Function core::intrinsics::copy

pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize)

Copies count * size_of::<T>() bytes from src to dst. The source and destination may overlap.

If the source and destination will never overlap, copy_nonoverlapping can be used instead.

copy is semantically equivalent to C’s memmove, but with the argument order swapped. Copying takes place as if the bytes were copied from src to a temporary array and then copied from the array to dst.

Safety

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

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

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

• Both src and dst must be properly aligned.

Like read, copy creates a bitwise copy of T, regardless of whether T is Copy. If T is not Copy, using both the values in the region beginning at *src and the region beginning at *dst can violate memory safety.

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

Examples

Efficiently create a Rust vector from an unsafe buffer:

use std::ptr;

/// # Safety
///
/// * `ptr` must be correctly aligned for its type and non-zero.
/// * `ptr` must be valid for reads of `elts` contiguous elements of type `T`.
/// * Those elements must not be used after calling this function unless `T: Copy`.
unsafe fn from_buf_raw<T>(ptr: *const T, elts: usize) -> Vec<T> {
    let mut dst = Vec::with_capacity(elts);

    // SAFETY: Our precondition ensures the source is aligned and valid,
    // and `Vec::with_capacity` ensures that we have usable space to write them.
    ptr::copy(ptr, dst.as_mut_ptr(), elts);

    // SAFETY: We created it with this much capacity earlier,
    // and the previous `copy` has initialized these elements.
    dst.set_len(elts);
    dst
}