src/libcore/mem.rs

   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.
   4 //
   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.
  10
  11 //! Basic functions for dealing with memory.
  12 //!
  13 //! This module contains functions for querying the size and alignment of
  14 //! types, initializing and manipulating memory.
  15
  16 #![stable(feature = "rust1", since = "1.0.0")]
  17
  18 use marker::Sized;
  19 use intrinsics;
  20 use ptr;
  21
  22 #[stable(feature = "rust1", since = "1.0.0")]
  23 pub use intrinsics::transmute;
  24
  25 /// Leaks a value into the void, consuming ownership and never running its
  26 /// destructor.
  27 ///
  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.
  31 ///
  32 /// There's only a few reasons to use this function. They mainly come
  33 /// up in unsafe code or FFI code.
  34 ///
  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.
  41 ///
  42 /// [swap]: fn.swap.html
  43 /// [ffi]: ../../book/ffi.html
  44 ///
  45 /// # Safety
  46 ///
  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.
  51 ///
  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.
  57 ///
  58 /// # Other forms of Leakage
  59 ///
  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
  62 /// leakage are:
  63 ///
  64 /// * `Rc` and `Arc` cycles
  65 /// * `mpsc::{Sender, Receiver}` cycles (they use `Arc` internally)
  66 /// * Panicking destructors are likely to leak local resources
  67 ///
  68 /// # Example
  69 ///
  70 /// Leak some heap memory by never deallocating it:
  71 ///
  72 /// ```rust
  73 /// use std::mem;
  74 ///
  75 /// let heap_memory = Box::new(3);
  76 /// mem::forget(heap_memory);
  77 /// ```
  78 ///
  79 /// Leak an I/O object, never closing the file:
  80 ///
  81 /// ```rust,no_run
  82 /// use std::mem;
  83 /// use std::fs::File;
  84 ///
  85 /// let file = File::open("foo.txt").unwrap();
  86 /// mem::forget(file);
  87 /// ```
  88 ///
  89 /// The `mem::swap` function uses `mem::forget` to good effect:
  90 ///
  91 /// ```rust
  92 /// use std::mem;
  93 /// use std::ptr;
  94 ///
  95 /// # #[allow(dead_code)]
  96 /// fn swap<T>(x: &mut T, y: &mut T) {
  97 ///     unsafe {
  98 ///         // Give ourselves some scratch space to work with
  99 ///         let mut t: T = mem::uninitialized();
 100 ///
 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);
 105 ///
 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.
 109 ///         mem::forget(t);
 110 ///     }
 111 /// }
 112 /// ```
 113 #[stable(feature = "rust1", since = "1.0.0")]
 114 pub fn forget<T>(t: T) {
 115     unsafe { intrinsics::forget(t) }
 116 }
 117
 118 /// Returns the size of a type in bytes.
 119 ///
 120 /// # Examples
 121 ///
 122 /// ```
 123 /// use std::mem;
 124 ///
 125 /// assert_eq!(4, mem::size_of::<i32>());
 126 /// ```
 127 #[inline]
 128 #[stable(feature = "rust1", since = "1.0.0")]
 129 pub fn size_of<T>() -> usize {
 130     unsafe { intrinsics::size_of::<T>() }
 131 }
 132
 133 /// Returns the size of the given value in bytes.
 134 ///
 135 /// # Examples
 136 ///
 137 /// ```
 138 /// use std::mem;
 139 ///
 140 /// assert_eq!(4, mem::size_of_val(&5i32));
 141 /// ```
 142 #[inline]
 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) }
 146 }
 147
 148 /// Returns the ABI-required minimum alignment of a type
 149 ///
 150 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 151 ///
 152 /// # Examples
 153 ///
 154 /// ```
 155 /// # #![allow(deprecated)]
 156 /// use std::mem;
 157 ///
 158 /// assert_eq!(4, mem::min_align_of::<i32>());
 159 /// ```
 160 #[inline]
 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>() }
 165 }
 166
 167 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 168 ///
 169 /// # Examples
 170 ///
 171 /// ```
 172 /// # #![allow(deprecated)]
 173 /// use std::mem;
 174 ///
 175 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 176 /// ```
 177 #[inline]
 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) }
 182 }
 183
 184 /// Returns the alignment in memory for a type.
 185 ///
 186 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 187 ///
 188 /// # Examples
 189 ///
 190 /// ```
 191 /// use std::mem;
 192 ///
 193 /// assert_eq!(4, mem::align_of::<i32>());
 194 /// ```
 195 #[inline]
 196 #[stable(feature = "rust1", since = "1.0.0")]
 197 pub fn align_of<T>() -> usize {
 198     unsafe { intrinsics::min_align_of::<T>() }
 199 }
 200
 201 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 202 ///
 203 /// # Examples
 204 ///
 205 /// ```
 206 /// use std::mem;
 207 ///
 208 /// assert_eq!(4, mem::align_of_val(&5i32));
 209 /// ```
 210 #[inline]
 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) }
 214 }
 215
 216 /// Creates a value initialized to zero.
