Android Build System
Status: Draft (as of May 18, 2006)
Objective
Principles and Use Cases and Policy
Multiple Targets
It needs to be possible to build the Android platform for multiple targets. This means:
- The build system will support building tools for the host platform, both ones that are used in the build process itself, and developer tools like the simulator.
- The build system will need to be able to build tools on Linux (definitely Goobuntu and maybe Grhat), MacOS, and to some degree on Windows.
- The build system will need to be able to build the OS on Linux, and in the short-term, MacOS. Note that this is a conscious decision to stop building the OS on Windows. We are going to rely on the emulator there and not attempt to use the simulator. This is a requirement change now that the emulator story is looking brighter.
Non-Recursive Make
To achieve the objectives, the build system will be rewritten to use make non-recursively. For more background on this, read Recursive Make Considered Harmful . For those that don't want PDF, here is the Google translated version .
Rapid Compile-Test Cycles
When developing a component, for example a C++ shared library, it must be possible to easily rebuild just that component, and not have to wait more than a couple seconds for dependency checks, and not have to wait for unneeded components to be built.
Both Environment and Config File Based Settings
To set the target, and other options, some people on the team like to have a configuration file in a directory so they do not have an environment setup script to run, and others want an environment setup script to run so they can run builds in different terminals on the same tree, or switch back and forth in one terminal. We will support both.
Object File Directory / make clean
Object files and other intermediate files will be generated into a directory that is separate from the source tree. The goal is to have make clean be "rm -rf " in the tree root directory. The primary goals of this are to simplify searching the source tree, and to make "make clean" more reliable.
SDK
The SDK will be a tarball that will allow non-OS-developers to write apps. The apps will actually be built by first building the SDK, and then building the apps against that SDK. This will hopefully (1) make writing apps easier for us, because we won't have to rebuild the OS as much, and we can use the standard java-app development tools, and (2) allow us to dog-food the SDK, to help ensure its quality. Cedric has suggested (and I agree) that apps built from the SDK should be built with ant. Stay tuned for more details as we figure out exactly how this will work.
Dependecies
Dependencies should all be automatic. Unless there is a custom tool involved (e.g. the webkit has several), the dependencies for shared and static libraries, .c, .cpp, .h, .java, java libraries, etc., should all work without intervention in the Android.mk file.
Hiding command lines
The default of the build system will be to hide the command lines being executed for make steps. It will be possible to override this by specifying the showcommands pseudo-target, and possibly by setting an environment variable.
Wildcard source files
Wildcarding source file will be discouraged. It may be useful in some scenarios. The default $(wildcard *) will not work due to the current directory being set to the root of the build tree.
Multiple targets in one directory
It will be possible to generate more than one target from a given subdirectory. For example, libutils generates a shared library for the target and a static library for the host.
Makefile fragments for modules
Android.mk is the standard name for the makefile fragments that control the building of a given module. Only the top directory should have a file named "Makefile".
Use shared libraries
Currently, the simulator is not built to use shared libraries. This should be fixed, and now is a good time to do it. This implies getting shared libraries to work on Mac OS.
Nice to Have
These things would be nice to have, and this is a good place to record them, however these are not promises.
Simultaneous Builds
The hope is to be able to do two builds for different combos in the same tree at the same time, but this is a stretch goal, not a requirement. Doing two builds in the same tree, not at the same time must work. (update: it's looking like we'll get the two builds at the same time working)
Deleting headers (or other dependecies)
Problems can arise if you delete a header file that is referenced in ".d" files. The easy way to deal with this is "make clean". There should be a better way to handle it. (from fadden)
One way of solving this is introducing a dependency on the directory. The problem is that this can create extra dependecies and slow down the build. It's a tradeoff.
Multiple builds
General way to perform builds across the set of known platforms. This would make it easy to perform multiple platform builds when testing a change, and allow a wide-scale "make clean". Right now the buildspec.mk or environment variables need to be updated before each build. (from fadden)
Aftermarket Locales and Carrier
We will eventually need to add support for creating locales and carrier customizations to the SDK, but that will not be addressed right now.
Usage
Your first build
The Building document describes how do do builds.
build/envsetup.sh functions
If you source the file build/envsetup.sh into your bash environment, . build/envsetup.sh you'll get a few helpful shell functions:
- printconfig - Prints the current configuration as set by the lunch and choosecombo commands.
- m - Runs
makefrom the top of the tree. This is useful because you can run make from within subdirectories. If you have theTOPenvironment variable set, it uses that. If you don't, it looks up the tree from the current directory, trying to find the top of the tree. - croot -
cdto the top of the tree. - sgrep - grep for the regex you provide in all .c, .cpp, .h, .java, and .xml files below the current directory.
Build flavors/types
When building for a particular product, it's often useful to have minor variations on what is ultimately the final release build. These are the currently-defined "flavors" or "types" (we need to settle on a real name for these).
| eng | This is the default flavor. A plain "
|
| user | " This is the flavor intended to be the final release bits.
|
| userdebug | " The same as
|
If you build one flavor and then want to build another, you should run "make installclean " between the two makes to guarantee that you don't pick up files installed by the previous flavor. "make clean " will also suffice, but it takes a lot longer.
