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 /// # Safety
  33 ///
  34 /// This function is not marked as `unsafe` as Rust does not guarantee that the
  35 /// `Drop` implementation for a value will always run. Note, however, that
  36 /// leaking resources such as memory or I/O objects is likely not desired, so
  37 /// this function is only recommended for specialized use cases.
  38 ///
  39 /// The safety of this function implies that when writing `unsafe` code
  40 /// yourself care must be taken when leveraging a destructor that is required to
  41 /// run to preserve memory safety. There are known situations where the
  42 /// destructor may not run (such as if ownership of the object with the
  43 /// destructor is returned) which must be taken into account.
  44 ///
  45 /// # Other forms of Leakage
  46 ///
  47 /// It's important to point out that this function is not the only method by
  48 /// which a value can be leaked in safe Rust code. Other known sources of
  49 /// leakage are:
  50 ///
  51 /// * `Rc` and `Arc` cycles
  52 /// * `mpsc::{Sender, Receiver}` cycles (they use `Arc` internally)
  53 /// * Panicking destructors are likely to leak local resources
  54 ///
  55 /// # When To Use
  56 ///
  57 /// There's only a few reasons to use this function. They mainly come
  58 /// up in unsafe code or FFI code.
  59 ///
  60 /// * You have an uninitialized value, perhaps for performance reasons, and
  61 ///   need to prevent the destructor from running on it.
  62 /// * You have two copies of a value (like `std::mem::swap`), but need the
  63 ///   destructor to only run once to prevent a double free.
  64 /// * Transferring resources across FFI boundries.
  65 ///
  66 /// # Example
  67 ///
  68 /// Leak some heap memory by never deallocating it.
  69 ///
  70 /// ```rust
  71 /// use std::mem;
  72 ///
  73 /// let heap_memory = Box::new(3);
  74 /// mem::forget(heap_memory);
  75 /// ```
  76 ///
  77 /// Leak an I/O object, never closing the file.
  78 ///
  79 /// ```rust,no_run
  80 /// use std::mem;
  81 /// use std::fs::File;
  82 ///
  83 /// let file = File::open("foo.txt").unwrap();
  84 /// mem::forget(file);
  85 /// ```
  86 ///
  87 /// The swap function uses forget to good effect.
  88 ///
  89 /// ```rust
  90 /// use std::mem;
  91 /// use std::ptr;
  92 ///
  93 /// fn swap<T>(x: &mut T, y: &mut T) {
  94 ///     unsafe {
  95 ///         // Give ourselves some scratch space to work with
  96 ///         let mut t: T = mem::uninitialized();
  97 ///
  98 ///         // Perform the swap, `&mut` pointers never alias
  99 ///         ptr::copy_nonoverlapping(&*x, &mut t, 1);
 100 ///         ptr::copy_nonoverlapping(&*y, x, 1);
 101 ///         ptr::copy_nonoverlapping(&t, y, 1);
 102 ///
 103 ///         // y and t now point to the same thing, but we need to completely
 104 ///         // forget `t` because we do not want to run the destructor for `T`
 105 ///         // on its value, which is still owned somewhere outside this function.
 106 ///         mem::forget(t);
 107 ///     }
 108 /// }
 109 /// ```
 110 #[stable(feature = "rust1", since = "1.0.0")]
 111 pub fn forget<T>(t: T) {
 112     unsafe { intrinsics::forget(t) }
 113 }
 114
 115 /// Returns the size of a type in bytes.
 116 ///
 117 /// # Examples
 118 ///
 119 /// ```
 120 /// use std::mem;
 121 ///
 122 /// assert_eq!(4, mem::size_of::<i32>());
 123 /// ```
 124 #[inline]
 125 #[stable(feature = "rust1", since = "1.0.0")]
 126 pub fn size_of<T>() -> usize {
 127     unsafe { intrinsics::size_of::<T>() }
 128 }
 129
 130 /// Returns the size of the type that `val` points to in bytes.
