android系统属性的初始化探索

系统属性中有一类是永久保存在文件中的：
bionic/libc/include/sys/_system_properties.h
#define PROP_SERVICE_NAME "property_service"
#define PROP_PATH_RAMDISK_DEFAULT  "/default.prop"
#define PROP_PATH_SYSTEM_BUILD     "/system/build.prop"
#define PROP_PATH_SYSTEM_DEFAULT   "/system/default.prop"
#define PROP_PATH_LOCAL_OVERRIDE   "/data/local.prop

那么我们在编译的时候，这些文件中的值是在何处进行初始化的呢？代码中又是在何时生成的这些永久文件的呢？本文以/default.prop进行探索。

在编译生成文件中有如下，这里第二行我们可以看到确实在根目录(/)下生成有default.prop文件。

./out/target/product/sangfei82_we_jb5/recovery/root/default.prop:2:# ADDITIONAL_DEFAULT_PROPERTIES
./out/target/product/sangfei82_we_jb5/root/default.prop:2:# ADDITIONAL_DEFAULT_PROPERTIES

我们打开default.prop查看其中的内容：

root@android:/ # cat default.prop                                             
#
# ADDITIONAL_DEFAULT_PROPERTIES
#
ro.secure=1
ro.allow.mock.location=0
persist.mtk.aee.aed=on
ro.debuggable=0
persist.sys.usb.config=mass_storage
persist.service.acm.enable=0
ro.mount.fs=EXT4

这之中我们需要关注的是ADDITIONAL_DEFAULT_PROPERTIES这个变量，这是makefile文件中定义的目标变量，在makefile文件中ro.secure这些变量都被定义在ADDITIONAL_DEFAULT_PROPERTIES这个目标变量上。我们在/build中搜索ADDITIONAL_DEFAULT_PROPERTIES：
grep -nr "ADDITIONAL_DEFAULT_PROPERTIES" .


./build/core/main.mk:330:  ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=1
./build/core/main.mk:359:  ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=0
./build/core/main.mk:368:  ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=0
./build/core/main.mk:370:  ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=1
./build/core/main.mk:375:ADDITIONAL_DEFAULT_PROPERTIES += persist.mtk.aee.aed=on
./build/core/main.mk:379:  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=1
./build/core/main.mk:384:  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=0
./build/core/main.mk:389:  ADDITIONAL_DEFAULT_PROPERTIES += persist.sys.usb.config=mass_storage
./build/core/main.mk:391:  ADDITIONAL_DEFAULT_PROPERTIES += persist.sys.usb.config=mtp
./build/core/main.mk:396:ADDITIONAL_DEFAULT_PROPERTIES += persist.service.acm.enable=1
./build/core/main.mk:398:ADDITIONAL_DEFAULT_PROPERTIES += persist.service.acm.enable=0
./build/core/Makefile:101:ADDITIONAL_DEFAULT_PROPERTIES :=
./build/core/Makefile:102:    $(call collapse-pairs, $(ADDITIONAL_DEFAULT_PROPERTIES))
./build/core/Makefile:103:ADDITIONAL_DEFAULT_PROPERTIES +=
./build/core/Makefile:105:ADDITIONAL_DEFAULT_PROPERTIES := $(call uniq-pairs-by-first-component,
./build/core/Makefile:106:    $(ADDITIONAL_DEFAULT_PROPERTIES),=)
./build/core/Makefile:112:            echo "# ADDITIONAL_DEFAULT_PROPERTIES" >> $@;
./build/core/Makefile:114:    $(hide) $(foreach line,$(ADDITIONAL_DEFAULT_PROPERTIES),

进入main.mk的这段代码：
## user/userdebug ##
 
user_variant := $(filter user userdebug,$(TARGET_BUILD_VARIANT))
enable_target_debugging := true
tags_to_install :=
ifneq (,$(user_variant))
  # Target is secure in user builds.
  ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=1
 
  ifeq ($(user_variant),userdebug)
    # Pick up some extra useful tools
    tags_to_install += debug
 
    # Enable Dalvik lock contention logging for userdebug builds.
    ADDITIONAL_BUILD_PROPERTIES += dalvik.vm.lockprof.threshold=500
  else
    # Disable debugging in plain user builds.
    enable_target_debugging :=
  endif
 
  # enable dex pre-optimization for all TARGET projects in default to
  # speed up device first boot-up
  WITH_DEXPREOPT := true
 
  # Turn on Dalvik preoptimization for user builds, but only if not
  # explicitly disabled and the build is running on Linux (since host
  # Dalvik isn't built for non-Linux hosts).
  ifneq (true,$(DISABLE_DEXPREOPT))
    ifeq ($(user_variant),user)
      ifeq ($(HOST_OS),linux)
        WITH_DEXPREOPT := true
      endif
    endif
  endif
 