More pseudotargets
Sometimes you want to just build one thing. The following pseudotargets are there for your convenience:
- droid -
make droidis the normal build. This target is here because the default target has to have a name. - all -
make allbuilds everythingmake droiddoes, plus everything whoseLOCAL_MODULE_TAGSdo not include the "droid" tag. The build server runs this to make sure that everything that is in the tree and has an Android.mk builds. - clean-$(LOCAL_MODULE) and clean-$(LOCAL_PACKAGE_NAME) - Let you selectively clean one target. For example, you can type
make clean-libutilsand it will delete libutils.so and all of the intermediate files, or you can typemake clean-Homeand it will clean just the Home app. - clean -
make cleandeletes all of the output and intermediate files for this configuration. This is the same asrm -rf out/<configuration>/ - clobber -
make clobberdeletes all of the output and intermediate files for all configurations. This is the same asrm -rf out/. - dataclean -
make datacleandeletes contents of the data directory inside the current combo directory. This is especially useful on the simulator and emulator, where the persistent data remains present between builds. - showcommands -
showcommandsis a modifier target which causes the build system to show the actual command lines for the build steps, instead of the brief descriptions. Most people don't like seeing the actual commands, because they're quite long and hard to read, but if you need to for debugging purposes, you can addshowcommandsto the list of targets you build. For examplemake showcommandswill build the default android configuration, andmake runtime showcommandswill build just the runtime, and targets that it depends on, while displaying the full command lines. Please note that there are a couple places where the commands aren't shown here. These are considered bugs, and should be fixed, but they're often hard to track down. Please let android-build-team know if you find any. - LOCAL_MODULE - Anything you specify as a
LOCAL_MODULEin an Android.mk is made into a pseudotarget. For example,make runtimemight be shorthand formake out/linux-x86-debug/system/bin/runtime(which would work), andmake libkjsmight be shorthand formake out/linux-x86-debug/system/lib/libkjs.so(which would also work). - targets -
make targetswill print a list of all of the LOCAL_MODULE names you can make.
How to add another component to the build - Android.mk templates
Apps
This template is pretty self-explanitory. See the variables below for more details.
Java Libraries
Use the templates/java_library file.
C/C++ Executables
Use the templates/executable file, or the templates/executable_host file.
By default, on the target these are built into /system/bin, and on the host, they're built into /host/bin. These can be overridden by setting LOCAL_MODULE_PATH . See Putting targets elsewhere for more.
Shared Libraries
Use the templates/shared_library file, or the templates/shared_library_host file.
Remember that on the target, we use shared libraries, and on the host, we use static libraries, since executable size isn't as big an issue, and it simplifies distribution in the SDK.
Static Libraries
Use the templates/static_library file, or the templates/static_library_host file.
Remember that on the target, we use shared libraries, and on the host, we use static libraries, since executable size isn't as big an issue, and it simplifies distribution in the SDK.
Using Custom Tools
Example 1
intermediates:= $(local-intermediates-dir)
GEN := $(intermediates)/chartables.c
$(GEN): PRIVATE_CUSTOM_TOOL = $(HOST_OUT_EXECUTABLES)/dftables $@
$(GEN): $(HOST_OUT_EXECUTABLES)/dftables
$(transform-generated-source)
LOCAL_GENERATED_SOURCES += $(GEN)
Example 2
intermediates:= $(local-intermediates-dir)
GEN := $(intermediates)/file.c
$(GEN): PRIVATE_INPUT_FILE := $(LOCAL_PATH)/input.file
$(GEN): PRIVATE_CUSTOM_TOOL = cat $(PRIVATE_INPUT_FILE) > $@
$(GEN): $(LOCAL_PATH)/file.c
$(transform-generated-source)
LOCAL_GENERATED_SOURCES += $(GEN)
Example 3
intermediates:= $(local-intermediates-dir)
GEN := $(addprefix $(intermediates)/kjs/, \
array_object.lut.h \
bool_object.lut.h \
)
$(GEN): PRIVATE_CUSTOM_TOOL = perl libs/WebKitLib/WebKit/JavaScriptCore/kjs/create_hash_table $< -i > $@
$(GEN): $(intermediates)/%.lut.h
: $(LOCAL_PATH)/%.cpp
$(transform-generated-source)
LOCAL_GENERATED_SOURCES += $(GEN)
Platform specific conditionals
For a device build, TARGET_OS is linux (we're using linux!), and TARGET_ARCH is arm .
| HOST_OS linux darwin (cygwin) |
HOST_ARCH x86 |
HOST_BUILD_TYPE release debug |
|
| TARGET_OS linux darwin (cygwin) |
TARGET_ARCH arm x86 |
TARGET_BUILD_TYPE release debug |
TARGET_PRODUCT sim dream sooner |
Some Examples
ifeq ($(TARGET_SIMULATOR),true)
LOCAL_CFLAGS += -DSIMULATOR
endif
ifeq ($(TARGET_BUILD_TYPE),release)
LOCAL_CFLAGS += -DNDEBUG=1
endif
# from libutils
ifeq ($(TARGET_OS),linux)
# Use the futex based mutex and condition variable
# implementation from android-arm because it's shared mem safe
LOCAL_SRC_FILES += futex_synchro.c
LOCAL_LDLIBS += -lrt -ldl
endif
Putting modules elsewhere
LOCAL_MODULE_PATH := $(TARGET_ROOT_OUT_SBIN)
LOCAL_UNSTRIPPED_PATH := $(TARGET_ROOT_OUT_SBIN_UNSTRIPPED)
For executables and libraries, you need to also specify aLOCAL_UNSTRIPPED_PATHlocation, because on target builds, we keep the unstripped executables so GDB can find the symbols.
