config.mk可以说是android编译系统中关于配置环境的一个总的makefile，定义了编译环境的方方面面。执行完config.mk就完成了android编译系统的所有准备工作，即准备好了所有的编译需要的全局变量，下一步直接执行make，即可产生镜像文件。

首先，定义了一些变量和目录，

# 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

然后调用pathmap.mk，包含一些hard code的路径，

# Various mappings to avoid hard-coding paths all over the place

include $(BUILD_SYSTEM)/pathmap.mk

下面的这些变量在我们写Android.mk时经常会用到，一般都是通过include调用，其实都是一些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的目标中包含showcommands，则会打印完整的命令。

# 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))

定义了一些全局变量，cflags，常用文件后缀等，

# ###############################################################

# 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还没研究咋用，-include可以看出不是强制必须存在的，

# ---------------------------------------------------------------

# 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，前面已经介绍，设置了很多编译相关的变量。

# ---------------------------------------------------------------

# Define most of the global variables. These are the ones that

# are specific to the user's build configuration.

include $(BUILD_SYSTEM)/envsetup.mk

在调用findleaves.py时设置的选项，

# 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太长，后面不一一列出来了，找重点的分析，中间主要都是些host和target 编译链相关的变量，还有一些编译需要用的应用程序，例如aapt，aidl等等。

其中调用select.mk主要是为了设置host和target编译链相关的变量。

#下面调用select.mk设置host端，即编译机的编译链，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

# 下面调用select.mk设置target端，即目标机的编译链，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，为了加快再次编译速度，关于ccache后续再研究。

include $(BUILD_SYSTEM)/ccache.mk

接着include了javac.mk，用来设置java编译器，

include $(BUILD_SYSTEM)/combo/javac.mk

同时在android6.0以上默认使用jack进行编译，在javac.mk中，如果ANDROID_COMPILE_WITH_JACK默认没设置，就设置为true。

ifndef ANDROID_COMPILE_WITH_JACK

# Defines if compilation with jack is enabled by default.

ANDROID_COMPILE_WITH_JACK := true

endif

接着包含了clang相关的makefile，关于clang后续再研究。

include $(BUILD_SYSTEM)/clang/config.mk

注：Clang是LLVM的前端，可以用来编译C，C++，ObjectiveC等语言。传统的编译器通常分为三个部分，前端(frontEnd)，优化器(Optimizer)和后端(backEnd)。在编译过程中，前端主要负责词法和语法分析，将源代码转化为抽象语法树；优化器则是在前端的基础上，对得到的中间代码进行优化，使代码更加高效；后端则是将已经优化的中间代码转化为针对各自平台的机器代码。Clang则是以LLVM为后端的一款高效易用，并且与IDE结合很好的编译前端。

最后include了dumpvar.mk，就是专为打印变量值的makefile，前面已经分析过。

include $(BUILD_SYSTEM)/dumpvar.mk

envsetup.sh中的一些函数，例如check_product，最终就是make config.mk，而make的目标为dumpvar-*。