vcpkg/docs/users/selecting-library-features.md

# Selecting library features

**The latest version of this documentation is available on [GitHub](https://github.com/Microsoft/vcpkg/tree/master/docs/users/selecting-library-features.md).**

## Installing a library

We will look at [llvm](https://llvm.org/) as an example. You could install it using:

```powershell
> vcpkg install llvm
```
or via a manifest with
```json
{
  "dependencies": ["llvm"]
}
```

With llvm now installed, we can execute:

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

we see:

```powershell
  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](https://llvm.org/docs/GettingStarted.html#local-llvm-configuration), 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:

```powershell
> 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:

```powershell
> vcpkg install llvm[target-arm] # Installs LLVM with the ARM target
```
```json
{
  "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":
```powershell
> 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:
```json
{
  "dependencies": [{
    "name": "llvm",
    "default-features": false,
    "features": ["target-arm"]
  }]
}
```

# Further reading
- The [Feature Packages](specifications/feature-packages.md) specification was the initial design for features.