Primitive Type pointer

1.0.0 · [−]
Expand description

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

See also the std::ptr module.

Working with raw pointers in Rust is uncommon, typically limited to a few patterns. Raw pointers can be unaligned or null. However, when a raw pointer is dereferenced (using the * operator), it must be non-null and aligned.

Storing through a raw pointer using *ptr = data calls drop on the old value, so write must be used if the type has drop glue and memory is not already initialized - otherwise drop would be called on the uninitialized memory.

Use the null and null_mut functions to create null pointers, and the is_null method of the *const T and *mut T types to check for null. The *const T and *mut T types also define 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;
Run

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;
Run

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));
}
Run

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. Create it using ptr::addr_of!

Instead of coercing a reference to a raw pointer, you can use the macros ptr::addr_of! (for *const T) and ptr::addr_of_mut! (for *mut T). These macros allow you to create raw pointers to fields to which you cannot create a reference (without causing undefined behaviour), such as an unaligned field. This might be necessary if packed structs or uninitialized memory is involved.

#[derive(Debug, Default, Copy, Clone)]
#[repr(C, packed)]
struct S {
    aligned: u8,
    unaligned: u32,
}
let s = S::default();
let p = std::ptr::addr_of!(s.unaligned); // not allowed with coercion
Run

4. Get it from C.

extern crate libc;

use std::mem;

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);
}
Run

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.

Implementations

Returns true if the pointer is null.

Note that unsized types have many possible null pointers, as only the raw data pointer is considered, not their length, vtable, etc. Therefore, two pointers that are null may still not compare equal to each other.

Behavior during const evaluation

When this function is used during const evaluation, it may return false for pointers that turn out to be null at runtime. Specifically, when a pointer to some memory is offset beyond its bounds in such a way that the resulting pointer is null, the function will still return false. There is no way for CTFE to know the absolute position of that memory, so we cannot tell if the pointer is null or not.

Examples

Basic usage:

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

Casts to a pointer of another type.

🔬 This is a nightly-only experimental API. (set_ptr_value #75091)

Use the pointer value in a new pointer of another type.

In case val is a (fat) pointer to an unsized type, this operation will ignore the pointer part, whereas for (thin) pointers to sized types, this has the same effect as a simple cast.

The resulting pointer will have provenance of self, i.e., for a fat pointer, this operation is semantically the same as creating a new fat pointer with the data pointer value of self but the metadata of val.

Examples

This function is primarily useful for allowing byte-wise pointer arithmetic on potentially fat pointers:

#![feature(set_ptr_value)]
let arr: [i32; 3] = [1, 2, 3];
let mut ptr = arr.as_ptr() as *const dyn Debug;
let thin = ptr as *const u8;
unsafe {
    ptr = thin.add(8).with_metadata_of(ptr);
    println!("{:?}", &*ptr); // will print "3"
}
Run
🔬 This is a nightly-only experimental API. (ptr_const_cast #92675)

Changes constness without changing the type.

This is a bit safer than as because it wouldn’t silently change the type if the code is refactored.

🔬 This is a nightly-only experimental API. (ptr_to_from_bits #91126)

Casts a pointer to its raw bits.

This is equivalent to as usize, but is more specific to enhance readability. The inverse method is from_bits.

In particular, *p as usize and p as usize will both compile for pointers to numeric types but do very different things, so using this helps emphasize that reading the bits was intentional.

Examples
#![feature(ptr_to_from_bits)]
let array = [13, 42];
let p0: *const i32 = &array[0];
assert_eq!(<*const _>::from_bits(p0.to_bits()), p0);
let p1: *const i32 = &array[1];
assert_eq!(p1.to_bits() - p0.to_bits(), 4);
Run
🔬 This is a nightly-only experimental API. (ptr_to_from_bits #91126)

Creates a pointer from its raw bits.

This is equivalent to as *const T, but is more specific to enhance readability. The inverse method is to_bits.

Examples
#![feature(ptr_to_from_bits)]
use std::ptr::NonNull;
let dangling: *const u8 = NonNull::dangling().as_ptr();
assert_eq!(<*const u8>::from_bits(1), dangling);
Run
🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Gets the “address” portion of the pointer.

This is similar to self as usize, which semantically discards provenance and address-space information. However, unlike self as usize, casting the returned address back to a pointer yields invalid, which is undefined behavior to dereference. To properly restore the lost information and obtain a dereferencable pointer, use with_addr or map_addr.

If using those APIs is not possible because there is no way to preserve a pointer with the required provenance, use expose_addr and from_exposed_addr instead. However, note that this makes your code less portable and less amenable to tools that check for compliance with the Rust memory model.

On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may perform a change of representation to produce a value containing only the address portion of the pointer. What that means is up to the platform to define.

This API and its claimed semantics are part of the Strict Provenance experiment, and as such might change in the future (including possibly weakening this so it becomes wholly equivalent to self as usize). See the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Gets the “address” portion of the pointer, and ‘exposes’ the “provenance” part for future use in from_exposed_addr.

