libffi 0.1.0 

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# libffi-rs

Rust bindings for [libffi](https://sourceware.org/libffi/).

## Purpose

Libffi provides two main facilities:

  - Assembling *calls* to functions dynamically.
  - Creating *closures* that can be called as ordinary C functions.

The former is useful mostly for implementing FFIs for untyped languages;
this library provides some support in the middle and low layers, but I’m
not sure how useful it is. The latter can be used to interface between
higher-order languages and C by making closures from the higher-order
language callable as C function pointers. In Rust, this means that we
can now, for example, pass a lambda as a callback to a C function.

Most users are likely interested in the high layer, which provides the
easiest interface to the closure facility.

## Organization

This library is organized in four layers, each of which attempts to
provide more safety and a simpler interface than the next layer
down. From top to bottom:

  - The high layer provides safe(?) and
    automatic marshalling of Rust closures into C function pointers.

  - The middle layer provides memory-managed
    abstractions for assembling calls and closures, but is unsafe
    because it doesn’t check argument types.

  - The low layer makes no attempts at safety,
    but provides a more idiomatically “Rusty” API than the underlying
    C library.

  - The raw layer is a direct mapping of the
    C libffi library into Rust, generated by [Rust
    Bindgen](https://github.com/crabtw/rust-bindgen).

It should be possible to use any layer without dipping into lower
layers (and it will be considered a bug to the extent that it
isn’t).

## Example

In this example, we convert a Rust lambda containing a free variable
into an ordinary C code pointer. The type of `fun` below is
`&extern "C" fn(u64, u64) -> u64`.

```rust
use libffi::high::Closure2;

let x = 5u64;
let f = |y: u64, z: u64| x + y + z;

let closure = Closure2::new(&f);
let fun     = closure.code_ptr();

assert_eq!(18, fun(6, 7));
```

## Building

I’m writing this section to document my build process.

  - Installed pkg-config: brew install pkg-config

  - Installed libffi 3.2.1 (rather than OS X system libffi, which is old)

      - brew install autoconf automake

  - Installed bindgen 0.16