最好的情况是每个项目组发布自己的打包.ar文件,这些.ar文件之间没有任何依赖关系, 然后由持续集成（ci）小组对这些包进行连接,不幸的是,这几乎是不可能的, 我们在连接时还是遇到了liba.ar和libb.ar相互依赖的情况。

因为gcc的库是个有点怪怪的特性,在看-l帮助时可以看到:
       -l library
           Search the library named library when linking. (The second alter-
           native 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.

    出于知识产权保护的考虑,每一个项目组可能只允许看到自己的代码和别的项目组的头文件,这给CI小组带来了很头痛的事情,很多时候你不得不把库顺序来回调 整。我也遇到了这样让人崩溃的情形，问题是对于liba.ar和libb.ar相互以来的情形，你可能最终采取丑陋的做法,将其中一个库在前后放两次:
gcc -o out.bin liba.ar libb.ar liba.ar -lrt
否则，您将不得不面对 "xx not referenced"之类的错误。

看看gcc的帮助,有下面的选项
-Xlinker option
           Pass option as an option to the linker. You can use this to supply
           system-specific linker options which GCC does not know how to rec-
           ognize.

           If you want to pass an option that takes an argument, you must use
           -Xlinker twice, once for the option and once for the argument. For
           example, to pass -assert definitions, you must write -Xlinker
           -assert -Xlinker definitions. It does not work to write -Xlinker
           "-assert definitions", because this passes the entire string as a
           single argument, which is not what the linker expects.

也就是说，-Xlinker是将连接选项传给连接器的，赶快看看ld的帮助有没有解决库顺序的选项吧:
-( archives -)
       --start-group archives --end-group
           The archives should be a list of archive files. They may be either
           explicit file names, or -l options.

           The specified archives are searched repeatedly until no new unde-
           fined references are created. Normally, an archive is searched
           only once in the order that it is specified on the command line.
           If a symbol in that archive is needed to resolve an undefined sym-
           bol referred to by an object in an archive that appears later on
           the command line, the linker would not be able to resolve that ref-
           erence. By grouping the archives, they all be searched repeatedly
           until all possible references are resolved.

           Using this option has a significant performance cost. It is best
           to use it only when there are unavoidable circular references
           between two or more archives.


不错，我们有个有点怪异的选项,-(和-),它能够强制"The specified archives are searched repeatedly"
god,这就是我们要找的啦。

最终的做法:
gcc -o output.bin -Xlinker "-(" liba.ar libb.ar -Xlinker "-)" -lrt

这样可以解决库顺序的问题了！问题是，如果你的库相互间的依赖如果错综复杂的话，可能会增加连接的时间，不过，做架构设计的都应该能考虑到这些问题吧。

原文地址：http://www.cppblog.com/findingworld/archive/2008/11/09/66408.html

http://hi.baidu.com/wangjianzhong0/item/6f3360d5b4e0534ffa5768f7