This is equivalent to self as usize, which semantically discards provenance and address-space information. Furthermore, this (like the as cast) has the implicit side-effect of marking the provenance as ‘exposed’, so on platforms that support it you can later call from_exposed_addr to reconstitute the original pointer including its provenance. (Reconstructing address space information, if required, is your responsibility.)

Using this method means that code is not following Strict Provenance rules. Supporting from_exposed_addr complicates specification and reasoning and may not be supported by tools that help you to stay conformant with the Rust memory model, so it is recommended to use addr wherever possible.

On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may not support this operation, since the ‘expose’ side-effect which is required for from_exposed_addr to work is typically not available.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer with the given address.

This performs the same operation as an addr as ptr cast, but copies the address-space and provenance of self to the new pointer. This allows us to dynamically preserve and propagate this important information in a way that is otherwise impossible with a unary cast.

This is equivalent to using wrapping_offset to offset self to the given address, and therefore has all the same capabilities and restrictions.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer by mapping self’s address to a new one.

This is a convenience for with_addr, see that method for details.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (ptr_metadata #81513)

Decompose a (possibly wide) pointer into its address and metadata components.

The pointer can be later reconstructed with from_raw_parts.

Returns None if the pointer is null, or else returns a shared reference to the value wrapped in Some. If the value may be uninitialized, as_uninit_ref must be used instead.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • The pointer must point to an initialized instance of T.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused! (The part about being initialized is not yet fully decided, but until it is, the only safe approach is to ensure that they are indeed initialized.)

Examples

Basic usage:

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

unsafe {
    if let Some(val_back) = ptr.as_ref() {
        println!("We got back the value: {val_back}!");
    }
}
Run
Null-unchecked version

If you are sure the pointer can never be null and are looking for some kind of as_ref_unchecked that returns the &T instead of Option<&T>, know that you can dereference the pointer directly.

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

unsafe {
    let val_back = &*ptr;
    println!("We got back the value: {val_back}!");
}
Run
🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a shared reference to the value wrapped in Some. In contrast to as_ref, this does not require that the value has to be initialized.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

Examples

Basic usage:

#![feature(ptr_as_uninit)]

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

unsafe {
    if let Some(val_back) = ptr.as_uninit_ref() {
        println!("We got back the value: {}!", val_back.assume_init());
    }
}
Run

Calculates the offset from a pointer.

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset, in bytes, cannot overflow an isize.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum, in bytes must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_offset instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

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);
}
Run

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 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_offset((y as isize) - (x as isize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to offset, this method basically delays the requirement of staying within the same allocated object: offset is immediate Undefined Behavior when crossing object boundaries; wrapping_offset produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. offset can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_offset(o).wrapping_offset(o.wrapping_neg()) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

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);
}
Run

Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes divided by mem::size_of::<T>().

This function is the inverse of offset.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and other pointer must be either in bounds or one byte past the end of the same allocated object.

  • Both pointers must be derived from a pointer to the same object. (See below for an example.)

  • The distance between the pointers, in bytes, must be an exact multiple of the size of T.

  • The distance between the pointers, in bytes, cannot overflow an isize.

  • The distance being in bounds cannot rely on “wrapping around” the address space.

Rust types are never larger than isize::MAX and Rust allocations never wrap around the address space, so two pointers within some value of any Rust type T will always satisfy the last two conditions. The standard library also generally ensures that allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so ptr_into_vec.offset_from(vec.as_ptr()) always satisfies the last two conditions.

Most platforms fundamentally can’t even construct such a large allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function. (Note that offset and add also have a similar limitation and hence cannot be used on such large allocations either.)

Panics

This function panics if T is a Zero-Sized Type (“ZST”).

Examples

Basic usage:

let a = [0; 5];
let ptr1: *const i32 = &a[1];
let ptr2: *const i32 = &a[3];
unsafe {
    assert_eq!(ptr2.offset_from(ptr1), 2);
    assert_eq!(ptr1.offset_from(ptr2), -2);
    assert_eq!(ptr1.offset(2), ptr2);
    assert_eq!(ptr2.offset(-2), ptr1);
}
Run

Incorrect usage:

let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
// Make ptr2_other an "alias" of ptr2, but derived from ptr1.
let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
assert_eq!(ptr2 as usize, ptr2_other as usize);
// Since ptr2_other and ptr2 are derived from pointers to different objects,
// computing their offset is undefined behavior, even though
// they point to the same address!
unsafe {
    let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
}
Run
🔬 This is a nightly-only experimental API. (ptr_sub_ptr #95892)

Calculates the distance between two pointers, where it’s known that self is equal to or greater than origin. The returned value is in units of T: the distance in bytes is divided by mem::size_of::<T>().

This computes the same value that offset_from would compute, but with the added precondition that that the offset is guaranteed to be non-negative. This method is equivalent to usize::from(self.offset_from(origin)).unwrap_unchecked(), but it provides slightly more information to the optimizer, which can sometimes allow it to optimize slightly better with some backends.

