# Unsafe\_wrap C pointer after first element

**URL:** <https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723>\
**Category:** General Usage\
**Created:** [April 17, 2020, 12:02am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723 "2020-04-17T00:02:57Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [April 17, 2020, 12:02am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723/1 "2020-04-17T00:02:57Z")

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Hi,

How to wrap the contents of a C pointer, not from its first element, which is done with

```julia
unsafe_wrap(Array, ptr, 100)

```

but for example, from its 50th element?

With C address it would be

```julia
unsafe_wrap(Array, &ptr[49], 50)

```

Thanks

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<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [April 17, 2020, 12:24am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723/2 "2020-04-17T00:24:30Z")

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Just do arithmetic on the pointer. The only difference is that you have to include the eltype size explicitly.

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<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [April 17, 2020, 12:32am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723/3 "2020-04-17T00:32:12Z")

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> [@yuyichao](#):
>
> Just do arithmetic on the pointer.

Thanks, but that was actually my question. How do I do that?  
(`ptr` is a pointer to double)

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**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [April 17, 2020, 12:33am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723/4 "2020-04-17T00:33:32Z")

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It should just be `unsafe_wrap(Array, ptr+49*8, 50)`

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<div class="post-metadata">

**Author:** ![joa-quim](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/joa-quim/32/227_2.png) [@joa-quim](https://discourse.julialang.org/u/joa-quim)\
**Post date:** [April 17, 2020, 12:37am UTC](https://discourse.julialang.org/t/unsafe-wrap-c-pointer-after-first-element/37723/5 "2020-04-17T00:37:31Z")

</div>

> [@Oscar\_Smith](#):
>
> ptr+49\*8

That’s simple indeed.  
Many thanks