Primitive Type pointer
1.0.0 ·Expand description
Raw, unsafe pointers, *const T
, and *mut T
.
Working with raw pointers in Rust is uncommon, typically limited to a few patterns.
Raw pointers can be unaligned or null
. However, when a raw pointer is
dereferenced (using the *
operator), it must be non-null and aligned.
Storing through a raw pointer using *ptr = data
calls drop
on the old value, so
write
must be used if the type has drop glue and memory is not already
initialized - otherwise drop
would be called on the uninitialized memory.
Use the null
and null_mut
functions to create null pointers, and the
is_null
method of the *const T
and *mut T
types to check for null.
The *const T
and *mut T
types also define the offset
method, for
pointer math.
§Common ways to create raw pointers
§1. Coerce a reference (&T
) or mutable reference (&mut T
).
let my_num: i32 = 10;
let my_num_ptr: *const i32 = &my_num;
let mut my_speed: i32 = 88;
let my_speed_ptr: *mut i32 = &mut my_speed;
RunTo 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;
RunThis 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));
}
RunNote that here the call to drop
is for clarity - it indicates
that we are done with the given value and it should be destroyed.
§3. Create it using ptr::addr_of!
Instead of coercing a reference to a raw pointer, you can use the macros
ptr::addr_of!
(for *const T
) and ptr::addr_of_mut!
(for *mut T
).
These macros allow you to create raw pointers to fields to which you cannot
create a reference (without causing undefined behaviour), such as an
unaligned field. This might be necessary if packed structs or uninitialized
memory is involved.
#[derive(Debug, Default, Copy, Clone)]
#[repr(C, packed)]
struct S {
aligned: u8,
unaligned: u32,
}
let s = S::default();
let p = std::ptr::addr_of!(s.unaligned); // not allowed with coercion
Run§4. Get it from C.
#[allow(unused_extern_crates)]
extern crate libc;
use std::mem;
unsafe {
let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>()) as *mut i32;
if my_num.is_null() {
panic!("failed to allocate memory");
}
libc::free(my_num as *mut libc::c_void);
}
RunUsually you wouldn’t literally use malloc
and free
from Rust,
but C APIs hand out a lot of pointers generally, so are a common source
of raw pointers in Rust.
Implementations§
source§impl<T: ?Sized> *const T
impl<T: ?Sized> *const T
const: unstable · sourcepub fn is_null(self) -> bool
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.
§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
let s: &str = "Follow the rabbit";
let ptr: *const u8 = s.as_ptr();
assert!(!ptr.is_null());
Run1.38.0 (const: 1.38.0) · sourcepub const fn cast<U>(self) -> *const U
pub const fn cast<U>(self) -> *const U
Casts to a pointer of another type.
const: unstable · sourcepub fn with_metadata_of<U>(self, meta: *const U) -> *const Uwhere
U: ?Sized,
🔬This is a nightly-only experimental API. (set_ptr_value
#75091)
pub fn with_metadata_of<U>(self, meta: *const U) -> *const Uwhere
U: ?Sized,
set_ptr_value
#75091)Use the pointer value in a new pointer of another type.
In case meta
is a (fat) pointer to an unsized type, this operation
will ignore the pointer part, whereas for (thin) pointers to sized
types, this has the same effect as a simple cast.
The resulting pointer will have provenance of self
, i.e., for a fat
pointer, this operation is semantically the same as creating a new
fat pointer with the data pointer value of self
but the metadata of
meta
.
§Examples
This function is primarily useful for allowing byte-wise pointer arithmetic on potentially fat pointers:
#![feature(set_ptr_value)]
let arr: [i32; 3] = [1, 2, 3];
let mut ptr = arr.as_ptr() as *const dyn Debug;
let thin = ptr as *const u8;
unsafe {
ptr = thin.add(8).with_metadata_of(ptr);
println!("{:?}", &*ptr); // will print "3"
}
Run1.65.0 (const: 1.65.0) · sourcepub const fn cast_mut(self) -> *mut T
pub const fn cast_mut(self) -> *mut T
Changes constness without changing the type.
This is a bit safer than as
because it wouldn’t silently change the type if the code is
refactored.
sourcepub fn to_bits(self) -> usizewhere
T: Sized,
👎Deprecated since 1.67.0: replaced by the expose_addr
method, or update your code to follow the strict provenance rules using its APIs🔬This is a nightly-only experimental API. (ptr_to_from_bits
#91126)
pub fn to_bits(self) -> usizewhere
T: Sized,
expose_addr
method, or update your code to follow the strict provenance rules using its APIsptr_to_from_bits
#91126)Casts a pointer to its raw bits.
This is equivalent to as usize
, but is more specific to enhance readability.
The inverse method is from_bits
.
In particular, *p as usize
and p as usize
will both compile for
pointers to numeric types but do very different things, so using this
helps emphasize that reading the bits was intentional.
§Examples
#![feature(ptr_to_from_bits)]
let array = [13, 42];
let p0: *const i32 = &array[0];
assert_eq!(<*const _>::from_bits(p0.to_bits()), p0);
let p1: *const i32 = &array[1];
assert_eq!(p1.to_bits() - p0.to_bits(), 4);
Runsourcepub fn from_bits(bits: usize) -> Selfwhere
T: Sized,
👎Deprecated since 1.67.0: replaced by the ptr::from_exposed_addr
function, or update your code to follow the strict provenance rules using its APIs🔬This is a nightly-only experimental API. (ptr_to_from_bits
#91126)
pub fn from_bits(bits: usize) -> Selfwhere
T: Sized,
ptr::from_exposed_addr
function, or update your code to follow the strict provenance rules using its APIsptr_to_from_bits
#91126)Creates a pointer from its raw bits.
This is equivalent to as *const T
, but is more specific to enhance readability.
The inverse method is to_bits
.
§Examples
#![feature(ptr_to_from_bits)]
use std::ptr::NonNull;
let dangling: *const u8 = NonNull::dangling().as_ptr();
assert_eq!(<*const u8>::from_bits(1), dangling);
Runsourcepub fn addr(self) -> usize
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn addr(self) -> usize
strict_provenance
#95228)Gets the “address” portion of the pointer.
This is similar to self as usize
, which semantically discards provenance and
address-space information. However, unlike self as usize
, casting the returned address
back to a pointer yields invalid
, which is undefined behavior to dereference. To
properly restore the lost information and obtain a dereferenceable pointer, use
with_addr
or map_addr
.
If using those APIs is not possible because there is no way to preserve a pointer with the
required provenance, then Strict Provenance might not be for you. Use pointer-integer casts
or expose_addr
and from_exposed_addr
instead. However, note that this makes your code less portable and less amenable to tools
that check for compliance with the Rust memory model.
On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may perform a change of representation to produce a value containing only the address portion of the pointer. What that means is up to the platform to define.
This API and its claimed semantics are part of the Strict Provenance experiment, and as such
might change in the future (including possibly weakening this so it becomes wholly
equivalent to self as usize
). See the module documentation for details.
sourcepub fn expose_addr(self) -> usize
🔬This is a nightly-only experimental API. (exposed_provenance
#95228)
pub fn expose_addr(self) -> usize
exposed_provenance
#95228)Gets the “address” portion of the pointer, and ‘exposes’ the “provenance” part for future
use in from_exposed_addr
.
This is equivalent to self as usize
, which semantically discards provenance and
address-space information. Furthermore, this (like the as
cast) has the implicit
side-effect of marking the provenance as ‘exposed’, so on platforms that support it you can
later call from_exposed_addr
to reconstitute the original pointer including its
provenance. (Reconstructing address space information, if required, is your responsibility.)
Using this method means that code is not following Strict
Provenance rules. Supporting
from_exposed_addr
complicates specification and reasoning and may not be supported by
tools that help you to stay conformant with the Rust memory model, so it is recommended to
use addr
wherever possible.
