Nick Desaulniers

The enemy's gate is down

Cross Compiling C/C++ for Android

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Let’s say you want to build a hello world command line application in C or C++ and run it on your Android phone. How would you go about it? It’s not super practical; apps visible and distributable to end users must use the framework (AFAIK), but for folks looking to get into developing on ARM it’s they likely have an ARM device in their pocket.

This post is for folks who typically invoke their compiler from the command line, either explicitly, from build scripts, or other forms of automation.

At work, when working on Android, we typically checkout the entire Android source code (which is huge), use lunch to configure a ton of environmental variables, then use Makefiles with lots of includes and special vars. We don’t want to spend the time and disk space checking out the Android source code just to have a working cross compiler. Luckily, the Android tools team has an excellent utility to grab a prebuilt cross compiler.

This assumes you’re building from a Linux host. Android is a distribution of Linux, which is much easier to target from a Linux host. At home, I’ll usually develop on my OSX machine, ssh’d into my headless Linux box. (iTerm2 and tmux both have exclusive features, but I currently prefer iTerm2.)

The first thing we want to do is fetch the Android NDK. Not the SDK, the NDK.

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➜  ~ curl -O \
  http://dl.google.com/android/repository/android-ndk-r12b-linux-x86_64.zip
➜  ~ unzip android-ndk-r12b-linux-x86_64.zip

It would be helpful to install adb and fastboot, too. This might be different for your distro’s package manager. Better yet may be to just build from source.

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➜  ~ sudo apt-get install android-tools-adb android-tools-fastboot

Now for you Android device that you want to target, you’ll want to know the ISA. Let’s say I want to target my Nexus 6P, which has an ARMv8-A ISA (the first 64b ARM ISA).

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➜  ~ ./android-ndk-r12b/build/tools/make_standalone_toolchain.py --arch arm64 \
  --install-dir ~/arm

This will create a nice standalone bundle in ~/arm. It will contain our cross compiler, linker, headers, libs, and sysroot (crt.o and friends). Most Android devices are ARMv7-A, so you’d use --arch arm. See the other supported architectures for cross compiling under table 4.

You might also want to change your install-dir and possible add it to your $PATH, or set $CC and $CXX.

Now we can compile hello_world.c.

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➜  ~ cat hello_world.c
#include <stdio.h>
int main () {
  puts("hello world");
}

➜  ~ ~/arm/bin/clang -pie hello_world.c
➜  ~ file a.out
a.out: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically
linked, interpreter /system/bin/linker64,
BuildID[sha1]=ecc46648cf2c873253b3b522c0d14b91cf17c70f, not stripped

Since Android Lollipop, Android has required that executables be linked as position independent (-pie) to help provide ASLR.

<install-dir>/bin/ also has shell scripts with more full featured names like aarch64-linux-android-clang if you prefer to have clearer named executables in your $PATH.

Connect your phone, enable remote debugging, and accept the prompt for remote debugging.

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➜  ~ adb push a.out /data/local/tmp/.
➜  ~ adb shell "./data/local/tmp/a.out"
hello world

We’ll use this toolchain in a follow post to start writing some ARMv8-A assembly. Stay tuned.

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