Android.mk

This page describes the syntax of the Android.mk build file used by ndk-build.

Overview

The Android.mk file resides in a subdirectory of your project's jni/ directory, and describes your sources and shared libraries to the build system. It is really a tiny GNU makefile fragment that the build system parses once or more. The Android.mk file is useful for defining project-wide settings that Application.mk, the build system, and your environment variables leave undefined. It can also override project-wide settings for specific modules.

The syntax of the Android.mk allows you to group your sources into modules. A module is either a static library, a shared library, or a standalone executable. You can define one or more modules in each Android.mk file, and you can use the same source file in multiple modules. The build system only places shared libraries into your application package. In addition, static libraries can generate shared libraries.

In addition to packaging libraries, the build system handles a variety of other details for you. For example, you don't need to list header files or explicit dependencies between generated files in your Android.mk file. The NDK build system computes these relationships automatically for you. As a result, you should be able to benefit from new toolchain/platform support in future NDK releases without having to touch your Android.mk file.

The syntax of this file is very close to that used in the Android.mk files distributed with the full Android Open Source Project. While the build system implementation that uses them is different, their similarity is an intentional design decision aimed at making it easier for application developers to reuse source code for external libraries.

Basics

Before exploring the syntax in detail, it is useful to start by understanding the basics of what a Android.mk file contains. This section uses the Android.mk file in the Hello-JNI sample toward that end, explaining the role that each line in the file plays.

An Android.mk file must begin by defining the LOCAL_PATH variable:

LOCAL_PATH := $(call my-dir)

This variable indicates the location of the source files in the development tree. Here, the macro function my-dir, provided by the build system, returns the path of the current directory (the directory containing the Android.mk file itself).

The next line declares the CLEAR_VARS variable, whose value the build system provides.

include $(CLEAR_VARS)

The CLEAR_VARS variable points to a special GNU Makefile that clears many LOCAL_XXX variables for you, such as LOCAL_MODULE, LOCAL_SRC_FILES, and LOCAL_STATIC_LIBRARIES. Note that it does not clear LOCAL_PATH. This variable must retain its value because the system parses all build control files in a single GNU Make execution context where all variables are global. You must (re-)declare this variable before describing each module.

Next, the LOCAL_MODULE variable stores the name of the module that you wish to build. Use this variable once per module in your application.

LOCAL_MODULE := hello-jni

Each module name must be unique and not contain any spaces. The build system, when it generates the final shared-library file, automatically adds the proper prefix and suffix to the name that you assign to LOCAL_MODULE. For example, the example that appears above results in generation of a library called libhello-jni.so.

The next line enumerates the source files, with spaces delimiting multiple files:

LOCAL_SRC_FILES := hello-jni.c

The LOCAL_SRC_FILES variable must contain a list of C and/or C++ source files to build into a module.

The last line helps the system tie everything together:

include $(BUILD_SHARED_LIBRARY)

The BUILD_SHARED_LIBRARY variable points to a GNU Makefile script that collects all the information you defined in LOCAL_XXX variables since the most recent include. This script determines what to build, and how to do it.

There are more complex examples in the samples directories, with commented Android.mk files that you can look at. In addition, Sample: native-activity provides a detailed explanation of that sample's Android.mk file. Finally, Variables and macros provides further information on the variables from this section.

Variables and macros

The build system provides many possible variables for use in the Android.mk file. Many of these variables come with preassigned values. Others, you assign.

In addition to these variables, you can also define your own arbitrary ones. If you do so, keep in mind that the NDK build system reserves the following variable names:

  • Names that begin with LOCAL_, such as LOCAL_MODULE.
  • Names that begin with PRIVATE_, NDK_, or APP. The build system uses these internally.
  • Lower-case names, such as my-dir. The build system uses these internally, as well.

If you need to define your own convenience variables in an Android.mk file, we recommend prepending MY_ to their names.

NDK-defined include variables

This section discusses the GNU Make variables that the build system defines before parsing your Android.mk file. Under certain circumstances, the NDK might parse your Android.mk file several times, using a different definition for some of these variables each time.

CLEAR_VARS

This variable points to a build script that undefines nearly all LOCAL_XXX variables listed in the "Developer-defined variables" section below. Use this variable to include this script before describing a new module. The syntax for using it is:

include $(CLEAR_VARS)

BUILD_EXECUTABLE

This variable points to a build script that collects all the information about the module you provided in your LOCAL_XXX variables, and determines how to build a target executable from the sources you listed. Note that using this script requires that you have already assigned values to LOCAL_MODULE and LOCAL_SRC_FILES, at a minimum (for more information about these variables, see Module-description variables).

