1 % The Rust Foreign Function Interface Guide
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)).
11 The following is a minimal example of calling a foreign function which will
12 compile if snappy is installed:
18 #[link(name = "snappy")]
20 fn snappy_max_compressed_length(source_length: size_t) -> size_t;
24 let x = unsafe { snappy_max_compressed_length(100) };
25 println!("max compressed length of a 100 byte buffer: {}", x);
29 The `extern` block is a list of function signatures in a foreign library, in
30 this case with the platform's C ABI. The `#[link(...)]` attribute is used to
31 instruct the linker to link against the snappy library so the symbols are
34 Foreign functions are assumed to be unsafe so calls to them need to be wrapped
35 with `unsafe {}` as a promise to the compiler that everything contained within
36 truly is safe. C libraries often expose interfaces that aren't thread-safe, and
37 almost any function that takes a pointer argument isn't valid for all possible
38 inputs since the pointer could be dangling, and raw pointers fall outside of
39 Rust's safe memory model.
41 When declaring the argument types to a foreign function, the Rust compiler can
42 not check if the declaration is correct, so specifying it correctly is part of
43 keeping the binding correct at runtime.
45 The `extern` block can be extended to cover the entire snappy API:
49 use libc::{c_int, size_t};
51 #[link(name = "snappy")]
53 fn snappy_compress(input: *const u8,
56 compressed_length: *mut size_t) -> c_int;
57 fn snappy_uncompress(compressed: *const u8,
58 compressed_length: size_t,
59 uncompressed: *mut u8,
60 uncompressed_length: *mut size_t) -> c_int;
61 fn snappy_max_compressed_length(source_length: size_t) -> size_t;
62 fn snappy_uncompressed_length(compressed: *const u8,
63 compressed_length: size_t,
64 result: *mut size_t) -> c_int;
65 fn snappy_validate_compressed_buffer(compressed: *const u8,
66 compressed_length: size_t) -> c_int;
71 # Creating a safe interface
73 The raw C API needs to be wrapped to provide memory safety and make use of higher-level concepts
74 like vectors. A library can choose to expose only the safe, high-level interface and hide the unsafe
77 Wrapping the functions which expect buffers involves using the `slice::raw` module to manipulate Rust
78 vectors as pointers to memory. Rust's vectors are guaranteed to be a contiguous block of memory. The
79 length is number of elements currently contained, and the capacity is the total size in elements of
80 the allocated memory. The length is less than or equal to the capacity.
84 # use libc::{c_int, size_t};
85 # unsafe fn snappy_validate_compressed_buffer(_: *const u8, _: size_t) -> c_int { 0 }
87 pub fn validate_compressed_buffer(src: &[u8]) -> bool {
89 snappy_validate_compressed_buffer(src.as_ptr(), src.len() as size_t) == 0
94 The `validate_compressed_buffer` wrapper above makes use of an `unsafe` block, but it makes the
95 guarantee that calling it is safe for all inputs by leaving off `unsafe` from the function
98 The `snappy_compress` and `snappy_uncompress` functions are more complex, since a buffer has to be
99 allocated to hold the output too.
101 The `snappy_max_compressed_length` function can be used to allocate a vector with the maximum
102 required capacity to hold the compressed output. The vector can then be passed to the
103 `snappy_compress` function as an output parameter. An output parameter is also passed to retrieve
104 the true length after compression for setting the length.
108 # use libc::{size_t, c_int};
109 # unsafe fn snappy_compress(a: *const u8, b: size_t, c: *mut u8,
110 # d: *mut size_t) -> c_int { 0 }
111 # unsafe fn snappy_max_compressed_length(a: size_t) -> size_t { a }
113 pub fn compress(src: &[u8]) -> Vec<u8> {
115 let srclen = src.len() as size_t;
116 let psrc = src.as_ptr();
118 let mut dstlen = snappy_max_compressed_length(srclen);
119 let mut dst = Vec::with_capacity(dstlen as uint);
120 let pdst = dst.as_mut_ptr();
122 snappy_compress(psrc, srclen, pdst, &mut dstlen);
123 dst.set_len(dstlen as uint);
129 Decompression is similar, because snappy stores the uncompressed size as part of the compression
130 format and `snappy_uncompressed_length` will retrieve the exact buffer size required.
