通过分析一个例子来了解NDK makefile文件的生成。例子"hello JNI" ,由NDK提供的例子
A. 目录结构
jni目录:包含本地源文件,eg:'jni/hello-jni.c',该源文件实现了一个简单的共享库,实现了一个简单的本地方法,返回字符串给java 虚拟机
src目录:包含了工程的java源文件
B. mk源文件
LOCAL_PATH := $(call my-dir) include $(CLEAR_VARS) LOCAL_MODULE := hello-jni LOCAL_SRC_FILES := hello-jni.c include $(BUILD_SHARED_LIBRARY)
第一行:
LOCAL_PATH := $(call my-dir)
必须的,调用系统方法,返回当前程序的目录
第二行:
include $(CLEAR_VARS)
必须得,该CLEAR_VARS变量由编译系统提供,指向一些特殊的GNU Makefile文件来清除一些LOCAL_XXX变量除了
LOCAL_PATH。因为所有的编译控制文件被一个单一的GUN Make执行时所有的变量时全局的。
第三行:
LOCAL_MODULE := hello-jni
用来指定你生成的动态库的名字,系统会自动为你添加前缀和后缀,生成后的so为 libhello-jni.so,系统自动添加了前缀“lib”和后缀“.so”
第四行:
LOCAL_SRC_FILES := hello-jni.c
其包含一系列的C or C++源文件,不需要添加.h文件,系统会自动为你添加
第五行:
include $(BUILD_SHARED_LIBRARY)
必须的,其由系统提供,指向一个GUN Makefile脚本,用来负责收集你定义的所有LOCAL_XXX变量,并确定该怎么构建,以及怎样做准确,同时也指定生成一个共享库
第二部分:参考资料
mk overviewOverview:概要 --------- An Android.mk file is written to describe your sources to the build system. More specifically:具体来说: - The file is really a tiny GNU Makefile fragment that will be parsed one or more times by the build system. As such, you should try to minimize the variables you declare there and do not assume that anything is not defined during parsing. - The file syntax is designed to allow you to group your sources into 'modules'. A module is one of the following: - a static library - a shared library Only shared libraries will be installed/copied to your application package. Static libraries can be used to generate shared libraries though. You can define one or more modules in each Android.mk file, and you can use the same source file in several modules. - The build system handles many details for you. For example, you don't need to list header files or explicit dependencies between generated files in your Android.mk. The NDK build system will compute these automatically for you. This also means that, when updating to newer releases of the NDK, you should be able to benefit from new toolchain/platform support without having to touch your Android.mk files. Note that the syntax is *very* close to the one used in Android.mk files distributed with the full open-source Android platform sources. While the build system implementation that uses them is different, this is an intentional design decision made to allow reuse of 'external' libraries' source code easier for application developers. Simple example: --------------- Before describing the syntax in details, let's consider the simple "hello JNI" example, i.e. the files under: apps/hello-jni/project Here, we can see: - The 'src' directory containing the Java sources for the sample Android project. - The 'jni' directory containing the native source for the sample, i.e. 'jni/hello-jni.c' This source file implements a simple shared library that implements a native method that returns a string to the VM application. - The 'jni/Android.mk' file that describes the shared library to the NDK build system. Its content is: ---------- cut here ------------------ LOCAL_PATH := $(call my-dir) include $(CLEAR_VARS) LOCAL_MODULE := hello-jni LOCAL_SRC_FILES := hello-jni.c include $(BUILD_SHARED_LIBRARY) ---------- cut here ------------------ Now, let's explain these lines: LOCAL_PATH := $(call my-dir) An Android.mk file must begin with the definition of the LOCAL_PATH variable. It is used to locate source files in the development tree. In this example, the macro function 'my-dir', provided by the build system, is used to return the path of the current directory (i.e. the directory containing the Android.mk file itself). include $(CLEAR_VARS) The CLEAR_VARS variable is provided by the build system and points to a special GNU Makefile that will clear many LOCAL_XXX variables for you (e.g. LOCAL_MODULE, LOCAL_SRC_FILES, LOCAL_STATIC_LIBRARIES, etc...), with the exception of LOCAL_PATH. This is needed because all build control files are parsed in a single GNU Make execution context where all variables are global. LOCAL_MODULE := hello-jni The LOCAL_MODULE variable must be defined to identify each module you describe in your Android.mk. The name must be *unique* and not contain any spaces. Note that the build system will automatically add proper prefix and suffix to the corresponding generated file. In other words, a shared library module named 'foo' will generate 'libfoo.so'. IMPORTANT NOTE: If you name your module 'libfoo', the build system