Look in config/envsetup.make for all of the variables defining places to build things.
Android.mk variables
These are the variables that you'll commonly see in Android.mk files, listed alphabetically.
But first, a note on variable naming:
- LOCAL_ - These variables are set per-module. They are cleared by the
include $(CLEAR_VARS)line, so you can rely on them being empty after including that file. Most of the variables you'll use in most modules are LOCAL_ variables. - PRIVATE_ - These variables are make-target-specific variables. That means they're only usable within the commands for that module. It also means that they're unlikely to change behind your back from modules that are included after yours. This link to the make documentation describes more about target-specific variables. Please note that there are a couple of these laying around the tree that aren't prefixed with PRIVATE_. It is safe, and they will be fixed as they are discovered. Sorry for the confusion.
- INTERNAL_ - These variables are critical to functioning of the build system, so you shouldn't create variables named like this, and you probably shouldn't be messing with these variables in your makefiles.
- HOST_ and TARGET_ - These contain the directories and definitions that are specific to either the host or the target builds. Do not set variables that start with HOST_ or TARGET_ in your makefiles.
- BUILD_ and CLEAR_VARS - These contain the names of well-defined template makefiles to include. Some examples are CLEAR_VARS and BUILD_HOST_PACKAGE.
- Any other name is fair-game for you to use in your Android.mk. However, remember that this is a non-recursive build system, so it is possible that your variable will be changed by another Android.mk included later, and be different when the commands for your rule / module are executed.
LOCAL_ASSET_FILES
In Android.mk files that include $(BUILD_PACKAGE) set this to the set of files you want built into your app. Usually:
LOCAL_ASSET_FILES += $(call find-subdir-assets)
This will probably change when we switch to ant for the apps' build system.
LOCAL_CC
If you want to use a different C compiler for this module, set LOCAL_CC to the path to the compiler. If LOCAL_CC is blank, the appropriate default compiler is used.
LOCAL_CXX
If you want to use a different C++ compiler for this module, set LOCAL_CXX to the path to the compiler. If LOCAL_CXX is blank, the appropriate default compiler is used.
LOCAL_CFLAGS
If you have additional flags to pass into the C or C++ compiler, add them here. For example:
LOCAL_CPPFLAGS
If you have additional flags to pass into only the C++ compiler, add them here. For example:
LOCAL_CPPFLAGS += -ffriend-injection
LOCAL_CPPFLAGS is guaranteed to be after LOCAL_CFLAGS on the compile line, so you can use it to override flags listed in LOCAL_CFLAGS .
LOCAL_CPP_EXTENSION
If your C++ files end in something other than ".cpp ", you can specify the custom extension here. For example:
LOCAL_CPP_EXTENSION := .cc
Note that all C++ files for a given module must have the same extension; it is not currently possible to mix different extensions.
LOCAL_NO_DEFAULT_COMPILER_FLAGS
Normally, the compile line for C and C++ files includes global include paths and global cflags. If LOCAL_NO_DEFAULT_COMPILER_FLAGS is non-empty, none of the default includes or flags will be used when compiling C and C++ files in this module. LOCAL_C_INCLUDES ,LOCAL_CFLAGS , and LOCAL_CPPFLAGS will still be used in this case, as will any DEBUG_CFLAGS that are defined for the module.
LOCAL_COPY_HEADERS
This will be going away.
The set of files to copy to the install include tree. You must also supply LOCAL_COPY_HEADERS_TO .
This is going away because copying headers messes up the error messages, and may lead to people editing those headers instead of the correct ones. It also makes it easier to do bad layering in the system, which we want to avoid. We also aren't doing a C/C++ SDK, so there is no ultimate requirement to copy any headers.
LOCAL_COPY_HEADERS_TO
This will be going away.
The directory within "include" to copy the headers listed in LOCAL_COPY_HEADERS to.
This is going away because copying headers messes up the error messages, and may lead to people editing those headers instead of the correct ones. It also makes it easier to do bad layering in the system, which we want to avoid. We also aren't doing a C/C++ SDK, so there is no ultimate requirement to copy any headers.
LOCAL_C_INCLUDES
Additional directories to instruct the C/C++ compilers to look for header files in. These paths are rooted at the top of the tree. Use LOCAL_PATH if you have subdirectories of your own that you want in the include paths. For example:
LOCAL_C_INCLUDES += extlibs/zlib-1.2.3
LOCAL_C_INCLUDES += $(LOCAL_PATH)/src
You should not add subdirectories of include to LOCAL_C_INCLUDES , instead you should reference those files in the #include statement with their subdirectories. For example:
#include <utils/KeyedVector.h>
not #include <KeyedVector.h>
There are some components that are doing this wrong, and should be cleaned up.
LOCAL_MODULE_TAGS
Set LOCAL_MODULE_TAGS to any number of whitespace-separated tags. If the tag list is empty or contains droid , the module will get installed as part of a make droid . Otherwise, it will only get installed by running make <your-module> or with the make all pseudotarget.
LOCAL_REQUIRED_MODULES
Set LOCAL_REQUIRED_MODULES to any number of whitespace-separated module names, like "libblah" or "Email". If this module is installed, all of the modules that it requires will be installed as well. This can be used to, e.g., ensure that necessary shared libraries or providers are installed when a given app is installed.
