1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Basic functions for dealing with memory.
13 //! This module contains functions for querying the size and alignment of
14 //! types, initializing and manipulating memory.
16 #![stable(feature = "rust1", since = "1.0.0")]
22 #[stable(feature = "rust1", since = "1.0.0")]
23 pub use intrinsics
::transmute
;
25 /// Leaks a value into the void, consuming ownership and never running its
28 /// This function will take ownership of its argument, but is distinct from the
29 /// `mem::drop` function in that it **does not run the destructor**, leaking the
30 /// value and any resources that it owns.
32 /// There's only a few reasons to use this function. They mainly come
33 /// up in unsafe code or FFI code.
35 /// * You have an uninitialized value, perhaps for performance reasons, and
36 /// need to prevent the destructor from running on it.
37 /// * You have two copies of a value (like when writing something like
38 /// [`mem::swap`][swap]), but need the destructor to only run once to
39 /// prevent a double `free`.
40 /// * Transferring resources across [FFI][ffi] boundaries.
42 /// [swap]: fn.swap.html
43 /// [ffi]: ../../book/ffi.html
47 /// This function is not marked as `unsafe` as Rust does not guarantee that the
48 /// `Drop` implementation for a value will always run. Note, however, that
49 /// leaking resources such as memory or I/O objects is likely not desired, so
50 /// this function is only recommended for specialized use cases.
52 /// The safety of this function implies that when writing `unsafe` code
53 /// yourself care must be taken when leveraging a destructor that is required to
54 /// run to preserve memory safety. There are known situations where the
55 /// destructor may not run (such as if ownership of the object with the
56 /// destructor is returned) which must be taken into account.
58 /// # Other forms of Leakage
60 /// It's important to point out that this function is not the only method by
61 /// which a value can be leaked in safe Rust code. Other known sources of
64 /// * `Rc` and `Arc` cycles
65 /// * `mpsc::{Sender, Receiver}` cycles (they use `Arc` internally)
66 /// * Panicking destructors are likely to leak local resources
70 /// Leak some heap memory by never deallocating it:
75 /// let heap_memory = Box::new(3);
76 /// mem::forget(heap_memory);
79 /// Leak an I/O object, never closing the file:
83 /// use std::fs::File;
85 /// let file = File::open("foo.txt").unwrap();
86 /// mem::forget(file);
89 /// The `mem::swap` function uses `mem::forget` to good effect:
95 /// # #[allow(dead_code)]
96 /// fn swap<T>(x: &mut T, y: &mut T) {
98 /// // Give ourselves some scratch space to work with
99 /// let mut t: T = mem::uninitialized();
101 /// // Perform the swap, `&mut` pointers never alias
102 /// ptr::copy_nonoverlapping(&*x, &mut t, 1);
103 /// ptr::copy_nonoverlapping(&*y, x, 1);
104 /// ptr::copy_nonoverlapping(&t, y, 1);
106 /// // y and t now point to the same thing, but we need to completely
107 /// // forget `t` because we do not want to run the destructor for `T`
108 /// // on its value, which is still owned somewhere outside this function.
113 #[stable(feature = "rust1", since = "1.0.0")]
114 pub fn forget
<T
>(t
: T
) {
115 unsafe { intrinsics::forget(t) }
118 /// Returns the size of a type in bytes.
125 /// assert_eq!(4, mem::size_of::<i32>());
128 #[stable(feature = "rust1", since = "1.0.0")]
129 pub fn size_of
<T
>() -> usize {
130 unsafe { intrinsics::size_of::<T>() }
133 /// Returns the size of the given value in bytes.
140 /// assert_eq!(4, mem::size_of_val(&5i32));
143 #[stable(feature = "rust1", since = "1.0.0")]
144 pub fn size_of_val
<T
: ?Sized
>(val
: &T
) -> usize {
145 unsafe { intrinsics::size_of_val(val) }
148 /// Returns the ABI-required minimum alignment of a type
150 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
155 /// # #![allow(deprecated)]
158 /// assert_eq!(4, mem::min_align_of::<i32>());
161 #[stable(feature = "rust1", since = "1.0.0")]
162 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
163 pub fn min_align_of
<T
>() -> usize {
164 unsafe { intrinsics::min_align_of::<T>() }
167 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
172 /// # #![allow(deprecated)]
175 /// assert_eq!(4, mem::min_align_of_val(&5i32));
178 #[stable(feature = "rust1", since = "1.0.0")]
179 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
180 pub fn min_align_of_val
<T
: ?Sized
>(val
: &T
) -> usize {
181 unsafe { intrinsics::min_align_of_val(val) }
184 /// Returns the alignment in memory for a type.
