1 % Foreign Function Interface
5 This guide will use the [snappy](https://github.com/google/snappy)
6 compression/decompression library as an introduction to writing bindings for
7 foreign code. Rust is currently unable to call directly into a C++ library, but
8 snappy includes a C interface (documented in
9 [`snappy-c.h`](https://github.com/google/snappy/blob/master/snappy-c.h)).
13 Many of these examples use [the `libc` crate][libc], which provides various
14 type definitions for C types, among other things. If you’re trying these
15 examples yourself, you’ll need to add `libc` to your `Cargo.toml`:
22 [libc]: https://crates.io/crates/libc
24 and add `extern crate libc;` to your crate root.
26 ## Calling foreign functions
28 The following is a minimal example of calling a foreign function which will
29 compile if snappy is installed:
36 #[link(name = "snappy")]
38 fn snappy_max_compressed_length(source_length: size_t) -> size_t;
42 let x = unsafe { snappy_max_compressed_length(100) };
43 println!("max compressed length of a 100 byte buffer: {}", x);
47 The `extern` block is a list of function signatures in a foreign library, in
48 this case with the platform's C ABI. The `#[link(...)]` attribute is used to
49 instruct the linker to link against the snappy library so the symbols are
52 Foreign functions are assumed to be unsafe so calls to them need to be wrapped
53 with `unsafe {}` as a promise to the compiler that everything contained within
54 truly is safe. C libraries often expose interfaces that aren't thread-safe, and
55 almost any function that takes a pointer argument isn't valid for all possible
56 inputs since the pointer could be dangling, and raw pointers fall outside of
57 Rust's safe memory model.
59 When declaring the argument types to a foreign function, the Rust compiler can
60 not check if the declaration is correct, so specifying it correctly is part of
61 keeping the binding correct at runtime.
63 The `extern` block can be extended to cover the entire snappy API:
68 use libc::{c_int, size_t};
70 #[link(name = "snappy")]
72 fn snappy_compress(input: *const u8,
75 compressed_length: *mut size_t) -> c_int;
76 fn snappy_uncompress(compressed: *const u8,
77 compressed_length: size_t,
78 uncompressed: *mut u8,
79 uncompressed_length: *mut size_t) -> c_int;
80 fn snappy_max_compressed_length(source_length: size_t) -> size_t;
81 fn snappy_uncompressed_length(compressed: *const u8,
82 compressed_length: size_t,
83 result: *mut size_t) -> c_int;
84 fn snappy_validate_compressed_buffer(compressed: *const u8,
85 compressed_length: size_t) -> c_int;
90 # Creating a safe interface
92 The raw C API needs to be wrapped to provide memory safety and make use of higher-level concepts
93 like vectors. A library can choose to expose only the safe, high-level interface and hide the unsafe
96 Wrapping the functions which expect buffers involves using the `slice::raw` module to manipulate Rust
97 vectors as pointers to memory. Rust's vectors are guaranteed to be a contiguous block of memory. The
98 length is number of elements currently contained, and the capacity is the total size in elements of
99 the allocated memory. The length is less than or equal to the capacity.
104 # use libc::{c_int, size_t};
105 # unsafe fn snappy_validate_compressed_buffer(_: *const u8, _: size_t) -> c_int { 0 }
107 pub fn validate_compressed_buffer(src: &[u8]) -> bool {
109 snappy_validate_compressed_buffer(src.as_ptr(), src.len() as size_t) == 0
114 The `validate_compressed_buffer` wrapper above makes use of an `unsafe` block, but it makes the
115 guarantee that calling it is safe for all inputs by leaving off `unsafe` from the function
118 The `snappy_compress` and `snappy_uncompress` functions are more complex, since a buffer has to be
119 allocated to hold the output too.
121 The `snappy_max_compressed_length` function can be used to allocate a vector with the maximum
122 required capacity to hold the compressed output. The vector can then be passed to the
123 `snappy_compress` function as an output parameter. An output parameter is also passed to retrieve
124 the true length after compression for setting the length.
129 # use libc::{size_t, c_int};
130 # unsafe fn snappy_compress(a: *const u8, b: size_t, c: *mut u8,
131 # d: *mut size_t) -> c_int { 0 }
132 # unsafe fn snappy_max_compressed_length(a: size_t) -> size_t { a }
134 pub fn compress(src: &[u8]) -> Vec<u8> {
136 let srclen = src.len() as size_t;
137 let psrc = src.as_ptr();
139 let mut dstlen = snappy_max_compressed_length(srclen);
140 let mut dst = Vec::with_capacity(dstlen as usize);
141 let pdst = dst.as_mut_ptr();
143 snappy_compress(psrc, srclen, pdst, &mut dstlen);
144 dst.set_len(dstlen as usize);
150 Decompression is similar, because snappy stores the uncompressed size as part of the compression
151 format and `snappy_uncompressed_length` will retrieve the exact buffer size required.
