rust/src/libcore/ptr.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

// FIXME: talk about offset, copy_memory, copy_nonoverlapping_memory

//! Operations on unsafe pointers, `*const T`, and `*mut T`.
//!
//! Working with unsafe pointers in Rust is uncommon,
//! typically limited to a few patterns.
//!
//! Use the [`null` function](fn.null.html) to create null pointers,
//! the [`is_null`](trait.PtrExt.html#tymethod.is_null)
//! methods of the [`PtrExt` trait](trait.PtrExt.html) to check for null.
//! The `PtrExt` trait is imported by the prelude, so `is_null` etc.
//! work everywhere. The `PtrExt` also defines the `offset` method,
//! for pointer math.
//!
//! # Common ways to create unsafe pointers
//!
//! ## 1. Coerce a reference (`&T`) or mutable reference (`&mut T`).
//!
//! ```
//! let my_num: int = 10;
//! let my_num_ptr: *const int = &my_num;
//! let mut my_speed: int = 88;
//! let my_speed_ptr: *mut int = &mut my_speed;
//! ```
//!
//! This does not take ownership of the original allocation
//! and requires no resource management later,
//! but you must not use the pointer after its lifetime.
//!
//! ## 2. Transmute an owned box (`Box<T>`).
//!
//! The `transmute` function takes, by value, whatever it's given
//! and returns it as whatever type is requested, as long as the
//! types are the same size. Because `Box<T>` and `*mut T` have the same
//! representation they can be trivially,
//! though unsafely, transformed from one type to the other.
//!
//! ```
//! # use std::boxed::Box;
//! use std::mem;
//!
//! unsafe {
//!     let my_num: Box<int> = Box::new(10);
//!     let my_num: *const int = mem::transmute(my_num);
//!     let my_speed: Box<int> = Box::new(88);
//!     let my_speed: *mut int = mem::transmute(my_speed);
//!
//!     // By taking ownership of the original `Box<T>` though
//!     // we are obligated to transmute it back later to be destroyed.
//!     drop(mem::transmute::<_, Box<int>>(my_speed));
//!     drop(mem::transmute::<_, Box<int>>(my_num));
//! }
//! ```
//!
//! Note that here the call to `drop` is for clarity - it indicates
//! that we are done with the given value and it should be destroyed.
//!
//! ## 3. Get it from C.
//!
//! ```
//! extern crate libc;
//!
//! use std::mem;
//!
//! fn main() {
//!     unsafe {
//!         let my_num: *mut int = libc::malloc(mem::size_of::<int>() as libc::size_t) as *mut int;
//!         if my_num.is_null() {
//!             panic!("failed to allocate memory");
//!         }
//!         libc::free(my_num as *mut libc::c_void);
//!     }
//! }
//! ```
//!
//! Usually you wouldn't literally use `malloc` and `free` from Rust,
//! but C APIs hand out a lot of pointers generally, so are a common source
//! of unsafe pointers in Rust.

#![stable]

use mem;
use clone::Clone;
use intrinsics;
use option::Option::{self, Some, None};
use marker::{Send, Sized, Sync};

use cmp::{PartialEq, Eq, Ord, PartialOrd};
use cmp::Ordering::{self, Less, Equal, Greater};

// FIXME #19649: intrinsic docs don't render, so these have no docs :(

#[unstable]
pub use intrinsics::copy_nonoverlapping_memory;

#[unstable]
pub use intrinsics::copy_memory;

#[unstable = "uncertain about naming and semantics"]
pub use intrinsics::set_memory;


/// Creates a null raw pointer.
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let p: *const int = ptr::null();
/// assert!(p.is_null());
/// ```
#[inline]
#[stable]
pub fn null<T>() -> *const T { 0 as *const T }

/// Creates a null mutable raw pointer.
///
/// # Examples
///
/// ```
/// use std::ptr;
///
/// let p: *mut int = ptr::null_mut();
/// assert!(p.is_null());
/// ```
#[inline]
#[stable]
pub fn null_mut<T>() -> *mut T { 0 as *mut T }