will not add another 'lib' prefix and will generate libfoo.so as well. This is to support Android.mk files that originate from the Android platform sources, would you need to use these. LOCAL_SRC_FILES := hello-jni.c The LOCAL_SRC_FILES variables must contain a list of C and/or C++ source files that will be built and assembled into a module. Note that you should not list header and included files here, because the build system will compute dependencies automatically for you; just list the source files that will be passed directly to a compiler, and you should be good. Note that the default extension for C++ source files is '.cpp'. It is however possible to specify a different one by defining the variable LOCAL_DEFAULT_CPP_EXTENSION. Don't forget the initial dot (i.e. '.cxx' will work, but not 'cxx'). include $(BUILD_SHARED_LIBRARY) The BUILD_SHARED_LIBRARY is a variable provided by the build system that points to a GNU Makefile script that is in charge of collecting all the information you defined in LOCAL_XXX variables since the latest 'include $(CLEAR_VARS)' and determine what to build, and how to do it exactly. There is also BUILD_STATIC_LIBRARY to generate a static library. There are more complex examples in the samples directories, with commented Android.mk files that you can look at. Reference: ---------- This is the list of variables you should either rely on or define in an Android.mk. You can define other variables for your own usage, but the NDK build system reserves the following variable names: - names that begin with LOCAL_ (e.g. LOCAL_MODULE) - names that begin with PRIVATE_, NDK_ or APP_ (used internally) - lower-case names (used internally, e.g. 'my-dir') If you need to define your own convenience variables in an Android.mk file, we recommend using the MY_ prefix, for a trivial example: ---------- cut here ------------------ MY_SOURCES := foo.c ifneq ($(MY_CONFIG_BAR),) MY_SOURCES += bar.c endif LOCAL_SRC_FILES += $(MY_SOURCES) ---------- cut here ------------------ So, here we go: NDK-provided variables: - - - - - - - - - - - - These GNU Make variables are defined by the build system before your Android.mk file is parsed. Note that under certain circumstances the NDK might parse your Android.mk several times, each with different definition for some of these variables. CLEAR_VARS Points to a build script that undefines nearly all LOCAL_XXX variables listed in the "Module-description" section below. You must include the script before starting a new module, e.g.: include $(CLEAR_VARS) BUILD_SHARED_LIBRARY Points to a build script that collects all the information about the module you provided in LOCAL_XXX variables and determines how to build a target shared library from the sources you listed. Note that you must have LOCAL_MODULE and LOCAL_SRC_FILES defined, at a minimum before including this file. Example usage: include $(BUILD_SHARED_LIBRARY) note that this will generate a file named lib$(LOCAL_MODULE).so BUILD_STATIC_LIBRARY A variant of BUILD_SHARED_LIBRARY that is used to build a target static library instead. Static libraries are not copied into your project/packages but can be used to build shared libraries (see LOCAL_STATIC_LIBRARIES and LOCAL_STATIC_WHOLE_LIBRARIES described below). Example usage: include $(BUILD_STATIC_LIBRARY) Note that this will generate a file named lib$(LOCAL_MODULE).a TARGET_ARCH Name of the target CPU architecture as it is specified by the full Android open-source build. This is 'arm' for any ARM-compatible build, independent of the CPU architecture revision. TARGET_PLATFORM Name of the target Android platform when this Android.mk is parsed. For example, 'android-3' correspond to Android 1.5 system images. For a complete list of platform names and corresponding Android system images, read docs/STABLE-APIS.TXT. TARGET_ARCH_ABI Name of the target CPU+ABI when this Android.mk is parsed. Two values are supported at the moment: armeabi For Armv5TE armeabi-v7a NOTE: Up to Android NDK 1.6_r1, this variable was simply defined as 'arm'. However, the value has been redefined to better match what is used internally by the Android platform. For more details about architecture ABIs and corresponding compatibility issues, please read docs/CPU-ARCH-ABIS.TXT Other target ABIs will be introduced in future releases of the NDK and will have a different name. Note