Add howto compile on android (#256)

src/SUMMARY.md

@@ -6,6 +6,7 @@
    * [Downloading radare2](first_steps/getting_radare.md)
    * [Compilation and Portability](first_steps/compilation_portability.md)
    * [Compilation on Windows](first_steps/windows_compilation.md)
+   * [Compilation on Android](first_steps/compilation_android.md)
    * [User Interfaces](first_steps/ui.md)
 * [First Steps](first_steps/intro.md)
    * [Command-line Flags](first_steps/commandline_flags.md)

src/first_steps/compilation_android.md

@@ -0,0 +1,102 @@
+## Android
+
+Radare2 can be cross-compiled for other architectures/systems as well, like Android.
+
+### Prerequisites
+
+* Python 3
+* Meson
+* Ninja
+* Git
+* Android NDK
+
+### Step-by-step
+
+#### Download and extract the Android NDK
+
+Download the Android NDK from the [official site](https://developer.android.com/ndk) and extract it somewhere on your system (e.g. `/tmp/android-ndk`)
+
+#### Make
+
+##### Specify NDK base path
+
+```
+$ echo NDK=/tmp/android-ndk  > ~/.r2androidrc
+```
+
+##### Compile + create tar.gz + push it to connected android device
+
+```
+./sys/android-build.sh arm64-static
+```
+
+You can build for different architectures by changing the argument to
+`./sys/android-build.sh`. Run the script without any argument to see the
+accepted values.
+
+#### Meson
+
+##### Create a cross-file for meson
+
+Meson needs a configuration file that describes the cross compilation environment (e.g. `meson-android.ini`).
+You can adjust it as necessary, but something like the following should be a good starting point:
+```
+[binaries]
+c       = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android28-clang'
+cpp     = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android28-clang++'
+ar      = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-ar'
+as      = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-as'
+ranlib  = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-ranlib'
+ld      = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-ld'
+strip   = '/tmp/android-ndk/toolchains/llvm/prebuilt/linux-x86_64/bin/aarch64-linux-android-strip'
+pkgconfig = 'false'
+
+[properties]
+sys_root = '/tmp/android-ndk/sysroot'
+
+[host_machine]
+system = 'android'
+cpu_family = 'arm'
+cpu = 'aarch64'
+endian = 'little'
+```
+
+##### Compile with meson + ninja
+
+Now setup the build directory with meson as usual:
+```
+$ CFLAGS="-static" LDFLAGS="-static" meson --default-library static --prefix=/tmp/android-dir -Dblob=true build --cross-file ./meson-android.ini
+```
+
+A bit of explanation about all the options:
+* `CFLAGS="-static"`, `LDFLAGS="-static"`, `--default-library static`: this
+  ensure that libraries and binaries are statically compiled, so you do not need
+  to properly set LD_* environment variables in your Android environment to make
+  it find the right libraries. Binaries have everything they need inside.
+* `-Dblob=true`: it tells meson to compile just one binary with all the needed
+  code for running `radare2`, `rabin2`, `rasm2`, etc. and creates symbolic links to
+  those names. This avoids creating many statically compiled large binaries and
+  just create one that provides all features. You will still have `rabin2`,
+  `rasm2`, `rax2`, etc. but they are just symlinks to `radare2`.
+* `--cross-file ./meson-android.ini`: it describes how to compile radare2 for Android
+
+Then compile and install the project:
+```
+$ ninja -C build
+$ ninja -C build install
+```
+
+##### Move files to your android device and enjoy
+
+At this point you can copy the generated files in /tmp/android-dir to your Android device and running radare2 from it.
+For example:
+```
+$ cd /tmp && tar -cvf radare2-android.tar.gz android-dir
+$ adb push radare2-android.tar.gz /data/local/tmp
+$ adb shell
+DEVICE:/ $ cd /data/local/tmp
+DEVICE:/data/local/tmp $ tar xvf radare2-android.tar.gz
+DEVICE:/data/local/tmp $ ./android-dir/bin/radare2
+Usage: r2 [-ACdfLMnNqStuvwzX] [-P patch] [-p prj] [-a arch] [-b bits] [-i file]
+          [-s addr] [-B baddr] [-m maddr] [-c cmd] [-e k=v] file|pid|-|--|=
+```