USING THE ANDROID TOOLCHAIN AS A STANDALONE COMPILER
======================================================
It is now possible to use the toolchains provided with the Android NDK as
standalone compilers. This can be useful if you already have your own build
system, and only need to ability to invoke the cross-compiler to add support
to Android for it.
A typical use case if invoking the 'configure' script of an open-source
library that expects a cross-compiler in the CC environment variable.
This document explains how to do that:
1/ Selecting your toolchain:
----------------------------
Before anything else, you need to decide whether your standalone toolchain
is going to target ARM-based devices, x86-based, or MIPS-based one.
Each architecture corresponds to a different toolchain name:
* arm-linux-androideabi-4.4.3 => targetting ARM-based Android devices
* x86-4.4.3 => targetting x86-based Android devices
* mipsel-linux-android-4.4.3 => targetting MIPS-based Android devices
2/ Selecting your sysroot:
--------------------------
The second thing you need to know is which Android native API level you want
to target. Each one of them provides a different various APIs, which are
documented under doc/STABLE-APIS.html, and correspond to the sub-directories
of $NDK/platforms.
This allows you to define the path to your 'sysroot', a GCC term for a
directory containing the system headers and libraries of your target.
Usually, this will be something like:
SYSROOT=$NDK/platforms/android-<level>/arch-<arch>/
Where <level> is the API level number, and <arch> is the architecture
("arm", "x86", and "mips" are the supported values). For example, if you're
targeting Android 2.2 (a.k.a. Froyo), you would use:
SYSROOT=$NDK/platforms/android-8/arch-arm
IMPORTANT: Note that only android-9 is supported for the x86 architecture.
Note that android-9 and later are supported for the MIPS architecture.
2/ Invoking the compiler (the hard way):
----------------------------------------
Invoke the compiler using the --sysroot option to indicate where the system
files for the platform you're targeting are located. For example, do:
export CC="$NDK/toolchains/<name>/prebuilt/<system>/bin/<prefix>gcc --sysroot=$SYSROOT"
$CC -o foo.o -c foo.c
Where <name> is the toolchain's name, <system> is the host tag for your system,
and <prefix> is a toolchain-specific prefix. For example, if you are on Linux
using the NDK r5 toolchain, you would use:
export CC="$NDK/toolchains/arm-linux-androideabi-4.4.3/prebuilt/linux-x86/bin/arm-linux-androideabi-gcc --sysroot=$SYSROOT"
As you can see, this is rather verbose, but it works!
IMPORTANT NOTE:
Using the NDK toolchain directly has a serious limitation:
You won't be able to use any C++ STL (either STLport or
the GNU libstdc++) with it. Also no exceptions and no RTTI.
3/ Invoking the compiler (the easy way):
----------------------------------------
The NDK allows you to create a "customized" toolchain installation to make
life easier. For example, consider the following command:
$NDK/build/tools/make-standalone-toolchain.sh --platform=android-5 --install-dir=/tmp/my-android-toolchain
This will create a directory named /tmp/my-android-toolchain containing a
copy of the android-5/arch-arm sysroot, and of the toolchain binaries.
Note that by default, the ARM-based toolchain will be selected by the script.
Use the '--arch=x86' option to specify the x86-based one,
use the '--arch=mips' option to specify the MIPS-based one, or alternatively
'--toolchain=<name>'.
You can later use it directly with something like:
export PATH=/tmp/my-android-toolchain/bin:$PATH
export CC=arm-linux-androideabi-gcc
Note that without the --install-dir option, make-standalone-toolchain.sh will
create a tarball in /tmp/ndk/<toolchain-name>.tar.bz2. This allows you to
archive and redistribute the binaries easily.
Another important benefit is that this standalone toolchain will contain a
working copy of the GNU libstdc++, with working exceptions and RTTI support
(as long as you link against libstdc++ or libsupc++)
Use --help for more options and details.
IMPORTANT: The toolchain binaries do not depend or contain host-specific paths,
in other words, they can be installed in any location, or even
moved if you need to.
NOTE: You can still use the --sysroot option with the new toolchain, but it
is now simply optional!
4/ ABI Compatibility:
---------------------
The machine code generated by the ARM toolchain should be compatible with
the official Android 'armeabi' ABI (see docs/CPU-ARCH-ABIS.html) by default.