186 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
193 /// assert_eq!(4, mem::align_of::<i32>());
196 #[stable(feature = "rust1", since = "1.0.0")]
197 pub fn align_of
<T
>() -> usize {
198 unsafe { intrinsics::min_align_of::<T>() }
201 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
208 /// assert_eq!(4, mem::align_of_val(&5i32));
211 #[stable(feature = "rust1", since = "1.0.0")]
212 pub fn align_of_val
<T
: ?Sized
>(val
: &T
) -> usize {
213 unsafe { intrinsics::min_align_of_val(val) }
216 /// Creates a value initialized to zero.
218 /// This function is similar to allocating space for a local variable and zeroing it out (an unsafe
221 /// Care must be taken when using this function, if the type `T` has a destructor and the value
222 /// falls out of scope (due to unwinding or returning) before being initialized, then the
223 /// destructor will run on zeroed data, likely leading to crashes.
225 /// This is useful for FFI functions sometimes, but should generally be avoided.
232 /// let x: i32 = unsafe { mem::zeroed() };
235 #[stable(feature = "rust1", since = "1.0.0")]
236 pub unsafe fn zeroed
<T
>() -> T
{
240 /// Creates a value initialized to an unspecified series of bytes.
242 /// The byte sequence usually indicates that the value at the memory
243 /// in question has been dropped. Thus, *if* T carries a drop flag,
244 /// any associated destructor will not be run when the value falls out
247 /// Some code at one time used the `zeroed` function above to
248 /// accomplish this goal.
250 /// This function is expected to be deprecated with the transition
251 /// to non-zeroing drop.
253 #[unstable(feature = "filling_drop", issue = "5016")]
254 pub unsafe fn dropped
<T
>() -> T
{
256 unsafe fn dropped_impl
<T
>() -> T { intrinsics::init_dropped() }
261 /// Bypasses Rust's normal memory-initialization checks by pretending to
262 /// produce a value of type T, while doing nothing at all.
264 /// **This is incredibly dangerous, and should not be done lightly. Deeply
265 /// consider initializing your memory with a default value instead.**
267 /// This is useful for FFI functions and initializing arrays sometimes,
268 /// but should generally be avoided.
270 /// # Undefined Behavior
272 /// It is Undefined Behavior to read uninitialized memory. Even just an
273 /// uninitialized boolean. For instance, if you branch on the value of such
274 /// a boolean your program may take one, both, or neither of the branches.
276 /// Note that this often also includes *writing* to the uninitialized value.
277 /// Rust believes the value is initialized, and will therefore try to Drop
278 /// the uninitialized value and its fields if you try to overwrite the memory
279 /// in a normal manner. The only way to safely initialize an arbitrary
280 /// uninitialized value is with one of the `ptr` functions: `write`, `copy`, or
281 /// `copy_nonoverlapping`. This isn't necessary if `T` is a primitive
282 /// or otherwise only contains types that don't implement Drop.
284 /// If this value *does* need some kind of Drop, it must be initialized before
285 /// it goes out of scope (and therefore would be dropped). Note that this
286 /// includes a `panic` occurring and unwinding the stack suddenly.
290 /// Here's how to safely initialize an array of `Vec`s.
296 /// // Only declare the array. This safely leaves it
297 /// // uninitialized in a way that Rust will track for us.
298 /// // However we can't initialize it element-by-element
299 /// // safely, and we can't use the `[value; 1000]`
300 /// // constructor because it only works with `Copy` data.
301 /// let mut data: [Vec<u32>; 1000];
304 /// // So we need to do this to initialize it.
305 /// data = mem::uninitialized();
307 /// // DANGER ZONE: if anything panics or otherwise
308 /// // incorrectly reads the array here, we will have
309 /// // Undefined Behavior.
