NDK帮助文档学习--ANDROID-MK.html

android-ndk-r7b\docs

ANDROID-MK.html

需要首先阅读docs/OVERVIEW.html

本文介绍了Android.mk build file的语法

Overview:---------An Android.mk file is written to describe your sources to thebuild system. More specifically:- 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.

这个文件有可能被build system build多次，所以，需要尽可能少的声明变量。- 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

该文件的语法被设计为允许你把源文件分组到modules中

  Only shared libraries will be installed/copied to your  application package. Static libraries can be used to generate  shared libraries though.

只有shared libraries可以被installed/copied到你的application package中。

Static library可以被用来生成shared libraries。  You can define one or more modules in each Android.mk file,  and you can use the same source file in several modules.

可以再一个Android.mk文件中定义一个或多个modules，也可以在几个modules中使用同一个source file。- 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.build system处理了很多细节。例如，不需要列出头文件和详尽的依赖关系。  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.

这样做的好处是当更新NDK到新的版本时，可以不需要改动Android.mk文件。Note that the syntax is *very* close to the one used in Android.mk filesdistributed with the full open-source Android platform sources. Whilethe build system implementation that uses them is different, this isan 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:先来看看最简单的hello JNI    apps/hello-jni/projectHere, we can see:  - The 'src' directory containing the Java sources for the    sample Android project.    src目录包含Java的源文件  - The 'jni' directory containing the native source for    the sample, i.e. 'jni/hello-jni.c'    jni目录包含native code    This source file implements a simple shared library that    implements a native method that returns a string to the    VM application.    它实现了一个简单的shared library，实现了一个native method，它向VM application返回一个string

  - The 'jni/Android.mk' file that describes the shared library    to the NDK build system. Its content is:   jni/Android.mk文件的内容如下：   ---------- 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 returnthe path of the current directory (i.e. the directory containing theAndroid.mk file itself).

一个Android.mk文件必须以对LOCAL_PATH variable的定义开始，它被用来定位source files。

在这个例子中，build system提供的宏my-dir，用来返回当前目录的路径  include $(CLEAR_VARS)The CLEAR_VARS variable is provided by the build system and points to aspecial 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 buildcontrol files are parsed in a single GNU Make execution context whereall variables are global.

清除LOCAL_XXX variables。这个是必须的，因为所有的build control files在一个单独的GNU Make execution context中解析，所有的变量都是全局的  LOCAL_MODULE := hello-jniThe LOCAL_MODULE variable must be defined to identify each module youdescribe in your Android.mk. The name must be *unique* and not containany spaces. Note that the build system will automatically add properprefix and suffix to the corresponding generated file. In other words,a shared library module named 'foo' will generate 'libfoo.so'.

LOCAL_MODULE variable用来标识每一个module。名称必须是唯一的，并且不能包含空格。build system将自动的添加合适的前缀和后缀，比如，一个叫做foo的shared library module将被生成libfoo.soIMPORTANT NOTE:If you name your module 'libfoo', the build system will notadd another 'lib' prefix and will generate libfoo.so as well.This is to support Android.mk files that originate from theAndroid platform sources, would you need to use these.

如果你的module名是libfoo，build system将不会添加前缀lib。  LOCAL_SRC_FILES := hello-jni.cThe LOCAL_SRC_FILES variables must contain a list of C and/or C++ sourcefiles that will be built and assembled into a module. Note that you shouldnot list header and included files here, because the build system willcompute dependencies automatically for you; just list the source filesthat will be passed directly to a compiler, and you should be good.

