Foreign Function Interface
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
).
A note about libc
Many of these examples use the libc
crate, which provides various
type definitions for C types, among other things. If you’re trying these
examples yourself, you’ll need to add libc
to your Cargo.toml
:
[dependencies]
libc = "0.2.0"
Calling foreign functions
The following is a minimal example of calling a foreign function which will compile if snappy is installed:
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 cannot 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 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 the 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.
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.
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.
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
}
}
}
Then, we can add some tests to show how to use them.
use libc::{c_int, size_t};
unsafe fn snappy_compress(input: *const u8,
input_length: size_t,
compressed: *mut u8,
compressed_length: *mut size_t)
-> c_int { 0 }
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_max_compressed_length(source_length: size_t) -> size_t { 0 }
unsafe fn snappy_uncompressed_length(compressed: *const u8,
compressed_length: size_t,
result: *mut size_t)
-> c_int { 0 }
unsafe fn snappy_validate_compressed_buffer(compressed: *const u8,
compressed_length: size_t)
-> c_int { 0 }
fn main() { }
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn valid() {
let d = vec![0xde, 0xad, 0xd0, 0x0d];
let c: &[u8] = &compress(&d);
assert!(validate_compressed_buffer(c));
assert!(uncompress(c) == Some(d));
}
#[test]
fn invalid() {
let d = vec![0, 0, 0, 0];
assert!(!validate_compressed_buffer(&d));
assert!(uncompress(&d).is_none());
}
#[test]
fn empty() {
let d = vec![];
assert!(!validate_compressed_buffer(&d));
assert!(uncompress(&d).is_none());
let c = compress(&d);
assert!(validate_compressed_buffer(&c));
assert!(uncompress(&c) == Some(d));
}
}
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).
For more about destructors, see the Drop trait.
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.
Rust side
First, we assume you have a lib crate named as rust_from_c
.
lib.rs
should have Rust code as following:
#[no_mangle] pub extern "C" fn hello_from_rust() { println!("Hello from Rust!"); } fn main() {}
The extern "C"
makes this function adhere to the C calling convention, as discussed above in "Foreign Calling Conventions".
The no_mangle
attribute turns off Rust's name mangling, so that it has a well defined symbol to link to.
Then, to compile Rust code as a shared library that can be called from C, add the following to your Cargo.toml
:
[lib]
crate-type = ["cdylib"]
(NOTE: We could also use the staticlib
crate type but it needs to tweak some linking flags.)
Run cargo build
and you're ready to go on the Rust side.
C side
We'll create a C file to call the hello_from_rust
function and compile it by gcc
.
C file should look like:
extern void hello_from_rust();
int main(void) {
hello_from_rust();
return 0;
}
We name the file as call_rust.c
and place it on the crate root.
Run the following to compile:
gcc call_rust.c -o call_rust -lrust_from_c -L./target/debug
-l
and -L
tell gcc to find our Rust library.
Finally, we can call Rust code from C with LD_LIBRARY_PATH
specified:
$ LD_LIBRARY_PATH=./target/debug ./call_rust
Hello from Rust!
That's it!
For more realistic example, check the cbindgen
.
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:
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:
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 a raw 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:
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:
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::sync::mpsc
) 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 macOS 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 library and framework dependencies are propagated to the final
artifact boundary, while static library dependencies are not propagated at
all, because the static libraries are integrated directly into the subsequent
artifact.
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 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 macOS, frameworks behave with the same semantics as a dynamic library.
Unsafe blocks
Some operations, like dereferencing raw 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:
#![allow(unused)] fn main() { unsafe fn kaboom(ptr: *const i32) -> i32 { *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:
#[link(name = "readline")]
extern {
static rl_readline_version: libc::c_int;
}
fn main() {
println!("You have readline version {} installed.",
unsafe { rl_readline_version as i32 });
}
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.