LOCAL_FORCE_STATIC_EXECUTABLE
If your executable should be linked statically, set LOCAL_FORCE_STATIC_EXECUTABLE:=true . There is a very short list of libraries that we have in static form (currently only libc). This is really only used for executables in /sbin on the root filesystem.
LOCAL_GENERATED_SOURCES
Files that you add to LOCAL_GENERATED_SOURCES will be automatically generated and then linked in when your module is built. See the Custom Tools template makefile for an example.
LOCAL_JAVA_LIBRARIES
When linking Java apps and libraries, LOCAL_JAVA_LIBRARIES specifies which sets of java classes to include. Currently there are two of these: core and framework . In most cases, it will look like this:
LOCAL_JAVA_LIBRARIES := core framework
Note that setting LOCAL_JAVA_LIBRARIES is not necessary (and is not allowed) when building an APK with "include $(BUILD_PACKAGE) ". The appropriate libraries will be included automatically.
LOCAL_LDFLAGS
You can pass additional flags to the linker by setting LOCAL_LDFLAGS . Keep in mind that the order of parameters is very important to ld, so test whatever you do on all platforms.
LOCAL_LDLIBS
LOCAL_LDLIBS allows you to specify additional libraries that are not part of the build for your executable or library. Specify the libraries you want in -lxxx format; they're passed directly to the link line. However, keep in mind that there will be no dependency generated for these libraries. It's most useful in simulator builds where you want to use a library preinstalled on the host. The linker (ld) is a particularly fussy beast, so it's sometimes necessary to pass other flags here if you're doing something sneaky. Some examples:
LOCAL_LDLIBS += -lcurses -lpthread
LOCAL_LDLIBS += -Wl,-z,origin
LOCAL_NO_MANIFEST
If your package doesn't have a manifest (AndroidManifest.xml), then set LOCAL_NO_MANIFEST:=true . The common resources package does this.
LOCAL_PACKAGE_NAME
LOCAL_PACKAGE_NAME is the name of an app. For example, Dialer, Contacts, etc. This will probably change or go away when we switch to an ant-based build system for the apps.
LOCAL_PATH
The directory your Android.mk file is in. You can set it by putting the following as the first line in your Android.mk:
LOCAL_PATH := $(my-dir)
The my-dir macro uses the MAKEFILE_LIST variable, so you must call it before you include any other makefiles. Also, consider that any subdirectories you inlcude might reset LOCAL_PATH, so do your own stuff before you include them. This also means that if you try to write several include lines that reference LOCAL_PATH , it won't work, because those included makefiles might reset LOCAL_PATH.
LOCAL_POST_PROCESS_COMMAND
For host executables, you can specify a command to run on the module after it's been linked. You might have to go through some contortions to get variables right because of early or late variable evaluation:
module := $(HOST_OUT_EXECUTABLES)/$(LOCAL_MODULE)
LOCAL_POST_PROCESS_COMMAND := /Developer/Tools/Rez -d __DARWIN__ -t APPL\
-d __WXMAC__ -o $(module) Carbon.r
LOCAL_PREBUILT_EXECUTABLES
When including $(BUILD_PREBUILT) or $(BUILD_HOST_PREBUILT), set these to executables that you want copied. They're located automatically into the right bin directory.
LOCAL_PREBUILT_LIBS
When including $(BUILD_PREBUILT) or $(BUILD_HOST_PREBUILT), set these to libraries that you want copied. They're located automatically into the right lib directory.
LOCAL_SHARED_LIBRARIES
These are the libraries you directly link against. You don't need to pass transitively included libraries. Specify the name without the suffix:
LOCAL_SHARED_LIBRARIES := \
libutils \
libui \
libaudio \
libexpat \
libsgl
LOCAL_SRC_FILES
The build system looks at LOCAL_SRC_FILES to know what source files to compile -- .cpp .c .y .l .java. For lex and yacc files, it knows how to correctly do the intermediate .h and .c/.cpp files automatically. If the files are in a subdirectory of the one containing the Android.mk, prefix them with the directory name:
LOCAL_SRC_FILES := \
file1.cpp \
dir/file2.cpp
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_STATIC_LIBRARIES := \
libutils \
libtinyxml
LOCAL_MODULE
LOCAL_MODULE is the name of what's supposed to be generated from your Android.mk. For exmample, for libkjs, the LOCAL_MODULE is "libkjs" (the build system adds the appropriate suffix -- .so .dylib .dll). For app modules, use LOCAL_PACKAGE_NAME instead of LOCAL_MODULE. We're planning on switching to ant for the apps, so this might become moot.
LOCAL_MODULE_PATH
Instructs the build system to put the module somewhere other than what's normal for its type. If you override this, make sure you also set LOCAL_UNSTRIPPED_PATH if it's an executable or a shared library so the unstripped binary has somewhere to go. An error will occur if you forget to.
See Putting modules elsewhere for more.
LOCAL_UNSTRIPPED_PATH
Instructs the build system to put the unstripped version of the module somewhere other than what's normal for its type. Usually, you override this because you overrode LOCAL_MODULE_PATH for an executable or a shared library. If you overrode LOCAL_MODULE_PATH , but not LOCAL_UNSTRIPPED_PATH , an error will occur.
See Putting modules elsewhere for more.
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.
