Android ndk android support

Android ndk android support

We’ve just released another update to Android Studio 1.3’s canary channel, Release Candidate 1.

This build contains a big new feature: Android NDK support, with support for editing, running and debugging C and C++ code! Note however that NDK support is only preview quality, and will remain in preview status for the upcoming final 1.3 release. There are many significant limitations (described below), but we hope that the preview will be useful for many Android NDK developers even in its current state!

Limitations

First, the Android NDK support only works with the new experimental Gradle plugin for Android (which in turn requires Gradle 2.5).

While the new gradle plugin delivers some major performance improvements (and Android NDK build support), note that it also requires changes to the build file “DSL” (the way your build is described in your build.gradle files.) Not only will you need to modify your build.gradle files (a process we hope to automate before the plugin graduates from experimental to stable), but we anticipate making some additional incompatible changes along the way.

Second, note also that Android Studio has not yet been updated to fully handle the experimental plugin. This means that for example the Project Structure Dialog, and various quick fixes which automatically update the build data, do not work correctly. You’ll need to edit your build.gradle files manually to configure your projects. As another example, the various templates which update the build files (such as New Module), have not yet been updated.

Finally, there are still some known bugs in this build; when debugging with LLDB breakpoints do not always work on Windows; if you run into this, you can switch to GDB debugging as a temporary workaround.

Bundled Android NDK

Historically, the Android NDK was distributed separately from the Android SDK, as a giant .zip file download, and installed in a separate location from where the Android SDK resides.

In this release we’ve tried to unify this; the NDK should now be installed under the SDK home in “ndk-bundle”. We’ve updated the IDE built-in SDK Manager to let you install the NDK in the right place, and the Project Structure Dialog’s SDK Location panel (where you can edit the locations), now also offers a quick link to install it directly; just click on the Download hyperlink shown below:

1 GB!) so unless you have a really great network connection, prepare to be patient.

Speaking of large downloads, note also that the IDE update patches from previous 1.3 builds to this one are really large: not only does this build contain all the new code to support Android NDK development, but it also contains LLDB binaries (

200 MB uncompressed). This is just a temporary situation; we’ll move these into the NDK.

How to get started

Since the Android NDK support depends on an experimental plugin, with an incompatible build file syntax which we know will change over the next few months, we have not added a simple way to create Android NDK projects. Instead, to explore the Android NDK support, either

Follow the Experimental Gradle Plugin document which describes how to modify your project to use the current version of the experimental plugin, or

Check out some of the existing Android NDK samples which have been updated to work with the new Gradle plugin.

To check out the samples, invoke VCS > Checkout from Version Control > Git, and in the resulting dialog point to https://github.com/googlesamples/android-ndk.git . Pick a location to check out the samples into. NOTE: At the end of the import the IDE will ask you whether you want to create a project from the checked out sources. Answer no. The android-ndk git project contains a large number of separate projects. Instead, once the import is done, go to File > Open. and navigate to (for example) the Teapot folder and select the file build.gradle in that folder. The project will now be opened and synced. This will take a while the first time as the new Gradle plugin and dependencies are downloaded. When you’re done you can look at the project sources and start exploring the IDE support for Android NDK.

For example, in the Project View you’ll see that the project has a “jni” source set next to the normal “java” source set; that’s where you’ll find the native code:

Источник

Android ABIs

Different Android devices use different CPUs, which in turn support different instruction sets. Each combination of CPU and instruction set has its own Application Binary Interface (ABI). An ABI includes the following information:

• The CPU instruction set (and extensions) that can be used.
• The endianness of memory stores and loads at runtime. Android is always little-endian.
• Conventions for passing data between applications and the system, including alignment constraints, and how the system uses the stack and registers when it calls functions.
• The format of executable binaries, such as programs and shared libraries, and the types of content they support. Android always uses ELF. For more information, see ELF System V Application Binary Interface.
• How C++ names are mangled. For more information, see Generic/Itanium C++ ABI.

This page enumerates the ABIs that the NDK supports, and provides information about how each ABI works.

ABI can also refer to the native API supported by the platform. For a list of those kinds of ABI issues affecting 32-bit systems, see 32-bit ABI bugs.

Supported ABIs

Table 1. ABIs and supported instruction sets.

ABI	Supported Instruction Sets	Notes
armeabi-v7a	armeabi Thumb-2 VFPv3-D16	Incompatible with ARMv5/v6 devices.
arm64-v8a	AArch64
x86	x86 (IA-32) MMX SSE/2/3 SSSE3	No support for MOVBE or SSE4.
x86_64	x86-64 MMX SSE/2/3 SSSE3 SSE4.1, 4.2 POPCNT

Note: Historically the NDK supported ARMv5 (armeabi), and 32-bit and 64-bit MIPS, but support for these ABIs was removed in NDK r17.

armeabi-v7a

This ABI is for 32-bit ARM-based CPUs. The Android variant includes Thumb-2 and the VFP hardware floating point instructions, specifically VFPv3-D16, which includes 16 dedicated 64-bit floating point registers.

For information about the parts of the ABI that aren’t Android-specific, see Application Binary Interface (ABI) for the ARM Architecture

The NDK’s build systems generate Thumb-2 code by default unless you use LOCAL_ARM_MODE in your Android.mk for ndk-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 all float values in integer registers and all double values in integer register pairs when making function calls.

arm64-v8a

This ABI is for ARMv8-A based CPUs, which support the 64-bit AArch64 architecture. 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. The Neon Programmer’s Guide for Armv8-A provides more information about Neon intrinsics and Neon programming in general.