/// Zeroes out `count * size_of::<T>` bytes of memory at `dst`. `count` may be
/// `0`.
///
/// # Safety
///
/// Beyond accepting a raw pointer, this is unsafe because it will not drop the
/// contents of `dst`, and may be used to create invalid instances of `T`.
#[inline]
#[unstable = "may play a larger role in std::ptr future extensions"]
pub unsafe fn zero_memory<T>(dst: *mut T, count: uint) {
    set_memory(dst, 0, count);
}

/// Swaps the values at two mutable locations of the same type, without
/// deinitialising either. They may overlap, unlike `mem::swap` which is
/// otherwise equivalent.
///
/// # Safety
///
/// This is only unsafe because it accepts a raw pointer.
#[inline]
#[stable]
pub unsafe fn swap<T>(x: *mut T, y: *mut T) {
    // Give ourselves some scratch space to work with
    let mut tmp: T = mem::uninitialized();
    let t: *mut T = &mut tmp;

    // Perform the swap
    copy_nonoverlapping_memory(t, &*x, 1);
    copy_memory(x, &*y, 1); // `x` and `y` may overlap
    copy_nonoverlapping_memory(y, &*t, 1);

    // y and t now point to the same thing, but we need to completely forget `tmp`
    // because it's no longer relevant.
    mem::forget(tmp);
}

/// Replaces the value at `dest` with `src`, returning the old
/// value, without dropping either.
///
/// # Safety
///
/// This is only unsafe because it accepts a raw pointer.
/// Otherwise, this operation is identical to `mem::replace`.
#[inline]
#[stable]
pub unsafe fn replace<T>(dest: *mut T, mut src: T) -> T {
    mem::swap(mem::transmute(dest), &mut src); // cannot overlap
    src
}

/// Reads the value from `src` without dropping it. This leaves the
/// memory in `src` unchanged.
///
/// # Safety
///
/// Beyond accepting a raw pointer, this is unsafe because it semantically
/// moves the value out of `src` without preventing further usage of `src`.
/// If `T` is not `Copy`, then care must be taken to ensure that the value at
/// `src` is not used before the data is overwritten again (e.g. with `write`,
/// `zero_memory`, or `copy_memory`). Note that `*src = foo` counts as a use
/// because it will attempt to drop the value previously at `*src`.
#[inline(always)]
#[stable]
pub unsafe fn read<T>(src: *const T) -> T {
    let mut tmp: T = mem::uninitialized();
    copy_nonoverlapping_memory(&mut tmp, src, 1);
    tmp
}

/// Reads the value from `src` and nulls it out without dropping it.
///
/// # Safety
///
/// This is unsafe for the same reasons that `read` is unsafe.
#[inline(always)]
#[unstable = "may play a larger role in std::ptr future extensions"]
pub unsafe fn read_and_zero<T>(dest: *mut T) -> T {
    // Copy the data out from `dest`:
    let tmp = read(&*dest);

    // Now zero out `dest`:
    zero_memory(dest, 1);

    tmp
}

/// Overwrites a memory location with the given value without reading or
/// dropping the old value.
///
/// # Safety
///
/// Beyond accepting a raw pointer, this operation is unsafe because it does
/// not drop the contents of `dst`. This could leak allocations or resources,
/// so care must be taken not to overwrite an object that should be dropped.
///
/// This is appropriate for initializing uninitialized memory, or overwriting
/// memory that has previously been `read` from.
#[inline]
#[stable]
pub unsafe fn write<T>(dst: *mut T, src: T) {
    intrinsics::move_val_init(&mut *dst, src)
}

/// Methods on raw pointers
#[stable]
pub trait PtrExt: Sized {
    type Target;

    /// Returns true if the pointer is null.
    #[stable]
    fn is_null(self) -> bool;