LOCAL_WHOLE_STATIC_LIBRARIES := \
libsqlite3_android
LOCAL_YACCFLAGS
Any flags to pass to invocations of yacc for your module. A known limitation here is that the flags will be the same for all invocations of YACC for your module. This can be fixed. If you ever need it to be, just ask.
LOCAL_YACCFLAGS := -p kjsyy
Implementation Details
You should never have to touch anything in the config directory unless you're adding a new platform, new tools, or adding new features to the build system. In general, please consult with the build system owner(s) (android-build-team ) before you go mucking around in here. That said, here are some notes on what's going on under the hood.
Environment Setup / buildspec.mk Versioning
In order to make easier for people when the build system changes, when it is necessary to make changes to buildspec.mk or to rerun the environment setup scripts, they contain a version number in the variable BUILD_ENV_SEQUENCE_NUMBER. If this variable does not match what the build system expects, it fails printing an error message explaining what happened. If you make a change that requires an update, you need to update two places so this message will be printed.
- In config/envsetup.make, increment the CORRECT_BUILD_ENV_SEQUENCE_NUMBER definition.
- In buildspec.mk.default, update the BUILD_ENV_SEQUENCE_DUMBER definition to match the one in config/envsetup.make
The scripts automatically get the value from the build system, so they will trigger the warning as well.
Additional makefile variables
You probably shouldn't use these variables. Please consult android-build-team before using them. These are mostly there for workarounds for other issues, or things that aren't completely done right.
LOCAL_ADDITIONAL_DEPENDENCIES
If your module needs to depend on anything else that isn't actually built in to it, you can add those make targets to LOCAL_ADDITIONAL_DEPENDENCIES . Usually this is a workaround for some other dependency that isn't created automatically.
LOCAL_BUILT_MODULE
When a module is built, the module is created in an intermediate directory then copied to its final location. LOCAL_BUILT_MODULE is the full path to the intermediate file. See LOCAL_INSTALLED_MODULE for the path to the final installed location of the module.
LOCAL_HOST
Set by the host_xxx.make includes to tell base_rules.make and the other includes that we're building for the host. Kenneth did this as part of openbinder, and I would like to clean it up so the rules, includes and definitions aren't duplicated for host and target.
LOCAL_INSTALLED_MODULE
The fully qualified path name of the final location of the module. See LOCAL_BUILT_MODULE for the location of the intermediate file that the make rules should actually be constructing.
LOCAL_REPLACE_VARS
Used in some stuff remaining from the openbinder for building scripts with particular values set,
LOCAL_SCRIPTS
Used in some stuff remaining from the openbinder build system that we might find handy some day.
LOCAL_MODULE_CLASS
Which kind of module this is. This variable is used to construct other variable names used to locate the modules. See base_rules.make and envsetup.make.
LOCAL_MODULE_NAME
Set to the leaf name of the LOCAL_BUILT_MODULE. I'm not sure, but it looks like it's just used in the WHO_AM_I variable to identify in the pretty printing what's being built.
LOCAL_MODULE_SUFFIX
The suffix that will be appended to LOCAL_MODULE to form LOCAL_MODULE_NAME . For example, .so, .a, .dylib.
LOCAL_STRIP_MODULE
Calculated in base_rules.make to determine if this module should actually be stripped or not, based on whether LOCAL_STRIPPABLE_MODULE is set, and whether the combo is configured to ever strip modules. With Iliyan's stripping tool, this might change.
LOCAL_STRIPPABLE_MODULE
Set by the include makefiles if that type of module is strippable. Executables and shared libraries are.
LOCAL_SYSTEM_SHARED_LIBRARIES
Used while building the base libraries: libc, libm, libdl. Usually it should be set to "none," as it is in $(CLEAR_VARS). When building these libraries, it's set to the ones they link against. For example, libc, libstdc++ and libdl don't link against anything, and libm links against libc. Normally, when the value is none, these libraries are automatically linked in to executables and libraries, so you don't need to specify them manually.
Android编译系统分析
编译脚本及系统变量
build/envsetup.sh脚本分析
在编译源代码之前通常需要在android源代码顶层目录执行 . ./build/envsetup.sh 目的是为了使用
脚本 envsetup.sh 里面定义了一些函数：
function help()
function get_abs_build_var()
function get_build_var()
function check_product()
function check_variant()
function setpaths()
function printconfig()
function set_stuff_for_environment()
function set_sequence_number()
function settitle()
function choosetype()
function chooseproduct()
function choosevariant()
function tapas()
function choosecombo()
function print_lunch_menu()
function lunch()
function gettop
function m()
function findmakefile()
function mm()
function mmm()
function croot()
function pid()
function gdbclient()
function jgrep()
function cgrep()
function resgrep()
function getprebuilt
function tracedmdump()
function runhat()
function getbugreports()
function startviewserver()
function stopviewserver()
function isviewserverstarted()
function smoketest()
function runtest()
function runtest_py()
function godir ()
choosecombo 命令分析：
function choosecombo()
{
choosesim $1
echo
echo
choosetype $2
echo
echo
chooseproduct $3
echo
echo
choosevariant $4
echo
set_stuff_for_environment
printconfig
}
会依次进行如下选择：
Build for the simulator or the device?
1. Device
2. Simulator
Which would you like? [1]
Build type choices are:
1. release
2. debug
Which would you like? [1]
Product choices are:
1. emulator
2. generic
3. sim
4. littleton
You can also type the name of a product if you know it.
