Different Android devices use different CPUs, which in turn support differentinstruction sets. Each combination of CPU and instruction set has its ownApplication Binary Interface (ABI). An ABI includes the following information:
This page enumerates the ABIs that the NDK supports, and provides informationabout how each ABI works.
ABI can also refer to the native API supported by the platform. For alist of those kinds of ABI issues affecting 32-bit systems, see32-bit ABI bugs.
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Table 1. ABIs and supported instruction sets.
ABI | Supported Instruction Sets | Notes |
---|---|---|
armeabi-v7a | Incompatible with ARMv5/v6 devices. | |
arm64-v8a | ||
x86 | No support for MOVBE or SSE4. | |
x86_64 |
Note: Historically the NDK supported ARMv5(armeabi), and 32-bit and 64-bit MIPS, but support for these ABIs was removed inNDK r17.
This ABI is for 32-bit ARM-based CPUs. The Android variant includes Thumb-2 andthe VFP hardware floating point instructions, specifically VFPv3-D16, whichincludes 16 dedicated 64-bit floating point registers.
For information about the parts of the ABI that aren't Android-specific, seeApplication Binary Interface (ABI) for the ARM Architecture
The NDK's build systems generate Thumb-2 code by default unless you useLOCAL_ARM_MODE
in your Android.mk
forndk-build or ANDROID_ARM_MODE
when configuring CMake.
Other extensions including Advanced SIMD (Neon) and VFPv3-D32 are optional.For more information, see Neon Support.
The armeabi-v7a
ABI uses -mfloat-abi=softfp
to enforce the rule that,although system can execute floating-point code, the compiler must pass allfloat
values in integer registers and all double
values in integer registerpairs when making function calls.
This ABI is for ARMv8-A based CPUs, which support the 64-bit AArch64architecture. It includes the Advanced SIMD (Neon) architecture extensions.
You can use Neon intrinsics in C and C++ code to take advantage of the Advanced SIMD extension. TheNeon Programmer's Guide for Armv8-A provides more information about Neon intrinsics and Neon programming in general.
See Arm'sLearn the Architecture for complete details of the parts of the ABI that aren't Android-specific. Armalso offers some porting advice in64-bit Android Development. /realtek-rtl8192cu-driver-windows-10.html.
On Android, the platform-specific x18 register is reserved forShadowCallStackand should not be touched by your code. Current versions of Clang default tousing the -ffixed-x18
option on Android, so unless you have hand-writtenassembler (or a very old compiler) you shouldn't need to worry about this.
This ABI is for CPUs supporting the instruction set commonly known as 'x86','i386', or 'IA-32'. Characteristics of this ABI include:
These flags target the Pentium Pro instruction set, along with thethe MMX,SSE,SSE2,SSE3, andSSSE3 instruction set extensions.The generated code is an optimization balanced across the top Intel 32-bitCPUs.
For more information on compiler flags, particularly related to performance optimization,refer toGCC x86 Performance Hints.
The ABI does not include any other optional IA-32 instruction setextensions, such as:
You can still use these extensions, as long as you use runtime feature-probing toenable them, and provide fallbacks for devices that do not support them.
The NDK toolchain assumes 16-byte stack alignment before a function call. The default tools andoptions enforce this rule. If you are writing assembly code, you must make sure to maintain stackalignment, and ensure that other compilers also obey this rule.
Refer to the following documents for more details: The cashflow quadrant free pdf.
This ABI is for CPUs supporting the instruction set commonly referred to as'x86-64.' It supports instructions that GCC typically generates with the followingcompiler flags:
These flags target the x86-64 instruction set, according to the GCCdocumentation. along with theMMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2, andPOPCNTinstruction-set extensions. The generated code is an optimization balancedacross the top Intel 64-bit CPUs.
For more information on compiler flags, particularly related to performance optimization,refer to GCCx86 Performance Hints.
