在分析Jelly Bean Audio Subsystem的时候，发现HAL层的库audio_policy.xxx.so与其依赖的静态库libaudiopolicy_legacy.a都有audio_policy_hal.cpp这个源文件而且两者都定义了一个HMI。当调用者引用HMI的时候，调用的究竟是哪个呢？
首先看audio_policy.xxx.so的Android.mk文件，在定义编译audio_policy.xxx.so的段落里面有这么一句：
LOCAL_STATIC_LIBRARIES :=
libaudiohw_legacy
libmedia_helper
libaudiopolicy_legacy
说明libaudiopolicy_legacy是以静态库的形式为audio_policy.xxx.so所用，而在通用的audio_policy库（也可以说是谷歌提供给厂商参考用的policy库）audio_policy.default.so的库里面是这么用的：
LOCAL_WHOLE_STATIC_LIBRARIES :=
libaudiopolicy_legacy
而audio_policy.default.so的源码里面就没有audio_policy_hal.cpp这个源文件，所以玄机应该就在LOCAL_WHOLE_STATIC_LIBRARIES和LOCAL_STATIC_LIBRARIES这两个宏的差异上面。先看下谷歌在build/core/build-system.html里面是怎么说的：
LOCAL_STATIC_LIBRARIES
These are the static libraries that you want to include in your module. Mostly, we use shared libraries, but there are a couple of places, like executables in sbin and host executables where we use static libraries instead.
LOCAL_WHOLE_STATIC_LIBRARIES
These are the static libraries that you want to include in your module without allowing the linker to remove dead code from them. This is mostly useful if you want to add a static library to a shared library and have the static library's content exposed from the shared library.
总的来说LOCAL_WHOLE_STATIC_LIBRARIES在连接静态连接库的时候不会移除"daed code"，何谓dead code呢，就是调用者模块永远都不会用到的代码段和变量，下面用几个小源码来说明。
Android.mk如下
1 LOCAL_PATH := $(call my-dir) 2 include $(CLEAR_VARS) 3 4 LOCAL_SRC_FILES := 5 libstone_1.cpp 6 7 LOCAL_MODULE := libstone_1 8 LOCAL_MODULE_TAGS := optional 9 10 include $(BUILD_STATIC_LIBRARY) 11 12 # ========================================= 13 include $(CLEAR_VARS) 14 15 LOCAL_SRC_FILES := 16 libstone_2.cpp 17 18 LOCAL_STATIC_LIBRARIES := libstone_1 19 #<span style="color:#ff0000;"> LOCAL_WHOLE_STATIC_LIBRARIES := libstone_1</span> 20 LOCAL_MODULE := libstone_2 21 LOCAL_MODULE_TAGS := optional 22 23 include $(BUILD_SHARED_LIBRARY)
静态库源文件（libstone_1.cpp）：
1 char hello[] = "this is a string in libstone_1"; 2 char str_2[] = "non static string 1"; 3 char str_3[] = "non static string 2"; 4 void func_1(){hello[0]+=1;}
调用者源文件（libstone_2.cpp）：
1 char hello[] = "this is a string in libstone_2";
case 1，按照上述的代码make之后，libstone_1的所有东西都没有被连接到动态库libstone_2里面：
readelf -Ws system/lib/libstone_2.so
Symbol table '.dynsym' contains 11 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 00002000 31 OBJECT GLOBAL DEFAULT 13 hello2
2: 000002e8 12 FUNC GLOBAL DEFAULT 7 __on_dlclose
3: 00000000 0 FUNC GLOBAL DEFAULT UND __cxa_finalize
4: 00000000 0 FUNC GLOBAL DEFAULT UND __aeabi_unwind_cpp_pr0
5: 00002020 0 NOTYPE GLOBAL DEFAULT 14 __dso_handle
6: 00001ee4 0 NOTYPE GLOBAL DEFAULT 9 __INIT_ARRAY__
7: 00001eec 0 NOTYPE GLOBAL DEFAULT 10 __FINI_ARRAY__
8: 0000201f 0 NOTYPE GLOBAL DEFAULT ABS _edata