The syntax for using this variable is:

include $(BUILD_EXECUTABLE)

BUILD_SHARED_LIBRARY

This variable points to a build script that collects all the information about the module you provided in your LOCAL_XXX variables, and determines how to build a target shared library from the sources you listed. Note that using this script requires that you have already assigned values to LOCAL_MODULE and LOCAL_SRC_FILES, at a minimum (for more information about these variables, see Module-description variables).

The syntax for using this variable is:

include $(BUILD_SHARED_LIBRARY)

A shared-library variable causes the build system to generate a library file with a .so extension.

BUILD_STATIC_LIBRARY

A variant of BUILD_SHARED_LIBRARY that is used to build a static library. The build system does not copy static libraries into your project/packages, but it can use them to build shared libraries (see LOCAL_STATIC_LIBRARIES and LOCAL_WHOLE_STATIC_LIBRARIES, below). The syntax for using this variable is:

include $(BUILD_STATIC_LIBRARY)

A static-library variable causes the build system to generate a library with a .a extension.

PREBUILT_SHARED_LIBRARY

Points to a build script used to specify a prebuilt shared library. Unlike in the case of BUILD_SHARED_LIBRARY and BUILD_STATIC_LIBRARY, here the value of LOCAL_SRC_FILES cannot be a source file. Instead, it must be a single path to a prebuilt shared library, such as foo/libfoo.so. The syntax for using this variable is:

include $(PREBUILT_SHARED_LIBRARY)

You can also reference a prebuilt library in another module by using the LOCAL_PREBUILTS variable. For more information about using prebuilts, see Use prebuilt libraries.

PREBUILT_STATIC_LIBRARY

The same as PREBUILT_SHARED_LIBRARY, but for a prebuilt static library. For more information about using prebuilts, see Use prebuilt libraries.

Target information variables

The build system parses Android.mk once per ABI specified by the APP_ABI variable, which is typically defined in your Application.mk file. If APP_ABI is all, then the build system parses Android.mk once per ABI the NDK supports. This section describes variables the build system defines each time it parses Android.mk.

TARGET_ARCH

The CPU family the build system is targeting as it parses this Android.mk file. This variable will be one of: arm, arm64, x86, or x86_64.

TARGET_PLATFORM

The Android API level number the build system is targeting as it parses this Android.mk file. For example, the Android 5.1 system images correspond to Android API level 22: android-22. For a complete list of platform names and corresponding Android system images, see Native APIs. The following example shows the syntax for using this variable:

ifeq ($(TARGET_PLATFORM),android-22)
    # ... do something ...
endif

TARGET_ARCH_ABI

The ABI the build system is targeting as it parses this Android.mk file. Table 1 shows the ABI setting used for each supported CPU and architecture.

Table 1. ABI settings for different CPUs and architectures.

CPU and architecture Setting
ARMv7 armeabi-v7a
ARMv8 AArch64 arm64-v8a
i686 x86
x86-64 x86_64

The following example shows how to check for ARMv8 AArch64 as the target CPU-and-ABI combination:

ifeq ($(TARGET_ARCH_ABI),arm64-v8a)
  # ... do something ...
endif

For more details about architecture ABIs and associated compatibility issues, refer to Android ABIs.

New target ABIs in the future will have different values.

TARGET_ABI

A concatenation of target Android API level and ABI. It is especially useful when you want to test against a specific target system image for a real device. For example, to check for a 64-bit ARM device running on Android API level 22:

ifeq ($(TARGET_ABI),android-22-arm64-v8a)
  # ... do something ...
endif

Module-description variables

The variables in this section describe your module to the build system. Each module description should follow this basic flow:

  1. Initialize or undefine the variables associated with the module, using the CLEAR_VARS variable.
  2. Assign values to the variables used to describe the module.
  3. Set the NDK build system to use the appropriate build script for the module, using the BUILD_XXX variable.

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 file. The following example shows how to do so:

LOCAL_PATH := $(call my-dir)

The script to which CLEAR_VARS points does not clear this variable. Therefore, you only need to define it a single time, even if your Android.mk file describes multiple modules.