This method can be though of as recovering the count that was passed to add (or, with the parameters in the other order, to sub). The following are all equivalent, assuming that their safety preconditions are met:

ptr.sub_ptr(origin) == count
origin.add(count) == ptr
ptr.sub(count) == origin
Run
Safety

  • The distance between the pointers must be non-negative (self >= origin)

  • All the safety conditions of offset_from apply to this method as well; see it for the full details.

Importantly, despite the return type of this method being able to represent a larger offset, it’s still not permitted to pass pointers which differ by more than isize::MAX bytes. As such, the result of this method will always be less than or equal to isize::MAX as usize.

Panics

This function panics if T is a Zero-Sized Type (“ZST”).

Examples
#![feature(ptr_sub_ptr)]

let a = [0; 5];
let ptr1: *const i32 = &a[1];
let ptr2: *const i32 = &a[3];
unsafe {
    assert_eq!(ptr2.sub_ptr(ptr1), 2);
    assert_eq!(ptr1.add(2), ptr2);
    assert_eq!(ptr2.sub(2), ptr1);
    assert_eq!(ptr2.sub_ptr(ptr2), 0);
}

// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.offset_from(ptr2)
Run
🔬 This is a nightly-only experimental API. (const_raw_ptr_comparison #53020)

Returns whether two pointers are guaranteed to be equal.

At runtime this function behaves like self == other. However, in some contexts (e.g., compile-time evaluation), it is not always possible to determine equality of two pointers, so this function may spuriously return false for pointers that later actually turn out to be equal. But when it returns true, the pointers are guaranteed to be equal.

This function is the mirror of guaranteed_ne, but not its inverse. There are pointer comparisons for which both functions return false.

The return value may change depending on the compiler version and unsafe code must not rely on the result of this function for soundness. It is suggested to only use this function for performance optimizations where spurious false return values by this function do not affect the outcome, but just the performance. The consequences of using this method to make runtime and compile-time code behave differently have not been explored. This method should not be used to introduce such differences, and it should also not be stabilized before we have a better understanding of this issue.

🔬 This is a nightly-only experimental API. (const_raw_ptr_comparison #53020)

Returns whether two pointers are guaranteed to be unequal.

At runtime this function behaves like self != other. However, in some contexts (e.g., compile-time evaluation), it is not always possible to determine the inequality of two pointers, so this function may spuriously return false for pointers that later actually turn out to be unequal. But when it returns true, the pointers are guaranteed to be unequal.

This function is the mirror of guaranteed_eq, but not its inverse. There are pointer comparisons for which both functions return false.

The return value may change depending on the compiler version and unsafe code must not rely on the result of this function for soundness. It is suggested to only use this function for performance optimizations where spurious false return values by this function do not affect the outcome, but just the performance. The consequences of using this method to make runtime and compile-time code behave differently have not been explored. This method should not be used to introduce such differences, and it should also not be stabilized before we have a better understanding of this issue.

Calculates the offset from a pointer (convenience for .offset(count as isize)).

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset, in bytes, cannot overflow an isize.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_add instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Examples

Basic usage:

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

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

Calculates the offset from a pointer (convenience for .offset((count as isize).wrapping_neg())).

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset cannot exceed isize::MAX bytes.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()).sub(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_sub instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Examples

Basic usage:

let s: &str = "123";

unsafe {
    let end: *const u8 = s.as_ptr().add(3);
    println!("{}", *end.sub(1) as char);
    println!("{}", *end.sub(2) as char);
}
Run

Calculates the offset from a pointer using wrapping arithmetic. (convenience for .wrapping_offset(count as isize))

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_add((y as usize) - (x as usize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to add, this method basically delays the requirement of staying within the same allocated object: add is immediate Undefined Behavior when crossing object boundaries; wrapping_add produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. add can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_add(o).wrapping_sub(o) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

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_add(6);

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

Calculates the offset from a pointer using wrapping arithmetic. (convenience for .wrapping_offset((count as isize).wrapping_neg()))

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_sub((x as usize) - (y as usize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to sub, this method basically delays the requirement of staying within the same allocated object: sub is immediate Undefined Behavior when crossing object boundaries; wrapping_sub produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. sub can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_add(o).wrapping_sub(o) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

Examples

Basic usage:

// Iterate using a raw pointer in increments of two elements (backwards)
let data = [1u8, 2, 3, 4, 5];
let mut ptr: *const u8 = data.as_ptr();
let start_rounded_down = ptr.wrapping_sub(2);
ptr = ptr.wrapping_add(4);
let step = 2;
// This loop prints "5, 3, 1, "
while ptr != start_rounded_down {
    unsafe {
        print!("{}, ", *ptr);
    }
    ptr = ptr.wrapping_sub(step);
}
Run

Reads the value from self without moving it. This leaves the memory in self unchanged.

See ptr::read for safety concerns and examples.

