1 % Writing Unsafe and Low-Level Code in Rust
5 Rust aims to provide safe abstractions over the low-level details of
6 the CPU and operating system, but sometimes one needs to drop down and
7 write code at that level. This guide aims to provide an overview of
8 the dangers and power one gets with Rust's unsafe subset.
10 Rust provides an escape hatch in the form of the `unsafe { ... }`
11 block which allows the programmer to dodge some of the compiler's
12 checks and do a wide range of operations, such as:
14 - dereferencing [raw pointers](#raw-pointers)
15 - calling a function via FFI ([covered by the FFI guide](guide-ffi.html))
16 - casting between types bitwise (`transmute`, aka "reinterpret cast")
17 - [inline assembly](#inline-assembly)
19 Note that an `unsafe` block does not relax the rules about lifetimes
20 of `&` and the freezing of borrowed data.
22 Any use of `unsafe` is the programmer saying "I know more than you" to
23 the compiler, and, as such, the programmer should be very sure that
24 they actually do know more about why that piece of code is valid. In
25 general, one should try to minimize the amount of unsafe code in a
26 code base; preferably by using the bare minimum `unsafe` blocks to
27 build safe interfaces.
29 > **Note**: the low-level details of the Rust language are still in
30 > flux, and there is no guarantee of stability or backwards
31 > compatibility. In particular, there may be changes that do not cause
32 > compilation errors, but do cause semantic changes (such as invoking
33 > undefined behaviour). As such, extreme care is required.
39 One of Rust's biggest features is memory safety. This is achieved in
40 part via [the ownership system](guide-ownership.html), which is how the
41 compiler can guarantee that every `&` reference is always valid, and,
42 for example, never pointing to freed memory.
44 These restrictions on `&` have huge advantages. However, they also
45 constrain how we can use them. For example, `&` doesn't behave
46 identically to C's pointers, and so cannot be used for pointers in
47 foreign function interfaces (FFI). Additionally, both immutable (`&`)
48 and mutable (`&mut`) references have some aliasing and freezing
49 guarantees, required for memory safety.
51 In particular, if you have an `&T` reference, then the `T` must not be
52 modified through that reference or any other reference. There are some
53 standard library types, e.g. `Cell` and `RefCell`, that provide inner
54 mutability by replacing compile time guarantees with dynamic checks at
57 An `&mut` reference has a different constraint: when an object has an
58 `&mut T` pointing into it, then that `&mut` reference must be the only
59 such usable path to that object in the whole program. That is, an
60 `&mut` cannot alias with any other references.
62 Using `unsafe` code to incorrectly circumvent and violate these
63 restrictions is undefined behaviour. For example, the following
64 creates two aliasing `&mut` pointers, and is invalid.
70 let ref_1: &mut u8 = &mut x;
71 let ref_2: &mut u8 = unsafe { mem::transmute(&mut *ref_1) };
73 // oops, ref_1 and ref_2 point to the same piece of data (x) and are
81 Rust offers two additional pointer types "raw pointers", written as
82 `*const T` and `*mut T`. They're an approximation of C's `const T*` and `T*`
83 respectively; indeed, one of their most common uses is for FFI,
84 interfacing with external C libraries.
86 Raw pointers have much fewer guarantees than other pointer types
87 offered by the Rust language and libraries. For example, they
89 - are not guaranteed to point to valid memory and are not even
90 guaranteed to be non-null (unlike both `Box` and `&`);
91 - do not have any automatic clean-up, unlike `Box`, and so require
92 manual resource management;
93 - are plain-old-data, that is, they don't move ownership, again unlike
94 `Box`, hence the Rust compiler cannot protect against bugs like
96 - are considered sendable (if their contents is considered sendable),
97 so the compiler offers no assistance with ensuring their use is
98 thread-safe; for example, one can concurrently access a `*mut int`
99 from two threads without synchronization.
100 - lack any form of lifetimes, unlike `&`, and so the compiler cannot
101 reason about dangling pointers; and
102 - have no guarantees about aliasing or mutability other than mutation
103 not being allowed directly through a `*const T`.
