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# 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.
|
