Function std::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
.
§Safety
Behavior is undefined if any of the following conditions are violated:
-
dst
must be valid for writes ofcount * 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 = ptr.read(); // This is fine, `ptr` points to a valid `bool`.
ptr.write_bytes(42u8, 1); // This function itself does not cause UB...
let _bool = ptr.read(); // ...but it makes this operation UB! ⚠️
}
Run§Examples
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]);
Run