android-cmake

CMake is great, and so is Android. This is a collection of CMake scripts that may be useful to the Android NDK community. It is based on experience from porting OpenCV library to Android: http://opencv.org/platforms/android.html

Main goal is to share these scripts so that devs that use CMake as their build system may easily compile native code for Android.

TL;DR

cmake -DCMAKE_TOOLCHAIN_FILE=android.toolchain.cmake \
      -DANDROID_NDK=<ndk_path>                       \
      -DCMAKE_BUILD_TYPE=Release                     \
      -DANDROID_ABI="armeabi-v7a with NEON"          \
      <source_path>
cmake --build .

One-liner:

cmake -DCMAKE_TOOLCHAIN_FILE=android.toolchain.cmake -DANDROID_NDK=<ndk_path> -DCMAKE_BUILD_TYPE=Release -DANDROID_ABI="armeabi-v7a with NEON" <source_path> && cmake --build .

android-cmake will search for your NDK install in the following order:

1. Value of ANDROID_NDK CMake variable;
2. Value of ANDROID_NDK environment variable;
3. Search under paths from ANDROID_NDK_SEARCH_PATHS CMake variable;
4. Search platform specific locations (home folder, Windows "Program Files", etc).

So if you have installed the NDK as ~/android-ndk-r10d then android-cmake will locate it automatically.

Getting started

To build a cmake-based C/C++ project for Android you need:

• Android NDK (>= r5) http://developer.android.com/tools/sdk/ndk/index.html
• CMake (>= v2.6.3, >= v2.8.9 recommended) http://www.cmake.org/download

The android-cmake is also capable to build with NDK from AOSP or Linaro Android source tree, but you may be required to manually specify path to libm binary to link with.

Difference from traditional CMake

Folowing the ndk-build the android-cmake supports only two build targets:

• -DCMAKE_BUILD_TYPE=Release
• -DCMAKE_BUILD_TYPE=Debug

So don't even try other targets that can be found in CMake documentation and don't forget to explicitly specify Release or Debug because CMake builds without a build configuration by default.

Difference from ndk-build

• Latest GCC available in NDK is used as the default compiler;
• Release builds with -O3 instead of -Os;
• Release builds without debug info (without -g) (because ndk-build always creates a stripped version but cmake delays this for install/strip target);
• -fsigned-char is added to compiler flags to make char signed by default as it is on x86/x86_64;
• GCC's stack protector is not used neither in Debug nor Release configurations;
• No builds for multiple platforms (e.g. building for both arm and x86 require to run cmake twice with different parameters);
• No file level Neon via .neon suffix;

The following features of ndk-build are not supported by the android-cmake yet:

• armeabi-v7a-hard ABI
• libc++_static/libc++_shared STL runtime

Basic options

Similarly to the NDK build system android-cmake allows to select between several compiler toolchains and target platforms. Most of the options can be set either as cmake arguments: -D<NAME>=<VALUE> or as environment variables:

• ANDROID_NDK - path to the Android NDK. If not set then android-cmake will search for the most recent version of supported NDK in commonly used locations;

• ANDROID_ABI - specifies the target Application Binary Interface (ABI). This option nearly matches to the APP_ABI variable used by ndk-build tool from Android NDK. If not specified then set to armeabi-v7a. Possible target names are:

  • armeabi - ARMv5TE based CPU with software floating point operations;
  • armeabi-v7a - ARMv7 based devices with hardware FPU instructions (VFPv3_D16);
  • armeabi-v7a with NEON - same as armeabi-v7a, but sets NEON as floating-point unit;
  • armeabi-v7a with VFPV3 - same as armeabi-v7a, but sets VFPv3_D32 as floating-point unit;
  • armeabi-v6 with VFP - tuned for ARMv6 processors having VFP;
  • x86 - IA-32 instruction set
  • mips - MIPS32 instruction set
  • arm64-v8a - ARMv8 AArch64 instruction set - only for NDK r10 and newer
  • x86_64 - Intel64 instruction set (r1) - only for NDK r10 and newer
  • mips64 - MIPS64 instruction set (r6) - only for NDK r10 and newer

• ANDROID_NATIVE_API_LEVEL - level of android API to build for. Can be set either to full name (example: android-8) or a numeric value (example: 17). The default API level depends on the target ABI:

  • android-8 for ARM;
  • android-9 for x86 and MIPS;
  • android-21 for 64-bit ABIs.