On most platforms this will produce a value with the same bytes as the original pointer,
because all the bytes are dedicated to describing the address. Platforms which need to store
additional information in the pointer may not support this operation, since the ‘expose’
side-effect which is required for from_exposed_addr
to work is typically not
available.
It is unclear whether this method can be given a satisfying unambiguous specification. This API and its claimed semantics are part of Exposed Provenance.
sourcepub fn with_addr(self, addr: usize) -> Self
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn with_addr(self, addr: usize) -> Self
strict_provenance
#95228)Creates a new pointer with the given address.
This performs the same operation as an addr as ptr
cast, but copies
the address-space and provenance of self
to the new pointer.
This allows us to dynamically preserve and propagate this important
information in a way that is otherwise impossible with a unary cast.
This is equivalent to using wrapping_offset
to offset
self
to the given address, and therefore has all the same capabilities and restrictions.
This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.
sourcepub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
strict_provenance
#95228)Creates a new pointer by mapping self
’s address to a new one.
This is a convenience for with_addr
, see that method for details.
This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.
const: unstable · sourcepub fn to_raw_parts(self) -> (*const (), <T as Pointee>::Metadata)
🔬This is a nightly-only experimental API. (ptr_metadata
#81513)
pub fn to_raw_parts(self) -> (*const (), <T as Pointee>::Metadata)
ptr_metadata
#81513)Decompose a (possibly wide) pointer into its data pointer and metadata components.
The pointer can be later reconstructed with from_raw_parts
.
1.9.0 (const: unstable) · sourcepub unsafe fn as_ref<'a>(self) -> Option<&'a T>
pub unsafe fn as_ref<'a>(self) -> Option<&'a T>
Returns None
if the pointer is null, or else returns a shared reference to
the value wrapped in Some
. If the value may be uninitialized, as_uninit_ref
must be used instead.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
The pointer must point to an initialized instance of
T
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
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
let ptr: *const u8 = &10u8 as *const u8;
unsafe {
if let Some(val_back) = ptr.as_ref() {
println!("We got back the value: {val_back}!");
}
}
Run§Null-unchecked version
If you are sure the pointer can never be null and are looking for some kind of
as_ref_unchecked
that returns the &T
instead of Option<&T>
, know that you can
dereference the pointer directly.
let ptr: *const u8 = &10u8 as *const u8;
unsafe {
let val_back = &*ptr;
println!("We got back the value: {val_back}!");
}
Runconst: unstable · sourcepub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>where
T: Sized,
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>where
T: Sized,
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a shared reference to
the value wrapped in Some
. In contrast to as_ref
, this does not require
that the value has to be initialized.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
This applies even if the result of this method is unused!
§Examples
#![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());
}
}
Runconst: 1.61.0 · sourcepub const unsafe fn offset(self, count: isize) -> *const Twhere
T: Sized,
pub const unsafe fn offset(self, count: isize) -> *const Twhere
T: Sized,
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
let s: &str = "123";
let ptr: *const u8 = s.as_ptr();
unsafe {
println!("{}", *ptr.offset(1) as char);
println!("{}", *ptr.offset(2) as char);
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_offset(self, count: isize) -> Self
pub const unsafe fn byte_offset(self, count: isize) -> Self
Calculates the offset from a pointer in bytes.
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using offset on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.16.0 (const: 1.61.0) · sourcepub const fn wrapping_offset(self, count: isize) -> *const Twhere
T: Sized,
pub const fn wrapping_offset(self, count: isize) -> *const Twhere
T: Sized,
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
// 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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_offset(self, count: isize) -> Self
pub const fn wrapping_byte_offset(self, count: isize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_offset on it. See that method
for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
sourcepub fn mask(self, mask: usize) -> *const T
🔬This is a nightly-only experimental API. (ptr_mask
#98290)
pub fn mask(self, mask: usize) -> *const T
ptr_mask
#98290)Masks out bits of the pointer according to a mask.
This is convenience for ptr.map_addr(|a| a & mask)
.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
§Examples
#![feature(ptr_mask, strict_provenance)]
let v = 17_u32;
let ptr: *const u32 = &v;
// `u32` is 4 bytes aligned,
// which means that lower 2 bits are always 0.
let tag_mask = 0b11;
let ptr_mask = !tag_mask;
// We can store something in these lower bits
let tagged_ptr = ptr.map_addr(|a| a | 0b10);
// Get the "tag" back
let tag = tagged_ptr.addr() & tag_mask;
assert_eq!(tag, 0b10);
// Note that `tagged_ptr` is unaligned, it's UB to read from it.
// To get original pointer `mask` can be used:
let masked_ptr = tagged_ptr.mask(ptr_mask);
assert_eq!(unsafe { *masked_ptr }, 17);
Run1.47.0 (const: 1.65.0) · sourcepub const unsafe fn offset_from(self, origin: *const T) -> isizewhere
T: Sized,
pub const unsafe fn offset_from(self, origin: *const T) -> isizewhere
T: Sized,
Calculates the distance between two pointers. The returned value is in
units of T: the distance in bytes divided by mem::size_of::<T>()
.
This is equivalent to (self as isize - origin as isize) / (mem::size_of::<T>() as isize)
,
except that it has a lot more opportunities for UB, in exchange for the compiler
better understanding what you are doing.
The primary motivation of this method is for computing the len
of an array/slice
of T
that you are currently representing as a “start” and “end” pointer
(and “end” is “one past the end” of the array).
In that case, end.offset_from(start)
gets you the length of the array.
All of the following safety requirements are trivially satisfied for this usecase.
§Safety
If any of the following conditions are violated, the result is Undefined Behavior:
-
Both
self
andorigin
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.)
The requirement for pointers to be derived from the same allocated object is primarily
needed for const
-compatibility: the distance between pointers into different allocated
objects is not known at compile-time. However, the requirement also exists at
runtime and may be exploited by optimizations. If you wish to compute the difference between
pointers that are not guaranteed to be from the same allocation, use (self as isize - origin as isize) / mem::size_of::<T>()
.
§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);
}
RunIncorrect 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
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize
pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize
Calculates the distance between two pointers. The returned value is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using offset_from
on it. See that method for
documentation and safety requirements.
For non-Sized
pointees this operation considers only the data pointers,
ignoring the metadata.
const: unstable · sourcepub unsafe fn sub_ptr(self, origin: *const T) -> usizewhere
T: Sized,
🔬This is a nightly-only experimental API. (ptr_sub_ptr
#95892)
pub unsafe fn sub_ptr(self, origin: *const T) -> usizewhere
T: Sized,
ptr_sub_ptr
#95892)Calculates the distance between two pointers, where it’s known that
self
is equal to or greater than origin
. The returned value is in
units of T: the distance in bytes is divided by mem::size_of::<T>()
.
This computes the same value that offset_from
would compute, but with the added precondition that the offset is
guaranteed to be non-negative. This method is equivalent to
usize::try_from(self.offset_from(origin)).unwrap_unchecked()
,
but it provides slightly more information to the optimizer, which can
sometimes allow it to optimize slightly better with some backends.
This method can be though of as recovering the count
that was passed
to add
(or, with the parameters in the other order,
to sub
). The following are all equivalent, assuming
that their safety preconditions are met:
ptr.sub_ptr(origin) == count
origin.add(count) == ptr
ptr.sub(count) == origin
Run§Safety
-
The distance between the pointers must be non-negative (
self >= origin
) -
All the safety conditions of
offset_from
apply to this method as well; see it for the full details.
Importantly, despite the return type of this method being able to represent
a larger offset, it’s still not permitted to pass pointers which differ
by more than isize::MAX
bytes. As such, the result of this method will
always be less than or equal to isize::MAX as usize
.