105 Fortunately, they come with a redeeming feature: the weaker guarantees
106 mean weaker restrictions. The missing restrictions make raw pointers
107 appropriate as a building block for implementing things like smart
108 pointers and vectors inside libraries. For example, `*` pointers are
109 allowed to alias, allowing them to be used to write shared-ownership
110 types like reference counted and garbage collected pointers, and even
111 thread-safe shared memory types (`Rc` and the `Arc` types are both
112 implemented entirely in Rust).
114 There are two things that you are required to be careful about
115 (i.e. require an `unsafe { ... }` block) with raw pointers:
117 - dereferencing: they can have any value: so possible results include
118 a crash, a read of uninitialised memory, a use-after-free, or
119 reading data as normal.
120 - pointer arithmetic via the `offset` [intrinsic](#intrinsics) (or
121 `.offset` method): this intrinsic uses so-called "in-bounds"
122 arithmetic, that is, it is only defined behaviour if the result is
123 inside (or one-byte-past-the-end) of the object from which the
124 original pointer came.
126 The latter assumption allows the compiler to optimize more
127 effectively. As can be seen, actually *creating* a raw pointer is not
128 unsafe, and neither is converting to an integer.
130 ### References and raw pointers
132 At runtime, a raw pointer `*` and a reference pointing to the same
133 piece of data have an identical representation. In fact, an `&T`
134 reference will implicitly coerce to an `*const T` raw pointer in safe code
135 and similarly for the `mut` variants (both coercions can be performed
136 explicitly with, respectively, `value as *const T` and `value as *mut T`).
138 Going the opposite direction, from `*const` to a reference `&`, is not
139 safe. A `&T` is always valid, and so, at a minimum, the raw pointer
140 `*const T` has to point to a valid instance of type `T`. Furthermore,
141 the resulting pointer must satisfy the aliasing and mutability laws of
142 references. The compiler assumes these properties are true for any
143 references, no matter how they are created, and so any conversion from
144 raw pointers is asserting that they hold. The programmer *must*
147 The recommended method for the conversion is
152 let p_imm: *const u32 = &i as *const u32;
155 let p_mut: *mut u32 = &mut m;
158 let ref_imm: &u32 = &*p_imm;
159 let ref_mut: &mut u32 = &mut *p_mut;
163 The `&*x` dereferencing style is preferred to using a `transmute`.
164 The latter is far more powerful than necessary, and the more
165 restricted operation is harder to use incorrectly; for example, it
166 requires that `x` is a pointer (unlike `transmute`).
170 ## Making the unsafe safe(r)
172 There are various ways to expose a safe interface around some unsafe
175 - store pointers privately (i.e. not in public fields of public
176 structs), so that you can see and control all reads and writes to
177 the pointer in one place.
178 - use `assert!()` a lot: since you can't rely on the protection of the
179 compiler & type-system to ensure that your `unsafe` code is correct
180 at compile-time, use `assert!()` to verify that it is doing the
181 right thing at run-time.
182 - implement the `Drop` for resource clean-up via a destructor, and use
183 RAII (Resource Acquisition Is Initialization). This reduces the need
184 for any manual memory management by users, and automatically ensures
185 that clean-up is always run, even when the task panics.
186 - ensure that any data stored behind a raw pointer is destroyed at the
189 As an example, we give a reimplementation of owned boxes by wrapping
190 `malloc` and `free`. Rust's move semantics and lifetimes mean this
191 reimplementation is as safe as the `Box` type.
194 #![feature(unsafe_destructor)]
197 use libc::{c_void, size_t, malloc, free};
200 # use std::boxed::Box;
202 // Define a wrapper around the handle returned by the foreign code.
203 // Unique<T> has the same semantics as Box<T>
204 pub struct Unique<T> {
205 // It contains a single raw, mutable pointer to the object in question.
209 // Implement methods for creating and using the values in the box.
211 // NB: For simplicity and correctness, we require that T has kind Send
212 // (owned boxes relax this restriction).
213 impl<T: Send> Unique<T> {
214 pub fn new(value: T) -> Unique<T> {
216 let ptr = malloc(mem::size_of::<T>() as size_t) as *mut T;
217 // we *need* valid pointer.
