# Function std::ptr::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 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 = 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! ⚠️
    }

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## 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]);

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