§Panics
This function panics if T
is a Zero-Sized Type (“ZST”).
§Examples
#![feature(ptr_sub_ptr)]
let a = [0; 5];
let ptr1: *const i32 = &a[1];
let ptr2: *const i32 = &a[3];
unsafe {
assert_eq!(ptr2.sub_ptr(ptr1), 2);
assert_eq!(ptr1.add(2), ptr2);
assert_eq!(ptr2.sub(2), ptr1);
assert_eq!(ptr2.sub_ptr(ptr2), 0);
}
// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.sub_ptr(ptr2)
Runconst: unstable · sourcepub fn guaranteed_eq(self, other: *const T) -> Option<bool>where
T: Sized,
🔬This is a nightly-only experimental API. (const_raw_ptr_comparison
#53020)
pub fn guaranteed_eq(self, other: *const T) -> Option<bool>where
T: Sized,
const_raw_ptr_comparison
#53020)Returns whether two pointers are guaranteed to be equal.
At runtime this function behaves like Some(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 None
for pointers that later actually turn out to have its equality known.
But when it returns Some
, the pointers’ equality is guaranteed to be known.
The return value may change from Some
to None
and vice versa depending on the compiler
version and unsafe code must not
rely on the result of this function for soundness. It is suggested to only use this function
for performance optimizations where spurious None
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.
const: unstable · sourcepub fn guaranteed_ne(self, other: *const T) -> Option<bool>where
T: Sized,
🔬This is a nightly-only experimental API. (const_raw_ptr_comparison
#53020)
pub fn guaranteed_ne(self, other: *const T) -> Option<bool>where
T: Sized,
const_raw_ptr_comparison
#53020)Returns whether two pointers are guaranteed to be inequal.
At runtime this function behaves like Some(self != other)
.
However, in some contexts (e.g., compile-time evaluation),
it is not always possible to determine inequality of two pointers, so this function may
spuriously return None
for pointers that later actually turn out to have its inequality known.
But when it returns Some
, the pointers’ inequality is guaranteed to be known.
The return value may change from Some
to None
and vice versa depending on the compiler
version and unsafe code must not
rely on the result of this function for soundness. It is suggested to only use this function
for performance optimizations where spurious None
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.
1.26.0 (const: 1.61.0) · sourcepub const unsafe fn add(self, count: usize) -> Selfwhere
T: Sized,
pub const unsafe fn add(self, count: usize) -> Selfwhere
T: Sized,
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
let s: &str = "123";
let ptr: *const u8 = s.as_ptr();
unsafe {
println!("{}", *ptr.add(1) as char);
println!("{}", *ptr.add(2) as char);
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_add(self, count: usize) -> Self
pub const unsafe fn byte_add(self, count: usize) -> Self
Calculates the offset from a pointer in bytes (convenience for .byte_offset(count as isize)
).
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using add on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const unsafe fn sub(self, count: usize) -> Selfwhere
T: Sized,
pub const unsafe fn sub(self, count: usize) -> Selfwhere
T: Sized,
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
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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_sub(self, count: usize) -> Self
pub const unsafe fn byte_sub(self, count: usize) -> Self
Calculates the offset from a pointer in bytes (convenience for
.byte_offset((count as isize).wrapping_neg())
).
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using sub on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const fn wrapping_add(self, count: usize) -> Selfwhere
T: Sized,
pub const fn wrapping_add(self, count: usize) -> Selfwhere
T: Sized,
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
// 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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_add(self, count: usize) -> Self
pub const fn wrapping_byte_add(self, count: usize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
(convenience for .wrapping_byte_offset(count as isize)
)
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_add on it. See that method for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const fn wrapping_sub(self, count: usize) -> Selfwhere
T: Sized,
pub const fn wrapping_sub(self, count: usize) -> Selfwhere
T: Sized,
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
// 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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_sub(self, count: usize) -> Self
pub const fn wrapping_byte_sub(self, count: usize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
(convenience for .wrapping_offset((count as isize).wrapping_neg())
)
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_sub on it. See that method for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.71.0) · sourcepub const unsafe fn read(self) -> Twhere
T: Sized,
pub const unsafe fn read(self) -> Twhere
T: Sized,
Reads the value from self
without moving it. This leaves the
memory in self
unchanged.
See ptr::read
for safety concerns and examples.
1.26.0 · sourcepub unsafe fn read_volatile(self) -> Twhere
T: Sized,
pub unsafe fn read_volatile(self) -> Twhere
T: Sized,
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.
1.26.0 (const: 1.71.0) · sourcepub const unsafe fn read_unaligned(self) -> Twhere
T: Sized,
pub const unsafe fn read_unaligned(self) -> Twhere
T: Sized,
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.
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_to(self, dest: *mut T, count: usize)where
T: Sized,
pub const unsafe fn copy_to(self, dest: *mut T, count: usize)where
T: Sized,
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)where
T: Sized,
pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)where
T: Sized,
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.
1.36.0 (const: unstable) · sourcepub fn align_offset(self, align: usize) -> usizewhere
T: Sized,
pub fn align_offset(self, align: usize) -> usizewhere
T: Sized,
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
use std::mem::align_of;
let x = [5_u8, 6, 7, 8, 9];
let ptr = x.as_ptr();
let offset = ptr.align_offset(align_of::<u16>());
if offset < x.len() - 1 {
let u16_ptr = ptr.add(offset).cast::<u16>();
assert!(*u16_ptr == u16::from_ne_bytes([5, 6]) || *u16_ptr == u16::from_ne_bytes([6, 7]));
} else {
// while the pointer can be aligned via `offset`, it would point
// outside the allocation
}
Runconst: unstable · sourcepub fn is_aligned(self) -> boolwhere
T: Sized,
🔬This is a nightly-only experimental API. (pointer_is_aligned
#96284)
pub fn is_aligned(self) -> boolwhere
T: Sized,
pointer_is_aligned
#96284)Returns whether the pointer is properly aligned for T
.
§Examples
#![feature(pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;
assert!(ptr.is_aligned());
assert!(!ptr.wrapping_byte_add(1).is_aligned());
Run§At compiletime
Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.
At compiletime, the compiler may not know where a value will end up in memory.
Calling this function on a pointer created from a reference at compiletime will only
return true
if the pointer is guaranteed to be aligned. This means that the pointer
is never aligned if cast to a type with a stricter alignment than the reference’s
underlying allocation.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
const _: () = {
let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;
assert!(ptr.is_aligned());
// At runtime either `ptr1` or `ptr2` would be aligned, but at compiletime neither is aligned.
let ptr1 = ptr.cast::<AlignedI64>();
let ptr2 = ptr.wrapping_add(1).cast::<AlignedI64>();
assert!(!ptr1.is_aligned());
assert!(!ptr2.is_aligned());
};
RunDue to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
const COMPTIME_PTR: *const AlignedI32 = &AlignedI32(42);
const _: () = assert!(!COMPTIME_PTR.cast::<AlignedI64>().is_aligned());
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).cast::<AlignedI64>().is_aligned());
// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
runtime_ptr.cast::<AlignedI64>().is_aligned(),
runtime_ptr.wrapping_add(1).cast::<AlignedI64>().is_aligned(),
);
RunIf a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
const _: () = {
let ptr = 40 as *const AlignedI32;
assert!(ptr.is_aligned());
// For pointers with a known address, runtime and compiletime behavior are identical.
let ptr1 = ptr.cast::<AlignedI64>();
let ptr2 = ptr.wrapping_add(1).cast::<AlignedI64>();
assert!(ptr1.is_aligned());
assert!(!ptr2.is_aligned());
};
Runconst: unstable · sourcepub fn is_aligned_to(self, align: usize) -> bool
🔬This is a nightly-only experimental API. (pointer_is_aligned
#96284)
pub fn is_aligned_to(self, align: usize) -> bool
pointer_is_aligned
#96284)Returns whether the pointer is aligned to align
.