LOCAL_SRC_FILES包含C和C++的source files列表，它们将被built和assembled到module中Note that the default extension for C++ source files is '.cpp'. It ishowever possible to specify a different one by defining the variableLOCAL_CPP_EXTENSION. Don't forget the initial dot (i.e. '.cxx' willwork, but not 'cxx').  include $(BUILD_SHARED_LIBRARY)The BUILD_SHARED_LIBRARY is a variable provided by the build system thatpoints to a GNU Makefile script that is in charge of collecting all theinformation you defined in LOCAL_XXX variables since the latest'include $(CLEAR_VARS)' and determine what to build, and how to do itexactly. There is also BUILD_STATIC_LIBRARY to generate a static library.
BUILD_SHARED_LIBRARY 指向一段有build system提供的GNU Makefile script，它用来搜集所有的LOCAL_XXX信息，来决定build什么和怎么去做
BUILD_STATIC_LIBRARY 用来生成static library
There are more complex examples in the samples directories, with commentedAndroid.mk files that you can look at.Reference:----------This is the list of variables you should either rely on or define inan Android.mk. You can define other variables for your own usage, butthe NDK build system reserves the following variable names:

NDK build system保留的名字：- 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.mkfile, we recommend using the MY_ prefix, for a trivial example:如果需要定义自己的变量名，建议使用MY_作为前缀，例如   ---------- cut here ------------------    MY_SOURCES := foo.c    ifneq ($(MY_CONFIG_BAR),)      MY_SOURCES += bar.c    endif    LOCAL_SRC_FILES += $(MY_SOURCES)   ---------- cut here ------------------

NDK-provided variables:- - - - - - - - - - - -NDK提供的变量名These GNU Make variables are defined by the build system beforeyour Android.mk file is parsed. Note that under certain circumstancesthe NDK might parse your Android.mk several times, each with differentdefinition 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)取消几乎所有的LOCAL_XXX变量的定义

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).soBUILD_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_WHOLE_STATIC_LIBRARIES described below).    Example usage:      include $(BUILD_STATIC_LIBRARY)    Note that this will generate a file named lib$(LOCAL_MODULE).a不会被复制到project/packages中，但是可以被用来build shared libraries

PREBUILT_SHARED_LIBRARY    Points to a build script used to specify a prebuilt shared library.    Unlike BUILD_SHARED_LIBRARY and BUILD_STATIC_LIBRARY, the value    of LOCAL_SRC_FILES must be a single path to a prebuilt shared    library (e.g. foo/libfoo.so), instead of a source file.    You can reference the prebuilt library in another module using    the LOCAL_PREBUILTS variable (see docs/PREBUILTS.html for more    information).PREBUILT_STATIC_LIBRARY    This is the same as PREBUILT_SHARED_LIBRARY, but for a static library    file instead. See docs/PREBUILTS.html for more.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.html.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.html    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 evaluatedby using '$(call <function>)'. They return textual information.my-dir    Returns the path of the last included Makefile, which typically is    the current Android.mk's directory. This is useful to define    LOCAL_PATH at the start of your Android.mk as with:        LOCAL_PATH := $(call my-dir)    IMPORTANT NOTE: Due to the way GNU Make works, this really returns    the path of the *last* *included* *Makefile* during the parsing of    build scripts. Do not call my-dir after including another file.实际上返回的是最近被included的Makefile的路径，所以，不要在including另一个文件后，调用my-dir    For example, consider the following example:        LOCAL_PATH := $(call my-dir)        ... declare one module        include $(LOCAL_PATH)/foo/Android.mk        LOCAL_PATH := $(call my-dir)        ... declare another module    The problem here is that the second call to 'my-dir' will define    LOCAL_PATH to $PATH/foo instead of $PATH, due to the include that    was performed before that.    For this reason, it's better to put additional includes after    everything else in an Android.mk, as in:        LOCAL_PATH := $(call my-dir)        ... declare one module        LOCAL_PATH := $(call my-dir)        ... declare another module        # extra includes at the end of the Android.mk        include $(LOCAL_PATH)/foo/Android.mk    If this is not convenient, save the value of the first my-dir call    into another variable, for example:        MY_LOCAL_PATH := $(call my-dir)        LOCAL_PATH := $(MY_LOCAL_PATH)        ... declare one module        include $(LOCAL_PATH)/foo/Android.mk        LOCAL_PATH := $(MY_LOCAL_PATH)        ... declare another moduleall-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.mkthis-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...import-module    A function that allows you to find and include the Android.mk    of another module by name. A typical example is:      $(call import-module,<name>)    And this will look for the module tagged <name> in the list of    directories referenced by your NDK_MODULE_PATH environment    variable, and include its Android.mk automatically for you.    Read docs/IMPORT-MODULE.html for more details.

