android config.mk,android编译分析之10—config.mk
config.mk可以說是android編譯系統(tǒng)中關于配置環(huán)境的一個總的makefile，定義了編譯環(huán)境的方方面面。執(zhí)行完config.mk就完成了android編譯系統(tǒng)的所有準備工作，即準備好了所有的編譯需要的全局變量，下一步直接執(zhí)行make，即可產生鏡像文件。
首先，定義了一些變量和目錄，
# Only use ANDROID_BUILD_SHELL to wrap around bash.
# DO NOT use other shells such as zsh.
ifdef ANDROID_BUILD_SHELL
SHELL := $(ANDROID_BUILD_SHELL)
else
# Use bash, not whatever shell somebody has installed as /bin/sh
# This is repeated from main.mk, since envsetup.sh runs this file
# directly.
SHELL := /bin/bash
endif
# Utility variables.
empty :=
space := $(empty) $(empty)
comma := ,
# Note that make will eat the newline just before endef.
define newline
endef
# Unfortunately you can't simply define backslash as \ or \\.
backslash := \a
backslash := $(patsubst %a,%,$(backslash))
# Tell python not to spam the source tree with .pyc files. This
# only has an effect on python 2.6 and above.
export PYTHONDONTWRITEBYTECODE := 1
# Standard source directories.
SRC_DOCS:= $(TOPDIR)docs
# TODO: Enforce some kind of layering; only add include paths
# when a module links against a particular library.
# TODO: See if we can remove most of these from the global list.
SRC_HEADERS := \
$(TOPDIR)system/core/include \
$(TOPDIR)system/media/audio/include \
$(TOPDIR)hardware/libhardware/include \
$(TOPDIR)hardware/libhardware_legacy/include \
$(TOPDIR)hardware/ril/include \
$(TOPDIR)libnativehelper/include \
$(TOPDIR)frameworks/native/include \
$(TOPDIR)frameworks/native/opengl/include \
$(TOPDIR)frameworks/av/include \
$(TOPDIR)frameworks/base/include
SRC_HOST_HEADERS:=$(TOPDIR)tools/include
SRC_LIBRARIES:= $(TOPDIR)libs
SRC_SERVERS:= $(TOPDIR)servers
SRC_TARGET_DIR := $(TOPDIR)build/target
SRC_API_DIR := $(TOPDIR)prebuilts/sdk/api
SRC_SYSTEM_API_DIR := $(TOPDIR)prebuilts/sdk/system-api
# Some specific paths to tools
SRC_DROIDDOC_DIR := $(TOPDIR)build/tools/droiddoc
然后調用pathmap.mk，包含一些hard code的路徑，
# Various mappings to avoid hard-coding paths all over the place
include $(BUILD_SYSTEM)/pathmap.mk
下面的這些變量在我們寫Android.mk時經常會用到，一般都是通過include調用，其實都是一些makefile文件，
# ###############################################################
# Build system internal files
# ###############################################################
BUILD_COMBOS:= $(BUILD_SYSTEM)/combo
CLEAR_VARS:= $(BUILD_SYSTEM)/clear_vars.mk
BUILD_HOST_STATIC_LIBRARY:= $(BUILD_SYSTEM)/host_static_library.mk
BUILD_HOST_SHARED_LIBRARY:= $(BUILD_SYSTEM)/host_shared_library.mk
BUILD_STATIC_LIBRARY:= $(BUILD_SYSTEM)/static_library.mk
BUILD_SHARED_LIBRARY:= $(BUILD_SYSTEM)/shared_library.mk