It is recommended to use the -mthumb compiler flag to force the generation
of 16-bit Thumb-1 instructions (the default being 32-bit ARM ones).
If you want to target the 'armeabi-v7a' ABI, you will need ensure that the
following two flags are being used:
CFLAGS='-march=armv7-a -mfloat-abi=softfp'
Note: The first flag enables Thumb-2 instructions, and the second one
enables H/W FPU instructions while ensuring that floating-point
parameters are passed in core registers, which is critical for
ABI compatibility. Do *not* use these flags separately!
If you want to use Neon instructions, you will need one more compiler flag:
CFLAGS='-march=armv7-a -mfloat-abi=softfp -mfpu=neon'
Note that this forces the use of VFPv3-D32, as per the ARM specification.
Also, is is *required* to use the following linker flags that routes around
a CPU bug in some Cortex-A8 implementations:
LDFLAGS='-Wl,--fix-cortex-a8'
If none of the above makes sense to you, it's probably better not to use
the standalone toolchain, and stick to the NDK build system instead, which
will handle all the details for you.
You don't have to use any specific compiler flag when targetting the x86 ABI
or the MIPS ABI.
5/ Warnings and Limitations:
--------------------------
5.1/ Windows support:
- - - - - - - - - - -
The Windows binaries do *not* depend on Cygwin. The good news is that they
are thus faster, the bad news is that they do not understand the Cygwin
path specification like /cygdrive/c/foo/bar (instead of C:/foo/bar).
The NDK build system ensures that all paths passed to the compiler from Cygwin
are automatically translated, and deals with other horrors for you. If you have
a custom build system, you may need to deal with the problem yourself.
NOTE: There is no plan to support Cygwin / MSys at the moment, but
contributions are welcome. Contact the android-ndk forum for details.
5.2/ wchar_t support:
- - - - - - - - - - -
As documented, the Android platform did not really support wchar_t until
Android 2.3. What this means in practical terms is that:
- If you target platform android-9 or higher, the size of wchar_t is
4 bytes, and most wide-char functions are available in the C library
(with the exception of multi-byte encoding/decoding functions and
wsprintf/wsscanf).
- If you target any prior API level, the size of wchar_t will be 1 byte
and none of the wide-char functions will work anyway.
We recommend any developer to get rid of any dependencies on the wchar_t type
and switch to better representations. The support provided in Android is only
there to help you migrate existing code.
5.3/ Exceptions, RTTI and STL:
- - - - - - - - - - - - - - -
The toolchain binaries *do* support C++ exceptions and RTTI by default.
They are enabled by default, so use -fno-exceptions and -fno-rtti if you
want to disable them when building sources with them (e.g. to generate
smaller machine code).
NOTE: You will need to explicitly link with libsupc++ if you use these
features. To do this, use -lsupc++ when linking binaries, as in:
arm-linux-androideabi-g++ .... -lsupc++
5.4/ C++ STL support:
- - - - - - - - - - -
The standalone toolchain also comes with a copy of the GNU libstdc++
library, which provides an implementation of the C++ Standard Template
Library. To use it, you however need to link with the proper library:
* Use -lstdc++ to link against the _static_ library version. This ensures
that all required C++ STL code is included into your final binary. This
is ideal if you are only generating a single shared library or executable.
This is the recommended way to do it.
* Use -lgnustl_shared to link against the _shared_ library version. This
is required if you have several related shared libraries or executables
that need to run in the same address space at runtime (some global variables
need to be defined uniquely, which is not possible if you link the static
libstdc++ against each one of your executables).
If you use this option, you need to ensure that libgnustl_shared.so is
also copied to your device for your code to load properly. The file is
at:
$TOOLCHAIN/arm-linux-androideabi/lib/ for ARM toolchains.
$TOOLCHAIN/i686-android-linux/lib/ for x86 ones.
$TOOLCHAIN/mipsel-linux-android/lib/ for MIPS toolchains.
IMPORTANT: The GNU libstdc++ is licensed under the GPLv3 with a
linking exception. See the following URL for details:
http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt01ch01s02.html
If you cannot comply with its requirements, i.e. you cannot redistribute
the shared library, do not use it in your project.
The reason the shared version of GNU libstdc++ is not called libstdc++.so is
because this would conflict at runtime with the system's own minimal C++
runtime, which is /system/lib/libstdc++.so. This enforces a new name for the
GNU ELF library. This is not a problem for the static library.