Module-description variables:- - - - - - - - - - - - - - -The following variables are used to describe your module to the buildsystem. You should define some of them between an 'include $(CLEAR_VARS)'and an 'include $(BUILD_XXXXX)'. As written previously, $(CLEAR_VARS) isa script that will undefine/clear all of these variables, unless explicitlynoted 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.    By default, the module name determines the name of generated files,    e.g. lib<foo>.so for a shared library module named <foo>. However    you should only refer to other modules with their 'normal'    name (e.g. <foo>) in your NDK build files (either Android.mk    or Application.mk)    You can override this default with LOCAL_MODULE_FILENAME (see below)LOCAL_MODULE_FILENAME    This variable is optional, and allows you to redefine the name of    generated files. By default, module <foo> will always generate a    static library named lib<foo>.a or a shared library named lib<foo>.so,    which are standard Unix conventions.    You can override this by defining LOCAL_MODULE_FILENAME, For example:        LOCAL_MODULE := foo-version-1        LOCAL_MODULE_FILENAME := libfoo    NOTE: You should not put a path or file extension in your    LOCAL_MODULE_FILENAME, these will be handled automatically by the    build system.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(s) of C++ source files. They must begin with a dot.    The default is '.cpp' but you can change it. For example:        LOCAL_CPP_EXTENSION := .cxx    Since NDK r7, you can list several extensions in this variable, as in:        LOCAL_CPP_EXTENSION := .cxx .cpp .ccLOCAL_CPP_FEATURES    This is an optional variable that can be defined to indicate    that your code relies on specific C++ features. To indicate that    your code uses RTTI (RunTime Type Information), use the following:        LOCAL_CPP_FEATURES := rtti    To indicate that your code uses C++ exceptions, use:        LOCAL_CPP_FEATURES := exceptions    You can also use both of them with (order is not important):        LOCAL_CPP_FEATURES := rtti features    The effect of this variable is to enable the right compiler/linker    flags when building your modules from sources. For prebuilt binaries,    this also helps declare which features the binary relies on to ensure    the final link works correctly.    It is recommended to use this variable instead of enabling -frtti and    -fexceptions directly in your LOCAL_CPPFLAGS definition.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    The LOCAL_C_INCLUDES path are also used automatically when    launching native debugging with ndk-gdb.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 additional 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).    It is possible to specify additional include paths with    LOCAL_CFLAGS += -I<path>, however, it is better to use LOCAL_C_INCLUDES    for this, since the paths will then also be used during native    debugging with ndk-gdb.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.LOCAL_WHOLE_STATIC_LIBRARIES    A variant of LOCAL_STATIC_LIBRARIES used to express that the corresponding    library module should be used as "whole archives" to the linker. See the    GNU linker's documentation for the --whole-archive flag.    This is generally useful when there are circular dependencies between    several static libraries. Note that when used to build a shared library,    this will force all object files from your whole static libraries to be    added to the final binary. This is not true when generating executables    though.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.html 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 targeting 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 learn more about this, please read the documentation at    docs/CPU-ARM-NEON.html and docs/CPU-FEATURES.html.    