Function alloc::slice::from_raw_parts

pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T]

Forms a slice from a pointer and a length.

The len argument is the number of elements, not the number of bytes.

Safety

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

• data must be valid for reads for len * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:

  • The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects. See below for an example incorrectly not taking this into account.
  • data must be non-null and aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().

• data must point to len consecutive properly initialized values of type T.

• The memory referenced by the returned slice must not be mutated for the duration of lifetime 'a, except inside an UnsafeCell.

• The total size len * mem::size_of::<T>() of the slice must be no larger than isize::MAX, and adding that size to data must not “wrap around” the address space. See the safety documentation of pointer::offset.

Caveat

The lifetime for the returned slice is inferred from its usage. To prevent accidental misuse, it’s suggested to tie the lifetime to whichever source lifetime is safe in the context, such as by providing a helper function taking the lifetime of a host value for the slice, or by explicit annotation.

Examples

use std::slice;

// manifest a slice for a single element
let x = 42;
let ptr = &x as *const _;
let slice = unsafe { slice::from_raw_parts(ptr, 1) };
assert_eq!(slice[0], 42);

Incorrect usage

The following join_slices function is unsound ⚠️

use std::slice;

fn join_slices<'a, T>(fst: &'a [T], snd: &'a [T]) -> &'a [T] {
    let fst_end = fst.as_ptr().wrapping_add(fst.len());
    let snd_start = snd.as_ptr();
    assert_eq!(fst_end, snd_start, "Slices must be contiguous!");
    unsafe {
        // The assertion above ensures `fst` and `snd` are contiguous, but they might
        // still be contained within _different allocated objects_, in which case
        // creating this slice is undefined behavior.
        slice::from_raw_parts(fst.as_ptr(), fst.len() + snd.len())
    }
}

fn main() {
    // `a` and `b` are different allocated objects...
    let a = 42;
    let b = 27;
    // ... which may nevertheless be laid out contiguously in memory: | a | b |
    let _ = join_slices(slice::from_ref(&a), slice::from_ref(&b)); // UB
}

FFI: Handling null pointers

In languages such as C++, pointers to empty collections are not guaranteed to be non-null. When accepting such pointers, they have to be checked for null-ness to avoid undefined behavior.

use std::slice;

/// Sum the elements of an FFI slice.
///
/// # Safety
///
/// If ptr is not NULL, it must be correctly aligned and
/// point to `len` initialized items of type `f32`.
unsafe extern "C" fn sum_slice(ptr: *const f32, len: usize) -> f32 {
    let data = if ptr.is_null() {
        // `len` is assumed to be 0.
        &[]
    } else {
        // SAFETY: see function docstring.
        unsafe { slice::from_raw_parts(ptr, len) }
    };
    data.into_iter().sum()
}

// This could be the result of C++'s std::vector::data():
let ptr = std::ptr::null();
// And this could be std::vector::size():
let len = 0;
assert_eq!(unsafe { sum_slice(ptr, len) }, 0.0);