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Primitive Type pointer

Raw, unsafe pointers, *const T, and *mut T.

Working with raw pointers in Rust is uncommon, typically limited to a few patterns.

Use the null function to create null pointers, and the is_null method of the *const T type to check for null. The *const T type also defines the offset method, for pointer math.

Common ways to create raw pointers

1. Coerce a reference (&T) or mutable reference (&mut T).

let my_num: i32 = 10;
let my_num_ptr: *const i32 = &my_num;
let mut my_speed: i32 = 88;
let my_speed_ptr: *mut i32 = &mut my_speed;

To get a pointer to a boxed value, dereference the box:

let my_num: Box<i32> = Box::new(10);
let my_num_ptr: *const i32 = &*my_num;
let mut my_speed: Box<i32> = Box::new(88);
let my_speed_ptr: *mut i32 = &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. Consume a box (Box<T>).

The into_raw function consumes a box and returns the raw pointer. It doesn't destroy T or deallocate any memory.

let my_speed: Box<i32> = Box::new(88);
let my_speed: *mut i32 = Box::into_raw(my_speed);

// By taking ownership of the original `Box<T>` though
// we are obligated to put it together later to be destroyed.
unsafe {
    drop(Box::from_raw(my_speed));
}

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 i32 = libc::malloc(mem::size_of::<i32>()) as *mut i32;
        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 raw pointers in Rust.

See also the std::ptr module.

Methods

impl<T> *const T where T: ?Sized [src]

Returns true if the pointer is null.

Examples

Basic usage:

let s: &str = "Follow the rabbit";
let ptr: *const u8 = s.as_ptr();
assert!(!ptr.is_null());

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.

Additionally, the lifetime 'a returned is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.

Examples

Basic usage:

let val: *const u8 = &10u8 as *const u8;

unsafe {
    if let Some(val_back) = val.as_ref() {
        println!("We got back the value: {}!", val_back);
    }
}

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

Both the starting and resulting pointer must be either in bounds or one byte past the end of an allocated object. If either pointer is out of bounds or arithmetic overflow occurs then any further use of the returned value will result in undefined behavior.

Examples

Basic usage:

let s: &str = "123";
let ptr: *const u8 = s.as_ptr();

unsafe {
    println!("{}", *ptr.offset(1) as char);
    println!("{}", *ptr.offset(2) as char);
}

Calculates the offset from a pointer using wrapping arithmetic. count is in units of T; e.g. a count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.

Safety

The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe).

Always use .offset(count) instead when possible, because offset allows the compiler to optimize better.

Examples

Basic usage:

// Iterate using a raw pointer in increments of two elements
let data = [1u8, 2, 3, 4, 5];
let mut ptr: *const u8 = data.as_ptr();
let step = 2;
let end_rounded_up = ptr.wrapping_offset(6);

// This loop prints "1, 3, 5, "
while ptr != end_rounded_up {
    unsafe {
        print!("{}, ", *ptr);
    }
    ptr = ptr.wrapping_offset(step);
}

impl<T> *mut T where T: ?Sized [src]

Returns true if the pointer is null.

Examples

Basic usage:

let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
assert!(!ptr.is_null());

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.

Additionally, the lifetime 'a returned is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.

Examples

Basic usage:

let val: *mut u8 = &mut 10u8 as *mut u8;

unsafe {
    if let Some(val_back) = val.as_ref() {
        println!("We got back the value: {}!", val_back);
    }
}

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 Behavior, regardless of whether the pointer is used.

Examples

Basic usage:

let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();

unsafe {
    println!("{}", *ptr.offset(1));
    println!("{}", *ptr.offset(2));
}

Calculates the offset from a pointer using wrapping arithmetic. count is in units of T; e.g. a count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.

Safety

The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe).

Always use .offset(count) instead when possible, because offset allows the compiler to optimize better.

Examples

Basic usage:

// Iterate using a raw pointer in increments of two elements
let mut data = [1u8, 2, 3, 4, 5];
let mut ptr: *mut u8 = data.as_mut_ptr();
let step = 2;
let end_rounded_up = ptr.wrapping_offset(6);

while ptr != end_rounded_up {
    unsafe {
        *ptr = 0;
    }
    ptr = ptr.wrapping_offset(step);
}
assert_eq!(&data, &[0, 2, 0, 4, 0]);

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, nor can it ensure that the lifetime 'a returned is indeed a valid lifetime for the contained data.

Examples

Basic usage:

let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
let first_value = unsafe { ptr.as_mut().unwrap() };
*first_value = 4;
println!("{:?}", s); // It'll print: "[4, 2, 3]".

Trait Implementations

impl<T> !Sync for *const T where T: ?Sized [src]

impl<T> !Sync for *mut T where T: ?Sized [src]

impl<T> PartialEq<*const T> for *const T where T: ?Sized [src]

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

impl<T> PartialEq<*mut T> for *mut T where T: ?Sized [src]

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

impl<T> Clone for *const T where T: ?Sized [src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl<T> Clone for *mut T where T: ?Sized [src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl<T> Eq for *const T where T: ?Sized [src]

impl<T> Eq for *mut T where T: ?Sized [src]

impl<T> Zeroable for *const T where T: ?Sized [src]

impl<T> Zeroable for *mut T where T: ?Sized [src]

impl<T> !Send for *const T where T: ?Sized [src]

impl<T> !Send for *mut T where T: ?Sized [src]

impl<T, U> CoerceUnsized<*mut U> for *mut T where T: Unsize<U> + ?Sized,
        U: ?Sized [src]

impl<T, U> CoerceUnsized<*const U> for *mut T where T: Unsize<U> + ?Sized,
        U: ?Sized [src]

impl<T, U> CoerceUnsized<*const U> for *const T where T: Unsize<U> + ?Sized,
        U: ?Sized [src]

impl<T> Pointer for *const T where T: ?Sized [src]

Formats the value using the given formatter.

impl<T> Pointer for *mut T where T: ?Sized [src]

Formats the value using the given formatter.

impl<T> Debug for *const T where T: ?Sized [src]

Formats the value using the given formatter.

impl<T> Debug for *mut T where T: ?Sized [src]

Formats the value using the given formatter.

impl<T> Ord for *const T where T: ?Sized [src]

This method returns an Ordering between self and other. Read more

impl<T> Ord for *mut T where T: ?Sized [src]

This method returns an Ordering between self and other. Read more

impl<T> Hash for *const T [src]

Feeds this value into the state given, updating the hasher as necessary.

Feeds a slice of this type into the state provided.

impl<T> Hash for *mut T [src]

Feeds this value into the state given, updating the hasher as necessary.

Feeds a slice of this type into the state provided.

impl<T> PartialOrd<*const T> for *const T where T: ?Sized [src]

This method returns an ordering between self and other values if one exists. Read more

This method tests less than (for self and other) and is used by the < operator. Read more

This method tests less than or equal to (for self and other) and is used by the <= operator. Read more

This method tests greater than (for self and other) and is used by the > operator. Read more

This method tests greater than or equal to (for self and other) and is used by the >= operator. Read more

impl<T> PartialOrd<*mut T> for *mut T where T: ?Sized [src]

This method returns an ordering between self and other values if one exists. Read more

This method tests less than (for self and other) and is used by the < operator. Read more

This method tests less than or equal to (for self and other) and is used by the <= operator. Read more

This method tests greater than (for self and other) and is used by the > operator. Read more

This method tests greater than or equal to (for self and other) and is used by the >= operator. Read more

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *const T
1.9.0
[src]

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *mut T
1.9.0
[src]

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/primitive.pointer.html