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% Foreign Function Interface

# Introduction

This guide will use the [snappy](https://github.com/google/snappy)
compression/decompression library as an introduction to writing bindings for
foreign code. Rust is currently unable to call directly into a C++ library, but
snappy includes a C interface (documented in
[`snappy-c.h`](https://github.com/google/snappy/blob/master/snappy-c.h)).

The following is a minimal example of calling a foreign function which will
compile if snappy is installed:

```no_run
extern crate libc;
use libc::size_t;

#[link(name = "snappy")]
extern {
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
}

fn main() {
    let x = unsafe { snappy_max_compressed_length(100) };
    println!("max compressed length of a 100 byte buffer: {}", x);
}
```

The `extern` block is a list of function signatures in a foreign library, in
this case with the platform's C ABI. The `#[link(...)]` attribute is used to
instruct the linker to link against the snappy library so the symbols are
resolved.

Foreign functions are assumed to be unsafe so calls to them need to be wrapped
with `unsafe {}` as a promise to the compiler that everything contained within
truly is safe. C libraries often expose interfaces that aren't thread-safe, and
almost any function that takes a pointer argument isn't valid for all possible
inputs since the pointer could be dangling, and raw pointers fall outside of
Rust's safe memory model.

When declaring the argument types to a foreign function, the Rust compiler can
not check if the declaration is correct, so specifying it correctly is part of
keeping the binding correct at runtime.

The `extern` block can be extended to cover the entire snappy API:

```no_run
extern crate libc;
use libc::{c_int, size_t};

#[link(name = "snappy")]
extern {
    fn snappy_compress(input: *const u8,
                       input_length: size_t,
                       compressed: *mut u8,
                       compressed_length: *mut size_t) -> c_int;
    fn snappy_uncompress(compressed: *const u8,
                         compressed_length: size_t,
                         uncompressed: *mut u8,
                         uncompressed_length: *mut size_t) -> c_int;
    fn snappy_max_compressed_length(source_length: size_t) -> size_t;
    fn snappy_uncompressed_length(compressed: *const u8,
                                  compressed_length: size_t,
                                  result: *mut size_t) -> c_int;
    fn snappy_validate_compressed_buffer(compressed: *const u8,
                                         compressed_length: size_t) -> c_int;
}
# fn main() {}
```

# Creating a safe interface

The raw C API needs to be wrapped to provide memory safety and make use of higher-level concepts
like vectors. A library can choose to expose only the safe, high-level interface and hide the unsafe
internal details.

Wrapping the functions which expect buffers involves using the `slice::raw` module to manipulate Rust
vectors as pointers to memory. Rust's vectors are guaranteed to be a contiguous block of memory. The
length is number of elements currently contained, and the capacity is the total size in elements of
the allocated memory. The length is less than or equal to the capacity.

```
# extern crate libc;
# use libc::{c_int, size_t};
# unsafe fn snappy_validate_compressed_buffer(_: *const u8, _: size_t) -> c_int { 0 }
# fn main() {}
pub fn validate_compressed_buffer(src: &[u8]) -> bool {
    unsafe {
        snappy_validate_compressed_buffer(src.as_ptr(), src.len() as size_t) == 0
    }
}
```

The `validate_compressed_buffer` wrapper above makes use of an `unsafe` block, but it makes the
guarantee that calling it is safe for all inputs by leaving off `unsafe` from the function
signature.

The `snappy_compress` and `snappy_uncompress` functions are more complex, since a buffer has to be
allocated to hold the output too.

The `snappy_max_compressed_length` function can be used to allocate a vector with the maximum
required capacity to hold the compressed output. The vector can then be passed to the
`snappy_compress` function as an output parameter. An output parameter is also passed to retrieve
the true length after compression for setting the length.

```
# extern crate libc;
# use libc::{size_t, c_int};
# unsafe fn snappy_compress(a: *const u8, b: size_t, c: *mut u8,
#                           d: *mut size_t) -> c_int { 0 }
# unsafe fn snappy_max_compressed_length(a: size_t) -> size_t { a }
# fn main() {}
pub fn compress(src: &[u8]) -> Vec<u8> {
    unsafe {
        let srclen = src.len() as size_t;
        let psrc = src.as_ptr();

        let mut dstlen = snappy_max_compressed_length(srclen);
        let mut dst = Vec::with_capacity(dstlen as usize);
        let pdst = dst.as_mut_ptr();

        snappy_compress(psrc, srclen, pdst, &mut dstlen);
        dst.set_len(dstlen as usize);
        dst
    }
}
```