For non-Sized
pointees this operation considers only the data pointer,
ignoring the metadata.
§Panics
The function panics if align
is not a power-of-two (this includes 0).
§Examples
#![feature(pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
assert!(ptr.wrapping_byte_add(2).is_aligned_to(2));
assert!(!ptr.wrapping_byte_add(2).is_aligned_to(4));
assert_ne!(ptr.is_aligned_to(8), ptr.wrapping_add(1).is_aligned_to(8));
Run§At compiletime
Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.
At compiletime, the compiler may not know where a value will end up in memory.
Calling this function on a pointer created from a reference at compiletime will only
return true
if the pointer is guaranteed to be aligned. This means that the pointer
cannot be stricter aligned than the reference’s underlying allocation.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
const _: () = {
let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
// At compiletime, we know for sure that the pointer isn't aligned to 8.
assert!(!ptr.is_aligned_to(8));
assert!(!ptr.wrapping_add(1).is_aligned_to(8));
};
RunDue to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
const COMPTIME_PTR: *const AlignedI32 = &AlignedI32(42);
const _: () = assert!(!COMPTIME_PTR.is_aligned_to(8));
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).is_aligned_to(8));
// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
runtime_ptr.is_aligned_to(8),
runtime_ptr.wrapping_add(1).is_aligned_to(8),
);
RunIf a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
const _: () = {
let ptr = 40 as *const u8;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
assert!(ptr.is_aligned_to(8));
assert!(!ptr.is_aligned_to(16));
};
Runsource§impl<T> *const [T]
impl<T> *const [T]
const: unstable · sourcepub fn len(self) -> usize
🔬This is a nightly-only experimental API. (slice_ptr_len
#71146)
pub fn len(self) -> usize
slice_ptr_len
#71146)Returns the length of a raw slice.
The returned value is the number of elements, not the number of bytes.
This function is safe, even when the raw slice cannot be cast to a slice reference because the pointer is null or unaligned.
§Examples
#![feature(slice_ptr_len)]
use std::ptr;
let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
assert_eq!(slice.len(), 3);
Runconst: unstable · sourcepub fn is_empty(self) -> bool
🔬This is a nightly-only experimental API. (slice_ptr_len
#71146)
pub fn is_empty(self) -> bool
slice_ptr_len
#71146)const: unstable · sourcepub fn as_ptr(self) -> *const T
🔬This is a nightly-only experimental API. (slice_ptr_get
#74265)
pub fn as_ptr(self) -> *const T
slice_ptr_get
#74265)sourcepub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Outputwhere
I: SliceIndex<[T]>,
🔬This is a nightly-only experimental API. (slice_ptr_get
#74265)
pub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Outputwhere
I: SliceIndex<[T]>,
slice_ptr_get
#74265)Returns a raw pointer to an element or subslice, without doing bounds checking.
Calling this method with an out-of-bounds index or when self
is not dereferenceable
is undefined behavior even if the resulting pointer is not used.
§Examples
#![feature(slice_ptr_get)]
let x = &[1, 2, 4] as *const [i32];
unsafe {
assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
}
Runconst: unstable · sourcepub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a shared slice to
the value wrapped in Some
. In contrast to as_ref
, this does not require
that the value has to be initialized.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be valid for reads for
ptr.len() * mem::size_of::<T>()
many bytes, and it must be properly aligned. This means in particular:-
The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.
-
The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as
data
for zero-length slices usingNonNull::dangling()
.
-
-
The total size
ptr.len() * mem::size_of::<T>()
of the slice must be no larger thanisize::MAX
. See the safety documentation ofpointer::offset
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
This applies even if the result of this method is unused!
See also slice::from_raw_parts
.
source§impl<T: ?Sized> *mut T
impl<T: ?Sized> *mut T
const: unstable · sourcepub fn is_null(self) -> bool
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.
§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
let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
assert!(!ptr.is_null());
Run1.38.0 (const: 1.38.0) · sourcepub const fn cast<U>(self) -> *mut U
pub const fn cast<U>(self) -> *mut U
Casts to a pointer of another type.
const: unstable · sourcepub fn with_metadata_of<U>(self, meta: *const U) -> *mut Uwhere
U: ?Sized,
🔬This is a nightly-only experimental API. (set_ptr_value
#75091)
pub fn with_metadata_of<U>(self, meta: *const U) -> *mut Uwhere
U: ?Sized,
set_ptr_value
#75091)Use the pointer value in a new pointer of another type.
In case meta
is a (fat) pointer to an unsized type, this operation
will ignore the pointer part, whereas for (thin) pointers to sized
types, this has the same effect as a simple cast.
The resulting pointer will have provenance of self
, i.e., for a fat
pointer, this operation is semantically the same as creating a new
fat pointer with the data pointer value of self
but the metadata of
meta
.
§Examples
This function is primarily useful for allowing byte-wise pointer arithmetic on potentially fat pointers:
#![feature(set_ptr_value)]
let mut arr: [i32; 3] = [1, 2, 3];
let mut ptr = arr.as_mut_ptr() as *mut dyn Debug;
let thin = ptr as *mut u8;
unsafe {
ptr = thin.add(8).with_metadata_of(ptr);
println!("{:?}", &*ptr); // will print "3"
}
Run1.65.0 (const: 1.65.0) · sourcepub const fn cast_const(self) -> *const T
pub const fn cast_const(self) -> *const T
Changes constness without changing the type.
This is a bit safer than as
because it wouldn’t silently change the type if the code is
refactored.
While not strictly required (*mut T
coerces to *const T
), this is provided for symmetry
with cast_mut
on *const T
and may have documentation value if used instead of implicit
coercion.
sourcepub fn to_bits(self) -> usizewhere
T: Sized,
👎Deprecated since 1.67.0: replaced by the expose_addr
method, or update your code to follow the strict provenance rules using its APIs🔬This is a nightly-only experimental API. (ptr_to_from_bits
#91126)
pub fn to_bits(self) -> usizewhere
T: Sized,
expose_addr
method, or update your code to follow the strict provenance rules using its APIsptr_to_from_bits
#91126)Casts a pointer to its raw bits.
This is equivalent to as usize
, but is more specific to enhance readability.
The inverse method is from_bits
.
In particular, *p as usize
and p as usize
will both compile for
pointers to numeric types but do very different things, so using this
helps emphasize that reading the bits was intentional.
§Examples
#![feature(ptr_to_from_bits)]
let mut array = [13, 42];
let mut it = array.iter_mut();
let p0: *mut i32 = it.next().unwrap();
assert_eq!(<*mut _>::from_bits(p0.to_bits()), p0);
let p1: *mut i32 = it.next().unwrap();
assert_eq!(p1.to_bits() - p0.to_bits(), 4);
}
Runsourcepub fn from_bits(bits: usize) -> Selfwhere
T: Sized,
👎Deprecated since 1.67.0: replaced by the ptr::from_exposed_addr_mut
function, or update your code to follow the strict provenance rules using its APIs🔬This is a nightly-only experimental API. (ptr_to_from_bits
#91126)
pub fn from_bits(bits: usize) -> Selfwhere
T: Sized,
ptr::from_exposed_addr_mut
function, or update your code to follow the strict provenance rules using its APIsptr_to_from_bits
#91126)Creates a pointer from its raw bits.
This is equivalent to as *mut T
, but is more specific to enhance readability.
The inverse method is to_bits
.
§Examples
#![feature(ptr_to_from_bits)]
use std::ptr::NonNull;
let dangling: *mut u8 = NonNull::dangling().as_ptr();
assert_eq!(<*mut u8>::from_bits(1), dangling);
}
Runsourcepub fn addr(self) -> usize
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn addr(self) -> usize
strict_provenance
#95228)Gets the “address” portion of the pointer.
