rust/doc/guide-ffi.md

% The Rust Foreign Function Interface Guide

Introduction

This guide will use the 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).

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

use std::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:

use std::libc::{c_int, size_t};

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

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 vec::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.

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.

pub fn compress(src: &[u8]) -> ~[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 uint);
        let pdst = dst.as_mut_ptr();

        snappy_compress(psrc, srclen, pdst, &mut dstlen);
        dst.set_len(dstlen as uint);
        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.

pub fn uncompress(src: &[u8]) -> Option<~[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 uint);
        let pdst = dst.as_mut_ptr();

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

For reference, the examples used here are also available as an library on GitHub.

Stack management

Rust tasks by default run on a "large stack". This is actually implemented as a reserving a large segment of the address space and then lazily mapping in pages as they are needed. When calling an external C function, the code is invoked on the same stack as the rust stack. This means that there is no extra stack-switching mechanism in place because it is assumed that the large stack for the rust task is plenty for the C function to have.

A planned future improvement (net yet implemented at the time of this writing) is to have a guard page at the end of every rust stack. No rust function will hit this guard page (due to Rust's usage of LLVM's __morestack). The intention for this unmapped page is to prevent infinite recursion in C from overflowing onto other rust stacks. If the guard page is hit, then the process will be terminated with a message saying that the guard page was hit.

For normal external function usage, this all means that there shouldn't be any need for any extra effort on a user's perspective. The C stack naturally interleaves with the rust stack, and it's "large enough" for both to interoperate. If, however, it is determined that a larger stack is necessary, there are appropriate functions in the task spawning API to control the size of the stack of the task which is spawned.

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 failure).

As an example, we give a reimplementation of owned boxes by wrapping malloc and free:

use std::cast;
use std::libc::{c_void, size_t, malloc, free};
use std::ptr;
use std::unstable::intrinsics;

// Define a wrapper around the handle returned by the foreign code.
// Unique<T> has the same semantics as ~T
pub struct Unique<T> {
    // It contains a single raw, mutable pointer to the object in question.
    priv ptr: *mut T
}

// Implement methods for creating and using the values in the box.
// NB: For simplicity and correctness, we require that T has kind Send
// (owned boxes relax this restriction, and can contain managed (GC) boxes).
// This is because, as implemented, the garbage collector would not know
// about any shared boxes stored in the malloc'd region of memory.
impl<T: Send> Unique<T> {
    pub fn new(value: T) -> Unique<T> {
        unsafe {
            let ptr = malloc(std::mem::size_of::<T>() as size_t) as *mut T;
            assert!(!ptr::is_null(ptr));
            // `*ptr` is uninitialized, and `*ptr = value` would attempt to destroy it
            // move_val_init moves a value into this memory without
            // attempting to drop the original value.
            intrinsics::move_val_init(&mut *ptr, value);
            Unique{ptr: ptr}
        }
    }

    // the 'r lifetime results in the same semantics as `&*x` with ~T
    pub fn borrow<'r>(&'r self) -> &'r T {
        unsafe { cast::copy_lifetime(self, &*self.ptr) }
    }

    // the 'r lifetime results in the same semantics as `&mut *x` with ~T
    pub fn borrow_mut<'r>(&'r mut self) -> &'r mut T {
        unsafe { cast::copy_mut_lifetime(self, &mut *self.ptr) }
    }
}

// The key ingredient for safety, we associate a destructor with
// Unique<T>, making the struct manage the raw pointer: when the
// struct goes out of scope, it will automatically free the raw pointer.
// NB: This is an unsafe destructor, because rustc will not normally
// allow destructors to be associated with parametrized types, due to
// bad interaction with managed boxes. (With the Send restriction,
// we don't have this problem.)
#[unsafe_destructor]
impl<T: Send> Drop for Unique<T> {
    fn drop(&mut self) {
        unsafe {
            let x = intrinsics::uninit(); // dummy value to swap in
            // We need to move the object out of the box, so that
            // the destructor is called (at the end of this scope.)
            ptr::replace_ptr(self.ptr, x);
            free(self.ptr as *c_void)
        }
    }
}

// A comparison between the built-in ~ and this reimplementation
fn main() {
    {
        let mut x = ~5;
        *x = 10;
    } // `x` is freed here

    {
        let mut y = Unique::new(5);
        *y.borrow_mut() = 10;
    } // `y` is freed here
}

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

A basic example is:

Rust code:

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

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

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

C code:

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 will Rust's main() will call do_callback() in C, which would call back to callback() in Rust.

Targetting 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 targetted 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 provide a unsafe possibility to access the referenced Rust object in callback.

Rust code:

struct RustObject {
    a: i32,
    // other members
}

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

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

fn main() {
    // Create the object that will be referenced in the callback
    let rust_object = ~RustObject{a: 5, ...};
     
    unsafe {
        // Gets a raw pointer to the object
        let target_addr:*RustObject = ptr::to_unsafe_ptr(rust_object);
        register_callback(target_addr, callback);
        trigger_callback(); // Triggers the callback
    }
}

C code:

typedef void (*rust_callback)(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 already 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 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 (and Rust task) that lead called the function which triggered the callback.

Things get more complicated when the external library spawns it's own threads and invokes callbacks from there. In these cases access to Rust data structures inside he 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 task.

If an asychronous 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 get's destroyed. This can be achieved by unregistering the callback it the object's destructor and designing the library in a way that guarantees that no callback will be performed after unregistration.

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:

#[link_args = "-foo -bar -baz"]
extern {}

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: *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:

use std::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 rust can mutate them.

use std::libc;
use std::ptr;

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

fn main() {
    do "[my-awesome-shell] $".as_c_str |buf| {
        unsafe { rl_prompt = buf; }
        // get a line, process it
        unsafe { rl_prompt = ptr::null(); }
    }
}

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:

#[cfg(target_os = "win32", target_arch = "x86")]
#[link_name = "kernel32"]
extern "stdcall" {
    fn SetEnvironmentVariableA(n: *u8, v: *u8) -> std::libc::c_int;
}

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

• stdcall
• aapcs
• cdecl
• fastcall
• Rust
• rust-intrinsic
• system
• C

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. A #[packed] attribute is available, which will lay out the struct members without padding. However, there are currently no guarantees about the layout of an enum.

Rust's owned and managed boxes 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 c_str::to_c_str function.

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.