Decompression is similar, because snappy stores the uncompressed size as part of the compression
format and `snappy_uncompressed_length` will retrieve the exact buffer size required.

```
# extern crate libc;
# use libc::{size_t, c_int};
# unsafe fn snappy_uncompress(compressed: *const u8,
#                             compressed_length: size_t,
#                             uncompressed: *mut u8,
#                             uncompressed_length: *mut size_t) -> c_int { 0 }
# unsafe fn snappy_uncompressed_length(compressed: *const u8,
#                                      compressed_length: size_t,
#                                      result: *mut size_t) -> c_int { 0 }
# fn main() {}
pub fn uncompress(src: &[u8]) -> Option<Vec<u8>> {
    unsafe {
        let srclen = src.len() as size_t;
        let psrc = src.as_ptr();

        let mut dstlen: size_t = 0;
        snappy_uncompressed_length(psrc, srclen, &mut dstlen);

        let mut dst = Vec::with_capacity(dstlen as usize);
        let pdst = dst.as_mut_ptr();

        if snappy_uncompress(psrc, srclen, pdst, &mut dstlen) == 0 {
            dst.set_len(dstlen as usize);
            Some(dst)
        } else {
            None // SNAPPY_INVALID_INPUT
        }
    }
}
```

For reference, the examples used here are also available as an [library on
GitHub](https://github.com/thestinger/rust-snappy).

# Destructors

Foreign libraries often hand off ownership of resources to the calling code.
When this occurs, we must use Rust's destructors to provide safety and guarantee
the release of these resources (especially in the case of panic).

# Callbacks from C code to Rust functions

Some external libraries require the usage of callbacks to report back their
current state or intermediate data to the caller.
It is possible to pass functions defined in Rust to an external library.
The requirement for this is that the callback function is marked as `extern`
with the correct calling convention to make it callable from C code.

The callback function can then be sent through a registration call
to the C library and afterwards be invoked from there.

A basic example is:

Rust code:

```no_run
extern fn callback(a: i32) {
    println!("I'm called from C with value {0}", a);
}

#[link(name = "extlib")]
extern {
   fn register_callback(cb: extern fn(i32)) -> i32;
   fn trigger_callback();
}

fn main() {
    unsafe {
        register_callback(callback);
        trigger_callback(); // Triggers the callback
    }
}
```

C code:

```c
typedef void (*rust_callback)(int32_t);
rust_callback cb;

int32_t register_callback(rust_callback callback) {
    cb = callback;
    return 1;
}

void trigger_callback() {
  cb(7); // Will call callback(7) in Rust
}
```

In this example Rust's `main()` will call `trigger_callback()` in C,
which would, in turn, call back to `callback()` in Rust.


## Targeting callbacks to Rust objects

The former example showed how a global function can be called from C code.
However it is often desired that the callback is targeted to a special
Rust object. This could be the object that represents the wrapper for the
respective C object.

This can be achieved by passing an unsafe pointer to the object down to the
C library. The C library can then include the pointer to the Rust object in
the notification. This will allow the callback to unsafely access the
referenced Rust object.

Rust code:

```no_run
#[repr(C)]
struct RustObject {
    a: i32,
    // other members
}

extern "C" fn callback(target: *mut RustObject, a: i32) {
    println!("I'm called from C with value {0}", a);
    unsafe {
        // Update the value in RustObject with the value received from the callback
        (*target).a = a;
    }
}

#[link(name = "extlib")]
extern {
   fn register_callback(target: *mut RustObject,
                        cb: extern fn(*mut RustObject, i32)) -> i32;
   fn trigger_callback();
}

fn main() {
    // Create the object that will be referenced in the callback
    let mut rust_object = Box::new(RustObject { a: 5 });

    unsafe {
        register_callback(&mut *rust_object, callback);
        trigger_callback();
    }
}
```

C code:

```c
typedef void (*rust_callback)(void*, int32_t);
void* cb_target;
rust_callback cb;

int32_t register_callback(void* callback_target, rust_callback callback) {
    cb_target = callback_target;
    cb = callback;
    return 1;
}

void trigger_callback() {
  cb(cb_target, 7); // Will call callback(&rustObject, 7) in Rust
}
```

## Asynchronous callbacks

In the previously given examples the callbacks are invoked as a direct reaction
to a function call to the external C library.
The control over the current thread is switched from Rust to C to Rust for the
execution of the callback, but in the end the callback is executed on the
same thread that called the function which triggered the callback.

