Rust Once, Run Everywhere

_posts/2015-04-24-Rust-Once-Run-Everywhere.md

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+---
+layout: post
+title: "Rust Once, Run Everywhere"
+author: Alex Crichton
+description: "Zero-cost and safe FFI in Rust"
+---
+
+Rust's quest for world domination was never destined to happen overnight, so
+Rust needs to be able to interoperate with the existing world just as easily as
+it talks to itself. For this reason, **Rust makes it easy to communicate with C
+APIs without overhead, and to leverage its ownership system to provide much
+stronger safety guarantees for those APIs at the same time**.
+
+To communicate with other languages, Rust provides a *foreign function
+interface* (FFI). Following Rust's design principles, the FFI provides a
+**zero-cost abstraction** where function calls between Rust and C have identical
+performance to C function calls. FFI bindings can also leverage language
+features such as ownership and borrowing to provide a **safe interface** that
+enforces protocols around pointers and other resources. These protocols usually
+appear only in the documentation for C APIs -- at best -- but Rust makes them
+explicit.
+
+In this post we'll explore how to encapsulate unsafe FFI calls to C in safe,
+zero-cost abstractions. Working with C is, however, just an example; we'll also
+see how Rust can easily talk to languages like Python and Ruby just as
+seamlessly as with C.
+
+### Rust talking to C
+
+Let's start with a simple example of calling C code from Rust and then
+demonstrate that Rust imposes no additional overhead. Here's a C program which
+will simply double all the input it's given:
+
+```c
+int double_input(int input) {
+    return input * 2;
+}
+```
+
+To call this from Rust, you might write a program like this:
+
+```rust
+extern crate libc;
+
+extern {
+    fn double_input(input: libc::c_int) -> libc::c_int;
+}
+
+fn main() {
+    let input = 4;
+    let output = unsafe { double_input(input) };
+    println!("{} * 2 = {}", input, output);
+}
+```
+
+And that's it! You can try this out for yourself by
+[checking out the code on GitHub][rust2c] and running `cargo run` from that
+directory. **At the source level we can see that there's no burden in calling an
+external function beyond stating its signature, and we'll see soon that the
+generated code indeed has no overhead, either.** There are, however, a few
+subtle aspects of this Rust program, so let's cover each piece in detail.
+
+[rust2c]: https://github.com/alexcrichton/rust-ffi-examples/tree/master/rust-to-c
+
+First up we see `extern crate libc`. [The libc crate][libc] provides many useful
+type definitions for FFI bindings when talking with C, and it makes it easy to
+ensure that both C and Rust agree on the types crossing the language boundary.
+
+[libc]: https://crates.io/crates/libc
+
+This leads us nicely into the next part of the program:
+
+```rust
+extern {
+    fn double_input(input: libc::c_int) -> libc::c_int;
+}
+```
+
+In Rust this is a **declaration** of an externally available function. You can
+think of this along the lines of a C header file. Here's where the compiler
+learns about the inputs and outputs of the function, and you can see above that
+this matches our definition in C. Next up we have the main body of the program:
+
+```rust
+fn main() {
+    let input = 4;
+    let output = unsafe { double_input(input) };
+    println!("{} * 2 = {}", input, output);
+}
+```
+
+We see one of the crucial aspects of FFI in Rust here, the `unsafe` block. The
+compiler knows nothing about the implementation of `double_input`, so it must
+assume that memory unsafety *could* happen whenever you call a foreign function.
+The `unsafe` block is how the programmer takes responsibility for ensuring
+safety -- you are promising that the actual call you make will not, in fact,
+violate memory safety, and thus that Rust's basic guarantees are upheld.  This
+may seem limiting, but Rust has just the right set of tools to allow consumers
+to not worry about `unsafe` (more on this in a moment).
+
+Now that we've seen how to call a C function from Rust, let's see if we can
+verify this claim of zero overhead. Almost all programming languages can call
+into C one way or another, but it often comes at a cost with runtime type
+conversions or perhaps some language-runtime juggling. To get a handle on what
+Rust is doing, let's go straight to the assembly code of the above `main`
+function's call to `double_input`:
+
+```
+mov    $0x4,%edi
+callq  3bc30 <double_input>
+```
+
+And as before, that's it! Here we can see that calling a C function from Rust
+involves precisely one call instruction after moving the arguments into place,
+exactly the same cost as it would be in C.