Performs a volatile read of the value from self without moving it. This leaves the memory in self unchanged.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

See ptr::read_volatile for safety concerns and examples.

Reads the value from self without moving it. This leaves the memory in self unchanged.

Unlike read, the pointer may be unaligned.

See ptr::read_unaligned for safety concerns and examples.

Copies count * size_of<T> bytes from self to dest. The source and destination may overlap.

NOTE: this has the same argument order as ptr::copy.

See ptr::copy for safety concerns and examples.

Copies count * size_of<T> bytes from self to dest. The source and destination may not overlap.

NOTE: this has the same argument order as ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

Computes the offset that needs to be applied to the pointer in order to make it aligned to align.

If it is not possible to align the pointer, the implementation returns usize::MAX. It is permissible for the implementation to always return usize::MAX. Only your algorithm’s performance can depend on getting a usable offset here, not its correctness.

The offset is expressed in number of T elements, and not bytes. The value returned can be used with the wrapping_add method.

There are no guarantees whatsoever that offsetting the pointer will not overflow or go beyond the allocation that the pointer points into. It is up to the caller to ensure that the returned offset is correct in all terms other than alignment.

Panics

The function panics if align is not a power-of-two.

Examples

Accessing adjacent u8 as u16

let x = [5u8, 6u8, 7u8, 8u8, 9u8];
let ptr = x.as_ptr().add(n) as *const u8;
let offset = ptr.align_offset(align_of::<u16>());
if offset < x.len() - n - 1 {
    let u16_ptr = ptr.add(offset) as *const u16;
    assert_ne!(*u16_ptr, 500);
} else {
    // while the pointer can be aligned via `offset`, it would point
    // outside the allocation
}
Run
🔬 This is a nightly-only experimental API. (slice_ptr_len #71146)

Returns the length of a raw slice.

The returned value is the number of elements, not the number of bytes.

This function is safe, even when the raw slice cannot be cast to a slice reference because the pointer is null or unaligned.

Examples
#![feature(slice_ptr_len)]

use std::ptr;

let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
assert_eq!(slice.len(), 3);
Run
🔬 This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to the slice’s buffer.

This is equivalent to casting self to *const T, but more type-safe.

Examples
#![feature(slice_ptr_get)]
use std::ptr;

let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
assert_eq!(slice.as_ptr(), ptr::null());
Run
🔬 This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to an element or subslice, without doing bounds checking.

Calling this method with an out-of-bounds index or when self is not dereferenceable is undefined behavior even if the resulting pointer is not used.

Examples
#![feature(slice_ptr_get)]

let x = &[1, 2, 4] as *const [i32];

unsafe {
    assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
}
Run
🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a shared slice to the value wrapped in Some. In contrast to as_ref, this does not require that the value has to be initialized.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be valid for reads for ptr.len() * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

    • The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.

    • The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().

  • The total size ptr.len() * mem::size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

See also slice::from_raw_parts.

Returns true if the pointer is null.

Note that unsized types have many possible null pointers, as only the raw data pointer is considered, not their length, vtable, etc. Therefore, two pointers that are null may still not compare equal to each other.

Behavior during const evaluation

When this function is used during const evaluation, it may return false for pointers that turn out to be null at runtime. Specifically, when a pointer to some memory is offset beyond its bounds in such a way that the resulting pointer is null, the function will still return false. There is no way for CTFE to know the absolute position of that memory, so we cannot tell if the pointer is null or not.

Examples

Basic usage:

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

Casts to a pointer of another type.

🔬 This is a nightly-only experimental API. (set_ptr_value #75091)

Use the pointer value in a new pointer of another type.

In case val is a (fat) pointer to an unsized type, this operation will ignore the pointer part, whereas for (thin) pointers to sized types, this has the same effect as a simple cast.

The resulting pointer will have provenance of self, i.e., for a fat pointer, this operation is semantically the same as creating a new fat pointer with the data pointer value of self but the metadata of val.

Examples

This function is primarily useful for allowing byte-wise pointer arithmetic on potentially fat pointers:

#![feature(set_ptr_value)]
let mut arr: [i32; 3] = [1, 2, 3];
let mut ptr = arr.as_mut_ptr() as *mut dyn Debug;
let thin = ptr as *mut u8;
unsafe {
    ptr = thin.add(8).with_metadata_of(ptr);
    println!("{:?}", &*ptr); // will print "3"
}
Run
🔬 This is a nightly-only experimental API. (ptr_const_cast #92675)

Changes constness without changing the type.

This is a bit safer than as because it wouldn’t silently change the type if the code is refactored.

While not strictly required (*mut T coerces to *const T), this is provided for symmetry with as_mut() on *const T and may have documentation value if used instead of implicit coercion.

🔬 This is a nightly-only experimental API. (ptr_to_from_bits #91126)

Casts a pointer to its raw bits.

This is equivalent to as usize, but is more specific to enhance readability. The inverse method is from_bits.

In particular, *p as usize and p as usize will both compile for pointers to numeric types but do very different things, so using this helps emphasize that reading the bits was intentional.