    /// Returns `None` if the pointer is null, or else returns a reference to
    /// the value wrapped in `Some`.
    ///
    /// # Safety
    ///
    /// While this method and its mutable counterpart are useful for
    /// null-safety, it is important to note that this is still an unsafe
    /// operation because the returned value could be pointing to invalid
    /// memory.
    #[unstable = "Option is not clearly the right return type, and we may want \
                  to tie the return lifetime to a borrow of the raw pointer"]
    unsafe fn as_ref<'a>(&self) -> Option<&'a Self::Target>;

    /// Calculates the offset from a pointer. `count` is in units of T; e.g. a
    /// `count` of 3 represents a pointer offset of `3 * sizeof::<T>()` bytes.
    ///
    /// # Safety
    ///
    /// The offset must be in-bounds of the object, or one-byte-past-the-end.
    /// Otherwise `offset` invokes Undefined Behaviour, regardless of whether
    /// the pointer is used.
    #[stable]
    unsafe fn offset(self, count: int) -> Self;
}

/// Methods on mutable raw pointers
#[stable]
pub trait MutPtrExt {
    type Target;

    /// Returns `None` if the pointer is null, or else returns a mutable
    /// reference to the value wrapped in `Some`.
    ///
    /// # Safety
    ///
    /// As with `as_ref`, this is unsafe because it cannot verify the validity
    /// of the returned pointer.
    #[unstable = "Option is not clearly the right return type, and we may want \
                  to tie the return lifetime to a borrow of the raw pointer"]
    unsafe fn as_mut<'a>(&self) -> Option<&'a mut Self::Target>;
}

#[stable]
impl<T> PtrExt for *const T {
    type Target = T;

    #[inline]
    #[stable]
    fn is_null(self) -> bool { self as uint == 0 }

    #[inline]
    #[stable]
    unsafe fn offset(self, count: int) -> *const T {
        intrinsics::offset(self, count)
    }

    #[inline]
    #[unstable = "return value does not necessarily convey all possible \
                  information"]
    unsafe fn as_ref<'a>(&self) -> Option<&'a T> {
        if self.is_null() {
            None
        } else {
            Some(&**self)
        }
    }
}

#[stable]
impl<T> PtrExt for *mut T {
    type Target = T;

    #[inline]
    #[stable]
    fn is_null(self) -> bool { self as uint == 0 }

    #[inline]
    #[stable]
    unsafe fn offset(self, count: int) -> *mut T {
        intrinsics::offset(self as *const T, count) as *mut T
    }

    #[inline]
    #[unstable = "return value does not necessarily convey all possible \
                  information"]
    unsafe fn as_ref<'a>(&self) -> Option<&'a T> {
        if self.is_null() {
            None
        } else {
            Some(&**self)
        }
    }
}

#[stable]
impl<T> MutPtrExt for *mut T {
    type Target = T;

    #[inline]
    #[unstable = "return value does not necessarily convey all possible \
                  information"]
    unsafe fn as_mut<'a>(&self) -> Option<&'a mut T> {
        if self.is_null() {
            None
        } else {
            Some(&mut **self)
        }
    }
}

// Equality for pointers
#[stable]
impl<T> PartialEq for *const T {
    #[inline]
    fn eq(&self, other: &*const T) -> bool {
        *self == *other
    }
    #[inline]
    fn ne(&self, other: &*const T) -> bool { !self.eq(other) }
}

#[stable]
impl<T> Eq for *const T {}

#[stable]
impl<T> PartialEq for *mut T {
    #[inline]
    fn eq(&self, other: &*mut T) -> bool {
        *self == *other
    }
    #[inline]
    fn ne(&self, other: &*mut T) -> bool { !self.eq(other) }
}

#[stable]
impl<T> Eq for *mut T {}

#[stable]
impl<T> Clone for *const T {
    #[inline]
    fn clone(&self) -> *const T {
        *self
    }
}

#[stable]
impl<T> Clone for *mut T {
    #[inline]
    fn clone(&self) -> *mut T {
        *self
    }
}

// Equality for extern "C" fn pointers
mod externfnpointers {
    use mem;
    use cmp::PartialEq;