LOCAL_MODULE

This variable stores the name of your module. It must be unique among all module names, and must not contain any spaces. You must define it before including any scripts (other than the one for CLEAR_VARS). You need not add either the lib prefix or the .so or .a file extension; the build system makes these modifications automatically. Throughout your Android.mk and Application.mk files, refer to your module by its unmodified name. For example, the following line results in the generation of a shared library module called libfoo.so:

LOCAL_MODULE := "foo"

If you want the generated module to have a name other than lib + the value of LOCAL_MODULE, you can use the LOCAL_MODULE_FILENAME variable to give the generated module a name of your own choosing, instead.

LOCAL_MODULE_FILENAME

This optional variable allows you to override the names that the build system uses by default for files that it generates. For example, if the name of your LOCAL_MODULE is foo, you can force the system to call the file it generates libnewfoo. The following example shows how to accomplish this:

LOCAL_MODULE := foo
LOCAL_MODULE_FILENAME := libnewfoo

For a shared library module, this example would generate a file called libnewfoo.so.

LOCAL_SRC_FILES

This variable contains the list of source files that the build system uses to generate the module. Only list the files that the build system actually passes to the compiler, since the build system automatically computes any associated dependencies. Note that you can use both relative (to LOCAL_PATH) and absolute file paths.

We recommend avoiding absolute file paths; relative paths make your Android.mk file more portable.

LOCAL_CPP_EXTENSION

You can use this optional variable to indicate a file extension other than .cpp for your C++ source files. For example, the following line changes the extension to .cxx. (The setting must include the dot.)

LOCAL_CPP_EXTENSION := .cxx

You can use this variable to specify multiple extensions. For instance:

LOCAL_CPP_EXTENSION := .cxx .cpp .cc

LOCAL_CPP_FEATURES

You can use this optional variable to indicate that your code relies on specific C++ features. It enables the right compiler and linker flags during the build process. For prebuilt binaries, this variable also declares which features the binary depends on, thus helping ensure the final linking works correctly. We recommend that you use this variable instead of enabling -frtti and -fexceptions directly in your LOCAL_CPPFLAGS definition.

Using this variable allows the build system to use the appropriate flags for each module. Using LOCAL_CPPFLAGS causes the compiler to use all specified flags for all modules, regardless of actual need.

For example, to indicate that your code uses RTTI (RunTime Type Information), write: 

LOCAL_CPP_FEATURES := rtti

To indicate that your code uses C++ exceptions, write:

LOCAL_CPP_FEATURES := exceptions

You can also specify multiple values for this variable. For example:

LOCAL_CPP_FEATURES := rtti features

The order in which you describe the values does not matter.

LOCAL_C_INCLUDES

You can use this optional variable to specify a list of paths, relative to the NDK root directory, to add 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)/<subdirectory>/foo

Define this variable before setting any corresponding inclusion flags via LOCAL_CFLAGS or LOCAL_CPPFLAGS.

The build system also uses LOCAL_C_INCLUDES paths automatically when launching native debugging with ndk-gdb.

LOCAL_ASFLAGS

Flags which will be passed to Clang when building .s or .S files.

LOCAL_ASMFLAGS

Flags which will be passed to yasm when building .asm files.

LOCAL_CFLAGS

Flags which will be passed to Clang when building building C, C++, and some assembly (.s and .S, but not .asm) source files. The ability to do so can be useful for specifying additional macro definitions or compile options. Use LOCAL_CPPFLAGS to specify flags for C++ only. Use LOCAL_CONLYFLAGS to specify flags for C only.

Try not to change the optimization/debugging level in your Android.mk file. The build system can handle this setting automatically for you, using the relevant information in the Application.mk file. Doing it this way allows the build system to generate useful data files used during debugging.

It is possible to specify additional include paths by writing:

LOCAL_CFLAGS += -I<path>,

It is better, however, to use LOCAL_C_INCLUDES for this purpose, since doing so also makes it possible to use the paths available for native debugging with ndk-gdb.

LOCAL_CONLYFLAGS

Flags which will be passed to Clang when compiling C sources. Unlike LOCAL_CFLAGS, LOCAL_CONLYFLAGS will not be passed to Clang when compiling C++ or assembly sources.

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. Use LOCAL_CFLAGS to specify flags for both C and C++.

LOCAL_STATIC_LIBRARIES

This variable stores the list of static libraries modules on which the current module depends.