 131 ///
 132 /// # Examples
 133 ///
 134 /// ```
 135 /// use std::mem;
 136 ///
 137 /// assert_eq!(4, mem::size_of_val(&5i32));
 138 /// ```
 139 #[inline]
 140 #[stable(feature = "rust1", since = "1.0.0")]
 141 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 142     unsafe { intrinsics::size_of_val(val) }
 143 }
 144
 145 /// Returns the ABI-required minimum alignment of a type
 146 ///
 147 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 148 ///
 149 /// # Examples
 150 ///
 151 /// ```
 152 /// use std::mem;
 153 ///
 154 /// assert_eq!(4, mem::min_align_of::<i32>());
 155 /// ```
 156 #[inline]
 157 #[stable(feature = "rust1", since = "1.0.0")]
 158 #[deprecated(reason = "use `align_of` instead", since = "1.2.0")]
 159 pub fn min_align_of<T>() -> usize {
 160     unsafe { intrinsics::min_align_of::<T>() }
 161 }
 162
 163 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 164 ///
 165 /// # Examples
 166 ///
 167 /// ```
 168 /// use std::mem;
 169 ///
 170 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 171 /// ```
 172 #[inline]
 173 #[stable(feature = "rust1", since = "1.0.0")]
 174 #[deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
 175 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 176     unsafe { intrinsics::min_align_of_val(val) }
 177 }
 178
 179 /// Returns the alignment in memory for a type.
 180 ///
 181 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 182 ///
 183 /// # Examples
 184 ///
 185 /// ```
 186 /// use std::mem;
 187 ///
 188 /// assert_eq!(4, mem::align_of::<i32>());
 189 /// ```
 190 #[inline]
 191 #[stable(feature = "rust1", since = "1.0.0")]
 192 pub fn align_of<T>() -> usize {
 193     unsafe { intrinsics::min_align_of::<T>() }
 194 }
 195
 196 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 197 ///
 198 /// # Examples
 199 ///
 200 /// ```
 201 /// use std::mem;
 202 ///
 203 /// assert_eq!(4, mem::align_of_val(&5i32));
 204 /// ```
 205 #[inline]
 206 #[stable(feature = "rust1", since = "1.0.0")]
 207 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
 208     unsafe { intrinsics::min_align_of_val(val) }
 209 }
 210
 211 /// Creates a value initialized to zero.
 212 ///
 213 /// This function is similar to allocating space for a local variable and zeroing it out (an unsafe
 214 /// operation).
 215 ///
 216 /// Care must be taken when using this function, if the type `T` has a destructor and the value
 217 /// falls out of scope (due to unwinding or returning) before being initialized, then the
 218 /// destructor will run on zeroed data, likely leading to crashes.
 219 ///
 220 /// This is useful for FFI functions sometimes, but should generally be avoided.
 221 ///
 222 /// # Examples
 223 ///
 224 /// ```
 225 /// use std::mem;
 226 ///
 227 /// let x: i32 = unsafe { mem::zeroed() };
 228 /// ```
 229 #[inline]
 230 #[stable(feature = "rust1", since = "1.0.0")]
 231 pub unsafe fn zeroed<T>() -> T {
 232     intrinsics::init()
 233 }
 234
 235 /// Creates a value initialized to an unspecified series of bytes.
 236 ///
 237 /// The byte sequence usually indicates that the value at the memory
 238 /// in question has been dropped. Thus, *if* T carries a drop flag,
 239 /// any associated destructor will not be run when the value falls out
 240 /// of scope.
 241 ///
 242 /// Some code at one time used the `zeroed` function above to
 243 /// accomplish this goal.
 244 ///
 245 /// This function is expected to be deprecated with the transition
 246 /// to non-zeroing drop.
 247 #[inline]
 248 #[unstable(feature = "filling_drop")]
 249 pub unsafe fn dropped<T>() -> T {
 250     #[inline(always)]
 251     unsafe fn dropped_impl<T>() -> T { intrinsics::init_dropped() }
 252
 253     dropped_impl()
 254 }
 255
 256 /// Bypasses Rust's normal memory-initialization checks by pretending to
 257 /// produce a value of type T, while doing nothing at all.
 258 ///
 259 /// **This is incredibly dangerous, and should not be done lightly. Deeply
 260 /// consider initializing your memory with a default value instead.**
 261 ///
 262 /// This is useful for FFI functions and initializing arrays sometimes,
 263 /// but should generally be avoided.
 264 ///
 265 /// # Undefined Behaviour
 266 ///
 267 /// It is Undefined Behaviour to read uninitialized memory. Even just an
 268 /// uninitialized boolean. For instance, if you branch on the value of such
 269 /// a boolean your program may take one, both, or neither of the branches.