311 /// // It's ok to mutably iterate the data, since this
312 /// // doesn't involve reading it at all.
313 /// // (ptr and len are statically known for arrays)
314 /// for elem in &mut data[..] {
315 /// // *elem = Vec::new() would try to drop the
316 /// // uninitialized memory at `elem` -- bad!
318 /// // Vec::new doesn't allocate or do really
319 /// // anything. It's only safe to call here
320 /// // because we know it won't panic.
321 /// ptr::write(elem, Vec::new());
324 /// // SAFE ZONE: everything is initialized.
327 /// println!("{:?}", &data[0]);
330 /// This example emphasizes exactly how delicate and dangerous doing this is.
331 /// Note that the `vec!` macro *does* let you initialize every element with a
332 /// value that is only `Clone`, so the following is semantically equivalent and
333 /// vastly less dangerous, as long as you can live with an extra heap
337 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
338 /// println!("{:?}", &data[0]);
341 #[stable(feature = "rust1", since = "1.0.0")]
342 pub unsafe fn uninitialized
<T
>() -> T
{
346 /// Swap the values at two mutable locations of the same type, without deinitializing or copying
359 /// assert_eq!(42, *x);
360 /// assert_eq!(5, *y);
363 #[stable(feature = "rust1", since = "1.0.0")]
364 pub fn swap
<T
>(x
: &mut T
, y
: &mut T
) {
366 // Give ourselves some scratch space to work with
367 let mut t
: T
= uninitialized();
369 // Perform the swap, `&mut` pointers never alias
370 ptr
::copy_nonoverlapping(&*x
, &mut t
, 1);
371 ptr
::copy_nonoverlapping(&*y
, x
, 1);
372 ptr
::copy_nonoverlapping(&t
, y
, 1);
374 // y and t now point to the same thing, but we need to completely
375 // forget `t` because we do not want to run the destructor for `T`
376 // on its value, which is still owned somewhere outside this function.
381 /// Replaces the value at a mutable location with a new one, returning the old value, without
382 /// deinitializing or copying either one.
384 /// This is primarily used for transferring and swapping ownership of a value in a mutable
389 /// A simple example:
394 /// let mut v: Vec<i32> = Vec::new();
396 /// mem::replace(&mut v, Vec::new());
399 /// This function allows consumption of one field of a struct by replacing it with another value.
400 /// The normal approach doesn't always work:
403 /// struct Buffer<T> { buf: Vec<T> }
405 /// impl<T> Buffer<T> {
406 /// fn get_and_reset(&mut self) -> Vec<T> {
407 /// // error: cannot move out of dereference of `&mut`-pointer
408 /// let buf = self.buf;
409 /// self.buf = Vec::new();
415 /// Note that `T` does not necessarily implement `Clone`, so it can't even clone and reset
416 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
417 /// `self`, allowing it to be returned:
420 /// # #![allow(dead_code)]
422 /// # struct Buffer<T> { buf: Vec<T> }
423 /// impl<T> Buffer<T> {
424 /// fn get_and_reset(&mut self) -> Vec<T> {
425 /// mem::replace(&mut self.buf, Vec::new())
430 #[stable(feature = "rust1", since = "1.0.0")]
431 pub fn replace
<T
>(dest
: &mut T
, mut src
: T
) -> T
{
432 swap(dest
, &mut src
);
436 /// Disposes of a value.
438 /// While this does call the argument's implementation of `Drop`, it will not
439 /// release any borrows, as borrows are based on lexical scope.
441 /// This effectively does nothing for
442 /// [types which implement `Copy`](../../book/ownership.html#copy-types),
443 /// e.g. integers. Such values are copied and _then_ moved into the function,
444 /// so the value persists after this function call.
