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.

The copy is “untyped” in the sense that data may be uninitialized or otherwise violate the requirements of T. The initialization state is preserved exactly.

Safety

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

• src must be valid for reads of count * size_of::<T>() bytes, and must remain valid even when dst is written for count * size_of::<T>() bytes. (This means if the memory ranges overlap, the two pointers must not be subject to aliasing restrictions relative to each other.)

• dst must be valid for writes of count * size_of::<T>() bytes, and must remain valid even when src is read for 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
}