134 # use libc::{size_t, c_int};
135 # unsafe fn snappy_uncompress(compressed: *const u8,
136 # compressed_length: size_t,
137 # uncompressed: *mut u8,
138 # uncompressed_length: *mut size_t) -> c_int { 0 }
139 # unsafe fn snappy_uncompressed_length(compressed: *const u8,
140 # compressed_length: size_t,
141 # result: *mut size_t) -> c_int { 0 }
143 pub fn uncompress(src: &[u8]) -> Option<Vec<u8>> {
145 let srclen = src.len() as size_t;
146 let psrc = src.as_ptr();
148 let mut dstlen: size_t = 0;
149 snappy_uncompressed_length(psrc, srclen, &mut dstlen);
151 let mut dst = Vec::with_capacity(dstlen as uint);
152 let pdst = dst.as_mut_ptr();
154 if snappy_uncompress(psrc, srclen, pdst, &mut dstlen) == 0 {
155 dst.set_len(dstlen as uint);
158 None // SNAPPY_INVALID_INPUT
164 For reference, the examples used here are also available as an [library on
165 GitHub](https://github.com/thestinger/rust-snappy).
169 Rust tasks by default run on a "large stack". This is actually implemented as a
170 reserving a large segment of the address space and then lazily mapping in pages
171 as they are needed. When calling an external C function, the code is invoked on
172 the same stack as the rust stack. This means that there is no extra
173 stack-switching mechanism in place because it is assumed that the large stack
174 for the rust task is plenty for the C function to have.
176 A planned future improvement (not yet implemented at the time of this writing)
177 is to have a guard page at the end of every rust stack. No rust function will
178 hit this guard page (due to Rust's usage of LLVM's `__morestack`). The intention
179 for this unmapped page is to prevent infinite recursion in C from overflowing
180 onto other rust stacks. If the guard page is hit, then the process will be
181 terminated with a message saying that the guard page was hit.
183 For normal external function usage, this all means that there shouldn't be any
184 need for any extra effort on a user's perspective. The C stack naturally
185 interleaves with the rust stack, and it's "large enough" for both to
186 interoperate. If, however, it is determined that a larger stack is necessary,
187 there are appropriate functions in the task spawning API to control the size of
188 the stack of the task which is spawned.
192 Foreign libraries often hand off ownership of resources to the calling code.
193 When this occurs, we must use Rust's destructors to provide safety and guarantee
194 the release of these resources (especially in the case of failure).
196 # Callbacks from C code to Rust functions
198 Some external libraries require the usage of callbacks to report back their
199 current state or intermediate data to the caller.
200 It is possible to pass functions defined in Rust to an external library.
201 The requirement for this is that the callback function is marked as `extern`
202 with the correct calling convention to make it callable from C code.
204 The callback function can then be sent through a registration call
205 to the C library and afterwards be invoked from there.
212 extern fn callback(a: i32) {
213 println!("I'm called from C with value {0}", a);
216 #[link(name = "extlib")]
218 fn register_callback(cb: extern fn(i32)) -> i32;
219 fn trigger_callback();
224 register_callback(callback);
225 trigger_callback(); // Triggers the callback
233 typedef void (*rust_callback)(int32_t);
236 int32_t register_callback(rust_callback callback) {
241 void trigger_callback() {
242 cb(7); // Will call callback(7) in Rust
246 In this example Rust's `main()` will call `trigger_callback()` in C,
247 which would, in turn, call back to `callback()` in Rust.