451 /// let v = vec![1, 2, 3];
453 /// drop(v); // explicitly drop the vector
456 /// Borrows are based on lexical scope, so this produces an error:
459 /// let mut v = vec![1, 2, 3];
462 /// drop(x); // explicitly drop the reference, but the borrow still exists
464 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
465 /// // borrowed as immutable
468 /// An inner scope is needed to fix this:
471 /// let mut v = vec![1, 2, 3];
476 /// drop(x); // this is now redundant, as `x` is going out of scope anyway
479 /// v.push(4); // no problems
482 /// Since `RefCell` enforces the borrow rules at runtime, `drop()` can
483 /// seemingly release a borrow of one:
486 /// use std::cell::RefCell;
488 /// let x = RefCell::new(1);
490 /// let mut mutable_borrow = x.borrow_mut();
491 /// *mutable_borrow = 1;
493 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
495 /// let borrow = x.borrow();
496 /// println!("{}", *borrow);
499 /// Integers and other types implementing `Copy` are unaffected by `drop()`
502 /// #[derive(Copy, Clone)]
507 /// drop(x); // a copy of `x` is moved and dropped
508 /// drop(y); // a copy of `y` is moved and dropped
510 /// println!("x: {}, y: {}", x, y.0); // still available
514 #[stable(feature = "rust1", since = "1.0.0")]
515 pub fn drop
<T
>(_x
: T
) { }
517 macro_rules
! repeat_u8_as_u32
{
518 ($name
:expr
) => { (($name
as u32) << 24 |
519 ($name
as u32) << 16 |
520 ($name
as u32) << 8 |
523 macro_rules
! repeat_u8_as_u64
{
524 ($name
:expr
) => { ((repeat_u8_as_u32
!($name
) as u64) << 32 |
525 (repeat_u8_as_u32
!($name
) as u64)) }
528 // NOTE: Keep synchronized with values used in librustc_trans::trans::adt.
530 // In particular, the POST_DROP_U8 marker must never equal the
531 // DTOR_NEEDED_U8 marker.
533 // For a while pnkfelix was using 0xc1 here.
534 // But having the sign bit set is a pain, so 0x1d is probably better.
536 // And of course, 0x00 brings back the old world of zero'ing on drop.
537 #[unstable(feature = "filling_drop", issue = "5016")]
538 #[allow(missing_docs)]
539 pub const POST_DROP_U8
: u8 = 0x1d;
540 #[unstable(feature = "filling_drop", issue = "5016")]
541 #[allow(missing_docs)]
542 pub const POST_DROP_U32
: u32 = repeat_u8_as_u32
!(POST_DROP_U8
);
543 #[unstable(feature = "filling_drop", issue = "5016")]
544 #[allow(missing_docs)]
545 pub const POST_DROP_U64
: u64 = repeat_u8_as_u64
!(POST_DROP_U8
);
547 #[cfg(target_pointer_width = "32")]
548 #[unstable(feature = "filling_drop", issue = "5016")]
549 #[allow(missing_docs)]
550 pub const POST_DROP_USIZE
: usize = POST_DROP_U32
as usize;
551 #[cfg(target_pointer_width = "64")]
552 #[unstable(feature = "filling_drop", issue = "5016")]
553 #[allow(missing_docs)]
554 pub const POST_DROP_USIZE
: usize = POST_DROP_U64
as usize;
556 /// Interprets `src` as `&U`, and then reads `src` without moving the contained
559 /// This function will unsafely assume the pointer `src` is valid for
560 /// `sizeof(U)` bytes by transmuting `&T` to `&U` and then reading the `&U`. It
561 /// will also unsafely create a copy of the contained value instead of moving
564 /// It is not a compile-time error if `T` and `U` have different sizes, but it
565 /// is highly encouraged to only invoke this function where `T` and `U` have the
566 /// same size. This function triggers undefined behavior if `U` is larger than
579 /// let foo_slice = [10u8];
582 /// // Copy the data from 'foo_slice' and treat it as a 'Foo'
583 /// let mut foo_struct: Foo = mem::transmute_copy(&foo_slice);
584 /// assert_eq!(foo_struct.bar, 10);
586 /// // Modify the copied data
587 /// foo_struct.bar = 20;
588 /// assert_eq!(foo_struct.bar, 20);
591 /// // The contents of 'foo_slice' should not have changed
592 /// assert_eq!(foo_slice, [10]);
595 #[stable(feature = "rust1", since = "1.0.0")]
596 pub unsafe fn transmute_copy
<T
, U
>(src
: &T
) -> U
{
597 ptr
::read(src
as *const T
as *const U
)