Prefab

Prefab is a tool for generating build system integrations for prebuilt C/C++ libraries. A Prefab package consists of a small amount of metadata and the prebuilt libraries it describes. Prefab is:

• Build system agnostic. Build system support is provided via a plugin API, so it’s simple to provide support for any build system not supported out of the box.

• Cross-platform capable. Prefab currently handles only Android libraries, but is designed to handle any number of platforms.

• Distribution agnostic. Prefab is only an archive format, and can be distributed with whatever package management infrastructure best fits your use case. Android packages will commonly be distributed within an AAR via Maven to be easily used in an Android Gradle project, but distribution as a tarball or a git submodule would work just as well.

Usage

Note: The Android Gradle Plugin natively supports Prefab packages. See Android Studio’s Using native dependencies page for more information.

Prefab is a command line tool that operates on the packages described in this document. At least one package path must be given, and each package path should point to the directory structure described in Package Structure.

Usage: prefab [OPTIONS] PACKAGE_PATH...

  https://google.github.io/prefab/

Android specific configuration options:
  --abi TEXT                       Target ABI.
  --os-version TEXT                Target OS version.
  --stl [c++_shared|c++_static|gnustl_shared|gnustl_static|none|stlport_shared|stlport_static|system]
                                   STL used by the application.
  --ndk-version INT                Major version of the NDK used by the
                                   application.

Options:
  --build-system TEXT  Generate integration for the given build system.
  --output DIRECTORY   Output path for generated build system integration.
  --platform TEXT      Target platform. Only 'android' is currently supported.
  -h, --help           Show this message and exit

For example, if your app is using CMake, c++_shared, NDK r27, and has a minSdkVersion of 21, you can generate CMake packages for your dependencies in the deps directory with the following commands. Note that the ‘riscv64’ ABI below is set to ‘–os-version 35’ as that is the minimum supported version for that ABI.

$ prefab --output out/arm64-v8a --build-system cmake --platform android \
    --abi arm64-v8a --os-version 21 --ndk-version 27 --stl c++_shared deps
$ prefab --output out/armeabi-v7a --build-system cmake --platform android \
    --abi armeabi-v7a --os-version 21 --ndk-version 27 --stl c++_shared deps
$ prefab --output out/x86 --build-system cmake --platform android \
    --abi x86 --os-version 21 --ndk-version 27 --stl c++_shared deps
$ prefab --output out/x86_64 --build-system cmake --platform android \
    --abi x86_64 --os-version 21 --ndk-version 27 --stl c++_shared deps
$ prefab --output out/riscv64 --build-system cmake --platform android \
    --abi riscv64 --os-version 35 --ndk-version 27 --stl c++_shared deps

This generates CMake packages that can be imported into your build with find_package. See the Build System Support in Prefab page for more information.

For an end-to-end example of a non-Gradle workflow, see example-workflow.md.

Package Structure

A complete Prefab artifact is called a “Package”, and each Package contains at least one Module. A Module is a distinct library to be consumed by the dependent project. Each Module has its own headers and at most one library per target platform.

For example, the OpenSSL package contains both libssl and libcrypto modules. The curl package contains just libcurl.

Note: Header-only libraries can be exposed by a Module with an empty libs directory.

A Package has the following directory layout:

<package directory>/
    prefab.json
    modules/
        <module name>/
            module.json
            include/
            libs/
                <platform>.<id>/
                    include/
                    <lib>
                ...
        ...

<package directory> can be anything. When the prefab package is contained within an Android AAR, the Android Gradle Plugin requires that the package directory is named prefab.

prefab.json is the package level metadata and module.json is the module level metadata. prefab.json is required, but module.json is optional.

The per-module include directory is optional. If present, the directory will be automatically added to the consumer’s header search path.

The libs directory contains a subdirectory for every supported platform. The <platform> portion of the directory name identifies the family of the platform (for example, “android”, “linux”, or “windows”) and more exact matching is performed in a platform-dependent manner. For example, Android libraries use an arbitrary identifier and read additional platform data such as ABI and minimum supported OS version from an abi.json file contained in the directory.

The libs directory is optional. A module without any libraries is a header only library and has no platform restrictions.

The file name of the contained library is determined in a platform-dependent manner. For Android, the format is lib${name}.so, but for Windows it would be ${name}.dll.

The include directory within each platform’s library directory is optional. If present, that include path will be used instead of the module headers for that platform.

For examples, see the test packages in the source.

Metadata

Both Packages and Modules contain metadata to describe their use.

Package Metadata

Package metadata describes the version of Prefab that the package was built for and any inter-package dependencies the package may have.

{
    "schema_version": 2,
    "name": "<package name>",
    "version": "<package version>",
    "dependencies": [
        "<dependency name>",
        ...
    ]
}

The schema_version describes the generation of the package format. Any incompatible change to either metadata format or the package layout will result in an increase in the schema version. Whenever possible, old package versions will continue to be supported when new schema versions are introduced. Currently there are only two schema versions (1 and 2). Packages using schema version 1 can be automatically migrated to version 2.

name identifies the Package and should match the name of the containing directory.

version contains the version number for the contained software. This field is optional. If absent, users will not be able to query the package version in their build scripts.

Note that for compatibility with CMake, versions must be specified as major[.minor[.patch[.tweak]]] with all components being numeric, even if that format does not match the native version of the software. For example, OpenSSL version 1.1.1a should be written as 1.1.1.1.

dependencies is a list of strings naming other Packages (as specified by each Package’s name) that this Package depends on. All dependent packages must be available to Prefab during build script generation.

For examples, see the prefab.json files in the test packages.

Module Metadata

Module metadata describes requirements for the module’s use. At present, the only requirements that may be exported to dependents are additional libraries that the dependent must link along with the module, but in the future this may be extended to include required compiler flags such as a minimum C++ version.

{
    "export_libraries": [
        "<library link specifier>",
    ],
    "library_name": "<library file name without file extension>",
    "android": {
      "export_libraries": [
          "<library link specifier>",
      ],
      "library_name": "<library file name without file extension>"
    }
}

export_libraries may specify either literal arguments to be used as-is, intra-package references, or inter-package references. This field is optional.

Value	Result
-lfoo	Consumers will use the -lfoo flag as-is when linking.
:foo	Consumers will link the foo module from this package.
//bar:baz	Consumers will link the baz module from the bar package.

Note that, except for the first case, the headers for the modules will also be made available to the consuming package.

library_name specifies the name of the library file without the file extension. This field is optional. If not specified, the library name is assumed to be lib<library_name>.

The android and field allows either export_libraries or library_name to be overridden dependening on the target platform. Each subfield follows the same rules of the main properties with the same name. If specified, the platform-specific properties override the generic properties.

For examples, see the module.json files in the test packages.