BUILD_EXECUTABLE:= $(BUILD_SYSTEM)/executable.mk
BUILD_HOST_EXECUTABLE:= $(BUILD_SYSTEM)/host_executable.mk
BUILD_PACKAGE:= $(BUILD_SYSTEM)/package.mk
BUILD_PHONY_PACKAGE:= $(BUILD_SYSTEM)/phony_package.mk
BUILD_HOST_PREBUILT:= $(BUILD_SYSTEM)/host_prebuilt.mk
BUILD_PREBUILT:= $(BUILD_SYSTEM)/prebuilt.mk
BUILD_MULTI_PREBUILT:= $(BUILD_SYSTEM)/multi_prebuilt.mk
BUILD_JAVA_LIBRARY:= $(BUILD_SYSTEM)/java_library.mk
BUILD_STATIC_JAVA_LIBRARY:= $(BUILD_SYSTEM)/static_java_library.mk
BUILD_HOST_JAVA_LIBRARY:= $(BUILD_SYSTEM)/host_java_library.mk
BUILD_DROIDDOC:= $(BUILD_SYSTEM)/droiddoc.mk
BUILD_COPY_HEADERS := $(BUILD_SYSTEM)/copy_headers.mk
BUILD_NATIVE_TEST := $(BUILD_SYSTEM)/native_test.mk
BUILD_NATIVE_BENCHMARK := $(BUILD_SYSTEM)/native_benchmark.mk
BUILD_HOST_NATIVE_TEST := $(BUILD_SYSTEM)/host_native_test.mk
BUILD_SHARED_TEST_LIBRARY := $(BUILD_SYSTEM)/shared_test_lib.mk
BUILD_HOST_SHARED_TEST_LIBRARY := $(BUILD_SYSTEM)/host_shared_test_lib.mk
BUILD_STATIC_TEST_LIBRARY := $(BUILD_SYSTEM)/static_test_lib.mk
BUILD_HOST_STATIC_TEST_LIBRARY := $(BUILD_SYSTEM)/host_static_test_lib.mk
BUILD_NOTICE_FILE := $(BUILD_SYSTEM)/notice_files.mk
BUILD_HOST_DALVIK_JAVA_LIBRARY := $(BUILD_SYSTEM)/host_dalvik_java_library.mk
BUILD_HOST_DALVIK_STATIC_JAVA_LIBRARY := $(BUILD_SYSTEM)/host_dalvik_static_java_library.mk
如果make的目標中包含showcommands，則會打印完整的命令。
# The 'showcommands' goal says to show the full command
# lines being executed, instead of a short message about
# the kind of operation being done.
SHOW_COMMANDS:= $(filter showcommands,$(MAKECMDGOALS))
定義了一些全局變量，cflags，常用文件后綴等，
# ###############################################################
# Set common values
# ###############################################################
# These can be changed to modify both host and device modules.
COMMON_GLOBAL_CFLAGS:= -DANDROID -fmessage-length=0 -W -Wall -Wno-unused -Winit-self -Wpointer-arith
COMMON_RELEASE_CFLAGS:= -DNDEBUG -UDEBUG
COMMON_GLOBAL_CPPFLAGS:= $(COMMON_GLOBAL_CFLAGS) -Wsign-promo -std=gnu++11
COMMON_RELEASE_CPPFLAGS:= $(COMMON_RELEASE_CFLAGS)
GLOBAL_CFLAGS_NO_OVERRIDE := \
-Werror=int-to-pointer-cast \
-Werror=pointer-to-int-cast \
GLOBAL_CPPFLAGS_NO_OVERRIDE :=
# Set the extensions used for various packages
COMMON_PACKAGE_SUFFIX := .zip
COMMON_JAVA_PACKAGE_SUFFIX := .jar
COMMON_ANDROID_PACKAGE_SUFFIX := .apk
# list of flags to turn specific warnings in to errors
TARGET_ERROR_FLAGS := -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point
ifdef TMPDIR
JAVA_TMPDIR_ARG := -Djava.io.tmpdir=$(TMPDIR)