  Building for android-L is possible only when it is explicitly selected.

• ANDROID_TOOLCHAIN_NAME - the name of compiler toolchain to be used. This option allows to select between different GCC and Clang versions. The list of possible values depends on the NDK version and will be printed by toolchain file if an invalid value is set. By default android-cmake selects the most recent version of GCC which can build for specified ANDROID_ABI.

  Example values are:

  • aarch64-linux-android-4.9
  • aarch64-linux-android-clang3.5
  • arm-linux-androideabi-4.8
  • arm-linux-androideabi-4.9
  • arm-linux-androideabi-clang3.5
  • mips64el-linux-android-4.9
  • mipsel-linux-android-4.8
  • x86-4.9
  • x86_64-4.9
  • etc.

• ANDROID_STL - the name of C++ runtime to use. The default is gnustl_static.

  • none - do not configure the runtime.
  • system - use the default minimal system C++ runtime library.
    • Implies -fno-rtti -fno-exceptions.
  • system_re - use the default minimal system C++ runtime library.
    • Implies -frtti -fexceptions.
  • gabi++_static - use the GAbi++ runtime as a static library.
    • Implies -frtti -fno-exceptions.
    • Available for NDK r7 and newer.
  • gabi++_shared - use the GAbi++ runtime as a shared library.
    • Implies -frtti -fno-exceptions.
    • Available for NDK r7 and newer.
  • stlport_static - use the STLport runtime as a static library.
    • Implies -fno-rtti -fno-exceptions for NDK before r7.
    • Implies -frtti -fno-exceptions for NDK r7 and newer.
  • stlport_shared - use the STLport runtime as a shared library.
    • Implies -fno-rtti -fno-exceptions for NDK before r7.
    • Implies -frtti -fno-exceptions for NDK r7 and newer.
  • gnustl_static - use the GNU STL as a static library.
    • Implies -frtti -fexceptions.
  • gnustl_shared - use the GNU STL as a shared library.
    • Implies -frtti -fno-exceptions.
    • Available for NDK r7b and newer.
    • Silently degrades to gnustl_static if not available.

• NDK_CCACHE - path to ccache executable. If not set then initialized from NDK_CCACHE environment variable.

Advanced android-cmake options

Normally android-cmake users are not supposed to touch these variables but they might be useful to workaround some build issues:

• ANDROID_FORCE_ARM_BUILD = OFF - generate 32-bit ARM instructions instead of Thumb. Applicable only for arm ABIs and is forced to be ON for armeabi-v6 with VFP;
• ANDROID_NO_UNDEFINED = ON - show all undefined symbols as linker errors;
• ANDROID_SO_UNDEFINED = OFF - allow undefined symbols in shared libraries;
  • actually it is turned ON by default for NDK older than r7
• ANDROID_STL_FORCE_FEATURES = ON - automatically configure rtti and exceptions support based on C++ runtime;
• ANDROID_NDK_LAYOUT = RELEASE - inner layout of Android NDK, should be detected automatically. Possible values are:
  • RELEASE - public releases from Google;
  • LINARO - NDK from Linaro project;
  • ANDROID - NDK from AOSP.
• ANDROID_FUNCTION_LEVEL_LINKING = ON - enables saparate putting each function and data items into separate sections and enable garbage collection of unused input sections at link time (-fdata-sections -ffunction-sections -Wl,--gc-sections);
• ANDROID_GOLD_LINKER = ON - use gold linker with GCC 4.6 for NDK r8b and newer (only for ARM and x86);
• ANDROID_NOEXECSTACK = ON - enables or disables stack execution protection code (-Wl,-z,noexecstack);
• ANDROID_RELRO = ON - Enables RELRO - a memory corruption mitigation technique (-Wl,-z,relro -Wl,-z,now);
• ANDROID_LIBM_PATH - path to libm.so (set to something like $(TOP)/out/target/product/<product_name>/obj/lib/libm.so) to workaround unresolved sincos.

