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。
http://www.cppblog.com/findingworld/archive/2011/07/12/66408.html
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