This ABI does not include any other optional x86-64 instruction setextensions, such as:
You can still use these extensions, as long as you use runtime feature probing toenable them, and provide fallbacks for devices that do not support them.
Refer to the following documents for more details:
Gradle (whether used via Android Studio or from the command line) builds forall non-deprecated ABIs by default. To restrict the set of ABIs that yourapplication supports, use abiFilters
. For example, to build for only64-bit ABIs, set the following configuration in your build.gradle
:
ndk-build builds for all non-deprecated ABIs by default. You can target aspecific ABIs by setting APP_ABI
in your Application.mk file. Thefollowing snippet shows a few examples of using APP_ABI
:
For more information on the values you can specify for APP_ABI
, seeApplication.mk.
With CMake, you build for a single ABI at a time and must specify your ABIexplicitly. You do this with the ANDROID_ABI
variable, which must bespecified on the command line (cannot be set in your CMakeLists.txt). Forexample:
For the other flags that must be passed to CMake to build with the NDK, seethe CMake guide.
The default behavior of the build system is to include the binaries for each ABIin a single APK, also known as a fat APK. A fat APK is significantly largerthan one containing only the binaries for a single ABI; the tradeoff is gainingwider compatibility, but at the expense of a larger APK. It is stronglyrecommended that you take advantage of either App Bundles or APK Splits toreduce the size of your APKs while still maintaining maximum devicecompatibility.
At installation time, the package manager unpacks only the most appropriatemachine code for the target device. For details, see Automatic extraction ofnative code at install time.
This section provides details about how the Android platform manages nativecode in APKs.
Both the Play Store and Package Manager expect to find NDK-generatedlibraries on filepaths inside the APK matching the following pattern:
Here, <abi>
is one of the ABI names listed under Supported ABIs,and <name>
is the name of the library as you defined it for the LOCAL_MODULE
variable in the Android.mk
file. SinceAPK files are just zip files, it is trivial to open them and confirm that the shared nativelibraries are where they belong.
If the system does not find the native shared libraries where it expects them, it cannot usethem. In such a case, the app itself has to copy the libraries over, and thenperform dlopen()
.
In a fat APK, each library resides under a directory whose name matches a corresponding ABI.For example, a fat APK may contain:
Note: ARMv7-based Android devices running 4.0.3 or earlierinstall native libraries from the armeabi
directory instead of the armeabi-v7a
directory if both directories exist. This is because /lib/armeabi/
comes after/lib/armeabi-v7a/
in the APK. This issue is fixed from 4.0.4.
The Android system knows at runtime which ABI(s) it supports, because build-specific systemproperties indicate:
This mechanism ensures that the system extracts the best machine code fromthe package at installation time.
For best performance, you should compile directly for the primary ABI. For example, atypical ARMv5TE-based device would only define the primary ABI: armeabi
. By contrast, atypical, ARMv7-based device would define the primary ABI as armeabi-v7a
and the secondaryone as armeabi
, since it can run application native binaries generated for each of them.
64-bit devices also support their 32-bit variants. Using arm64-v8a devicesas an example, the device can also run armeabi and armeabi-v7a code. Note,however, that your application will perform much better on 64-bit devices if ittargets arm64-v8a rather than relying on the device running the armeabi-v7aversion of your application.
Many x86-based devices can also run armeabi-v7a
and armeabi
NDK binaries. Forsuch devices, the primary ABI would be x86
, and the second one, armeabi-v7a
.
You can force install an apk for a specific ABI. This is useful for testing. Use the following command:
When installing an application, the package manager service scans the APK, and looks for anyshared libraries of the form:
If none is found, and you have defined a secondary ABI, the service scans for shared libraries ofthe form:
When it finds the libraries that it's looking for, the package manager copiesthem to /lib/lib<name>.so
, under the application's native library directory(<nativeLibraryDir>/
). The following snippets retrieve the nativeLibraryDir
:
If there is no shared-object file at all, the application builds and installs, but crashes atruntime.