218 assert!(!ptr.is_null());
219 // `*ptr` is uninitialized, and `*ptr = value` would
220 // attempt to destroy it `overwrite` moves a value into
221 // this memory without attempting to drop the original
223 ptr::write(&mut *ptr, value);
228 // the 'r lifetime results in the same semantics as `&*x` with
230 pub fn borrow<'r>(&'r self) -> &'r T {
231 // By construction, self.ptr is valid
232 unsafe { &*self.ptr }
235 // the 'r lifetime results in the same semantics as `&mut *x` with
237 pub fn borrow_mut<'r>(&'r mut self) -> &'r mut T {
238 unsafe { &mut *self.ptr }
242 // A key ingredient for safety, we associate a destructor with
243 // Unique<T>, making the struct manage the raw pointer: when the
244 // struct goes out of scope, it will automatically free the raw pointer.
246 // NB: This is an unsafe destructor, because rustc will not normally
247 // allow destructors to be associated with parameterized types, due to
248 // bad interaction with managed boxes. (With the Send restriction,
249 // we don't have this problem.) Note that the `#[unsafe_destructor]`
250 // feature gate is required to use unsafe destructors.
252 impl<T: Send> Drop for Unique<T> {
255 // Copy the object out from the pointer onto the stack,
256 // where it is covered by normal Rust destructor semantics
257 // and cleans itself up, if necessary
258 ptr::read(self.ptr as *const T);
260 // clean-up our allocation
261 free(self.ptr as *mut c_void)
266 // A comparison between the built-in `Box` and this reimplementation
269 let mut x = Box::new(5i);
271 } // `x` is freed here
274 let mut y = Unique::new(5i);
275 *y.borrow_mut() = 10;
276 } // `y` is freed here
280 Notably, the only way to construct a `Unique` is via the `new`
281 function, and this function ensures that the internal pointer is valid
282 and hidden in the private field. The two `borrow` methods are safe
283 because the compiler statically guarantees that objects are never used
284 before creation or after destruction (unless you use some `unsafe`
289 For extremely low-level manipulations and performance reasons, one
290 might wish to control the CPU directly. Rust supports using inline
291 assembly to do this via the `asm!` macro. The syntax roughly matches
295 asm!(assembly template
303 Any use of `asm` is feature gated (requires `#![feature(asm)]` on the
304 crate to allow) and of course requires an `unsafe` block.
306 > **Note**: the examples here are given in x86/x86-64 assembly, but
307 > all platforms are supported.
311 The `assembly template` is the only required parameter and must be a
312 literal string (i.e `""`)
317 #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
325 #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
326 fn foo() { /* ... */ }
335 (The `feature(asm)` and `#[cfg]`s are omitted from now on.)
337 Output operands, input operands, clobbers and options are all optional
338 but you must add the right number of `:` if you skip them:
342 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
343 # fn main() { unsafe {
344 asm!("xor %eax, %eax"
352 Whitespace also doesn't matter:
356 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
357 # fn main() { unsafe {
358 asm!("xor %eax, %eax" ::: "eax");
364 Input and output operands follow the same format: `:
365 "constraints1"(expr1), "constraints2"(expr2), ..."`. Output operand
366 expressions must be mutable lvalues:
370 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
371 fn add(a: int, b: int) -> int {
381 # #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]
382 # fn add(a: int, b: int) -> int { a + b }
385 assert_eq!(add(3, 14159), 14162)
391 Some instructions modify registers which might otherwise have held
392 different values so we use the clobbers list to indicate to the
393 compiler not to assume any values loaded into those registers will
398 # #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
399 # fn main() { unsafe {
400 // Put the value 0x200 in eax
401 asm!("mov $$0x200, %eax" : /* no outputs */ : /* no inputs */ : "eax");
405 Input and output registers need not be listed since that information
406 is already communicated by the given constraints. Otherwise, any other
407 registers used either implicitly or explicitly should be listed.
409 If the assembly changes the condition code register `cc` should be
410 specified as one of the clobbers. Similarly, if the assembly modifies
411 memory, `memory` should also be specified.
415 The last section, `options` is specific to Rust. The format is comma
416 separated literal strings (i.e `:"foo", "bar", "baz"`). It's used to
417 specify some extra info about the inline assembly:
419 Current valid options are:
421 1. **volatile** - specifying this is analogous to `__asm__ __volatile__ (...)` in gcc/clang.
422 2. **alignstack** - certain instructions expect the stack to be
423 aligned a certain way (i.e SSE) and specifying this indicates to
424 the compiler to insert its usual stack alignment code
425 3. **intel** - use intel syntax instead of the default AT&T.
