selecting-library-features.md

Selecting library features

The latest version of this documentation is available on GitHub.

Installing a library

We will look at llvm as an example. You could install it using:

> vcpkg install llvm

or via a manifest with

{
  "dependencies": ["llvm"]
}

With llvm now installed, we can execute:

> installed\x86-windows\bin\llc.exe --version

we see:

  Registered Targets:
    x86    - 32-bit X86: Pentium-Pro and above
    x86-64 - 64-bit X86: EM64T and AMD64

Installing additional features

But llvm supports many more targets, from ARM to SPARC to SystemZ. However, clearly our current installation doesn't include ARM as a target; thus, we need to learn how vcpkg allows us to install other LLVM targets. The llvm port allows this via the "target-*" features.

If we do:

> vcpkg search llvm

We can see:

llvm                 10.0.0#6         The LLVM Compiler Infrastructure
llvm[clang]                           Build C Language Family Front-end.
llvm[clang-tools-extra]               Build Clang tools.
...
llvm[target-all]                      Build with all backends.
llvm[target-amdgpu]                   Build with AMDGPU backend.
llvm[target-arm]                      Build with ARM backend.
...

We can install any of these targets by using the install-feature syntax:

> vcpkg install llvm[target-arm] # Installs LLVM with the ARM target

{
  "dependencies": [{ "name": "llvm", "features": ["target-arm"] }]
}

Opting out of default features

The llvm port includes a few default features that you as a user may not want: for example, the clang feature is default, which means that vcpkg install llvm will also build and install clang. If you are writing a compiler that uses LLVM as a backend, you're likely not interested in installing clang as well, and we can do that by disabling default features with the special core "feature":

> vcpkg install llvm[core,target-arm] # removing the default-feature with "core" also removes all of the default targets you get

or in manifest files:

{
  "dependencies": [{
    "name": "llvm",
    "default-features": false,
    "features": ["target-arm"]
  }]
}

Further reading

• The Feature Packages specification was the initial design for features.