1 <html><body><pre>Android Native CPU ABI Management 2 3 4 Introduction: 5 ============= 6 7 Every piece of native code generated with the Android NDK matches a given 8 "Application Binary Interface" (ABI) that defines exactly how your 9 application's machine code is expected to interact with the system at 10 runtime. 11 12 A typical ABI describes things in *excruciating* details, and will typically 13 include the following information: 14 15 - the CPU instruction set that the machine code should use 16 17 - the endianness of memory stores and loads at runtime 18 19 - the format of executable binaries (shared libraries, programs, etc...) 20 and what type of content is allowed/supported in them. 21 22 - various conventions used to pass data between your code and 23 the system (e.g. how registers and/or the stack are used when functions 24 are called, alignment constraints, etc...) 25 26 - alignment and size constraints for enum types, structure fields and 27 arrays. 28 29 - the list of function symbols available to your machine code at runtime, 30 generally from a very specific selected set of libraries. 31 32 This document lists the exact ABIs supported by the Android NDK and the 33 official Android platform releases. 34 35 36 I. Supported ABIs: 37 ================== 38 39 Each supported ABI is identified by a unique name. 40 41 42 I.1. 'armeabi' 43 -------------- 44 45 This is the name of an ABI for ARM-based CPUs that support *at* *least* 46 the ARMv5TE instruction set. Please refer to following documentation for 47 more details: 48 49 - ARM Architecture Reference manual (a.k.a ARMARM) 50 - Procedure Call Standard for the ARM Architecture (a.k.a. AAPCS) 51 - ELF for the ARM Architecture (a.k.a. ARMELF) 52 - ABI for the ARM Architecture (a.k.a. BSABI) 53 - Base Platform ABI for the ARM Architecture (a.k.a. BPABI) 54 - C Library ABI for the ARM Architecture (a.k.a. CLIABI) 55 - C++ ABI for the ARM Architecture (a.k.a. CPPABI) 56 - Runtime ABI for the ARM Architecture (a.k.a. RTABI) 57 58 - ELF System V Application Binary Interface 59 (DRAFT - 24 April 2001) 60 61 - Generic C++ ABI (http://www.codesourcery.com/public/cxx-abi/abi.html) 62 63 Note that the AAPCS standard defines 'EABI' as a moniker used to specify 64 a _family_ of similar but distinct ABIs. Android follows the little-endian 65 ARM GNU/Linux ABI as documented in the following document: 66 67 http://www.codesourcery.com/gnu_toolchains/arm/arm_gnu_linux_abi.pdf 68 69 With the exception that wchar_t is only one byte. This should not matter 70 in practice since wchar_t is simply *not* really supported by the Android 71 platform anyway. 72 73 This ABI does *not* support hardware-assisted floating point computations. 74 Instead, all FP operations are performed through software helper functions 75 that come from the compiler's libgcc.a static library. 76 77 Thumb (a.k.a. Thumb-1) instructions are supported. Note that the NDK 78 will generate thumb code by default, unless you define LOCAL_ARM_MODE 79 in your Android.mk (see docs/ANDROID-MK.html for all details). 80 81 82 I.2. 'armeabi-v7a' 83 ------------------ 84 85 This is the name of another ARM-based CPU ABI that *extends* 'armeabi' to 86 include a few CPU instruction set extensions as described in the following 87 document: 88 89 - ARM Architecture v7-a Reference Manual 90 91 The instruction extensions supported by this Android-specific ABI are: 92 93 - The Thumb-2 instruction set extension. 94 - The VFP hardware FPU instructions. 95 96 More specifically, VFPv3-D16 is being used, which corresponds to 16 97 dedicated 64-bit floating point registers provided by the CPU. 98 99 Other extensions described by the v7-a ARM like Advanced SIMD (a.k.a. NEON), 100 VFPv3-D32 or ThumbEE are optional to this ABI, which means that developers 101 should check *at* *runtime* whether the extensions are available and provide 102 alternative code paths if this is not the case. 103 104 (Just like one typically does on x86 systems to check/use MMX/SSE2/etc... 105 specialized instructions). 106 107 You can check docs/CPU-FEATURES.html to see how to perform these runtime 108 checks, and docs/CPU-ARM-NEON.html to learn about the NDK's support for 109 building NEON-capable machine code too. 110 111 IMPORTANT NOTE: This ABI enforces that all double values are passed during 112 function calls in 'core' register pairs, instead of dedicated FP ones. 113 However, all internal computations can be performed with the FP registers 114 and will be greatly sped up. 115 116 This little constraint, while resulting in a slight decrease of 117 performance, ensures binary compatibility with all existing 'armeabi' 118 binaries. 119 120 IMPORTANT NOTE: The 'armeabi-v7a' machine code will *not* run on ARMv5 or 121 ARMv6 based devices. 122 123 124 I.3. 