Things get more complicated when the external library spawns its own threads
and invokes callbacks from there.
In these cases access to Rust data structures inside the callbacks is
especially unsafe and proper synchronization mechanisms must be used.
Besides classical synchronization mechanisms like mutexes, one possibility in
Rust is to use channels (in `std::comm`) to forward data from the C thread
that invoked the callback into a Rust thread.

If an asynchronous callback targets a special object in the Rust address space
it is also absolutely necessary that no more callbacks are performed by the
C library after the respective Rust object gets destroyed.
This can be achieved by unregistering the callback in the object's
destructor and designing the library in a way that guarantees that no
callback will be performed after deregistration.

# Linking

The `link` attribute on `extern` blocks provides the basic building block for
instructing rustc how it will link to native libraries. There are two accepted
forms of the link attribute today:

* `#[link(name = "foo")]`
* `#[link(name = "foo", kind = "bar")]`

In both of these cases, `foo` is the name of the native library that we're
linking to, and in the second case `bar` is the type of native library that the
compiler is linking to. There are currently three known types of native
libraries:

* Dynamic - `#[link(name = "readline")]`
* Static - `#[link(name = "my_build_dependency", kind = "static")]`
* Frameworks - `#[link(name = "CoreFoundation", kind = "framework")]`

Note that frameworks are only available on OSX targets.

The different `kind` values are meant to differentiate how the native library
participates in linkage. From a linkage perspective, the rust compiler creates
two flavors of artifacts: partial (rlib/staticlib) and final (dylib/binary).
Native dynamic libraries and frameworks are propagated to the final artifact
boundary, while static libraries are not propagated at all.

A few examples of how this model can be used are:

* A native build dependency. Sometimes some C/C++ glue is needed when writing
  some rust code, but distribution of the C/C++ code in a library format is just
  a burden. In this case, the code will be archived into `libfoo.a` and then the
  rust crate would declare a dependency via `#[link(name = "foo", kind =
  "static")]`.

  Regardless of the flavor of output for the crate, the native static library
  will be included in the output, meaning that distribution of the native static
  library is not necessary.

* A normal dynamic dependency. Common system libraries (like `readline`) are
  available on a large number of systems, and often a static copy of these
  libraries cannot be found. When this dependency is included in a rust crate,
  partial targets (like rlibs) will not link to the library, but when the rlib
  is included in a final target (like a binary), the native library will be
  linked in.

On OSX, frameworks behave with the same semantics as a dynamic library.

## The `link_args` attribute

There is one other way to tell rustc how to customize linking, and that is via
the `link_args` attribute. This attribute is applied to `extern` blocks and
specifies raw flags which need to get passed to the linker when producing an
artifact. An example usage would be:

``` no_run
#![feature(link_args)]

#[link_args = "-foo -bar -baz"]
extern {}
# fn main() {}
```

Note that this feature is currently hidden behind the `feature(link_args)` gate
because this is not a sanctioned way of performing linking. Right now rustc
shells out to the system linker, so it makes sense to provide extra command line
arguments, but this will not always be the case. In the future rustc may use
LLVM directly to link native libraries in which case `link_args` will have no
meaning.

It is highly recommended to *not* use this attribute, and rather use the more
formal `#[link(...)]` attribute on `extern` blocks instead.

# Unsafe blocks

Some operations, like dereferencing unsafe pointers or calling functions that have been marked
unsafe are only allowed inside unsafe blocks. Unsafe blocks isolate unsafety and are a promise to
the compiler that the unsafety does not leak out of the block.

Unsafe functions, on the other hand, advertise it to the world. An unsafe function is written like
this:

```
unsafe fn kaboom(ptr: *const int) -> int { *ptr }
```

This function can only be called from an `unsafe` block or another `unsafe` function.