Which would you like? [littleton]
Variant choices are:
1. user
2. userdebug
3. eng
Which would you like? [eng] user
默认选择以后会出现：
TARGET_PRODUCT=littleton
TARGET_BUILD_VARIANT=user
TARGET_SIMULATOR=false
TARGET_BUILD_TYPE=release
TARGET_ARCH=arm
HOST_ARCH=x86
HOST_OS=linux
HOST_BUILD_TYPE=release
BUILD_ID=
＝＝＝＝＝＝＝＝＝＝
function chooseproduct()函数分析：
choices=(`/bin/ls build/target/board/*/BoardConfig.mk vendor/*/*/BoardConfig.mk 2> /dev/null`)
读取 build/target/board/* 目录下的板配置文件：BoardConfig.mk
读取 vendor/*/*/目录下的板配置文件：BoardConfig.mk
choices 的值为：
build/target/board/emulator/BoardConfig.mk
build/target/board/generic/BoardConfig.mk
build/target/board/sim/BoardConfig.mk
vendor/marvell/littleton/BoardConfig.mk
经过：
for choice in ${choices[@]}
do
# The product name is the name of the directory containing
# the makefile we found, above.
prodlist=(${prodlist[@]} `dirname ${choice} | xargs basename`)
done
的处理，prodlist的值为：
emulator generic sim littleton
所以选择菜单为：
Product choices are:
1. emulator
2. generic
3. sim
4. littleton
如果选择 4，那么 TARGET_PRODUCT 被赋值为： littleton。
board_config_mk := \
$(strip $(wildcard \
$(SRC_TARGET_DIR)/board/$(TARGET_DEVICE)/BoardConfig.mk \
vendor/*/$(TARGET_DEVICE)/BoardConfig.mk \
))
怎样添加一个模块
LOCAL_PATH:= $(call my-dir)
#编译静态库
include $(CLEAR_VARS)
LOCAL_MODULE = libhellos
LOCAL_CFLAGS = $(L_CFLAGS)
LOCAL_SRC_FILES = hellos.c
LOCAL_C_INCLUDES = $(INCLUDES)
LOCAL_SHARED_LIBRARIES := libcutils
LOCAL_COPY_HEADERS_TO := libhellos
LOCAL_COPY_HEADERS := hellos.h
include $(BUILD_STATIC_LIBRARY)
#编译动态库
include $(CLEAR_VARS)
LOCAL_MODULE = libhellod
LOCAL_CFLAGS = $(L_CFLAGS)
LOCAL_SRC_FILES = hellod.c
LOCAL_C_INCLUDES = $(INCLUDES)
LOCAL_SHARED_LIBRARIES := libcutils
LOCAL_COPY_HEADERS_TO := libhellod
LOCAL_COPY_HEADERS := hellod.h
include $(BUILD_SHARED_LIBRARY)
BUILD_TEST=true
ifeq ($(BUILD_TEST),true)
#使用静态库
include $(CLEAR_VARS)
LOCAL_MODULE := hellos
LOCAL_STATIC_LIBRARIES := libhellos
LOCAL_SHARED_LIBRARIES :=
LOCAL_LDLIBS += -ldl
LOCAL_CFLAGS := $(L_CFLAGS)
LOCAL_SRC_FILES := mains.c
LOCAL_C_INCLUDES := $(INCLUDES)
include $(BUILD_EXECUTABLE)
#使用动态库
include $(CLEAR_VARS)
LOCAL_MODULE := hellod
LOCAL_MODULE_TAGS := debug
LOCAL_SHARED_LIBRARIES := libc libcutils libhellod
LOCAL_LDLIBS += -ldl
LOCAL_CFLAGS := $(L_CFLAGS)
LOCAL_SRC_FILES := maind.c
LOCAL_C_INCLUDES := $(INCLUDES)
include $(BUILD_EXECUTABLE)
endif # ifeq ($(WPA_BUILD_SUPPLICANT),true)
########################
#local_target_dir := $(TARGET_OUT)/etc/wifi
#include $(CLEAR_VARS)
#LOCAL_MODULE := wpa_supplicant.conf
#LOCAL_MODULE_TAGS := user
#LOCAL_MODULE_CLASS := ETC
#LOCAL_MODULE_PATH := $(local_target_dir)
#LOCAL_SRC_FILES := $(LOCAL_MODULE)
#include $(BUILD_PREBUILT)
########################
系统变量解析
LOCAL_MODULE － 编译的目标对象
LOCAL_SRC_FILES － 编译的源文件
LOCAL_C_INCLUDES － 需要包含的头文件目录
LOCAL_SHARED_LIBRARIES － 链接时需要的外部库
LOCAL_PRELINK_MODULE － 是否需要prelink处理
BUILD_SHARED_LIBRARY － 指明要编译成动态库
LOCAL_PATH - 编译时的目录
$(call 目录，目录….) 目录引入操作符
如该目录下有个文件夹名称 src，则可以这样写 $(call src)，那么就会得到 src 目录的完整路径
include $(CLEAR_VARS) -清除之前的一些系统变量
CLEAR_VARS:= $(BUILD_SYSTEM)/clear_vars.mk
