xref: /ndk/docs/text/ANDROID-MK.text

`Android.mk` file syntax specification

Introduction:
-------------

This document describes the syntax of `Android.mk` build file
written to describe your C and C++ source files to the Android
NDK. To understand what follows, it is assumed that you have
read the [OVERVIEW](OVERVIEW.html) file that explains their role and
usage.

Overview:
---------

An `Android.mk` file is written to describe your sources to the
build 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.

- 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.
    - A standalone executable.

  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:

> `samples/hello-jni`

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_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_WHOLE_STATIC_LIBRARIES described below).
>    Example usage:

            include $(BUILD_STATIC_LIBRARY)

> Note that this will generate a file named `lib$(LOCAL_MODULE).a`.

- - - - - - - - - - -
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 [PREBUILTS](PREBUILTS.html) for more
>    information).

- - - - - - - - - - -
PREBUILT_STATIC_LIBRARY
>    This is the same as PREBUILT_SHARED_LIBRARY, but for a static library
>    file instead. See [PREBUILTS](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 [STABLE APIS](STABLE-APIS.html).

- - - - - - - - - - -
TARGET_ARCH_ABI
>    Name of the target CPU+ABI when this `Android.mk` is parsed.
>    Four values are supported at the moment:

       armeabi
            For ARMv5TE

       armeabi-v7a
            For ARMv7

       x86
            For i686

       mips
            For mips32 (r1)

>    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 [CPU ARCH ABIS](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 evaluated
by 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.

>    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 module


- - - -
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...

- - - -
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 [IMPORT-MODULE](IMPORT-MODULE.html) for more details.
- - - -


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 explicitly
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.
>
>    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 relative to LOCAL_PATH and
>    you can use path components, e.g.:

            LOCAL_SRC_FILES := foo.c \
                               toto/bar.c

>    Absolute file paths are also supported:

            LOCAL_SRC_FILES := /home/user/mysources/foo.c

>    or on Windows:

            LOCAL_SRC_FILES := c:/Users/user/sources/foo.c

>    Avoiding absolute file paths is recommended, this makes your
>    `Android.mk` easy to reuse on a different machine / system.
>
>    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 .cc

- - - -
LOCAL_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 that the current module depends
>    on.
>
>    If the current module is a shared library or an executable, this will
>    force these libraries to be linked into the resulting binary.
>
>    If the current module is a static library, this simply tells that
>    another other module that depends on the current one will also
>    depend on the listed libraries.

- - - -
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
>    shared library or executable. 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 [STABLE-APIS](STABLE-APIS.html) for the list of exposed system libraries you
>    can linked against with this NDK release.
>
>    NOTE: This is ignored for static libraries, and ndk-build will print
>          a warning if you define it in such a module.

- - - -
LOCAL_LDFLAGS
>    The list of other linker flags to be used when building your shared
>    library or executable. For example, the following will use the `ld.bfd`
>    linker on ARM/X86 GCC 4.6+ where `ld.gold` is the default

            LOCAL_LDFLAGS += -fuse-ld=bfd

>    NOTE: This is ignored for static libraries, and ndk-build will print
>          a warning if you define it in such a module.

- - - -
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.
>
>    *NOTE*: This is ignored for static libraries, and ndk-build will print
>          a warning if you define it in such a module.

- - - -
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
>    [CPU-ARM-NEON](CPU-ARM-NEON.html) and [CPU-FEATURES](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:
>
>    * [Wikipedia: NX bit](http://en.wikipedia.org/wiki/NX_bit)
>    * [The GNU stack kickstart](http://www.gentoo.org/proj/en/hardened/gnu-stack.xml)

- - - -
LOCAL_DISABLE_RELRO
>    By default, NDK compiled code is built with read-only relocations
>    and GOT protection.  This instructs the runtime linker to mark
>    certain regions of memory as being read-only after relocation,
>    making certain security exploits (such as GOT overwrites) harder
>    to perform.
>
>    It is enabled by default, but you can disable it if you *really*
>    need to by setting this variable to '`true`'.
>
>    NOTE: These protections are only effective on newer Android devices
>          ("Jelly Bean" and beyond). The code will still run on older
>          versions (albeit without memory protections).
>
>    For more information, see:
>
>    * [RELRO: RELocation Read-Only](http://isisblogs.poly.edu/2011/06/01/relro-relocation-read-only/)
>    * [Security enhancements in RedHat Enterprise Linux (section 6)](http://www.akkadia.org/drepper/nonselsec.pdf)

- - - -
LOCAL_DISABLE_FORMAT_STRING_CHECKS
>    By default, NDK compiled code is compiled with format string
>    protection. This forces a compiler error if a non-constant format
>    string is used in a printf style function.
>
>    It is enabled by default, but you can disable it if you *really*
>    need to by setting this variable to '`true`'.

- - - -
LOCAL_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_SHORT_COMMANDS
>    Set this variable to '`true`' when your module has a very high number of
>    sources and/or dependent static or shared libraries. This forces the
>    build system to use an intermediate list file, and use it with the
>    library archiver or static linker with the `@$(listfile)` syntax.

>    This can be useful on Windows, where the command-line only accepts
>    a maximum of 8191 characters, which can be too small for complex
>    projects.

>    This also impacts the compilation of individual source files, placing
>    nearly all compiler flags inside list files too.

>    Note that any other value than '`true`' will revert to the default
>    behaviour. You can also define APP_SHORT_COMMANDS in your
>    Application.mk to force this behaviour for all modules in your
>    project.

>    *NOTE*: We do not recommend enabling this feature by default, since it
>          makes the build slower.

- - - -
LOCAL_THIN_ARCHIVE
>    Set this variable to '`true`' when building static libraries. This will
>    generate a 'thin archive', i.e. a library file (e.g. `libfoo.a`) which
>    doesn't contain object files, but simply file paths to the actual
>    objects that it should normally contain.

>    This is useful to reduce the size of your build output. The drawback
>    is that such libraries _cannot_ be moved to a different location
>    (all paths inside them are relative).

>    Valid values are '`true`', '`false`' or empty. A default value can be
>    set in your Application.mk through APP_THIN_ARCHIVE.

>    *NOTE*: This is ignored for non-static library modules, or prebuilt
>    static library ones.

- - - -
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