Primitive Type pointer

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.

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;

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.

Methods

impl<T> *const T where
    T: ?Sized, 

pub fn is_null(self) -> bool

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.

Examples

Basic usage:

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

pub unsafe fn as_ref<'a>(self) -> Option<&'a T>

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

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

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);
}

pub unsafe fn offset(self, count: isize) -> *const T

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);
}

pub fn wrapping_offset(self, count: isize) -> *const T

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

The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe). In particular, the resulting pointer may not be used to access a different allocated object than the one self points to. In other words, x.wrapping_offset(y.wrapping_offset_from(x)) is not the same as y, and dereferencing it is undefined behavior unless x and y point into the same allocated object.

Always use .offset(count) instead when possible, because offset allows the compiler to optimize better. If you need to cross object boundaries, cast the pointer to an integer and do the arithmetic there.

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);
}

pub unsafe fn offset_from(self, origin: *const T) -> isize

🔬 This is a nightly-only experimental API. (ptr_offset_from #41079)

Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes is 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.

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

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

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

The compiler and standard library generally try 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 ptr_into_vec.offset_from(vec.as_ptr()) 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_from instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Panics

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

Examples

Basic usage:

#![feature(ptr_offset_from)]

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);
}

pub fn wrapping_offset_from(self, origin: *const T) -> isize

🔬 This is a nightly-only experimental API. (ptr_wrapping_offset_from #41079)

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

If the address different between the two pointers is not a multiple of mem::size_of::<T>() then the result of the division is rounded towards zero.

Though this method is safe for any two pointers, note that its result will be mostly useless if the two pointers aren't into the same allocated object, for example if they point to two different local variables.

Panics

This function panics if T is a zero-sized type.

Examples

Basic usage:

#![feature(ptr_wrapping_offset_from)]

let a = [0; 5];
let ptr1: *const i32 = &a[1];
let ptr2: *const i32 = &a[3];
assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);
assert_eq!(ptr1.wrapping_offset_from(ptr2), -2);
assert_eq!(ptr1.wrapping_offset(2), ptr2);
assert_eq!(ptr2.wrapping_offset(-2), ptr1);

let ptr1: *const i32 = 3 as _;
let ptr2: *const i32 = 13 as _;
assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);

pub unsafe fn add(self, count: usize) -> *const T

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

pub unsafe fn sub(self, count: usize) -> *const T

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_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";

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

pub fn wrapping_add(self, count: usize) -> *const T

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

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

Always use .add(count) instead when possible, because add 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_add(6);

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

pub fn wrapping_sub(self, count: usize) -> *const T

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

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<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 .sub(count) instead when possible, because sub allows the compiler to optimize better.

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);
}

pub unsafe fn read(self) -> T

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

See ptr::read for safety concerns and examples.

pub unsafe fn read_volatile(self) -> T

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.

pub unsafe fn read_unaligned(self) -> T

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.

pub unsafe fn copy_to(self, dest: *mut T, count: usize)

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.

pub unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)

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.

pub fn align_offset(self, align: usize) -> usize

🔬 This is a nightly-only experimental API. (align_offset #44488)

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_value().

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

There are no guarantees whatsover 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[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
}

impl<T> *mut T where
    T: ?Sized, 

pub fn is_null(self) -> bool

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.

Examples

Basic usage:

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

pub unsafe fn as_ref<'a>(self) -> Option<&'a T>

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 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);
    }
}

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);
}

pub unsafe fn offset(self, count: isize) -> *mut T

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));
}

pub fn wrapping_offset(self, count: isize) -> *mut T

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

The resulting pointer does not need to be in bounds, but it is potentially hazardous to dereference (which requires unsafe). In particular, the resulting pointer may not be used to access a different allocated object than the one self points to. In other words, x.wrapping_offset(y.wrapping_offset_from(x)) is not the same as y, and dereferencing it is undefined behavior unless x and y point into the same allocated object.

Always use .offset(count) instead when possible, because offset allows the compiler to optimize better. If you need to cross object boundaries, cast the pointer to an integer and do the arithmetic there.

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]);

pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T>

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]".

pub unsafe fn offset_from(self, origin: *const T) -> isize

🔬 This is a nightly-only experimental API. (ptr_offset_from #41079)

Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes is 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.

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

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

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

The compiler and standard library generally try 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 ptr_into_vec.offset_from(vec.as_ptr()) 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_from instead if these constraints are difficult to satisfy. The only advantage of this method is that it enables more aggressive compiler optimizations.

Panics

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

Examples

Basic usage:

#![feature(ptr_offset_from)]

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);
}

pub fn wrapping_offset_from(self, origin: *const T) -> isize

🔬 This is a nightly-only experimental API. (ptr_wrapping_offset_from #41079)

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

If the address different between the two pointers is not a multiple of mem::size_of::<T>() then the result of the division is rounded towards zero.

Though this method is safe for any two pointers, note that its result will be mostly useless if the two pointers aren't into the same allocated object, for example if they point to two different local variables.

Panics

This function panics if T is a zero-sized type.