250 ## Targeting callbacks to Rust objects
252 The former example showed how a global function can be called from C code.
253 However it is often desired that the callback is targeted to a special
254 Rust object. This could be the object that represents the wrapper for the
257 This can be achieved by passing an unsafe pointer to the object down to the
258 C library. The C library can then include the pointer to the Rust object in
259 the notification. This will allow the callback to unsafely access the
260 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 RustObject { a: 5 };
292 register_callback(&mut *rust_object, callback);
301 typedef void (*rust_callback)(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 (and Rust task) that lead called the function which triggered
325 Things get more complicated when the external library spawns its own threads
326 and invokes callbacks from there.
327 In these cases access to Rust data structures inside the callbacks is
328 especially unsafe and proper synchronization mechanisms must be used.
329 Besides classical synchronization mechanisms like mutexes, one possibility in
330 Rust is to use channels (in `std::comm`) to forward data from the C thread
331 that invoked the callback into a Rust task.
333 If an asynchronous callback targets a special object in the Rust address space
334 it is also absolutely necessary that no more callbacks are performed by the
335 C library after the respective Rust object gets destroyed.
336 This can be achieved by unregistering the callback in the object's
337 destructor and designing the library in a way that guarantees that no
338 callback will be performed after deregistration.
342 The `link` attribute on `extern` blocks provides the basic building block for
343 instructing rustc how it will link to native libraries. There are two accepted
344 forms of the link attribute today:
346 * `#[link(name = "foo")]`
347 * `#[link(name = "foo", kind = "bar")]`
349 In both of these cases, `foo` is the name of the native library that we're
350 linking to, and in the second case `bar` is the type of native library that the
351 compiler is linking to. There are currently three known types of native
354 * Dynamic - `#[link(name = "readline")]`
355 * Static - `#[link(name = "my_build_dependency", kind = "static")]`
356 * Frameworks - `#[link(name = "CoreFoundation", kind = "framework")]`
358 Note that frameworks are only available on OSX targets.
360 The different `kind` values are meant to differentiate how the native library
361 participates in linkage. From a linkage perspective, the rust compiler creates
362 two flavors of artifacts: partial (rlib/staticlib) and final (dylib/binary).
363 Native dynamic libraries and frameworks are propagated to the final artifact
364 boundary, while static libraries are not propagated at all.
366 A few examples of how this model can be used are:
368 * A native build dependency. Sometimes some C/C++ glue is needed when writing
369 some rust code, but distribution of the C/C++ code in a library format is just
370 a burden. In this case, the code will be archived into `libfoo.a` and then the
371 rust crate would declare a dependency via `#[link(name = "foo", kind =
374 Regardless of the flavor of output for the crate, the native static library
375 will be included in the output, meaning that distribution of the native static
376 library is not necessary.
378 * A normal dynamic dependency. Common system libraries (like `readline`) are
379 available on a large number of systems, and often a static copy of these
380 libraries cannot be found. When this dependency is included in a rust crate,
381 partial targets (like rlibs) will not link to the library, but when the rlib
382 is included in a final target (like a binary), the native library will be
385 On OSX, frameworks behave with the same semantics as a dynamic library.
387 ## The `link_args` attribute
389 There is one other way to tell rustc how to customize linking, and that is via
390 the `link_args` attribute. This attribute is applied to `extern` blocks and
391 specifies raw flags which need to get passed to the linker when producing an
392 artifact. An example usage would be:
395 #![feature(link_args)]
397 #[link_args = "-foo -bar -baz"]
402 Note that this feature is currently hidden behind the `feature(link_args)` gate
403 because this is not a sanctioned way of performing linking. Right now rustc
404 shells out to the system linker, so it makes sense to provide extra command line
405 arguments, but this will not always be the case. In the future rustc may use
406 LLVM directly to link native libraries in which case `link_args` will have no
409 It is highly recommended to *not* use this attribute, and rather use the more
410 formal `#[link(...)]` attribute on `extern` blocks instead.
414 Some operations, like dereferencing unsafe pointers or calling functions that have been marked
415 unsafe are only allowed inside unsafe blocks. Unsafe blocks isolate unsafety and are a promise to
416 the compiler that the unsafety does not leak out of the block.
