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 /// # Example
  56 ///
  57 /// ```rust,no_run
  58 /// use std::mem;
  59 /// use std::fs::File;
  60 ///
  61 /// // Leak some heap memory by never deallocating it
  62 /// let heap_memory = Box::new(3);
  63 /// mem::forget(heap_memory);
  64 ///
  65 /// // Leak an I/O object, never closing the file
  66 /// let file = File::open("foo.txt").unwrap();
  67 /// mem::forget(file);
  68 /// ```
  69 #[stable(feature = "rust1", since = "1.0.0")]
  70 pub fn forget<T>(t: T) {
  71     unsafe { intrinsics::forget(t) }
  72 }
  73
  74 /// Returns the size of a type in bytes.
  75 ///
  76 /// # Examples
  77 ///
  78 /// ```
  79 /// use std::mem;
  80 ///
  81 /// assert_eq!(4, mem::size_of::<i32>());
  82 /// ```
  83 #[inline]
  84 #[stable(feature = "rust1", since = "1.0.0")]
  85 pub fn size_of<T>() -> usize {
  86     unsafe { intrinsics::size_of::<T>() }
  87 }
  88
  89 /// Returns the size of the type that `val` points to in bytes.
  90 ///
  91 /// # Examples
  92 ///
  93 /// ```
  94 /// use std::mem;
  95 ///
  96 /// assert_eq!(4, mem::size_of_val(&5i32));
  97 /// ```
  98 #[cfg(not(stage0))]
  99 #[inline]
 100 #[stable(feature = "rust1", since = "1.0.0")]
 101 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
 102     unsafe { intrinsics::size_of_val(val) }
 103 }
 104
 105 /// Returns the size of the type that `_val` points to in bytes.
 106 ///
 107 /// # Examples
 108 ///
 109 /// ```
 110 /// use std::mem;
 111 ///
 112 /// assert_eq!(4, mem::size_of_val(&5i32));
 113 /// ```
 114 #[cfg(stage0)]
 115 #[inline]
 116 #[stable(feature = "rust1", since = "1.0.0")]
 117 pub fn size_of_val<T>(_val: &T) -> usize {
 118     size_of::<T>()
 119 }
 120
 121 /// Returns the ABI-required minimum alignment of a type
 122 ///
 123 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
 124 ///
 125 /// # Examples
 126 ///
 127 /// ```
 128 /// use std::mem;
 129 ///
 130 /// assert_eq!(4, mem::min_align_of::<i32>());
 131 /// ```
 132 #[inline]
 133 #[stable(feature = "rust1", since = "1.0.0")]
 134 pub fn min_align_of<T>() -> usize {
 135     unsafe { intrinsics::min_align_of::<T>() }
 136 }
 137
 138 /// Returns the ABI-required minimum alignment of the type of the value that `val` points to
 139 ///
 140 /// # Examples
 141 ///
 142 /// ```
 143 /// use std::mem;
 144 ///
 145 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 146 /// ```
 147 #[cfg(not(stage0))]
 148 #[inline]
 149 #[stable(feature = "rust1", since = "1.0.0")]
 150 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
 151     unsafe { intrinsics::min_align_of_val(val) }
 152 }
 153
 154 /// Returns the ABI-required minimum alignment of the type of the value that `_val` points to
 155 ///
 156 /// # Examples
 157 ///
 158 /// ```
 159 /// use std::mem;
 160 ///
 161 /// assert_eq!(4, mem::min_align_of_val(&5i32));
 162 /// ```
 163 #[cfg(stage0)]
 164 #[inline]
 165 #[stable(feature = "rust1", since = "1.0.0")]
 166 pub fn min_align_of_val<T>(_val: &T) -> usize {
 167     min_align_of::<T>()
 168 }
 169
 170 /// Returns the alignment in memory for a type.
 171 ///
 172 /// This function will return the alignment, in bytes, of a type in memory. If the alignment
 173 /// returned is adhered to, then the type is guaranteed to function properly.