Examples

Basic usage:

#![feature(ptr_wrapping_offset_from)]

let mut a = [0; 5];
let ptr1: *mut i32 = &mut a[1];
let ptr2: *mut i32 = &mut a[3];
assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);
assert_eq!(ptr1.wrapping_offset_from(ptr2), -2);
assert_eq!(ptr1.wrapping_offset(2), ptr2);
assert_eq!(ptr2.wrapping_offset(-2), ptr1);

let ptr1: *mut i32 = 3 as _;
let ptr2: *mut i32 = 13 as _;
assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);

pub unsafe fn add(self, count: usize) -> *mut T

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

pub unsafe fn sub(self, count: usize) -> *mut T

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_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";

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

pub fn wrapping_add(self, count: usize) -> *mut T

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

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

Always use .add(count) instead when possible, because add 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_add(6);

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

pub fn wrapping_sub(self, count: usize) -> *mut T

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

count is in units of T; e.g., a count of 3 represents a pointer offset of 3 * size_of::<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 .sub(count) instead when possible, because sub allows the compiler to optimize better.

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);
}

pub unsafe fn read(self) -> T

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

See ptr::read for safety concerns and examples.

pub unsafe fn read_volatile(self) -> T

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.

pub unsafe fn read_unaligned(self) -> T

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.

pub unsafe fn copy_to(self, dest: *mut T, count: usize)

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.

pub unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)

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.

pub unsafe fn copy_from(self, src: *const T, count: usize)

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.

pub unsafe fn copy_from_nonoverlapping(self, src: *const T, count: usize)

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.

pub unsafe fn drop_in_place(self)

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

See ptr::drop_in_place for safety concerns and examples.

pub unsafe fn write(self, val: T)

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

See ptr::write for safety concerns and examples.

pub unsafe fn write_bytes(self, val: u8, count: usize)

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.

pub unsafe fn write_volatile(self, val: T)

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.

pub unsafe fn write_unaligned(self, val: T)

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.

pub unsafe fn replace(self, src: T) -> T

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

See ptr::replace for safety concerns and examples.

pub unsafe fn swap(self, with: *mut T)

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.

pub fn align_offset(self, align: usize) -> usize

🔬 This is a nightly-only experimental API. (align_offset #44488)

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_value().

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

There are no guarantees whatsover 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[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
}

Trait Implementations

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

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

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

impl<T> Clone for *const T where
    T: ?Sized, 

fn clone(&self) -> *const T

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> Clone for *mut T where
    T: ?Sized, 

fn clone(&self) -> *mut T

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T> PartialOrd<*const T> for *const T where
    T: ?Sized, 

fn partial_cmp(&self, other: &*const T) -> Option<Ordering>

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

fn lt(&self, other: &*const T) -> bool

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

fn le(&self, other: &*const T) -> bool

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

fn gt(&self, other: &*const T) -> bool

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

fn ge(&self, other: &*const T) -> bool

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

impl<T> PartialOrd<*mut T> for *mut T where
    T: ?Sized, 

fn partial_cmp(&self, other: &*mut T) -> Option<Ordering>

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

fn lt(&self, other: &*mut T) -> bool

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

fn le(&self, other: &*mut T) -> bool

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

fn gt(&self, other: &*mut T) -> bool

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

fn ge(&self, other: &*mut T) -> bool

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

impl<T> Ord for *mut T where
    T: ?Sized, 

fn cmp(&self, other: &*mut T) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl<T> Ord for *const T where
    T: ?Sized, 

fn cmp(&self, other: &*const T) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

Compares and returns the minimum of two values.

impl<T> Pointer for *const T where
    T: ?Sized, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Pointer for *mut T where
    T: ?Sized, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T, U> DispatchFromDyn<*const U> for *const T where
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<T, U> DispatchFromDyn<*mut U> for *mut T where
    T: Unsize<U> + ?Sized,
    U: ?Sized, 

impl<T> Eq for *mut T where
    T: ?Sized, 

impl<T> Eq for *const T where
    T: ?Sized, 

impl<T> Debug for *mut T where
    T: ?Sized, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> Debug for *const T where
    T: ?Sized, 

fn fmt(&self, f: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.

impl<T> PartialEq<*const T> for *const T where
    T: ?Sized, 

fn eq(&self, other: &*const T) -> bool

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

#[must_use]

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl<T> PartialEq<*mut T> for *mut T where
    T: ?Sized, 

fn eq(&self, other: &*mut T) -> bool

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

#[must_use]

fn ne(&self, other: &Rhs) -> bool

This method tests for !=.

impl<T> Copy for *const T where
    T: ?Sized, 

impl<T> Copy for *mut T where
    T: ?Sized, 

impl<T> Hash for *const T where
    T: ?Sized, 

fn hash<H>(&self, state: &mut H) where
    H: Hasher, 

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

impl<T> Hash for *mut T where
    T: ?Sized, 

fn hash<H>(&self, state: &mut H) where
    H: Hasher, 

Feeds this value into the given [Hasher].

fn hash_slice<H>(data: &[Self], state: &mut H) where
    H: Hasher, 

Feeds a slice of this type into the given [Hasher].

impl<T> !Sync for *mut T where
    T: ?Sized, 

impl<T> !Sync for *const T where
    T: ?Sized, 

impl<T> !Send for *const T where
    T: ?Sized, 

impl<T> !Send for *mut T where
    T: ?Sized, 

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *const T

impl<T: RefUnwindSafe + ?Sized> UnwindSafe for *mut T