1 <html><body><pre>USING THE ANDROID TOOLCHAIN AS A STANDALONE COMPILER 2 ====================================================== 3 4 It is now possible to use the toolchains provided with the Android NDK as 5 standalone compilers. This can be useful if you already have your own build 6 system, and only need to ability to invoke the cross-compiler to add support 7 to Android for it. 8 9 A typical use case if invoking the 'configure' script of an open-source 10 library that expects a cross-compiler in the CC environment variable. 11 12 13 This document explains how to do that: 14 15 1/ Selecting your toolchain: 16 ---------------------------- 17 18 Before anything else, you need to decide whether your standalone toolchain 19 is going to target ARM-based devices, x86-based, or MIPS-based one. 20 Each architecture corresponds 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 * mipsel-linux-android-4.4.3 => targetting MIPS-based Android devices 25 26 2/ Selecting your sysroot: 27 -------------------------- 28 29 The second thing you need to know is which Android native API level you want 30 to target. Each one of them provides a different various APIs, which are 31 documented under doc/STABLE-APIS.html, and correspond to the sub-directories 32 of $NDK/platforms. 33 34 This allows you to define the path to your 'sysroot', a GCC term for a 35 directory containing the system headers and libraries of your target. 36 Usually, this will be something like: 37 38 SYSROOT=$NDK/platforms/android-<level>/arch-<arch>/ 39 40 Where <level> is the API level number, and <arch> is the architecture 41 ("arm", "x86", and "mips" are the supported values). For example, if you're 42 targeting Android 2.2 (a.k.a. Froyo), you would use: 43 44 SYSROOT=$NDK/platforms/android-8/arch-arm 45 46 IMPORTANT: Note that only android-9 is supported for the x86 architecture. 47 Note that android-9 and later are supported for the MIPS architecture. 48 49 2/ Invoking the compiler (the hard way): 50 ---------------------------------------- 51 52 Invoke the compiler using the --sysroot option to indicate where the system 53 files for the platform you're targeting are located. For example, do: 54 55 export CC="$NDK/toolchains/<name>/prebuilt/<system>/bin/<prefix>gcc --sysroot=$SYSROOT" 56 $CC -o foo.o -c foo.c 57 58 Where <name> is the toolchain's name, <system> is the host tag for your system, 59 and <prefix> is a toolchain-specific prefix. For example, if you are on Linux 60 using the NDK r5 toolchain, you would use: 61 62 export CC="$NDK/toolchains/arm-linux-androideabi-4.4.3/prebuilt/linux-x86/bin/arm-linux-androideabi-gcc --sysroot=$SYSROOT" 63 64 As you can see, this is rather verbose, but it works! 65 66 IMPORTANT NOTE: 67 68 Using the NDK toolchain directly has a serious limitation: 69 You won't be able to use any C++ STL (either STLport or 70 the GNU libstdc++) with it. Also no exceptions and no RTTI. 71 72 73 3/ Invoking the compiler (the easy way): 74 ---------------------------------------- 75 76 The NDK allows you to create a "customized" toolchain installation to make 77 life easier. For example, consider the following command: 78 79 $NDK/build/tools/make-standalone-toolchain.sh --platform=android-5 --install-dir=/tmp/my-android-toolchain 80 81 This will create a directory named /tmp/my-android-toolchain containing a 82 copy of the android-5/arch-arm sysroot, and of the toolchain binaries. 83 84 Note that by default, the ARM-based toolchain will be selected by the script. 85 Use the '--arch=x86' option to specify the x86-based one, 86 use the '--arch=mips' option to specify the MIPS-based one, or alternatively 87 '--toolchain=<name>'. 88 89 You can later use it directly with something like: 90 91 export PATH=/tmp/my-android-toolchain/bin:$PATH 92 export CC=arm-linux-androideabi-gcc 93 94 Note that without the --install-dir option, make-standalone-toolchain.sh will 95 create a tarball in /tmp/ndk/<toolchain-name>.tar.bz2. This allows you to 96 archive and redistribute the binaries easily. 97 98 Another important benefit is that this standalone toolchain will contain a 99 working copy of the GNU libstdc++, with working exceptions and RTTI support 100 (as long as you link against libstdc++ or libsupc++) 101 102 Use --help for more options and details. 103 104 IMPORTANT: The toolchain binaries do not depend or contain host-specific paths, 105 in other words, they can be installed in any location, or even 106 moved if you need to. 107 108 NOTE: You can still use the --sysroot option with the new toolchain, but it 109 is now simply optional! 110 111 112 4/ ABI Compatibility: 113 --------------------- 114 115 The machine code generated by the ARM toolchain should be compatible with 116 the official Android 'armeabi' ABI (see docs/CPU-ARCH-ABIS.html) by default. 117 118 It is recommended to use the -mthumb compiler flag to force the generation 119 of 16-bit Thumb-1 instructions (the default being 32-bit ARM ones). 