 174 ///
 175 /// # Examples
 176 ///
 177 /// ```
 178 /// use std::mem;
 179 ///
 180 /// assert_eq!(4, mem::align_of::<i32>());
 181 /// ```
 182 #[inline]
 183 #[stable(feature = "rust1", since = "1.0.0")]
 184 pub fn align_of<T>() -> usize {
 185     // We use the preferred alignment as the default alignment for a type. This
 186     // appears to be what clang migrated towards as well:
 187     //
 188     // http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20110725/044411.html
 189     unsafe { intrinsics::pref_align_of::<T>() }
 190 }
 191
 192 /// Returns the alignment of the type of the value that `_val` points to.
 193 ///
 194 /// This is similar to `align_of`, but function will properly handle types such as trait objects
 195 /// (in the future), returning the alignment for an arbitrary value at runtime.
 196 ///
 197 /// # Examples
 198 ///
 199 /// ```
 200 /// use std::mem;
 201 ///
 202 /// assert_eq!(4, mem::align_of_val(&5i32));
 203 /// ```
 204 #[inline]
 205 #[stable(feature = "rust1", since = "1.0.0")]
 206 pub fn align_of_val<T>(_val: &T) -> usize {
 207     align_of::<T>()
 208 }
 209
 210 /// Creates a value initialized to zero.
 211 ///
 212 /// This function is similar to allocating space for a local variable and zeroing it out (an unsafe
 213 /// operation).
 214 ///
 215 /// Care must be taken when using this function, if the type `T` has a destructor and the value
 216 /// falls out of scope (due to unwinding or returning) before being initialized, then the
 217 /// destructor will run on zeroed data, likely leading to crashes.
 218 ///
 219 /// This is useful for FFI functions sometimes, but should generally be avoided.
 220 ///
 221 /// # Examples
 222 ///
 223 /// ```
 224 /// use std::mem;
 225 ///
 226 /// let x: i32 = unsafe { mem::zeroed() };
 227 /// ```
 228 #[inline]
 229 #[stable(feature = "rust1", since = "1.0.0")]
 230 pub unsafe fn zeroed<T>() -> T {
 231     intrinsics::init()
 232 }
 233
 234 /// Creates a value initialized to an unspecified series of bytes.
 235 ///
 236 /// The byte sequence usually indicates that the value at the memory
 237 /// in question has been dropped. Thus, *if* T carries a drop flag,
 238 /// any associated destructor will not be run when the value falls out
 239 /// of scope.
 240 ///
 241 /// Some code at one time used the `zeroed` function above to
 242 /// accomplish this goal.
 243 ///
 244 /// This function is expected to be deprecated with the transition
 245 /// to non-zeroing drop.
 246 #[inline]
 247 #[unstable(feature = "filling_drop")]
 248 pub unsafe fn dropped<T>() -> T {
 249     #[inline(always)]
 250     unsafe fn dropped_impl<T>() -> T { intrinsics::init_dropped() }
 251
 252     dropped_impl()
 253 }
 254
 255 /// Creates an uninitialized value.
 256 ///
 257 /// Care must be taken when using this function, if the type `T` has a destructor and the value
 258 /// falls out of scope (due to unwinding or returning) before being initialized, then the
 259 /// destructor will run on uninitialized data, likely leading to crashes.
 260 ///
 261 /// This is useful for FFI functions sometimes, but should generally be avoided.
 262 ///
 263 /// # Examples
 264 ///
 265 /// ```
 266 /// use std::mem;
 267 ///
 268 /// let x: i32 = unsafe { mem::uninitialized() };
 269 /// ```
 270 #[inline]
 271 #[stable(feature = "rust1", since = "1.0.0")]
 272 pub unsafe fn uninitialized<T>() -> T {
 273     intrinsics::uninit()
 274 }
 275
 276 /// Swap the values at two mutable locations of the same type, without deinitialising or copying
 277 /// either one.