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:
#[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
vectorcall
This is currently hidden behind theabi_vectorcall
gate and is subject to change.Rust
rust-intrinsic
system
C
win64
sysv64
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 only 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 libc
crate on crates.io includes type aliases and function
definitions for the C standard library in the libc
module, and Rust links
against libc
and libm
by default.
Variadic functions
In C, functions can be 'variadic', meaning they accept a variable number of arguments. This can
be achieved in Rust by specifying ...
within the argument list of a foreign function declaration:
extern {
fn foo(x: i32, ...);
}
fn main() {
unsafe {
foo(10, 20, 30, 40, 50);
}
}
Normal Rust functions can not be variadic:
#![allow(unused)] fn main() { // This will not compile fn foo(x: i32, ...) {} }
The "nullable pointer optimization"
Certain Rust types are defined to never 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, which would seem to
require some messy transmute
s and/or unsafe code to handle conversions to/from Rust types.
However, the language provides a workaround.
As a special case, an enum
is eligible for the "nullable pointer optimization" if it contains
exactly two variants, one of which contains no data and the other contains a field of one of the
non-nullable types listed above. This means no extra space is required for a discriminant; rather,
the empty variant is represented by putting a null
value into the non-nullable field. This is
called an "optimization", but unlike other optimizations it is guaranteed to apply to eligible
types.
The most common type that takes advantage of the nullable pointer optimization is Option<T>
,
where None
corresponds to null
. So Option<extern "C" fn(c_int) -> c_int>
is a correct way
to represent a nullable function pointer using the C ABI (corresponding to the C type
int (*)(int)
).
Here is a contrived example. Let's say some C library has a facility for registering a callback, which gets called in certain situations. The callback is passed a function pointer and an integer and it is supposed to run the function with the integer as a parameter. So we have function pointers flying across the FFI boundary in both directions.
use libc::c_int;
#[cfg(hidden)]
extern "C" {
/// Registers the callback.
fn register(cb: Option<extern "C" fn(Option<extern "C" fn(c_int) -> c_int>, c_int) -> c_int>);
}
unsafe fn register(_: Option<extern "C" fn(Option<extern "C" fn(c_int) -> c_int>,
c_int) -> c_int>)
{}
/// This fairly useless function receives a function pointer and an integer
/// from C, and returns the result of calling the function with the integer.
/// In case no function is provided, it squares the integer by default.
extern "C" fn apply(process: Option<extern "C" fn(c_int) -> c_int>, int: c_int) -> c_int {
match process {
Some(f) => f(int),
None => int * int
}
}
fn main() {
unsafe {
register(Some(apply));
}
}
And the code on the C side looks like this:
void register(int (*f)(int (*)(int), int)) {
...
}
No transmute
required!
FFI and unwinding
It’s important to be mindful of unwinding when working with FFI. Each
non-Rust
ABI comes in two variants, one with -unwind
suffix and one without. If
you expect Rust panic
s or foreign (e.g. C++) exceptions to cross an FFI
boundary, that boundary must use the appropriate -unwind
ABI string (note
that compiling with panic=abort
will still cause panic!
to immediately
abort the process, regardless of which ABI is specified by the function that
panic
s).
Conversely, if you do not expect unwinding to cross an ABI boundary, use one of
the non-unwind
ABI strings (other than Rust
, which always permits
unwinding). If an unwinding operation does encounter an ABI boundary that is
not permitted to unwind, the behavior depends on the source of the unwinding
(Rust panic
or a foreign exception):
panic
will cause the process to safely abort.- A foreign exception entering Rust will cause undefined behavior.
Note that the interaction of catch_unwind
with foreign exceptions is
undefined, as is the interaction of panic
with foreign exception-catching
mechanisms (notably C++'s try
/catch
).
Rust panic
with "C-unwind"
#[no_mangle]
extern "C-unwind" fn example() {
panic!("Uh oh");
}
This function (when compiled with panic=unwind
) is permitted to unwind C++
stack frames.
[Rust function with `catch_unwind`, which stops the unwinding]
|
...
|
[C++ frames]
| ^
| (calls) | (unwinding
v | goes this
[Rust function `example`] | way)
| |
+--- rust function panics --+
If the C++ frames have objects, their destructors will be called.
