1<html><body><pre>USING THE ANDROID TOOLCHAIN AS A STANDALONE COMPILER 2====================================================== 3 4It is now possible to use the toolchains provided with the Android NDK as 5standalone compilers. This can be useful if you already have your own build 6system, and only need to ability to invoke the cross-compiler to add support 7to Android for it. 8 9A typical use case if invoking the 'configure' script of an open-source 10library that expects a cross-compiler in the CC environment variable. 11 12 13This document explains how to do that: 14 151/ Selecting your toolchain: 16---------------------------- 17 18Before anything else, you need to decide whether your standalone toolchain 19is going to target ARM-based devices, or x86-based one. Each architecture 20corresponds to a different toolchain name: 21 22 * arm-linux-androideabi-4.4.3 => targetting ARM-based Android devices 23 * x86-4.4.3 => targetting x86-based Android devices 24 252/ Selecting your sysroot: 26-------------------------- 27 28The second thing you need to know is which Android native API level you want 29to target. Each one of them provides a different various APIs, which are 30documented under doc/STABLE-APIS.html, and correspond to the sub-directories 31of $NDK/platforms. 32 33This allows you to define the path to your 'sysroot', a GCC term for a 34directory containing the system headers and libraries of your target. 35Usually, this will be something like: 36 37 SYSROOT=$NDK/platforms/android-<level>/arch-<arch>/ 38 39Where <level> is the API level number, and <arch> is the architecture 40("arm" and "x86" are the supported values). For example, if you're targeting 41Android 2.2 (a.k.a. Froyo), you would use: 42 43 SYSROOT=$NDK/platforms/android-8/arch-arm 44 45IMPORTANT: Note that only android-9 is supported for the x86 architecture. 46 472/ Invoking the compiler (the hard way): 48---------------------------------------- 49 50Invoke the compiler using the --sysroot option to indicate where the system 51files for the platform you're targeting are located. For example, do: 52 53 export CC="$NDK/toolchains/<name>/prebuilt/<system>/bin/<prefix>gcc --sysroot=$SYSROOT" 54 $CC -o foo.o -c foo.c 55 56Where <name> is the toolchain's name, <system> is the host tag for your system, 57and <prefix> is a toolchain-specific prefix. For example, if you are on Linux 58using the NDK r5 toolchain, you would use: 59 60 export CC="$NDK/toolchains/arm-linux-androideabi-4.4.3/prebuilt/linux-x86/bin/arm-linux-androideabi-gcc --sysroot=$SYSROOT" 61 62As you can see, this is rather verbose, but it works! 63 64IMPORTANT NOTE: 65 66 Using the NDK toolchain directly has a serious limitation: 67 You won't be able to use any C++ STL (either STLport or 68 the GNU libstdc++) with it. Also no exceptions and no RTTI. 69 70 713/ Invoking the compiler (the easy way): 72---------------------------------------- 73 74The NDK allows you to create a "customized" toolchain installation to make 75life easier. For example, consider the following command: 76 77 $NDK/build/tools/make-standalone-toolchain.sh --platform=android-5 --install-dir=/tmp/my-android-toolchain 78 79This will create a directory named /tmp/my-android-toolchain containing a 80copy of the android-5/arch-arm sysroot, and of the toolchain binaries. 81 82Note that by default, the ARM-based toolchain will be selected by the script. 83Use the '--arch=x86' option to specify the x86-based one, or alternatively 84'--toolchain=<name>'. 85 86You can later use it directly with something like: 87 88 export PATH=/tmp/my-android-toolchain/bin:$PATH 89 export CC=arm-linux-androideabi-gcc 90 91Note that without the --install-dir option, make-standalone-toolchain.sh will 92create a tarball in /tmp/ndk/<toolchain-name>.tar.bz2. This allows you to 93archive and redistribute the binaries easily. 94 95Another important benefit is that this standalone toolchain will contain a 96working copy of the GNU libstdc++, with working exceptions and RTTI support 97(as long as you link against libstdc++ or libsupc++) 98 99Use --help for more options and details. 