that all ARM-based ABIs will have 'TARGET_ARCH' defined to 'arm', but may have different 'TARGET_ARCH_ABI' TARGET_ABI The concatenation of target platform and abi, it really is defined as $(TARGET_PLATFORM)-$(TARGET_ARCH_ABI) and is useful when you want to test against a specific target system image for a real device. By default, this will be 'android-3-armeabi' (Up to Android NDK 1.6_r1, this used to be 'android-3-arm' by default) NDK-provided function macros: - - - - - - - - - - - - - - - The following are GNU Make 'function' macros, and must be evaluated by using '$(call <function>)'. They return textual information. my-dir Returns the path of the current Android.mk's directory, relative to the top of the NDK build system. This is useful to define LOCAL_PATH at the start of your Android.mk as with: LOCAL_PATH := $(call my-dir) all-subdir-makefiles Returns a list of Android.mk located in all sub-directories of the current 'my-dir' path. For example, consider the following hierarchy: sources/foo/Android.mk sources/foo/lib1/Android.mk sources/foo/lib2/Android.mk If sources/foo/Android.mk contains the single line: include $(call all-subdir-makefiles) Then it will include automatically sources/foo/lib1/Android.mk and sources/foo/lib2/Android.mk This function can be used to provide deep-nested source directory hierarchies to the build system. Note that by default, the NDK will only look for files in sources/*/Android.mk this-makefile Returns the path of the current Makefile (i.e. where the function is called). parent-makefile Returns the path of the parent Makefile in the inclusion tree, i.e. the path of the Makefile that included the current one. grand-parent-makefile Guess what... Module-description variables: - - - - - - - - - - - - - - - The following variables are used to describe your module to the build system. You should define some of them between an 'include $(CLEAR_VARS)' and an 'include $(BUILD_XXXXX)'. As written previously, $(CLEAR_VARS) is a script that will undefine/clear all of these variables, unless explicitely noted in their description. LOCAL_PATH This variable is used to give the path of the current file. You MUST define it at the start of your Android.mk, which can be done with: LOCAL_PATH := $(call my-dir) This variable is *not* cleared by $(CLEAR_VARS) so only one definition per Android.mk is needed (in case you define several modules in a single file). LOCAL_MODULE This is the name of your module. It must be unique among all module names, and shall not contain any space. You MUST define it before including any $(BUILD_XXXX) script. The module name determines the name of generated files, e.g. lib .so for a shared library module named . However you should only refer to other modules with their 'normal' name (e.g. ) in your NDK build files (either Android.mk or Application.mk) LOCAL_SRC_FILES This is a list of source files that will be built for your module. Only list the files that will be passed to a compiler, since the build system automatically computes dependencies for you. Note that source files names are all relative to LOCAL_PATH and you can use path components, e.g.: LOCAL_SRC_FILES := foo.c / toto/bar.c NOTE: Always use Unix-style forward slashes (/) in build files. Windows-style back-slashes will not be handled properly. LOCAL_CPP_EXTENSION This is an optional variable that can be defined to indicate the file extension of C++ source files. The default is '.cpp' but you can change it. For example: LOCAL_CPP_EXTENSION := .cxx LOCAL_C_INCLUDES An optional list of paths, relative to the NDK *root* directory, which will be appended to the include search path when compiling all sources (C, C++ and Assembly). For example: LOCAL_C_INCLUDES := sources/foo Or even: LOCAL_C_INCLUDES := $(LOCAL_PATH)/../foo These are placed before any corresponding inclusion flag in LOCAL_CFLAGS / LOCAL_CPPFLAGS LOCAL_CFLAGS An optional set of compiler flags that will be passed when building C *and* C++ source files. This can be useful to specify additionnal macro definitions or compile options. IMPORTANT: Try not to change the optimization/debugging level in your Android.mk, this can be handled automatically for you by specifying the appropriate information in your Application.mk, and will let the NDK generate useful data files used during debugging. NOTE: In android-ndk-1.5_r1, the