Primitive Type pointer [−]
Raw, unsafe pointers, *const T, and *mut T.
Working with raw pointers in Rust is uncommon, typically limited to a few patterns.
Use the null function to create null pointers, and the is_null method
of the *const T type to check for null. The *const T type also defines
the offset method, for pointer math.
Common ways to create raw pointers
1. Coerce a reference (&T) or mutable reference (&mut T).
fn main() {
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;
}let my_num: i32 = 10; let my_num_ptr: *const i32 = &my_num; let mut my_speed: i32 = 88; let my_speed_ptr: *mut i32 = &mut my_speed;
To get a pointer to a boxed value, dereference the box:
fn main() { 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; }let my_num: Box<i32> = Box::new(10); let my_num_ptr: *const i32 = &*my_num; let mut my_speed: Box<i32> = Box::new(88); let my_speed_ptr: *mut i32 = &mut *my_speed;
This does not take ownership of the original allocation and requires no resource management later, but you must not use the pointer after its lifetime.
2. Consume a box (Box<T>).
The into_raw function consumes a box and returns
the raw pointer. It doesn't destroy T or deallocate any memory.
let my_speed: Box<i32> = Box::new(88); let my_speed: *mut i32 = Box::into_raw(my_speed); // By taking ownership of the original `Box<T>` though // we are obligated to put it together later to be destroyed. unsafe { drop(Box::from_raw(my_speed)); }
Note that here the call to drop is for clarity - it indicates
that we are done with the given value and it should be destroyed.
3. Get it from C.
#![feature(libc)] extern crate libc; use std::mem; fn main() { unsafe { let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>() as libc::size_t) as *mut i32; if my_num.is_null() { panic!("failed to allocate memory"); } libc::free(my_num as *mut libc::c_void); } }extern crate libc; use std::mem; fn main() { unsafe { let my_num: *mut i32 = libc::malloc(mem::size_of::<i32>() as libc::size_t) as *mut i32; if my_num.is_null() { panic!("failed to allocate memory"); } libc::free(my_num as *mut libc::c_void); } }
Usually you wouldn't literally use malloc and free from Rust,
but C APIs hand out a lot of pointers generally, so are a common source
of raw pointers in Rust.
Methods
impl<T> *mut T where T: ?Sized
fn is_null(self) -> bool1.0.0
Returns true if the pointer is null.
Examples
Basic usage:
fn main() { let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); assert!(!ptr.is_null()); }let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); assert!(!ptr.is_null());
unsafe fn as_ref(self) -> Option<&'a T>1.9.0
Returns None if the pointer is null, or else returns a reference to
the value wrapped in Some.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
Additionally, the lifetime 'a returned is arbitrarily chosen and does
not necessarily reflect the actual lifetime of the data.
Examples
Basic usage:
fn main() { let val: *mut u8 = &mut 10u8 as *mut u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } } }let val: *mut u8 = &mut 10u8 as *mut u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } }
unsafe fn offset(self, count: isize) -> *mut T1.0.0
Calculates the offset from a pointer. count is in units of T; e.g. a
count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.
Safety
The offset must be in-bounds of the object, or one-byte-past-the-end.
Otherwise offset invokes Undefined Behavior, regardless of whether
the pointer is used.
Examples
Basic usage:
fn main() { let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); unsafe { println!("{}", *ptr.offset(1)); println!("{}", *ptr.offset(2)); } }let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); unsafe { println!("{}", *ptr.offset(1)); println!("{}", *ptr.offset(2)); }
unsafe fn as_mut(self) -> Option<&'a mut T>1.9.0
Returns None if the pointer is null, or else returns a mutable
reference to the value wrapped in Some.
Safety
As with as_ref, this is unsafe because it cannot verify the validity
of the returned pointer, nor can it ensure that the lifetime 'a
returned is indeed a valid lifetime for the contained data.
Examples
Basic usage:
fn main() { let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr(); }let mut s = [1, 2, 3]; let ptr: *mut u32 = s.as_mut_ptr();
impl<T> *const T where T: ?Sized
fn is_null(self) -> bool1.0.0
Returns true if the pointer is null.
Examples
Basic usage:
fn main() { let s: &str = "Follow the rabbit"; let ptr: *const u8 = s.as_ptr(); assert!(!ptr.is_null()); }let s: &str = "Follow the rabbit"; let ptr: *const u8 = s.as_ptr(); assert!(!ptr.is_null());
unsafe fn as_ref(self) -> Option<&'a T>1.9.0
Returns None if the pointer is null, or else returns a reference to
the value wrapped in Some.
Safety
While this method and its mutable counterpart are useful for null-safety, it is important to note that this is still an unsafe operation because the returned value could be pointing to invalid memory.
Additionally, the lifetime 'a returned is arbitrarily chosen and does
not necessarily reflect the actual lifetime of the data.
Examples
Basic usage:
fn main() { let val: *const u8 = &10u8 as *const u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } } }let val: *const u8 = &10u8 as *const u8; unsafe { if let Some(val_back) = val.as_ref() { println!("We got back the value: {}!", val_back); } }
unsafe fn offset(self, count: isize) -> *const T1.0.0
Calculates the offset from a pointer. count is in units of T; e.g. a
count of 3 represents a pointer offset of 3 * sizeof::<T>() bytes.
Safety
Both the starting and resulting pointer must be either in bounds or one byte past the end of an allocated object. If either pointer is out of bounds or arithmetic overflow occurs then any further use of the returned value will result in undefined behavior.
Examples
Basic usage:
fn main() { let s: &str = "123"; let ptr: *const u8 = s.as_ptr(); unsafe { println!("{}", *ptr.offset(1) as char); println!("{}", *ptr.offset(2) as char); } }let s: &str = "123"; let ptr: *const u8 = s.as_ptr(); unsafe { println!("{}", *ptr.offset(1) as char); println!("{}", *ptr.offset(2) as char); }