If the current module is a shared library or an executable, this variable will force these libraries to be linked into the resulting binary.

If the current module is a static library, this variable simply indicates that other modules depending on the current one will also depend on the listed libraries.

LOCAL_SHARED_LIBRARIES

This variable is the list of shared libraries modules on which this module depends at runtime. This information is necessary at link time, and to embed the corresponding information in the generated file.

LOCAL_WHOLE_STATIC_LIBRARIES

This variable is a variant of LOCAL_STATIC_LIBRARIES, and expresses that the linker should treat the associated library modules as whole archives. For more information on whole archives, see the GNU ld documentation for the --whole-archive flag.

This variable is useful when there are circular dependencies among several static libraries. When you use this variable to build a shared library, it will force the build system to add all object files from your static libraries to the final binary. The same is not true, however, when generating executables.

LOCAL_LDLIBS

This variable contains the list of additional linker flags for use in building your shared library or executable. It enables you to use the -l prefix to pass the name of specific system libraries. For example, the following example tells the linker to generate a module that links to /system/lib/libz.so at load time: 

LOCAL_LDLIBS := -lz

For the list of exposed system libraries against which you can link in this NDK release, see Native APIs.

LOCAL_LDFLAGS

The list of other linker flags for the build system to use when building your shared library or executable. For example, to use the ld.bfd linker on ARM/X86:

LOCAL_LDFLAGS += -fuse-ld=bfd

LOCAL_ALLOW_UNDEFINED_SYMBOLS

By default, when the build system encounters an undefined reference encountered while trying to build a shared, it will throw an undefined symbol error. This error can help you catch bugs in your source code.

To disable this check, set this variable to true. Note that this setting may cause the shared library to load at runtime.

LOCAL_ARM_MODE

By default, the build system generates ARM target binaries in thumb mode, where each instruction is 16 bits wide and linked with the STL libraries in the thumb/ directory. Defining this variable as arm forces the build system to generate the module's object files in 32-bit arm mode. The following example shows how to do this:

LOCAL_ARM_MODE := arm

You can also instruct the build system to only build specific sources in arm mode by appending .arm suffix to the source filenames. For example, the following example tells the build system to always compile bar.c in ARM mode, but to build foo.c according to the value of LOCAL_ARM_MODE.

LOCAL_SRC_FILES := foo.c bar.c.arm

LOCAL_ARM_NEON

This variable only matters when you are targeting the armeabi-v7a ABI. It allows the use of ARM Advanced SIMD (NEON) compiler intrinsics in your C and C++ sources, as well as NEON instructions in Assembly files.

Note that not all ARMv7-based CPUs support the NEON instruction set extensions. For this reason, you must perform runtime detection to be able to safely use this code at runtime. For more information, see Neon support and CPU features.

Alternatively, you can use the .neon suffix to specify that the build system only compile specific source files with NEON support. In the following example, the build system compiles foo.c with thumb and neon support, bar.c with thumb support, and zoo.c with support for ARM and NEON:

LOCAL_SRC_FILES = foo.c.neon bar.c zoo.c.arm.neon

If you use both suffixes, .arm must precede .neon.

LOCAL_DISABLE_FORMAT_STRING_CHECKS

By default, the build system compiles code with format string protection. Doing so forces a compiler error if a non-constant format string is used in a printf-style function. This protection is on by default, but you can disable it by setting the value of this variable to true. We do not recommend doing so without a compelling reason.

LOCAL_EXPORT_CFLAGS

This variable records a set of C/C++ compiler flags to add to the LOCAL_CFLAGS definition of any other module that uses this one via the LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES variables.

For example, consider the following pair of modules: foo and bar, which depends on foo:

include $(CLEAR_VARS)
LOCAL_MODULE := foo
LOCAL_SRC_FILES := foo/foo.c
LOCAL_EXPORT_CFLAGS := -DFOO=1
include $(BUILD_STATIC_LIBRARY)


include $(CLEAR_VARS)
LOCAL_MODULE := bar
LOCAL_SRC_FILES := bar.c
LOCAL_CFLAGS := -DBAR=2
LOCAL_STATIC_LIBRARIES := foo
include $(BUILD_SHARED_LIBRARY)

Here, the build system passes the flags -DFOO=1 and -DBAR=2 to the compiler when building bar.c. It also prepends exported flags to your your module's LOCAL_CFLAGS so you can easily override them.