See Arm’s Learn the Architecture for complete details of the parts of the ABI that aren’t Android-specific. Arm also offers some porting advice in 64-bit Android Development.

On Android, the platform-specific x18 register is reserved for ShadowCallStack and should not be touched by your code. Current versions of Clang default to using the -ffixed-x18 option on Android, so unless you have hand-written assembler (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:

Instructions normally generated by GCC with compiler flags such as the following:

These flags target the Pentium Pro instruction set, along with the the MMX, SSE, SSE2, SSE3, and SSSE3 instruction set extensions. The generated code is an optimization balanced across the top Intel 32-bit CPUs.

For more information on compiler flags, particularly related to performance optimization, refer to GCC x86 Performance Hints.

Use of the standard Linux x86 32-bit calling convention, as opposed to the one for SVR. For more information, see section 6, «Register Usage», of Calling conventions for different C++ compilers and operating systems.

The ABI does not include any other optional IA-32 instruction set extensions, such as:

You can still use these extensions, as long as you use runtime feature-probing to enable 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 and options enforce this rule. If you are writing assembly code, you must make sure to maintain stack alignment, and ensure that other compilers also obey this rule.

Refer to the following documents for more details:

x86_64

This ABI is for CPUs supporting the instruction set commonly referred to as «x86-64.» It supports instructions that GCC typically generates with the following compiler flags:

These flags target the x86-64 instruction set, according to the GCC documentation. along with the MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, and POPCNT instruction-set extensions. The generated code is an optimization balanced across the top Intel 64-bit CPUs.

For more information on compiler flags, particularly related to performance optimization, refer to GCC x86 Performance Hints.

This ABI does not include any other optional x86-64 instruction set extensions, such as:

You can still use these extensions, as long as you use runtime feature probing to enable them, and provide fallbacks for devices that do not support them.

Refer to the following documents for more details:

Generate code for a specific ABI

Gradle

Gradle (whether used via Android Studio or from the command line) builds for all non-deprecated ABIs by default. To restrict the set of ABIs that your application supports, use abiFilters . For example, to build for only 64-bit ABIs, set the following configuration in your build.gradle :

ndk-build

ndk-build builds for all non-deprecated ABIs by default. You can target a specific ABIs by setting APP_ABI in your Application.mk file. The following snippet shows a few examples of using APP_ABI :

For more information on the values you can specify for APP_ABI , see Application.mk.

CMake

With CMake, you build for a single ABI at a time and must specify your ABI explicitly. You do this with the ANDROID_ABI variable, which must be specified on the command line (cannot be set in your CMakeLists.txt). For example:

For the other flags that must be passed to CMake to build with the NDK, see the CMake guide.

The default behavior of the build system is to include the binaries for each ABI in a single APK, also known as a fat APK. A fat APK is significantly larger than one containing only the binaries for a single ABI; the tradeoff is gaining wider compatibility, but at the expense of a larger APK. It is strongly recommended that you take advantage of either App Bundles or APK Splits to reduce the size of your APKs while still maintaining maximum device compatibility.

At installation time, the package manager unpacks only the most appropriate machine code for the target device. For details, see Automatic extraction of native code at install time.

ABI management on the Android platform

This section provides details about how the Android platform manages native code in APKs.

Native code in app packages

Both the Play Store and Package Manager expect to find NDK-generated libraries on filepaths inside the APK matching the following pattern:

Here, is one of the ABI names listed under Supported ABIs, and is the name of the library as you defined it for the LOCAL_MODULE variable in the Android.mk file. Since APK files are just zip files, it is trivial to open them and confirm that the shared native libraries are where they belong.

If the system does not find the native shared libraries where it expects them, it cannot use them. In such a case, the app itself has to copy the libraries over, and then perform 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 earlier install 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.

Android platform ABI support

The Android system knows at runtime which ABI(s) it supports, because build-specific system properties indicate:

• The primary ABI for the device, corresponding to the machine code used in the system image itself.
• Optionally, secondary ABIs, corresponding to other ABI that the system image also supports.

This mechanism ensures that the system extracts the best machine code from the package at installation time.

For best performance, you should compile directly for the primary ABI. For example, a typical ARMv5TE-based device would only define the primary ABI: armeabi . By contrast, a typical, ARMv7-based device would define the primary ABI as armeabi-v7a and the secondary one 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 devices as 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 it targets arm64-v8a rather than relying on the device running the armeabi-v7a version of your application.

Many x86-based devices can also run armeabi-v7a and armeabi NDK binaries. For such 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:

Automatic extraction of native code at install time

When installing an application, the package manager service scans the APK, and looks for any shared libraries of the form:

If none is found, and you have defined a secondary ABI, the service scans for shared libraries of the form:

When it finds the libraries that it’s looking for, the package manager copies them to /lib/lib .so , under the application’s native library directory ( / ). The following snippets retrieve the nativeLibraryDir :

Kotlin

If there is no shared-object file at all, the application builds and installs, but crashes at runtime.

Content and code samples on this page are subject to the licenses described in the Content License. Java is a registered trademark of Oracle and/or its affiliates.

Источник