Enable VNDK

The Vendor Native Development Kit (VNDK) requires several changes to a codebase to separate concerns between vendor and system. Use the following guide to enable VNDK in a vendor/OEM codebase.

Build system libraries

The build system contains several types of objects including libraries (shared, static, or header) and binaries.

Figure 1. Build system libraries.

core libraries are used by the system image, on the system image. These libraries can't be used by vendor, vendor_available, vndk, or vndk-sp libraries.
```
cc_library {
    name: "libThatIsCore",
    ...
}
```

vendor-only (or proprietary) libraries are used by the vendor image, on the vendor image.

cc_library {
    name: "libThatIsVendorOnly",
    proprietary: true,
    # or: vendor: true, # (for things in AOSP)
    ...
}

vendor_available libraries are used by the vendor image, on the vendor image (may contain duplicates of core).
```
cc_library {
    name: "libThatIsVendorAvailable",
    vendor_available: true,
    ...
}
```

vndk libraries are used by the vendor image, on the system image.

cc_library {
    name: "libThatIsVndk",
    vendor_available: true,
    vndk: {
        enabled: true,
    }
    ...
}

vndk-sp libraries are used by the vendor image, and also by the system image indirectly.

cc_library {
    name: "libThatIsVndkSp",
    vendor_available: true,
    vndk: {
        enabled: true,
        support_system_process: true,
    }
    ...
}

llndk libraries are used by both the system and vendor images.

cc_library {
    name: "libThatIsLlndk",
    llndk: {
        symbol_file: "libthatisllndk.map.txt"
    }
    ...
}

When a lib is marked as vendor_available:true, it's built twice:

Once for platform (and thus installed to /system/lib)
Once for vendor (and thus installed to /vendor/lib or VNDK APEX)

The vendor versions of libs are built with -D__ANDROID_VNDK__. Private system components that may change significantly in future versions of Android are disabled with this flag. In addition, different libraries export a different set of headers (such as liblog). Options specific to a vendor variant of a target can be specified in an Android.bp file in:

target: { vendor: { … } }

Enable VNDK for a codebase

To enable the VNDK for a codebase:

Determine eligibility by calculating the required sizes of vendor.img and system.img partitions.
Enable BOARD_VNDK_VERSION=current. You can add to BoardConfig.mk or build components with it directly (for example, m -j BOARD_VNDK_VERSION=current MY-LIB).

After enabling BOARD_VNDK_VERSION=current, the build system enforces the following dependency and header requirements.

Manage dependencies

A vendor object that depends on a core component that doesn't exist in vndk or as a vendor object must be resolved using one of the following options:

The dependency can be removed.
If the core component is owned by vendor, it can be marked as vendor_available or vendor.
A change making the core object part of the vndk may be upstreamed to Google.

In addition, if a core component has dependencies on a vendor component, the vendor component must be made into a core component or the dependency must be removed in another way (for example, by removing the dependency or by moving the dependency into a vendor component).

Manage headers

Global header dependencies must be removed to enable the build system to know whether to build the headers with or without -D__ANDROID_VNDK__. For example, libutils headers such as utils/StrongPointer.h can still be accessed using the header library libutils_headers.

Some headers (such as unistd.h) can no longer be included transitively but can be included locally.

Finally, the public part of private/android_filesystem_config.h has been moved to cutils/android_filesystem_config.h. To manage these headers, do one of the following:

Remove the dependency to private/android_filesystem_config.h by replacing all AID_* macros with getgrnam/ getpwnam calls if possible. For example:
- (uid_t)AID_WIFI becomes getpwnam("wifi")->pw_uid.
- (gid_t)AID_SDCARD_R becomes getgrnam("sdcard_r")->gr_gid.
For details, refer to private/android_filesystem_config.h.
For hard-coded AIS, include cutils/android_filesystem_config.h.