 270 ///
 271 /// Note that this often also includes *writing* to the uninitialized value.
 272 /// Rust believes the value is initialized, and will therefore try to Drop
 273 /// the uninitialized value and its fields if you try to overwrite the memory
 274 /// in a normal manner. The only way to safely initialize an arbitrary
 275 /// uninitialized value is with one of the `ptr` functions: `write`, `copy`, or
 276 /// `copy_nonoverlapping`. This isn't necessary if `T` is a primitive
 277 /// or otherwise only contains types that don't implement Drop.
 278 ///
 279 /// If this value *does* need some kind of Drop, it must be initialized before
 280 /// it goes out of scope (and therefore would be dropped). Note that this
 281 /// includes a `panic` occurring and unwinding the stack suddenly.
 282 ///
 283 /// # Examples
 284 ///
 285 /// Here's how to safely initialize an array of `Vec`s.
 286 ///
 287 /// ```
 288 /// use std::mem;
 289 /// use std::ptr;
 290 ///
 291 /// // Only declare the array. This safely leaves it
 292 /// // uninitialized in a way that Rust will track for us.
 293 /// // However we can't initialize it element-by-element
 294 /// // safely, and we can't use the `[value; 1000]`
 295 /// // constructor because it only works with `Copy` data.
 296 /// let mut data: [Vec<u32>; 1000];
 297 ///
 298 /// unsafe {
 299 ///     // So we need to do this to initialize it.
 300 ///     data = mem::uninitialized();
 301 ///
 302 ///     // DANGER ZONE: if anything panics or otherwise
 303 ///     // incorrectly reads the array here, we will have
 304 ///     // Undefined Behaviour.
 305 ///
 306 ///     // It's ok to mutably iterate the data, since this
 307 ///     // doesn't involve reading it at all.
 308 ///     // (ptr and len are statically known for arrays)
 309 ///     for elem in &mut data[..] {
 310 ///         // *elem = Vec::new() would try to drop the
 311 ///         // uninitialized memory at `elem` -- bad!
 312 ///         //
 313 ///         // Vec::new doesn't allocate or do really
 314 ///         // anything. It's only safe to call here
 315 ///         // because we know it won't panic.
 316 ///         ptr::write(elem, Vec::new());
 317 ///     }
 318 ///
 319 ///     // SAFE ZONE: everything is initialized.
 320 /// }
 321 ///
 322 /// println!("{:?}", &data[0]);
 323 /// ```
 324 ///
 325 /// This example emphasizes exactly how delicate and dangerous doing this is.
 326 /// Note that the `vec!` macro *does* let you initialize every element with a
 327 /// value that is only `Clone`, so the following is semantically equivalent and
 328 /// vastly less dangerous, as long as you can live with an extra heap
 329 /// allocation:
 330 ///
 331 /// ```
 332 /// let data: Vec<Vec<u32>> = vec![Vec::new(); 1000];
 333 /// println!("{:?}", &data[0]);
 334 /// ```
 335 #[inline]
 336 #[stable(feature = "rust1", since = "1.0.0")]
 337 pub unsafe fn uninitialized<T>() -> T {
 338     intrinsics::uninit()
 339 }
 340
 341 /// Swap the values at two mutable locations of the same type, without deinitialising or copying
 342 /// either one.
 343 ///
 344 /// # Examples
 345 ///
 346 /// ```
 347 /// use std::mem;
 348 ///
 349 /// let x = &mut 5;
 350 /// let y = &mut 42;
 351 ///
 352 /// mem::swap(x, y);
 353 ///
 354 /// assert_eq!(42, *x);
 355 /// assert_eq!(5, *y);
 356 /// ```
 357 #[inline]
 358 #[stable(feature = "rust1", since = "1.0.0")]
 359 pub fn swap<T>(x: &mut T, y: &mut T) {
 360     unsafe {
 361         // Give ourselves some scratch space to work with
 362         let mut t: T = uninitialized();
 363
 364         // Perform the swap, `&mut` pointers never alias
 365         ptr::copy_nonoverlapping(&*x, &mut t, 1);
 366         ptr::copy_nonoverlapping(&*y, x, 1);
 367         ptr::copy_nonoverlapping(&t, y, 1);
 368
 369         // y and t now point to the same thing, but we need to completely
 370         // forget `t` because we do not want to run the destructor for `T`
 371         // on its value, which is still owned somewhere outside this function.
