Function core::intrinsics::write_bytes

1.0.0 (const: unstable) · source · []
pub unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize)
Expand description

Sets count * size_of::<T>() bytes of memory starting at dst to val.

write_bytes is similar to C’s memset, but sets count * size_of::<T>() bytes to val.


Behavior is undefined if any of the following conditions are violated:

  • dst must be valid for writes of count * size_of::<T>() bytes.

  • dst must be properly aligned.

Note that even if the effectively copied size (count * size_of::<T>()) is 0, the pointer must be non-null and properly aligned.

Additionally, note that changing *dst in this way can easily lead to undefined behavior (UB) later if the written bytes are not a valid representation of some T. For instance, the following is an incorrect use of this function:

unsafe {
    let mut value: u8 = 0;
    let ptr: *mut bool = &mut value as *mut u8 as *mut bool;
    let _bool =; // This is fine, `ptr` points to a valid `bool`.
    ptr.write_bytes(42u8, 1); // This function itself does not cause UB...
    let _bool =; // ...but it makes this operation UB! ⚠️


Basic usage:

use std::ptr;

let mut vec = vec![0u32; 4];
unsafe {
    let vec_ptr = vec.as_mut_ptr();
    ptr::write_bytes(vec_ptr, 0xfe, 2);
assert_eq!(vec, [0xfefefefe, 0xfefefefe, 0, 0]);