# Accessing foreign globals

Foreign APIs often export a global variable which could do something like track
global state. In order to access these variables, you declare them in `extern`
blocks with the `static` keyword:

```no_run
extern crate libc;

#[link(name = "readline")]
extern {
    static rl_readline_version: libc::c_int;
}

fn main() {
    println!("You have readline version {} installed.",
             rl_readline_version as int);
}
```

Alternatively, you may need to alter global state provided by a foreign
interface. To do this, statics can be declared with `mut` so we can mutate
them.

```no_run
extern crate libc;

use std::ffi::CString;
use std::ptr;

#[link(name = "readline")]
extern {
    static mut rl_prompt: *const libc::c_char;
}

fn main() {
    let prompt = CString::new("[my-awesome-shell] $").unwrap();
    unsafe {
        rl_prompt = prompt.as_ptr();

        println!("{:?}", rl_prompt);

        rl_prompt = ptr::null();
    }
}
```

Note that all interaction with a `static mut` is unsafe, both reading and
writing. Dealing with global mutable state requires a great deal of care.

# Foreign calling conventions

Most foreign code exposes a C ABI, and Rust uses the platform's C calling convention by default when
calling foreign functions. Some foreign functions, most notably the Windows API, use other calling
conventions. Rust provides a way to tell the compiler which convention to use:

```
extern crate libc;

#[cfg(all(target_os = "win32", target_arch = "x86"))]
#[link(name = "kernel32")]
#[allow(non_snake_case)]
extern "stdcall" {
    fn SetEnvironmentVariableA(n: *const u8, v: *const u8) -> libc::c_int;
}
# fn main() { }
```

This applies to the entire `extern` block. The list of supported ABI constraints
are:

* `stdcall`
* `aapcs`
* `cdecl`
* `fastcall`
* `Rust`
* `rust-intrinsic`
* `system`
* `C`
* `win64`

Most of the abis in this list are self-explanatory, but the `system` abi may
seem a little odd. This constraint selects whatever the appropriate ABI is for
interoperating with the target's libraries. For example, on win32 with a x86
architecture, this means that the abi used would be `stdcall`. On x86_64,
however, windows uses the `C` calling convention, so `C` would be used. This
means that in our previous example, we could have used `extern "system" { ... }`
to define a block for all windows systems, not just x86 ones.

# Interoperability with foreign code

Rust guarantees that the layout of a `struct` is compatible with the platform's
representation in C only if the `#[repr(C)]` attribute is applied to it.
`#[repr(C, packed)]` can be used to lay out struct members without padding.
`#[repr(C)]` can also be applied to an enum.

Rust's owned boxes (`Box<T>`) use non-nullable pointers as handles which point
to the contained object. However, they should not be manually created because
they are managed by internal allocators. References can safely be assumed to be
non-nullable pointers directly to the type.  However, breaking the borrow
checking or mutability rules is not guaranteed to be safe, so prefer using raw
pointers (`*`) if that's needed because the compiler can't make as many
assumptions about them.

Vectors and strings share the same basic memory layout, and utilities are
available in the `vec` and `str` modules for working with C APIs. However,
strings are not terminated with `\0`. If you need a NUL-terminated string for
interoperability with C, you should use the `CString` type in the `std::ffi`
module.

The standard library includes type aliases and function definitions for the C
standard library in the `libc` module, and Rust links against `libc` and `libm`
by default.

# The "nullable pointer optimization"

Certain types are defined to not be `null`. This includes references (`&T`,
`&mut T`), boxes (`Box<T>`), and function pointers (`extern "abi" fn()`).
When interfacing with C, pointers that might be null are often used.
As a special case, a generic `enum` that contains exactly two variants, one of
which contains no data and the other containing a single field, is eligible
for the "nullable pointer optimization". When such an enum is instantiated
with one of the non-nullable types, it is represented as a single pointer,
and the non-data variant is represented as the null pointer. So
`Option<extern "C" fn(c_int) -> c_int>` is how one represents a nullable
function pointer using the C ABI.

# Calling Rust code from C

You may wish to compile Rust code in a way so that it can be called from C. This is
fairly easy, but requires a few things:

```
#[no_mangle]
pub extern fn hello_rust() -> *const u8 {
    "Hello, world!\0".as_ptr()
}
# fn main() {}
```

The `extern` makes this function adhere to the C calling convention, as
discussed above in "[Foreign Calling
Conventions](ffi.html#foreign-calling-conventions)". The `no_mangle`
attribute turns off Rust's name mangling, so that it is easier to link to.