 372         forget(t);
 373     }
 374 }
 375
 376 /// Replaces the value at a mutable location with a new one, returning the old value, without
 377 /// deinitialising or copying either one.
 378 ///
 379 /// This is primarily used for transferring and swapping ownership of a value in a mutable
 380 /// location.
 381 ///
 382 /// # Examples
 383 ///
 384 /// A simple example:
 385 ///
 386 /// ```
 387 /// use std::mem;
 388 ///
 389 /// let mut v: Vec<i32> = Vec::new();
 390 ///
 391 /// mem::replace(&mut v, Vec::new());
 392 /// ```
 393 ///
 394 /// This function allows consumption of one field of a struct by replacing it with another value.
 395 /// The normal approach doesn't always work:
 396 ///
 397 /// ```rust,ignore
 398 /// struct Buffer<T> { buf: Vec<T> }
 399 ///
 400 /// impl<T> Buffer<T> {
 401 ///     fn get_and_reset(&mut self) -> Vec<T> {
 402 ///         // error: cannot move out of dereference of `&mut`-pointer
 403 ///         let buf = self.buf;
 404 ///         self.buf = Vec::new();
 405 ///         buf
 406 ///     }
 407 /// }
 408 /// ```
 409 ///
 410 /// Note that `T` does not necessarily implement `Clone`, so it can't even clone and reset
 411 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
 412 /// `self`, allowing it to be returned:
 413 ///
 414 /// ```
 415 /// use std::mem;
 416 /// # struct Buffer<T> { buf: Vec<T> }
 417 /// impl<T> Buffer<T> {
 418 ///     fn get_and_reset(&mut self) -> Vec<T> {
 419 ///         mem::replace(&mut self.buf, Vec::new())
 420 ///     }
 421 /// }
 422 /// ```
 423 #[inline]
 424 #[stable(feature = "rust1", since = "1.0.0")]
 425 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
 426     swap(dest, &mut src);
 427     src
 428 }
 429
 430 /// Disposes of a value.
 431 ///
 432 /// While this does call the argument's implementation of `Drop`, it will not
 433 /// release any borrows, as borrows are based on lexical scope.
 434 ///
 435 /// # Examples
 436 ///
 437 /// Basic usage:
 438 ///
 439 /// ```
 440 /// let v = vec![1, 2, 3];
 441 ///
 442 /// drop(v); // explicitly drop the vector
 443 /// ```
 444 ///
 445 /// Borrows are based on lexical scope, so this produces an error:
 446 ///
 447 /// ```ignore
 448 /// let mut v = vec![1, 2, 3];
 449 /// let x = &v[0];
 450 ///
 451 /// drop(x); // explicitly drop the reference, but the borrow still exists
 452 ///
 453 /// v.push(4); // error: cannot borrow `v` as mutable because it is also
 454 ///            // borrowed as immutable
 455 /// ```
 456 ///
 457 /// An inner scope is needed to fix this:
 458 ///
 459 /// ```
 460 /// let mut v = vec![1, 2, 3];
 461 ///
 462 /// {
 463 ///     let x = &v[0];
 464 ///
 465 ///     drop(x); // this is now redundant, as `x` is going out of scope anyway
 466 /// }
 467 ///
 468 /// v.push(4); // no problems
 469 /// ```
 470 ///
 471 /// Since `RefCell` enforces the borrow rules at runtime, `drop()` can
 472 /// seemingly release a borrow of one:
 473 ///
 474 /// ```
 475 /// use std::cell::RefCell;
 476 ///
 477 /// let x = RefCell::new(1);
 478 ///
 479 /// let mut mutable_borrow = x.borrow_mut();
 480 /// *mutable_borrow = 1;
 481 ///
 482 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
 483 ///
 484 /// let borrow = x.borrow();
 485 /// println!("{}", *borrow);
 486 /// ```
 487 #[inline]
 488 #[stable(feature = "rust1", since = "1.0.0")]
 489 pub fn drop<T>(_x: T) { }
 490
 491 macro_rules! repeat_u8_as_u32 {
 492     ($name:expr) => { (($name as u32) << 24 |
 493                        ($name as u32) << 16 |
 494                        ($name as u32) <<  8 |
 495                        ($name as u32)) }
 496 }
 497 macro_rules! repeat_u8_as_u64 {
 498     ($name:expr) => { ((repeat_u8_as_u32!($name) as u64) << 32 |
 499                        (repeat_u8_as_u32!($name) as u64)) }
 500 }
 501
 502 // NOTE: Keep synchronized with values used in librustc_trans::trans::adt.