 217 ///
 218 /// This function is similar to allocating space for a local variable and zeroing it out (an unsafe
 219 /// operation).
 220 ///
 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.
 224 ///
 225 /// This is useful for FFI functions sometimes, but should generally be avoided.
 226 ///
 227 /// # Examples
 228 ///
 229 /// ```
 230 /// use std::mem;
 231 ///
 232 /// let x: i32 = unsafe { mem::zeroed() };
 233 /// ```
 234 #[inline]
 235 #[stable(feature = "rust1", since = "1.0.0")]
 236 pub unsafe fn zeroed<T>() -> T {
 237     intrinsics::init()
 238 }
 239
 240 /// Creates a value initialized to an unspecified series of bytes.
 241 ///
 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
 245 /// of scope.
 246 ///
 247 /// Some code at one time used the `zeroed` function above to
 248 /// accomplish this goal.
 249 ///
 250 /// This function is expected to be deprecated with the transition
 251 /// to non-zeroing drop.
 252 #[inline]
 253 #[unstable(feature = "filling_drop", issue = "5016")]
 254 pub unsafe fn dropped<T>() -> T {
 255     #[inline(always)]
 256     unsafe fn dropped_impl<T>() -> T { intrinsics::init_dropped() }
 257
 258     dropped_impl()
 259 }
 260
 261 /// Bypasses Rust's normal memory-initialization checks by pretending to
 262 /// produce a value of type T, while doing nothing at all.
 263 ///
 264 /// **This is incredibly dangerous, and should not be done lightly. Deeply
 265 /// consider initializing your memory with a default value instead.**
 266 ///
 267 /// This is useful for FFI functions and initializing arrays sometimes,
 268 /// but should generally be avoided.
 269 ///
 270 /// # Undefined Behavior
 271 ///
 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.
 275 ///
 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.
 283 ///
 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.
 287 ///
 288 /// # Examples
 289 ///
 290 /// Here's how to safely initialize an array of `Vec`s.
 291 ///
 292 /// ```
 293 /// use std::mem;
 294 /// use std::ptr;
 295 ///
 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];
 302 ///
 303 /// unsafe {
 304 ///     // So we need to do this to initialize it.
 305 ///     data = mem::uninitialized();
 306 ///
 307 ///     // DANGER ZONE: if anything panics or otherwise
 308 ///     // incorrectly reads the array here, we will have
 309 ///     // Undefined Behavior.
 310 ///
 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!
 317 ///         //
 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());
 322 ///     }
 323 ///
 324 ///     // SAFE ZONE: everything is initialized.
 325 /// }
 326 ///
 327 /// println!("{:?}", &data[0]);
 328 /// ```
 329 ///
 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
 334 /// allocation:
 335 ///
 336 /// ```
 337 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
 338 /// println!("{:?}", &data[0]);
 339 /// ```
 340 #[inline]
 341 #[stable(feature = "rust1", since = "1.0.0")]
 342 pub unsafe fn uninitialized<T>() -> T {
 343     intrinsics::uninit()
 344 }
 345
 346 /// Swap the values at two mutable locations of the same type, without deinitializing or copying
 347 /// either one.
 348 ///
 349 /// # Examples
 350 ///
 351 /// ```
 352 /// use std::mem;
 353 ///
 354 /// let x = &mut 5;
 355 /// let y = &mut 42;
 356 ///
 357 /// mem::swap(x, y);
 358 ///
 359 /// assert_eq!(42, *x);
 360 /// assert_eq!(5, *y);
 361 /// ```
 362 #[inline]
 363 #[stable(feature = "rust1", since = "1.0.0")]
 364 pub fn swap<T>(x: &mut T, y: &mut T) {
 365     unsafe {
 366         // Give ourselves some scratch space to work with
 367         let mut t: T = uninitialized();
 368
 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);
 373
 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.
 377         forget(t);
 378     }
 379 }
 380
 381 /// Replaces the value at a mutable location with a new one, returning the old value, without
 382 /// deinitializing or copying either one.
 383 ///
 384 /// This is primarily used for transferring and swapping ownership of a value in a mutable
 385 /// location.