 278 ///
 279 /// # Examples
 280 ///
 281 /// ```
 282 /// use std::mem;
 283 ///
 284 /// let x = &mut 5;
 285 /// let y = &mut 42;
 286 ///
 287 /// mem::swap(x, y);
 288 ///
 289 /// assert_eq!(42, *x);
 290 /// assert_eq!(5, *y);
 291 /// ```
 292 #[inline]
 293 #[stable(feature = "rust1", since = "1.0.0")]
 294 pub fn swap<T>(x: &mut T, y: &mut T) {
 295     unsafe {
 296         // Give ourselves some scratch space to work with
 297         let mut t: T = uninitialized();
 298
 299         // Perform the swap, `&mut` pointers never alias
 300         ptr::copy_nonoverlapping(&*x, &mut t, 1);
 301         ptr::copy_nonoverlapping(&*y, x, 1);
 302         ptr::copy_nonoverlapping(&t, y, 1);
 303
 304         // y and t now point to the same thing, but we need to completely forget `t`
 305         // because it's no longer relevant.
 306         forget(t);
 307     }
 308 }
 309
 310 /// Replaces the value at a mutable location with a new one, returning the old value, without
 311 /// deinitialising or copying either one.
 312 ///
 313 /// This is primarily used for transferring and swapping ownership of a value in a mutable
 314 /// location.
 315 ///
 316 /// # Examples
 317 ///
 318 /// A simple example:
 319 ///
 320 /// ```
 321 /// use std::mem;
 322 ///
 323 /// let mut v: Vec<i32> = Vec::new();
 324 ///
 325 /// mem::replace(&mut v, Vec::new());
 326 /// ```
 327 ///
 328 /// This function allows consumption of one field of a struct by replacing it with another value.
 329 /// The normal approach doesn't always work:
 330 ///
 331 /// ```rust,ignore
 332 /// struct Buffer<T> { buf: Vec<T> }
 333 ///
 334 /// impl<T> Buffer<T> {
 335 ///     fn get_and_reset(&mut self) -> Vec<T> {
 336 ///         // error: cannot move out of dereference of `&mut`-pointer
 337 ///         let buf = self.buf;
 338 ///         self.buf = Vec::new();
 339 ///         buf
 340 ///     }
 341 /// }
 342 /// ```
 343 ///
 344 /// Note that `T` does not necessarily implement `Clone`, so it can't even clone and reset
 345 /// `self.buf`. But `replace` can be used to disassociate the original value of `self.buf` from
 346 /// `self`, allowing it to be returned:
 347 ///
 348 /// ```
 349 /// use std::mem;
 350 /// # struct Buffer<T> { buf: Vec<T> }
 351 /// impl<T> Buffer<T> {
 352 ///     fn get_and_reset(&mut self) -> Vec<T> {
 353 ///         mem::replace(&mut self.buf, Vec::new())
 354 ///     }
 355 /// }
 356 /// ```
 357 #[inline]
 358 #[stable(feature = "rust1", since = "1.0.0")]
 359 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
 360     swap(dest, &mut src);
 361     src
 362 }
 363
 364 /// Disposes of a value.
 365 ///
 366 /// This function can be used to destroy any value by allowing `drop` to take ownership of its
 367 /// argument.