156 # use libc::{size_t, c_int};
157 # unsafe fn snappy_uncompress(compressed: *const u8,
158 # compressed_length: size_t,
159 # uncompressed: *mut u8,
160 # uncompressed_length: *mut size_t) -> c_int { 0 }
161 # unsafe fn snappy_uncompressed_length(compressed: *const u8,
162 # compressed_length: size_t,
163 # result: *mut size_t) -> c_int { 0 }
165 pub fn uncompress(src: &[u8]) -> Option<Vec<u8>> {
167 let srclen = src.len() as size_t;
168 let psrc = src.as_ptr();
170 let mut dstlen: size_t = 0;
171 snappy_uncompressed_length(psrc, srclen, &mut dstlen);
173 let mut dst = Vec::with_capacity(dstlen as usize);
174 let pdst = dst.as_mut_ptr();
176 if snappy_uncompress(psrc, srclen, pdst, &mut dstlen) == 0 {
177 dst.set_len(dstlen as usize);
180 None // SNAPPY_INVALID_INPUT
186 For reference, the examples used here are also available as a [library on
187 GitHub](https://github.com/thestinger/rust-snappy).
191 Foreign libraries often hand off ownership of resources to the calling code.
192 When this occurs, we must use Rust's destructors to provide safety and guarantee
193 the release of these resources (especially in the case of panic).
195 For more about destructors, see the [Drop trait](../std/ops/trait.Drop.html).
197 # Callbacks from C code to Rust functions
199 Some external libraries require the usage of callbacks to report back their
200 current state or intermediate data to the caller.
201 It is possible to pass functions defined in Rust to an external library.
202 The requirement for this is that the callback function is marked as `extern`
203 with the correct calling convention to make it callable from C code.
205 The callback function can then be sent through a registration call
206 to the C library and afterwards be invoked from there.
213 extern fn callback(a: i32) {
214 println!("I'm called from C with value {0}", a);
217 #[link(name = "extlib")]
219 fn register_callback(cb: extern fn(i32)) -> i32;
220 fn trigger_callback();
225 register_callback(callback);
226 trigger_callback(); // Triggers the callback
234 typedef void (*rust_callback)(int32_t);
237 int32_t register_callback(rust_callback callback) {
242 void trigger_callback() {
243 cb(7); // Will call callback(7) in Rust
247 In this example Rust's `main()` will call `trigger_callback()` in C,
248 which would, in turn, call back to `callback()` in Rust.
251 ## Targeting callbacks to Rust objects
253 The former example showed how a global function can be called from C code.
254 However it is often desired that the callback is targeted to a special
255 Rust object. This could be the object that represents the wrapper for the
258 This can be achieved by passing an raw pointer to the object down to the
259 C library. The C library can then include the pointer to the Rust object in
260 the notification. This will allow the callback to unsafely access the
261 referenced Rust object.
272 extern "C" fn callback(target: *mut RustObject, a: i32) {
273 println!("I'm called from C with value {0}", a);
275 // Update the value in RustObject with the value received from the callback
280 #[link(name = "extlib")]
282 fn register_callback(target: *mut RustObject,
283 cb: extern fn(*mut RustObject, i32)) -> i32;
284 fn trigger_callback();
288 // Create the object that will be referenced in the callback
289 let mut rust_object = Box::new(RustObject { a: 5 });
292 register_callback(&mut *rust_object, callback);
301 typedef void (*rust_callback)(void*, int32_t);
305 int32_t register_callback(void* callback_target, rust_callback callback) {
306 cb_target = callback_target;
311 void trigger_callback() {
312 cb(cb_target, 7); // Will call callback(&rustObject, 7) in Rust
316 ## Asynchronous callbacks
318 In the previously given examples the callbacks are invoked as a direct reaction
319 to a function call to the external C library.
320 The control over the current thread is switched from Rust to C to Rust for the
321 execution of the callback, but in the end the callback is executed on the
322 same thread that called the function which triggered the callback.
324 Things get more complicated when the external library spawns its own threads
325 and invokes callbacks from there.