This is similar to self as usize
, which semantically discards provenance and
address-space information. However, unlike self as usize
, casting the returned address
back to a pointer yields invalid
, which is undefined behavior to dereference. To
properly restore the lost information and obtain a dereferenceable pointer, use
with_addr
or map_addr
.
If using those APIs is not possible because there is no way to preserve a pointer with the
required provenance, then Strict Provenance might not be for you. Use pointer-integer casts
or expose_addr
and from_exposed_addr
instead. However, note that this makes your code less portable and less amenable to tools
that check for compliance with the Rust memory model.
On most platforms this will produce a value with the same bytes as the original pointer, because all the bytes are dedicated to describing the address. Platforms which need to store additional information in the pointer may perform a change of representation to produce a value containing only the address portion of the pointer. What that means is up to the platform to define.
This API and its claimed semantics are part of the Strict Provenance experiment, and as such
might change in the future (including possibly weakening this so it becomes wholly
equivalent to self as usize
). See the module documentation for details.
sourcepub fn expose_addr(self) -> usize
🔬This is a nightly-only experimental API. (exposed_provenance
#95228)
pub fn expose_addr(self) -> usize
exposed_provenance
#95228)Gets the “address” portion of the pointer, and ‘exposes’ the “provenance” part for future
use in from_exposed_addr
.
This is equivalent to self as usize
, which semantically discards provenance and
address-space information. Furthermore, this (like the as
cast) has the implicit
side-effect of marking the provenance as ‘exposed’, so on platforms that support it you can
later call from_exposed_addr_mut
to reconstitute the original pointer including its
provenance. (Reconstructing address space information, if required, is your responsibility.)
Using this method means that code is not following Strict
Provenance rules. Supporting
from_exposed_addr_mut
complicates specification and reasoning and may not be supported
by tools that help you to stay conformant with the Rust memory model, so it is recommended
to use addr
wherever possible.
On most platforms this will produce a value with the same bytes as the original pointer,
because all the bytes are dedicated to describing the address. Platforms which need to store
additional information in the pointer may not support this operation, since the ‘expose’
side-effect which is required for from_exposed_addr_mut
to work is typically not
available.
It is unclear whether this method can be given a satisfying unambiguous specification. This API and its claimed semantics are part of Exposed Provenance.
sourcepub fn with_addr(self, addr: usize) -> Self
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn with_addr(self, addr: usize) -> Self
strict_provenance
#95228)Creates a new pointer with the given address.
This performs the same operation as an addr as ptr
cast, but copies
the address-space and provenance of self
to the new pointer.
This allows us to dynamically preserve and propagate this important
information in a way that is otherwise impossible with a unary cast.
This is equivalent to using wrapping_offset
to offset
self
to the given address, and therefore has all the same capabilities and restrictions.
This API and its claimed semantics are an extension to the Strict Provenance experiment, see the module documentation for details.
sourcepub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
🔬This is a nightly-only experimental API. (strict_provenance
#95228)
pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
strict_provenance
#95228)Creates a new pointer by mapping self
’s address to a new one.
This is a convenience for with_addr
, see that method for details.
This API and its claimed semantics are part of the Strict Provenance experiment, see the module documentation for details.
const: unstable · sourcepub fn to_raw_parts(self) -> (*mut (), <T as Pointee>::Metadata)
🔬This is a nightly-only experimental API. (ptr_metadata
#81513)
pub fn to_raw_parts(self) -> (*mut (), <T as Pointee>::Metadata)
ptr_metadata
#81513)Decompose a (possibly wide) pointer into its data pointer and metadata components.
The pointer can be later reconstructed with from_raw_parts_mut
.
1.9.0 (const: unstable) · sourcepub unsafe fn as_ref<'a>(self) -> Option<&'a T>
pub unsafe fn as_ref<'a>(self) -> Option<&'a T>
Returns None
if the pointer is null, or else returns a shared reference to
the value wrapped in Some
. If the value may be uninitialized, as_uninit_ref
must be used instead.
For the mutable counterpart see as_mut
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
The pointer must point to an initialized instance of
T
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
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
let ptr: *mut u8 = &mut 10u8 as *mut u8;
unsafe {
if let Some(val_back) = ptr.as_ref() {
println!("We got back the value: {val_back}!");
}
}
Run§Null-unchecked version
If you are sure the pointer can never be null and are looking for some kind of
as_ref_unchecked
that returns the &T
instead of Option<&T>
, know that you can
dereference the pointer directly.
let ptr: *mut u8 = &mut 10u8 as *mut u8;
unsafe {
let val_back = &*ptr;
println!("We got back the value: {val_back}!");
}
Runconst: unstable · sourcepub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>where
T: Sized,
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>where
T: Sized,
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a shared reference to
the value wrapped in Some
. In contrast to as_ref
, this does not require
that the value has to be initialized.
For the mutable counterpart see as_uninit_mut
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
This applies even if the result of this method is unused!
§Examples
#![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());
}
}
Runconst: 1.61.0 · sourcepub const unsafe fn offset(self, count: isize) -> *mut Twhere
T: Sized,
pub const unsafe fn offset(self, count: isize) -> *mut Twhere
T: Sized,
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
let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
unsafe {
println!("{}", *ptr.offset(1));
println!("{}", *ptr.offset(2));
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_offset(self, count: isize) -> Self
pub const unsafe fn byte_offset(self, count: isize) -> Self
Calculates the offset from a pointer in bytes.
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using offset on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.16.0 (const: 1.61.0) · sourcepub const fn wrapping_offset(self, count: isize) -> *mut Twhere
T: Sized,
pub const fn wrapping_offset(self, count: isize) -> *mut Twhere
T: Sized,
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
// 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]);
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_offset(self, count: isize) -> Self
pub const fn wrapping_byte_offset(self, count: isize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_offset on it. See that method
for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
sourcepub fn mask(self, mask: usize) -> *mut T
🔬This is a nightly-only experimental API. (ptr_mask
#98290)
pub fn mask(self, mask: usize) -> *mut T
ptr_mask
#98290)Masks out bits of the pointer according to a mask.
This is convenience for ptr.map_addr(|a| a & mask)
.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
§Examples
#![feature(ptr_mask, strict_provenance)]
let mut v = 17_u32;
let ptr: *mut u32 = &mut v;
// `u32` is 4 bytes aligned,
// which means that lower 2 bits are always 0.
let tag_mask = 0b11;
let ptr_mask = !tag_mask;
// We can store something in these lower bits
let tagged_ptr = ptr.map_addr(|a| a | 0b10);
// Get the "tag" back
let tag = tagged_ptr.addr() & tag_mask;
assert_eq!(tag, 0b10);
// Note that `tagged_ptr` is unaligned, it's UB to read from/write to it.
// To get original pointer `mask` can be used:
let masked_ptr = tagged_ptr.mask(ptr_mask);
assert_eq!(unsafe { *masked_ptr }, 17);
unsafe { *masked_ptr = 0 };
assert_eq!(v, 0);
Run1.9.0 (const: unstable) · sourcepub unsafe fn as_mut<'a>(self) -> Option<&'a mut T>
pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T>
Returns None
if the pointer is null, or else returns a unique reference to
the value wrapped in Some
. If the value may be uninitialized, as_uninit_mut
must be used instead.
For the shared counterpart see as_ref
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
The pointer must point to an initialized instance of
T
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.
This applies even if the result of this method is unused! (The part about being initialized is not yet fully decided, but until it is, the only safe approach is to ensure that they are indeed initialized.)
§Examples
let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
let first_value = unsafe { ptr.as_mut().unwrap() };
*first_value = 4;
println!("{s:?}"); // It'll print: "[4, 2, 3]".