corresponding flags only applied to C source files, not C++ ones. This has been corrected to match the full Android build system behaviour. (You can use LOCAL_CPPFLAGS to specify flags for C++ sources only now). LOCAL_CXXFLAGS An alias for LOCAL_CPPFLAGS. Note that use of this flag is obsolete as it may disappear in future releases of the NDK. LOCAL_CPPFLAGS An optional set of compiler flags that will be passed when building C++ source files *only*. They will appear after the LOCAL_CFLAGS on the compiler's command-line. NOTE: In android-ndk-1.5_r1, the corresponding flags applied to both C and C++ sources. This has been corrected to match the full Android build system. (You can use LOCAL_CFLAGS to specify flags for both C and C++ sources now). LOCAL_STATIC_LIBRARIES The list of static libraries modules (built with BUILD_STATIC_LIBRARY) that should be linked to this module. This only makes sense in shared library modules. LOCAL_SHARED_LIBRARIES The list of shared libraries *modules* this module depends on at runtime. This is necessary at link time and to embed the corresponding information in the generated file. Note that this does not append the listed modules to the build graph, i.e. you should still add them to your application's required modules in your Application.mk LOCAL_LDLIBS The list of additional linker flags to be used when building your module. This is useful to pass the name of specific system libraries with the " -l" prefix. For example, the following will tell the linker to generate a module that links to /system/lib/libz.so at load time: LOCAL_LDLIBS := -lz See docs/STABLE-APIS.TXT for the list of exposed system libraries you can linked against with this NDK release. LOCAL_ALLOW_UNDEFINED_SYMBOLS By default, any undefined reference encountered when trying to build a shared library will result in an " undefined symbol" error. This is a great help to catch bugs in your source code. However, if for some reason you need to disable this check, set this variable to ' true'. Note that the corresponding shared library may fail to load at runtime. LOCAL_ARM_MODE By default, ARM target binaries will be generated in 'thumb' mode, where each instruction are 16-bit wide. You can define this variable to 'arm' if you want to force the generation of the module's object files in 'arm' (32-bit instructions) mode. E.g.: LOCAL_ARM_MODE := arm Note that you can also instruct the build system to only build specific sources in arm mode by appending an '.arm' suffix to its source file name. For example, with: LOCAL_SRC_FILES := foo.c bar.c.arm Tells the build system to always compile 'bar.c' in arm mode, and to build foo.c according to the value of LOCAL_ARM_MODE. NOTE: Setting APP_OPTIM to 'debug' in your Application.mk will also force the generation of ARM binaries as well. This is due to bugs in the toolchain debugger that don't deal too well with thumb code. LOCAL_ARM_NEON Defining this variable to ' true' allows the use of ARM Advanced SIMD (a.k.a. NEON) GCC intrinsics in your C and C++ sources, as well as NEON instructions in Assembly files. You should only define it when targetting the 'armeabi-v7a' ABI that corresponds to the ARMv7 instruction set. Note that not all ARMv7 based CPUs support the NEON instruction set extensions and that you should perform runtime detection to be able to use this code at runtime safely. To lean more about this, please read the documentation at docs/CPU-ARM-NEON.TXT and docs/CPU-FEATURES.TXT. Alternatively, you can also specify that only specific source files may be compiled with NEON support by using the '.neon' suffix, as in: LOCAL_SRC_FILES = foo.c.neon bar.c zoo.c.arm.neon In this example, 'foo.c' will be compiled in thumb+neon mode, 'bar.c' will be compiled in 'thumb' mode, and 'zoo.c' will be compiled in 'arm+neon' mode. Note that the '.neon' suffix must appear after the '.arm' suffix if you use both (i.e. foo.c.arm.neon works, but not foo.c.neon.arm !) LOCAL_DISABLE_NO_EXECUTE Android NDK r4 added support for the " NX bit" security feature. It is enabled by default, but you can disable it if you *really* need to by setting this variable to ' true'. NOTE: This feature does not modify the ABI and is only enabled on kernels targetting ARMv6+ CPU devices. Machine code generated with this feature enabled will run unmodified on devices running earlier CPU architectures.