100 101IMPORTANT: The toolchain binaries do not depend or contain host-specific paths, 102 in other words, they can be installed in any location, or even 103 moved if you need to. 104 105NOTE: You can still use the --sysroot option with the new toolchain, but it 106 is now simply optional! 107 108 1094/ ABI Compatibility: 110--------------------- 111 112The machine code generated by the ARM toolchain should be compatible with 113the official Android 'armeabi' ABI (see docs/CPU-ARCH-ABIS.html) by default. 114 115It is recommended to use the -mthumb compiler flag to force the generation 116of 16-bit Thumb-1 instructions (the default being 32-bit ARM ones). 117 118If you want to target the 'armeabi-v7a' ABI, you will need ensure that the 119following two flags are being used: 120 121 CFLAGS='-march=armv7-a -mfloat-abi=softfp' 122 123Note: The first flag enables Thumb-2 instructions, and the second one 124 enables H/W FPU instructions while ensuring that floating-point 125 parameters are passed in core registers, which is critical for 126 ABI compatibility. Do *not* use these flags separately! 127 128If you want to use Neon instructions, you will need one more compiler flag: 129 130 CFLAGS='-march=armv7-a -mfloat-abi=softfp -mfpu=neon' 131 132Note that this forces the use of VFPv3-D32, as per the ARM specification. 133 134Also, is is *required* to use the following linker flags that routes around 135a CPU bug in some Cortex-A8 implementations: 136 137 LDFLAGS='-Wl,--fix-cortex-a8' 138 139If none of the above makes sense to you, it's probably better not to use 140the standalone toolchain, and stick to the NDK build system instead, which 141will handle all the details for you. 142 143You don't have to use any specific compiler flag when targetting the x86 ABI. 144 1455/ Warnings and Limitations: 146-------------------------- 147 1485.1/ Windows support: 149- - - - - - - - - - - 150 151The Windows binaries do *not* depend on Cygwin. The good news is that they 152are thus faster, the bad news is that they do not understand the Cygwin 153path specification like /cygdrive/c/foo/bar (instead of C:/foo/bar). 154 155The NDK build system ensures that all paths passed to the compiler from Cygwin 156are automatically translated, and deals with other horrors for you. If you have 157a custom build system, you may need to deal with the problem yourself. 158 159NOTE: There is no plan to support Cygwin / MSys at the moment, but 160 contributions are welcome. Contact the android-ndk forum for details. 161 162 1635.2/ wchar_t support: 164- - - - - - - - - - - 165 166As documented, the Android platform did not really support wchar_t until 167Android 2.3. What this means in practical terms is that: 168 169 - If you target platform android-9 or higher, the size of wchar_t is 170 4 bytes, and most wide-char functions are available in the C library 171 (with the exception of multi-byte encoding/decoding functions and 172 wsprintf/wsscanf). 173 174 - If you target any prior API level, the size of wchar_t will be 1 byte 175 and none of the wide-char functions will work anyway. 176 177We recommend any developer to get rid of any dependencies on the wchar_t type 178and switch to better representations. The support provided in Android is only 179there to help you migrate existing code. 180 181 1825.3/ Exceptions, RTTI and STL: 183- - - - - - - - - - - - - - - 184 185The toolchain binaries *do* support C++ exceptions and RTTI by default. 186They are enabled by default, so use -fno-exceptions and -fno-rtti if you 187want to disable them when building sources with them (e.g. to generate 188smaller machine code). 189 190NOTE: You will need to explicitly link with libsupc++ if you use these 191 features. To do this, use -lsupc++ when linking binaries, as in: 192 193 arm-linux-androideabi-g++ .... -lsupc++ 194 195 196The toolchain also comes with a working GNU libstdc++ implementation, which 197provides a working C++ Standard Template Library implementation. You will 198need to explicitly link against -lstdc++ to use it. 199 200Proper toolchain configuration to avoid these explicit link flags is 201planned for the future. 202 203</pre></body></html>