  # Disallow mock locations by default for user builds
  ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=0
 
else # !user_variant
  # Turn on checkjni for non-user builds.
  # Kirby: turn off it temporarily to gain performance {
  # ADDITIONAL_BUILD_PROPERTIES += ro.kernel.android.checkjni=1
  ADDITIONAL_BUILD_PROPERTIES += ro.kernel.android.checkjni=0
  # } Kirby
  # Set device insecure for non-user builds.
  ADDITIONAL_DEFAULT_PROPERTIES += ro.secure=0
  # Allow mock locations by default for non user builds
  ADDITIONAL_DEFAULT_PROPERTIES += ro.allow.mock.location=1
endif # !user_variant
 
 
# always enable aed
ADDITIONAL_DEFAULT_PROPERTIES += persist.mtk.aee.aed=on
 
ifeq (true,$(strip $(enable_target_debugging)))
  # Target is more debuggable and adbd is on by default
  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=1
  # Include the debugging/testing OTA keys in this build.
  INCLUDE_TEST_OTA_KEYS := true
else # !enable_target_debugging
  # Target is less debuggable and adbd is off by default
  ADDITIONAL_DEFAULT_PROPERTIES += ro.debuggable=0
endif # !enable_target_debugging
 
# default usb function
ifeq ($(strip $(MTK_MASS_STORAGE)),yes)
  ADDITIONAL_DEFAULT_PROPERTIES += persist.sys.usb.config=mass_storage
else
  ADDITIONAL_DEFAULT_PROPERTIES += persist.sys.usb.config=mtp
endif
 
# serial port open or not
ifeq ($(strip $(MTK_SERIAL_PORT_DEFAULT_ON)),yes)
ADDITIONAL_DEFAULT_PROPERTIES += persist.service.acm.enable=1
else
ADDITIONAL_DEFAULT_PROPERTIES += persist.service.acm.enable=0
endif
可以得出，在这里我们开始了定义ADDITIONAL_DEFAULT_PROPERTIES(default.prop)里的"有用内容"(因为这里并没有生成default.prop文件，也没有往其中写入任何内容，但定义的这些内容是系统真正会用到的内容)，并赋了初值。

进入makefile文件：

# -----------------------------------------------------------------
# default.prop
INSTALLED_DEFAULT_PROP_TARGET := $(TARGET_ROOT_OUT)/default.prop
ALL_DEFAULT_INSTALLED_MODULES += $(INSTALLED_DEFAULT_PROP_TARGET)
ADDITIONAL_DEFAULT_PROPERTIES :=
    $(call collapse-pairs, $(ADDITIONAL_DEFAULT_PROPERTIES))
ADDITIONAL_DEFAULT_PROPERTIES +=
    $(call collapse-pairs, $(PRODUCT_DEFAULT_PROPERTY_OVERRIDES))
ADDITIONAL_DEFAULT_PROPERTIES := $(call uniq-pairs-by-first-component,
    $(ADDITIONAL_DEFAULT_PROPERTIES),=)
 
$(INSTALLED_DEFAULT_PROP_TARGET):
    @echo Target buildinfo: $@
    @mkdir -p $(dir $@)
    $(hide) echo "#" > $@;
            echo "# ADDITIONAL_DEFAULT_PROPERTIES" >> $@;
            echo "#" >> $@;
    $(hide) $(foreach line,$(ADDITIONAL_DEFAULT_PROPERTIES),
        echo "$(line)" >> $@;)
    build/tools/post_process_props.py $@

重点关注$(INSTALLED_DEFAULT_PROP_TARGET):之后的这一部分内容，我们可以发现这里的格式和default.prop文件中开头的格式一样。如果熟悉makefile文件的编写语法的话，那么就好理解了，简单的作下说明：
:= , += 都表示赋值
makefile的执行语法为：
目标：条件
(Tab键)命令
我们只需要关注命令即可：

    @echo Target buildinfo: $@                                       @表示不显示此行内容，$@ 等同于目标，echo是shell命令，显示内容到控制台。如果没有"@"default.prop文件中

                                                                                              会多此行内容： Target buildinfo: default.prop

    @mkdir -p $(dir $@)                                                      shell命令：创建文件，因此default.prop在此处创建的
    $(hide) echo "#" > $@;                                         
            echo "# ADDITIONAL_DEFAULT_PROPERTIES" >> $@;                   shell重定向输入到文件，这里就是default.prop开头处的三行内容
            echo "#" >> $@;
    $(hide) $(foreach line,$(ADDITIONAL_DEFAULT_PROPERTIES),           makefile的函数循环方法，即输出每一行重定向到文件中
        echo "$(line)" >> $@;)
    build/tools/post_process_props.py $@                                                            一个python脚本，做了一些其它操作。

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