Run§Null-unchecked version
If you are sure the pointer can never be null and are looking for some kind of
as_mut_unchecked
that returns the &mut T
instead of Option<&mut T>
, know that
you can dereference the pointer directly.
let mut s = [1, 2, 3];
let ptr: *mut u32 = s.as_mut_ptr();
let first_value = unsafe { &mut *ptr };
*first_value = 4;
println!("{s:?}"); // It'll print: "[4, 2, 3]".
Runconst: unstable · sourcepub unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>where
T: Sized,
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_mut<'a>(self) -> Option<&'a mut MaybeUninit<T>>where
T: Sized,
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a unique reference to
the value wrapped in Some
. In contrast to as_mut
, this does not require
that the value has to be initialized.
For the shared counterpart see as_uninit_ref
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be properly aligned.
-
It must be “dereferenceable” in the sense defined in the module documentation.
-
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.
This applies even if the result of this method is unused!
const: unstable · sourcepub fn guaranteed_eq(self, other: *mut T) -> Option<bool>where
T: Sized,
🔬This is a nightly-only experimental API. (const_raw_ptr_comparison
#53020)
pub fn guaranteed_eq(self, other: *mut T) -> Option<bool>where
T: Sized,
const_raw_ptr_comparison
#53020)Returns whether two pointers are guaranteed to be equal.
At runtime this function behaves like Some(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 None
for pointers that later actually turn out to have its equality known.
But when it returns Some
, the pointers’ equality is guaranteed to be known.
The return value may change from Some
to None
and vice versa depending on the compiler
version and unsafe code must not
rely on the result of this function for soundness. It is suggested to only use this function
for performance optimizations where spurious None
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.
const: unstable · sourcepub fn guaranteed_ne(self, other: *mut T) -> Option<bool>where
T: Sized,
🔬This is a nightly-only experimental API. (const_raw_ptr_comparison
#53020)
pub fn guaranteed_ne(self, other: *mut T) -> Option<bool>where
T: Sized,
const_raw_ptr_comparison
#53020)Returns whether two pointers are guaranteed to be inequal.
At runtime this function behaves like Some(self != other)
.
However, in some contexts (e.g., compile-time evaluation),
it is not always possible to determine inequality of two pointers, so this function may
spuriously return None
for pointers that later actually turn out to have its inequality known.
But when it returns Some
, the pointers’ inequality is guaranteed to be known.
The return value may change from Some
to None
and vice versa depending on the compiler
version and unsafe code must not
rely on the result of this function for soundness. It is suggested to only use this function
for performance optimizations where spurious None
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.
1.47.0 (const: 1.65.0) · sourcepub const unsafe fn offset_from(self, origin: *const T) -> isizewhere
T: Sized,
pub const unsafe fn offset_from(self, origin: *const T) -> isizewhere
T: Sized,
Calculates the distance between two pointers. The returned value is in
units of T: the distance in bytes divided by mem::size_of::<T>()
.
This is equivalent to (self as isize - origin as isize) / (mem::size_of::<T>() as isize)
,
except that it has a lot more opportunities for UB, in exchange for the compiler
better understanding what you are doing.
The primary motivation of this method is for computing the len
of an array/slice
of T
that you are currently representing as a “start” and “end” pointer
(and “end” is “one past the end” of the array).
In that case, end.offset_from(start)
gets you the length of the array.
All of the following safety requirements are trivially satisfied for this usecase.
§Safety
If any of the following conditions are violated, the result is Undefined Behavior:
-
Both
self
andorigin
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.)
The requirement for pointers to be derived from the same allocated object is primarily
needed for const
-compatibility: the distance between pointers into different allocated
objects is not known at compile-time. However, the requirement also exists at
runtime and may be exploited by optimizations. If you wish to compute the difference between
pointers that are not guaranteed to be from the same allocation, use (self as isize - origin as isize) / mem::size_of::<T>()
.
§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);
}
RunIncorrect 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
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize
pub const unsafe fn byte_offset_from<U: ?Sized>(self, origin: *const U) -> isize
Calculates the distance between two pointers. The returned value is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using offset_from
on it. See that method for
documentation and safety requirements.
For non-Sized
pointees this operation considers only the data pointers,
ignoring the metadata.
const: unstable · sourcepub unsafe fn sub_ptr(self, origin: *const T) -> usizewhere
T: Sized,
🔬This is a nightly-only experimental API. (ptr_sub_ptr
#95892)
pub unsafe fn sub_ptr(self, origin: *const T) -> usizewhere
T: Sized,
ptr_sub_ptr
#95892)Calculates the distance between two pointers, where it’s known that
self
is equal to or greater than origin
. The returned value is in
units of T: the distance in bytes is divided by mem::size_of::<T>()
.
This computes the same value that offset_from
would compute, but with the added precondition that the offset is
guaranteed to be non-negative. This method is equivalent to
usize::try_from(self.offset_from(origin)).unwrap_unchecked()
,
but it provides slightly more information to the optimizer, which can
sometimes allow it to optimize slightly better with some backends.
This method can be though of as recovering the count
that was passed
to add
(or, with the parameters in the other order,
to sub
). The following are all equivalent, assuming
that their safety preconditions are met:
ptr.sub_ptr(origin) == count
origin.add(count) == ptr
ptr.sub(count) == origin
Run§Safety
-
The distance between the pointers must be non-negative (
self >= origin
) -
All the safety conditions of
offset_from
apply to this method as well; see it for the full details.
Importantly, despite the return type of this method being able to represent
a larger offset, it’s still not permitted to pass pointers which differ
by more than isize::MAX
bytes. As such, the result of this method will
always be less than or equal to isize::MAX as usize
.
§Panics
This function panics if T
is a Zero-Sized Type (“ZST”).
§Examples
#![feature(ptr_sub_ptr)]
let mut a = [0; 5];
let p: *mut i32 = a.as_mut_ptr();
unsafe {
let ptr1: *mut i32 = p.add(1);
let ptr2: *mut i32 = p.add(3);
assert_eq!(ptr2.sub_ptr(ptr1), 2);
assert_eq!(ptr1.add(2), ptr2);
assert_eq!(ptr2.sub(2), ptr1);
assert_eq!(ptr2.sub_ptr(ptr2), 0);
}
// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.offset_from(ptr2)
Run1.26.0 (const: 1.61.0) · sourcepub const unsafe fn add(self, count: usize) -> Selfwhere
T: Sized,
pub const unsafe fn add(self, count: usize) -> Selfwhere
T: Sized,
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
let s: &str = "123";
let ptr: *const u8 = s.as_ptr();
unsafe {
println!("{}", *ptr.add(1) as char);
println!("{}", *ptr.add(2) as char);
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_add(self, count: usize) -> Self
pub const unsafe fn byte_add(self, count: usize) -> Self
Calculates the offset from a pointer in bytes (convenience for .byte_offset(count as isize)
).
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using add on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const unsafe fn sub(self, count: usize) -> Selfwhere
T: Sized,
pub const unsafe fn sub(self, count: usize) -> Selfwhere
T: Sized,
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
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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const unsafe fn byte_sub(self, count: usize) -> Self
pub const unsafe fn byte_sub(self, count: usize) -> Self
Calculates the offset from a pointer in bytes (convenience for
.byte_offset((count as isize).wrapping_neg())
).
