howto-standalone-toolchain.md

HOWTO Cross compiling on Android

5W1H

What is NDK

NDK (Native Develop Toolkit) is a toolchain from Android official, originally for users who writes native C/C++ code as JNI library. It's not designed for compiling standalone programs (./a.out) and not compatible with automake/cmake etc.

What is Standalone Toolchain

"Standalone" refers to two meanings:

1. The program is standalone (has nothing connect to NDK, and don't need helper scripts to run it)
2. The toolchain is made for building standalone programs and libs, and which can used by automake etc.

(Optional) Why NDK is hard to use

By default, NDK uses android flavor directory structure when it's finding headers and libs, which is different from GNU flavor, so the compiler cannot find them. For Example:

/home/tyeken8/Desktop/elab/geo/android-ndk-r9d/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld: error: cannot open crtbegin_dynamic.o: No such file or directory
/home/tyeken8/Desktop/elab/geo/android-ndk-r9d/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld: error: cannot open crtend_android.o: No such file or directory
/home/tyeken8/Desktop/elab/geo/android-ndk-r9d/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld: error: cannot find -lc
/home/tyeken8/Desktop/elab/geo/android-ndk-r9d/toolchains/arm-linux-androideabi-4.8/prebuilt/linux-x86_64/bin/../lib/gcc/arm-linux-androideabi/4.8/../../../../arm-linux-androideabi/bin/ld: error: cannot find -ldl
collect2: error: ld returned 1 exit status

Although we can manuall specify the path (someone wrote a program called "agcc" to handle this automatically, but still not good), it's really annoying.

Howto

1. Download Android NDK
   from https://developer.android.com/tools/sdk/ndk/index.html
2. Extract the NDK
   tar xf android-ndk-r9d-*.tar.bz2 && cd android-ndk-r9d
3. Make GNU Android Toolchain from NDK
   build/tools/make-standalone-toolchain.sh --toolchain=arm-linux-androideabi-4.8 --platform=android-19 --install-dir=../toolchain
4. Delete the NDK (Yes, we don't need it any more)
   cd .. && rm -rf android-ndk-r9d
5. Test the native toolchain
   cd toolchain/bin
echo "main(){}" | ./arm-linux-androideabi-gcc -x c -
file a.out # a.out: ELF 32-bit LSB executable, ARM, version 1 (SYSV), dynamically linked (uses shared libs), not stripped
6. (Optional) Now you can use it as a standard GNU toolchain
   For example: ./configure --prefix=/opt/android --host=arm-linux-androideabi && make && make install