 386 ///
 387 /// # Examples
 388 ///
 389 /// A simple example:
 390 ///
 391 /// ```
 392 /// use std::mem;
 393 ///
 394 /// let mut v: Vec<i32> = Vec::new();
 395 ///
 396 /// mem::replace(&mut v, Vec::new());
 397 /// ```
 398 ///
 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:
 401 ///
 402 /// ```rust,ignore
 403 /// struct Buffer<T> { buf: Vec<T> }
 404 ///
 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();
 410 ///         buf
 411 ///     }
 412 /// }
 413 /// ```
 414 ///
 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:
 418 ///
 419 /// ```
 420 /// # #![allow(dead_code)]
 421 /// use std::mem;
 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())
 426 ///     }
 427 /// }
 428 /// ```
 429 #[inline]
 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);
 433     src
 434 }
 435
 436 /// Disposes of a value.
 437 ///
 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.
 440 ///
 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.
 445 ///
 446 /// # Examples
 447 ///
 448 /// Basic usage:
 449 ///
 450 /// ```
 451 /// let v = vec![1, 2, 3];
 452 ///
 453 /// drop(v); // explicitly drop the vector
 454 /// ```
 455 ///
 456 /// Borrows are based on lexical scope, so this produces an error:
 457 ///
 458 /// ```ignore
 459 /// let mut v = vec![1, 2, 3];
 460 /// let x = &v[0];
 461 ///
 462 /// drop(x); // explicitly drop the reference, but the borrow still exists
 463 ///
 464 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
 465 ///            // borrowed as immutable
 466 /// ```
 467 ///
 468 /// An inner scope is needed to fix this:
 469 ///
 470 /// ```
 471 /// let mut v = vec![1, 2, 3];
 472 ///
 473 /// {
 474 ///     let x = &v[0];
 475 ///
 476 ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
 477 /// }
 478 ///
 479 /// v.push(4); // no problems
 480 /// ```
 481 ///
 482 /// Since `RefCell` enforces the borrow rules at runtime, `drop()` can
 483 /// seemingly release a borrow of one:
 484 ///
 485 /// ```
 486 /// use std::cell::RefCell;
 487 ///
 488 /// let x = RefCell::new(1);
 489 ///
 490 /// let mut mutable_borrow = x.borrow_mut();
 491 /// *mutable_borrow = 1;
 492 ///
 493 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
 494 ///
 495 /// let borrow = x.borrow();
 496 /// println!("{}", *borrow);
 497 /// ```
 498 ///
 499 /// Integers and other types implementing `Copy` are unaffected by `drop()`
 500 ///
 501 /// ```
 502 /// #[derive(Copy, Clone)]
 503 /// struct Foo(u8);
 504 ///
 505 /// let x = 1;
 506 /// let y = Foo(2);
 507 /// drop(x); // a copy of `x` is moved and dropped
 508 /// drop(y); // a copy of `y` is moved and dropped
 509 ///
 510 /// println!("x: {}, y: {}", x, y.0); // still available
 511 /// ```
 512 ///
 513 #[inline]
 514 #[stable(feature = "rust1", since = "1.0.0")]
 515 pub fn drop<T>(_x: T) { }
 516
 517 macro_rules! repeat_u8_as_u32 {
 518     ($name:expr) => { (($name as u32) << 24 |
 519                        ($name as u32) << 16 |
 520                        ($name as u32) <<  8 |
 521                        ($name as u32)) }
 522 }
 523 macro_rules! repeat_u8_as_u64 {
 524     ($name:expr) => { ((repeat_u8_as_u32!($name) as u64) << 32 |
 525                        (repeat_u8_as_u32!($name) as u64)) }
 526 }
 527
 528 // NOTE: Keep synchronized with values used in librustc_trans::trans::adt.
 529 //
 530 // In particular, the POST_DROP_U8 marker must never equal the
 531 // DTOR_NEEDED_U8 marker.
 532 //
 533 // For a while pnkfelix was using 0xc1 here.
 534 // But having the sign bit set is a pain, so 0x1d is probably better.
 535 //
 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);
 546
 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;
 555
 556 /// Interprets `src` as `&U`, and then reads `src` without moving the contained
 557 /// value.
 558 ///
 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
 562 /// out of `src`.
 563 ///
 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
 567 /// `T`.
 568 ///
 569 /// # Examples
 570 ///
 571 /// ```
 572 /// use std::mem;
 573 ///
 574 /// #[repr(packed)]
 575 /// struct Foo {
 576 ///     bar: u8,
 577 /// }
 578 ///
 579 /// let foo_slice = [10u8];
 580 ///
 581 /// unsafe {
 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);
 585 ///
 586 ///     // Modify the copied data
 587 ///     foo_struct.bar = 20;
 588 ///     assert_eq!(foo_struct.bar, 20);
 589 /// }
 590 ///
 591 /// // The contents of 'foo_slice' should not have changed
 592 /// assert_eq!(foo_slice, [10]);
 593 /// ```
 594 #[inline]
 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)
 598 }