Fine-tuning CMakeLists.txt for android-cmake

Recognizing Android build

android-cmake defines ANDROID CMake variable which can be used to add Android-specific stuff:

if (ANDROID)
    message(STATUS "Hello from Android build!")
endif()

The recommended way to identify ARM/MIPS/x86 architecture is examining CMAKE_SYSTEM_PROCESSOR which is set to the appropriate value:

• armv5te - for armeabi ABI
• armv6 - for armeabi-v6 with VFP ABI
• armv7-a - for armeabi-v7a, armeabi-v7a with VFPV3 and armeabi-v7a with NEON ABIs
• aarch64 - for arm64-v8a ABI
• i686 - for x86 ABI
• x86_64 - for x86_64 ABI
• mips - for mips ABI
• mips64 - for mips64 ABI

Other variables that are set by android-cmake and can be used for the fine-grained build configuration are:

• NEON - set if target ABI supports Neon;
• ANDROID_NATIVE_API_LEVEL - native Android API level we are building for (note: Java part of Andoid application can be built for another API level)
• ANDROID_NDK_RELEASE - version of the Android NDK
• ANDROID_NDK_HOST_SYSTEM_NAME - "windows", "linux-x86" or "darwin-x86" depending on the host platform
• ANDROID_RTTI - set if rtti is enabled by the runtime
• ANDROID_EXCEPTIONS - set if exceptions are enabled by the runtime

Finding packages

When crosscompiling CMake find_* commands are normally expected to find libraries and packages belonging to the same build target. So android-cmake configures CMake to search in Android-specific paths only and ignore your host system locations. So

find_package(ZLIB)

will surely find libz.so within the Android NDK.

However sometimes you need to locate a host package even when cross-compiling. For example you can be searching for your documentation generator. The android-cmake recommends you to use find_host_package and find_host_program macro defined in the android.toolchain.cmake:

find_host_package(Doxygen)
find_host_program(PDFLATEX pdflatex)

However this will break regular builds so instead of wrapping package search into platform-specific logic you can copy the following snippet into your project (put it after your top-level project() command):

# Search packages for host system instead of packages for target system
# in case of cross compilation these macro should be defined by toolchain file
if(NOT COMMAND find_host_package)
  macro(find_host_package)
    find_package(${ARGN})
  endmacro()
endif()
if(NOT COMMAND find_host_program)
  macro(find_host_program)
    find_program(${ARGN})
  endmacro()
endif()

Compiler flags recycling

Make sure to do the following in your scripts:

set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${my_cxx_flags}")
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${my_cxx_flags}")

The flags will be prepopulated with critical flags, so don't loose them. Also be aware that android-cmake also sets configuration-specific compiler and linker flags.

Troubleshooting

Building on Windows

First of all cygwin builds are NOT supported and will not be supported by android-cmake. To build natively on Windows you need a port of make but I recommend http://martine.github.io/ninja/ instead.

To build with Ninja you need:

• Ensure you are using CMake newer than 2.8.9;
• Download the latest Ninja from https://github.com/martine/ninja/releases;
• Put the ninja.exe into your PATH (or add path to ninja.exe to your PATH environment variable);
• Pass -GNinja to cmake alongside with other arguments (or choose Ninja generator in cmake-gui).
• Enjoy the fast native multithreaded build :)

But if you still want to stick to old make then:

• Get a Windows port of GNU Make:
  • Android NDK r7 (and newer) already has make.exe on board;
  • mingw-make should work as fine;
  • Download some other port. For example, this one: http://gnuwin32.sourceforge.net/packages/make.htm.
• Add path to your make.exe to system PATH or always use full path;
• Pass -G"MinGW Makefiles" and -DCMAKE_MAKE_PROGRAM="<full/path/to/>make.exe"
  • It must be MinGW Makefiles and not Unix Makefiles even if your make.exe is not a MinGW's make.
• Run make.exe or cmake --build . for single-threaded build.

Projects with assembler files

The android-cmake should correctly handle projects with assembler sources (*.s or *.S). But if you still facing problems with assembler then try to upgrade your CMake to version newer than 2.8.5

Copying

android-cmake is distributed under the terms of BSD 3-Clause License