+
+### Safe Abstractions
+
+Most features in Rust tie into its core concept of ownership, and the FFI is no
+exception. When binding a C library in Rust you not only have the benefit of zero
+overhead, but you are also able to make it *safer* than C can! **Bindings can
+leverage the ownership and borrowing principles in Rust to codify comments
+typically found in a C header about how its API should be used.**
+
+For example, consider a C library for parsing a tarball. This library will
+expose functions to read the contents of each file in the tarball, probably
+something along the lines of:
+
+```c
+// Gets the data for a file in the tarball at the given index, returning NULL if
+// it does not exist. The `size` pointer is filled in with the size of the file
+// if successful.
+const char *tarball_file_data(tarball_t *tarball, unsigned index, size_t *size);
+```
+
+This function is implicitly making assumptions about how it can be used,
+however, by assuming that the `char*` pointer returned cannot outlive the input
+tarball. When bound in Rust, this API might look like this instead:
+
+```rust
+pub struct Tarball { raw: *mut tarball_t }
+
+impl Tarball {
+    pub fn file(&self, index: u32) -> Option<&[u8]> {
+        unsafe {
+            let mut size = 0;
+            let data = tarball_file_data(self.raw, index as libc::c_uint,
+                                         &mut size);
+            if data.is_null() {
+                None
+            } else {
+                Some(slice::from_raw_parts(data as *const u8, size as usize))
+            }
+        }
+    }
+}
+```
+
+Here the `*mut tarball_t` pointer is *owned by* a `Tarball`, which is
+responsible for any destruction and cleanup, so we already have rich knowledge
+about the lifetime of the tarball's memory. Additionally, the `file` method
+returns a **borrowed slice** whose lifetime is implicitly connected to the
+lifetime of the source tarball itself (the `&self` argument). This is Rust's way
+of indicating that the returned slice can only be used within the lifetime of
+the tarball, statically preventing dangling pointer bugs that are easy to
+make when working directly with C. (If you're not familiar with this kind of
+borrowing in Rust, have a look at Yehuda Katz's [blog post on ownership].)
+
+[blog post]: http://blog.skylight.io/rust-means-never-having-to-close-a-socket/
+
+A key aspect of the Rust binding here is that it is a safe function, meaning
+that callers do not have to use `unsafe` blocks to invoke it! Although it has an
+`unsafe` *implementation* (due to calling an FFI function), the *interface* uses
+borrowing to guarantee that no memory unsafety can occur in any Rust code that
+uses it. That is, due to Rust's static checking, it's simply not possible to
+cause a segfault using the API on the Rust side. And don't forget, all of this
+is coming at zero cost: the raw types in C are representable in Rust with no
+extra allocations or overhead.
+
+Rust's amazing community has already built some substantial safe bindings around
+existing C libraries, including [OpenSSL][rust-openssl], [libgit2][git2-rs],
+[libdispatch][dispatch], [libcurl][curl-rust], [sdl2][sdl2], [Unix APIs][nix],
+and [libsodium][sodiumoxide]. This list is also growing quite rapidly on
+[crates.io][crates-io], so your favorite C library may already be bound or will
+be bound soon!
+
+[rust-openssl]: https://crates.io/crates/openssl
+[git2-rs]: https://crates.io/crates/git2
+[curl-rust]: https://crates.io/crates/curl
+[dispatch]: https://crates.io/crates/dispatch
+[sdl2]: https://crates.io/crates/sdl2
+[nix]: https://crates.io/crates/nix
+[sodiumoxide]: https://crates.io/crates/sodiumoxide
+[crates-io]: https://crates.io
+
+### C talking to Rust
+
+**Despite guaranteeing memory safety, Rust does not have a garbage collector or
+runtime, and one of the benefits of this is that Rust code can be called from C
+with no setup at all.** This means that the zero overhead FFI not only applies
+when Rust calls into C, but also when C calls into Rust!