C++ throw
with "C-unwind"
#[link(...)]
extern "C-unwind" {
// A C++ function that may throw an exception
fn may_throw();
}
#[no_mangle]
extern "C-unwind" fn rust_passthrough() {
let b = Box::new(5);
unsafe { may_throw(); }
println!("{:?}", &b);
}
A C++ function with a try
block may invoke rust_passthrough
and catch
an
exception thrown by may_throw
.
[C++ function with `try` block that invokes `rust_passthrough`]
|
...
|
[Rust function `rust_passthrough`]
| ^
| (calls) | (unwinding
v | goes this
[C++ function `may_throw`] | way)
| |
+--- C++ function throws ----+
If may_throw
does throw an exception, b
will be dropped. Otherwise, 5
will be printed.
panic
can be stopped at an ABI boundary
#![allow(unused)] fn main() { #[no_mangle] extern "C" fn assert_nonzero(input: u32) { assert!(input != 0) } }
If assert_nonzero
is called with the argument 0
, the runtime is guaranteed
to (safely) abort the process, whether or not compiled with panic=abort
.
Catching panic
preemptively
If you are writing Rust code that may panic, and you don't wish to abort the
process if it panics, you must use catch_unwind
:
use std::panic::catch_unwind; #[no_mangle] pub extern "C" fn oh_no() -> i32 { let result = catch_unwind(|| { panic!("Oops!"); }); match result { Ok(_) => 0, Err(_) => 1, } } fn main() {}
Please note that catch_unwind
will only catch unwinding panics, not
those that abort the process. See the documentation of catch_unwind
for more information.
Representing opaque structs
Sometimes, a C library wants to provide a pointer to something, but not let you know the internal details of the thing it wants.
A stable and simple way is to use a void *
argument:
void foo(void *arg);
void bar(void *arg);
We can represent this in Rust with the c_void
type:
extern "C" {
pub fn foo(arg: *mut libc::c_void);
pub fn bar(arg: *mut libc::c_void);
}
fn main() {}
This is a perfectly valid way of handling the situation. However, we can do a bit
better. To solve this, some C libraries will instead create a struct
, where
the details and memory layout of the struct are private. This gives some amount
of type safety. These structures are called ‘opaque’. Here’s an example, in C:
struct Foo; /* Foo is a structure, but its contents are not part of the public interface */
struct Bar;
void foo(struct Foo *arg);
void bar(struct Bar *arg);
To do this in Rust, let’s create our own opaque types:
#[repr(C)] pub struct Foo { _data: [u8; 0], _marker: core::marker::PhantomData<(*mut u8, core::marker::PhantomPinned)>, } #[repr(C)] pub struct Bar { _data: [u8; 0], _marker: core::marker::PhantomData<(*mut u8, core::marker::PhantomPinned)>, } extern "C" { pub fn foo(arg: *mut Foo); pub fn bar(arg: *mut Bar); } fn main() {}
By including at least one private field and no constructor,
we create an opaque type that we can't instantiate outside of this module.
(A struct with no field could be instantiated by anyone.)
We also want to use this type in FFI, so we have to add #[repr(C)]
.
The marker ensures the compiler does not mark the struct as Send
, Sync
and Unpin
are
not applied to the struct. (*mut u8
is not Send
or Sync
, PhantomPinned
is not Unpin
)
But because our Foo
and Bar
types are
different, we’ll get type safety between the two of them, so we cannot
accidentally pass a pointer to Foo
to bar()
.
Notice that it is a really bad idea to use an empty enum as FFI type.
The compiler relies on empty enums being uninhabited, so handling values of type
&Empty
is a huge footgun and can lead to buggy program behavior (by triggering
undefined behavior).
NOTE: The simplest way would use "extern types". But it's currently (as of June 2021) unstable and has some unresolved questions, see the RFC page and the tracking issue for more details.