427 # Avoiding the standard library
429 By default, `std` is linked to every Rust crate. In some contexts,
430 this is undesirable, and can be avoided with the `#![no_std]`
431 attribute attached to the crate.
437 # // fn main() {} tricked you, rustdoc!
440 Obviously there's more to life than just libraries: one can use
441 `#[no_std]` with an executable, controlling the entry point is
442 possible in two ways: the `#[start]` attribute, or overriding the
443 default shim for the C `main` function with your own.
445 The function marked `#[start]` is passed the command line parameters
446 in the same format as C:
450 #![feature(lang_items)]
452 // Pull in the system libc library for what crt0.o likely requires
455 // Entry point for this program
457 fn start(_argc: int, _argv: *const *const u8) -> int {
461 // These functions and traits are used by the compiler, but not
462 // for a bare-bones hello world. These are normally
463 // provided by libstd.
464 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
465 #[lang = "eh_personality"] extern fn eh_personality() {}
466 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
467 # // fn main() {} tricked you, rustdoc!
470 To override the compiler-inserted `main` shim, one has to disable it
471 with `#![no_main]` and then create the appropriate symbol with the
472 correct ABI and the correct name, which requires overriding the
473 compiler's name mangling too:
478 #![feature(lang_items)]
482 #[no_mangle] // ensure that this symbol is called `main` in the output
483 pub extern fn main(argc: int, argv: *const *const u8) -> int {
487 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
488 #[lang = "eh_personality"] extern fn eh_personality() {}
489 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
490 # // fn main() {} tricked you, rustdoc!
494 The compiler currently makes a few assumptions about symbols which are available
495 in the executable to call. Normally these functions are provided by the standard
496 library, but without it you must define your own.
498 The first of these three functions, `stack_exhausted`, is invoked whenever stack
499 overflow is detected. This function has a number of restrictions about how it
500 can be called and what it must do, but if the stack limit register is not being
501 maintained then a task always has an "infinite stack" and this function
502 shouldn't get triggered.
504 The second of these three functions, `eh_personality`, is used by the
505 failure mechanisms of the compiler. This is often mapped to GCC's
506 personality function (see the
507 [libstd implementation](std/rt/unwind/index.html) for more
508 information), but crates which do not trigger a panic can be assured
509 that this function is never called. The final function, `panic_fmt`, is
510 also used by the failure mechanisms of the compiler.
514 > **Note**: the core library's structure is unstable, and it is recommended to
515 > use the standard library instead wherever possible.
517 With the above techniques, we've got a bare-metal executable running some Rust
518 code. There is a good deal of functionality provided by the standard library,
519 however, that is necessary to be productive in Rust. If the standard library is
520 not sufficient, then [libcore](core/index.html) is designed to be used
523 The core library has very few dependencies and is much more portable than the
524 standard library itself. Additionally, the core library has most of the
525 necessary functionality for writing idiomatic and effective Rust code.
527 As an example, here is a program that will calculate the dot product of two
528 vectors provided from C, using idiomatic Rust practices.
533 #![feature(lang_items)]
538 use core::prelude::*;
543 pub extern fn dot_product(a: *const u32, a_len: u32,
544 b: *const u32, b_len: u32) -> u32 {
545 use core::raw::Slice;
547 // Convert the provided arrays into Rust slices.
548 // The core::raw module guarantees that the Slice
549 // structure has the same memory layout as a &[T]
552 // This is an unsafe operation because the compiler
553 // cannot tell the pointers are valid.
554 let (a_slice, b_slice): (&[u32], &[u32]) = unsafe {
556 Slice { data: a, len: a_len as uint },
557 Slice { data: b, len: b_len as uint },
561 // Iterate over the slices, collecting the result
563 for (i, j) in a_slice.iter().zip(b_slice.iter()) {
569 #[lang = "panic_fmt"]
570 extern fn panic_fmt(args: &core::fmt::Arguments,
576 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
577 #[lang = "eh_personality"] extern fn eh_personality() {}
578 # #[start] fn start(argc: int, argv: *const *const u8) -> int { 0 }
582 Note that there is one extra lang item here which differs from the examples
583 above, `panic_fmt`. This must be defined by consumers of libcore because the
584 core library declares panics, but it does not define it. The `panic_fmt`
585 lang item is this crate's definition of panic, and it must be guaranteed to
588 As can be seen in this example, the core library is intended to provide the
589 power of Rust in all circumstances, regardless of platform requirements. Further
590 libraries, such as liballoc, add functionality to libcore which make other
591 platform-specific assumptions, but continue to be more portable than the
592 standard library itself.