120 121 If you want to target the 'armeabi-v7a' ABI, you will need ensure that the 122 following two flags are being used: 123 124 CFLAGS='-march=armv7-a -mfloat-abi=softfp' 125 126 Note: The first flag enables Thumb-2 instructions, and the second one 127 enables H/W FPU instructions while ensuring that floating-point 128 parameters are passed in core registers, which is critical for 129 ABI compatibility. Do *not* use these flags separately! 130 131 If you want to use Neon instructions, you will need one more compiler flag: 132 133 CFLAGS='-march=armv7-a -mfloat-abi=softfp -mfpu=neon' 134 135 Note that this forces the use of VFPv3-D32, as per the ARM specification. 136 137 Also, is is *required* to use the following linker flags that routes around 138 a CPU bug in some Cortex-A8 implementations: 139 140 LDFLAGS='-Wl,--fix-cortex-a8' 141 142 If none of the above makes sense to you, it's probably better not to use 143 the standalone toolchain, and stick to the NDK build system instead, which 144 will handle all the details for you. 145 146 You don't have to use any specific compiler flag when targetting the x86 ABI 147 or the MIPS ABI. 148 149 5/ Warnings and Limitations: 150 -------------------------- 151 152 5.1/ Windows support: 153 - - - - - - - - - - - 154 155 The Windows binaries do *not* depend on Cygwin. The good news is that they 156 are thus faster, the bad news is that they do not understand the Cygwin 157 path specification like /cygdrive/c/foo/bar (instead of C:/foo/bar). 158 159 The NDK build system ensures that all paths passed to the compiler from Cygwin 160 are automatically translated, and deals with other horrors for you. If you have 161 a custom build system, you may need to deal with the problem yourself. 162 163 NOTE: There is no plan to support Cygwin / MSys at the moment, but 164 contributions are welcome. Contact the android-ndk forum for details. 165 166 167 5.2/ wchar_t support: 168 - - - - - - - - - - - 169 170 As documented, the Android platform did not really support wchar_t until 171 Android 2.3. What this means in practical terms is that: 172 173 - If you target platform android-9 or higher, the size of wchar_t is 174 4 bytes, and most wide-char functions are available in the C library 175 (with the exception of multi-byte encoding/decoding functions and 176 wsprintf/wsscanf). 177 178 - If you target any prior API level, the size of wchar_t will be 1 byte 179 and none of the wide-char functions will work anyway. 180 181 We recommend any developer to get rid of any dependencies on the wchar_t type 182 and switch to better representations. The support provided in Android is only 183 there to help you migrate existing code. 184 185 186 5.3/ Exceptions, RTTI and STL: 187 - - - - - - - - - - - - - - - 188 189 The toolchain binaries *do* support C++ exceptions and RTTI by default. 190 They are enabled by default, so use -fno-exceptions and -fno-rtti if you 191 want to disable them when building sources with them (e.g. to generate 192 smaller machine code). 193 194 NOTE: You will need to explicitly link with libsupc++ if you use these 195 features. To do this, use -lsupc++ when linking binaries, as in: 196 197 arm-linux-androideabi-g++ .... -lsupc++ 198 199 200 5.4/ C++ STL support: 201 - - - - - - - - - - - 202 203 The standalone toolchain also comes with a copy of the GNU libstdc++ 204 library, which provides an implementation of the C++ Standard Template 205 Library. To use it, you however need to link with the proper library: 206 207 * Use -lstdc++ to link against the _static_ library version. This ensures 208 that all required C++ STL code is included into your final binary. This 209 is ideal if you are only generating a single shared library or executable. 210 211 This is the recommended way to do it. 212 213 * Use -lgnustl_shared to link against the _shared_ library version. This 214 is required if you have several related shared libraries or executables 215 that need to run in the same address space at runtime (some global variables 216 need to be defined uniquely, which is not possible if you link the static 217 libstdc++ against each one of your executables). 218 219 If you use this option, you need to ensure that libgnustl_shared.so is 220 also copied to your device for your code to load properly. The file is 221 at: 222 223 $TOOLCHAIN/arm-linux-androideabi/lib/ for ARM toolchains. 224 $TOOLCHAIN/i686-android-linux/lib/ for x86 ones. 225 $TOOLCHAIN/mipsel-linux-android/lib/ for MIPS toolchains. 226 227 228 IMPORTANT: The GNU libstdc++ is licensed under the GPLv3 with a 229 linking exception. See the following URL for details: 230 231 http://gcc.gnu.org/onlinedocs/libstdc++/manual/bk01pt01ch01s02.html 232 233 If you cannot comply with its requirements, i.e. you cannot redistribute 234 the shared library, do not use it in your project. 235 236 The reason the shared version of GNU libstdc++ is not called libstdc++.so is 237 because this would conflict at runtime with the system's own minimal C++ 238 runtime, which is /system/lib/libstdc++.so. This enforces a new name for the 239 GNU ELF library. This is not a problem for the static library. 240 241 </pre></body></html> 242
