93 lines
2.5 KiB
Markdown
93 lines
2.5 KiB
Markdown
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# Selecting Library Features
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**The latest version of this documentation is available on [GitHub](https://github.com/Microsoft/vcpkg/tree/master/docs/users/selecting-library-features.md).**
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## Installing a library
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We will look at [llvm](https://llvm.org/) as an example. You could install it using:
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```powershell
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> vcpkg install llvm
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```
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or via a manifest with
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```json
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{
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"dependencies": ["llvm"]
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}
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```
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With llvm now installed, we can execute:
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```powershell
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> installed\x86-windows\bin\llc.exe --version
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```
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we see:
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```powershell
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Registered Targets:
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x86 - 32-bit X86: Pentium-Pro and above
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x86-64 - 64-bit X86: EM64T and AMD64
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```
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## Installing additional features
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But [llvm supports many more targets](https://llvm.org/docs/GettingStarted.html#local-llvm-configuration), from ARM to SPARC to SystemZ.
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However, clearly our current installation doesn't include ARM as a target;
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thus, we need to learn how vcpkg allows us to install other LLVM targets.
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The llvm port allows this via the "target-*" features.
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If we do:
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```powershell
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> vcpkg search llvm
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```
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We can see:
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```
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llvm 10.0.0#6 The LLVM Compiler Infrastructure
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llvm[clang] Build C Language Family Front-end.
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llvm[clang-tools-extra] Build Clang tools.
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...
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llvm[target-all] Build with all backends.
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llvm[target-amdgpu] Build with AMDGPU backend.
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llvm[target-arm] Build with ARM backend.
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...
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```
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We can install any of these targets by using the install-feature syntax:
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```powershell
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> vcpkg install llvm[target-arm] # Installs LLVM with the ARM target
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```
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```json
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{
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"dependencies": [{ "name": "llvm", "features": ["target-arm"] }]
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}
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```
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## Opting out of default features
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The llvm port includes a few default features that you as a user may not want: for example,
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the `clang` feature is default, which means that `vcpkg install llvm` will also build and install clang.
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If you are writing a compiler that uses LLVM as a backend,
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you're likely not interested in installing clang as well,
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and we can do that by disabling default features with the special `core` "feature":
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```powershell
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> vcpkg install llvm[core,target-arm] # removing the default-feature with "core" also removes all of the default targets you get
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```
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or in manifest files:
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```json
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{
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"dependencies": [{
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"name": "llvm",
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"default-features": false,
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"features": ["target-arm"]
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}]
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}
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```
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# Further reading
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- The [Feature Packages](../specifications/feature-packages.md) specification was the initial design for features.
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