Embedded Software

Android.mk Documentation

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 docs/OVERVIEW.TXT 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

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 under apps/, 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 now, only ‘android-3′ is supported, which corresponds to the
Android 1.5 platform.

TARGET_ARCH_ABI
Name of the target CPU+ABI when this Android.mk is parsed.
For now, only ‘arm’ is supported, which really means the following:

ARMv5TE or higher CPU, with ‘softfloat’ floating point support

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-arm’

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<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)

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.

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