build.prop 文件编译生成追踪

build.prop是什么

在Android系统中有一个类似Windows系统注册表的文件build.prop。这个文件内定义了系统初始（或永久）的一些参数属性、功能的开放等。通过调整/增加参数可以达到调校系统性能偏重点和附加功能开启的作用。

编译流程

代码路径build/core/Makefile

$(INSTALLED_BUILD_PROP_TARGET): $(BUILDINFO_SH) $(INTERNAL_BUILD_ID_MAKEFILE) $(BUILD_SYSTEM)/version_defaults.mk $(system_prop_file)
        @echo Target buildinfo: $@
        @mkdir -p $(dir $@)
        $(hide) echo > $@
ifneq ($(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_OEM_PROPERTIES),)
        $(hide) echo "#" >> $@; \
                echo "# PRODUCT_OEM_PROPERTIES" >> $@; \
                echo "#" >> $@;
        $(hide) $(foreach prop,$(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_OEM_PROPERTIES), \
                echo "import /oem/oem.prop $(prop)" >> $@;)
endif
        $(hide) TARGET_BUILD_TYPE="$(TARGET_BUILD_VARIANT)" \
                        TARGET_BUILD_FLAVOR="$(TARGET_PRODUCT)-$(TARGET_BUILD_VARIANT)" \
                        TARGET_DEVICE="$(TARGET_DEVICE)" \
                        PRODUCT_NAME="$(TARGET_PRODUCT)" \
                        PRODUCT_BRAND="$(PRODUCT_BRAND)" \
                        PRODUCT_DEFAULT_LANGUAGE="$(call default-locale-language,$(PRODUCT_LOCALES))" \
                        PRODUCT_DEFAULT_REGION="$(call default-locale-region,$(PRODUCT_LOCALES))" \
                        PRODUCT_DEFAULT_WIFI_CHANNELS="$(PRODUCT_DEFAULT_WIFI_CHANNELS)" \
                        PRODUCT_MODEL="$(PRODUCT_MODEL)" \
                        PRODUCT_MANUFACTURER="$(PRODUCT_MANUFACTURER)" \
                        PRIVATE_BUILD_DESC="$(PRIVATE_BUILD_DESC)" \
                        BUILD_ID="$(BUILD_ID)" \
                        BUILD_DISPLAY_ID="$(BUILD_DISPLAY_ID)" \
                        BUILD_DISPLAY_ID_MASK="$(BUILD_DISPLAY_ID_MASK)" \
                        BUILD_INSIDE_ID_MASK="$(BUILD_INSIDE_ID_MASK)" \
                        BUILD_NUMBER="$(BUILD_NUMBER)" \
                        PLATFORM_VERSION="$(PLATFORM_VERSION)" \
                        PLATFORM_SDK_VERSION="$(PLATFORM_SDK_VERSION)" \
                        PLATFORM_VERSION_CODENAME="$(PLATFORM_VERSION_CODENAME)" \
                        PLATFORM_VERSION_ALL_CODENAMES="$(PLATFORM_VERSION_ALL_CODENAMES)" \
                        BUILD_VERSION_TAGS="$(BUILD_VERSION_TAGS)" \
                        TARGET_BOOTLOADER_BOARD_NAME="$(TARGET_BOOTLOADER_BOARD_NAME)" \
                        BUILD_FINGERPRINT="$(BUILD_FINGERPRINT)" \
                        $(if $(OEM_THUMBPRINT_PROPERTIES),BUILD_THUMBPRINT="$(BUILD_THUMBPRINT)") \
                        TARGET_BOARD_PLATFORM="$(TARGET_BOARD_PLATFORM)" \
                        TARGET_CPU_ABI_LIST="$(TARGET_CPU_ABI_LIST)" \
                        TARGET_CPU_ABI_LIST_32_BIT="$(TARGET_CPU_ABI_LIST_32_BIT)" \
                        TARGET_CPU_ABI_LIST_64_BIT="$(TARGET_CPU_ABI_LIST_64_BIT)" \
                        TARGET_CPU_ABI="$(TARGET_CPU_ABI)" \
                        TARGET_CPU_ABI2="$(TARGET_CPU_ABI2)" \
                        TARGET_AAPT_CHARACTERISTICS="$(TARGET_AAPT_CHARACTERISTICS)" \
                bash $(BUILDINFO_SH) >> $@
        $(hide) $(foreach file,$(system_prop_file), \
                if [ -f "$(file)" ]; then \
                        echo "#" >> $@; \
                        echo Target buildinfo from: "$(file)"; \
                        echo "# from $(file)" >> $@; \
                        echo "#" >> $@; \
                        cat $(file) >> $@; \
                fi;)
       >>==$(ADDITIONAL_BUILD_PROPERTIES), \
                $(hide) echo >> $@; \
                        echo "#" >> $@; \
                        echo "# ADDITIONAL_BUILD_PROPERTIES" >> $@; \
                        echo "#" >> $@; )
        $(hide) $(foreach line,$(ADDITIONAL_BUILD_PROPERTIES), \
                echo "$(line)" >> $@;)
        $(hide) build/tools/post_process_props.py $@ $(PRODUCTS.$(INTERNAL_PRODUCT).PRODUCT_SYSTEM_PROPERTY_BLACKLIST)==<<

同一个变量的覆盖规则

代码路径build/core/Makefile

INSTALLED_BUILD_PROP_TARGET := $(TARGET_OUT)/build.prop
ALL_DEFAULT_INSTALLED_MODULES += $(INSTALLED_BUILD_PROP_TARGET)
ADDITIONAL_BUILD_PROPERTIES := \
    $(call collapse-pairs, $(ADDITIONAL_BUILD_PROPERTIES))
ADDITIONAL_BUILD_PROPERTIES := $(call uniq-pairs-by-first-component, \
    $(ADDITIONAL_BUILD_PROPERTIES),=)

代码路径build/make/common/strings.mk

110define uniq-pairs-by-first-component
111$(eval _upbfc_fc_set :=)\
112$(strip $(foreach w,$(1), $(eval _first := $(word 1,$(subst $(2),$(space),$(w))))\
113    $(if $(filter $(_upbfc_fc_set),$(_first)),,$(w)\
114        $(eval _upbfc_fc_set += $(_first)))))\
115$(eval _upbfc_fc_set :=)\
116$(eval _first:=)
117endef

collapse-pairs函数是用来格式化赋值语句的，去掉=两边的空格，例如a = b或a =b，都会格式化为a=b

uniq-pairs-by-first-component函数是用来剔除这些赋值语句中多次重复对一个属性进行赋值的语句，规则为保留第一个赋值的值。后边的赋值无效。

build.prop的快速编译生成

http://blog.sina.com.cn/s/blog_a42d507e01019kyc.html

参考文献：
https://blog.csdn.net/beijing2008lm/article/details/84672933