CODEWITHSUNDEEP
androidc++android-ndknativecross-compiling
Suman
Suman

Reputation: 4251

Android NDK compilation without sysroot

I am trying to compile the source code for multiple architectures and multiple API levels of Android NDK based on the compilation flags (by dynamically passing path of Application.mk to the ndk-build command). It was going fine till r12. Now I was trying to update my build system to latest NDK r15c and found that it's taking headers from "android-ndk/sysroot/..." path instead of depending on the APP_PLATFORM flag from Application.mk file

APP_PLATFORM                   := android-19

Logs for r12b NDK:

-I/data/android/android-ndk-r10d/platforms/android-19/arch-arm/usr/include

Above include path is present when I try verbose logs for ndk-build (V=1) .

Logs for r15c NDK:

--sysroot
/home/airwatch/Android/android-ndk/sysroot
-isystem
/home/airwatch/Android/android-ndk/sysroot/usr/include/arm-linux-androideabi

Above include path is present when I try verbose logs for r15c NDK.

It will not be possible to configure Standalone Toolchain due to my project requirements. Is there anyway to get rid of this sysroot?

Upvotes: 2

Views: 5970

Answers (1)

Alexander Ushakov
Alexander Ushakov

Reputation: 5397

As mentioned in comments above here is full explain: https://android.googlesource.com/platform/ndk/+/master/docs/UnifiedHeaders.md

Unified Headers

Before NDK r14, we had a set of libc headers for each API version. In many cases these headers were incorrect. Many exposed APIs that didn‘t exist, and others didn’t expose APIs that did.

In NDK r14 (as an opt in feature) we unified these into a single set of headers, called unified headers. This single header path is used for every platform level. API level guards are handled with #ifdef. These headers can be found in prebuilts/ndk/headers.

Unified headers are built directly from the Android platform, so they are up to date and correct (or at the very least, any bugs in the NDK headers will also be a bug in the platform headers, which means we're much more likely to find them).

In r15 unified headers are used by default. In r16, the old headers have been removed.

Known Issues

  • Standalone toolchains using GCC are not supported out of the box. To use GCC, pass -D__ANDROID_API__=$API when compiling. Note: this is not something we will be fixing.

Supporting Unified Headers in Your Build System

  1. Unified headers require only a few changes compared to using the deprecated NDK headers. For reference, this patch added support to ndk-build: https://android-review.googlesource.com/c/239934/

The compile time sysroot is now $NDK/sysroot. Previously this was $NDK/platforms/android-$API/arch-$ARCH.

  1. Pass -isystem $NDK/sysroot/usr/include/$TRIPLE when compiling. The triple has the following mapping:

Arch Triple
ARM arm-linux-androideabi
ARM64 aarch64-linux-android
MIPS mipsel-linux-android
MIPS64 mips64el-linux-android
x86 i686-linux-android
x86_64 x86_64-linux-android

This is needed for architecture specific headers such as those in asm/ and machine/. We plan to teach Clang's driver to automatically search the architecture specific include directory, but that has yet to be done.

  1. Pass -D__ANDROID_API__=$API when compiling. This define used to be provided by <android/api-level.h>, but with only one set of headers this is no longer possible. In the future we will look in to adding -mandroid-version or similar to Clang so this is automatic.

  2. At link time, change nothing. All link time build behavior should match the deprecated headers behavior. --sysroot should still point to $NDK/platforms/android-$API/arch-$ARCH/.

Upvotes: 6

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