1 <html><body><pre>Android Native CPU ABI Management
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
12 A typical ABI describes things in *excruciating* details, and will typically
13 include the following information:
15 - the CPU instruction set that the machine code should use
17 - the endianness of memory stores and loads at runtime
19 - the format of executable binaries (shared libraries, programs, etc...)
20 and what type of content is allowed/supported in them.
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...)
26 - alignment and size constraints for enum types, structure fields and
29 - the list of function symbols available to your machine code at runtime,
30 generally from a very specific selected set of libraries.
32 This document lists the exact ABIs supported by the Android NDK and the
33 official Android platform releases.
39 Each supported ABI is identified by a unique name.
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
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)
58 - ELF System V Application Binary Interface
59 (DRAFT - 24 April 2001)
61 - Generic C++ ABI (http://www.codesourcery.com/public/cxx-abi/abi.html)
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:
67 http://www.codesourcery.com/gnu_toolchains/arm/arm_gnu_linux_abi.pdf
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
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.
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).
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
89 - ARM Architecture v7-a Reference Manual
91 The instruction extensions supported by this Android-specific ABI are:
93 - The Thumb-2 instruction set extension.
94 - The VFP hardware FPU instructions.
96 More specifically, VFPv3-D16 is being used, which corresponds to 16
97 dedicated 64-bit floating point registers provided by the CPU.
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.
104 (Just like one typically does on x86 systems to check/use MMX/SSE2/etc...
105 specialized instructions).
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.
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.
116 This little constraint, while resulting in a slight decrease of
117 performance, ensures binary compatibility with all existing 'armeabi'
120 IMPORTANT NOTE: The 'armeabi-v7a' machine code will *not* run on ARMv5 or
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
132 Intel IA-32 Intel Architecture Software Developer's Manual
133 volume 2: Instruction Set Reference
136 IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT IMPORTANT:
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.
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.
147 II. Generating code for a specific ABI:
148 =======================================
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:
154 APP_ABI := armeabi-v7a
156 It is also possible to build machine code for *two* distinct ABIs by using:
158 APP_ABI := armeabi armeabi-v7a
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.
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.
171 III. ABI Management on the Android platform:
172 ============================================
174 This section provides specific details about how the Android platform manages
175 native code in application packages.
178 III.1. Native code in Application Packages:
179 -------------------------------------------
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:
184 lib/<abi>/lib<name>.so
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
190 System.loadLibrary("<name>");
192 Since .apk files are just zip files, you can trivially list their content
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.
203 In the case of a "fat" binary, two distinct libraries are thus placed in
204 the .apk, for example at:
206 lib/armeabi/libfoo.so
207 lib/armeabi-v7a/libfoo.so
210 III.2. Android Platform ABI support:
211 ------------------------------------
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
217 - the 'primary' ABI for the device, corresponding to the machine
218 code used in the system image itself.
220 - an optional 'secondary' ABI, corresponding to another ABI that
221 is also supported by the system image.
223 For example, a typical ARMv5TE-based device would only define
224 the primary ABI as 'armeabi' and not define a secondary one.
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
232 III.3. Automatic extraction of native code at install time:
233 -----------------------------------------------------------
235 When installing an application, the package manager service will scan
236 the .apk and look for any shared library of the form:
238 lib/<primary-abi>/lib<name>.so
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.
243 If none is found, and a secondary ABI is defined, the service will
244 then scan for shared libraries of the form:
246 lib/<secondary-abi>/lib<name>.so
248 If anything is found, then it is copied under $APPDIR/lib/lib<name>.so
250 This mechanism ensures that the best machine code for the target
251 device is automatically extracted from the package at installation