Examples
#![feature(ptr_to_from_bits)]
let mut array = [13, 42];
let mut it = array.iter_mut();
let p0: *mut i32 = it.next().unwrap();
assert_eq!(<*mut _>::from_bits(p0.to_bits()), p0);
let p1: *mut i32 = it.next().unwrap();
assert_eq!(p1.to_bits() - p0.to_bits(), 4);
Run
🔬 This is a nightly-only experimental API. (ptr_to_from_bits #91126)

Creates a pointer from its raw bits.

This is equivalent to as *mut T, but is more specific to enhance readability. The inverse method is to_bits.

Examples
#![feature(ptr_to_from_bits)]
use std::ptr::NonNull;
let dangling: *mut u8 = NonNull::dangling().as_ptr();
assert_eq!(<*mut u8>::from_bits(1), dangling);
Run
🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Gets the “address” portion of the pointer.

This is similar to self as usize, which semantically discards provenance and address-space information. However, unlike self as usize, casting the returned address back to a pointer yields invalid, which is undefined behavior to dereference. To properly restore the lost information and obtain a dereferencable pointer, use with_addr or map_addr.

If using those APIs is not possible because there is no way to preserve a pointer with the required provenance, use expose_addr and from_exposed_addr_mut instead. However, note that this makes your code less portable and less amenable to tools that check for compliance with the Rust memory model.

On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may perform a change of representation to produce a value containing only the address portion of the pointer. What that means is up to the platform to define.

This API and its claimed semantics are part of the Strict Provenance experiment, and as such might change in the future (including possibly weakening this so it becomes wholly equivalent to self as usize). See the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Gets the “address” portion of the pointer, and ‘exposes’ the “provenance” part for future use in from_exposed_addr.

This is equivalent to self as usize, which semantically discards provenance and address-space information. Furthermore, this (like the as cast) has the implicit side-effect of marking the provenance as ‘exposed’, so on platforms that support it you can later call from_exposed_addr_mut to reconstitute the original pointer including its provenance. (Reconstructing address space information, if required, is your responsibility.)

Using this method means that code is not following Strict Provenance rules. Supporting from_exposed_addr_mut complicates specification and reasoning and may not be supported by tools that help you to stay conformant with the Rust memory model, so it is recommended to use addr wherever possible.

On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may not support this operation, since the ‘expose’ side-effect which is required for from_exposed_addr_mut to work is typically not available.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer with the given address.

This performs the same operation as an addr as ptr cast, but copies the address-space and provenance of self to the new pointer. This allows us to dynamically preserve and propagate this important information in a way that is otherwise impossible with a unary cast.

This is equivalent to using wrapping_offset to offset self to the given address, and therefore has all the same capabilities and restrictions.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer by mapping self’s address to a new one.

This is a convenience for with_addr, see that method for details.

This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.

🔬 This is a nightly-only experimental API. (ptr_metadata #81513)

Decompose a (possibly wide) pointer into its address and metadata components.

The pointer can be later reconstructed with from_raw_parts_mut.

Returns None if the pointer is null, or else returns a shared reference to the value wrapped in Some. If the value may be uninitialized, as_uninit_ref must be used instead.

For the mutable counterpart see as_mut.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • The pointer must point to an initialized instance of T.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused! (The part about being initialized is not yet fully decided, but until it is, the only safe approach is to ensure that they are indeed initialized.)

Examples

Basic usage:

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

unsafe {
    if let Some(val_back) = ptr.as_ref() {
        println!("We got back the value: {val_back}!");
    }
}
Run
Null-unchecked version

If you are sure the pointer can never be null and are looking for some kind of as_ref_unchecked that returns the &T instead of Option<&T>, know that you can dereference the pointer directly.

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

unsafe {
    let val_back = &*ptr;
    println!("We got back the value: {val_back}!");
}
Run
🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a shared reference to the value wrapped in Some. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_mut.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

Examples

Basic usage:

#![feature(ptr_as_uninit)]

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

unsafe {
    if let Some(val_back) = ptr.as_uninit_ref() {
        println!("We got back the value: {}!", val_back.assume_init());
    }
}
Run

Calculates the offset from a pointer.

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset, in bytes, cannot overflow an isize.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum, in bytes must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_offset instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

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));
}
Run

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 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_offset((y as isize) - (x as isize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to offset, this method basically delays the requirement of staying within the same allocated object: offset is immediate Undefined Behavior when crossing object boundaries; wrapping_offset produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. offset can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_offset(o).wrapping_offset(o.wrapping_neg()) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

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]);
Run

Returns None if the pointer is null, or else returns a unique reference to the value wrapped in Some. If the value may be uninitialized, as_uninit_mut must be used instead.

For the shared counterpart see as_ref.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • The pointer must point to an initialized instance of T.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.

This applies even if the result of this method is unused! (The part about being initialized is not yet fully decided, but until it is, the only safe approach is to ensure that they are indeed initialized.)

