> For the complete documentation index, see [llms.txt](https://www.crystalforrubyists.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.crystalforrubyists.com/book/chapter-10-c-bindings.md).

# Chapter 10: C Bindings

There are lots of useful C libraries out there. It’s important that we make use of them instead of rewriting every single of them.

In Crystal, It’s super easy to use existing C libraries with bindings. Even Crystal itself uses C libraries.

For example Crystal uses `libpcre` for it’s `Regex` implementation.

Like I said it’s super easy to write bindings for C. Crystal itself links to `libpcre` like this

```ruby
@[Link("pcre")]
lib LibPCRE
...
end
```

With just 3 lines of code you we’re linked to `libpcre` :) We use `lib` keyword to group functions and types that belong to a library. And it’s a good convetion to start with `Lib` for your C library declarations.

Next we bind to C functions with the `fun` keyword.

```ruby
@[Link("pcre")]
lib LibPCRE
  type Pcre = Void*
  fun compile = pcre_compile(pattern : UInt8*, options : Int, errptr : UInt8**, erroffset : Int*, tableptr : Void*) : Pcre
end
```

Here we binded to `libpcre`s compile function with the matching types. Now we can easily access this function in our Crystal code.

```ruby
LibPCRE.compile(..)
```