418 Unsafe functions, on the other hand, advertise it to the world. An unsafe function is written like
422 unsafe fn kaboom(ptr: *const int) -> int { *ptr }
425 This function can only be called from an `unsafe` block or another `unsafe` function.
427 # Accessing foreign globals
429 Foreign APIs often export a global variable which could do something like track
430 global state. In order to access these variables, you declare them in `extern`
431 blocks with the `static` keyword:
436 #[link(name = "readline")]
438 static rl_readline_version: libc::c_int;
442 println!("You have readline version {} installed.",
443 rl_readline_version as int);
447 Alternatively, you may need to alter global state provided by a foreign
448 interface. To do this, statics can be declared with `mut` so rust can mutate
455 #[link(name = "readline")]
457 static mut rl_prompt: *const libc::c_char;
461 "[my-awesome-shell] $".with_c_str(|buf| {
462 unsafe { rl_prompt = buf; }
463 // get a line, process it
464 unsafe { rl_prompt = ptr::null(); }
469 # Foreign calling conventions
471 Most foreign code exposes a C ABI, and Rust uses the platform's C calling convention by default when
472 calling foreign functions. Some foreign functions, most notably the Windows API, use other calling
473 conventions. Rust provides a way to tell the compiler which convention to use:
478 #[cfg(target_os = "win32", target_arch = "x86")]
479 #[link(name = "kernel32")]
480 #[allow(non_snake_case)]
482 fn SetEnvironmentVariableA(n: *const u8, v: *const u8) -> libc::c_int;
487 This applies to the entire `extern` block. The list of supported ABI constraints
500 Most of the abis in this list are self-explanatory, but the `system` abi may
501 seem a little odd. This constraint selects whatever the appropriate ABI is for
502 interoperating with the target's libraries. For example, on win32 with a x86
503 architecture, this means that the abi used would be `stdcall`. On x86_64,
504 however, windows uses the `C` calling convention, so `C` would be used. This
505 means that in our previous example, we could have used `extern "system" { ... }`
506 to define a block for all windows systems, not just x86 ones.
508 # Interoperability with foreign code
510 Rust guarantees that the layout of a `struct` is compatible with the platform's representation in C
511 only if the `#[repr(C)]` attribute is applied to it. `#[repr(C, packed)]` can be used to lay out
512 struct members without padding. `#[repr(C)]` can also be applied to an enum.
514 Rust's owned boxes (`Box<T>`) use non-nullable pointers as handles which point to the contained
515 object. However, they should not be manually created because they are managed by internal
516 allocators. References can safely be assumed to be non-nullable pointers directly to the type.
517 However, breaking the borrow checking or mutability rules is not guaranteed to be safe, so prefer
518 using raw pointers (`*`) if that's needed because the compiler can't make as many assumptions about
521 Vectors and strings share the same basic memory layout, and utilities are available in the `vec` and
522 `str` modules for working with C APIs. However, strings are not terminated with `\0`. If you need a
523 NUL-terminated string for interoperability with C, you should use the `c_str::to_c_str` function.
525 The standard library includes type aliases and function definitions for the C standard library in
526 the `libc` module, and Rust links against `libc` and `libm` by default.
528 # The "nullable pointer optimization"
530 Certain types are defined to not be `null`. This includes references (`&T`,
531 `&mut T`), boxes (`Box<T>`), and function pointers (`extern "abi" fn()`).
532 When interfacing with C, pointers that might be null are often used.
533 As a special case, a generic `enum` that contains exactly two variants, one of
534 which contains no data and the other containing a single field, is eligible
535 for the "nullable pointer optimization". When such an enum is instantiated
536 with one of the non-nullable types, it is represented as a single pointer,
537 and the non-data variant is represented as the null pointer. So
538 `Option<extern "C" fn(c_int) -> c_int>` is how one represents a nullable
539 function pointer using the C ABI.