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]".
Run
Null-unchecked version

If you are sure the pointer can never be null and are looking for some kind of as_mut_unchecked that returns the &mut T instead of Option<&mut T>, know that you can dereference the pointer directly.

let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
let first_value = unsafe { &mut *ptr };
*first_value = 4;
println!("{s:?}"); // It'll print: "[4, 2, 3]".
Run
🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a unique reference to the value wrapped in Some. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_ref.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be properly aligned.

  • It must be “dereferenceable” in the sense defined in the module documentation.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.

This applies even if the result of this method is unused!

🔬 This is a nightly-only experimental API. (const_raw_ptr_comparison #53020)

Returns whether two pointers are guaranteed to be equal.

At runtime this function behaves like self == other. However, in some contexts (e.g., compile-time evaluation), it is not always possible to determine equality of two pointers, so this function may spuriously return false for pointers that later actually turn out to be equal. But when it returns true, the pointers are guaranteed to be equal.

This function is the mirror of guaranteed_ne, but not its inverse. There are pointer comparisons for which both functions return false.

The return value may change depending on the compiler version and unsafe code might not rely on the result of this function for soundness. It is suggested to only use this function for performance optimizations where spurious false return values by this function do not affect the outcome, but just the performance. The consequences of using this method to make runtime and compile-time code behave differently have not been explored. This method should not be used to introduce such differences, and it should also not be stabilized before we have a better understanding of this issue.

🔬 This is a nightly-only experimental API. (const_raw_ptr_comparison #53020)

Returns whether two pointers are guaranteed to be unequal.

At runtime this function behaves like self != other. However, in some contexts (e.g., compile-time evaluation), it is not always possible to determine the inequality of two pointers, so this function may spuriously return false for pointers that later actually turn out to be unequal. But when it returns true, the pointers are guaranteed to be unequal.

This function is the mirror of guaranteed_eq, but not its inverse. There are pointer comparisons for which both functions return false.

The return value may change depending on the compiler version and unsafe code might not rely on the result of this function for soundness. It is suggested to only use this function for performance optimizations where spurious false return values by this function do not affect the outcome, but just the performance. The consequences of using this method to make runtime and compile-time code behave differently have not been explored. This method should not be used to introduce such differences, and it should also not be stabilized before we have a better understanding of this issue.

Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes divided by mem::size_of::<T>().

This function is the inverse of offset.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and other pointer must be either in bounds or one byte past the end of the same allocated object.

  • Both pointers must be derived from a pointer to the same object. (See below for an example.)

  • The distance between the pointers, in bytes, must be an exact multiple of the size of T.

  • The distance between the pointers, in bytes, cannot overflow an isize.

  • The distance being in bounds cannot rely on “wrapping around” the address space.

Rust types are never larger than isize::MAX and Rust allocations never wrap around the address space, so two pointers within some value of any Rust type T will always satisfy the last two conditions. The standard library also generally ensures that allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so ptr_into_vec.offset_from(vec.as_ptr()) always satisfies the last two conditions.

Most platforms fundamentally can’t even construct such a large allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function. (Note that offset and add also have a similar limitation and hence cannot be used on such large allocations either.)

Panics

This function panics if T is a Zero-Sized Type (“ZST”).

Examples

Basic usage:

let mut a = [0; 5];
let ptr1: *mut i32 = &mut a[1];
let ptr2: *mut i32 = &mut a[3];
unsafe {
    assert_eq!(ptr2.offset_from(ptr1), 2);
    assert_eq!(ptr1.offset_from(ptr2), -2);
    assert_eq!(ptr1.offset(2), ptr2);
    assert_eq!(ptr2.offset(-2), ptr1);
}
Run

Incorrect usage:

let ptr1 = Box::into_raw(Box::new(0u8));
let ptr2 = Box::into_raw(Box::new(1u8));
let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
// Make ptr2_other an "alias" of ptr2, but derived from ptr1.
let ptr2_other = (ptr1 as *mut u8).wrapping_offset(diff);
assert_eq!(ptr2 as usize, ptr2_other as usize);
// Since ptr2_other and ptr2 are derived from pointers to different objects,
// computing their offset is undefined behavior, even though
// they point to the same address!
unsafe {
    let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
}
Run
🔬 This is a nightly-only experimental API. (ptr_sub_ptr #95892)

Calculates the distance between two pointers, where it’s known that self is equal to or greater than origin. The returned value is in units of T: the distance in bytes is divided by mem::size_of::<T>().

This computes the same value that offset_from would compute, but with the added precondition that that the offset is guaranteed to be non-negative. This method is equivalent to usize::from(self.offset_from(origin)).unwrap_unchecked(), but it provides slightly more information to the optimizer, which can sometimes allow it to optimize slightly better with some backends.

This method can be though of as recovering the count that was passed to add (or, with the parameters in the other order, to sub). The following are all equivalent, assuming that their safety preconditions are met:

ptr.sub_ptr(origin) == count
origin.add(count) == ptr
ptr.sub(count) == origin
Run
Safety

  • The distance between the pointers must be non-negative (self >= origin)

  • All the safety conditions of offset_from apply to this method as well; see it for the full details.