在 build/core/config.mk 定义 CLEAR_VARS:= $(BUILD_SYSTEM)/clear_vars.mk
通过include 包含自定义的.mk文件（即是自定义编译规则）或是引用系统其他的.mk文件（系统定义的编译规则）。
LOCAL_SRC_FILES － 编译的源文件
可以是.c, .cpp, .java, .S（汇编文件）或是.aidl等格式
不同的文件用空格隔开。如果编译目录子目录，采用相对路径，如子目录/文件名。也可以通过$(call 目录)，指明编译某目录
下所有.c/.cpp/.java/.S/ .aidl文件.追加文件 LOCAL_SRC_FILES += 文件
LOCAL_C_INCLUDES － 需要包含的头文件目录
可以是系统定义路径，也可以是相对路径. 如该编译目录下有个include目录，写法是include/*.h
LOCAL_SHARED_LIBRARIES － 链接时需要的外部共享库
LOCAL_STATIC_LIBRA RIES － 链接时需要的外部外部静态
LOCAL_JAVA_LIBRARIES 加入jar包
LOCAL_MODULE － 编译的目标对象
module 是指系统的 native code，通常针对c,c++代码
./system/core/sh/Android.mk:32:LOCAL_MODULE:= sh
./system/core/libcutils/Android.mk:71:LOCAL_MODULE := libcutils
./system/core/cpio/Android.mk:9:LOCAL_MODULE := mkbootfs
./system/core/mkbootimg/Android.mk:8:LOCAL_MODULE := mkbootimg
./system/core/toolbox/Android.mk:61:LOCAL_MODULE:= toolbox
./system/core/logcat/Android.mk:10:LOCAL_MODULE:= logcat
./system/core/adb/Android.mk:65:LOCAL_MODULE := adb
./system/core/adb/Android.mk:125:LOCAL_MODULE := adbd
./system/core/init/Android.mk:20:LOCAL_MODULE:= init
./system/core/vold/Android.mk:24:LOCAL_MODULE:= vold
./system/core/mountd/Android.mk:13:LOCAL_MODULE:= mountd
LOCAL_PACKAGE_NAME
Java 应用程序的名字用该变量定义
./packages/apps/Music/Android.mk:9:LOCAL_PACKAGE_NAME := Music
./packages/apps/Browser/Android.mk:14:LOCAL_PACKAGE_NAME := Browser
./packages/apps/Settings/Android.mk:8:LOCAL_PACKAGE_NAME := Settings
./packages/apps/Stk/Android.mk:10:LOCAL_PACKAGE_NAME := Stk
./packages/apps/Contacts/Android.mk:10:LOCAL_PACKAGE_NAME := Contacts
./packages/apps/Mms/Android.mk:8:LOCAL_PACKAGE_NAME := Mms
./packages/apps/Camera/Android.mk:8:LOCAL_PACKAGE_NAME := Camera
./packages/apps/Phone/Android.mk:11:LOCAL_PACKAGE_NAME := Phone
./packages/apps/VoiceDialer/Android.mk:8:LOCAL_PACKAGE_NAME := VoiceDialer
BUILD_SHARED_LIBRARY － 指明要编译成动态库。
编译的目标，用include 操作符
UILD_STATIC_LIBRARY来指明要编译成静态库。
如果是java文件的话，会用到系统的编译脚本host_java_library.mk，用BUILD_PACKAGE来指明。三个编译
－－－－－－－－－－－－－－－－－－－
include $(BUILD_STATIC_LIBRARY)
BUILD_STATIC_LIBRARY:= $(BUILD_SYSTEM)/static_library.mk
－－－－－－－－－－－－－－－－－－－
include $(BUILD_SHARED_LIBRARY)
./build/core/config.mk:50:BUILD_SHARED_LIBRARY:= $(BUILD_SYSTEM)/shared_library.mk
－－－－－－－－－－－－－－－－－－－
include $(BUILD_HOST_SHARED_LIBRARY)
BUILD_HOST_SHARED_LIBRARY:= $(BUILD_SYSTEM)/host_shared_library.mk
－－－－－－－－－－－－－－－－－－－
include $(BUILD_EXECUTABLE)
build/core/config.mk:51:BUILD_EXECUTABLE:= $(BUILD_SYSTEM)/executable.mk
－－－－－－－－－－－－－－－－－－－
include $(BUILD_HOST_EXECUTABLE)
./build/core/config.mk:53:BUILD_HOST_EXECUTABLE:= $(BUILD_SYSTEM)/host_executable.mk
－－－－－－－－－－－－－－－－－－－
BUILD_HOST_JAVA_LIBRARY:= $(BUILD_SYSTEM)/host_java_library.mk
－－－－－－－－－－－－－－－－－－－
BUILD_JAVA_LIBRARY
./build/core/config.mk:58:BUILD_JAVA_LIBRARY:= $(BUILD_SYSTEM)/java_library.mk
－－－－－－－－－－－－－－－－－－
BUILD_STATIC_JAVA_LIBRARY 编译静态JAVA库
./build/core/config.mk:59:BUILD_STATIC_JAVA_LIBRARY:= $(BUILD_SYSTEM)/static_java_library.mk
－－－－－－－－－－－－－－－－－－
BUILD_HOST_JAVA_LIBRARY 编译本机用的JAVA库
./build/core/config.mk:60:BUILD_HOST_JAVA_LIBRARY:= $(BUILD_SYSTEM)/host_java_library.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_RAW_STATIC_LIBRARY := $(BUILD_SYSTEM)/raw_static_library.mk