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 targeting ARMv6+ CPU devices. Machine code generated          with this feature enabled will run unmodified on devices          running earlier CPU architectures.    For more information, see:        http://en.wikipedia.org/wiki/NX_bit        http://www.gentoo.org/proj/en/hardened/gnu-stack.xmlLOCAL_EXPORT_CFLAGS    Define this variable to record a set of C/C++ compiler flags that will    be added to the LOCAL_CFLAGS definition of any other module that uses    this one with LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES.    For example, consider the module 'foo' with the following definition:        include $(CLEAR_VARS)        LOCAL_MODULE := foo        LOCAL_SRC_FILES := foo/foo.c        LOCAL_EXPORT_CFLAGS := -DFOO=1        include $(BUILD_STATIC_LIBRARY)    And another module, named 'bar' that depends on it as:        include $(CLEAR_VARS)        LOCAL_MODULE := bar        LOCAL_SRC_FILES := bar.c        LOCAL_CFLAGS := -DBAR=2        LOCAL_STATIC_LIBRARIES := foo        include $(BUILD_SHARED_LIBRARY)    Then, the flags '-DFOO=1 -DBAR=2' will be passed to the compiler when    building bar.c    Exported flags are prepended to your module's LOCAL_CFLAGS so you can    easily override them. They are also transitive: if 'zoo' depends on    'bar' which depends on 'foo', then 'zoo' will also inherit all flags    exported by 'foo'.    Finally, exported flags are *not* used when building the module that    exports them. In the above example, -DFOO=1 would not be passed to the    compiler when building foo/foo.c.LOCAL_EXPORT_CPPFLAGS    Same as LOCAL_EXPORT_CFLAGS, but for C++ flags only.LOCAL_EXPORT_C_INCLUDES    Same as LOCAL_EXPORT_CFLAGS, but for C include paths.    This can be useful if 'bar.c' wants to include headers    that are provided by module 'foo'.LOCAL_EXPORT_LDLIBS    Same as LOCAL_EXPORT_CFLAGS, but for linker flags. Note that the    imported linker flags will be appended to your module's LOCAL_LDLIBS    though, due to the way Unix linkers work.    This is typically useful when module 'foo' is a static library and has    code that depends on a system library. LOCAL_EXPORT_LDLIBS can then be    used to export the dependency. For example:        include $(CLEAR_VARS)        LOCAL_MODULE := foo        LOCAL_SRC_FILES := foo/foo.c        LOCAL_EXPORT_LDLIBS := -llog        include $(BUILD_STATIC_LIBRARY)        include $(CLEAR_VARS)        LOCAL_MODULE := bar        LOCAL_SRC_FILES := bar.c        LOCAL_STATIC_LIBRARIES := foo        include $(BUILD_SHARED_LIBRARY)    There, libbar.so will be built with a -llog at the end of the linker    command to indicate that it depends on the system logging library,    because it depends on 'foo'.LOCAL_FILTER_ASM    Define this variable to a shell command that will be used to filter    the assembly files from, or generated from, your LOCAL_SRC_FILES.    When it is defined, the following happens:      - Any C or C++ source file is generated into a temporary assembly        file (instead of being compiled into an object file).      - Any temporary assembly file, and any assembly file listed in        LOCAL_SRC_FILES is sent through the LOCAL_FILTER_ASM command        to generate _another_ temporary assembly file.      - These filtered assembly files are compiled into object file.    In other words, If you have:      LOCAL_SRC_FILES  := foo.c bar.S      LOCAL_FILTER_ASM := myasmfilter    foo.c --1--> $OBJS_DIR/foo.S.original --2--> $OBJS_DIR/foo.S --3--> $OBJS_DIR/foo.o    bar.S                                 --2--> $OBJS_DIR/bar.S --3--> $OBJS_DIR/bar.o    Were "1" corresponds to the compiler, "2" to the filter, and "3" to the    assembler. The filter must be a standalone shell command that takes the    name of the input file as its first argument, and the name of the output    file as the second one, as in:        myasmfilter $OBJS_DIR/foo.S.original $OBJS_DIR/foo.S        myasmfilter bar.S $OBJS_DIR/bar.S