使用gcc生成可執行文件時,大部分時候我們需要連接我們自己打包(AR)好的一些庫文件,對于中大型(50萬代碼行以上)項目上，你將面對數個項目組,最好的情況是每個項目組發布自己的打包.ar文件,這些.ar文件之間沒有任何依賴關系, 然后由持續集成（ci）小組對這些包進行連接,不幸的是,這幾乎是不可能的, 我們在連接時還是遇到了liba.ar和libb.ar相互依賴的情況。

因為gcc的庫是個有點怪怪的特性,在看-l幫助時可以看到:
       -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.

    出于知識產權保護的考慮,每一個項目組可能只允許看到自己的代碼和別的項目組的頭文件,這給CI小組帶來了很頭痛的事情,很多時候你不得不把庫順序來回調整。我也遇到了這樣讓人崩潰的情形，問題是對于liba.ar和libb.ar相互以來的情形，你可能最終采取丑陋的做法,將其中一個庫在前后放兩次:
gcc -o out.bin liba.ar libb.ar liba.ar -lrt
否則，您將不得不面對 "xx not referenced"之類的錯誤。

看看gcc的幫助,有下面的選項
-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.

也就是說，-Xlinker是將連接選項傳給連接器的，趕快看看ld的幫助有沒有解決庫順序的選項吧:
 -( 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.


不錯，我們有個有點怪異的選項,-(和-),它能夠強制"The specified archives are searched repeatedly"
god,這就是我們要找的啦。

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

這樣可以解決庫順序的問題了！問題是，如果你的庫相互間的依賴如果錯綜復雜的話，可能會增加連接的時間，不過，做架構設計的都應該能考慮到這些問題吧。