# Julia alignas: is there a way to specify the alignment of Julia objects in memory?

**URL:** <https://discourse.julialang.org/t/julia-alignas-is-there-a-way-to-specify-the-alignment-of-julia-objects-in-memory/57501>\
**Category:** General Usage\
**Tags:** question, simd, array, vector, cache\
**Created:** [March 18, 2021, 10:40pm UTC](https://discourse.julialang.org/t/julia-alignas-is-there-a-way-to-specify-the-alignment-of-julia-objects-in-memory/57501 "2021-03-18T22:40:59Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![stevengj](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stevengj/32/71_2.png) [@stevengj](https://discourse.julialang.org/u/stevengj)\
**Post date:** [March 18, 2021, 11:04pm UTC](https://discourse.julialang.org/t/julia-alignas-is-there-a-way-to-specify-the-alignment-of-julia-objects-in-memory/57501/2 "2021-03-18T23:04:42Z")

</div>

You can allocate an `Array` using [`posix_memalign`](https://pubs.opengroup.org/onlinepubs/9699919799/functions/posix_memalign.html), for example:

```julia
function alignedvec(::Type{T}, n::Integer, alignment::Integer=sizeof(Int)) where {T}
    @static if Sys.iswindows()
        return Array{T}(undef, n)
    else
        ispow2(alignment) || throw(ArgumentError("$alignment is not a power of 2"))
        alignment ≥ sizeof(Int) || throw(ArgumentError("$alignment is not a multiple of $(sizeof(Int))"))
        isbitstype(T) || throw(ArgumentError("$T is not a bitstype"))
        p = Ref{Ptr{T}}()
        err = ccall(:posix_memalign, Cint, (Ref{Ptr{T}}, Csize_t, Csize_t), p, alignment, n*sizeof(T))
        iszero(err) || throw(OutOfMemoryError())
        return unsafe_wrap(Array, p[], n, own=true)
    end
end

```

but [`posix_memalign` is unavailable on Windows](https://stackoverflow.com/questions/33696092/whats-the-correct-replacement-for-posix-memalign-in-windows) so it has to fall back to unaligned memory in that case (Julia defaults to 16-byte alignment). If the alignment is just an optimization, however, that may not be so terrible.

It doesn’t seem crazy to add an `alignment` keyword argument to the `Array` constructor, but the difficulty of supporting Windows is (as usual) a pain point. (You can have an aligned-memory allocator on Windows, but then you can’t use `free` to deallocate it, and so you’d need lower-level hacks for garbage-collection to work.)

---

_[View the full topic](https://discourse.julialang.org/t/julia-alignas-is-there-a-way-to-specify-the-alignment-of-julia-objects-in-memory/57501)._