9: 0000201f 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
10: 00002030 0 NOTYPE GLOBAL DEFAULT ABS _end
case 2，修改Android.mk使用LOCAL_WHOLE_STATIC_LIBRARIES 宏，这个时候libstone_2.cpp中的hello要改成hello2，因为libstone_1里面的东西全部链接进来了：
readelf -Ws system/lib/libstone_2.so
Symbol table '.dynsym' contains 16 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000037d 20 FUNC GLOBAL DEFAULT 7 _Z6func_3v
2: 0000201f 31 OBJECT GLOBAL DEFAULT 13 hello
3: 00000000 0 FUNC GLOBAL DEFAULT UND __aeabi_unwind_cpp_pr0
4: 00002000 31 OBJECT GLOBAL DEFAULT 13 hello2
5: 00000390 12 FUNC GLOBAL DEFAULT 7 __on_dlclose
6: 00000000 0 FUNC GLOBAL DEFAULT UND __cxa_finalize
7: 00002070 0 NOTYPE GLOBAL DEFAULT 14 __dso_handle
8: 00001ee0 0 NOTYPE GLOBAL DEFAULT 9 __INIT_ARRAY__
9: 00001ee8 0 NOTYPE GLOBAL DEFAULT 10 __FINI_ARRAY__
10: 000003a1 16 FUNC GLOBAL DEFAULT 7 _Z6func_1v
11: 00002052 20 OBJECT GLOBAL DEFAULT 13 str_3
12: 0000203e 20 OBJECT GLOBAL DEFAULT 13 str_2
13: 00002066 0 NOTYPE GLOBAL DEFAULT ABS _edata
14: 00002066 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
15: 00002080 0 NOTYPE GLOBAL DEFAULT ABS _end
case 3，仍使用LOCAL_STATIC_LIBRARIES宏，libstone_2.cpp的代码修改为：
1 char hello2[] = "this is a string in libstone_2"; 2 3 extern char str_2[]; 4 5 void func_3(){str_2[0]+=1;}
这个时候readelf的结果是：
readelf -Ws system/lib/libstone_2.so
Symbol table '.dynsym' contains 16 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 00000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000037d 20 FUNC GLOBAL DEFAULT 7 _Z6func_3v
2: 0000201f 31 OBJECT GLOBAL DEFAULT 13 hello
3: 00000000 0 FUNC GLOBAL DEFAULT UND __aeabi_unwind_cpp_pr0
4: 00002000 31 OBJECT GLOBAL DEFAULT 13 hello2
5: 00000390 12 FUNC GLOBAL DEFAULT 7 __on_dlclose
6: 00000000 0 FUNC GLOBAL DEFAULT UND __cxa_finalize
7: 00002070 0 NOTYPE GLOBAL DEFAULT 14 __dso_handle
8: 00001ee0 0 NOTYPE GLOBAL DEFAULT 9 __INIT_ARRAY__
9: 00001ee8 0 NOTYPE GLOBAL DEFAULT 10 __FINI_ARRAY__
10: 000003a1 16 FUNC GLOBAL DEFAULT 7 _Z6func_1v
11: 00002052 20 OBJECT GLOBAL DEFAULT 13 str_3
12: 0000203e 20 OBJECT GLOBAL DEFAULT 13 str_2
13: 00002066 0 NOTYPE GLOBAL DEFAULT ABS _edata
14: 00002066 0 NOTYPE GLOBAL DEFAULT ABS __bss_start
15: 00002080 0 NOTYPE GLOBAL DEFAULT ABS _end
可以看到，不仅仅是被引用到的str_2，所有在libstone_1中的global变量都被链接到libstone_2中。
所以可以得到如下的结论：
| 是否链接到调用者模块 | 使用了静态库的global变量 | 不使用 |
| LOCAL_STATIC_LIBRARIES | Y | N |
| LOCAL_WHOLE_STATIC_LIBRARIES | Y | Y |
至于android的编译系统是怎么处理这两个宏的，在./build目录发现其实使用了gcc的参数来区分(in core/definitions.mk )：
1161 -Wl,--whole-archive
1162 $(call normalize-host-libraries,$(PRIVATE_ALL_WHOLE_STATIC_LIBRARIES))
1163 -Wl,--no-whole-archive
1164 $(if $(PRIVATE_GROUP_STATIC_LIBRARIES),-Wl$(comma)--start-group)
综上所述，audio_policy.xxx.so这个policy库使用的是自己的audio_policy_hal.cpp源文件。