'x86' 125 ---------- 126 127 This is the name of an ABI for CPUs supporting the instruction set 128 commonly named 'x86' or 'IA-32'. More specifically, this targets the 129 instruction set commonly referenced as 'i686' or 'Pentium Pro' in 130 documents such as: 131 132 Intel IA-32 Intel Architecture Software Developer's Manual 133 volume 2: Instruction Set Reference 134 135 136 IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT: 137 138 THE 'x86' ABI IS AN EXPERIMENTAL FEATURE THAT IS NOT FULLY 139 SUPPORTED YET BY THIS NDK RELEASE. TRYING TO USE IT WILL RESULT 140 IN AN ERROR DURING THE BUILD PROCESS. 141 142 Note that optional features like MMX/SSE2/SSE3/3DNow!/KVM must be 143 explicitly tested at runtime by the generated machine code and 144 cannot be assumed to be everywhere. 145 146 147 II. Generating code for a specific ABI: 148 ======================================= 149 150 By default, the NDK will generate machine code for the 'armeabi' ABI. 151 You can however add the following line to your Application.mk to generate 152 ARMv7-a compatible machine code instead: 153 154 APP_ABI := armeabi-v7a 155 156 It is also possible to build machine code for *two* distinct ABIs by using: 157 158 APP_ABI := armeabi armeabi-v7a 159 160 This will instruct the NDK to build two versions of your machine code: one for 161 each ABI listed on this line. Both libraries will be copied to your application 162 project path and will be ultimately packaged into your .apk. 163 164 Such a package is called a "fat binary" in Android speak since it contains 165 machine code for more than one CPU architecture. At installation time, the 166 package manager will only unpack the most appropriate machine code for the 167 target device. See below for details. 168 169 170 171 III. ABI Management on the Android platform: 172 ============================================ 173 174 This section provides specific details about how the Android platform manages 175 native code in application packages. 176 177 178 III.1. Native code in Application Packages: 179 ------------------------------------------- 180 181 It is expected that shared libraries generated with the NDK are stored in 182 the final application package (.apk) at locations of the form: 183 184 lib/<abi>/lib<name>.so 185 186 Where <abi> is one of the ABI names listed in section II above, and <name> 187 is a name that can be used when loading the shared library from the VM 188 as in: 189 190 System.loadLibrary("<name>"); 191 192 Since .apk files are just zip files, you can trivially list their content 193 with a command like: 194 195 unzip -l <apk> 196 197 to verify that the native shared libraries you want are indeed at the 198 proper location. You can also place native shared libraries at other 199 locations within the .apk, but they will be ignored by the system, or more 200 precisely by the steps described below; you will need to extract/install 201 them manually in your application. 202 203 In the case of a "fat" binary, two distinct libraries are thus placed in 204 the .apk, for example at: 205 206 lib/armeabi/libfoo.so 207 lib/armeabi-v7a/libfoo.so 208 209 210 III.2. Android Platform ABI support: 211 ------------------------------------ 212 213 The Android system knows at runtime which ABI(s) it supports. More 214 precisely, up to two build-specific system properties are used to 215 indicate: 216 217 - the 'primary' ABI for the device, corresponding to the machine 218 code used in the system image itself. 219 220 - an optional 'secondary' ABI, corresponding to another ABI that 221 is also supported by the system image. 222 223 For example, a typical ARMv5TE-based device would only define 224 the primary ABI as 'armeabi' and not define a secondary one. 225 226 On the other hand, a typical ARMv7-based device would define the 227 primary ABI to 'armeabi-v7a' and the secondary one to 'armeabi' 228 since it can run application native binaries generated for both 229 of them. 230 231 232 III.3. Automatic extraction of native code at install time: 233 ----------------------------------------------------------- 234 235 When installing an application, the package manager service will scan 236 the .apk and look for any shared library of the form: 237 238 lib/<primary-abi>/lib<name>.so 239 240 If one is found, then it is copied under $APPDIR/lib/lib<name>.so, 241 where $APPDIR corresponds to the application's specific data directory. 242 243 If none is found, and a secondary ABI is defined, the service will 244 then scan for shared libraries of the form: 245 246 lib/<secondary-abi>/lib<name>.so 247 248 If anything is found, then it is copied under $APPDIR/lib/lib<name>.so 249 250 This mechanism ensures that the best machine code for the target 251 device is automatically extracted from the package at installation 252 time. 253 </pre></body></html>