BUILD_SHARED_LIBRARY:= $(BUILD_SYSTEM)/shared_library.mk
BUILD_EXECUTABLE:= $(BUILD_SYSTEM)/executable.mk
BUILD_RAW_EXECUTABLE:= $(BUILD_SYSTEM)/raw_executable.mk
BUILD_HOST_EXECUTABLE:= $(BUILD_SYSTEM)/host_executable.mk
BUILD_PACKAGE:= $(BUILD_SYSTEM)/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_KEY_CHAR_MAP := $(BUILD_SYSTEM)/key_char_map.mk
＝＝＝＝＝＝＝＝＝＝＝＝
LOCAL_PRELINK_MODULE
Prelink利用事先链接代替运行时链接的方法来加速共享库的加载，它不仅可以加快起动速度，还可以减少部分内存开销，
是各种Linux架构上用于减少程序加载时间、缩短系统启动时间和加快应用程序启动的很受欢迎的一个工具。程序运行时的
动态链接尤其是重定位(relocation)的开销对于大型系统来说是很大的。
动态链接和加载的过程开销很大，并且在大多数的系统上, 函数库并不会常常被更动, 每次程序被执行时所进行的链接
动作都是完全相同的，对于嵌入式系统来说尤其如此。因此，这一过程可以改在运行时之前就可以预先处理好，即花一些时间
利用Prelink工具对动态共享库和可执行文件进行处理，修改这些二进制文件并加入相应的重定位等信息，节约了本来在程序
启动时的比较耗时的查询函数地址等工作，这样可以减少程序启动的时间，同时也减少了内存的耗用。
Prelink的这种做法当然也有代价：每次更新动态共享库时，相关的可执行文件都需要重新执行一遍Prelink才能保
证有效，因为新的共享库中的符号信息、地址等很可能与原来的已经不同了，这就是为什么 android framework代码一改动，
这时候就会导致相关的应用程序重新被编译。
这种代价对于嵌入式系统的开发者来说可能稍微带来一些复杂度，不过好在对用户来说几乎是可以忽略的。
－－－－－－－－－－－－－－－－－－－－
变量设置为false那么将不做prelink操作
LOCAL_PRELINK_MODULE := false
默认是需要prlink的，同时需要在 build/core/prelink-linux-arm.map 中加入
libhellod.so 0x96000000
这个map文件好像是制定动态库的地址的，在前面注释上面有一些地址范围的信息，注意库与库之间的间隔数，
如果指定不好的话编译的时候会提示说地址空间冲突的问题。另外，注意排序，这里要把数大的放到前面去，
按照大小降序排序。
解析 LOCAL_PRELINK_MODULE 变量
build/core/dynamic_binary.mk:94:ifeq ($(LOCAL_PRELINK_MODULE),true)
ifeq ($(LOCAL_PRELINK_MODULE),true)
$(prelink_output): $(prelink_input) $(TARGET_PRELINKER_MAP) $(APRIORI)
$(transform-to-prelinked)
transform-to-prelinked定义：
./build/core/definitions.mk:1002:define transform-to-prelinked
define transform-to-prelinked
@mkdir -p $(dir $@)
@echo "target Prelink: $(PRIVATE_MODULE) ($@)"
$(hide) $(APRIORI) \
--prelinkmap $(TARGET_PRELINKER_MAP) \
--locals-only \
--quiet \
$< \
--output $@
endef
./build/core/config.mk:183:APRIORI := $(HOST_OUT_EXECUTABLES)/apriori$(HOST_EXECUTABLE_SUFFIX)
prelink工具不是常用的prelink而是apriori，其源代码位于” <your_android>/build/tools/apriori”
参考文档：
动态库优化——Prelink（预连接）技术
http://www.eefocus.com/article/09-04/71629s.html
＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝
LOCAL_ARM_MODE := arm
目前Android大部分都是基于Arm处理器的，Arm指令用两种模式Thumb(每条指令两个字节)和arm指令（每条指令四个字节）
LOCAL_CFLAGS += -O3 -fstrict-aliasing -fprefetch-loop-arrays
通过设定编译器操作，优化级别，-O0表示没有优化,-O1为缺省值，-O3优化级别最高
LOCAL_CFLAGS += -W -Wall
LOCAL_CFLAGS += -fPIC -DPIC
LOCAL_CFLAGS += -O2 -g -DADB_HOST=1 -Wall -Wno-unused-parameter
LOCAL_CFLAGS += -D_XOPEN_SOURCE -D_GNU_SOURCE -DSH_HISTORY
LOCAL_CFLAGS += -DUSEOVERLAY2
根据条件选择相应的编译参数
ifeq ($(TARGET_ARCH),arm)
LOCAL_CFLAGS += -DANDROID_GADGET=1
LOCAL_CFLAGS := $(PV_CFLAGS)
endif
ifeq ($(TARGET_BUILD_TYPE),release)
LOCAL_CFLAGS += -O2
endif
LOCAL_LDLIBS := -lpthread
LOCAL_LDLIBS += -ldl
ifdef USE_MARVELL_MVED
LOCAL_WHOLE_STATIC_LIBRARIES += lib_il_mpeg4aspdecmved_wmmx2lnx lib_il_h264decmved_wmmx2lnx
LOCAL_SHARED_LIBRARIES += libMrvlMVED
else
LOCAL_WHOLE_STATIC_LIBRARIES += lib_il_h264dec_wmmx2lnx lib_il_mpeg4aspdec_wmmx2lnx
endif
＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝＝
本文转自： http://blogold.chinaunix.net/u3/90973/showart_2164116.html