Importantly, despite the return type of this method being able to represent a larger offset, it’s still not permitted to pass pointers which differ by more than isize::MAX bytes. As such, the result of this method will always be less than or equal to isize::MAX as usize.

Panics

This function panics if T is a Zero-Sized Type (“ZST”).

Examples
#![feature(ptr_sub_ptr)]

let mut a = [0; 5];
let p: *mut i32 = a.as_mut_ptr();
unsafe {
    let ptr1: *mut i32 = p.add(1);
    let ptr2: *mut i32 = p.add(3);

    assert_eq!(ptr2.sub_ptr(ptr1), 2);
    assert_eq!(ptr1.add(2), ptr2);
    assert_eq!(ptr2.sub(2), ptr1);
    assert_eq!(ptr2.sub_ptr(ptr2), 0);
}

// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.offset_from(ptr2)
Run

Calculates the offset from a pointer (convenience for .offset(count as isize)).

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset, in bytes, cannot overflow an isize.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_add instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Examples

Basic usage:

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

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

Calculates the offset from a pointer (convenience for .offset((count as isize).wrapping_neg())).

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

If any of the following conditions are violated, the result is Undefined Behavior:

  • Both the starting and resulting pointer must be either in bounds or one byte past the end of the same allocated object.

  • The computed offset cannot exceed isize::MAX bytes.

  • The offset being in bounds cannot rely on “wrapping around” the address space. That is, the infinite-precision sum must fit in a usize.

The compiler and standard library generally tries to ensure allocations never reach a size where an offset is a concern. For instance, Vec and Box ensure they never allocate more than isize::MAX bytes, so vec.as_ptr().add(vec.len()).sub(vec.len()) is always safe.

Most platforms fundamentally can’t even construct such an allocation. For instance, no known 64-bit platform can ever serve a request for 263 bytes due to page-table limitations or splitting the address space. However, some 32-bit and 16-bit platforms may successfully serve a request for more than isize::MAX bytes with things like Physical Address Extension. As such, memory acquired directly from allocators or memory mapped files may be too large to handle with this function.

Consider using wrapping_sub instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Examples

Basic usage:

let s: &str = "123";

unsafe {
    let end: *const u8 = s.as_ptr().add(3);
    println!("{}", *end.sub(1) as char);
    println!("{}", *end.sub(2) as char);
}
Run

Calculates the offset from a pointer using wrapping arithmetic. (convenience for .wrapping_offset(count as isize))

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_add((y as usize) - (x as usize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to add, this method basically delays the requirement of staying within the same allocated object: add is immediate Undefined Behavior when crossing object boundaries; wrapping_add produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. add can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_add(o).wrapping_sub(o) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

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_add(6);

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

Calculates the offset from a pointer using wrapping arithmetic. (convenience for .wrapping_offset((count as isize).wrapping_neg()))

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<T>() bytes.

Safety

This operation itself is always safe, but using the resulting pointer is not.

The resulting pointer “remembers” the allocated object that self points to; it must not be used to read or write other allocated objects.

In other words, let z = x.wrapping_sub((x as usize) - (y as usize)) does not make z the same as y even if we assume T has size 1 and there is no overflow: z is still attached to the object x is attached to, and dereferencing it is Undefined Behavior unless x and y point into the same allocated object.

Compared to sub, this method basically delays the requirement of staying within the same allocated object: sub is immediate Undefined Behavior when crossing object boundaries; wrapping_sub produces a pointer but still leads to Undefined Behavior if a pointer is dereferenced when it is out-of-bounds of the object it is attached to. sub can be optimized better and is thus preferable in performance-sensitive code.

The delayed check only considers the value of the pointer that was dereferenced, not the intermediate values used during the computation of the final result. For example, x.wrapping_add(o).wrapping_sub(o) is always the same as x. In other words, leaving the allocated object and then re-entering it later is permitted.

Examples

Basic usage:

// Iterate using a raw pointer in increments of two elements (backwards)
let data = [1u8, 2, 3, 4, 5];
let mut ptr: *const u8 = data.as_ptr();
let start_rounded_down = ptr.wrapping_sub(2);
ptr = ptr.wrapping_add(4);
let step = 2;
// This loop prints "5, 3, 1, "
while ptr != start_rounded_down {
    unsafe {
        print!("{}, ", *ptr);
    }
    ptr = ptr.wrapping_sub(step);
}
Run

Reads the value from self without moving it. This leaves the memory in self unchanged.

See ptr::read for safety concerns and examples.

Performs a volatile read of the value from self without moving it. This leaves the memory in self unchanged.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

See ptr::read_volatile for safety concerns and examples.

Reads the value from self without moving it. This leaves the memory in self unchanged.

Unlike read, the pointer may be unaligned.

See ptr::read_unaligned for safety concerns and examples.