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using sub on it. See that method for documentation
and safety requirements.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const fn wrapping_add(self, count: usize) -> Selfwhere
T: Sized,
pub const fn wrapping_add(self, count: usize) -> Selfwhere
T: Sized,
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
// 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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_add(self, count: usize) -> Self
pub const fn wrapping_byte_add(self, count: usize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
(convenience for .wrapping_byte_offset(count as isize)
)
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_add on it. See that method for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.61.0) · sourcepub const fn wrapping_sub(self, count: usize) -> Selfwhere
T: Sized,
pub const fn wrapping_sub(self, count: usize) -> Selfwhere
T: Sized,
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
// 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);
}
Run1.75.0 (const: 1.75.0) · sourcepub const fn wrapping_byte_sub(self, count: usize) -> Self
pub const fn wrapping_byte_sub(self, count: usize) -> Self
Calculates the offset from a pointer in bytes using wrapping arithmetic.
(convenience for .wrapping_offset((count as isize).wrapping_neg())
)
count
is in units of bytes.
This is purely a convenience for casting to a u8
pointer and
using wrapping_sub on it. See that method for documentation.
For non-Sized
pointees this operation changes only the data pointer,
leaving the metadata untouched.
1.26.0 (const: 1.71.0) · sourcepub const unsafe fn read(self) -> Twhere
T: Sized,
pub const unsafe fn read(self) -> Twhere
T: Sized,
Reads the value from self
without moving it. This leaves the
memory in self
unchanged.
See ptr::read
for safety concerns and examples.
1.26.0 · sourcepub unsafe fn read_volatile(self) -> Twhere
T: Sized,
pub unsafe fn read_volatile(self) -> Twhere
T: Sized,
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.
1.26.0 (const: 1.71.0) · sourcepub const unsafe fn read_unaligned(self) -> Twhere
T: Sized,
pub const unsafe fn read_unaligned(self) -> Twhere
T: Sized,
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.
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_to(self, dest: *mut T, count: usize)where
T: Sized,
pub const unsafe fn copy_to(self, dest: *mut T, count: usize)where
T: Sized,
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)where
T: Sized,
pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)where
T: Sized,
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.
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_from(self, src: *const T, count: usize)where
T: Sized,
pub const unsafe fn copy_from(self, src: *const T, count: usize)where
T: Sized,
1.26.0 (const: 1.63.0) · sourcepub const unsafe fn copy_from_nonoverlapping(self, src: *const T, count: usize)where
T: Sized,
pub const unsafe fn copy_from_nonoverlapping(self, src: *const T, count: usize)where
T: Sized,
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.
1.26.0 · sourcepub unsafe fn drop_in_place(self)
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.
1.26.0 (const: unstable) · sourcepub unsafe fn write(self, val: T)where
T: Sized,
pub unsafe fn write(self, val: T)where
T: Sized,
Overwrites a memory location with the given value without reading or dropping the old value.
See ptr::write
for safety concerns and examples.
1.26.0 (const: unstable) · sourcepub unsafe fn write_bytes(self, val: u8, count: usize)where
T: Sized,
pub unsafe fn write_bytes(self, val: u8, count: usize)where
T: Sized,
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.
1.26.0 · sourcepub unsafe fn write_volatile(self, val: T)where
T: Sized,
pub unsafe fn write_volatile(self, val: T)where
T: Sized,
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.
1.26.0 (const: unstable) · sourcepub unsafe fn write_unaligned(self, val: T)where
T: Sized,
pub unsafe fn write_unaligned(self, val: T)where
T: Sized,
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.
1.26.0 · sourcepub unsafe fn replace(self, src: T) -> Twhere
T: Sized,
pub unsafe fn replace(self, src: T) -> Twhere
T: Sized,
Replaces the value at self
with src
, returning the old
value, without dropping either.
See ptr::replace
for safety concerns and examples.
1.26.0 (const: unstable) · sourcepub unsafe fn swap(self, with: *mut T)where
T: Sized,
pub unsafe fn swap(self, with: *mut T)where
T: Sized,
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.
1.36.0 (const: unstable) · sourcepub fn align_offset(self, align: usize) -> usizewhere
T: Sized,
pub fn align_offset(self, align: usize) -> usizewhere
T: Sized,
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
use std::mem::align_of;
let mut x = [5_u8, 6, 7, 8, 9];
let ptr = x.as_mut_ptr();
let offset = ptr.align_offset(align_of::<u16>());
if offset < x.len() - 1 {
let u16_ptr = ptr.add(offset).cast::<u16>();
*u16_ptr = 0;
assert!(x == [0, 0, 7, 8, 9] || x == [5, 0, 0, 8, 9]);
} else {
// while the pointer can be aligned via `offset`, it would point
// outside the allocation
}
Runconst: unstable · sourcepub fn is_aligned(self) -> boolwhere
T: Sized,
🔬This is a nightly-only experimental API. (pointer_is_aligned
#96284)
pub fn is_aligned(self) -> boolwhere
T: Sized,
pointer_is_aligned
#96284)Returns whether the pointer is properly aligned for T
.
§Examples
#![feature(pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
let mut data = AlignedI32(42);
let ptr = &mut data as *mut AlignedI32;
assert!(ptr.is_aligned());
assert!(!ptr.wrapping_byte_add(1).is_aligned());
Run§At compiletime
Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.
At compiletime, the compiler may not know where a value will end up in memory.
Calling this function on a pointer created from a reference at compiletime will only
return true
if the pointer is guaranteed to be aligned. This means that the pointer
is never aligned if cast to a type with a stricter alignment than the reference’s
underlying allocation.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
#![feature(const_mut_refs)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
const _: () = {
let mut data = AlignedI32(42);
let ptr = &mut data as *mut AlignedI32;
assert!(ptr.is_aligned());
// At runtime either `ptr1` or `ptr2` would be aligned, but at compiletime neither is aligned.
let ptr1 = ptr.cast::<AlignedI64>();
let ptr2 = ptr.wrapping_add(1).cast::<AlignedI64>();
assert!(!ptr1.is_aligned());
assert!(!ptr2.is_aligned());
};
RunDue to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
// Also, note that mutable references are not allowed in the final value of constants.
const COMPTIME_PTR: *mut AlignedI32 = (&AlignedI32(42) as *const AlignedI32).cast_mut();
const _: () = assert!(!COMPTIME_PTR.cast::<AlignedI64>().is_aligned());
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).cast::<AlignedI64>().is_aligned());
// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
runtime_ptr.cast::<AlignedI64>().is_aligned(),
runtime_ptr.wrapping_add(1).cast::<AlignedI64>().is_aligned(),
);
RunIf a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);
const _: () = {
let ptr = 40 as *mut AlignedI32;
assert!(ptr.is_aligned());
// For pointers with a known address, runtime and compiletime behavior are identical.
let ptr1 = ptr.cast::<AlignedI64>();
let ptr2 = ptr.wrapping_add(1).cast::<AlignedI64>();
assert!(ptr1.is_aligned());
assert!(!ptr2.is_aligned());
};
Runconst: unstable · sourcepub fn is_aligned_to(self, align: usize) -> bool
🔬This is a nightly-only experimental API. (pointer_is_aligned
#96284)
pub fn is_aligned_to(self, align: usize) -> bool
pointer_is_aligned
#96284)Returns whether the pointer is aligned to align
.
For non-Sized
pointees this operation considers only the data pointer,
ignoring the metadata.
§Panics
The function panics if align
is not a power-of-two (this includes 0).
§Examples
#![feature(pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
let mut data = AlignedI32(42);
let ptr = &mut data as *mut AlignedI32;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
assert!(ptr.wrapping_byte_add(2).is_aligned_to(2));
assert!(!ptr.wrapping_byte_add(2).is_aligned_to(4));
assert_ne!(ptr.is_aligned_to(8), ptr.wrapping_add(1).is_aligned_to(8));
Run§At compiletime
Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.
At compiletime, the compiler may not know where a value will end up in memory.
