Cross Compilation of CGAL for Android

Table of Contents

• Requirements
• Generating a Standalone Toolchain
• Building GMP
• Building MPFR
• Configuring CGAL
• Build an Example

Requirements

• GMP sources
• MPFR sources
• Boost headers
• Android NDK
• CGAL sources
• For Windows, MSYS2 with gcc, make, autoconf and automake, installed with the command

> pacman -S gcc make autoconf automake

Generating a Standalone Toolchain

The first step for cross-compiling a library to Android is to create a toolchain from the NDK. A python script is given to do so:

> ${ANDROID_NDK}/build/tools/make_standalone_toolchain.py --arch ${ARCH} --install-dir ${TOOLCHAIN_PATH}

• ${ANDROID_NDK} is the root directory of your NDK.
• ${ARCH} is the architecture you are targeting. For example: arm, arm64, ...
• ${TOOLCHAIN_PATH} is the directory where the toolchain will be installed.

This will generate all the files you need in ${TOOLCHAIN_PATH}.

Building GMP

We will now build a version of GMP for Android, using the previously generated toolchain. In the GMP sources dir, do the following:

> export CC=${TOOLCHAIN_PATH}/bin/clang
> export PATH="${TOOLCHAIN_PATH}:${PATH}"
> ./configure --prefix=${GMP_PATH} --enable-shared --host=${TARGET_TRIPLET}

> make
> make install

• ${TARGET_TRIPLET} represents the machine you are targeting. It is of the form cpu-company-system. For example,
  • aarch64-linux-android for a 64bit arm Android device, or
  • arm-linux-androideabihf for a 32bit arm Android device (ARMv7 or later, with a hard floating-points unit).

Building MPFR

This is very similar to building GMP. In the MPFR sources dir, do the following:

> export CC=${TOOLCHAIN_PATH}/bin/clang
> export PATH="${TOOLCHAIN_PATH}:${PATH}"
> ./configure --prefix=${MPFR_PATH} --enable-shared --host=${TARGET_TRIPLET} --with-gmp=${GMP_PATH}

> make
> make install

Configuring CGAL

Now that we have the dependencies built, we can configure CGAL. First we will create a toolchain.cmake file, containing the paths needed by CMake. Create this file at ${TOOLCHAIN_FILE_PATH}:

#Target system
set(CMAKE_SYSTEM_NAME  Android)
set(CMAKE_SYSTEM_VERSION 21)
set(CMAKE_ANDROID_ARCH_ABI arm64-v8a) # Change arm64-v8a to armeabi-v7a for 32bit phones
set(CMAKE_ANDROID_NDK /home/gimeno/Android/android-ndk-r15c)
set(CMAKE_ANDROID_NDK_TOOLCHAIN_VERSION clang)

set(GMP_INCLUDE_DIR /home/gimeno/Android/3rdPartyLibs/gmp-arm64/include)
set(GMP_LIBRARIES /home/gimeno/Android/3rdPartyLibs/gmp-arm64/lib/libgmp.so)

set(MPFR_INCLUDE_DIR /home/gimeno/Android/3rdPartyLibs/mpfr-arm64/include)
set(MPFR_LIBRARIES /home/gimeno/Android/3rdPartyLibs/mpfr-arm64/lib/libmpfr.so)

set(Boost_INCLUDE_DIR /home/gimeno/Android/3rdPartyLibs/boost/include)

This is an example and you obviously have to use the right paths instead of those.

Once we have this, we call cmake:

> cmake -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_FILE_PATH} -DWITH_CGAL_Core=FALSE -DCMAKE_CXX_FLAGS=-std=c++11 -DCGAL_test_cpp_version_RUN_RES=0 -DCGAL_HEADER_ONLY=TRUE -DWITH_CGAL_Qt5=FALSE /path/to/cgal/sources

CGAL is now ready to be used with an Android Application.

Notes:

• You have to run cmake twice, because of a bug of CGAL CMake scripts in case of a cross-compilation.
• On Windows, we add the option -G "Unix Makefiles".
• The options -DCMAKE_CXX_FLAGS=-std=c++11 -DCGAL_test_cpp_version_RUN_RES=0 allow to use the C++11 standard. When CGAL is compiled with a C++11 compiler, the library Boost.Thread is no longer required, and Boost libraries can be used header-only. There is one exception: CGAL_Core still requires Boost.Thread. That is why we use the cmake option -DWITH_CGAL_Core=FALSE, to disable CGAL_Core. If you need CGAL_Core, then you will have to cross-compile Boost, and then set the appropriate CMake variables. Some packages have more dependencies. If you wish to use them, you will have to cross-compile them too.
• With the option -DCGAL_HEADER_ONLY=TRUE, CGAL is used header-only.

Build an Example

Since Android 5.0 (API level 21), an executable needs to be PIE (Position independent executable) to work. To make your executable PIE, you need to add -fPIE to the CXX flags, and -fPIE -pie to the linker flags

> cmake -DCMAKE_CXX_FLAGS="-std=c++11 -fPIE" -DCMAKE_EXE_LINKER_FLAGS="-fPIE -pie" .