326 In these cases access to Rust data structures inside the callbacks is
327 especially unsafe and proper synchronization mechanisms must be used.
328 Besides classical synchronization mechanisms like mutexes, one possibility in
329 Rust is to use channels (in `std::sync::mpsc`) to forward data from the C
330 thread that invoked the callback into a Rust thread.
332 If an asynchronous callback targets a special object in the Rust address space
333 it is also absolutely necessary that no more callbacks are performed by the
334 C library after the respective Rust object gets destroyed.
335 This can be achieved by unregistering the callback in the object's
336 destructor and designing the library in a way that guarantees that no
337 callback will be performed after deregistration.
341 The `link` attribute on `extern` blocks provides the basic building block for
342 instructing rustc how it will link to native libraries. There are two accepted
343 forms of the link attribute today:
345 * `#[link(name = "foo")]`
346 * `#[link(name = "foo", kind = "bar")]`
348 In both of these cases, `foo` is the name of the native library that we're
349 linking to, and in the second case `bar` is the type of native library that the
350 compiler is linking to. There are currently three known types of native
353 * Dynamic - `#[link(name = "readline")]`
354 * Static - `#[link(name = "my_build_dependency", kind = "static")]`
355 * Frameworks - `#[link(name = "CoreFoundation", kind = "framework")]`
357 Note that frameworks are only available on OSX targets.
359 The different `kind` values are meant to differentiate how the native library
360 participates in linkage. From a linkage perspective, the Rust compiler creates
361 two flavors of artifacts: partial (rlib/staticlib) and final (dylib/binary).
362 Native dynamic library and framework dependencies are propagated to the final
363 artifact boundary, while static library dependencies are not propagated at
364 all, because the static libraries are integrated directly into the subsequent
367 A few examples of how this model can be used are:
369 * A native build dependency. Sometimes some C/C++ glue is needed when writing
370 some Rust code, but distribution of the C/C++ code in a library format is
371 a burden. In this case, the code will be archived into `libfoo.a` and then the
372 Rust crate would declare a dependency via `#[link(name = "foo", kind =
375 Regardless of the flavor of output for the crate, the native static library
376 will be included in the output, meaning that distribution of the native static
377 library is not necessary.
379 * A normal dynamic dependency. Common system libraries (like `readline`) are
380 available on a large number of systems, and often a static copy of these
381 libraries cannot be found. When this dependency is included in a Rust crate,
382 partial targets (like rlibs) will not link to the library, but when the rlib
383 is included in a final target (like a binary), the native library will be
386 On OSX, frameworks behave with the same semantics as a dynamic library.
390 Some operations, like dereferencing raw pointers or calling functions that have been marked
391 unsafe are only allowed inside unsafe blocks. Unsafe blocks isolate unsafety and are a promise to
392 the compiler that the unsafety does not leak out of the block.
394 Unsafe functions, on the other hand, advertise it to the world. An unsafe function is written like
398 unsafe fn kaboom(ptr: *const i32) -> i32 { *ptr }
401 This function can only be called from an `unsafe` block or another `unsafe` function.
403 # Accessing foreign globals
405 Foreign APIs often export a global variable which could do something like track
406 global state. In order to access these variables, you declare them in `extern`
407 blocks with the `static` keyword:
413 #[link(name = "readline")]
415 static rl_readline_version: libc::c_int;
419 println!("You have readline version {} installed.",
420 rl_readline_version as i32);
424 Alternatively, you may need to alter global state provided by a foreign
425 interface. To do this, statics can be declared with `mut` so we can mutate
432 use std::ffi::CString;
435 #[link(name = "readline")]
437 static mut rl_prompt: *const libc::c_char;
441 let prompt = CString::new("[my-awesome-shell] $").unwrap();
443 rl_prompt = prompt.as_ptr();
445 println!("{:?}", rl_prompt);
447 rl_prompt = ptr::null();
452 Note that all interaction with a `static mut` is unsafe, both reading and
453 writing. Dealing with global mutable state requires a great deal of care.
