Unified Headers

Issue #120

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. In r15 these are used by default. In r16, the old headers have been removed. 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 be 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).

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

App developers can stop reading here. The following information is only relevant to build system engineers.

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/

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

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

    ArchTriple
    ARMarm-linux-androideabi
    ARM64aarch64-linux-android
    MIPSmipsel-linux-android
    MIPS64mips64el-linux-android
    x86i686-linux-android
    x86_64x86_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.

  3. 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.

  4. 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/.