    #[stable]
    impl<_R> PartialEq for extern "C" fn() -> _R {
        #[inline]
        fn eq(&self, other: &extern "C" fn() -> _R) -> bool {
            let self_: *const () = unsafe { mem::transmute(*self) };
            let other_: *const () = unsafe { mem::transmute(*other) };
            self_ == other_
        }
    }
    macro_rules! fnptreq {
        ($($p:ident),*) => {
            #[stable]
            impl<_R,$($p),*> PartialEq for extern "C" fn($($p),*) -> _R {
                #[inline]
                fn eq(&self, other: &extern "C" fn($($p),*) -> _R) -> bool {
                    let self_: *const () = unsafe { mem::transmute(*self) };

                    let other_: *const () = unsafe { mem::transmute(*other) };
                    self_ == other_
                }
            }
        }
    }
    fnptreq! { A }
    fnptreq! { A,B }
    fnptreq! { A,B,C }
    fnptreq! { A,B,C,D }
    fnptreq! { A,B,C,D,E }
}

// Comparison for pointers
#[stable]
impl<T> Ord for *const T {
    #[inline]
    fn cmp(&self, other: &*const T) -> Ordering {
        if self < other {
            Less
        } else if self == other {
            Equal
        } else {
            Greater
        }
    }
}

#[stable]
impl<T> PartialOrd for *const T {
    #[inline]
    fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
        Some(self.cmp(other))
    }

    #[inline]
    fn lt(&self, other: &*const T) -> bool { *self < *other }

    #[inline]
    fn le(&self, other: &*const T) -> bool { *self <= *other }

    #[inline]
    fn gt(&self, other: &*const T) -> bool { *self > *other }

    #[inline]
    fn ge(&self, other: &*const T) -> bool { *self >= *other }
}

#[stable]
impl<T> Ord for *mut T {
    #[inline]
    fn cmp(&self, other: &*mut T) -> Ordering {
        if self < other {
            Less
        } else if self == other {
            Equal
        } else {
            Greater
        }
    }
}

#[stable]
impl<T> PartialOrd for *mut T {
    #[inline]
    fn partial_cmp(&self, other: &*mut T) -> Option<Ordering> {
        Some(self.cmp(other))
    }

    #[inline]
    fn lt(&self, other: &*mut T) -> bool { *self < *other }

    #[inline]
    fn le(&self, other: &*mut T) -> bool { *self <= *other }

    #[inline]
    fn gt(&self, other: &*mut T) -> bool { *self > *other }

    #[inline]
    fn ge(&self, other: &*mut T) -> bool { *self >= *other }
}

/// A wrapper around a raw `*mut T` that indicates that the possessor
/// of this wrapper owns the referent. This in turn implies that the
/// `Unique<T>` is `Send`/`Sync` if `T` is `Send`/`Sync`, unlike a
/// raw `*mut T` (which conveys no particular ownership semantics).
/// Useful for building abstractions like `Vec<T>` or `Box<T>`, which
/// internally use raw pointers to manage the memory that they own.
#[unstable = "recently added to this module"]
pub struct Unique<T>(pub *mut T);

/// `Unique` pointers are `Send` if `T` is `Send` because the data they
/// reference is unaliased. Note that this aliasing invariant is
/// unenforced by the type system; the abstraction using the
/// `Unique` must enforce it.
#[unstable = "recently added to this module"]
unsafe impl<T:Send> Send for Unique<T> { }

/// `Unique` pointers are `Sync` if `T` is `Sync` because the data they
/// reference is unaliased. Note that this aliasing invariant is
/// unenforced by the type system; the abstraction using the
/// `Unique` must enforce it.
#[unstable = "recently added to this module"]
unsafe impl<T:Sync> Sync for Unique<T> { }

impl<T> Unique<T> {
    /// Returns a null Unique.
    #[unstable = "recently added to this module"]
    pub fn null() -> Unique<T> {
        Unique(null_mut())
    }

    /// Return an (unsafe) pointer into the memory owned by `self`.
    #[unstable = "recently added to this module"]
    pub unsafe fn offset(self, offset: int) -> *mut T {
        self.0.offset(offset)
    }
}