Enabling the VNDK

The 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. Used by the system image, on the system image. These libraries cannot be used by vendor, vendor_available, vndk, or vndk-sp libraries.
```
cc_library {
    name: "libThatIsCore",
    ...
}
```

vendor-only (or proprietary). Used by the vendor image, on the vendor image.

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

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

vndk. Used by the vendor image, on the system image (a subset of vendor_available).

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

vndk-sp. Used by the system image indirectly, on the system image (subset of core).

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

llndk. Used by both the system and vendor images.
```
llndk_library {
    name: "libThasIsLlndk",
}
```

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

Once for platform (and thus installed to /system/lib).
Once for vendor (and thus installed to /vendor/lib, /system/lib/vndk, or /system/lib/vndk-sp).

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: { … } }

Enabling 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 (i.e. m -j BOARD_VNDK_VERSION=current MY-LIB).

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

Managing dependencies

A vendor object that depends on a core component that doesn't exist in the 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).

Managing 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 transitively included 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.