 368 ///
 369 /// # Examples
 370 ///
 371 /// ```
 372 /// use std::cell::RefCell;
 373 ///
 374 /// let x = RefCell::new(1);
 375 ///
 376 /// let mut mutable_borrow = x.borrow_mut();
 377 /// *mutable_borrow = 1;
 378 ///
 379 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
 380 ///
 381 /// let borrow = x.borrow();
 382 /// println!("{}", *borrow);
 383 /// ```
 384 #[inline]
 385 #[stable(feature = "rust1", since = "1.0.0")]
 386 pub fn drop<T>(_x: T) { }
 387
 388 macro_rules! repeat_u8_as_u32 {
 389     ($name:expr) => { (($name as u32) << 24 |
 390                        ($name as u32) << 16 |
 391                        ($name as u32) <<  8 |
 392                        ($name as u32)) }
 393 }
 394 macro_rules! repeat_u8_as_u64 {
 395     ($name:expr) => { ((repeat_u8_as_u32!($name) as u64) << 32 |
 396                        (repeat_u8_as_u32!($name) as u64)) }
 397 }
 398
 399 // NOTE: Keep synchronized with values used in librustc_trans::trans::adt.
 400 //
 401 // In particular, the POST_DROP_U8 marker must never equal the
 402 // DTOR_NEEDED_U8 marker.
 403 //
 404 // For a while pnkfelix was using 0xc1 here.
 405 // But having the sign bit set is a pain, so 0x1d is probably better.
 406 //
 407 // And of course, 0x00 brings back the old world of zero'ing on drop.
 408 #[unstable(feature = "filling_drop")]
 409 pub const POST_DROP_U8: u8 = 0x1d;
 410 #[unstable(feature = "filling_drop")]
 411 pub const POST_DROP_U32: u32 = repeat_u8_as_u32!(POST_DROP_U8);
 412 #[unstable(feature = "filling_drop")]
 413 pub const POST_DROP_U64: u64 = repeat_u8_as_u64!(POST_DROP_U8);
 414
 415 #[cfg(target_pointer_width = "32")]
 416 #[unstable(feature = "filling_drop")]
 417 pub const POST_DROP_USIZE: usize = POST_DROP_U32 as usize;
 418 #[cfg(target_pointer_width = "64")]
 419 #[unstable(feature = "filling_drop")]
 420 pub const POST_DROP_USIZE: usize = POST_DROP_U64 as usize;
 421
 422 /// Interprets `src` as `&U`, and then reads `src` without moving the contained
 423 /// value.
 424 ///
 425 /// This function will unsafely assume the pointer `src` is valid for
 426 /// `sizeof(U)` bytes by transmuting `&T` to `&U` and then reading the `&U`. It
 427 /// will also unsafely create a copy of the contained value instead of moving
 428 /// out of `src`.
 429 ///
 430 /// It is not a compile-time error if `T` and `U` have different sizes, but it
 431 /// is highly encouraged to only invoke this function where `T` and `U` have the
 432 /// same size. This function triggers undefined behavior if `U` is larger than
 433 /// `T`.
 434 ///
 435 /// # Examples
 436 ///
 437 /// ```
 438 /// use std::mem;
 439 ///
 440 /// let one = unsafe { mem::transmute_copy(&1) };
 441 ///
 442 /// assert_eq!(1, one);
 443 /// ```
 444 #[inline]
 445 #[stable(feature = "rust1", since = "1.0.0")]
 446 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
 447     // FIXME(#23542) Replace with type ascription.
 448     #![allow(trivial_casts)]
 449     ptr::read(src as *const T as *const U)
 450 }
 451
 452 /// Transforms lifetime of the second pointer to match the first.
 453 #[inline]
 454 #[unstable(feature = "core",
 455            reason = "this function may be removed in the future due to its \
 456                      questionable utility")]
 457 pub unsafe fn copy_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
 458                                                         ptr: &T) -> &'a T {
 459     transmute(ptr)
 460 }
 461
 462 /// Transforms lifetime of the second mutable pointer to match the first.
 463 #[inline]
 464 #[unstable(feature = "core",
 465            reason = "this function may be removed in the future due to its \
 466                      questionable utility")]
 467 pub unsafe fn copy_mut_lifetime<'a, S: ?Sized, T: ?Sized + 'a>(_ptr: &'a S,
 468                                                                ptr: &mut T)
 469                                                               -> &'a mut T
 470 {
 471     transmute(ptr)
 472 }