else
JAVA_TMPDIR_ARG :=
endif
buildspec.mk還沒研究咋用，-include可以看出不是強制必須存在的，
# ---------------------------------------------------------------
# Try to include buildspec.mk, which will try to set stuff up.
# If this file doesn't exist, the environment variables will
# be used, and if that doesn't work, then the default is an
# arm build
ifndef ANDROID_BUILDSPEC
ANDROID_BUILDSPEC := $(TOPDIR)buildspec.mk
endif
-include $(ANDROID_BUILDSPEC)
include了envsetup.mk，前面已經介紹，設置了很多編譯相關的變量。
# ---------------------------------------------------------------
# Define most of the global variables. These are the ones that
# are specific to the user's build configuration.
include $(BUILD_SYSTEM)/envsetup.mk
在調用findleaves.py時設置的選項，
# Pruned directory options used when using findleaves.py
# See envsetup.mk for a description of SCAN_EXCLUDE_DIRS
FIND_LEAVES_EXCLUDES := $(addprefix --prune=, $(OUT_DIR) $(SCAN_EXCLUDE_DIRS) .repo .git)
由于config.mk太長，后面不一一列出來了，找重點的分析，中間主要都是些host和target 編譯鏈相關的變量，還有一些編譯需要用的應用程序，例如aapt，aidl等等。
其中調用select.mk主要是為了設置host和target編譯鏈相關的變量。
#下面調用select.mk設置host端，即編譯機的編譯鏈，gcc ar等
combo_target := HOST_
combo_2nd_arch_prefix :=
include $(BUILD_SYSTEM)/combo/select.mk
# Load the 2nd host arch if it's needed.
ifdef HOST_2ND_ARCH
combo_target := HOST_
# 2ND_
combo_2nd_arch_prefix := $(HOST_2ND_ARCH_VAR_PREFIX)
include $(BUILD_SYSTEM)/combo/select.mk
endif
# on windows, the tools have .exe at the end, and we depend on the
# host config stuff being done first
# 下面調用select.mk設置target端，即目標機的編譯鏈，gcc ar等
combo_target := TARGET_
combo_2nd_arch_prefix :=
include $(BUILD_SYSTEM)/combo/select.mk
# Load the 2nd target arch if it's needed.
ifdef TARGET_2ND_ARCH
combo_target := TARGET_
combo_2nd_arch_prefix := $(TARGET_2ND_ARCH_VAR_PREFIX)
include $(BUILD_SYSTEM)/combo/select.mk
endif
然后include了ccache，為了加快再次編譯速度，關于ccache后續(xù)再研究。
include $(BUILD_SYSTEM)/ccache.mk
接著include了javac.mk，用來設置java編譯器，
include $(BUILD_SYSTEM)/combo/javac.mk
同時在android6.0以上默認使用jack進行編譯，在javac.mk中，如果ANDROID_COMPILE_WITH_JACK默認沒設置，就設置為true。
ifndef ANDROID_COMPILE_WITH_JACK
# Defines if compilation with jack is enabled by default.
ANDROID_COMPILE_WITH_JACK := true
endif
接著包含了clang相關的makefile，關于clang后續(xù)再研究。
include $(BUILD_SYSTEM)/clang/config.mk
注：Clang是LLVM的前端，可以用來編譯C，C++，ObjectiveC等語言。傳統(tǒng)的編譯器通常分為三個部分，前端(frontEnd)，優(yōu)化器(Optimizer)和后端(backEnd)。在編譯過程中，前端主要負責詞法和語法分析，將源代碼轉化為抽象語法樹；優(yōu)化器則是在前端的基礎上，對得到的中間代碼進行優(yōu)化，使代碼更加高效；后端則是將已經優(yōu)化的中間代碼轉化為針對各自平臺的機器代碼。Clang則是以LLVM為后端的一款高效易用，并且與IDE結合很好的編譯前端。
最后include了dumpvar.mk，就是專為打印變量值的makefile，前面已經分析過。
include $(BUILD_SYSTEM)/dumpvar.mk
envsetup.sh中的一些函數，例如check_product，最終就是make config.mk，而make的目標為dumpvar-*。
總結
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