In addition, the relationship among modules is transitive: If zoo depends on bar, which in turn depends on foo, then zoo also inherits all flags exported from foo.

Finally, the build system does not use exported flags when building locally (i.e., building the module whose flags it is exporting). Thus, in the example above, it does not pass -DFOO=1 to the compiler when building foo/foo.c. To build locally, use LOCAL_CFLAGS instead.

LOCAL_EXPORT_CPPFLAGS

This variable is the same as LOCAL_EXPORT_CFLAGS, but for C++ flags only.

LOCAL_EXPORT_C_INCLUDES

This variable is the same as LOCAL_EXPORT_CFLAGS, but for C include paths. It is useful in cases where, for example, bar.c needs to include headers from module foo.

LOCAL_EXPORT_LDFLAGS

This variable is the same as LOCAL_EXPORT_CFLAGS, but for linker flags.

LOCAL_EXPORT_LDLIBS

This variable is the same as LOCAL_EXPORT_CFLAGS, telling the build system to pass names of specific system libraries to the compiler. Prepend -l to the name of each library you specify.

Note that the build system appends imported linker flags to the value of your module's LOCAL_LDLIBS variable. It does this due to the way Unix linkers work.

This variable is typically useful when module foo is a static library and has code that depends on a system library. You can then use LOCAL_EXPORT_LDLIBS to 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)

In this example, the build system puts -llog at the end of the linker command when it builds libbar.so. Doing so tells the linker that, because libbar.so depends on foo, it also depends on the system logging library.

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. Doing so forces the build system to use @ syntax for archives containing intermediate object files or linking libraries.

This feature can be useful on Windows, where the command line accepts a maximum of only of 8191 characters, which can be too small for complex projects. It also impacts the compilation of individual source files, placing nearly all compiler flags inside list files, too.

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

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. Doing so will generate a thin archive, a library file that does not contain object files, but instead just file paths to the actual objects that it would 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 file through the APP_THIN_ARCHIVE variable.

LOCAL_FILTER_ASM

Define this variable as a shell command that the build system will use to filter the assembly files extracted or generated from the files you specified for LOCAL_SRC_FILES. Defining this variable causes the following things to occur:

  1. The build system generates a temporary assembly file from any C or C++ source file, instead of compiling them into an object file.
  2. The build system executes the shell command in LOCAL_FILTER_ASM on any temporary assembly file and on any assembly file listed in LOCAL_SRC_FILES, thus generating another temporary assembly file.
  3. The build system compiles these filtered assembly files into an object file.

For example:

LOCAL_SRC_FILES  := foo.c bar.S
LOCAL_FILTER_ASM :=

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

"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. For example:

myasmfilter $OBJS_DIR/foo.S.original $OBJS_DIR/foo.S
myasmfilter bar.S $OBJS_DIR/bar.S

NDK-provided function macros

This section explains GNU Make function macros that the NDK provides. Use $(call <function>) to evaluate them; they return textual information.

my-dir

This macro returns the path of the last included makefile, which typically is the current Android.mk's directory. my-dir is useful for defining LOCAL_PATH at the start of your Android.mk file. For example:

LOCAL_PATH := $(call my-dir)

Due to the way GNU Make works, what this macro really returns is the path of the last makefile that the build system included when parsing the build scripts. For this reason, you should 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 defines LOCAL_PATH as $PATH/foo instead of $PATH, because that was where its most recent include pointed.

You can avoid this problem by putting additional includes after everything else in the Android.mk file. For example:

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 file
include $(LOCAL_PATH)/foo/Android.mk

If it is not feasible to structure the file in this way, 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 the list of Android.mk files located in all subdirectories of the current my-dir path.

You can use this function to provide deep-nested source directory hierarchies to the build system. By default, the NDK only looks for files in the directory containing the Android.mk file.

this-makefile

Returns the path of the current makefile (from which the build system called the function).

parent-makefile

Returns the path of the parent makefile in the inclusion tree (the path of the makefile that included the current one).

grand-parent-makefile

Returns the path of the grandparent makefile in the inclusion tree (the path of the makefile that included the current one).

import-module

A function that allows you to find and include a module's Android.mk file by the name of the module. A typical example is as follows: 

$(call import-module,<name>)

In this example, the build system looks for the module tagged <name> in the list of directories referenced that your NDK_MODULE_PATH environment variable references, and includes its Android.mk file automatically for you.