+
+Let's take the example above, but reverse the roles of each language. As before,
+all the code below is [available on GitHub][c2rust]. First we'll start off with
+our Rust code:
+
+[c2rust]: https://github.com/alexcrichton/rust-ffi-examples/tree/master/c-to-rust
+
+```rust
+#[no_mangle]
+pub extern fn double_input(input: i32) -> i32 {
+    input * 2
+}
+```
+
+As with the Rust code before, there's not a whole lot here but there are some
+subtle aspects in play. First off, we've labeled our function definition with a
+`#[no_mangle]` attribute. This instructs the compiler to not mangle the symbol
+name for the function `double_input`. Rust employs name mangling similar to C++
+to ensure that libraries do not clash with one another, and this attribute
+means that you don't have to guess a symbol name like
+`double_input::h485dee7f568bebafeaa` from C.
+
+Next we've got our function definition, and the most interesting part about
+this is the keyword `extern`. This is a specialized form of specifying the [ABI
+for a function][abi-fn] which enables the function to be compatible with a C
+function call.
+
+[abi-fn]: http://doc.rust-lang.org/reference.html#extern-functions
+
+Finally, if you [take a look at the `Cargo.toml`][cargo-toml] you'll see that
+this library is not compiled as a normal Rust library (rlib) but instead as a
+static archive which Rust calls a 'staticlib'. This enables all the relevant
+Rust code to be linked statically into the C program we're about to produce.
+
+[cargo-toml]: https://github.com/alexcrichton/rust-ffi-examples/blob/master/c-to-rust/Cargo.toml#L8
+
+Now that we've got our Rust library squared away, let's write our C program
+which will call Rust.
+
+```c
+#include <stdint.h>
+#include <stdio.h>
+
+extern int32_t double_input(int32_t input);
+
+int main() {
+    int input = 4;
+    int output = double_input(input);
+    printf("%d * 2 = %d\n", input, output);
+    return 0;
+}
+```
+
+Here we can see that C, like Rust, needs to declare the `double_input` function
+that Rust defined. Other than that though everything is ready to go! If you run
+`make` from the [directory on GitHub][c2rust] you'll see these examples getting
+compiled and linked together and the final executable should run and print
+`4 * 2 = 8`.
+
+Rust's lack of a garbage collector and runtime enables this seamless transition
+from C to Rust. The external C code does not need to perform any setup on Rust's
+behalf, making the transition that much cheaper.
+
+### Beyond C
+
+Up to now we've seen how FFI in Rust has zero overhead and how we can use Rust's
+concept of ownership to write safe bindings to C libraries. If you're not using
+C, however, you're still in luck! These features of Rust enable it to also be
+called from [Python][py2rust], [Ruby][rb2rust], [Javascript][js2rust], and many
+more languages.
+
+[py2rust]: https://github.com/alexcrichton/rust-ffi-examples/tree/master/python-to-rust
+[rb2rust]: https://github.com/alexcrichton/rust-ffi-examples/tree/master/ruby-to-rust
+[js2rust]: https://github.com/alexcrichton/rust-ffi-examples/tree/master/node-to-rust
+
+When writing code in these languages, you sometimes want to speed up some
+component that's performance critical, but in the past this often required
+dropping all the way to C, and thereby giving up the memory safety, high-level
+abstractions, and ergonomics of these languages.
+
+The fact that Rust can talk to easily with C, however, means that it is also
+viable for this sort of usage. One of Rust's first production users,
+[Skylight](https://www.skylight.io), was able to improve the performance and
+memory usage of their data collection agent almost instantly by just using Rust,
+and the Rust code is all published as a Ruby gem.
+
+Moving from a language like Python and Ruby down to C to optimize performance is
+often quite difficult as it's tough to ensure that the program won't crash in a
+difficult-to-debug way. Rust, however, not only brings zero cost FFI, but *also*
+makes it possible to retain the same safety guarantees as the original source
+language. In the long run, this should make it much easier for programmers in
+these languages to drop down and do some systems programming to squeeze out
+critical performance when they need it.
+
+FFI is just one of many tools in the toolbox of Rust, but it's a key component
+to Rust's adoption as it allows Rust to seamlessly integrate with existing code
+bases today. I'm personally quite excited to see the benefits of Rust reach as
+many projects as possible!