455 # Foreign calling conventions
457 Most foreign code exposes a C ABI, and Rust uses the platform's C calling convention by default when
458 calling foreign functions. Some foreign functions, most notably the Windows API, use other calling
459 conventions. Rust provides a way to tell the compiler which convention to use:
465 #[cfg(all(target_os = "win32", target_arch = "x86"))]
466 #[link(name = "kernel32")]
467 #[allow(non_snake_case)]
469 fn SetEnvironmentVariableA(n: *const u8, v: *const u8) -> libc::c_int;
474 This applies to the entire `extern` block. The list of supported ABI constraints
482 This is currently hidden behind the `abi_vectorcall` gate and is subject to change.
489 Most of the abis in this list are self-explanatory, but the `system` abi may
490 seem a little odd. This constraint selects whatever the appropriate ABI is for
491 interoperating with the target's libraries. For example, on win32 with a x86
492 architecture, this means that the abi used would be `stdcall`. On x86_64,
493 however, windows uses the `C` calling convention, so `C` would be used. This
494 means that in our previous example, we could have used `extern "system" { ... }`
495 to define a block for all windows systems, not only x86 ones.
497 # Interoperability with foreign code
499 Rust guarantees that the layout of a `struct` is compatible with the platform's
500 representation in C only if the `#[repr(C)]` attribute is applied to it.
501 `#[repr(C, packed)]` can be used to lay out struct members without padding.
502 `#[repr(C)]` can also be applied to an enum.
504 Rust's owned boxes (`Box<T>`) use non-nullable pointers as handles which point
505 to the contained object. However, they should not be manually created because
506 they are managed by internal allocators. References can safely be assumed to be
507 non-nullable pointers directly to the type. However, breaking the borrow
508 checking or mutability rules is not guaranteed to be safe, so prefer using raw
509 pointers (`*`) if that's needed because the compiler can't make as many
510 assumptions about them.
512 Vectors and strings share the same basic memory layout, and utilities are
513 available in the `vec` and `str` modules for working with C APIs. However,
514 strings are not terminated with `\0`. If you need a NUL-terminated string for
515 interoperability with C, you should use the `CString` type in the `std::ffi`
518 The [`libc` crate on crates.io][libc] includes type aliases and function
519 definitions for the C standard library in the `libc` module, and Rust links
520 against `libc` and `libm` by default.
522 # The "nullable pointer optimization"
524 Certain types are defined to not be `null`. This includes references (`&T`,
525 `&mut T`), boxes (`Box<T>`), and function pointers (`extern "abi" fn()`).
526 When interfacing with C, pointers that might be null are often used.
527 As a special case, a generic `enum` that contains exactly two variants, one of
528 which contains no data and the other containing a single field, is eligible
529 for the "nullable pointer optimization". When such an enum is instantiated
530 with one of the non-nullable types, it is represented as a single pointer,
531 and the non-data variant is represented as the null pointer. So
532 `Option<extern "C" fn(c_int) -> c_int>` is how one represents a nullable
533 function pointer using the C ABI.
535 # Calling Rust code from C
537 You may wish to compile Rust code in a way so that it can be called from C. This is
538 fairly easy, but requires a few things:
542 pub extern fn hello_rust() -> *const u8 {
543 "Hello, world!\0".as_ptr()
548 The `extern` makes this function adhere to the C calling convention, as
549 discussed above in "[Foreign Calling
550 Conventions](ffi.html#foreign-calling-conventions)". The `no_mangle`
551 attribute turns off Rust's name mangling, so that it is easier to link to.
555 It’s important to be mindful of `panic!`s when working with FFI. A `panic!`
556 across an FFI boundary is undefined behavior. If you’re writing code that may
557 panic, you should run it in another thread, so that the panic doesn’t bubble up
564 pub extern fn oh_no() -> i32 {
565 let h = thread::spawn(|| {
577 # Representing opaque structs
579 Sometimes, a C library wants to provide a pointer to something, but not let you
580 know the internal details of the thing it wants. The simplest way is to use a
588 We can represent this in Rust with the `c_void` type:
595 pub fn foo(arg: *mut libc::c_void);
596 pub fn bar(arg: *mut libc::c_void);
601 This is a perfectly valid way of handling the situation. However, we can do a bit
602 better. To solve this, some C libraries will instead create a `struct`, where
603 the details and memory layout of the struct are private. This gives some amount
604 of type safety. These structures are called ‘opaque’. Here’s an example, in C:
607 struct Foo; /* Foo is a structure, but its contents are not part of the public interface */
609 void foo(struct Foo *arg);
610 void bar(struct Bar *arg);
613 To do this in Rust, let’s create our own opaque types with `enum`:
620 pub fn foo(arg: *mut Foo);
621 pub fn bar(arg: *mut Bar);
626 By using an `enum` with no variants, we create an opaque type that we can’t
627 instantiate, as it has no variants. But because our `Foo` and `Bar` types are
628 different, we’ll get type safety between the two of them, so we cannot
629 accidentally pass a pointer to `Foo` to `bar()`.