 503 //
 504 // In particular, the POST_DROP_U8 marker must never equal the
 505 // DTOR_NEEDED_U8 marker.
 506 //
 507 // For a while pnkfelix was using 0xc1 here.
 508 // But having the sign bit set is a pain, so 0x1d is probably better.
 509 //
 510 // And of course, 0x00 brings back the old world of zero'ing on drop.
 511 #[unstable(feature = "filling_drop")]
 512 #[allow(missing_docs)]
 513 pub const POST_DROP_U8: u8 = 0x1d;
 514 #[unstable(feature = "filling_drop")]
 515 #[allow(missing_docs)]
 516 pub const POST_DROP_U32: u32 = repeat_u8_as_u32!(POST_DROP_U8);
 517 #[unstable(feature = "filling_drop")]
 518 #[allow(missing_docs)]
 519 pub const POST_DROP_U64: u64 = repeat_u8_as_u64!(POST_DROP_U8);
 520
 521 #[cfg(target_pointer_width = "32")]
 522 #[unstable(feature = "filling_drop")]
 523 #[allow(missing_docs)]
 524 pub const POST_DROP_USIZE: usize = POST_DROP_U32 as usize;
 525 #[cfg(target_pointer_width = "64")]
 526 #[unstable(feature = "filling_drop")]
 527 #[allow(missing_docs)]
 528 pub const POST_DROP_USIZE: usize = POST_DROP_U64 as usize;
 529
 530 /// Interprets `src` as `&U`, and then reads `src` without moving the contained
 531 /// value.
 532 ///
 533 /// This function will unsafely assume the pointer `src` is valid for
 534 /// `sizeof(U)` bytes by transmuting `&T` to `&U` and then reading the `&U`. It
 535 /// will also unsafely create a copy of the contained value instead of moving
 536 /// out of `src`.
 537 ///
 538 /// It is not a compile-time error if `T` and `U` have different sizes, but it
 539 /// is highly encouraged to only invoke this function where `T` and `U` have the
 540 /// same size. This function triggers undefined behavior if `U` is larger than
 541 /// `T`.
 542 ///
 543 /// # Examples
 544 ///
 545 /// ```
 546 /// use std::mem;
 547 ///
 548 /// let one = unsafe { mem::transmute_copy(&1) };
 549 ///
 550 /// assert_eq!(1, one);
 551 /// ```
 552 #[inline]
 553 #[stable(feature = "rust1", since = "1.0.0")]
 554 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
 555     // FIXME(#23542) Replace with type ascription.
 556     #![allow(trivial_casts)]
 557     ptr::read(src as *const T as *const U)
 558 }
 559
 560 /// Transforms lifetime of the second pointer to match the first.
 561 #[inline]
 562 #[unstable(feature = "copy_lifetime",
 563            reason = "this function may be removed in the future due to its \
 564                      questionable utility")]
 565 #[deprecated(since = "1.2.0",
 566              reason = "unclear that this function buys more safety and \
 567                        lifetimes are generally not handled as such in unsafe \
 568                        code today")]
 569 pub unsafe fn copy_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
 570                                                         ptr: &T) -> &'a T {
 571     transmute(ptr)
 572 }
 573
 574 /// Transforms lifetime of the second mutable pointer to match the first.
 575 #[inline]
 576 #[unstable(feature = "copy_lifetime",
 577            reason = "this function may be removed in the future due to its \
 578                      questionable utility")]
 579 #[deprecated(since = "1.2.0",
 580              reason = "unclear that this function buys more safety and \
 581                        lifetimes are generally not handled as such in unsafe \
 582                        code today")]
 583 pub unsafe fn copy_mut_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
 584                                                                ptr: &mut T)
 585                                                               -> &'a mut T
 586 {
 587     transmute(ptr)
 588 }