Primitive Type pointer

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());

Casts to a pointer of another type.

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 “dereferencable” 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, for the duration of this lifetime, 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);
    }
}

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);
}

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 “dereferencable” 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, for the duration of this lifetime, 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());
    }
}

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);
}

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);
}

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.

• 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);
}

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
}

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.

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 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);
}

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);
}

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);
}

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);
}

Sets the pointer value to ptr.

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

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

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 = ptr.set_ptr_value(thin.add(8));
    println!("{:?}", &*ptr); // will print "3"
}

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
}

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());

Casts to a pointer of another type.

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 “dereferencable” 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, for the duration of this lifetime, 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);
    }
}

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);
}

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 “dereferencable” 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, for the duration of this lifetime, 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());
    }
}

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));
}

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]);

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 “dereferencable” 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, for the duration of this lifetime, 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]".

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]".

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 “dereferencable” 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, for the duration of this lifetime, 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!

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.

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 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.

• 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);
}

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
}

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);
}

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);
}

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);
}

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);
}

Sets the pointer value to ptr.

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

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

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 = ptr.set_ptr_value(thin.add(8));
    println!("{:?}", &*ptr); // will print "3"
}

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
}

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);

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(), 0 as *const i8);

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 dereferencable 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));
}

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, for the duration of this lifetime, 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 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);

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(), 0 as *mut i8);

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 dereferencable 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));
}

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, for the duration of this lifetime, 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 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, for the duration of this lifetime, 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.