Copies count * size_of<T> bytes from self to dest. The source and destination may overlap.

NOTE: this has the same argument order as ptr::copy.

See ptr::copy for safety concerns and examples.

Copies count * size_of<T> bytes from self to dest. The source and destination may not overlap.

NOTE: this has the same argument order as ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

Copies count * size_of<T> bytes from src to self. The source and destination may overlap.

NOTE: this has the opposite argument order of ptr::copy.

See ptr::copy for safety concerns and examples.

Copies count * size_of<T> bytes from src to self. The source and destination may not overlap.

NOTE: this has the opposite argument order of ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

Executes the destructor (if any) of the pointed-to value.

See ptr::drop_in_place for safety concerns and examples.

Overwrites a memory location with the given value without reading or dropping the old value.

See ptr::write for safety concerns and examples.

Invokes memset on the specified pointer, setting count * size_of::<T>() bytes of memory starting at self to val.

See ptr::write_bytes for safety concerns and examples.

Performs a volatile write of a memory location with the given value without reading or dropping the old value.

Volatile operations are intended to act on I/O memory, and are guaranteed to not be elided or reordered by the compiler across other volatile operations.

See ptr::write_volatile for safety concerns and examples.

Overwrites a memory location with the given value without reading or dropping the old value.

Unlike write, the pointer may be unaligned.

See ptr::write_unaligned for safety concerns and examples.

Replaces the value at self with src, returning the old value, without dropping either.

See ptr::replace for safety concerns and examples.

Swaps the values at two mutable locations of the same type, without deinitializing either. They may overlap, unlike mem::swap which is otherwise equivalent.

See ptr::swap for safety concerns and examples.

Computes the offset that needs to be applied to the pointer in order to make it aligned to align.

If it is not possible to align the pointer, the implementation returns usize::MAX. It is permissible for the implementation to always return usize::MAX. Only your algorithm’s performance can depend on getting a usable offset here, not its correctness.

The offset is expressed in number of T elements, and not bytes. The value returned can be used with the wrapping_add method.

There are no guarantees whatsoever that offsetting the pointer will not overflow or go beyond the allocation that the pointer points into. It is up to the caller to ensure that the returned offset is correct in all terms other than alignment.

Panics

The function panics if align is not a power-of-two.

Examples

Accessing adjacent u8 as u16

let x = [5u8, 6u8, 7u8, 8u8, 9u8];
let ptr = x.as_ptr().add(n) as *const u8;
let offset = ptr.align_offset(align_of::<u16>());
if offset < x.len() - n - 1 {
    let u16_ptr = ptr.add(offset) as *const u16;
    assert_ne!(*u16_ptr, 500);
} else {
    // while the pointer can be aligned via `offset`, it would point
    // outside the allocation
}
Run
🔬 This is a nightly-only experimental API. (slice_ptr_len #71146)

Returns the length of a raw slice.

The returned value is the number of elements, not the number of bytes.

This function is safe, even when the raw slice cannot be cast to a slice reference because the pointer is null or unaligned.

Examples
#![feature(slice_ptr_len)]
use std::ptr;

let slice: *mut [i8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 3);
assert_eq!(slice.len(), 3);
Run
🔬 This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to the slice’s buffer.

This is equivalent to casting self to *mut T, but more type-safe.

Examples
#![feature(slice_ptr_get)]
use std::ptr;

let slice: *mut [i8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 3);
assert_eq!(slice.as_mut_ptr(), ptr::null_mut());
Run
🔬 This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to an element or subslice, without doing bounds checking.

Calling this method with an out-of-bounds index or when self is not dereferenceable is undefined behavior even if the resulting pointer is not used.

Examples
#![feature(slice_ptr_get)]

let x = &mut [1, 2, 4] as *mut [i32];

unsafe {
    assert_eq!(x.get_unchecked_mut(1), x.as_mut_ptr().add(1));
}
Run
🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a shared slice to the value wrapped in Some. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_slice_mut.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be valid for reads for ptr.len() * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

    • The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.

    • The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().

  • The total size ptr.len() * mem::size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

See also slice::from_raw_parts.

🔬 This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns None if the pointer is null, or else returns a unique slice to the value wrapped in Some. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_slice.

Safety

When calling this method, you have to ensure that either the pointer is null or all of the following is true:

  • The pointer must be valid for reads and writes for ptr.len() * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

    • The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.

    • The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().

  • The total size ptr.len() * mem::size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.

  • You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.

This applies even if the result of this method is unused!

See also slice::from_raw_parts_mut.

Trait Implementations

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Formats the value using the given formatter. Read more

Converts a *mut T into an AtomicPtr<T>.

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

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

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

Restrict a value to a certain interval. Read more

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

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

Restrict a value to a certain interval. Read more

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

This method tests for !=.

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

This method tests for !=.

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

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

Formats the value using the given formatter.

Formats the value using the given formatter.

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.

This documentation is an old archive. Please see https://rust.docs.kernel.org instead.