16 KiB
% 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
}
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. Borrowed pointers 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.