Calling this function on a pointer created from a reference at compiletime will only
return true
if the pointer is guaranteed to be aligned. This means that the pointer
cannot be stricter aligned than the reference’s underlying allocation.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
#![feature(const_mut_refs)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
const _: () = {
let mut data = AlignedI32(42);
let ptr = &mut data as *mut AlignedI32;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
// At compiletime, we know for sure that the pointer isn't aligned to 8.
assert!(!ptr.is_aligned_to(8));
assert!(!ptr.wrapping_add(1).is_aligned_to(8));
};
RunDue to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);
// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
// Also, note that mutable references are not allowed in the final value of constants.
const COMPTIME_PTR: *mut AlignedI32 = (&AlignedI32(42) as *const AlignedI32).cast_mut();
const _: () = assert!(!COMPTIME_PTR.is_aligned_to(8));
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).is_aligned_to(8));
// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
runtime_ptr.is_aligned_to(8),
runtime_ptr.wrapping_add(1).is_aligned_to(8),
);
RunIf a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.
#![feature(pointer_is_aligned)]
#![feature(const_pointer_is_aligned)]
const _: () = {
let ptr = 40 as *mut u8;
assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));
assert!(ptr.is_aligned_to(8));
assert!(!ptr.is_aligned_to(16));
};
Runsource§impl<T> *mut [T]
impl<T> *mut [T]
const: unstable · sourcepub fn len(self) -> usize
🔬This is a nightly-only experimental API. (slice_ptr_len
#71146)
pub fn len(self) -> usize
slice_ptr_len
#71146)Returns the length of a raw slice.
The returned value is the number of elements, not the number of bytes.
This function is safe, even when the raw slice cannot be cast to a slice reference because the pointer is null or unaligned.
§Examples
#![feature(slice_ptr_len)]
use std::ptr;
let slice: *mut [i8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 3);
assert_eq!(slice.len(), 3);
Runconst: unstable · sourcepub fn is_empty(self) -> bool
🔬This is a nightly-only experimental API. (slice_ptr_len
#71146)
pub fn is_empty(self) -> bool
slice_ptr_len
#71146)sourcepub unsafe fn split_at_mut(self, mid: usize) -> (*mut [T], *mut [T])
🔬This is a nightly-only experimental API. (raw_slice_split
#95595)
pub unsafe fn split_at_mut(self, mid: usize) -> (*mut [T], *mut [T])
raw_slice_split
#95595)Divides one mutable raw slice into two at an index.
The first will contain all indices from [0, mid)
(excluding
the index mid
itself) and the second will contain all
indices from [mid, len)
(excluding the index len
itself).
§Panics
Panics if mid > len
.
§Safety
mid
must be in-bounds of the underlying allocated object.
Which means self
must be dereferenceable and span a single allocation
that is at least mid * size_of::<T>()
bytes long. Not upholding these
requirements is undefined behavior even if the resulting pointers are not used.
Since len
being in-bounds it is not a safety invariant of *mut [T]
the
safety requirements of this method are the same as for split_at_mut_unchecked
.
The explicit bounds check is only as useful as len
is correct.
§Examples
#![feature(raw_slice_split)]
#![feature(slice_ptr_get)]
let mut v = [1, 0, 3, 0, 5, 6];
let ptr = &mut v as *mut [_];
unsafe {
let (left, right) = ptr.split_at_mut(2);
assert_eq!(&*left, [1, 0]);
assert_eq!(&*right, [3, 0, 5, 6]);
}
Runsourcepub unsafe fn split_at_mut_unchecked(self, mid: usize) -> (*mut [T], *mut [T])
🔬This is a nightly-only experimental API. (raw_slice_split
#95595)
pub unsafe fn split_at_mut_unchecked(self, mid: usize) -> (*mut [T], *mut [T])
raw_slice_split
#95595)Divides one mutable raw slice into two at an index, without doing bounds checking.
The first will contain all indices from [0, mid)
(excluding
the index mid
itself) and the second will contain all
indices from [mid, len)
(excluding the index len
itself).
§Safety
mid
must be in-bounds of the underlying [allocated object].
Which means self
must be dereferenceable and span a single allocation
that is at least mid * size_of::<T>()
bytes long. Not upholding these
requirements is undefined behavior even if the resulting pointers are not used.
§Examples
#![feature(raw_slice_split)]
let mut v = [1, 0, 3, 0, 5, 6];
// scoped to restrict the lifetime of the borrows
unsafe {
let ptr = &mut v as *mut [_];
let (left, right) = ptr.split_at_mut_unchecked(2);
assert_eq!(&*left, [1, 0]);
assert_eq!(&*right, [3, 0, 5, 6]);
(&mut *left)[1] = 2;
(&mut *right)[1] = 4;
}
assert_eq!(v, [1, 2, 3, 4, 5, 6]);
Runconst: unstable · sourcepub fn as_mut_ptr(self) -> *mut T
🔬This is a nightly-only experimental API. (slice_ptr_get
#74265)
pub fn as_mut_ptr(self) -> *mut T
slice_ptr_get
#74265)sourcepub unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Outputwhere
I: SliceIndex<[T]>,
🔬This is a nightly-only experimental API. (slice_ptr_get
#74265)
pub unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Outputwhere
I: SliceIndex<[T]>,
slice_ptr_get
#74265)Returns a raw pointer to an element or subslice, without doing bounds checking.
Calling this method with an out-of-bounds index or when self
is not dereferenceable
is undefined behavior even if the resulting pointer is not used.
§Examples
#![feature(slice_ptr_get)]
let x = &mut [1, 2, 4] as *mut [i32];
unsafe {
assert_eq!(x.get_unchecked_mut(1), x.as_mut_ptr().add(1));
}
Runconst: unstable · sourcepub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]>
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a shared slice to
the value wrapped in Some
. In contrast to as_ref
, this does not require
that the value has to be initialized.
For the mutable counterpart see as_uninit_slice_mut
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be valid for reads for
ptr.len() * mem::size_of::<T>()
many bytes, and it must be properly aligned. This means in particular:-
The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.
-
The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as
data
for zero-length slices usingNonNull::dangling()
.
-
-
The total size
ptr.len() * mem::size_of::<T>()
of the slice must be no larger thanisize::MAX
. See the safety documentation ofpointer::offset
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except insideUnsafeCell
).
This applies even if the result of this method is unused!
See also slice::from_raw_parts
.
const: unstable · sourcepub unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]>
🔬This is a nightly-only experimental API. (ptr_as_uninit
#75402)
pub unsafe fn as_uninit_slice_mut<'a>(self) -> Option<&'a mut [MaybeUninit<T>]>
ptr_as_uninit
#75402)Returns None
if the pointer is null, or else returns a unique slice to
the value wrapped in Some
. In contrast to as_mut
, this does not require
that the value has to be initialized.
For the shared counterpart see as_uninit_slice
.
§Safety
When calling this method, you have to ensure that either the pointer is null or all of the following is true:
-
The pointer must be valid for reads and writes for
ptr.len() * mem::size_of::<T>()
many bytes, and it must be properly aligned. This means in particular:-
The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.
-
The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as
data
for zero-length slices usingNonNull::dangling()
.
-
-
The total size
ptr.len() * mem::size_of::<T>()
of the slice must be no larger thanisize::MAX
. See the safety documentation ofpointer::offset
. -
You must enforce Rust’s aliasing rules, since the returned lifetime
'a
is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.
This applies even if the result of this method is unused!
See also slice::from_raw_parts_mut
.
Trait Implementations§
source§impl<T: ?Sized> Ord for *const T
impl<T: ?Sized> Ord for *const T
source§impl<T: ?Sized> Ord for *mut T
impl<T: ?Sized> Ord for *mut T
source§impl<T: ?Sized> PartialOrd for *const T
impl<T: ?Sized> PartialOrd for *const T
source§fn le(&self, other: &*const T) -> bool
fn le(&self, other: &*const T) -> bool
self
and other
) and is used by the <=
operator. Read more