594 # Interacting with the compiler internals
596 > **Note**: this section is specific to the `rustc` compiler; these
597 > parts of the language may never be fully specified and so details may
598 > differ wildly between implementations (and even versions of `rustc`
601 > Furthermore, this is just an overview; the best form of
602 > documentation for specific instances of these features are their
603 > definitions and uses in `std`.
605 The Rust language currently has two orthogonal mechanisms for allowing
606 libraries to interact directly with the compiler and vice versa:
608 - intrinsics, functions built directly into the compiler providing
609 very basic low-level functionality,
610 - lang-items, special functions, types and traits in libraries marked
611 with specific `#[lang]` attributes
615 > **Note**: intrinsics will forever have an unstable interface, it is
616 > recommended to use the stable interfaces of libcore rather than intrinsics
619 These are imported as if they were FFI functions, with the special
620 `rust-intrinsic` ABI. For example, if one was in a freestanding
621 context, but wished to be able to `transmute` between types, and
622 perform efficient pointer arithmetic, one would import those functions
623 via a declaration like
626 # #![feature(intrinsics)]
629 extern "rust-intrinsic" {
630 fn transmute<T, U>(x: T) -> U;
632 fn offset<T>(dst: *const T, offset: int) -> *const T;
636 As with any other FFI functions, these are always `unsafe` to call.
640 > **Note**: lang items are often provided by crates in the Rust distribution,
641 > and lang items themselves have an unstable interface. It is recommended to use
642 > officially distributed crates instead of defining your own lang items.
644 The `rustc` compiler has certain pluggable operations, that is,
645 functionality that isn't hard-coded into the language, but is
646 implemented in libraries, with a special marker to tell the compiler
647 it exists. The marker is the attribute `#[lang="..."]` and there are
648 various different values of `...`, i.e. various different "lang
651 For example, `Box` pointers require two lang items, one for allocation
652 and one for deallocation. A freestanding program that uses the `Box`
653 sugar for dynamic allocations via `malloc` and `free`:
657 #![feature(lang_items, box_syntax)]
665 #[lang = "owned_box"]
666 pub struct Box<T>(*mut T);
668 #[lang="exchange_malloc"]
669 unsafe fn allocate(size: uint, _align: uint) -> *mut u8 {
670 let p = libc::malloc(size as libc::size_t) as *mut u8;
679 #[lang="exchange_free"]
680 unsafe fn deallocate(ptr: *mut u8, _size: uint, _align: uint) {
681 libc::free(ptr as *mut libc::c_void)
685 fn main(argc: int, argv: *const *const u8) -> int {
691 #[lang = "stack_exhausted"] extern fn stack_exhausted() {}
692 #[lang = "eh_personality"] extern fn eh_personality() {}
693 #[lang = "panic_fmt"] fn panic_fmt() -> ! { loop {} }
696 Note the use of `abort`: the `exchange_malloc` lang item is assumed to
697 return a valid pointer, and so needs to do the check internally.
699 Other features provided by lang items include:
701 - overloadable operators via traits: the traits corresponding to the
702 `==`, `<`, dereferencing (`*`) and `+` (etc.) operators are all
703 marked with lang items; those specific four are `eq`, `ord`,
704 `deref`, and `add` respectively.
705 - stack unwinding and general failure; the `eh_personality`, `fail`
706 and `fail_bounds_checks` lang items.
707 - the traits in `std::markers` used to indicate types of
708 various kinds; lang items `send`, `sync` and `copy`.
709 - the marker types and variance indicators found in
710 `std::markers`; lang items `covariant_type`,
711 `contravariant_lifetime`, `no_sync_bound`, etc.
713 Lang items are loaded lazily by the compiler; e.g. if one never uses
714 `Box` then there is no need to define functions for `exchange_malloc`
715 and `exchange_free`. `rustc` will emit an error when an item is needed
716 but not found in the current crate or any that it depends on.